Small-area Silicon Research Articles

Optimization of silicon solar cell design for use under concentrated solar irradiation

The task of reducing the cost of a unit of photoelectric-generated electricity is still relevant today. One of the most effective ways to do that is to use concentrator photovoltaic energy conversion systems with cheaper, small-area silicon solar cells (SC), because they require significantly less semiconductor material to make. In this study, the authors develop solutions to optimize the design and improve the manufacturing technology of silicon SCs of the combined diffusion-field type. Such SCs are used to concentrate solar irradiation. The authors propose design and technological solutions for the development and manufacture of a concentrator unit based on the Fresnel lens, which is designed to measure the photoelectric characteristics of SCs when concentrating solar irradiation. Next, the photoelectric characteristics of the combined diffusion-field type SCs were investigated under concentrated solar irradiation in natural sun conditions. The degree of concentration varied from 1X to 100X. Measuring the light I –V characteristics allowed determining photoelectric parameters of the cells — shortcircuit current, open-circuit voltage, fill factor and photoconversion efficiency. The obtained experimental results are in good agreement with the results of theoretical modeling. It is shown that due to the minimization of the specific series resistance, the samples of such SCs have high values of operational parameters in the range of natural solar irradiation concentration K=1X–100X.

Development of a large array of Silicon Drift Detectors for high-rate synchrotron fluorescence spectroscopy

An array of small-area Silicon Drift Detectors (SDD), named Hera, has been developed at Brookhaven National Laboratory (BNL) in the past few years. Its primary application is high-rate spectroscopy at synchrotrons. Each SDD pixel is a 1 mm × 1 mm square to match the footprint of the original diode-based pixel detector called Maia, which was developed for the same application. The replacement of the diode with an SDD allows for better energy resolution at short shaping times and an increased stability of the detector. 32, 96, and 384-channel arrays were designed and fabricated and achieved a Full Width Half Maximum (FWHM) as low as 176 eV at 5.9 keV with peaking time of 1 μs at -13°C. The sensor design, simulation, fabrication, its readout system, and its spectroscopic performance are reported.

Optimizing the Spatial Nonuniformity of Irradiance in a Large-Area LED Solar Simulator

The solar simulator has allowed all photovoltaic devices to be developed and tested under laboratory conditions. Filtered xenon arc lamps were the gold-standard source for solar simulation of small-area silicon photovoltaic devices; however, scaling these devices to illuminate large areas is neither efficient nor practical. Large-area solar simulation to meet appropriate spectral content and spatial nonuniformity of irradiance (SNI) standards has traditionally been difficult and expensive to achieve, partly due to the light sources employed. LED-based solar simulation allows a better electrical efficiency and uniformity of irradiance while meeting spectral intensity requirements with better form factors. This work details the design based on optical modeling of a scalable, large-area, LED-based, solar simulator meeting Class AAA performance standards formed for inline testing of printed solar cells. The modular design approach employed enables the illuminated area to be expanded in quanta of ~260 cm2 to any preferred illumination area. A 640 cm2 area illuminated by two adjacent PCB units has a measured total emission of 100 mW/cm2, with a SNI of 1.7% and an excellent approximation to the AM1.5G spectrum over the wavelength range of 350–1100 nm. The measured long-term temporal instability of irradiance (TIE) is <0.5% for a 550-min continuous run. This work identifies the design steps and details the development and measurement of a scalable large-area LED-based solar simulator of interest to the PV testing community, and others using solar simulators.

Extinction of random telegraph switching in small area silicon metal-oxide-semiconductor transistors

Random telegraph switching (RTS) noise showing a slow decay in the switching rate at cryogenic temperatures which leads to the eventual extinction of the discrete noise fluctuations has been observed in the drain-source current (IDS) of small area (<0.05 μm2) Si n-channel metal-oxide-semiconductor field-effect transistors. The RTS noise was characterized by current fluctuations between two discrete current levels spaced ΔIDS apart with relative fluctuation amplitude ΔIDS/IDS from 2% to 76%, depending on the device, over a finite interval of gate bias. In all devices showing RTS, the average switching rate gradually diminished to zero over a time of 1 to 2 h at 15 K while maintaining the nearly constant fluctuation amplitude so that the RTS eventually ceased with IDS staying in its lower current state. This decay in the switching rate may be due to a metastable oxygen vacancy defect that gradually repairs itself after repeated capture and emission of charge, deactivating the trap defect. Once gone, RTS noise did not reappear in any subsequent measurements of a given device even after bias and temperature cycling, suggesting a mechanism to deactivate at least some forms of RTS through a “cryogenic anneal.”

