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Photodiffusion kinetic studies by means of elemental analysis of Ag doped As2S3 glass
Journal of Non.CrystaUineSolids 56 (1983) 325-330 North-Holland Publishing Company
325
P H O T O D I F F U S I O N K I N E T I C STUDIES BY MEANS OF E L E M E N T A L ANALYSIS OF Ag DOPED As2S 3 GLASS Janusz
ul.
PXocharski,
Jan PrzyXuski,
Henryk
D e p a r t m e n t of C h e m i s t r y Warsaw Technical University N o a k o w s k i e g o 3, 00-664 Warszawa,
Wyci~lik
Poland
P h o t o d i f f u s i o n has been observed in light irradiated A s 2 S 3 / A g two layer samples. X-ray fluorescence analysis has been u t i l i z e d to d e t e r m i n e the chemical c o m p o s i t i o n of the p h o t o r e a c t i o n p r o d u c t giving the formula A g 2 . 6 A s 2 S 3. The values of the p h o t o r e a c t i o n rate e s t i m a t e d from the analytical results s a t i s f a c t o r i l y agree w i t h the analogous values d e r i v e d from the electrical r e s i s t a n c e measurements. The p h o t o d i f f u s i o n p h e n o m e n o n models are also discussed.
1.
INTRODUCTION
Since first reported (i), light e n h a n c e d reactions between some metals and amorphous c h a l c o g e n i d e semiconductors, called p h o t o d i f f u sion, p h o t o d o p i n g or p h o t o d i s s o l u t i o n , have been the subject of a c o n s i d e r a b l e number of papers. Two systems, A s 2 S 3 - A g (2,3) and G e x S e l - x - A g (4,5), have been i n v e s t i g a t e d w i t h emphasis on the p o t e n t i a l a p p l i c a t i o n of these m a t e r i a l s as inorganic resists for u l t r a - h i g h r e s o l u t i o n x-ray lithography. As a result, remarkable progress in inorganic resist technology has been achieved, but the p r o b l e m of the p h o t o d i f f u s i o n p h e n o m e n o n m e c h a n i s m still seems to be unsolved. To date, no e l e m e n t a l analysis of the p h o t o d i f f u s i o n products has ever been reported. X-ray f l u o r e s c e n c e is believed to be a convenient tool for the direct d e t e r m i n a t i o n of the chemical c o m p o s i t i o n of the products created upon p h o t o d i f f u s i o n and is expected to be helpful in the v e r i f i c a t i o n of the p h o t o d i f f u s i o n models. 2.
EXPERIMENTAL
Thin film samples were p r e p a r e d by a successive, quartz m o n i t o r controlled, e v a p o r a t i o n of Ag and As2S 3 (both of 4N purity) in an oil free high v a c u u m system (pressure of 10-4pa). Silver films (5 to 22 nm thick) and 350 nm thick As2S 3 films have been d e p o s i t e d onto (iii) m o n o c r y s t a l l i n e Si w a f e r s or soda-lime glass substrates. The samples were i r r a d i a t e d with white light from a m e r c u r y lamp e q u i p p e d w i t h a glass bulb. The total intensity of i l l u m i n a t i o n was ii n W / c m 2 over the w a v e l e n g t h range of 400 - 760 nm. All the samples d e p o s i t e d on the silicon substrates were c h e m i c a l l y etched, just after irradiation, in 1 N NaOH solution for 30 seconds at ~20°C. This time was s u f f i c i e n t l y long to assure the complete etching of the u p p e r layer of the u n d o p e d As2S 3. The silicon plates with the p h o t o d i f f u s i o n p r o d u c t and the rest of the m e t a l l i c silver were then analyzed by an X-ray f l u o r e s c e n c e m e t h o d to d e t e r m i n e the q u a n t i t i e s of sulphur,
0022-3093/83/0000-0000/$03.00 © 1983 North-Holland
J. Pfocharski et al. / Photodiffusion kinetic studies
326
arsenic and silver. was used.
A VRA-20
(Carl Zeiss Jena)
type s p e c t r o m e t e r
The samples deposited on glass were a d d i t i o n a l l y equipped with electrical contacts to the silver layer. The electrical resistance of the metallic film was m e a s u r e d c o n t i n u o u s l y during the irradiation. This e n a b l e d the calculation of the changes in the m e t a l l i c silver film thickness according to the procedure reported by G o l d s c h m i d t and Rudman (6). 3.
