Heterostructures and superlattices
A method based on the weak-binding
approximation is proposed for calculating the energy spectrum and the carrier
wave functions in semiconductor heterostructures. It is shown that this
method can be effectively used for a wide range of heterostructures in
the absence and in the presence of the external magnetic field oriented
along the axis of the heterostructure. The structural features of the hole
spectrum in superlattices consisting of a very thin layer of one semiconductor
and a thick layer of another semiconductor are studied. It is shown that
in such superlattices the hole wave function is a superposition of several
bulk states of light and heavy holes in which their mixing at the heterojunction
is taken into account. The hole spectrum of the superlattices, which consist
of asymmetric triangular wells, is calculated and analyzed.
A multi-component structure in the
excitonic reflectivity line shape originating from large-scale monolayer-thick
fluctuations within short-period GaAs/AlAs superlattices has been revealed
and studied. The evolution of the reflectivity and photoluminescence spectra
with changing superlattice period, from an integral number N of monolayers
to the next integral number, N + 1, has been followed. In contrast to a
reflectivity spectrum, the intensity and shape of the excitonic photoluminescence
line shape is shown to be governed by the dynamics of localized-state occupation.
The localized states which appear in
a superlattice as a result of thickness variations of barriers or quantum
wells are calculated in the framework of the tight-binding approximation.
We obtain very simple formulas in terms of single-quantum-well parameters.
There is always a localized state for an enlarged or narrowed quantum well.
Two localized states exist in the case of decreased barrier thickness.
The range of validity of the formulas is discussed as a function of the
superlattice period.
This paper considers how the transformation
of heavy holes into boundary states and then into light holes influences
the tunnel transparency of a single barrier in a semiconductor heterostructure.
The tunnel transparency is shown to be of a resonant nature, the barrier
becoming absolutely transparent at definite angles of incidence of the
heavy hole onto the heteroboundary.
A detailed analysis is made of the
surface states at an ideal heterojunction of the GaAs/AlxGa1-xAs
type when the spin-orbit interaction is of arbitrary strength. A strong
dependence of the dispersion, localization, and quality of the surface
states on the spin-orbit interaction is demonstrated.
It is shown that a redistribution of
an electric field within the i-type layer of P-i-N heterostructures due
to the drift of photocarriers formed as a result of electroabsorption of
light gives rise to a strong optical nonlinearity including the appearance
of an optical bistability. In principle, such a P-i-N heterostructure can
be used to ensure simultaneous operation over its area of a large number
of components of the self-electrooptic-effect device (SEED) type without
separation of the heterostructure in two separate parts: each independent
component can be selected quite simply by its own illumination spot.
A calculation is reported of the electrical
conductivity tensor of a semiconductor superlattice in a magnetic field
which is perpendicular to the superlattice axis and which creates intraminiband
Landau levels; it is assumed also that the superlattice is subjected to
a weak electric field. The Hall resistivity of the superlattice is found
to be less than that of ordinary semiconductors and does not obey the laws 
and 
;
moreover, the magnetoresistance is high along the superlattice axis; it
is negative along the superlattice layers.
Other publications
[1]
Internal
redistribution of the electric field and optical non-linearity in p-i-p
heterostructure by electroabsorption of light
D. M. Butusov, G. G. Gotsadze, B. S. Ryvkin, R. A. Suris
Proc. SPIE, vol. 1280, pp. 238-244, 1990
[2]
Heavy-hole
exciton transport in short-period superlattices
P. S. Kop'ev, R. A. Suris, I. N. Uraltsev, A. M Vasiliev
Sol. State Commun. vol. 72, no. 5, pp. 401-404, Nov., 1989
Terahertz emission accompanying heating
of two-dimensional electrons by a strong electric field applied along size-quantized
GaAs/AlGaAs layers is observed and investigated. The emission is due to
indirect optical transitions of hot electrons in the bottom size-quantization
band. The experimentally obtained emission spectra are compared with the
spectra calculated taking into account scattering of electrons by optical
phonons, impurities, and interfacial roughness and electron-electron scattering.
Satisfactory agreement is obtained. The temperature of the hot electrons
is determined from a comparison of the spectra.
It is shown that in a semiconductor
with quantum wells intraband absorption of long-wavelength radiation due
to Coulomb interaction of the electrons is possible as a result of band
nonparabolicity. Analytical expressions for the absorption are found for
the limiting cases of non degenerate and strongly degenerate, two-dimensional
electron, gas. At high carrier densities the absorption due to electron-electron
interaction can be much stronger than absorption due to electron-phonon
interaction.
A combined exciton-cyclotron resonance
is found in photoluminescence excitation and reflectivity spectra of semiconductor
quantum wells containing an electron gas of low density. In external magnetic
fields, an incident photon creates an exciton in the ground state and simultaneously
excites one of the resident electrons from the lowest to one of the upper
Landau levels. A theoretical model is developed, which gives a good quantitative
description of the energy position and the intensity of the combined exciton-cyclotron
resonance.
A combined exciton-cyclotron resonance
line is found in photoluminescence excitation and reflectivity spectra
of semiconductor quantum wells with electron gas of low density. This line
appears in a magnetic field due to the combine transition when an incident
photon creates an exciton in the ground state and simultaneously excites
an electron between Landau levels. An inelastic process of exciton-electron
scattering is found in quantum wells under external magnetic fields. The
process is spin-dependent and is determined by the exchange interaction
of excitons with free electrons. A theoretical model is developed to describe
these effects.
The momentum and spin distribution
functions for electrons excited by polarized light incident normally to
the heterostructure plane are determined for a quantum well with infinitely
high walls. It is shown that for a low ratio of the light and heavy hole
masses, an anomalously rapid growth of momentum alignment and a decrease
of electron spin orientation occur with increase of their energy of motion
in the plane of the well. The dependences on the exciting photon energy
of the plane and circular polarizations of the hot luminescence in the
pumping direction at it shortwave edge are found.
A theoretical investigation is made
of the influence of valence-band corrugations on the linear polarization
of the photoluminescence emitted by hot electrons in a symmetric quantum
well. The degree of anisotropy of the polarization of the luminescence
resulting from band-band recombination is shown to be governed by the photon
energy of the exciting light and by the temperature of holes. When hot
electrons recombine with holes at acceptor levels the degree of the polarization
anisotropy changes greatly within the limits of a zero-phonon peak.
The depolarization of hot photoluminescence emitted in a magnetic field as a result of recombination of electrons with holes at acceptors has been analyzed. The effect of the valence-band warping on the Hanle curve, when different assumptions are made about the structure of an acceptor in a quantum well, has been studied.
Other publications
[1]
Depolarization
of the photoluminescence as a result of emission of optical phonons by
hot electrons in quantum wells
M. E. Portnoi
Semiconductors, vol. 27, no. 3, pp. 294-295, Mar. 1993
[2]
Theory
of Optical Orientation and Alignment in Quantum Wells
I. A. Merkulov, V. I. Perel, and M. E. Portnoi
Superlat. Microstruct., vol. 10, no. 3, pp. 371-374, Mar. 1991
Auger recombination
in bulk semiconductors and quantum wells
The main mechanisms for the Auger recombination
of nonequilibrium carriers in semiconductor quantum-well heterostructures
are investigated. It is shown for the first time that there are three fundamentally
different Auger recombination mechanisms in quantum wells: 1) a threshold-free
mechanism, 2) a quasithreshold mechanism, and 3) a threshold mechanism.
The rate of the threshold-free process has a weak temperature dependence.
The rate of the quasithreshold Auger process exhibits an exponential temperature
dependence. However, the threshold energy depends significantly on the
quantum-well width and is close to zero for narrow quantum wells. It is
shown that the threshold-free and quasithreshold processes are dominant
in fairly narrow quantum wells, while the quasithreshold and threshold
Auger processes are dominant in wide quantum wells. The limiting transition
to a three-dimensional Auger process is accomplished for a quantum-well
width tending to infinity. The value of the critical quantum-well width,
at which the quasithreshold and threshold Auger processes combine to form
a single three-dimensional Auger recombination process, is found.
The principal mechanisms of Auger recombination
of nonequilibrium carriers in semiconductor heterostructures with quantum
wells (QWs) are investigated. It is shown for the first time that there
exist three fundamentally different Auger recombination mechanisms of (i)
thresholdless, (ii) quasi-threshold, and (iii) threshold types. The rate
of the thresholdless Auger process depends on temperature only slightly.
The threshold energy of the quasi-threshold process essentially varies
with QW width and is close to zero for narrow QWs. It is shown that the
thresholdless and the quasi-threshold Auger processes dominate in narrow
QWs, while the threshold and the quasi-threshold processes prevail in wide
QWs. The limiting case of a three-dimensional (3D) Auger process is reached
for infinitely wide QWs. The critical QW width is found at which the quasi-threshold
and threshold Auger processes merge into a single 3D Auger process. Also
studied is phonon-assisted Auger recombination in QWs. It is shown that
for narrow QWs the act of phonon emission becomes resonant, which in turn
increases substantially the coefficient of phonon-assisted Auger recombination.
The effect of strain on thresholdless
Auger recombination in quantum wells has been studied theoretically. A
detailed analysis of overlap integrals bet -ween the initial and final
states of carriers has been carried out. It is shown that the strain affects
both qualitatively and quantitatively the overlap integral between the
electron and hole states. The Auger recombination coefficient is calculated
for InAlAsSb quantum well and its dependence on quantum well parameters,
strain, and temperature is analyzed.
A non threshold mechanism for auger
recombination of nonequilibrium carrier in quantum wells with strained
layers is investigated theoretically. It is shown that dependence of the
Auger recombination rate on the magnitude of the strain and the height
of the heterobarriers forelectrons and holes can be analyzed only by calculating
the overlap integrals between initial and final particle state microscopically.
In quantum wells with strained layers the presence of strain affects qualitatively
the electron-hole overlap integral. The dependence of the Auger recombination
rate on the quantum well parameters, the magnitude of the stress, and temperature
are analyzed for heterostructures based on InGaAsP/InP andInGaAlAs/InP.
Thresholdless Auger recombination in
InAlAsSb strained quantum wells is studied theoretically.Analytical formulas
for the Auger transi-tion matrix element have been derived in the framework
of the Kane model. A detailed analysis of overlap integrals between initial
and fi-nal states of carriere has shown that the strain and lightheavy
hole mixing affect both qualitatively and quantitatively the overlap integ-ral
between the electron and hole states. The Auger recombination coefficient
is found to have a strong dependence on strain, quantum well width, and
emission wavelength, but weak dependence on tem-perature. The Auger coefficient
temperature depen-dence is shown to be very sensitive to the bandgap variation
with temperature.
A theory of the recombination of noneqilibrium
carriers in type-II semiconductor heterostructures with quantum wells is
devised for the first time. Analitical expressions for the radiative recombination
and Auger recombination rates are obtained. It is shown that the Auger
recombination mechanisms in type-I and type-II heterostructures differ
fundamentally. A fundamentally new result is obtained: the Auger recombination
rate has a minimum at certain values of the heterostructure parameters.
Such effective suppression of the Auger recombination processes in type-II
heterostructures is associated with the short-range character of the Coulomb
interaction of the electrons participating in the recombination process.
