Electron Trap Levels Research Articles

Defect level distributions and atomic relaxations induced by charge trapping in amorphous silica

We compute the distribution of electronic levels of native defects in amorphous silica from total energy differences of charge-state density functional theory calculations over an ensemble of atomic structures. The predicted distributions reproduce results from trap spectroscopy by charge injection experiments, validating the calculations. Furthermore, our study characterizes the experimentally inaccessible contributions of individual defect types to the overall distribution. Computed electron and hole trapping levels provide insight into the positive charge buildup in bulk silica observed in negative-bias-temperature-instability, an important degradation mechanism of metal-oxide-semiconductor devices.

Radiation dose response correlation between thermoluminescence and optically stimulated luminescence in quartz

The fast, linearly modulated optically stimulated luminescence (LM-OSL) component in quartz is the main dosimetric signal used for the dating applications of this material. Since the blue light stimulation (470nm, 40mWcm −2) time needed to obtain the fast LM-OSL component is less than 50s the electron trapping levels responsible for it are still highly populated. In this way an active radiation history is created which could play an important role in the dosimetric characteristics of the fast OSL signal. In the present work the dose response behavior of the fast OSL signal is investigated in quartz samples with an annealed radiation history and quartz samples possessing an artificial radiation history. A computerized curve de-convolution analysis of the LM-OSL curves for 50s stimulation time showed that it consists of three individual OSL components. The faster component C1 with peak maximum time around 5s has a linear dose response in virgin samples, which turns to a slight superlinearity as a function of the artificial radiation history. On the other hand the component C2 with peak maximum time at 12s is slightly superlinear which turns into strong superlinearity as a function of artificial radiation history. Finally, component C3 with peak maximum time at about 45s is strongly superlinear for both virgin samples and as a function of artificial radiation history. The implications to practical application are discussed.

Reconstruction of thermally quenched glow curves in quartz

The experimentally measured thermoluminescence (TL) glow curves of quartz samples are influenced by the presence of the thermal quenching effect, which involves a variation of the luminescence efficiency as a function of temperature. The real shape of the thermally unquenched TL glow curves is completely unknown. In the present work an attempt is made to reconstruct these unquenched glow curves from the quenched experimental data, and for two different types of quartz samples. The reconstruction is based on the values of the thermal quenching parameter W (activation energy) and C (a dimensionless constant), which are known from recent experimental work on these two samples. A computerized glow-curve deconvolution (CGCD) analysis was performed twice for both the reconstructed and the experimental TL glow curves. Special attention was paid to check for consistency between the results of these two independent CGCD analyses. The investigation showed that the reconstruction attempt was successful, and it is concluded that the analysis of reconstructed TL glow curves can provide improved values of the kinetic parameters E, s for the glow peaks of quartz. This also leads to a better evaluation of the half-lives of electron trapping levels used for dosimetry and luminescence dating. 2012 Published by Elsevier Ltd.

A hybrid density functional study on the electron and hole trap states in anatase titanium dioxide

We present a theoretical study on electron and hole trap states in the bulk and (001) surface of anatase titanium dioxide using screened hybrid density functional calculations. In both the bulk and surface, calculations suggest that the neutral and ionized oxygen vacancies are possible electron traps. The doubly ionized oxygen vacancy is the most stable in the bulk, and is a candidate for a shallow donor in colorless anatase crystals. The hole trap states are localized at oxygen anions in both the bulk and surface. The self-trapped electron centered at a titanium cation cannot be produced in the bulk, but can be formed at the surface. The electron trap level at the surface oxygen vacancy is consistent with observations by photoelectron spectroscopy. The optical absorptions and luminescence in UV-irradiated anatase nanoparticles are found to come from the surface self-trapped hole and the surface oxygen vacancy.

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00at.%) films on GaSe (0001) lamellas and amorphous quartz substrates, obtained by annealing, at 700K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00−2.70eV. Al doping induces an additional recombination level at 1.13eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22eV and 0.26eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10−1.40eV, are created.