Small-area Si Photovoltaics for Low-Flux Infrared Energy Harvesting.

Silicon photovoltaics are prospective candidates to power mm-scale implantable devices. These applications require excellent performance for small-area cells under low-flux illumination condition, which is not commonly achieved for silicon cells due to shunt leakage and recombination losses. Small area (1-10 mm2) silicon photovoltaic cells are studied in this work, where performance improvements are demonstrated using a surface n-type doped emitter and Si3N4 passivation. A power conversion efficiency of more than 17% is achieved for 660 nW/mm2 illumination at 850 nm. The silicon cells demonstrate improved power conversion efficiency and reduced degradation under low illumination conditions in comparison to conventional crystalline silicon photovoltaic cells available commercially.

ENUBET: Enhanced NeUtrino BEams from kaon Tagging

A reduction of the neutrino flux uncertainty by one order of magnitude in conventional neutrino beams can be achieved monitoring the positron production in the decay tunnel originating from the Ke3 decays of charged kaons. This novel approach will be developed in the framework of the ERC ENUBET Project. In this talk we present the aims of the project and the ongoing R&D for the instrumentation of the decay tunnel. In particular, we describe a specialized shashlik calorimeter (iron-scintillator) with a compact readout based on small-area silicon photo multipliers that allows for a very effective longitudinal segmentation of the detector to enhance electron/hadron separation. The expected performance of the detector estimated from a full GEANT4 simulation of the neutrino decay tunnel are presented. We also discuss preliminary results on a prototype composed by 12 ultra compact modules exposed to pions and electrons at CERN-PS.

A new-concept calorimeter for future neutrino beams based on Kaon tagging

Neutrino cross-section measurements are an essential requirement for the next generation of neutrino oscillation experiments and they are presently limited by uncertainties on neutrino fluxes. In [1] we propose to instrument a neutrino decay tunnel to detect large angle positrons and tag the three-body semileptonic K+→e+π0νe decays. In such a facility the absolute electron neutrino flux could be determined with unprecedented precision (O(1%)). An e+/π+ separation capability of about 2% as well as a high e+ efficiency is required for a diffuse particle source over a length of several tens of meters. Additional constraints, due to the harsh beam environment, involve radiation hardness and fast response. For this purpose we propose a specialized shashlik calorimeter (copper-scintillator) with a compact readout based on small-area Silicon PhotoMultipliers coupled to WLS fibers. The setup would allow an effective longitudinal segmentation for electron/hadron separation, reducing the dead zones introduced by fiber bundling. Detailed Monte Carlo simulations are in progress. The construction of a small prototype and exposures to pion and electron beams are foreseen.

Fabrication, characterization and modeling of a silicon solar cell optimized for concentrated photovoltaic applications

Silicon is still an interesting material for developing Concentration Photovoltaic (CPV) cells working at low and medium concentration range. In this work we describe modeling, design, fabrication technology and functional characterization of a small-area silicon solar cell suitable for CPV applications up to 200 suns. Two-dimensional (2-D) numerical simulations by a state-of-the-art Technology Computer Aided Design (TCAD) tool adopting calibrated physical models have been performed for both cell design and deep understanding of its performance. Specifically, the simulations have allowed the development and optimization of front contact grid scheme and design of the cell operating under medium concentration. The cell has been tested by means of a novel indoor concentrator system up to 300 suns and a conversion efficiency higher than 22% has been measured, according to numerical simulations. The dependence of short-circuit current on concentration factor has been observed to be super-linear. An excess of short-circuit current at 300 suns of approximately 8% has been measured. The super-linear effect has been investigated by means of numerical simulations and explained in terms of enhanced carrier diffusion length under concentrated light. The dependence of the super-linear effect on the incoming photon wavelength was also observed and discussed, showing that the super-linearity is due to the spectrum portion above 600nm only.

A Fully Digital 8$\,\times\,$16 SiPM Array for PET Applications With Per-Pixel TDCs and Real-Time Energy Output

An 8 × 16 pixel array based on CMOS small-area silicon photomultipliers (mini-SiPMs) detectors for PET applications is reported. Each pixel is 570 × 610 μm2 in size and contains four digital mini-SiPMs, for a total of 720 SPADs, resulting in a full chip fill-factor of 35.7%. For each gamma detection, the pixel provides the total detected energy and a timestamp, obtained through two 7-b counters and two 12-b 64-ps TDCs. An adder tree overlaid on top of the pixel array sums the sensor total counts at up to 100 Msamples/s, which are then used for detecting the asynchronous gamma events on-chip, while also being output in real-time. Characterization of gamma detection performance with an 3 × 3 × 5 mm3 LYSO scintillator at 20°C is reported, showing a 511-keV gamma energy resolution of 10.9% and a coincidence timing resolution of 399 ps.

