Charge Trapping Centers Research Articles

Structural and electrical properties of polyaniline/silver nanocomposites

Polyaniline (PANI)/Ag nanocomposites were prepared by separate synthesis of silver nanoparticles by inert gas condensation, incorporating in the 1-methyl-2-pyrrolidinone (NMP) solution of polyaniline emeraldine base (PANIEB) and then cast into films at 120 °C. X-ray diffraction confirmed the presence of ∼67 nm silver nanoparticles in the polyaniline matrix. From the thermogravimetric analysis it is observed that the nanocomposite films have a higher degradation temperature than the pure PANI film. Scanning electron microscopy showed a uniform distribution, with spherical and granular morphology for low concentration of Ag nanoparticles, whereas for higher concentration (1.0% Ag) nanorods are formed. The impedance spectroscopic studies of NMP plasticized nanocomposite films suggest microphase separation into reduced and oxidized repeat units. Incorporation of silver nanoparticles in PANI reduces the charge trapping centres and increases the conducting channels, which causes a tenfold decrease in the real part of impedance.

Organic bistable memory device using MoO 3 nanocrystal as a charge trapping center

Organic bistable memory devices (OBDs) with MoO 3 as a nanocrystal inside organic layer were developed and bistability of MoO 3 based OBDs was investigated. High on/off ratio over 200 was obtained at a low reading voltage of 1 V. MoO 3 OBDs could be electrically switched between high conductance state and low conductance state over more than 100 cycles and space charge limited conduction mechanism dominated switching behavior in MoO 3 OBDs.

Crystal growth and scintillation properties of YAlO 3:Pr co-doped with Mo 3+ and Ga 3+ ions

In this work, we present results of studies aimed at optimization of scintillation performance of fast but low-light yield YAlO 3:Pr scintillator. We applied co-doping with trivalent ions Mo 3+ and Ga 3+ in attempts to reduce the concentration of charge trapping centers that may exist in this material. Systematic study was performed by using the micro-pulling-down method for single crystal growth. Square-shaped YAlO 3:(Pr,Mo) and YAlO 3:(Pr,Ga) crystals with selected co-dopant concentrations were grown. Experimental results including absorption spectra, luminescence under X-ray excitation and scintillation decay are presented. No positive contribution of the co-dopant ions was found. Complicated co-dopant behavior makes the improvement of scintillation parameters difficult.

MIMC reliability and electrical behavior defined by a physical layer property of the dielectric

Metal–insulator–metal capacitor (MIMC) reliability and electrical properties are defined by the TDDB lifetime, breakdown voltage and leakage current. In this article, the correlation is determined between these electrical properties and the physical and chemical properties of the SiN dielectric layer. It is demonstrated how a SiN dielectrics with a high refractive index have high Si content and show an increased initial leakage current. However, contradictory to the high leakage current, these dielectrics also show high lifetimes. It is shown that SiN dielectrics with a high Si content contain high numbers of charge trapping centers. Over time, a high concentration of trapped charges is build up to such an extend that the local electric field over the dielectric is significantly decreased. This results in the observed reliability improvement of the dielectric. The final intrinsic quality and reliability of MIMC capacitors can therefore be determined by measurable physical properties of the MIMC dielectric at the time of the deposition of this layer.

Observation of interfacial electrostatic field‐induced changes in the silicon dielectric function using spectroscopic ellipsometry

AbstractThis work investigates the capability of spectroscopic ellipsometry to measure charge trapping centers in thin dielectric films. Specific interfacial electrostatic fields, induced by electrons injected into charge trapping states at the interface, have been identified that directly affect the underlying silicon substrate critical points. The effect of a field‐induced change in the silicon fundamental absorption edge due to different processing conditions affecting the oxygen vacancy defects at the interface is presented. Measuring the field‐induced change in the silicon dielectric function between a sample with a 2 nm HfO2 film as‐deposited and the same sample after a 1000 °C anneal in an N2 ambient reveals that a stronger interfacial field is present for the as‐deposited HfO2 film. These results are consistent with the understanding that high temperature anneals work well to passivate oxygen vacancy defects at the silicon/HfO2 interface. Finally, we compare our results with Second Harmonic Generation where specific resonant features are identifiable with electric field enhancements at the same interface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron Spin Resonance and Photoluminescence Study of Charge Trap Centers in Silicon Nitride Films and Fabrication of Proposed Oxide–Nitride–Oxide Sidewall 2-bit/Cell Nonvolatile Memories

