Reduction Of Carrier Lifetime Research Articles

Accelerated light-induced degradation for detecting copper contamination in p-type silicon

Copper is a harmful metal impurity that significantly impacts the performance of silicon-based devices if present in active regions. In this contribution, we propose a fast method consisting of simultaneous illumination and annealing for the detection of copper contamination in p-type silicon. Our results show that, within minutes, such method is capable of producing a significant reduction of the minority carrier lifetime. A spatial distribution map of copper contamination can then be obtained through the lifetime values measured before and after degradation. In order to separate the effect of the light-activated copper defects from the other metastable complexes in low resistivity Cz-silicon, we carried out a dark anneal at 200 °C, which is known to fully recover the boron-oxygen defect. Similar to the boron-oxygen behavior, we show that the dark anneal also recovers the copper defects. However, the recovery is only partial and it can be used to identify the possible presence of copper contamination.

Modeling of radiation damage recovery in particle detectors based on GaN

The pulsed characteristics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift–diffusion models. The drift–diffusion current simulations have been implemented by employing the commercial software package Synopsys TCAD Sentaurus. The bipolar drift regime has been analyzed. The possible internal gain in charge collection through carrier multiplication processes determined by impact ionization has been considered in order to compensate carrier lifetime reduction due to radiation defects introduced into GaN material of detector.

Effects of 340 Kev Proton Irradiation on InGaN/GaN Blue Light-Emitting Diodes

The development of GaN-based light-emitting diodes (LEDs) holds promise for various applications such as televisions, mobile phones, traffic signals, automotive lighting and indoor/outdoor lighting. Recently, the application of GaN-based LEDs has been extended to satellite communication systems for weather forecasting or broadband data transmission due to their high radiation hardness. GaN-based devices with small lattice constant (a = 3.189 Å, c = 5.186 Å for wurtzite structure) show superior radiation resistance due to the higher displacement energies compared with other semiconductor systems such as GaAs (a = 5.653 Å). In this work, the low energy (340 keV) proton irradiation effects on optical and electrical performances of InGaN/GaN blue LEDs were investigated. The commercial InGaN/GaN blue LEDs with the emission peak of 441nm were used. The epitaxial structures consisted of p- and n-type GaN layer and InGaN/GaN multi quantum wells grown by metal organic chemical vapor deposition on sapphire substrates. Different proton doses from 5 × 1010 to 1 × 1014 /cm2 were employed to investigate the effects of irradiation doses on the device performances. The irradiated samples were intentionally tilted 7° away from the normal direction of the proton beam to prevent the channeling effect. The Stopping and Range of Ions in Matter (SRIM) simulation was used to estimate the penetration depth of protons and the distribution of vacancies generated by proton irradiations. The L-I-V characteristics of InGaN/GaN blue LEDs were measured with Si photodiode connected to a HP 4156 parameter analyzer. The electroluminescence (EL) spectra were obtained by an optical fiber connected to the Ocean Optics USB 4000 spectrometer. The reverse recovery characteristics were measured with oscilloscope and pulse generator. Figure 1 shows the current-voltage (I-V) of InGaN/GaN blue LEDs before and after proton irradiations with various doses, respectively. The LED diode I-V characteristics of InGaN/GaN blue LEDs were drastically degraded after high dose (1 × 1014 /cm2) proton irradiation. This degradation was caused by the displacement damage, resulting in the reduction of carrier concentration and mobility. However, the degradation of light output was much faster than that of I-V characteristics as shown in Figure 2. This faster degradation of light output indicates that defects generated by energetic protons can act as non-radiative recombination centers. The reduction of light output was also confirmed using EL spectra. In addition, the reverse recovery time, which is related to the minority carrier life time, was also measured by switching the current from 0mA to 50mA. The reverse recovery time before and after 1 × 1013 and 1 × 1014 /cm2 proton irradiations were 80, 79 and 73 ns, respectively. This degradation resulted from the reduction of minority carrier lifetime caused by the introduction of irradiation-induced defects in InGaN/GaN blue LEDs after proton irradiation. In conclusion, the effects of 340 keV proton irradiations on InGaN/GaN blue LEDs were investigated using L-I-V characteristics, EL spectra and reverse recovery characteristics. The optical performances severely degraded compared with the electrical performances due to the radiation-induced defects served as the non-radiative recombination centers. Figure 1

Simulations of operation dynamics of different type GaN particle sensors.

The operation dynamics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift-diffusion models. The drift-diffusion current simulations have been implemented by employing the software package Synopsys TCAD Sentaurus. The monopolar and bipolar drift regimes have been analyzed by using dynamic models based on the Shockley-Ramo theorem. The carrier multiplication processes determined by impact ionization have been considered in order to compensate carrier lifetime reduction due to introduction of radiation defects into GaN detector material.

