Deep Trapping Centers Research Articles

Theoretical studies on nonvolatile holographic recording for LiNbO3:Cu:Ce crystals

The steady-state nonvolatile two-step, two-color holographic recording performance is studied theoretically for LiNbO3:Cu:Ce based on the two-center model, with taking into account the direct electron transfer between the deep-trap center Cu+/Cu2+ and the shallow-trap center Ce3+/Ce4+ due to the tunneling effect. The results show that the total space-charge field is determined by the space-charge field on the deep-trap center, and the direct electron exchange between the Cu+/Cu2+ and the Ce3+/Ce4+ levels through the tunneling effect dominates the charge-transfer process in the two-step, two-color holographic recording. Therefore, the material parameters related to this direct tunneling process play a key role in the two-step, two-color holography performance.

Effect of [Li]/[Nb] ratios on the photorefraction and scattering properties in In: Fe: Cu: LiNbO 3 crystals at 488 nm wavelength

The high sensitivity, fast response and the high quality reconstructions are observed in various [Li]/[Nb] ratios In:Fe:Cu:LiNbO 3 crystals at 488 nm wavelength based on the two-beam coupling experiment. The strong blue photorefraction is contributed by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The blue photorefraction is enhanced significantly with the increasing of [Li]/[Nb] ratios under the same experimental conditions. The sensitivity S" is reduced to 0.46 J/cm, simultaneously the response time is as fast as 4.4 s and the erase phenomenon is not obvious in In:Fe:Cu:LiNbO 3 crystals which [Li]/[Nb] ratio is 0.986 in crystal. Increasing [Li]/[Nb] ratios improve the damage-resistant ability of the crystals, but lead to a more serious beam fanning. Experimental results definitely show that the near-stoichiometric In: Fe: Cu: LiNbO 3 crystal becomes a promising candidate for blue photorefractive holographic recording.

Dark current mechanisms in stabilized amorphous selenium based n-i detectors for x-ray imaging applications

The dark current behavior under operating bias is one of the important selection criteria for an x-ray photoconductor to be usable in a practical x-ray image detector. The authors have developed an analytical model for describing the transient and steady-state behavior of dark current in n-i-type amorphous selenium (a-Se) detectors by considering carrier injections from the metal contacts and thermally generated carriers. It has been found that the thermal generation current is almost two orders of magnitude smaller than the total steady-state dark current in n-i-type a-Se detectors. The main source of dark current is the injection of holes from the metal/n-layer interface which is described by the diffusion theory. The hole injection from the metal depends on the blocking layer (n-layer) thickness, the concentration of trap centers in the blocking layer, the characteristic carrier release time, and the effective barrier height. The fitting of the first principles model with the experimental results estimates the concentration of deep hole trap center in the n-layer, the trap depth from the valence band edge, and the effective barrier heights for the injecting carriers. The electron injection varies with the work function of the contact metal.

The effect of light illumination in photoionization of deep traps in GaN MESFETs buffer layer using an ensemble Monte Carlo simulation

Deep traps can produce serious degradation in the drain current and consequently the output power of GaN based FETs. This current collapse phenomenon represents a significant impediment to the incorporation of these devices in electronic systems. In this article trapping of hot electron behavior by deep trap centers located in buffer layer of a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. The simulated results show the trap centers are responsible for current collapse in GaN MESFET at low temperatures. These electrical traps degrade the performance of the device at low temperature. On the opposite, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built in electric field associated with the trapped charge distribution.

Reliability studies on Ta2O5 high-κ dielectric metal-insulator-metal capacitors prepared by wet anodization

The temperature dependence of leakage current for tantalum oxide metal-insulator-metal capacitors has been investigated over the temperature range 20–160 °C. The leakage current shows an increase with temperature and the conduction mechanism at medium to high electric fields is in agreement with the modified Poole–Frenkel model. The activation energy of the dominant deep trapping center in the oxide is calculated using this model. Constant voltage and constant current stress have been applied to the devices and the effect of stress conditions on leakage current, breakdown voltage, and high frequency capacitance-voltage have been investigated. Early oxide breakdown or time-dependent dielectric breakdown was observed during constant voltage and constant current stress, in which the former is a function of stress time and applied voltage or current. There is an increase in leakage current with time during the constant voltage stress, presumably due to generation of positive defect states. This is also apparent from the decrease in voltage with time during constant current stress. Degradation of the oxides by constant current stress and its effect on the oxide leakage current (or stress induced leakage current) at various constant current values and stress times has also been investigated and is discussed in this article.

