Thermally Stimulated Current Research Articles

Design, Fabrication, Characterization, and Evaluation of X-ray Detectors Based on n-type 4H-SiC Epitaxial Layer

The Schottky barrier diode (SBD) radiation detectors on n-type 4H-SiC epitaxial layer have been designed, fabricated, and evaluated for low energy x-rays detection. The detectors were found to be highly sensitive to x-ray flux in the 50 eV to few keV range and showed significantly improved response compared to the commercial of-the-shelf (COTS) SiC UV photodiodes. Thermally stimulated current (TSC) measurements performed in wide temperature range (94 - 550 K) revealed low density of deep level centers in the epitaxial layer. The high quality of the epitaxial layer was confirmed by XRD rocking curve measurements and defect delineating chemical etching. The detectors fabricated on 4H-SiC epitaxial layers exhibited low leakage current at 475 K (< 1 nA at 200 V) revealing great possibility of high temperature operation.

Thermostimulated currents in low-density polyethylene and its blends with various components after high-pressure plastic deformation

It is established that the plastic deformation of low-density polyethylene (LDPE) under a pressure 0.5–2.0 GPa on a high-pressure apparatus of the Bridgman anvil type leads to the appearance of thermostimulated currents in samples, indicating that the samples contain trapped electrons. It is shown that two peaks are present on the temperature dependences of the currents; one of these is most probably related to the cold crystallization of the polymer, its structure being destroyed upon high-pressure deformation, while the other is related to the melting of the polymer. It is noted that the peaks were absent on temperature dependences of the currents for LDPE blends with some components; this could be due to the formation of a conducting state at interfaces. It is found that the electroconductivity of some blends after processing under pressure was higher than that in LDPE itself by a factor of 25.

Study of traps in P3HT:PCBM based organic solar cells using fractional thermally stimulated current (FTSC) technique

Thermally Stimulated Current (TSC) measurement, well known technique for trap level detection, was applied to derive the density of occupied states in organic solar cells. The device structure is Glass/ITO/P3HT:PCBM/Al. Analysis of the TSC spectra allows to determine the Fermi level position from the edge of the nearest band (LUMO or HOMO) (~0.42eV). At that time, TSC peaks cannot be evaluated due to multiple traps that contribute to the current signal. In order to achieve a rough picture of the trap structure, Fractional TSC (FTSC) method was applied. The procedure is based on multiple temperature cycles which evaluate activation energies for different ranges of temperatures. The results reveal several plateaus, pointing out the presence of four trap levels (0.12eV, 0.16eV, 0.20eV and 0.35eV). Additionally, a curve-fitting procedure is done in order to characterize the trap parameters and achieved with five trap levels (0.059eV, 0.12eV, 0.14eV, 0.20eV and 0.35eV) in Gaussian or exponential distribution. Four of them are in complete agreement with measured FTSC ones. Those results are very well correlated with our other characterizations by Admittance Spectroscopy (AS). The results presented here show that, in spite of the chemical differences between inorganic materials and conjugated polymers, FTCS technique can still be applied to determine the density of occupied states in these materials, as long as care is taken. Polymers are low-mobility materials with large concentrations of deep impurities.

Mechanisms of Difficulty to Correlate the Leakage Current of High-k Capacitor Structures with Defect States Detected Spectroscopically by the Thermally Stimulated Current Technique

Historically, it has been difficult to correlate the leakage current of capacitor structures involving high-k dielectric materials and defect states detected spectroscopically by the thermally stimulated current (TSC) technique. Four mechanisms are proposed and solutions are explained with tantalum oxide as an example. One of the mechanisms is the limitation of the TSC technique itself because of the presence of a parasitic current due to the bias voltage used. This can be solved by migrating to more advanced versions of TSC like zero-bias TSC or zero-temperature-gradient zero-bias TSC. In addition, another possible mechanism is that some defect states may have an electron repulsive energy barrier. Furthermore, another possible mechanism is that the leakage current may be insensitive to the presence of defect states under some situations; a unified Schottky-Poole-Frenkel model is proposed by the author to explain such a situation. Finally, another mechanism is due to the non-uniform distribution of defect states. Sometimes, this can be solved by using a 2-zone model proposed by the author.

