Optical properties of the mixed amorphous system As2SxSe3-x

Solid State Communications, Vol. 5, PP. 555-558, 1967. Pergamon Press Ltd. Printed in Great Britain

OPTICAL PROPERTIES OF THE MIXED AMORPHOUS SYSTEM As2S1 Se3

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E. J. Felty, G. Lucovsky and M. B. Myers Research Laboratories, Xerox Corporation, Rochester, N. Y., U. S. A. (Received 4 May 1967 by E. Burstein)

We have obtained an effective electronic energy gap by studying optical absorption, photoconductivity and the variation of electrical resistance with temperature for the mixed amorphous systemAs2S, Se3 -x and have found a linear increase in this energy gap in going from As2 Se3 to As2 S~ Infra-red reflectivity measurements indicate two reststrahlen bands (one at about 215 cm’ due to an As-Se vibration and another at 300 cm’~, due to an As-S vibration) whose osciUator strengths vary with the formula weight fraction of the end member compounds. .

Introduction

Composition and homogeneity were checked by electron probe microanalysis using the bulk glasses as standards. Electron microscopy and electron diffraction also showed all samples to be uniform and non-crystalline.

THE WORK reported in this paper was stimulated by the recent interest in the electronic and vibrational energy states of mixed crystals. We have extended the scope of this work and have studied the optical properties, at room temperature, of the mixed amorphous system As2 S~Se3 In particular we report here the behavior of the electronic energy gap as a function of x, as deduced from measurements of the absorption constant, photoconductivity, the eleëtrical resistivity the steady state and the and behavior of the infra-red active optical lattice modes as deduced from reflectivity studies,

Measurements and results

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Optical transmission in the region of the absorption edge was measured using a Cary Model 14 spectrophotometer. Absorption con-4cm’ stants either were determined 1 cm’ using paired filmsfrom or, in the casea of10 polished thick samples, measured values of the reflectivity. For the samples studied, the absorption constant was exponential for values of a ranging from approximately 10 cm~ to at least 10~cm1. The reflectivity spectrum did not indicate any structure indicative of a direct

Sample preparation Starting materials of stoichiometry of As 2 5, Se3 with 0 x <3 were prepared by direct synthesis from high purity (99. 999% or better) elements in evacuated silica ampuls which were rocked for 24 hr at 500°Cto insure homogeniety of the resulting glasses. Thin (0. 02 to 504 films were prepared by flash evaporation of these materials from a 500°C boat at 5 x 10- e torr and condensation on a000i.i) substrate held atcut55° C. the Thick samplesingots (500 and to l polished were from quenched by standard metallurgical techniques, -

energy gap. We therefore have arbitrarily defined an energy gap, for purposes of comparison, as that energy at which the absorption constant is equal to 10~ cm’. The values of this effective energy gap appear in Table 1.

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Gold electrodes were avaporated onto a temperature 500 ~ wafers and range resistivity determined data by wasthe taken glassover transition temperature (onset of sample flow) at high temperatures and by a limiting measurable 555

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THE MIXED AMORPHOUS SYSTEM

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Vol. 5, No. 7

THE MIXED AMORPHOUS SYSTEM

resistance of — 1018 ohm at low temperatures. The absence of electrode effects was demonstrated by four-point probe measurements on bulk As2 Se3 samples. An exponential behavior of resistance as a function of reciprocal temperature was observed, Assuming an intrinsic conduction mechanism, and a mobility which is independent of temperature over our range of measurement, we calculated an effective or conductivity energy gap from the slope of the resistancereciprocal temperature curves. The energy gap deduced from this analysis agrees very well with that determined from our absorption studies and is indicated in Table 1. Steady state photoconductivity was measured on evaporated films with semi-transparent gold electrodes (.... 50%) using a sandwich cell geometry. Spectral measurements were made at room temperature with fields of .-~ 10W/cm with light incident on the gold electrode. Spectral resolution was approximately 0. 015 eV.

