Strain distribution in the core and the shell of a semiconductor nanowire (NW) and its effect on band structures including carrier recombination dynamics of individual Wurtzite (WZ) In1-xGaxAs/InP and Zincblende (ZB) GaAs1-xSbx/InP strained core-shell NWs are investigated using room temperature Raman scattering and transient Rayleigh scattering (TRS) optical spectroscopy techniques. In addition, the electrical transport properties of individual ZB InP NWs are explored using gate-dependent current-voltage (I-V) measurements.
Micro-Raman scattering from individual In1-xGaxAs NWs show InAs like TO and GaAs like TO modes with frequencies which are consistent with the 35% Ga concentration determined from the growth parameters. Calculations showed that the In0.65Ga0.35As core is under compressive strain of 0.26% while the InP shell is in tensile strain of 0.42% in In0.65Ga0.35As/InP NWs. TRS measurements of single NWs show clear evidence for a strong band resonance in the WZ In0.65Ga0.35As NW at 0.819 eV which is estimated to be a ~186 meV blue-shift with respect to bulk ZB In0.65Ga0.35As. Furthermore, both Raman scattering and TRS measurements are on excellent agreement with the band gap shift of In0.65Ga0.35As/InP core-shell NWs with respect to the core only NW by 46~48 meV which experimentally confirmed the InP shell induced compression of the core. The time decays of the resonance are observed to be long (~125 ps) for core-shell NWs while it is short (~31 ps) for core only NWs consistent with a larger nonradiative recombination rate.
Optical phonon modes of GaAs1-xSbx are observed to be red-shifted with increasing Antimony fraction in GaAs1-xSbx NWs which can be expected in an alloy with increasing concentration of a heavier atom in the lattice. Using TRS measurements, the GaAs0.71Sb0.29 band gap for the core-shell NW is observed to be reduced by 0.04 eV with respect to the core only NW because of the tensile strain in the core. Raman experiments show a blue-shift of the InP phonons and a red-shift of the GaAs1-xSbx phonons in individual GaAs0.71Sb0.29/InP NWs, which is consistent with the tensile core strain inferred from TRS results. The recombination life times in GaAs0.71Sb0.29, GaAs0.71Sb0.29/InP NWs are found to be 31 ps and 127 ps respectively reflecting the effectiveness of the InP shell surface passivation.
Individual InP NW field effect transistors are fabricated using photolithography to investigate the electrical transport properties of InP NWs. Gate-dependent I-V plots showed that the InP NWs are n-type and displayed typical non-Ohmic behavior due to the contact resistance between NW and metal electrodes. Carrier mobility determined for the InP NWs is as high as 655 cm2/(V.s) for the carrier density of 4.08 x 1017 cm-3 which is comparable to n-type InP thin film materials with similar carrier densities. An equivalent circuit model of the metal-semiconductor-metal structure is used to extract the carrier density and mobility of the NW as 1.00 x 1017 cm-3 and 511 cm2/(V.s). This model makes it possible to determine the barrier heights of the NW device while providing a good agreement with the experimental results.