Thermal analysis of bulk amorphous arsenic triselenide

Journal of Non-Crystalline Solids 17 (1975) 2-8 © North-Holland Publishing Company

THERMAL ANALYSIS OF BULK AMORPHOUS

ARSENIC TRISELENIDE

D .D. THORNBURG and R.I. JOHNSON Xerox Palo Alto Research Center, Palo Alto, CA. 94304, USA

Received 29 August 1974 Through the use of differential scanning calorimetry, the heat capacity and crystallization parameters of amorphous As2 Se3 have been studied. It is found that the heat capacity above the glass transition temperature (T~) exceeds, by a factor of 1.6, the Dulong-Petit limiting value found below Tg. The ~rystallization process is found to obey first-order kinetics with an activation enthalpy of 1.24 eV and an enthalpy of crystallization of 0.36 eV. molecule-1 . The effect of sample age on the rate constant is also reported.

1. Introduction

Of the large number of chalcogenide glasses studied to date, As2Se 3 has received a great amount o f consideration. Interest in this material has been stimulated because it is a good glass-former, it typifies a saturated-bond covalent network [ I ], it displays interesting photo-darkening [2, 3] and switching [ 4 - 8 ] effects and, as a wide band gap intrinsic semiconductor, it has potential for application as a panchromatic xerographic photoreceptor [9]. Since As2Se 3 is a single phase compound whose glass structure is believed to be not far rer~oved from that of the crystal [10], experiments on this material are far more amenable to theoretical interpretation than experiments on more complicated glass-forming systems. Previous studies o f the thermal properties of amorphous (a-) As 2 Se 3 have included measurements o f the glass transition phenomenon [ 11, 12 ], heat capacity [11 ], thermal conductivity [13], viscosity [14, 15] and an estimate o f the heat o f condensation from the vapor [16]. The present work was undertaken to determine the crystallization kinetics o f bulk quenched a-As 2 Se3, since these data provide insight to the energetics o f the glass bonding network as well as being of considerable practical interest. The method employed uses differential scanning calorimetry (DSC) and is readily applicable to other glasses as well.

D.D. Thornburg, R.L Johnson/Bulk amorphous arsenic triselenide

3

2. Experimental methods 2.1. Sample preparation Bulk amorphous arsenic triselenide was prepared by homogenization of a 10 g batch of commercially available starting material (Alfa/Ventron) in a thin-walled quartz ampoule sealed at 10 -6 torr. Homogenization was carried out under continuous agitation at 600°C for 24 h at which time the ampoule was quenched in an ice-water bath. The glass sample was removed and crushed to coarse powder in a Wiley mill. Two runs of a-As2Se 3 were prepared from the same lot of starting material. Thermal analysis was started on one sample within a day of preparation, and on another sample which had been aged for 800 days at room temperature. 2.2. Thermal analysis All thermal analysis procedures were carried out on a DuPont 990 thermal analyser with the DSC attachment. During all runs the sample chamber was purged with dry N 2 (60 ml" min- 1 at 760 torr). The samples consisted of roughly 20 mg of a-As2Se 3 powder in crimped (but not hermetically sealed) A1 sample pans. All measurements were referenced to an empty A1 pan. The heat capacity of a-As2Se 3 was measured in the vicinity of the glass transition temperature (Tg) at a heating rate of 10°C "rain -1 . The cell was calibrated with an A1203 standard. Enthalpies of crystallization and melting were measured at a heating rate of 2°C •min -1 after calibrating the cell against the known enthalpies of melting for In, Sn and Zn.

