David Forbes

David Forbes
Rochester Institute of Technology | RIT · NanoPower Research Laboratory

Ph.D. Materials Science and Engineering

About

155
Publications
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Publications

Publications (155)
Article
Full-text available
The cover image, by Brittany L. Smith et al., is based on the Short Communication InAlAs photovoltaic cell design for high device efficiency, DOI: 10.1002/pip.2895.
Conference Paper
Full-text available
Electrical transport properties (resistivity, scattering time, carrier concentration, mobility, effective mass) can be deduced from the Drude model for free carrier optical absorption. Optical response of materials may include simultaneous contributions from free carriers, phonon modes, and electronic transitions, however. Improved sensitivity to f...
Article
Full-text available
This study presents a new design for a single-junction InAlAs solar cell, which reduces parasitic absorption losses from the low band-gap contact layer while maintaining a functional window layer by integrating a selective etch stop. The etch stop is then removed prior to depositing an anti-reflective coating. The final cell had a 17.9% efficiency...
Article
Full-text available
An InGaAsP quantum well with a type-II band alignment is studied using continuous wave power and temperature dependent photoluminescence (PL) spectroscopy. The small energy separation between the ground state and first excited state results in significant thermal carrier redistribution and excited state occupation, particularly, with increasing exc...
Article
The growth of InAlAsSb by metal-organic vapor phase epitaxy has been demonstrated, with a controllable antimony fraction exceeding 6%. Calculations have shown that InAlAsSb with Sb contents greater than 5-7% in the quaternary are within the miscibility gap, however this work demonstrates specific growth conditions that allow compositions well withi...
Article
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 combin...
Article
Models based on continuum elasticity theory are discussed to calculate the necessary thickness of a strain compensation (SC) layer for a superlattice (SL) of strained quantum wells (QW) or quantum dots (QD). These models are then expanded to cover material systems (substrates, QW or QD, and SC) composed of AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InA...
Article
In order to expand the technology of III-V semiconductor devices with quantum structures to both terrestrial and space use, radiation induced defects as well as native defects generated in the quantum structures should be clarified. Electrically active defects in GaAs p+n diodes with embedded ten layers of InAsquantum dots(QDs) are investigated usi...
Article
Photoluminescence measurements on a series of GaAs double heterostructures demonstrate a rapid quenching of carriers in the GaAs layer at irradiance levels below 0.1 W/cm2 in samples with a GaAs-on-InGaP interface. These results indicate the existence of non-radiative defect centers at or near the GaAs-on-InGaP interface, consistent with previous r...
Conference Paper
Significant development work has been completed in recent years to improve experimental results reaching a record efficiency of 9.14% under one sun AM0 conditions with no anti-reflection coating. The nipi solar cell utilizes epitaxial regrowth contacts to ensure carrier selective contacts to the alternating n and p-type doped layers, forming select...
Conference Paper
Transfer-printing is an important, commercial technology for manufacturing state of the art CPV modules, and has emerged recently as a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III-V solar cells with low cost. This paper presents the latest results for microscale CPV cells grown on GaAs, InP and GaSb...
Article
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...
Conference Paper
Steady improvements have been demonstrated in the fabrication process for GaAs doping superlattice solar cells, reaching a maximum efficiency of 9.0% for a non anti-reflection coated device under AM0 illumination. Numerous opportunities remain for improvement to reach a simulated one sun AM0 efficiency of 14.4%. Devices are fabricated by contacting...
Article
Baseline and quantum dot (QD) GaAs pn-junction diodes were characterized by deep level transient spectroscopy before and after both 1MeV electron irradiation and 140 keV proton irradiation. Prior to irradiation, the addition of quantum dots appeared to have introduced a higher density of defects at EC-0.75 eV. After 1 MeV electron irradiation the w...
Article
Gallium arsenide diodes with and without indium arsenide quantum dots were electron irradiated to investigate radiation induced defects. Baseline and quantum dot gallium arsenide pn-junction diodes were characterized by capacitance-voltage measurements, and deep level transient spectroscopy. Carrier accumulation was observed in the gallium arsenide...
Article
The effects of delta-doping InAs quantum-dot (QD)-enhanced GaAs solar cells were studied both through modeling and device experimentation. Delta doping of two, four, and eight electrons per QD, as well as nine holes per QD, was used in this study. It was observed that QD doping reduced Shockley–Read–Hallrecombination in the QDs, which results in a...
Conference Paper
Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked II-IV solar cells with low cost. In this work, we describe the development of InGaAs solar cells, designed to harvest long wavelength photons when stacked in tandem with a high efficiency InGaP/GaAs/InGaAsNSb triple junction solar cell....
