Charge Trapping Processes Research Articles

Gamma Radiation‐Induced Changes in Trombay Nuclear Waste Glass Containing Iron

Alkali‐based barium borosilicate glasses having similar composition to the Trombay nuclear research reactor waste base glass containing iron were prepared and irradiated by gamma rays. The radiation‐induced defect centers were investigated using electron paramagnetic resonance (EPR) technique. The results showed the formation of silicon hole centers and electron trap centers apart from boron‐based oxy hole centers in the glass due to the irradiation. The EPR Hamiltonian parameters for these radicals were evaluated by simulation technique using Bruker SIMFONIA computer program. Glasses with varying iron content were irradiated with the same dose to evaluate the effect of iron content on the defect center concentration. A spin counting technique was employed to evaluate the number of defect centers produced in each glass. It was observed that the defect center concentration reduced as a function of increasing iron content. This was attributed to the charge trapping process of ferric ions in the amorphous system.

Transport properties of triarylamine based dendrimers studied by space charge limited current transients

We have studied hole transport in triarylamine based dendrimer using space-charge-limited current transient technique. A mobility of 8×10−6cm2/(Vs) and a characteristic detrapping time of about 100ms have been obtained. We found that quasi-ohmic contact is formed with gold. The obtained mobility differs from the apparent one given by the analysis of stationary current–voltage characteristics because of a limited contact efficiency. The comparison between transients obtained from fresh and aged samples reveals no change in mobility with aging. The deterioration of electrical properties is exclusively caused by trap formation and accumulation of ionic conducting impurities. Finally, repeated transient measurements have been applied to analyze the dynamics of charge trapping process.

Bias-Stress Effect in 1,2-Ethanedithiol-Treated PbS Quantum Dot Field-Effect Transistors

We investigate the bias-stress effect in field-effect transistors (FETs) consisting of 1,2-ethanedithiol-treated PbS quantum dot (QD) films as charge transport layers in a top-gated configuration. The FETs exhibit ambipolar operation with typical mobilities on the order of μ(e) = 8 × 10(-3) cm(2) V(-1) s(-1) in n-channel operation and μ(h) = 1 × 10(-3) cm(2) V(-1) s(-1) in p-channel operation. When the FET is turned on in n-channel or p-channel mode, the established drain-source current rapidly decreases from its initial magnitude in a stretched exponential decay, manifesting the bias-stress effect. The choice of dielectric is found to have little effect on the characteristics of this bias-stress effect, leading us to conclude that the associated charge-trapping process originates within the QD film itself. Measurements of bias-stress-induced time-dependent decays in the drain-source current (I(DS)) are well fit to stretched exponential functions, and the time constants of these decays in n-channel and p-channel operation are found to follow thermally activated (Arrhenius) behavior. Measurements as a function of QD size reveal that the stressing process in n-channel operation is faster for QDs of a smaller diameter while stress in p-channel operation is found to be relatively invariant to QD size. Our results are consistent with a mechanism in which field-induced nanoscale morphological changes within the QD film result in screening of the applied gate field. This phenomenon is entirely recoverable, which allows us to repeatedly observe bias stress and recovery characteristics on the same device. This work elucidates aspects of charge transport in chemically treated lead chalcogenide QD films and is of relevance to ongoing investigations toward employing these films in optoelectronic devices.

Charge trapping process of nonvolatile memory devices based on CdTe and CdTe–CdSe core-shell nanoparticles/poly(methylmethacrylate) nanocomposites

Nonvolatile memory devices based on CdTe and CdTe–CdSe core-shell nanoparticles embedded in a poly(methylmethacrylate) (PMMA) layer were fabricated to investigate the variation in the carrier transport mechanisms due to a CdSe shell. Capacitance-voltage (C-V) curves for Al/CdTe nanoparticles embedded in PMMA/p-Si and Al/CdTe–CdSe nanoparticles embedded in PMMA/p-Si devices at 300 K showed that the flatband voltage shift of the C-V curve for the device with the CdTe–CdSe nanoparticles was relatively smaller than that for the device with the CdTe nanoparticle. Carrier transport mechanisms of the memory devices are described by using the C-V results, energy band diagrams, and capacitance-time retentions.