Influence of ionic Bismuth on intrinsic defects in ZnO varistors

The relationship between the dielectric dissipation factor D(tan δ) and frequency in the varistors of ZnO–Sb 2O 3–BaO and ZnO–Bi 2O 3–Sb 2O 3–BaO system ceramics has been investigated. A new dielectric dissipation peak was found near 1.5 MHz at room temperature in the ZnO–Sb 2O 3–BaO system, and the corresponding electron trapping level is about 0.18 eV. The dissipation peak was considered to result from the intrinsic defect Zn i . The additive Bi 2O 3 may play a suppressing effect on the formation of Zn i . This assumption was consistent with the experimental fact that degradation phenomenon of varistor ceramics under long-term load voltage is improved by the increasing amount of Bi 2O 3 additive.

Electron Spin Resonance of Chromium–Platinum Pair in Silicon

We have investigated a chromium–platinum pair in silicon by electron spin resonance measurement. A new ESR spectrum originating from a chromium–platinum pair has been detected in both n- and p-type silicons diffused with chromium and platinum. The anisotropic g-tensor obtained by analyzing the angular dependence of the ESR spectrum shows a monoclinic (C1h) symmetry with g-values of g1=4.67, g2=2.99, and g3=1.80. The g1 axis is along the <110 > direction. The g2 and g3 axes are perpendicular to the g1 axis, and the g2 axis is rotated from the <100 > direction to the <111 > direction at an angle of 20°. The anisotropic character of monoclinic (C1h) symmetry results from the nearest-neighbor configuration consisting of a Pt atom at a substitutional site distorted by the displacement of Pt along the <100 > direction and a Cr atom at the nearest-neighbor interstitial site. The ESR measurement under illumination suggests that a chromium–platinum pair forms a donor like electron trap level.

Structural and electrical characteristics of high-k Tb2O3 and Tb2TiO5 charge trapping layers for nonvolatile memory applications

In this study, we investigated the structural properties and electrical characteristics of metal/oxide/high-k material/oxide/silicon (MOHOS)-type memory devices incorporating Tb2O3 and Tb2TiO5 films as charge storage layers for nonvolatile memory applications. X-ray diffraction and x-ray photoelectron spectroscopy revealed the structural and chemical features of these films after they had been subjected to annealing at various temperatures. From capacitance-voltage measurements, we found that the MOHOS-type memory devices incorporating the Tb2TiO5 film and that had been annealed at 800 °C exhibited a larger flatband voltage shift of 2.94 V (Vg=9 V for 0.1 s) and lower charge loss of 8.5% (at room temperature), relative to those of the systems that had been subjected to other annealing conditions. This result suggests that Tb2TiO5 films featuring a thinner silicate layer and a higher dielectric constant provide a higher probability for trapping of the charge carrier and deeper electron trapping levels.

Oxygen vacancy levels and electron transport in Al2O3

The energy levels of the oxygen vacancy in α- and θ-Al2O3 were calculated using the screened exchange hybrid functional, and explain the electron hopping and trapping levels seen in deposited Al2O3 at ∼1.8 eV below its conduction band edge. The vacancy supports five accessible charge states, from 2+ to 2−. Electron hopping corresponds to the 0/− level, which lies 1.8 eV below the conduction band edge in θ-Al2O3. This level lies much deeper than it does HfO2. The +/0 level lies at 2.8 eV above oxide valence band in θ-Al2O3 and thus below the Si valence band top.

Shallow trapping center parameters in as‐grown AgIn5S8 crystals determined by thermally stimulated current measurements

AbstractThermally stimulated current measurements were carried out on as‐grown AgIn5S8 single crystals. The investigations were performed in temperatures ranging from 10 to 70 K with heating rate of 0.2 Ks–1. The analysis of the data revealed the electron trap level located at 5 meV. The activation energies of the traps have been determined using various methods of analysis, and they agree with each other. The calculation for these traps yielded 2.2 × 10–25 cm2 for capture cross section and 6.1 × 1012 cm–3 for the concentration. It was concluded that in this center retrapping was negligible, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Comparison of the structural properties and electrical characteristics of Pr2O3, Nd2O3 and Er2O3 charge trapping layer memories

The structural properties and electrical characteristics of metal-oxide-high-k material-oxide- silicon (MOHOS)-type memory devices using praseodymium oxide (Pr2O3), neodymium oxide (Nd2O3) and erbium oxide (Er2O3) films as the charge storage layer were investigated. X-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy were employed to analyze the structural and morphological features of these high-k films. It was found that MOHOS-type memory devices prepared under the Nd2O3 film and annealed at 800 °C exhibited a larger flatband voltage shift of 3 V (Vg = 10 V for 1 ms) and a lower charge loss of ∼11% at room temperature as compared to other conditions. This result suggests that the Nd2O3 film having a rougher surface, higher ratio of silicate to silica and larger conduction band offset causes the higher probability for trapping the charge carrier and the deep electron trap level in the film.

