A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 x 10(17), 1.5 x 10(17) and 4.7 x 10(17) cm(-3) are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm(2). In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700 degrees C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.