Influence of trapping states at the dielectric–dielectric interface on the stability of organic field-effect transistors with bilayer gate dielectric

Wet-chemical treatment on the gate dielectric, i.e. tuning the chemical properties via the choice of the buffer dielectric, has been widely employed to study the morphology and growth mode of organic semiconductors (OSCs). Through these studies, further understanding of charge transport mechanisms is obtained, and the electrical performances of organic field-effect transistors (OFETs), e.g. charge carrier mobility, on/off current ratio and subthreshold swing, thus have been greatly improved. In order to achieve a useful device, its stability becomes extremely important. In this article, the study on the charge trapping at the dielectric–dielectric interface by using the combination of poly(methyl methacrylate) (PMMA) and silicon dioxide (SiO 2) dual-dielectric was carried out under dark and illuminated conditions. Our results showed that the thickness of the PMMA layer, determining the film uniformity and the magnitude of charge injection barrier, plays a decisive role in the charge trapping process. With the thickness optimized, the device stability is greatly enhanced. Our findings bring about a low-cost, easy-to-realize fabrication method to produce high-performance and stable/reliable OFETs.

Metastability mechanisms in thin film transistors quantitatively resolved using post-stress relaxation of threshold voltage

A new approach is presented to resolve bias-induced metastability mechanisms in hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). The post stress relaxation of threshold voltage (VT) was employed to quantitatively distinguish between the charge trapping process in gate dielectric and defect state creation in active layer of transistor. The kinetics of the charge de-trapping from the SiN traps is analytically modeled and a Gaussian distribution of gap states is extracted for the SiN. Indeed, the relaxation in VT is in good agreement with the theory underlying the kinetics of charge de-trapping from gate dielectric. For the TFTs used in this work, the charge trapping in the SiN gate dielectric is shown to be the dominant metastability mechanism even at bias stress levels as low as 10 V.

Optically stimulated luminescence: A brief overview

The use of optically stimulated luminescence (OSL) in radiation dosimetry has been conditioned by the availability of suitable dosimetry (OSLD) materials. The crucial property dictating the suitability of a material as an OSLD is the material’s defect structure. This paper reviews some of the recent developments in radiation dosimetry that have been enabled by knowledge of the charge trapping and recombination processes occurring in OSLD materials during irradiation and stimulation. Although many materials have been and are currently being studied, this short review focuses on just two, namely carbon-doped aluminum oxide (Al2O3:C) and europium-doped potassium bromide (KBr:Eu). The defect structure and trapping/recombination mechanisms in these materials have led to application in several areas of radiation dosimetry and dose imaging. Two recent areas of development are in space dosimetry (for Al2O3:C) and in medical dosimetry (for KBr:Eu). This overview briefly describes these latter modern applications and relates the functionality of the OSLDs to their basic defect properties.

False multiple exciton recombination and multiple exciton generation signals in semiconductor quantum dots arise from surface charge trapping

Multiple exciton recombination (MER) and multiple exciton generation (MEG) are two of the main processes for assessing the usefulness of quantum dots (QDs) in photovoltaic devices. Recent experiments, however, have shown that a firm understanding of both processes is far from well established. By performing surface-dependent measurements on colloidal CdSe QDs, we find that surface-induced charge trapping processes lead to false MER and MEG signals resulting in an inaccurate measurement of these processes. Our results show that surface-induced processes create a significant contribution to the observed discrepancies in both MER and MEG experiments. Spectral signatures in the transient absorption signals reveal the physical origin of these false signals.