3. Results and discussion 3.1. General thermogram results A typical thermogram of a fresh a-As2Se 3 sample heated at 2°C • min -1 is shown in fig. 1. This graph shows the three phenomena of interest: the glass transition (at temperature Tg), the crystallization exotherm (with maximum crystallization rate occurring at temperature Tc) and the melting endotherm (with melting point Tin). While Tm showed no dependence on heating rate, and that of Tg was slight, the crystallization exotherm location changed markedly as the heating rate or sample age was varied. As will be shown later, the heating rate dependence of T c allowed the kinetics of the crystallization of a-As2Se 3 to be determined. Our determination of Tg (173°C) agrees well with that of Schnaus et al. [ 11 ] (175°C) and with that of Dembovsky [12] (177°C). Our results are in variance with those of E1-Fouly and Edmond [17], in that we observed crystallization phenomena for samples quenched from 600°C whereas they only see a crystallization

4

D.D. Thornburg, R.L Johnson/Bulk amorphous arsenic triselenide i

i

a-As2Se3 2 C min -1

T

Tg

EXO

-f

ENDO

1 mcal sec-1 ±

J

100

'

2 0 300 400 T (C) Fig. 1. Differential scanning calorimeter trace of bulk a-As2Se3 at 2°C • min-1 showing glass transition, crystallization and melting phenomena as described in the text. exotherm for samples quenched from 800°C. This variance probably results from other differences in sample preparation and will be discussed later.

3.2. Heat capacity results The heat capacity at constant pressure (Cp) o f a fresh sample o f a-As 2 Se 3 is shown in fig. 2 for a temperature range spanning Tg. The data represent the average o f three runs, with a typical error bar shown on the first point. Below Tg the data agree well with the classical limit o f Dulong and Petit for a five atom molecule (125 J" mo1-1 • K - I ) . As the sample is heated past Tg the heat capacity increases b y a factor o f 1.6. The peak in Cp is an artifact resulting from the structural relaxation times being on the same order as the time scale o f the experiment. Below Tg the fictive temperature lags behind the measured temperature. When the structural re260

,

,

I 100

i

,

,

r

~

~ 180

i

,

,

220

Cp J rno1-1 K-1180 140 3R100

I 140

i

k 220

L 260

T (C)

Fig. 2. Heat capacity at constant pressure (Cp) for a-As2 Se 3 as a function of temperature. Below T_ the data agree well with the classical limit of Dulong-Petit, while the pronounced increase above Tg results from the addition of translational and/or rotational modes.

D.D. Thornburg, R.L Johnson/Bulk amorphous arsenic triselenide

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laxation time becomes short enough for the fictive temperature to catch up with the r0easured temperature, the glass absorbs the extra heat lost at lower temperatures. This shows up as an apparent maximum in the heat capacity [11 ]. Since, below Tg, the value of Cp indicates that the glass network is fully bonded, the increased value above Tg must result from the addition of translational and/or rotational modes made available by the breakage of weak (highly strained) bonds. Our Cp values are in good agreement with those of Schnaus et al. [11 ] below Tg, but are higher than theirs above Tg by about 10%. 3. 3. Kinetics o f the crystallization process The kinetics of crystallization of a-As2Se 3 was determined from analysis of DSC data taken as a function of heating rate. The method used was derived by Kissinger [18] for first-order rate processes. Prior studies on a-CdGeAs2 [19] and a-As2SeTe 2 [20] have shown the utility of this technique for studying thermally activated crystallization phenomena. Suppose the rate of crystallization is governed by the equation dr= c(1 - f ) dt

(1)

at constant temperature, in which f is the fraction of crystallized material and c is the rate constant given by c = co e x p ( - h * / k T ) ,

