Photodoping of Ag in AgAsS glasses

J O U R N A L OF

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Journal of Non-Crystalline SoLids170 (1994) 27-31

Photodoping of Ag in Ag-As-S glasses Keiji T a n a k a * Department of Applied Physics, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan

(Received 3 August 1993; revised manuscript received 29 November 1993)

Abstract

The photodoping rate of Ag in As-S and Ag-As-S bulk glasses has been studied as a function of composition. The rate is evaluated from the resistance change of Ag layers. In the Ag:AsxS100_x system, the rate shows a maximum at x = 30. In the mg:(Ag2S)x(AS2S3)]_ x system, with an increase in the Ag content, the rate increases first, then decreases, and it becomes approximately zero in the glass containing k 25 at.% Ag. These results are compared with thermal diffusion phenomena and are discussed in the light of the Owen's idea for photodoping.

I. Introduction

The photoinduced dissolution of metals such as Ag into glassy chalcogenide semiconductors, hereafter referred to as 'photodoping', was discovered by Kostyshin et al. [1], and extensive studies of fundamentals and applications have continued (see Ref. [2], as a review). Nonetheless, the microscopic mechanism of the photodoping is still unknown. At present, it may be fruitful to consider the photodoping process from two aspects [3]; the first concerns the mechanism responsible for the photoinduced Ag + motion, and the second concerns chemical problems. Regarding the chemical problems, Owen et al. have proposed a thermodynamic model [4] which provides a unified interpretation of the chemical composition of the Agphotodoped region and the composition depen-

* Corresponding author. Tel: + 81-11 716 2111. Telefax: + 8111 726 4336.

dence of the photodoping rate in A g : A s - S systems. Ewen et al. [5], Wagner and co-workers [6-9] and Kawaguchi and Maruno [10] have demonstrated that the thermodynamic model is applicable to Ag:As(Ge)-S(Se) systems. The model seems to apply also to the photo-surface deposition p h e n o m e n o n [2,4,11] and the photoinduced chemical modification [12]. However, for the photodoping, previous studies relating to Owen's idea are not necessarily sufficient, at least, in three respects. First, most of the previous experiments have dealt with deposited chalcogenide films [2-10,13], while Owen's idea is based upon the glass-forming region of bulk glasses. It is well known that atomic structures in glasses are dependent on preparation procedures [14] and, hence, in order to obtain insight into compositional problems, it may be preferred to examine the characteristics of photodoping in bulk glasses. Second, almost all previous studies seem to have been performed for Ag-photodoping in Ag-free chalcogenide glasses [2], but Ag-photodoping in Ag-containing glasses

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K. Tanaka/Journal of Non-Crystalline Solids 170 (1994) 27-31

may afford a valuable insight. Last, since Owen's model is thermodynamic, its application to thermal diffusion phenomena is an interesting experiment. Actually, thermal diffusion of a substantial amount of Ag in As-S can produce Ag-As-S glasses [15], and this process is seemingly in harmony with Owen's model. Hence, a comparative study of photodoping and thermal diffusion under similar experimental conditions may be intriguing. In the present paper, therefore, the composition dependence of the photodoping rate and the thermal diffusion of Ag in As-S and Ag-As-S glasses is studied.

3. Results

2. Experiments

As2S 3 films, an induction period was not ob-

To compare the photoinduced dissolution with thermal diffusion, the composition dependence of Ag thermal diffusion was studied through isochronal experiments. That is, the resistance of Ag films was measured after successive annealing for 1 h at increasing temperatures, and the temperature at which the resistance became twice the initial values was defined as the diffusion temperature, which may be a measure of thermal diffusion; the lower the diffusion temperature, the higher the diffusivity.

It is noted first that in all samples, except

Bulk glasses with compositions ASxS10o_x, where x = 17 to 43, and As2S3-Ag2S, in which the Ag content y = 0-35 at.%, were prepared using conventional melt-quenching techniques [14]. The compositions are shown in Fig. 1 with the reported glass-forming regions [16]. The bulk glasses were polished with alumina powders and annealed in order to reduce mechanical strain. As a reference, As2S 3 films were prepared by vacuum evaporation. Then, semi-transparent Ag films (200-300 ,~ thick) were deposited onto the bulk and film samples. The photodoping rate was evaluated at room temperature from the change in the dc resistance of the Ag film, the change which was caused by the photoinduced dissolution of Ag into (Ag)As-S glasses. This method was originally developed by Goldschmidt and Rudman [17] and has been widely utilized to monitor the photodoping kinetics in a simple way [2,3,18]. The light source employed for inducing the photodoping was a 100 W ultrahigh-pressure Hg lamp, which irradiated the Ag:(Ag)-As-S samples from the Ag side. The light intensity was around 10 m W / c m 2, and the reciprocity law between the light intensity and exposure time was retained, as previously demonstrated [17]. The photodoping rate was defined as the inverse of the exposure time (> 2 min) needed to double the resistance of initial values (~ 5 fl).

