Photo-effect on crystallization kinetics of amorphous selenium doped with sulphur

Journal of Non-Crystalline Solids 33 (1979) 13-22 © North-Holland Publishing Company

PHOTO-EFFECT ON CRYSTALLIZATION KINETICS OF AMORPHOUS SELENIUM DOPED WITH SULPHUR M.F. KOTKATA, F.M. AYAD and M.K. EL-MOUSLY Physics Department, Faculty of Science, Ain Shams University, Cairo

Received 6 February 1978 Revised manuscript received 4 December 1978

The method of change in electrical conductivity has been applied to investigate the illumination effect on the crystallization of amorphous selenium containing 4.26 at.% sulphur. Kinetic analysis of the crystallization process in the temperature range of 98-144°C together with microstructure studies have been used to illustrate that the light has two effects: a pure thermal one that appears when increasing the temperature of the sample, and an optical effect which leads to the destruction of the Se-Se bonds, reducing the Se chain length and opposing the crystal growth or the formation of the ordered chain structure. A value of 9.96 -+0.32 Kcal mole -1 has been obtained for the activation energy of the crystal growth under purely thermal or with photo-effect consistent with the suggested mechanism.

1. Introduction The use of the change in electrical conductivity to investigate the crystallization process in selenium and selenium-like structure has proved to be an effective method [1,2]. During the crystallization process in selenium the spherulites grow outward from a nucleation centre. When the spherulites become sufficient large and touch each other a process of welding takes place providing a continuous path of electric current across the sample [3]. The introduction of 3 - 5 at.% sulphur into selenium does not lead to the formation of any new compound [4], but considerably decreases the crystallization rate of selenium [5]. During the crystal growth process caused by decreasing temperature, sulphur partially moves outside the spherulites and collects on the boundaries hindering the welding process as suggested by Crystal [3]. Light, on the other hand, can produce a marked enhancement of the Crystallization kinetics over that obtained from the purely thermal effect [ 6 - 8 ] . This effect has been used to write holograms [9] and discrete images [8] with lasers in other chalcogenide alloys. In this paper, the crystallization of selenium containing 4.26 at.% sulphur, namely SSe22.s, has been studied by continuously following the changes in the electrical conducttivity during simultaneous thermal annealing and photo-illumination. 13

14

M.F. Kotkata et aL/ Photo-effect on crystallization kinetics

The obtained results have been interpreted in view of the current theories of crystallization.

2. Experimental Bulk 58e22.5 samples have been prepared in the amorphous phase by heating the constituents at 280°C for 2 h and quenching in air at 18-20°C inside evacuated (10 -4 mm Hg) pyrex-cells having parallel faces (~2 mm apart) and provided with two tungesten electrodes. Crystallization has been conducted in preheated ovens (0.2°C max. fluctuations) under dark and illuminated conditions. Illumination was achieved by a 250 W ultrahigh pressure Hg-quartz lamp. The IR part of the emitted spectrum was filtered out to reduce sample heating. The crystallization was monitored by measuring the electronic conduction periodicaUy (½ min interval) during several isothermal transformations in the range of 98-144°C using an electrometer with error less than 2%. The remarkable increase of the electrical conductivity accompanying the amorphous to crystalline phase change in absence or presence of light implies that the measured conductivity o at any time t is the result of two conductivities Oa and oc corresponding to a doublephase system, amorphous and crystalline. 3. Results and discussion 3.1. Electrical conductivity measurements

The use of conductivity-structure characterization for a proper description of the volume fraction which has crystallized depends on the specific regimes of percent transformation. Fig. 1. shows the time-dependence of the electrical conductivity of SSe22.s measured at four different temperatures in the range of 98-144°C under the effect of 2.13 eV mean energy incident photons. Fig. 2 illustrates variation of the function o =f(t) for two SSe22.s samples annealed at 121°C in the dark and under photo-illumination. During the transformation process, whether under purely thermal or with the photon effect, there appear to be three regimes of o versus percent transformation: the conductivity remains at first approximately constant, but after a certain time, depending on annealing temperatures, it increases abruptly by several orders to attain a certain limiting value. Such a strong increase of a is due mainly to transformation of the very low conductivity amorphous phase into a rather better conducting crystalline SSe22.s. The constancy of the maximum conductivity attained at a given temperature indicates the completion of possible transformation process. The limiting value of o depends on the crystallization temperature due to the change in degree of structural perfection defining the number of grains per cm a [10].

