Heavy Ga Doping of Ge and Ge_1-xSi_x Semiconductors by CVD Using GaH₃·N(CH₃)₃

We describe a new chemical method for doping and hyper-doping Ge and Ge_1-xSi_x with Ga acceptors for potential applications in high-speed transistors. We employ the volatile GaH₃·N(CH₃)₃ Ga source as a better alternative to metalorganic precursors for low temperature deposition of epitaxial layers grown directly on Si and Ge-buffered Si substrates. Proof-of-concept samples were initially produced at 380-400 °C using a single-wafer epitaxy reactor operating at 10^-5 Torr and designed for small substrates to test GaH₃·N(CH₃)₃ as a dopant and demonstrate feasibility. The process was subsequently transitioned to a multiwafer UHV CVD system which supports larger substrates suitable for device fabrication. This system enabled depositions at lower-temperature (<360 °C) and higher-pressure for better tuning of Ga substitution, improved film homogeneity, and enhanced stoichiometric control. We found that in both cases GaH₃·N(CH₃)₃ reacts cleanly with Ge₄H₁₀ and Si₄H₁₀ sources to deliver pure Ga atoms without carbon contaminants and preventing Ga surface segregation issues. Epitaxial growth of p-type Ge and Ge_1-xSi_x layers is achieved, resulting in optimal crystal quality and minimal defect levels, as evidenced by Rutherford backscattering (RBS), X-ray diffraction, (XRD) and cross-sectional transmission electron microscopy (XTEM). The Si contents in the Ge_1-xSi_x films range from x = 0.06 to 0.22, as determined by RBS and XRD. The Ga concentrations are measured by secondary ion mass spectrometry (SIMS) and found to be in the range of (1.5 × 10²⁰)-(1 × 10²¹)/cm³. These values closely follow the active carrier concentrations of (1 × 10²⁰)-(9 × 10²⁰)/cm³ determined by spectroscopic ellipsometry (and, in a few cases, by Hall effect measurements), indicating high levels of activation and heavy doping in the as grown samples, without the need of additional postgrowth thermal treatments. Resistivities as low as (4-6) × 10^-4 Ω cm have been measured. Only background levels of C, ranging from 6.0 × 10¹⁶/cm³ to 9.0 × 10¹⁶/cm³, are determined by SIMS. This confirms that our doping method is compatible with the goal of low contact resistance in transistors. Photoluminescence spectra of Ga:Ge samples show the characteristic emission due to direct and indirect bandgap recombination as well additional signatures characteristic of p-type Ge. Functional p-i-n diodes featuring Ga-doped Ge components were successfully demonstrated, highlighting the potential of this approach for device applications.