Charge Separation and Trapping in N-Doped TiO2 Photocatalysts: A Time-Resolved Microwave Conductivity Study

We studied charge separation and trapping processes in N-doped TiO2 (N-TiO2) photocatalysts by means of time-resolved microwave conductivity (TRMC). We observed that the trapping rate increased with N-doping due to the increase in the oxygen vacancy. Furthermore, the trapping rate increased markedly when Cu was loaded onto the photocatalysts, and this increase indicated a rapid electron transfer to this Cu cocatalyst. For N-TiO2, the charge separation efficiency under visible light excitation (450 nm, Ti 3d ← N 2p transition) was found to be one-third as high as the efficiency observed under ultraviolet excitation (Ti 3d ← O 2p transition). We discuss possible mechanisms for the charge separation and trapped processes, which is essential for the design of efficient doped-TiO2 photocatalysts.

(Invited) Charge Trapping and the Negative Bias Temperature Instability

During the last couple of years new measurement techniques have provided insight into the physics behind the negative bias temperature instability (NBTI) and indicate that the recoverable component of NBTI is due to some kind of charge trapping. As a consequence, charge trapping processes have been investigated and modeled in detail. We review the evolution of the latest charge trapping model by focussing on the correct temperature- and field dependence as well as on the quality of agreement with experimental stress/relaxation curves.

Donor/Acceptor Adsorbates on the Surface of Metal Oxide Nanoporous Films: A Spectroscopic Probe for Different Electron Transfer Pathways

A composite model system which consists of a molecular donor/acceptor pair coupled to the surface of metal oxide nanoporous films is proposed to study the contribution of the surface trap states and their influence on the interfacial ET as well as charge trapping and spatial diffusion processes in films. The photophysics of this donor/acceptor system is investigated by time-resolved transient absorbance spectroscopy in the UV/Vis (347-675 nm) spectral region. Variation of the band gap allows one to disentangle the ET pathways. Adsorption of the donor/acceptor pair on the nonreactive Al(2)O(3) surface shows that coupling to the surface assists electron transfer between adsorbed donor and acceptor molecules, resulting in ultrafast intermolecular electron transfer of approximately 100 fs. On the other hand, a competition between interfacial and intermolecular electron transfer is observed for the donor/acceptor pair coupled to a reactive TiO(2) surface. The subsequent transfer of the conduction band electron to the electron acceptor is examined by monitoring the free charge carrier absorption in the mid-IR (approximately 5 microm) spectral region.

Near infrared long-persistent phosphorescence in SrAl2O4:Eu2+,Dy3+,Er3+ phosphors based on persistent energy transfer

Eu 2 + , Dy3+, and Er3+ ions codoped SrAl2O4 phosphors with near infrared (NIR) (at 1530 nm) long-persistent phosphorescence were synthesized by a combustion method. The NIR phosphorescence emission was realized through a persistent energy transfer process from Eu2+ ions to Er3+ ions. Optimal Er3+ concentration for the maximum energy transfer efficiency was determined and NIR phosphorescence persisting for more than 10 min was achieved. A model of charge trapping and energy transfer process among Eu, Dy, and Er ions in SrAl2O4 host was proposed.

Single-charge tunneling in uncoupled boron-doped silicon nanochains

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements have been performed on boron-doped silicon nanochains (BDSNs). STM results suggest BDSNs may be an excellent nanostructure to build single-charge tunneling transistors. STS measurements clearly reveal Coulomb blockade (CB) effects in BDSN at 25 K with a large charging energy, which should make CB effect observable even at RT. The CB characteristics are attributed to boron- or impurity-induced local states at the surface or interface. The experimental results suggest the possibility of realizing single-charge nanodevices by exploiting boron- or defect-induced charge trapping and de-trapping processes at the nanoparticles of BDSNs.

The study of molecular movements in dielectrics using isothermal and non-isothermal current measurements

The electrical methods used to study the molecular movements in dielectric materials are based on the movement of the dipoles, which are parts of the component molecules, under a convenient external applied DC or AC electric field. We have proposed recently a combined isothermal and non-isothermal measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in low mobility materials. During an electric polarization process or an electric charging process, besides the polarization process, electric charge is injected into the material. This charge is partially transported and partially trapped in the material, depending on the material properties and on the experimental conditions. As a consequence of the charge trapping process, the structure is decorated with space charge and during the subsequent thermally stimulated discharge current experiment we are observing an apparent peak and the genuine peaks that are related to dipole randomization and charge de-trapping determined by the molecular movements. The aim of this paper is to present a combined measuring protocol that allows to separate the apparent peak and the genuine peaks in polar and non-polar dielectric materials. The method is very sensitive, very selective and allows to reveal, in a single run, details that are not observed when other electrical and non-electrical techniques are used.

