Microstructure of Epitaxial Ge Films Deposited on (111) CaF2 Substrates

Abstract

A more comprehensive experimental investigation of the microstructure of single-crystal Ge films deposited on polished (111) CaF2 substrates in a vacuum of 10−9 to 4×10−7 Torr has been made as a function of deposition rate, substrate temperature during deposition, and film thickness. Comparisons have also been made using cleaved substrates and a vacuum of 10−5 Torr. Transmission electron microscopy of thin films has shown that the density and complexity of defects increased with deposition rate and decreasing substrate temperature. The initial defect density was 109-1011 per cm2 for cleaved or polished substrates and for deposition in both vacuum systems. Little change occurred up to thicknesses of 2000–4000 Å. Electron microscopy of surface replicas for thicker films indicated that the density of faults intersecting the surface decreased by three orders of magnitude over the 5100- to 171 000-Å thickness range. The results for thin films, consistent with the growth mechanism associated with nucleation theory, indicated that for this unlike substrate-film combination the principal source of defects was the intergrowth of neighboring nuclei in other than perfect register and the decrease in density with thickness a result of interactions of defects as they are propagated with film growth.