GaAs Nanowire Arrays Research Articles

Theoretical exploration of photoemission characteristics of GaAs nanowire array cathode based on photon-enhanced thermionic emission

The theoretical photoemission models of single GaAs nanowire and nanowire array cathode based on photon-enhanced thermionic emission (PETE) mechanism are respectively developed by utilizing two-dimensional continuity equations. Based on the built models in this work, the effect of several key factors on the quantum efficiency are also discussed. Results show that GaAs nanowire cathode exhibits the superiority of photoemission performance compared with GaAs film cathode at the spectral range of 300∼600 nm. Besides, the optimal wire diameter of GaAs nanowire cathode is in the range of 340∼360 nm. In addition, the nanowire cathode with low electron affinity of 0.3 eV achieves PETE effect under low temperature of 400 K, which is conducive to improve the stability of cathode. Moreover, increasing the incident light angle or decreasing the array spacing provides an effective method of obtaining high quantum efficiency of nanowire array cathode. Furthermore, the numerical difference between quantum efficiency and effective quantum efficiency implies that the low collection efficiency of emitted electrons is the main issue that hinders the application of PETE cathodes of GaAs nanowire array. The proposed GaAs nanowire array model would provide theoretical guidance for optimum design of PETE cathodes with nanoarray structure.

Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices.

III-V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array-geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.

Inverse design of GaAs nanowire array solar cell structures with nonuniform diameters

Non-uniform diameter GaAs nanowire array solar cell structures are designed using the particle swarm optimized inverse design method. The results show that unlike the uniform diameter structure, which has a series of absorption valleys, the absorption spectra of the non-uniform structures are much flatter, which is attributed to a combination of absorption spectra from nanowires of different diameters. For the same height, the optimized structure with non-uniform diameter yields a remarkable ultimate efficiency of 34.3%, 1.1% higher than that the structure with uniform diameter, mainly due to the enhanced absorption at long wavelengths. For the same volume, an even higher ultimate efficiency of 34.6% is obtained, due to the enhanced absorption over almost the entire wavelength range. This work could pave the way for the design of low-cost high-efficiency nanoscale solar cells and other photoelectric conversion devices.

Theoretical research and simulation of GaAs nanowire arrays in reflection-type photon-enhanced thermionic emission solar converters

In this paper, GaAs was used as the cathode of the photon-enhanced thermionic emission (PETE) solar converter. Based on the one-dimensional continuity equation and boundary conditions, a reflection-type PETE theoretical model was established to simulate the effects of solar concentration, electron affinity, cathode temperature and electron recombination velocity on the current density and the conversion efficiency. For optimizing the performance of the solar converter, we used GaAs nanowire (NW) arrays as the cathode and calculated the influences of NW arrays’ period, duty ratio, and height on the absorbance and the conversion efficiency by combining the theoretical model with the three-dimensional finite-difference time-domain (3D FDTD) method. The results were used to design the structure size of NW arrays, which improved the conversion efficiency of the solar converter to 23.07%. It shows that the GaAs NW arrays are promising PETE cathode materials.

Effect of Si(111) Surface Modification by Ga Focused Ion Beam at 30 kV on GaAs Nanowire Growth

This paper presents the results of experimental studies of the effect of Si(111) surface modification by Ga-focused ion beam (FIB) at 30 kV accelerating voltage on the features of the epitaxial GaAs nanowire (NW) growth processes. We experimentally established the regularities of the Ga ions' dose effect during surface modification on the structural characteristics of GaAs NW arrays. Depending on the Ga ion dose value, there is one of three modes on the surface for subsequent GaAs NW growth. At low doses, the NW growth is almost completely suppressed. The growth mode of high-density (up to 6.56 µm-2) GaAs NW arrays with a maximum fraction (up to 70%) of nanowires normally oriented to the substrate is realized in the medium ion doses range. A continuous polycrystalline base with a dense array of misoriented short (up to 0.9 µm) and thin (up to 27 nm) GaAs NWs is formed at high doses. We assume that the key role is played by the interaction of the implanted Ga ions with the surface at various process stages and its influence on the surface structure in the modification region and on GaAs NW growth conditions.

