Showing papers in "Philosophical Magazine Part B in 1982"
TL;DR: In this article, it was shown that the crystal lattice expansion of microcrystalline silicon leads, in the limit of small crystallite size of about 30 A, to an instability of the diamond structure with respect to the amorphous phase.
Abstract: It is shown that the crystal lattice expansion of microcrystalline silicon leads, in the limit of small crystallite size of about 30 A, to an instability of the diamond structure with respect to the amorphous phase. Simple thermodynamic considerations are presented which support the idea of the crystalline-to-amorphous transformation being a discontinuous order—disorder phase transition.
225 citations
TL;DR: In this paper, a simple model explains how multiple trapping (MT) in a continuous distribution of localized states gives rise to dispersion, which is then used to show what can be learned about the material from a complete study of its dispersive transport when MT is known to be the origin of the dispersion.
Abstract: Amorphous semiconductors and insulators display the phenomenon of dispersive transport: the average mobility of the carriers decreases with time after pulsed excitation. A simple model explains how multiple trapping (MT) in a continuous distribution of localized states gives rise to dispersion. This model is then used to show what can be learned about the material from a complete study of its dispersive transport when MT is known to be the origin of the dispersion. Transient photo-induced optical absorption (PA) provides a direct test of the presence of the MT mechanism. Transient photocurrent (PC) then provides the spectrum of the localized states when examined in the time regime before recombination begins. Using this density of states, many of the parameters that characterize transport and recombination, both monomolecular (MR) and bimolecular (BR), can be determined. The assumption that thermal excitation from localized states to higher-energy transport states limits both thermalization and r...
180 citations
TL;DR: In this article, it was shown that neutral defects D0, generated by the reverse reaction of 2D0 → D+ + D−, can be important for a.c. conduction.
Abstract: Although it is now fairly well established that defects in amorphous chalcogenide semiconductors are normally charged (D+ and D−), it is demonstrated that neutral defects D0, generated by the reverse reaction of 2D0 → D+ + D−, can be important for a.c. conduction. The contribution to [sgrave](ω, T) from correlated barrier hopping of single polarons exceeds that of bipolarons at high temperatures (T). An expression for [sgrave](ω, T), which includes terms due to electrons hopping between D0 and D+ and holes between D0 and D−, as well as two electrons hopping between D− and D+ (bipolarons), is able to account for all the features observed in chalcogenide glass semiconductors. Effects on the a.c. conductivity of the addition of impurities to chalcogenides are also discussed in terms of the present model. The energy levels and densities of charged defects are estimated for given materials, and these are supported by values deduced from photoluminescence, drift mobility and d.c. conductivity studies.
178 citations
TL;DR: Amorphous carbon films have been prepared from gas mixtures containing small predetermined amounts of phosphine, diborane or nitrogen as discussed by the authors, and it is concluded that conduction is predominantly by hopping in a region of fairly...
Abstract: Amorphous carbon films have been prepared in a r.f. glow discharge from various hydrocarbon gases. Electrical conductivity and optical absorption measurements show a strong dependence on the preparation gas and on the substrate temperature T d. Particularly for specimens prepared from ethylene, varying T d from 230 to 300°C increased the conductivity by ten orders of magnitude and decreased the optical gap from 2·0 to 0·8 eV. The hydrogen content of the specimens, measured by the thermal evolution technique, lies between 30% and 45% and shows no systematic dependence on T d or the preparation gas. Results are also presented for films which have been prepared from gas mixtures containing small predetermined amounts of phosphine, diborane or nitrogen. The addition of each dopant gas increases the specimen conductivity by between one and three orders of magnitude, depending on T d. It is concluded that for both undoped and doped specimens, conduction is predominantly by hopping in a region of fairly...
151 citations
TL;DR: In this paper, the authors investigated the SCLC flow as a function of applied potential, temperature and specimen thickness in amorphous silicon films prepared by the glow-discharge technology.
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 techn...
146 citations
TL;DR: The influence of adsorbates such as water and ammonia on the electrical conductance of hydrogenated amorphous silicon, a-Si: H, has been studied as a function of doping, temperature, thickness, and sur... as discussed by the authors.
