States In Amorphous Silicon Research Articles

Inelastic resonant tunneling in amorphous silicon microstructures

We have observed strong electron-phonon interaction on deep centers in the process of resonant tunneling conduction through localized states in amorphous silicon. Our data support the theory of Glazman and Matveev pertaining to inelastic resonant tunneling in mesoscopic junctions.

Correlated hopping through thin disordered insulators.

We present data from Mo/a-Si/Mo tunnel junctions together with calculations that show that hopping transport via localized states in amorphous silicon is highly correlated. Localized states whose single-particle energies lie well below the Fermi level participate in transport due to the large on-site Coulomb interaction U. The results also imply that the density of these states is roughly constant over a wide energy range of order U.

Density of Deep Bandgap States in Amorphous Silicon From the Temperature Dependence of Thin Film Transistor Current

ABSTRACTWe have measured the current-voltage characteristics of amorphous silicon thin film transistors (a-Si TFTs) over a wide range of temperatures (20 to 160°C) and determined the activation energy of the channel current as a function of gate bias with emphasis on the leakage current and subthreshold regimes. We propose a new method for estimating the density of localized states (DOS) from the dependence of the derivative of activation energy with respect to gate bias. This differential technique does not require knowledge of the flat-band voltage (VFB) and does not incorporate integration over gate bias. Using this Method, we have characterized the density of localized states with energies in the range 0.15–1.2 eV from the bottom of the conduction band and have found a wide peak in the DOS in the range of 0.8–0.95 eV below the conduction band. We have also observed that the DOS peak in the lower half of the bandgap increases in magnitude and shifts towards the conduction band as a result of thermal and bias stress. We also measured an overall increase in the DOS in the upper half of the energy gap and an additional peak, centered at 0.2 eV below the conduction band, which appear due to the applied stress. These results are in qualitative agreement with the defect pool Model [1,2].

Measurement of Urbach edge and midgap states in amorphous silicon p-i-n devices

Abstract Urbach energy of valence band tails and mid-gap defect densities are important parameters for determining the performance of amorphous silicon (a-Si:H) devices. In this paper, we examine a technique which allows one to determine these parameters in device structures, as opposed to in films. The technique consists of measuring sub-gap quantum efficiency as a function of reverse bias and photon wavelength. We show that there is a distinction between the response of tail states and mid-gap states to the application of reverse bias, and that by analyzing these differences, one can locate the energies of the mid-gap states in the device. The technique gives an accurate measurement of Urbach edge of tail states, even on textured substrates, but only estimates mid-gap states within a factor of 2.

Improved defect-pool model for charged defects in amorphous silicon.

We have developed an improved defect-pool model for the calculation of the density of dangling-bond states in amorphous silicon. The results of this improved defect-pool model are contrasted with earlier work, from which we have eliminated some errors and approximations. We show that the calculated energy-dependent density of states depends on the specific microscopic reaction involving hydrogen, in contrast to previous conclusions. We calculate the bulk density ofstates, using the best input parameters drawn from experiment, and conclude that the best agreement with experimental results is found for a rather wide defect pool and for a microscopic model where two Si-H bonds break for every weak bond converted into two dangling bonds

Defect chemical potential and the density of states in amorphous silicon.

We derive a general expression for the defect chemical potential of amphoteric silicon dangling bond states in amorphous silicon. We show that a single expression is valid, irrespective of the charge state of the dangling bond. We apply this result in a defect pool model for the formation of dangling bonds by the breaking of weak silicon-silicon bonds, with and without hydrogen motion. We calculate the resulting energy dependent density of states.

Molybdenum impurity states in amorphous silicon and related materials

Abstract The role of molybdenum as an electronically active impurity in amorphous silicon and other tetrahedrally bonded amorphous semiconductors is discussed. The presence of Mo can be detected via the paramagnetic Mo5 + ion, which gives rise to an electron spin resonance with a g value of 1.92–1.93. The formation of this ion is enhanced by the presence of oxygen during deposition of thin films. We present experimental evidence that molybdenum in amorphous silicon can form an impurity donor level and thus can have an influence on the electronic properties of nominally undoped samples. Potential sources of Mo during thin-film depositiion are also discussed.

