Characterization of Gallium-Doped Zinc Oxide Contact on n-Type Gallium Nitride Epitaxial Layers

This study demonstrates the electrical characteristics of Ga-doped ZnO (GZO) thin films deposited onto n-type GaN (n-GaN) epitaxial layers with a variety of electron concentrations. The ohmic characteristics could be obtained from the GZO/n-GaN samples after a thermal annealing process was performed at 800 degrees C in a nitrogen atmosphere for 1 min when the n-GaN layers had a carrier concentration of approximately 1x10(19) cm(-3). The specific contact resistance (rho(c)) was as low as 2.6x10(-4) cm(2). The ohmic characteristics can be attributed to the fact that the heavy doping of n-GaN layers and the low resistivity GZO film can result in a carrier transport via a tunneling mechanism at the GZO/GaN interface. However, the GZO/n-GaN samples exhibited nonlinear current-voltage (I-V) characteristics (i.e., a Schottky contact) when the annealing temperature (T-A) was lower than or equal to 600 degrees C even though the carrier concentration of n-GaN reached 1x10(19) cm(-3). These GZO/n-GaN Schottky contacts were due to the low carrier concentration of GZO films and/or n-GaN layers. The effective Schottky barrier heights of the 700 degrees C annealed GZO/n-GaN (n similar to 10(16) cm(-3)) samples were around 0.73 and 0.75 eV, which were deduced from the I-V and capacitance-voltage characteristics, respectively.