Non-uniform Devices Research Articles

Multilayer-Grown Ultrathin Nanostructured GaAs Solar Cells as a Cost-Competitive Materials Platform for III–V Photovoltaics

Large-scale deployment of GaAs solar cells in terrestrial photovoltaics demands significant cost reduction for preparing device-quality epitaxial materials. Although multilayer epitaxial growth in conjunction with printing-based materials assemblies has been proposed as a promising route to achieve this goal, their practical implementation remains challenging owing to the degradation of materials properties and resulting nonuniform device performance between solar cells grown in different sequences. Here we report an alternative approach to circumvent these limitations and enable multilayer-grown GaAs solar cells with uniform photovoltaic performance. Ultrathin single-junction GaAs solar cells having a 300-nm-thick absorber (i.e., emitter and base) are epitaxially grown in triple-stack releasable multilayer assemblies by molecular beam epitaxy using beryllium as a p-type impurity. Microscale (∼500 × 500 μm2) GaAs solar cells fabricated from respective device layers exhibit excellent uniformity (<3% relative) of photovoltaic performance and contact properties owing to the suppressed diffusion of p-type dopant as well as substantially reduced time of epitaxial growth associated with ultrathin device configuration. Bifacial photon management employing hexagonally periodic TiO2 nanoposts and a vertical p-type metal contact serving as a metallic back-surface reflector together with specialized epitaxial design to minimize parasitic optical losses for efficient light trapping synergistically enable significantly enhanced photovoltaic performance of such ultrathin absorbers, where ∼17.2% solar-to-electric power conversion efficiency under simulated AM1.5G illumination is demonstrated from 420-nm-thick single-junction GaAs solar cells grown in triple-stack epitaxial assemblies.

High power LED assemblies for solid state lighting – Thermal analysis

We report on the Chip-on-Plate (COP) packaged high power LED assemblies for solid state lighting (SSL) and backlighting display applications from thermal analysis perspective. Three different light modules with different elements (devices) distribution and heatsink design were considered. Thermal resistance circuit (TRC) model and heat diffusion equation were employed in this study of thermal behavior of the assembly elements. Finite element analysis along with 3D modeling and simulations were used for considered lighting modules and assemblies. Also, the effect of thermal uniformity on the emission spectra was evaluated for different operating powers. Comparative study showed that conventional packages with circularly symmetric having non-uniform devices’ distribution per unit area will have temperature differences ∼7°C among the chip-elements operating around 1.25W power level. Also, it was predicted that high elements temperature difference could result in over 2nm redshift in the emission spectra accompanied by ∼ 4% drop in the output power. However, temperature difference was found to be less than 1°C for the uniform rectangular array module distribution, which ensures uniform output intensity for all devices. Such findings are extremely useful for designing larger area LED packaging where thermal non-uniformity effects are expected to be more severe for devices’ longevity and performance.

Research on MRF Shear Stress Measurement Method under Non-Uniform Magnetic

Magnetorheological fluid as an important branch of the emerging intelligent material, its excellent characteristics make it widely used in electrical system and mechanical system. Common MRF system environment is limited to a uniform magnetic field, the practical application process to the vast of magneto rheological characteristics devices has shown the phenomenon, which does not match the uniform magnetic field theory, the reason is that the actual work environment is the non-uniform magnetic [1-2]. To study the characteristics of MRF shear stress in a non-uniform magnetic field, this paper established a non-uniform field magneto-rheological fluid shear stress measurement device. Comparing the experimental shear stress curves with the uniform magnetic field measured shear stress curves, we found that the non-uniform magnetic field strength of the environmental equivalent shear stress values ​​compared with the uniform magnetic field data have large deviation, this indicates that the existing uniform magnetic field theory is not well to explain non-uniform magnetic field shear properties of MRF, we need to build the introduction of non-uniform magnetic field theory.

Local electric field screening in bi-layer graphene devices

We present experimental studies of both local and macroscopic electrical effects in uniform single- (1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing amounts of 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.

Sampling based on timing: Time encoding machines on shift-invariant subspaces

Sampling information using timing is an approach that has received renewed attention in sampling theory. The question is how to map amplitude information into the timing domain. One such encoder, called time encoding machine, was introduced by Lazar and Tóth (2004 [23]) for the special case of band-limited functions. In this paper, we extend their result to a general framework including shift-invariant subspaces. We prove that time encoding machines may be considered as non-uniform sampling devices, where time locations are unknown a priori. Using this fact, we show that perfect representation and reconstruction of a signal with a time encoding machine is possible whenever this device satisfies some density property. We prove that this method is robust under timing quantization, and therefore can lead to the design of simple and energy efficient sampling devices.