Photon-accelerated negative bias instability involving subgap states creation in amorphous In–Ga–Zn–O thin film transistor

We investigated the visible photon accelerated negative bias instability (NBI) in amorphous In–Ga–Zn–O (a-IGZO) thin film transistor (TFT). As reported in previous works, the rigid shift in transfer curves with insignificant changes in field-effect mobility and subthreshold swing was observed. On the other hand, there is substantial change in capacitance-voltage characteristics caused by created subgap states. The suggested nature of created states is the ionized oxygen vacancy (VO2+) by the combination of visible light and negative bias. The generated VO2+ states enhance the NBI under illumination as increased deep hole trapping centers. Furthermore, the photoexcitation of VO to stable VO2+ yields excess free carriers in conduction band. The increased carrier density also enhances the negative shift in turn-on voltage of a-IGZO TFT.

Characterization of silicon polycrystalline solar cells at cryogenic temperatures with ion beam-induced charge

We present a spatial analysis of biased polycrystalline silicon solar cell samples at cryogenic temperatures by the ion beam-induced charge technique. Performing the measurements at cryogenic temperatures leads to freezing out of deep trapping centers, a reduction of the specimen’s capacitance, an increase in the mobility of carriers, and an improvement in the signal to noise ratio and charge collection efficiency. At this low temperature, it permits a higher sensitivity analysis when compared to room temperature measurements, which are limited to zero bias due to leakage current that occurs via surface carrier trapping and detrapping along the grain boundaries. As a result we find the images are significantly more sensitive to subtle differences in charge collection efficiency arising from intra-grain defects.

Control of Interface Traps in HfO2 Gate Dielectric on Silicon

The effects of postdeposition annealing (PDA) on the interface between HfO2 high-k dielectric and bulk silicon were studied in detail. The key challenges of successfully adopting the high-k dielectric/Si gate stack into advanced complementary metal–oxide–semiconductor (CMOS) technology are mostly due to interfacial properties. We have proposed a PDA treatment at 600°C for several different durations (5 min to 25 min) in nitrogen or oxygen (95% N2 + 5% O2) ambient with a 5-nm-thick HfO2 film on a silicon substrate. We found that oxidation of the HfO2/Si interface, removal of the deep trap centers, and crystallization of the film take place during the postdeposition annealing (PDA). The optimal PDA conditions for low interface trap density were found to be dependent on the PDA duration. The formation of an amorphous interface layer (IL) at the HfO2/Si interface was observed. The growth was due to oxygen incorporated during thermal annealing that reacts with the Si substrate. The interface traps of the bonding features, defect states, and hysteresis under different PDA conditions were studied using x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), transmission electron microscopy (TEM), and leakage current density–voltage (J–V) and capacitance–voltage (C–V) techniques. The results showed that the HfO2/Si stack with PDA in oxygen showed better physical and electrical performance than with PDA in nitrogen. Therefore, PDA can improve the interface properties of HfO2/Si and the densification of HfO2 thin films.

Photoconductivity characteristics of ZnO nanoparticles assembled in nanogap electrodes for portable photodetector applications

In the present report, ZnO nanoparticles of size ∼9 nm have been assembled by optimized dielectrophoresis process on the pre-fabricated nanogap (60 nm) electrodes. The fabricated ZnO nanoparticles based nano-device was studied for its I– V characteristic and typical non-linear semiconducting behavior was observed. When illuminated with ultraviolet (UV) radiation of wavelength 365 nm, a tremendous change in conductivity of almost one order of magnitude was observed indicating a high sensitivity of the fabricated nano-device. Temporal photoresponse characteristics were studied at a fixed level of UV illumination intensity (1.0 mW/cm 2) and the response time under 10 ms was recorded. The photoresponse curve exhibited a perfect rise and recovery without any slow component. Absence of slow component in the photoresponse characteristic suggested that shallow trapping centers were inactive and the photoconductivity could be due to the true quantum yield of photogenerated charge carriers through interband and deep trapping centers transitions.

Improvement of blue photorefractive properties in In-doped LiNbO3 : Fe : Cu crystals

A high sensitivity and large dynamic range are observed in LiNbO3 : Fe : Cu and In-doped LiNbO3 : Fe : Cu crystals at 488 nm wavelength based on the two-beam coupling experiment. The strong blue photorefraction is contributed by the two-centre effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centres. The sensitivity S is measured up to 6.24 cm J−1, and simultaneously the dynamic range M/# is achieved to 34.2 in the In-doped LiNbO3 : Fe : Cu crystal, which are enhanced significantly as compared with those of the LiNbO3 : Fe : Cu crystal under the same experimental conditions. The damage-resistant dopants such as In3+ ions are no longer damage resistant, but they enhance the photorefractive properties at 488 nm wavelength. Experimental results definitely show that In-doped two-centre LiNbO3 : Fe : Cu crystal is a promising candidate as a blue photorefraction material for volume holographic storage.