It is also that the radiative recombination processes take place with equal
efficiency in type-I and type-II heterostructures and that their rates
are comparable. The possibility of regulating the Auger recombination rate
in type-II heterostructures by varing the parameters of the structure is
demonstrated. The effectiveness of using type-II heterostructures as opposed
to type-I heterostructures to create optoelectronic devices is also demonstrated.
The mechanism for Auger recombination
in type-Ii heterostructures is studied theoretically for the first time.
The rate of this recombination is a power-law function of the temperature,
not an exponential function, as it is in the bulk material. A suppression
of Auger recombination in type-II heterostructures is Predictrd. This suppression
would stem from the short-range nature of the Coulomb interaction of the
electronnns involved in the recombination. A suppression of Auger recombination
at type-II heterostructures has recently been observed experimentally.
The mechsnism of Auger recombination
in type-II heterostructures is studied theoretically. It is shown that
the Auger recombination rate is a power function of temperature rather
than an exponential function as in bulk materials. The feasibility of suppression
of the Auger recombination process in type-II heterostructures is demonstrated.
The possibility of controlling the Auger recombination rate is shown to
be very important for development of optoelectronic device with improved
characteristics.
Auger recombination in narrow-bandgap
semiconductors in the presense of a quantizing magnetic field is studied
theoretically. It is shown that for certain values of the magnetic field
the Auger process is a thresholdless process. The absence of a threshold
is related directly to Landau transitions of electrons from a lower Landau
level into a highly excited level is of a resonance character. The Auger
recombination rate is an oscillating function of the magnetic fiel. The
effect of Auger recombination process on the emission spectra of narrow-bandgap
semiconductors at low temperatures in quantizing magnetic field is analyzed.
Recombination rates are calculated for InSb and HgCdTe at different temperatures.
The effect of a heteroboundary on nonradiative recombination of non -equilibrium carriers in semiconductor structures was investigated. It is shown that the presence of the heteroboundary results in the appearance of a new no-threshold mechanism of Auger recombination: an electron-hole pair is annihilated in the volume of the narrow-band semiconductor and a fast Auger particle is ejected from the subbarrier-motion region located next to the heteroboundary. It was established that the new recombination mecha -nism has a week power-law temperature dependence and, owing to the efficient long-range action, it is the dominant process of nonradiative recom -bination were calculated for heterostructures with different values of the parameters.
AB: The variable-phase approach is
applied to scattering and bound states in an attractive Coulomb potential,
statically screened by a two-dimensional (2D) electron gas. A 2D formulation
of Levinson's theorem is used for bound-state counting and a hitherto undiscovered,
simple relationship between the screening length and the number of bound
states is found. As the screening length is increased, sets of bound states
with differing quantum numbers appear degenerately.
AB: We consider scattering state contributions to the partition function of a two-dimensional (2D) plasma in addition to the bound-state sum. A partition function continuity requirement is used to provide a statistical mechanical heuristic proof of Levinson's theorem in two dimensions. We show that a proper account of scattering eliminates singularities in thermodynamic properties of the nonideal 2D gas caused by the emergence of additional bound states as the strength of an attractive potential is increased. The bound-state contribution to the partition function of the 2D gas, with a weak short-range attraction between its particles, is found to vanish logarithmically as the binding energy decreases. A consistent treatment of bound and scattering states in a screened Coulomb potential allowed us to calculate the quantum-mechanical second virial coefficient of the dilute 2D electron-hole plasma and to establish the difference between the nearly ideal electron-hole gas in GaAs and the strongly correlated exciton/free-carrier plasma in wide-gap semiconductors such as ZnSe or GaN.
Other publications
[1]
Scattering
of two-dimensional particles by a short-range potential
M. E. Portnoi
Sov. Tech. Phys. Lett., vol. 14, no. 7, pp. 547-548, July 1988
AB: We propose a model of an anyon
exciton consisting of a hole and several anyons, and apply it to the spectroscopy
of an incompressible quantum liquid. Fractionalization of the electron
charge makes properties of such entities quite different from those of
usual magnetoexcitons. The model describes a number of properties established
by few-particle simulations, including an abrupt change in emission vs
electron-hole asymmetry of the system. The attractive field of a hole may
eliminate the hard core constraint for anyons. The effect of exciton-magnetoroton
coupling is discussed.
AB: We propose a model of an anyon
exciton (AE) consisting of a hole and several anyons [E. I. Rashba and
M. E. Portnoi, Bull. Amer. Phys. Soc. 38, 136 (1993)] and apply it to the
spectroscopy of an incompressible quantum liquid (IQL). Fractionalization
of the electron charge makes properties of such entities different from
those of usual magnetoexcitons. The model describes a number of properties
established by few-particle simulations, including an abrupt change in
the emission spectra vs electron-hole asymmetry of the system.
AB: Excitons acquire a peculiar structure
under the conditions of the fractional quantum Hall effect, since elementary
excitations of Laughlin incompressible quantum liquids possess fractional
electric charges. We discuss the manifestation of the fractional charges
in emission spectra and methods for determining the parameters of these
liquids from the spectra.
AB: A theory of anyon excitons consisting
of a valence hole and three quasielectrons with electric charges -e/3 is
presented. A full symmetry classification of the k=0 states is given, where
k is the exciton momentum. The energy levels of these states are expressed
by quadratures of confluent hypergeometric functions. It is shown that
the angular momentum L of the exciton ground state depends on the distance
between the electron and hole confinement planes and takes the values L=3n,
where n is an integer. With increasing k the electron density shows a spectacular
splitting on bundles. At first a single anyon splits off of the two-anyon
core, and finally all anyons become separated.
AB: Energy spectrum of an exciton and
electron density distribution in it under the fractional quantum Hall regime
are calculated within the framework of an anyon exciton model.
AB: Elementary excitations of incompressible quantum liquids (IQL's) are anyons, i.e., quasiparticles carrying fractional charges and obeying fractional statistics. To find out how the properties of these quasiparticles manifest themselves in the optical spectra, we have developed the anyon exciton model (AEM) and compared the results with the finite-size data for excitons of nu=1/3 and nu=2/3 IQL's. The model considers an exciton as a neutral composite consisting of three quasielectrons and a single hole. The AEM works well when the separation between electron and hole confinement planes, h, is larger than the magnetic length l. In the framework of the AEM an exciton possesses momentum k and two internal quantum numbers, one of which can be chosen as the angular momentum, L, of the k=0 state. Existence of the internal degrees of freedom results in the multiple branch energy spectrum, crater-like electron density shape and 120 degrees density correlations for k=0 excitons, and the splitting of the electron shell into bunches for non-zero k excitons. For h larger than 2l the bottom states obey the superselection rule L=3m (m are integers starting from 2), all of them are hard core states. For h nearly 2l there is one-to-one correspondence between the low-energy spectra found for the AEM and the many-electron exciton spectra of the nu=2/3 IQL, whereas some states are absent from the many-electron spectra of the nu=1/3 QL. We argue that this striking difference in the spectra originates from the different populational statistics of the quasielectrons of charge conjugate IQL's and show that the proper account of the statistical requirements eliminates excessive states from the spectrum. Apparently, this phenomenon is the first manifestation of the exclusion statistics in the anyon bound states.
The threshold characteristics of InGaN
multiquantum well lasers are investigated. A detailed analysis of the dependence
of the threshold current on the quantum-well parameters and the temperature
is performed. It is shown that, in comparison with long-wavelength lasers,
InGaN lasers have a qualitatively different dependence of the threshold
current on the quantum-well parameters (well width and number of quantum
wells). The possibility of optimizing a InGaN laser structure is analyzed
with the aim of improving the threshold characteristics and increasing
the peak radiated power.
A microscopic quantum-mechanical analysis
of the intervalence band absorption of radiation (IVA) with hole transition
into the spin-orbit split-off band has been made. It was found that IVA
can heavily influence the threshold characteristics and quantum efficiency
of heterolasers based on InAs. A detailed study of the threshold characteristics
as functions of temperature and heterostructure parameters has been analyzed
taking into account IVA.
The spatial carrier distribution function
is derived for a single heterojunction at high injection rates. It is found
that, because of the inhomogeneous distribution, a thin layer exists in
the n-type region near the interface. The width of this layer is
about the same as that of the active region and its permittivity has a
negative imaginary part. The wave field is localized in this layer. A relationship
is derived between the carrier density and the current density at the lasing
threshold.
A theoretical investigation is made
of fluctuation-induced excitation of side longitudinal modes in the emission
spectra of semiconductor lasers, including those with an external mirror.
It is shown that nonlinear refraction of light in the active region of
a semiconductor laser may result in a noise redistribution of the radiation
between longitudinal resonator modes and can be responsible for the multimode
nature of the average emission spectrum. An analysis is made of the influence
of selectivity of an external mirror on the stability of cw operation,
minimum line width, and mode composition of the emission spectra of semiconductor
lasers. The conditions for maximum narrowing of the emission spectrum of
a semiconductor laser with an external selective mirror are identified.
Semiconductor lasers emitting in the mid-infrared (IR) range (2-5 m m) have been intensely investigated recently. Interest in this type of lasers is aroused by outlooks for their extensive application to different branches of science and engineering.
First of all, mid-infrared lasers as components of semiconductor gas analyzers are of great importance in environmental protection and ecology monitoring because of their wavelength range (2-5 m m) where a number of industrial toxic gases have strong absorption lines. Moreover, in this wavelength range there are three atmospheric transmission windows, which enable widespread use of lasers emitting in a spectral window as elements of laser radars and for illumination. Application of mid-infrared lasers to medicine is also very promising.
A number of materials, such as the III-V semiconductor compounds, II-VI and lead-salts, have been used in creating mid-infrared lasers. For example, lead-salt lasers with emission wavelengths of ~4 m m can operate in pulsed mode at the temperatures up to 290 K and in CW mode up to 200 K. The advantages of the compounds III-V over other materials that might be used in creating mid-infrared lasers were analyzed by H. K. Choi and D. W. Turner in 1995.
There are two different approaches to creating mid-infrared lasers based on III-V alloys. They are aimed at developing lasers based on intraband transions (quantum cascade lasers) and lasers using interband transitions. Lasers with interband transitions usually have lower limits of operating temperature than lead-salt lasers. For example, at l ~4 m m, the InAsSb/InAlAsSb quantum-well lasers operate in pulsed mode up to 165 K and in CW mode up to 123 K. A maximum operating temperature of about 180 K for mid-infrared lasers based on the InAsSb(P)/InAsSbP system was obtained in Ioffe Institute (A. H. Baranov, A. N. Imenkov, V. V. Sherstrov, and Yu. P. Yakovlev, 1994).
Mid-infrared III-V lasers based on
interband transitions are studied in this chapter. Up to now, there has
been no lasers of this kind emitting in the wavelength range of l>4
m
m at room temperature. The purpose of this chapter is to study fundamental
physical processes governing the operation of mid-infrared lasers based
on strained quantum wells. We will also study the limits of their operation
at room temperature. We will describe a variety of new effects due to the
carriers-at the interface. The most important of these are: a new non-threshold
channel of Auger recombination, intraband absorption, and leakage. These
and other processes discussed in the present chapter are to be taken into
account in creating mid-infrared diode lasers. And finally, we will propose
a new fundamental approach to development of mid-infrared lasers operating
at room temperature.
