Excess Carrier Lifetime Research Articles

Microstructural, electrical, and optoelectronic properties of BaSi2 epitaxial films grown on Si substrates by close-spaced evaporation

BaSi2, an earth-abundant photovoltaic material with a limiting efficiency of 32 %, has the interesting property that grain boundaries do not significantly degrade the film properties. In this study, we investigated the microstructure and its effect on the electrical and optoelectronic properties of the BaSi2 films grown by close-spaced evaporation. Microstructural analysis confirmed the (100)-oriented epitaxial growth on both Si(100) and Si(111) substrates. In addition to conventional epitaxial relationships, lattice matching with the BaSi2(01¯3) plane parallel to Si(110) was observed on Si(100) substrates. The density of the epitaxial domain boundary was found to decrease with increasing film thickness, increasing growth temperature, and using Si(100) substrates instead of Si(111). Hall effect measurements revealed the conductivity type transition at a temperature between 900 and 1000 ∘C and a decreasing tendency of carrier density with film thickness. There was no obvious correlation between mobility and grain boundary density, consistent with previous reports. Carrier recombination kinetics were evaluated by the microwave-detected photoconductivity decay method and the photoconductivity measurement under constant optical irradiation. Both analyses revealed weak negative correlations between excess carrier lifetime and grain boundary density, which clarified the superiority of large-grained BaSi2 films for photovoltaic applications.

A Review of Minority Carrier Recombination Lifetime Measurements

Abstract: The recombination lifetime of minority carriers is a critical parameter in semiconductor devices such as photovoltaic cells since it controls the efficiency of such devices. Many techniques have been developed to accomplish recombination measurements and thereby test semiconductor devices' efficiencies. Recombination lifetime average values differ according to semiconductor device type; thus, choosing an appropriate technique is important. This paper studies the concept of excess minority carrier lifetime and its calculations. It also investigates the advantages, limitations, and capabilities of the most common recombination lifetime measurement techniques. A chart was drawn with all known measurement methods to make it easier to understand the relation between these techniques.

Field Stop (FS) Type IGBT Transient Temperature Characteristics: E-Thermal Simulation Approach

This research presents an electro-thermal simulation method for analyzing the transient temperature characteristics of Field Stop (FS) type Insulated Gate Bipolar Transistors (IGBTs). The method combines IGBT working principles and semiconductor physical principles to investigate the influence of internal excess carrier lifetime on the IGBT's transient temperature behavior. By conducting actual tests on an FS type IGBT switching transient working process, the research establishes an electro-thermal simulation method with improved accuracy and simplified parameter calculation.

Simultaneous Characterization of Optical, Electronic, and Thermal Properties of Perovskite Single Crystals Using a Photoacoustic Technique

Metal halide perovskite possesses many excellent properties beneficial to its potential applications in optoelectronics and pyroelectricity. Surface recombination velocity, electronic diffusivity, and excess carrier lifetime accounting for the photoexcited carrier recombination and electronic transport features are key for improving the performance of perovskite-based optoelectronic devices. Meanwhile, the thermal conductivity and thermal diffusivity in halide perovskite have been paid limited attention despite their potential practical applications such as heat management and thermoelectric materials. To date, lots of techniques have been developed to extract these physical properties, while very few of them can effectively and nondestructively receive the results. The photoacoustic (PA) technique based on photothermal conversion is a powerful method to study the optical, electronic, and thermal properties of various materials, especially semiconductor materials. Optical absorption spectrum, surface recombination velocity, electronic diffusivity, photoexcited carrier lifetime, and thermal diffusivity can be obtained simultaneously without the destruction and contact of the samples. Here, for the first time, we utilized the PA technique in the perovskite single crystal. Optical absorption of MAPbBr3 and MAPbI3 (MA = methylammonium) single crystals was investigated under a reflection detection configuration (RDC), and the electronic and thermal properties were measured under a transmission detection configuration (TDC). Comparing the results with previous reports and other characterizations, the PA technique has been verified to be an efficient and convenient method to study the optical, electronic, and thermal properties of perovskite single crystals.

