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Its piezo- and potentially ferroelectric properties make the metastable kappa polymorph of Ga$_2$O$_3$ an interesting material for multiple applications, while In-incorporation into any polymorphs of Ga$_2$O$_3$ allows to lower their bandgap. In this work, we provide a guideline to achieve single phase $\kappa$-, $\beta$-Ga$_2$O$_3$ as well as their (In$_x$Ga$_{1-x}$)$_2$O$_3$ alloys up to x = 0.14 and x = 0.17 respectively, using In-mediated metal exchange catalysis in plasma assisted molecular beam epitaxy (MEXCAT-MBE). The polymorph transition from $\kappa$ to $\beta$ is also addressed, highlighting the fundamental role played by the thermal stability of the $\kappa$-Ga$_2$O$_3$. Additionally, we also demonstrate the possibility to grow ($\bar{2}$01) $\beta$-Ga$_2$O$_3$ on top of $\alpha$-Al$_2$O$_3$ (0001) at temperatures at least 100 {\deg}C above those achievable with conventional non-catalyzed MBE, opening the road for increased crystal quality in heteroepitaxy. The role of the substrate, as well as strain and structural defects in the growth of $\kappa$-Ga$_2$O$_3$ is also investigated by growing simultaneously on three different materials: (i) $\alpha$-Al$_2$O$_3$ (0001), (ii) 20 nm of ($\bar{2}$01) $\beta$-Ga$_2$O$_3$ on $\alpha$-Al$_2$O$_3$ (0001) and (iii) ($\bar{2}$01) $\beta$-Ga$_2$O$_3$ single crystal., Comment: Main text: 7 pages, 4 figures; Supplementary: 6 pages, 9 figures

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) is addressed by using solid SiO as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux ($\Phi_{SiO}$) from the source at different temperatures ($T_{SiO}$) confirmed SiO molecules to sublime with an activation energy of 3.3eV. The $T_{SiO}$-dependent $\Phi_{SiO}$ was measured in vacuum before and after subjecting the source material to an O$_{2}$-background of $10^{-5}$ mbar (typical oxide MBE regime). The absence of a significant $\Phi_{SiO}$ difference indicates negligible source oxidation in molecular O$_{2}$. Mounted in an oxygen plasma-assisted MBE, Si-doped $\beta$-Ga2O3 layers were grown using this source. The $\Phi_{SiO}$ at the substrate was evaluated [from 2.9x10$^{9}$ cm$^{-2}$s$^{-1}$ ($T_{SiO}$=700{\deg}C) to 5.5x10$^{13}$ cm$^{-2}$s$^{-1}$ (T$_{SiO}$=1000{\deg}C)] and Si-concentration in the $\beta$-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., $N_{Si}$ from 4x10$^{17}$ cm$^{-3}$ ($T_{SiO}$=700{\deg}C) up to 1.7x10$^{20}$ cm$^{-3}$ ($T_{SiO}$=900{\deg}C). For a homoepitaxial $\beta$-Ga2O3 layer an Hall charge carrier concentration of 3x10$^{19}$ cm$^{-3}$ in line with the provided $\Phi_{SiO}$ ($T_{SiO}$=800{\deg}C) is demonstrated. No SiO-incorporation difference was found between $\beta$-Ga2O3(010) layers homoepitaxially grown at 750{\deg}C and $\beta$-Ga2O3(-201) layers heteroepitaxially grown at 550{\deg}C. The presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease of doping concentration (particularly at $T_{SiO}$<800{\deg}C) which has been tentatively explained with a simple model. Degassing the source at 1100{\deg}C reverted the oxidation.

We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by X-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, elongate along the c-axis, and are randomly oriented in plane. The luminescence of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence spectroscopies, is characterized by two bands, the nearband-edge emission and the characteristic defect-related green luminescence of ZnO. The cross-correlation of scanning electron micrographs and monochromatic cathodoluminescence intensity maps reveals that: (i) coalescence joints act as a source of non-radiative recombination, and (ii) the luminescence of ZnO nanowires is inhomogeneously distributed at the single nanowire level. Specifically, the near-band-edge emission arises from the nanowire cores, while the defect-related green luminescence originates from the volume close to the nanowire sidewalls. Two-dimensional simulations of the optical guided modes supported by ZnO nanowires allow us to exclude waveguiding effects as the underlying reason for the luminescence inhomogeneities. We thus attribute this observation to the formation of a core-shell structure in which the shell is characterized by a high concentration of green-emitting radiative point defects with respect to the core.

