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GaSb grown on GaAs through interfacial misfit (IMF) arrays grown via molecular beam epitaxy has been heavily studied; there is limited research, however, on IMF growth through metal-organic vapor phase epitaxy. To demonstrate viability for integration in a multijunction solar cell for terrestrial use, it is imperative to demonstrate high quality GaSb grown on GaAs through metal-organic vapor phase epitaxy. The preferred gallium precursors for n-type and p-type GaSb for longest minority carrier diffusion length were determined to be trimethylgallium and triethylgallium, respectively. A heteroepitaxial GaSb-on-GaAs device attained an open-circuit voltage of 190 mV and an efficiency of 2.2%. Extracted threading dislocation density from the minority carrier lifetime for the heteroepitaxial GaSb-on-GaAs device was determined to be 7.5 × 10 6 cm − 2 . In a modeled multijunction solar cell, this device attributes to an overall efficiency of 33.1% under AM1.5g illumination. [ABSTRACT FROM AUTHOR]

Spin polarized photocathodes are key to the future operation of electron accelerators such as the ones at Thomas Jefferson National Accelerator Facility and Brookhaven National Laboratory. Currently, these photocathodes come in short supply due to limited production by molecular beam epitaxy. By developing a process to implement similar structures using metal organic chemical vapor deposition, the availability of these devices can be increased. In this paper, we detail the implementation of recent photocathode advancements via metal organic chemical vapor deposition process and show an improvement in both polarization and quantum efficiency of our devices compared to those fabricated via molecular beam epitaxy, with devices reaching 82% polarization and 2.9% quantum efficiency. [ABSTRACT FROM AUTHOR]

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k.p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate (effectively zero at 5 kV/cm and 7.9 × 106 s-1 at 50 kV/cm) but also doubles the thermal escape rate (2.2 × 1011 s-1 at 5 kV/cm and 4.1 × 1011 s-1 at 50 kV/cm). Temperature-dependent external quantum efficiency measurements were performed to verify that the increasing electric field decreases the effective barrier height. Additionally, the electric field dependent radiative lifetimes of the ground state were characterized with time-resolved photoluminescence experiments. This study showed that the increasing electric field extended the radiative recombination lifetime in the ground state of the QDs as a consequence of the reduced wave-function overlap between the electrons and holes. The balance of carrier escape and recombination determines the probability of TSPA. [ABSTRACT FROM AUTHOR]

We present a low-cost and scalable approach for the synthesis of wafer-scale InAs nanowire (NW) arrays on photolithographically patterned, reusable Si wafers using a localized self-assembly (LSA) epitaxial growth technique. Conventional i-line lithography is used to define arrays of 500 nm diameter pores through 50 nm thick SiO2 layers, which serve as the LSA mask. A two-step, flowrate-modulated growth sequence is implemented to optimize selective-area self-assembly of NW arrays with over 80% yield and excellent control over the placement of one NW, with a mean diameter of 130 nm, inside each 500 nm pore. As-grown NW arrays are delaminated from the growth substrate, enabling fabrication of flexible membrane devices as well as reuse of Si wafers and growth masks while preserving the template pattern fidelity. Reuse of Si substrates for III–V epitaxy is demonstrated with and without pre-growth substrate restoration treatments. In both cases, the yield of NWs on reused wafers is comparable to that achieved in the original growth run. Without substrate restoration procedures, the remnant base segments of NWs on parent wafers act as preferential sites for regrowth of vertical NWs. Transmission electron microscopy analysis reveals that the InAs lattice is coherently extended from the remnant NW base segments during regrowth. The delaminated InAs NW arrays are transferred to carrier wafers for the fabrication of substrate-free photodetectors through the use of an anchoring procedure, which preserves the original NW position and orientation. Under broadband illumination, the NW array-based photodetectors produce a photo-to-dark current ratio of 102, demonstrating the utility of the fabrication procedure employed. This work establishes a low-cost route toward III–V semiconductor-based flexible optoelectronics via LSA epitaxial growth of NW arrays on reusable Si wafers. [ABSTRACT FROM AUTHOR]

In0.53Ga0.47As photodiodes were irradiated by 2.0- and 3.5-MeV protons at low temperature to study the effects of displacement damage. Defect spectroscopy and dark current measurements were performed in situ before room-temperature annealing could occur. It was found that annealing of the dominant mid-gap trap began as low as at 150 K. At room temperature, significant annealing of this trap occurred rapidly, annihilating more than half the traps on a time scale of tens of minutes. Because of annealing, the dark current at 250 K, a typical operating temperature for these devices, increased by up to twice the rate versus proton fluence than it did at 300 K. The results demonstrate that irradiation testing at room temperature is not the ideal approach to predicting in-flight device performance. [ABSTRACT FROM AUTHOR]