Role of twin defects on growth dynamics and size distribution of undoped and Si-doped GaAs nanowires by selective area epitaxy

We report the effects of Si doping on the growth dynamics and size distribution of entirely catalyst-free GaAs nanowire (NW) arrays grown by selective area molecular beam epitaxy on SiO2-masked Si (111) substrates. Surprising improvements in the NW-array uniformity are found with increasing Si doping, while the growth of undoped NWs appears in a metastable regime, evidenced by large size and shape distributions, and the simultaneous presence of crystallites with tetrahedral termination. Correlating scanning electron microscopy and transmission electron microscopy investigations, we propose that the size and shape distributions are strongly linked to the underlying twin defect formation probabilities that govern the growth. Under the present growth conditions, Si-doping of GaAs NWs leads to a very high twin defect formation probability (∼0.4), while undoped NWs exhibit a nearly threefold decreased probability (∼0.15). By adopting a model for facet-mediated growth, we describe how the altered twin formation probabilities impact the competing growth of the relevant low-index NW facets, and hence, NW size and shape. Our model is further supported by a generic Monte Carlo simulation approach to highlight the role of twin defects in reproducing the experimentally observed size distributions.

Disorder induced absorption enhancement of light in GaAs nanowire array

Light absorption characteristics of vertically aligned GaAs nanowire (NW) arrays with disordered diameters and heights are investigated in this work using finite difference time domain (FDTD) analysis technique. By varying the random height ranges, an optimum variation range of 1000–2000 nm is obtained that provides the maximum average absorbance at different fill-factors of the arrays. An array having random heights of the NWs within the optimized range is found to have better absorbance for both normal and oblique incidence of light compared to the uniform height structure. Even for 45° incidence angle of light, average absorbance is obtained to be 2% higher for the random-height array, compared to the case of absorbance obtained for the uniform height structure. The proposed arrays having random diameters provide up to 12.8% improvement in short circuit current density whereas the random-height structure enhances the short circuit current density by 1.1% compared to the arrays having uniform diameter and height. The present work also provides an effective medium theory based theoretical model taking into account the random height variation of the arrays. The theoretically calculated values are found to be in good agreement with FDTD simulation results, thereby providing further guidelines for designing random array based high performance photonic devices.

Core–Shell Structures Prepared by Atomic Layer Deposition on GaAs Nanowires

GaAs nanowire arrays have been prepared by anodization of GaAs substrates. The nanowires produced on (111)B GaAs substrates were found to be oriented predominantly perpendicular to the substrate surface. The prepared nanowire arrays have been coated with thin ZnO or TiO2 layers by means of thermal atomic layer deposition (ALD), thus coaxial core–shell hybrid structures are being fabricated. The hybrid structures have been characterized by scanning electron microscopy (SEM) for the morphology investigations, by Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis for the composition and crystal structure assessment, and by photoluminescence (PL) spectroscopy for obtaining an insight on emission polarization related to different recombination channels in the prepared core–shell structures.

Spatial Modulation of Vibrational and Luminescence Properties of Monolayer MoS₂ Using a GaAs Nanowire Array

The integration of transition-metal dichalcogenides (TMDs) with non-planar substrates such as nanopillars provides a way to spatially modify the optical properties mainly through the localized strain. Similar studies to date have utilized insulating SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanopillars. Here, we combine monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with free standing GaAs nanowires (NWs), in views of coupling their semiconducting properties. We find that monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> exhibits three different configurations: pierced, wrapped and tent-like. We demonstrate how to identify the configurations by optical microscopy and elucidate the impact on the vibrational and luminescence characteristics by confocal spectroscopy mapping. In particular, we highlight the increase of intensity and shift due to the photonic properties of nanowires and increase in dielectric screening associated with the GaAs NW. This work signifies the first step towards the use of vertical III-V NW arrays as a versatile platform for spatially engineering the optical properties of TMDs.

