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Zinc-blende and wurtzite phase separation in catalyst-free molecular beam epitaxy vapor–liquid–solid-grown Si-doped GaAs nanowires on a Si(111) substrate induced by Si doping

The effect of Si-doping on the phase separation of wurtzite (WZ) and zinc-blende (ZB) phases in catalyst-free Si-doped GaAs nanowires (NWs) grown on a Si(111) substrate was investigated using transmission electron microscope (TEM), high-resolution X-ray diffraction (HR-XRD), low-temperature photoreflectance (PR), and photoluminescence (PL) techniques. The appearance of WZ structure with an increase in the amount of Si dopant was observed through TEM, and the results showed that the thicknesses of ZB and WZ structures were random. Furthermore, all NW samples exhibited HR-XRD diffraction peaks at the (0002) and (111) planes, which correspond to the WZ and ZB structures, respectively. Their peak intensity ratio [WZ/(WZ + ZB)] increased with the amount of Si doping. The PR modulus and PL spectra at 4 K for the sample with the middle amount of Si doping in three samples exhibited peaks at 1.43, 1.49, and 1.51 eV. The peaks at 1.51 and 1.49 eV were presumed to result from band-to-band and conduction-band-to-Si-acceptor transitions, respectively. In accordance with the prediction by a theoretical band alignment calculation of the conduction- and valence-bands discontinuities, the transition energy of 1.43 eV was due to the interband transition at the WZ-ZB interface. We also found that the 1.43 eV PR and PL peaks became dominant when the amount of Si doping increased. This indicate that this interband transition became significant when the amount of WZ phase increased, which resulted from the increased Si doping. The appearance of type-II band structures induced by Si doping was also confirmed.