Ge(001) gas-source molecular beam epitaxy on Ge(001)2×1 and Si(001)2×1 from Ge2H6: Growth kinetics and surface roughening

Abstract

The growth rates RGe of epitaxial Ge films deposited on Ge(001)2×1 and Si(001)2×1 substrates from Ge2H6 by gas-source molecular beam epitaxy were determined over a wide range of temperatures Ts (300–800 °C) and impingement fluxes JGe2H6(0.1–1×1016 cm−2 s−1). Steady-state RGe(Ts, JGe2H6) curves were well described at both low and high growth temperatures (Ts≤325 °C and Ts≳500 °C) using a model based upon dissociative Ge2H6 chemisorption followed by a series of surface decomposition reactions with the rate-limiting step being first-order hydrogen desorption from Ge monohydride for which the activation energy was found to be 1.56 eV. At intermediate temperatures, however, experimental RGe results exhibited a large positive deviation from model predictions due, as demonstrated by temperature programmed desorption measurements and transmission electron microscopy (TEM) observations, to kinetic surface roughening. Extensive (113) faceting resulted in both an increase in the number of active surface sites and higher reactive sticking probabilities. With increased growth temperatures, the facets became more rounded and film surfaces appeared sinusoidal in cross section. The zero-coverage Ge2H6 reactive sticking probability on Ge(001) in the high-temperature flux-limited regime was found to be 0.052, more than two orders of magnitude higher than that for GeH4. In situ reflection high-energy electron diffraction and post-deposition TEM examinations showed that Ge films deposited on Ge(001) at Ts≤325 °C grew in a layer-by-layer mode exhibiting a smooth flat surface.