Energy Distribution Of Trap States Research Articles

(Invited) Combining Machine Learning, DFT, EFM, and Modeling to Design Nanodielectric Behavior

Polymer Nanodielectrics are class of materials with intriguing combinations of properties. Predicting and designing the properties, however, is complex due to the number of parameters controlling the properties. This makes it difficult to compare results across groups, validate models, and develop a design methodology. This presentation will share a recent approach to developing a data driven design methodology grounded in physics-based models and experimental calibration. We combine finite element modeling of dielectric constant and loss functions with a Monte Carlo multi-scale simulation of carrier hopping to predict break down strength. Filler dispersion, filler geometry, isotropy and interface properties are explicitly taken into to account to compute objective functions for ideal nanodielectric insulators. Further, we have used machine learning to develop an EFM technique for directly measuring the dielectric constant of the nanoscale interfacial region as critical input to our models. Ultimately, we calculate the Pareto frontier with respect to nanocomposite constitute properties and geometry to optimize properties.The finite element approach can be used to forward predict properties based on the properties of the interfacial region and filler dispersion, or as an inverse tool to calculate interface properties or develop optimized filler morphologies. The breakdown strength model critically depends on the energy distribution of trap states that inhibit space charge motion. We have developed an ab initio approach to determine the trap states at amorphous interfaces, and have used that to do a systematic analyses of trap distributions in composites with functionalized particle interfaces. The models all use 3D particle distributions based on 2D imaging (TEM) of the composites and publicly available tools for binarization and characterization of filler morphology. Finally, we use a design of experiment approach (Latin Hybercube Design) to sample the complete experimental design space, and using calibrated models and morphologies, create a data set spanning the full set of parameters. We use the data from the DOE to train a Gaussian Process metamodel and a genetic algorithm to determine which designs fulfill the design parameters. The talk will present several case studies as examples.The data for this work was accessed from a new data resource, MaterialsMine, using FAIR (Findable, Accessible, Interoperable, and Reusable) principles. MaterialsMine is an open source data repository, and resource. It includes unique visualization tools as well as modeling and characterization resources.

Characterization of Low-Dielectric Constant Silicon Carbonitride (SiCN) Dielectric Films for Charge Trapping Nonvolatile Memories

We investigated the charge retention characteristics of the charge-trapping type memory capacitors with blocking oxide-SiCN-tunneling oxide stacked films at four different temperatures. Long-term data retention more than 10 years was estimated at 85 °C for the SiCN memory capacitors. We presented a simple method to analyze the energy distribution of trap states inside the SiCN layer. The trapped electrons were estimated to be distributed from 0.8 to 1.3 eV below the conduction band edge in the SiCN band gap.

Evaluation of the energy distribution of trap states in polymer films

This paper reports on the results of an investigation of polymer thin films by the methods of thermally stimulated currents and current-voltage characteristics. It has been found using these methods that, in the band gap of the poly(diphenylene phthalide) films, there are three groups of electron traps with maxima in the density of states at energies of 0.50, 1.06, and 2.40 eV with respect to the bottom of the conduction band. These results correlate well with the data obtained in our investigation of the fluorescence excitation spectra.

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.

Effects of oxygen impurity on the energy distribution of gap states in hydrogenated amorphous silicon studied by post-transit photocurrent spectroscopy

Oxygen is normally present in the highest concentration (${10}^{18}--{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ range) among the impurities in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ and affects the electrical properties and light-induced changes in this material. However, little is known about how the energy distribution of trap states, $g(E)$, in undoped $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ changes with the presence of oxygen. In this study, we prepare undoped $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ samples in an ultrahigh-vacuum plasma-enhanced chemical-vapor-deposition chamber and intentionally introduce oxygen atoms at a concentration of $3\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ by adding $\mathrm{C}{\mathrm{O}}_{2}$ to the source gases. By comparing the results obtained for high-purity and oxygen-doped samples by post-transit photocurrent spectroscopy, we discuss the effects of oxygen on the energy distribution of electron and hole trap states before and after long exposure to light. It is found that the presence of oxygen atoms enhances the defect reactions during prolonged illumination, while the shapes of $g(E)$ in the as-deposited state are similar between the high-purity and oxygen-doped samples in the energy range studied (from ${E}_{c}\ensuremath{-}0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{c}\ensuremath{-}0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for electron traps and from ${E}_{v}+0.38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{c}+0.78\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for hole traps). A light-induced decrease of the hole trap density in the energy range from ${E}_{v}+0.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{v}+0.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is observed and discussed.

Numerical study of the dependence of thermally stimulated currents in disordered solids on initial trap occupancy

In the paper a numerical investigation of thermally stimulated currents (TSCs) in disordered solids, which are characterized by continuous energy distribution of trapping states, is presented. The set of stiff differential equations governing TSCs is solved with the use of Gear's algorithm. The dependence of TSC curves on the initial trap occupancy as well as on the carrier recombination time is examined. It is concluded that the Fritzsche-Ibaraki method of TSC analysis, ameliorated by other authors, applies solely for almost complete trap filling and high carrier recombination rate. In the case of small trap filling the method of analysis the TSCs proposed by the present authors may be more adequate.

