Excess Carrier Generation Research Articles

Prediction of betavoltaic battery output parameters based on SEM measurements and Monte Carlo simulation

An approach for a prediction of 63Ni-based betavoltaic battery output parameters is described. It consists of multilayer Monte Carlo simulation to obtain the depth dependence of excess carrier generation rate inside the semiconductor converter, a determination of collection probability based on the electron beam induced current measurements, a calculation of current induced in the semiconductor converter by beta-radiation, and SEM measurements of output parameters using the calculated induced current value. Such approach allows to predict the betavoltaic battery parameters and optimize the converter design for any real semiconductor structure and any thickness and specific activity of beta-radiation source.

Research on MeV pulsed radiation detection based on refractive index modulaiton

High time resolution detecting systems for MeV pulsed radiation are essential for inertial confinement fusion diagnostics. Traditional detection of system time resolution is restricted by cable bandwidth. Based on recording excess carrier dynamics in semiconductors, a new detecting mechanism, called RadOptic, was developed by Lawrence Livermore National Laboratory (LLNL). The variation of intensity of pulsed radiation with time was converted into the variation of intensity of infrared laser probe by using this mechanism. The sensing material was InGaAsP quantum wells with severalmicrometer thickness. Picosecond time resolution for several keV pulsed radiation has been demonstrated. The reported system is not suitable for MeV pulses due to its low efficiency to MeV photons. Multiple cascaded structure for MeV photon to electron transformation was proposed by LLNL. Applying bulk material with several-hundredmicrometer thickness is an alternative. Based on transient free carrier absorption, a system recording bulk materials' instantaneous refractive index change is established. The system consists of a probe laser, an interferometer module, a signal transmission module and a signal recording module. The probe is a tunable infrared continuous wave laser whose wavelength is ~1453 nm, guided by single mode fiber to the interferometer. The interferometer consists of a single mode fiber head coupled directly with the polished face of a bulk semiconductor. The interference pattern forms by multiple beams reflected from the front face and the back face of the bulk. Part of interference light is coupled to the single mode fiber and forms the output signal. Pulsed radiation will deposit energy and generate excess carriers in the bulk material. The refractive index of the bulk material changes therewith according to the Drude model. The interference pattern and the light coupled to the single mode fiber also change therewith. The signal is transmitted by a long single mode fiber. The signal recording module consists of photoelectric detectors and a digital oscilloscope. The signal generation process and the time resolution of the system are analyzed. Intrinsic GaAs refractive index change is exploited under electron pulses and X ray pulses. The analysis of signal generation process shows that when the excess carriers recombine much faster/much slower than the pulse width, the output signal/output signal differential can be viewed as a measure of intensity variation with time of the incident pulse. For this prototype system, the time resolution is restricted by the digital oscilloscope to 1 GHz. Bulk intrinsic GaAs demonstrates 30 ns refractive index response time, which is longer than the incident pulse width. The differential signal can be viewed as a measure of incident pulse intensity when GaAs is exposed to 1 ns~0.2 MeV electrons pulses. The differential signal width is shorter than the pulse width when GaAs is exposed to 5 ns~0.2 MeV electrons pulses. Auger recombination process may occur in the pulse duration under this situation. The differential signal width is longer than the pulse width when GaAs is exposed to 1 ns~0.2 MeV X ray pulses. The poor signal to noise ratio affects the signal. The excess carrier generation process may be longer than theoretically estimated one under X ray pulse incident situation. The generation process and recombination process of excess carriers in GaAs show very different characteristics compared with optical excitation. The relationship between the system output signal and the incident pulsed radiation depends on the type of the incident radiation. With carefully considering the effects from incident pulse type and transient carriers density, the system can be used to detect ~MeV pulsed radiation. With an upgraded recording module, the system would demonstrate much higher time resolution.

