Reduced-pressure chemical vapor deposition of boron-doped Si and Ge layers

Abstract

We have studied the in-situ boron (B) doping of germanium (Ge) and silicon (Si) in Reduced Pressure-Chemical Vapor Deposition. Three growth temperatures have been investigated for the B-doping of Ge: 400, 600 and 750°C at a constant growth pressure of 13300Pa (i.e. 100Torr). The B concentration in the Ge:B epilayer increases linearly with the diborane concentration in the gaseous phase. Single-crystalline Ge:B layers with B concentrations in-between 9∙1017 and 1∙1020cm−3 were achieved. For the in-situ B doping of Si at 850°C, two dichlorosilane mass flow ratios (MFR) have been assessed: F[SiH2Cl2]/F[H2]=0.0025 and F[SiH2Cl2]/F[H2]=0.0113 at a growth pressure of 2660Pa (i.e. 20Torr). Linear boron incorporation with the diborane concentration in the gas phase has been observed and doping levels in-between 3.5∙1017 and 1∙1020cm−3 were achieved. We almost kept the same ratio of B versus Si atoms in the gas phase and in the Si epilayer. By contrast, roughly half of the B atoms present in the gas phase were incorporated in the Ge:B layers irrespective of the growth temperature. X-Ray Diffraction (XRD) allowed us to extract from the angular position of the Ge:B layer diffraction peak the substitutional B concentration. Values close to the B concentrations obtained by 4-probe resistivity measurements were obtained. Ge:B layers were smooth (<1m root mean square roughness associated with 20×20μm2 Atomic Force Microscopy images). Only for high F[B2H6]/F[GeH4] MFR (3.2 10−3) did the Ge:B layers became rough; they were however still mono-crystalline (XRD). Above this MFR value, Ge:B layers became polycrystalline.