Conduction Band Tail Research Articles

Recombination traffic in highly crystallized undoped microcrystalline Si films studied by steady state photoconductivity

The dependences of photoconductivity ( σ ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films on the temperature and the intensity of light were measured in a temperature range 15–325 K. The variation in light intensity exponent ( γ) is found to be 0.5 ≤ γ ≤ 0.9 for thin samples, and 0.15 ≤ γ ≤ 0.8 in a thick fully crystallized sample. Based on our simulation and experimental results, we have been able to construct density of state map for both the samples of different microstructure. In particular, we found that the fully crystallized thick sample has a very steep conduction band tail compared to thin sample. The valence band tails (VBT) are classified into two parts: one with sharper tail near the edge originating from grain boundary defects and other one with less steep tail associated with the defects in columnar boundary regions. Capture cross section for the deeper tail is smaller than the shallower one. The shallower VBT states extend up to more depth in thin sample. Our simulation results for thick sample show the transformation of recombination traffic taking place from shallower states to deeper tail states in the temperature region where γ < 0.5. Therefore, these deeper tail in thick sample works as a “safe hole traps”.

Pre-exponential factor ina-Se75In25-x Pb x thin films

Temperature dependence of d.c. conductivity is studied ina- Se75In25-x Pb x thin films wherex is varied from 0–10. From these measurements, the values of the pre-exponential factor (σ0) and activation energy (ΔE) are calculated for each glassy alloy. An approximate linear dependence of ln σ0 on AE is observed in this glassy system with good agreement between the expected and calculated σ0 values using Meyer-Neldel rule. Linear dependence of ln σ0 on ΔE in case of amorphous materials indicates that the conduction band tails a finite energy distance towards the valence band and Fermi level is controlled by fixed dominant hole levels deeper in the gap.

Field-effect mobility of amorphous silicon thin-film transistors under strain

We applied strain ranging from 1% compressive to ∼0.3% tensile to a-Si:H TFTs on polyimide foils by bending them inward or outward, or by stretching them in a microstrain tester. We also applied strain to a-Si:H TFTs by deforming a flat substrate into a spherical dome. In each case, compression lowered and tension raised the on-current and hence the electron field-effect mobility. We conclude that compressive strain broadens both the valence and conduction band tails of the a-Si:H channel material, and thus reduces the effective electron mobility. We show that the mobility can be used as an indicator of local mechanical strain.

A pulsed EDMR study of hydrogenated microcrystalline silicon at low temperatures

Experimental results of pulsed electrically detected magnetic resonance (pulsed EDMR) measurements are reviewed with regard to the question for microscopic mechanisms that determine the lifetimes of excess charge carriers in hydrogenated microcrystalline silicon. The results show that two qualitatively different dangling bond (db) recombination processes occur. (i) Distant pair recombination through shallow conduction band tail states (CE centers) and dbs and (ii) a db direct capture (dc) recombination where charge carriers localize directly at dbs. For the db–dc process, it is shown that an intermediate, excited triplet state of the db exists which lies about 50meV below the conduction band edge. The recombination probabilities of this path are quantified by means of pulsed EDMR. The temperature and light intensity dependencies suggest that under the experimental conditions given, the db–dc process is not only electronically more active in comparison to the CE–db process, it is even the determining process for the lifetime of excess charge carriers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

State distribution in hydrogenated microcrystalline silicon

We have been able to determine the density of states map in the band gap of a semiconductor by the measurement of the phototransport properties of its majority and minority carriers. In particular we found that the carrier recombination in single-phase hydrogenated microcrystalline silicon $(\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ is significantly different from the one in hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ and that it is controlled only by its two band tails. The comparison of the observed temperature dependence of the phototransport properties of this material with model simulations further suggests that, while the conduction-band tail has an exponential distribution of states, the valence-band-tail states have a Gaussian-like distribution. This, in turn, meets the challenge of the determination of the analytical shape of the density of states distribution from experimental data. Our experimental procedure implies then that this distribution is associated with the route through which the transport and phototransport take place and thus we conclude that both the recombination and transport in undoped single-phase $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ take place in the disordered layer that wraps the crystallites. We further conclude that, from the transport and phototransport points of view, the single-phase $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ is, in general, different from both polycrystalline silicon and $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ The polycrystalline-silicon-like behavior, when found, appears to be an asymptotic case in which the crystallites are large enough, while the $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ behavior prevails only when there is a significant content of its phase within the system.

