Intrinsic and dopant-enhanced solid-phase epitaxy in amorphous germanium

The kinetics of intrinsic and dopant-enhanced solid-phase epitaxy (SPE) is studied in amorphous germanium (a-Ge) layers formed by ion implantation on 100 Ge substrates. The SPE rates were measured with a time-resolved reflectivity (TRR) system between 300 and 540°C and found to have an activation energy of (2.15±0.04) eV. To interpret the TRR measurements the refractive indices of the a-Ge layers were measured at the two wavelengths used, 1.152 and 1.532 μm. For the first time, SPE rate measurements on thick a-Ge layers (>3 μm) have also been performed to distinguish between bulk and near-surface SPE growth rate behavior. Possible effects of explosive crystallization on thick a-Ge layers are considered. When H is present in a-Ge it is found to have a considerably greater retarding effect on the SPE rate than for similar concentrations in a-Si layers. Hydrogen is found to reduce the preexponential SPE velocity factor but not the activation energy of SPE. However, the extent of H indiffusion into a-Ge surface layers during SPE is about one order of magnitude less than that observed for a-Si layers. This is thought to be due to the lack of a stable surface oxide on a-Ge. Dopant-enhanced kinetics was measured in a-Ge layers containing uniform concentration profiles of implanted As or Al spanning the concentration regime 1-10× 1019 / cm-3. Dopant compensation effects are also observed in a-Ge layers containing equal concentrations of As and Al, where the SPE rate is similar to the intrinsic rate. Various SPE models are considered in light of these data.