Jihyun Paek

Probability of twin formation on self-catalyzed GaAs nanowires on Si substrate

We attempted to control the incorporation of twin boundaries in self-catalyzed GaAs nanowires (NWs). Self-catalyzed GaAs NWs were grown on a Si substrate under various arsenic pressures using molecular beam epitaxy and the vapor-liquid-solid method. When the arsenic flux is low, wurtzite structures are dominant in the GaAs NWs. On the other hand, zinc blende structures become dominant as the arsenic flux rises. We discussed this phenomenon on the basis of thermodynamics and examined the probability of twin-boundary formation in detail.

Effect of Silicon Doping on the Photoluminescence and Photoreflectance Spectra of Catalyst-Free Molecular Beam Epitaxy–Vapor Liquid Solid Grown GaAs Nanowires on (111)Si Substrate

The optical properties of catalyst-free GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by low-temperature photoreflectance (PR) and photoluminescence (PL) techniques. Although the bandgap energy (Eg) of non-doped NWs agreed well with that of liquid-encapsulated Czochralski-grown semi-insulating bulk, a distinctive PL peak caused by a carbon acceptor to an unspecified donor recombination was observed. Because this recombination was also observed in the Si-doped NW sample, we concluded that a new type of donor was introduced during the NW growth processes. Owing to Si doping, the arsenic vacancy-Si acceptor complex was introduced in the NWs, which showed a broad but large PL band of approximately 1.4 eV. Another important finding was that Eg of the Si-doped NW sample was estimated to be 20 meV lower than that of the non-doped NW sample. This implies that the Si-related donor impurity band in NWs was caused by Si doping.

Stacking Faults and Luminescence Property of InGaN Nanowires

InGaN nanowires (NWs) were grown on (111)Si substrate using radio-frequency plasma-assisted molecular beam epitaxy, and the density of the stacking faults (SFs) in the InGaN NWs was estimated. High-density SFs were observed in the scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) images of the InGaN NWs, and a few zincblende layers appeared in the wurtzite structure. When growth temperature increased, the density of the SFs in the InGaN NW, the photoluminescence (PL) peak wavelength, and the full width at half maximum (FWHM) of PL spectra decreased, whereas the integrated PL intensity increased. These results suggest that a high growth temperature is effective for decreasing the density of SFs in InGaN NWs, and InGaN NWs grown at high temperature have strong PL luminescence due to the low In composition and the corresponding low SFs density.

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.

Optical properties of Be-doped GaAs nanowires on Si substrate grown by a catalyst-free molecular beam epitaxy vapor–liquid–solid method

The effect of Be doping on the crystal structure and the optical properties of catalyst-free Be-doped GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and photoreflectance (PR) and photoluminescence (PL) techniques. Be-doped NWs sample showed a striped pattern in STEM images. Owing to a high arsenic flux under the growth condition, the sample did not have a wurtzite but a zinc-blend (ZB) structure, and the observed striped pattern in STEM images suggests the presence of twin boundaries and stacking faults. The bandgap energy of the Be-doped NWs was lower than that of the nondoped NWs sample in the entire temperature range. In addition, the deviation from the conventional Varshni curve was found to be large in the low-temperature region. However, this is well explained by considering the strong electron–phonon interaction and high average phonon temperature. Therefore, we concluded that the induced strain increased the linear thermal expansion coefficient. This is because the Be atom has a smaller covalent radius than the Ga and As atoms.

Thermoelectric power of catalyst-free GaAs nanowires grown by MBE-VLS method

Compound semiconductor nanowires (NWs) have been investigated because of their potential as the new application in electrical and optical devices of the next generation. NWs have been extensively studied by thermoelectric power measurement owing to their higher thermoelectric figure of merit, ZT, than the bulk [1]. The carrier density of NWs could be estimated from their Seebeck coefficient. Moreover, the ZT’s and carrier densities of InSb, ZnO and GaN NWs measured from thermoelectric power have already been reported [2, 3], however, the thermoelectric power of catalyst-free GaAs NWs has not been reported yet. In this study, we determined the thermoelectric power of catalyst-free Si-doped GaAs NWs grown on a Si substrate by MBE-VLS method which is combined molecular beam epitaxy (MBE) with a vapor-liquid-solid (VLS) method.