COMBINING TRANSMISSION ELECTRON-MICROSCOPY WITH FOCUSED ION-BEAM SPUTTERING FOR MICROSTRUCTURAL INVESTIGATIONS OF ALGAAS GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS/

Transmission electron microscopy (TEM) and focused ion beam sputtering have been used for investigations of microstructure and defect format ion in AlGaAs/GaAs heterojunction bipolar transistors. The use of focu sed ion beam sputtering to prepare nearly ideal thin membranes within the active region of transistors for cross-sectional electron microsco py is described. Results are obtained from as-fabricated devices, devi ces annealed at temperatures of 400 degrees C, and devices for which ' 'aging'' is accelerated by operating at high-bias conditions and heati ng in combination. We find that electrical operation leads to signific ant microstructural changes which are distinct from those observed in devices which are annealed. TEM images of the metal/semiconductor cont act reveal ''metal spiking'' and facetted structures associated with a lloy interpenetration. Characterization of the semiconductor heterojun ctions reveal precipitates at the emitter-base junction. Mechanisms fo r the formation of these defects under high-biased operation are discu ssed. (C) 1995 American Vacuum Society.