OPTIMIZATION OF MULTIJUNCTION SOLAR CELL BY WAFER RAY TRACER FOR DEVELOPMENT OF HIGH PHOTOGENERATED CURRENT

Optical losses limit the excess carriers generation in absorber part of multijuction (MJ) solar cell. The generation of excess carriers is directly proportional to photogenerated current solar cell. Therefore, reduction of optical losses is fundamentally important for improving the power conversion efficiency. Thickness of layers strongly influences the performance of MJ solar cell. In this study we simulated a MJ solar cell of Air/ZnO/SiC/c-Si/a-Si(n)/Al structure using Wafer Ray Tracer (WRT) simulation software and optimized the thicknesses of the layers for photogenerated current. The simulation result shows that without SiC layer, only 57.48% of incident light is absorbed and generates 26.85 mA/cm2 photogenerated current in solar cell. A 70 nm thickness of optimized SiC layer is increasing the light absorption 22.16% and photogenerated current 38.54%. Result shows that there is no transmission of light through the absorber layer. The MJ solar cell without Back Surface Field (BSF) layer of a-Si(n) shows photogenerated current of 37.05 mA/cm2 which can be improved to 37.24 mA/cm2 with a 100 nm thickness of a-Si(n). The c-Si absorber layer shows highest absorptance within 500 nm-1000 nm wavelength of light spectrum with 100 nm thickness of aSi(n). An a-Si(n) BSF layer at the back surface minimizes the effective back-surface recombination velocity and improves the collection probability of minority carriers of solar cell. Furthermore a 100 nm Al rear contact improves the photogenerated current of MJ solar cell to 37.25 mA/cm2. An Al rear contact layer improves the mechanical strength of c-Si absorber layer. The electrical property of Al improves the excess carriers’ collection probability of MJ solar cell.

A study of nonlinear lasing dynamics of an InGaAs/AlGaAs/GaAs heterostructure power laser-thyristor emitting at 905 nm

The results obtained demonstrate that the lasing dynamics reflects the current dynamics formed as a result of complex nonlinear couplings within the laser-thyristor heterostructure. The observed specific features mainly result from the appearance of new channels for generation of excess carriers in the p-base. These channels enhance the main optical activation channel formed by the photogeneration due to the absorption of the spontaneous emission from the active region of the laser part of the heterostructure. The additional channels of excess carrier generation may have an optical nature in the case of scattered laser light upon appearance of new high-Q modes. For nearly critical blocked voltages, generation of carriers can be initiated by an avalanche multiplication of photogenerated carriers.

On plasmon-induced photocurrent and doping of metal-patterned graphene

Patterning graphene with noble metal plasmonic nanostructures to enhance and to manipulate the optical and electronic properties of graphene promises a variety of technological innovations in the field of nano-optoelectronics. In this report, we briefly revisit photoconduction experiments done recently on graphene plasmonic sensors and show that the excess electrical current generated in response to spatially non-uniform optical excitation is primarily induced by a photo-thermo-electric effect in the graphene itself. As this mechanism has nothing to do with the excess free carrier generation common with conventional semiconductors, the plasmonic nanostructures cannot be utilized to regulate the free carrier density and doping of graphene, in contrast to what has been proposed recently.

Theory of a Directive Optical Leaky Wave Antenna Integrated into a Resonator and Enhancement of Radiation Control

We provide for the first time the detailed study of the radiation performance of an optical leaky wave antenna (OLWA) integrated into a Fabry-P\'erot resonator. We show that the radiation pattern can be expressed as the one generated by the interference of two leaky waves counter-propagating in the resonator leading to a design procedure for achieving optimized broadside radiation, i.e., normal to the waveguide axis. We thus report a realizable implementation of the OLWA made of semiconductor and dielectric regions. The theoretical modeling is supported by full-wave simulation results, which are found to be in good agreement. We aim to control the radiation intensity in the broadside direction via excess carrier generation in the semiconductor regions. We show that the presence of the resonator can provide an effective way of enhancing the radiation level modulation, which reaches values as high as 13.5 dB, paving the way for novel promising control capabilities that might allow the generation of very fast optical switches, as an example.

Excess carrier generation in femtosecond-laser processed sulfur doped silicon by means of sub-bandgap illumination

With Fourier-transform photocurrent spectroscopy and spectral response measurements, we show that silicon doped with sulfur by femtosecond laser irradiation generates excess carriers, when illuminated with infrared light above 1100 nm. Three distinct sub-bandgap photocurrent features are observed. Their onset energies are in good agreement with the known sulfur levels S+, S0, and S20. The excess carriers are separated by a pn-junction to form a significant photocurrent. Therefore, this material likely demonstrates the impurity band photovoltaic effect.

Determination of Bulk Lifetime and Surface Recombination Velocity of Silicon Ingots From Dynamic Photoluminescence

This paper elaborates upon the theory of self-consistent minority carrier bulk lifetime measurements of silicon ingots via time-modulated photoluminescence and presents an experimental proof of concept. For silicon ingots, the solution of the continuity equation at harmonic time modulation of excess carrier generation is shown to generally reveal a remarkably pronounced contrast with respect to minority carrier bulk lifetime. We combine our dynamic self-consistent approach with an analysis of surface recombination velocity from photoluminescence intensity ratios upon irradiation with different laser wavelengths. This combined analysis enables an accurate simultaneous determination of bulk lifetime and surface recombination velocity. For sufficiently high or accurately known ingot surface recombination velocities, this approach could likewise be used for an accurate determination of the minority carrier diffusion coefficient and of minority carrier mobility in novel silicon materials.

