Carrier Lifetime Measurements Research Articles

Impact of different SiO2 diffusion barrier layers on lifetime distribution in multi-crystalline silicon ingots

Abstract Three different SiO2-based barrier layers, used in industry for prevention of metallic impurity diffusion into directionally solidified silicon ingots, were investigated in detail by G1 experiments in order to find out the most relevant barrier properties responsible for the blocking behaviour. Minority carrier lifetime and interstitial iron measurements on the grown silicon ingots show significant differences between the barriers regarding the red zone extension as well as the maximum lifetime/minimum iron content in the ingot centre. Structural analysis of the barriers by optical microscopy and Raman spectroscopy reveal a clear correlation to the porosity and thickness of the barrier layers. Further, indications were found that the barriers become effective only during the process and that this should occur as early as possible to obtain an optimal impact of the barrier. Only one of the tested barriers shows an overall positive impact on the lifetime, coming close to using a high purity quartz glass SiO2 crucible instead of the standard ceramic SiO2 crucible.

Many-Body Effects in Strongly Disordered III-Nitride Quantum Wells: Interplay Between Carrier Localization and Coulomb Interaction

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly-disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the 6-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of InGaN quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight in the physics of modern light-emitting diodes.

Effects of damages induced by indium-tin-oxide reactive plasma deposition on minority carrier lifetime in silicon crystal

A reactive-plasma deposition (RPD) process damage is evaluated for an indium-tin-oxide (ITO)/SiO2/Si structure in terms of the recombination properties at the interface. To calculate the surface recombination velocity (SRV), the defect density at the SiO2/Si interface (Dit), the effective total charge (Qtot) which is the sum of the charges in the SiO2 and the trapped charge at the SiO2/Si interface, and the workfunction of ITO at the ITO/SiO2 interface (ϕITO), are extracted by capacitance-voltage (C-V) analysis as a function of a postdeposition annealing (PDA) temperature. The Dit and Qtot significantly increase by the RPD process to 1.4×1012 cm-2eV-1 and 1.5×1012/q cm-2, respectively, and decrease by the PDA at 200 °C in N2 ambient to 1.4×1011 cm-2eV-1 and 1.1×1011/q cm-2, respectively. The ϕITO also varies from 4.80 eV to 4.35 eV by the PDA. The correlation is examined between the SRV values calculated based on the extended SRH model using the extracted Dit, Qtot, and ϕITO values [SRV(CV)], and the SRV ones obtained from the minority carrier lifetime measurements [SRV(LT)]. It is found that multiple sets of capture cross-sections were required for SRV(CV) values to coincide with SRV(LT) ones. The effect of variation in the Qtot and ϕITO on the band bending is almost canceled out in the present PDA condition. It is thus considered that the increase in Dit is the main cause to decrease the minority carrier lifetime.

Phonon-Suppressed Auger Scattering of Charge Carriers in Defective Two-Dimensional Transition Metal Dichalcogenides.

Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain ab initio simulations. The simulations show that phonon-driven trapping competes successfully with the Auger process. On the one hand, trap states are relatively close to band edges, and phonons accommodate efficiently the electronic energy during the trapping. On the other hand, trap states localize around defects, and the overlap of trapped and free carriers is small, decreasing carrier-carrier interactions. At low carrier densities, phonons provide the main charge trapping mechanism, decreasing carrier lifetimes compared to defect-free samples. At high carrier densities, phonons suppress Auger processes and lower the dependence of the trapping rate on carrier density. Our results provide theoretical insights into the diverse roles played by phonons and Auger processes in TMDs and generate guidelines for defect engineering to improve device performance at high carrier densities.

