Intrinsic and dopant enhanced solid phase epitaxy in amorphous germanium

TL;DR

This study investigates the solid phase epitaxy (SPE) process in amorphous germanium, revealing how intrinsic and dopant-enhanced rates depend on temperature, hydrogen presence, and dopant types, with new measurements on thick layers and dopant effects.

Contribution

It provides the first measurements of SPE rates on thick amorphous germanium layers and analyzes the effects of hydrogen and dopants on SPE kinetics.

Findings

SPE rate has an activation energy of 2.15 eV.

Hydrogen retards SPE more in germanium than in silicon.

Dopant effects vary with concentration and compensation.

Abstract

The kinetics of intrinsic and dopant-enhanced solid phase epitaxy (SPE) is stud- ied 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 degC 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 {\mu}m. For the first time, SPE rate measurements on thick a-Ge layers (>3 {\mu}m) 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 affect on the SPE rate than for similar concentrations in a-Si layers. Hydrogen is found to reduce the pre-exponential 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 that 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 were measured in a-Ge layers containing uniform concentration profiles of implanted As or Al spanning the concentration regime 1-10 x1019 /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.