Thermally Stimulated Current Curves Research Articles

Deep-Level Traps Responsible for Persistent Photocurrent in Pulsed-Laser-Deposited β-Ga2O3 Thin Films

Gallium oxide (β-Ga2O3) is emerging as a promising wide-bandgap semiconductor for optoelectronic and high-power electronic devices. In this study, deep-level defects were investigated in pulsed-laser-deposited epitaxial films of β-Ga2O3. A deep ultraviolet photodetector (DUV) fabricated on β-Ga2O3 film showed a slow decay time of 1.58 s after switching off 250 nm wavelength illumination. Generally, β-Ga2O3 possesses various intentional and unintentional trap levels. Herein, these traps were investigated using the fractional emptying thermally stimulated current (TSC) method in the temperature range of 85 to 473 K. Broad peaks in the net TSC curve were observed and further resolved to identify the characteristic peak temperature of individual traps using the fractional emptying method. Several deep-level traps having activation energies in the range of 0.16 to 1.03 eV were identified. Among them, the trap with activation energy of 1.03 eV was found to be the most dominant trap level and it was possibly responsible for the persistent photocurrent in PLD-grown β-Ga2O3 thin films. The findings of this current work could pave the way for fabrication of high-performance DUV photodetectors.

ГЛУБОКИЕ УРОВНИ В P-N-СТРУКТУРАХ НА ОСНОВЕ GaAsP

Research of deep levels in p-n-structures based on a GaAsP solid solution has been done. It is shown that a significant spread in the efficiency of the studied LEDs is due to the difference in the composition of GaAsP in the active region. It has been determined that the main parameter that determines the efficiency of a light-emitting diode is its internal quantum efficiency, which is associated with the probability of radiative and nonradiative transitions. It is connected with deep levels caused by various types of defects. The ionization energies of deep levels in the depletion layer of p-n-junctions are determined using the method of thermally stimulated currents (TSC). It is shown that the presence of such levels can lead to a significant increase in the series resistance of the LED, which in turn will lead to a decrease in efficiency and additional heating of the diode. The structure and parameters the samples that investigated are described. Information about distribution of impurities in p-n-junctions of light-emitting diodes based on GaAsP was obtained by measuring the capacitance-voltage characteristics. Device that used to determine the TSС is described. The results of calculations of deep impurity levels and their concentration from the TSC curves for various LEDs based on GaAsP are presented

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.

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Effect of modification of copper electrode on the space charge accumulation in LDPE

In this paper, titanium and tungsten films were coated on copper sheets, respectively, by plasma chemical vapor deposition (PCVD). The copper sheets before and after coating were, respectively, injected as charge into the low-density polyethylene (LDPE), the space charge accumulation in LDPE was tested by the thermally stimulated current (TSC), and the trap level of the dielectric was calculated by the TSC curve. The results show that the work function improves after the modification of the electrode material, the space charge accumulation in LDPE decreases, and the trap level of the polymer reduces. Because of the oxygen vacancies, the titanium coating electrode has significant suppression on the charge injection in LDPE under 30 kV/mm and the tungsten shows excellent inhibition under 70 kV/mm.

A modified thermally stimulated current analysis method for direct determination of trap energy distribution

A modified method is proposed to directly determine the trap energy distribution in dielectrics after high electric field charge injection based on thermally stimulated current (TSC) measurement. Firstly, through a series of equation derivation, it is found that obtaining trap energy distribution from measured TSC data becomes solving a nonnegative least squares problem. Then, an iteration algorithm is introduced to solve the problem in detail. Finally, the modified method is applied to analyze TSC experiment results of polyethylene terephthalate (PET) film, which are compared with other researchers' work. The experiment results show that there is a wide apparent peak in TSC curves, extending from ~325 K to ~400 K. After analyzing with the method, the peak is composed of two components formed by released charges previously trapped in the traps of two major depths of ~1.09 eV and ~1.18 eV in the range of 0.5-1.3 eV. Altogether, these results generally agree with other researchers' analysis of PET, which acquired by various measurements and methods. Moreover, in order to check the reliability of the algorithm, R-square values are calculated. The values are close to 1 which manifests that the numerical solutions are almost coincide with experiment data and the method is proved to be trustworthy, realistic and stable.

Trap center study in hybrid organic-inorganic perovskite using thermally stimulated current (TSC) analysis

This paper presents results of a study that allowed identifying states of traps in thin films of hybrid organic-inorganic perovskite compounds based on methylammonium lead halide with different compositions CH3NH3PbX3 (X is Cl, Br or I) prepared by spin-coating, through Thermally Stimulated Current (TSC) measurements. Special emphasis was done in studying the influence of the composition and adsorption of oxygen on the depth of traps and on the density of states associated with trapping centers. Deconvolution of the TSC curves obtained from measurements made inside a vacuum chamber under different oxygen partial pressures, revealed the presence of traps centers whose activation energies are affected by the oxygen concentration. It was also found that TSC peaks do not appear in TSC measurements carried out at pressures less than 0.1 mbar, indicating that the possible nature of the identified traps centers is related to oxygen adsorbed superficially and/or located into the grain boundaries.