Valence band density of states in amorphous silicon deduced from spd orbitals

Hybrid spd orbitals adapted to the local environment of individual atoms are used as a basis for a tight-binding calculation of the valence-band density of states for several structural models of amorphous silicon. The results for truly random network models are in good agreement with experiment.

Comments on Electron and Hole Transport in the Band Tail States of Amorphous Silicon at Low Temperatures

ABSTRACTRecent experimental results on the low temperature drift mobility in amorphous silicon are examined on the basis of the approach to hopping transport developed by Silver and Bässler. It is shown on general grounds that the main features of the experimental results cannot be explained by a purely exponential tail state distribution, but are consistent with the distribution used by Spear and Cloude (1988) in model calculations.

Electron and hole transport in the band-tail states of amorphous silicon at low temperatures

Abstract In this paper a detailed investigation is presented of the new low-temperature transport regime in amorphous silicon (a-Si) recently reported by the authors. Electron and hole drift mobility results on reverse-biased Cr-i-n+ barriers and p+-i-n+ junctions were extended to 10 K. Activation energies below 30 K are found to lie between 1 and 2meV. Both carrier mobilities, μe and μb are systematically dependent on the excitation intensity and this aspect has been further investigated. The discussion of these results leads to the conclusion that the observed electron hopping transport takes place near the bottom of the tail states where the thermalization of excess electrons has become sufficiently slow for well defined transit signals; transport is constrained to hopping paths in a region of sufficiently strong wavefunction overlap between sites. These conclusions are tested by model calculations of thermalization and hopping transport in two tail-state distributions. An exponential distribution cann...

Photoconductivity measurements as a tool for the evaluation of the density of states in amorphous silicon

Photoconductivity measurements as a function of the temperature and light intensity were carried out on samples of hydrogenated and halogenated silicon films. The samples were deposited by r.f. glow discharge starting from a mixture of silicon tetrachloride and hydrogen.Experimental data were analysed by using a theoretical model which assumes a density of states distribution having two exponential band tails and two gaussian distribution of states in the mobility gap.The model predicts a dependence of the photoconductivity as a function of the light intensity which agrees very well with the experimental one.

Determination of the density of states in amorphous silicon from thermostimulated conductivity measurements

The density of states between the Fermi level and the conduction band in glow-discharge a-Si:H has been determined by measuring thermostimulated currents and photo-currents simultaneously. During the TSC measurements the sample is exposed to light pulses of low intensity. From the measurements both g(E) and the μτ product can be calculated. Results are shown from samples grown at different glow-discharge powers. Minimum values of g(E) at 0.3 eV below E c range from 10 16 to 10 17 eV −1 cm −3.

Gap states in amorphous silicon—dangling and floating bonds

It is shown that the primitive intrinsic defects in amorphous Si (a-Si) are threefold- and fivefold-coordinated Si atoms. A variety of experimental and theoretical information indicates that both these defects are likely to play a role in determining the properties of a-Si. Both have localized states in the energy gap. Electron-spin-resonance (ESR) data show that the dominant paramagnetic defect is more likely to be fivefold-coordinated Si (floating bond). A luminescence peak at ∼0.9 eV, on the other hand, is likely to be due to threefold-coordinated Si (dangling bond). In hydrogenated a-Si (a-Si:H), interstitial H (either at a “bond center” or “channel” site) may also have a state in the gap.

Photoemission and density of states in amorphous silicon

The density of occupied states in the band gap and the position of the Fermi level of a-Si:H films of different doping levels have been measured. Undoped samples reveal a disordered surface layer with surface defects at 0.6 eV above the valence band maximum and with a density between 3−15×10 12 cm −2. The work function is 4.3 eV and nearly independent of doping. Oxidation reduces the defect density, etching by HF creates 6×10 12 cm −2 defects.

