Reduction Of Carrier Lifetime Research Articles

Suppression of stacking-fault expansion in 4H-SiC diodes by helium implantation

Abstract Bipolar degradation is a critical problem in Silicon carbide devices, caused by the expansion of single Shockley stacking faults (1SSFs) from basal plane dislocations. This paper presents the effects of helium implantation on the suppression of 1SSFs expansion. The fabricated PiN diodes were analyzed using current–voltage characteristics and electroluminescence imaging. The results show that helium implantation can effectively suppress 1SSFs expansion without significantly degrading diode performance. We consider that this suppression is due to the pinning of dislocations by point defects introduced during helium implantation, and the reduction of carrier lifetime may also play a role in suppressing 1SSFs expansion.

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr3 Perovskite Nanocrystals.

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

The role of hydrogen in nonradiative recombination in CsPbI3: a first-principles investigation

Defect-induced nonradiative recombination is the main factor hindering efficiency improvement in CsPbI3 perovskite solar cells. It has been recently claimed that the unintentionally incorporated H impurity can potentially cause nonradiative losses due to the deep levels. Using first-principles approaches, we show that, as a matter of fact, the H impurity has a negligible effect on carrier recombination in CsPbI3 due to its small nonradiative capture coefficient and low density. This insight rationalizes why the addition of hydriodic acid does not reduce carrier lifetime and could prevent acidic additives being discarded as potential candidates for assisting film growth.

Enhanced Modulation Bandwidth by Integrating 2D Semiconductor and Quantum Dots for Visible Light Communication

Light‐emitting devices present a tremendous potential for visible light communication (VLC) due to their dual functionality as both communication and lighting devices. Herein this study, the significant enhancement in VLC modulation bandwidth by integrating two‐dimensional (2D) semiconductor and quantum dots (QDs) emitter is reported. Generally, the modulation bandwidth of CdSe‐based QDs is limited to only less than 25 MHz; however, with the proposed hybrid emitter, a maximum modulation bandwidth of 130 MHz for CdSe/ZnS QDs emitter is able to be achieved. The WSe2 monolayer is integrated into an Au–nanorod–decorated CdSe/ZnS QDs emitter to achieve high modulation performance. The modulation bandwidth of the hybrid QD–Au–WSe2 emitter (130 MHz) is found to be higher than those of the pristine QDs and QD–WSe2 heterostructure without Au nanorods (79 and 91 MHz, respectively). A significant increase is observed in the transition rate of QDs excitons when they are integrated with Au nanorods and WSe2 monolayer, which is substantiated by a reduction in average carrier lifetime from time‐resolved photoluminescence analysis. This approach and the findings open an opportunity to apply 2D semiconductors into the next‐generation miniature VLC devices, for high‐speed optical communications.

Is carrier mobility a limiting factor for charge transfer in TiO[formula omitted]/Si devices? A study by transient reflectance spectroscopy

TiO2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiOx, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 104 cm s−1 for TiO2 heat-treated at 300 °C, and even faster for amorphous TiO2 if the interfacial SiOx layer was removed using HF before TiO2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO2 away from the TiO2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO2 due to back recombination via the interfacial SiOx, which is detrimental to TiO2/Si device performance.

Cat-CVD SiN x as a gas barrier for application to perovskite solar cells

We investigated a silicon nitride (SiN x ) film prepared by catalytic chemical vapor deposition as a gas barrier for further improving stability of mixed-cation-halide perovskite (PVK) Cs0.05FA0.85MA0.1PbI2Br under dark air conditions. The SiN x film can be formed on the PVK capped with carrier transport layers such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxylphenyl)amino]−9,9′-spirobifluorene (Spiro-OMeTAD) and phenyl-C61-butyric-acid-methyl-ester (PCBM)/aluminum-doped-zinc-oxide (AZO) with less degradation in its optical transmittance property and crystal structure. The PVK/Spiro-OMeTAD encapsulated by SiN x exhibits a slower reduction in average carrier lifetime after storage for 330 h at room temperature with ∼65%RH. The PVK/PCBM/AZO covered with SiN x shows a small blue-shift (8–10 nm) in the absorption band-edge of PVK and with less decrease in the transmittance in the long wavelength range for ∼500 h at 85 °C with ∼2%RH. These results demonstrate the effect of SiN x in preventing the degradation of PVK due to the interactions with moisture and oxygen in the air.