Edge electroluminescence in small-area silicon p +-n diodes heavily doped with boron: Analysis of model representations

The experimental results and model representations of the edge electroluminescence of two published studies for small-area silicon p +-n diodes heavily doped with boron are analyzed. In one of these studies it was assumed that edge electroluminescence appears in the p + region of the diode, and in the other, in the n region of the diode. In the latter case, it was demonstrated that electroluminescence indeed arose in the n region and was caused predominantly by the radiative recombination of free excitons. It is shown that similar model concepts are also applicable to the other study. Based on several independent experimental studies (of edge photoluminescence, electroluminescence, and radiation absorption by free carriers), it is demonstrated that the linear or close-to-linear dependences of the edge-luminescence intensity on the excitation intensity, observed in single-crystal silicon at high injection levels, are caused by the close-to-linear dependences of the exciton concentration on the free-carrier concentration. The results of this study extend the capability of luminescence methods for determining the carrier lifetimes to the region of high injection levels.

Fabrication of a metal mold with microstructures using a novel passive-alignment recombining technique

This study presents a novel scheme for manufacturing a large area molds with alignment microstructures electroplated from lateral joining of small area silicon plates. Conventional methods for manufacturing metal molds with nano or microstructures are extremely challenging. Semiconductor fabrication procedures that utilize photolithographic processes to generate nano-scale features easily limit wafer scale. However, a large area silicon mold with small features has been developed. However, its drawbacks were serious misalignment, tilting, and brittleness. This work presents a novel approach for fabricating a metal mold with precisely aligned microstructures. The gap, alignment precision, tilt, and height difference between the two joined plates joined laterally by passive alignment recombining techniques were all on a micro scale. Furthermore, the measurements of the metal molds and Polyvinylchloride (PVC) replicas were extremely similar. Moreover, the scalability of the technique was demonstrated using four small area silicon plates. Consequently, this approach has significant potential for bridging the technological gap between conventional precision machining and photolithography-based micromachining for metal molds exceeding typical wafer size with small features.

Modeling of silicon deposition process scale-up employing axisymmetric ring nozzle sources. I

The scale-up of a new silicon thin film deposition technique employing supersonic molecular beams is investigated. Small area silicon thin film deposition was proven successful previously. This study aims to investigate the possibility of a larger scale deposition using axisymmetric ring sources through the numerical approach. It is found that the use of axisymmetric ring nozzle sources can significantly increase the deposition area and improve the film uniformity. The simulations show a uniform silicon growth over an area 18 cm in diameter at ∼155 Å/min. The geometrical effects of the ring nozzle sources and the substrate to nozzle distance are studied. The influence of the source flow rate and pumping capacity is also examined.

Discrimination between Low Atomic Number Materials from their Characteristic Scattering of X-ray Radiation

There are many applications within the agricultural, food and medical sciences where it is required to detect targets that absorb X-rays in a similar way to the background material and these are therefore not easy to detect in a transmission image. Scattered X-rays provide useful additional information as they have interacted with the material in question. Furthermore, the incident and scatter angles can be defined so as to characterize a small volume element without recourse to complex computer tomography techniques. The potential of scattering techniques has been partially recognized within the medical sciences but remains largely unstudied in other application areas. This paper investigates the potential of distinguishing between different low atomic number targets from their scattering characteristics with a view to applying these techniques to food and agricultural applications. An experimental and simulation study are presented. The investigations use a polychromatic X-ray source to irradiate several low atomic number samples and a small area silicon photodiode to detect the scattered radiation. This geometry is reproduced within a simulation study based on a Monte-Carlo-based program. The results of the simulation and experimental study are shown, qualitatively, to be in close agreement with each other. These studies show that the intensity of scattered radiation, over a range of scattering angles, gives a greater contrast measure between low atomic number targets than a traditional transmission image.

