GaAs Nanowire Arrays Research Articles

Optical simulation and geometrical optimization of P3HT/GaAs nanowire hybrid solar cells for maximal photocurrent generation via enhanced light absorption

The implications of using the combination of inorganic and organic materials in the active layer of solar cells have motivated researchers to find a new pathway for flexible, low cost, and highly efficient future generation solar cells. III-V material based nanowires (NWs) with subwavelength scale diameters have shown excellent light harvesting and charge transport properties and can be easily combined with organic polymer materials and thin substrates to design hybrid solar cells (HSCs). To obtain optimum design requirements for GaAs nanowire array (NWA) HSCs, an optical simulation of poly(3-hexylthiophene) P3HT/ GaAs (NWA) hybrid solar cell is investigated using finite-difference-time-domain (FDTD) method. In comparison to planar substrate, P3HT coated NWA have shown superior optical properties owing to its intrinsic anti-reflection, broad absorption spectra, and efficient excitation of resonance modes. A steady improvement in light absorption of P3HT/GaAs NWA HSCs is observed as the polymer coating thickness increased up to a certain limit and beyond which there is a significant degradation in the exciton generation. The geometrical parameters like diameter (D) of NW and filling ratio or periodicity are also optimized to achieve superior light absorption and maximum short circuit current density (Jsc). Under AM1.5G illumination, maximum photocurrent obtained for optimized structure with conformal coating of P3HT is almost 12% and 15% higher than its fully infiltrated and uncoated counterparts respectively

Photovoltaic Light Funnels Grown by GaAs Nanowire Droplet Dynamics

GaAs nanowire (NW) arrays were grown on Si substrates by selective-area molecular beam epitaxy (MBE) using the self-assisted vapor-liquid-solid (VLS) method. In the self-assisted VLS method, the GaAs NW diameter is determined by the size of a Ga droplet at the top of the NW. Absorption of the solar spectrum for photovoltaic devices is optimum with a GaAs NW diameter of ∼170 nm, requiring a Ga droplet during the growth of a similar size. Control of the Ga droplet size, and therefore the NW diameter, is demonstrated by controlling the V/III flux ratio during the MBE growth. A V/III flux ratio near unity rapidly increased the Ga droplet volume during the NW growth, thereby creating an inverse-tapered (funnel-shaped) NW morphology. Numerical simulations indicated that the inverse-tapered NW showed strong broadband absorption of the solar spectrum due to the absorption of consecutively longer wavelengths toward the base of the NW.

Modeling of conductive states in polymer nanocomposites with disordered GaAs nanowire array

We provide numerical Monte Carlo modeling of conductive properties of polymer composites, comprised by doping a polymer matrix with carbon nanotubes and III-IV GaAs nanowires. We apply the basic Su-Schrieffer-Heeger model, which is considered universal for every component of such ternary systems, and in a general case with required parameters. Such modeling of conductive characteristics is motivated by requirements of optimization of geometry and structure of novel photovoltaic devices.

Impact of the Shadowing Effect on the Crystal Structure of Patterned Self-Catalyzed GaAs Nanowires.

The growth of regular arrays of uniform III-V semiconductor nanowires is a crucial step on the route toward their application-relevant large-scale integration onto the Si platform. To this end, not only does optimal vertical yield, length, and diameter uniformity have to be engineered, but also, control over the nanowire crystal structure has to be achieved. Depending on the particular application, nanowire arrays with varying area density are required for optimal device efficiency. However, the nanowire area density substantially influences the nanowire growth and presents an additional challenge for nanowire device engineering. We report on the simultaneous in situ X-ray investigation of regular GaAs nanowire arrays with different area density during self-catalyzed vapor-liquid-solid growth on Si by molecular-beam epitaxy. Our results give novel insight into selective-area growth and demonstrate that shadowing of the Ga flux, occurring in dense nanowire arrays, has a crucial impact on the evolution of nanowire crystal structure. We observe that the onset of Ga flux shadowing, dependent on array pitch and nanowire length, is accompanied by an increase of the wurtzite formation rate. Our results moreover reveal the paramount role of the secondary reflected Ga flux for VLS NW growth (specifically, that flux that is reflected directly into the liquid Ga droplet).

Combining 1D and 2D waveguiding in an ultrathin GaAs NW/Si tandem solar cell.