Conference Paper
Epitaxial layers of InAlAs are prime candidates for the top cell in triple-junction photovoltaics (PV). Growth conditions during metalorganic vapor phase epitaxy (MOVPE) of InAlAs affect the material properties and subsequently the device characteristics of the epilayers. Impurity concentrations in InAlAs epilayers grown under various conditions ar...
Conference Paper
The use of nanostructures such as quantum dots (QD) offers tremendous potential to realize high-efficiency photovoltaic (PV) cells. The optimization of the electronic structure of the layers within the QD region should lead to improved PV performance. This includes the QD layer itself, but also the surrounding barrier and/or strain balancing layers...
Conference Paper
The confinement depth of bound states in InAs quantum dots was studied as a function of QD doping. k·p simulation predicted a decrease of up to 50 meV in absolute confinement depth as QDs are doped due to local band bending from QD charging. Photoreflectance of solar cells containing QDs doped to several levels was used to determine optical transit...
Conference Paper
The response of InGaAs quantum well solar cells (QWSCs) to proton irradiation is presented. The QWSCs consisted of sixteen layers of 5 nm InGaAs QWs / 10 nm InAlGaAs barriers were embedded into the i-region of a 1.0 eV InAlGaAs solar cell, and the results were compared to a 1.0 eV InAlGaAs control solar cell. We report the results of 3 MeV proton i...
Conference Paper
The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electri...
Conference Paper
Triple junction solar cells lattice matched to InP have the potential to exceed 50% efficiency under AM1.5 500× illumination with a bandgap stack of 1.74 / 1.1 / 0.7 eV. A top cell having 1.74 eV requires development of high-quality InAlAsSb, which currently has very little development effort reported. Preliminary deep-level transient spectroscopy...
Article
Full-text available
Space exploration missions and space electronic equipment require improvements in solar cell efficiency and radiation hardness. Triple-junction photovoltaic (TJ PV) cell is one of the most widely used PV for space missions due to it high efficiency. A proper models and simulation techniques are needed to speed-up the development on novel solar cell...
Conference Paper
Solar cells utilizing doping superlattices in the active region of the device have been proposed as an alternative design to increase radiation hardness. Multiple diodes are connected together in parallel, where each diode can be as thin or thick as the design requires. Thinning the doped layers reduces the diffusion length requirements ensuring ef...
Article
The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in a p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when elect...
Article
InGaAs quantum well / InAlGaAs barrier solar cells were grown and tested in order to evaluate their solar cell performance. These samples were grown with five layers of QWs at varying depths in the intrinsic region of the n-i-p devices. An external quantum efficiency measurement was used to determine the sub-bandgap spectral responsivity, and showe...
Article
Full-text available
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 def...
Article
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 experimenta...
Article
Dense arrays of indium arsenide (InAs) nanowires material have been grown by selective-area metal organic vapor phase epitaxy (SA-MOVPE) using Polystyrene-b-Polymethyl Methacrylate (PS/PMMA) Diblock Copolymer (DBC) nano-patterning technique, which is a catalyst-free approach. Nano-scale openings were defined in a thin (~10nm) SiNx layer deposited o...
Article
Full-text available
InAs/GaAs strain-balanced quantum dot (QD) n-i-p solar cells were fabricated by epitaxial lift-off (ELO), creating thin and flexible devices that exhibit an enhanced sub-GaAs bandgap current collection extending into the near infrared. Materials and optical analysis indicates that QD quality after ELO processing is preserved, which is supported by...
Article
The organometallic vapor phase epitaxy of InAs quantum dots has been investigated by comparing the effect the underlying surface has on the quantum dot physical characteristics. Atomic force microscopy measurements were used to identify the InAs QDs coalesce to significantly larger size when deposited on an InGaP surface compared to a GaAs surface....
Conference Paper
Lattice matched, triple-junction solar cells with strain-compensated quantum dots (QDs) in the GaAs middle cell were grown by Metal-Organic Chemical Vapor Deposition (MOCVD). Devices with different numbers of QD layers are compared to baseline devices with no QDs. Quantum efficiency and light I-V measurements show an increase in short circuit curre...
Conference Paper
Optical transitions and carrier transport in InAs/GaAs self-assembled quantum dots solar cells (QDSC) were investigated via temperature dependent photoluminescence (TDPL) spectroscopy, charge collection efficiency calculated from bias dependent spectral response, and low temperature photo-current measurements. Band structure simulations were used t...
Article
The incorporation of nanostructures, such as quantum dots (QD), into the intrinsic region of III-V solar cells has been proposed as a potential route towards boosting conversion efficiencies with immediate applications in concentrator photovoltaic and space power systems. Necessary to the optimization process of this particular class of solar cells...
Article
Quantum dot triple junction solar cells (QD TJSCs) have potential for higher efficiency for space and terrestrial applications. Extended absorption in the QD layers can increase efficiency by increasing the short circuit current density of the device, as long as carrier extraction remains efficient and quality of the bulk material remains high. Exp...
