Low resistance Ohmic contacts to graded InGaN

Ohmic contacts to polarization doped, compositionally graded indium gallium nitride (InGaN) films have been studied. The strong spontaneous and piezoelectric polarizations exhibited by the III-nitride materials result in effective doping in the graded layers without the use of impurity dopants. Two samples were grown by grading the In composition from ∼5% to 21.5% or 28% over 90 nm on top of GaN by molecular beam epitaxy, resulting in carrier concentrations of 2.13 × 1018 cm−3 and 2.87 × 1018 cm−3, respectively. Both films of Ni and Cr metals were tested for contact quality. Transmission line measurements demonstrated a minimum specific contact resistance of 3.19 × 10−4 Ωcm2 resulting from room temperature deposition of Cr. The methods and materials characterization will be discussed.