Thermally stable ohmic contacts to n-type GaAs. II. MoGeInW contact metal

Abstract

It was previously found that a small amount of In impurity was able to convert MoGeW contacts from Schottky to ohmic behavior yielding thermally stable, low-resistance ohmic contacts n-type GaAs. In the present experiment transport measurements and materials studies were carried out for MoGeInW contacts in which a thin layer of In was directly added to the MoGeW contacts during deposition. The transition from Schottky to ohmic behavior was observed by adding an In layer as thin as ∼1 nm to the MoGeW. Contact resistances were found to be very sensitive to the deposition sequence, the annealing method, the annealing temperature, and the In layer thickness. Low resistances of ∼0.5 Ω mm were obtained in the MoGeInW contacts with 2-nm-thick In layers, annealed by the heat-pulse method at temperatures in the range of 880-960 °C for 2 s. Contact resistances were stable during subsequent annealing at 400 °C for 100 h. Evidence of formation of the parallel diode areas with various barrier heights was obtained for these contacts after annealing at elevated temperatures. These low-barrier-height areas are believed to be the interfaces between the contact metals and InGaAs phases. The composition within the ternary phases was uniform, and no composition gradient was observed. The composition was determined by small-probe x-ray energy dispersive spectrum to be close to In0.2Ga0.8As. The distribution of these ternary phases, influenced by the contact fabrication process parameters, strongly affected the contact resistance.