Performance and Stability Characterization of Bottom Gated Amorphous Indium Gallium Zinc Oxide Thin Film Transistors Grown by RF and DC Sputtering

In this paper, the performance and the gate bias stability of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) with different channel sputtering conditions, RF and DC are studied. DC devices show excellent electrical characteristics but larger charge trapping and slower detrapping in relaxation period under the same gate bias stress. To understand the cause of stress behavior, traps are extracted by subthreshold slop and low-high frequency dependent capacitance measurement and compared before and after gate bias stress. The extracted trap densities well explain the process-dependent device properties like as threshold voltage, mobility and on/off current ratio, but are not dependent on bias stress. From current–voltage (I–V) and capacitance–voltage (C–V) measurement, we can conclude that the threshold voltage instability arises due to the process of temporary charge trapping which is dominant only at gate bias stress and the trapping/detrapping behavior is strongly dependent on the channel layer deposition condition.

Numerical model of multilayer organic light-emitting devices

A numerical model of multilayer organic light-emitting devices is presented in this article. This model is based on the drift-diffusion equations which include charge injection, transport, space charge effects, trapping, heterojunction interface and recombination process. The device structure in the simulation is ITO/CuPc (20 nm)/NPD (40 nm)/Alq3 (60 nm)/LiF/Al. There are two hetero junctions which should be dealt with in the simulation. The I–V characteristics, carrier distribution and recombination rate of a device are calculated. The simulation results and measured data are in good agreement.

New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials

The thermally stimulated discharge current (TSDC) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. The experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature Tma shifted with about 10 to 30 K above the charging temperature Tc. The shift of Tma with respect to Tc depends on the experimental conditions. The peak width at the half maximum intensity decreases as Tc increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

Comparative Study of Plasma Source-Dependent Charging Polarity in Metal–Oxide–Semiconductor Field Effect Transistors with High-k and SiO2 Gate Dielectrics

The polarities of charging damage in n- and p-channel metal–oxide–semiconductor field effect transistors (MOSFETs) with Hf-based high-k gate stack (HfAlOx/SiO2) were studied for two different plasma sources (Ar- and Cl-based gas mixtures) and found to depend on plasma conditions, in contrast to those with conventional SiO2. It was also found that high-k devices were more susceptible to plasma charging damage than SiO2 devices. For Ar-plasma, which was confirmed to induce a larger charging damage, both n- and p-channel MOSFETs with high-k gate stacks suffer from negative charge trapping, whereas for Cl-plasma confirmed to induce less damage, both n- and p-channel MOSFETs with high-k gate stacks suffer from positive charge trapping. The above-mentioned plasma-source-dependent charging damage was also compared on the basis of the results obtained by the plasma diagnostics. From the results of constant-current stress tests, the unique charging polarity observed for the high-k gate stack was attributed to the characteristic hole and electron trapping phenomena, in accordance with the injected stress current and stress time, implying the necessity of taking the intrinsic charge trapping process into consideration for accurate evaluations of charging damage on high-k gate dielectrics.

