Modeling and Simulation Study of Electrical Properties of Ge-on-Si Diodes with Nanometer-thin PureGaB Layer

Abstract

Deposition of a nanometer-thin layer-stack of pure gallium and boron (PureGaB) on arsenic (As)-doped epitaxial germanium (Ge) forms a shallow-junction photodiode, reported to have almost ideal I-V characteristics, low saturation current densities, and high responsivity down to 255 nm wavelengths. In this work, different physical mechanisms that could explain the high anode Gummel number in PureGaB-Ge-on-Si diode have been examined. A model for point-defect-mediated diffusion of B and Ga in Ge has been developed. Formation of a shallow pn junction has been modeled using 1D process simulations of B and/or Ga drive-in from the PureGaB layer. B diffusion resulted in junction depths less than a nanometer deep, while Ga formed a highly doped p+ regions with peak concentrations> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> and junction depths from 31 nm to 123 nm, depending on the applied sets of diffusion parameter. Both approaches have been used to fit the I-V characteristics of a fabricated PureGaB Ge-on-Si diode: B-only diffusion model with negative interface charge concentration of 1.9 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> for suppression of electron injection and Ga diffusion model, self-sufficient for the explanation of low electron current densities. Both proposed models give possibilities to obtain a Gummel number of ≈ 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> s/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , matching the value extracted from I-V characteristics of a fabricated device.