Search

The scalability of nanostructured, self-assembled antireflection (AR) coatings has been demonstrated on 6-inch glass and silicon wafers. Ultra-high transmittance through these large-area coatings has been confirmed by measuring the short circuit current of a CIGS-based thin film photovoltaic (PV) device placed below the large-area AR-coated glass wafer. At normal light incidence, the light transmitted through the AR coated glass wafer yields 5% more short-circuit current compared to the uncoated glass wafer. At off-angle incidence, the light transmitted through the AR-coated wafer yields nearly 20% higher short-circuit current compared to light transmitted through an uncoated glass wafer. The large-area AR coating preserves ultra-high transmittance over a wide range of incident angles and has the potential to enhance PV device performance from dawn to dusk.

Flexible photovoltaic cells with a specific power of more than 275 W/kg have been demonstrated by depositing copper indium gallium diselenide (CIGS) absorber layers on ultra lightweight and highly durable titanium foil. Advanced device designs employing nanostructured optical coatings and exploiting optical cavity effects provide a pathway to further increase power-generating capability. Single-junction CIGS devices can potentially outperform multi-junction III-V structures in some environments, including under high air mass terrestrial spectrums.

Nano-enhanced solar cells incorporating III-V quantum wells or quantum dots have the potential to revolutionize the performance of photovoltaic devices. Extended spectral response characteristics have been widely demonstrated in both quantum well and quantum dot solar cells using a variety of different III-V materials. To fully leverage the increased spectral response of nano-enhanced solar cells, new device designs are discussed that can both maximize the current generating capability of the limited volume of narrow band-gap material and minimize the unwanted carrier recombination that degrades the voltage output.

We present the performance of a unipolar barrier long-wave type-II InAs/GaSb superlattice (SLS) photodetector with a 50% cut-off wavelength of approximately 8.7 microns. In this study, the ability to lower dark current densities over traditional PIN diodes is presented by way of hetero-structure engineering of a pBiBn structure utilizing superlattice Ptype (P) and N-type (N) contacts, Intrinsic (I) superlattice active (absorber) region, and unipolar superlattice electron and hole blocking (b) layers. The spectral response of this pBiBn detector structure was determined using a Fourier Transform Infrared (FTIR) Spectrometer and the quantum efficiency (QE) was determined using a narrow 6250 nm narrow band filter and a 500K blackbody source. A diode structure designed, grown, and fabricated in this study yielded a dark current density of less than one (1) mA/cm 2 at a reverse bias of 150 mV and a specific detectivity value of greater than 10 11 Jones at 77K. In addition to single point temperature measurements, a variable temperature study (80K-300K) of the dark current is presented for a diode demonstrating diffusion limited dark current from 160K down to 80K.

Oblique-angle deposition of indium tin oxide (ITO) is used to fabricate optical thin-film coatings with a porous, columnar nanostructure. Indium tin oxide is a material that is widely used in industrial applications because it is both optically transparent and electrically conductive. The ITO coatings are fabricated, using electron-beam evaporation, with a range of deposition angles between 0 degrees (normal incidence) and 80 degrees. As the deposition angle increases, we find that the porosity of the ITO film increases and the refractive index decreases. We measure the resistivity of the ITO film at each deposition angle, and find that as the porosity increases, the resistivity increases superlinearly. A new theoretical model is presented to describe the relationship between the ITO film's resistivity and its porosity. The model takes into account the columnar structure of the film, and agrees very well with the experimental data.

Ultra-high, omnidirectional transmittance through a coated glass window is demonstrated over the entire accessible portion of the solar spectrum. The average broadband transmittance has been increased to greater than 98.5% at normal incidence, and exceeds 97.8% at all wavelengths between 440 nm and 1800 nm, significantly outperforming conventional MgF 2 coated glass. The measured improvement in transmittance results from coating the window with a new class of materials consisting of porous SiO 2 nanorods. The step-graded antireflection structure also exhibits excellent omnidirectional performance, enabling average broadband transmittance in excess of 96% at incident angles as high as 70°.

