States In Amorphous Silicon Research Articles

Effects of hydrogenation and doping on the conductivity and density of defect states in amorphous silicon

The temperature dependence of the dc conductivity σ has been measured in the temperature range T=100–500 K for a series of a-SiHx:B thin-film specimens prepared by magnetron reactive sputtering. Two groups of specimens were measured: (a) undoped specimens containing hydrogen concentration varying from zero to about 30 at. %, and (b) doped specimens containing 10 at. % hydrogen and boron concentration in the range 0.1–10 at. %. Activated band conduction (log σ∼T−1) was observed at high temperatures in undoped specimens containing about 5–10 at. % hydrogen. For all other specimens there is an extended temperature range of log σ∼T−1/4 consistent with charge transport by Mott’s variable range hopping mechanism. The density of states N(EF) at the Fermi level undergoes appreciable changes with increasing concentrations of hydrogen and boron. These changes are interpreted as evidence of substantial structural disorder in hydrogen-rich and heavily doped a-SiHx:B films.

Space-charge-limited conduction for the determination of the midgap density of states in amorphous silicon: Theory and experiment

The density of states (DOS) in amorphous silicon is a key parameter in assessing the performances of photocells made of this material. The principle of the determination of the DOS by the study of the space-charge-limited current (SCLC) had first been given by den Boer in an approximate but very physical model. We have found that a precise determination of the DOS in amorphous silicon by this method requires special precautions, both theoretical and experimental: it is only after elimination of most of the pollution by the electrodes and walls of the chamber that we have found that the scaling law is valid with good precision, and only for films thicker than $d=1.5$ \ensuremath{\mu}m; in the usual experimental conditions (current density 1 ${\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}$), the situation is intermediate between the low-injection condition (Ohm's law) and the high-injection condition, so that the asymptotic solutions given by the regional approximation, as used by previous authors, are not valid. By comparing the experimental curves with the exact solutions obtained by numerical integration of the SCLC equations, we have determined the DOS in amorphous silicon films with an estimated uncertainty of 15%. The application of the method to a series of films produced by capacitive glow discharge shows the following: (a) the DOS at the Fermi level is very sensitive to the quality of the pumping system (pollution by air, ${\mathrm{H}}_{2}$O, pumping oil), the best value obtained in our films being $5\ifmmode\times\else\texttimes\fi{}{10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$${\mathrm{eV}}^{\ensuremath{-}1}$; (b) for films produced in identical conditions, there is a well-defined minimum of the DOS at a preparation temperature ${T}_{s}=260\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, which explains why most of the best photocells are prepared at this temperature; (c) the DOS in the first 2000-4000 \AA{} of the films is larger by a factor of 3-10 than that in the rest of the film. This has a direct implication for the fabrication of photocells which have a thickness only 2-3 times this perturbed region. The cause of this effect, whether it is an intrinsic surface effect or external effect, corrected by self-cleaning after a few minutes of plasma, remains to be determined.

Localized density of states in amorphous silicon determined by electrophotography

We report that the distribution of localized density of states g(E) for hydrogenated amorphous silicon can be estimated by the electrophotographic (xerographic) method. The density of states near midgap can be evaluated to be about 1×1016 cm−3 eV−1 for undoped films. The general trend and the shape of g(E) are similar to those obtained by the deep level transient spectroscopy and the photothermal deflection spectroscopy.

The relationship between space-charge-limited current and density of states in amorphous silicon

Abstract This paper is concerned with the nature of the current-voltage (J-V) characteristics of thin amorphous silicon layers under space-charge-limited conditions. J-V curves have been calculated by using a computer program which employs any arbitrary form of the density-of-states function N(E) as input. The use of simple analytical forms of N(E) illustrates the way in which J(V) depends on the form of N(E) and shows that, under certain conditions, it should be possible to observe the effects of carrier trapping both in deep states near the Fermi level and in tail states remote from the Fermi level. The possibility of observing tail-state trapping at attainable current densities is enhanced by low values of N(E), by working at low temperatures and by using thin samples. Calculations have also been made by using two experimentally determined density-of-states functions, one derived from field-effect conductance and the other from deep-level transient spectroscopy (DLTS) measurements. The corresponding J-V characteristics show wide variations. Comparison with experimental results in the literature shows no evidence for the deep minimum in N(E) derived from DLTS measurements.

New paramagnetic states in amorphous silicon and germanium

New paramagnetic centers were observed by electron spin resonance at low temperatures in a-Si:H, a-Ge:H and related alloys. Phosphorus-doped samples show a pair of hyperfine lines with spin densities up to 10 17 cm −3 centered around the well known conduction band tail resonance. A second resonance is found in many samples regardless of the doping level and is tentatively ascribed to surface states.

