C: Solid State Phys., 16 (1983) 6241-6262. Printed in Great ... The application of polycrystalline silicon in the fabrication of electronic devices is well known.
J. Phys. C: Solid State Phys., 16 (1983) 6241-6262. Printed in Great Britain
Properties of microcrystalline silicon: IV. ?Electrical conductivity, electron spin resonance and the effect of gas adsorption S Vepiek, Z Iqbal, R 0 Kuhne, P Capezzuto, F-A Sarott and J K Gimzews ki Institute of Inorganic Chemistry, University of Zurich, 8057 Zurich, Switzerland
Received 11 September 1981, in final form 28 April 1983
Abstract. Measured values of the electrical conductivity, U, and electron spin density (g = 2.0057) of microcrystalline silicon can be essentially determined by the extent of the contamination of the samples by oxygen unless special precautions are taken as regards the sample preparation and/or handling. For samples deposited at a floating potential, two kinds of oxygen incorporation are identified: irreversible formation of Si-0 bonds on the grain boundaries (and o n the sample surface) and a reversible adsorption which is probably T associated with a non-dissociative 0:- (ads) state. The latter results in a decrease of ~ R by up to five orders of magnitude, an increase of the activation energy, E ~ and , of the preexponential factor, @, as well as in an increase of the electron spin density. A reversible desorption of oxygen leads to an increase of ~ R Tup to not less than about R-' cm-' and In order to avoid a decrease of the EPR signal below the detection limit of less than 1016(31-r~~. such effects a negative bias has to be applied to the substrate during deposition. Samples of undoped yc-Si deposited in this way show neither the incorporation of oxygen into the bulk nor significant changes in the dark conductivity even after long-term exposure to air.
1. Introduction
The application of polycrystalline silicon in the fabrication of electronic devices is well known. Such a material is usually prepared by the thermal decomposition of silane at temperatures above about 620 "C and its crystallite size varies from several hundred angstroms to the micron range, depending on the deposition temperature. The deposition temperature can be significantly lowered if an intense glow discharge is used. The resulting material with crystallite sizes ranging between about 20 and less than about 200 A is referred to as 'microcrystalline silicon', p s i . The most convenient method of preparation of pc-Si thin films which provides optimum control of the deposition is chemical transport in a hydrogen plasma at a pressure between 0.1-1 Torr and substrate temperature of between about 65 and 500 "C. Plasma-activated decomposition of silane diluted with hydrogen or 'sputtering' can also yield pc-Si provided the discharge parameters are properly chosen so that the deposition takes place close to chemical equilibrium. The chemistry of the heterogeneous system Si(s) + H2 under plasma conditions and the discharge parameters controlling the deposition of amorphous and microcrystalline silicon (a-Si and pc-Si) have been discussed in
+ Parts I, I1 and 111: Vepfek et a1 (1981c), Iqbal and Veptek (1982), Iqbal era1 (1983), respectively. @ 1983 The Institute of Physics
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several papers (Webb and Vepiek 1979, Vepiek 1980, Vepiek er a1 1981a, Wagner and Vepiek 1982, 1983) and summarised in recent reviews (Vepiek 1982, 1983), where references to the publications of other workers can also be found. The optical absorption, hydrogen content and thermal evolution, purity, Raman spectra, x-ray and electron diffraction, and preliminary data on electrical conductivity of pc-Si films have been reported by Vepiek et a1 (1981~).In more recent papers we have studied in some detail the Raman scattering (Iqbal et a1 1981a, Iqbal and Vepiek 1982), the crystalline-to amorphous transition (Vepiek er a1 1982), the effect of the substrate bias (Sarott er a1 1982) and the optical absorption (Iqbal et a1 1983). The electronic properties of pc-Si have been studied by Spear and his collaborators (Spear er a1 1981, Willeke er a1 1982) who concluded that the large increase of conductivity over a-Si is almost entirely caused by the increased carrier density resulting from delocalised tail states. They also found that the si n of the Hall constant remains normal down to a ‘critical’ crystallite size of about 20 and extrapolates to the opposite sign for a-Si. This result is of considerable interest since this ‘critical’ size compares well with the lower limit of the stability of pcSi for samples deposited under negative bias (Vepiek er a1 1982). In the present paper we shall restrict ourselves to a discussion of the effect of the constituents of air on the measured value of the electrical conductivity and electron spin density of undoped pc-Si. It will be shown that, unless special precautions are taken, the measured data can be significantly influenced by incorporated oxygen. Two kinds of oxygen adsorption in the samples, a reversible and an irreversible one, are identified and studied in some detail. The organisation of the paper is as follows: first, we shall give some additional information on the preparation of the samples and their properties in order to update earlier data. Next, electrical conductivity measurements will be reported vis-a-uis a study of oxygen incorporation using infrared absorption and EPR spectroscopy and supplemented by x-ray photoelectron spectroscopy (XPS).
