Infrared absorption of Ge epitaxial films on a GaAs substrate

Germanium films were deposited on GaAs (100) substrates with or without an epiready surface oxide at temperatures between room temperature (RT) and 500 degrees C using an ultrahigh-vacuum e-beam deposition system. The film at 100 degrees C on a substrate with a surface oxide had a flat absorption curve over the wave-number range investigated, 500-4000 cm(-1), with an absorption of less than 10/cm at 1000 cm(-1) (10 mu m wavelength). Films deposited at RT and 50 degrees C on substrates with a surface oxide had comparable low absorption, but they contained an absorption peak at 830 cm(-1) associated with the Ge-O bonds. Although all three films were amorphous, the films deposited at the lower temperatures were more porous. This enabled oxygen to percolate in from the atmosphere to form the Ge-O bonds. The films deposited at 150 degrees C and above on substrates with a surface oxide and at 100 degrees C with the surface oxide removed thermally in situ prior to deposition the Ge films, and the single crystal films deposited at 400 and 500 degrees C on oxide-free substrates, had strong absorption in the vicinity of the Ge/GaAs interface with the characteristic of two-dimensional free-carrier absorption. (C) 1996 American Institute of Physics.