Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one-dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments, these structures emit visible light, which is optically polarized up to 86% at 10 K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2 nm to 8 nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarized along the length of these nanostructures.

Semiconductor nanowires suffer from significant non-radiative surface recombination; however, heavy p-type doping has proven to be a viable option to increase the radiative recombination rate and, hence, quantum efficiency of emission, allowing the demonstration of room-temperature lasing. Using a large-scale optical technique, we have studied Zn-doped GaAs nanowires to understand and quantify the effect of doping on growth and lasing properties. We measure the non-radiative recombination rate ( knr) to be (0.14 ± 0.04) ps-1 by modeling the internal quantum efficiency (IQE) as a function of doping level. By applying a correlative method, we identify doping and nanowire length as key controllable parameters determining lasing behavior, with reliable room-temperature lasing occurring for p ≳ 3 × 1018 cm-3 and lengths of ≳4 μm. We report a best-in-class core-only near-infrared nanowire lasing threshold of ∼10 μJ cm-2, and using a data-led filtering step, we present a method to simply identify subsets of nanowires with over 90% lasing yield.

The photoluminescence spectra of a zincblende GaN epilayer grown via metal-organic chemical vapour deposition upon 3C-SiC/Si (001) substrates were investigated. Of particular interest was a broad emission band centered at 3.4 eV, with a FWHM of 200 meV, which extends above the bandgap of both zincblende and wurtzite GaN. Photoluminescence excitation measurements show that this band is associated with an absorption edge centered at 3.6 eV. Photoluminescence time decays for the band are monoexponential, with lifetimes that reduce from 0.67 ns to 0.15 ns as the recombination energy increases. TEM measurements show no evidence of wurtzite GaN inclusions which are typically used to explain emission in this energy range. However, dense stacking fault bunches are present in the epilayers. A model for the band alignment at the stacking faults was developed to explain this emission band, showing how both electrons and holes can be confined adjacent to stacking faults. Different stacking fault separations can change the carrier confinement energies sufficiently to explain the width of the emission band, and change the carrier wavefunction overlap to account for the variation in decay time.

The luminescence properties of cubic GaN films grown upon 3C-SiC/Si (001) substrates by MOCVD were investigated. The spectra show luminescence peaks which are associated with donor bound exciton recombination and donor acceptor pair recombination. A reduced peak energy for the (DX)-X-0 emission compared with values reported in the literature suggests a tensile-strain-reduced bandgap of approximately 3.27eV, which is consistent with the absorption edge in photoluminescence-excitation spectroscopy. The presence of hexagonal material introduces a broad emission band at 3.40eV with a FWHM of 190meV, extending to energies up to 3.60eV. The intensity of this emission scales linearly with excitation power, its peak energy and width remaining unchanged. This band is associated with an absorption edge below 3.70eV and therefore is not caused by absorption into phase-pure cubic or hexagonal GaN. The photoluminescence lifetimes measured across this band reduce from 0.40 to 0.20ns with increasing emission energy. All these observations can be explained by considering a type-II-band alignment adjacent to stacking faults within the cubic GaN. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Gallium nitride (GaN) is a direct bandgap semiconductor used for high-frequency and high-power optoelectronic devices that are typically based upon wurtzite GaN grown along the [0001] (c-axis) direction. However, strong spontaneous and piezoelectric polarisations exist along the c-axis and result in large internal electrostatic fields [1] that have been shown to reduce the efficiency of devices [2]. To avoid these internal fields, GaN can be grown along the [112̅0] (a-axis) and [11̅00] (m-axis), which lie perpendicular to the c-axis and consequently possess no polarisation field [3]. The non-polar planes do not therefore suffer from internal electrostatic fields but since the c-axis also defines the optic axis of the wurtzite crystal structure, a-plane and m-plane GaN exhibit birefringence. This leads to a polarisation-dependent anisotropy in the dielectric properties that has been the focus of recent studies in the optical [4,5] and mid-infrared [6,7] spectral regions. The characterisation of the dielectric properties of non-polar GaN within the terahertz regime is however lacking, despite a great deal of recent interest in its use in the development of optoelectronic devices within this spectral range [8-10]. Here we report on the characterisation of the dielectric properties of both a-plane and m-plane GaN wafers in the spectral region from 0.5 – 5.0 THz, using terahertz time-domain spectroscopy. Terahertz radiation was generated using 50 fs laser pulses incident on a gallium arsenide photoconductive antenna and aligned through a 4F confocal geometry of parabolic mirrors, creating focal spots at the sample position and at a 400 μm thick gallium phosphide crystal in a standard electro-optic detection scheme. The wafers were commercially-available, semi-insulating a-plane and m-plane GaN crystals, grown by hydride vapour phase epitaxy (Nanowin Ltd.), with a room temperature resistivity of greater than 10 6 .cm and thicknesses of 284 μm and 298 μm respectively. They were attached over an aperture in a rotation mount to allow measurement of the transmitted terahertz electric field (E) polarised both parallel and perpendicular to the c-axis. By taking a corresponding reference measurement through an identical clear aperture, the complex transmission function for both a-plane and m-plane GaN was determined, enabling extraction of the refractive index and absorption coefficient. Figure 1 shows the refractive index of mplane GaN and as expected for a positive, uniaxial crystal, the extraordinary refractive index (E ∥ c) was greater than the ordinary refractive index (E ⊥ c), with a difference of 0.18 across the measured frequency range. The results were wellfitted with a pseudo-harmonic approximation of the phonon contribution to the dielectric function, using phonon frequencies obtained from Raman scattering measurements. Furthermore, with an absorption coefficient below 10 cm -1 and an average transmission of greater than 50% across the measured frequency range for both a-plane and mplane wafers, we have demonstrated the potential for their use in future terahertz optoelectronic devices.

Energy levels of the krypton atom in the range 23-28 eV have been observed using low-energy electron impact excitation and ejected-electron spectroscopy. The levels observed belong to the configurations 4s4p6ns, 4s4p6np, 4s4p6nd and 4s24p4nln'l'. Those of the former kind have been identified but an analysis of the doubly excited levels has not been possible. The 1S and 3S terms of the 4s4p65s configuration have been studied in detail in order to determine their energies, their widths and the behaviour of their shape parameters as a function of incident energy, angle of ejection and final ion state.

The electron impact excitation of the configurations 2s2p63s and 2s2p63p in neon has been observed in the energy-loss mode over a range of impact energies and scattering angles. The spectra of the 3p configuration have been analysed to obtained the variation with angle and energy of the line shape parameters and intensities for the 1P and 3P terms. Spectra of the 3s configuration show predominantly the 1S term but a detailed analysis has enabled a value to be obtained for the energy of the 3S term which is in disagreement with previous results. The variation of line shape and intensity for the 1S term is given. A very broad feature at an excitation energy of about 44 eV has been observed for the first time and its behaviour is discussed.

Electron impact excitation of the configurations 3s3p64s, 4p, 3d and 5s in argon has been observed in the energy-loss mode over a range of scattering angles and impact energies. The energy-loss spectra have been analysed to obtain the variation with angle and energy of the line shape parameters and intensities for individual terms of these configurations. A number of accurate values of energy and width for terms of the 3s3p64s, 4p, 3d, 5s, 4d and 6s configurations have also been derived.