Transmission electron microscopy of GaAs grown over submicrometer-period tungsten gratings

The GaAs overgrowth of tungsten gratings, the most critical step in the fabrication of the permeable base transistor (PBT), has been characterized by transmission electron microscopy (TEM). The 3200-Å-period tungsten base gratings for the PBT are very uniform in linewidth and spacing before the GaAs overgrowth, and this uniformity is preserved after the overgrowth. Both voids and stacking faults are observed in the epitaxial GaAs grown over the gratings. An unexpected result of this study is the observation that a considerable density of such defects can be present in a PBT wafer that yields uniform, high-frequency devices.