Carrier Lifetime Measurements Research Articles

Investigation of physical and electrical properties of a suboxide layer at Si/Si-hexafluoride interface

The silicon suboxide SiOx (x < 2.0) offers promising industrial application possibilities ranging from electrodes in lithium-ion batteries, which are used widely in electrical vehicles and portable devices to sensing applications. Therefore, a low cost, environmental friendly and high performance silicon oxide materials are required for an appropriate operation of any electronic gadget. In this work, we report on the physical and electrical properties of a suboxide layer of up to 1 μm, which was grown on silicon during the formation of a dielectric layer, namely the ammonium silicon hexafluoride. It is a stable oxide exhibiting light emission from 400 to 1700 nm offering scalable and cost-effective large area processing capability. The measurement results reveal interesting properties, which are required to be understood clearly before proceeding with any suitable application. The results have been analyzed using state-of-the-art physical and electrical characterization techniques such as ellipsometry, AFM, SEM, FTIR, photoluminescence lifetime and resistive switching measurements to determine structural, optical and electrical properties. At 300 K the carrier lifetime measurements reveal the lifetime values ranging from about few tens of picosecond up to 4500 picoseconds. Scanning probe analysis indicate a surface roughness of about 30 Å. Resistive memory forming was observed also in these layers at relatively low power thresholds. We provide a comprehensive description of the physical and electrical properties in order to clarify the origin of the observed features. The wavelength dependent real ε1(ω)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\varepsilon }_{1}(\\omega )$$\\end{document} and the imaginary ε2ω\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\varepsilon }_{2}\\left(\\omega \\right)$$\\end{document} dielectric functions provided useful insights on optical properties. A lookout is given for the possible applications of this special SiOx dielectric oxide layer.

Mitigation of carrier trapping effects on carrier lifetime measurements with continuous-wave laser illumination for Pb-based metal halide perovskite materials

We investigated the impact of carrier trapping on the carrier lifetime of metal halide perovskite materials, which are key to solar cell production. We examined NH3CH3PbI3 (MAPbI3), NH3CH3PbBr3 (MAPbBr3), and CsPbBr3 using continuous-wave (CW) laser illumination during microwave photoconductivity decay (μ-PCD) measurements. Traditional pulsed light excitation falls short of mirroring solar cell operating conditions, owing to carrier trapping. Implementing CW laser illumination provides a more accurate estimation of the carrier lifetimes under operational conditions. With an increased photon flux from the CW laser, the μ-PCD decay curves changed, indicating reduced recombination via traps. The experiments revealed extended carrier lifetimes under continuous light for the MAPbI3 polycrystal. This suggests that CW lasers can mitigate trapping effects on carrier lifetime measurements. For the other samples, carrier trapping had a negligible effect on the measured carrier lifetimes. We believe that these findings will aid in the design of perovskite-based devices.

Passivation-free high performance self-powered photodetector based on Si nanostructure-PEDOT:PSS hybrid heterojunction

Hybrid heterojunction between organic PEDOT:PSS and inorganic Silicon (Si) nanostructures is promising for high-performance self-powered photodetectors due to their favourable band energetics and ease of processing. Traditionally, Si nanostructures require additional passivation layers to reduce surface defect states, which adds complexity and involves the use of harmful organic solvents. Here in, we demonstrate a simple chemical polishing process to reduce defects and fabricate a conformable heterojunction between Si nanostructures and PEDOT:PSS. Si nanostructures fabricated by metal-assisted chemical etching (MACE) technique and subsequently treated by the chemical polishing process are spin-coated with PEDOT:PSS to form heterojunction and employed as self-powered broadband photodetector. The optimized device shows superior performance, such as high responsivity of 555.34 mA/W, quick rise/fall times of 79 ms/81 ms, high External Quantum Efficiency (EQE) of 0.8 at zero bias (0 V) and high photostability up to 500 illumination cycles. Dark I-V characteristics and carrier lifetime measurements reveal that the enhanced performance of chemically polished devices is attributed to the formation of a conformable heterojunction and reduction in defects in the Si nanostructures. Given the scalability and simplicity of the demonstrated passivation-free approach, this work may aid the fabrication of high-performance hybrid Si nanostructured photodetectors.

