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In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitorthe growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface ac-cumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly showsthat, even at low growth temperaturesof ~550 °C, not all of the supplied indium isincorporated into the quantumwell (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of theQW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior.The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. ThePL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at~423 nm forN2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to ex-plain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs 

Crystalline zinc blende GaAs has been grown on a trigonal c-plane sapphire substrate by molecular beam epitaxy. The initial stage of GaAs thin film growth has been investigated extensively in this paper. When grown on c-plane sapphire, it takes (111) crystal orientation with twinning as a major problem. Direct growth of GaAs on sapphire results in three-dimensional GaAs islands, almost 50% twin volume, and a weak in-plane correlation with the substrate. Introducing a thin AlAs nucleation layer results in complete wetting of the substrate, better in-plane correlation with the substrate, and reduced twinning to 16%. Further, we investigated the effect of growth temperature, pregrowth sapphire substrate surface treatment, and in-situ annealing on the quality of the GaAs epilayer. We have been able to reduce the twin volume below 2% and an X-ray diffraction rocking curve line width to 223 arcsec. A good quality GaAs on sapphire can result in the implementation of microwave photonic functionality on a photonic chip. 

The near-bandgap optical properties of Ge1-xSnx alloys were characterized by photovoltage spectroscopy and spectral ellipsometry measurements. Contributions of Urbach tailing as well as direct and indirect optical transitions were observed. The compositional dependence of direct bandgaps of strained GeSn films grown on a Ge buffered Si substrate was studied for up to 15% Sn content. The contribution to the photovoltage spectra of Ge1-xSnx alloys (x < 6%) from indirect optical transitions was observed at lower energies than from direct bandgaps. Using bowing parameters, a correlation was detected between calculated and measured indirect and direct bandgaps at 82 K. As the Sn content was increased, the difference between the energies of the indirect and direct bandgaps decreased, resulting in a smaller contribution of the indirect transitions due to competition with direct transitions and Urbach tails. Two sublayers with different Sn content, strain values and bandgaps were observed for samples with x ~12%. The results indicated that strain relaxation in films with thicknesses exceeding a critical value occurs via formation of a Sn-rich top layer with higher direct bandgap. These findings have important implications when designing IR photodetectors or solar cells.