Strained Ge films and nanostructures for optoelectronic devices

Strained Ge films and their nanostructures are attractive for high performance electronic and photonic devices. A review of our work on direct band gap light emission from strained Ge films, nanocrystals and nanowire heterostructures will be presented. For low injected current density, the electroluminescence of strained Ge from metal-insulator-semiconductor structure is dominated by the direct band gap optical transition from conduction band Γ-valley to strained induced splitting of heavy hole and light hole valence bands. The structural and optical properties of Ge quantum dots grown on Si (001) and Ge nanocrystals embedded in oxide matrices for memory and light emitting devices will be presented. Ge quantum dots were grown on Si (001) substrates by solid source molecular beam epitaxy. Varieties of morphological shape with different sizes were evolved by tuning the growth parameters. We observed infra-red photoluminescence signal from Ge islands, associated with the radiative recombination of holes confined in Ge islands and electrons localized in the Si buffer layer. Ge clusters of different sizes and densities were embedded in oxide matrices using radio frequency magnetron sputtering and pulsed laser deposition. The light emission in the visible wavelength range from Ge nanocrystals embedded in different high band gap matrices was attributed to quantum confinement of carriers. The observed electroluminescence (EL) around 730 nm is attributed to electron hole recombination in quantum confined Ge nanocrystals. We shall also present the results of a novel graphene oxide (GO)/n-Si heterojunction device with excellent rectification characteristics up to 1.0 MHz. The transient photocurrent measurements indicate that the GO based heterojunction diodes can be useful for UV and broadband photodetectors, compatible to silicon device technology. (author)