557

the gold electrode. The energies corresponding to the maximum photoresponse agree very well with the positions of the energy gap as defined in the absorption edge and resistivity studies and are included in Table 1. Infra-red reflectivity measurements were made over a frequency range from 150 to 550 cm” using a Perkin Elmer Model 301 Spectrophotometer. Measurements were made at approximately normal incidence (—. 9° ) with a spectral resolution of at least one hundred. Figure 2 contains the infra-red reflectivity spectra. For both As2 53 and As2 Se3, we observed only one piece of structure in the reflectivity spectra. For the three mixed samples we observed two distinct reflectivity peaks at frequencies corresponding quite closely to the respective peaks observed in As2 S3 and As2 Se3 . No additional structure, which might have been attributed for example to Se-S, Se-Se or S-S bonds, was evident. Conclusions

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To a first approximation the energy gap as deduced from the absorption, resistivity and photoconductivity measurements increases linearly as the Se concentration decreases. Similar behavior has been observed in mixed crystal systems where the energy gap occurs at the same point in k-space in each of2.theHowever, end member in ancomamorphous is not a good quantum pounds, e.g.material In,Ga,...,k As number so that the absorption constant is a convolution of valence and conduction band density of states functions. A decrease in slope of the exponential absorption edge observed in going from As 2S3 to As2Se3 may be related to either a change in this effective density of states or to phonon assisted transitions. A study of temperatare dependence of the absorption edge should resolve this point.

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2.20 180 PHOTON ENERGY

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FIG, 1 Plot of normalized photoconductivity as a function of Photon energy, Figure 1 contains a plot of the normalized spectral response corrected for the transmission of

Reflectivity spectra for the As2 5, Se3 1.

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sys~emLor 3x Table u. i.,, ‘.)U~ ~. ~ an 2 contains the oscillator parameters were in synthesized using awith harmonic used this model along a comparison of formalism. several of the features of the experimental data and the synthesized spectra. The model assumes that the oscillator strength for a given vibrational model i. e. the As-S mode at .-~ 300 cm” or the As-Se mode at 215 cm’, was proportional to the formula fraction of that constituent. The general agreement between the observed and synthesized spectrum supports this assumption. From Table 2 we observe that each of the fitted

• 558

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Infra-red reflectivity spectra for the mixed amorphous system As2S, Se3

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frequencies in the mixed glasses increases slightly from that of the end member compounds. For the experimental data, the minimum at the high frequency side of the reflectivity peak is considerably broader than that obtained in the harmonic oscillator synthetic spectrum. This is tentatively attributed to structural disorder inherent In the glassy phase. We observe that the

behavior of the reststrahlen bands in the amorphous As2 S,, Se~ system is quite similar to that of the mixed crystalline systems, e. g. GaAs, P, -z and CdSX Se, For these semiconductor systems the reststrahlen bands of the end member compounds do not overlap in frequency space as is also the case for As25, Se3 .,

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References 1.

FELTY E.J., LUCOVSKY G., and MYERS M.B., Bull. Am. Phys. Soc. 12, (1964).

2.

WOOLLEY J.C., GILLETT C.M. and EVANS J.A., Proc. Phys. Soc. 78, 354 (1961).

3.

SPITZER W.G. and KLEINMAN D.A., Phys. Rev. 121, 1324 (1961).

4.

VERLEUR H.W. and BARKER A.S. Jr., Phys. Rev. 149, 715 (1966).

5.

BALKANSKI M., BESERMAN R. and BESSON J, M., Solid State Comm. 4, 201 (1966),

Die wirksame Energielucke fUr das amorphe System As2 5,, Se8 wurde mittels Messungen der optlschen Absorption, Photoleitf~hlgkeit, und des elektrischen Wiederstandes als Funktion von Temperatur, bestimmt. Die Energieltlcke w~chstlinear von As2 Se3 mit wachsendem As2 53 Gehalt. Infrarot Reflexionsmessungen zeigen zwei Reststrahlen Bander, das erste bei zlrka 215 cm 1 wlrd durch eine As-Se Schwingung hervorgerufen, das zwelte bei 300 cm’ durch eine As-S Schwingung. Die Oszillatorstärke ~ndert sich mit der Zusammensetzung. -,