Cold-wall ultrahigh vacuum chemical vapor deposition of doped and undoped Si and Si1−xGex epitaxial films using SiH4 and Si2H6

Abstract

Doped and undoped silicon homoepitaxy and Si1−xGex-on-Si heteroepitaxy have been achieved by cold-wall ultrahigh vacuum chemical vapor deposition using disilane (Si2H6), silane (SiH4), digermane (Ge2H6), and germane (GeH4) as the reactant gases. Boron and phosphorus doping were achieved by flowing B2H6 and PH3 dopant gases, respectively, into the reactor. The growth mechanism and film quality of intrinsic Si, B/P-doped films, and Si1−xGex alloys using SiH4 and Si2H6 are comparatively studied in this article. The crystallinity of the films was examined by Nomarski microscopy after Schimmel etching, transmission electron microscopy (TEM), and in situ reflection high energy electron diffraction (RHEED). The defect density of the intrinsic Si films grown by Si2H6 and SiH4 was found to be below the TEM detection limit (105/cm2). B-doped and P-doped single-crystal Si were deposited using Si2H6 or SiH4 with a partial pressure of 10 mTorr at substrate temperatures from 550 to 600 °C. The B- and P-doping concentrations were determined by secondary ion mass spectroscopy (SIMS). From plan-view TEM analysis, it was found that the defect density of the B- and P-doped Si films grown using Si2H6 is lower than that grown using SiH4 at even lower substrate temperatures. It was also found that the phosphine (PH3) ‘‘poisoning effect’’ for growth rate reduction and crystallinity degradation in Si2H6 chemical vapor deposition (CVD) is not as serious as in SiH4 CVD. A model for the PH3 poisoning effect was developed to explain the growth rate reduction in SiH4 CVD with high PH3 flux. The morphology of both P-doped and B-doped Si films using Si2H6 was found to be smoother than for those using SiH4, as indicated by plan-view TEM and by RHEED analysis. Si1−xGex alloys were deposited by using various reactant gases. The thickness of the Si1−xGex alloy films and the Ge mole fraction were measured by SIMS. A 50-period Si1−xGex–Si superlattice was grown using Si2H6 and GeH4. The transition layers were found to be extremely sharp and layer thickness control using Si2H6 was found to be excellent by cross-sectional TEM analysis. In this study, Si2H6 was found to be a better Si precursor compared to SiH4 under our growth conditions. From the growth rate study, the activation energies for the Si growth rate were found to be 1.47 and 1.60 eV for Si2H6 and SiH4, respectively. The activation energies for the Ge growth rate are 0.93 and 1.0 eV using Ge2H6 and GeH4, respectively. A growth kinetic model was developed to predict the growth rate of undoped Si for various reactant gas partial pressures and substrate temperatures. The adsorption rate preexponential constant for SiH4 was found to be about twice that of Si2H6. The model can also predict the Ge mole fraction in Si1−xGex alloys and B/P doping concentrations in the Si films.