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The electrical properties of W contacts to both n-and p-GaN were investigated. W contacts to p-type GaN doped with Mg to a level of 10 18 cm Ϫ3 were annealed for 1 min at temperatures from 350 to 900°C. The contact resistivity was found to decrease with increasing annealing temperature parallel to an increase in the GaN sheet resistance. The contacts were rectifying after all of the heat treatments. Measurements at higher temperatures ͑up to 400°C͒ resulted in I -V characteristics becoming almost linear and a decrease in contact resistivity with temperature down to 10 Ϫ2 ⍀ cm 2 . These results are attributed to the ionization of more Mg acceptors as the temperature increases. In this system thermionic emission combined with tunneling through deep energy levels was found to be the transport mechanism. W contacts to heavily Si implanted (N d ϳ10 20 cm Ϫ3 ) n-GaN annealed at 750-1050°C for 10 s produced ohmic behavior with no significant dependence of the contact resistivity on the annealing temperature. The observed weak dependence of the contact resistivity on the measurement temperature is attributed to the dominance of the field emission mechanism.

W and WSi ohmic contacts on both p-and n-type GaN have been annealed at temperatures fiom 300 -1000 "C. There is minimal reaction ( S 100 A broadening of the metal/GaN interface) even at 1000 "C. Specific contact resistances in the SZ-cm2 range are obtained for WSi, on Si-implanted GaN with a peak doping concentration of -5 x lo2' ~m -~, after annealing at 950 "C. On p-GaN, leaky Schottky diode behavior is observed for W, WSi, and Ni/Au contacts at room temperature, but true ohmic characteristics are obtained at 250 -300 "C, where the specific contact resistances are typically in the Gcm2 range. The best contacts for W and WSi, are obtained after 700 "C annealing for periods of 30 -120 secs. The formation of P-W2N interfacial phases appear to be important in determining the contact quality.