Abstract: The influence of adsorbates such as water and ammonia on the electrical conductance of hydrogenated amorphous silicon, a-Si: H, has been studied as a function of doping, temperature, thickness, sur...
122 citations
TL;DR: In this paper, the optical bandgap of glow-discharge and sputter-deposited a-Si was deduced from measurements of the absorption coefficient α using a linear extrapolation of (αhvn)1/3 as a function of the photon energy h v. The exponent 1/3 is used instead of 1/2, resulting in a much better fit to the data.
Abstract: The optical bandgap of glow-discharge and sputter-deposited a-Si has been deduced from measurements of the absorption coefficient α using a linear extrapolation of (αhvn)1/3 as a function of the photon energy h v. The exponent 1/3 is used instead of 1/2, resulting in a much better fit to the data. The influence of the method of extrapolation on the resulting value for the optical bandgap has been assessed.
121 citations
TL;DR: In this article, a quasi-equilibrium transport model for disordered materials is proposed, in which the localized states are assumed to be distributed over some energy region in the gap, and the drift term is characterized by the equilibrium trap-controlled mobility.
Abstract: A concept of mobile and localized states is used for a description of carrier transport in disordered materials. The localized states are assumed to be distributed over some energy region in the gap. Two regimes of transport are possible: quasi-equilibrium and non-equilibrium. In the present work we concentrate on the quasi-equilibrium regime. Quasi-equilibrium transport takes place under conditions of thermal equilibrium between mobile and localized carrier fractions. The quasi-equilibrium transport equation obtained has the form of a conventional Fokker-Planck equation with drift and diffusion terms. The drift term is characterized by the equilibrium trap-controlled mobility. The diffusion term is determined by a coefficient which in its turn consists of two parts. The first describes conventional trap-controlled diffusion, while the second represents the so-called ‘field diffusion coefficient’ proportional to the second power of the electric field. The field diffusion is shown to have an anoma...
102 citations
TL;DR: In this article, a new approach to the structure of amorphous semiconductors is presented, where regular structures (polytopes) defined in three-dimensional curved space are used to describe an ideal order.
Abstract: This paper presents a new approach to the structure of amorphous semiconductors. Regular structures (‘polytopes') defined in three-dimensional curved space are used to describe an ideal order. We assume that disorder in real material is comparable to the disorder induced by the mapping of the polytope onto Euclidian space. This mapping has two effects: elastic distortions and internal cut surfaces. These surfaces can be relaxed by disclination lines. A systematic generation of amorphous Si: H structural models can be achieved by the saturation, with hydrogen atoms, of the dangling bonds arising near the defects.
98 citations
TL;DR: In this paper, it is suggested that the current transport of well-crystallized film is dominated by conduction in the crystallites, and a nucleation mechanism of microcrystallization is discussed in conjunction with in-situ optical emission spectroscopy of the silane plasma.
Abstract: Microcrystalline silicon composed of crystalline and amorphous phases has been prepared by the glow discharge of a SiH4 + H2 gas mixture. The volume fraction of the crystallites and dark conductivity are simultaneously increased either by increasing the r.f. power or by decreasing the silane concentration. As the proportional content of the microcrystallites increases, only the number of crystallites is increased without any appreciable accompanying change in the grain size. On the basis of the structural model of microcrystalline silicon, it is suggested that the current transport of well-crystallized film is dominated by conduction in the crystallites. A nucleation mechanism of microcrystallization is discussed in conjunction with in-situ optical emission spectroscopy of the silane plasma.
81 citations
TL;DR: In this article, the pinning force exerted by an isolated grain boundary upon the flux line lattice (FLL) of a type-II superconductor has been computed, based upon the assumption that electron scattering at the boundary is negligible.
Abstract: The pinning force exerted by an isolated grain boundary upon the flux line lattice (FLL) of a type-II superconductor has been computed, based upon the assumption that electron scattering at the bou
TL;DR: In this article, the longitudinal and transverse resistivity of n-type indium antimonide in magnetic fields up to 40 kG and for temperatures down to 30 mK are described.
