Deep Level Defects Research Articles

Photoinduced Current Transient Spectroscopy on Metal Halide Perovskites: Electron Trapping and Ion Drift.

Metal halide perovskites (MHPs) are disruptive materials for a vast class of optoelectronic devices. The presence of electronic trap states has been a tough challenge in terms of characterization and thus mitigation. Many attempts based on electronic spectroscopies have been tested, but due to the mixed electronic-ionic nature of MHP conductivity, many experimental results retain a large ambiguity in resolving electronic and ionic charge contributions. Here we adapt a method, previously used in highly resistive inorganic semiconductors, called photoinduced current transient spectroscopy (PICTS) on lead bromide 2D-like ((PEA)2PbBr4) and standard "3D" (MAPbBr3) MHP single crystals. We present two conceptually different outcomes of the PICTS measurements, distinguishing the different electronic and ionic contributions to the photocurrents based on the different ion drift of the two materials. Our experiments unveil deep level trap states on the 2D, "ion-frozen" (PEA)2PbBr4 and set new boundaries for the applicability of PICTS on 3D MHPs.

Influence of trap-assisted and intrinsic Auger–Meitner recombination on efficiency droop in green InGaN/GaN LEDs

We study the impact of deep-level defects on trap-assisted Auger–Meitner recombination in c-plane InGaN/GaN LEDs using a small-signal electroluminescence (SSEL) method and deep-level optical spectroscopy (DLOS). Carrier dynamics information, including carrier lifetime, recombination rate, and carrier density, is obtained from SSEL, while DLOS is used to obtain the deep-level defect density. Through fitting the nonradiative recombination rates of wafers with different deep-level defect densities, we obtain the Shockley–Read–Hall (SRH) and trap-assisted Auger–Meitner recombination (TAAR) coefficients. We show that defect-related nonradiative recombination, including both SRH and TAAR, accounts for a relatively small fraction of the total nonradiative recombination, which is dominated by intrinsic Auger–Meitner recombination. The interplay between carrier localization and Coulomb enhancement has a different impact on radiative and intrinsic Auger–Meitner recombination. Evidence is presented that the imbalance between the change of radiative and intrinsic Auger–Meitner recombination is the primary cause of the efficiency droop at high carrier densities in the samples studied.

Oxygen vacancies and local amorphization introduced by high fluence neutron irradiation in β -Ga2O3 power diodes

Beta phase gallium oxide (β-Ga2O3) is emerging as a promising material for space applications due to its unique properties and potential high performance in extreme environments. In this work, we systematically study the impact of β-Ga2O3 Schottky barrier diodes (SBDs) under a high fluence neutron irradiation to explore the degradation mechanism of the devices. After irradiated by neutrons with an average energy of 1–2 MeV and a dose rate of 1.3 × 1012 cm−2 s−1, SBDs with a homoepitaxial layer suffered serious performance degradation. The main manifestation of this degradation was a substantial increase in on-resistance, which rose from 3.9 to 3.5 × 108 mΩ·cm2 under the aforementioned irradiation conditions. The appearance of amorphous/polycrystalline striped lattice damage in the epitaxial layer as well as the presence of deep-level defects caused by oxygen vacancies are factors related to this phenomenon. The simulation revealed that the capture reaction of neutrons and Ga elements is the primary cause of neutron irradiation. This reaction generates high-energy beta- particles (β-particles) resulting in the formation of defects. This paper reveals the degradation mechanism of β-Ga2O3 SBDs under neutron irradiation and provides a possible design roadmap for radiation-resistant β-Ga2O3 power devices. Moreover, a high-temperature oxygen annealing process was implemented, which proved to be in restoring the device performance.

Enhancement in the Efficiency of Sb2 Se3 Solar Cells by Triple Function of Lithium Hydroxide Modified at the Back Contact Interface.

The efficiency of antimony selenide (Sb2 Se3 ) solar cells is still limited by significant interface and deep-level defects, in addition to carrier recombination at the back contact surface. This paper investigates the use of lithium (Li) ions as dopant for Sb2 Se3 films, using lithium hydroxide (LiOH)as a dopant medium. Surprisingly, the LiOH solution not only reacts at the back surface of the Sb2 Se3 film but also penetrate inside the film along the (Sb4 Se6 )n molecular chain. First, the Li ions modify the grain boundary's carrier type and create an electric field between p-type grain interiors and n-type grain boundary. Second, a gradient band structure is formed along the vertical direction with the diffusion of Li ions. Third, carrier collection and transport are improved at the surface between Sb2 Se3 and the Au layer due to the reaction between the film and alkaline solution. Additionally, the diffusion of Li ions increases the crystallinity, orientation, surface evenness, and optical electricity. Ultimately, the efficiency of Sb2 Se3 solar cells is improved to 7.57% due to the enhanced carrier extraction, transport, and collection, as well as the reduction of carrier recombination and deep defect density. This efficiency is also a record for CdS/Sb2 Se3 solar cells fabricated by rapid thermal evaporation.

