Photoconductivity and absorption in amorphous Si

The paper deals with photoconductivity and absorption in aSi specimens, prepared mainly by the decomposition of silane in a glow discharge. Substrate temperatures, T_d, between 300 K and 650 K were used during deposition. The normalised photoresponse was measured at room temperature as a function of photon energy and T_d in a spectral range from 0.5 eV to 3 eV. The absorption coefficient was determined for evaporated, sputtered and glow discharge specimens.The main features of the results are in agreement with conclusions drawn from previous electrical transport and field effect measurements and can be interpreted on the basis of the proposed model for the localised state distribution. It is confirmed that ?_c??_v is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ?_c. At room temperature the steady photocurrent is carried predominantly by electrons in states above ?_c, whether excitation is from localized or extended states. Specimens prepared at T_d > 500 K are highly photosensitive, with electron recombination lifetimes, τ, of up to 10^?5s. Rise and decay times of the signal lie in the millisecond range. For T_d < 500 K there is a drastic decrease in τ, which falls to 10^?11 s at $\text{T}{}_{\mathrm{<mtext>d</mtext>}}\text{? 300}\text{K}$ and is even less for evaporated specimens. These results are discussed in some detail.     

Localized valence states in the forbidden gap of non-crystalline tetracene

Employing the thermally stimulated current technique the existence of three distinct gaussian trap distributions in vapor-deposited tetracene layers has been established. In layers formed at T_f ? 130 K the valence band is split into a distribution of localized states, the width being σ ? 0.06 eV, and the density N_t equal to the molecular density. Upon increasing T_f from 130 to 180 K, both N_t and σ decrease, indicating a decrease of structural disorder. For $\text{T}{}_{\mathrm{<mtext>f</mtext>}}\text{? 180}\text{K}$ the center of the trap distribution is at E₀ = 0.07 eV above the valence band. At T_f = 180 K an optimum quasi amorphous structure is formed which still lacks long-range order, but in which short-range order is present to a degree that in local regions a narrow valence band can be established. Trap distributions centered near 0.4 and 0.7 eV with a width of 0.07 and 0.1 eV, respectively, and containing about 10¹⁵ states/cm^?3, are almost independent of film formation condition, and are probably also of structural origin.     

Electrical conductivity and thermoelectric power of liquid AgSb Te2

The electrical conductivity and thermoelectric power of liquid AgSb Te₂ have been investigated as a function of temperature. Experimental data are analyzed in terms of a recent model proposed by Mott. The activation energy for electrical conductivity and thermoelectric power is found to be approximately 0.50 eV with a large temperature coefficient γ ～ 7 × 10^?4 eV/deg K. The gradual transition from a semiconducting to a metallic behaviour has been observed at high temperature.     

The temperature dependence f photoconductivity in a-Si

The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ?_A is situated about 0.18 eV below ge_C in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ?_V. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ?_A. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.     

Electrical resistivity,magnetic susceptibility and thermoelectric power of amorphous niobium-nickel alloys synthesized by rf-sputtering

Thin film samples (10–20 μ thick) of niobium-nickel alloys in the composition range Nb-5 to 95% Ni were vapour quenched by rf sputtering onto fused quartz substrates held at a temperature of 450 K. At room temperature, the electrical resistivity of these alloys lies between 176–210 μΩ cm, and the absolute thermoelectric power S between 2.20–2.52 μV/K. Magnetic susceptibility for Ni_0.5Nb_0.5 and Ni_0.4NB_.6 amorphous alloys show a Pauli magnetic behaviour with values of x of about 1.5 × 10^?4 and 1.8 × 10^?4 emu g^?1, respectively.     

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.     

Electrical and optical properties of amorphous boron and amorphous concept for ß-rhombohedral boron

Measurements of the conductivity (σ), thermoelectric power (S) and thermal conductivity (κ) of amorphous boron are made over wide temperature ranges (T = 77–850 K for σ, T = 300–850 K for S and T = 80–1100 K for κ). The room temperature spectral dependencies of the reflection (R) and absorption (α) coefficients are determined for the wavelength intervals 2–25 μm and 1.3–25 μm respectively. The I–V characteristics are also studied and shown to be consistent with the Poole-Frenkel law.The value obtained for the thermal energy gap of amorphous boron (1.3 eV) is slightly smaller than that of crystal ß-rhombohedral boron (1.4 eV). The temperature dependence of the electrical conductivity can be satisfactorily described by the Mott law $\text{ln}\text{σ ≈ ?(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, where $\text{T}{}_0\text{? 10}{}^8\text{K}$. This gives an estimate, N ≈ 10¹⁸ cm^?3, for the concentration of trapping levels responsible for the hopping conduction. The value $\text{?}{}_0\text{? 9}$ is found from the spectral dependence of R while α has Urbach-like character $\text{? α ≈}\text{exp}\text{(}\text{h}\text{?}\text{ω/Δ)}$, where $\text{Δ ? 0.19}\text{eV}$.A comparison is made between amorphous boron and crystalline ß-rhombohedral boron. Because of the very complex crystal structure and the large dimensions of the unit cell of ß-boron, some of its physical properties could be qualitatively described on the basis of the so-called ‘amorphous concept’.     

