Novel Type of Ohmic Contacts to P-Doped GaN Using Polarization Fields in Thin InxGa-xN Capping Layers

The performance of devices fabricated from GaN and related compounds is strongly affected by the resistances caused by electrical contacts. To avoid excessive heating resulting in a failure of the device, speciŽc contact resistances less than ,10 Wcm for light-emitting diodes (LEDs) and less than ,10 Wcm for laser diodes are required. This applies in particular to ohmic contacts on p-doped GaN (p-GaN) due to the resistive nature of GaN obtained by standard p-type doping techniques. Several methods have been used to attain low, speciŽc contact resistances to p-GaN, such as deposition of high work-function metals, growth of AlGaN/GaN superlattices, and tunnel-diode structures on top of p-GaN. In this publication, the use of thin InxGa12xN capping layers, pseudomorphically grown on top of pdoped GaN, is presented. Strain-induced piezoelectric as well as spontaneous polarization Želds in the InGaN capping layer and the lower p-GaN layer cause band bending that leads to the formation of a two-dimensional (2-D) hole gas (HG). As a result, the concentration of free holes at the InGaN/GaN interface is greatly increased, and the tunneling barrier width of the metal-semiconductor contact is reduced, thereby allowing for a high hole-tunneling probability through the barrier. The theory describing ohmic contacts to p-GaN by using thin InxGa12xN capping layers will be presented together with experimental results, demonstrating the capability of the new approach in achieving low contact resistances.