Electronic effects of light ion damage in hydrogenated amorphous silicon

Light ions in the range 11–20 amu have been implanted at low energies into hydrogenated amorphous silicon. Based on Schottky barrier measurements it is shown that for low doses the residual disorder following annealing above the equilibration temperature is approximately proportional to the energy dissipated per unit volume by the ion beam. This is manifest as a decrease in the effective Schottky barrier height arising from an increase in the strength of the conduction band tail and the Urbach energy and the associated decrease in mobility gap. At high doses the change in band gap saturates when the damage reaches a well defined value independent of ion mass. Chemical effects are detected following N and O implants both of which introduce a small number of donor-like states. Flourine has the remarkable ability to passivate defect states remaining after bombardment and annealing and improve the quality of the material.

Magnetic resonance measurements in hydrogenated amorphous silicon

Abstract Nuclear magnetic resonance (NMR) and electron spin resonance (ESR) are useful techniques for obtaining information about local structural order and defects in hydrogenated amorphous silicon (a-Si:H). In particular, NMR is a useful probe of the bonding of hydrogen to silicon and of the trapping of molecular hydrogen (H 2 ) in a-Si:H. ESR is a useful probe of localized electronic states, such as silicon dangling bond defects and optically excited electrons and holes trapped in the conduction and valence band tails, respectively. Recent NMR measurements have shown that H 2 exists in two environments, one clustered and one dilute. The fraction of H 2 , which is greater than previously thought, is approximately 10% of the total hydrogen concentration. ESR measurements of the growth and decay of silicon dangling bonds after optical excitation at low temperatures (one probe of the Staebler–Wronski effect) show that these defects are created at a slower rate and anneal at lower temperatures than those created at 300 K. Recent NMR measurements suggest the presence of a paired-hydrogen site, which may be the stabilizing center for the defects that control the Staebler–Wronski effect. ESR results show that the decay of optically excited, band-tail electrons and holes is a universal feature of amorphous semiconductors for which the electron–lattice interaction can be neglected. Although the research is less developed in microcrystalline silicon, NMR and ESR are proving to be useful probes of local order and defects in this material as well.

Theory of amorphous silicon solar cell (b): a five layer analytical model

Abstract An analytical model for amorphous silicon p–i–n solar cell is developed by dividing the i-layer into five regions. The region close to p+ layer is dominated by charge in the tail states, whereas the region next to it has a space charge due to positively charged dangling bonds. This is followed by a quasi-neutral region with a nearly constant electric field. Then, there is a space charge arising due to negatively charged dangling bonds with a conduction band tail dominated space charge region adjacent to n+ layer. Poisson's equation and continuity equation for electrons and holes are solved to obtain the current–voltage characteristic of the cell. An interesting finding of the analysis is that the electric field in the quasi-neutral region increases significantly in short circuit condition when illuminated by light. This is due to neutralization of dangling bonds in the space charge region. When the voltage across the cell is allowed to develop as one moves from short circuit to open-circuit condition, the electric field in the central part of the cell reduces resulting in a loss of collection efficiency and hence a poorer fill factor.

A Study of Electronic Defects in Hydrogenated Amorphous Silicon Prepared by the Expanding Thermal Plasma Technique

AbstractThe electronic properties of amorphous silicon films prepared by the expanding thermal plasma technique have been studied using steady-state and transient photoconductivity measurements. It is found that films deposited at a substrate temperature of 400°C have a conduction band tail slope of 29 meV, deep defect density of order 3×1016 cm-3, an Urbach tail slope of 65 meV, defect absorption of 5-10 cm-1, and a mobility-lifetime product of 1.3×10-7 cm2 V-1. Aslight increase in defect density and reduction in mobility-lifetime product is observed on moderate light-soaking. The overall optoelectronic quality is somewhat poorer than commercial PECVD material, but there is scope for improvement as deposition conditions are further optimised.

Localised States in Microcrystalline Silicon Photovoltaic Structures Studied by Post-Transit Time-of-Flight Spectroscopy

AbstractPost-transit time-of-flight spectroscopy has been used to study the density of states distribution in hot-wire CVD microcrystalline silicon pin solar cell structures. For an absorber layer Raman scattering intensity ratio ICRS of 0.4 or less, behaviour consistent with multiple-trapping carrier transport is observed and may be interpreted in terms of a conduction-band tail of some 18 meV slope plus a broad defect bump of order 1017 cm-3 centered at 0.55 eV relative to the mobility edge. As ICRS is increased beyond 0.4, the temperature-dependence of the photocurrent transient becomes inconsistent with multiple-trapping and above 0.6 the decays are almost temperature-independent. By comparing data taken at 300 K, it may be inferred from the multiple-trapping model that localised states between 0.35 and 0.5 eV are associated with the presence of columns or clusters of nanocrystals and those deeper than 0.5 eV with the amorphous tissue. Results are compared with previous work on coplanar and sandwich structures.

