Friederich Limbach

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

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.

Current path in light emitting diodes based on nanowire ensembles

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.

Influence of the adatom diffusion on selective growth of GaN nanowire regular arrays

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.

Highly polarized Raman scattering anisotropy in single GaN nanowires

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.

Height self-equilibration during the growth of dense nanowire ensembles: Order emerging from disorder

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.

Enhanced light scattering of the forbidden longitudinal optical phonon mode studied by micro-Raman spectroscopy on single InN nanowires

In the literature, there are controversies on the interpretation of the appearance in InN Raman spectra of a strong scattering peak in the energy region of the unscreened longitudinal optical (LO) phonons, although a shift caused by the phonon-plasmon interaction is expected for the high conductance observed in this material. Most measurements on light scattering are performed on ensembles of InN nanowires (NWs). However, it is important to investigate the behavior of individual nanowires and here we report on micro-Raman measurements on single nanowires. When changing the polarization direction of the incident light from parallel to perpendicular to the wire, the expected reduction of the Raman scattering was observed for transversal optical (TO) and E(2) phonon scattering modes, while a strong symmetry-forbidden LO mode was observed independently on the laser polarization direction. Single Mg- and Si-doped crystalline InN nanowires were also investigated. Magnesium doping results in a sharpening of the Raman peaks, while silicon doping leads to an asymmetric broadening of the LO peak. The results can be explained based on the influence of the high electron concentration with a strong contribution of the surface accumulation layer and the associated internal electric field.

Understanding peculiarities in the optoelectronic characteristics of light emitting diodes based on (In,Ga)N/GaN nanowires

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.

Properties of uniform diameter InN nanowires obtained under Si doping

High quality, well-separated, homogeneous sizes and high aspect ratio Si-doped InN nanowires (NWs) were grown by catalyst-free molecular beam epitaxy (MBE) after optimization of the growth conditions. To this end, statistical analysis of NW density and size distribution was performed. The high crystal quality and smooth NW surfaces were observed by high resolution transmission electron microscopy. Spectral photoluminescence has shown the increase of the band filling effect with Si flux, indicating successful n-type doping. A Raman LO scattering mode appears with a pronounced low energy tail, also reported for highly doped InN films.

Cathodoluminescence spectroscopy on selectively grown GaN nanowires

GaN nanowires (NWs) were grown selectively on Si(111) substrate without catalyst by plasma-assisted molecular-beam epitaxy under N-rich conditions. The selective growth was obtained using regular arrays of holes patterned in a silicon oxide layer on top of a thin AlN buffer. The optical properties of the selectively grown GaN NWs have been studied using cathodoluminescence (CL) spectroscopy. Both, CL spectra measured on NW ensembles and spatially resolved monochromatic images were investigated. From a comparison of morphology and CL studies it emerges that NW coalescence is responsible for the appearance of the defect related emission.

Nitride nanowire structures for LED applications

This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.