served. Hence, we can assume that the evaluation procedure of the photodoping rate provides a qualitative compositional dependence. The induction phenomenon may be unique to molecular glasses as previously mentioned [18]. Fig. 2(a) shows the composition dependence of the photodoping rate in the Ag:AsxS10o_x system. We note that, in the Ag:As17S83 system, chemical reaction between Ag and As17S83 proceeded even in the dark at room temperature, and accordingly

Ag

AgAs~~ ~0 As

60 As2S 3

80

>S

Fig. 1. The glass-forming region in the Ag-As-S system (hatched), and the bulk glasses (solid points) investigatedin the present study. Three compounds (As2S3, AgAsS2 and Ag2S) are indicated, and the tie-line connecting Ag and ms30S70 is drawn.

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K. Tanaka/Journal of Non-Crystalline Solids 170 (1994) 27-31

the photodoping rate could not be measured. The reaction product was probably AgzS , and the observation is consistent with the fact that S molecules are contained in AsxSt00_ x glasses with x < 20 [19]. In Fig. 2(a), previous results obtained using A s - S films are also plotted as Refs. [5-7,13]. We see that all the results, except the line a, exhibit the highest photodoping rates at x - - 3 0 , while there exist some compositional differences. The detailed features may be dependent upon experimental variations such as measurements of the photodoping rate, sample thicknesses and light sources employed. These details can be neglected in the present study. We also note that, although the bandgap energy in glassy A s - S decreases from 2.8 to 2.4 eV with an increase in the As content [20], this effect in the present experiments is small. Fig. 2(a) shows that the effects of composition in the bulk and film samples are similar. How-

ever, under the same illumination condition, the rate of photodoping was a factor of two to three higher in as-deposited AszS 3 films than in bulk materials. The reason for the difference is not known. A similar tendency was also found in the glassy G e - S system; the details will be published elsewhere. Fig. 2(b) shows the composition dependence of the photodoping rate of Ag in A g - A s - S glasses. As shown in the figure, with an increase in the Ag content, y, the photodoping is a maximum at y = 2, and then dramatically decreases at y > 25. In samples with y = 30, the Ag films form clusters upon illumination, and thus we could not evaluate the photodoping rate. In samples with y = 35, photoinduced effects are hardly observed. A comparative study between the photodoping rate and the thermal diffusion may give a deeper insight. I suggest that the result shown in Fig. 2(a) for the A s - S system indicates that the thermal

(a)

(b) Td ~ I0~...-'{ Td

100

E 20C

?>.--~'...x... i \ gr,ia

i,4f r\ I,I""

Z(

(3r~ C

"5. 0

d//

"6 r-

J

"0 0

\'

t-

CL 0 "I0 0 c

Q.

0

'

2'0

'

x (at. °/o)

4'0

0

20

y (at.%)

40

Fig. 2. The photodoping rate, the diffusion temperature, Ta, and the glass-transition temperature, Tg, [19,22,23] in bulk As-S glasses as a function of the As content x (a) and in bulk Ag-As-S glasses as a function of the Ag content y (b). Ta and Tg are plotted downward for easy comparison of the photodoping and thermal characteristics. In (a), previous results of the photodoping rate obtained using deposited films are summarized: a, Ref. [13]; b, Ref. [5], c, Ref. [6]; d, Ref. [7], in which maxima of the photodoping rates are normalized to unity. In (b), the photodopingrate at y = 0 (As2S3) is normalized to unity. All lines are drawn as a guide for the eye, and error bars represent scatter of experimental results.

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K. Tanaka/Journal of Non-Crystalline Solids 170 (1994) 27-31

diffusion correlates with the glass transition temperature, which linearly increases with x when x < 40 [19]. Dependence of the diffusion temperature and the photodoping rate on x differ. By contrast, the result shown in Fig. 2(b) indicates that the thermal diffusion is efficient in samples with y = 20-30, the result being in harmony with conventional diffusion experiments [15,21]. In this system, the thermal diffusion does not exhibit a close correspondence with the glass transition temperature, which gradually decreases with an increase in the Ag content [22,23]. No correlation exists between the composition dependences of the diffusion temperature and the photodoping rate.