Radiation effects in ultra-thin GaAs solar cells

Ultra-thin solar cells are of significant interest for use in space due to their intrinsic radiation tolerance, which may allow them to be used in particularly harsh radiation environments, where thicker cells would degrade rapidly and enable reduction in cover glass thickness to reduce launch mass. In this study, devices with an 80 nm GaAs absorber layer were irradiated with 3 MeV protons. It is shown that integrated light management in these ultra-thin devices offers enhanced efficiency, in addition to extended lifetime through radiation resilience. Time-resolved cathodoluminescence is employed to map the introduction of radiation-induced defects with increasing proton fluence and characterize a decrease in carrier lifetime from 198 ± 5 ps pre-radiation to 6.2±0.6 ps, after irradiation to 2×1014 cm−2 fluence. Despite the substantial reduction in carrier lifetime, short-circuit current does not degrade up to a proton fluence of 1 × 1015 cm−2, beyond which a collapse in short-circuit current is observed. This exposure correlates with the point at which the carrier lifetime, extrapolated from cathodoluminescence, becomes comparable to the transit time for carriers to cross the ultra-thin device. Variation in current–voltage behavior with carrier lifetime and fluence shows that the recombination statistics are similar to those of a Shockley–Read–Hall single deep-level trap model, but that bimolecular recombination does not fully describe the observed behavior. An implication of these highly radiation tolerant cells for space power systems is shown to offer significant savings in cover glass mass, compared with a thicker cell.

Suppression of stacking fault expansion in a 4H-SiC epitaxial layer by proton irradiation

SiC bipolar degradation, which is caused by stacking fault expansion from basal plane dislocations in a SiC epitaxial layer or near the interface between the epitaxial layer and the substrate, is one of the critical problems inhibiting widespread usage of high-voltage SiC bipolar devices. In the present study, we investigated the stacking fault expansion behavior under UV illumination in a 4H-SiC epitaxial layer subjected to proton irradiation. X-ray topography observations revealed that proton irradiation suppressed stacking fault expansion. Excess carrier lifetime measurements showed that stacking fault expansion was suppressed in 4H-SiC epitaxial layers with proton irradiation at a fluence of 1 × 1011 cm−2 without evident reduction of the excess carrier lifetime. Furthermore, stacking fault expansion was also suppressed even after high-temperature annealing to recover the excess carrier lifetime. These results implied that passivation of dislocation cores by protons hinders recombination-enhanced dislocation glide motion under UV illumination.

Ultra-small size (1–20 μ m) blue and green micro-LEDs fabricated by laser direct writing lithography

Ultra-small micro-LEDs are essential for next-generation display technology. However, micro-LEDs below 5 μm have been seldom reported. In this work, we demonstrate InGaN-based blue and green micro-LEDs from 1 to 20 μm by using laser direct writing lithography. The 1-μm blue micro-LEDs show a peak external quantum efficiency of 13.02%, which is 9.57% for green ones. By characterizing the size-dependent external quantum efficiency and simply assuming that this variety is dominantly determined by the dry-etching induced dead zone, we deduce that the dead zone sizes of carrier injection at the edge of chips are 0.18 and 0.15 μm in blue and green ones, respectively. A time-resolved photoluminescence measurement also shows that carrier lifetime reduction at the edge of blue ones is more serious than that of green ones, reflecting the easier carrier lateral diffusion in the former than the latter. These results exhibit the ability of laser direct writing lithography on micro-LED fabrication and also provide a reference for predicting the limit of their chip size scaling-down.

Investigation and Mitigation of Sputter Damage on Co‐Evaporated Cu(In,Ga)Se2 Absorbers for Photovoltaic Applications

Herein, the detrimental impact of radio frequency (RF)‐sputtering on bare Cu(In,Ga)Se2 photovoltaic absorbers in view of vacuum‐deposited buffer layers is evaluated, and the possible mitigation strategies are explored. Carrier lifetimes are measured using time‐resolved photoluminescence before and after buffer deposition and exposure to the plasma environment. When directly applied on bare Cu(In,Ga)Se2 absorbers, RF‐sputtering is severely limiting the device performance, with oxygen ions emanating from the target having a stronger effect than argon ions from the process gas. The possibilities to avoid sputter damage by atomic layer deposited Zn1−xMgxO and chemical bath deposited CdS buffer layers are shown, and the ability of the latter to restore damaged surfaces is highlighted. Absorber performance is also investigated for absorbers stored in air, N2, or ultra‐high vacuum. The reduction in carrier lifetime is reflected in the reduced open‐circuit voltage of solar cells. SCAPS simulations are used to investigate possibilities to minimize the effect of sputter damage. Finally, options on how to minimize sputter damage during buffer deposition as well as on how to modify the absorber to be less sensitive are discussed.