Twist Angle Effects on the Absorbance, Carrier Lifetime, and Diffusion Properties in Low Dimension Type‐II MoS2/WS2 Heterobilayers

AbstractAtomically thin two‐dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for photovoltaic (PV) applications. Their self‐terminated nature and strong absorption characteristics introduce an unprecedented possibility for high voltages to bandgap ratios, with secondary benefits including the potential for high internal quantum efficiencies/low recombination and strong absorption coefficients coupled with stability in a range of environments. However, despite the promise of such material systems, their PV performances still lag behind the conventional 3D materials. In principle, one possible way to manipulate the behavior of a 2D heterobilayer structure is to change its interlayer twist angle. In this study, the effects of twist angle between vertically stacked type‐II MoS2/WS2 heterobilayers on fundamental optical properties such as light absorbance, excess carrier lifetime, and diffusion are reported. These properties can have a direct effect on the final PV performance of these heterobilayers. It is found that the interlayer twist in MoS2/WS2 heterobilayers does not affect their absorbance. However, the carrier lifetime and photon emission across the heterobilayers are modulated with the interlayer twist. These findings could be useful to facilitate the optimization of monolayer TMD‐based optoelectronic devices.

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.

Carrier lifetimes in high-lifetime silicon wafers and solar cells measured by photoexcited muon spin spectroscopy

Photoexcited muon spin spectroscopy (photo-μSR) is used to study excess charge carrier lifetimes in silicon. Experiments are performed on silicon wafers with very high bulk lifetimes with the surface passivation conditions intentionally modified to control the effective lifetime. When the effective lifetime is low (<500 μs), implanting the muons to different depths enables the reliable measurement of carrier lifetime as a function of distance from a surface. It is also demonstrated that the photo-μSR technique can measure effective carrier lifetimes in completed commercial gallium doped silicon passivated emitter and rear cell devices, with results validated with harmonically modulated photoluminescence imaging. It is discovered, however, that prolonged muon irradiation of samples with very long effective lifetimes (>10 ms) results in detectable degradation of the measured lifetime. Re-passivation of degraded samples with a temporary room temperature superacid-based passivation scheme demonstrates that degradation occurs in the silicon bulk. Deep-level transient spectroscopy measurements reveal the existence of several defect-related traps near the muon-exposed surface in concentrations of order 1010 cm−3 that are not present near the surface not exposed to muons. In contrast to the common perception of the μSR technique, our results demonstrate that muons are not inert probes and that beam-induced recombination activity modifies the bulk lifetime significantly in samples with high effective carrier lifetimes.

Self-powered silicon PIN neutron detector based on triboelectric nanogenerator

Silicon PIN neutron detectors play an important role in nuclear science with advantages such as small size, light weight, low cost, and mature manufacturing process. This paper proposes a self-powered neutron detection system based on a triboelectric nanogenerator (TENG) and silicon PIN diode for the first time. The proposed environmentally efficient and low-cost TENG is connected to the detector through a rectifier bridge circuit. When the TENG powered silicon neutron detector is irradiated, the PIN diode generates defects in the silicon body, which leads to a decrease in the lifetime of excess carriers. The neutron dose can be measured by monitoring the change of the terminal voltage of the PIN diode.

Implications of grain boundaries on quasi-steady-state photoconductance measurements in multicrystalline and cast-mono silicon

Quasi-steady-state photoconductance is commonly used for measuring the effective lifetime of excess carriers in silicon wafers. A known artifact, unrelated to the sample's lifetime in eddy current based photoconductance measurements, is a strong increase in lifetime at low carrier densities. It is commonly observed in multicrystalline and cast-mono crystalline silicon. This artifact is often attributed to bulk defects changing their charge state and is referred to as trapping.

Calculation of Minority Carriers’ Lifetime in HgCdTe Structures by Using the Method Based on Mutual Correlation Between the Concentration of Electrical Particles

We perform a theoretical study of the bulk generation-recombination mechanisms, and the carrier lifetime in n- and p-type long- and mid-wavelength infrared HgCdTe structures at liquid nitrogen (LN) temperature has been done. This article is a continuation of our previous work and presents a method for calculating the excess minority carrier lifetime considering the mutual correlation between the concentration of electrons, holes, ionized impurities, and trap states resulting from the principle of charge conservation. It has been shown that recombination involving shallow impurity states in HgCdTe structures with low intrinsic concentration has a significant impact on minority carriers’ lifetime. This applies to both p- and n-type materials. The results of calculations of the carrier lifetime determined by the interband processes: radiative, Auger 1 and 7, recombination involving impurities, as well as and Shockley–Read–Hall (SRH) mechanisms related to mercury vacancies have been compared with experimental data presented by other authors. By analyzing the published calculations made by American researchers, the values of the overlap integrals <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert {F}_{1}{F}_{2} \vert $ </tex-math></inline-formula> for Auger processes were determined, which allowed to obtain a good compliance of our calculations with the experimental values. In this article, the cross sections for SRH recombination with the participation of mercury vacancies were estimated. These values are lower than those previously presented by other researchers. By verifying the results of the calculations with the experimental data, we confirmed the effectiveness of the proposed method.