Characterization of defect states in Ga-rich gallium arsenide crystals by thermally stimulated current

The trap levels in Ga-rich GaAs crystals were studied in the temperature range of 10–300 K using thermally stimulated currents (TSC) technique. During the experiments we utilized a constant heating rate of 0.2 K/s. Experimental evidence is found for two electron trapping centers in the crystal with activation energies of 44 and 50 meV. The analysis of the experimental TSC curve suggests slow retrapping. The capture cross sections of the traps were determined as 8.8 × 10−25 and 1.0 × 10−25 cm2 with concentrations of 2.7 × 1011 and 1.3 × 1011 cm−3, respectively.

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu3Ga5Se9 layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46eV and the temperature range of 15–115K (PL) and 10–170K (TSC). An infrared PL band centered at 1.42eV was revealed at T=15K. Radiative transitions from shallow donor level placed at 20meV to moderately deep acceptor level at 310meV were suggested to be the reason of the observed PL band. TSC curve of Cu3Ga5Se9 crystal exhibited one broad peak at nearly 88K. The thermal activation energy of traps was found to be 22meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.

Combined low-temperature photoluminescence and thermally stimulated current studies in Cu3In5S9 layered single crystals

Photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu3In5S9 layered crystals grown by Bridgman method have been studied in the photon energy region of 1.20–1.65eV and the temperature range of 15–70K (PL) and in the temperature region of 10–130K (TSC). A broad PL band centered at 1.484eV was observed at T=15K. Radiative transitions from shallow donor level located at 16meV to moderately deep acceptor level located at 120meV were suggested to be responsible for the observed PL band. TSC curve of Cu3In5S9 crystal exhibited one broad peak at about 60K. The activation energy of trap level was found to be 17meV. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of PL and TSC measurements carried out below room temperature. Moreover, the absorption edge of Cu3In5S9 crystals have been studied through transmission and reflection measurements in the photon energy range 1.13–1.85eV. The band gap energy was determined as 1.61eV.

Comparative study of low-temperature photoluminescence and thermally stimulated current in quinary Tl4Ga3InSe6S2 layered crystals

Photoluminescence (PL) and thermally stimulated current (TSC) spectra of Tl4Ga3InSe6S2 layered crystals grown by Bridgman method have been studied in the wavelength region of 560–690nm and the temperature range of 15–45K (PL) and in the temperature region of 10–90K (TSC). A broad PL band centered at 620nm (2.00eV) was observed at T=15K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.7–36.5mWcm−2 range. Radiative transitions from donor level located at 150meV below the bottom of conduction band to shallow acceptor level located at 10meV above the top of the valence band were suggested to be responsible for the observed PL band. TSC curve of Tl4Ga3InSe6S2 crystal exhibited one broad peak at about 41K. The activation energy of trap level was found to be 11meV. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of PL and TSC measurements carried out below room temperature.

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.

Non-Homogeneity of Distribution of Transport and Trapping States in Poly[2-methoxy-5-(3′,7′- dimethyloctyloxy)-1,4-phenylenevinylene

Carrier transport and trapping was investigated in poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) layers by thermally stimulated currents (TSC) depending on the exciting light spectral range. The upper edge of the spectra ranged from 1.77 eV up to 3.1 eV to assure selective excitation of the defect states. The TSC curves were shown to be a superposition of several thermally activated processes, i.e., carrier generation from trapping states and thermally stimulated mobility growth according to the Gaussian disorder model. The extrinsic excitation resulted in 0.15 eV photoconductivity effective activation energy values, which decreased down to 0.05 eV for the intrinsic excitation. The deeper states with activation energies of 0.28-0.3 eV and 0.8-0.85 eV were identified too. The results are direct indication by photo-thermo-electrical methods of distributed in energy trapping and transport states with the standard deviation of the density of states of about 0.015 eV.

Detection of nitrogen related traps in nitrided/reoxidized silicon dioxide films with thermally stimulated current and maximum entropy method

We constructed the new method to analyze thermally stimulated current (TSC) using maximum entropy method. This method allows us to extract trap density depending on trap energy level [Nt(Et)] from a TSC curve. Using this to analyze TSC of as-deposited, nitrided, and reoxidized thin silicon dioxide films, we were able to determine Nt(Et) of hole traps with very highly energy resolution and to observe the generation of nitrogen related hole traps.