M.F. Kotkata et al.

/Photo-effect on

ar xloT(.~-lcm -1) 100~

b

crystallization kinetics

15

Q 132 C

/

80

60b 4020-

~

124°C

b

0 a r r t

. i

i

i

i

0

~

98°C

2 a i

40mln r" j r

r10

i

i

i

i

i

i

i

i

Time i

1

i

Fig. 1. The time-dependence of the electrical conductivity of SSe22.5 conducted at four different isotherms under the effect of photo-illumination of 2.13 eV mean photon energy.

40- cr XIO?(~--L-Icm-I)

b

b

20-

///

10-

llummated 5Se22.5

f 0

~ I"

I I

I

I

20rain

Non-illuminated SSe 22.5 Both annealed at 121°C I I

Time I

1

I

I

-/

Fig. 2. Variation of the function a = f(t) for two SSe22.5 samples isothermally annealed at 121°C in the dark and under photo-illumination.

16

M.F. Kotkata et aL / Photo-effect on crystallization kinetics "50-

A~(min)

25"

o

O

, 90

1 110

,

i 130

,

i Tern p.(°C) 150

Fig. 3. The temperature dependence of the increment of total time of amorphous to crystalline transformation under purely thermal conditions and that with photo-effect.

The total time for the amorphous to crystalline transformation r shows a decrease with increasing temperature and it is longer for the samples conducting under illumination. The increment of time Ar is given in fig. 3 and indicate that the increase in temperature leads to the decrease of the effect of illumination on the crystallization process. Such effect tends to zero at temperature about 150°C. Changing the energy of incident photons in the range of 1.91-3.04 eV has no great effect on the nature and total time of the transformation process in the temperature range studied. This is easy to understand as the dissociation energy of S e Se bond is 1.8 eV [ 1 1 ] , - 1 . 9 eV [12]. 3.2. Micrograph patterns o f crystallization

Plates 3 and 4 are typical reflection micrograph patterns taken for SSe22.5 film isothermally annealed at 100°C for 1 and 2 h with 2.13 eV incident photons. This value is greater than the known value of the dissociation energy of the Se-Se bond. The plates 1 and 2 are those for identical SSe22.s films annealed in the dark. The plates show that the growth is faster in .the dark and leads to a more ordered crystalline network. The illuminated samples are characterized by the presence of highly dispersed fine crystallites due to light effects. Further annealing of the samples in the presence of light leads to the same result of the less ordered products compared to those annealed in the dark. Plate 5 represent s the dark annealed 2 h sample (plate 2) after subjecting it to a thermal treatment for two-additional hours under the effect of light. The last sample becomes more ordered when re-annealed in the dark, plate 6.

M.F. Kotkata et al. /Photo-effect on crystallization kinetics

17

3.3. Crystallization kinetics To study the kinetics of transformation the experimental data should be expressed in terms of the transformed fraction, a, at different crystallization time. In the present work, a = f ( t ) is evaluated for the S-doped Se sample by using a power

Plate 1. SSe22.5 film isothermally annealed at 100°C for 1 hr, X 450. Plate 2. SSe22.5 film isothermally annealed at 100°C for 2 hrs, X 450.

Plate 3. SSe22.5 film isothermally annealed for 1 hr at 100°C under photo-illumination, × 450. Plate 4. SSe22.5 film isothermally annealed for 2 hrs at I00°C under photo-illumination, x 450.

18

M.F. Kotkata et al. / Photo-effect on crystallization kinetics

Plate 5. Sample of photo No. 2 after annealing for 2 hrs at 100°C under photo-effect, X 450. Plate 6. Sample of photo No. 5 after annealing for 2 hrs at 100°C in the dark, × 450.

conductivity dependence according to Odelevsky [13]. It gives for a first-order approximation that,

O(t) = (oc - ot)/(oc

-

Oa),

where, 0 is the fraction left uncrystallized at a time t. The subscripts a and c are refer to values at the beginning and at the end o f the transformation process, respectively. These correspond to points a and b on the curves o f figs. 1 and 2. The results of a = f ( t ) at the two temperature 117 and 144°C are given in fig. 4 for both illuminated and non-illuminated cases. Such crystallization curves have been found to shift toward lower time scales with increasing temperature consistent with the formal theory o f crystallization, see Tamman [14].

