In this paper, we present GaAs/Ge heteroepitaxy grown by RIBER32 solid-source Molecular Beam Epitaxy (SSMBE) with initial GaAs nucleation by migration-enhanced epitaxy (MEE) technique. We look at the influence of substrate temperature during the MEE and the influence of Ge surface orientation to the quality of the GaAs layer. Three samples were grown for this study; the first two samples, sample A and B, have surface orientation of (100) 6° offcut towards (111) plane. This surface orientation was chosen as to achieve double-atomic steps surface that is crucial to suppress anti-phase domain (APD) formation. These samples were then subjected to different temperature during the MEE process, 450°C for sample A and 250°C for sample B. The third sample, sample C, has the same MEE substrate temperature as sample A, which is 450°C but with nominal (100) surface orientation. The growth conditions and structure of the layers after the MEE process were kept constant across the three samples.
We examine the structures and the optical quality of the samples by cross-sectional Transmission Electron Microscope (XTEM) and 5K photoluminescence (PL). Analyzing the XTEM images of sample A and B, it is found that the APD still appear in sample A while it is suppressed totally in sample B. It is also observed from sample A XTEM image that two APDs propagating with 45° and 135° angle, as measured from the GaAs/Ge interface, could meet with each other at certain layer thickness leading to self-annihilation. However, some of the self-annihilations took place only after the APDs propagated hundreds of nanometers in the GaAs layer, producing large undulation in the layer. As for sample B, we believe that the APD suppression is mainly due to the much lower nucleation temperature. At such low temperature, As dimers are adsorbed onto the substrate surface more readily with negligible re-evaporation. This ensures complete As coverage on the double-atomic steps Ge surface and minimize As vacancies that may act as defect initiation centers. Furthermore, the low substrate temperature shortens the migration distance of Ga adatoms, minimizing their adsorption into the kinks and step edges, resulting in two-dimensional growth mode instead of step-flow growth mode.
Meanwhile, XTEM image of sample C shows APDs that propagated almost perpendicularly to the GaAs/Ge interface, making it unlikely for the self-annihilation mechanism to take event. It is understood that the direction of propagation of the APD relies on the atomic steps reconstruction of the Ge starting surface. In the case of single atomic-step surface, the APDs propagate almost perpendicularly to the interface. Finally, the 5K PL spectra of the embedded InGaAs single quantum-well in all the samples clearly demonstrate that the best optical quality comes from sample B; which we believe mainly because of the total suppression of APD, leading to much improved layer quality.