Minority carrier lifetime in indium doped silicon for photovoltaics

AbstractFor photovoltaics, switching the p‐type dopant in silicon wafers from boron to indium may be advantageous as boron plays an important role in the light‐induced degradation mechanism. With the continuous Czochralski crystal growth process it is now possible to produce indium doped silicon substrates with the required doping levels for solar cells. This study aims to understand factors controlling the minority carrier lifetime in such substrates with a view to enabling the quantification of the possible benefits of indium doped material. Experiments are performed using temperature‐dependent Hall effect and injection‐dependent carrier lifetime measurements. The recombination rate is found to vary linearly with the concentration of un‐ionized indium which exists in the sample at room temperature due to indium's relatively deep acceptor level at 0.15 eV from the valence band. Lifetime in indium doped silicon is also shown to degrade rapidly under illumination, but to a level substantially higher than in equivalent boron doped silicon samples. A window of opportunity exists in which the minority carrier lifetime in degraded indium doped silicon is higher than the equivalent boron doped silicon, indicating it may be suitable as the base material for front contact photovoltaic cells.

Role of Impurities in Silicon Solidification and Electrical Properties Studied by Complementary In Situ and Ex Situ Methods

All silicon (Si) ingot fabrication processes share challenges to control grain structure, defect, and impurity contamination during the solidification step to improve the material properties. The final grain structure and inherent structural defects issued from the solidification step are responsible for the photovoltaic (PV) properties for a large part, all the more as they are often associated with impurity distribution. Impurities play a major role as they not only can modify the development of the grain structure formation but can also interact with structural defects creating the regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes are selected and analyzed as‐grown or observed by X‐ray imaging during resolidification from as‐grown seeds. The growth features and relative crystallographic orientation of neighbor grains are characterized. Moreover, minority carrier lifetime measurements are performed and correlated with the growth features. The complementarity of the different techniques improves the understanding of phenomena at stake during the formation of grains and twins, the effect of impurities, and their impact on photovoltaic properties. The results show the significant influence of light and metallic impurities, such as copper, on the grain structure and on the electrical properties.

Light-induced lifetime degradation effects at elevated temperature in Czochralski-grown silicon beyond boron-oxygen-related degradation

The effect of ‘Light and elevated Temperature Induced Degradation’ (LeTID) of the carrier lifetime is well known from multicrystalline silicon (mc-Si) wafers and solar cells. In this contribution, we perform a series of carrier lifetime measurements to examine, whether the same effect may also be observable in boron-doped Czochralski-grown silicon (Cz-Si). The Cz-Si samples of our study are illuminated (i) at room temperature, (ii) under standard regeneration conditions eliminating the boron-oxygen (BO) related defect (i.e. at 185 °C) and (iii) at a temperature of 80 °C, typical for the examination of the LeTID effect in mc-Si. We observe the typical decay of the carrier lifetime due to the activation of the BO-related defect. Beyond the BO degradation, applying standard solar cell processes, there is no indication for the activation of a second defect. On samples, whose surfaces are passivated by fired hydrogen-rich silicon nitride layers, an additional bulk lifetime degradation effect on a long timescale is observed in the Cz-Si material. However, defect generation rate and injection dependence of the lifetime suggest another defect type than the mc-Si-specific LeTID defect. We conclude that by applying processing steps that trigger LeTID in mc-Si, the same defect does not occur in the Cz-Si samples examined in this study. On a long timescale, however, a hitherto unknown type of defect is activated, which is different from the mc-Si-specific LeTID defect. A careful differentiation between the various kinds of recombination centres which may form during illumination at elevated temperatures is hence of utmost importance.

Minority Carrier Lifetime Measurements on 4H-SiC Epiwafers by Time-Resolved Photoluminescence and Microwave Detected Photoconductivity

Temperature dependent microwave detected photoconductivity MDP and time-resolved photoluminescence TRPL were employed to investigate the carrier lifetime in CVD grown 4H-SiC epilayers of different thickness. The minority carrier lifetime may be found from both theMDP and defect PL decay at room temperature for all epilayers, whereas the near bandedge emission (NBE) decay is much faster for thin epilayers (<17 μm) due to the substrate proximity and only follows the minority carrier lifetime for thicker samples at lower excess carrier concentrations.