Thermally stimulated current observation of trapping centers and their distribution in as-grown TlGaSeS layered single crystals

Thermally stimulated current (TSC) measurements were carried out in as-grown TlGaSeS layered single crystals. The investigations were performed in temperatures ranging from 10 to 160 K with heating rate of 0.8 K s −1. The analysis of the data revealed three electron trap levels at 13, 20 and 50 meV. The activation energies of the traps have been determined using various methods of analysis, and they agree with each other. The calculation for these traps yielded 3.3 × 10 −24, 1.0 × 10 −24, and 1.1 × 10 −24 cm 2 for capture cross-sections and 1.9 × 10 12, 2.9 × 10 11 and 4.5 × 10 10 cm −3 for the concentrations, respectively. It was concluded that in these centers retrapping was negligible, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. An exponential distribution of electron traps was also revealed from the analysis of the TSC data obtained at different light excitation temperatures. This experimental technique provided values of 9 and 77 meV/decade for traps distribution of peaks A and C, respectively.

Time-delayed transformation of defects in zinc oxide layers grown along the zinc-face using a hydrothermal technique

A study of deep level defects in a hydrothermally grown, intrinsically n-type zinc oxide (ZnO) device has been carried out using conventional deep level transient spectroscopy (DLTS). Performed under variable measurement conditions, DLTS demonstrates two electron trap levels, E1 (dominant) and E2, with activation energies Ec−0.22±0.02 eV and Ec−0.47±0.05 eV, respectively. A time-delayed transformation of shallow donor defects zincinterstitial and vacancyoxygen (Zni-VO) into the E1 level has been observed. While the x-ray diffraction measurements reveal that the preferred direction of ZnO growth is along the (101¯0) plane, i.e., the (Zni-VO) complex, it is assumed that the (Zni-VO) complex is transformed into a zinc antisite (ZnO) under favorable conditions. As a result, the free carrier concentration decreases with increasing trap concentration. Henceforth, the E1 level exhibiting the increase in concentration is attributed to ZnO.

Study of trap states in polyspirobifluorene based devices: Influence of chromophore addition

The defect states in spiro copolymer based light emitting diodes were investigated by charge based deep level transient spectroscopy (Q-DLTS). Two types of polymers have been studied: blue emitting spiro copolymer and white emitting spiro blend polymer. The white emitting spiro polymer was obtained by adding green and red chromophores into the host blue copolymer. The devices are composed of indium-tin oxide–polyethylene dioxythiophene:polystyrene sulfonate-spiro copolymer-Ba–Al. Q-DLTS measurements were performed on these diodes with various conditions of charging time, charging voltage, and temperature aiming at determining the role of chromophores in the defect formation process. Analysis of the Q-DLTS spectra obtained in both devices revealed at least five trap levels. The mean activation energies of traps are distributed in the range 0.17–0.85 eV within the band gap of the copolymers with capture cross sections of the order of 10−16–10−20 cm2. The trap densities are in the range of 1015–1016 cm−3. The results show that incorporation of dyes into the copolymer resulted in creation of an additional electron trap level and an increase in the density of the existing trap levels, indicating a more disordered state of the emitting material containing chromophores

Electron Trap Level of Cu-Doped ZnO

The electron trap level of Cu-doped ZnO has been investigated. Films with Cu contents from 0.01 to 1 at. % were deposited by RF magnetron sputtering onto (0001)-oriented sapphire substrates at 300 °C, and annealed at 900 °C. The resistivity of these films was high and increased with increasing Cu content. Hall measurements carried out at high temperatures of approximately 400 °C showed exponential temperature dependence. The results were analyzed assuming that donors and electron traps coexist. The electron trap energy level was determined to be in the 0.30–0.35 eV range below the conduction band edge.