Local current transport and surface potential of photovoltaic Cu(In,Ga)Se2 thin films probed by multi-scale imaging methods**Invited talk at the 6th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2012, 30 October–2 November 2012, Ha Long, Vietnam.

Microstructural alteration induces non-uniform device characteristics in polycrystalline thin films. In thin-film solar cells based on Cu(In,Ga)Se2 (CIGS), local electrical properties are investigated by Raman scattering spectroscopic imaging and scanning probe microscopic tools. Localized and uneven intensity of phonon modes, which represent different orientation and phases, elucidate the nature of non-uniformity of crystallinity, composition and defects in the films. Surface potential mapping at nanoscale is performed by Kelvin probe force microscopy, showing ∼40 mV of band-bending at the grain boundaries. Externally biased-current mapping, which is obtained by conductive atomic force microscopy, shows preferred current path in the films.

Influence of catalyst choices on transport behaviors of InAs NWs for high-performance nanoscale transistors

The influence of the catalyst materials on the electron transport behaviors of InAs nanowires (NWs) grown by a conventional vapor transport technique is investigated. Utilizing the NW field-effect transistor (FET) device structure, ~20% and ~80% of Au-catalyzed InAs NWs exhibit strong and weak gate dependence characteristics, respectively. In contrast, ~98% of Ni-catalyzed InAs NWs demonstrate a uniform n-type behavior with strong gate dependence, resulting in an average OFF current of ~10(-10) A and a high I(ON)/I(OFF) ratio of >10(4). The non-uniform device performance of Au-catalyzed NWs is mainly attributed to the non-stoichiometric composition of the NWs grown from a different segregation behavior as compared to the Ni case, which is further supported by the in situ TEM studies. These distinct electrical characteristics associated with different catalysts were further investigated by the first principles calculation. Moreover, top-gated and large-scale parallel-array FETs were fabricated with Ni-catalyzed NWs by contact printing and channel metallization techniques, which yield excellent electrical performance. The results shed light on the direct correlation of the device performance with the catalyst choice.

Performance Implications of Nonuniform Device Topologies in Scalable Heterogeneous Architectures

This article considers trends in heterogeneous system design, particularly for GPUs. Using the Keeneland Initial Delivery System, the authors examine the performance implications of increased parallelism and specialized hardware on parallel scientific applications. They examine how nonuniform data-transfer performance across the node-level topology can impact performance. Finally, they help users of GPU-based systems avoid performance problems related to this nonuniformity.

Modeling of Non-Uniform Thermoelement Devices Subject to Lateral Heat Convection

In this paper, we draw attention to the importance of the lateral convection heat transfer effect on the operation of thermoelements with variable cross-sectional area. The significance of lateral heat convection arises from a new application, in which thermoelements form the walls of a MEMS-based convergent-divergent micronozzle in order to pump heat from the divergent part of the micronozzle to the convergent part to enhance overall performance. To assess the effect of thermoelement, we develop a quasi-one-dimensional model for thermoelements, in which, the physical phenomena of Peltier effect, Joule heating, Seebeck effect, longitudinal heat conduction, and lateral heat convection are considered. The general energy equation of the thermoelement with variable material properties is formulated with heat convection modeled as a lateral heat source. We used the model to study thermoelements with rectangular cross section and uniform thickness, but variable width. The width of the element is maximum at the location forming the micronozzle throat. Several geometries have been investigated; a piece-wise linear, parabolic and piece-wise sine wave. Two parameters which play important role in the thermal performance of thermoelement are identified. These are the thermal resistance ratio; ratio of longitudinal conduction resistance to lateral convection resistance, and heating ratio; ratio of Joule heating to Fourier conducted heat. The effects of varying these two parameters as well as thermoelement geometry have been investigated thoroughly and the results are presented in the form of charts to assist the design and material selection of the thermoelement.

