Process technology for InGaAs/InAlAs modulation doped field effect transistors on InP substrates

Abstract

We present a process for fabricating lattice-matched InGaAs/InAlAs modulation doped field effect transistors (MODFETs) on InP wafers including molecular beam epitaxial growth of a high electron mobility transistor structure consisting of In0.53Ga0.47As and In0.52Al0.48As layers, and electron-beam lithography for gate definition. For selective gate recessing we investigated both wet and dry etch processes. Viable procedures have been found with a citric acid: H2O2:H2O wet etching solution and with an HBr/Ar gas mixture for reactive ion etching (RIE). The selectivities obtained for InGaAs with respect to InAlAs were 14:1 for the wet etchant and 6.7:1 for RIE. Another crucial process step is the MODFET isolation. Earlier work by other groups has shown that implant isolation is difficult on InGaAs [S. J. Pearton et al., Mater. Res. Soc. Symp. Proc. 144, 433 (1989)]. Therefore, we studied both oxygen ion implantation as well as wet-chemical mesa etching for device isolation on the same wafer. Although the isolation sheet resistance achieved with ion implantation is inferior to that obtained in a mesa process, we found similar MODFET performance for both approaches. For devices with a 0.3-μm gate length and 1.3-μm source–drain distance, a transconductance of more than 600 mS/mm and threshold voltages of −1.3 and −0.6 V for wet and dry recessed transistors, respectively, were obtained. Wafer mapping measurements showed that the MODFET data are uniform over an entire 2-in. wafer and also from wafer-to-wafer within a batch.