Point defect formation near the epitaxial Ge(001) growth surface and the impact on phosphorus doping activation

Abstract

First-principles calculations are used to systematically investigate the impact of surface effects on the energetic cost to incorporate point defects near the growing surface [Ge(001)] and its consequence on the dopant activation in P-doped Ge layers. We illustrate the formation enthalpy & UDelta; H f for the incorporation of a mono-vacancy, which is the dominant point defect responsible for the electrically inactive dopant incorporation in P-doped Ge. The calculated values point toward a significant lowering of & UDelta; H f near the growing surface in comparison to the bulk cubic Ge supercell. The impact of the surface almost vanishes beyond the sixth atomic monolayer located below the surface and the calculated & UDelta; H f values overlap with the ones computed for a bulk cubic Ge supercell. For epitaxial Ge:P layers, grown at low growth temperatures ( < 450 & DEG;C) using the deposition method such as the Chemical Vapor Deposition, the dopant-vacancy clusters are formed within the first few monolayers close to the growing surface. The low & UDelta; H f values for the incorporation of a mono-vacancy near the growing surface suggest that the concentration of vacancies can be significantly enhanced with respect to the bulk Ge, which can subsequently trap more dopants and deactivate them at the surface itself. Our first-principles calculation results are in line with previously reported experimental positron annihilation spectroscopy analysis on Ge:P layers grown at 440 & DEG;C using GeH 4 as gas precursor. At P concentrations above 1 x 10 20 cm - 3, the mono-vacancy sized open-volume defects are passivated by at least three P atoms.