Accurate determination of minority carrier mobility in silicon from quasi-steady-state photoluminescence

Minority carrier mobility is a crucial transport property affecting the performance of semiconductor devices such as solar cells. Compensation of dopant species and novel multicrystalline materials call for accurate knowledge of minority carrier mobility for device simulation and characterization. Yet, measurement techniques of minority carrier mobility are scarce, and published data scatter significantly even on monocrystalline material. In this paper, the determination of minority carrier mobility from self-consistent quasi-steady-state photoluminescence measurements of effective carrier lifetime is presented. The measurement design is distinguished by a limitation of carrier recombination through minority carrier transport—with excess carrier generation and recombination confined to opposite interfaces, respectively. Minority carrier mobility is inferred from the minority carrier diffusion coefficient via the Einstein relation. An experimental proof of concept on monocrystalline p-type material is provided, showing good agreement with state-of-the-art data and models. Considerations for the applicability of the method to compensated and multicrystalline silicon materials are discussed.

Mixed Oxide Thin Film Transistors under Combinatory Optical Irradiation and Electrical Bias

This study reports the operational instability and failure causing defect mechanisms of indium gallium zinc oxides based thin film transistors (TFTs) under practical, combined bias and illumination stress conditions. The effect of the illumination stress with light source of different wavelengths (410 nm, 467 nm, 532 nm, and 632 nm) has been explained. Wavelengths shorter than green light (532 nm) demonstrate creation of ionized vacancies and subsequent generation of excess carriers that alter TFT functions. Recovery evaluations performed upon removal of combinatory stresses exhibits incomplete recovery due to incapacity in regenerating neutral oxygen vacancies. The Ion/Ioff values show an increase by an order of magnitude when compared to previous studies where the Ion/Ioff is reduced. This can be contributed to the reduced defect and vacancy densities due to the effective post anneal process.

An optical leaky wave antenna with Si perturbations inside a resonator for enhanced optical control of the radiation

We investigate the directive radiation at 1550 nm from an optical leaky wave antenna (OLWA) with semiconductor perturbations made of silicon (Si). We study the radiation pattern dependence on the physical dimensions, number of perturbations and carrier densities in these semiconductor perturbations through optical excitations at a visible wavelength, 625 nm. In this detailed theoretical study we show the correlation between the pump power absorbed in the perturbations, the signal guided in the waveguide and the radiation through leakage. To overcome the limited control of the radiation intensity through excess carrier generation in Si, we present a new design with the OLWA integrated with a Fabry-Pérot resonator (FPR). We provide analytical and numerical studies of the enhanced radiation performance of the OLWA antenna inside the FPR, and derive closed-form formulas accounting for LW reflection at the edges of the FPR. A discussion on the constructive and destructive radiation by the direct and reflected leaky waves in the FPR resonator is provided. Results shown in this paper exhibit 3 dB variation of the radiation and pave the way for further optimization and theoretical developments.

Three-dimensional transport imaging for the spatially resolved determination of carrier diffusion length in bulk materials

A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (μτ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Green's function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity. The approach enables measurement of spatial variations in the μτ product with a high degree of spatial resolution.

A Novel Double HBT-Based Capacitorless 1T DRAM Cell With Si/SiGe Heterojunctions

We propose a novel double heterojunction bipolar transistor (DHBT)-based capacitorless one-transistor (1T) DRAM cell employing a narrow bandgap SiGe body and Si/SiGe heterojunction for a possible next-generation DRAM cell. It has a body with a narrow bandgap and a valence band offset between the source/drain and the body. Through an extended investigation via TCAD simulation, we verified the advantages of the proposed DHBT-based 1T DRAM cell, including an improved excess carrier generation rate, a high current gain, a large sensing margin, and a suppressed sensitivity to the bandgap-narrowing effect in the heavily doped source and drain.

A Two-Dimensional (2D) Analytical Model for the Potential Distribution and Threshold Voltage of Short-Channel Ion-Implanted GaAs MESFETs under Dark and Illuminated Conditions

A two-dimensional (2D) analytical model for the potential distribution and threshold voltage of short-channel ion-implanted GaAs MESFETs operating in the sub-threshold regime has been presented. A double-integrable Gaussian-like function has been assumed as the doping distribution profile in the vertical direction of the channel. The Schottky gate has been assumed to be semi-transparent through which optical radiation is coupled into the device. The 2D potential distribution in the channel of the shortchannel device has been obtained by solving the 2D Poisson’s equation by using suitable boundary conditions. The effects of excess carrier generation due to the incident optical radiation in channel region have been included in the Poisson’s equation to study the optical effects on the device. The potential function has been utilized to model the threshold voltage of the device under dark and illuminated conditions. The proposed model has been verified by comparing the theoretically predicted results with simulated data obtained by using the commercially available ATLAS™ 2D device simulator.

