Microstructural studies of epitaxial Ge films grown on [100] GaAs by laser photochemical vapor deposition

Abstract

Ge films grown on [100] GaAs by laser photochemical vapor deposition (LPVD) in parallel geometry at temperatures (Ts) ranging from ∼240 to 415 °C have been examined by transmission electron microscopy. For 285 ≤Ts≲330 °C, a thin (250–700 Å) epitaxial film is grown initially but a switch to amorphous material is subsequently observed. At higher substrate temperatures (Ts ≳400 °C), thicker (≳800 Å) epitaxial Ge films are grown before the transition to polycrystalline material takes place. In the absence of external 193-nm laser radiation (i.e., growing by conventional low-pressure chemical vapor deposition), the Ge films are completely amorphous (285 ≤Ts≤330 °C) or heavily defected polycrystalline (Ts∼400 °C). The >100 °C temperature reduction for the growth of epi Ge films made possible by LPVD is attributed to the direct production of a species (GeH3) by the laser which is collisionally converted to Ge2H6 en route to the substrate. Upon reaching the [100] GaAs surface, the digermane is pyrolyzed. Experiments with [100] substrates tilted 3° toward [110] yielded thinner (∼120 Å), but smooth, epitaxial films which is attributed to the higher density of available nucleation sites. Films grown at 280–330 °C on [111] oriented GaAs were completely amorphous which appears to arise from reduced adatom mobilities on [111] surfaces. These results clearly demonstrate the feasibility of photochemically generating a species which migrates to the surface and alters the chemistry at a substrate (which is itself not illuminated by the optical source) so as to permit the growth of epitaxial semiconductor films.