%

40 204

144°C

117°C

dark

~'j 20rain

/ j

~

w;,h ,,~..

40min

Fig. 4. Crystallinity % versus annealing time for SSe22.S crystallized at two different temperatures in both cases, dark and with light-effect.

M.F. Kotkata et al. I Photo-effect on crystallization kinetics

19

The crystallization of organic polymers has been described by the Avrami formula [15], -In(1 -- a) = K t ' , where K is the temperature-dependent rate constant and n is a parameter which depends on the nucleation and growth mode [16]. To fit the Avrami equation, a plot of In f - I n ( I - a t ) ] versus In (t) must yield a straight line whose slope is n. Figs. 5 and 6 are just such plots for SSe22.s transformed under purely thermal and with photo-illumination, respectively. At some temperatures, the plot takes on two distinct slopes during the isothermal transformation which, therefore, can be described by two different values for n. The obtained results for n are given in table 1 for both illuminated and non-illuminated samples. At lower temperatures (<117°C for illuminated and <124°C for non-illuminated samples), the kinetic calculations indicate that the crystallization process is characterized by a single value of n decreasing with the increase of temperature. Such single values indicates a sporadic nucleation together with nearly 2-dimensional crystal growth (n ~ 2 - 3 ) . The increase of temperature leads to a predetermined nucleation process with crystal growth of order about 1 (n2 ~ 1-2). Moreover, the presence of the two values of n at higher temperature, table 1, means that sulphur cannot prevent the welding of spherulites. In the temperature range of 121-132°C,

1-

O-

Ln[-Ln (1-~t)]

Ln(t) '

jfl

i 7

7'5

J/ 14&*C

//, 1320C

12{ :.124°I:

.i o ,~ 0 . 9 ~o#c

98 c

Fig. 5. Kinetics of the crystallization of SSe22.S in the dark on the basis of Odelevsky approach.

M.F. Kotkata et aL / Photo-effect on crystallization kinetics

20 ,_

Ln[-LoC~-*'t>] .*

144°C

1320C 126°C

12dC

0 .~- O. 1

121%117°C 10~°C980C

Fig. 6. Kinetics of the crystallization of SSe22.5 conducted with photo-illumination and calculated on basis of Odelevsky approach.

the values o f n~ are greater in the case o f t r a n s f o r m a t i o n u n d e r p h o t o - i l l u m i n a t i o n indicating the possibility o f increasing s p o r a d i c n u c l e a t i o n during the process. The m i c r o s t r u c t u r e p a t t e r n o f plate 5 c o n f i r m s this idea as well as the a p p e a r a n c e o f two.values o f n at l o w e r t e m p e r a t u r e t h a n in the dark. The value o f n2 w h i c h indicates nearly 1-dimensional g r o w t h c o r r e s p o n d s to the welding process suggested b y Crystal [3]. The greater values o f n and their reversing, n2 > n l , at t e m p e r a t u r e

Table 1 Values of Avrami constant n calculated on the basis of the Odelevsky approach for an amorphous SSe22.5 crystallized at different isotherms in the dark and under photo-illumination. Temp., °C

Crystallization in the dark

Crystallization under photo-illumination

98 108 117 121 124 126 132 144

n n n n nl nl nl nl

n --- 2.76 n = 2.24 nl -- 2.70, n2 = 2.00 nl = 2 . 7 5 , n 2 = 1 . 9 0 nl -- 3.20, n2 = 1.72 nl -- 3.80, n2 = 0.98 nl ---4.20, n2 = 1.04 n 1 ~ 1.56, n 2 = 4.10

= 3.20 = 2.20 = 1.50 =1.80 = 2.40, n2 = 2.42, n2 = 2.48, n2 = 2.14, n2

= 1.14 -- 1.12 = 1.16 = 6.60

M.F. Kotkata et al. / Photo-effect on crystallization kinetics -Log

21

(l/T)

2.1

i

~

o

•

~,,~

,,oh,

~o" A)-I I

24

L

i

I

I

25

26

27

]

Fig. 7. Log(I/r) versus T-1 for both cases of crystanization, non-illuminated and illuminated.

