Minority Carrier Trapping Research Articles

Effects of Transition Energy on Intra-Band Photoluminescence of Zinc Oxide (ZnO) Semiconductor under Low injection Level

This paper presents the effects of the transition energies on photoluminescence intensities in Zinc Oxide compound semiconductor due to the intra-band transition of free carriers. The excitation of free carriers from the valence band to conduction band and from different localized state to the conduction band by the illumination of sufficient energy is considered. A theoretical model for minority carrier trapping is also investigated to explain the dependence of the photoluminescence on the trap energy. Variation of photoluminescence intensities along with localized state energy and transition energy is considered at different temperatures. As temperature increases the photoluminescence due to the transition of free electrons from the conduction band to the valence band, from the conduction band to the localized states and from the localized states to the valence band are increasing.

Minority Carrier Trap in n-Type 4H–SiC Schottky Barrier Diodes

We present preliminary results on minority carrier traps in as-grown n-type 4H–SiC Schottky barrier diodes. The minority carrier traps are crucial for charge trapping and recombination processes. In this study, minority carrier traps were investigated by means of minority carrier transient spectroscopy (MCTS) and high-resolution Laplace-MCTS measurements. A single minority carrier trap with its energy level position at Ev + 0.28 eV was detected and assigned to boron-related defects.

Deep defect levels in CuInSe2 single crystals using DLTS, MCTS and photoacoustic spectroscopy

In this paper, we investigate the presence of deep defect levels in as-grown p-conducting CuInSe2 single crystals using deep level transient spectroscopy, minority carrier transient spectroscopy and photoacoustic spectroscopy. The samples were cut from the middle of ingots grown by employing a vertical displaced Bridgman furnace. This material is attracting increasing interest in solar cells fabrication as the absorber layer. Aluminum Schottky contacts were deposited on the as-grown and etched material in an evaporation system pumped down to less than 10−6 mbar. Three majority carrier traps were observed using deep level transient spectroscopy, and one minority carrier trap was detected by minority carrier transient spectroscopy. The concentrations of the traps have been estimated. The results hence obtained are compared to deep defect levels observed in the measured spectral dependence of the samples prior to junction’s fabrication by using the technique of photoacoustic spectroscopy in the photon energy range 0.6 to 1.3 eV. The study shows a good agreement between the results of DLTS/MCTS to those of photoacoustic and to published data.

Time-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence

ABSTRACT Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley–Read–Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation.

Method to extract parameters of power law for nano-scale SiON pMOSFETs under negative bias temperature instability

This paper proposes a fast and accurate method to extract parameters of the power law for nano-scale SiON pMOSFETs under negative bias temperature instability (NBTI), which is useful for an accurate estimation of NBTI lifetime. Experimental results show that accurate extraction of the time exponent n of the power law was obstructed by either fast trapping of minority carriers or damage recovery during measurement of threshold voltage Vth. These obstructing effects were eliminated using ΔVths obtained from fast and slow measurement-stress-measurement (MSM) procedures. The experimental SiON pMOSFETs had n ≈ 1/4, an activation energy Ea = 0.04 eV for the fast recoverable degradation, and Ea = 0.2 eV for the slow permanent degradation. Based on these experimental observations, a method to estimate NBTI lifetime is proposed.

Conductive poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonate) polymer glue as an ohmic and rectifying electrical contact for H‐terminated n‐Si and p‐Si wafers

AbstractAn organic conductive glue based on a blend of poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and d‐sorbitol was examined for laminating conductors to crystalline silicon. The PEDOT:PSS glue functions as a high‐work‐function solution processable conductor and exhibits an ohmic contact on p‐type silicon and a rectifying contact on n‐type silicon. Under illumination, the n‐Si/PEDOT:PSS:d‐sorbitol junctions exhibit current–voltage characteristics suggesting minority carrier trap states, leading to charge recombination at the silicon/polymer interface. Conductive glue for laminating to crystalline silicon is desirable for making electrical contacts to flexible materials such as molecular semiconductors, graphene or transparent conductive oxides. These materials could eliminate the need for metal contacts to the front face of silicon solar cells. Conductive glue could prove especially useful for laminating to textured silicon or novel micro‐ or nanostructured silicon materials. © 2018 Society of Chemical Industry