RESULTS AND D I S C U S S I O N
It is well known that the product created during the irradiation of the two-layer system of Ag and As2S 3 is resistant to etching. Thus, the high etching rate for pure As2S 3 in, for instance, alkaline solutions can be exploited to remove the u n r e a c t e d chalcogenide. Since the analytical m e t h o d applied ensures the d e t e r m i n a t i o n of the total amount of S, As, Ag in any analyzed sample, one can expect that the u n i r r a d i a t e d samples will contain only silver. The irradiated samples will contain, in addition, varying amounts of sulphur and/or arsenic depending on the irradiation time. The o b s e r v e d As to S ratio should be c h a r a c t e r i s t i c of the p h o t o r e a c t i o n product stoichiometry. Figures i, 2 and 3 show the results of the d e t e r m i n a t i o n of sulphur, arsenic and silver contents, respectively, versus irradiation time and for different initial (directly evaporated) quantities of Ag. The c o n s i d e r a b l e d i s p e r s i o n of the m e a s u r e d values might indicate that the analytical method used was near the limits of its applicability and the m e t h o d of sample p r e p a r a t i o n was not s u f f i c i e n t l y r e p r o d u c i b l e due to the weak adhesion of the films. Despite this, a few clear conclusions can be d e r i v e d from these results.
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327
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J. PIocharski et aL / Photodiffusion kinetic studies
328
Ag content in the samples and the irradiation time. No deposited silver dissolves in the etching solution. This strongly indicates that the silver c o n c e n t r a t i o n in p h o t o d i f f u s e d samples exhibits a step-like depth d i s t r i b u t i o n (see the Appendix). //
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irradiation.
Summarizing, it can be stated that the product of the p h o t o r e a c t i o n of metallic Ag with amorphous As2S3 contains ca. 53 wt.% Ag and the As to S molar ratio is 1:1.5. Such a substance has a formula of Ag2.6As2S3. This is in good agreement with a suggestion by Janai (7) who assumed that a relatively stable amorphous phase is being created during the p h o t o d i f f u s i o n process. If one accepts this idea, the rate of m e t a l l i c silver consumption resulting from its c o n v e r s i o n into Ag2.6As2S3 can be estimated from the curves in Figs. l a n d 2. These values, calculated from the m a x i m u m slope of curves (i) and (2) from Figs. 1 and 2, are listed in Table i.
I Pl~ocharski et al. / Photodiffusion kinetic studies
329
It is w o r t h w h i l e to compare the silver c o n s u m p t i o n rate estimates based on the e l e m e n t a l analysis and s u p p l e m e n t e d by the above mentioned a s s u m p t i o n s for analogous values d e r i v e d from another m e t h o d of p h o t o d i f f u s i o n k i n e t i c testing. The m e t h o d d e s c r i b e d by G o l d s c h m i d t and Rudman (6) seems to be the most adequate for this purpose. The continuous m e a s u r e m e n t (during irradiation) of the sheet resistance of the m e t a l l i c silver film is the crucial p o i n t of this method; the r e s i s t i v i t y of m e t a l l i c Ag is several orders of m a g n i t u d e lower than that of both pure and A g - d o p e d As2S 3. The values o b t a i n e d are compared with the e x p e r i m e n t a l l y d e t e r m i n e d resistance vs Ag thickness curve. Figure 5 shows the effect of such a t r e a t m e n t for three A s 2 S 3 / A g samples d e p o s i t e d on glass substrates and i r r a d i a t e d through the c h a l c o g e n i d e layer. The rates of metallic Ag c o n s u m p t i o n c a l c u l a t e d from the m a x i m u m slope of the curves from Fig. 5 are listed on Table i. Table
1.
The
comparison
of the silver
Ag c o n s u m p t i o n rates calculated from elemental analysis (nm/min.)
consumption
rates.
Ag c o n s u m p t i o n rates calculated from resistance m e a s u r e m e n t s (nm/min.)
S analysis Fig. 1
curve (i) 5.8
curve (2) 3.3
curve Fig.
As analysis Fig. 2
curve (i) 5.9
curve (2) 4.3
3.7
(i) 5
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(2) 5
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(3 5
3.9
It can be seen that the rates of the p h o t o d i f f u s i o n process estimated by two s u b s t a n t i a l l y d i f f e r e n t m e t h o d s give fairly similar values and the o b s e r v e d deviations are within the limit of the e x p e r i m e n t a l error. All o b s e r v a t i o n s p r e s e n t e d in this paper agree with the p h o t o d i f f u s i o n models p r o p o s e d by Janai (7) and Yamaguchi, et al. (8). Janai believes that the a b s o r p t i o n of light in the m e t a l layer is the d r i v i n g force of photodiffusion. Silver atoms e x c i t e d in this process react w i t h As2S 3 and create a new relatively stable phase A g 5 A s 4 S 6. In o p p o s i t i o n to this idea, Yamaguchi et al. suggest that the a b s o r p t i o n of light in the c h a l c o g e n i d e layer plays the m a i n role, but only 20% of the total quantity of Ag atoms would be mobile under illumination. The remaining 80% are tightly bonded inside the c h a l c o g e n i d e lattice. The exact v e r i f i c a t i o n of the p h o t o d i f f u s i o n m o d e l s needs more work. We believe that the i n v e s t i g a t i o n of the t e m p e r a t u r e d e p e n d e n c e of the p h o t o d i f f u s i o n kinetics and the relation between the induction period and other p h o t o d i f f u s i o n p a r a m e t e r s w o u l d be fairly fruitful. 4.