The electroluminescence appearing upon
pulsed excitation in a solitary p-GaInAsSb/p-InAs type-II (broken-gap)
heterostructure is investigated at T = 77 K. It is shown that just as in
the case of excitation by a constant current, two emission bands with maxima
corresponding to 384 and 311 meV, respectively, are observed in the spectra.
The half-widths of the emission bands equal 1819 meV. Time-resolved spectroscopy
is used to evaluate the relaxation time of the nonequilibrium carriers,
which amounts to 67 s. The emission spectrum is calculated on the basis
of the Kane model, and the radiative recombination time is estimated. The
theoretical estimates are in reasonable agreement with experiment.
The electroluminescence of the single
unconfined type-II heterojunction p-GaInAsSb/p-InAs was investigated in
the temperature range T = 4. 277 K. As the temperature was reduced below
T = 77 K, the luminescence bands with maxima at 311 meV (band A) and 384
meV (band B) were found to shift toward higher energies. At 4. 2 K, the
short-wave band split into two bands, B1 and B2. These results are explained
in terms of a model involving recombinations of electrons from the conduction
band to an acceptor level of InAs, and also recombinations of electrons
and holes localized in self-consistent quantum wells on either side of
the heterojunction.
The mechanism of interband absorption
of radiation with hole transition into the spin-orbit split-off band, or
so-called intervalence-band absorption (IVA), is subjected to microscopic
analysis. It is shown that this IVA mechanism significantly influences
the threshold characteristics and quantum efficiency of InAs-based heterolasers.
The dependences of the laser threshold characteristics on the temperature
and the parameters of the laser heterostructure are analyzed in detail,
taking into account the new (IVA) channel of interband absorption by holes.
A first observation of electroluminenescence
in typeII broken-gap isotype p-GaInAsSb/ p-InAs single heterojunction
is reported. Two narrow luminescence peaks were observed in the spectral
range of l =3-4 m
m at T=77 K with full width athalf maximum (FWHM) about 10-20 meV.
Spectral position and intensity of these emission bands can be changed
by drive current, and "blue" shift was observed with increasing current.
Intensive spontaneous emission was obtained up to room temperature. It
wasfound that unusual electroluminescence is due to indirect (tunnel) radiative
recom -bination of spatially separatedelectrons and holes localized in
deep adjacent quantum wells at the different sides of the interface. A
new physical approach for the design of mid-infrared lasers using a typeII
broken-ap p-p hetero -junction as an active layer is proposed.
An electroluminescence has been observed
for the time at single p-GaInAsSb/ p-InAs isotypic brokrn-gap
type-II heterojunctions in an external electric field. Two narrow luminescence
peaks, with a full-width at half-maximum ~ 10-20 meV, are observed in the
spectral interval 3-5 m
m at T = 77 K. The spectral position and intensities of these peaks
vary with the pump current. An intense electoluminescence is observrd up
to room temperature. It is shown that these narrow electroluminescence
bands are due to an indirect tunneling radiative recombination of spatially
separeted electrons and holes, which are confined in deep quantum wells
on different sides of the heterojunction. It should be possible to develop
a novel tunable IR light source which uses an isotype p-p broken-gap
type-II heterojunction in its active region.
Experimental studies of the polarization
of the emission from double-hetero -structure InAsSb/InAsSbP lasers show
that in the spontaneous and laser regimes the emission originates near
the heterojunction and is predominantly TM-polarized. It was shown
theoretically that the optical tranitions near the heterojunction are indirect
(in k - space), and that the high degree of TM po -larization
of the emission is associated with light and heavy holes trans -forming
into one another as they interact with the heterojunction.
The temperature dependence of the threshold
current density of double-heterostructure lasers, based on the solid solution
GaInAsSb, with a thin active region is studied theoretically. It is shown
that a new Auger recom -bination mechanism, which is due to the interaction
of current carriers with the heteroboundary, contributes to the threshold
current in double-heterostructures with a thin active region It is also
shown that at low temperatures the new Auger recombination channel is the
main mechanism for non -radiative recombination of carriers. The theoretically
computed temperature dependence of the threshold current density is compared
with the experimental data.
The threshold current of InAsSbP/InAs
and InAsSbP/InAsSb double-hetero -structure lasers were investigated theoretically
and experimentally at tempe -ratures 4. 2-150 K. Theoretical calculations
were made using the exact values of the overlap integrals, occurring in
the expression for the rate of the Auger recombination process involving
the spin-orbit splitt-off band (CHHS pro -cess), and allowing for the nonparabolicity
of the cattier spectrum at high values of the wave vector. Interband radiative
recombination occured in the temperature range 4. 2-150 K, whereas the
dominant process at higher tempe -ratures was CHHS and it governed not
only the quantum efficiency but the upper limit of the working temperatures
of the lasers.
A study is reported of the mechanism
of generation of electromagnetic radiation in a heterojunction laser formed
by direct-gap semiconductors and by injection mechanism. It is shown that,
in the case of direct-gap semiconductors, a new mechanism of generation
of radiation is possible: it results in the emission of photons of energy
less than the band gap and the process is assisted by optical phonons.
A calculation is reported of the gain for transitions for this kind and
a comparison is made with the results of experimental investigations.
The temperature dependence of the threshold
current density was calculated and determined experimentally for double-heterostructure
lasers made of GaInAsSb solid solutions. In the range 77-310 K this temperature
dependence consisted of three sections, corresponding to three mechanisms
of nonequilibrium carrier recombination in the active region. At the temperatures
77-200 K the dependence was J ?T
3/2
and it was governed by the radiative recombination rate. At highter temperatures
(200-300 K) the dependence changed to and
J?T
9/2
was governed by the Auger recombination rate (CHHS process). Above room
temperature the threshold current rose exponentially because of the Auger
CHCC process.
An analytical dependence of the density
of the injection current on the temperature of a semiconductor laser is
determined near the lasing threshold. The ratio of the electron quasi-Fermi
level Fc to the temperature T at the inversion
threshold, and at the lasing threshold is shown to be constant. This constant
is governed solely by the properties of the semiconductor.
The influence of spontaneous fluctuations
on a semiconductor laser emission spectrum is reviewed within the framework
of linearized theory. The main focus of attention is the noise excitation
of side longitudinal modes near and far above the laser threshold. Both
'noise amplifier' and 'developed oscillation' models of this effect are
discussed. Their results are shown to be quite different, at least for
long-cavity lasers: the latter model predicts strong mode interaction within
some frequency region around the main mode, which can lead to an increase
in side-mode power. The theory of spectral linewidth of single-mode compound-cavity
lasers, and noise excitation of external cavity modes for lasers with external
mirror in case of stationary generation are also briefly discussed.
TE and TM optical gains were measured in AlGaAs/GaAs single- quantum-well (SQW) diode lasers at 300 K. Both the TE and TM gain spectra were step-like. The long and short wavelength steps in the experimental spectra were attributed to the onset of transitions from the first (e1) and second (e2) electronic subbands respectively. With increasing pumping current, the growth of the gain amplitude in the e1 transition slows down, the peak gain being approximately the same for TE and TM polarizations. On the contrary, the TE gain dominates over the TM gain in the e2 transition, which shows up at higher injection. Most of the polarization features of the observed gain spectra are easily interpreted within a model of direct optical transitions with the band mixing, relaxation broadening and bandgap shrinkage effects taken into account. At the same time, the gain saturation in the e1 transition observed in experiment can hardly be explained by the theory.
Other publications
[1]
Theory
of InGaN multi-quantum well laser diodes
G. G. Zegrya
Proc. SPIE, vol. 3001, 1997
[2]
Calculation
of Threshold Characteristics of Mid-infrared
Laser Based on Type II Heterostructure
with QWs
G. G. Zegrya and A. D. Andreev
Proc. SPIE, vol. 2682, 1996
[3]
Array
of lasers based on a multiple-pass p-n heterostructure
U. A. Bekirev, S. A. Bondar, D. V. Galchenkov, R. A. Suris, M.
A. Grankin, G. V. Ershova, V. N. Inkin, M. A. Malyshkin
Sov. Tech. Phys. Lett. , vol. 14, no. 23, pp. 2140-2144, Dec., 1988
[4]
Estimate
of the lifetimes of nonequilibrium carriers in a semiconductor irradiated
with heavy ions
E. A. Avrutin and M. E. Portnoi
Sov. Phys. Semicond., vol. 22, no.8, pp. 968-970, Aug. 1988
Detailed theoretical analysis of the
temperature dependence of threshold current density of a semiconductor
quantum dot (QD) laser is given. Temperature dependences of the threshold
current density components associated with the radiative recombination
in QD's and in the optical confinement layer (OCL) are calculated. Violation
of the charge neutrality in QD's is shown to give rise to the slight temperature
dependence of the current density component associated with the recombination
in QD's. The temperature is calculated (as a function of the parameters
of the structure)at which the components of threshold current density become
equal to each other. Temperature dependences of the optimum surface density
of QD's and the optimum thickness of the OCL, minimizing the threshold
current density, are obtained. The characteristic temperature of QD laser,
T0, is calculated for the first time considering carrier recombination
in the OCL (barrier regions) and violation of the charge neutrality in
QD's. The inclusion of violation of the charge neutrality is shown to be
critical for the correct calculation of T0. The characteristic
temperature is shown to fall off profoundly with increasing temperature.
A drastic decrease in T0 is shown to occur in passing from temperature
conditions wherein the threshold current density is controlled by radiative
recombination in QD's to temperature conditions wherein the threshold current
density is controlled by radiative recombination in the OCL. The dependences
of T0 on the root mean square of relative QD size fluctuations,
total losses, and surface density of QD's are obtained.
Theory of quantum-dot (QD) lasers is
augmented to include, in a self-consistent manner, the QD-layer charge.
The electron- and hole-level occupancies in QD's are obtained through the
solution of the problem for the electrostatic-field distribution across
the junction. They are shown to differ from each other. As a result, the
local neutrality is broken down in each QD, i. e. , the QD layer is charged.
The key dimensionless parameters controlling the difference of the hole-
and electron-level occupancies are revealed. The detailed analysis of the
gain and spontaneous radiative recombination current density is given,
having regard to the fact of violation of the charge neutrality in QD's.
The gain-current density dependence is calculated. The voltage dependences
of the electron and hole level occupancies, gain, and current density are
obtained. Particular emphasis is given to the transparency and lasing threshold
characteristics. Optimization of the QD-laser structure is carried out.
The optimum surface density of QD's, minimizing the threshold current density,
is shown to be distinctly higher than that calculated without regard for
the lack of the charge neutrality in QD's.
The characteristic temperature of a
quantum dot (QD) laser, T0, has been calculated for the first
time considering carrier recombination in the optical confinement layer
and violation of the charge neutrality in QDs. T0 is shown to
fall off profoundly with increasing temperature, which is in line with
the available experimental results.
