Low-resistance ohmic contacts to p-type GaN

The specific contact resistance of two types of ohmic contacts to p-type Ga N is analyzed. First, an ohmic contact formed by a metal electrode deposite d on a highly doped p-type GaN layer. Second, an ohmic contact formed by a metal electrode deposited on a thin GaN layer with an internal electric fie ld caused by polarization effects. It is shown that contacts mediated by po larization effects can result, for typical materials parameters, in low con tact resistances comparable or better than contacts mediated by dopant-indu ced surface fields. A type of contact is proposed and demonstrated. These c ontacts employ polarization charges to enhance tunneling transport as well as high doping. Experimental results on Ni contacts to p-type AlxGa1-xN/GaN doped superlattices are presented. The contacts have linear current-voltag e characteristics with contact resistances of 9.3x10(-4) Omega cm(2), as in ferred from linear transmission-line method measurements. The influence of annealing at temperatures ranging from 400 to 500 degrees C on the contact resistance is studied. (C) 2000 American Institute of Physics. [S0003-6951( 00)01219-5].