4. Discussion

The fact that the photodoping rate is high in samples with x = 30 (Fig. 2(a)) appears to be consistent with the idea proposed by Owen et al. [4] and developed by the other researchers [5-10]. They emphasize that a tie-line connecting As30S70 and Ag in the Ag-As-S composition triangle (Fig. 1) passes near AgAsS2, and hence the photodoping rate is maximum at As30S70 [4-7]. The idea seems to rely on an underlying assumption that AgAsS 2 forms a stable glass with minimal free energy. This thermodynamic assumption may be supported with the following two observations. First, AgAsS 2 lies at a central position of the glass-forming region and consists of equal amounts of AseS 3 and AgeS (Fig. 1). Conventional thermodynamics imply that, at this central composition, mixing entropy of the two components (As2S 3 and AgeS) may be highest, and accordingly the free energy will be lowest [24]. Second, it is known that AgAsS e can be solidified into crystalline phases, called smithite [16], which could possess low free energies. It is known that short-range structures in a glass are similar to that in the corresponding crystal, and hence AgAsS 2 will be a stable glass if viscosity problems can be overcome [14]. If AgAsS 2 glass possesses minimal free energy, we can assume, taking the phase-transformation

concept into account [24], that dissolution of Ag into As30570 may be favored thermodynamically. The photodoping can then become efficient at x = 30. Here, application of thermodynamic concepts to non-equilibrium glassy systems remains a fundamental problem; nonetheless, it is argued that the free energy can be defined within an experimental timescale [25]. The composition dependence of the photodoping rate shown in Fig. 2(b) appears also to be in harmony with Owen's idea [4], if we may assume the stability of AgAsS 2 (y = 25) and As2S 3 (y = 0). In the sample with y = 2, the free energy may be higher than that in As2S3, and hence conversion of Ag:Ag-As-S to AgAsS z can occur efficiently. By contrast, in the Ag-As-S glasses with x > 15, the free energy may be lower, and the chemical reaction is impeded. In the glasses with x > 30, the reaction will be completely suppressed, because the phase change may increase the free energy. However, the differing composition dependences of the photodoping and the thermal diffusion shown in Figs. 2(a) and (b) indicate that the phase transformations are governed by different factors. We first note that atomic diffusion occurs when energetic and kinetic conditions are satisfied [24]. That is, the diffusion is a manifestation of a phase change from a non-equilibrium to an equilibrium system, and the rate of the diffusion is governed by kinetic factors. As discussed above, the photodoping characteristics can be understood thermodynamically, and accordingly we may assume that the kinetic factors are overcome by illumination. By contrast, the thermal diffusion in Ag:As-S seems to reflect the kinetic factor, since the diffusion temperature shows correlation with the glass transition temperature, which is a function of the rigidity of glassy networks [14]. In Ag:Ag-As-S, the thermal diffusion may be governed by kinetics at y < 25, and by thermodynamics at y >_.25. The reason why the kinetic factor appears to be ineffective with illumination is speculative. The phenomenon may be related to experimental observations which indicate stress relaxation in chalcogenide glasses under illumination [26]. The stress relaxation may be responsible for atomic

K. Tanaka/Journal of Non-Crystalline Solids 170 (1994) 27-31

(ionic) diffusion. These phenomena are related to electron-lattice interactions, and the details will be a subject of further studies. We finally note that the present argument for the composition dependence may apply to Ag(Cu):As(P,Ge)-S(Se) systems exhibiting the photodoping phenomenon [2,27], since Ag(Cu)As(P,Ge)-S(Se) ternary systems have glass-forming regions at similar compositions to that in Ag-As-S [16], i.e., the regions where the Ag content is less than ~ 30 at.% and the As(P,Ge) content is less than ~ 50 at.%. The reasons these glasses have similar glass-forming regions are not known [28].

5. Conclusions An account for the composition dependence of the photodoping rate in Ag:(Ag)-As-S is based on the thermodynamic model originally proposed by Owen et al. Thermal diffusion of Ag in (Ag)A s - S glasses have different composition dependences, which are affected by both thermodynamic and kinetic factors. The present work was partially supported by Izumi Science and Technology Foundation.

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