Optical Spectroscopy of Single Nanowires

Abstract: This thesis describes optical spectroscopy on III-V semiconductor nanowires. The nanowires were grown by metal-organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE). Photoluminescence and photocurrent spectroscopy are used as tools to investigate issues such as the size of the band gap, the effects of surface states, and the charge carrier transport in core-shell nanowires. The band gap of InAs1-xPx nanowires with wurtzite crystal structure is measured as a function of the composition for 0.15<x<0.48. The band gap is measured using photocurrent spectroscopy on single InAs nanowires with a centrally placed InAs1-xPx segment. The wurtzite band gap is found to be about 120 meV larger than the corresponding zinc blende band gap over the entire composition range. The photocurrent spectrum is measured for excitation polarized parallel and perpendicular to the nanowire axis. The nanowires are found to have a large polarization dependence of the photocurrent, which is explained by the difference in dielectric constant of the nanowire and the surrounding air. The large polarization dependence in combination with the tunable band gap and the low dark current due to the band edge offset in the heterostructure, makes such nanowires possible candidates for polarization-sensitive photodetectors in the infrared. The effect on the optical properties of the crystal structure is further investigated by comparing the spectral excitation power dependence of InP nanowires with zinc blende crystal structure and InP nanowires with a high density of rotational twins. The difference in excitation power dependence is explained by interpreting the rotational twins as monolayer thick wurtzite segments. The rotationally twinned structure responds to the light as a type II heterostructure due to the type II offset between the zinc blende and wurtzite energy bands. p- and n-doped InP nanowires are studied with photoluminescence spectroscopy. The radial band bending caused by the Fermi level pinning at the surface, causes the electrons and holes to be separated radially and this is observed as a lowering of the photoluminescence energy. This is further investigated by applying a gate voltage on the nanowire sample to change the band bending, and observe the changes in the photoluminescence signal. This could potentially be used for investigating the doping concentration in such nanowires. Core-shell nanowires with GaAs core and a larger band gap GaxIn1-xP shell are studied by photoluminescence and time-resolved photoluminescence spectroscopy. It is observed that the photoluminescence decay is fast, indicating that the decay is dominated by non-radiative recombination also with a passivating shell on the nanowire. The charge carrier transport from the shell to the core is partially hindered at the low temperatures used (10~K). The photoluminescence decay is modelled by simple rate equations, with qualitative agreement with the experiments. It is also studied how the strain from the lattice mismatched shell, and the choice of substrate (Si or GaP) affects the photoluminescence intensity and decay time. It is found that the maximum PL intensity is obtained for unstrained nanowires. A smaller part of the thesis describes photoluminescence measurements on the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]). The measurements are performed on single polymer chains dispersed in a PMMA matrix. The polymer spectra acquired at room temperature and 20~K are compared to obtain information about the conformational dynamics of the polymer chain. It is observed that at 20 K, the photoluminescence spectrum has a narrow line width and there is a large spread in the distribution of the spectral maxima. This was explained by assuming that at this low temperature, the thermal energy was not enough to allow conformational changes, and each single chain is frozen in a specific conformation. At room temperature conformational changes are possible, resulting in the single chain spectra being broad with only small inhomogeneous broadening of the ensemble spectrum.