The mechanism of photoconductivity in polycrystalline CdS has been studied over the temperature range 100–300 K using Hall-effect and conductivity measurements in the dark and under white light illumination. Samples were prepared in thin film form by spray pyrolysis and as power-binder mixtures. Dark conductivities covered the range 10−9–101 Ω−1 cm−1. Dark conductivity is interpreted in terms of a two-dimensional version of the grain-boundary barrier model developed by Seto for polycrystalline Si. Except at very low carrier densities, Hall mobilities are found to be thermally activated, and intergrain barrier heights φb are derived for spray pyrolysis layers with doping levels covering the range N = 1014–1018 cm−3. A maximum barrier φbmax ≊0.2 Ev is found at a corresponding doping level, Nmax ≊2×1016 cm−3, which represents the situation where the barrier depletion layers just extend through the whole grain. From this we derive a mean grain diameter of 0.3 μm in good agreement with the result of transmission electron microscopy. For samples having N<Nmax the average free carrier density n̄≪N and photoconductivity occurs entirely through an increase in n̄ up to the point where the depletion regions begin to contract away from the center of the grains. For samples having N≳Nmax both μ and n̄ increase. The detailed variations of μ and n̄ are interpreted in terms of the Seto model with the added hypothesis that photogenerated holes are all trapped at grain boundaries. Hall-effect measurements are interpreted on the assumption that the Hall coefficient R measures the average carrier density in the grain, i.e., R = (n̄e)−1, and we note that n̄ may differ significantly from the doping level N, even when N⩾Nmax. Photo-Hall results provide evidence in support of this hypothesis.
The Petritz theory of Hall mobility in polycrystalline semiconductor films predicts that μH=μ0 exp(−φb/kT) where φb is the intergrain barrier height and μ0 depends only weakly on temperature. Careful examination of experimental results in the literature reveals that, in practice, μ0 varies strongly with φb, an anomaly which can be understood if φb is temperature dependent. We present the results of Hall and resistivity measurements on CdS films which support this interpretation and yield a value for the temperature coefficient α=(1/φb)∂φb/∂T=−0.0022 K−1. This compares with a value of α≃−0.003 K−1 reported by Seager and Pike for silicon bicrystals.
Refractive index measurements on fluid-xenon isochores in the density range 1-12 millimoles cm" 3 indicate that the temperature and density dependence of the LorentzLorenz function (LL) is small. The mean value of LL for the coexistence curve is 10.527±0.07 cm 3 mole"" 1 at 5893 A.Dielectric-constant (e) and refractive-index (n) data have recently been used to establishp-V-T relationships for fluids, for example the slope of critical isotherms, 1 and the shape of the coexistence curve near the critical point. 2 Such procedures are valid only if the Lorentz-Lorenz function (w 2 -l)/(« 2 +2)p or the dielectric-constant analog (€-l)/(e+2)p remains constant throughout the region under study. In particular it is assumed that the Lorentz-Lorenz function (LL), and hence the molar polarizability a = 3LL/47r, are independent of both density and temperature.Previous investigations 3 suggest that although the molar polar iz ability of a spherical nonpolar dielectric is nearly constant, small but possibly significant deviations do occur. At low densities LL increases slowly with density, passes through a maximum at about 100-200 amagats, and then decreases more rapidly. This behavior has been examined theoretically in terms of the effect of statistical fluctuations on the induced dipole moment, 4 combined with a change of polarizability due to "caging" of the molecules at higher densities. 5 In addition there appears to be some experimental evidence 6 for an increase in the critical region, greater than might be expected from theoretical considerations. 7 We have measured the refractive indices of fluid-xenon isochores in the range 1-12 millimoles cm"" 3 by the spectrometric method of minimum deviation. The refractive index is given by n = sin[i(A +D)]/sin(iA), where A is the prism angle and D is the angle of minimum deviation. Samples were isolated at temperatures (»T C ) corresponding to well-established isotherms. 8 These were then examined over a wide range of temperature. Such a procedure has allowed us to match the density and optical data with greater certainty than was possible in earlier measurements, 9 and also to determine the specific temperature dependence of the molar polarizability. The cryostat 10 and experimental techniques 9 have been described elsewhere.It was found that LL decreased slightly with increasing temperature at constant density. The variation for a near-critical isochore density of 8.46 millimoles cm™" 3 is shown in Fig. 1 (p c = 8.42 millimoles cm"" 3 ). A change of 0.1 % in LL is observed as the temperature is varied from 20 to 78°C. Since A and p (corrected for thermal expansion of the cell) remain constant, the relative accuracy of n is ±0.01 % and thus LL is determined to ±0.08 %. Values of LL taken from the sets of isochoric measurements at temperatures which correspond to liquid-vapor equilibrium (the coexistence curve) are shown in Fig. 2. The mean value of these points is 10.527 cm 3 mole"* 1 (standard deviation 0.2 cm 3 mole"" 1 ). We estimate the maximum error in n to be ±0.02 %
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