W. E. Spear

Substitutional doping of amorphous silicon

It is shown that the electrical conductivity of a tetrahedral amorphous semiconductor can be controlled over many orders of magnitude by doping with substitutional impurities. Experiments were carried out on a-Si specimens prepared by the glow discharge technique, with phosphorus and boron impurity levels between 5 × 10−6 and 10−2 ppv.

Electronic properties of substitutionally doped amorphous Si and Ge

Abstract It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...

Electrical and optical properties of amorphous silicon carbide, silicon nitride and germanium carbide prepared by the glow discharge technique

Abstract Amorphous specimens of silicon carbide, silicon nitride and germanium carbide have been prepared by decomposition of suitable gaseous mixtures in a r.f. glow discharge. Substrates were held at a temperature T d between 400 and 800 K during deposition. In all three of the above materials the results of optical absorption and of d.c. conductivity measurements show a systematic variation with T d and with the volume ratio of the gases used. Electron microprobe results on silicon carbide specimens indicate that a wide range of film compositions can be prepared. The optical gap has a pronounced maximum at the composition Si00–32C0–68 where it is 2·8 eV for a sample deposited at T d = 500 K, but shifts to lower energies with increasing T d. The conductivity above about 400 K has a single activation energy approximately equal to half the optical gap and extended state conduction predominates if the silicon content exceeds 32%. If the latter is reduced, hopping transport takes over and it is suggested th...

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

Abstract The electronic properties of amorphous solids are largely determined by the distribution of localized states N (ϵ) in the mobility gap. In this paper, the field effect technique is applied to the experimental study of N (ϵ) 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. N (ϵ) 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 N (ϵ), 0.18 eV below ϵ 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 N (ϵ) The effects of annealing on the state distribution have been investigated, showing that N ( ϵ 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.

Drift mobility techniques for the study of electrical transport properties in insulating solids

Abstract The paper reviews in some detail the principles and experimental techniques involved in drift mobility measurements. These are particularly suitable for transport studies in highly resistive, low mobility solids and examples of their application to crystalline and non-crystalline materials are given. A platelet specimen is fitted with electrodes on opposite sides and charge carriers are generated near the top electrode by a fast excitation pulse. Both light and electron pulses have been used and the particular advantages of electron beam excitation are discussed. A steady or pulsed applied field draws one type of carrier across the specimen and the transit time tt is determined either by charge integration or from the observed current transient. This leads directly to the drift mobility. A section of the paper deals with the effects of trapping on the measurements. Shallow centers, possessing a release time constant τ r « t t , lead at lower temperatures to a transport controlled by multiple trapping and release. Measurements in this range give information about such centres. Deep traps introduce disturbing polarisation effects and ultimately limit the applicability of the method. Details of a space charge neutralisation technique are given. A different, more “static” method developed by Davis is briefly described.

Investigation of the localised state distribution in amorphous Si films

Abstract Field effect techniques have been used to determine the distribution function N (ϵ3) of the localised states in amorphous Si films prepared by glow discharge decomposition of silane. It was found possible to sweep the surface potential through about 0.5 eV and to determine N (e) to within 0.18 eV of the extended states. N (e) curves show a pronounced structure which largely depends on the substrate temperature during deposition of the films (400–630 K). Localised state densities increase with decreasing substrate temperatures. The equilibrium Fermi level generally lies close to a peak in the distribution and N (eF) ∼ 17 cm−3 eV−1. The analysis of the field effect experiments is described in some detail and relevant information from drift mobility and conductivity measurements are discussed. In particular it is found that the distribution of occupied states calculated from the N (e) curves agrees with predictions from the transport experiments. This supports our contention that the N (e) curves represent a volume rather than a surface property of the films.

Electronic transport and state distribution in amorphous Si films

The specimens used in this investigation were prepared by the decomposition of silane in an r.f. glow discharge. Substrate temperatures, Td, between 310 K and 670 K were used during deposition. The temperature dependence of both the conductivity and the drift mobility was measured on the same specimens and was studied as a function of Td. An electron beam technique was used in the mobility experiments. In specimens prepared at Td>350 K, current flow above 250 K is associated with electron transport in the extended states near ec. An electron mobility of about 10 cm2 s−1 V−1 is deduced from the experiments. Below 250 K, phonon-assisted hopping of electrons through localized states, about 0.18 eV below ec, becomes the predominant transport mechanism. When specimens are deposited at Td⪝350 K, the Fermi energy satisfies ec−eF⪞0.8 eV and a transition to predominant hopping transport by holes occurs. Based on the transport results and previous field effect measurements, a model for the electronic state distribution is put forward, which also appears to be applicable to the interpretation of transport results in amorphous Si specimens prepared by vacuum evaporation or sputtering.

Application of amorphous silicon field effect transistors in addressable liquid crystal display panels

It is shown that thin-film field effect transistors (FETs) made from amorphous (a-) silicon deposited by the glow-discharge technique have considerable potential as switching elements in addressable liquid crystal display panels. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed in some detail. Two important points are stressed: (i) a-Si device arrays can be produced by well-established photolithographic techniques, and (ii) satisfactory operation at applied voltages below 15VV is possible. Small experimental 7×5 transistor panels have been investigated and it is shown that with the present design up to 250-way multiplexing could be achieved. The reproducibility of FET characteristics is good and in tests so far no change has been observed after more than 109 switching operations.

Photoconductivity and recombination in doped amorphous silicon

Abstract The photoconductivity αph and its dependence on incident light intensity and temperature have been investigated in a series of doped amorphous silicon specimens at a photon energy of 2 eV. Specimens were prepared by the glow discharge decomposition of silane at a substrate temperature between 500 and 550 K. Doping was achieved by the addition of controlled amounts of phosphine or diborane during deposition. The primary aim of the work has been to explore the recombination process and its dependence on the known density of state distribution using substitutional doping to control the dark Fermi levels position over a range of 0·8 eV. It is shown that [sgrave]ph attains its optimum level when at a given temperature and intensity the steady-state electron Fermi level has been moved to an energy between 0·35 and 0·30 eV below the mobility edge, ∊ c . At this stage there also occurs a transition from predominantly monomolecular to bimolecular recombination. Both effects appear to be associated with th...

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, Td, 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 Td 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 ϵc−ϵv is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ϵc. At room temperature the steady photocurrent is carried predominantly by electrons in states above ϵc, whether excitation is from localized or extended states. Specimens prepared at Td > 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 Td < 500 K there is a drastic decrease in τ, which falls to 10−11 s at Td ⋍ 300 K and is even less for evaporated specimens. These results are discussed in some detail.