Thermal diffused post-selenization for enhancing the photovoltaic performance of thermally evaporated Sb2Se3 solar cell

Sb2Se3 is a promising absorber material for photovoltaic applications owing to its suitable bandgap and high absorption coefficient. However, the initial deposition of the precursor Sb2Se3 thin films using thermal evaporation with an annealing process, results in poor crystallinity and inadequate Se content, leading to significantly low device efficiency and some deep-level detrimental defects. To address these issues, herein, we propose a facile thermal diffusion post-selenization method to supplement Se into post-deposited Sb2Se3 thin films. This process not only achieves stoichiometric composition but also improves the crystallinity of the samples, thereby reducing defect density and carrier recombination. As a result, the efficiency of the fabricated Mo/Sb2Se3/CdS/ITO/Ag solar cell is significantly enhanced from 0.66% to 3.29%.

DLTS Study of Defects in HgCdTe Heterostructure Photodiode

Deep-level transient spectroscopy (DLTS) measurements were performed on HgCdTe heterostructure photodiode grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrate. In order to extract defects from individual layers of the heterostructure, three consecutive etchings were performed. In the first experiment, the N+/T/p/T/P+/n+ structure was chemically etched to the N+ bottom contact to obtain a mesa-type detector. Six localized defects were extracted across the entire photodiode. In the second experiment, the bottom contact was made to the p-type absorber. Two localized defects were found in the p/T/P+/n+ structure. In the third experiment, the top layers were removed and N+/T/p type detector was made—the cap contact was made to the p-type absorber, the bottom to the N+ layer. Five defect levels were identified, three of which overlap with the first experiment. A deep-trap level located at 183 meV above the top of the valence band was identified within the absorber bandgap. This energy coincides with the activation energy determined from the Arrhenius plot for the dark currents. A defect at the level of ∼0.5Eg suggests that the dark current at low reverse bias voltage is dominated by the Shockley–Read–Hall (SRH) mechanism.

Carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN platform with ultra-low resistivity

In this work, the carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN with a very low specific contact resistivity of 1.98 × 10−5 Ω cm2 is investigated. Secondary ion mass spectroscopy measurement results show that the Mg concentration near the p-GaN surface increases form 8 × 1019 to 7 × 1020/cm3 for Mg/Pt/Au contact after annealing. It indicates that Mg atoms from the Mg/Pt/Au metal stack diffuse into the p-GaN during annealing, forming a heavily Mg doped p++-GaN layer with a depth of about 3 nm. The sheet resistance Rsh depends on temperature for Mg/Pt/Au contacts on p-GaN/GaN, indicating that the influence of 2DHG on carrier transport mechanisms at the metal/p-GaN interface was eliminated. For Mg/Pt/Au contacts at ≥360 K, specific contact resistivity reasonably follows T−1, which indicates that the band conduction of Schottky theory dominates the carrier transport. For Mg/Pt/Au contacts at 200–360 K, the electrical resistivity reasonably follows T−1/4, indicating variable-range hopping (VRH) conduction through Mg-related deep-level defects (DLD). Based on the VRH conduction model, the effective barrier height (qφ) of the Mg-related DLD band is extracted as 0.265 eV, which well matches the excellent Ohmic contact.

Sb2S3 Thin-Film Solar Cells Fabricated from an Antimony Ethyl Xanthate Based Precursor in Air.

