Electronic and Mechanical Properties of Ge Films Grown on Glass Substrates

As germanium is closed lattice matched to GaAs, it a suitable substrate for epitaxial growth. In the quest for inexpensive substrates, thin-film Ge grown on glass is an attractive candidate if suitable grain growth can be achieved. Here we will describe Ge films that are deposited by an e-beam evaporator on glass and are approximately 2000 angstroms thick. The films were annealed at 500 °C and 600 °C to improve the quality of the material. The growth was done in three steps with 1000 angstroms of Ge, 70 angstroms of Sb, and followed by another 1000 angstroms of Ge. Sb is an n-type dopant in Ge and is included to enhance grain growth. The best films contained the Sb layer and hole concentrations between 1.4×10¹⁵ to 1.6×10¹⁷ cm^-3. The largest hole mobility measured was 30.6 cm²/Vs in the 1.4×10¹⁵ p-type sample. The electron lifetime was measured by ultra-high frequency photoconductive decay and the best lifetimes were in the 30- to 40-ns range. Scanning-electron microscope and transmission-electron microscope studies indicated a polycrystalline grain structure with grain size comparable to the film thickness.