Abstract: Measurements of the longitudinal and transverse resistivity of n-type indium antimonide in magnetic fields up to 40 kG and for temperatures down to 30 mK are described. The resistivity is given by the relation p1 exp (E 1/kT) + p 3 exp (E 3/kT) + p 0 exp (T 0/T) x where for T > 1 K the first term, arising from the freeze-out of the electrons, is important; for 1 K T < 0·3 K the second term, associated with activated hopping, dominates; and for T < 0·3 variable-range hopping, given by the third term, is important. Expressions for x and T 0 in the variable-range-hopping region have been derived assuming the density of states at the Fermi energy (a) is a constant and (b) varies with energy as C ϵ 2. Donor wavefunctions given either by Yafet, Keyes and Adams (1956) or by Hasegawa and Howard (1961) are considered in both cases. The best fit to the results is obtained with a constant density of states and Yafet, Keyes and Adams wavefunctions. The measured values of T 0 are, however, much lower than the...
TL;DR: In this article, the infrared spectra of synthetic diamonds were analyzed and it was shown that there is an excellent correlation between the strengths of the nitrogen-induced absorption at 1130 cm−1 and of the local mode absorption peak at 1344 cm −1.
Abstract: Careful measurements of infrared spectra of synthetic diamonds bring to light two apparently contradictory observations. On the one hand, it is shown that there is an excellent correlation between the strengths of the nitrogen-induced absorption at 1130 cm−1 and of the local-mode absorption peak at 1344 cm−1. On the other hand, our results confirm another earlier report that, whilst the 1130 cm−1 band is shifted towards lower wavenumbers by the incorporation of 15N into the crystals, the 1344 cm−1 peak is unaffected. This suggests that nitrogen atoms do not participate in the 1344 cm−1 vibration. It is difficult to understand the second observation in the light of the first. A small peak at ∼ 2688 cm−1 in the absorption of one of the specimens studied is thought to be the first overtone of the 1344 cm−1 vibration.
TL;DR: In this paper, the initial stages of oxidation of clean, high-purity, iron foil have been studied using a Kratos ES 300 electron spectrometer with monochromatic X-radiation.
Abstract: The initial stages of oxidation of clean, high-purity, iron foil have been studied using a Kratos ES 300 electron spectrometer with monochromatic X-radiation. Through careful observation of the Fe ...
TL;DR: In this paper, the authors compared the low-temperature 1·4 eV luminescence of a-Si: H with the distant-pair model, showing that the decay curve will be independent of the excitation pulse intensity when the pulse creates fewer than ∼ 1018 cm−3 carriers.
Abstract: The kinetics of the recombination of separately trapped electrons and holes are treated theoretically, and the results are compared with the low-temperature 1·4 eV luminescence of a-Si: H. The carriers are assumed to be distributed at random spatially, and the recombination transition rate is assumed to depend exponentially on the electron-hole separation. We obtain an exact solution, in the form of a non-linear integral equation, for the case of luminescence decay following a single excitation pulse; and we give approximate solutions for repetitively pulsed excitation (as in time-resolved spectroscopy) and for continuous excitation. Good agreement is found between the experimentally observed behaviour of a-Si: H and the predictions of the distant-pair model, without any adjustable parameters. In particular, the model predicts that the decay curve will be independent of the excitation pulse intensity when the pulse creates fewer than ∼ 1018 cm−3 carriers. We show, therefore, that frequency-respon...
TL;DR: In this article, the influence of Tl, I, Te, Sb and Ge impurities in terms of local bonding was discussed in pure and doped liquid selenium.
Abstract: Viscosity measurements have been performed on pure and doped liquid selenium. The influence of Tl, I, Te, Sb and Ge impurities is discussed in terms of local bonding. Two main factors dominate the ...
TL;DR: In this paper, a model of electronic filling and emptying of dangling bonds and tail states by the carriers supplied by the doping atoms was proposed to analyze the effect of hydrogen incorporation on conductivity and E.P.R.
