Thermally Stimulated Current Peak Research Articles

Revealing defect centers in PbWO4 single crystals using thermally stimulated current measurements

The trap centers have a significant impact on the electronic properties of lead tungstate (PbWO4), suggesting their crucial role in optoelectronic applications. In the present study, we investigated and revealed the presence of shallow trap centers in PbWO4 crystals through the utilization of the thermally stimulated current (TSC) method. TSC experiments were performed in the 10–280 K range by applying a constant heating rate. The TSC spectrum showed the presence of a total of four peaks, two of which were overlapped. As a result of analyzing the TSC spectrum using the curve fit method, the activation energies of revealed centers were found as 0.03, 0.11, 0.16, and 0.35 eV. The trapping centers were associated with hole centers according to the comparison of TSC peak intensities recorded by illuminating the opposite polarity contacts. Our findings not only contribute to the fundamental understanding of the charge transport mechanisms in PbWO4 crystals but also hold great promise for enhancing their optoelectronic device performance. The identification and characterization of these shallow trap centers provide valuable insights for optimizing the design and fabrication of future optoelectronic devices based on PbWO4.

Shallow trapping centers in Bi12GeO20 single crystals by thermally stimulated current measurements

Bi 12 GeO 20 single crystals were investigated by thermally stimulated current (TSC) experiments performed in the temperature range of 10-290 K. Recorded TSC glow curve exhibited six distinctive peaks with maxima at around 90, 105, 166, 209, 246, 275 K. The analyses of the obtained glow curve were accomplished by curve fitting and initial rise methods. The analysis results were in good agreement that the TSC peaks appeared in the glow curve due to existence of trapping levels with activation energies of 0.10, 0.18, 0.23, 0.53, 0.68 and 0.73 eV. These trapping levels were estimated to be hole traps above valence band. The heating rate dependent TSC glow curves were also obtained for various rates between 0.30 and 0.45 K/s. The changes of TSC intensity, peak maximum temperature and full-widths-half-maximum values with heating rates were studied in detail. TSC intensity decreased and peak maximum temperature increased with increasing heating rate. Determination of defects and trapping/stimulation mechanism of those are significant for technological applications since local states in these materials take critical role for device performance.

Investigation of traps distribution in GaS single crystals by thermally stimulated current measurements

Thermally stimulated current (TSC) investigations of p-GaS (gallium sulfide) single crystals grown by Bridgman method were achieved by virtue of consecutive experiments carried out at various heating rates in between 0.4 and 1.0 K/s in the temperature range of 10–280 K. One single TSC peak around 148 K and overlapped, incomplete peaks in the end limit temperature of the experiments were observed in the spectrum recorded at constant heating rate of 1.0 K/s. Individual peak was analyzed utilizing curve fitting method. Existence of one trapping level centered at 0.11 eV was revealed by the analyses. Heating rate dependency of obtained TSC curve was also studied and it was shown that TSC intensity decreased besides increase of peak maximum temperature with heating rate. Characteristics feature of trapping mechanism was investigated in detail by employing different stopping temperature between 50 and 110 K. Analyses on Tm–Tstop dependency resulted in a presence of quasi-continuously distributed traps with activation energies ranging from 0.11 to 0.55 eV. The revealed trap was thought to be arising from intrinsic defect possibly created by VGa or antisite SGa.

Thermally Stimulated Current Study and Relaxation Behaviour of Annealed Copolymer P(VDF-TrFE) Films for Potential Pyroelectric Energy Harvesting

The effects of annealing treatment on the polarization of copolymer P(VDF-TrFE) free-standing films were thoroughly studied by the thermally stimulated current (TSC) method using decomposition analysis and first-order kinetic theory. The TSC measurement mainly revealed three depolarization peaks, which are known as the β, α and Curie mode (ρ) peaks in P(VDF-TrFE). The origin of the TSC peaks and their relaxation behaviour were correlated with structural, morphological and electrical properties such as ferroelectricity, dielectricity and pyroelectricity. Spin-coated copolymer thin films with thicknesses of 300 nm were also prepared from 5 wt.% solutions. Both free-standing and thin films were initially annealed in the range of their Curie temperature up to the melting point [80–140°C] to increase their crystallinity. A remnant polarization current of 76.7 mC m−2, which gives a pyroelectric coefficient of 31 μC m−2K−1 and a figure of merit FD of 86 μC m−2K−1 (1 kHz), was obtained from the sample annealed at 100°C, where the percentage of crystallinity was maximum and the phase was about to transition from ferroelectric to paraelectric.