Characterization of deep levels in n-type and semi-insulating 4H-SiC epitaxial layers by thermally stimulated current spectroscopy

We have investigated deep level centers in n-type and semi-insulating (SI) 4H-SiC epitaxial layers by thermally stimulated current (TSC) spectroscopy. The epitaxial layers were grown using chemical vapor deposition utilizing a dichlorosilane precursor. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D-centers. A deep level center with an activation energy of 1.1 eV, peaked at ∼400 K, was detected in the n-type epitaxial layer and correlated with the IL2 level and the 1.1 eV center in a high purity bulk SI 4H-SiC. The TSC spectra of the SI epitaxial layer was dominated by the peaks at 525–585 K that we attributed to intrinsic defects and their complexes with energy levels close to the middle of the bandgap. The TSC spectra of SI epitaxial layer exhibited peaks with different current polarity which is explained by thermoelectric effect and the built-in electric field reversal. The results of the transfer length method measurements of the SI epitaxial layer and the room temperature current-voltage (I-V) characteristics of both epitaxial layers are also reported.

Magneto electret State of Porous Polypropylene and Investigation on Surface Charge Characteristics

This paper reports preparation of porous polypropylene magneto electret and its investigation on its surface charge characteristics. The electrets were prepared under the combined action of heat and magnetic field. Influences of polarizing temperature and magnetic field on MEs state of polypropylene were studied. Five different forming temperatures and five different magnetic field strength were used for the surface charge study of magnetoelectrets. Surface charge was found to be of the order of 10 -10 C. Negative iso charge was found in all the magnetoelectrets. The decay of surface charge was studied continuously for 30 days and corresponding time constant were calculated. Keywords - Dielectric, Magnetoelectrets (MEs), porous polypropylene, magnetic field, surface charge. I. INTRODUCTION Dielectric samples showing persistent polarization is termed as 'Electrets' (1) in analogy with magnet because in many ways an electret is the electrostatic counterpart of magnet. The first electret resembling Heaviside's was prepared by Eguchi (2) by solidifying the molten mixture of carnauba wax and resin in an intense electric field. In an electret, the dielectric is in state of persistent polarization, i.e. the polarized state continues to stay in the dielectric for considerably long time even after the removal of polarizing field. Electrets are classified depending on the physical agency that produces polarization in dielectric under the influence of an external field. The various types of electrets known so far are thermo, photo, radio, cathode, electro and magneto etc. In 1964, Bhatnagar (3) while observing the effect of magnetic field on carnauba wax for detecting Hall Effect, found that magnetic field in conjunction with heat energy (physical agency), could bring about the polarization in carnauba wax. The study of initial charge, i.e surface charge measured immediately after the electret formation, its subsequent reversal in polarity and decay with time and the amount of residual charge (final charge) almost permanently retained by it, has helped a great deal in understanding the electret state in the dielectric. The method of study of charge trapped within the electreted dielectric, by the modern technique of thermally stimulated current (TSC), (4) has added much more to our knowledge about the electret state. Thus the study of surface charge is one of the basic and important studies; hence it is undertaken for the porous polymers polypropylene for the present work. From the study of surface charge on magnetoelectret, Khare and Bhatnagar propounded a 'Two charge Theory (5) analogous to the two charge theory of Gross and Gemant (6) on thermoelectret. According to the two charge theory on magnetoelectret, the net surface charge is the resultant of two types of charges known as the 'isocharge' and the 'idiocharge'. The isocharge is the charge of the same sign on the surface of electret which may be +ve or -ve depending upon the substance while idiocharge is a charge due to polarization of the dielectric which has opposite charges on the opposite surfaces of electret. The two charge theory which explains the magnetoelectret surface charge is later found to be capable of explaining several phenomena associated with the magnetoelectret state in the dielectric, such as change in dielectric constant, refractive index susceptibility etc. In present study, The surface charge study on porous polypropylene magneto electrets consists in the study of initial charge on either surface of the sample and its dependence on various forming parameters, such as strength of the magnetic field, the constant elevated temperature subsequent measurement of surface charge is done continuously for about a month, to study the possible reversible of charge, its decay with time and the charge finally retained by the sample. These studies give an assessment of strength, stability and retentivity of charge, besides providing us with information regarding the polarization process responsible for the induction of magnetoelectret state in the substance. The results of surface charge study are discussed in the following sections.

Structural, optical, and electrical properties of In2S3:Sn thin films grown by chemical bath deposition on Pyrex

Sn-doped In2S3 thin films are deposited on Pyrex. Tin is incorporated in the solution using SnCl2·2H2O. The properties of the films are investigated by X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, spectrophotometry, and thermally stimulated current (TSC) measurements. XRD analysis reveals that the films become almost amorphous when tin exceeds well determined optimum quantity. The investigation of optical properties shows that the band gap depends on the value of the Sn concentration. It is also observed that upon adding the optimum quantity of tin, the maximum of the TSC intensity increased by 2 orders of magnitude. These results are discussed with respect to introduce the effect of tin incorporated in the physical properties.