We have proposed a novel oxide–nitride–oxide (ONO)-sidewall 2-bit/cell nonvolatile memory and fabricated 70-nm-node nonvolatile memory devices. For low-pressure chemical-vapor-deposition (LPCVD)-SiN films, with increasing SiH4/NH3 mixture gas ratio, we have found from ESR and PL evaluation that the paramagnetic defect density increases and some PL emission energy levels become deeper. We consider that the energy-level shift is due to the effects of trap potential overlapping, where the trap centers are generated at the excess silicon atoms in the SiN films. In this study, a SiH4/NH3 mixture gas ratio of less than 1:100 was used to suppress the potential overlapping. As a result, we have also shown that the proposed memory device has high performance and excellent scalability.

Modelling the light induced metastable effects in amorphous silicon

AbstractWe present results of computer simulations of the light induced degradation of amorphous silicon solar cells. It is now well established that when amorphous silicon is illuminated the density of dangling bond states increases. Dangling bond states produce amphoteric electronic mid‐gap states which act as efficient charge trapping and recombination centres. The increase in dangling bond states causes a decrease in the performance of amorphous silicon solar cells. To show this effect, a modelling approach has been developed which uses the density of localised states with exponentially increasing band‐tails and dangling bond defect states distribution chosen according to the defect pool model. The calculation of the evolution of dangling bond state density during illumination has been achieved through a dynamic scaling relation derived from a defect creation model. The approach considers the amphoteric nature of the dangling bond state and thus accounts for the contributions of the different charge states of the dangling bond during the degradation process. This paper attempts to describe the simulation approach which calculates the defect density as a function of energy, position in the solar cell and illumination time. In excellent agreement with other workers, our simulation results show that the increase in the density of neutral dangling bond states during illumination is higher than of the charged states. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of Mo Silicide Nanodot Memory by Rapid Thermal Annealing Dual Electron-Gun Evaporated Mo–Si Layer

In this study, distributed charge trapping centers molybdenum silicide nanodots, are demonstrated. The Mo silicide nanodots were formed in the rapid thermal annealed mixed Mo-Si layer deposited by a dual electron gun system. A significant memory effect is observed through the electrical measurements, which is attributed to the formation of Mo silicide nanodots. In addition, the memory window is large enough to be identified as logic level 0 and 1 for the application on nonvolatile memory. The manufacture processes are simple and compatible with the conventional semiconductor manufacturing processes.

Bulk and Interfacial Oxygen Defects in HfO2 Gate Dielectric Stacks: A Critical Assessment

Oxygen-related point defects can provide fixed charge or act as charge trapping centers in high-k gate stacks and, therefore, their origins and properties are of great interest. In this paper, reported experimental and theoretical results related to oxygen defects in HfO2 gate dielectrics are reviewed critically to assess the relative importance of different defect species in terms of their electrical activity and their contributions to important kinetic processes. In addition to point defects in the "bulk" of HfO2 thin films, defects resulting from differences in oxygen coordination across the internal hafnium oxide/silicon oxide interface are considered.

Ab initio study on intrinsic defect properties of germanium nitride considered for gate dielectric

First-principles calculations based on density-functional theory and local-density approximation were carried out to investigate intrinsic defect properties in β-Ge3N4. It was found that nitrogen vacancies are the main source of intrinsic defects in Ge3N4 due to their low formation energy. The N vacancies might become charge trapping centers since they induce energy levels near the Ge conduction band edge and in the middle of the Ge3N4 band gap. The formation energy of intrinsic defects in Ge3N4 is sensitive to N chemical potential, and N-rich ambient is favorable to reduce the concentration of N vacancies.

