An indium-free mount for GaAs substrate heating during molecular beam epitaxial growth

Abstract

The substrate mounting system for a Varian GEN/II molecular beam epitaxy (MBE) system was modified to allow the growth of device quality GaAs and AlGaAs epitaxial layers on GaAs substrates without the use of indium bonding. The indium-free holders are compatible and interchangeable with the standard molybdenum blocks that use indium bonding. The wafer is heated by direct radiation from the unmodified spiral filaments with no intermediate absorbers or backside wafer coatings. A commercial pyrometer with a silicon detector was used to measure the absolute temperature. The silicon detector is sensitive to radiation from 0.8 to 1.1 μm, which is primarily above the band gap energy of GaAs at substrate temperatures above 550 °C. For photon energies above the band gap the spectral emittance of GaAs is nearly independent of the substrate doping, thickness, and temperature. With this pyrometer a maximum temperature difference of 3 °C was measured over the central two-thirds of a 2 in. diam semi-insulating GaAs wafer. In contrast, the maximum temperature difference on n+GaAs substrates was significantly larger (30 °C over the central two-thirds of the wafer) and showed the same pattern as the filament configuration. This large difference is explained by a simple model which takes into account the smaller thermal conductivity and larger total emittance of n+GaAs compared to undoped GaAs.