Low Injection Levels Research Articles

Dynamics of Photoinduced Charge Carrier and Photothermal Effect in Pulse-Illuminated Narrow Gap and Moderate Doped Semiconductors

When a sample of semiconducting material is illuminated by monochromatic light, in which the photon energy is higher than the energy gap of the semiconductor, part of the absorbed electromagnetic energy is spent on the generation of pairs of quasi-free charge carriers that are bound by Coulomb attraction. Photo-generated pairs diffuse through the material as a whole according to the density gradients established, carrying part of the excitation energy and charge through the semiconducting sample. This energy is indirectly transformed into heat, where the excess negatively charged electron recombines with a positively charged hole and causes additional local heating of the lattice. The dynamic of the photoexcited charge carrier is described by a non-linear partial differential equation of ambipolar diffusion. In moderate doped semiconductors with a low-level injection of charge carriers, ambipolar transport can be reduced to the linear parabolic partial differential equation for the transport of minority carriers. In this paper, we calculated the spectral function of the photoinduced charge carrier distribution based on an approximation of low-level injection. Using the calculated distribution and inverse Laplace transform, the dynamics of recombination photoinduced heat sources at the surfaces of semiconducting samples were studied for pulse optical excitations of very short and very long durations. It was shown that the photoexcited charge carriers affect semiconductor heating depending on the pulse duration, velocity of surface recombination, lifetime of charge carriers, and their diffusion coefficient.

Influence of Material Composition and Wafer Thickness on the Performances of Electron Irradiated Gallium‐Doped Silicon Heterojunction Solar Cells

Past studies have underlined the importance of silicon material composition for optimum space solar cells performances. However, the maturity and performances of silicon cells have evolved over the last decades. Due to the increasing space photovoltaic power demand, it becomes crucial to assess modern silicon radiation hardness. Herein, the influence of material composition (resistivity and interstitial oxygen, gallium, and thermal donor concentrations) of modern gallium‐doped silicon wafers on their electronic properties after electron irradiation is investigated. Results demonstrate stable majority carrier concentrations and mobilities within the doping ranges and fluences investigated. Regarding the post‐irradiation carrier recombinations, the higher the resistivity the higher the carrier lifetime is at low injection level. Similarly, the electron diffusion length is six times higher for the 60 Ω.cm samples compared to the 0.9 Ω.cm ones. The Shockley–Read–Hall recombination signature of a vacancy‐related defect (reported in boron‐doped silicon) reproduces well this trend. Then, complete heterojunction solar cells are processed from these materials. While highest resistivity samples feature better carrier lifetimes after irradiation, the best conversion efficiencies are obtained for intermediate resistivity samples (15 Ω.cm). It is shown that it is essentially due to the positive effect of higher majority carrier concentration on the open‐circuit voltage.

A Pilot Study to Investigate Peripheral Low-Level Chronic LPS Injection as a Model of Neutrophil Activation in the Periphery and Brain in Mice.

Lipopolysaccharide-induced (LPS) inflammation is used as model to understand the role of inflammation in brain diseases. However, no studies have assessed the ability of peripheral low-level chronic LPS to induce neutrophil activation in the periphery and brain. Subclinical levels of LPS were injected intraperitoneally into mice to investigate its impacts on neutrophil frequency and activation. Neutrophil activation, as measured by CD11b expression, was higher in LPS-injected mice compared to saline-injected mice after 4 weeks but not 8 weeks of injections. Neutrophil frequency and activation increased in the periphery 4-12 h and 4-8 h after the fourth and final injection, respectively. Increased levels of G-CSF, TNFa, IL-6, and CXCL2 were observed in the plasma along with increased neutrophil elastase, a marker of neutrophil extracellular traps, peaking 4 h following the final injection. Neutrophil activation was increased in the brain of LPS-injected mice when compared to saline-injected mice 4-8 h after the final injection. These results indicate that subclinical levels of peripheral LPS induces neutrophil activation in the periphery and brain. This model of chronic low-level systemic inflammation could be used to understand how neutrophils may act as mediators of the periphery-brain axis of inflammation with age and/or in mouse models of neurodegenerative or neuroinflammatory disease.

High Efficiency of 5 μm‐Diameter Blue Micro‐Light‐Emitting Diodes

Small‐size pixels and high efficiency under low injection levels are required for high‐resolution microdisplay. Efficient blue InGaN micro‐light‐emitting diodes (μLEDs) with 5 μm diameter are fabricated using AuSn flip‐chip bonding, high reflection electrodes, and large‐area N electrodes surrounding the mesas. The peak external quantum efficiency (EQE) measured in an integrating sphere is as high as 13.67% at a current density of 5.4 A cm−2. Moreover, at a current density of 0.1 A cm−2, EQE can still reach 11.69%. The electrical efficiency approaches 1, and the differential slope of log L versus log I is close to 1 at low current density. These results suggest significant progress in exploring high‐efficiency 5 μm InGaN blue μLEDs.