Control of Polaronic Behavior and Carrier Lifetimes via Metal and Anion Alloying in Chalcogenide Perovskites

Transition-metal perovskite chalcogenides (TMPCs) have emerged as lead-free alternatives to lead-halide perovskites and have been currently of increasing interest for optoelectronic applications because of their suitable band gaps, high carrier mobility, strong light absorption, and high stability. Here, we systematically report a study on the effects of Ti- and Se-alloying strategies on polaron behavior and carrier lifetimes in nonradiative recombination. Although such alloying can effectively tune the band gap of BaZrS3, we observe localized small polaron formation upon Ti alloying and large polarons generating in Se alloying. Ti-alloying strengthens the electron-phonon coupling, leading to a reduced carrier lifetime. Remarkably, Se-alloying weakens the electron-phonon coupling and prolongs the nonradiative electron-hole recombination lifetime by up to 60% compared to that in pristine BaZrS3 material. The simulations rationalize the difference in carrier lifetimes in TMPC alloys and provide guidelines for further improvements in TMPC-based photoelectronic devices.

Gettering in PolySi/SiOx Passivating Contacts Enables Si-Based Tandem Solar Cells with High Thermal and Contamination Resilience.

Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as the bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high-temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe), and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 °C, we show that increasing the heavily doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by 1 order of magnitude, with final lifetimes above 500 μs uniformly across areas up to 20 cm2. In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

Single Event Burnout Hardening Technique for High-Voltage p-i-n Diodes With Field Limiting Rings Termination Structure

High-voltage p-i-n diodes for space applications are sensitive to single event burnout (SEB), which may cause the failure of the entire electronic system. This article describes the SEB failure mechanism of high-voltage p-i-n diodes with field limiting rings (FLRs) termination structure, based on the electro-thermal coupled transient simulation model in the Sentaurus TCAD simulator. The simulation results show that the catastrophic failures result from the intense local heating at the edge of the main junction, because the high electric field strength at this region induces strong impact ionization of the charge carriers. The strike at the edge of the main junction shows the strongest sensitivity to SEB since the energetic particle-induced carriers are deposited exactly at the high electric field region. Then, a novel SEB hardening technique of localized carrier lifetime control at the edge of the main junction region is proposed based on the analysis on the failure mechanism. Our simulation results demonstrate that the reduction of the localized carrier lifetime can alleviate the peak electric field and reduce the local heat accumulation, thereby improving the SEB performance of high-voltage p-i-n diodes.

Simulation of Ionizing/Displacement Synergistic Effects on NPN Bipolar Transistors Irradiated by Mixed Neutrons and Gamma Rays

Transistors working in complex radiation environments such as space are simultaneously irradiated by neutrons and gamma rays. But the mechanism of the synergistic radiation effect between the two rays is still unclear. Based on TCAD, the synergistic radiation effects of ionizing/displacement damage caused by mixed neutrons and gamma rays are simulated. The results demonstrate that the synergistic effects are more serious than the simple sum of the two radiation effects due to their mutual enhancement. The change of the carrier recombination rate in the device at different positions shows that the displacement effects increase the peak value of surface recombination rate; meanwhile, the ionizing dose effects enhance the recombination process in bulk silicon. The mechanism of this phenomenon is that positive charges from the oxide layer and interface enhance the recombination of carriers in bulk, and the reduced carrier lifetime caused by defects from bulk makes carriers more likely to be trapped by the interface traps. In addition, the simulation result which shows the influence of temperature on the synergistic effects indicates that the synergistic effects are more sensitive to the lower temperature.