Evaluation of correction factors for transmittance measurements in single-beam integrating spheres

An integrating sphere for transmittance measurements at normal and oblique angles of incidence has been constructed. The sphere is a single-beam instrument that uses a small-area silicon diode as the detector. The entry port is only 0.37% of the total wall area and has an oblong shape to permit measurements at high angles of incidence for scattering samples. A small beam size has been made possible by using a low-noise preamplifier system for the detector circuit. The oblong port shape and a small beam size make it possible to perform simulated double-beam measurements at near-normal incidence. Modified correction factors for the sample reflectance have been derived. Special attention has been paid to the separation into a diffuse and a specular component of the transmitted light. Results have been compared with the results of measurements on a double-beam instrument, and the correction factors for specular and diffuse samples have been experimentally verified. The importance of using the right correctionfactors for different types of samples has been evaluated together with the influence of the sphere parameters.

Room temperature X-ray spectroscopy with a silicon diode detector and an ultra low noise preamplifier

An X-ray spectrograph operating at room-temperature has been designed and tested. It consists of a small area (1 mm/sup 2/) silicon diode detector on a high resistivity bulk and an ultra low noise preamplifier of new conception. A resolution of 61 RMS electrons (517 eV FWHM) was measured at 297 K, 34 RMS electrons (288 eV FWHM) at 223 K. The limits and perspectives for room temperature operation of silicon planar diode detectors and related front-end electronics are highlighted. >

Radiobiology of α Particles: II. Dosimetry of Low-Energy α Particles Using Parallel-Plate Ionization Chambers and a Surface Barrier Detector

Three small parallel-plate ionization chambers were developed for measuring dose rates, of primarily low-energy alpha particles in the energy range 0.4-3.5 MeV, at a defined cell-Mylar interface. Spectral energy distributions of these alpha particles were also measured at the same position using a specially designed small-area silicon surface barrier detector. Dose rates were derived from the spectral distributions and compared with those derived from the ionization chambers. Different alpha-particle energies were obtained using a 144-MBq 238Pu collimated source and a variety of Mylar moderator foils of different thicknesses. These measurements, extended to mean alpha-particle energies as low as 0.4 MeV, will enable us to correlate radiobiological data with effects of alpha particles terminating in different regions of cell nuclei.

Investigation of drain current RTS noise in small area silicon MOS transistors

An investigation of the amplitude drain current RTS fluctuations in small area silicon MOS transistors is presented. A simple theoretical model for the interpretation of the RTS amplitude variations with gate and drain voltages is proposed. The limits of application of the model are established and discussed with respect to the experimental results.

An analysis of positive and negative resistance characteristics in the high-current-density region of Schottky diodes

A number of different diode characteristics are observed in small-area silicon Schottky diodes under forward-biased and high-current-density conditions. These characteristics, such as positive resistance, negative resistance, and normal diode characteristics, are analyzed. The analysis demonstrates that the variations are due to the minority-carrier accumulation in the epitaxial region of the diodes, which is mainly controlled by Schottky barrier heights and the resistivity of the semiconductor epitaxial region. >

Developmental experiments on large-area silicon solar cells

A comprehensive design/material/technological study was conducted with the aim of obtaining high-efficiency back-surface-field- (BSF-) type bifacial solar cells with reproducible parameters on large-area n- and p-silicon wafers. Practical ways of attenuating the severe limitations imposed by different kinds of areal inhomogeneities on the electro-optical performance of large-area BSF single-crystal silicon solar cells are described and tested experimentally. Various procedures leading to a substantial increase of both the emitter and the base contributions to the generated photocurrent are implemented and discussed in detail. The p+-n-n+ and n+-p-p+ cells were processed as bifacial devices and tested under both front and backside AM1 illumination. The trial devices were fabricated on 2-in and 3-in. commercially available lower-grade silicon and the minimization of the cell cost was one of the sought-after objectives. The combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, high-efficiency (17%–18%) large-area silicon solar cells with good overall electro-optical performance as bifacial devices. The results of this work show clearly that simple, but adequately designed/processed cells fabricated on an industrial scale on large-area silicon wafers could possess parameters similar to those of sophisticated laboratory samples elaborated on small-area silicon. With the design/technological approaches outlined in this work, the use of even cheaper, i.e., lower-grade single-crystal silicon, could still yield 2- and 3-in. cells with fairly acceptable conversion efficiencies.

High-efficiency, low-cost solar cells on 3-inch silicon wafers

The present work describes practical ways of attenuating the severe limitations imposed by different kinds of areal inhomogeneities on the performance of large-area p+−n−n+ single-crystal silicon solar cells. The test devices were processed on 3-inch, lower-grade n-wafers and the minimization of the cells' cost was one of the desired objectives. A combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, high-efficiency (> 17%) 3-inch silicon solar cells with improved overall electro-optical performance. These devices possess short-circuit current densities similar to those of sophisticated laboratory samples prepared on high-quality, small-area silicon.