Influence of different excitation spectra on the measured carrier lifetimes in quasi-steady-state photoconductance measurements

The interpretation of lifetime measurements on unpassivated silicon wafers by means of the quasi-steady-state photoconductance (QSSPC) technique is a practically highly relevant, but challenging task. We investigate QSSPC measurements on damage-etched samples with five different resistivities from 0.25 Ω cm p-type to 100 Ω cm n-type float-zone material with four different thicknesses each, ranging from 105 to 250 μm. The influence of different excitation spectra, realized by adding different long pass filters to the standard QSSPC Xenon flash spectrum, on the injection-dependent effective excess carrier lifetime is studied in detail. We observed that the measured effective lifetime is significantly dependent upon the excitation spectrum in use and is augmented by increasing the long pass filter cut-off wavelength, the latter being equivalent to more symmetrical excess carrier density profiles. Furthermore, the size and the injection dependency of the dominating surface recombination channels are investigated. Finally, two different approaches to accurately calculate the excess carrier generation rate within the sample are presented and compared.

Simulation of XBIC Contrast of Precipitates in Si

Simulation of contrast for small spherical defects in the X-ray beam-induced current (XBIC) mode has been carried out. Under simulations the excess carrier generation function is described by the rigid cylinder with the constant generation rate inside it. The dependence of maximum contrast value on the precipitate depth, diffusion length and effective beam radius is calculated. The XBIC contrast profile as a function of diffusion length, of beam radius and of precipitate depth has been calculated that allows to evaluate the spatial resolution of the technique. The results obtained are compared with those calculated for the EBIC contrast of the same defect. It is shown that in the semiconductor materials with the small diffusion length the XBIC contrast could be comparable with the EBIC one.

ZnO-nanorod arrays for solar cells with extremely thin sulfidic absorber

Solar cells with an extremely thin sulfidic absorber have been prepared by spray ion layer gas reaction (ILGAR) of In 2S 3 on ZnO-nanorod arrays. As transparent hole conductor, CuSCN was deposited on the coated ZnO nanorods by impregnation. Surface photovoltage spectroscopy was applied to characterize states contributing to excess carrier generation and charge separation. The charge-selective contact is formed at the In 2S 3/CuSCN interface region the states of which also contribute significantly to the photocurrent. The influence of annealing temperature and annealing time of the In 2S 3/CuSCN contact region on the open-circuit potential ( V OC), short-circuit current ( I SC) and fill factor (FF) was studied in detail. For solar cells based on ZnO-nanorod arrays (rod length 1.5 μm), efficiency of 2.8% is obtained at AM1.5.

3D Study of Bifacial Silicon Solar Cell under Intense Light Concentration and Under External Constant Magnetic Field

This work presents a three-dimensional study of bifacial silicon solar cell under intense light concentration and under constant magnetic field. This approach is based on the resolution of the minority continuity equation, taking into account the distribution of the electric field in the bulk evaluated as a function of both majority and minority carrier densities. In this approach, new analytical expressions of diffusion length, diffusion coefficient and excess minority carrier density was established for front illumination. The effect of magnetic field on excess minority carrier generation, carriers mobility, carriers density and electric parameters profile are then analysed.

Influence of electric field on the photoelectric effect in a Schottky barrier based on n-type cadmium diphosphide

The results of a study on photoelectric properties of a Schottky barrier based on n-type CdP2 are considered. The effect of the barrier electric field on photocurrents caused by photoelectron emission from metal and optical generation of excess charge carriers in the semiconductor was studied. It was found that the dependence of the photocurrent on the light-modulation frequency is controlled by level recharging times at the interfaces between the space-charge and quasi-neutrality regions and between semiconductor and metal. Good agreement between the calculated and experimental results was achieved.

Generation of excess charge carriers in molecularly doped polymers by electron-beam irradiation

The processes of generation and transport of excess charge carriers in molecularly doped polycarbonate and polystyrene were experimentally studied at room temperature. The polymers were ionized by 7-and 50-keV electrons. The radiation-chemical yield of free ions was determined by means of the universal method based on the combination of the time-of-flight technique in two carrier generation modes (surface and bulk generation) with the measurement of nonsteady-state conductivity. It was shown that the radiation-chemical yield of free ions under irradiation by 7-keV electrons is almost the same as that in the case of 50-keV electrons, despite the substantially different values of the linear energy transfer for these electrons.