Quantum-structured photovoltaic devices incorporating III-V quantum wells or quantum dots have the potential to dramatically increase the performance of energy harvesting devices. In this work, the dark current of high-voltage InGaAs quantum well structures is characterized, and the underlying saturation current density analyzed as a function of effective energy gap. Analysis of the current-voltage characteristics suggests that these advanced quantum well device structures are operating in a regime of suppressed radiative recombination. High-voltage output from quantum-structured energy harvesting devices, coupled with advances in the field of light trapping, provides a pathway for achieving ultra-high conversion efficiencies.

High resolution imaging in the UV band has a lot of applications in defense and commercial systems. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss model that can predict sensor performance in the UV band using APD's with various gain and other parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APD's. We will present recent data on the GaN based APD's for their gain, detector response, dark current noise and the 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APD's in wide band gap semiconductors. The paper will also discuss state-of-the-art in UV APDs and the future directions for small unit cell size and gain in the APD's.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

While radiative recombination is a well-known intrinsic loss mechanism in photovoltaic devices, nonradiative recombination mechanisms typically dominate compound semiconductor diode currents and limit the performance of even state-of-the-art devices. However, recent advances in device structure design have allowed quantum well structures to begin reaching the radiative limits of dark current operation. In this work, a novel extended heterojunction structure is employed in InGaAs quantum well devices to reduce non-radiative recombination and expose the limiting n=1 radiative component of the diode current. Short circuit current versus open circuit voltage curves derived from illuminated currentvoltage measurements indicate that the underlying dark diode currents of the InGaAs quantum well devices vary with well thickness and emission energy. Analysis of the extracted n=1 saturation current densities indicate that these high-voltage InGaAs quantum well devices are operating in a regime of suppressed radiative recombination.

InGaP/GaInAsN double heterojunction bipolar transistors (HBTs) with compositionally graded bases are presented which exhibit superior dc and radio frequency performance. Reducing the average base layer energy gap and optimizing the emitter-base (e-b) and base-collector (b-c) heterojunctions leads to a 100-mV reduction in the turn-on voltage compared to a baseline InGaP/GaAs process. Simultaneously grading the base layer energy band-gap results in over a 66% improvement in the dc current gain and up to a 35% increase in the unity gain cutoff frequency. DC current gains as high as 250 and cutoff frequencies of 70 GHz are demonstrated. In addition, the InGaP/GaInAsN DHBT structure significantly reduces the common emitter offset and knee voltages, as well as improves the dc current gain temperature stability relative to standard InGaP/GaAs HBTs.

Photon absorption, and thus current generation, is hindered in conventional thin-film solar cell designs, including quantum well structures, by the limited path length of incident light passing vertically through the device structure. Optical scattering into lateral waveguide structures provides a physical mechanism to dramatically increase photocurrent generation through in-plane light trapping. However, the insertion of wells of high refractive index material with lower energy gap into the device structure often results in lower voltage operation, and hence lower photovoltaic power conversion efficiency. In this work, we demonstrate that the voltage output of an InGaAs quantum well waveguide photovoltaic device can be increased by employing a novel III-V material structure with an extended wide band gap emitter heterojunction. Analysis of the light IV characteristics from small area test devices reveals that nonradiative recombination components of the underlying dark diode current have been reduced, exposing the limiting radiative recombination component and providing a pathway for realizing solar-electric conversion efficiency of 30% or more in single-junction cells.

InGaAs quantum well solar cells are demonstrated with record-high open circuit voltages. Higher open circuit voltages result from the use of a novel structure incorporating a wide band gap barrier layer within a heterojunction depletion region and a step-graded InGaAs well to suppress both non-radiative and radiative recombination. Analysis of the open circuit voltage as a function of well energy suggests these devices are operating in a regime that exceeds traditional radiative limits on the performance of photovoltaic devices.