Mobility overestimation due to minority carrier injection and trapping in organic field-effect transistors

Abstract Donor–acceptor (D–A) polymers are promising candidates as semiconductors in flexible transistors because of their high mobility. However, in recent studies on D–A polymer transistors, device characteristics deviate from the idealized transistor model, which is commonly used for extracting mobility, and show an abrupt turn-on in the drain current when measured as a function of gate voltage. To examine the validity of the reported mobility values of D–A polymers, we investigate the detailed mechanism of the nonideal electrical characteristics by device simulation based on a split-channel model, where the gate voltage swing on the drain side is higher than that on the source side. We demonstrate that minority carriers injected into low-bandgap D–A polymers (electrons in the case of usual D–A polymers) at high drain voltages and low gate voltages cause insufficient pinch off, resulting in an overestimation of mobility if the conventional mobility extraction method is used. A more reliable method for estimating mobility from the nonideal transfer characteristics is developed and applied to recent reports. It was determined that the mobility values of D–A polymers in the recent reports were overestimated by a factor of 1.7–94 or more.

Verification of minority carrier traps in Cu(In,Ga)Se2 and Cu2ZnSnSe4 by means of time-resolved photoluminescence

The decay of the room-temperature time-resolved photoluminescence (TRPL) on thin-film semiconductors such as Cu(In,Ga)Se2 and Cu2ZnSnSe4 often is bi-exponential. This can be traced back either to fluctuations of the electrostatic potential or to minority charge carrier trapping. We show by means of simulations that both effects can be discriminated by a measurement of the TRPL decay at different excitation intensities and temperatures. Application of the standard semiconductor theory yields, that the bi-exponential photoluminescence decay in Cu(In,Ga)Se2 and Cu2ZnSnSe4 must result from a strong minority carrier trapping. By simulation of experimental TRPL decay curves we can determine the minority carrier lifetime, the trap energy, the trap density, and the doping density of these materials with values in the ranges of 1–10ns, 200meV, 1015 −1016cm−3, and 1015 −1017cm−3. These yield reasonable solar cell parameters and they also explain the non-correlation of the open-circuit voltage and the luminescence decay time.

Potassium post deposition treatment of solution-processed kesterite solar cells

Potassium post deposition treatment (K-PDT) has been a major breakthrough for CIGSSe (Cu(In,Ga)(S,Se)2) solar cells yielding world record efficiencies. K-PDT yields significant improvements in open circuit voltage, fill factor and allows thinning CdS thus leading to efficiencies above 20% for CIGSSe material. Here we present a similar K-PDT approach for solution processed kesterite solar cells. K-PDT improves the open circuit voltage of kesterite solar cells, however a severe blocking of the short circuit current and reduction of fill factor reduces the overall device performance. Furthermore, quantum efficiency measurements indicate that K-PDT alters the properties of the CdS layer whereas time-resolved photoluminescence measurements exhibit an additional third exponential component, which can be attributed to minority carrier trapping at the interface.

Extrinsic photoresponse enhancement under additional intrinsic photoexcitation in organic semiconductors

Dual light beam photoresponse experiments are employed to explore the photoresponse under simultaneous extrinsic and intrinsic photoexcitation of organic semiconductors. The photoresponse of a red modulated light extrinsic photoexcitation is found that can be significantly enhanced under an additional blue bias-light intrinsic photoexcitation in two terminal pentacene films on glass substrates. From the frequency resolved photoresponse, it is deduced that the phenomenon of photoresponse enhancement can be attributed to an increase in the extrinsic photogeneration rate of the red modulated light and/or an improvement of the drift velocity of carriers under an additional blue light intrinsic photoexcitation. The possible predominant extrinsic photogeneration mechanism, which can be compatible with the observed dependence of the photoresponse enhancement on the frequency and on the light intensities of the red and blue light excitation, is the singlet exciton dissociation through electron transfer to acceptor-like traps. Moreover, an improvement in the drift velocity of carriers traversing grain boundaries with potential energy barriers, which may be reduced by trapping of minority carriers created from the intrinsic photoexcitation, may partly contribute to the photoresponse enhancement.