Investigation of Properties Limiting Efficiency in Cu<sub>2</sub>ZnSnSe<sub>4</sub>-Based Solar Cells

We have investigated different nonidealities in Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ZnSnSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -CdS-ZnO solar cells with 9.7% conversion efficiency, in order to determine what is limiting the efficiency of these devices. Several nonidealities could be observed. A barrier of about 300 meV is present for electron flow at the absorber-buffer heterojunction leading to a strong crossover behavior between dark and illuminated current-voltage curves. In addition, a barrier of about 130 meV is present at the Mo-absorber contact, which could be reduced to 15 meV by inclusion of a TiN interlayer. Admittance spectroscopy results on the devices with the TiN backside contact show a defect level with an activation energy of 170 meV. Using all parameters extracted by the different characterization methods for simulations of the two-diode model including injection and recombination currents, we come to the conclusion that our devices are limited by the large recombination current in the depletion region. Potential fluctuations are present in the devices as well, but they do not seem to have a special degrading effect on the devices, besides a probable reduction in minority carrier lifetime through enhanced recombination through the band tail defects.

Direct Femtosecond Observation of Charge Carrier Recombination in Ternary Semiconductor Nanocrystals: The Effect of Composition and Shelling

Heavy-metal free ternary semiconductor nanocrystals are emerging as key materials in photoactive applications. However, the relative abundance of intra-bandgap defect states and lack of understanding of their origins within this class of nanocrystals are major factors limiting their applicability. To remove these undesirable defect states which considerably shorten the lifetimes of photogenerated excited carriers, a detailed understanding about their origin and nature is required. In this report, we monitor the ultrafast charge carrier dynamics of CuInS2 (CIS), CuInSSe (CISSe), and CuInSe2 (CISe) nanocrystals, before and after ZnS shelling, using state-of-the-art time-resolved laser spectroscopy with broadband capabilities. The experimental results demonstrate the presence of both electron and hole trapping intra-bandgap states in the nanocrystals which can be removed significantly by ZnS shelling, and the carrier dynamics is slowed down. Another important observation remains the reduction of carrier lifetime in the presence of Se, and the shelling strategy is observed to be less effective at suppressing trap states. This study provides quantitative physical insights into the role of anion composition and shelling on the charge carrier dynamics in ternary CIS, CISSe, and CISe nanocrystals which are essential to improve their applicability for photovoltaics and optoelectronics.

Laser Enhanced Hydrogen Passivation of Silicon Wafers

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite.

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

Steady state minority carrier lifetime and defect level occupation in thin film CdTe solar cells

A model consisting of Shockley Read Hall (SRH) recombination under steady state conditions of constant photon injection is proposed in this work to study the steady state minority carrier lifetime in CdS/CdTe thin film solar cells. The SRH recombination rate versus optical injection level is analytically approximated in the junction and neutral regions. In the neutral region, it is found that the recombination rate through certain defect levels has one constant value under lower optical injection conditions and another constant value under higher optical injection conditions with the transition occurring at a critical optical injection level. By simultaneously solving the equations of charge neutrality, charge conservation and SRH recombination in the neutral region, it is found that the compensation of doping and the reduction of minority carrier lifetime by donors in the p-type semiconductor can each be remedied by optical injection. It is also demonstrated that this optical-dependent SRH recombination is significant in large bandgap thin films. The measured minority carrier diffusion length in a CdS/CdTe solar cells, as determined from the steady-state photo-generated carrier collection efficiency, shows the predicted transition of minority carrier lifetime versus optical injection level. A numerical fitting of the indirectly-measured minority carrier lifetime by assuming the minority carrier mobility gives a non-intuitive picture of the p–n junction with a low free hole concentration but a narrow depletion region width.

Quantum-Well Solar Cells for Space: The Impact of Carrier Removal on End-of-Life Device Performance

In this paper, a detailed analysis on the radiation response of solar cells with multi quantum wells (MQW) included in the quasi-intrinsic region between the emitter and the base layer is presented. While the primary source of radiation damage of photovoltaic devices is minority carrier lifetime reduction, we found that in the case of MQW devices, carrier removal (CR) effects are also observed. Experimental measurements and numerical simulations reveal that with increasing radiation dose, CR can cause the initially quasi-intrinsic background doping of the MQW region to become specifically n- or p-type. This can result in a significant narrowing and even the collapse of the electric field between the emitter and the base where the MQWs are located. The implications of the CR-induced modification of the electric field on the current-voltage characteristics and on the collection efficiency of carriers generated within the emitter, the MQW region, and the base are discussed for different radiation dose conditions. This paper concludes with a discussion of improved radiation hard MQW device designs.