Trap States of Tris-8-(Hydroxyquinoline)Aluminum Degraded by Blue Laser Using Thermally Stimulated Current Method

We investigated the photodegradation phenomena of tris-8-(hydroxyquinoline)aluminum (Alq3) film after irradiation with a blue laser of 442 nm to identify the contributions of the excited states of Alq3 molecules. The photodegraded Alq3 film showed decreases in fluorescence lifetime and carrier transport ability markedly. These photodegradation phenomena are considered to be due to an increase in the energy distribution of trap states detected from the analysis by a thermally stimulated current (TSC) method, in connection with the orientation of neighboring Alq3 molecules, e.g., the disordered aggregation states of Alq3 molecules.

High-field transport and noise properties of sputter-deposited amorphous carbon–silicon heterojunctions

The electrical conductivity of heterojunctions of amorphous carbon (a-C) films (25 and 75 nm thick) grown on silicon by magnetron sputtering has been studied as a function of the applied electric field and temperature. At low electric fields and high temperatures, the conductivity exhibits thermally activated ohmic behaviour with activation energy 0.14 eV. At high electric fields, photoconductance measurements indicate that the conductivity is primarily due to a field-activated mobility with its activation energy decreasing as the electric field increases. At very high electric fields, band-to-band tunnelling is the dominant conduction mechanism. The mobility field-activated conduction model indicates an energy distribution of trapping states consisting of two exponential distributions. The exponential distributions correspond to tail states arising from clustering of sp2 sites and to deep states caused by isolated sp2 sites. Low-frequency noise measurements show that thicker a-C films contain a higher concentration of the trapping states. This result was explained by an increase of the sp2/sp3 bonding ratio found from the analysis of Raman spectroscopic measurements.

Energy distribution of trapping states at grain boundaries in polycrystalline silicon

A method to obtain the energy distribution of traps at grain boundaries in polycrystalline silicon films from the activation energy of resistivity measured as a function of doping concentration is proposed. The energy distribution of trapping states in the energy gap for plasma-hydrogenated and nonhydrogenated polycrystalline silicon films doped with boron are determined: nonhydrogenated films have a Gaussian distribution with a peak at the midgap, and after plasma hydrogenation the peak at the midgap has diminished. The validity of the monoenergetic trap model applied to the conduction in nonhydrogenated and plasma-hydrogenated polycrystalline silicon films is discussed.

Effects of grain-boundary trapping-state energy distribution on the activation energy of resistivity of polycrystalline-silicon films

The trapping model assuming a δ-function energy distribution of trapping states has been accepted as an effective first-order approximation for modeling the electrical properties of polysilicon films. However, it predicts that as the doping concentration N is smaller than a critical level N ∗ , the activation energy of resistivity E a is independent of N. This is inconsistent with experimental observations. In this paper a trapping model using a Gaussian energy distribution of trapping states is introduced to calculate E a vs N. The results demonstrate a good agreement with the experimental data of boron-doped polysilicon films. The physical bases of such an improvement and the existence of a Gaussian energy distribution of trapping states have been addressed.

Energy distribution of trapping states in polycrystalline silicon

The electronic density of states in the forbidden gap of polycrystalline silicon has been determined from an analysis of capacitance and conductance of a Metal/SiO2 (∼60 Å)/polycrystalline silicon(∼250 Å)/Si(111) (MOSS) structure. In this structure the thickness of the polycrystalline silicon is comparable to its grain size. Net density of trapped charges in the polycrystalline silicon is enough to terminate the electric field penetrating from the oxide layer. Then, two-terminal admittance of the MOSS structure is dominated by charging or discharging of the trapping states in a wide range of applied gate bias. The U-shaped distribution of trapping state density has been found for thin polycrystalline silicon films.

Phenomenological Model of Electret State in Carnauba Wax Inferred from Electrical Conduction Measurements

Measurements of current-voltage charactristics, isothermal discharge currents, and thermally stimulated current have been used to clarify the mechanism of formation of electret state in Carnauba wax. The results of such measurements are interpreted with the aid of an energy band model in which a set of localized energy levels, located in the gap of energy bands, acts as charge carrier traps. The gap is about 7.4 eV wide. In the neighborhood of electrode contact the traps have a uniform energy distribution which extends as far as 0.3 eV from the edge of the bands. In the bulk of the material the energy distribution of trapping states becomes exponential and extends all over the gap. The charges (electrons and holes) have a double origin: the injection from the electrodes and a thermally activated process in which electron-hole pairs are produced. The homocharges arise because of trapping of the injected charges near the electrodes and the heterocharge because of trapping of electron-hole pairs in the bulk of the material.