Deposition and etching of refractory ohmic contacts to GaAs for self-aligned technology

Abstract

Refractory ohmic contacts to n-type GaAs have been developed for heterojunction bipolar transistors (HBTS) processed with self-aligned technology and including high-temperature anneals. We report the results on W and MoGe deposition, annealing and the reactive ion etching (RIE) operations necessary for HBTS' self-aligned processing. These refractory metallizations were fabricated on n-type epitaxial GaAs layers with a carrier concentration of 1018 cm-3. Ge was evaporated initially by an electron beam technique, while Mo and W films were deposited separately by sputtering in RF plasma. The MoGe ohmic contact formation is due to both an n+ layer created at the GaAs contact interface by Ge overdoping and molybdenum germanide stable phase formation. Low specific contact resistivities were obtained with experiments combining As-doped Ge and As overpressure. It was then possible to reduce the resistivity to 2*10-7 Omega cm2. Reactive ion etching of W, Mo and Ge is also described. Etch rates are compared using either photoresists or Pt masking materials. A Pt mask was found to be much more suitable than photoresist because it allows higher selectivities and etch rates. Surface contamination on the GaAs surface is a)so discussed and could be minimized for optimal device performance.