A study of P-type ohmic contacts to InAlAs/InGaAs heterostructures

Optical modulators operating at near-infrared wavelengths are of interest for a variety of applications including bi-directional communications and optical interconnects. The fabrication of 1.06 mu m and 1.32 mu m operating wavelength strained-layer superlattice vertical-cavity optoelectronic modulators requires the formation of a p-type ohmic contact to the InAlAs/InGaAs quarter-wave bottom mirror stack. In this study, BeAu and TiPtAu p-type ohmic contact metallization schemes were evaluated for use on molecular beam epitaxy grown In0.10Al0.90As/In0.12Ga0.88As and In0.32Al0.68As/In0.33Ga0.67As device heterostructures. Recessed and non-recessed transmission line measurement structures were fabricated and evaluated as a function of rapid thermal anneal temperatures over the range of 360-420 degrees C. Atomic force microscopy was used to determine the surface morphology of each sample for evidence of metal or material degradation. For contacts directly on InGaAs layers, TiPtAu contacts had relatively high specific contact resistance values of rho(c) similar to 3 x 10(-4) Omega cm(2) and displayed no dependence on the anneal. The BeAu contacts had minimum specific contact resistance values of rho(c) similar to 5 x 10(-7) Omega cm(2) but showed evidence of degradation at higher temperatures. Contacts directly made to InAlAs layers had minimum specific contact resistances of rho(c) similar to 4 x 10(-5) Omega cm(2) and were improved slightly with the addition of a thin GaAs layer.