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2. Preparation of samples and some additional data on their structural properties Thin films of pc-Si were deposited on various substrates by means of chemical transport in a hydrogen discharge as described elsewhere (Vepiek and MareEek 1968, Webb and ) summarised recently (Vepiek 1982). The apparVepiek 1979, Vepiek er a1 1 9 8 1 ~and atus used in the present work is shown schematically in figure 1.An oil-free, base vacuum of about 2 X lo-’ Torr was provided by a 301s-’ ion pump. During deposition the chosen pressure and gas flow were controlled by a rotary pump with a zeolite trap and appropriate valves. The bent shape of the discharge tube and polycrystalline silicon powder (Alfa Products 99.99,50 mesh) covering the anode ensured that no impurities from the electrodes could contaminate the deposit. A DC discharge was maintained between the anode and the metallic valve, K. The temperature of the substrate was controlled to within less than 2 2 “C by means of a temperature controller which was carefully insulated from the ground. Thus, the surface of the substrate was at a floating potential. Whenever desirable, the potential of the substrate could be varied, in a controlled way, between that of the cathode and anode using an auxiliary power supply. Unless specified otherwise all samples to be reported on here were deposited at a floating potential. The effect of the bias on the deposition has been briefly described elsewhere (Vepiek er a1 1981a, b, Vepiek 1982, Sarott et a1 1982). The deposition rate was typically 60 to 300 min-’ depending on the deposition conditions.
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Figure 1. Schematic view of the apparatus used for the deposition of samples: 1, discharge tube q = 5 cm (silica glass); 2, substrate holder; 3, silica glass tube protecting the substrate holder from plasma; 4, extension of the discharge tube (silica glass); 5, silica-to-pyrex-tometal seal; K, all-metal valve (cathode); 6 , metallic flange with a differentially pumped double O-ring seal; A, anode; TC, thermocouple; H, substrate heater; S , substrate; CH, charge zone.
2.1. Crystallite size and preferential orientation As mentioned earlier (Vepiek et a1 1981c, Iqbal et a1 1981b) the crystallite size and the extent of the preferential orientation were essentially independent of the nature of the substrate (silica glass, metallic glasses, polycrystalline Al, MO and stainless steel, single-crystal sapphire and silicon). For samples deposited at a floating potential the crystallite size, D ,depends essentially only on the deposition temperature and it varies between about 30 and 200 8, for Td = 65 "C and Td 450 "C respectively, as shown by both x-ray diffraction and transmission electron microscopic data. The application of a negative bias to the substrate during deposition results in a large decrease of D (for example from 100 A to -30 8, at Td = 260 "C and Vb = 0 and - 200 V, respectively; Sarott e t a f 1982). Under a strong negative bias, v b IS -50 V, a compressive stress is built up in the films that reaches a value of about 40 kbar for v b - 650 V (Iqbal and Vepfek 1983b). The crystallite size is independent of thickness between a few hundred angstroms and more than about 40 pm. The films deposited at v b 0 show a preferential orientation with (111)-planes parallel to the substrate surface at Td IS 100 "C which changes to a (110)-orientation at Td = 450 "C. At Td = 200 "C the crystallites are randomly oriented.
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2.2. Nature of grain boundaries The films of undoped yc-Si to be discussed here are essentially free of any extended amorphous interconnecting tissue (Iqbal and Vepi-ek 1982). The interconnection between the individual crystallites seems to be more appropriately described in terms of low-angle grain boundaries. In the case of extended boundaries of this kind there are atoms belonging to both grains which form a coincidence-site-lattice with an array of
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dislocations in between. The bond lengths are somewhat elongated in the vicinity of the grain boundaries. Theoretical calculations and experimental studies show that such distortions of the periodic crystalline lattice extend over a distance of 3-4 lattice planes, i.e. typically about 10-15 8, around the boundary (Pond et a1 1974,Pond and Vitek 1977, Pond 1979). Figure 2, which is a high-resolution transmission electron photomicrograph of a
Figure 2. ( a ) Transmission electron photomicrograph of p s i showing a direct image of the (111) lattice planes (distance 3.15 A). The dark region in the middle of the figure corresponds to two overlapping crystallites. The remarkable orientation of the film is obvious from the well-resolved (111) planes in relatively large regions over the whole area of the figure of about 600 x lo00 A' and, in particular in the region of the overlapping crystallites. The samples were deposited at T 300 "Con a polycrystalline molybdenum substrate. (b) Part of (a) in greater detail, showing part of the grain boundary.
