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In molecular beam epitaxy, the spontaneous formation of GaN nanowires on Si(111) substrates at elevated temperatures is limited by the long incubation time that precedes nanowire nucleation. In this work, we present three unconventional growth approaches to minimize the incubation time and thus facilitate significantly higher growth temperatures (up to 875$^{\circ}$C). We achieve this by: (i) using III/V flux ratios larger than one to compensate for Ga desorption, (ii) introducing a two-step growth procedure, and (iii) using an AlN buffer layer to favor GaN nucleation. The GaN nanowire ensembles grown at so far unexplored substrate temperatures exhibit excitonic transitions with sub-meV linewidths and the low-temperature photoluminescence spectra are comparable to those of state-of-the-art free-standing GaN layers grown by hydride vapor phase epitaxy.

We investigate the strain state of ensembles of thin and nearly coalescence-free self-assembled GaN nanowires prepared by plasma-assisted molecular beam epitaxy on Ti/Al$_{2}$O$_{3}(0001)$ substrates. The shifts of Bragg peaks in high-resolution X-ray diffraction profiles reveal the presence of a homogeneous tensile strain in the out-of-plane direction. This strain is inversely proportional to the average nanowire radius and results from the surface stress acting on the nanowire sidewalls. The superposition of strain from nanowires with different radii in the same ensemble results in a broadening of the Bragg peaks that mimics an inhomogeneous strain on a macroscopic scale. The nanowire ensembles show a small blueshift of the bound-exciton transitions in photoluminescence spectra, reflecting the existence of a compensating in-plane compressive strain, as further supported by grazing incidence x-ray diffraction measurements carried out at a synchrotron. By combining X-ray diffraction and photoluminescence spectroscopy, the surface stress components $f_{x}$ and $f_{z}$ of the air-exposed GaN$\{1\bar100\}$ planes that constitute the nanowire sidewalls are determined experimentally to be 2.25 and $-0.7$~N/m, respectively., Comment: 22 pages, 8 figures

GaN nanowires grown by molecular beam epitaxy generally suffer from dominant nonradiative recombination, which is believed to originate from point defects. To suppress the formation of these defects, we explore the synthesis of GaN nanowires at temperatures up to 915 ${\deg}C$ enabled by the use of thermally stable TiN$_x$/Al$_2$O$_3$ substrates. These samples exhibit indeed bound exciton decay times approaching those measured for state-of-the-art bulk GaN. However, the decay time is not correlated with the growth temperature, but rather with the nanowire diameter. The inverse dependence of the decay time on diameter suggests that the nonradiative process in GaN nanowires is not controlled by the defect density, but by the field ionization of excitons in the radial electric field caused by surface band bending. We propose a unified mechanism accounting for nonradiative recombination in GaN nanowires of arbitrary diameter.

Piezoelectric semiconductor III-Nitride nanostructures have received increasing interest as an alternative material for energy harvesters, sensors, and self-sustainable electronics, demanding well-clarification of their piezoelectric behavior. Despite the feasibility of piezoresponse force microscopy (PFM) to resolve piezo-responses at the nanoscale, several difficulties arise when the measurements are performed on low piezo-coefficient materials due to various artifacts. This work shows that semi-quantitative PFM on low piezo-coefficient III-Nitrides can be achieved in high-aspect-ratio nanostructures such as nanowires or nanorods. For conventional bulks and thin films, accurate determination of their piezoresponses is limited because of clamping and bending effects which can occur simultaneously during PFM measurements. While the clamping effect only reduces the piezoresponse amplitude, the bending motion either increases or decreases this amplitude and can also rotate the phase by 180{\deg}. Improved electric field distribution in nanowires minimizes both artifacts, allowing correct determinations of crystal polarities and piezo-coefficients. In contrast to the reports in the literature, we do not observe giant piezoelectricity in III-N nanowires with a diameter in the range of 30-80 nm. This work provides an access to fundamental parameters for developing III-N based piezoelectric nano-devices.

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

The strong light-matter interaction in transition Metal dichalcogenides (TMDs) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation, which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS$_2$ and MoSe$_2$ MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then we cover them with exfoliated hBN to finally obtain an encapsulated sample : exfoliated hBN/TMD ML/MBE hBN. We observe an improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO$_2$ substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge free substrate for fabricating TMD-based heterostructures on a larger scale., Comment: 5 pages, 3 figures

In the experimental electroluminescence (EL) spectra of light-emitting diodes (LEDs) based on N-polar (In,Ga)N/GaN nanowires (NWs), we observed a double peak structure. The relative intensity of the two peaks evolves in a peculiar way with injected current. Spatially and spectrally resolved EL maps confirmed the presence of two main transitions in the spectra, and suggested that they are emitted by the majority of single nano-LEDs. In order to elucidate the physical origin of this effect, we performed theoretical calculations of the strain, electric field, and charge density distributions both for planar LEDs and NW-LEDs. On this basis, we simulated also the EL spectra of these devices, which exhibit a double peak structure for N-polar heterostructures, both in the NW and the planar case. In contrast, this feature is not observed when Ga-polar planar LEDs are simulated. We found that the physical origin of the double peak structure is a stronger quantum-confined Stark effect occurring in the first and last quantum well of the N-polar heterostructures. The peculiar evolution of the relative peak intensities with injected current, seen only in the case of the NW-LED, is attributed to the three-dimensional strain variation resulting from elastic relaxation at the free sidewalls of the NWs. Therefore, this study provides important insights on the working principle of N-polar LEDs based on both planar and NW heterostructures.