(2)

in which c o is a constant and h* is the activation enthalpy. Suppose the sample is being heated at a constant rate. When the reaction rate is a maximum, its derivative with respect to time (i.e., d(df/dt)/dt) is zero, and h*r c o exp(-h*/kTc) = ~_--~-~, (3) kT c in which Tc is the temperature of maximum crystallization rate (in K) and r is the constant heating rate (K. sec-1). A plot of ln(r/T 2) versus T c 1 yields a straight line from which c o and h* can be determined. Crystallization data from fresh and aged samples of a-As2Se 3 are shown in fig. 3. The straight line behavior shows that the crystallization process obeys first-order kinetics. This result is in accord with certain diffusion-limited growth processes [21 ]. The activation enthalpy (1.24 eV) is independent of sample age, although c o was lower for the fresh sample (1.2 × 108 sec-1) than for the sample aged 800 days (1.8 × 108 sec-1). The difference between the crystallization rate of the two samples is thus attributed to an increase in the number of nuclei with sample age. Since the observation of first-order kinetics implies that crystallization is diffusion limited, it is interesting to compare the activation enthalpy we obtained with that determined from viscosity data. Viscosity data taken well into the liquid phase

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D.D. Thornburg, R.1. Johnson/Bulk amorphous arsenic triselenide -13

-14

-15

-17

-18

O -As2s~3 -FRESHSAMPLE

~k'

r~ SAMPLE AGED 800 DAYS 1.5

q

1.6

I

1.7

I

1.8

.9

103/Tc Fig. 3. Plot of the crystallization data for a fresh (o) and aged (~) sample of a-As2 Se 3 from which the activation enthalpy of the crystallization process can be determined, See text for description.

[14] yield an activation enthalpy of 1 eV, while data taken in the vicinity of Tg [15] yield an activation enthalpy of 1.94 eV, which is in close agreement with the estimated energy of the As-Se bond (1.83 eV). Our activation enthalpy (1.24 eV) falls between these two values, as it should. The non-Arrhenius nature of the diffusive process over 13 decades of viscosity is not unexpected. It may be hypothesized that, above Tg, the As2Se 3 network has broken up into macro-molecular dusters whose size decreases with increasing temperature. Such a reduction in duster size would not only be consistent with a decreasing value of h*, but would also explain the observation of E1-Fouly and Edmond [17] that a-As2Se 3 samples quenched from 800°C show more pronounced crystallization on reheating than do samples quenched from 600°C. This result is probably a fictive temperature effect, since the glass quenched from 800°C may contain vestiges of lower molecular weight (and hence higher mobility) clusters than the glass quenched from 600°C. The driving force for crystallization of a supercooled liquid above Tg is given by the difference between the Gibbs free energy per molecule of the amorphous (ga) and crystalline (gc) phases, evaluated at the crystallization temperature. This quantity is given by

D.D. Thornburg, R.L Johnson/Bulk amorphous arsenic triselenide

Ague = ga - gc -

Ahm(T m - Tc) Tm ,

7

(4)

in which Ah m is the enthalpy of melting per molecule [21 ]. Integration of the melting endotherm on five DSC traces made at 2°C • min -1 and plotted versus time, yielded an average value for Ah m of 0.39 eV-molecule -1 . Using the measured value of T m (372°C), the driving force, Agac, was found to be 0.04 eV" molecule 1, which is close to k T c. The transformation rate is then approximately given by

d f ~ A ( A gac/k T) exp (-h* /k T) ( 1 - f ) , dt

( 5)

in which A is a constant [21]. The temperature dependence in the prefactor to the exponential term is too small to alter the pronounced exponential dependence shown in fig. 3.

3.4. Enthalpy o f crystallization Crystalline As 2 Se 3 has a structure consisting of layers, each of which is composed of puckered rings with each As atom bonded to three Se atoms and each Se atom bonded to two As atoms. The large interlayer separation suggests that van der Waals bonding holds the layers together [22]. Amorphous As2Se 3 has been shown to have the same nearest neighbor configuration as that of the crystal [23], but with no long-range order, even though the puckered sheet structure is believed to be preserved. In this case, interlayer bonding is attributed to the presence of occasional octahedrally coordinated As atoms which are bonded to three Se atoms in each layer [23]. The concentration of such atoms must be small, however, and the major energy difference between the amorphous and crystalline forms must reside in differences in As-Se angular bond strains. The enthalpy of crystallization (Ahc) was determined for both fresh and aged samples heated at 2°C • min -1 . The fresh sample had an average value of Ah c = 0.37 eV" molecule -1 and the sample aged 800 days had an average value of Ah c = 0.36 eV" molecule -1 . The difference between the two values is less than the scatter for each determination (three specimens of each sample), and shows that negligible crystallization had occurred during the aging at room temperature for 800 days. If this enthalpy is attributed to bond strain energy differences, then a strain energy difference of Ahc/6 = 0.061 eV is obtained per As-Se bond.