Detailed balance analysis of vertical GaAs nanowire array solar cells: exceeding the Shockley Queisser limit.

We performed detailed balance analysis using rigorous coupled-wave analysis (RCWA) on vertical GaAs nanowire (NW) arrays. Both freestanding NW arrays as well as NW arrays on a perfect back reflector are assessed. Both types of vertical NW arrays demonstrate efficiencies that exceed the Shockley Queisser (SQ) or radiative efficiency limit when the NWs are sufficiently long. The use of a back reflector enhances the efficiency of NW solar cells by increasing solar absorption and suppressing emission from the backside of the solar cell. We study the light trapping and material reduction advantages of NWs. Furthermore, we compare simulations that evaluate detailed balance efficiency with ultimate efficiency and show that ultimate efficiency studies can determine near-optimal solar cells while vastly reducing the number of simulations that need to be performed. While open circuit voltages above the radiative limit can be achieved, tradeoffs with short circuit current must be carefully considered. We also compare our simulation results to other claims in the literature that NWs are capable of exceeding the SQ limit.

Thermal Conductivity of GaAs Nanowire Arrays Measured by the 3ω Method.

Vertical nanowire (NW) arrays are the basis for a variety of nanoscale devices. Understanding heat transport in these devices is an important concern, especially for prospective thermoelectric applications. To facilitate thermal conductivity measurements on as-grown NW arrays, a common NW-composite device architecture was adapted for use with the 3ω method. We describe the application of this technique to obtain thermal conductivity measurements on two GaAs NW arrays featuring ~130 nm diameter NWs with a twinning superlattice (TSL) and a polytypic (zincblende/wurtzite) crystal structure, respectively. Our results indicate NW thermal conductivities of 5.2 ± 1.0 W/m-K and 8.4 ± 1.6 W/m-K in the two samples, respectively, showing a significant reduction in the former, which is the first such measurements on TSL NWs. Nearly an order of magnitude difference from the bulk thermal conductivity (~50 W/m-K) is observed for the TSL NW sample, one of the lowest values measured to date for GaAs NWs.

Second-harmonic generation tuning by stretching arrays of GaAs nanowires.

We present a wearable device with III–V nanowires in a flexible polymer, which is used for active mechanical tuning of the second-harmonic generation intensity. An array of vertical GaAs nanowires was grown with metalorganic vapour-phase epitaxy, then embedded in polydimethylsiloxane and detached from the rigid substrate with mechanical peel off. Experimental results show a tunability of the second-harmonic generation intensity by a factor of two for 30% stretching which matches the simulations including the distribution of sizes. We studied the impact of different parameters on the band dispersion and tunability of the second-harmonic generation, such as the pitch, the length, and the diameter. We predict at least three orders of magnitude active mechanical tuning of the nonlinear signal intensity for nanowire arrays. The flexibility of the array together with the resonant wavelength engineering make such structures perspective platforms for future bendable or stretchable nanophotonic devices as light sources or sensors.

GaAs/AlGaAs Nanowire Array Solar Cell Grown on Si with Ultrahigh Power-per-Weight Ratio

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.

Study on the optoelectronic properties of Ag, Pt, Na and Li particles adsorbed on GaAs nanowire arrays

In this paper, COMSOL Multiphysics software based on finite element method was used to design a GaAs nanowire array simulation model absorbed with metal nanoparticles. We studied the effects of the size, position and number of metal nanoparticles on their light absorption, and in this experiment, precious metal (Ag, Pt) and alkali metal (Na, Li) particles adsorbed on nanowires. The results show that the adsorption of metal particles is helpful to improve the light binding capacity of short wavelength nanowire arrays. With the change of the radius, position and number of particles, the nanowires absorbed by Li and Ag nanoparticles showed better light absorption characteristics. The optical trapping effect can be effectively enhanced by increasing the size of particles, the number of surrounding particles and the number of particles which distributed along the nanowire direction appropriately.