We investigate the radiative and nonradiative recombination processes in planar (In,Ga)N/GaN(0001) quantum wells and (In,Ga)N quantum disks embedded in GaN$(000\bar{1})$ nanowires using photoluminescence spectroscopy under both continuous-wave and pulsed excitation. The photoluminescence intensities of these two samples quench only slightly between 10 and 300 K, which is commonly taken as evidence for high internal quantum efficiencies. However, a side-by-side comparison shows that the absolute intensity of the Ga-polar quantum wells is two orders of magnitude higher than that of the N-polar quantum disks. A similar difference is observed for the initial decay time of photoluminescence transients obtained by time-resolved measurements, indicating the presence of a highly efficient nonradiative decay channel for the quantum disks. In apparent contradiction to this conjecture, the decay of both samples is observed to slow down dramatically after the initial rapid decay. Independent of temperature, the transients approach a power law for longer decay times, reflecting that recombination occurs between individual electrons and holes with varying spatial separation. Employing a coupled system of stochastic integro-differential equations taking into account both radiative and nonradiative Shockley-Read-Hall recombination of spatially separate electrons and holes as well as their diffusion, we obtain simulated transients matching the experimentally obtained ones. The results reveal that even dominant nonradiative recombination conserves the power law decay for (In,Ga)N/GaN{0001} quantum wells and disks.

We study the nature of excitons bound to I1 basal plane stacking faults in ensembles of ultrathin GaN nanowires by continuous-wave and time-resolved photoluminescence spectroscopy. These ultrathin nanowires, obtained by the thermal decomposition of spontaneously formed GaN nanowire ensembles, are tapered and have tip diameters down to 6 nm. With decreasing nanowire diameter, we observe a strong blue shift of the transition originating from the radiative decay of stacking fault-bound excitons. Moreover, the radiative lifetime of this transition in the ultrathin nanowires is independent of temperature up to 60 K and significantly longer than that of the corresponding transition in as-grown nanowires. These findings reveal a zero-dimensional character of the confined exciton state and thus demonstrate that I1 stacking faults in ultrathin nanowires act as genuine quantum dots.

We investigated the origin of the high reverse leakage current in light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. To this end, capacitance deep level transient spectroscopy (DLTS) and temperature-dependent current-voltage (I-V) measurements were performed on a fully processed NW-LED. The DLTS measurements reveal the presence of two distinct electron traps with high concentrations in the depletion region of the p-i-n junction. These band gap states are located at energies of $570\pm20$ and $840\pm30$ meV below the conduction band minimum. The physical origin of these deep level states is discussed. The temperature-dependent I-V characteristics, acquired between 83 and 403 K, show that different conduction mechanisms cause the observed leakage current. On the basis of all these results, we developed a quantitative physical model for charge transport in the reverse bias regime. By taking into account the mutual interaction of variable range hopping and electron emission from Coulombic trap states, with the latter being described by phonon-assisted tunnelling and the Poole-Frenkel effect, we can model the experimental I-V curves in the entire range of temperatures with a consistent set of parameters. Our model should be applicable to planar GaN-based LEDs as well. Furthermore, possible approaches to decrease the leakage current in NW-LEDs are proposed., Comment: 10 pages, 9 figures

We investigate the structural and optical properties of spontaneously formed GaN nanowires with different degrees of coalescence. This quantity is determined by an analysis of the cross-sectional area and perimeter of the nanowires obtained by plan-view scanning electron microscopy. X-ray diffraction experiments are used to measure the inhomogeneous strain in the nanowire ensembles as well as the orientational distribution of the nanowires. The comparison of the results obtained for GaN nanowire ensembles prepared on bare Si(111) and AlN buffered 6H-SiC(000-1) reveals that the main source of the inhomogeneous strain is the random distortions caused by the coalescence of adjacent nanowires. The magnitude of the strain inhomogeneity induced by nanowire coalescence is found not to be determined solely by the coalescence degree, but also by the mutual misorientation of the coalesced nanowires. The linewidth of the donor-bound exciton transition in photoluminescence spectra does not exhibit a monotonic increase with the coalescence degree. In contrast, the comparison of the root mean square strain with the linewidth of the donor-bound exciton transition reveals a clear correlation: the higher the strain inhomogeneity, the larger the linewidth., Comment: 18 pages, 7 figures

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.