Density Of Localized Gap States Research Articles

Temperature-Dependent Drain Current Characteristics and Low Frequency Noises in Indium Zinc Oxide Thin Film Transistors**Supported by the National Natural Science Foundation of China under Grant Nos 61204112, 61204089 and 61306099, and the Guangdong Provincial Natural Science Foundation under Grant No 2014A030313656.

The I–V characteristics and low frequency noises for indium zinc oxide thin film transistor are measured between 250 K and 430 K. The experimental results show that drain currents are thermally activated following the Meyer–Neldel rule, which can be explained by the multiple-trapping process. Moreover, the field effect electron mobility firstly increases, and then decreases with the increase of temperature, while the threshold voltage decreases with increasing the temperature. The activation energy and the density of localized gap states are extracted. A noticeable increase in the density of localized states is observed at the higher temperatures.

Density of States of a-InGaZnO From Temperature-Dependent Field-Effect Studies

Temperature-dependent field-effect measurements were performed on radio-frequency sputtered amorphous In-Ga-Zn-O thin film transistors (TFTs). We studied the effect of temperature on the TFT electrical properties. We observed that the field-effect mobility (mu) increases and the threshold voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) shifts negatively with temperature, while the current on-off ratio and subthreshold slope (S) remain almost unchanged. We also observed that the TFT drain current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) is thermally activated, and the relation between the prefactor (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D0</sub> ) and activation energy ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) obeys the Meyer-Neldel rule. The density of localized gap states (DOS) was then calculated by using a self-consistent method based on the experimentally obtained <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> . The result shows good agreement with the DOS distribution calculated from SPICE simulations.

Re-examination of the post-transit photocurrents in expanding-thermal-plasma-grown a-Si : H

The interpretation of post-transit photocurrents in a time-of-flight experiment, in terms of the underlying density of localized gap states in the sample, is questioned for the case of previously examined hydrogenated amorphous silicon cells prepared by the expanding thermal plasma technique. Part of the observed current is not generated by re-emission of trapped photo-generated charge and should, therefore, not be used for density-of-states calculations.

Post-transit Analysis of Transient Photocurrents from High-Deposition-Rate a-Si:H Samples

AbstractAn interpretation of post-transit photocurrents in a time-of-flight experiment in terms of the underlying density of localized gap states in the sample is presented for the case of hydrogenated amorphous silicon cells prepared by the expanding thermal plasma technique. It is pointed out that part of the observed current is not generated by re-emission of trapped photo-generated charge and should, therefore, not be used for density-of-states calculations.

Comparison of transport and defects properties in hydrogenated polymorphous and amorphous silicon

Due to its specific microstructure with nanocrystals embedded in an amorphous matrix, polymorphous silicon (pm-Si:H) has been shown to present interesting properties for large area electronic applications. In particular, this material exhibits a lower density of localized gap states and better transport properties than standard amorphous silicon (a-Si:H) after light-soaking. In this paper, we focus our attention on the comparison of electronic transport and defect properties of both materials. The temperature-dependent photocurrent is similar to that of high-quality a-Si:H: it shows thermal quenching in the temperature range around 250 K and becomes temperature-independent at temperatures smaller than about 40 K. Electrically detected magnetic resonance (EDMR) under illumination was performed from 296 K down to 10 K to cover a very wide temperature range wherein the photocurrent is dominated either by free electrons in the band (higher temperatures) or by energy-loss hopping (lower temperatures). The comparison with the EDMR properties of a-Si:H and μc-Si:H assists in the interpretation of the results, thereby clarifying the influence of the incorporated nanocrystals on the recombination physics in pm-Si:H.

Charged and neutral defect states in a-Si:H determined from improved analysis of the constant photocurrent method

In order to obtain information about the density of localized gap states in a-Si:H, i.e., the valence band tail, the integrated defect density, the energetic defect distribution and the charge state of the defect states, a numerical model has been developed to simulate constant photocurrent method spectra. It takes into account the full set of optical transitions between localized and extended states under sub-bandgap illumination, capture, emission and recombination processes as well as the energetic position of the Fermi level. We compare measured and simulated CPM spectra of doped and undoped a-Si:H. In the annealed state the defect absorption of n- and p-type as well as undoped a-Si:H is dominated by a charged defect states. The simulation reveals that in undoped a-Si:H light soaking, causing an enhanced defect density, does not alter the charged-to-neutral defect ratio.

Improved analysis of the constant photocurrent method

Abstract A numerical model has been developed to simulate constant photocurrent method spectra. It takes into account the position of the Fermi energy and the full set of optical transitions between localized and extended states under subbandgap illumination, capture, emission and recombination processes. The comparison of simulated and measured spectra yields information about the density of localized gap states in amorphous silicon, that is the valence-band tail, the integrated defect density, the defect distribution in energy and the charge state of the defect states. For n- and p-type hydrogenated amorphous silicon (a-Si:H) we achieved good agreement between simulation and experimental data. In the annealed state the defect absorption is dominated by a single defect peak which can be attributed to D − states in n-type material and D + states in p-type a-Si: H. In undoped a-Si: H we observed more charged than neutral defect states, confirming the predictions of the defect-pool model. Furthermore, the simulations reveal that the defect chemical potential depends on the Fermi level as postulated by the defect-pool model. In the light-soaked state, undoped material shows an enhanced defect density. We observe an increase in the density of both neutral and charged defect states. The charged-to-neutral defect ratio does not change upon light soaking.