144°C compared to other temperatures in both illuminated and non-illuminated transformations may indicate the presence of a completely different mechanism of grow th. 3.4. Activation energy o f crystallization

The activation energy may be calculated from the temperature dependence of the rate of linear-crystal growth K(T). Actually, in the fluctuation process of formation of crystal grains, one may write for the probability of the process that, K(T) = Ko e x p ( - E / R T ) .

Consider that the rate is proportional to 1/r. The relation between log(I/T) and 1/T(K -1) is given in fig. 7 for the non-illuminated and illuminated transformation of SSe22.s where the result is a linear dependence for both cases in the range of 98-132°C with nearly the same slope. This gives an average value for the activation energy of linear-crystal growth of I~ = 9.96 -+ 0.32 Kcal mole -1. This value is consistent with that of Dresner and Stringfellow [6] for crystallization of pure Se under photo-illumination with mean photon energy close to 3 eV. They found a dependence of growth rate on light intensity, and E = 11.27 Kcal mole -1 in their temperature range (49-80°C) at the low intensities.

4. Conclusions The overall effect of photo-illumination on the amorphous to crystalline transformation of SSe22.s can be understood as a sum of two separate and maybe opposite effects. First; the effect of light reduced to a pure thermal effect. That is, it leads to increase of temperature. This is confirmed with appearance of two values

22

M.F. Kotkata et a L / Photo-effect on crystallization kinetics

for n in the case o f illuminated transformations at a lower temperature than that in the dark, 117°C compared with 124°C. Second; the pure photo-effect due to the absorbed energy of incident quanta. This leads mainly to destruction o f the chemical S e - S e bonds, reducing the length o f Se-chains, and opposing the crystal growth or the formation o f ordered chain structure. This second effect decreases with increasing temperature as is clear from fig. 3 considering the first effect independent of temperature. To check the value of the first effect, the temperature of the illuminated sample was measured and compared with the non-illuminated part. A difference of about 2°C has been recorded. The constancy of the activation energy I~ = 9.96 -+ 0.32 Kcal mole -~ whether under purely thermal or with the light-effect indicates that the light has no great influence on the activation energy of linearcrystal growth o f SSe22.s in the considered temperature range.

References [ 1 ] C.H. Champness and R.H. Hoffmann, J. of Non-Crystalline Solids 4 (t970) 138. [2] M.K. E1-Mously and M.M. E1-Zaidia, J. of Non-Crystalline Solids 27 (1978) 265. [3] R.G. Crystal, J. Polym. Sci. A-2,8 (1970) 1755. [4] J.W. Melor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 10 (1946). [5] M.K. E1-Mously, J. Neorg. Mat. USSR 13 (1977) 801. [6] J. Dresner and G.B. Stringfellow, J. Phys. Chem. Solids 29 (1968) 303. [71 1.A. Paribok-Alexandrovich, Soviet Phys.-Solid State 11 (1970) 1631. [8] J. Feinleib, J. de Neufville, S.G. Moss and S.R. Ovshinsky, Appl. Phys. Letters 18 (1971) 254. [9] R.G. Brandes, F.B. Laming and A.D. Pearson, Appl. Optics 9 (1970) 1712. [10] M.K. E1-Mously, A.H. Waf'hkand S.A. Saleh, Indian J. of Tech. 14 (1976) 565. [ 11] T.L. Cottrell, The Strength of Chemical Bonds, 2nd ed. (Butterworths, London, 1958) p. 258. [12] L. Pauling, The Nature of the Chemical Bond, 3rd ed. (Corneil Univ. Press, Ithaca, 1960) p. 85. [13] V.I. Odelevsky, J. Tecla. Phys. USSR 21,6 (1951) 673. [14] G. Tamman, The States of Aggregation, Trans. R.F. (Mehl, New York, Van Nostrand, 1925). [15] M. Avrami, J. Chem. Phys. 7 (1939) 1103;lbid 8 (1940) 213; Ibid. 9 (1941) 117. [ 16] L. Mandelkern, Crystallization of Polymers (McGraw-Hill, New York, 1964).