Electron spin resonance of shallow defect states in amorphous silicon and germanium

The nature of shallow states in a-Si:H and a-Ge:H is discussed. Spin resonance techniques can be used to distinguish between intrinsic tail states and extrinsic dopant levels. A quantitative analysis of the spin resonance data allows one to construct models for the density of states close to the mobility edges in doped samples and to explore the role of these states for carrier trapping.

Experimental determination of the density of gap states in amorphous silicon by Schottky barrier admittance

Abstract Experimental admittance measurements are presented for an n-type doped amorphous-silicon Schottky barrier. The measurements are shown to be quite consistent with the theory described by Archibald and Abram (1983, 1986) and, using this theory, an estimate of the density of states in the upper half of the mobility gap is calculated. The average value is ∼ 1017 cm−3 eV−1 and there is a minimum situated approximately 0˙4 eV below the conduction-band mobility edge. This result is in approximate agreement with the density of states deduced by Lang, Cohen and Harbison (1982) using deep-level transient spectroscopy. The test-fit value for the gap-state capture/emission time constant τ0 is found to be 10−12 s.

Amorphous-crystalline silicon heterojunction: Theoretical studies of the dark current and spectral response

All the terms contributing to the dark current for an n-amorphous/ p-crystalline silicon heterojunction have been evaluated. The abrupt heterojunction model with the presence of interface states has been adopted and flat quasi-Fermi levels have been assumed across both depletion regions. The gap density of states in amorphous silicon has been approximated by a V-distribution. The dominant current mechanisms at forward and reverse biases have been determined and the zero-bias spectral response has been calculated.

The localized density of states in amorphous silicon determined by electrophotography

The localized density of states g(E) near midgap for undoped, B2H6-doped, and PH3-doped hydrogenated amorphous silicon is evaluated from the electrophotographic (xerographic) method. The density of hole traps decreases with increasing B2H6 concentration and that of electron traps decreases with increasing PH3. These features are qualitatively similar to those deduced from the deep-level transient spectroscopy, the photothermal deflection spectroscopy, the time-of-flight measurements, and the isothermal capacitance transient spectroscopy. The energy positions of neutral dangling bonds D0 and doubly occupied D− are estimated to be 0.85–0.88 eV from the valence band and 0.62–0.67 eV from the conduction band, respectively. Comparison of the energy peaks of electron and hole traps shows that the positive correlation energy of dangling bonds is about 0.3–0.4 eV.

Application of picosecond optical parametric amplifiers to semiconductor studies

The radiation of a picosecond parametric amplifier based on KDP crystal pumped by the second harmonic of a mode-locked Nd:YAG laser was used to investigate excitation and relaxation processes in different semiconductors on a subnanosecond time scale using time-resolved reflection, transmission, and photoconductance measurements. The excitation wavelength tunability (400-1500 nm) was used both for distinction of bulk and surface recombination and of diffusion processes in silicon and for investigation of carrier excitation processes in GaAs and CdS. Furthermore, the tunability was used to excite carriers into different states in amorphous silicon in order to separate different relaxation channels.

Thermally stimulated current studies of the density of gap states in amorphous silicon

Careful thermally stimulated current experiments were carried out on undoped amorphous hydrogenated silicon. We observe two peaks in the TSC profile, one at 120°K and the other at 300°K. We construct a theoretical framework based on the retrapping mechanism to understand this phenomena. The agreement between experiment and the theoretical prediction is good. In contrast the non-retrapping, high field formulation provides a poor fit to the experiment. We deduce that the density of states (DOS) in the gap has a maxima at 0.45 eV in broad agreement with the results of field effect measurements. Deep level transient spectroscopy (DLTS) measurements, which predict a minima in the DOS at 0.4 eV, find no support from our calculations.