Long‐Term Degradation of Passivated Emitter and Rear Contact Silicon Solar Cell under Light and Heat

Advanced designs enable high‐efficiency solar cells; however, more complex structures create new long‐term stability concerns. Herein, the long‐term degradation processes affecting advanced silicon solar cells using laboratory‐based illumination and heating over hundreds of hours are investigated. The activation energy for the degradation of voltage is estimated and the degradation rates to normal solar cell operating temperature ranges are extrapolated. The cell degradation observed at high temperatures in the lab is kinetically similar to the process affecting field‐deployed modules contributing to 0.37% year−1 of annualized degradation. Electroluminescence and photoluminescence mapping show that the degradation is dominated by minority carrier lifetime reduction. Suns−open‐circuit voltage and light beam‐induced current results indicate that the degradation could result from passivation degradation at the surface or defect formation in the near‐subsurface region, leading to increased minority carrier recombination. This work highlights a long‐term degradation process under elevated temperature and illumination that may continue to affect cells in an irreversible manner that is separate from recoverable light‐induced degradation and light‐ and elevated temperature‐induced degradation.

Temperature and band gap dependence of GaAsBi p-i-n diode current–voltage behaviour

The dark current characteristics of two series of bulk GaAsBi p-i-n diodes are analysed as functions of temperature and band gap. Each temperature dependent measurement indicates that recombination current dominates in these devices. The band gap dependence of the dark currents is also consistent with recombination dominated current for the devices grown at a common growth temperature, indicating that the presence of Bi does not directly adversely affect the dark currents. However, the devices grown at different growth temperatures exhibit a faster increase in dark current with decreasing device band gap, suggesting that a reduced growth temperature causes a reduction in minority carrier lifetime.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Metal contamination of silicon from the furnace atmosphere after crystallization

The contamination of silicon by the furnace atmosphere is investigated based on lab-scale annealing experiments in a carbon-based furnace. The analysis comprises measurements of charge carrier lifetime and interstitial iron as well as elemental analysis. An approach to obtain information on the concentration of the metal species involved in the contamination process based on charge carrier lifetime profiles is established. A significant reduction of the charge carrier lifetime below the sample surface is observed. This reduction is governed by iron and at least one (cobalt and/or nickel) faster diffusing species. The contributions from the furnace parts and the purging gas are investigated and possibilities to reduce the gas phase contamination are elaborated. The results indicate a possible relevance of gas phase contamination in crystallization experiments.

Reliability of PERC Solar Cells under Alpha Irradiation: A Simulation-Based Study for Space Applications

The effects of alpha particles’ irradiation on the performance of simplified PERC silicon solar cells are studied through technology computer aided design (TCAD) simulations. The methodology adopted in this work incorporates the non-ionizing energy loss (NIEL) phenomenon to understand the radiation effects, which does not depend on extensive irradiation experimentations. Simulation results show that alpha particles in the range of ∼10 MeV and with fluence >1014 cm−2 lead to maximum performance degradation of the solar cell. Moreover, it is found that upon irradiating the solar cell with 1 MeV alpha particles, the output power reduces proportionally with increasing fluence due to the reduction of minority carrier lifetime in the base. On the other hand, the fill factor (FF) degrades by only <10% up to a fluence of ∼1013 cm−2, after which it decreases sharply. This trend reveals that the FF deteriorates significantly only when the irradiation process creates a Frenkel pair density that approaches the majority carrier density in the base. The methodology presented in this work opens new research avenues for potentially optimizing the solar cell design not only for better efficiency but also for improved reliability in radiation rich environments.

Electrical Characterization of Thermally Activated Defects in n-Type Float-Zone Silicon

Float-zone (FZ) silicon is usually assumed to be bulk defect-lean and stable. However, recent studies have revealed that detrimental defects can be thermally activated in FZ silicon wafers and lead to a reduction of carrier lifetime by up to two orders of magnitude. A robust methodology which combines different characterization techniques and passivation schemes is used to provide new insight into the origin of degradation of 1 Ω·cm n-type phosphorus doped FZ silicon (with nitrogen doping during growth) after annealing at 500 °C. Carrier lifetime and photoluminescence experiments are first performed with temporary room temperature surface passivation which minimizes lifetime changes which can occur during passivation processes involving thermal treatments. Temperature- and injection-dependent lifetime spectroscopy is then performed with a more stable passivation scheme, with the same samples finally being studied by deep level transient spectroscopy (DLTS). Although five defect levels are found with DLTS, detailed analysis of injection-dependent lifetime data reveals that the most detrimental defect levels could arise from just two independent single-level defects or from one two-level defect. The defect parameters for these two possible scenarios are extracted and discussed.