P. G. Le Comber

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 ...

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.

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.

Analogue memory and ballistic electron effects in metal-amorphous silicon structures

Abstract We present experimental results showing that p+ amorphous silicon memory structures exhibit polarity-dependent analogue memory switching. The effect is non-volatile and we propose that it is associated with changes in a tunnelling barrier within the structure. It is also observed that conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can, under certain conditions, travel ballistically. As a conseauence, auantized resistance levels associated with ballistic electron transport are observed under certain circumstances. In the presence of a magnetic field, additional steps in the auantized resistance levels occur. A particular feature of this auantized resistance is that the effect can be observed at relatively high temperatures (up to about 190 K).

Present and future applications of amorphous silicon and its alloys

Abstract Thin films of amorphous silicon (a-Si:H) and its alloys prepared by the glow discharge decomposition of silane and appropriate gases, are presently incorporated into six commercial products and approximately twenty other applications have been proposed for these materials. This presentation will review the present applications and a number of those proposed for the future. Finally, some of the material and technological issues of current importance will be discussed.

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

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...

Amorphous silicon p‐n junction

The preparation of an all‐amorphous thin‐film p‐n junction is described. The a‐Si films were produced by the glow discharge decomposition of silane, mixed with trace amounts of donor or acceptor impurities. The current‐voltage characteristics of the junction were investigated in the range from 150 to 300 K.

Hall effect and impurity conduction in substitutionally doped amorphous silicon

Abstract Hall mobility and conductivity measurements have been made as a function of temperature on a series of n- and p-type a—Si specimens prepared by the glow discharge technique and doped with phosphorus or boron atoms. The Hall effect exhibits an interesting double reversal in sign : n-type samples show a Hall potential in the direction normally expected for holes and the opposite applies to p-type specimens. In lightly doped n-type samples, the observed Hall mobility μmH has a magnitude of about 0.1 cm2V−1see−1, essentially independent of temperature; this is in agreement with theoretical predictions for transport in the extended states. On increasing the donor density, μmH begins to show a temperature dependence which varies systematically with the doping level. The conductivity and Hall data for n-type specimens have been analysed in terms of a model with two parallel conduction paths, one in the extended electron states and the other through the localized impurity levels where transport takes pla...