Measurement of energy levels in a silicon detector damaged by neutrons

The level of defects in a semiconductor silicon detector diode made of high resistivity N type material and exposed to neutrons in a research nuclear reactor was examined by measuring the thermally stimulated current (TSC). A modified TSC method was employed where the released charge was measured in the reverse direction on a diode with zero bias voltage. Electrons captured in cooled traps due to the photoelectric effect are released when the material is heated. The detector was irradiated with an integral neutron flux of 7.63 × 1015 n/cm2.

Influence of Rubber Annealing on Behavior of the Three-Phase Model in PET Study by the Thermally Stimulated Current (TSC) Method

Polymers are generally formed by two phases: a crystalline phase where chains are arranged in parallel, one over the other, and an amorphous phase where there is absence of order. Several recent works have supposed the existence of a three-phase model where there coexist simultaneously a crystalline phase, a rigid amorphous phase, and a mobile amorphous phase. The objective of our work is to put in evidence the existence of three phases separately within polyethylene terephtalate (PET). This polymer is widely used in electrical insulation and especially in packaging. To this effect, samples of PET have undergone a heat treatment of isothermal annealing time from 10 to 240 min. The samples thus treated were studied by the thermally stimulated current method, which tracks the variation of the depolarization current as a function of sample temperature. Recordings of thermocurrent are made after increasing temperature with a linear heating rate of 7°C/min. Then we can calculate the areas under the thermocurrent peak and hence estimate the corresponding degrees for the various phases present in the annealed material, χc and χam. Analysis of the values obtained for these degrees introduce a third component, denoted χar, which characterizes the rigid amorphous phase. Monitoring the evolution of these degrees with duration of annealing is realized. It is clear from all results that PET annealed rubber has a three-phase behavior with simultaneous presence of three phases, a crystalline phase χc, an amorphous mobile phase χam, and a rigid amorphous phase χar.

Series of Porous 3-D Coordination Polymers Based on Iron(III) and Porphyrin Derivatives

A new series of 3-D coordination polymers based on iron(III) and nickel(II) tetracarboxylate porphyrin (Ni-TCPP) have been produced using solvothermal conditions. MIL-141(A) solids (MIL stands for Material from Institut Lavoisier), formulated Fe(Ni-TCPP)A•(DMF)x (A = Li, Na, K, Rb, Cs, DMF = N,N-dimethylformamide, x ∼ 3), are built up from three anionic interpenetrated PtS-type networks charge-balanced by alkali cations (A) entrapped inside the pores. MIL-141(A) thus includes three types of cations, two of which may act as coordinatively unsaturated metal sites (Ni2+ and A+). These solids all present a permanent porosity with a reasonably high surface area (SBET = 510–860 m2 g–1) as well as some structural flexibility toward adsorption/desorption processes, modulated in both cases by the nature of A. Thermally Stimulated Current (TSC) measurements indicated that alkali cations are rather homogeneously distributed within the pores, while their interaction with the framework is stronger in MIL-141(A) than i...

Effect of Internal Electrode Materials in Multilayer Ceramic Capacitors on Electrical Properties

The influence of internal electrode materials on electrical properties in multilayer ceramic capacitors (MLCCs) fabricated by low-temperature co-firing was discussed. The lifetime of MLCCs is considerably improved by using copper rather than nickel internal electrodes. The leakage current density for various dc electric fields (I–V characteristics) and thermally stimulated current (TSC) were measured to investigate the lifetime improvement mechanism. The I–V characteristics demonstrated that the leakage current in a high dc electric field was suppressed in the case of copper internal electrodes. The TSC spectra demonstrated that the internal electrode materials hardly influenced the quantity of polarization charge formed by the electromigration of oxygen vacancies. Therefore, we considered that the improved lifetime by using copper internal electrodes was mainly due to the suppressed leakage current in a high dc electric field, not the quantity of polarization charge formed by the electromigration of oxygen vacancies in the MLCCs.