Aging effect in Er3+‐doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 ceramics

AbstractUsually, aging in poled ferroelectrics leads to degradation of certain physical properties. In this study, we found a remarkable aging effect in tetragonal Er3+‐doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT‐BCT) ceramics after poling. It is observed that the domains can spontaneously rotate to keep their spontaneous polarization direction similar to that of the poling electric field during aging for the poled ceramics. Furthermore, compared with freshly poled ceramics, the thermally stimulated current (TSC) peak of the aged ones shifts toward a higher temperature (10°C). And the temperature of the TSC peak in the aged ceramics is exactly equal to their Curie temperature. Such features indicate that aging for the poled ceramics could stabilize the alignment of ordered ferroelectric domains. Additionally, a downward TSC peak above Curie temperature is obtained in both poled and aged ceramics, demonstrating that poling and aging can lead to ordered alignment of defect dipoles. The aging mechanism of poled Er3+‐doped BZT‐BCT ceramics has proposed and discussed in this article.

Identification of Trap States in Perovskite Solar Cells.

Thermally stimulated current (TSC) measurements are used to characterize electronic trap states in methylammonium lead iodide perovsite solar cells. Several TSC peaks were observed over the temperature range from 20 K to room temperature. To elucidate the origins of these peaks, devices with various organic charge transport layers and devices without transport layers were tested. Two peaks appear at very low temperatures, indicating shallow trap states that are mainly attributed to the PCBM/C60 electron transport bilayer. However, two additional peaks appear at higher temperatures, that is, they are deeper in energy, and are assigned to the perovskite layer. At around T = 163 K, a sharp peak, also present in the dark TSC measurements, is assigned to the orthorhombic-tetragonal phase transition in the perovskite. However, a peak at around T = 191 K is assigned to trap states with activation energies of around 500 meV but with a rather low concentration of 1 × 10(21) m(-3).

Effect of Electrical Aging on the Trap Parameter of HVAC XLPE Cable Insulation

The effect of electrical aging on the trap parameter of HVAC XLPE cable insulation was investigated in this work. Thermally Stimulated Current (TSC) was used to measure depolarization current. The variation of trap parameter was calculated by means of start-up method based on the Gaussian fitting curve of TSC data. It was found that, the activation energy and the amount of trap charge obtained from TSC peak at 243K and 348K were increased after electrical aging, which may be benefit to characterize the degree of aging. The amount of trap charge in the two peaks increases as the increase of oxide dipole after electrical aging. And charge trapped described by the TSC peak at melting temperature increased after aging. It is considered that the increase of charge in the TSC peak can be attributed to the release of trap charge in the melt crystallization process, which corresponds to the α relaxation process in XLPE insulation.

Analysis of electron traps formed in organic films with a sputtered cathode

To understand why performance degradation is reduced for sputtered cathodes on organic devices when the electron transport layer (ETL) is doped with Li, we analyze electron-only devices using the thermally stimulated current (TSC) technique and modeling of temperature-dependent current–voltage characteristics with a trapped-charge-limited current (TCLC) model. The combined results suggest that the trap density measured by TSC might also include a portion of the density of the hopping sites in the lowest unoccupied molecular orbital levels, which contributes to charge transport. Compared to undoped devices, doped devices maintain a high density of hopping sites even when the Al is sputtered. We propose that the reduced effect of sputtering on electron injection and transport properties is because radical anions of Alq3 might still be formed by the strong reducer Li even if the organic material is partially damaged. An additional TSC peak and increased driving voltage for doped tris(8-hydroxyquinoline)aluminum (Alq3) as an ETL with a sputtered cathode suggests the formation of new traps possibly because of damage even though the transport is better compared to the undoped device. Such traps are not found in doped bathophenanthroline (Bphen) as an ETL, which shows no change in driving voltage.

Characteristics of traps in AgIn5S8 single crystals

Characteristics of charge traps in AgIn5S8 single crystals are investigated by the use of Thermally Stimulated Current (TSC) technique. The TSC spectra of the unintentionally doped sample recorded at a constant heating rate from 80K to 300K reveals a single broad peak at ∼90K. The shift of the TSC peak continuously due to post-illumination preheating process confirmed the continuous trap distribution, which is Gaussian in shape. The trap density distribution changes from ∼40meV on the shallow side to ∼120meV on the deep side with the appearance of a maximum at ∼72meV.

