Growth of Inclined GaAs Nanowires by Molecular Beam Epitaxy: Theory and Experiment

Geometrical Selection of GaN Nanowires Grown by Plasma-Assisted MBE on Polycrystalline ZrN Layers

GaN nanowires grown on metal substrates have attracted increasing interest for a wide range of applications. Herein, we report GaN nanowires grown by plasma-assisted molecular beam epitaxy on thin polycrystalline ZrN buffer layers, sputtered onto Si(111) substrates. The nanowire orientation was studied by X-ray diffraction and scanning electron microscopy, and then described within a model as a function of the Ga beam angle, nanowire tilt angle, and substrate rotation. We show that vertically aligned nanowires grow faster than inclined nanowires, which leads to an interesting effect of geometrical selection of the nanowire orientation in the directional molecular beam epitaxy technique. After a given growth time, this effect depends on the nanowire surface density. At low density, the nanowires continue to grow with random orientations as nucleated. At high density, the effect of preferential growth induced by the unidirectional supply of the material in MBE starts to dominate. Faster growing nanowires with smaller tilt angles shadow more inclined nanowires that grow slower. This helps to obtain more regular ensembles of vertically oriented GaN nanowires despite their random position induced by the metallic grains at nucleation. The obtained dense ensembles of vertically aligned GaN nanowires on ZrN/Si(111) surfaces are highly relevant for device applications. Importantly, our results are not specific for GaN nanowires on ZrN buffers, and should be relevant for any nanowires that are epitaxially linked to the randomly oriented surface grains in the directional molecular beam epitaxy.

Non-〈111〉-oriented semiconductor nanowires: growth, properties, and applications

In recent years, non-〈111〉-oriented semiconductor nanowires have attracted increasing interest in terms of fundamental research and promising applications due to their outstanding crystal quality and distinctive physical properties. Here, a comprehensive overview of recent advances in the study of non-〈111〉-oriented semiconductor nanowires is presented. We start by introducing various growth techniques for obtaining nanowires with certain orientations, for which the growth energetics and kinetics are discussed. Attention is then given to the physical properties of non-〈111〉 nanowires, as predicted by theoretical calculations or demonstrated experimentally. After that, we review the advantages and challenges of non-〈111〉 nanowires as building blocks for electronic and optoelectronic devices. Finally, we discuss the possible challenges and opportunities in the research field of non-〈111〉 semiconductor nanowires.

Low growth rate synthesis of GaAs nanowires with uniform size

The growth of nanowires (NWs) with uniform sizes is crucial for future NW-based electronics. In this work, an efficient one-step process is introduced for the growth of uniform gallium arsenide NWs on the native oxide surface of Si, which could be even considered as an alternative for expensive and sophisticated patterning approaches. The proposed strategy considers a Ga pre-deposition step leading to the formation of droplets with homogeneous sizes. That is followed by controlled nucleation of gallium arsenide from those droplets only. Our key to controlling the nucleation of gallium arsenide is to perform the NW growth at temperatures above 580 ± 10 °C and low Ga fluxes. By this method, the statistical distribution of the length and diameter of the vertically grown NWs decreased to about 3%–6% of their averaged values. Moreover, 100% epitaxial growth was realized. Besides, the growth of undesired parasitic islands is addressed and accordingly suppressed. Our study focuses on NW low growth rates, which is so far not investigated in the literature and, could be of great interest e.g. for in situ growth studies.

Kinetics and mechanism of planar nanowire growth

Surface-guided growth of planar nanowires offers the possibility to control their position, direction, length, and crystallographic orientation and to enable their large-scale integration into practical devices. However, understanding of and control over planar nanowire growth are still limited. Here, we study theoretically and experimentally the growth kinetics of surface-guided planar nanowires. We present a model that considers different kinetic pathways of material transport into the planar nanowires. Two limiting regimes are established by the Gibbs-Thomson effect for thinner nanowires and by surface diffusion for thicker nanowires. By fitting the experimental data for the length-diameter dependence to the kinetic model, we determine the power exponent, which represents the dimensionality of surface diffusion, and results to be different for planar vs. nonplanar nanowires. Excellent correlation between the model predictions and the data is obtained for surface-guided Au-catalyzed ZnSe and ZnS nanowires growing on both flat and faceted sapphire surfaces. These data are compared with those of nonplanar nanowire growth under similar conditions. The results indicate that, whereas nonplanar growth is usually dominated by surface diffusion of precursor adatoms over the nanowire walls, planar growth is dominated by surface diffusion over the substrate. This mechanism of planar nanowire growth can be extended to a broad range of material-substrate combinations for higher control toward large-scale integration into practical devices.

Comparison of photoemission characteristics between square and circular wire array GaAs photocathodes

Two types of negative electron affinity gallium arsenide (GaAs) wire array photocathodes were fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material. High density GaAs wire arrays with high periodicity and good morphology were verified using scanning electron microscopy, and photoluminescence spectra confirmed the wire arrays had good crystalline quality. Reflection spectra showed that circular GaAs wire arrays had superior light trapping compared with square ones. However, after Cs/O activation, the square GaAs wire array photocathodes showed enhanced spectral response. The integral sensitivity of the square wire array photocathodes was approximately 2.8 times that of the circular arrays.

Enhanced photovoltaic performance of an inclined nanowire array solar cell

An innovative solar cell based on inclined p-i-n nanowire array is designed and analyzed. The results show that the inclined geometry can sufficiently increase the conversion efficiency of solar cells by enhancing the absorption of light in the active region. By tuning the nanowire array density, nanowire diameter, nanowire length, as well as the proportion of intrinsic region of the inclined nanowire solar cell, a remarkable efficiency in excess of 16% can be obtained in GaAs. Similar results have been obtained in InP and Si nanowire solar cells, demonstrating the universality of the performance enhancement of inclined nanowire arrays.