Ohmic contact formation mechanism of nonalloyed Pd contacts to p-type GaN observed by positron annihilation spectroscopy

The change of the Fermi energy level at the interface of Pd/p-type GaN by s urface treatment was investigated using positron annihilation spectroscopy, and the results were used to provide interpretation of the electrical prop erties of the contact. Changes in the positron parameters at the interface in the aqua regia-treated GaN are more pronounced than that in the HCl-trea ted one. This provides evidence that the surface treatment with aqua regia prior to Pd metal deposition removes surface oxides, resulting in the shift of the Fermi level position from a middle of the bandgap to near the valen ce band. Thus, the barrier for hole injection from metal to p-type GaN is l owered, subsequent to the decrease of contact resistivity by two orders of magnitude. (C) 1999 American Institute of Physics. [S0003-6951(99)01016-5].