The rapidly expanding demand for photovoltaics (PVs) requires stable, quick, and easy to manufacture solar cells based on socioeconomically and ecologically viable earth-abundant resources. Sb2S3 has been a potential candidate for solar PVs and the efficiency of planar Sb2S3 thin-film solar cells has witnessed a reasonable rise from 5.77% in 2014 to 8% in 2022. Herein, the aim is to bring new insight into Sb2S3 solar cell research by investigating how the bulk and surface properties of the Sb2S3 absorber and the current-voltage and deep-level defect characteristics of solar cells based on these films are affected by the ultrasonic spray pyrolysis deposition temperature and the molar ratio of thiourea to SbEX in solution. The properties of the Sb2S3 absorber are characterized by bulk- and surface-sensitive methods. Solar cells are characterized by temperature-dependent current-voltage, external quantum efficiency, and deep-level transient spectroscopy measurements. In this paper, the first thin-film solar cells based on a planar Sb2S3 absorber grown from antimony ethyl xanthate (SbEX) by ultrasonic spray pyrolysis in air are demonstrated. Devices based on the Sb2S3 absorber grown at 200 °C, especially from a solution of thiourea and SbEX in a molar ratio of 4.5, perform the best by virtue of suppressed surface oxidation of Sb2S3, favorable band alignment, Sb-vacancy concentration, a continuous film morphology, and a suitable film thickness of 75 nm, achieving up to 4.1% power conversion efficiency, which is the best efficiency to date for planar Sb2S3 solar cells grown from xanthate-based precursors. Our findings highlight the importance of developing synthesis conditions to achieve the best solar cell device performance for an Sb2S3 absorber layer pertaining to the chosen deposition method, experimental setup, and precursors.

Managing Interface Defects via Mixed-Salt Passivation toward Efficient and Stable Perovskite Solar Cells.

Iodine vacancies and uncoordinated Pb0 defects existing at the perovskite surface have been widely demonstrated to induce deep-level defects, which can greatly limit improvement of the efficiency and stability of perovskite solar cells (PSCs). In this work, a novel strategy is proposed for functionalizing perovskite surface by using trimethylsulfoxonium iodide (TMSI), which can enhance the defect formation energy and inhibit Pb0 defects. Meanwhile, TMSI modification also can fill the iodine vacancies of perovskite surface-terminating ends. Consequently, the optimized device shows the improved charge dynamics and the reduced energy losses, achieving a champion efficiency of up to 24.03% along with excellent air-storage and thermal stabilities. This work offers guidelines for more efficient and stable PSCs based on the management of interface defects.

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics.

Influence of deep level trap charges on the reliability of asymmetric doped double gate JunctionLess transistor (AD-DG-JLT)

A study based on the numerical simulation of Double Gate JunctionLess Transistor DG-JLT has been presented under the influence of Trap Charges (TC) inside the oxide or at the oxide/channel interface using ATLAS TCAD software. These trap charges may be generated due to various effects like aging and exposure to radiation and chemical environment etc. and thus leading to variability in the device behavior. To evaluate the impact of TC, the forward and reverse threshold voltage were calculated by altering the gate voltage from zero to positive and subsequently returning to zero, while employing various gate bias durations. Asymmetric doping has also been employed to study the variability in DG-JLT with TC as compared to Uniformly Doped DG-JLT (UD-DG-JLT). Furthermore, the investigation is expanded to evaluate how the performance of AD-DG-JLT is affected by the Dual Material Gate (DMG) in the presence of generated TC. To explore the robustness of the device, breakdown voltage (BV) has been examined with varying TC and device specifications. The impact of the trap energy level (EC-ET) on the BV of the devices has also been explored. Extensive numerical simulations have also been performed to showcase the results regarding the change in drain current caused by Single Event Upset (SEU).

Influence of S-content ratios on the defect properties of Sb2(Sx, Se1–x)3 thin-film solar cells

Sb2(S, Se)3 has widely emerged as an absorber material for solar cells because of its favourable optical properties and abundant raw materials. Although the Sb2(S, Se)3 film comprises simple binary elements and has a lesser complex phase than that of quaternary copper indium gallium selenide, the distribution of atoms within its structure significantly affects its performance. More specifically, this effect is attributed to three non-equivalent positions of selenium or sulphur atoms and two non-equivalent positions of antimony atoms inside its unique quasi-one-dimensional asymmetric structure. Consequently, the composition ratios inside Sb2(Sx, Se1–x)3 significantly impact the film's properties, especially the defect properties. The underlying origins of this effect remain elusive. Along these lines, in this work, defect measurements of the Sb2(Sx, Se1–x)3 solar cells with different S-content ratios were conducted using deep-level transient spectroscopy and then discussed in detail. The analysis confirmed that deep-level defects were always present, and their lower density favoured the device's performance. Moreover, the solar cells consisting of Sb2(Sx, Se1–x)3 thin films with 0.3 S-content ratios exhibit better conversion efficiency with lower deep-level defects density and capture cross-section.