Charge Recombination Induced Luminescence of Chemically Modified Cross-Linked Polyethylene Materials

Very little is known on the chemical nature of the deep trapping centres that play a role in charge storage in insulating polymeric materials. The analysis of the luminescence emitted during a process of charge recombination can be helpful to unravel the nature of such centres. The spectral features of the luminescence induced by charge recombination are analyzed for three different cross-linked polyethylene (XLPE) materials, chemically-modified or not. In two XLPE formulations, a controlled amount of chemical groups containing an aromatic ring has been grafted to the polymer chains. The other formulation is the same XLPE without chemical modification. Photoluminescence is investigated to settle a background to identify the species involved in charge recombination. It is excited on films at room and liquid nitrogen temperature providing fluorescence and phosphorescence spectra. The luminescence excited by the recombination of trapped electrical charges is subsequently analyzed at low temperature and compared to the photoluminescence emission. The results obtained on chemically-modified XLPE provide a basis to identify the species involved in charge recombination in the non modified material. It turns out that aromatic compounds are involved in photo- and charge recombination-induced luminescence in all the tested XLPE, and that the luminescence has the same spectral features for the two stimulations, contrary to what is observed for low density polyethylene.

Enhancement of nonvolatile blue photorefractive properties in LiNbO3:In:Fe:Cu crystals

The nonvolatile photorefractive characteristics of LiNbO3:Fe:Cu and In-doped LiNbO3:Fe:Cu crystals are investigated. The stronger nonvolatile blue photorefraction observed can be ascribed to its remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers, which is one or two orders of magnitude higher than those obtained in conventional two-color recordings under the same recording conditions. Furthermore, it is interesting that, compared with LiNbO3:Fe:Cu, the recording properties, such as the saturation refractive index change, nonvolatile sensitivity and response time at 488 nm wavelength are enhanced in LiNbO3:In:Fe:Cu crystals under the same recording conditions. The so-called damage-resistant dopants such as In3+ ions in red photorefraction are not damage resistant at 488 nm wavelength but they enhance the blue photorefraction.

Charge transport in polycrystalline and single crystal synthetic diamond using ion beam induced charge imaging

Ion beam induced charge (IBIC) imaging has been used to investigate charge transport in high quality polycrystalline CVD diamond and single crystal synthetic diamond. In polycrystalline material the intra-crystallite charge transport within single crystals was studied. Micrometer resolution images of charge signal amplitude in different regions of a diamond crystallite are obtained as a function of the electric field strength. The charge signal amplitude, which is directly related to charge carrier drift length, is correlated to the distribution of radiative centres in the diamond identified using cathodoluminescence imaging. By comparison, IBIC imaging of single crystal synthetic diamond was also carried out. The sample, which contained deliberately introduced nitrogen lines, showed electron and hole CCE values close to 100%, in the low nitrogen regions. By comparison with CL and UV luminescence images, direct evidence is observed for the role of nitrogen and dislocations as deep trapping centres and their reduction of both electron and hole lifetimes.

Charge trapping properties at silicon nitride/silicon oxide interface studied by variable-temperature electrostatic force microscopy

Charge trapping properties of electrons and holes in ultrathin nitride-oxide-silicon (NOS) structures were quantitatively determined by variable-temperature electrostatic force microscopy (EFM). From charge retention characteristics obtained at temperatures between 250 and 370°C and assuming that the dominant charge decay mechanism is thermal emission followed by oxide tunneling, we find that there are considerable deep trap centers at the nitride-oxide interface. For electron, the interface trap energy and density were determined to be about 1.52eV and 1.46×1012cm−2, respectively. For hole, these are about 1.01eV and 1.08×1012cm−2, respectively. In addition, the capture cross section of electron can be extracted to be 4.8×10−16cm2. The qualitative and quantitative determination of charge trapping properties and possible charge decay mechanism reported in this work can be very useful for the characterization of oxide-nitride-silicon based charge storage devices.

Multiple relaxation processes in high-energy ion irradiated kapton-H polyimide: Thermally stimulated depolarization current study

High-energy ion irradiation effects on the thermally stimulated depolarization current (TSDC) behaviour of kapton-H samples (12.5 μm) irradiated with 50 MeV Li ion (fluence 5 × 10 4, 10 5 and 5 × 10 5 ions/cm 2) have been investigated. The TSDC spectra of the irradiated samples reveal that the β-peak (appearing around 80–110 °C) associated with dipolar relaxation has been significantly affected owing to the demerization of carbonyl groups due to irradiation. The TSDC spectra also reveal a new relaxation process (termed as γ-relaxation) around 30 °C, due to increased water absorptivity in irradiated samples. The peak around 200 °C (α-peak) associated with space charge relaxation process also shows a behavioural change with ion irradiation. The peak not only shifts towards the higher temperature with increasing fluence but also show an increase in its activation energy (0.33–0.99 eV) with increasing polarizing field. The creation of new deep energy trap centers due to the formation of conjugated bonds after irradiation is responsible for this modification. The Cole–Cole distribution curves show the formation of new sub-polar group with different characteristic relaxation time.

