Heteroepitaxy of GaAs on (001) ⇒ 6° Ge substrates at high growth rates by hydride vapor phase epitaxy

Abstract

The growth of GaAs on (001) ⇒ 6° Ge substrates by hydride vapor phase epitaxy has been investigated. The effects of varying deposition temperature and gas phase supersaturation on growth rate and material quality as determined by atomic force microscopy measured surface roughness and x-ray diffraction were established. GaAs growth rates up to 44 μm/hr were achieved. The deposition temperature has a strong effect on growth rate under the investigated range of growth conditions indicating that growth is typically limited by surface kinetic processes. An apparent activation energy of 35.1 ± 2.0 kcal/mol was determined for growth on these Ge substrates, agreeing well with past kinetic data for GaAs growth on GaAs substrates. The deposition temperature also had a significant effect on both root mean square surface roughness and x-ray full width at half maximum, with minima of 0.92 nm and 26 arcsec occurring for samples grown at temperatures of 725 °C and 750 °C, respectively. These values are comparable to or better than values measured for GaAs on Ge layers grown by metalorganic vapor phase epitaxy. The use of a thin Si3N4 coating on the Ge substrate backside mitigated the observed Ge gas phase autodoping effect. With back surface passivation, GaAs background doping levels within the GaAs epilayer of n = 1.2 × 1016 cm−3 were achieved 2.3 μm from the heterointerface. The heterointerfaces of the samples grown at 725 °C and 775 °C were imaged by transmission electron microscopy. Anti-phase domain boundaries (APBs) were observed near the heterointerface of the 775 °C sample. These APBs self-annihilated after roughly 100 nm of epilayer thickness. The 725 °C sample exhibited no APBs in the vicinity of the interface or elsewhere in the film, indicating a more optimal growth temperature. Ge diffusion through the GaAs/Ge interface was profiled by secondary ion mass spectrometry and multiple regions of diffusion behavior were observed. In the region of high Ge concentration ([Ge] > 5 × 1019 cm−3) closest to the heterointerface, the concentration vs. position data fit a vacancy-assisted diffusion mechanism. The data between 0.05 and 0.20 μm from the heterointerface were fit to a concentration independent, semi-infinite diffusion model with a constant diffusion coefficient. These models indicate that complex mechanisms control diffusion during growth at these temperatures.