4H-SiC Schottky barrier diodes as radiation detectors: A role of Schottky contact area

Large-area SiC Schottky barrier diodes can significantly improve the sensitivity in radiation detection due to the increased interacting SiC volume. In this work, we tested a large SiC detector with an area of 1 × 1 cm2. Extensive electrical characterization was performed at temperatures ranging from 150 K to 390 K, demonstrating the impact of barrier inhomogeneities on the electrical performance of the diode. Forward current-voltage (I-V) measurements of the diodes revealed two distinct regions caused by Schottky barrier inhomogeneity present throughout the entire temperature range. The barrier heights in the low- and high-current forward voltage regions were extracted from Richardson plots corrected for the Gaussian distribution of barrier heights, yielding values of 1.52 eV and 1.79 eV, respectively. Deep-level transient spectroscopy (DLTS) revealed only one deep-level defect, Z1/2, with an activation energy for electron emission of 0.67 eV, which was assigned to the known carbon vacancy. The DLTS study showed no correlation between electrically active defects and barrier inhomogeneity. An excellent energy resolution of 3.2 % was measured using a large area 241Am radiation source, consistent with values for small area SiC detectors that exhibited no barrier height inhomogeneities. The impact of temperature on the alpha particle radiation response was determined within a temperature range of 200 K to 390 K.

Efficient and bending durable flexible Cu2ZnSn(S,Se)4 solar cells achieved by Ga doping stress regulation

Ga-doped CZTSSe is achieved by evaporating Ga layer in precursor combined with subsequent selenization to balance the efficiency and bending durability of flexible CZTSSe solar cells in this paper. Ga doping can simultaneously alleviate the residual stress and reinforce the microstructure of CZTSSe, retaining the original lattice structure of CZTSSe and suppressing Sn-related deep level defects. The gratifying residual stress reduction from -5.31 GPa to -3.82 GPa with decreased porosity from 6.24% to 3.24% of CZTSSe thin film spontaneously promotes the device performance after evaporating the 20 nm Ga layer. A desirable efficiency enhancement from 2.61% to 5.04% with preferable bending durability of flexible device is achieved, which strategy provides substantial assistance for the potential application of flexible solar cells.

Temperature-dependent ion migration and mobile-ion-induced degradation of perovskite solar cells under illumination

Effects of temperature-dependent ion migration on power reduction of perovskite solar cells (PSCs) under illumination were investigated at temperatures between 15 °C and 45 °C. Mobile-ion concentrations increased with temperature and the power reduction under illumination became faster at high temperatures due to an increase in the number of slow mobile ions and a decrease in the number of fast mobile ions, which was confirmed through ion migration currents. This indicates formation of deep-level defects with low mobility from the fast mobile ions (halide vacancies). Moreover, the transient power responses during maximum-power-point tracking (MPPT) became slower at high temperatures upon illumination, reflecting an increase in the number of slow mobile ions. Furthermore, the light–dark cycling test clarified that the energy yield during 8 h of illumination was reduced by ∼10 rel% per light–dark cycle at 45 °C, which was accompanied by an increase in the concentration of, particularly slow, mobile ions. These results indicate that transient ion-migration analysis for evaluating the concentration of mobile ions (mobile defects) can be effective in understanding the mobile-ion-induced degradation of PSCs.

Work function modulation and charge transport studies of 100 MeV Au7+ ions irradiated TiO2 films for optoelectronic devices

This work demonstrates the impact of 100 MeV Au7+ ion irradiation on the work function and charge-transport properties of TiO2 thin films. The sol-gel synthesized TiO2 films were irradiated by 100 MeV Au7+ ions with varying fluences from 1 × 1011 ion-cm−2 to 1 × 1013 ion-cm−2. The surface potential of these films is found to vary from 139 mV to 469 mV for pristine to ion-irradiated TiO2 thin films. This alternatively tuned the work function of the TiO2 films from −4.87 eV to −4.53 eV which may be due to the higher oxygen vacancies and surface defects created by ion irradiation. The optical transmittance and band gap of the TiO2 thin films are found to be increased after ion irradiation. Photoluminescence and Raman measurements illustrate that various irradiation-induced defect levels such as self-trapped states, oxygen vacancies, color centers, and deep-level emission defects have altered in TiO2 films after ion irradiation. Further, important electrical parameters such as the Schottky barrier height, sheet resistance, and ideality factor have been evaluated to understand the impact of ion irradiation on the charge carrier transport properties of TiO2 films by fabricating the diode in the device configuration FTO/TiO2/Ag.

High Performance Pd/4H-SiC Epitaxial Schottky Barrier Radiation Detectors for Harsh Environment Applications.