Heterogeneous self-assembly of III–V nanostructures on inert two-dimensional monolayer materials enables novel hybrid nanosystems with unique properties that can be exploited for low-cost and low-weight flexible optoelectronic and nanoelectronic device applications. Here, the pseudo-van der Waals epitaxy (vdWE) growth parameter space for heterogeneous integration of InAs nanowires (NWs) with continuous films of single layer graphene (SLG) via metalorganic chemical vapor deposition (MOCVD) is investigated. The length, diameter, and number density of NWs, as well as areal coverage of parasitic islands, are quantified as functions of key growth variables including growth temperature, V/III ratio, and total flow rate of metalorganic and hydride precursors. A compromise between self-assembly of high aspect ratio NWs comprising high number density arrays and simultaneous minimization of parasitic growth coverage is reached under a selected set of optimal growth conditions. Exploration of NW crystal structures formed under various growth conditions reveals that a characteristic polytypic and disordered lattice is invariant within the explored parameter space. A growth evolution study reveals a gradual reduction in both axial and radial growth rates within the explored timeframe for the optimal growth conditions, which is attributed to a supply-limited competitive growth regime. Two strategies are introduced for further growth optimization. Firstly, it is shown that the absence of a pre-growth in situ arsine surface treatment results in a reduction of parasitic island coverage by factor of ∼0.62, while NW aspect ratio and number densities are simultaneously enhanced. Secondly, the use of a two-step flow-modulated growth procedure allows for realization of dense fields of high aspect ratio InAs NWs. As a result of the applied studies and optimization of the growth parameter space, the highest reported axial growth rate of 840 nm min−1 and NW number density of ∼8.3 × 108 cm−2 for vdWE of high aspect ratio (＞80) InAs NW arrays on graphitic surfaces are achieved. This work is intended to serve as a guide for vdWE of self-assembled III–V semiconductor NWs such as In-based ternary and quaternary alloys on functional two-dimensional monolayer materials, toward device applications in flexible optoelectronics and tandem-junction photovoltaics. [ABSTRACT FROM AUTHOR]

Intermediate band solar cells promise improved efficiencies beyond the Shockley-Queisser limit by utilizing an intermediate band formed within the bandgap of a single junction solar cell. InP quantum dots (QDs) in an In0.49Ga0.51P host are a promising material system for this application, but two-step photon absorption has not yet been demonstrated. InP QDs were grown via metalorganic chemical vapor deposition, and a density, a diameter, and a height of 0.7 × 1010 cm−2, 56 ± 10 nm, and 18 ± 2.8 nm, respectively, were achieved. Time-resolved photoluminescence measurements show a long carrier lifetime of 240 ns, indicating a type-II band alignment of these InP quantum dots. Several n-i-p In0.49Ga0.51P solar cells were grown with both 3 and 5 layers of InP QDs in the i-region. While the solar cells showed an overall loss in short circuit current compared to reference cells due to emitter degradation, a sub-bandgap enhancement of 0.11 mA/cm2 was clearly observed, due to absorption and collection from the InP QDs. Finally, two-step photon absorption experiments have shown unambiguous photocurrent generation involving an intermediate band within the bandgap at temperatures up to 250 K. [ABSTRACT FROM AUTHOR]

InAs quantum dots (QDs) were grown in an AlAs0.56Sb0.44/GaAs matrix in the unintentionally doped (uid) region of an In0.52Al0.48As solar cell, establishing a variety of optical transitions both into and out of the QDs. The ultimate goal is to demonstrate sequential absorption, where one photon is absorbed, promoting an electron from the valence band into the QD, and a second photon is absorbed in order to promote the trapped electron from a QD state into the host conduction band. In this study, we directly investigate the optical properties of the solar cell using photoreflectance and evaluate the possibility of sequential absorption by measuring spectral responsivity with broadband infrared illumination. [ABSTRACT FROM AUTHOR]

ABSTRACT InAs 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. [ABSTRACT FROM AUTHOR]

The effect of the position of InAs quantum dots (QD) within the intrinsic region of pin-GaAs solar cells is reported. Simulations suggest placing the QDs in regions of reduced recombination enables a recovery of open-circuit voltage (VOC). Devices with the QDs placed in the center and near the doped regions of a pin-GaAs solar cell were experimentally investigated.While the VOC of the emitter-shifted device was degraded, the center and base-shifted devices exhibited VOC comparable to the baseline structure. This asymmetry is attributed to background doping which modifies the recombination profile and must be considered when optimizing QD placement. [ABSTRACT FROM AUTHOR]

Abstract: During quantum dot (QD) growth, substrate misorientation has been shown to play a role in the QD growth mechanism, changing their size, shape and density. Since various misorientation angles are used in production of solar cells, this work investigates QD enhanced GaAs p-i-n solar cells grown using the Stranski–Krastanov (SK) growth method on substrates misoriented either 2° or 6° off the (100) in the direction. Results of this work show that 2° misoriented samples have a lower critical thickness for InAs QD formation as compared to the 6° misorientation: ∼1.7 monolayers (ML) versus∼1.8ML, respectively. In addition, the 6° substrates showed a more uniform QD density and size distribution of QDs without significant QD coalescence. Photoluminescence of both substrate types shows that the QD ground state transitions are similar in wavelength. Results of the solar cells under one sun illumination show that QD cells grown on both 2° and 6° substrates have higher short-circuit current density than comparable cells without QD and maintain a high open-circuit voltage. The QD contributed short-circuit current density was normalized for QD size and density for both the 2° and 6° samples. Values of 360A/cm2 per cm3 of InAs and 400A/cm2 per cm3 of InAs were found for the 2° and 6° samples, respectively. For both substrate types, the reduced number of coalesced QDs promoted effective strain balancing, while the increased QD density lead to strong QD absorption. [Copyright &y& Elsevier]

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x10 19 cm -3 . Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x10 19 cm -3 , respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.