On the Application of Lifetime-Equivalent Defect Densities on Solar Cell Level

The concept of lifetime-equivalent defect density based on lifetime measurements as a measure of electrically active defect density is extended to solar cells using Suns-V $_{\mathrm{oc}}$ measurements. It is found to be superior to previous approaches, and offers defect-specific injection-dependent information. However, the correct application of the presented method requires some knowledge of or reasonable assumptions on sample properties and measurement conditions, in particular, effective intrinsic carrier concentration, doping level, and measurement temperature. Furthermore, it is found that samples with high effective excess carrier lifetime can be susceptible to nonequilibrium related issues when the measurement duration is too short compared with lifetime. The use of series-resistance affected current-voltage curves is found to be error-prone.

Bright mid-infrared photoluminescence from high dislocation density epitaxial PbSe films on GaAs

We report on photoluminescence in the 3–7 µm mid-wave infrared (MWIR) range from sub-100 nm strained thin films of rocksalt PbSe(001) grown on GaAs(001) substrates by molecular beam epitaxy. These bare films, grown epitaxially at temperatures below 400 °C, luminesce brightly at room temperature and have minority carrier lifetimes as long as 172 ns. The relatively long lifetimes in PbSe thin films are achievable despite threading dislocation densities exceeding 109 cm−2 arising from island growth on the nearly 8% lattice- and crystal-structure-mismatched GaAs substrate. Using quasi-continuous-wave and time-resolved photoluminescence, we show that the Shockley–Read–Hall recombination is slow in our high dislocation density PbSe films at room temperature, a hallmark of defect tolerance. Power-dependent photoluminescence and high injection excess carrier lifetimes at room temperature suggest that degenerate Auger recombination limits the efficiency of our films, although the Auger recombination rates are significantly lower than equivalent III–V bulk materials and even a bit slower than expectations for bulk PbSe. Consequently, the combined effects of defect tolerance and low Auger recombination rates yield an estimated peak internal quantum efficiency of roughly 30% at room temperature, unparalleled in the MWIR for a severely lattice-mismatched thin film. We anticipate substantial opportunities for improving performance by optimizing crystal growth as well as understanding Auger processes in thin films. These results highlight the unique opportunity to harness the unusual chemical bonding in PbSe and related IV–VI semiconductors for heterogeneously integrated mid-infrared light sources constrained by tight thermal budgets in new device designs.

Delay of Regeneration by Adding Aluminum in Boron‐Doped Crystalline Si

Here, two B‐doped Cz‐grown Si materials with different Al concentrations are investigated concerning the long‐term behavior of excess charge carrier lifetime under injection at elevated temperature. By determining the defect density and the surface saturation current density, a delay in regeneration and a delay in the onset of surface‐related degradation is found in the material containing an order of magnitude more Al. Investigations under constant excess carrier concentration reveal that the effect of the delay is still significant, but less pronounced compared with constant generation conditions, so the effect causing the delay seems to be injection dependent. The findings can be explained by the higher activation energy for the splitting of Al−H pairs compared with splitting of B−H pairs, which might cause a delayed release in H from the dopant H configuration. Assuming that regeneration depends on this released H, the delay in regeneration can be explained by this model.

Epitaxial Ge-on-Nothing and Epitaxial Ge on Si-on-Nothing as Virtual Substrates for 3D Device Stacking Technologies

Two new templates for strain-relaxed detachable Ge are introduced. The first one is based on epitaxial growth of Ge on a Si-on-Nothing (SiON) platform while the second consists of Ge-on-Nothing (GeON). The SiON and GeON templates are obtained from the reorganization of macro-porous Si and Ge, respectively, fabricated on regular Si substrates using conventional lithography, dry-etching and annealing routines. Both templates contain similar densities of threading dislocations as typically obtained for conventional strain relaxed epitaxial Ge grown on bulk Si. In the GeON case, the excess carrier lifetime (32 ns) as extracted from time-resolved photoluminescence was > 4 times higher than the values measured for Ge grown on bulk Si with a similar thickness of the Ge layer. Both templates can be considered for novel device applications relying on thin Ge films and as starting material for layer transfer and 3-dimensional device stacking technologies, provided that a successful detachment from the parent substrate can be demonstrated. The reduction of non-radiative electron-hole recombinations makes the GeON also attractive for the fabrication of optical devices used in the fields of communication and imagers, such as Ge photodetectors and modulators, as lower dark currents are expected.

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.

