Optical Studies of Iii-V Semiconductor Heterostructures
Abstract
We investigated the optical properties of rm In_ xGa_{1-x}As/GaAs single quantum well structures, GaAs/Si and InSb/GaAs heteroepitaxial films. The electrolyte electroreflectance, photoreflectance, photoluminescence, and Raman scattering techniques were used to study the effects of strains caused by: (i) lattice mismatch between the epilayer and the substrate and (ii) mismatch in their thermal expansion coefficients. The electronic transitions of rm In_ xGa_{1-x}As/GaAs were determined experimentally by fitting the photoreflectance spectra and theoretically by solving the finite quantum well problem. The results show that the rm In_ xGa_{1-x}As layers are fully strained even for thicknesses greater than the critical thicknesses predicted theoretically. Strain in the InGaAs quantum wells is compressive and in the range of 2.2 times 10^{ -3} to 9.7 times 10 ^{-3}, increasing with increasing In mole fraction. The valence band offset in the strained rm In_ xGa_{1-x}As/GaAs hetero-interface is between 35-55% for 0.1 <=q x <=q 0.2 and well thicknesses in the range of 10-35 nm. The strain in GaAs/Si and InSb/GaAs was studied as a function of distance from the film-substrate interface by successive chemical etch of the epilayer. The E_1 and E_1 + Delta_1 gaps of GaAs shift to higher energies as the layer thickness decreases. Tensile strain, due to thermal mismatch, changes to compressive, due to lattice mismatch, when the GaAs thickness is ~95 nm. Compression dominates in the layers close to the interface (<70 nm). The GaAs crystal quality degrades as we approach the interface due to high dislocation density and disorder. The InSb/GaAs heterostructure has a larger lattice mismatch than GaAs/Si. It showed, however, smaller changes as a function of InSb thickness. The crystal quality is worse for the InSb layers closer to the interface due to dislocations. The use of an undoped InSb interface layer improves the quality of the epilayer. The critical thickness of this system is small (~2 nm), much smaller than the depth probed in our experiments. Furthermore, the thermal mismatch is not sufficient to cause significant strain in the bulk of InSb as in GaAs/Si. The InSb epilayer shows no disorder and grows with the same (100) orientation of the GaAs substrate.
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1990
- Bibcode:
- 1990PhDT.......172K
- Keywords:
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- PHOTOLUMINESCENCE;
- Physics: Condensed Matter; Engineering: Materials Science