Abstract: Conductivity and E.P.R. experiments are reported for boron- and phosphorus-doped chemical-vapour-deposited amorphous silicon film. In as-deposited films a large increase in the conductivity above a threshold dopant concentration is observed in both types of doped sample. Three resonances with g factors at respectively 2.0058, 2.0043 and 2.013 are observed. These results are interpreted by a simple model of electronic filling and emptying of the dangling bonds and tail states by the carriers supplied by the doping atoms. The effect on these properties of ‘adjustable’ hydrogen incorporation is analysed. For phosphorus doping, and for low hydrogenation in boron doping, the passivation of defects depends on the quantity of hydrogen, which explains the experimental results. The most striking result is the decrease in doping efficiency for strong hydrogenation observed in boron-doped films.
TL;DR: In this article, a.c. loss measurements in sputtered hydrogenated amorphous germanium films made over the temperature range 30-150 K are described. But the measurements show similar features to those previously reported for evaporated films, namely a frequency exponent which decreases with temperature and a low-frequency loss peak which has a simple activated temperature dependence.
Abstract: Audio frequency a.c. loss measurements in sputtered hydrogenated amorphous germanium films made over the temperature range 30–150 K are described. The measurements show similar features to those previously reported for evaporated films, namely a frequency exponent which decreases with temperature and a low-frequency loss peak which has a simple activated temperature dependence. The activation energy W’ of the loss peak increases with the hydrogen content of the films, and to a first approximation all features of the a.c. loss scale with temperature and hydrogen content according to the variation of the parameter W'/kT. The data is found to be most readily explicable in terms of carrier hopping between states in the films around which polaron wells are formed. The main effect of hydrogen incorporation is to increase the dielectric constant in the neighbourhood of the wells and thereby increase the polaron activation energy. The d.c. conductivity is found to scale with hydrogen content in a similar...
TL;DR: In this article, a theory of multiple trapping with an arbitrary continuous energy distribution of trap density is used to interpret time-of-flight experiments, and it is shown that a so-called "normal" current shape occurs if the thermal equilibrium between mobile and localized carriers corresponds to a thermal equilibrium function, and a quasi-equilibrium carrier packet moves through a sample.
Abstract: A theory of multiple trapping with an arbitrary continuous energy distribution of trap density is used to interpret time-of-flight experiments. It is shown that a so-called ‘normal’ current shape occurs if an energy distribution of localized carriers corresponds to a thermal equilibrium function, and a quasi-equilibrium carrier packet moves through a sample. If the thermal equilibrium between mobile and localized carrier fractions has not time to be established earlier than that at which most of the carriers leave the sample, the current is caused by the drift of a non-equilibrium very-dispersive carrier packet. Propagation of such a packet results in so-called ‘anomalous’ behaviour of transient current characteristics. As a result of theoretical analysis of non-equilibrium and quasi-equilibrium transient current curves, a method for interpretation of experimental transient current data is suggested. This method may provide important information on the energy distribution of localized states.
TL;DR: In this paper, an amorphous thin plate of vanadium pentoxide, V2O5, prepared by ultrafast quenching from the melt (splat-cooling) has been examined by large-angle X-ray scattering (LAXS) and its structure thereby investigated.
Abstract: An amorphous thin plate of vanadium pentoxide, V2O5, prepared by ultrafast quenching from the melt (splat-cooling) has been examined by large-angle X-ray scattering (LAXS) and its structure thereby investigated. The intensities at 450 points within the θ range 2° < θ < 70° were measured at room temperature (22°C). The oxygen coordination around the vanadium atoms is tetrahedral, with an average V—O distance of 1·75 A. The proposed structure is based on sub-units of three VO4 tetrahedra sharing corners, the relative orientation of which has been determined. The V2O5 formula is retained by following the rule that each tetrahedron shares three corners with other VO4 tetrahedra. Experimental and theoretical intensity and radial distribution curves have been compared to support the model. A proposal is made concerning the transition of the crystal structure of V2O5 to the molten and then amorphous states phases, which are supposed to be strongly structurally correlated.
TL;DR: The photofield effect has been shown to be a promising new tool for studying both electron and hole photoconductivity in the same sample as mentioned in this paper, and the first direct evidence has been obtained for an increase in N F(E) as a-Si: H is annealed above T A ∼ 300°C or after prolonged exposure to light.