TL and TSC studies on TlGaSe2 layered single crystals

Defects in – as grown – TlGaSe2 layered single crystals were investigated using Thermoluminescence (TL) and Thermally Stimulated Currents (TSC) techniques in the temperature range 10–300K. TL and TSC curves of samples illuminated using a light with energy greater than the band gap of the material, i.e. blue light (∼470nm) at 10K, exhibited peaks around 27 and 28K, respectively, when measured by heating up the samples at a rate of 1K/s. TL and TSC curves were analyzed to characterize the defects responsible for the peaks. Both TL and TSC peaks were observed to be obeying first order kinetics. Thermal activation energies of the peaks were determined using various methods: curve fitting, initial rise, peak shape and different heating rates. For both TL and TSC peaks, thermal activation energy was determined as around 8meV, implying that they may originate from similar kinds of trapping centers. A distribution of traps (in terms of energy) was experimentally verified by illuminating the sample at different temperatures and measuring the TL curves. As a result of this, the apparent thermal energies were observed to be shifted from ∼8 to ∼17meV by increasing the illumination temperature from 10 to 16K.

Effect of doping on trapping center parameters in nanocrystalline CdSe thin films

Thin films of the undoped nanocrystalline CdSe (nc-CdSe) and doped (Zn and In) thin films have been prepared by physical vapor deposition in the presence of argon as inert gas at room temperature. The influence of the doping materials on the stoichiometry, grain size and trap parameters has been explored by means of Energy Dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM) and thermally stimulated current (TSC) measurements. The TEM results show that small particles of nanometer range are formed in thin films and that size increases slightly with Zn doping and decreases when In is used as the doping material. TSC measurements in the temperature range from 225 to 350K are performed at different heating rates. Well defined TSC peaks have been observed. The peaks shift to higher temperatures as heating rate is increased. Trap parameters, such as trap depth and density of traps have been determined. The trap parameters have been evaluated by using Gaussian fit and different heating rates methods. Good agreement has been obtained between trap parameters calculated from these two methods. It is observed that the trap depth decreases and trap density increases in doped samples compared to undoped ones.

Study of traps in P3HT:PCBM based organic solar cells using fractional thermally stimulated current (FTSC) technique

Thermally Stimulated Current (TSC) measurement, well known technique for trap level detection, was applied to derive the density of occupied states in organic solar cells. The device structure is Glass/ITO/P3HT:PCBM/Al. Analysis of the TSC spectra allows to determine the Fermi level position from the edge of the nearest band (LUMO or HOMO) (~0.42eV). At that time, TSC peaks cannot be evaluated due to multiple traps that contribute to the current signal. In order to achieve a rough picture of the trap structure, Fractional TSC (FTSC) method was applied. The procedure is based on multiple temperature cycles which evaluate activation energies for different ranges of temperatures. The results reveal several plateaus, pointing out the presence of four trap levels (0.12eV, 0.16eV, 0.20eV and 0.35eV). Additionally, a curve-fitting procedure is done in order to characterize the trap parameters and achieved with five trap levels (0.059eV, 0.12eV, 0.14eV, 0.20eV and 0.35eV) in Gaussian or exponential distribution. Four of them are in complete agreement with measured FTSC ones. Those results are very well correlated with our other characterizations by Admittance Spectroscopy (AS). The results presented here show that, in spite of the chemical differences between inorganic materials and conjugated polymers, FTCS technique can still be applied to determine the density of occupied states in these materials, as long as care is taken. Polymers are low-mobility materials with large concentrations of deep impurities.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Thermally Stimulated Current Separation of Hole and Acceptor Trap Density in 4H-SiC MOS Devices Using Gamma Ray Irradiation

Thermally stimulated current (TSC) measurements on epitaxial and implanted 4H-SiC MOS capacitors are presented. The effect of gamma ray irradiation on the TSC spectra of epitaxial 4H-SiC MOSCAP devices is discussed. On non-irradiated samples, two TSC peaks are observed near 55 K and 80 K. Due to the generated oxide charge during irradiation, the 80 K emission split into two constituent peaks. These have been attributed to hole traps and Al acceptors.