Determination of the gap density of states in amorphous silicon by phase shift analysis of the modulated photocurrent

The density of localized states in the mobility gap of evaporated amorphous silicon films has been measured over a range of 250 meV between the conduction band and the Fermi level. The method employed is based on the analysis of the phase shift between an intensity modulated exciting light and the associated photocurrent induced in the semiconductor. The density of states falls off almost exponentially with energy away from the conduction band. It suggests an overlap of the conduction and valence band tails, a result consistent with the Cohen, Fritzsche, and Ovshinsky model.

Doping and gap states in amorphous silicon

The author shows that the decline in doping efficiency in a-Si:H with increased doping level arises because the dopants act as negative U centres and pin Ef during deposition. He also shows that only deep states from dangling bonds and dopants respond to coordination changes so unlike in chalcogenides, Ef is pinned in a finite density of shallow tail states. Defect bonding can now be expressed in a single scheme valid for all amorphous semiconductors.

Theory of amorphous-silicon charge-coupled devices

The theory of operation of amorphous-silicon charge-coupled devices has been studied numerically and analytically under the assumption that the localized states in amorphous-silicon are distributed exponentially with respect to energy. The transfer inefficiency ε is found to depend not only on the localized state density but also on the transit time and initial density of signal electrons. The approximate analysis shows that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\ln(\epsilon)</tex> is a linear function of logarithmic clock frequency, and that its coefficient is given by the characteristic temperature which represents the steepness of the localized state density distribution in amorphous-silicon.

On the analysis of space-charge-limited current—voltage characteristics and the density of states in amorphous silicon

Abstract An expression is derived for the current-voltage characteristic under space-charge-limited conditions when the density of deep states increases exponentially with energy above the Fermi level. This shows that the slope of a logJ - log V curve increases slowly and only reaches the limiting value of (l + 1) (where l = T c/T and T c characterizes the steepness of the exponential) at voltages above those used in practical experiments. This invalidates the procedure of deriving values for T c by fitting straight lines to experimental log J - log V plots and suggests an explanation of some anomalous results published recently. Some brief comments are made on the values of T c to be expected for a-Si: H.

Reduction in the localized band-gap states in amorphous silicon by annealing and hydrogen implantation

The effect of in situ thermal annealing prior to hydrogen implantation is reported on vacuum evaporated amorphous silicon. By performing a 400 °C anneal for 4 h immediately following film deposition the film porosity is greatly reduced. The film is then implanted with hydrogen to a dose of 5×1016/cm2. A field-effect conductance change of six orders of magnitude was observed which yielded a density of localized states near the Fermi level of 4×1017/cm3 eV.

Defect states in amorphous silicon

AbstractThe nature of defects in amorphous silicon is calculated by using several methods for clusters with varying size. The wave function of the dangling bond state has an approximately exponential decay with a decay length of about 0.2 nm. The correlation energy of electrons and the hyperfine structure constant with 29Si for the dangling bond state are evaluated. The correlation energy and the g‐values are calculated both for a weak bond and for a two‐fold coordinated defect and are compared with experiments.

Physical Properties of Two Metastable States of Amorphous Silicon

ABSTRACTCharacterization of the two metastable states of amorphous Si produced by ion implantation is extended to include electron paramagnetic resonance, fundamental absorption edge, and density measurements in addition to infrared reflection. It is found that the properties of the two a-Si states are not dependent upon the mass of the incident ion (12C, 29Si, 31p, 120Sn) or upon the anneal temperature for 400°≤TA≤600°C. The dangling-bond density drops about a factor of 2, the absorption coefficient drops by more than a factor of 5, but the density does not change when the a-Si makes a transition between the two states.

The density of states in amorphous silicon determined by space-charge-limited current measurements

Abstract Space-charge-limited current (SCLC) flow has been investigated as a function of applied potential, temperature and specimen thickness in amorphous silicon films prepared by the glow-discharge technique. Specimens had symmetric n+-i-n+ or n+-n-n+ structures in which the thin n+ surface layers gave strong electron injection. From the analysis of the current—voltage characteristics in the SCLC regime it is shown that the current flow is controlled by localized states situated at the quasi-Fermi level. Specimens in a thickness range from 0·7–2 μm satisfy the scaling law for SCLCs, confirming that the measurements are representative of a bulk-controlled property. Three methods for the determination of the density-of-state distribution, g(ϵ), from the SCLC characteristics were investigated and gave results agreeing within a factor of two. g(ϵ) curves were determined in a range from 0·75–0·46 eV below ϵc. Their shape agrees with that of typical field-effect curves, but their magnitude lies below the field-effect results by a factor of 3 to 5, in the above range. This suggests that the density of states in the surface region probed by the field effect may exceed the volume density. However, the volume distribution deduced here from the SCLCs differs basically in shape and magnitude from that found by DLTS measurements. g(ϵ) curves obtained by the three experimental methods are compared and discussed in some detail.