Threshold Voltage Model of Short-Channel FD-SOI MOSFETs With Vertical Gaussian Profile

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel approximation of 2-D potential function perpendicular to the channel for fully depleted (FD) silicon-on-insulator (SOI) MOSFETs on films with vertical Gaussian profile is proposed in the paper, then an analytical threshold voltage model is derived. The model agrees well with the MEDICI numerical simulation results. It represents a feasible way to find the threshold voltage and gives some reference points in developing new 2-D models for nonuniform FD-SOI devices. </para>

Limits of Heat Removal in Microelectronic Systems

Thermal management issues play an increasingly prominent role in microelectronic system design. The constraints on heat removal are a major factor limiting the performance of a microelectronic system. This work presents the thermodynamic limit of performance for a thermal solution utilizing air cooling to reject thermal energy as the inverse of its mass flow-heat capacity product. The minimum resistance to heat flow offered by a thermal solution is further refined by including the effects of thermal interface materials, substrate materials, and the impact of nonuniform device layer heating. Active cooling solutions may offer additional needed cooling for microelectronics systems, but the system thermal resistances limit its applicability. This work describes the minimum efficiency that an active cooling solution must provide to offer a thermal advantage over passive cooling. This minimum efficiency is dictated by the thermal resistances involved in drawing heat into the active cooler and expelling heat to the ambient environment. Knowledge of the fundamental limitations of thermal solutions gives system designers realistic expectations to set roadmaps, define architecture specifications, and evaluate the validity of thermal system performance claims

New fundamental insights into capacitance modeling of laterally nonuniform MOS devices

In compact transistor modeling for circuit simulation, the capacitances of conventional MOS devices are commonly determined as the derivatives of terminal charges, which in their turn are obtained from the so-called Ward-Dutton charge partitioning scheme. For devices with a laterally nonuniform channel doping profile, however, it is shown in this paper that no terminal charges exist from which the capacitances can be derived. Instead, for such devices, a new model is presented for the capacitances themselves. Furthermore, a method is given to incorporate such a capacitance model into circuit simulators, which are traditionally based on terminal charge models. Comparison with two-dimensional device simulations and a segmentation model shows that for a constant mobility, the new capacitance model provides an accurate description for a MOSFET with a laterally diffused channel doping profile. Through a comparison with high-frequency measurements, the agreement between model and experimental results is discussed.

Generation of Greenberger–Horne–Zeilinger entangled states with three SQUID qubits: a scheme with tolerance to non-uniform device parameters

We propose a scheme for creating Greenberger–Horne–Zeilinger (GHZ) type of entangled states with three superconducting quantum interference device (SQUID) qubits in a microwave cavity. The scheme operates essentially by an auxiliary SQUID built in the cavity as a microwave photon generator, and by performing a phase shift with the aid of the cavity photon. We show that the present scheme does not require identical cavity-SQUID coupling constants for each SQUID. Therefore, our scheme can be readily implemented since neither uniform device parameters nor exact placement of SQUIDs in the cavity is required. We note that the method can in principle be applied to prepare many SQUIDs in a GHZ state. In addition, how to prepare multiple SQUIDs in W-class entangled states is discussed.

Correlation of laser-beam-induced current with current-voltage measurements in HgCdTe photodiodes

Laser-beam-induced current (LBIC) is being investigated as an alternative to electrical measurements of individual photodiodes in a two-dimensional array. This is possible because LBIC only requires two electrical contacts to an array and the two-dimensional scanning of a focused laser beam across the array to image the entire array. The measured LBIC profiles, obtained from linear arrays of HgCdTe photodiodes, will be used to study the uniformity of photodiodes in the array and to extract the R0A of the photodiodes. It will be shown that the shape of the LBIC signal is correlated to the electrical performance of the photodiode, with R0A related to the spreading length of the photodiodes. Linear arrays of n-on-p, mid-wavelength infrared (MWIR) and long wave-length infrared (LWIR) devices were formed in liquid-phase epitaxy HgCdTe epilayers using a plasma junction-formation technique. The LBIC profiles were measured on each of the devices at various temperatures. For the MWIR devices, the extracted spreading length shows no correlation with R0A. However, the LBIC signal does detect nonuniform devices within the array. For the case of the LWIR devices, the spreading length is extracted as a function of temperature, with the R0A subsequently calculated from the spreading length. The calculated R0A, obtained without requiring contact to each photodiode in the array, agrees well with electrical measurements. Asymmetry of the LBIC signals for certain devices in the arrays is shown to be a result of localized leakage at the photodiode junction or from the contact pads through the passivation layers. These results are confirmed by numerical modeling of the device structures.

Quantum information transfer and entanglement with SQUID qubits in cavity QED: a dark-state scheme with tolerance for nonuniform device parameter.