As an effective means to surpass the Shockley-Queisser efficiency limit, tandem solar cells have been successfully designed and used for years. However, there are still economical and design set-backs hampering the terrestrial implementation of tandem solar cells. Introducing high efficiency, thin Si-based tandem cells that are flexible in design (shape and curvature) will be the next major step towards integrating highly efficient solar cells into fashionable designs of today's buildings and technologies. In this work we present an optically coupled tandem cell that consists of a GaAs nanowire array on a 2μm-thick Si film as the top and bottom cells, respectively. By performing FDTD simulations, we show that coupling the incident light to guided modes of the 1D wires not only boosts the absorption in the wires, but also efficiently transfers the below bandgap photons to the Si bottom cell. Due to diffraction by the nanowire array the momentum of the transmitted light is matched to that of guided modes of the 2D Si thin film. Consequently, infrared light is up to four times more efficiently trapped in the Si bottom cell compared to when the film is not covered by the nanowires.

Enhanced photovoltaic performance of nanowire array solar cells with multiple diameters.

A multi-diameter p-i-n junction GaAs nanowire (NW) array architecture is proposed for high-performance solar cells. Coupled three-dimensional optoelectronic simulations are performed to investigate the photovoltaic properties. The NW diameters are randomly selected within the range of 220-400 nm, following the Gaussian distribution. The results show that the absorption strongly depends on the diameter, and the multi-diameter NW array exhibits higher optical absorption, in comparison with the uniform-diameter counterpart. This is because of the superposition of multiple absorption peaks. Moreover, the multi-diameter NW array can efficiently enhance the effective absorption; that is, the depletion region absorption, which directly leads to increased photocurrent. A remarkable efficiency of 17% is obtained for a 16-diameter NW array solar cell with a full width at half maximum of the diameter distribution of 75 nm, higher than the best value (16.1%) of uniform-diameter device with an optimum diameter of 310 nm. This work demonstrates that the native diameter nonuniformity of self-organized nanowires is beneficial for high-performance photovoltaics with low cost and a simple fabrication process.

Inductively coupled plasma etching of the GaAs nanowire array based on self-assembled SiO2 nanospheres

We reported a method combining the gas–liquid interface method and inductively coupled plasma (ICP) to fabricate highly-ordered nanowire arrays on a 2 inch GaAs substrate. A large-area and highly-ordered SiO2 nanosphere monolayer was fabricated and the GaAs nanowire array was etched by ICP. The flow rates and ratios of the etching gases were studied. Finally, highly-ordered GaAs nanowire arrays with high quality surface morphology were obtained with the volumetric flow rates at 9 sccm and 21 sccm for BCl3 and N2 gases, respectively. Reflectance of the samples was found to be consistently below 10%, the lowest being 3%.

High density GaAs nanowire arrays through substrate processing engineering

GaAs nanowires (NWs) vertically aligned were successfully fabricated through substrate processing engineering. High-density vertical GaAs NWs are grown on n-type Si (111) substrate by molecular beam epitaxy. Systematic experiments indicate that substrate pretreatment is crucial to vertical epitaxial growth of one-dimensional (1D) nanomaterials. The substrates etched using diluted buffered oxide etch (BOE) were explored to improve the NW density and vertical. We also find that the substrate processing engineering strongly affect the morphology of GaAs NWs. Finally, we demonstrate fabrication of GaAs NW arrays on Si surface by field-emission scanning electron microscopy (FE-SEM). This single-step process indeed offers a simple and cost-effective way to obtain a large area of GaAs NW arrays without using e-beam lithography (EBL) and/or nanoimprint lithography (NIL) processes. This work provided a new approach for hight density NW arrays.

An Optimized Structure for Enhancing Optical Absorption of Solar Energy in Elliptical GaAs Nanowire Array Solar Cell

The enhanced optical absorption of solar energy in a solar cell using nanostructured materials is a very demanding and important research area. Hence, an optimized structure to increase the optical absorption of solar energy in an elliptical GaAs (Gallium Arsenide) nanowire array solar cell was proposed in this paper. The influence of geometric parameters on the optical absorption of elliptical GaAs nanowire solar cells was investigated by exploiting the finite difference in time domain simulations. Based on the design and analysis, the elliptical GaAs nanowire array performed better than any circular GaAs nanowire. It was found that the enhanced performance was due to the reduced reflection from the top surface and a reduced transmission from the bottom surface of the elliptical GaAs nanowire array. The structural parameters of the elliptical GaAs nanowire were optimized by calculating the short circuit current density (Jsc) for different geometries over the solar spectrum. It was demonstrated that there was an increase of 9.4% in the Jsc compared with the circular GaAs nanowire under the optimized conditions.

