Effects of surface treatment of n-GaN on the formation of Hf ohmic contacts as a function of the chemical solutions and immersion time are studied. Hf ohmic contacts to n-GaN were readily formed without postdeposition annealing by surface treatment of n-GaN films in a KOH solution at 70 degreesC for 60 min prior to Hf deposition. The specific contact resistance as low as 7.5 x 10(-5) Ohm cm(2) can be reached. X-ray photoelectron spectroscopy investigation showed that the oxide and carbon contaminants present on the surface of n-GaN could be substantially removed by treatment in the 70 degreesC KOH solution for 60 min, resulting in the formation of Hf ohmic contacts to n-GaN without postdeposition annealing. For improving the formation of ohmic contacts to n-GaN the performance of surface treatment of n-GaN in the 70 degreesC KOH solution is superior to that in the commonly used HF and HCl solutions.