Conference Paper
Doping superlattice devices have been pursued in part because of their inherent radiation hardness which results from long lifetimes and minimum diffusion length requirements in the range of nanometers. Diffusion length requirements are reduced because of the multiple closely spaced doped layers in the superlattice. Higher doping levels in conjunct...
Article
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 subst...
Article
Full-text available
The simulation and fabrication of a multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping superlattice solar cell have been demonstrated. Devices have been fabricated and characterized, demonstrating a proof of concept for a nipi device contacted via epitaxial regrowth. Current-voltage measurements in the dark and under one sun illumina...
Article
Ten-, 20-, and 40-layer InAs/GaAs quantum-dot (QD)-embedded superlattice solar cells were compared with a baseline GaAs p-i-n solar cell. Proper strain balancing and a reduction of InAs coverage value in the superlattice region of the QD embedded devices enabled the systematic increase in short-circuit current density with QD layers (0.02-mA/cm$tex...
Conference Paper
Heterojunction emitter InAs/GaAs quantum dot solar cells (QDSC) with an In0.48Ga0.52P (InGaP) n-type emitter and p-type GaAs base were fabricated along with homojunction nip solar cells in order to enable sub-cell polarity compatibility of InAs/GaAs QDSCs with current state-of-the-art monolithic InGaP/GaAs/Ge triple junction solar cells for space a...
Conference Paper
A comparison of quantum dot (QD) triple junction solar cells (TJSCs) under tensile strain are compared to those under compressive strain and baseline devices to examine the effects of strain induced by the QD layers. It is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1 MeV electrons tri...
Conference Paper
The use of nanostructures, such as quantum dots (QD) or quantum wells within photovoltaic (PV) devices has demonstrated enhanced current generation, but often at the expense of open-circuit voltage. QD morphology and optical quality have a direct impact on PV performance and optimizing the epitaxial growth of QDs is critical to achieve the desired...
Conference Paper
The low temperature operation of GaAs solar cells incorporating a superlattice of InAs quantum dots (QD) was studied to explore carrier removal mechanisms. Dark current-voltage characteristics of QD devices did not show a temperature dependence, which suggests a tunneling-dominated recombination mechanism. Two-photon extraction was investigated usi...
Conference Paper
In order to enhance understanding of the short circuit improvement in InAs/GaAs quantum dot (QD) solar cells, the thermally assisted and tunneling mechanisms of carrier escape from the QD quantum confinement are investigated. The dependence of voltage biased spectral responsivity for QD solar cells at room temperature is studied to analyze carrier...
Conference Paper
Substrate misorientation was investigated as a mechanism to control InAs quantum dot density and size. Results have shown that both 2° and 6° substrate misorientation toward the ; direction can lead to high InAs QD density (4.8×1010 cm-2) as well as suppress QD ripening. In the 6° misoriented samples the critical thickness for QD formation was high...
Conference Paper
Quantum dot nipi test structures consisting of n-type / intrinsic / p-type / intrinsic layers have been developed to probe the effects of placing quantum dots within a doping superlattice. Coupling between the doping superlattice states and quantum dots that can be grown within the superlattice results in increases in carrier lifetimes within the q...
Conference Paper
Intermediate band solar cells using quantum dots (QDs) have been proposed as an approach to increasing solar cell efficiency [1]. This paper presents results of attempts to grow QDs in the GaAs middle cell of lattice matched triple junction solar cells by Metal-Organic Chemical Vapor Deposition (MOCVD). QD size and density as a function of number o...
Article
Radiation tolerance of quantum dot (QD) enhanced solar cells has been measured and modeled. GaAs solar cells enhanced with 10, 20, 40, 60, and 100X layers of strain compensated QDs are compared to baseline devices without QDs. Radiation resistance of the QD layers is higher than the bulk material. Increasing the number of QD layers does not lead to...
Conference Paper
The nipi photovoltaic device is a doping superlattice-based device, that uses iterative n-type / intrinsic / p-type / intrinsically doped GaAs layers to minimize the effect of minority carrier diffusion length. Following photon absorption, carriers are quickly swept vertically by drift into majority doped layers. Carriers are collected in the later...
Article
Bulk ion implantations of AlxGa1-xAs (x = 0.6 or 0.85) were performed at 77 K with 1.5 MeV Kr+, 1 MeV Ar+ or 1.5 MeV++ ions, and the resulting damage state examined by using Rutherford backscattering spectrometry-channeling at 77 K and 293 K and transmission electron microscopy at 300 K. Amorphization of a portion of the A1xGa1-x As layer occurred...