CONCLUSIONS
(i) Chemical analysis indicates that the As:S ratio in the photor e a c t i o n p r o d u c t remains the same as in pristine As2S 3. (2) The o v e r a l l formula c o r r e s p o n d i n g to the reaction p r o d u c t can be e x p r e s s e d as A g 2 . 6 A s 2 S 3 and agrees w i t h the p o s s i b l e formation of an amorphous phase of the same stoichiometry. (3) The results p r e s e n t e d are consistent w i t h at least two different p h o t o d i f f u s i o n models (7,8), but more e x p e r i m e n t a l work is needed to e l u c i d a t e the detailed m e c h a n i s m of this phenomenon.
Z P2Ocharski et al. / Photodiffusion kinetic studies
330
Appendix: Etching rate of A g - d o p e d As2S 3 glass. It should be noted that the etching rate varies with the Ag content and its precise d e t e r m i n a t i o n could be helpful for i n t e r p r e t i n g some of the results p r e s e n t e d in this work. In order to estimate the etching rate, a set of bulk Ag-As2S 3 alloy samples was prepared. Known amounts of Ag and As2S 3 of 4N purity were melted and h o m o g e n i z e d in evacuated quartz ampoules and then quenched in air. Ingots were cut and polished and then part of the surface was masked with picien wax. The samples were etched in 1 N NaOH at %20°C and the height of each step created by the selective etching was m e a s u r e d by the stylus method. These results formed the basis of the etching rate calculations. The r e l a t i o n s h i p obtained is depicted in Fig. 6. All the prepared ingots were analyzed, by x-ray diffraction, also. Unfortunately, the samples whose Ag c o n c e n t r a t i o n exceeded ca. 20 wt.% were at least partially crystalline. Therefore, these results should be applied with caution to usually amorphous products of photodiffusion. However, one can assume that the chemical properties of amorphous phases do not differ s i g n i f i c a n t l y from those of crystalline phases.
~, l.Of° Fig. 6 Etching rate of A s 5 - A S bulk alloys in 1 ~3aOH solution at ~20oc.
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40
60
80
Ag CONCENTRATION (wt.%) References: i. 2. 3. 4. 5. 6. 7. 8.
Kostyshin, M.T., Mikhailovskaya, E.V., and Romanenko, P.F., Fiz. Tverd. Tela. 8/1966/571. Chang, M.S. and Chen, J.T., Appl. Phys. Lett., 33/1978/892 Kolwicz, K.D. and Chang, M.S., J. Electrochem. Soc., 127/1980/ 135. Yoshikawa, A., Ochi, 0., and Mizushima, Y., Appl. Phys. Lett., 36/1980/107. Yoshikawa, A., Hirota, S., Ochi, 0., Takeda, A., and Mizushima, Y., Jap. J. Appl. Phys., 20/1981/L81. Goldschmidt, D. and Rudman, P.S., J. Non-Cryst. Solids, 22/1976/ 229. Janai, M., Phys. Rev. Lett., 47/1981/726. Yamaguchi, M., Shimizu, I., and Inoue, E., J. Non-Cryst. Solids, 47/1982/341.
325
P H O T O D I F F U S I O N K I N E T I C STUDIES BY MEANS OF E L E M E N T A L ANALYSIS OF Ag DOPED As2S 3 GLASS Janusz
ul.
PXocharski,
Jan PrzyXuski,
Henryk
D e p a r t m e n t of C h e m i s t r y Warsaw Technical University N o a k o w s k i e g o 3, 00-664 Warszawa,
Wyci~lik
Poland
P h o t o d i f f u s i o n has been observed in light irradiated A s 2 S 3 / A g two layer samples. X-ray fluorescence analysis has been u t i l i z e d to d e t e r m i n e the chemical c o m p o s i t i o n of the p h o t o r e a c t i o n p r o d u c t giving the formula A g 2 . 6 A s 2 S 3. The values of the p h o t o r e a c t i o n rate e s t i m a t e d from the analytical results s a t i s f a c t o r i l y agree w i t h the analogous values d e r i v e d from the electrical r e s i s t a n c e measurements. The p h o t o d i f f u s i o n p h e n o m e n o n models are also discussed.