Deep centers in semi‐insulating Fe‐doped native GaN substrates grown by hydride vapour phase epitaxy

AbstractElectrical properties, Fe concentration, and deep centers in semi‐insulating Fe‐doped GaN substrates grown by hydride vapor phase epitaxy (HVPE) were characterized by temperature‐dependent Hall‐effect measurements, secondary ion mass spectroscopy, and thermally stimulated current (TSC) spectroscopy. Five adjacent samples from a low‐[Fe] wafer displayed very high resistivity, dominated by a center at 0.94 eV. At least six traps were observed in the samples by TSC, with trap B (0.56‐0.60 eV) being dominant. A metastable trap A1 at ∼0.82 eV appeared after white‐light illumination at 300 K. A sample from a high‐[Fe] wafer displayed a lower resistivity, dominated by a center at 0.58 eV. The largest TSC peak in this sample was trap A1, although trap B also appeared. These TSC traps are compared with deep‐level‐transient‐spectroscopy traps reported in conductive epitaxial and bulk HVPE‐GaN. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Light Irradiation on Electrical Properties of Lead Zirconate Titanate Film Capacitors

Pb(Zr,Ti)O3 (PZT) thin film capacitors have been widely investigated, due to their various applications, including ferroelectric memories and micro-electromechanical systems. However, degradation of electrical properties of the films is caused by crystal defects, and the exact mechanism for this has not been fully elucidated. We have attempted to investigate defects in the films by thermally stimulated current (TSC) measurements, and the identification of the observed TSC peaks and the relationships between the degradation and TSC signal. However, the behavior of carriers in the films, such as recombination, has not been fully elucidated, and quantitative evaluation of carrier movement and defect density has been difficult. We have therefore investigated the influence of UV-light irradiation on electrical properties of PZT film capacitors. The leakage current and capacitance measurements under UV light (= 250 nm) irradiation were carried out. The current and capacitance were gradually changed by the irradiation, revealing that the relaxation time and lifetime of carriers in the films were sufficiently high, above 100 s, and the decrease in the observed current in TSC by recombination seems to be small. The relaxation time slightly decreased with increasing temperature and applied voltage because of carrier drift. The drift properties were also discussed.

Thermally Stimulated Current (TSC) Studies on Resistance Degradation of Ni-MLCC

To avoid the influence of spontaneous polarization, thermally stimulated current (TSC) measurements of BaTiO3-based multilayer ceramic capacitors with Ni internal electrodes were performed within the paraelectric phase. We observed two TSC peaks and inferred that one of them was related to the resistance degradation according to the electromigration of oxygen vacancies. Further, we proposed that the two peaks resulted from the relaxation phenomenon of the oxygen vacancies, which migrated under dc electrical field stresses, in the grains and over the grain boundaries.

Thermally stimulated currents in α-HgI2 polycrystalline films

A study of thermally stimulated currents (TSC) is applied to α-HgI2 polycrystalline films grown by physical vapour deposition with various thermal boundary conditions. Five TSC peaks are clearly observed and numerically fitted. The activation energy and the density of the trapping centre that corresponds to each TSC peak are then calculated. Finally, the effects of the deposition conditions on the TSC results are discussed.

Study of traps in polydiacetylene based devices using TSC technique

Trap parameters in poly(1-(3,4-difluorophenyl)-2-(4-pentylcyclohexylphenyl)acetylene) (PDPA-2F) based devices have been investigated by using the thermally stimulated current (TSC) technique. The device structure is ITO-PEDOT-(PDPA-2F)-M, where M stands for the cathode metal (Al, Ca/Al, and Au). The results reveal at least three TSC peaks in devices denoted as peaks A, B and C. Comparing trap parameters in ITO-PEDOT-(PDPA-2F)-Au hole-only device and ITO-PEDOT-(PDPA-2F)-Ca Al (Al) bipolar devices, we assigned A and B trap types to hole-like traps and C type traps to electron-like traps. The trap densities are in the range of 1015−1017 cm−3 and the trap levels are 0.12 eV (A type traps), 0.36 eV (B type traps), and 0.25 eV (C type traps).

Thermally stimulated currents in amorphous barium titanate thin films deposited by rf magnetron sputtering

Thermally stimulated currents (TSCs) are measured in amorphous barium titanate thin films deposited by the rf sputtering technique. The TSC global curve is composed of three overlapping peaks in the 0–200°C temperature range. At 50°C, a first TSC peak is observed that can be related to a shallow-trap level. A second peak due to a dipolar polarization process is observed at 95°C. Finally, a third peak appears at 140°C that is ascribed to the oxygen vacancy motion and their accumulation at electrodes. The different peaks constituting the global TSC spectrum are separately studied by the fractional polarization technique in order to analyze their fine structures and to determine their activation energies.