Article
InAs quantum dots have been incorporated into the middle junction of an InGaP/(In)GaAs/Ge triple junction solar cell (TJSC) on four inch wafers, in aims of band gap engineering a high efficiency solar cell to even higher limits. Results of QD growth on 4” diameter Ge templates gave densities near 1×1011 cm−3 and QD height between 2–5 nm. Arrays of...
Conference Paper
The simulation and fabrication of a multi-period GaAs n-type / intrinsic / p-type / intrinsic (nipi) doping superlattice solar cell has been demonstrated. A fabrication procedure has been developed using regrown contacts in wet etched V-grooves. Devices have been fabricated and characterized. Current-Voltage measurements in the dark and under one s...
Article
In order to increase thermal stability of solar cells for high temperature applications, wide bandgap semiconductors such as GaP are being investigated. The addition of nanostructures, such as quantum wells to the solar cell is expected to extend sub-host bandgap absorption and photocurrent generation. Increasing current generation in wide bandgap...
Article
Control of InAs critical thickness (θ) is important to realizing optimized growth in quantum dot (QD) devices. Substrate misorientation can change the value of θc but also creates more uniform QDs in both size and distribution. This work explores GaAs p-i-n photovoltaic (PV) devices grown via organometallic vapor phase epitaxy (OMVPE) using the Str...
Conference Paper
Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (Jsc) and open circuit voltage (Voc) comparable to a control GaAs p-i-n solar cell are reported. One-sun Jsc is enhanced by 3.5% compared to that of the GaAs control, while the open circuit voltage is maintained at 994 mV. Results were achieved using optimize...
Article
Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (Jsc) and open circuit voltage (Voc) comparable to a control GaAs p-i-n solar cell are reported. 1 sun Jsc is enhanced by 3.5% compared to that of the GaAs control, while the Voc is maintained at 994 mV. Results were achieved using optimized InAs QD coverage...
Article
GaAs p-i-n solar cells embedded with varying number of QD layers (0-60) were grown by OMVPE. 1x1 cm2 cells were fabricated and standard solar cell testing was performed. Illuminated AM0 current-voltage characteristics were measured of both a baseline and 10-layer quantum dot (QD) embedded GaAs p-i-n. The QD solar cell (QDSC) gave an short circuit c...
Conference Paper
The simulation and characterization of multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping structure solar cells has been demonstrated. The nipi device depends almost exclusively on drift rather than diffusion currents to collect the carriers. This architecture has been proposed to increase the radiation hardness of a device due to a...
Article
State of the art photovoltaics exhibiting conversion efficiency in excess of 30% (1-sun) utilize epitaxially grown multijunction III-V materials. Increasing photovoltaic efficiency is critically important to the space power, and more recently, the terrestrial concentrator PV communities The use of nanostructured materials within photovoltaic device...
Conference Paper
A series of InAs QD enhanced solar cells has been grown with arrays of 10, 20, 40, 60 and 100 layers of QDs in the GaAs i-region. An enhancement in short circuit current density (J<sub>SC</sub>) using QDs was observed. QD cells with up to 40 layers show a 1.4 mA/cm<sup>2</sup> improvement in J<sub>SC</sub> compared to a GaAs control cell grown with...
Conference Paper
GaAs solar cells with varying layers of QDs and varying cell size (to vary perimeter to area ratio) were fabricated. Cells were tested using an I<sub>SC</sub>-V<sub>OC</sub> method to determine local ideality factors, as well as to extract recombination currents at low and high bias. Baseline devices without QDs were also simulated using SILVACO to...
Conference Paper
The InAs/GaAs quantum dot system has been shown to improve device performance when included in single junction GaAs solar cells. The management of electrons and holes in the barrier material, well material and between the two is important for the successful function of such nanostructured devices. Since carriers produced in these confined materials...
Conference Paper
The state of the art in space solar cells utilizes epitaxially grown III-V multijunction cells with champion devices exceeding 30% efficiency under 1-sun. An array of self-organized InAs quantum dots (QD) within a GaAs matrix is one approach to enable significant efficiency improvements by extending the spectral bandwidth of the GaAs cell. In this...
Article
Semiconducting nanowires have been intensively studied in order to exploit unique optical and electrical properties that develop at the nano-scale. Commonly, nanowires are produced using Au nanoparticles (∼50 nm diameter) as seed particles on the surface applied prior to epitaxy. The Au particle acts as a preferred nucleation site for nanowire grow...
Article
Full-text available
The impact of strain-balancing quantum dot superlattice arrays is critical to device performance. InAs/GaAs/GaP strain-balanced quantum dot arrays embedded in p-i-n diodes were investigated via high resolution x-ray diffraction (HRXRD) and photoluminescence (PL) as a function of the GaP thickness. A three-dimensional modification of the continuum e...
Conference Paper
A review of state-of-the-art solar cell efficiencies and their application to future space array utilization is discussed. New emerging technologies are presented and the impact on cost and efficiency is projected for space use.