Quantum structured solar cells seek to harness a wide spectrum of photons at high voltages by embedding low energy-gap wells or dots within a high energy-gap matrix. Quantum well and quantum dot solar cells have the potential to deliver ultra-high power conversion efficiencies in single junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide range of environmental conditions. In this paper, we will briefly review the theoretical underpinnings of quantum well and quantum dot photovoltaic devices, and summarize recent experimental efforts developing quantum-structured solar cell devices. In a specific example, test devices utilizing radiation-hard, III-V nitride materials have been built using both bulk and multiple quantum well (MQW) structures. Photovoltaic devices with an InGaN MQW structure are shown to outperform devices employing a thicker, bulk InGaN layer. These results, along with the underlying theoretical foundations, suggest that quantum well and quantum dot structures can enhance the performance of photovoltaic devices for a variety of defense applications.

Next Generation EO/IR focal plane arrays using nanostructure materials are being developed for a variety of Defense and Homeland Security Sensor Applications. Several different nanomaterials are being evaluated for these applications. These include ZnO nanowires, GaN Nanowires and II-VI nanowires, which have demonstrated large signal to noise ratio as a wide band gap nanostructure material in the UV band. Similarly, the work is under way using Carbon Nanotubes (CNT) for a high speed detector and focal plane array as two-dimensional array as bolometer for IR bands of interest, which can be implemented for the sensors for homeland security applications. In this paper, we will discuss the sensor design and model predicting performance of an EO/IR focal plane array and Sensor that can cover the UV to IR bands of interest. The model can provide a robust means for comparing performance of the EO/IR FPA's and Sensors that can operate in the UV, Visible-NIR (0.4- 1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR bands (8-14μ). This model can be used as a tool for predicting performance of nanostructure arrays under development. We will also discuss our results on growth and characterization of ZnO nanowires and CNT's for the next generation sensor applications. We also present several approaches for integrated energy harvesting using nanostructure based solar cells and Nanogenerators that can be used to supplement the energy required for nanostructure based sensors.

Ultra-high, broadband transmittance through coated glass windows is demonstrated over a wide range of incident angles. Near perfect 100% transmittance through a glass substrate has been achieved over select spectral bands, and the average transmittance increased to over 97% for photons incident between 0° and 75° with wavelengths between 400 nm and 1600 nm. The measured improvements in transmittance result from coating the windows with a new class of materials consisting of porous SiO2 nanorods.

Nitride semiconductors possess a number of unique material properties applicable to energy harvesting photovoltaic devices, including a large range of energy gaps, superior radiation resistance, and tolerance to high temperatures. We present here our experimental results related to the self-assembled InN quantum dots formed on Si substrates. We have been successful at synthesizing InN quantum dots using the metal-organic chemical vapor deposition (MOCVD) process. We demonstrate the synthesis of a high density of InN dots exhibiting excellent structural and optical properties. An unprecedented range of absorption energies, ranging from the infrared to the ultraviolet, can be obtained by embedding InN-based quantum dots in a wide band gap GaN barrier. The combination of energy-gaps accessible to III-V nitride materials may be used to reap the benefits of advance quantum dot device concepts involving hot carrier effects or multiple carrier generation processes.

EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers from 0.4 to 1.6 micron. One of the critical technologies that will enhance the EO/IR sensor performance is the development of high quality nanostructure based antireflection coating. Prof. Fred Schubert and his group have used the TiO2 and SiO2 graded-index nanowires / nanorods deposited by obliqueangle deposition, and, for the first time, demonstrated their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN-air interface over the UV band. This was achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05, the lowest value so far reported In this paper, we will discuss our modeling approach and experimental results for using oblique angle nanowires growth technique for extending the application for UV, Visible and NIR sensors and their utility for longer wavelength application. The AR coating is designed by using a genetic algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the wavelength range of 400 nm to 2500 nm and 0° to 40° angle of incidence. The measured average optical transmittance of an uncoated glass substrate between 1000 nm and 2000 nm is improved from 92.6% to 99.3% at normal incidence by using a two-layer nanostructured AR coating deposited on both surfaces of the glass substrate.