Independence of photoconductivity of p-type a-Si:H films on doping level and defect concentration

The temperature dependence of the photoconductivity of boron doped a-Si:H films was studied in detail before and after intense light soaking. At temperatures from 200 to 330 K the lack of dependence of the photoconductivity of p-type a-Si:H films on doping level and light induced defects was observed. These results are explained by a model based on the concept that non-equilibrium occupation of dangling bonds (DB) in p-type a-Si:H differs essentially from equilibrium one. This is mainly caused from asymmetry of the density of states distribution in conductance- and valence-band tails. According to the proposed model, the non-equilibrium concentration of D 0-centers in p-type a-Si:H is determined not by the doping level and total DB concentration, but by the parameters of the effective trap level in the valence band tail. Numerical calculations taking into account the direct capture of free carriers by DB and tunneling recombination between trapped electrons in the conduction band tail and trapped holes in the valence band tail confirm our findings.

Tail widths in hydrogenated amorphous silicon and amorphous silicon carbon alloys measured by photomodulated infrared spectroscopy

We have measured the photoinduced infrared absorption of hydrogenated amorphous silicon a-Si:H and silicon-carbon alloys $a\ensuremath{-}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}:\mathrm{H},$ using the sensitive technique of photomodulated infrared spectroscopy. The observed spectra are quantitatively accounted for by a model, which allows one to determine the width (energy spreading) of the conduction and valence band tail states separately.

Properties of polymorphous silicon–germanium alloys deposited under high hydrogen dilution and at high pressure

Hydrogenated polymorphous silicon–germanium samples with Ge content x up to 0.35 have been deposited in a plasma-enhanced chemical vapor deposition system from silane and germane precursors highly diluted in hydrogen and in a high pressure range. The optical, defect-related, and transport properties of both types of carriers have been studied using a set of complementary techniques before light soaking, after light soaking, and after subsequent annealing. Before light soaking, we observe a small deterioration of the electron transport properties with increasing Ge content. This is related to the widening of the conduction band tail and to the increase of a deep defect band with emission energies at about 0.4 eV below the conduction transport path, which is attributed to Ge dangling bonds. The hole transport properties are much less sensitive to the incorporation of Ge, which we attribute to the unchanged Urbach tail parameter. Light soaking results in a deterioration of the transport properties due to an increase of the density of states, the relative effect being less pronounced for the highest Ge contents due to the higher native defect concentrations. Again, the electron transport properties are more affected than the hole transport properties, which appear quite stable against light soaking. Annealing at 460 K following light soaking is found to restore the initial state more easily in the low Ge content samples. Compared to samples deposited under more conventional conditions (no or less hydrogen dilution and low pressure) the striking features of our alloys are that (i) they exhibit an enhanced stability of the transport properties against light soaking and (ii) the characteristic time of the light-soaking kinetics is shorter. These features reveal the polymorphous character of our alloys, related to a specific hydrogen microstructure observed by infrared spectroscopy and hydrogen effusion, which indicate that the amorphous matrix exhibits an improved order like in hydrogenated polymorphous silicon.

A simplified joint density of states analysis of hydrogenated amorphous silicon

We propose a simplified empirical model for the density of state functions of hydrogenated amorphous silicon that neglects the conduction band tail electronic states. The corresponding joint density of states function is then computed. We find, while this analysis is considerably simplified, that the resultant joint density of states function compares favorably with that determined from an empirical model for the density of states functions with the conduction band tail taken into account. Analytical and asymptotic results, relating the parameters characterizing the underlying density of states functions with the joint density of states function, are developed. The density of states parameters corresponding to hydrogenated amorphous silicon are then determined through an analysis of some hydrogenated amorphous silicon joint density of states experimental data. It is suggested that this simplified empirical model for the density of states functions will prove of greater utility to the experimentalist.

Violet luminescence emitted from ZnO films deposited on Si substrate by rf magnetron sputtering

Highly orientated polycrystalline ZnO films have been deposited on Si substrate at room temperature (RT) by rf magnetron sputtering. A strong violet photoluminescence (PL) located at 402 nm and a weak ultraviolet (UV), PL located at 384 nm are observed when excited with 300 nm light. The former PL originated from the electron transition from conduction band tail states to valence band tail states and the latter is produced due to electron transition from conduction band to valence band. With an increase in intensity of the excitation light, the violet emission peak increases super-linearly and the UV emission increases linearly. After high temperature annealing in air, the crystallinity of obtained films is improved, the violet emission becomes weak and the UV emission gets strong.

