Low-resistance contacts with chemical vapor deposited tungsten on GaAs grown by molecular layer epitaxy

Approximately 100 Å thick, mirror-like, ordered tungsten layers were fabricated on (100) GaAs with chemical vapor deposition, using a W(CO)₆ precursor. Native oxides on GaAs were reduced with AsH₃ pretreatment in a chemical vapor deposition chamber of tungsten. Secondary ion mass spectrometry and Rutherford backscattering spectroscopy analyses indicate tungsten-GaAs mixed layer not thicker than 20 Å. The chemical vapor deposited tungsten was applied to fabrication of state of the art, low resistanc'e contacts to GaAs layers grown with molecular layer epitaxy. Optimization of GaAs doping, pretreatment, and tungsten chemical vapor deposition conditions were necessary to obtain low contact resistance. Contact resistance, measured with the transmission line method, was 3 × 10^-7 Ω cm² to Te doped n-type GaAs and below 2 × 10^-8 Ωcm² to C doped p-type GaAs. Electrically active donor/acceptor concentrations in GaAs layers grown by molecular layer epitaxy were about one order of magnitude lower than expected from literature trend lines for these contact resistance.