We investigate the experimental feasibility of realizing quantum information transfer (QIT) and entanglement with SQUID qubits in a microwave cavity via dark states. Realistic system parameters are presented. Our results show that QIT and entanglement with two-SQUID qubits can be achieved with a high fidelity. The present scheme is tolerant to device parameter nonuniformity. We also show that the strong coupling limit can be achieved with SQUID qubits in a microwave cavity. Thus, cavity-SQUID systems provide a new way for production of nonclassical microwave source and quantum communication.

Design of multi-finger HBTs with a thermal–electrical model

Multi-finger heterojunction bipolar transistors (HBTs) with uniform spacing exhibit a higher temperature at the center of devices. The temperature distribution on the emitter fingers of the HBT is studied with a three-dimensional thermal–electrical model. Using this model, multi-finger HBTs are designed with non-uniform spacing to improve temperature distribution. Depending on the number of emitter fingers, different design approaches are demonstrated. For a six-finger or 12-finger HBT, the design is more straightforward. For a complex structure such as a 26-finger HBT, an efficient design procedure is necessary. In all of these cases, the calculated results show significant temperature reduction on non-uniform spacing devices.

Influence of nonuniform charge-coupled device pixel response on aperture photometry

The extensive use of charge-coupled devices (CCDs) in astronomical imaging and spectroscopic applications has resulted in techniques being developed to correct for sensitivity variations attributable to the focal plane array detector. One normally overlooked effect is that these sensitivity variations also include intrapixel spatial effects that can influence measurements made with CCDs in certain circumstances. We demonstrate the influence of CCD pixel response nonuniformity on photometric measurements. An investigation is presented of how the relation between pixel size and the image size affects the photometric accuracy. Results are presented for a front-illuminated CCD array using an experimentally measured pixel sensitivity functions.

Modeling and optimization of drug release from laminated polymer matrix devices

AbstractThe design and modeling of drug delivery devices using laminated layers to produce spatially nonuniform matrix devices leading to desired release rates were investigated with the focus on their optimal laminated hydrogel matrices for the diffusion‐controlled release of dissolved drugs. The model consists of polymer layers laminated together through a photopolymerization process to form matrices with a spatially nonuniform initial drug distribution and/or nonuniform drug diffusivities. The model solution establishes that the drug diffusion behavior, especially the early time release behavior, can be manipulated by altering spatially nonuniform initial drug distributions and drug diffusivities in the assembly. Furthermore, optimal control theory and calculus of variation were used to determine a set of initial drug concentrations in the layers to attain a system that exhibits a drug release profile as close to required profile as possible for all time.

High-capacity neural networks on nonideal hardware

We present a new training-out algorithm for neural networks that permits good performance on nonideal hardware with limited analog neuron and weight accuracy. Optical neural networks are emphasized with the error sources including nonuniform beam illumination and nonlinear device characteristics. We compensate for processor nonidealities during gated learning (off-line training); thus our algorithm does not require real-time neural networks with adaptive weights. This permits use of high-accuracy nonadaptive weights and reduced hardware complexity. The specific neural network we consider is the Ho-Kashyap associative processor because it provides the largest storage capacity. Simulation results and optical laboratory data are provided. The storage measure we use is the ratio M/N of the number of vectors stored (M) to the dimensionality of the vectors stored (N). We show a storage capacity of M/N = 1.5 on our optical laboratory system with excellent recall accuracy, > 95%. The theoretical maximum storage is M/N = 2 (as N approaches infinity), and thus the storage and performance we demonstrate are impressive considering the processor nonidealities we present. Our techniques can be applied to other neural network algorithms and other nonideal processing hardware.

Comparison of variable birefringence devices and Fabry-Perot etalons for use as tunable filters

A comparison is made of three device configurations which can be used as tunable filters: a Fabry-Perot etalon used in transmission (FPT), a Fabry-Perot etalon used in reflection (FPR), and a variable birefringence filter used in transmission (VBF). Initial conditions are defined for the achievement of a contrast ratio of 100:1, and the implications of these results on material choice discussed. The effect of nonuniform device thickness on contrast ratio is analyzed for all three cases. Analytic expressions are derived which predict the behavior of the contrast ratio under differing device conditions and which give rule-of-thumb values for the maximum acceptable thickness variation for a given contrast ratio. Using these expressions it is shown that the configuration least sensitive to variations in thickness is the VBF, with the FPT coming second and the FPR last. Experimental studies were carried out on three devices-a piezoelectric tunable air-spaced Fabry-Perot etalon, an electrooptically tunable liquid-crystal Fabry-Perot etalon, and an electrooptically tunable liquid-crystal variable birefringence filter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>