Four vanadium-based contacts to n-type Al 0.6 Ga 0.4 N were compared in this work. Both V/Al/Pd/Au and V/Al/V/Au contacts with optimized layer thicknesses provided lower specific-contact resistances than did the previously reported V/Al/Pt/Au ohmic contact. Specific contact resistances of the V/Al/Pd/Au (15 nm/85 nm/20 nm/95 nm) and V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts were 3 ϫ 10 Ϫ6 Ω·cm 2 and 4 ϫ 10 Ϫ6 Ω·cm 2 , respectively. On the other hand, an analogous V/Al/Mo/Au contact never became ohmic, even after it was annealed at 900°C for 30 sec. Compared to the V/Al/Pd/Au contact, the V/Al/V/Au contact required a less severe annealing condition (30 sec at 700°C instead of 850°C). The V/Al/V/Au contact also provided a smoother surface, with a root-mean-square (RMS) roughness of 39 nm.

Cross-plane electronic and thermal transport properties of p-type La0.67Sr0.33MnO3/LaMnO3 perovskite oxide metal/semiconductor superlattices J. Appl. Phys. 112, 063714 (2012) Polarization Coulomb field scattering in In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors J. Appl. Phys. 112, 054513 (2012) Modulation doping to control the high-density electron gas at a polar/non-polar oxide interface Appl. Phys. Lett. 101, 111604 (2012) Ultra low-resistance palladium silicide Ohmic contacts to lightly doped n-InGaAsIn recent years, Au/Ni/p-GaN ohmic contacts annealed in air have been studied extensively because they provide low specific contact resistances and high transparency. In this article we focus on the environmental and thermal degradation that we have observed in these contacts. When the contacts were not protected from the environment, degradation of the contacts always occurred over a period of days, and after sitting 30 days in the laboratory at room temperature, the current-voltage characteristics of the originally ohmic contacts were nonlinear and the contacts were significantly more resistive. To pinpoint the degradation mechanism, samples were stored at room temperature in dry nitrogen, dry oxygen, dry air, air saturated with water vapor, nitrogen gas saturated with water vapor, or vacuum. These experiments revealed that water vapor was the cause of the room temperature degradation. Since no change in the sheet resistance of the p-type GaN was observed upon aging, four point probe measurements and x-ray photoelectron spectroscopy depth profiles were carried out to determine the interaction between water vapor and the p-type NiO present in the annealed contact metallization. The measurements indicated that hydroxyl groups were incorporated in the NiO, leading to a reduction in its conductivity and presumably a decrease in its hole concentration. Aging studies at 200°C further revealed poor thermal stability of the contacts not only in water vapor but also in nitrogen gas and vacuum, and this degradation was again linked to a degradation in the conductivity of the NiO component of the contact metallization.

Pt contacts to the wide band gap icosahedral boride semiconductor B 12 As 2 have been studied. All Pd and Pt contacts exhibited nonlinear I-V characteristics, while Cr/ Pt contacts were Ohmic. The specific contact resistance was reduced from 6 ⍀ cm 2 as-deposited to 3 ϫ 10 −4 ⍀ cm 2 after the Cr/ Pt contacts were annealed at 750°C for 30 s in Ar. Annealing at 600°C or higher drastically reduced the semiconductor sheet resistance, whether annealing was performed before or after metallization. This apparent activation of the semiconductor is a likely cause for the improvement in the Ohmic contacts with annealing.