Theoretical analysis of the gain and threshold current of a semiconductor quantum dot (QD) laser is given which takes account of the line broadening caused by fluctuations in quantum dot sizes. The following processes are taken into consideration together with the main process of radiative recombination of carriers in QDs: band-to-band radiative recombination of carriers in the waveguide region, carrier capture into QDs and thermally excited escape from QDs, photoexcitation of carriers from QDs to continuous-spectrum states. For an arbitrary QD size distribution, expressions for the threshold current density as a function of the root mean square of relative QD size fluctuations, total losses in the waveguide region, surface density of QDs and thickness of the waveguide region have been obtained in an explicit form. The minimum threshold current density and optimum parameters of the structure (surface density of QDs and thickness of the waveguide region) are calculated as universal functions of the main dimensionless parameter of the theory developed. This parameter is the ratio of the stimulated transition rate in QDs at the lasing threshold to the spontaneous transition rate in the waveguide region at the transparency threshold. Theoretical estimations presented in the paper confirm the possibility of a significant reduction of the threshold currents of QD lasers as compared with the conventional quantum well lasers.
Other publications
[1]
Effect
of spatial hole burning and multi-mode generation threshold in quantum
dot lasers
L. V. Asryan and R. A. Suris
Proc. of Int. Symp. "Nanostructures: Physics and Technology". June
22-26, 1998, St. Petersburg, Russia., pp. 390-393
[2]
Temperature
sensitivity of threshold current density of a quantum dot laser
L. V. Asryan and R. A. Suris
Proc. of SPIE's Int. Symp.
PHOTONICS WEST'98, January 24-31, 1998. San Jose, California USA. vol.
3283, рр. 816-827
[3]
Spatial
hole burning in quantum dot lasers
L. V. Asryan and R. A. Suris
Proc. of the 24rd Int. Conf. on the Physics of Semiconductors. Jerusalem,
Israel, August 2-7, 1998
[4]
Spatial
hole burning and multimode generation threshold in quantum dot lasers
L. V. Asryan and R. A. Suris
Proc. of IEEE LEOS 11th Annual Meeting, Orlando, FL, Dec. 1-4, 1998
[5]
Theory
of quantum dot lasers
R. A. Suris and L. V. Asryan
Bulletin of Stefan University. Series II. Book of Abstracts. vol.
10, no. 11, pp. 71-72, Sept., 1998
[6]
Effect
of carrier escapes from quantum dots on the multi-mode eneration threshold
in quantum dot lasers
L. V. Asryan and R. A. Suris
Proc. of the 11th Int. Conf. on Superlattice, Microstructures and
Microdevices. Hurgada, Egypt, July 27-August 1, 1998
[ 7]
To
the theory of temperature dependence of threshold current density of a
quantum dot laser
L. V. Asryan and R. A. Suris
Proc. of 1997 Int. Semiconductor Device Research Symp. -ISDRS'97.
Omni Charlottesville Hotel, Charlottesville, VA, December 10-13, 1997,
pp. 433-436
[ 8]
Theoretical
Analysis of the Temperature Dependence of Threshold Current Density of
a Quantum Dot Laser
L. V. Asryan and R. A. Suris
Proc. of IEEE LEOS 10th Annual Meeting, San Francisco, CA, Nov.
10-13, 1997, vol. 2, pp. 496-497
[ 9]
Temperature
dependence of threshold current density of quantum dot laser
L. V. Asryan and R. A. Suris
Digest of 10th Int. Conf. on Superlattices, Microstructures and
Microdevices. Lincoln, Nebraska, July 8-11, 1997
[10]
Effect
of carrier recombination in the optical confinement layer on the temperature
dependence of threshold current density of a quantum dot laser
L. V. Asryan and R. A. Suris
Proc. of Int. Symp. "Nanostructures: Physics and Technology". June
23-27, 1997, St. Petersburg, Russia. pp. 176-179
[11]
Threshold
current density of a semiconductor quantum dot laser
R. A. Suris and L. V. Asryan
Proc. of the 8th Seoul Int. Symp. on the Physics of Semiconductors
and Applications - ISPSA'96, Seoul, Korea, Oct. 21-22, 1996, p. 42
[12]
To
the Theory of Quantum Dot Lasers: Self-consistent Consideration of Quantum
Dot Charge
L. V. Asryan and R. A. Suris
Digest of 15th IEEE Int. Semiconductor Laser Conf. Haifa, Israel,
Oct. 13-18, 1996, pp. 107-108
[13]
Charge
neutrality violation in quantum dot lasers
L. V. Asryan and R. A. Suris
Proc. of the 23rd Int. Conf. on the Physics of Semiconductors. Berlin,
Germany, July 21-26, 1996. Editors M. Scheffler, R. Zimmermann, World Scientific,
Singapore, vol. 2, pp. 1369-1372
[14]
Gain
and Current Density of Quantum Dot Laser
L. V. Asryan and R. A. Suris
Proc. of Int. Symp. "Nanostructures: Physics and Technology". June
24-28, 1996, St. Petersburg, Russia. pp. 354-357
[15]
Quantum-Dot
Laser: Gain Spectrum Inhomogeneous Broadening and Threshold Current
R. A. Suris and L. V. Asryan
Proc. of SPIE's 1995 Int. Symp. on Optoelectronic, Microphotonic,
& Laser Technologies. PHOTONICS WEST'95, 4-10 February, 1995.
San Jose, California USA. vol. 2399, pp. 433-444
[16]
Linewidth
Broadening and Threshold Current Density of Quantum-Box Laser
L. V. Asryan and R. A. Suris
Proc. of Int. Symp. "Nanostructures: Physics and Technology". June
20-24, 1994, St. Petersburg, Russia. pp. 181-184
Simulation
of nanostructure growth
We study the evolution of two-dimensional
nuclei and holes on the isotropic surface during both crystal growth and
growth interruption. We show that there are fundamental differences between
the nuclei evolution in the absence of an outer source of condensing atoms,
which is described by the theory of Lifshitz and Slezov, and that in the
presence of such a source, i. e. during epitaxial growth. We also consider
the evolution of the nuclei size distribution function, which is also different
for these two cases.
Growth of two-dimentional nuclei on
the isotropic surface is considered. Studies are made of all the stages
of their evolution up to the monolayer completion. The dependence of the
nucleus growth rate on its size is obtained. The evolution of the nuclei
size distribution function is considered.
The form and the energy of a two-dimensional critical nucleus (CN), arising on the As-stabilized (001) surface of GaAs, are discussed. It is shown that the CN, which consists of Ga and As consecutive layers, is strongly stretched along the [110] direction and the free energy of the CN is much less compared with the free energy of an isotropic nucleus.
Other publications
[1]
A
novel approach to theoretical treatment with 3D island formation during
epitaxial growth
A. Yu. Kaminski and R. A. Suris
Proc. of the 23rd Int. Conf. on the Physics of Semiconductors. Berlin,
Germany, July 21-26, 1996. Editors M. Scheffler, R. Zimmermann, World Scientific,
Singapore, vol. 2, pp. 1337-1340
[2]
The
shape and activation energy of critical two-dimensional nuclei of the (001)
surface of III-V crystal during epitaxial growth
I. L. Aleiner, R. A. Suris
Sov. Techn. Phys. Lett. vol. 16, no. 7, pp. 547-548, July, 1990
The kinetics of intrachain collisions
in polymer molecules of arbitrary configuration described in the Gaussian
chain approximation are investigated. An accurate exact value of the mean
collision time of two reactive segments is found in the limit of a small
reaction volume. It is shown that the problem can be reduced to calculating
the resistance between two corresponding points in an electrical circuit
having the same topology as the molecule under consideration.
The kinetics of Brownian particle annihilation
on randomly distributed correlated traps is considered. It is proved that
trap attraction (repulsion) leads to a slowdown (acceleration) of the process,
as compared to the case of noncorrelated traps. The importance of trap
correlation length is demonstrated.
A theory of the death of Brownian panicles
on correlaled traps is proposed. It is demonstrated that trap repulsion
leads to an acceleration of particle death in comparison with the case
of uncorrelated traps. In contrast, trap attraction leads to a slowdown
of the process.
The problem of Brownian particle death
on correlated traps is considered. The one-dimensional case is analysed
in detail. It is shown that trap repulsion (attraction) leads to acceleration
(deceleration) of the process in comparison with the case of noninteracting
traps.
We review a novel approach to treating
many-body effects in diffusion-limited kinetics. The derivation of lhe
general expression for the survival probability of a Brownian particle
in the presence of randomly disiributed traps is given. The reduction of
this expression to both the Smoluchuwski solution and the well-known asymptotic
behavior is demonstrated. It is shown that the Smoluchowski solution gives
a lower bound for the particle survival probability. The correction to
the Smoluchowski solution which takes into account the particle death slowdown
in the initial process stage is described. The steady-stale rate-constant
concentration dependence and the reflection of many-body effects in it
are discussed in detail.
Local bending or compression of chemical
bonds in trans-polyacetylene chain are considered with respect to their
influence on movement of a soliton along the chain. Compression of double
and single bond by an equal length (bending by an equal angle) results
in different ground state energies of the system. Therefore while passing
along the chain with a strained double or single bond the soliton can be
reflected, decelerated or accelerated. Being placed between strained double
bonds, the soliton appears to be in potential well.
The time dependence of the mean square
displacement of Brownian particle in a space with randomly disturbuted
static traps is analysed. A time invertal in which the mean square displacement
decreases with time but not increases, as usual, is discovered.
We have obtained the tune dependence
of the mean square displacement of a Brownian particle in the presence
of randomly distributed traps in the onc-dimensional case. It turned out
that devialions from the traditional relation <x2 (t)>tr
=
2Dt occur as early as the initial slage of the process.
The influence of trap diffusion on
the fluctuation slow-down of death of Brownian particles, discovered earlier
in the case of stationary traps, is analysed, it is shown that fluctuation
slow-down also takes place with movable traps if the diffusion is slow
enough.
The statistical characteristics of
a spatial region visited by a spherical Brownian particle during time t
(Wiener sausage) are investigated. The expectation value and dispersion
of this quantity are obtained for a space of arbitrary dimension. In the
one-dimensional case the distribution of probability density and the moments
of any order are determined for this quantity.
We suggest a new approach to the problem
оf taking account of many-body effects in the kinetics of diffusion-controlled
reactions of the type A+B
Pr with
an excess of one of the reagents wliosc particles are at rest (nB"nA,
DB=0). A new expression for the survival probability of particles
A which uniformly describes the process within the entire time interval
is obtained. It is shown that the conventional expression which was derived
without regarding many-body effects always underestimates the probability
of survival of panicles A. A correction lo the conventional expression
is found accounting for such lowering at the initial stage of the process.
At the asymptotically large times when the conventional expression appreciably
lowers the survival probability the fluctuation asymptotics takes place.
Other publications
[1]
О
кинетике гибели броуновских частиц на случайно расположеннvх стоках
А. М. Бережковский, Ю. А. Махновский, Р. А. Сурис
Химическая физика т. 8, вып. 6, стр. 827-833, 1989
High-temperature
superconductivity
A parametric resonance due to temperature
oscillations in superconducting micron-scale waveguides is considered.
Oscillations of superconductor temperature are assumed to be induced by
the irradiation of the waveguide with a laser beam. The laser power and
parameters of the waveguide providing a possibility of parametric excitation
have been calculated. It is shown that for a waveguide made of a YBa2Cu3O7
microstrip
with resonant frequency of 10 GHz a laser with a power of about 70 W/cm2
is needed to excite oscillations. The effect can be used for the creation
of high-sensitivity tuneable filters and optoelectric transformers on superconducting
microstrips in the GHz range.