Light absorption properties of a nanowire/quantum-dot hybrid nanostructure array

The light absorption properties of a nanowire/quantum-dot hybrid nanostructure array are investigated. By growing multilayer InAs quantum dots on the sidewalls of GaAs nanowires, not only the absorption spectrum of GaAs nanowires is extended by quantum dots, but also the light absorption of quantum dots is dramatically enhanced due to the light-trapping effect of the nanowire array. By optimizing the nanowire and quantum dot parameters, a maximum photocurrent increment of 2.8 mA/cm2 is obtained by incorporating 5 layer InAs quantum dots into a 3 μm high GaAs nanowire array, 4.6 times of its thin-film counterpart. Mode analysis demonstrates that the fundamental mode in the nanowire has a dominate influence on the QD absorption in the wavelength range beyond GaAs bandgap.

Exploring time-resolved photoluminescence for nanowires using a three-dimensional computational transient model.

Time-resolved photoluminescence (TRPL) has been implemented experimentally to measure the carrier lifetime of semiconductors for decades. For the characterization of nanowires, the rich information embedded in TRPL curves has not been fully interpreted and meaningfully mapped to the respective material properties. This is because their three-dimensional (3-D) geometries result in more complicated mechanisms of carrier recombination than those in thin films and analytical solutions cannot be found for those nanostructures. In this work, we extend the intrinsic power of TRPL by developing a full 3-D transient model, which accounts for different material properties and drift-diffusion, to simulate TRPL curves for nanowires. To show the capability of the model, we perform TRPL measurements on a set of GaAs nanowire arrays grown on silicon substrates and then fit the measured data by tuning various material properties, including carrier mobility, Shockley-Read-Hall recombination lifetime, and surface recombination velocity at the GaAs-Si heterointerface. From the resultant TRPL simulations, we numerically identify the lifetime characteristics of those material properties. In addition, we computationally map the spatial and temporal electron distributions in nanowire segments and reveal the underlying carrier dynamics. We believe this study provides a theoretical foundation for interpretation of TRPL measurements to unveil the complex carrier recombination mechanisms in 3-D nanostructured materials.

Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate

Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.

Photoelectrochemical Water Oxidation by GaAs Nanowire Arrays Protected with Atomic Layer Deposited NiO x Electrocatalysts

Photoelectrochemical (PEC) hydrogen production makes possible the direct conversion of solar energy into chemical fuel. In this work, PEC photoanodes consisting of GaAs nanowire (NW) arrays were fabricated, characterized, and then demonstrated for the oxygen evolution reaction (OER). Uniform and periodic GaAs nanowire arrays were grown on a heavily n-doped GaAs substrates by metal–organic chemical vapor deposition selective area growth. The nanowire arrays were characterized using cyclic voltammetry and impedance spectroscopy in a non-aqueous electrochemical system using ferrocene/ferrocenium (Fc/Fc+) as a redox couple, and a maximum oxidation photocurrent of 11.1 mA/cm2 was measured. GaAs NW arrays with a 36 nm layer of nickel oxide (NiO x ) synthesized by atomic layer deposition were then used as photoanodes to drive the OER. In addition to acting as an electrocatalyst, the NiO x layer served to protect the GaAs NWs from oxidative corrosion. Using this strategy, GaAs NW photoanodes were successfully used for the oxygen evolution reaction. This is the first demonstration of GaAs NW arrays for effective OER, and the fabrication and protection strategy developed in this work can be extended to study any other nanostructured semiconductor materials systems for electrochemical solar energy conversion.

(Invited) Orientation Dependent GaAs Nanowire Schottky Solar Cells with 16% Efficiency

Nowadays, high efficiency and low cost solar cell is still technological challenging. In this study, crystalline GaAs NWs with the uniform, pure <110>, <111> orientations and other different mixture ratios can be successfully prepared by tuning the catalyst thickness, nucleation and growth temperatures in the two-step chemical vapor deposition. The single GaAs NWs are fabricated into Au-Al asymmetric Schottky contact solar cells 1 by a simple ultraviolet photolithography process. The low-cost NW solar cell shows a record high efficiency of ~16% under air mass 1.5 global (AM 1.5G) illumination, which produces another record in addition to the vertically aligned p-i-n GaAs NW solar cells previously reported (40%), where the superior photovoltaic performance is attributed to the light concentrating effect of NWs with diameter in the order of light wavelength 2. Employing lift-off resists, three-layer NW parallel arrays can be easily attained for X-ray diffraction in order to evaluate their growth orientation along with the fabrication of NW parallel arrays based Schottky photovoltaic devices for the subsequent performance assessment3. Notably, the open circuit voltage of purely <111>-oriented NW arrayed cells is far higher than that of <110>-oriented NW arrayed counterparts, which can be interpreted by the different surface Fermi level pinning existed on various NW crystal surface planes due to the different As dangling bond densities.Fig1. (a) Output IV curve of a single GaAs NW solar cell and (b) the orientation dependent PV property of printed GaAs NW array solar cellsRef. 1. Han, N.; Yang, Z.; Wang, F.; Dong, G.; Yip, S.; Liang, X.; Hung, T. F.; Chen, Y.; Ho, J. C., High performance GaAs nanowire solar cells for flexible and transparent photovoltaics. ACS Appl. Mater. Interfaces 2015, 7, 20454-20459. 2. Krogstrup, P.; Jorgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Morral, A. F. I., Single-nanowire solar cells beyond the Shockley-Queisser limit. Nat. Photonics 2013, 7 (4), 306-310. 3. Han, N.; Yang, Z. X.; Wang, F. Y.; Yip, S.; Li, D. P.; Hung, T. F.; Chen, Y. F.; Ho, J. C., Crystal Orientation Controlled Photovoltaic Properties of Multilayer GaAs Nanowire Arrays. ACS Nano 2016, 10 (6), 6283-6290. Figure 1

Engineering the Size Distributions of Ordered GaAs Nanowires on Silicon.

Reproducible integration of III-V semiconductors on silicon can open new path toward CMOS compatible optoelectronics and novel design schemes in next generation solar cells. Ordered arrays of nanowires could accomplish this task, provided they are obtained in high yield and uniformity. In this work, we provide understanding on the physical factors affecting size uniformity in ordered GaAs arrays grown on silicon. We show that the length and diameter distributions in the initial stage of growth are not much influenced by the Poissonian fluctuation-induced broadening, but rather are determined by the long incubation stage. We also show that the size distributions are consistent with the double exponential shapes typical for macroscopic nucleation with a large critical length after which the nanowires grow irreversibly. The size uniformity is dramatically improved by increasing the As4 flux, suggesting a new path for obtaining highly uniform arrays of GaAs nanowires on silicon.

Top-down fabricated tapered GaAs nanowires with sacrificial etching of the mask

A novel fabrication method using controlled sacrificial etching of the mask is utilized to fabricate tapered vertical GaAs nanowire arrays. Experimental measurements of the absorption characteristics show that the tapered nanowires absorb over a broadband range as compared to cylindrical ones. The broadband characterization is verified by using optical modeling and results from improved coupling of the nanowires due to distinct radial HE modes being excited separately in the taper and the cylindrical part. The absorption is found to be more broadband as compared to conical nanowires studied so far.

Room Temperature Sensing Achieved by GaAs Nanowires and oCVD Polymer Coating.

Novel structures comprised of GaAs nanowire arrays conformally coated with conducting polymers (poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4-ethylenedioxythiophene-co-3-thiophene acetic acid) display both sensitivity and selectivity to a variety of volatile organic chemicals. A key feature is room temperature operation, so that neither a heater nor the power it would consume, is required. It is a distinct difference from traditional metal oxide sensors, which typically require elevated operational temperature. The GaAs nanowires are prepared directly via self-seeded metal-organic chemical deposition, and conducting polymers are deposited on GaAs nanowires using oxidative chemical vapor deposition (oCVD). The range of thickness for the oCVD layer is between 100 and 200 nm, which is controlled by changing the deposition time. X-ray diffraction analysis indicates an edge-on alignment of the crystalline structure of the PEDOT coating layer on GaAs nanowires. In addition, the positive correlation between the improvement of sensitivity and the increasing nanowire density is demonstrated. Furthermore, the effect of different oCVD coating materials is studied. The sensing mechanism is also discussed with studies considering both nanowire density and polymer types. Overall, the novel structure exhibits good sensitivity and selectivity in gas sensing, and provides a promising platform for future sensor design.