Charging of dielectrics under focused ion beam irradiation

We present a detailed experimental and theoretical study of the charging of silicon nitride and silicon oxide thin films following focused ion beam irradiation. The samples were irradiated using 30keV Ga+ ions at different ion doses and their consequent work function changes were measured by Kelvin probe force microcopy. The surface potential of both samples increased following the ion irradiation up to a critical ion dose, and then moderately decreased. The dependence of the sample surface potential on the irradiated ion dose is analyzed by taking into account all the main factors affecting charging in dielectric thin films: electron-hole generation by the incident fast ions, secondary ion-electron emission, sputtering of surface atoms, electron-hole recombination, electron recombination with the incident stopped ions, hole leakage current to the Si substrate, and various charge trapping processes. It was found that the much larger surface potential induced in Si3N4 in comparison to SiO2 is associated with the different resistance to the Ga+ ion bombardment. Under equal ion irradiation dose, a larger concentration of shallow traps is created in SiO2 than in Si3N4. This leads to an increased hole capture in shallow traps versus deep traps, and a consequent decrease in the surface potential.

Mobile Ionic Impurities in Poly(vinyl alcohol) Gate Dielectric: Possible Source of the Hysteresis in Organic Field‐Effect Transistors

Threshold voltage shifts in organic field effect transistors (OFETs) have been reported frequently. Cyclic sweeps of the gate voltage in OFETs reveal a hysteresis in the transfer characteristics (drain-source current versus gate-source voltage) thereby unfolding an electrical instability of the transistor element. On the one hand bistable transistors with a nonvolatile hysteresis in the transfer characteristics may be used in organic memory elements, whereas on the other hand hysteresis free transistors are desired in integrated organic circuits. Therefore understanding the causes of these electrical bior instabilities in organic field-effect devices is of primary interest. OFETs using charged electrets or ferroelectric-like gate dielectrics (e.g., see [7,8,12,14]) show a hysteresis which is normally attributed to the intrinsic properties of these materials. In polymer dielectrics without an intrinsic hysteresis such as poly(vinyl alcohol) (PVA), gate voltage induced hysteresis are often interpreted in terms of electrostatic screening of trapped charge carriers released under the influence of the electric field between the gate and the source/drain electrodes. Experimental results so far revealed a fairly complex picture of this “bias-stress” effect where the detailed mechanisms involved are not yet fully understood. Several publications report evidences of a charge trapping process in the organic semiconductor close to the semiconductor/dielectric interface. Lindner et al. supposed that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., a direct polaron-bipolaron reaction or a complex formation reaction of polarons / bipolarons with counterions. Bias-stress experiments with pentacene on various inorganic dielectrics indicate a reversible structural change of the semiconductor. A substantial number of experiments demonstrate an influence of the gate dielectric material on the formation of hysteresis effects. Transistors with a polymer gate dielectric are more likely to show a pronounced hysteresis than transistors with an inorganic gate dielectric. It has been reported that hydroxyl groups in the form of silanols at the SiO2-dielectric/semiconductor interface can work as electron traps which can be eliminated by a thin alkane interlayer. In many organic dielectrics hydroxyl groups are present in large numbers, either as integral part of the chemical structure and/or as impurities remaining from the process of synthesis. Such hydroxyl groups therefore were suspected of being responsible for the hysteresis effect in many OFET configurations. Lee et al. conclude that the increase of hydroxyl groups in polymer dielectrics equally increases electron trapping sites which in turn cause a large hysteresis in OFETs, but as they also discussed, the drain current as well as the gate leakage current increased with increasing hydroxyl density, which may be in contradiction with the proposed idea of immobilized carriers. Polymer dielectrics are often hosts for mobile ions. It has been demonstrated for example that mobile Na ions may diffuse from an underlying substrate into organic semiconductors, like pentacene or poly(3-hexylthiophene) under the influence of an applied voltage. Thereby they cause an increase of the current through the semiconductor and, additionally, a current-voltage hysteresis. The finding that mobile ions in semiconductors alter their electrical performance is not new. p-n junction devices, called flexodes, with a variable current-voltage (I–V) characteristics resulting from a reversible drift of Li ions were suggested in 1963. Two years later, small traces of mobile alkali ions in SiO2 gate dielectrics were reported to cause significant problems in semiconductor devices. In fact, the practical application of MOSFETs was delayed in the early 1960s because of severe gate bias instability problems caused by mobile ionic oxide charges like Na, C O M M U N IC A TI O N