Density of states in amorphous silicon produced by microwave glow discharge

Capacitance-voltage characteristics of a metal-oxide-amorphous silicon structure are analyzed in order to obtain the density of states N(E) in microwave glow discharge amorphous silicon films. Our measurement and deconvolution techniques reveal some structure in the density of states profile. At the midgap N(E) is found to be lower than 1017 cm−3eV−1.

Correlation effects and the density of states in amorphous silicon

The evaluation of the density of gap states from the charge density measured in field-effect experiments is considered in terms of a model which incorporates electron correlation effects. Due to the positive Hubbard U a density of states is deduced which differs appreciably from the “field effect” densities published previously. Besides the lack of the E x and E y peaks the minimum density is found to be ∼ 10 15 − 10 16 cm −3 eV −1.

Amorphous silicon-silicon nitride thin-film transistors

A thin-film field-effect transistor has been fabricated using glow-discharge amorphous silicon as the semiconductor and silicon nitride as the insulator. The transistor operates in the electron (n type) accumulation mode and by changing the gate potential from zero to only 3 V a change in the source-drain conductance of greater than four orders of magnitude is obtained. The results imply upper limits to the density of gap states in amorphous silicon and interface states at the amorphous silicon-silicon nitride interface of 3×1016 cm−3 eV−1 and 5×1011 cm−2 eV−1, respectively.

Electron spin resonance measurements on amorphous silicon

Electron spin resonance (ESR) and related spin-dependent measurements have been very useful probes of defect states in amorphous silicon (a-Si). In this paper we briefly describe spin resonance techniques ( e.g. ESR, light-induced ESR, spin-dependent luminescence and photoconductivity, and electron-nuclear double resonance) and the relevant experimental parameters ( e.g. spin density, line shape, hyperfine coupling, exchange interaction etc.). We then review what has been learned about a-Si usually from the coupled results of spin resonance and other experiments ( e.g. luminescence, conductivity, hydrogen effusion etc.) concerning the nature of states and recombination kinetics of non-equilibrium carriers. We also indicate the potential applications and limitations of the techniques for characterizing material and devices.

Density of states in amorphous silicon determined from transport experiments

Conductivity and thermopower has been studied for a variety of glow discharge and evaporated aSi films doped by alkali ions, phosphorus, and by boron. It is shown that for all dopants and doping levels the electronic transport over a wide temperature range can be described by an activated mobility. The unusual temperature dependence of both conductivity and thermopower observed in the experiments must, therefore, be ascribed to the statistical shift of the Fermi level. A selfconsistent density of states model is proposed that accounts for the statistical temperature shift of the Fermi level as well as for the changes of the Fermi level position due to doping. It shows a deep minimum near midgap for undoped material and for evaporated films a higher density of gap states which is independent of energy.

Hydrogenation and the density of defect states in amorphous silicon

We discuss the nature and density of defects in pure amorphous silicon, under the light of recent hydrogenation experiments. It is argued that a reliable measure of defect densities can be obtained by counting the number of hydrogen atoms that can be incorporated in a pure starting material by subsequent plasma treatment. This leads to interpretation of hopping conductivities in terms of a Fermi level density of states N f in excess of 10 20 cm −3 eV −1 and wave function spatial extent of the order of 2.5 Å. This figure is shown to be reasonable for a dangling bond wavefunction.

Defect states in amorphous silicon

Abstract A novel mechanism for spin-pairing at defect centres in amorphous silicon is proposed and discussed in terms of simple chemical bond arguments. Unlike the case of chalcogenide glasses (or amorphous arsenic), the presence of non-bonding (lone-pair) orbitals is found to be unnecessary in producing a negative effective Hubbard energy in this material. Instead, a mechanism is proposed in which dehybridization occurs at the negatively charged centre, the predominant bonding changing from sp3-like to p-like. Various experimental data strongly point to the presence of such spin-paired defects in amorphous silicon, and these observations are discussed in the light of the present model. The implications for a.c. conduction are considered, and it is concluded that, as in chalcogenide glasses, bipolaron hopping is the predominant mechanism.