Local measurements of structural characteristics such as intrinsic microstrain
along
the c axis of the lattice e
= d c/c and
its mean square fluctuation e
, oxygen deficiency x, cation composition, etc. were performed on
epitaxial YBa2Cu3O7 films
grown on various substrates (MgO, BaSrTiO3/MgO, SrTiO3,
LaAlO3, ZrO2/Si, Al2O3). A
number of film microstrips were fabricated and the normalized flicker noise
intensity (Hooge parameter a
) and the resistivity r
at 300 K were measured at each characterized point. A theoretical model
was developed that explains the observed first growth of a
with e and the well-known
high level of the normal-phase flicker noise in various high temperature
superconducting compounds. Comparison of the experimental and simulated
dependence of a on
e,
frequency, and temperature permits one to determine numerically the theoretical
parameters of the double-well potential with minima located at the chain
(O1) and empty (O5) oxygen lattice positions of the CuO plane.
Errors inherent in electron probe microanalysis
of YBa2Cu3O7 films and the atomic composition
of films with a resolution of 2 m
m have been found. Critical temperature values have been determined from
the temperature dependence of the electron beam induced voltage (EBIV).
Having plotted these results on a triple phase diagram of oxides, we found
two tie lines with highest Tc(ridges) and two tie lines with
lowest Tc (valleys). A mechanism of cation defect formation
was proposed which accounts for the presence of this topology. The mechanism
was verified by reconstructing cation defects observed in TEM images.
Scanning SQLUD- microscopy has been
extended to record current-induced magnetic field maps of superconductor
topologies based on YBCO film structures. The technique described can be
used as a powerful tool for the diagnostics of superconductor integrated
circuits and highly sensitive bolometers. This kind of diagnostics predicts
the degree of integration which can be achieved at the modern level of
HTSC microelectronics technology. One would expect an overall yield of
100-element 1 mm2 chips at a level of ~0. 1-0. 5% in the production
of superconducting YBCO/MgO bolometers.
Burning of superconducting thin film
bridges by large transport currents (up to densities of 2*107
A/cm2) is investigated by magneto-optical imaging of flux distribution
and low-temperature scanning electron microscopy providing Tc
maps. It is shown that the destruction is preceded by significant penetration
of magnetic field inside a weak-pinning region. In bridges containing extended
defects magneto-optic investigation is sufficient to locate the incipient
burning region. In high-quality bridges free from such defects only a combination
of the two techniques will allow prediction of the place of fatal destruction.
The Tc -mapping method using
low temperature scanning electron microscopy (LTSEM) has been developed
to study the spatial distribution of the critical temperature in HTSC films
with a spatial resolution approaching 2 m
m. To achieve so high a spatial resolution, a numerical deconvolution method
was applied that eliminated distorting effects associated with the thermal
diffusion and with the contribution from the absorbed beam current. The
Tc -mapping method was used to investigate modification by water
of YBa2Cu3O7 films grown on (100) MgO
and (110) LaAlO3 substrates. The rate of modification of a [110]
-oriented YBa2Cu3O7/LaAlO3
film is found to be 40 times that of a c-axis oriented YBa2Cu3O7/MgO
epitaxial film. It is argued that water-initiated modification of the films
results from penetration of hydrogen into the films, rather than from outdiffusion
of oxygen.
The voltage noise and current inhomogeneities
in a thin c-oriented YBa2Cu3O7-x film
on MgO substrate have been investigated. An unusually sharp peak of low-frequency
noise was observed 7 K below Tc with width of less than 1 K
and strongly nonmonotonous dependence on external magnetic field. The appearance
of this peak is found to be related to a single defect which leads to the
formation of a weak link in the bottleneck of the current percolation path.
This weak link is latent one; it is undetectable by the standard characterization
techniques and has no effect on the integral transport properties of the
film. We were able to reveal it using the low - temperature scanning electron
microscopy which allowed us to determine the current density distribution
across the weak link with resolution of 1 m
m. The critical current density of the weak link is found to be jc(77K)
> 5x106 A/cm2 which is comparable with the critical
current density of high-quality YBaCuO films. Analysis of experimental
dependences in terms of the resistively shunted junction model shows that
it is the Abrikosov vortex motion resulting in the fluctuations of the
phase difference across the weak link that leads to the peak of voltage
noise at the temperature at which the current through the weak link is
close to its critical current.
Noise properties of c-oriented YBa2Cu3O7
films near Tc have been
investigated both experimentally and theoretically. The films studied are
divided into two groups. The resistance noise and transport properties
for the films (1) are controlled by the grain boundary junctions. The films
(2) are of high structural perfection and have very low flicker noise level.
Their SR(T)-characteristics near Tc are quantitatively
described by a novel percolation model. The model takes into account static
Tc -inhomogeneities of the film and the presence of the defects
that modulate the local Tc. Distribution functions of Tc
were obtained and defect density of states as well as a scale of Tc
-relief were estimated. The spatial current distribution is studied by
the low temperature SEM. The method is originally used for Tc
-mapping of the films. A special algorithm for the LTSEM data treatment
for inhomogeneous films using the effective medium approach and allowing
us to derive local transition curves is proposed.
It is shown that strong TT-inhomogeneity
of high temperature superconductors (HTSC) leads to a modification of the
mechanism of the thermodynamic noise in these materials. In spatially inhomogeneous
materials, along with classical thermodynamic noise associated with random
heat exchange between superconductor and its environment, there exists
an excess thermodynamic noise associated with heat exchange inside the
superconductor. Besides, due to inhomogeneity, the effective volume which
makes a dominant contribution to the fluctuations of sample resistance
can be far less than the total volume of the sample. Using a simple model
we derive the expression for the spectral density of resistance noise for
a thin superconducting film. It is shown that spatial Tc-inhomogeneity
can lead to a substantial enhancement of noise.
The transverse thermoelectric effect
in HTSC films was analysed theoretically concerning a plausible anisotropic
electron-band structure which was supposed to be b weakly binding in Cu-O
layers and tightly binding in the directions close to the c-axis. We have
found interesting peculiarities in the effect value as a function of Fermy
energy in the vicinity in the points, which correspond to the boundaries
of the Brilllouin-zone along the c-axis direction. At the first-zone boundary
the effect is 5 times greater than its initial value. Then, if many zones
are taken into account, it demonstrates a nonmonotone behaviour strongly
depending on the band parameters (energy band widths and gaps) with a possible
change in the sign of the effect.
We report on some specific properties
of multi-connected micron-scale waveguides with a thinb superconducting
wire in a form of a strip o as a inner core of a coaxial line. With decreasing
wire cross section, beginning from a size of the order of squared London
penetration depth in a superconductor, one may observe two effects: the
signal slowing-down and the related increase of the line wave impedance.
Prospects for microelectronics of high-impedance waveguides as interconnections
and delay lines are discussed. Its noted, that the effect become stronger
for high-T superconductors, provided, that their anisotropy axis is oriented
along the waveguide.
A concept of new device: bolometer (infrared detector) on superconductor in zero-resistance state using the resonance effect is proposed. In bolometers operating at T<Tc modulations of kinetic inductance of superconductor caused by temperature modulations are detected. It is proposed to use a superconductor microstrip waveguide as a sensitive element of such a bolometer. Its resonance frequency depends on kinetic inductance which provides a high sensitivity of the device. Signal to noise ratio is estimated theoretically and compared to that of conventional bolometers. The new device is shown to be more promising due to the absence of Johnson noise at T<Tc.
Other publications
[1]
Noise
properties of inhomogeneous non-linear medium. Application to high- Tc
Superconductors
A. V. Bobyl, M. E. Gaevski, O. Shalaev, D. V. Shantsev, R. A.
Suris
Proc. of 14th Int. Conf. on Noise in Physical Systems and 1/f Fluctuations,
ICNF'97 (Leuven, Belgium, July, 1997). ed. by C. Claeys and E. Simoen (World
Scientific, Singapoure), pp. 317-321
[2]
Spatially-Resolved
Investigation of Magnetic Flux Fluctuations in Current-biased High- Tc
Films
M. E. Gaevski, V. A . Solov'ev, S. G. Konnikov, D. V. Shantsev,
A. V. Bobyl, S. F. Karmanenko, T. H. Johansen, H. Bratsberg, Yu. Galperin
Proc. of 14th Int. Conf. on Noise in Physical Systems and 1/f Fluctuations,
ICNF'97 (Leuven, Belgium, July, 1997). ed. by C. Claeys and E. Simoen (World
Scientific, Singapoure), pp. 321-325
[3]
Model
for Description of R(T,H)-dependence in Tc inhomogeneous HTSC
Films Near Tc
D. V. Shantsev, A. V. Bobyl,
M. E. Gaevski, V. Gasumyants, O. L. Shalaev, R. A. Suris
Abstracts of X Trilateral German-Russian-Ukrainian Seminar on
HTSC, (N. Novgorod, Russia, Sept., 1997), p. 35
[4]
Количественна
низкотемпературна растрова электронна микроскопи тонких ВТСП пленок
В. А. Соловьев, М. Е. Гаевский, Д. В. Шанцев, С. Г. Конников
Известия АН, тoм 60, вып. 2, стр. 32-40, 1995
[5]
Analysis
of signal scattering by inhomogeneities in HTSC communication lines
R. A. Suris, N. V Fomin
Sov. Techn. Phys. Lett. , vol. 16, no. 9, pp. 669-671, Sept., 1990
[6]
On
the possibility of high-Tc coaxial lines for interconnections
R. A. Suris, N. V. Fomin
Sov. Techn. Phys. Lett. , vol. 15, no. 12, pp. 965-966, Dec., 1989
The stressed state of a finite-thickness
film of a solid solution is analyzed in the isotropic approximation. It
is shown that if spatial redistribution of the components of the solid
solution is postulated, as is necessary in order to achieve thermodynamic
equilibrium, the resulting new stressed state is described by equations
of the theory of elasticity with renormalized elastic moduli. As a result,
the formation of misfit dislocations in solid solutions takes place more
easily than in bulk materials. , i. e. , for smaller mismatches between
the lattice periods of the film and the substrate. The planar elasticity
problem of the energy of formation of a misfit dislocation in a film of
finite thickness is solved for Frenkel'-Kontorova boundary conditions (boundary
atoms are situated in a cosine potential generated by the substrate). The
energy of formation of a misfit dislocation is calculated, and phase diagrams
are plotted in the coordinates lattice-period mismatch d
vs film thickness and mismatch d
vs temperature. Situations are found to be possible where the sign of the
static Poisson ratio becomes negative, and dislocations are formed for
an arbitrarily small mismatch of the film and substrate periods.
The Frenkel - Kontorova theory of phase
transitions into an incommensurate phase with the formation of a superlattice
of misfit dislocations on a surface is extended to the case of alloys for
which the lattice mismatch depends on the local composition. A model is
considered
that takes into account the possibility of atomic diffusion between the
surface and the bulk. Peculiarities of the conditions at the surface are
accounted for by introducing a chemical potential for one type of the atoms
of the alloy which leads to a difference in composition between the bulk
and the surface layer. We calculated the average mismatch between the surface
and the bulk lattices and the critical value of chemical potential at which
the phase transition into an incommensurate phase with misfit dislocations
occurs. A phase diagram of the system in coordinates of temperature and
chemical potential is presented for the alloy InxGa1-xAs.