Correlation between the electrooptical properties and the composition of α-Si1−xGex:H (x&lt;0.1) thin films

Abstract Amorphous silicon-germanium alloys α- Si 1–x Ge x : H (x have been prepared by RF magnetron reactive co-sputtering of Si and Ge in Ar+H2 gas mixture with 5% hydrogen concentration. The total pressure, the substrate temperature and RF frequency were kept constant at 0.5 Pa, 200°C and 13.56 MHz, respectively. The RF power was varied from 100 to 200 W. The H concentration in the films have been investigated by elastic recoil detection (ERD) analysis. The Si and Ge composition and the density of the films have been determined by the Rutherford back scattering method. The properties have been measured as a function of the RF power. The results are reported of the optical gap, refractive index, density, and dark conductivity at room temperature, thermal activation energy of the dark conductivity, IR absorption and Raman spectra as function of the RF power as well as the film composition. It has been found that the properties are influenced by the H2 content in the films rather than by the Ge content. The small quantity of Ge reduces the density of localized gap states.

Post-transit-time analysis of time-of-flight photocurrents

It is shown how the density of localized gap states in amorphous semiconductors can be deduced from time-of-flight photocurrents on the basis of the trap-limited band transport model, post-transit currents giving information on deep-state densities; and how strong deep-trapping of the photo-induced excess carriers invalidates the analysis. Results from a-Si:H, p-type a-Si:H, a-Si,S:H and a-Si,C:H samples are used to illustrate these points.

FIELD EFFECT STUDIES ON a-Si:H FILMS

The density of localized gap states N(E) is calculated using a new scheme, which computes the charge density ρ (V) without explicit solution of Poisson's equation directly from the experimental field effect data avoiding fitting procedures and loops. - In a second step N(E) is computed from ρ (V) considering finite temperature statistics. This method is applied to measurements on undoped glow discharge deposited a-Si:H films. It is found that high temperature annealing as well as strong illumination raise the density of states near midgap by about one order of magnitude.

Adsorption effects on ESR and conductance of rf sputtered a-Si films

Distinct adsorption effects on ESR and conductance have been observed for doped amorphous silicon films prepared by rf sputtering under high Ar gas pressure. Donor- and acceptor-like adsorbates cause the change of the occupation profile of electrons in localized gap states, resulting in the changes of ESR and conductance. These results would be due to a relatively small density of localized gap states in these films and also to the largeness of the effective surface area because of the porosity of the sample films.

Analysis of field-effect and capacitance–voltage measurements in amorphous semiconductors

Abstract We describe the details of an iterative computer method for deriving N(E), the density of gap states, from field-effect or capacitance–voltage measurements without using simplifying approximations. The method of analysis has been tested by fitting artificially constructed data derived from an arbitrarily chosen N(E). It shows that, although gross features of N(E) can be determined, the presence of detailed structure or narrow peaks cannot be verified. Several different N(E) can yield good fits to the same set of data. The determination of experimental parameters needed for this analysis is discussed. Field-effect and capacitance–voltage measurements are complementary but each is particularly useful in a different range of field voltage values. The field effect probes a layer less than 100 Å thick at the interface. This has significant implications for the interpretation of the results. We question whether N(E) deduced from field-effect measurements represents the density of localized gap states in the bulk.

Doping Effects of Group-III and -V Element on a-Si Prepared by High Pressure rf Sputtering

Doping effects of group-III and -V element on the a-Si films prepared by “high” pressure rf sputtering were studied. Three different types of doping effects on conductivity were observed for different elements. One is the increase of conductivity due to the shift of the Fermi level, another is the increase of conductivity due to the increase of hopping sites contributing to variable range hopping conduction and the third is that doping is less effective on conductivity. Distinct adsorption effects on conductance were observed for the cases of B- or Al-doping. These phenomena indicate that the band-bending is caused in surface regions by adsorbates because of a relatively small density of localized gap states, so the formation of built-in potential due to Schottky barrier or p-n junction would be possible for those a-Si films prepared by “high” pressure rf sputtering.

高アルゴン圧ｒｆスパッタによるアモルファスシリコンの不純物効果と吸着効果

Substitutional doping was attained by co-sputtering B or P with Si under high Ar gas pressure. Adsorption effects on conductance and ESR were found in doped samples. The changes of conductance and ESR signal due to the adsorption of gases can be consistently explained by the changes of the occupation profile of electrons in the gap states which were caused by electrons or holes donated from adsorbates. Successful substitutional doping and distinct adsorption effects are considered to be due to a relatively small density of localized gap states in those a-Si which were prepared by rf sputtering under high Ar gas pressure.

Optical and electrical energy gaps in amorphous semiconductors

The experiments are analyzed which yield information about the density of localized gap states g l (E) in amorphous semiconductors. A conflict is found between optical absorption data which reveal only a very small g l (E) and electrical measurements, which predict orders of magnitude larger g l (E) . A model proposed resolves this conflict by assuming that because of spatial potential fluctuations electrons and holes are trapped in different regions and that near the mobility edge the extended electron wavefunctions are somewhat excluded from regions in which electrons are localized. The consequences of this model regarding the interpretation of the optical and the electrical gap are discussed.