Boosting image denoising effect via low-level noise injection

Boosting image denoising effect via low-level noise injection

Bias-dependent degradation of single quantum well on InGaN-based light emitting diode

This paper investigates the factors that influence the efficiency and the reliability of InGaN-based light emitting diodes. The devices under test have a single InGaN quantum well, with a nominal emission wavelength of 495 nm. From the preliminary characterization, the L-I curve shows an anomalous slope mainly due to: i) high Shockley Read Hall recombination rate, and ii) low electron densities in the explored range of injection, as also confirmed by simulation. The reliability of the devices was investigated by performing a constant current stress test at J = 80 A/cm2 at room temperature. Optical degradation occurs with different trends, depending on the injection regime. For low injection levels, the optical power decay is monotonic. In the high injection range, for I > 10 mA, we can recognize different degradation phases. The optical degradation can be ascribed to the generation and relocation of defects in the active region, as also confirmed by DLOS measurement. The origin of degradation was also investigated by a stress test with photoluminescence measurement. The results suggest that the variation of the optical characteristics can be related to the variation of the intrinsic characteristics of the material.

Study on the series resistance of betavoltaic batteries

Series resistance (Rs ) is an essential factor that affects the performance of betavoltaic batteries. However, the Rs value of betavoltaic batteries tends to be anomaly high when it is extracted from the IV characteristic curve. To explore the reasons for this phenomenon, different injection conditions and their impacts on Rs of betavoltaic and photovoltaic cells were compared and analyzed, since photovoltaic cells have been studied in-depth in respect of Rs and have similar principles to betavoltaic batteries. The results show that Rs will be significantly affected by the injection conditions, and the high resistance of the depletion region under low-level radiation injection of betavoltaic batteries is the main reason for their high Rs . The effect of high Rs on betavoltaic battery performance was further explored, and the results show that the IV characteristic curve will be distorted obviously when Rs is close to or higher than the shunt resistance. This thus leads to a significant decrease in the battery’s fill factor and energy conversion efficiency. The effect of the input power on Rs should be considered in the future analysis of betavoltaic batteries. This work provides theoretical support for the parameter analysis of betavoltaic batteries.

Partial Gaussian-Approximation Soft Demapper for the Core Layer of MIMO-LDM in ATSC 3.0

In this paper, we propose a new partial Gaussian approximation (PGA) soft demapper for the core layer (CL) signal in the multiple-input-multiple-output layered-division-multiplexing (MIMO-LDM) of ATSC 3.0 systems. Unlike a conventional GA demapper, where enhanced layer (EL) signals are intentionally modelled as additive Gaussian noise, the proposed PGA jointly decodes the CL signal by assuming the EL signals to be QPSK signals added with the Gaussian noise irrespective of their modulation order and type. The assumed QPSK constellations are used in such a way that the residual signal power in the EL signals should be minimized. We verify the performance gains from this new PGA demapper by analyzing the variation of the received injection-level with respect to MIMO channel gains. Simulation results show that this new PGA outperforms conventional GA at the cost of an acceptable increase in decoding complexity, especially when dealing with low injection-levels or high code rates.

Bulk defect characterization in metalized solar cells using temperature-dependent Suns-Voc measurements

Extracting the parameters, energy level and electron-to-hole capture cross-section ratio, of efficiency-limiting bulk defects in silicon solar cells is a critical step in identifying those defects and potentially eliminating their impact. Typically, this is achieved on specially prepared test structures. However, in some cases, this is not possible, especially in mass production lines when only completed solar cells are available. In this study, a method that is based on temperature-dependent Suns-V oc measurements is introduced to extract the defect parameters in metalized solar cells. The method is validated by comparing the parameters of the boron-oxygen-related defect extracted from cells and those extracted from wafers using the commonly used temperature- and injection-dependent lifetime spectroscopy. It is shown that this method has the benefit of a more accurate lifetime at low injection levels compared with photoconductance-based lifetime measurement since it is not impacted by minority carrier traps. The proposed technique is then applied to determine the parameters of the defect causing light-induced degradation in gallium-doped silicon solar cells. We determined an energy level, with respect to the intrinsic level, of −0.26 ± 0.04 eV and a capture cross-section ratio of 34 ± 2 for this defect. Finally, a sensitivity analysis is performed by considering the system’s limited measurement temperature range. The findings demonstrate the potential of the temperature-dependent Suns-V oc method as a fast and easy-to-apply method for defect characterization in metalized cells. • Parameters of bulk defects in cells can be extracted using Sun-V oc (T) method. • The method is validated by extracting the parameters of BO-related defects. • The parameters of LID-related defects in Ga-doped cells are extracted with the method. • Sensitivity analysis is performed to assess the limitations of the method.

