Nonalloyed ohmic contacts to n-Si using a strained Si_0.50Ge_0.50 buffer layer

We have grown an 80-Å-thick strained Si_0.50Ge_0.50 layer on n-Si by molecular-beam epitaxy. The strained layer is used to lower the Schottky barrier height for making a nonalloyed shallow ohmic contact to the n-Si. X-ray photoelectron spectroscopy was employed to investigate the Si 2p and Ge 3d core-level binding energies of the strained and the relaxed Si_0.50Ge_0.50 and to determine their relative Fermi-level positions. Rutherford backscattering and Auger depth profiling were employed to determine the contact reactions using Ti, W, or Pt as contact metals. In the case of Pt, a 500-Å W diffusion barrier can protect the ohmic behavior up to 550 °C for 30 min. The specific contact resistance of the metal/Si_0.50Ge_0.50/n-Si contact extracted from the D-type cross-bridge Kelvin resistor was 3.5×10^-5 Ω cm².