Deviations from square-root distributions of electronic states in hydrogenated amorphous silicon and their impact upon the resultant optical properties

We study the distributions of conduction band and valence band electronic states associated with hydrogenated amorphous silicon. We find that there are substantial deviations from square-root distributions, particularly deep within the bands and within the gap region. The impact of these deviations is assessed through a determination of the spectral dependence of both the joint density of states function and the imaginary part of the dielectric function. These deviations are found to have a considerable effect upon the determination of the corresponding Tauc optical gap, the optical gap obtained for the case of hydrogenated amorphous silicon being 220 meV lower than the energy difference between the valence band and conduction band band edges. We suggest that the standard interpretation for the Tauc optical gap, as the energy difference between these band edges, should be reconsidered in light of these results.     

Advances in understanding the defects contributing to the tail states in pure amorphous silicon

The tight-binding molecular dynamics simulations and reverse Monte Carlo structural modeling method were applied in order to investigate the existence of small bond angles (like those in triangles and squares) in amorphous silicon networks. The influence of small bond angles on the electronic density of states was analyzed. The presence of a number of smaller bond angles is necessary for a proper reproduction of the neutron diffraction data of amorphous silicon. Semiempirical Hartree–Fock calculations show that these arrangements provide higher energy levels in electronic density of states which are localized on these local structures. Accepting this result we must reconsider the electronic density of states of amorphous semiconductors. The localized mobility gap has structure i.e. two characteristic peaks can be found inside the tail. First larger peak belongs to the squares while the peak at larger energy is formed by triangles.     

Examination of the electronic energy band model for one-dimensional disordered thin films

The nature of the electronic states in a disordered one-dimensional finite system subject to traveling wave boundary conditions is examined. A method is developed whereby these states can be characterized as either “localized” or extended. The concept of localization of states is modified so as to be applicable to amorphous thin films. The extent to which the disorder modifies the band structure of the ordered system is investigated as a function of disorder and the nature of the electronic states is related to the elastic tunneling transmission coefficient for a model metal-semiconductor-metal heterojunction. The results seem to corroborate the Mott-CFO model.     

Photo-induced phenomena in chalcogenide glass: Comparison with those in oxide glass and polymer

Two topics, the both being related with localized states in non-crystalline solids, are studied. One is a comparison of photo-induced phenomena in oxide, chalcogenide, and organic materials. Despite of different inorganic and organic structures, there exist many similarities in photo-induced phenomena, which can be ascribed to excitation of localized electronic states. The other topic concerns photo-induced phenomena induced by linear and non-linear excitation. A result on As₂S₃ demonstrates that band-gap excitation by one- and two-photon processes provides different changes. It is suggested that the two-photon process occurs resonantly at around localized states in amorphous materials, and the process plays important roles in the structural change.     

Chemical bond approach to the electronic structure of amorphous semiconductors

The electronic structure of amorphous semiconductors with various compositions is described by the assembly of bonding, antibonding and lone-pair orbitals. Energy levels for these orbitals are calculated by extended Hückel theory. Broadening of the levels due to the interaction among chemical bonds and/or lone-pairs is determined empirically. The present calculation is free from detailed atomic structures. The changes of densities of valence states, average energy gaps and optical gaps with composition are calculated for various amorphous semiconductors. The results agree rather well with the observed ones.     

Charge transport between localized states in lithium-intercalated amorphous tungsten oxide

We report on electron transport in amorphous tungsten oxide films, prepared by dc magnetron sputtering and subsequently electrochemically intercalated with lithium. The ion–electron intercalation process allows us to change the position of the Fermi level in a controlled way and to determine the density-of-states in the conduction band. We present a new method to determine the localization length of the electron states in disordered materials. The method is based on measurements of the electronic density-of-states together with electrical resistance in the variable range hopping regime. We find that the electronic states of amorphous tungsten oxide are localized up to about 1.3 eV into the conduction band, where an insulator-metal transition occurs. The localization length was determined on the insulating side of the transition and the estimated scaling exponent is consistent with the scaling theory of localization.     

Magnetic susceptibility and atomic structure of paramagnetic Zr–(Co, Ni, Cu) amorphous alloys

The experimental results for the magnetic susceptibility and superconducting transition temperature of amorphous Zr–(Co, Ni, Cu) alloys extending over a wide composition range are analyzed in some detail. By combining the results of this analysis with the literature results for the electronic density of states at E_F for the same alloy systems we obtained a set of parameters associated with the electronic structure of the amorphous Zr. The comparison of these parameters with the results of the band structure calculations for different crystalline phases of Zr and with the results of the atomic structure and crystallization studies of the same alloy systems indicates an fcc-like local atomic structure for amorphous Zr.     

The case for sharp band edges in amorphous Ge and Si

Must the absence of long range order alone result in the loss of well defined valence and conduction band edges in the density of states? The available experimental data for amorphous Ge and Si is examined. It is concluded that, for samples where the density of microvoids and other extrinsic defects are kept sufficiently small, the experimental data strongly suggests sharp band edges.     