Impact of Carriers Injection Level on Transients of Discrete and Paralleled Silicon and 4H-SiC NPN BJTs

The 4H-SiC vertical NPN BJTs are attractive power devices with potentials to be used as high power switching devices with high voltage ratings in range of 1.7 kV and high operating temperatures. In this paper, the advantages of the 4H-SiC NPN BJTs in terms of switching transients and current gain over their silicon counterparts is illustrated by means of extensive experimental measurements and modelling, including investigation of high level injection, as a common phenomenon in bipolar devices that influences the switching rates and DC current gain. The two device types have been tested at 800 V with maximum temperature of 175 °C and maximum collector current of 8 A. The turn-ON and turn-OFF transition in Silicon BJT is seen to be much slower than that of the SiC BJT while the transient duration will increase with increasing temperature and decreases with larger collector currents. The common-emitter current gain of SiC BJT is also found to be much higher than silicon counterparts, increasing with temperature in low injection levels but decreasing in higher injection levels in both devices. The rate of increase of current gain slows down toward stability as the collector current increases, known as the high-level injection. Current sharing imbalance among parallel connected devices is also investigated, which are shown to be evidently dependant on temperature and base resistance in Silicon BJT, while the current collapse in also seen in SiC BJT at high injection levels with high base resistance. The turn-OFF delay is seen to be temperature dependant in single and paralleled Silicon BJTs while almost non-existent in SiC device.

Optical Pump–Terahertz Probe Study of HR GaAs:Cr and SI GaAs:EL2 Structures with Long Charge Carrier Lifetimes

The time dynamics of nonequilibrium charge carrier relaxation processes in SI GaAs:EL2 (semi-insulating gallium arsenide compensated with EL2 centers) and HR GaAs:Cr (high-resistive gallium arsenide compensated with chromium) were studied by the optical pump–terahertz probe technique. Charge carrier lifetimes and contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of surface and volume Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that, in most cases for HR GaAs:Cr and SI GaAs:EL2, Auger recombination mechanisms make the largest contribution to the recombination rate of nonequilibrium charge carriers at injection levels above ~(0.5–3)·1018 cm−3, typical of pump–probe experiments. At a lower photogenerated charge carrier concentration, the SRH recombination prevails. The derived charge carrier lifetimes, due to the SRH recombination, are approximately 1.5 and 25 ns in HR GaAs:Cr and SI GaAs:EL2, respectively. These values are closer to but still lower than the values determined by photoluminescence decay or charge collection efficiency measurements at low injection levels. The obtained results indicate the importance of a proper experimental data analysis when applying terahertz time-resolved spectroscopy to the determination of charge carrier lifetimes in semiconductor crystals intended for the fabrication of devices working at lower injection levels than those at measurements by the optical pump–terahertz probe technique. It was found that the charge carrier lifetime in HR GaAs:Cr is lower than that in SI GaAs:EL2 at injection levels > 1016 cm−3.

Effect of RF Power on the Physical Properties of Sputtered ZnSe Nanostructured Thin Films for Photovoltaic Applications.

Zinc selenide (ZnSe) thin films were deposited by RF magnetron sputtering in specific conditions, onto optical glass substrates, at different RF plasma power. The prepared ZnSe layers were afterwards subjected to a series of structural, morphological, optical and electrical characterizations. The obtained results pointed out the optimal sputtering conditions to obtain ZnSe films of excellent quality, especially in terms of better optical properties, lower superficial roughness, reduced micro-strain and a band gap value closer to the one reported for the ZnSe bulk semiconducting material. Electrical characterization were afterwards carried out by measuring the current–voltage (I-V) characteristics at room temperature, of prepared “sandwich”-like Au/ZnSe/Au structures. The analysis of I-V characteristics have shown that at low injection levels there is an Ohmic conduction, followed at high injection levels, after a well-defined transition voltage, by a Space Charge Limited Current (SCLC) in the presence of an exponential trap distribution in the band gap of the ZnSe thin films. The results obtained from all the characterization techniques presented, demonstrated thus the potential of ZnSe thin films sputtered under optimized RF plasma conditions, to be used as alternative environmentally-friendly Cd-free window layers within photovoltaic cells manufacturing.

