Systematic characterization of Cl2 reactive ion etching for improved ohmics in AlGaN/GaN HEMTs

Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFNI). The contact resistance decreased from about 0.45 Ohmmm for unetched ohmics to a minimum of 0.27 Ohmmm for 70 A etched ohmics. The initial thickness of the AlGaN layer was 250 Angstrom. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs