GaAs Junction Research Articles

Analysis of InAs/GaAs quantum dot solar cells using Suns-Voc measurements

The performance of InAs/GaAs quantum dot solar cells was investigated up to an optical concentration of 500-suns. A high temperature spacer layer between successive layers of quantum dots was used to reduce the degradation in the open circuit voltage relative to a control device without quantum dots. This improvement is explained using optical data while structural imaging of quantum dot stacks confirm that the devices are not limited by strain. The evolution of the open circuit voltage as a function of number of suns concentration was observed to be nearly ideal when compared with a high performance single junction GaAs solar cell. Analysis of Suns-Voc measurements reveal diode ideality factors as low as 1.16 which is indicative of a low concentration of defects in the devices.

Efficient Schottky-like junction GaAs nanowire photodetector with 9 GHz modulation bandwidth with large active area

Efficient, low capacitance density GaAs/Indium-Tin-Oxide Schottky-like junction photodetectors with a 50 μm square active are fabricated for operation in the gigahertz range. Modulation bandwidth is experimentally measured up to 10 GHz at various applied reverse biases and optical intensities to explore the effects of photo-generated carrier screening on modulation bandwidth. Last, the bandwidth dependence on applied reverse bias and optical intensity is simulated as a means to quantify average carrier velocities in nanowire material systems.

Modeling of Tunnel Junction (GaAs) in the Cascade Solar Cell

In this paper describes a simple model for tunnel junction (GaAs) between the top cell (GaAs) and bottom cell (Ge) of cascade solar cells. We theoretically studied the electrical characteristics (IV) of GaAs tunnel diode with the accounting program MATLAB for doping concentration of the junction after Using this model between two cascaded solar cell (GaAs / Ge) and we calculate the electrical characteristics and performance using AMPS-1D software. The conduction properties of this tunnel diode show good ohmic behavior and low contact resistance.

Modeling of Tunnel Junction (GaAs) in the Cascade Solar Cell

Modeling of Tunnel Junction (GaAs) in the Cascade Solar Cell

Non-Lithographic Growth of Core–Shell GaAs Nanowires on Si for Optoelectronic Applications

We demonstrated a nonlithographic method for integrating GaAs nanowire (NW) array-based light-emitting diodes (LEDs) on silicon (Si) substrates. A sub-100 nm hole array on a deposited SiO2 layer was patterned on an entire 2 in. Si wafer based on a sacrificed self-assembled InAs NW array. Then, a core–shell n–p junction GaAs NW array was grown on exposed Si windows via the selective-area growth method. The electrical properties of the core–shell n–p junction GaAs NW has been measured and compared to those of the core–shell junction NWs formed via the self-assembled growth method. Room temperature electroluminescence was successfully observed from the fabricated GaAs NW array-based LEDs. The core–shell junction III–V NW array epitaxially grown on a ubiquitous Si platform could be applied to future low-cost optical devices.

Strain Effects on Radiation Tolerance of Triple-Junction Solar Cells With InAs Quantum Dots in the GaAs Junction

A comparison of quantum dot (QD) triple-junction solar cells (TJSCs) with the QD superlattice under tensile strain are compared with those under compressive strain and baseline devices to examine the effects of strain induced by the InAs QD layers in the middle junction. Theoretical results show samples with tensile-strained InAs QDs have lower defect formation energy while compressive-strained QDs have the greatest. Experimentally, it is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1-MeV electrons, TJSCs with tensile strain exhibit a faster degradation in Isc of the QD samples and slower degradation in Voc but overall faster degradation in efficiency compared with baseline TJSCs, regardless of the magnitude of tensile strain. Compressively strained QD TJSCs have similar degradation in Isc and slower degradation in Voc compared with baseline TJSCs. From this study, it is determined that a slightly compressive strain in the QD superlattice allows for the best performance pre- and postirradiation for QD TJSCs based upon AM0 IV and quantum efficiency measurements and analysis. Fabricating devices with improvements determined from samples with varying strain leads to QD TJSCs with better radiation tolerance in terms of power output for 5, 10, 15, and 20 layers of QDs.

High-Efficiency Nanostructured Window GaAs Solar Cells

Nanostructures have been widely used in solar cells due to their extraordinary optical properties. In most nanostructured cells, high short circuit current has been obtained due to enhanced light absorption. However, most of them suffer from lowered open circuit voltage and fill factor. One of the main challenges is formation of good junction and electrical contact. In particular, nanostructures in GaAs only have shown unsatisfactory performances (below 5% in energy conversion efficiency) which cannot match their ideal material properties and the record photovoltaic performances in industry. Here we demonstrate a completely new design for nanostructured solar cells that combines nanostructured window layer, metal mesa bar contact with small area, high quality planar junction. In this way, we not only keep the advanced optical properties of nanostructures such as broadband and wide angle antireflection, but also minimize its negative impact on electrical properties. High light absorption, efficient carrier collection, leakage elimination, and good lateral conductance can be simultaneously obtained. A nanostructured window cell using GaAs junction and AlGaAs nanocone window demonstrates 17% energy conversion efficiency and 0.982 V high open circuit voltage.

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

ABSTRACTInAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm2. Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a Voc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 1011 cm−2, which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.

Modeling of Cascade Solar Cell Ga0.5In0.5P /GaAs Using AMPS-1D

The primary objective of this modeling investigation is to optimize a multijunction cascade device under the AM1.5G spectrum. Based on previous studies, GaInP and GaAs cells between them tunnel junction GaAs, because of their energy band gaps, can be combined together to achieve high-efficiency double-junction devices. In this study, the top cell is made of Ga0.5In0.5P (1.74 eV) while the bottom cell is made of GaAs (1.42 eV). In order to avoid the losses and design constraints observed in two-terminal and four-terminal devices, the tandem cell GaInP /GaAs is designed with tunnel junction. In order to determine the optimal structure of the device, the top and bottom junctions were investigated and optimized with regard to the thicknesses. The optimum configuration of the device shows an efficiency of 36.4% under the AM1.5G spectrum and one sun, which is higher than the efficiency of an optimized single-junction Si cell under the same illumination conditions

Single-nanowire solar cells beyond the Shockley–Queisser limit

Light management is of great importance to photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal pn-junction combined with an optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At one sun illumination a short circuit current of 180 mA/cm^2 is obtained, which is more than one order of magnitude higher than what would be predicted from Lambert-Beer law. The enhanced light absorption is shown to be due to a light concentrating property of the standing nanowire as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III-V based nanowire solar cells under one sun illumination.

Influences of InGaP Conical Frustum Nanostructures on the Characteristics of GaAs Solar Cells

Conical frustums with quasihexagonal nanostructures are fabricated on an InGaP window layer of single junction GaAs solar cells using a polystyrene nanosphere lithography technique followed by anisotropic etching processes. The optical and photovoltaic characteristics of the conical frustum nanostructured solar cells are investigated. Reflectance of the conical frustum nanostructured solar cells is significantly reduced in a wide range of wavelengths compared to that of the planar sample. The measured reflectance reduction is attributed to the gradual change in the refractive index of the InGaP conical frustum window layer. An increase of 15.2% in the power conversion efficiency has been achieved in the fabricated cell with an optimized conical frustum nanostructure compared to that of the planar cell.

Lattice-matched InGaAs on InP thermophovoltaic cells

We report the fabrication and characterization of lattice matched single junction InGaAs thermophotovoltaic cells grown on InP substrates with an EQE >90% over a broad spectral range. The I–V characteristics of the cells are examined for a range of operating temperatures and illumination conditions. An accurate model of the cell performance, including the flat-spot behaviour exhibited by the cells, is developed using the commercial PC1D package. We use this model to estimate the output of these cells with an example low-cost spectral control system, which suggest that these cells would produce system efficiencies >10%, with the potential for efficiencies >20%.

Reuse of GaAs substrates for epitaxial lift-off by employing protection layers

We demonstrate repeated use of GaAs wafers for multiple growths by employing lattice-matched epitaxial protection layers to preserve the wafer surface in its original condition following their etch removal after growth. The protection layers provide a regrowth surface that eliminates the need for repolishing prior to subsequent growth. Between growths, the protection layers are removed by wet chemical etching. The resulting surface quality is examined using atomic force microscope and energy dispersive spectrometry. We show that the surface roughness, chemical composition, morphology, and electronic properties of the GaAs surface after protection-layer removal are comparable to that of the original substrate surface. We show that p-n junction GaAs solar cells grown on original and reused wafers have nearly identical performance with power conversion efficiencies of ∼23%, under simulated 1 sun illumination, AM1.5 G. The high power conversion efficiency of GaAs solar cells combined with reduced costs associated with multiple parent wafer reuses promise cost competitiveness with incumbent solar cell technologies.