Effect of mechanical stress on the traps in silicon nitride thin films

Silicon nitride (SiNx) films were prepared by plasma enhanced chemical vapor deposition (PECVD) and different traps were induced in the films by tuning the RF power ratio during film deposition. Different initial stresses were applied to study the relationship between the mechanical stress and trap density. In-situ stress measurements showed that the film densification occurred above 400 °C during the first thermal cycling. The trap density extracted by the capacitance-voltage measurements showed its lowest value (−0.10 × 1011 cm−2eV−1) at the initial compressive stress (−730 MPa) of the film, which was explained by the hydrogen elimination due to the ion bombardment during the deposition in the compressive film. The increase in trap density (1.60 × 1011 cm−2eV−1) for the tensile films (480 MPa) is attributed to the lowering of the ion bombardment, which increases the formation of Si-H bonds. Interestingly, the performed carrier lifetime measurements for the compressive stressed SiNx films are higher (1200 μs) than the tensile stressed SiNx films (750 μs) which validate the trap density calculations. Fourier transform infrared spectra and refractive index measurements showed that the Si/N ratio increased with the increasing initial stress. Overall, we predict that our current study facilitates the formation of highly endured nitride films for different electrical and optoelectronic applications.

Measuring low doping level and short carrier lifetime in indium arsenide with a contactless terahertz technique at room temperature

Non-destructive, reliable, and accurate measurements of low doping levels and carrier lifetimes in small direct-bandgap semiconductors such as indium arsenide (InAs) at room temperature are challenging due to intrinsic limitations of conventional experimental techniques. In this article, we introduce a contactless optical-terahertz (THz) pump–probe technique in a continuous-wave regime measuring low doping levels in the 1015−1016cm−3 range and lifetimes down to 8 ns at room temperature in InAs. We show that a single-parameter material model can be used to retrieve the free carrier density from THz transmission measurements of InAs layers. Additionally, we show that lifetime of photogenerated excess carriers can be measured by modulating this carrier density using an amplitude-modulated optical pump.

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

Simultaneous measurement of charge carrier concentration, mobility, and lifetime

Simultaneous measurement of charge carrier concentration, mobility, and lifetime

Kinetically regulated growth of Cs3Cu2I5 single-crystalline thin films for highly responsive and stable deep-ultraviolet photodetectors

The newly emerging copper halide Cs3Cu2I5 is a promising candidate for deep-ultraviolet photodetector applications. However, the device performance available today is plagued by massive grain boundaries in Cs3Cu2I5 polycrystalline films and unnecessary thicker volume in bulk single crystals. In this study, for the first time, highly crystalline and stable Cs3Cu2I5 single-crystalline thin films (SCFs) were prepared by facile supersaturation-controlled growth strategy, with adjustable thickness ranging from 45 nm to 27.9 µm. By in situ monitoring the growth process, the crystallization kinetics of Cs3Cu2I5 SCFs were elucidated, and the crystallization driving force comes from the slow inverse temperature nucleation and evaporation crystallization. Moreover, the substrate-independent growth feature of Cs3Cu2I5 SCFs is intriguing for the direct on-chip fabrication of diverse photoelectric devices. The structural and electric characterizations reveal that the Cs3Cu2I5 SCFs have a low trap density of 6.42 × 1011 cm−3, and the measured carrier lifetime and diffusion length are as long as 1.32 μs and 1.85 µm, respectively, which enable high deep-ultraviolet light detection capability of the proposed photoconductive detector based on Cs3Cu2I5 SCFs. Encouraged by the robust operating stability of the fabricated device, the above results obtained could catalyze further research on on-chip manufacturing of deep-ultraviolet photodetectors.

Silicon surface passivation with a-Si:H by PECVD: growth temperature effects on defects and band offset

The surface passivation of crystalline silicon (c-Si) is studied during growth of hydrogenated amorphous silicon (a-Si:H) by means of plasma-enhanced CVD. The surface passivation is characterized by an in situ method of the photocurrent measurement of c-Si during the growth of an a-Si:H passivation layer at various growth temperatures. The passivation is also characterized by an ex situ method of the carrier lifetime measurement performed at RT in air. According to both the in situ and ex situ characterization results, the surface passivation is optimized around a growth temperate of 200 °C, where the defect reduction and the band offset formation at the a-Si:H/c-Si interface play important roles.