Effects of Internal Gain and Illumination-Induced Stored Charges in MgZnO Metal–Semiconductor–Metal Photodetectors

The effects of the internal gain originated from the trapping of minority carriers and illumination-induced stored charges have been investigated systematically in MgZnO metal–semiconductor–metal (MSM) photodetectors (PDs) by a proposed semianalytical equivalent circuit model with the basic semiconductor equations and the external parasitic elements taken into account. To verify the accuracy and effectiveness of this model, the simulated steady and transient results are compared with the reported experimental data. It is found that a fall time of 400 ns, much longer than the response time with no internal gain, is attributed to the hole trapping process, inducing an internal gain with a value of 181. In addition, for the high input optical pulse energy (above 10 pJ), the time-delay of the response is caused by the stored charges. This paper indicates that it is very important to improve the MgZnO thin-film quality and balance the optical gain with the response speed. Meanwhile, a reasonable compromise should be found between the energy level of the optical pulse and the bias voltage to minimize the stored charges effect for practical applications. This paper is helpful for the design of high-speed and high-power MgZnO MSM PDs.

Electrical mechanisms for carrier compensation in homoepitaxial nonpolar m-ZnO doped with nitrogen

By combining photoluminescence, capacitance-voltage profiling and deep level optical spectroscopy, the optical and electrical signatures of the deep levels induced by N in MBE-grown homoepitaxial m-ZnO layers are identified and correlated to different physical origins. The films are electrically compensated, with carrier concentrations that decrease from ∼1 · 1016 cm−3 to ∼2 · 1015 cm−3 as a result of increasing N incorporation. Regardless of the presence of N, an intrinsic trap is found in all films at EV + 0.25 eV, most likely related to VZn defects. More interestingly, N induces three new deep levels close to the valence band whose bandgap position is electrically observed to be at EV + 0.48 eV, EV + 0.17 eV and EV + 0.12 eV. The deepest trap at EV + 0.48 eV correlates well with a N-induced level observed in previous studies on both ZnMgO and ZnO films. The EV + 0.17 eV trap behaves as a minority (hole) carrier trap, and can be uniquely correlated with the acceptor level involved in the N-induced DAP emission observed in the photoluminescence spectra. Finally, the shallowest level at EV + 0.12 eV shows an electrical signature completely distinguishable from the EV + 0.17 eV level, and dominates the deep level spectra after N-incorporation with a trap concentration of ∼1.2 · 1015 cm−3.

Specific features of nonequilibrium depletion accompanied by the trapping of minority carriers by surface states in metal–insulator–semiconductor structures

Specific features (abrupt changes in current caused by voltage variation) of nonequilibrium depletion in metal–insulator–semiconductor structures based on gallium arsenide and silicon with a 40-nm-thick film of yttria-stabilized zirconia were revealed. These features may help extend the range of application of the nonequilibrium depletion effect in microelectronics.

High-performance organic broadband photomemory transistors exhibiting remarkable UV-NIR response.

The electrical and optical properties of organic semiconductors have improved rapidly in recent years, rendering them highly promising for various optoelectronic applications owing to low-cost and lightweight potential in combination with spectral tunability and long photocarrier lifetimes. Organic photomemory has emerged as an innovative application to achieve optical data storage. However, practical operation requires universal device design with broader spectral response in terms of related materials, interfaces and architecture, a task that remains a significant challenge. Herein, we present a universal strategy to fabricate organic broadband photomemories featuring remarkable UV-NIR response, thereby providing optical switching ability with a controllable memory window. To the best of our knowledge, this study demonstrates an excellent performance with the broadest response spectra and the highest photomemory efficiency of up to 593%. We systematically study the charge trapping mechanism and photoinduced injection enhancement by combining an energy level model with theoretical calculations, characterizing conceivable photogenerated minority carrier trapping and accumulation kinetics. Thus, it is anticipated that the proposed approach will be a starting point for further research, resulting in high-performance organic photomemory ideal for digital commutation between optical and electric signals.

