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GaNAs and GaAsBi : structural and electronic properties of two resonant state semiconductor alloys

University of British Columbia

Semiconductor alloys that are lattice matched to GaAs but have a smaller energy band gap are of interest for numerous applications, including infrared lasers for telecommunications, high efficiency solar cells, and high electron mobility transistors. For high optoelectronic efficiency, these materials must be highly perfect single crystals with low defect densities. In this thesis, two substitutional GaAs-based alloy families, nitrides and bismides, are investigated experimentally. In the first alloy, GaNAs, the addition of N results in a large band gap reduction, though the small size of the N relative to As introduces tensile strain into the lattice, and the high electronegativity of N attracts electrons. The second alloy, GaAsBi, also has a smaller band gap and is formed by the addition of the very large Bi atom to GaAs, which introduces compressive strain and tends to attract holes. The experimental investigations of these alloys focused on elucidating the relationships between the growth process, atomic structure, and electronic properties. Films were grown by molecular beam epitaxy (MBE) with in-situ process monitoring and subject to post-growth structural and electronic characterization. For GaNAs and a related alloy. InGaNAs, degradation in luminescence efficiency, mobility and structural integrity were observed as the nitrogen content of the alloy was increased. A comprehensive study of strain relaxation in compressively strained InGaNAs and InGaAs quantum wells revealed that the nitrogen alloying did not have an effect on the critical thickness for dislocation formation, or the dislocation density in relaxed films. At large lattice mismatch, InGaNAs quantum wells were observed to relax by means of unusually oriented pure edge-type misfit dislocations aligned with [left angle bracket]100[right angle bracket] directions, likely due to the high stress associated with the large misfit. Use of bismuth as a non-incorporating surfactant during growth was successfully applied to improve the material quality of the nitrides. The Bi surface layer during growth was investigated using in-situ electron diffraction intensity measurements, and was found to improve both the smoothness of nitride films, by promoting a layer-by-layer growth mode, as well as increasing the photoluminescence (PL) intensity by a factor of 2.4. The improvement in PL is attributed to a reduction in nitrogen clusters with the surfactant. In addition, an increase in nitrogen content of up to 50% was observed in films grown with Bi over films grown without the surfactant. The increase in the nitrogen incorporation scales with the Bi flux, and saturates at full Bi coverage. A modified Langmuir model was applied to describe the behaviour of Bi on the surface, as well as the increase in nitrogen incorporation. The bismide alloy family requires atypical MBE growth conditions such as low substrate temperature and low As overpressure in order to achieve Bi incorporation. The conditions are close to those where Ga and Bi droplets form. However, in-situ light scattering was used to identify and avoid growth with droplets, resulting in films with high structural quality as determined by x-ray diffraction, and strong photoluminescence. 1% Bi alloying in GaAs was also found to result in a minimal 15% decrease in electron mobility, as compared with a 94% reduction for 1% N alloying. Finally, a preliminary investigation was made into the concept of co-doping GaAs with both N and Bi. GaNAsBi films showed room temperature PL at long wavelengths commensurate with contributions to band gap reduction from both N and Bi. Since lattice matching to GaAs can be achieved with a Bi/N ratio of 1.7, GaNAsBi and GaAsBi are promising new alloys for the applications described.

Text

2011-02-10

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http://hdl.handle.net/2429/31188

Materials Engineering

University of British Columbia

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