The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between "core" bulk (quadratic) and "shell" sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Molecular beam epitaxy (MBE) on patterned Si/AlN/Si(111) substrates was used to obtain regular arrays of uniform-size GaN nanowires (NWs). The silicon top layer has been patterned with e-beam lithography, resulting in uniform arrays of holes with different diameters (dh) and periods (P). While the NW length is almost insensitive to the array parameters, the diameter increases significantly with dh and P till it saturates at P values higher than 800 nm. A diffusion induced model was used to explain the experimental results with an effective diffusion length of the adatoms on the Si, estimated to be about 400 nm.

Light emitting diodes (LEDs) have been fabricated using ensembles of free-standing (In, Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron-beam-induced current analysis, cathodoluminescence as well as biased μ-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In, Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect.

We show that the growth kinetics of dense arrays of self-induced GaN nanowires involves the exchange of Ga atoms between nanowires: Ga atoms desorbed from the side surfaces of nanowires readsorb on neighboring nanowires. This process favors the growth of shorter nanowires and gives rise to a narrow nanowire height distribution during the late stages of growth. We propose a stochastic differential equation model which describes the growth of dense nanowire ensembles. The model calculations are in good agreement with the experiments.

GaN nanowires were grown without any catalyst by plasma-assisted molecular beam epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of wires increases to a higher value, and nanowire coalescence is more pronounced than without Mg. During nanowire nucleation the Ga desorption was monitored in-situ by line-of-sight quadrupole mass spectrometry for various substrate temperatures. Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were deduced, respectively. This effect has to be taken into account for the fabrication of nanowire devices and could be employed to tune the NW areal density

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...

Single GaN nanowires and larger GaN ensembles are investigated by Raman spectroscopy. Spectra of nanowire ensembles prove the high crystal quality and are in agreement with selection rules for the wurtzite structure. Single nanowires are studied with a spatial resolution of the order of 400 nm for different polarization directions of the incident laser beam relative to the nanowire axis. In the single wire spectrum, only the A1(TO) was observed and the Raman intensity was suppressed for perpendicular polarization. These results confirm that Raman scattering in isolated GaN nanowires is governed by size effects.