In this study we used Photoluminescence (PL), Time-resolved photoluminescence (TRPL) and Photoluminescence excitation (PLE) spectroscopy to investigate optical and electronic properties of individual Zincblend GaAs/AlGaAs core-shell and Wurtzite InP nanowires (NWs) at low temperatures (10K).
GaAs/AlGaAs core-shell NWs were prepared by using Au catalyst-assisted metal organic chemical vapor deposition (MOCVD) method and a titanium-Sapphire laser was used to excite the nanowire sample. PL emission from single NWs exhibit an exciton peak at ~1.515eV. PL and TRPL spectroscopies exhibit high quantum efficiency and exciton lifetimes of ~1ns, which is equivalent to high quality two-dimensional heterostructures. State filling and many-body interaction effects were observed by increasing the carrier densities using pulsed laser excitation.
Further, polarized TRPL spectroscopy was used to study exciton dynamics in these nanowires at 10K. The polarization of the emitted PL was monitored at the exciton emission peak (1.515~eV) as a function of time after excitation by a polarized pulse. With no quantum confinement effects, in thermal equilibrium the density of excitons dipoles parallel and perpendicular to the NW should be equal. This investigation revealed at low excitation intensities the excitons are created out of thermal equilibrium, but relax within several hundred picoseconds (~200 ps). At higher excitation powers, the exciton dipoles relax much more rapidly within a time less than our temporal system response of 80ps. This suggests that exciton dipole relaxation is very sensitive to carrier-carrier scattering.
PLE spectroscopy was used to investigate the electronic band structure of wurtzite InP NWs at 10K with nominal diameters of 50 and 100nm, along with PL and TRPL. The NWs were prepared by Au catalyst-assisted MOCVD growth with 420 °C growth temperature and a precursor flow rate (V/III ratio) of 700.
PLE spectra show three main peaks for band-to-band transitions between the A, B and C hole bands to conduction band at energies of 1.504, 1.534 and 1.665eV in the 100nm diameter NW sample. From our PLE data we determined the energy splitting between A and B hole-bands as 30 meV and the energy splitting between A and C hole-bands as 161 meV. These measurements allow one to extract a crystal field splitting of 52 meV and spin-orbit interaction energy of 139 meV for these WZ InP nanowires. Polarized PLE measurements probe the optical selection rules for these band-to-band transitions, which are expected not to be isotropic as for zincblende InP. PLE measurements were extended to probe transitions between the A, B and C valence bands to the higher-level conduction band using pulsed super continuum fiber excitation. The transitions from A, B and C hole-bands to first conduction band were clearly identified in this experiment and we interpret the resonances seen at 1.737, 1.780 and 1.906 eV as the transitions from the A, B and C hole-bands to second conduction band respectively. From these measurements we estimated that the second conduction band is 236 ±6 meV away from the usual WZ band edge.