The Frenkel-Kontorova theory of phase transition onto an incommensurate phase with formation of superlattice of misfit dislocations on a surface is extended to the case of solid solutions, for which the lattice mismatch depends on the local composition. In the result, incommensurate phases having spatially modulated composition are described. The few features of the model considered are follows: 1) possibility of a phase transition with arbitrary small average mismatch of the periods and 2) power-low dependence of the interaction between Frenkel-Kontorowa dislocations on the distance between the dislocations an, consequently, the possibility of describing a phase transition on the basis of the Ginsburg-Landau theory of second-order phase transitions.
Group of the Theory and Computer Simulation of Radiation Defects and Processes in Multicomponent Materials
(complete list of publications see on the page of group)
Neutron irradiation of HTSCs creates
a wide spectrum of defect sizes. In this work, the influence of small defects
on the critical temperature and critical current density Jc is investigated.
The characteristic behaviour of Tc and Jc under this treatment is satisfactorily
explained by a theoretical model considering the creation, migration and
annihilation of small defects in the oxygen sublattice.
Изложены основы физики дефектов в кристаллах
и радиационной физики твердого тела. Дано математическое описание физических
процессов на разных стадиях радиационной повреждаемости материалов. Приведены
оригиеальные результаты последних лет. Пособие соответствует государственному
образовательному стандарту подготовки бакалавров по дисциплине "прикладная
физика: физическое материаловедение; радиационная физика твердого тела:
взаимодействие излучени с веществом" направление 553100 "Tехническая физика".
Experimental studies of concentration
profiles in Ni/C and W/C multilayers prepared by pulsed laser deposition
are compared with ballistic simulations of the deposition process by means
of the computer code TRIDYN. One part of the deposited particles processes
kinetic energies of about 100 eV and leads to a ballistic mixing of the
deposited layers. As a consequence, diffisive interface concentration profiles
arise and the concentrations within the individual layers depend on the
layer thickness. The concentration profiles can be highly asymmetric between
adjacent interfaces as observed e. g. in W/C multilayers. Simulations predict
that the interface width for the deposition of W onto C is up to 3. 5 times
larger than in the opposite case. Differences between simulation results
and HREM, AES, X-ray and XPS studies suggest that the resulting interface
concentration are essentially influenced by compound formation as weii
as by demixing of components occurring during deposition.
The (SiC) (AlN) system is being extensively
investigated due to the full miscibility of the two constituents, SiC and
AlN, their good thermal and lattice matches, and the the possibility of
modifying the band gap of the resulting structure over a wide range of
2. 9 eV (6H-SiC) to 6. 2 eV (2H-AlN). From a practical viewpoint, the solid
solutions of SiC and AlN are promising materials for advanced high-temperature
electronic and optoelectronic devices. One novel method of producing thin
layers of (SiC)(AlN) potentially suitable for microelectronic applications
is the use of N and Al co-implantation into 6H-SiC at elevated temperatures
followed by annealing, i. e. ion-beam synthesis. Hitherto, to the best
of our knowledge, there has been only one report on the formation of buried
(SiC)(AlN) layers in 6H-SiC by ion-beam synthesis. This work is an attempt
to model the fundamental processes that occur when 6H-SiC is implanted
at elevated substrate temperatures with high doses of N+ and
Al+ ions to form thin buried layers of (SiC)(AlN) having predetermined
composition and dimensions. Results from the calculations have been correlated
with those obtained by Rutherford backscattering/ channelling spectrometry
(RBS/C).
This work is an attempt to model the
processes that occur when 6H-SiC is implanted at elevated substrate temperatures
with high doses of N+ and Al+ ions to form thin buried
layers of (SiC)(AlN). The theoretical treatment has involved ballistic
calculations of the distribution of the above species and the resulting
ion-induced defects by means of computer codes (developed specially for
modelling multi-elemental targets) that take into account the effect of
cascade overlapping at high ion doses. Results from the computer simulations
have been correlated with data obtained by Rutherford backscattering spectrometry/ion
channeling (RBS/C) and elastic recoil detection (ERD) techniques, and good
agreement has been achieved between theory and experiment. The analysys
of the theoretical and experimental findings has enabled five specific
regions to be identified in the implanted material which are discus-sed
in detail.
A theoretical analysis of the evolution
of the defect structure in silicon carbide (6H-SiC) implanted with N+
and Al+ ions of various energies. Satisfactory agreement was
achieved between the calculated defect distributions and experimental data.
The following kinetic parameters of silicon carbide were estimated numerically:
the migration energy of interstitial silicon atoms and the recombination
parameters of vacancies and interstitial sites in the carbon and silicon
subsystems.
A physical model is proposed for the
influence of oversize impurities on the characteristics of radiation-defect
formation in neutron-irradiated iron alloys at low and high irradiation
temperatures. A comparison is made between the theoretical expressions
obtained for radiation hardening at different temperatures and the existing
experimental data. For low temperatures, the agreement was satisfactory,
so we can estimate one of the microscopic parameters of impurity atoms
the relative size of the region of influence of the impurity atoms. For
high irradiation temperatures, the behavior of the radiation hardening
of an alloy with oversize impurities is predicted as a funcdon of impurity
size.
The novel binary collision approximation
Monte Carlo (BCA-MC) computer codes TRIRS and DYTRIRS for simulating ion
sputtering of polyatomic nonuniform amorphous targets are presented. TRIRS
simulates the collision cascade in a target and related secondary processes,
including sputtering, damage generation etc. , being more realistic than
similar MC-BCA codes in modeling low-energy interatomic collisions. These
improvements ensure better simulation of low-energy atomic collision processes
in nonuniform targets, like sputtering, and ultra-low energy ion implantation.
DYTRIRS is the extension of TRIRS that simulates the dynamics of the ballistic
stage of ion-induced modification and sputtering for a target under high
fluence ion irradiation. The efficiency of DYTRIRS is verified by comparing
of the simulation of sputtering and secondary-ion mass-spectrometry in-depth
compositional profiling of molecular-beam epitaxy grown two-dimensional
(Al,Ga)As-GaAs (001) heterostructures, including structures with silicon
and aluminum marker layers.
A model based on original experimental
data is proposed to describe the transition of the structure from hexagonal
to cubic during the growth of boron nitride layers. It is postulated that
boron and nitrogen ions entering the growing layer form additional atomic
planes parallel to the (ab) planes of the hexagonal structure or complete
existing dislocations in the growing crystal, causing the boron nitride
to undergo a transition to a cubic structure.
A physical model is presented for the
formation of a structure consisting of "micropoints" and "cavities" on
the surface of pyrolitic graphite bombarded by 210 MeV Kr+ ions. This structure
may be explained in terms of the depth distribution of the energy deposited
by the bombardment.
A series of experimental and theoretical
investigations has been initiated for 6H-SiC samples sequentially implanted
with high doses of N (65 keV) + N+(120keV) + Al+(100keV) + Al+ (160 keV)
ions at temperatures between 200 and 800 C. Nytrogen and carbon distribution
profiles are measured by ERD and structural defect disributions are measured
by Rutherford backscattering with channeling. A comparison between the
experimental data and the results of computer simulation yields a physical
model to describe the relaxation processes opf the implanted SiC structure,
where the entire implanted volume is devided into regions of different
depth, having different guiding kinetics mechanisms.
The general theory is developed of
radiation processes in multicomponent materials. The complex of computer
codes is created, which allows to calculate self-ccordinated an evolution
of radiation defect distrbutions in polyatomic and multilayered structures
under high-fluence irradiation. The computer program DYTRIRS is created
for calculations of point defect distributions and sputtering coefficients
under high-fluence irradiation. This code provides 10 sorts of atoms and
8 layers of deferent composition. Also it is possible to set energetical
parameters for each atom sort in each layer. Testing of this code is carried
out using experimental scattering profiles onspecially grown heterostructures
with different compositions. On the base of modificated computer code MGEAR
the program for IBM PC is created which allows to calculate kinetics of
defects in multicomponent materials] . The coordination of this code with
ballistic program DYTRIRS is carried out which gives the possibility to
calculate self-coordinated the evolution of defects in such irradiated
complex system as YBaCuO, BN, SiC.
Neutron irradiation of HTCs creates
a wide spectrum of defect sizes. In this work, the influence of small defects
on the critical temperature is investigated by sequential reactor neutron
irradiation and annealing of a YBCO single crystal. Thecharacteristic behaviour
of Tc under this treatment is satisfactorily explained by a theoretical
model considering the creation, migration and annihilation of small defects
in the oxygen sublattice.
A method is developed for computer
simulation of the ballistic stage of ion sputtering. With this method it
is possible to describe the dynamic variation of the composition of a layered
material as the result of spatial overlap of cascades at high bombardment
doses. The method is modified from the algorithms of existing dynamical
programs. In particular, this method takes account of how the volumes of
the point defects that arc formed and the volumes of the implanted ions
affect the change in the parameters of the material during the bombardment.
Test calculations are carried out for ion etching of targets of homogeneous
GaAs and GaAs with a d
-layer of silicon. The results of the calculations arc in qualitative agreement
with the data from secondary-ion mass spectroscopy of profiled GaAs samples
d-doped with silicon.
The special dynamic computer code DYTRIRS
for the simulation of the sputtering and modification phenomena in multilayered
compound materials under high-fluence ion irradiation was developed on
the basis of the Monte Carlo computer code TRIRS for the collision cascade
calculation. In the computer simulation technique, the spatial overlap
of ion-induced atomic collision cascades in a modified material are taken
into account. This simulation technique was tried out on the specially
grown multilayered structures. The results of the simulation of the sputtering
phenomena of the Fe and Cu targets protected by carbon layer under high
fluence ion irradiation are demonstrated.
T главе I Обсуждаются общие представления
о развитии радиационной повреждаемости металлов и сплавов. Подробно анализируется
динамика радиационной повреждаемости, включая спектры первично выбитых
атомов, процессы рекомбинации разноименных дефектов, послекаскадные распределени
дефектов. Рассматриваются фазовые превращения в сплавах, стимулированные
облучением. Описывается радиационно индуцированные и радиационно-ускоренные
фазовоструктурные превращения в сплавах. Обсуждается явление усиления рекомбинации
радиационных дефектов в распадающихся твердых растворах.
A model is presented for the evolution
of point defects generated by neutron irradiation in YBaCuO. The concentrations
of point defects generated by the neutron irradiaion are calculated. It
is shown that by annealing the irradiated sample at room temperature (in
the reactor channel) it is possible to form small clusters of point defects
in concentrations sufficient for effective pinning of the magnetic flux.
This model can explain the anisotropy in the lowering of the critical current
density by annealing of irradiated samles and shows that the critical current
in the presence of a magnetic field parallel to ab-plane is proportional
to the concentration of pinning defects.
The influence of processes occuring
in the oxygen subsystem of YBaCuO thin films under gamma-irradiation on
the critical temperature and on the temperature dependence of the film
resistance has been investigated theoretically. The values of parameters
such as sink strength values, the energy barrier for oxygen escape from
sinks and cross-section of oxygen-vacancy formation under gamma-irradiation
have been estimated.
