Effect of heating on the structure of Au/GaAs encapsulated with SiO₂

The structural effects of heating 1500 Å Au/GaAs (001) encapsulated with 2000 Å of SiO₂ were examined by scanning electron microscopy and X-ray diffraction. It was observed that SiO₂/Au/GaAs (capped) in vacuum up to 500°C remained shiny and gold in color, whereas similar heating of Au/GaAs (uncapped) caused a change of color from shiny gold to dull silver. Furthermore, mass spectroscopy showed that the amount of arsenic vapor evolved was much less for the capped sample. However, X-ray diffraction showed that Au₇Ga₂ formed abundantly in both types of samples after heating at 500°C, though the epitaxial relationship was mainly Au₇Ga₂ (001) {norm of matrix} GaAs (001) for capped and Au₇Ga₂ (100) {norm of matrix} GaAs (001) for uncapped. SEM revealed gold-rich aligned rectangular protrusions on the surfaces of SiO₂/Au/GaAs as well as Au/GaAs after heating at 500°C, though the average length of these rectangles was 1.5 μm for the capped sample and 6.7 μm for the uncapped sample. Moreover, new morphological features absent in Au/GaAs were observed in SiO₂/Au/GaAs. These features are a gold-rich maze with a line width of μm and gold-rich protruded lines with a line width of 9 μm. The gold-rich protruded lines were formed by the growth and joining together of some gold-rich aligned rectangular protrusions. The gold-rich maze was observed in SiO₂/Au/GaAs after heating in vacuum, but was not observed in SiO₂/Au/GaAs after heating in nitrogen.