Exploring hafnium oxide's potential for passivating contacts for silicon solar cells

We investigate the potential of ultra-thin HfO2 films grown by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness and post-deposition annealing temperature. A peak in passivation quality – as assessed by carrier lifetime measurements – is reported for 2.2 nm thick films annealed at 475 °C, for which a surface recombination velocity <1 cm/s is determined. For films <2.2 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic to amorphous HfO2 as film thickness decreases from 12 nm to 2.2 nm. Kelvin probe results indicate that as-deposited 2.2–12 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon post-deposition annealing in vacuum, all films exhibit a reduction in effective work function at temperatures coincident with the onset of passivation in air-annealed samples. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong post-deposition annealing temperature dependence, with the lowest resistance achieved below 375 °C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 °C). Limitations of the contact structure used are discussed.

Evaluation of electron lifetime for Te inclusions free CdZnTe

Te secondary phase defects are known as major obstacle for improving charge carriers transport properties of CdTe compounds material. The enhanced detector performance was reported for Te inclusions free CZT detector implemented through two-step annealing (i.e., first in Cd and second in Te). We applied the time-of-flight (TOF) technique to measure the electron lifetime in as-grown and two-step annealed CZT material. TOF has several advantages over Hecht equation fitting such as independence of the electric field distribution, direct measurement of carrier lifetime, and high accuracy for high mobility-lifetime product CZT material. The detector performance improved drastically for the two-step annealed CZT detector except which containing inordinate number and size of prismatic punching defects. The average electron lifetime increased more than 10 times after successful two-step annealing.

The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Laser-Patterned Alumina Mask and Mask-Less Dry Etch of Si for Light Trapping with Photonic Crystal Structures

Ultra-short 230 fs laser pulses of a 515 nm wavelength were tightly focused onto 700 nm focal spots and utilised in opening ∼0.4-1 μm holes in alumina Al2O3 etch masks with a 20-50 nm thickness. Such dielectric masks simplify the fabrication of photonic crystal (PhC) light-trapping patterns for the above-Lambertian performance of high-efficiency solar cells. The conditions of the laser ablation of transparent etch masks and the effects sub-surface Si modifications were revealed by plasma etching, numerical modelling, and minority carrier lifetime measurements. Mask-less patterning of Si is proposed using fs laser direct writing for dry plasma etching of Si.

Toward White Light Random Lasing Emission Based on Strained Nano Polygermanium Doped with Tin via Metal-Induced Crystallization (MIC)

Solubility of Sn in Ge network gives it a preference for photonic applications, because of the direct transition in GeSn alloy. Here, we employed the metal-induced crystallization (MIC) process of amorphous Ge and Si via Sn as a novel mechanism to incorporate Sn inside Ge and Si networks. (Al/Si/Sn/Ge/Sn) and (Al/Ge/Sn/Ge/Sn) multilayers are deposited by thermal vacuum evaporation on different substrates. The devices are annealed under low vacuum at 500 °C to incorporate the oxygen for band-gap tuning. The structure of Ge-doped nanocrystals is investigated. The direct transition and band-gap values have been estimated using diffuse reflectance spectroscopy and photoluminescence (PL) measurements. PL indicated that the junctions have emissions from visible to NIR regions that make them promising as optically pumped white-light sources as well as waveguide applications, and the impact of the base substrate on enhancing the emission has been investigated via PL measurements. Electroluminescence measurements show that the prepared heterostructures on fluorine-doped tin oxide (FTO) substrate have sharp random lasing spikes over the range of PL samples spectra as the sample can lase randomly by light scattering through the Ge-doped nanocrystalline materials. The charge carrier lifetime measurements show high lifetime for the prepared sample. These give them the chance to be a candidate for white-light random laser diode applications.

