High purity and chrome-doped GaAs buffer layers grown by liquid phase epitaxy for mesfet application

High purity GaAs buffer layers of carrier concentration in the low (l-5)×l0l4/cm3 range with 77K electron mobility over 100,000 cm2/V-sec and 300K mobility around 8000 cm?/ V-sec have been grown by liquid phase epitaxy on Cr-doped GaAs substrates using the graphite sliding boat method. The high purity has been achieved with systematic and concurrent long term bake-outs (24 hrs) of both LPE melt and substrate, both exposed to the H2 ambient gas stream at 775‡C, prior to epitaxial growth at 700‡C. Substrate surface degradation was reduced by using Ga:GaAs etch melts that were undersaturated at 700‡C by 5‡ to 40‡C. Best buffer layer morphologies with regard to surface planarity were obtained using etch melts that were saturated by near 85% of weight of GaAs at 700°C. The importance of substrate preconditioning in order to achieve the low ( 1 -2)×l014 was examined and found to be critical. Melt and substrate bake outs at 800‡C, and use of a 40‡C undersaturated etch melt prior to epitaxial growth at 800‡C resulted in a p-type layer of carrier concentration, 1 .9×l0l2/cm3 and resistivity 1×105 ohm-cm. Chromium doping at 700‡C resulted in buffer layers with sheet resistivities greater than 10 ohms/sq and low pinhole densities.