4. Summary The heat capacity and crystallization parameters of a-As2Se 3 have been determined by differential scanning calorimetry. Below the glass transition temperature (Tg), the heat capacity (Cp) had the Dulong-Petit classical limit. Above Tg addi-

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D.D. Thornburg, R.L Johnson/Bulk amorphous arsenic triselenide

tional translational modes became available and Cp increased by a factor of 1.6. The kinetics o f crystallization above Tg was studied and found to be first order with an activation enthalpy of 1.24 eV. The rate constant was found to be smaller for a freshly quenched sample than for a sample aged 800 days at room temperature. The crystallization data was interpreted to imply that the crystal growth process was diffusion limited. The additional bond strain on amorphization was calculated from the measured enthalpy of crystallization to be 0.06 eV. bond -1 . The results were consistent with the generally accepted model that a - A s 2 S e 3 has the same shortrange order as the crystal.

References [1] N.F. Mott and E.A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1971) p. 324ff. [2] J.S. Berkes, S.W. Ing and W.J. Hillegas, J. Appl. Phys. 42 (1971) 4908. [3] J.P. DeNeufville, S.C. Moss and S.R. Ovshinsky, J. Non-Crystalline Solids 13 (1973/74) 191. [4] J.M. Marshall and A.E. Owen, Phil. Mag. 21 (1971) 1281. [5] D.D. Thornburg, J. Non-Crystalline Solids 11 (1972) 113. [6] D.D. Thornburg and R.M. White, J. Appl. Phys. 43 (1972) 4609. [7] D.D. Thornburg, J. Electron. Mat. 2 (1973) 3. [8] J.R. Bosnell and J.A. Savage, J. Mat. Sci. 7 (1972) 1235. [9] M.D. Tabak, S.W. Ing and M.E. Scharfe, IEEE Trans. Elect. Dev. ED-20 (1973) 132. [10] c.f., D.D. Thornburg, J. Electron. Mat. 2 (1973) 495. [11] U.E. Schnaus, C.T. Moynihan, R.W. Gammon and P.B. Macedo, Phys. Chem. Glasses 11 (1970) 213. [12] S.A. Dembovsky, Phys. Chem. Glasses. 10 (1969) 73. [13] R. Flasck and H.K. Rockstad, J. Non-Crystalline Solids 12 (1973) 353. [14] B.T. Kolomiets, Phys. Stat. Sol. 7 (1964) 359. [15] S.V. Nemilov, Soy. Phys.-Solid State 6 (1964) 1075. [16] D.D. Thornburg, Thin Solid Films 11 (1972) 219. [17] M.H. E1-Fouly and J.T. Edmond, Phys. Stat. Sol. (a) 21 (1974) K43. [18] H.E. Kissinger, J. Res. Nat. Bur. Stand. 57 (1956) 217. [19] S. Risbud, J. Amer. Ceram. Soc. 56 (1973) 440. [20] D.D. Thornburg, Mat. Res. Bull., in press. [21 ] J.W. Christian, The Theory of Transformations in Metals and Alloys (Pergamon Press, Oxford, 1965) chs. 11 and 12. [22] E.I. Yarembash, E.S. Vigelevaand N.P. Luzhnaya, Russ. J. Inorg. Chem. 7 (1962) 177. [23] A.A. Vaipolin and E.A. Porai-Koshits, Sov. Phys.-Solid State 5 (1963) 178.