Growth of high-crystallinity uniform GaAs nanowire arrays by molecular beam epitaxy* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61674021, 11674038, 61704011, 61904017, 11804335, and 12074045), the Developing Project of Science and Technology of Jilin Province, China (Grant No. 20200301052RQ), the Project of Education Department of Jilin Province, China (Grant No. JJKH20200763KJ), and the Youth Foundation

The self-catalyzed growth of GaAs nanowires (NWs) on silicon (Si) is an effective way to achieve integration between group III–V elements and Si. High-crystallinity uniform GaAs NW arrays were grown by solid-source molecular beam epitaxy (MBE). In this paper, we describe systematic experiments which indicate that the substrate treatment is crucial to the highly crystalline and uniform growth of one-dimensional nanomaterials. The influence of natural oxidation time on the crystallinity and uniformity of GaAs NW arrays was investigated and is discussed in detail. The GaAs NW crystallinity and uniformity are maximized after 20 days of natural oxidation time. This work provides a new solution for producing high-crystallinity uniform III–V nanowire arrays on wafer-scale Si substrates. The highly crystalline uniform NW arrays are expected to be useful for NW-based optical interconnects and Si platform optoelectronic devices.

Improving the yield of GaAs nanowires on silicon by Ga pre-deposition

GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor−liquid−solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume.

Simulation of optical absorption in conical nanowires.

The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE1n modes, the red-shift of the HE1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm-2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.

Theoretical study on the improvement of optoelectronic properties of Al_xGa_(1-x) As nanowires with variable Al content adsorbed by metal nanoparticles

GaAs nanowire arrays with variable Al content can effectively improve the photoelectric emission of nanowires due to the existence of internal electric field. Metal particles adsorb nanowires, which can excite local plasmons and enhance the optical absorption of nanowires. In this paper, the Al content change of Al_xGa_ (1-x) As material and metal particle adsorption are combined to study the light absorption and quantum efficiency of the Al_xGa_ (1-x) As nanowire array with metal particles adsorbed on the surface. Based on the finite element method and COMSOL multiphysics software, the effects of the number of metal nanoparticles, the distribution of particles, array period and particle size on the optical properties of the array are analyzed. The results show that metal particle adsorption can enhance the light capture ability of Al_xGa_(1-x) As nanowires arrays, the absorption of short wave light can be improved by adding adsorption particles layer by layer. Higher quantum efficiency can be obtained by appropriately increasing the array period and the diameter of metal particles of the structure.

Defect-Tolerant TiO2-Coated and Discretized Photoanodes for &gt;600 h of Stable Photoelectrochemical Water Oxidation

Arrays of GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition (MOCVD) onto photoactive planar Si substrates. This tandem, vertical-wire-array-on-planar absorber...

Anodically Induced Chemical Etching of GaAs Wafers for a GaAs Nanowire-Based Flexible Terahertz Wave Emitter.

A generic top-down approach for the preparation of extended arrays of high-aspect ratio GaAs nanowires (NWs) with different crystallographic orientations (i.e., [100] or [111]) and morphologies (i.e., porous, nonporous, tapered, or awl-like NWs) is reported. The method is based on the anodically induced chemical etching (AICE) of GaAs wafers in an oxidant-free aqueous HF solution at room temperature by using a patterned metal mesh and allows us to overcome the drawbacks of conventional metal-assisted chemical etching (MACE) processes. Local oxidative dissolution of GaAs in contact with a metal is achieved by externally injecting holes (h+) into the valence band (VB) of GaAs through the metal mesh. It is found that injection of holes (h+) through direct GaAs contact, rather than the metal mesh, does not yield uniform nanowires but porosify GaAs wafers due to the high cell potential. On the basis of experiments and numerical simulation for the spatial distribution of an electric field, a phenomenological model that explains the formation of GaAs NWs and their porosification behaviors is proposed. GaAs NWs exhibit excellent terahertz (THz) wave emission properties, which vary with either the length or the shape of the nanowires. By taking advantage of controlled porosification and easy transfer of GaAs NWs to foreign substrates, a flexible THz wave emitter is realized.