In this paper, we describe the design and characterization of 400-nm-long (88 periods) AlxGa1-xN/AlN (0 < x < 0.1) quantum dot superlattices deposited on self-assembled GaN nanowires for application in electron-pumped ultraviolet sources. The optical performance of GaN/AlN superlattices on nanowires is compared with the emission of planar GaN/AlN superlattices with the same periodicity and thickness grown on bulk GaN substrates along the N-polar and metal-polar crystallographic axes. The nanowire samples are less sensitive to nonradiative recombination than planar layers, attaining internal quantum efficiencies (IQE) in excess of 60% at room temperature even under low injection conditions. The IQE remains stable for higher excitation power densities, up to 50 kW/cm2. We demonstrate that the nanowire superlattice is long enough to collect the electron-hole pairs generated by an electron beam with an acceleration voltage VA = 5 kV. At such VA, the light emitted from the nanowire ensemble does not show any sign of quenching under constant electron beam excitation (tested for an excitation power density around 8 kW/cm2 over the scale of minutes). Varying the dot/barrier thickness ratio and the Al content in the dots, the nanowire peak emission can be tuned in the range from 340 to 258 nm. Keywords: GaN, AlN, nanowire, ultraviolet

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al deficient core and an Al rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies, but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2019), copyright (C) American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.9b01521, the supporting information is available (free of charge) under the same link

We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride (h-BN) film grown on Ni by molecular beam epitaxy. After transfer to a SiO$_2$/Si substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multi-layer islands that form at the nucleation centers of the h-BN film. The comparison of their energy, polarization and phonon replica emission with previous theoretical predictions suggest that the N$_\mathrm{B}$V$_\mathrm{N}$ anti-site could be one of the light emitters present in our sample. Moreover, we have also observed evidences of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources.

N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 {\deg}C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 {\deg}C. This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells exhibit a spatially homogeneous green luminescence band up to room temperature, but the intensity of this band is observed to strongly quench with temperature. Temperature-dependent photoluminescence transients show that this thermal quenching is related to a high density of nonradiative Shockley-Read-Hall centers with large capture coefficients for electrons and holes., Comment: 10 pages, 2 figures, 1 table

Several ten $\mu$m GaN have been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates have been covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features allow for an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear, and the luminescence of this part of the GaN layer is dominated by the donor-bound exciton. For templates consisting of both a thin AlN buffer and GaN nanostructures, a silicon incorporation into the GaN grown by HVPE is not observed. Moreover, the growth mode of the (high-T) HVPE step depends on the specific structure of the AlN/GaN template, where in a first case, the epitaxy is dominated by the formation of slowly growing facets, while in a second case, the epitaxy proceeds directly along the c-axis., Comment: 19 pages, 11 figures

This paper assesses intersubband transitions in the 1 to 10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular quantum wells in the two perpendicular inplane directions, with high-resolution images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects introduced by the epitaxial process. Low-temperature intersubband absorption from 1.5 to 9 THz is demonstrated, covering part of the 7 to 10 THz band forbidden to GaAs-based technologies.

This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells grown on bulk GaN for intersubband optoelectronics in the short- and mid-wavelength infrared ranges. The characterization results are compared to those for reference samples grown on the polar c-plane, and are verified by self-consistent Schr\"odinger-Poisson calculations. The best results in terms of mosaicity, surface roughness, photoluminescence linewidth and intensity, as well as intersubband absorption are obtained from m-plane structures, which display room-temperature intersubband absorption in the range from 1.5 to 2.9 um. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells were designed to determine the accessible spectral range in the mid-infrared. These samples exhibit tunable room-temperature intersubband absorption from 4.0 to 5.8 um, the long-wavelength limit being set by the absorption associated with the second order of the Reststrahlen band in the GaN substrates.

We investigate the nature of excitons bound to I1 basal-plane stacking faults [(I1;X)] in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i. e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the (I1;X) and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the (I1;X) recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of 450 cm/s considering the nanowires mean diameter of 105 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than 2x10^16 cm^-3. As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state.