Infrared optical anisotropic properties and photocarriers lifetime in 2D black phosphorus by angle-resolved absorption and reflectometry

In this article, angle and polarization-resolved absorption and time-resolved pump-probe measurements have been carried out on multilayer black phosphorus (BP) at infrared wavelength (around 1550 nm), which is an extension of the previous studies of the BP behavior done at higher frequency [Lazar et al., Nanoscale 10(19), 8979–8988 (2018)]. Exfoliated down to a few layers, BP reveals the properties of a 2D material with anisotropic response, high mobility (up to 10 000 cm2/V/s), and absorption, which are very relevant to optoelectronics. BP bandgap is tunable according to the number of layers, allowing applications from UV to IR. The bandgap height of the single and thicker layers has been measured through absorption and reflection spectra. We report the measurement of the ambipolar carrier lifetime, which has been determined to be about 15 ps. The structure-induced anisotropy is studied. The results confirm BP as a suitable material for ultrafast devices such as microwave photoswitches, samplers, terahertz antenna, for instance.

End Group Tuning in Acceptor–Donor–Acceptor Nonfullerene Small Molecules for High Fill Factor Organic Solar Cells

AbstractHigh fill factors have only recently become commonplace in nonfullerene‐based organic solar cells, with the balance of charge carrier mobilities often cited as the contributing factor. Here an end‐group modification to a commonly used nonfullerene acceptor (O‐IDTBR) is reported, in which the rhodanine end groups are replaced with dicyano moieties, resulting in the acceptor O‐IDTBCN. This new acceptor affords significant improvement in the fill factor (73%) and photocurrent (19.8 mA cm−2) in organic solar cells with the low bandgap polymer PTB7‐Th. A narrowing of the bandgap, as a result of greater push–pull hybridization, allows complementary absorption to the donor and thus improved photon harvesting. Additionally, the measurement of charge carrier mobilities and lifetimes in both systems reveal that the PTB7‐Th:O‐IDTBCN blend possesses more balanced charge carrier mobilities, and longer lifetimes. Morphology studies reveal a slightly greater degree of molecular mixing of the O‐IDTBCN when blended with PTB7‐Th, despite the greater and more balanced charge carrier mobilities in this blend.

Terahertz pulse emission from GaInAsBi

Quaternary GaInAsBi alloy epitaxial layers were grown on InP substrates with 6% Bi. It was found that the thick layers remain fully strained. The measured carrier lifetimes were of the order of a few picoseconds. The terahertz (THz) emission was investigated using a GaInAsBi layer as an unbiased surface emitter and as a substrate for photoconductive antenna. It was observed that fabricated THz emitters were sensitive to the optical pulses with wavelengths longer than 2 μm. The demonstrated spectral characteristics of THz pulses obtained when using an Er-doped fiber laser for photoexcitation were comparable with those observed in other emitters used for THz-time-domain spectroscopy systems.

Reassessments of Minority Carrier Traps in Silicon With Photoconductance Decay Measurements

In photoconductance (PC) based carrier lifetime measurement, artificially high values at low-to-medium injection levels are often observed because of the presence of minority carrier traps in the sample. In this paper, we demonstrate that the traps in n -type Czochralski silicon are in a transient regime under the typical quasi-steady-state measurement condition. The trap-associated apparent lifetime in such a transient regime is different from the one in the steady-state condition; thus, traps should be analyzed according to their actual measurement condition. We demonstrate and subsequently propose a method to extract the majority carrier capture the cross section of traps from transient PC decay measurements. Based on different assumptions, the proposed method is complementary to the method proposed by Hornbeck and Haynes. The proposed method is demonstrated with transient PC measurements of a trap in n -type Czochralski silicon wafers.

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method.

This work presents a protocol employing the microwave photoconductivity decay (μ-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the μ-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the μ-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

Measurement of carrier lifetime in micron-scaled materials using resonant microwave circuits

The measurement of minority carrier lifetimes is vital to determining the material quality and operational bandwidth of a broad range of optoelectronic devices. Typically, these measurements are made by recording the temporal decay of a carrier-concentration-dependent material property following pulsed optical excitation. Such approaches require some combination of efficient emission from the material under test, specialized collection optics, large sample areas, spatially uniform excitation, and/or the fabrication of ohmic contacts, depending on the technique used. In contrast, here we introduce a technique that provides electrical readout of minority carrier lifetimes using a passive microwave resonator circuit. We demonstrate >105 improvement in sensitivity, compared with traditional photoemission decay experiments and the ability to measure carrier dynamics in micron-scale volumes, much smaller than is possible with other techniques. The approach presented is applicable to a wide range of 2D, micro-, or nano-scaled materials, as well as weak emitters or non-radiative materials.