Quantum-well based solar cells have the potential to deliver ultra-high efficiencies in single-junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide range of environmental conditions. In this work, thin-film GaAs-based quantum well solar cells are demonstrated that operate at voltages higher than previous state-of-the-art GaAs devices. Higher open circuit voltages result from the use of a novel structure incorporating a wide band gap barrier layer within a heterojunction depletion region. Ultra-low dark current is observed from this structure as result of the simultaneous reduction of carrier diffusion and space charge recombination. Efficient carrier extraction of photogenerated carries over the potential barriers within the structure is achieved by increasing the field strength and tailoring the barrier profile to enhance thermionic emission and tunneling. High open circuit voltages (>1.1 V @ 25 A/cm2) and fill factors (> 80%) are demonstrated in multi-layer depletion region structures incorporating both an extended region of wide band gap material and an InGaAs quantum well within a GaAs base layer. These ground breaking results indicate that it is possible to simultaneously increase both the current and voltage output of GaAs-based solar cells.

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.

EO/IR nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers performance up to 1.6 micron. The use of nanowires for developing high quality antireflection coatings that allows minimizing the reflection loss is discussed. We have explored the possibility of using nanostructures grown by oblique angle deposition technique. A graded-index coating with different index profiles has been investigated for broadband antireflection properties, particularly with air as the ambient medium. In this paper, we present, modeling and experimental results for nanostructure AR coatings for UV, Visible and calculations for NIR sensors and also their utility for longer wavelength application.

Quantum solar cell structures seek to harness a wide spectrum of photons by embedding low energy-gap wells or dots within a high energy-gap matrix. Contrary to previous reports and expectations, we find that an extended region of narrow band-gap material can be inserted into the depletion region of a wide band-gap p-n junction without significantly degrading the dark diode characteristics. In particular, thin InGaP barrier layers are found to be extremely effective at reducing the diode dark current of GaAs-based diodes by several orders of magnitude. Our results imply a clear separation in the quasi-Fermi level at the InGaP/GaAs heterointerface, as well as efficient photogenerated carrier transport over the potential energy barrier. This novel GaAs-based device structure has produced over a 100 mV increase in the open circuit voltage without any degradation in the short circuit current.

We report time resolved measurements of spin and carrier relaxation in InAs films with carrier densities of 1.3 x 10(16) and 1.6 x 10(16) cm(-3) grown on (001) and (111) GaAs, respectively. We used standard pump-probe and magneto-optical Kerr effect spectroscopy at different excitation wavelengths, power densities, and temperatures. We observed sensitivity of carrier and spin relaxation time to the photoinduced carrier density but not to the variation in temperature. We explain our results using the Elliot-Yafet picture of spin relaxation process in narrow gap semiconductors. (c) 2008 American Institute of Physics. National Science Foundation - Grant No. DMR-0507866 AFOSR Young Investigator Program - 06NE231 Jeffress Trust Fund J748 Advance-VT National Science Foundation - STTR Program (Award No. 0638227)

GaN / Al 1-x Ga x N-based hetero-structures have demonstrated a versatility in RF electronic applications which is practically unmatched by any other material system. There are many device structures under consideration for use in RF and Power amplifiers, suitable for both commercial and military applications. In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating SiC substrates. Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance mapping, optical reflectance, and high-resolution X-ray diffraction. Several of the wafers were fabricated into HEMT devices, and the results on these devices are also presented.

We experimentally investigate the details of the minority electron transport across the base of a GaAs heterojunction bipolar transistor (HBT). A stepwise doping change in the base layer is employed to locally accelerate the electrons and enhance the dc current gain. The position of the step change is varied to probe the electron velocity across the base layer in multiple sample sets with different base thicknesses. Positioning the doping change within 200 /spl Aring/ of the collector provides the largest velocity increase (/spl sim/15%), independent of base thickness. This indicates that the electrons are moving slower as they approach the collector, an observation which sharply contrasts with the conventional quasi-ballistic drift-diffusion models of base transport that predict the highest electron velocities are reached near the collector.