Perspectives on the electrically induced properties of electrospun cellulose/liquid crystal devices

Abstract A mat of electrospun cellulose fibers are deposed on transparent conductive oxide covered glass, and two such plates enclose a nematic liquid crystal. Thus two new types of Cellulose based Polymer Dispersed Liquid Crystal devices, based on hydroxypropylcellulose and Cellulose Acetate and the nematic liquid crystal E7 have been obtained. The current–voltage characteristics indicates ionic type conduction. Heating–cooling cycles have been applied on the samples and the activation energies have been determined. Simultaneously with the thermo-stimulated currents, the optical transmission dependence on the d.c. electric field and temperature was registered. ON–OFF switching times have been determined for different control voltages.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

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.

Determination of the charge trap energy distribution in conjugated polymer MDMO-PPV

The trap distribution depending on the exciting light spectral range and applied electrical field in the MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene) was investigated by the thermally stimulated current (TSC) technique. To ensure selective excitation of the defect states we have used the long-pass colour filters with cut-on energies from 1.77 up to 3.10 eV. In order to evaluate the trap parameters of MDMO-PPV, curve fitting was done. The analysis revealed the Gaussian distribution of the defect states; the estimated effective activation energies were about 0.22 and 0.40 eV.

Electrospun nanofibers for microfluidic analytical systems

Nanofibers embedded with the functional polymers exhibited a charged surface. These fibers were incorporated into microfluidic channels to provide high surface area and functional surfaces within a microfluidic system. The positively or negatively charged nanofibers were fabricated by blending of hydrophilic PVA with functional polymers with available amine groups or carboxyl groups, respectively. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to confirm the presence and location of blend components in the spun fibers. Thermally stimulated current (TSC) measurements confirmed that surface-charged nanofibers showed positive or negative currents according to the functional polymers embedded with the PVA polymer. The surface-charged nanofibers were incorporated into microfluidic channels. Poly(vinyl alcohol) (PVA) blend nanofibers formulated to create variations in fiber surface chemistry were electrospun to form patterns around gold microelectrodes on a poly(methyl methacrylate) (PMMA) chip surface. These nanofiber patterns were integrated into polymer-based microfluidic channels to create a functionalized microfluidic system with potential applications in bioanalysis. Spinning conditions and microelectrodes were optimized to enable an alignment of the nanofibers across the microfluidic channel. Importantly, nanofibers within the assembled microfluidic channels were robust and did not break or wash out of the channel under extreme fluid flow conditions at linear velocities up to 13.6 mm/s.

Mechanisms of Difficulty to Correlate the Leakage Current of High-k Capacitor Structures with Defect States Detected Spectroscopically by the Thermally Stimulated Current Technique

Historically, it has been difficult to correlate the leakage current of capacitor structures involving high-k dielectric materials and defect states detected spectroscopically by the thermally stimulated current (TSC) technique. Four mechanisms are proposed and solutions are explained with tantalum oxide as an example. One of the mechanisms is the limitation of the TSC technique itself because of the presence of a parasitic current due to the bias voltage used. This can be solved by migrating to more advanced versions of TSC like zero-bias TSC.

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.

Transport and interface states in high- κ LaSiO x dielectric

The paper presents the results of capacitance–voltage, conductance-frequency and current–voltage characterization in the wide temperature range (140–300 K) as well as results of low temperature (5–20 K) thermally stimulated currents (TSC) measurements of metal–oxide-semiconductor (MOS) structures with a high- κ LaSiO x dielectric deposited on p- and n-type Si(1 0 0) substrate. Interface states ( D it) distribution determined by several techniques show consistent result and demonstrates the adequacy of techniques used. Typical maxima of interface states density were found as 4.6 × 10 11 eV −1cm −2 at 0.2 eV and 7.9 × 10 11 eV −1cm −2 at 0.77 eV from the silicon valence band. The result of admittance spectroscopy showed the presence of local states in bandgap with activation energy E a = 0.38 eV from silicon conductance band, which is in accord with interface states profile acquired by conductance method. Low-temperature TSC spectra show the presence of shallow traps at the interface with activation energies ranging from 15 to 32 meV. The charge carrier transport through the dielectric film was found to occur via Poole–Frenkel mechanism at forward bias.

TRAP DISTRIBUTION NEAR THE ENERGY BAND GAP EDGE IN MDMO-PPV

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 light spectra was varied from 1.77 eV up to 3.1 eV to assure selective excitation of the defect states. We had shown that material conductivity is affected by several thermally activated processes, i.e., carrier generation from trapping states and thermally stimulated mobility growth. If the below band gap excitation was used, the effective photoconductivity activation energy values of 0.13–0.15 eV were obtained. After the above band gap excitation, the effective photoconductivity activation energy values decreased to 0.05 eV. The energy distribution of the trapping state density was shown to follow the Gaussian distribution function. 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.