Numerical study of dependence of thermally stimulated currents in disordered solids at step heating regime on initial trap occupancy

In the present paper numerical investigation of thermally stimulated currents (TSCs) in disordered solids, measured at step heating regime, is performed. The utilized energy distribution of trapping states is the superposition of exponential and Gaussian distributions. The monomolecular carrier recombination regime is assumed. The resulting set of stiff differential equations governing TSCs is solved with the use of Gear's procedure. The calculations are made for several carrier generation and recombination rates. The TSC curves are next analyzed by the methods developed in previous papers and the calculated trap distributions are compared with the input distributions. It is concluded that the methods have satisfactory accuracy solely in the cases of week carrier retrapping or small trap occupancy.

Negative signals of photoinduced current transient and thermally stimulated current spectroscopy in GaAs due to negative-U properties of oxygen

Bulk GaAs grown under high oxygen potential shows an intermediate resistivity (ρ∼3×103 Ω cm at room temperature) with an electron activation energy E=0.42 eV. The relevant defect has been identified in literature as off-centered substitutional oxygen with three different charge states. Photoelectrical measurements on such GaAs material exhibit distinctly anomalous signals. Photoinduced current transient spectroscopy (PICTS) shows a signal with a negative sign, whereas thermally stimulated currents (TSCs) are clearly lower than the dark current. By means of a set of coupled differential equations, the carrier exchange between the defect levels and the conduction band is described. Solving these rate equations, the PICTS and TSC curves are modeled. In this way, it could be demonstrated that the anomalous PICTS and TSC signals are caused by the negative-U behavior of the off-center oxygen. This is in agreement with results of infrared spectroscopy.

Determination of Activation Energies and Electric Dipole Moment of In‐Plane Highly Electrically Conductive Hetero‐C12TCNQ/C20 LB Films Using Thermally Stimulated Current

AbstractThe activation energies of in‐plane highly electrically conductive 2‐dodecyl‐7,7,8,8‐tetracyanoquinodimethane (C12 TCNQ)/arachidic acid (C20) hetero‐LB films and C12TCNQ homo‐LB films were determined by thermally stimulated current (TSC) measurement. The TSC curves for the upper Al electrode/C12TCNQ/C20/lower Al electrode were nearly of the same shape as those for the upper Al electrode/C12TCNQ/lower Al electrode. The activation energies of the upper Al electrode/C12TCNQ/C20/lower Al electrode and upper Al electrode/C12TCNQ/lower Al electrode were 0.9–1.3 eV at 55–80°C and 1.6–1.8 eV at 105–120°C, respectively. The TSC curves for the upper Al electrode/C12TCNQ/C20/lower Al electrode and upper Al electrode/C12TCNQ/lower Al electrode had two peaks at 55–80°C owing to the depolarization of the electric dipole and two peaks at 105–120°C owing to the movement of space charges in the C12TCNQ molecule. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Thermally stimulated currents in disordered solids at fractional heating

Thermally stimulated currents (TSCs) in disordered solids, registered at fractional sample heating, are investigated theoretically. The Gobrecht–Hofmann method of determining trap distribution in energy gap is extended to the case of significant carrier retrapping. Two specific cases, when the TSCs are limited either by carrier recombination or by carrier neutralization on collecting electrode, are considered. The accuracy of proposed method is estimated from the analysis of TSC curves, calculated numerically for exponential and Gaussian trap distributions.

Numerical study of the dependence of thermally stimulated currents in disordered solids on initial trap occupancy

In the paper a numerical investigation of thermally stimulated currents (TSCs) in disordered solids, which are characterized by continuous energy distribution of trapping states, is presented. The set of stiff differential equations governing TSCs is solved with the use of Gear's algorithm. The dependence of TSC curves on the initial trap occupancy as well as on the carrier recombination time is examined. It is concluded that the Fritzsche-Ibaraki method of TSC analysis, ameliorated by other authors, applies solely for almost complete trap filling and high carrier recombination rate. In the case of small trap filling the method of analysis the TSCs proposed by the present authors may be more adequate.

Application of thermally stimulated current measurement to the polymorphic characterization of drug substances

The thermal stimulated current measurement was used as an innovative analytical equipment to evaluate the polymorphic properties of terfenadine and Compound A, being developed by Takeda Pharmaceutical Company, Limited. At first, terfenadine, which is known to have polymorphs, was used as a model sample for thermally stimulated current (TSC) analyses. The TSC curves of amorphous and two polymorphs were distinctly different from each other. Therefore, it was considered that TSC measurement could be a useful technique to evaluate the crystalline properties of drug substances. The polymorphs of compound A were difficult to distinguish the characteristics of polymorphs from conventional powder X-ray diffractometry and also differential scanning calorimetry. Forms A and B of compound A were clearly differentiated by the thermal stimulated current properties that were adequate to characterize each form. Thus, it was shown that TSC was extremely useful and powerful tool for identification of complicated polymorphs, which were not distinguished by conventional methods.