States In Amorphous Silicon Research Articles

Investigation of Thermal Behavior on High-Performance Organic TFTs Using Phase Separated Organic Semiconductors

This study investigates the thermal behavior of small molecule and polymer blend (TMTES-Pentacene based) organic thin-film transistors, which exhibit high mobility and excellent on/off ratios. Solution-processed TMTES-Pentacene was formulated with a high-k polymer binder to achieve both high mobility and excellent device to device uniformity. The activation energy was extracted using the Arrhenius equation from the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\text {D}}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\text {G}}$ </tex-math></inline-formula> transfer characteristics measured over a range of temperatures. Different activation energies were observed in distinct temperature regions. The corresponding transfer mechanism was explained by the unstable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\pi }-\boldsymbol {\pi } $ </tex-math></inline-formula> packing caused by steric effects in TMTES-Pentacene, which is similar to the strain-bond-induced defect states in amorphous silicon bonding.

Control of Single-Electron Charging of Metallic Nanoparticles onto Amorphous Silicon Surface

Sequential single-electron charging of iron oxide nanoparticles encapsulated in oleic acid/oleyl amine envelope and deposited by the Langmuir-Blodgett technique onto Pt electrode covered with undoped hydrogenated amorphous silicon film is reported. Single-electron charging (so-called quantized double-layer charging) of nanoparticles is detected by cyclic voltammetry as current peaks and the charging effect can be switched on/off by the electric field in the surface region induced by the excess of negative/positive charged defect states in the amorphous silicon layer. The particular charge states in amorphous silicon are created by the simultaneous application of a suitable bias voltage and illumination before the measurement. The influence of charged states on the electric field in the surface region is evaluated by the finite element method. The single-electron charging is analyzed by the standard quantized double layer model as well as two weak-link junctions model. Both approaches are in accordance with experiment and confirm single-electron charging by tunnelling process at room temperature. This experiment illustrates the possibility of the creation of a voltage-controlled capacitor for nanotechnology.

Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices

We present the first measurements of optical nonlinearity due to free carrier effects in amorphous silicon films using z-scan technique, demonstrating enhanced nonlinearity due to existence of midgap localized states. We also introduce, fabricate and experimentally characterized a new composite waveguide structure consisting of amorphous and crystalline silicon. The fabricated composite rib waveguide confirms to have enhanced free-carrier nonlinearity at the estimated value of 4cm/GW, i.e., seven times larger than that of a crystalline silicon waveguide. Due to existence of the midgap localized states in amorphous silicon, the measured free-carrier lifetime in the composite rib waveguide was about approximately 300ps which is shorter than the values reported in the literature for similar geometries made of silicon.

Thermal Conductivity and Specific Heat of Thin-Film Amorphous Silicon

We report the thermal conductivity and specific heat of amorphous silicon thin films measured from 5-300 K using silicon-nitride membrane-based microcalorimeters. Above 50 K the thermal conductivity of thin-film amorphous silicon agrees with values previously reported by other authors. However, our data show no plateau, with a low T suppression of the thermal conductivity that suggests that the scattering of long wavelength, low Q vibrations goes as Q2. The specific heat shows Debye-like behavior below 15 K, with theta(D) = 487 +/- 5 K, and is consistent with a very small contribution of tunneling states in amorphous silicon. Above 15 K, the specific heat deviates less from Debye behavior than does its crystalline allotrope, indicating no significant excess modes (boson peak) in amorphous silicon.

Systematic study of electron localization in an amorphous semiconductor

We investigate the electronic structure of gap and band tail states in amorphous silicon. Starting with two 216-atom models of amorphous silicon with defect concentration close to the experiments, we systematically study the dependence of electron localization on basis set, density functional and spin polarization using the first principles density functional code Siesta. We briefly compare three different schemes for characterizing localization: information entropy, inverse participation ratio and spatial variance. Our results show that to accurately describe defect structures within self consistent density functional theory, a rich basis set is necessary. Our study revealed that the localization of the wave function associated with the defect states decreases with larger basis sets and there is some enhancement of localization from GGA relative to LDA. Spin localization results obtained via LSDA calculations, are in reasonable agreement with experiment and with previous LSDA calculations on a-Si:H models.

Geometric characterization of metastable states in tetrahedral bonded amorphous semiconductors

We introduce a new structural order parameter called the S vector that can be associated with each atom to describe short-range order in a solid, and use it to characterize the defects (floating bonds and dangling bonds) in amorphous silicon, based on a structural model using molecular dynamics (Dyrting et al., Europhys. Lett. 48 (1999) 403). The S vector fields have statistical characteristics that are generic for all the samples produced for amorphous silicon with our existing method. The S vector at an atomic site associated with defects typically has a large magnitude, which will decay exponentially as one moves away from the defects. The S vector has an orientation with a sense when coupled to the S vectors of neighboring atomic sites that can be described by the concept of chirality. The usefulness of this chirality associated with the defect is manifested in samples with multiple defects, where the neutrality condition for chirality is statistically obeyed. In a given sample with multiple defects, once we assign the chirality to the dangling bond, we find that all dangling bonds will have same chirality and the floating bonds will have the opposite chirality. The observation of absence of odd number of defects is explained. We also find that defects do interact in a tensorial manner. Experimental relevance of the geometric characterization of metastable states in amorphous silicon is discussed in the context of electron spin resonance and the relation of the S field with the strain field.