Abstract: The field effect has been examined as a technique for determining the density of gap states in a-Si: H. It gives the field-effect density of states N F(E) to an accuracy of a factor of two and is therefore most useful for studying changes in a sample as it undergoes a series of treatments. Using the field effect the first direct evidence has been obtained for an increase in N F(E) as a-Si: H is annealed above T A ∼ 300°C or after prolonged exposure to light. The photofield effect is shown to be a promising new tool for studying both electron and hole photoconductivity in the same sample.
TL;DR: In this paper, the electrical resistivity and the Hall coefficient have been measured from small-angle X-ray scattering measurements, and the origin of the resistivity maximum is discussed, based on the theory proposed by Hillel, Edwards and Wilkes.
Abstract: In order to examine the scattering of conduction electrons from Guinier-Preston (GP) zones, the electrical resistivity (ρ) and the Hall coefficient have been measured. Quantitative information on the microstructure of aged alloys was obtained from small-angle X-ray scattering measurements. The resistivity behaves anomalously during isothermal ageing; it increases to a maximum, then decreases and reaches a value corresponding to the steady state of the growth of GP zones. A further decrease occurs when the θ˝ phase begins to precipitate. During these changes, the zone size increases, and reaches a constant value which is slightly larger at higher ageing temperatures. The change in volume fraction (V f) of precipitated zones is quite similar to the change in zone size. It was found that the function (ρ - ρAsQ)/V f depends on the zone size. The origin of the resistivity maximum is discussed, based on the theory proposed by Hillel, Edwards and Wilkes.
TL;DR: In this paper, the dual-beam photoconductivity modulation was used to elucidate the recombination mechanisms which affect d.D.c. photoconductivities and dualbeam photometric measurements on intrinsic a-Si:H are reported.
Abstract: D.c. photoconductivity and dual-beam photoconductivity modulation measurements on intrinsic a-Si: H are reported. The dual-beam technique is used to elucidate the recombination mechanisms which affect d.c. photoconductivity. In the dual-beam photoconductivity technique two light beams are used, a steady pump beam with hv > 1·5 eV and a chopped monochromatic beam with 0·6 eV hv < 2·0 eV. The pump beam creates free electrons and holes and thus changes the occupation of the semiconductor gap states through which recombination occurs. The second, monochromatic, beam is used to probe recombination processes by altering the occupation of selected gap states, thereby modulating the photoconductivity. The photoconductivity modulation spectrum, which is dependent on temperature and the intensity of the pump beam, is rich in information about gap states that act as recombination centres. Infrared quenching of photoconductivity is observed below T = 200 K. Analysis of this effect in terms of two classes of ...
TL;DR: In this article, it was shown that annealed, nominally pure and nearly stoichiometric NiO single crystals normally have a low resistivity surface layer which masks the behaviour of the bulk in most electrical measurements.
Abstract: From the measurement of three-probe resistivities, which are much larger than two-probe resistivities, we conclude that annealed, nominally pure and nearly stoichiometric NiO single crystals normally have a low-resistivity surface layer which masks the behaviour of the bulk in most electrical measurements. Bulk resistivities larger than 1016 Ω cm have been measured below 100°C. It is possible that the results of nearly all previous Hall, resistivity and thermoelectric-power measurements made on high-resistivity samples are indicative of the surface rather than the bulk properties of NiO.
TL;DR: In this paper, the thickness dependence of photo and dark conductivities, optical absorption and hydrogen vibrational spectra have been investigated for undoped glow-discharge a-Si: H films.
Abstract: The thickness dependence of photo- and dark-conductivities, E.S.R., optical absorption and hydrogen vibrational spectra have been investigated for undoped glow-discharge a-Si: H films. The measurements of the photoconductivity were carried out at wavelengths of 5145 and 6328 and 8500 A. Reductions of photoconductivity and activation energy for dark conductivity, and increases of E.S.R. spin density, the optical energy gap and hydrogen concentration, were found to occur for decreasing film thickness. These results are analysed in terms of a two-layer model and, as a consequence, by the existence of high densities of recombination and E.S.R. centres near the surface(s). The effects of plasma hydrogenation on the photoconductivity and E.S.R., and the photo-induced change of the photoconductivity are discussed in terms of the two-layer model. The photoinduced change is interpreted as a bulk rather than a surface effect.
