Optical properties of Ge nanowires grown on silicon (100) and (111) substrates

We report Raman scattering and photoluminescence (PL) measurements of Ge nanowires (NWs) grown by chemical vapor deposition on silicon substrates with (100) and (111) crystallographic orientations. A sharp and narrow Raman peak at ∼ 300 cm ^-1 indicates single-crystal quality of Ge NWs ∼40 nm in diameter and approximately a micrometer in length. The absence of Si-Ge vibrations in Raman spectra shows that SiGe interdiffusion is insignificant for most of the NW volume. Low temperature PL spectra and the PL intensity-temperature dependence strongly indicate that the observed emission originates mostly at Ge NW Si substrate interfaces, where Si-Ge intermixing has been detected. We found that such interfaces are formed differently for (111) and (100) oriented Si substrates due to the strongly oriented <111> preferential growth direction of Ge NWs.