Amorphous silicon-electrolyte interface

Amorphous silicon-electrolyte (AS-E) interface has been studied to determine the density of localized states in evaporated amorphous silicon films. Due to the large density of such states in amorphous semiconductors, very large fields are required to cause changes in the space-charge layer. AS-E system presents a more convenient system to meet this need for large fields than the usual MOS structure. A study of cpacitance as a function of bias shows that the density of localized states around E_F is of the order of 10¹⁸/cm³ · eV and that it is fairly uniform in the midgap region.     

Modulated photoconductivity method for investigation of band gap states distribution in silicon-based thin films

It has been demonstrated that the well known modulated photocurrent technique can be modified to escape imperfect data in intrinsic parameters of amorphous and microcrystalline silicon films and to simplify measurements of the density of localized states distribution in the band gaps of these semiconductors. The information on the density-of-states distribution can be extracted from temperature dependence measurements of the constant and modulated components of the photoconductivity in the film illuminated by the light modulated with some selected frequencies. The modified method has been applied to microcrystalline hydrogenated silicon films with n- and p-type conduction. The study has demonstrated that the tail of the density-of-states distribution near the valence band of microcrystalline hydrogenated silicon is less steep than that near the conduction band.     

Principles of electrical behavior of amorphous semiconductor alloys

A new approach to understanding amorphous semiconductors is provided by a model whose novel concepts are necessary consequences of transport and recombination fundamentals. The amorphous semiconductors are examples of the ‘relaxation semiconductor’: When dielectric relaxation is slow compared with recombination, familiar transport behavior is drastically altered and the recombination characterized by cross sections for carrier capture which are extremely large. At thermal equilibrium, ‘equality recombination’ obtains, with which local space-charge neutrality is maintained through equal total capture rates of electrons and holes per unit volume. In the alloys, such recombination after single free paths gives equal electron and hole conductivities, and thus pins the Fermi level at the position of minimum conductivity for a given band gap. Both mean free paths are equal and of the order of the electron wavelength. They determine the concentrations of centers positively and negatively ionized, which are equal and large compared with mobile carrier concentrations. Nearly all the centers are neutral, are donors near the valence band and acceptors near the conduction band, and may be in fairly short band tails. The recombination is mainly at the deeper levels of the acceptor centers, which act also as shallow electron traps. On a simple and unified electronic basis, this model accounts in detail for the various aspects of observed behavior. The threshold switching is due to characteristics ‘recombinative injection’ of minority electrons. This produces a region of injected space charge in which the net recombination rate is zero, and puts at the anode an adjoining ‘recombination front’ of a type in which carriers are activated by field equal to the threshold field. Other aspects treated include, for example: the small negative Hall coefficient; the positive thermoelectric power; preswitching; delay time; integrated current or total charge during preswitching in its quantitative dependence on delay time; the ‘blocked ON state’; and the field effect, analysis of which gives the ionized concentration. The model has further implications for theory of the band structure.     

Substitutional doping of chemically vapor-deposited amorphous silicon

Dc conductivity, absorption coefficient, photoconductivity, and magnetoresistance of phosphorus-doped amorphous Si films prepared by chemical vapor deposition (CVD) have been measured as a function of doping ratio. These results indicate that phosphorus doping reduces localized states in the mobility gap, narrows the tailing width below the extended states, and that phosphorus donors form the impurity band at 0.15 eV below E_c at a doping ratio of about 1×10^-2. It is also found that electronic properties of CVD amorphous Si can be controlled in a wide range by substitutional doping of phosphorus atoms.     

Luminescence gap in hydrogenated amorphous silicon

The “luminescence gap” is used instead of the thermalization gap and the hopping-gap because the gap is obtained from the luminescence measurement. The luminescence gaps in hydrogenated amorphous silicon (a-Si:H) are observed in the temperature range from 4.2 to 225 K for the films prepared at different substrate temperatures 170 to 300 °C by plasma CVD. It is shown from the temperature dependence of the luminescence gap that the luminescence edges are at the localized band tail states at which the waiting time for the hopping is equal to the life time of the luminescence. The excitation energy dependence of the luminescence peak energy similar to that of the porous Si has been observed.     

Cu/Ag单掺及Cu-Ag共掺对ZnO光催化性能影响的第一性原理研究

运用第一性原理方法研究了Cu/Ag单掺及Cu-Ag共掺对ZnO稳定性、电子结构和光学性质的影响。Cu_Zn2和Cu_Zn1-Cu_Zn3的形成能接近0.0 eV,Cu_Zn1-Cu_Zn2的形成能为负值,表明Cu较易掺杂且掺杂时有聚集的趋势。Ag_Zn2的形成能为2.5 eV,Ag_Zn1-Ag_Zn2和Ag_Zn1-Ag_Zn3的形成能高于4.5 eV,说明高浓度的Ag掺杂在实际合成中不易出现。Cu_Zn1-Ag_Zn2和Cu_Zn1-Ag_Zn3的形成能与Ag_Zn2的相近。Cu_Zn2、Cu_Zn1-Cu_Zn2和Cu_Zn1-Cu_Zn3模型的Cu3d与O2p轨道有较大程度的杂化,价带...     