TL;DR: In this paper, a consistent and quantitatively correct interpretation of the temperature and doping dependence of S can be given on the basis of crystalline transport theory, where it is shown that the effective density of states at ϵs is proportional to Tv, where v varies systematically with doping level.
Abstract: The paper deals with transport measurements and their interpretation in microcrystalline silicon specimens deposited directly in a silane glow discharge, strongly diluted with hydrogen. Also included are the results of field-effect experiments. The thermoelectric power S and the Hall effect have been measured on a series of undoped and n-type specimens between 470 and 170 K. Using the carrier densities derived from the Hall effect, it is shown that a consistent and quantitatively correct interpretation of the temperature and doping dependence of S can be given on the basis of crystalline transport theory. From the analysis it is found that the effective density of states at ϵs is proportional to Tv , where v varies systematically with doping level. With an average value for the heat of transport of A c = 2·5, the Fermi level position is deduced from the thermoelectric power data as a function of temperature and doping level. ϵc - ϵf vanishes at a donor density of about 5 × 1019 cm−1. The results ...
TL;DR: The spectrum of the yellow cathodoluminescence emitted by platelets in diamond is shown to be a featureless broad band superimposed on the shortwavelength tail of the well-known infrared luminescence from platelets as discussed by the authors.
Abstract: The spectrum of the yellow cathodoluminescence emitted by ‘giant’ platelets in diamond is shown to be a featureless broad band superimposed on the shortwavelength tail of the well-known infrared luminescence from platelets. ‘Giant’ platelets are definitely not associated with the H3 system as has been suggested earlier. Also, strongly polarized emission in a vibronic band with a zero-phonon line at 490·7 nm (2·526 eV) has been observed to originate at regions of diamond that are plastically deformed. The 490·7 nm system has also been observed in volume distributions in growth bands.
PARC1
TL;DR: In this article, it was shown that, with increasing E.R.S. spin density, the photoconductivity decreases, but its spin dependence increases, while the r...
Abstract: Studies of spin-dependent photoconductivity in a-Si: H are reported. It is shown that, with increasing E.S.R. spin density, the photoconductivity decreases, but its spin dependence increases. The r...
TL;DR: In this paper, the authors studied the recrystallization behavior of a buried amorphous layer formed by P+ implantation in (111) Si using transmission electron microscopy (TEM), secondary-ion mass spectroscopy (SIMS) and Hall measurements.
Abstract: The recrystallization behaviour of a buried amorphous layer formed by P+ implantation in (111) Si has been studied using transmission electron microscopy (TEM), secondary-ion mass spectroscopy (SIMS) and Hall measurements. Special TEM specimen preparation techniques were used to study individual sub-surface damage regions in the as-implanted as well as the annealed samples. The TEM results showed that the recrystallization of the amorphous layer began slowly, increased rapidly above 550°C and was complete at 600°C. At temperatures betweeen 750 and 1000°C, two discrete layers of interstitial a/2 ⟨110⟩ and a/3⟨111⟩ dislocation loops formed below the specimen surface, and the surface region concontained stacking-fault tetrahedra. The SIMS results showed that pronounced segregation of phosphorus atoms occurred in the defect-rich regions at 750°C. At higher annealing temperatures P segregation still occurred, but the magnitude of the segregation was reduced because of diffusive effects. The segregatio...
TL;DR: In this article, the frequency-dependent differential capacitance of an amorphous silicon Schottky barrier is calculated as a function of d.c. bias using a simple model of the barrier and an assumed form for the density of electronic states of the silicon.
Abstract: The frequency-dependent differential capacitance of an amorphous silicon Schottky barrier is calculated as a function of d.c. bias using a simple model of the Schottky barrier and an assumed form for the density of electronic states, n(E), of the silicon. The results show that at frequencies as low as 1 Hz the capacitance of the Schottky barrier can be considerably different from the static capacitance because of the slow response of the electron occupancy of localized states. A detailed development of the theory is presented to demonstrate the way in which information on n(E) is contained within the capacitance-voltage (CV) results. The deduction of n(E) from the CV results is possible if the static capacitance is known but, when relaxation effects are significant, there appears to be no simple way of obtaining n(E) directly.