Formation of thermally stable low-resistance Ti/W/Au ohmic contacts on n-type GaN

A Ti(12 nm)/W(20 nm)/Au(50 nm) metallization scheme has been investigated for obtaining thermally stable low-resistance ohmic contacts to n-type GaN (4.0 x 10(18) cm(-3)). It is shown that the current-voltage (I-V) characteristics of the samples are abnormally dependent on the annealing temperature. For example, the samples that were annealed at temperatures below 750 degrees C for 1 min in a N-2 ambient show rectifying behavior. However, annealing the samples at temperatures in excess of 850 degrees C results in linear I-V characteristics. The contact produces a specific contact resistance as low as 8.4 x 10(-6) Omega cm(2) when annealed at 900 degrees C. It is further shown that the contacts are fairly thermally stable even after annealing at 900 degrees C; annealing the samples at 900 degrees C for 30 min causes insignificant degradation of the electrical and structural properties. Based on glancing angle X-ray diffraction and Auger electron microscopy results, the abnormal temperature dependence of the ohmic behavior is described and discussed.