Effects of KrF excimer laser irradiation on metal contacts to n-type and p-type GaN

Electrical properties of metal contacts on laser-irradiated n-type and p-type GaN surfaces were investigated using current-voltage, capacitance-voltage, and synchrotron radiation photoemission spectroscopy. After the irradiation of a KrF excimer laser pulse (600 mJ/cm2 at 248 nm for 38 ns) onto Si-doped GaN, a nonalloyed Ti/Al metallization formed an ohmic contact with the specific contact resistivity of 1.7x10-6 Ω cm2. The laser irradiation decomposed GaN into metallic Ga and nitrogen gas. The decomposed metallic Ga reacted with oxygen in air to form a Ga oxide layer with the thickness of ∼40 Aa, producing a large number of N vacancies near the surface. The formation of a degenerated n-type GaN layer resulted in the low contact resistivity. For Mg-doped GaN, the laser irradiation increased the effective acceptor concentration. Simultaneously, the activation efficiency of Mg dopants was enhanced by the photon-assisted breaking of Mg-H bonds and/or the removal of hydrogen atoms in the presence of oxygen, producing the p-type GaN with an increased hole concentration. As a result, the contact resistivity of an oxidized Ni/Au contact could be reduced from 1.3x10-3 to 3.6x10-4 Ω cm2