Femtosecond nonlinear optical response and minority carrier lifetime of F4TCNQ-doped bismuthene for optoelectronic and ultrafast photonic applications

Bismuthene has received a lot of interest because of its unusual electronic and photonic properties. To widen the use of bismuthene in optoelectronic and ultrafast photonic devices, its electronic and optical characteristics should be tuned. Herein, for the first time, a strong p-type molecular dopant, such as tetrafluoro-tetracyanoquinodimethane (F4TCNQ), is used to achieve efficient surface charge transfer doping (SCTD) of bismuthene. The minority carrier lifetime measurements of bismuthene shifted to 6.5 µs after doping, confirming that F4TCNQ has the opportunity to boost bismuthene's p-type conductivity due to an unusual charge transfer interaction between bismuthene and F4TCNQ molecules. The thermally generated intermolecular charge transfer influence between bismuthene and F4TCNQ resulted in an improved nonlinear optical and optical limiting response, which was estimated by femtosecond z-scan measurements. Our findings suggest that SCTD is an appropriate means for tuning the linear and nonlinear optical properties of bismuthene via modulating their band gap and minority carrier lifetime. This alternating approach expands its potential in electronic, optoelectronic, and ultrafast photonic devices.

Improvement of amorphous silicon/crystalline silicon heterojunction solar cells by light-thermal processing

It has been known that amorphous silicon/crystalline silicon heterojunction solar cells (HJT cells) can be improved by light-thermal processes. The present work aims to acquire a further understanding of the effect of illumination intensity and temperatures in a broader range and the roles of light and heat in the improvement. Naked HJT cells without grids were used for more reliable measurement of minority carrier lifetimes. Experiments of light-thermal processes of the naked HJT cells, with lighting from red LED arrays and heating/cooling from a thermal stage/air fan, have been carried out, with the illumination intensity ranging in 1–8 kW/m2 and cell temperature ranging in 160–220 °C. The higher illumination intensity results in longer lifetimes of the cells, the optimum cell temperature appeared to be around 180 °C, with the lower temperature needing a longer processing time to reach peak lifetime increment, while the higher temperature tends to degrade the passivation of the cell. The cell temperature of 180 °C was then chosen for the light-thermal processes of commercial HJT cells of ∼23.8% efficiency on a pilot line, with 12 kW/m2 red illumination. An average efficiency gain of 0.77%abs and a maximum gain of 0.87%abs have been achieved, with a processing time of 80 s. It is encouraging that the light intensity required is much lower than the previously reported level for similar improvement, indicating much lower power consumption for industrial application of the process. Through semi-shading experimentation of the light-thermal process, it is postulated that the effect of illumination does not rely on the direct effect of the injected photons, but possibly resulted from the effect of light-generated carriers.

Impact of doped barriers on the recombination coefficients of c-plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). Despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

Origins of p-Doping and Nonradiative Recombination in CsSnI3.

It is commonly believed that the spontaneous p-doping in Sn-based perovskites is caused by Sn vacancies. By performing rigorous first-principles calculations for a prototypical Sn-based perovskite CsSnI3 , we reveal that, in fact, the defects dominating p-doping are Cs vacancies. The reason that adding extra Sn2+ could reduce p-doping is that Cs and Sn present the same changing trend in terms of chemical potentials, and thus inhibiting the formation of Sn vacancies will also limit the formation of Cs vacancies. Moreover, we show that I vacancies are the dominant nonradiative recombination centers, and can result in sizable nonradiative losses, which explains why the experimentally measured carrier lifetime is only a few nanoseconds even if p-doping is suppressed. This work provides new insights into the origins of p-doping and nonradiative recombination in CsSnI3 , and suggests that minimizing the formation of Cs and I vacancies is critical to realizing the best device performance.

Origins of p‐Doping and Nonradiative Recombination in CsSnI3

AbstractIt is commonly believed that the spontaneous p‐doping in Sn‐based perovskites is caused by Sn vacancies. By performing rigorous first‐principles calculations for a prototypical Sn‐based perovskite CsSnI3, we reveal that, in fact, the defects dominating p‐doping are Cs vacancies. The reason that adding extra Sn2+ could reduce p‐doping is that Cs and Sn present the same changing trend in terms of chemical potentials, and thus inhibiting the formation of Sn vacancies will also limit the formation of Cs vacancies. Moreover, we show that I vacancies are the dominant nonradiative recombination centers, and can result in sizable nonradiative losses, which explains why the experimentally measured carrier lifetime is only a few nanoseconds even if p‐doping is suppressed. This work provides new insights into the origins of p‐doping and nonradiative recombination in CsSnI3, and suggests that minimizing the formation of Cs and I vacancies is critical to realizing the best device performance.

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.