Investigation of long lifetimes in Cu(In,Ga)Se2 by time-resolved photoluminescence

The main objective of time-resolved photoluminescence (TRPL) is to characterize minority carrier recombination in semiconductors. However, trap states in the band gap can lead to artificially long decay times thus distorting the measured minority carrier lifetime. In this work, we propose to measure TRPL under elevated temperature and excitation in order to reduce minority carrier trapping. Taking three Cu(In,Ga)Se2 layers as examples, we show that the decay time decreases with increasing temperature—in accordance with simulations. Under increasing excitation, the decay time can become smaller due to trap saturation but also can become larger due to asymmetric hole and electron lifetimes. By comparison of simulation and experiment, we can find the energy, the density, and the electron capture cross-section of the trap which in the present example of Cu(In,Ga)Se2 films gives values of ∼200 meV, ∼1015 cm−3, and ∼10−13 cm2, respectively.

Design rules for organic donor-acceptor heterojunctions: pathway for charge splitting and detrapping.

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge-transfer state formed on a molecular donor-acceptor pair. These charge-transfer states are strongly bound by Coulomb interactions and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.

An accurate method for calibrating photoluminescence-based lifetime images on multi-crystalline silicon wafers

We present a method for converting photoluminescence images into carrier lifetime images for silicon wafers with inhomogeneous lifetime distributions, such as multi-crystalline silicon wafers, based on a calibration factor extracted from a separate, homogeneous, mono-crystalline calibration wafer and simple optical modelling of the photoluminescence signal from both the calibration wafer and the test wafer. The method is applicable to planar wafers with uniform carrier profiles depth-wise. A multi-crystalline wafer is used to demonstrate the difference between the conventional calibration approach, where the photoluminescence signal is calibrated against a quasi-steady-state photoconductance measurement on the test sample itself, and our proposed method. The lifetimes calibrated by our method are consistent, in contrast with the lifetime calibrated by the conventional approach, in which the magnitude and injection-dependence of the lifetime is observed to be sensitive to the choice of reference area. The error in the conventional calibration method mainly originates from measurement artifacts in the quasi-steady-state photoconductance measurements on multi-crystalline wafers, which we propose to be mainly due to minority carrier trapping, radial sensitivity of the quasi-steady-state photoconductance sensor coil and overestimation of the carrier mobility sum. We also show that the proposed new method is effectively insensitive to the lifetime, doping density, reflectance and wafer thickness of the calibration wafer (provided it is below 500µm).

Detection of minority carrier traps in p-type 4H-SiC

Contrarily to the case of n-type 4H-SiC, very little is known about the presence of minority carrier traps in p-type epilayers. In this study, we performed the electrical characterization of as-grown, electron irradiated, and thermally oxidized p-type 4H-SiC, by using minority carrier transient spectroscopy. Four minority carrier traps are reported in 1.6–2.3 eV energy range above the valence band edge (EV). Particular emphasis is given to the mid-gap minority carrier trap (EH6∕7) and to its correlation to an energetically close mid-gap majority carrier trap (HK4).

Minority Carrier Transient Spectroscopy of As-Grown, Electron Irradiated and Thermally Oxidized p-Type 4H-SiC

A total of nine electrically active levels have been detected in as-grown and electron irradiated p-type 4H-SiC epilayers. These traps are found in the 0.32-2.26 eV energy range, above the valence band edge (EV). Of these, six are majority carrier traps whereas three are minority carrier traps. We found that thermal oxidation affects the concentrations of two midgap levels, the majority carrier trap, labeled HK4 and the minority carrier trap identified as EH6/7. The analysis of the irradiation energy and dose dependence of the concentration of these two traps, rules out the possibility that they may share the same origin.

Anomalously high lifetimes measured by quasi-steady-state photoconductance in advanced solar cell structures

Quasi-Steady-State Photoconductance is widely used in photovoltaics industry to measure the effective minority carrier lifetime of silicon wafers, a key material parameter affecting final solar cell efficiency. When interpreting photoconductance based lifetime measurements, it is important to account for various artefacts that can cause an over-estimation of the carrier lifetime, such as minority carrier trapping. This paper provides experimental evidence for another artefact in photoconductance lifetime measurements, affecting samples that have a conductive layer that is interrupted by lines of the opposite polarity doping, forming laterally alternating regions of p/n doping. This structure often appears in the emitter region of samples used to monitor the lifetime of interdigitated back contact cells. The cause of this artefact is linked to a reduction in the measured dark conductance. Experimental data are presented that suggest this is due to the formation of a phototransistor type structure on the samples surface, resulting in variations in conductivity under different illumination levels.