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Background and Justification
Recent progress in synthesis and fabrication of devices based on the (In,Ga)N-(Al,Ga)N system allows to create light emitters working in blue and ultraviolet ranges. The growing interest in this area all over the world is evident. Lasers in this system are realized and close to appear on the market. These lasers are based on thin InGaN layers embedded in a GaN matrix.
Currently, only lasers for UV and blue spectral range were created. Attempts to shift the emission wavelength towards green by increasing the In content in the InGaN layer failed due to severe degradation of optical properties of the material with increase in the lattice mismatch. Creation of red lasers is currently thought to be impossible as this would require highly lattice-mismatched pure InN in the active region. However, lasers and LED for green and red fields based on one material system are very promising for different applications ( e.g. color laser-based TV etc.). This motivate active research of different approaches to shift the emission wavelength. One of these approach is use concept of formation of quantum dots via structural transformation thin InGaN strained layer. Structural investigations of InGaAsN-GaN systems show the existance of such kind of inhomogeneities. Additionally in InGaAsN composition modulations take place. This allows to shift emission wavelength toward long wavelength. But control of this morphological transformation is not developed for this system.
Early it was demonstrated theoretically and experimentally that GaAs1-xNx-GaAs and GaAsxN1-x-GaN system allows very strong wavelength shifts towards long wavelength range opening wide perspectives for various applications. Long wavelength lasers based on InGaAsN system on GaAs substrate have been demonstrated recently and exhibited lasing at 1.3 µm at room temperature and electroluminescence up to 1.45 µm far beyond the range available for conventional QWs. These results are demonstrated for the GaAs-rich corner of the Ga-As-N system and cubic material. Very recently, first publication related to luminescence properties of N-rich hexagonal GaAsN structures. The authors demonstrated strong increase of band gap emission and bright PL up to room temperature, but failed to demonstrate significant shifts of the PL line, as the maximum As concentration reported was only 0.5%. At the same time ultralarge shift of PL normalized to 1% is clearly demonstrated (~140 meV/%). Another bright result of As incorporation into GaN is improvement of the electrical properties of GaN. These results can be used for fabrication of the contact layers of LED.
Thus, motivation for As-containing lattice mismatched nitrides as an alternative (addition) to conventional InGaN/GaN system is following.
- Narrower band gaps. Using of GaAsN and InGaAsN provides a possibility to realise narrower effective band gap of the active layer for the same lattice mismatch and, respectively, larger spectral shift as compare with the InGaN material.
- Refractive indices. Significant difference between refractive indices of GaAsN and GaN (
 n  15·10-3/%GaAs) in comparison with the InGaN-GaN system (n6·10-3/%InN) can lead to efficient optical confinement despite on small As content. It is important to get lower threshold current density and realise some of the device application (e.g. vertical cavity lasers).
- AlGaN cladding layers with minor As concentrations. As AlGaN has large difference in lattice constant with GaN this leads to dislocation formation when thick AlGaN layers are deposited. Small additions of As (low than ~1 %) leads to compensate this difference without dramatic narrowing of AlGaN band gap that allows to improve quality of the thick AlGaN cladding layers.
- p-doping. It is currently understood, that the problem of p-doping of (Al,Ga)N layers is associated with formation of electrically inactive complexes between Mg dopant and H atoms during the growth process. These complexes of hydrogen and MgGa-N (Mg is the main p-dopant in the MOCVD technology) result in high resistivity of p-layers. As opposite, GaAs can be heavy doped by Mg in hydrogen atmosphere up to ~1021 cm-3. One can assume that finite As concentration in the GaN layer (where As atoms will replace part of N atoms) will lead to improvement in p-doping for the same Mg concentration. If only several per cent of Mg dopant atoms will be activated due to even minor As concentration, it gives a good chance to get p-layers with reasonable conductivity.
Because we are going to utilize new system to fabricate light emitting devices experimental work will be accompanied by theoretical consideration. As was mentioned above self-organized processes which lead to the formation of nanoscale QDs. Conventional theoretical approaches developed in high-frequency physics of semiconductors are inappropriate to predict high-frequency properties of semiconductor inhomogeneous structures with nanoscale incorporations of another material than host material like QDs or irregular or regular corrugated interfaces between different components of the semiconductor HSs. Thus, to describe the high-frequency properties of inhomogeneous light-emitting devices based on InGaAsN-GaN double heterostructures, a theory of two- and three-dimensional heterostructures should be developed and methods of macroscopic averaging or numerical simulations of multi-dimensional inhomogeneous systems should be elaborated as applied to nanograins in semiconductor matrices, as well as nanosized irregularities of the interfaces.
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