Evaluation of impedance spectroscopy as a tool to characterize degradation mechanisms in silicon photovoltaics

Advancements in photovoltaic technologies are hindered by degradation mechanisms such as potential induced degradation (PID) and current-induced degradation (CID). In this work, impedance spectroscopy is used to examine passivated emitter and rear cell (PERC) silicon modules with PID and CID. A comparison between control and degraded modules is done to identify key differences in the impedance spectra and determine the extent of the degradation. PID was observed at the module level as a dramatic reduction in shunt resistance, with a small amount of spatial inhomogeneity present in the degradation. It was found that accurate characterization of CID via measurement of the minority carrier lifetime requires a high bias voltage at the module level that exceeds the capabilities of a standard impedance spectrometer. Because of this, CID was also examined at the cell level, where reductions in minority carrier lifetimes could be accurately measured. A correlation between the reduction in minority carrier lifetime due to CID and a reduction in the power conversion efficiency was observed. Thus the PID and CID mechanisms studied here induce unique changes in the impedance spectroscopy results, making them distinguishable and quantifiable. Finally, the ability to mitigate CID through the use of different silicon wafers and a current induced regeneration process was characterized by impedance spectroscopy.

Influence of additional insulation block on multi-crystalline silicon ingot growth process for PV applications

Directional solidification process is the main process for producing the multi-crystalline silicon ingots. It is economically beneficial to grow the good quality mc-Si ingots with lower power consumption. Here we have carried out numerical simualtion for analysing the impact of addition of an insulation block on the DS furnace. The addition of the insulation block has been implemented experimentally and the quality of the grown ingot was evaluated by the minority carrier lifetime measurement. Though the temperature gradient and hence the growth rate are higher in the insulation block added DS furnace, the generated dislocation density has not increased beyond the acceptable limit 108 1/m2. The power used by the DS furnace has been reduced considerably by the addition of the insulation block. The good quality mc-Si ingot with lower power consumption has been obtained as a result of insulation block addition.

H2O/O2 Vapor Annealing Effect on Spin Coating Alumina Thin Films for Passivation of Silicon Solar Cells

Aluminum acetylacetonate-based AlOx thin films were introduced as a low-cost, high-quality passivation layers for crystalline silicon solar cells. Films were formed by a spin coating method on p-type silicon substrates at 450°C in ambient air, O2, or water vapor (H2O/O2) for 15 or 120 min. XPS analysis confirms the AlOx formation and reveals a high intensity of interfacial SiOx at the AlOx/Si interface of processed wafers. Ambient H2O/O2 was found to be more beneficial for the activation of introduced AlOx passivation films which offers high lifetime improvements with a low thermal budget. Carrier lifetime measurements provides that symmetrically coated wafers reach 119.3 μs and 248.3 μs after annealing in ambient H2O/O2 for 15 min and 120 min, respectively.

Quantum Efficiency of III-Nitride Emitters: Evidence for Defect-Assisted Nonradiative Recombination and its Effect on the Green Gap

Carrier lifetime measurements reveal that, contrary to common expectations, the high-current non-radiative recombination (droop) in III-Nitride light emitters is comprised of two contributions which scale with the cube of the carrier density: an intrinsic recombination --most likely standard Auger scattering-- and an extrinsic recombination which is proportional to the density of point defects. This second droop mechanism, which hasn't previously been observed, is likely caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point defect recombinations, in turn causing an increase in the extrinsic droop process. It is proposed that this effect leads to the green gap, and that point defect reduction is a strategy to both vanquish the green gap and more generally improve quantum efficiency at high current.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)