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 growth approaches to minimize the incubation time and to thus facilitate significantly higher growth temperatures (up to 875°C). We achieve this advancement by (i) using III/V flux ratios of >1 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 comparable to those of state-of-the-art free-standing GaN layers grown by hydride vapor phase epitaxy.

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 ± 20 and 840 ± 30 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 phononassisted 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. a) M. Musolino and D. van Treeck contributed equally to this work. b) Author to whom correspondence should be addressed. Electronic mail: treeck@pdi-berlin.de dependent current-voltage (I-V) measurements. On the basis of these data, we have developed a quantitative physical model able to describe the experimental I-V curves of NW-LEDs in the reverse bias regime for a wide range of temperatures. The assumptions made in this study should remain valid also for planar devices based on III-N heterostructures, thus making our model applicable also to conventional planar LEDs.The NW-LED structure employed in this work was grown by molecular beam epitaxy (MBE) on an AlN-buffered ndoped Si(111) substrate with the help of self-assembly processes. The active region of the NW-LED consists of four axial (In,Ga)N quantum wells (QWs) with an average In content of approximately 25%, separated by three GaN barriers. The last QW is immediately followed by a Mg-doped (Al,Ga)N electron blocking layer (EBL). The active region is embedded between two doped GaN segments designed such that an n-i-p diode doping profile is created. A schematic Si x O y n-Si(111) FIG. 1. (Color online) Schematic of the employed NW-LED structure. Note that the various dimensions are not to scale.

We investigate the effect of the p-type top contact on the optoelectronic characteristics of light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. We compare devices fabricated with either Ni/Au or indium tin oxide (ITO) top contact. The NW-LEDs with ITO exhibit a number density of NWs emitting electroluminescence about ten times higher, significantly lower turn-on voltage and series resistance, and a relative external quantum efficiency more than one order of magnitude higher than the sample with Ni/Au. These results show that limitations in the performance of such devices reported so far can be overcome by improving the p-type top-contact.III-N nanowires (NWs) are an attractive alternative to conventional planar layers as the basis for light-emitting diodes (LEDs) 1-3 because they offer several conceptual advantages. The NW geometry enables the elastic relaxation of the strain induced by lattice mismatch at the free sidewalls, 4 thus permitting the growth of high quality (In,Ga)N/GaN heterostructures with high In content on Si substrates. Furthermore, the high aspect ratio of NWs inhibits the vertical propagation of extended defects, 5 and light extraction from arrays of NWs can be enhanced compared to planar devices. 2 In combination, these benefits could lead to cost-effective phosphorless monolithic white LEDs. 6 In practice, LEDs based on GaN NW ensembles on Si substrates have been fabricated by several groups, 1,7-14 and significant limitations in device performance have been reported. In particular, careful investigations showed that only about 1 % of the NWs in the ensemble may emit electroluminescence (EL). 8,13,15 Also, in many cases high turn-on voltages in the range of 4.5-8 V were measured, 12,13,15,16 while for more complex NW structures lower values were obtained. 12,14,17 Thus, it seems fair to say that the actual implementation of the above conceptual advantages in device performance still remains to be demonstrated. Naturally, the processing of such LEDs is rather complex because of the three-dimensional morphology of NW ensembles. Therefore, it is at present unclear whether the reported limitations are peculiar to LEDs based on NW ensembles on Si substrates or such devices simply need further advances in processing technology.One peculiarity of such NW-LEDs is that for typical device sizes they contain millions of NWs. Hence, the macroscopic LED actually consists of very many individual NWLEDs contacted in parallel, and the overall device characteristics are determined by the properties of all the individual NW-LEDs. For example, NW-to-NW fluctuations in series resistance inevitably lead to a filamentation of the current path in the NW ensemble, and this phenomenon was in fact identified as the reason for the very low fraction of electroluminescent NWs. 13 Such fluctuations in series resistance a) Author to whom correspondence should be addressed. Electronic mail: musolino@pdi-berlin.de could be caused either by non-unif...

AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with short nucleation time. Such an AlN buffer layer was successfully employed, together with a patterned SiOx mask, for the selective-area growth (SAG) of vertical GaN NWs. In addition, we fabricated light-emitting diodes (LEDs) from NW ensembles that were grown by means of self-organization phenomena on bare and on AlN-buffered Si substrates. A careful characterization of the optoelectronic properties of the two devices showed that the performance of NW-LEDs on bare and AlN-buffered Si is similar. Electrical conduction across the AlN buffer is facilitated by a high number of grain boundaries that were revealed by transmission electron microscopy. These results demonstrate that grainy AlN buffer layers on Si are compatible both with the SAG of GaN NWs and LED operation. Therefore, this study is a first step towards the fabrication of LEDs on Si substrates based on homogeneous NW ensembles.

In the experimental electroluminescence (EL) spectra of light-emitting diodes (LEDs) based on N-polar (In,Ga)N/GaN nanowires (NWs), we observe 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 confirm the presence of two main transitions in the spectra and suggest that they are emitted by a majority of the single nano LEDs. In order to elucidate the physical origin of this effect, we perform theoretical calculations of the strain, electric field, and charge-density distributions for both planar LEDs and NW LEDs. On this basis, we simulate also the EL spectra of these devices, which exhibit a double-peak structure for N-polar heterostructures, in both the NW and the planar case. By contrast, this feature is not observed when Ga-polar planar LEDs are simulated. We find 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