Radiation hardness studies in a CCD with high-speed column parallel readout

Charge Coupled Devices (CCDs) have been successfully used in several high energy physics experiments over the past two decades. Their high spatial resolution and thin sensitive layers make them an excellent tool for studying short-lived particles. The Linear Collider Flavour Identification (LCFI) collaboration is developing Column-Parallel CCDs (CPCCDs) for the vertex detector of the International Linear Collider (ILC). The CPCCDs can be read out many times faster than standard CCDs, significantly increasing their operating speed. The results of detailed simulations of the charge transfer inefficiency (CTI) of a prototype CPCCD including the variation of model parameters are reported. The effects of bulk radiation damage on the CTI of a CPCCD are studied by simulating the effects of two electron trap levels, 0.17 and 0.44 eV, at different concentrations and operating temperatures. The dependence of the CTI on different occupancy levels (percentage of hit pixels) and readout frequencies is also studied. The optimal operating temperature for the CPCCD, where the effects of the charge trapping are at a minimum, is found to be about 230 K for the range of readout speeds proposed for the ILC. The results of the full simulation have been compared with an analytic model.

21.5L: Late‐News Paper: Measurement of Exo‐Electron Emission from MgO Thin Film of AC‐PDP

AbstractExoelectron emission from MgO thin film was measured by attaching a high precision current sensor to the address electrode of rear plate of test panels of AC‐PDP. The measured results revealed that the exoelectron emission currents vary very sensitively with the type of doping elements used and measuring temperatures. The activation energy of the exo‐electron emissions estimated from the emission curves indicated that the trap levels lies between 0.05∼0.32eV below the bottom of its conduction band. This suggests that shallow electron trap levels within MgO film are mainly responsible for the exo‐electron emissions, rather than deep trap levels like F‐type centers.

Simulation of the charge transfer inefficiency of column parallel CCDs

Charge-Coupled Devices (CCDs) have been successfully used in several high-energy physics experiments over the past two decades. Their high spatial resolution and thin sensitive layer make them an excellent tool for studying short-lived particles. The Linear Collider Flavour Identification (LCFI) collaboration is developing Column Parallel CCDs (CPCCDs) for the vertex detector of the International Linear Collider (ILC). The CPCCDs can be read out many times faster than standard CCDs, significantly increasing their operating speed. The results of detailed simulations of the Charge Transfer Inefficiency (CTI) of a prototype CPCCD chip are reported. The effects of the radiation damage on the CTI of a Si-based CCD particle detector are studied by simulating the effects of two electron trap levels Ec-0.17 and Ec-0.44 eV at different concentrations and operating temperatures. The dependence of the CTI on different occupancy levels (percentage of hit pixels) and readout frequencies is also studied. The optimal operating temperature—where the effects of the trapping are at a minimum—is found to be ∼230 K for the range of readout speeds proposed for the ILC.

A study of electron induced defects in n‐type germanium by deep level transient spectroscopy (DLTS)

AbstractThis paper presents the electrical characteristics of electron irradiation induced defects in n‐type germanium doped with antimony (Sb) by deep level transient spectroscopy (DLTS) and Laplace‐ DLTS. The following electrons traps at 0.15 eV, 0.20 eV, 0.21 eV 0.23 eV, 0.31 eV and 0.38 eV below the conduction band were observed and two hole traps at 0.09 eV and 0.31 eV above the valence band were also recorded. The electron trap level at 0.38 eV is the E – center and is the most dominant electron trap for our measurements. The capacitance versus temperature and current versus temperature characteristics of our samples showed good correlation as the irradiation dose is increased. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Sensitivity of PbSnTe:In films to submillimeter radiation under conditions of field electron injection

Based on the concept of injection currents in the PbSnTe:In alloy, the mechanism of photosignal formation in PbSnTe:In films in the submillimeter spectral range at liquid-helium temperatures was considered. It was shown that the existence of energy-distributed electron trap levels controlling the features in current-voltage characteristics can cause extrinsic photoconductivity. For such sensitivity to submillimeter radiation, its spectral dependence should be related to the injection level controlling the filling of traps with different energies. The sensitivity initiation mechanism caused by the interaction of submillimeter radiation with the PbSnTe:In phonon subsystem was considered. Such a process can result in an increase in the static permittivity and, hence, an increase in the injection current.