Tricolor resonant cavity organic light-emitting diodes using dielectric distributed Bragg reflector in resonant cavity

Various-thick indium-tin oxide films and dielectric distributed Bragg reflector (DDBR) were utilized in resonant cavity to obtain the tricolor resonant cavity organic light-emitting diodes (RCOLEDs). The Commission Internationale d’Eclairage (CIE) coordinate of the resulting red, green, and blue RCOLEDs was (0.528, 0.393), (0.315, 0.575), and (0.186, 0.076), respectively. Compared with the conventional white OLEDs, these tricolor RCOLEDs revealed higher luminance efficiency. Since the resonant effect was caused from the resonant cavity structure with properly designed DDBR, the color purity and cavity quality factor of the tricolor RCOLEDs were also improved by narrowing the emission spectrum band and reducing carrier lifetime.

The structure and properties of (aluminum, oxygen) defect complexes in silicon

The atomic structure and electronic properties of aluminum (Al)-related defect complexes in silicon (Si) are investigated using first-principles calculations. Individual substitutional Al (AlSi), interstitial Al (Ali) and their possible complex configurations with oxygen (O) atoms are studied. We find a unique stable complex configuration consisting of an Ali and an oxygen dimer, Ali-2Oi, which introduces deep levels in the band gap of Si. The formation energies of the Ali-2Oi complexes could be lower than that of individual Ali atoms under oxygen-rich conditions. The formation of Ali-2Oi complexes may explain the experimental observation that the coexistence of Al and O results in reduced carrier lifetime in Si wafers.

Modeling Charge Loss in CMOS MAPS Exposed to Non-Ionizing Radiation

A model, approximating minority carrier diffusion with a discrete random walk and accounting for radiation induced reduction of minority carrier lifetime, is proposed to predict the effects of neutron irradiation on the charge collection properties of monolithic active pixel sensors (MAPS) in CMOS technology. The model has been implemented in a Monte Carlo code to simulate MAPS operation in minimum ionizing particle detection systems. For the purpose of validating it, the results from the characterization of monolithic sensors irradiated up to an integrated fluence of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> 1- MeV-neutron equivalent/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have been compared with the outcomes of the Monte Carlo simulations. The monolithic sensors taken into consideration for the model validation are based on two different CMOS processes, one featuring a triple well option, the other one featuring a quadruple well structure and a standard (10 Ω·cm) or high (1 kΩ·cm) resistivity epitaxial layer. Simulation results are shown to be in good agreement with experimental data. The consistency between the model and the measurement results seems to confirm that radiation induced increase in the recombination rate is the main source of charge collection degradation in neutron-irradiated MAPS.

Impact of the transparent conductive oxide work function on injection-dependent a-Si:H/c-Si band bending and solar cell parameters

An analysis of the contact formation between degenerated n-type transparent conductive oxide (TCO) and p-type amorphous silicon (a-Si:H) as it is used for front side contacts in high efficiency a-Si:H/crystalline silicon (c-Si) heterojunction solar cells is presented. It is shown that the deposition of a TCO on a (p)a-Si:H emitter layer causes a reduction of charge carrier lifetime in low injection levels which leads to a lowering of the implied fill factor. Simulation based analysis of charge carrier lifetime and direct measurements by surface photovoltage reveals that TCO deposition induces a change of the c-Si band bending. The magnitude of this change depends on the (p)a-Si:H doping level. Both observations are explained by the impact of the TCO/a-Si:H work function difference on the c-Si band bending. Based on numerical simulations, the reduced injection-dependent band bending is identified as the reason for the reduced fill factor of final solar cells.

A comparative study based on optical and electrical performance of micro- and nano-textured surfaces for silicon solar cells

In this contribution, we report on the optical and electrical properties of textured silicon surfaces usually incorporated in solar cell fabrication. In order to study the influence of different texturisation processes, nano-texturing was accomplished by soft UV-NIL process while the micro-textured surfaces were prepared using TMAH solution. The nano-textured surfaces were found to be superior in terms of optical performance but the passivation of these structures was solely limited by the fabrication process itself. Any defect or damage caused during the pattern transfer (plasma etch) process acts as a recombination centre which significantly reduces carrier lifetime. By systematically removing the plasma induced damage from the surface, it was possible to improve the effective lifetime from 24.4μs to 1092μs. We report that, despite of the enlarged surface area compared to micro-textured surface, our nano-texture surface possess excellent optical properties and higher lifetime.