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100-200 8,thick pc-Si film representative of the material discussed in the present paper, shows that this concept of polycrystalline material applies also to pc-Si prepared by chemical transport in a plasma. Two points have to be emphasised. (a) In order to obtain their direct image all the lattice planes have to be perfectly aligned with respect to the electron beam since a small distortion of the alignment (more than a few tenths of a degree) and/or the periodicity (such as that to be expected at the grain boundaries) already leads to an apparent loss of the image contrast. ( b )The typical width of the contrastless boundaries between two well-resolvedgrains amounts to about 8-13 8, as expected for the low-angle boundaries. Several examples are seen in figure 2 and we have also obtained such data from a series of photomicrographs taken on different samples.
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It has been shown previously (Vepiek et a1 1981c, Iqbal and Vepiek 1982) that the hydrogen incorporated into the films during deposition is located at the grain boundaries occupying sites with a well-defined surrounding. It is natural to expect that this saturates dangling bonds at the dislocations. In addition, there must be some weak Si-Si bonds into which oxygen can be incorporated during long-term exposure of the film to air since no significant interaction between the oxygen and hydrogen is seen in the infrared absorption spectra (see § 3). These data substantiate the view that pc-Si prepared by chemical transport and discussed in the present paper is essentially free of amorphous tissue. This of course does not exclude the possibility that such tissue might be present in material prepared by another method which does not enable sufficient control of the deposition. 3. Infrared absorption study of oxygen incorporation into the films
It will be shown later in the paper that both the measured electrical conductivity and electron spin density of pc-Si can be strongly influenced by irreversibly absorbed and reversibly adsorbed oxygen. Therefore, in this section we shall present data on the optical and infrared absorption studies of these phenomena. Unless specified otherwise all the results to be presented were obtained with samples deposited at a floating potential. For optical absorption measurements in the range from 185 nm to about 3 pm fused silica substrates were used. The infrared absorption studies were performed between 200 and 4000 cm-' with pc-Si films of thickness ranging from about 10 to 40 pm deposited on ( I l l ) & chips which were polished on both sides (the intrinsic IR absorption of the substrate was balanced with another chip in the reference beam of the spectrometer).
3.1. Irreversible oxygen incorporation As a typical example, three relevant regions of the infrared spectra of a 23.7 pm thick pc-Si film are shown in figure3. First, one notices the absorption bands due to the stretching, bending and wagging modes of SiH, groups which were already reported in our earlier paper (Vepiek et a1 1981~).In addition, a weak absorption between about 1020 and 1050 cm-' is observed after a few minutes of exposure of the samples to air which, upon prolonged exposure, increases in intensity, broadens and its centre of gravity shifts towards about 1060 cm-'. After an exposure time of about 7 days a second component centred at about 1120 cm-' appears and it grows somewhat faster than that at about 1060 cm-' (see figure 3, showing spectra taken at various times). According to the literature, the feature at about 1060 cm-' is assigned to a Si-0 valence vibration, and that at about 1120 cm-' to a stretching-bending mode of bridging oxygen between two Si atoms, Si-0-Si (Simon and McMahon 1953 and references therein). Similar absorption bands due to Si-0 vibrations were also observed by Freeman and Paul (1978) and Yacobi et a1 (1981) in RF sputtered films of a-SiH which was exposed to air for about five months. Figure 4 ( a ) , curve A , shows the time dependence of the absorption coefficient corresponding to the band maximum and the integrated intensity of the absorption band associated with chemically bonded oxygen. The relative amount of oxygen, as evaluated from the absorption using calibration from thermally grown S O z films of known thickness, is shown in figure 4(b).
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The oxygen content given in figure 4(b) should be considered as an upper limit because of a possible, but as yet unknown, difference in the oscillator strengths of the Si-0 and Si-0-Si bonds in thermally grown Si02 and at the grain boundaries in our films. Using a calibration reported for a-Si :H (Yacobi et a1 1981) and for crystalline Si (Sari eta1 1978) the oxygen content at saturation (-5.8 at.% in figure 4(b)) should be about 1.1and 0.26 at. %, respectively. Thus, additional data on the oscillator strengths of the Si-0 and Si-0-Si bonds in pc-Si are necessary in order to evaluate the exact absolute value of the oxygen content. SI-H,
Si-H, stretch