Muonium in InSb: Shallow acceptor versus deep trap or recombination center

The bound state of a muonium atom has been detected in both $n$-type and $p$-type InSb using a high-field $\ensuremath{\mu}\mathrm{SR}$ technique. The hyperfine constant obtained for this isotropic center $({A}_{\mathrm{T}}=2464\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})$, roughly half that of a Mu atom in vacuum, is characteristic of deep level ${\mathrm{Mu}}^{0}$ centers at tetrahedral interstitial sites in other cubic semiconductors, which typically ionize above $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In contrast, the ${\mathrm{Mu}}^{0}$ signal in InSb begins to disappear near $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is more characteristic of ionization of a shallow level impurity. The charge-state dynamics of Mu in InSb can be understood in terms of the predicted shallow acceptor behavior for H in InSb versus a deep hole trap or recombination center, rather than as ionization via electron promotion to the conduction band.

Deep muonium state in InSb: Recombination center vs. trapping center

The bound state of an isotropic muonium atom has been detected in both n- and p-type InSb using a high-field μ SR technique. The hyperfine constant obtained for this center ( A = 2464 ± 1 MHz ) is characteristic of deep-level Mu 0 centers at tetrahedral interstitial sites in other cubic semiconductors, which typically ionize above 300 K . In contrast, the Mu 0 center in InSb disappears above about 30 K , which is more characteristic of ionization of a shallow-level impurity. The charge-state dynamics of Mu in InSb is discussed in terms of a deep trap or recombination center, rather than as electron ionization.

Sensitivity of stabilized a-Se based X-ray photoconductors

A model explaining the ghosting mechanisms in amorphous selenium based X-ray image detectors is described by considering deep trapping of charge carriers, trapped charges arising from previous exposures, trap filling effects, recombination between drifting and trapped carriers, generation of X-ray induced new deep trap centers, space charge effects, carrier detrapping, and electric field dependent electron–hole pair creation energy. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson’s equation across the photoconductor for a pulse X-ray exposure by the finite difference method. The numerical results are compared with Monte Carlo simulations. The relative sensitivity decreases with increasing accumulative X-ray exposure and decreasing applied electric field. The sensitivity reduction at negative bias is greater than at positive bias. The theoretical model is fitted to experimental data. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers, electric field dependent charge carrier generation and X-ray induced new deep trap centers are mainly responsible for the sensitivity reduction in biased amorphous selenium based X-ray detectors.

The role of interfaces in the dielectric strength of polymeric films under high intensity fields

This paper examines the role of interfaces in the polymeric insulating films under the scope of their dielectric strength. PET (Polyethylene terephthalate) films of different thickness are put together forming a planar sample of a typical thickness of m and subjected to AC ramp voltage, until the dielectric breakdown. The role of interface as a potential barrier for electrons was examined as well as the influence of interface number for a standard thickness. Experiments were also executed for investigating the role of the position of the first, the second and the last interface in the sample. All the experiments were executed with the same voltage rising rate, the breakdown AC voltage V b was measured and the dielectric strength F b was calculated. It was shown that interfaces act as deep trapping centers for electrons giving rise to the sample dielectric strength for sort time experiments. Moreover the thinner the first film, the higher the dielectric strength for the sample. It was also found that the position of the second interface does not play an important role in the process.

Theoretical studies on nonvolatile two-step, two-color holographic recording sensitivity for LiNbO3:Fe

The effects of material and experimental parameters on the nonvolatile two-step, two-color holographic recording sensitivity for LiNbO3:Fe have been studied theoretically based on a two-center model. We have taken into account the direct electron transfer between the deep-trap center Fe2+/Fe3+ and the shallow-trap center NbLi4+/NbLi5+ due to the tunneling effect. The results show that the direct electron transfer between the Fe2+/Fe3+ and the NbLi4+/NbLi5+ centers plays a key role in the improvement of two-step, two-color holographic recording sensitivity. The near-stoichiometric LiNbO3:Fe has been confirmed theoretically to be of higher recording sensitivity and larger dynamic range than the congruent one in the low intensity region. A further improvement of the recording sensitivity can be achieved by employing appropriate doping concentration and thermal reduction treatment of Fe and/or selecting appropriate gating and recording wavelengths with large photo-excitation cross-sections.