The photoluminescence and thermoluminescence properties of novel green long-lasting phosphorescence materials Ca8Mg(SiO4)4Cl2:Eu2+,Nd3+

The novel green long-lasting phosphorescence (LLP) material Ca8Mg(SiO4)4Cl2:Eu2+,Nd3+ was systematically investigated by means of photoluminescence (PL) excitation and emission, a thermal quenching process, long-lasting phosphorescent emission, decay curves, chromaticity and thermoluminescence (TL) spectra. It is clearly observed that there exist two distinguishable Eu sites, EuI for 426-nm and EuII for 504-nm emissions, but only the green LLP appears at the EuII site. The emissions for both PL and LLP are due to the 5d→4f transitions. The green phosphorescence with the chromaticity coordination x=0.196, y=0.506 can still be observed in the dark for no less than 5 h by the naked eye. The results of TL show that Nd3+ ions induce the predominating band at 335 K, associated with the charge-trapping centers that are responsible for the appearance of intense green phosphorescence.

Determination of aggregates as charge trapping and recombination centers in poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] by time-resolved electroluminescence spectroscopy

A presence of aggregates can give critical influences on photophysical properties of conjugated polymer. For poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene], the authors found that the aggregates can serve as charge traps and recombination centers by using time-resolved electroluminescence spectroscopy, and that the emissions from isolated chains and aggregates show different dynamics. The authors also found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of aggregates lie within the band gap of isolated chains. Thus, the aggregates act as shallow traps for both electron and hole so that a reexcitation to HOMO and LUMO of isolated chains is possible.

Effect of H 2O adsorption on the electrical transport properties of double-walled carbon nanotubes

Water molecules adsorbed on a double-walled carbon nanotube (DWCNT) serve as charge trapping centers when present in low density and as electron donors when present in high density. There is a discontinuous change between the low- and high-density regions. H 2O molecules are apt to be adsorbed on the outer surface of DWCNTs, and in this case the electrical transport properties are extremely sensitive to environment, which suggests that DWCNTs are hole doped and act as an electric dipole with the inner tube.

Synchrotron–laser interactions in hexagonal boron nitride: an examination of charge trapping dynamics at the boron K-edge

Synchrotron–laser pump-probe (PP) luminescence measurements have been undertaken in hexagonal-phase boron nitride. For non-resonant excitation below the boron K-edge, the dynamics of the transients induced by the application of laser probes at 1.49, 2.33 and 3.07 eV are determined, as well as the signal recovery to equilibrium after the probes are removed. The transients are shown to be entirely related to the presence of trapping states within the material. By varying the synchrotron pump energy, the system dynamics are shown to change significantly, due to alteration in competition between the defect centres. As the synchrotron energy is scanned across the near-edge x-ray absorption (NEXAFS) region of the boron K-edge (190–205 eV), PP NEXAFS can be extracted directly from the trapping rate changes, and this opens the possibilities for directly linking particular charge trapping centres with their chemical environments. This is the first report where such processes have been used to obtain luminescence-based PP x-ray absorption spectra.

42.2: Invited Paper: Influence of Lanthanide Level Locations on the Performance of Phosphors

Divalent and trivalent lanthanides are used in luminescent and storage phosphors where they play the role of luminescence center but also the role of charge trapping center. The performance of these phosphors critically depends on the location of lanthanide energy levels relative to the conduction band and valence band of the host crystal. This paper reviews the methods to find the absolute location of energy levels and then demonstrates how they influence the behavior of the phosphors.