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.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.     

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E_c, the mobility edge at the conduction band.     

Effect of γ-irradiation on non-linear I-V behaviour and thermoelectric measurements in amorphous semiconducting AsSeTe system

An amorphous semiconducting compound of the composition As₂SeTe₂ is studied for its non-linear current-voltage characteristics, conductivity and thermoelectric measurements. It has been observed that γ-irradiation of the sample gives a lower threshold voltage and greater thermoelectric power. The threshold voltage shifts to a lower voltage value for the irradiated sample. The non-linear I-V behaviour and p-type nature of unirradiated and γ-irradiated As₂SeTe₂ are discussed on the basis of charged defect states existing and also created after γ-irradiation in the material. The thermoelectric power for unirradiated and γ-irradiated As₂SeTe₂ is discussed on the basis of a quantity, related to conductivity and thermoelectric power which explains to a great extent the effect observed experimentally.     

Electrical conductivity of GeTe glasses under hydrostatic pressure

The electrical conductivity of some GeTe bulk glasses has been measured between 10 and 80°C under hydrostatic pressure up to 3000 bar. The electrical conductivity (σ) of as-prepared, amorphous samples can be expressed by an equation: σ = A exp (?B/kT). For Ge₁₇Te₈₃ glass, the pressure dependences of the constants, A and B, are: (d ln A/dp) = ?3.2 × 10^?4 bar^?1 and (dB/dp) = ? 2.1 × 10^?5 eV · bar^?1. The results are analysed in terms of the low-mobility band model of Mott-CFO for amorphous semiconductors.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Electrical,structural and optical properties of amorphous carbon

The planar and transverse electrical resistivity of amorphous carbon (a-C) films getter-sputtered at low temperature (77–95 K) is well-fitted by the expression $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ The exponent T₀ being approximately the same in both cases (≈ 7 × 10⁷ K) suggests that the amorphous films are isotropic. Films thinner than 600 Å display a two-dimensional hopping conductivity from which one deduces a density of states N(E_F) at the Fermi level of 10¹⁸ eV^?1 cm^?3 and a radius of the localized wave functions (a) of 12 Å. Tunneling experiments and optical absorption measurements are consistent with a pseudogap of approximately 0.8 eV. Electron diffraction experiments indicate that a-C films consist of a mixture of diamond and graphite bonds; this fact taken in the light of the other experiments would suggest that the graphite bonds act as the localized conduction states.     

Hall resistivity of a magnetic amorphous alloy — Effects of thermal cycling and annealing

We have studied the effects of thermal cycling and annealing on the Hall resistivity ?_H and electrical resistivity ? of an amorphous magnetic material, Metglas 2826A. Thermal cycling up to 600 K has no significant effect on ? but changes ?_H, especially at T > 300 K. Annealing at higher temperatures T_a affects ?_H more strongly that it influences ?. For T_a = 650 K the Curie temperature jumps by ～300 K and this, combined with X-ray data, is regarded as being symptomatic of the onset of crystallization. It is demonstrated that, for a fixed T, the variation of the extraordinary Hall coefficient R₁ is simply correlated with that of ? for both the amorphous material and the annealed (crystallized) specimens. Furthermore, for a given T_a, the variations of R₁ and ? as functions of temperature exhibit the same formal relationship. The temperature dependence of ? in the annealed specimens is rendered plausible in terms of recent theories of disordered materials. Room-temperature studies of electron spin resonance and the field dependence of the magnetization are also reported.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Study of amorphous Ge–SiO2–P+Si tunnel junctions

A new method to determine ac conductivity of amorphous Ge using Al-amorphous Ge–SiO₂–P⁺Si tunnel junctions is presented. Frequency dependence of ac conductivity is found to satisfy the power law in the frequency range between 1 and 50 kHz and the density of localized states at the Fermi level is estimated to be ～ 1.7 × 10²⁰ cm^?3 eV^?1 which decreases to ～ 4.5 × 10¹⁹ cm^?3 eV^?1 after annealing at 175°C.Temperature dependence of tunneling conductance of Al-amorphous Ge–SiO₂–P⁺Si junctions is appreciable only near zero bias. Zero bias conductance of the junctions obeys the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law of Mott; the density of localized states obtained from the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law is one order of magnitude smaller than that obtained by ac conductivity measurements, being insensitive to annealing. This behavior of the tunnel junctions differes in many respects from those of Al–Al₂O₃-amorphous Ge tunnel juntions.     

Microsegregation in liquid Pb-based eutectics

Temperature dependence of electrical conductivity, σ(T), thermoelectric power, S(T), and viscosity, η(T), of Pb-based eutectic systems was studied. Anomalous changes of thermophysical properties in liquid binary Pb_26.1Sn_73.9, Pb₄₄Bi₅₆, Pb₈₃Mg₁₇ and ternary Bi₄₆Pb₂₉Sn₂₅ eutectics occurred well above the liquidus. The temperature range of anomalies reached hundreds of degrees. The obtained results are interpreted assuming that microsegregation areas exist in the eutectic systems.