Tunnel collector HBTs employ a thin layer of GaInP between the GaAs base and collector regions, in order to suppress hole injection when the base-collector junction is forward biased. Devices with a low offset voltage of 30 mV, and low knee voltage of /spl sim/0.3 V, while still maintaining high current gain (170), and good RF performance with f/sub T/=54 GHz and f/sub MAX/=68 GHz have been demonstrated. The devices exhibit increased output conductance as compared to conventional GaInP/GaAs SHBTs, due to a residual barrier at the base-collector junction. This paper presents an experimental method for the determination of such barriers, by analyzing the collector current as a function of applied base-collector voltage, V/sub CB/.

The base transport characteristics of InP n-p-n DHBTs grown using metal-organic vapor phase epitaxy (MOVPE) were examined in both constant base and graded base structures. Constant-base InP structures demonstrated an inverse square dependence of DC current gain on base thickness, in contrast to the linear inverse dependence measured for GaAs HBTs. The different gain dependence in the InP DHBTs is attributed to a more diffusive, compared to a quasi-ballistic, nature of the electron transport in the base. The use of a compositionally graded InGaAs base increased the DC current gain by /spl ap/ 1.6 times compared to constant base InP DHBTs. The higher DC gain is attributed to reduced neutral base recombination resulting from the grading-induced increase in base electron velocity. The RF cutoff frequency increased by 9% at high current density in the graded-base HBTs compared to the constant base, due to the reduced base transit time.

The performance of a long-wave infrared type-II InAs/GaSb superlattice photodetector with a 50% cut-off wavelength of approximately 8.7 μm is presented. The ability to lower dark current densities over traditional P-type-Intrinsic-N-type diodes is offered by way of hetero-structure engineering of a pBiBn structure utilizing superlattice p-type (p) and n-type (n) contacts, an intrinsic (i) superlattice active (absorber) region, and unipolar superlattice electron and hole blocking (B) layers. The spectral response of this pBiBn detector structure was determined using a Fourier transform infrared spectrometer, and the quantum efficiency was determined using a 6250 nm narrow band filter and a 500 K blackbody source. A diode structure designed, grown, and fabricated in this study yielded a dark current density of 1.05×10−5 A/cm2 at a reverse bias of −50 mV and a specific detectivity value of greater than 1011 Jones at 77 K. Theoretical fittings of the diode dark currents at 77 K were used in this study to help isolate the contributing current components observed in the empirical dark current data. A variable temperature study (80 to 300 K) of the dark current is presented for a diode demonstrating diffusion-limited dark current down to 77 K.

High-voltage InGaAs quantum well devices are shown to operate in a regime of suppressed radiative recombination. A novel extended heterojunction structure is employed to reduce non-radiative recombination and expose the limiting n=1 component of the diode current. Short circuit current versus open circuit voltage curves derived from illuminated current-voltage measurements on several sets of InGaAs quantum well solar cells are analyzed, and the underlying saturation current densities extracted. Diode currents lower than those expected from detailed balance theory are observed, and possible mechanisms for the apparent suppression in radiative recombination are discussed.

The design, fabrication, and characterization of a broadband, omnidirectional, graded-index anti-reflection (AR) coating on a glass substrate, fabricated by using nanostructured low-refractive-index (n = 1.05–1.40) silica, is reported. The AR coating is designed by using a genetic algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the wavelength range of 400 to 2500 nm and 0 to 40° angle of incidence. The measured average optical transmittance between 1000 and 2000 nm is improved from 92.6 to 99.3% at normal incidence by using a two-layer AR coating deposited on both surfaces of the glass substrate.

Electron radiation induced hydrocarbon contamination can be either a problem or a useful tool in electron beam analyses and lithographies. We have used atomic force microscopy to study electron beam written contamination structures. Contamination is shown not only to arise from the primary electron beam but also from the energy scattered outside of the direct impingement area. The size of the contamination structures correlates well with that expected from electron scattering theory. By varying the geometry of the written structures, the rate at which the electron dose is deposited, and the nearby hydrocarbon surface density, we show that surface diffusion of hydrocarbon molecules plays a primary role in the formation of the contamination structures.