Diode Characteristics Approaching Bulk Limits in GaAs Nanowire Array Photodetectors.

We present the electrical properties of p-n junction photodetectors comprised of vertically oriented p-GaAs nanowire arrays on the n-GaAs substrate. We measure an ideality factor as low as n = 1.0 and a rectification ratio >108 across all devices, with some >109, comparable to the best GaAs thin film photodetectors. An analysis of the Arrhenius plot of the saturation current yields an activation energy of 690 meV-approximately half the bandgap of GaAs-indicating generation-recombination current from midgap states is the primary contributor to the leakage current at low bias. Using fully three-dimensional electrical simulations, we explain the lack of a recombination current dominated regime at low forward bias, as well as some of the issues related to analysis of the capacitance-voltage characteristics of nanowire devices. This work demonstrates that, through proper design and fabrication, nanowire-based devices can perform as well as their bulk device counterparts.

Evolution of microstructures during rapid crystallization of liquid GaAs

The technological importance of compound semiconductor GaAs are increasing because of their use in optoelectronic and microelectronic applications. Due to the high conversion efficiency and carrier mobility, GaAs can also be applied in solar cells and the recent study upon GaAs nanowires and their heterostructures has revealed that the conversion efficiency of GaAs nanowire array solar cells conversion is high up to 15.3%. Early the liquid and amorphous properties of GaAs were investigated by employing the first-principles calculations. The emergence of semi-empirical potential and the improvement of computer level have promoted the research and application of molecular dynamics (MD) simulation. MD simulation has now become one of the typical modeling methods at the molecular scale. The simulation is based on the known physical approximation of all particles in the system to solve the equation of motion, and obtain the atomic motion trajectory. Analytical potentials is very important in MD simulation as it is not feasible to solve the Hamiltonian by means of quantum-mechanical methods with huge computational complexity. Abell-Tersoff potential function is a short-ranged bond-order algorithm, which depends on bond lengths and bond angles and hence accesses information about the atomic structure. So it is suitable for simulating covalent bond species. Generally used for the IV elements and compounds like silicon, carbon, and others, but for the III-V compound semiconductor it is not very accurate due to the ionic bonds. Usually the modified tersoff potential, by the addition of Coulomb term, the modified exclusion potential and the truncation parameter, is used to simulate such semiconductor materials. Many studies on the bulk, surface and elastic properties of GaAs by means of MD method, are in good agreement with the experimental results. In this paper Karsten Albe’s Tersoff potential is adopted as it allows one to model a wide range of properties of GaAs compound structure. GaAs has two kinds of tetrahedral crystal structure, namely, Zinc-blende and Wurtzite, the former structure is more stable under normal conditions. But when reduced to a nanoscale scale, Wurtzite structure becomes stable. Different structures have distinct properties, similar to carbon and grapheme. But so far, there is no report on the evolution of the microstructure and the specific crystalline structure of GaAs during crystallization under rapid cooling. In this study, MD simulation was performed for liquid GaAs at the cooling rate 1×1010 K/s. The pair distribution function, the total energy per atom, the bond angle distribution function, the dihedral angle distribution and visualization method were used to analyze the variations of microstructure during the solidification process. Results show that the onset temperature of crystallization of GaAs liquid is 1460 K. The random network is the essential structural feature of liquid. The rapidly cooled crystallization is Zinc-blende based polycrystalline structure, with the grain boundary in a eutectic twin structure is a layer of wurtzite structure. At temperature below 520 K, part of As atoms segregate into simple cubic structure As8.

Optically excited THz generation from ordered arrays of GaAs nanowires

THz generation under excitation by ultrashort optical pulses from ordered arrays of GaAs nanowires is reported. It was found that the efficiency of THz generation is determined by the geometrical parameters of nanostructures and has a resonant character. Furthermore, it is shown that the terahertz generation efficiency at optimum geometrical parameters of an array of semiconductor nanowires is greater than the corresponding value for bulk semiconductor p-InAs which is the most effective THz emitter.