A theoretical investigation is made
of the influence of processes taking place in the oxygen subsystem of thin
YBaCuO films under the influence of gamma irradiation on the value of crytical
temperature of superconducting transition and the dependence of the resistivity
on the temperature. Estimates of the numerical values are obtained for
the following parameters of oxygen subsystem of thin YBaCuO films: the
sink strength, the energy of binding of the oxygen to a sink, and the cross
section for the formation of an oxygen vacancy by ionization during gamma
irradiation.
A technique for analytical calculation
of the radiation swelling of both pure materials and alloys is demonstrated.
It is shown that the use of a simplified expression for the bias factor
Q is incorrect.
A general theory is derived for the
radiation swelling of a heterogeneous material. A study is made of how
the swelling of a solid solution undergoing decomposition as a result of
irradiation is affected by precipitates of a secondary phase that are found
in a coherent state.
A physical model is proposed for the
anomalous recombination of unlike radiation defects in solid solutions
undergoing decomposition as the result of irradiation. The precipitating
coherent pre-precipitates initiate the formation of small interstitial
clusters, which act as recombination centers in these materials. The rate
of radiation swelling decreases as a result of the anomalous recombination.
An analysis is made of the feasibility
of doing field-ion microscopic studies of the stabilization (trapping)
of interstitial atoms by impurity element atoms and of determining the
temperature and energy para-meters of this process, the stability of complexes
of interstitial atoms with impurity atoms, and their mobility. An experimental
procedure is developed for such studies. A theoretical method for analyzing
the experimental results, which allows one to calculate the migration energies
and the binding energies of complexes of interstitial atoms and impurities,
is described. Methods for treating the evaporating action of the electric
field in such experiments, adsorption of gas pa-rticles, thermal-field
molding of the sample surface, and possible fe-atures of the field-ion
microscopic contrast created by the impurity atoms are studied and developed.
Possible effects of sinks within the investigated volume of the sample
on the quantitative results of the measurements are discussed. A philosophy
of such investigations is elaborated, and their practical applications
and significance are discussed.
A direct method of measuring of absorption
rate and efficiency of point defects capturing by various structural sinks
is proposed. Irradiated needle-shaped specimens are isochronaly annealed
in field ion microscope. During anneal a persistent calculation of point
defects number N(t), appearing on the specimen surface is carried out.
Presence of structural sinks in the specimen volume, results in reducing
N(t). This makes possible to estimate quantitatively the absorption properties
(sinks strength, rate and efficiency of point defects absorption) of sinks
of various nature.
We propose a model for the evolution
of clusters of radiation point defects with different mobilities in irradiated
crystals. As an example, we apply the model to the case of alkali halide
crystals. In so doing, we calculate the time dependence of the defect densities
and defect cluster radii. The calculated results are compared to experimental
data.
A short survey is made of the works
which have contributed lo a theoretical description of the main physical
processes occurring in irradiated multicomponent materials in three interrelated
stages of radiation damage. The application of the theory of radiation-induced
processes in these materials is considered, in particular, for determining
the energy parameters of point defects and the adsorption characteristics
of structural sinks.
The rate of radiation swelling is estimated
for a material with various types of sinks by considering the steady-state
fluxes of point defects. Retardation of the rate of radiation swelling
by precipitate formation is considered.
Fundamental equations are derived and
solved which are suitable for the development of a self-consistent theory
of radiation processes in crystals. The theoretical results are compared
with experimental data on the growth kinetics for precipitates of secondary
phase and for radiation swelling.
The rate of radiation swelling for
a material with various types of sinks is evaluated with allowance for
the stationary point-defect flow. The precipitates are considered to provoke
a loss in the radiation swelling rate.
A general theoretical description is
presented for the process of radiation damage in materials. Some analytical
dependences of physical quantities are given, which were obtained from
a general approach. It is shown that it is necessary to calculate the characteristics
self-consistently and to take into account inhomogeneity in the point-defect
distribution around sinks. The analytical dependences are compared to the
results of physical and computer experiments.
Irradiation of solid solutions by fast
ions can induce decomposition with precipitation of a secondary phase.
We will illustrate this point for a binary solid solution irradiated by
hard radiation. The precipitates of the secondary phase that form during
the decomposition pass through two main stages: coherent pre-precipitation
followed by precipitation. The first stage is of the greatest interest
from the standpoint of reducing the radiation swelling.
Путем обобщения имеющихся теоретических
работ, описывающих физические процессы в чистых и примесных металлах, сформулирована
система самосогласованных уравнений для трех стадий процессов радиационного
повреждения - динамической, диффузионной и стадии эволюции стоков. Рассмотрены
некоторые конкретные случаи, в частности, задачи о зависимости радиуса
выделений от времени. Дано сравнение с экспериментальными данными. Указаны
перспективы дальнейших исследований в области теоретического прогнозирования
поведения материалов под воздействием проникающих излучений.
Теоретически рассмотрены предельные случаи истощения твердых растворов под облучением. Получены аналитические зависимости для распределений точечных дефектов около выделений и для радиусов сферических преципитатов как функций времени. Проведено сравнение последних с имеющимися экспериментальными данными по кинетике роста выделений вторичной фазы под облучением в твердых растворах.
Radiation swelling is known experimentally
to be suppressed in materials that decompose rapidly and uniformly . In
our model explaining this effect, the reduced swelling is attributed to
enhanced recombination of unlike defects in these materials. For this to
occur, one must have additional recombination centers such as, for instance,
dislocation dipoles 8 in deformed materials, or serniinterstitials and
semivacancies at incoherent boundaries.
The performance and parameters of materials
that are used under conditions of intense irradiation are determined by
the interaction of radiation-induced point defects both with structural
imperfections of these materials and with the specific defect structure
formed under irradiation. The most fundamental characteristics of these
interactions are the adsorption capacities of the structural imperfections.
They are quite difficult to determine, and until recently this problem
was solved by indirect analysis of the experimental data on radiation-induced
swelling of constructional materials. However this method for obtaining
physical data on the absorption of point defects by sinks (dislocations,
pores, precipitates, etc.) is incorrect, because it is based on model concepts
about the physical nature of the changes occurring in the macroscopic properties
studied.
For determination of the concentration
profiles of an impurity as a function of the depth into the sample, various
methods are used which make use of ion etching with subsequent (or simultaneous)
analysis of the surface by the methods of secondary ion mass spectrometry
(SIMS), Auger electron spectroscopy (AES), etc. However, in the course
of sputtering of the surface, mass transport can occur in the sample and
lead to distortion of the initial profile of the impurity distribution
and degradation of the resolution of the method.
Развита теория радиационных процессов в металлических твердых растворах замещения на различных стадиях повреждаемости и распада. Получены аналитические выражения и зависимости для таких характеристик, как каскадная функция, длина пробега динамических краудионов, концентрация точечных дефектов, скорость и эффективность поглощения стоками точечных дефектов, размеры формирующихся выделений второй фазы, скорость радиационного распухания гетерогенного материала. Подчеркнута необходимость постановки комплексных аналитических и ЭВМ-расчетов для описания изменения свойств облученных материалов.
Развитие современных радиационных технологий, совершенствование имеющихся энергетических установок и проектирование термоядерных аппаратов требует, чтобы конструкционные материалы вырабатывали свой ресурс при различных внешних воздействиях, в том числе и при обучения быстрыми частицами. Конструкционные материалы, используемые и создаваемые с применением радиационных технологий и для нужд современной энергетики, являются сплавами, в основе получения которых лежит их легирование различными примесями. Для разработки научно-обоснованных рекомендаций по совершенствованию имеющиеся и созданию новых композиций необходимо изучить комплекс физических механизмов радиационных процессов в гетерогенных материалах для установления закономерностей изменения их свойств под облучением и отыскания физических путей повышения их радиационной стойкости.
Analytical estimates are obtained for
the boundary concentrations for cases of homogeneous and selective nucleation
of second-phase precipitates.
Recent years have seen detailed research
on the kinetics of the ge-neration of point defects and the formation of
clusters during irra-diation. The system of balance equations for the average
con-centrations (Cj of point defects of species j (j = i for interstitial
atoms, j = v for vacancies, and j = a for impurity atoms) and of small
clus-ters (j = 2i, ai, etc.) is the following system of nonlinear ordinary
differential equations.
The recombination of unlike radiation-induced
defects is an important process in the evolution of the defect structure
of crystals during bombardment by fast particles. Experimental data1-4
show that recombination in solid solutions accelerates if the latter contain
coherent preprecipitates of a second phase. Descriptions of the kinetics
of radiation-induced defects by means of balance equations for vacancies
(v) and interstitial atoms (i) are usually restricted to consideration
of mutual recombination, v +i =0, and the escape of vacancies and interstitial
atoms to sinks: dislocations, grain boundaries, phase boundaries. Analysis
of the experimental data on the basis of such models forces one to assume
that in addition to these sinks there are also certain "hidden" sinks which
influence the recombination rate. Such sinks might be shallow interstitial
clusters of subcritical size. No theory has incorporated the role played
by such clusters.
In the present work the influence of
the special deposition procedure of LPVD on the initial kinetics of film
formation is investigated.
A study is made of the influence of
the elastic fields of spherically symmetric precipitation nuclei on the
concentration profiles and steady-state diffusion fluxes of intrinsic point
defects in the elastic field on an edge dislocation. Relatively simple
analytical expressions are obtained for the efficiency of absorption of
vacancies and interstitial atoms by an edge dislocation. It is predicted
that the fields of the precipitation nuclei should tend to screen the fluxes
of interstitial atoms toward the dislocation.
В работе рассмотрена процедура расчета скорости радиационного распухания при учете гетерогенности материала в виде когеррентных предвыделений второй фазы.
Изложены основы моделирования физических процессов на ЭВМ. Обсуждены основные методы моделирования, особенности граничных условий. Приведен набор несложных задач, описывающих радиационные процессы в твердых телах и решаемых методами машинного моделирования.
Other publications
[1]
Theory
of Radiation Processes in Metal Solid Solutions
Yu. V. Trushin
Nova Science Publishers Inc. New York, pp. 405, 1996
[2]
Предисловие к сборнику "Моделирование на ЭВМ дефектов в металлах
Ю. В. Трушин
Л. , Наука, стр. 3-5, 1990
Fullerenes
and carbon nanoclusters:Energetics and electronic structure
В работе теоретически исследованы высокочастотные возбуждения молекулярного диэлектрика C60. Модель сферически свернутой квантовой ямы применена для расчета дипольных (мультипольных в общем случае) мод отдельного кластера C60. Зная спектр и силы осцилляторов коллективных мод отдельного кластера, микроскопический континуальный подход применим для расчета спектра делокализованных возбуждений в кластерном кристалле. Затем, обычный формализм диэлектрической проницаемости позволяет рассчитать оптические характеристики материала в изучаемой области вакуумного УФ.
The two-parameter phenomenological
model allows to calculate formation energies of different nanotubes. The
energy, geometry and size of the optimal tube were found. This tube has
the minimal formation energy at the definite number of atoms. A region
of energetical stability of tubes in respect with planar fragment of graphite
sheet was investigated. We accounted for the possible dangling bond energy
passivation, which essentially changes the energetics of formation process.