Statistical considerations on electronic behavior of the gap states in amorphous semiconductors

The equilibrium distribution functions of the gap states taking into account the effective intrasite electronic correlation energy have been employed to discuss the electronic behaviors of amorphous semiconductors, especially to interpret the temperature dependence of dc and ac conductivity and also the effect of foreign impurity atoms on the conductivity in chalcogenide glasses. The dc conductivity of thermal activation type has been explained without assuming a negligible density of states near the Fermi level, and the gradual decrease in activation energy of conductivity with increasing impurity concentration, even in melt-quenched chalcogenide glasses, has been deduced.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

Chemische Bindung und Struktur von Halbleitern und Isolatoren

Modern semiempirical substance and structure theories are very useful for the investigation of chemical trends of material properties of semiconductors and insulators. On the basis of the conception of the ionicity of mixed ionic-covalent chemical bonds, chemical trends of selected energetic, electric and elastic properties are described. The ionicity parameter, which can be derived from the static electronic dielectric constant or from valence band spectra, is built into models for different effects (e.g. the chemical shift of electronic binding energies). In this way chemical information is taken into account in an explicit manner and good agreement with experimental results is obtained. With the help of the ionicity and related parameters the separation of fourfold from sixfold coordinated crystal structures, the relative stability of the wurtzite and zincblende structure, and the problem of different bonding angles in different SiO₂ modifications are investigated. An extension of this conception with the help of the pseudopotential theory leads to some interesting conclusions concerning structural disorder effects in amorphous semiconductors     

Comparison of dispersion models in the optical characterization of As–S chalcogenide thin films

In this paper, the optical analysis of the As–S thin films prepared under different conditions is performed using three dispersion models for amorphous materials typically employed in practice. The Tauc–Lorentz (TL) model, Urbach–Cody–Lorentz (UCL) model and our model based on the parameterization of the density of electronic states (PDOS model) are namely used to determine the optical parameters of these films. Within the structural model of the As–S films, the existence of the optical constants profiles and overlayers on to the upper boundaries of these films are included. Dispersion and structural models are employed within the analysis based on a combination of variable angle spectroscopic ellipsometry and transmission spectrophotometry in conjunction with multi-sample modification. It is demonstrated that the TL model is not suitable for the optical characterization of the As–S thin films because of the absence of the Urbach tail. Furthermore, it is shown that both the UCL and PDOS models are satisfactory ones for the optical characterization of these films. Moreover, it is also shown that, using the PDOS dispersion model, one can evaluate the material parameters of these As–S films that correspond to the electronic band structure.     

Investigation of the density of localized states in a-Si using the field effect technique

The electronic properties of amorphous solids are largely determined by the distribution of localized states $\text{N}$ (?) in the mobility gap. In this paper, the field effect technique is applied to the experimental study of $\text{N}$ (?) in specimens of a-Si prepared by the glow discharge method and by vacuum evaporation. The experimental approach and the analysis of the results are discussed in some detail. $\text{N}$ (?) curves, extending over an energy range of up to 0.5 eV have been obtained for a series of glow discharge specimens, deposited at substrate temperatures between 310 and 570 K. The results show structure in the gap states, a well-defined minimum almost in the centre of the mobility gap and a rapid rise in $\text{N}$ (?), 0.18 eV below ?_c, which is identified with the onset of band tail states. The field effect data confirm that the predominant conduction mechanism at room temperature changes from hole hopping to transport in extended electron states, as the Fermi level is moved through the minimum in $\text{N}$ (?) The effects of annealing on the state distribution have been investigated, showing that $\text{N}$ (?_f) can be reduced by one or two orders of magnitude. The nature of the gap states is discussed and the divacancy is suggested as a basic model for the electronic states involved.     

Electrical and optical properties of binary glasses from the arsenictelluriumselenium system

An investigation of binary glasses derived from the arsenicseleniumtellurium system is described in which measurements were made of d.c. conductivity over a range of temperatures, space charge limited current, density and absorption in the near infrared region. For the higher temperature region the electrical results were consistent with conduction being due to carriers excited into extended states. While conduction at lower temperatures may in some cases be due to carriers hopping in localized states at the band edges, an alternative approach in which the localized states were considered to act as donors and acceptors gave satisfactory agreement with the experimentally determined conductivity versus temperature and activation energy versus temperature relationships and with the value of the pre-exponental constant in the conductivity expression. An anomalous field dependence of conductivity was found for annealed specimens of two glasses and it is tentatively suggested that this is associated with the creation of an impurity band. Measurements of space charge limited current showed that gold gave ohmic contacts to the glass, whereas silver and copper did not. The measurements also provided evidence for the localized states having an exponential distribution. Changes of electrical properties and density as a function of glass composition are discussed in the light of possible structural changes taking place in the glasses. The absorption edges of the glasses in the infrared region were found to be exponential in form and had slopes that did not very greatly for the range of compositions studied.