First principles molecular dynamics study of amorphous AlxGa1−xN alloys

We have systematically investigated atomic structures, electronic and dynamical properties of amorphous aluminum–gallium–nitride alloys (a-AlGaN) by performing first principles local basis molecular dynamics simulations. The network topology and defects of the amorphous 216-atom model system have been analyzed with the radial distribution function, the angular distribution function, the ring statistics, and the local coordination. It was found that the models have mixed threefold and fourfold coordinations, and the number of threefold (fourfold) coordinated atoms in alloys decreased (increased) with increasing Al composition. No odd rings are found, indicating that no wrong bonds (homonuclear bonds) appear in the a-AlxGa1−xN alloys. The Ga–N and Al–N bond lengths show a small variation with the Al composition, which is in agreement with recent extended x-ray absorption fine structure experimental measurements. The electronic properties examined by the electronic density of states and local bonding character demonstrate that no mid-band-gap states exist. The band-gap dependence on Al fraction x in a-AlxGa1−xN alloys shows a nearly linear variation with Al composition, and exhibits a small downward bowing behavior. It was also shown that valence band tail states are mostly localized on the threefold coordinated N sites, while the conduction band tail states are mostly localized on the threefold coordinated Ga and Al sites, and the electronic localization tends to become weaker with the addition of Al. We find a mixture of sp3 and sp2 bonds present in the network and their interaction plays a key role in the dynamical properties of a-AlxGa1−xN alloys.

Electrons in feldspar II: a consideration of the influence of conduction band-tail states on luminescence processes

Most natural feldspars contain many charged impurities, and display a range of bond angles, distributed about the ideal. These effects can lead to complications in the structure of the conduction band, giving rise to a tail of energy states (below the high-mobility conduction band) through which electrons can travel, but with reduced mobility: transport through these states is expected to be thermally activated. The purpose of this article is twofold. Firstly, we consider what kind of lattice perturbations could give rise to both localized and extended conduction band-tail states. Secondly, we consider what influence the band tails have on the luminescence properties of feldspar, where electrons travel through the sample prior to recombination. The work highlights the dominant role that 0.04–0.05-eV phonons play in both the luminescence excitation and emission processes of these materials. It also has relevance in the dating of feldspar sediments at elevated temperatures.

Role of interstitial hydrogen and voids in light-induced metastable defect formation in hydrogenated amorphous silicon: A model

Densities of states of a large set of samples have been measured. Samples were deposited by radio-frequency powered glow discharge under various conditions of temperature, power, gas pressure, and dilution of silane. Density of states was studied in the as-deposited, light-soaked, and annealed states. For all the samples light soaking resulted in an increase of both the deep defect density and of the conduction-band tail states. For samples deposited on the edge of crystallinity and polymorphous materials irreversible modifications of the density of states were observed after light soaking followed by annealing. Since none of the existing models of the metastability can account for this behavior we propose a model. In this model, light induced creation of dangling bonds is mediated by interstitial hydrogen. Hydrogen coming from the breaking of Si-H bonds is trapped into voids or platelets during light soaking and released in the lattice during annealing. This model fully explains our experimental results and also many other experimental observations found in the literature.

Universal distribution of optically excited carriers in tetrahedral amorphous semiconductors

The decay of optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors is a universal property of these amorphous solids [Phys. Rev. Lett. 84 (2000) 4180]. The experimental measurements employ a seldom-used electron spin resonance (ESR) detection technique that surpasses the sensitivity of previous measurements by several orders of magnitude. This technique employs second harmonic detection (SH-ESR) of the ESR. Measurements have been performed over a wide range of excitation intensities (nW/cm 2 to W/cm 2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). Using the SH-ESR technique, the kinetics can be studied down to saturated carrier densities as low as approximately 10 14 cm −3 . In addition to the long-time decay curves, both the saturated densities of carriers after long-time irradiation and the most probable recombination lifetimes for the optically excited carriers in a-Si:H agree well with models for the universal diffusion and recombination of carriers at low temperatures. Similar results are obtained for samples of a-Ge:H with germanium dangling-bond densities ≲10 17 cm −3 . A comparison of the ESR lineshapes for the electrons and holes trapped in the conduction and valence band tails, respectively, with electrons and holes trapped in a divacancy in crystalline silicon shows that the asymmetries and localizations of the wavefunctions are similar in both cases.

Defect-related absorption in hydrogenated silicon films

Recently introduced interference spectroscopy technique (IST) is used to investigate the absorption spectrum in the sub-gap energy range (0.6–1.7 eV) of hydrogenated silicon films prepared under a wide range of plasma conditions. The shape of the sub-gap absorption in several sets of samples, including amorphous, polymorphous (pm-Si:H) and microcrystalline films has been analyzed. In particular, for a series of polymorphous films deposited at 200 °C in the pressure range of 800–1800 mTorr a clear minimum of the sub-gap absorption is obtained at a pressure of 1400 mTorr. In the spectra of the best samples, a dip is observed in the conduction band tail in the region of 1.4–1.65 eV. The rise in absorption coefficient with the threshold around 1–1.1 eV in highly crystallized samples may be an indication of the crystallinity. Light-soaking effects on spectra and on the evolution of the dip at 1.5 eV are also studied in this work. The detailed analysis of results is presented.