Electrical properties and microstructure of vanadium-based contacts on ICP plasma etched n-type AlGaN:Si and GaN:Si surfaces

Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type AlxGa1?xN contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10?6?? cm2?to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al0.4Ga0.6N:Si and GaN:Si surfaces. On Al0.4Ga0.6N, the lowest specific contact resistivity, (2.3 ? 0.2)??10?6?? cm2, was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ? 0.7)??10?3?? cm2. Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al0.4Ga0.6N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.