Process integration and nanometer-scale electrical characterization of crystalline high- k gate dielectrics

Crystalline praseodymium oxide (Pr 2O 3) high- k gate dielectric has been successfully integrated into a polysilicon gate CMOS technology. Fully functional MOSFETs with an equivalent oxide thickness (EOT) of 1.8 nm and gate leakages below 10 −6 A/cm 2 have been fabricated. However, at this early stage of development the transistors show Vt-instabilities and unusual high gate leakage for L > 10 μm. As a first attempt to explain the observed macroscopic device characteristics, topographical and electrical measurements at the nanometer scale have been performed directly on the Pr 2O 3 surface by Conductive Atomic Force Microscopy (C-AFM). This technique allows to discriminate between structural defect sites and charge trapping centers.

Charge trapping at high doses in an active matrix flat panel dosimeter

Active matrix flat panel dosimeters (AMFPDs) have a high density of charge trapping centers (/spl sim/3/spl times/10/sup 15//cm/sup 3/) which lead to both lag and ghosting in reconstructed dose measurements. Lag is due to charge release within the detector and is a function of the lifetime of the trapping centers within the detector. Ghosting, defined as excess charge not produced by charge release, is a phenomenon believed to be caused by reduced charge trapping probability due to the trapped charge population density being greater after exposure to a radiation field than previous to the exposure. This paper examines the characteristics of a large AMFPD (26 cm /spl times/ 26 cm) detector for long exposure times (up to 1 minute) and high doses (up to 3 Gy). This detector was operated in a direct detection mode. Nonlinear response functions were mapped as a function of exposure time and dose for an X-ray beam where the accelerating potential of the electrons was 6 megavolts (MV). Additionally, the impulse response functions (IRFs) were measured at 6 MV for the same set of exposure times and doses used for the measurement of the nonlinear mapping. These IRFs are shown to be functions of both exposure time and dose delivered to the detector. The reproducibility of dose measurements using procedures derived from this paper is discussed. The temporal transfer function (TTF), defined as the absolute value of the Fourier transform of the IRF, is introduced and several examples of the TTFs are shown and discussed.

Trapping parameters of repulsive centers in SbSI single crystals

Charge trapping centers in antimony sulphide iodide (SbSI) single crystals have been investigated by the use of thermally stimulated current (TSC) technique. The TSC spectrum consists of only one apparent peak which is found to be associated with a single trapping level. Those traps are experimentally found to obey the monomolecular kinetics. The trapping parameters as the energy depth, temperature dependent frequency factor and capture cross section together with the concentrations of the corresponding discrete trapping level are determined. The TSC signal is found to be strongly dependent on illumination temperature of the sample and this is explained by the model in which the traps are considered to be surrounded by repulsive potential barriers.

Scaling laws between seismo‐electric/magnetic fields and earthquake magnitude

AbstractThe mechanisms generating the charges/potentials and abnormal electromagnetic radiation that accompany earthquakes have been discussed based on observational, theoretical and laboratory studies, in particular over the last few decades. We previously conducted stick‐slip tests using precut granite cores with a contact electrode on the sample side, and proposed that surface charges released from charge trapping centres on sheared asperities are a possible source of the above‐mentioned charges or potentials. Here we discuss seismo‐electric and magnetic fields formed by the surface charges, and derive their scaling laws with the earthquake magnitude at a distance. According to our model, detectability of these fields at a point on the ground surface depends strongly on wet/dry condition at a fault zone (sheared asperities).

Trapping parameters of repulsive centers in SbSI single crystals

Charge trapping centers in antimony sulphide iodide (SbSI) single crystals have been investigated by the use of thermally stimulated current (TSC) technique. The TSC spectrum consists of only one apparent peak which is found to be associated with a single trapping level. Those traps are experimentally found to obey the monomolecular kinetics. The trapping parameters as the energy depth, temperature dependent frequency factor and capture cross section together with the concentrations of the corresponding discrete trapping level are determined. The TSC signal is found to be strongly dependent on illumination temperature of the sample and this is explained by the model in which the traps are considered to be surrounded by repulsive potential barriers.