FULLERENES: CHEMISTRY, PHYSICS AND
NEW DIRECTIONS XI
Frenkel-exciton on finite icosahedral
lattice
S. F. Harlapenko, V. V. Rotkin
Frenkel Exciton model is solved for
a single fullerene cluster,which icosahedral lattice has a very high symmetry.
That allowsto find analytically spectrum and dipole moments of 5 modes
whichare optically active.
The modified three parameter phenomenological
model of the energetics of the carbon cluster growth is applied to the
pure carbon nanocluster formation process as well as to the formation of
clusters with the passivated bonds. The results in these two cases are
distinct in principle. The closed spherical cluster with no dangling bonds
is always energetically favourable in the original model. While the relative
instability region of spheres in respect with the tubes (and moreover,
with the planar graphite) opens at some critical bond softening (the analytical
formulas are presented).
The standard theory of Frenkel-exciton
(small radius exciton) is applied to the fullerene 2D solid. It is the
dipole collective electron excitation of a single cluster which forms the
delocalized plasmon-Frenkel-exciton (PFE) in a crystal. The PFE retarded
interaction is taken into account. We present transverse PFE-polariton
dispersion curves along with the Coulomb problem solution for longitudinal
excitation in the 2D plane.
Рассмотрен кулоновский гамильтониан экситона малого радиуса, заданный на кластере, атомы которого расположены в узлах групповой решетки. Методами гармонического анализа решеточного гамильтониана спектр экситонов Френкеля может быть получен аналитически для определенных мод. В качестве примера приводится углеродный кластер C60 икосаэдрической симметрии.
It is shown that the van-der-Waals
energy of the cluster-cluster interaction in the fullerene systems is described
well via the change of the plasmon energy. The energetical reason gives
a guess that the solution of the single C60 cluster is not favorable.
Contrary, the dimer solution energy is negative, which results in the supposition
that the dissolving occurs via the dimerization. The possible experiment
to proof the result is discussed.
The theoretical investigation of the
electronic structure of the fullerene cluster solid predicts that in the
high frequency region the optical response of the chain and layered cluster
structures has the polariton phenomena like in the insulator solid. The
dispersion of the dipole collective modes in 1D and 2D systems is presented.
The analogy between these fullerene excitations and the low-dimensional
exciton-polariton is traced.
The dielectric function approach was
used to evaluate the van-der-Waals energy of C60 cluster interaction
in different systems through the calculation of the change of the collective
mode energy. Within the same approach the analytic expression for the attraction
in the fullerene solid, in the fullerene dimer, in a solution, in a compound
material is obtained. The correct mean field result for the plasmon frequency
of C60 cluster in the insulating uniform medium is presented.
Диссертация основана на теоретических исследованиях, проведенных в период 1993-1997 гг. , по расчету и моделированию электронной структуры кластера фуллерена C60, энергий образования и процессов взаимодействия наноразмерных кластеров на основе углерода, исследованию вклада высокочастотных коллективных мод кластера в оптические характеристики кластерного материала, в диэлектрическую проницаемость кристаллического фуллерена, в энергию ван-дер-ваальсовского взаимодействия.
Диссертация состоит из введения, пяти глав, заключения и библиографии. Объем диссертации составляет 142 страницы текста, в том числе 19 рисунков и список литературы, включающий 138 публикаций, отдельно приводится список работ автора по теме диссертации: 27 наименований.
Основные результаты диссертационной работы:
Предложена модель свернутой в сферу квантовой ямы для квантово-механического расчета электронной структуры C60, показано, что решения сферически симметричной задачи описывают замкнутую электронную структуру кластера. В рамках модели ССКЯ получены в приближении случайных фаз динамическая поляризуемость отдельного кластера и диэлектрическая проницаемость кристалла; проанализированы статический и высокочастотный пределы. Показано, что в высокочастотном пределе поляризуемость кластера определяется коллективной электронной поверхностной плазменной модой, получена зависимость частоты плазмона от его углового момента. Продемонстрировано, что классическое описание плазмона в рамках гидродинамики заряженной электронной жидкости на поверхности кластера полностью согласуется с результатом квантово-механической модели в высокочастотном пределе; в рамках гидродинамического подхода исследовано взаимодействие плазменных колебаний двух компонент, s и p валентных электронов кластера. Изучено кулоновское взаимодействие двух кластеров фуллерена; показано, что в димере происходит смешивание плазмонов, рассчитан спектр элементарных возбуждений, проанализирована применимость теории возмущений в мультипольном разложении потенциала взаимодействия для кластера C119. Рассчитан спектр высокочастотных коллективных возбуждений типа экситона Френкеля в кристалле, пленке или полимеризованной цепочке фуллереновых кластеров, в которых роль элементарного возбуждения играет поверхностный плазмон отдельного кластера, обладающий дипольным моментом. Получена зависимость частоты плазмона в отдельном кластере, находящемся в изотропной диэлектрической среде, исправлен известный ранее феноменологический результат для частоты плазменной моды C60 в изотропном жидком диэлектрике. Рассчитаны энергии ван-дер-Ваальсовского взаимодействия фуллереновых кластеров в твердом теле, кластера с изотропной диэлектрической средой. Предложена континуальная модель для оценки энергии кривизны углеродной поверхности различных фуллереновых кластеров, использующая три параметра, которые могут быть определены методами квантовой химии или извлечены из данных эксперимента. В рамках модели рассчитаны энергии образования сфероидальных, полиэдрических, цилиндрических и эллипсоидальных замкнутых кластеров. Найдено, что существует цилиндрический кластер минимальной энергии при фиксированном числе атомов, определено соотношение между длиной и диаметром такого кластера; Модель применена для энергетического анализа процесса образования сфероидальных кластеров из плоского фрагмента листа графита. В рамках модели установлено, что для фуллереновых кластеров определенного размера энергетически выгоднее образование фасетированной полиэдрической структуры, чем сфероида постоянной кривизны;
Energies of formation of CN
carbon clusters (CC) shaped as cylinders, spheres, icosahedral polihedra,
capsules, partially opened spheres and capsules were calculated in frame
of phenomenological model.
High frequency plasma excitation of
C60, which was detected by optical methods and EELS, is found
to consist of two coupled components. A comparison with a graphite planar
structure shows that electrons of s
and p valence bands
with different symmetry participate in common oscillations. This oscillation
energy is calculated within a simple model. We investigated some possible
mechanisms of a strengthening of lower collective excitation frequency
and found it to be similar to the acoustic plasma branch in the solids.
The frequencies of two branches depend on the plasmon multipole index like
the two-dimensional plasmon frequency depends on the wavevector.
Model of coupled spherical quantum
wells is applied to C119 molecule, which consists of two globes
having dumb-bell shape. Within the model a dipole collective excitation
series obtained. Two types of coupled oscillation exist with energy shifted
up and down from C60 molecule plasmon energy depending on dipole
polarization. Recently observed photopolymerization and chemical polymerization
of fullerene molecules gives an example of chain of connected globes. In
tight-binding approximation the energy band for composed collective oscillations
of chain of fullerene molecules is obtained. Optically active, transverse
collective excitation with energy about 28 eV slightly disperged is obtained.
A phenomenological model for calculation
of formation energy of (CNC) carbon nano-clusters of definite shape was
suggested. The model uses three energetic parameters, Ec and
W5, being determined from comparison with experimental data
and results of computer simulation for various CNC, and dangling bound
energy. We showed that there is a optimal opened tube having minimal energy
keeping number of atoms N constant. Both tube length L and diameter D are
determined uniquely by N and co-dependent as L~D2 so L >>
D. We calculate energies corresponded to spheroidal CNC and polyhedra-shape
clusters of Y- symmetry, being topological equivalent to spheroids. First
are energetically favorable for the bigger N but it was shown that under
proper parameters chosen a such region of cluster size exists where fullerenes
of polyhedral configuration have lower energy.
Model of coupled spherical shell quantum
wells is applied to C119 molecule. In a frame of the model the
frequencies of the collective excitation series are calculated. The lowermost
excitation, which could be detected by optical methods, is founded to have
essentially dipolar character. Two types of the coupled oscillations exist
with a different parity, which frequencies shifted up and down from C60
molecule plasmon frequency. For a dipole mode the two plasmon peaks with
a gap about 3. 5 eV have different dipole polarization. A transition to
the usual coupled plasmons in two metal planes is fulfilled.
The Spherical Quantum Well model for
C60 has been put forward. It reveals the reasonable values of
the energy gap, static permittivity, the transmitted beam EELS plasma frequency
in the solid fullerene. The single molecule plasma oscillations frequencies
are discussed in connection with the reflected beam EELS data.
Представлена модель свернутой в сферу квантовой ямы, позволяющая рассчитать ширину запрещенной зоны фуллерена, частоты молекулярных коллективных возбуждений, диэлектрическую функцию кристалла в дипольном приближении. Поляризуемость молекулы, определяемая в рамках модели методом самосогласованного поля, содержит большой фактор деполяризации.
Представлено современное состояние некоторых аспектов исследования C60 и его производных. Рассмотрена симметрия отдельной молекулы, структура молекулярного кристалла, электронные свойства, особенности динамики решетки и, как следствие этого, существование различных фаз кристаллического C60. Дан краткий обзор строения фуллеридов с металлической проводимостью.
Luminescence
in semiconductors and nanostructures
A mechanism is proposed by which inverse
distribution of Er3+ f-electrons can be created in semiconductors
with quantum wells. It is shown that, if the electrons are localized in
quantum wells, the Coulomb excitation of Er3+ f-electrons
by electrons of the semiconductor has resonant nature. The double Coulomb
excitation of f-electrons, I 15/2 
I
11/2
and
I
15/2 I
13/2
,
produces electron population inversion for the I
13/2
level.
A possibility is demonstrated of developing a laser with wavelength 
=1.
54 microns based on I 13/2 I
15/2
transitions.
A model is presented which explains
the high degree of linear polarization of luminescence, which was observed
in porous silicon under non-resonant excitation. Porous silicon is supposed
to be composed of elongated nanocrystals. We show that because of the anisotropy
of the depolarizing field in silicon needles polarized light excites preferentially
those nanocrystals which emit light with the same polarization.
A model is presented which explains
the high degree of polarization of luminescence observed in porous silicon
under non resonant polarized excitation. It is assumed that the luminescence
comes from Si ellipsoids or Si chains in Si0. The polarization of light
is obtained by a two-step process, selection of ellipsoids or chains elongated
along the polarization direction of light and preferential emission along
the same direction.
The possible mechanisms for the increase in the f - f emission efficiency in semiconductors doped with rare-earth elements, including when the f shell of the impurity center is excited as a result of Auger recombination of an electron-hole pair localized on an impurity atom, were considered. Two new mechanisms for the excitation of f - f emission were analyzed. For the first mechanism, impurities are introduced into the heterostructure with quantum wells. For the second mechanism an extrinsic semicoductor is inserted into a quantizing magnetic field. It was shown that in each case the Coulomb exitation of an f electron of the impurity atom in an electron-hole pair in the semiconductor is of a resonance character. In the process the f - f emission excitation increases by several orders of magnitude.
Other puplications
[1]
Two
nowel mechanism of f - f luminescence resonance xitation in semiconductors
G. G. Zegrya and V. F. Masterov
Proc. SPIE, vol. 2706, 1996
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