Atomistic Simulation of the Finite-Temperature Anderson Localization Problem

We present a simulation of the electron dynamics of localized edge states in amorphous silicon (a-Si) at room temperature by integrating the time dependent Schrodinger equation using a Crank-Nicholson method and a first-principles local basis Hamiltonian. We study the character of the spatial and spectral diffusion of the localized states and directly simulate and reveal the nature of thermally driven hopping. Phonon-induced resonant mixing leads to rapid electronic diffusion if states are available nearby in energy and real-space. We believe that many of the results we obtain are crucial for modeling transport phenomena involving localized states.

Bandwidth considerations in modulated and transient photoconductivity measurements to determine localized state distributions

This work examines the influence of limited instrumental bandwidth on the accuracy of recovery of the density of localized states in semiconductors from transient and modulated photoconductivity data. Paradoxically, knowledge of the short-time transient photoresponse can be vital in the estimation, via a Fourier transform, of the density of deep-lying states. We demonstrate that retention of the natural response of a bandwidth limited system, although subject to distortion at short times, can lead to much improved accuracy in density of states determination than simple truncation of the short-time response. It is shown that this improvement arises simply from the integrating effect of a bandwidth limited system over short time intervals, which makes it possible to access and exploit information originating at times much shorter than the instrumentation rise time. These concepts are exemplified using computer simulated transient photoconductivity for several model systems including one which mimics the expected density of states in amorphous silicon.

Anderson transition and thermal effects on electron states in amorphous silicon

I discuss the properties of electron states in amorphous Si based on large scale calculations with realistic several thousand atom models. A relatively simple model for the localized to extended (Anderson) transition is reviewed. Then, the effect of thermal disorder on localized electron states is considered. It is found that under readily accessible conditions, localized (midgap or band tail) states and their conjugate energies may fluctuate. The possible importance of non-adiabatic atomic dynamics to doped or photo-excited systems is briefly discussed.

Low-temperature anomalous specific heat without tunneling modes: A simulation fora−Siwith voids

Using empirical potential molecular dynamics we compute dynamical matrix eigenvalues and eigenvectors for a 4096 atom model of amorphous silicon and a set of models with voids of different sizes based on it. This information is then employed to study the localization properties of the low-energy vibrational states, calculate the specific heat $C(T),$ and examine the low-temperature properties of our models usually attributed to the presence of tunneling states in amorphous silicon. The results of our calculations for $C(T)$ and ``excess specific-heat bulge'' in the ${C(T)/T}^{3}$ vs T graph for voidless $a\ensuremath{-}\mathrm{Si}$ appear to be in good agreement with experiment; moreover, our investigation shows that the presence of localized low-energy excitations in the vibrational spectrum of our models with voids strongly manifests itself as a sharp peak in ${C(T)/T}^{3}$ dependence at $T&lt;3 \mathrm{K}.$ To our knowledge this is the first numerical simulation that provides adequate agreement with experiment for the very-low-temperature properties of specific heat in disordered systems within the limits of harmonic approximation.

Correlation between bond distortion and the band-tail electronic density of states in amorphous silicon: a tight-binding recursion study

We study the spatial structure of the electronic states in amorphous silicon using a tight-binding Hamiltonian and the recursion method. A realistic cluster consisting of 4096 atoms in a periodic cubic supercell, obtained using reverse Monte–Carlo methods, is used as a model of amorphous silicon [B.R. Djordjevic, M.F. Thorpe, F. Wooten, Phys. Rev. B 52 (1995) 5685]. The local density of states for each of the 16 384 orbitals is computed. Our aim is to provide some insight into the nature of the band tail states and we particularly address the following issues. We examine the relative contributions of the p-orbitals and the s-orbitals to the density of states in the region of the band tail and gap. We investigate the geometric nature of these states. Finally, we study the correlation between the structural distortions in the amorphous silicon cluster and the spatial distribution of the band tail and gap states. Our results show that the p-orbitals contribute a large fraction of the density of states in the valence band tail. This fraction decreases as the energy increases across the gap into the conduction band. We observe the expected change from extended states deep in the valence and conduction band to localized states in the band gap. An attempt was made to characterize this change in terms of the fraction of the total density of states required for a percolating nearest-neighbor path in the cluster as a function of the energy. No abrupt change was observed in this quantity for energies in the region of the band tails. Although there are 4096 atoms in the cluster, 90% of the density of states in the band tail and gap is localized on only 416 atoms, that is, we find a backbone of atoms in the cluster responsible for a large fraction of the states in this energy range. We also find a strong correlation between atoms contributing a high density of states in the band tail and gap region and atoms with a large bond angle distortion.