Phosphorous Diffusion Gettering of Trapping Centers in Upgraded Metallurgical‐Grade Solar Silicon

Experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers is shown, as observed by means of inductively coupled photoconductance (PC) decay and lifetime measurements carried out on upgraded metallurgical‐grade silicon (UMG‐Si) wafers. The presence of trapping species dominating the long time range of the PC decay of UMG material (slow traps), which is effectively removed after a PDG conducted at 780 °C, is detected. Notwithstanding, a second trapping mechanism, characterized by a shorter time constant, still governs the response at very low injection levels after the gettering. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18‐fold, up to the range of 70 μs. Thereby, the way for developing gettering strategies capable of successfully removing trap centers and improving the bulk lifetime of unconventional Si material is paved.

Extraction of PEDOT:PSS/c‐Si Junction Properties by Modeling of Injection‐Dependent Lifetime Measurements Including Depletion Region Modulation

The carrier lifetime of n‐type silicon wafers coated with the conducting polymer PEDOT:PSS as a function of the excess carrier concentration Δp within the wafer is characterized using the quasi‐steady‐state photoconductance (QSSPC) method, and a drastic increase in the measured apparent lifetime τapp with decreasing Δp is observed. The observed increase with the depletion region modulation (DRM) effect is explained, as PEDOT:PSS‐coated p‐type silicon wafers do not show any increase in lifetime toward low injection levels. By modeling the measured τapp(Δp) curves on n‐type silicon including interface recombination as well as the DRM effect, the interface recombination velocity as well as the band bending Ψs within the silicon induced by PEDOT:PSS are able to be extracted. The impact of adding sorbitol to the PEDOT:PSS dispersion on the τapp(Δp) curves is examined, and it is demonstrated that the admixture of sorbitol improves chemical interface passivation but leaves the band bending within the silicon bulk toward the PEDOT:PSS/c‐Si interface unaffected.

A Simple Method to Differentiate between Free‐Carrier Recombination and Trapping Centers in the Bandgap of the p‐Type Semiconductor

In this research, the ranges of the localized states in which the recombination and the trapping rates of free carriers dominate the entire transition rates of free carriers in the bandgap of the p‐type semiconductor are described. Applying the Shockley–Read–Hall model to a p‐type material under a low injection level, the expressions for the recombination rates, the trapping rates, and the excess carrier lifetimes (recombination and trapping) were described as functions of the localized state energies. Next, the very important quantities called the excess carriers’ trapping ratios were described as functions of the localized state energies. Variations of the magnitudes of the excess carriers’ trapping ratios with the localized state energies enable us to categorize the localized states in the bandgap as the recombination, the trapping, the acceptor, and the donor levels. Effects of the majority and the minority carriers’ trapping on the excess carrier lifetimes are also evaluated at different localized energy levels. The obtained results reveal that only excess minority trapping affects the excess carrier lifetimes, and excess majority carrier trapping has no effect.

Variations in Minority Carrier-Trapping Effects Caused by Hydrogen Passivation in Multicrystalline Silicon Wafer

In a multicrystalline silicon (mc-Si) wafer, trapping effects frequently occur in the carrier lifetime measurement based on the quasi-steady-state photoconductance (QSSPC) technique. This affects the accurate measurement of the carrier lifetime of an mc-Si solar cell by causing distortions at a low injection level close to the Pmax point. Therefore, it is necessary to understand this effect and effectively minimize the trapping-center density. In this study, the variations in the minority carrier-trapping effect of hydrogen at different annealing temperatures in an mc-Si were observed using QSSPC, time-of-flight secondary ion mass spectroscopy, and atom probe tomography. A trapping effect was confirmed and occurred in the grain boundary area, and the effect was reduced by hydrogen. Thus, in an mc-Si wafer, effective hydrogen passivation on the grain area and grain boundary is crucial and was experimentally proven to minimize the distortion of the carrier lifetime.

Transmitter Identification Signal Detection Algorithm for ATSC 3.0 Single Frequency Networks

To effectively operate and maintain an existing single frequency network (SFN), the advanced television systems committee (ATSC) 3.0 transmission system has utilized a transmitter identification (TxID) technique. The TxID technique aims is to obtain the channel impulse response (CIR) of each transmitter independently to support the adjustment of the relative power level and delay offset of individual transmitters. The professional receiver can detect such CIR, but a commercial ATSC 3.0 receiver does not need to be necessarily decoded the TxID signal. Also, according to the ATSC 3.0 standard, the TxID signal is embedded into the preamble signal with an appropriately low injection level (IL), such that the impact on the preamble reception is negligible. This paper proposes an efficient TxID signal detection algorithm to enhance the individual CIR detection performance during the TxID analyzing process. The simulation results indicate that the proposed TxID signal detection algorithm can offer better individual CIR detection performance as compared to the conventional TxID signal detection algorithms, even though the IL of TxID signal is low.