Comparison of single junction GaAs and In0.2Ga0.8N based solar cells at various temperatures

In this paper we focus on the effect of the temperature on two different solar cell semiconductor materials, one of them is the GaAs based solar cell and the other one is the In0.2Ga0.8N based solar cell. After simulating and extracting the electrical parameters of these devices in various temperatures from ATLAS (the two dimensional device simulator software), we compare those parameters between two different solar cell materials so as to introduce one of them as a better semiconductor material working in high temperatures.

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Growth and Characterization of Radial pn Junction Gaas Nanowire by MOCVD

Radial pn-junction GaAs nanowires were fabricated and investigated in detail. These nanowires were grown on GaAs (111)B substrate by metal-organic chemical vapor deposition via Au-catalyzed vapor-liquid-solid mechanism. Two types of nanowire p-n junctions were fabricated by growing a n(p)-doped GaAs shell outside a p(n) GaAs core. P-type doping was provided by diethyl zinc, while silane was introduced for n-type doping. The morphology, crystal structure and doping characteristics were investigated by FESEM, TEM and EDS. The results showed that both the two structures were of good morphology and both dopants were successfully incorporated into the nanowires.

Movement of the boundary of a p-n junction in GaAs:Si under gyrotronic irradiation

It is shown that, by using a gyratron, it is possible to control the position of a p-n junction in an already fabricated light-emitting structure. A shift of the compensated region in the emitting structure based on GaAs:Si is caused by the motion of impurities in the field of thermoelastic stresses appearing in the course of sample cooling after gyrotronic irradiation.

Electrical Transport Properties and Spin Injection in $\hbox{Co}_{2}\hbox{FeAl}_{0.5}\hbox{Si}_{0.5}/\hbox{GaAs}$ Junctions

We investigated the electrical transport properties in Co2FeAl0.5Si0.5 (CFAS)/GaAs junctions. From current density-voltage characteristics, the formation of a Schottky tunnel barrier in the CFAS/GaAs interface was indicated. Moreover, junction resistance of 2×10-9 Ω·m2 which is adequate for high magnetoresistance ratio attributable to high spin injection efficiency was obtained. Comparing the bias dependencies of conductance with samples, which CFAS ordering is lower, indicated that L21 ordered CFAS contributes to electrical transport. Finally, the spin injection signal was observed with 3-terminal Hanle measurement, and spin relaxation time was estimated to be 380 ps at 5 K.

Hybrid GaAs-Nanowire–Carbon-Nanotube Flexible Photovoltaics

A simple method is described for the preparation of carbon nanotube (CNT) composite films containing Au nanoparticles, which act as adatom collection agents promoting the growth of core-shell p-n junction GaAs nanowires (NW), according to the vapor-liquid-solid mechanism. This approach for composite film formation requires no covalent modification of the CNTs. GaAs NWs obtained are randomly aligned with respect to the CNT substrate in a densely packed arrangement. Nanohybrid films, incorporating coaxial GaAs NWs as the active, light-harvesting medium are used in the fabrication of photovoltaic cells, exhibiting conversion efficiencies up to 0.32%. Doping experiments confirm that conduction is not dominated by Schottky barriers at contact interfaces. The NW/CNT solar cells are shown to retain function following bending up to a curve radius of 12.5 mm, illustrating the potential of these novel materials in flexible device applications.

Design optimization of GaAs betavoltaic batteries

GaAs junctions are designed and fabricated for betavoltaic batteries. The design is optimized according to the characteristics of GaAs interface states and the diffusion length in the depletion region of GaAs carriers. Under an illumination of 10 mCi cm−2 63Ni, the open circuit voltage of the optimized batteries is about ∼0.3 V. It is found that the GaAs interface states induce depletion layers on P-type GaAs surfaces. The depletion layer along the P+PN+ junction edge isolates the perimeter surface from the bulk junction, which tends to significantly reduce the battery dark current and leads to a high open circuit voltage. The short circuit current density of the optimized junction is about 28 nA cm−2, which indicates a carrier diffusion length of less than 1 µm. The overall results show that multi-layer P+PN+ junctions are the preferred structures for GaAs betavoltaic battery design.