The structure of electronic states in amorphous silicon

We illustrate the structure and dynamics of electron states in amorphous Si. The nature of the states near the gap at zero temperature is discussed and especially the way the structure of the states changes for energies ranging from midgap into either band tail (Anderson transition). We then study the effect of lattice vibrations on the eigenstates, and find that electronic states near the optical gap can be strongly influenced by thermal modulation of the atomic positions. Finally, we show the structure of generalized Wannier functions for amorphous Si, which are of particular interest for efficient ab initio calculation of electronic properties and forces for first principles dynamic simulation.

Current Noise Measurements of Surface Defect States in Amorphous Silicon

AbstractMeasurements of conductance fluctuations in coplanar hydrogenated amorphous silicon (a-Si:H) are reported as a function of surface etching treatments. The noise power spectrum displays a broadened Lorentzian peak, associated with surface damage by CF4 reactive ion etching (RIE), whereas surface etches using ion milling or wet chemicals remove the Lorentzian spectral feature and only a 1/f spectral form for frequency f is observed. The Lorentzian spectral feature can be explained by trapping-detrapping from surface states induced by the RIE etch, which cause fluctuations in the depletion width of the space charge region near the film surface. The thermally activated Lorentzian comer frequency is a measure of the degree of band bending and the Fermi energy at the thin film surface.

Atomistic Structure of Band-Tail States in Amorphous Silicon

We compute accurate approximations of the electronic states near the gap in a very large and realistic model of $a\ensuremath{-}\mathrm{Si}$. The spatial structure of the states is computed explicitly and discussed. The character of the local to the extended (Anderson) transition in amorphous Si is described. The density of states, the conductivity, and doping are discussed.

Determination of Hydrogen Density of States in Amorphous Silicon Using Fractional Evolution Experiments

ABSTRACTHydrogen plays an important role in the electronic behavior, structure and stability of amorphous silicon films. Therefore, determination of the hydrogen density of states (DOS) and correlation of the hydrogen DOS with the electronic film properties are important research goals. We have developed a novel method for determination of hydrogen DOS in silicon films, based on fractional evolution experiments. Fractional evolution experiments are performed by subjecting a silicon film to a series of linear, alternating heating and cooling ramps, while monitoring the hydrogen evolution rate. The fractional evolution data can be analyzed using two complementary memods, the fixed frequency factor approach and Arrhenius analysis. Using a rigorous, mean-field evolution model, we demonstrate the applicability of the two approaches to obtaining the hydrogen DOS in silicon films. We further validate both methods by analyzing experimental fractional evolution data foran amorphous silicon carbide film. Both types of analysis yield a similar double peaked density of states for the a-Si:C:H:D film.

Anharmonic Decay of Vibrational States in Amorphous Silicon

Anharmonic decay rates are calculated for a realistic atomic model of amorphous silicon. The results show that the vibrational states decay on picosecond time scales and their decay rates increase with increasing frequency. These results disagree with a recent experiment. In contrast to predictions of the fracton model, we find no evidence that the anharmonic decay is inhibited in the region of localized states.

Investigation of interface and band gap states in amorphous silicon p—i—n solar cells using current deep level transient spectroscopy

Abstract Current deep level transient spectroscopy (DLTS) studies were performed on as-deposited and reverse biased annealed samples of a-Si:H p—i—n solar cells. These studies indicate interface states at p/i and i/n junctions, in addition to the band gap states in the i-layer. The hole and electron trap states are located at 0.8 and 1.12 eV, respectively, above the valence band, whereas the interface states at the p/i junction are located at 0.55 eV and those at the i/n junction at 1.2 eV above the valence band. A decrease in the interface states with reverse biased annealing treatment has been observed.

Localized vibrational states in amorphous silicon

Using a valence-force-field model as interatomic potential, vibrational eigenstates have been computed in the harmonic approximation. The density of vibrational states and their inverse participation ratio are compared for crystalline silicon, fully-coordinated amorphous silicon (a-Si) and a-Si with voids. Voids of various sizes and concentrations have been introduced into an a-Si structure that was generated with a vacancy model. The presence of voids increases the local strain in the a-Si network and causes substantial changes in the vibrational density of states. Localization occurs not only for high frequency modes but also for band edge states. At low frequencies, the deviation from a Debye density of states is due to such localized extra modes depending on void size and concentration.

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.

Effect of carbon impurities on the density of states and the stability of hydrogenated amorphous silicon.

The effect of trace carbon impurities on the density of states in amorphous silicon has been studied. The deep-defect densities and mobility-gap electronic structure were characterized with electron-spin resonance, drive-level capacitance profiling, and transient photocurrent measurements. Good quantitative agreement in the defect densities deduced from these three methods has been found. This implies a ratio between charged and neutral dangling bonds of at most 2 to 1. Light-induced changes in the mobility-gap electronic structure were also investigated in these films. A small but significant increase in the density of light-induced defects was observed for samples with carbon impurities at the 1 at. % level. The time to saturation of the light-induced degradation for the carbon containing samples was also significantly increased. We discuss the interpretation of these results in terms of two possible mechanisms: either from the presence of carbon-related precursor sites, or by widening of the band gap with carbon alloying.