ВЛИЯНИЕ ДВИЖЕНИЯ ПРИМЕСНЫХ ИОНОВ НА СТАБИЛЬНОСТЬ СВЕТОДИОДОВ

The influence of the motion of impurity centers on the stability of GaInAsP-based light- emitting diodes and their efficiency are studied. The stability and efficiency of the electroluminescence of LEDs is mainly determined by the ratio between the intensities of the radiative and non-radiative recombination of charge carriers. We studied the electroluminescent and electrical characteristics of LEDs. To clarify the degradation mechanism of LEDs, we studied the effect of their current training for 3000 hours at various current densities for stability and efficiency and for their electrical characteristics. It was shown that the degradation of LEDs at low injection levels is associated with the drift of impurity centers near the inhomogeneities of р-л-junctions. It is shown that during the degradation of LEDs, the magnitude of the radiative current component at a fixed voltage varies little. At the same time, nonradiative current components increase significantly. It was established that the growth of nonradiative current components is associated with the drift of mobile impurities to inhomogeneities of the р-л-junction. The kinetics of LED degradation was calculated using certain assumptions. The diffusion coefficient of the ions responsible for the degradation of the diodes is estimated. It is shown that “sudden” LED failures are of the same nature as their gradual degradation. They can occur at a sufficiently high concentration of mobile impurities. The obtained dependences of the radiation intensity on the duration of degradation can be used to estimate the diffusion coefficient of a mobile impurity.

The Measurement of Charge Carrier Lifetime in SIGaAs: Cr and EL2-GaAs by Pump-Probe Terahertz Spectroscopy

In the present work, the temporal dynamics of relaxation of a nonequilibrium concentration of charge carriers in SI-GaAs:Cr and EL2-GaAs semiconductor crystals has been studied using pump-probe terahertz spectroscopy. The obtained experimental data were analyzed taking into account the mechanisms of surface and bulk Shockley–Reed–Hall recombination, radiative recombination, interband and trap assisted Auger recombination. It was found that at injection levels arising upon excitation of samples by laser radiation with a pulse duration of 35 fs and an energy of 0.1 mJ per pulse at a central wavelength of 791 nm, Auger recombination mechanisms have a significant effect. Auger recombination mechanisms make a dominant contribution to the recombination rate of nonequilibrium charge carriers at injection levels above 2·1018 cm–3 for SI-GaAs:Cr and above 1018 cm–3 for EL2-GaAs. At lower injection levels, the bulk and surface Shockley-Reed-Hall recombination are the dominant recombination mechanisms.

The impact of interstitial Fe contamination on n-type Cz-Silicon for high efficiency solar cells

In this work, we have investigated the impact of interstitial Fe contamination on the effective minority carrier lifetime of n-type Cz silicon bulk material for high efficiency solar cells. The study covers a Fe concentration in the silicon bulk from 3.5 × 1012 cm-3 to 2.7 × 1014cm-3. We have added 5 different concentrations (30, 100, 300, 1000 and 3000 ppb) of Fe intentionally to a wet chemical process tank and measured the transfer to the silicon wafer surface mimicking a possible contamination during wet chemical processing. In order to fabricate carrier lifetime test vehicles, the silicon wafer is then passivated with thermal silicon oxide from both sides. The surface contamination is driven into the bulk by mimicking a high temperature process during solar cell manufacturing. Effective minority carrier lifetime is measured at injection levels from 1 × 1013 cm-3 to 3 × 1015cm-3. We have fitted the theoretical curve for interstitial Fe derived from the SRH theory to the measured values and extracted the Fe contamination concentration. This value is comparable to the calculated value extracted from the surface contamination measurement. For low level injection (LLI), we extracted the capture cross section for interstitial Fe to be 6.45 × 10-17 cm/s ± 2.23 × 10-17 cm/s. The measured Fe contamination levels are used for the conversion efficiency fitting of a n-type bifacial silicon solar cell using QUOKKA simulations. The simulations show that very low Fe contamination concentrations of [Fe]bulk = 3.5 × 1012 cm-3 ([Fe]surf = 6 × 1010cm-2) already degrade the solar cell efficiency by 10% relative.