Study on inertial characteristics of photoresistors in a physical workshop

The article describes a measuring complex based on the 32-bit STM32F103VET6 microcontroller for studying the lux-ampere (light), frequency and inertial characteristics of a photoresistor with different laws of recombination of nonequilibrium charge carriers arising under the influence of light. The developed complex is connected to a personal computer (smartphone) and allows to analyze the rise and fall edges of the photoresistor current, as well as to determine the lifetime of excess charge carriers at different illumination levels. Unlike traditional methods for measuring the parameters of photoresistors operating in a dynamic mode, the proposed measuring complex does not use an oscilloscope and a separate modulator of the luminous flux (generator or light interrupter), this made it possible to significantly reduce the size and cost of the complex, as well as automate the measurement process. To determine the frequency response of the photoresistor, it is proposed to use a frequency synthesizer, which allows, together with a digital-to-analog converter of the microcontroller, to form an amplitude-modulated light flux of the required frequency and modulation depth. The complex described in the work can be connected to the Internet using a Wi-Fi module based on an ESP8266 microcontroller, which allows conducting research in a remote mode. It is also possible to determine the parameters of the photoresistor at different values of the load resistance.

Epitaxial Ge-on-Nothing Virtual Substrates for 3D Device Stacking Technologies

We introduce a fabrication scheme for strain-relaxed Ge, epitaxially grown on a Si-on-Nothing (SiON) template formed from macro-porous Si. The SiON template is fabricated on regular Si substrates using conventional lithography, dry-etching and annealing routines. The material properties of the virtual Ge substrate grown on this detachable template are identical to those obtained for Ge grown on bulk Si. Strain-relaxed, suspended, and detachable Ge-on-Nothing (GeON) was fabricated using a similar fabrication scheme except for the epitaxial growth of Ge on Si, which was performed before the patterning of the substrate. For GeON, the extracted excess carrier lifetime using time-resolved photoluminescence (32 ns) was > 4 times higher than the values measured for Ge grown on bulk Si with a similar thickness of the Ge layer. Both templates can be considered as starting material for layer transfer and 3D device stacking technologies provided that successful detachment from the parent substrate can be demonstrated.

Effect of Electron Beam Excitation Pulses on the Lasing Threshold of the Laser Semiconductor Target

The dependence of the threshold intensity of the electron beam (EB) on the pulse shape and duration upon lasing excitation in a laser semiconductor target (ST) is considered. It is shown that a minimum EB threshold intensity can be reached before the pulse end in the case of sinusoidal pulses. The possibility of decreasing the EB threshold intensity and laser radiation duration under the ST excitation by a train of high-frequency EB pulses. The results obtained can be used in calculating the EB threshold intensity, to excite generation of subnanosecond laser radiation pulses for the time t ≤ τ, where τ is the lifetime of excess carriers in the ST.

Detailed description of the transient effect of radiative carrier transition in the bulk of semiconductor: The case of gallium antimonide

Upon solving the differential equation related to the rate of accumulation of photo-generated carriers in the respective bands of GaSb that involves radiative generation-recombination mechanisms under constant illumination in the entire bulk of the material, the transient response (time dependence) of the photo-generated carrier density and the excess carrier lifetimes during illumination and after switching-off illumination are described for a system under different injection levels. The rates of variations of the photo-generated carrier density and the radiative excess carrier lifetimes at the initial stages of the illumination and after switching-off the illumination are evaluated at different injection levels. The injection level dependences of the effective values of the photo-generated carrier density and the radiative excess carrier lifetimes are described. The competition of the thermal and the illumination effects in the band to band radiative recombination mechanism is also described thoroughly.

High efficiency photomodulators for millimeter wave and THz radiation

Photomodulators for mm-wave and THz radiation are an essential component for many imaging and signal processing applications. While a myriad of schemes have been devised to enhance photomodulation by enhancing the light-matter interaction, there has been less focus on the photoconductive materials themselves, which are often the limiting factor. Here, we present an approach to increase the photomodulation efficiency of silicon by orders of magnitude, using post treatment of off-the-shelf silicon wafers. The increase in efficiency removes the need for bulky and costly amplified laser sources, and creates the potential for compact and cost-effective modulators for real-world applications. By passivating the surfaces of long bulk-lifetime silicon wafers with Al2O3, the recombination of the photoexcited carriers at the surfaces is mostly eliminated. This results in vastly longer excess carrier lifetimes (up to ~50 ms), with corresponding increases in photoconductivity. The resulting modulators are highly efficient, with the transmission through them being reduced from ~90% to <10% over a narrow frequency band with a continuous wave excitation intensity of just 10 Wm−2, whilst modulation factors of greater than 80% can be achieved over a broad band with similar intensities. We also discuss the limitations of such long-lifetime modulators for applications where the switching speed or spatial resolution of a modulator may be critical.