Research of Single-Event Burnout in Power UMOSFETs

This brief presents 2-D numerical simulation results of single-event burnout (SEB) in power UMOSFETs (trench-gate MOSFET) and investigates hardening solutions to SEB such as carrier lifetime reduction, emitter doping decrease, and p+ plug modification. We find that the linear energy transfer (LET) does not have an important influence on the occurrence of SEB. In addition, we present the effect of a varied ion strike position, and the result is that the position in the middle of the neck is easier to SEB than the other positions. In addition, the single-event gate rupture (SEGR) threshold voltages in different LETs are given in order to compare with SEB.

Fabrication of a Cu(InGa)Se2Thin Film Photovoltaic Absorber by Rapid Thermal Annealing of CuGa/In Precursors Coated with a Se Layer

Cu(InGa)Se2 (CIGS) thin film absorbers are prepared using sputtering and selenization processes. The CuGa/In precursors are selenized during rapid thermal annealing (RTA), by the deposition of a Se layer on them. This work investigates the effect of the Cu content in precursors on the structural and electrical properties of the absorber. Using X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, and Hall effect measurement, it is found that the CIGS thin films produced exhibit facetted grains and a single chalcopyrite phase with a preferred orientation along the (1 1 2) plane. A Cu-poor precursor with a Cu/() ratio of 0.75 demonstrates a higher resistance, due to an increase in the grain boundary scattering and a reduced carrier lifetime. A Cu-rich precursor with a Cu/() ratio of 1.15 exhibits an inappropriate second phase () in the absorber. However, the precursor with a Cu/() ratio of 0.95 exhibits larger grains and lower resistance, which is suitable for its application to solar cells. The deposition of this precursor on Mo-coated soda lime glass substrate and further RTA causes the formation of a MoSe2 layer at the interface of the Mo and CIGS.

A neutron radiation-hardened superluminescent diode

We present a novel superluminescent diode (SLD) with high optical performances for hardened neutron irradiation. The degradation of the light output from the SLDs is caused by a reduction of the minority carrier lifetime resulting from displacement damage after high-energy neutron irradiation. The SLDs with a higher pre-irradiation light output will be less sensitive to radiation. We have selected an InGaAsP/InP multi-quantum well (MQW) as the active region structure for its performance, its high optical gain and minute active region. Graded-index separate-confinement-heterostructure (GRIN-SCH) has been applied for the waveguide structure. A specific absorbing region and anti-reflective coatings have been designed and optimized. Moreover, the radiation test results indicate that the SLD with an InGaAsP/InP MQW structure has better neutron hardening ability than the SLD with DH structures after a 6 × 1013 − 1 × 1014 n/cm2 1 MeV neutron irradiation.

Laser induced lifetime degradation in p-type crystalline silicon

Pulsed, green laser irradiation of uncoated p-type silicon leads to a significant reduction of the effective minority carrier lifetime. The reason for the lifetime drop lies in the introduction of recombination centres into the laser melted and recrystallized surface layer, leading to a low local minority carrier lifetime τ ≈ 10 ns inside this surface layer. The laser treatment introduces the impurities oxygen, carbon and nitrogen into the silicon and further leads to an n-type doping of the surface layer. There are strong indications that these impurities are responsible for the observed n-type doping, as well as the lifetime reduction after irradiation. Both effects are removed by thermal annealing. An estimate shows that the low local lifetime does nevertheless not affect the performance of industrial or contacted selective solar cell emitter structures.

Thermal annealing effect on the properties of GaBiAs

AbstractThe influence of thermal annealing on optical properties and carrier lifetime of dilute GaBixAs1‐x layers was investigated. Thick (0.5‐1.5 μm) relaxed and thin (30‐100 nm) strained bismide layers were grown onto semi‐insulating (100)‐oriented GaAs substrates by molecular beam epitaxy (MBE) using As4 and As2, respectively. The post‐growth annealing was carried out in the rapid thermal annealing (RTA) oven at the temperatures ranging from 550 °C to 750 °C under nitrogen atmosphere. The high resolution X‐ray diffraction (HRXRD), photoluminescence (PL) and optical pump‐THz probe measurements revealed two different GaBixAs1‐x anneal mechanisms. The annealing at temperatures higher than 600 °C caused a significant reduction of carrier lifetime of relaxed bismide layers with x &gt; 0.05. Meanwhile, thick layers with a smaller content of Bi (x &lt; 0.05) and thin strained layers annealed up to 600 °C still exhibited long carrier lifetimes. On the other hand, the significant photoluminescence signal increase at room and liquid nitrogen temperatures was observed for the GaBiAs layers grown using As2 (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)