Although many refractory metals have been investigated as the choice of contact metal in 4H-SiC devices, palladium (Pd) as a Schottky barrier contact for 4H-SiC radiation detectors for harsh environment applications has not been investigated adequately. Pd is a refractory metal with high material weight-to-thickness ratio and a work function as high as nickel, one of the conventional metal contacts for high performing 4H-SiC Schottky barrier detectors (SBDs). In this article, Pd/4H-SiC epitaxial SBDs have been demonstrated for the first time as a superior self-biased (0 V applied bias) radiation detector when compared to benchmark Ni/4H-SiC SBDs. The Pd/4H-SiC SBD radiation detectors showed a very high energy resolution of 1.9% and 0.49% under self- and optimized bias, respectively, for 5486 keV alpha particles. The SBDs demonstrated a built-in voltage (Vbi) of 2.03 V and a hole diffusion length (Ld) of 30.8 µm. Such high Vbi and Ld led to an excellent charge collection efficiency of 76% in the self-biased mode. Capacitance mode deep level transient spectroscopy (DLTS) results revealed that the "lifetime-killer" Z1/2 trap centers were present in the 4H-SiC epilayer. Another deep level trap was located at 1.09 eV below the conduction band minimum and resembles the EH5 trap with a concentration of 1.98 × 1011 cm-3 and capture cross-section 1.7 × 10-17 cm-2; however, the detector performance was found to be limited by charge trapping in the Z1/2 center. The results presented in this article revealed the unexplored potential of a wide bandgap semiconductor, SiC, as high-efficiency self-biased radiation detectors. Such high performance self-biased radiation detectors are poised to address the longstanding problem of designing self-powered sensor devices for harsh environment applications e.g., advanced nuclear reactors and deep space missions.

Modulating Crystallization and Defect Passivation by Butyrolactone Molecule for Perovskite Solar Cells.

The attainment of a well-crystallized photo-absorbing layer with minimal defects is crucial for achieving high photovoltaic performance in polycrystalline solar cells. However, in the case of perovskite solar cells (PSCs), precise control over crystallization and elemental distribution through solution processing remains a challenge. In this study, we propose the use of a multifunctional molecule, α-amino-γ-butyrolactone (ABL), as a modulator to simultaneously enhance crystallization and passivate defects, thereby improving film quality and deactivating nonradiative recombination centers in the perovskite absorber. The Lewis base groups present in ABL facilitate nucleation, leading to enhanced crystallinity, while also retarding crystallization. Additionally, ABL effectively passivates Pb2+ dangling bonds, which are major deep-level defects in perovskite films. This passivation process reduces recombination losses, promotes carrier transfer and extraction, and further improves efficiency. Consequently, the PSCs incorporating the ABL additive exhibit an increase in conversion efficiency from 18.30% to 20.36%, along with improved long-term environmental stability. We believe that this research will contribute to the design of additive molecular structures and the engineering of components in perovskite precursor colloids.

2D/2D Cs0.32 WO3 /CuS Nano-Heterojunctions for Simultaneous High-Efficiency Solar Desalination, Photocatalytic Decontamination, and Electricity Generation.

Desalination and power generation through solar energy harvesting is a crucial technology that can effectively address freshwater shortages and energy crises. However, owing to the complexity of the actual water environment, the thermal output capability of the photothermal material and the functional integration of the evaporation system need urgent improvement, to obtain high-quality fresh water and sufficient electricity. Herein, a 2D/2D cesium tungsten bronze/copper sulfide (2D/2D Cs0.32 WO3 /CuS) nano-heterojunction is developed and it is loaded into a cellulose-based hybrid hydrogel to construct a multifunctional evaporator. Benefiting from the more nonradiative recombination centers from deep-level defects, as well as shorter carrier migration distances and higher redox potentials in the Cs0.32 WO3 /CuS nano-heterojunction, this evaporator has a significant improvement in thermal output capacity, enabling both super-efficient seawater evaporation (4.22kg m-2 h-1 ) and photodegradation of organic pollutants (removal rate ≈ 99%). Moreover, the evaporator exhibits long-term stability and sustainable self-cleaning property against salt accumulation. Remarkably, the thermoelectric module based on the Cs0.32 WO3 /CuS nano-heterojunction shows promising electricity generation performance (4.85W m-2 ), which can power small appliances durably and stably, exceeding previously reported similar devices. This 2D/2D heterojunction-based solar evaporation system will provide a more reliable solution for efficient and sustainable freshwater-electricity co-generation in resource-limited areas.

Characterization of deep-level defects in highly-doped silicon with asymmetric structure by transient capacitance spectroscopy

Characterization of deep-level defects in highly-doped silicon with asymmetric structure by transient capacitance spectroscopy