1.
INTRODUCTION
Since first reported (i), light e n h a n c e d reactions between some metals and amorphous c h a l c o g e n i d e semiconductors, called p h o t o d i f f u sion, p h o t o d o p i n g or p h o t o d i s s o l u t i o n , have been the subject of a c o n s i d e r a b l e number of papers. Two systems, A s 2 S 3 - A g (2,3) and G e x S e l - x - A g (4,5), have been i n v e s t i g a t e d w i t h emphasis on the p o t e n t i a l a p p l i c a t i o n of these m a t e r i a l s as inorganic resists for u l t r a - h i g h r e s o l u t i o n x-ray lithography. As a result, remarkable progress in inorganic resist technology has been achieved, but the p r o b l e m of the p h o t o d i f f u s i o n p h e n o m e n o n m e c h a n i s m still seems to be unsolved. To date, no e l e m e n t a l analysis of the p h o t o d i f f u s i o n products has ever been reported. X-ray f l u o r e s c e n c e is believed to be a convenient tool for the direct d e t e r m i n a t i o n of the chemical c o m p o s i t i o n of the products created upon p h o t o d i f f u s i o n and is expected to be helpful in the v e r i f i c a t i o n of the p h o t o d i f f u s i o n models. 2.
EXPERIMENTAL
Thin film samples were p r e p a r e d by a successive, quartz m o n i t o r controlled, e v a p o r a t i o n of Ag and As2S 3 (both of 4N purity) in an oil free high v a c u u m system (pressure of 10-4pa). Silver films (5 to 22 nm thick) and 350 nm thick As2S 3 films have been d e p o s i t e d onto (iii) m o n o c r y s t a l l i n e Si w a f e r s or soda-lime glass substrates. The samples were i r r a d i a t e d with white light from a m e r c u r y lamp e q u i p p e d w i t h a glass bulb. The total intensity of i l l u m i n a t i o n was ii n W / c m 2 over the w a v e l e n g t h range of 400 - 760 nm. All the samples d e p o s i t e d on the silicon substrates were c h e m i c a l l y etched, just after irradiation, in 1 N NaOH solution for 30 seconds at ~20°C. This time was s u f f i c i e n t l y long to assure the complete etching of the u p p e r layer of the u n d o p e d As2S 3. The silicon plates with the p h o t o d i f f u s i o n p r o d u c t and the rest of the m e t a l l i c silver were then analyzed by an X-ray f l u o r e s c e n c e m e t h o d to d e t e r m i n e the q u a n t i t i e s of sulphur,
0022-3093/83/0000-0000/$03.00 © 1983 North-Holland
J. Pfocharski et al. / Photodiffusion kinetic studies
326
arsenic and silver. was used.
A VRA-20
(Carl Zeiss Jena)
type s p e c t r o m e t e r
The samples deposited on glass were a d d i t i o n a l l y equipped with electrical contacts to the silver layer. The electrical resistance of the metallic film was m e a s u r e d c o n t i n u o u s l y during the irradiation. This e n a b l e d the calculation of the changes in the m e t a l l i c silver film thickness according to the procedure reported by G o l d s c h m i d t and Rudman (6). 3.
RESULTS AND D I S C U S S I O N
It is well known that the product created during the irradiation of the two-layer system of Ag and As2S 3 is resistant to etching. Thus, the high etching rate for pure As2S 3 in, for instance, alkaline solutions can be exploited to remove the u n r e a c t e d chalcogenide. Since the analytical m e t h o d applied ensures the d e t e r m i n a t i o n of the total amount of S, As, Ag in any analyzed sample, one can expect that the u n i r r a d i a t e d samples will contain only silver. The irradiated samples will contain, in addition, varying amounts of sulphur and/or arsenic depending on the irradiation time. The o b s e r v e d As to S ratio should be c h a r a c t e r i s t i c of the p h o t o r e a c t i o n product stoichiometry. Figures i, 2 and 3 show the results of the d e t e r m i n a t i o n of sulphur, arsenic and silver contents, respectively, versus irradiation time and for different initial (directly evaporated) quantities of Ag. The c o n s i d e r a b l e d i s p e r s i o n of the m e a s u r e d values might indicate that the analytical method used was near the limits of its applicability and the m e t h o d of sample p r e p a r a t i o n was not s u f f i c i e n t l y r e p r o d u c i b l e due to the weak adhesion of the films. Despite this, a few clear conclusions can be d e r i v e d from these results.
/
.
.
.
.
v
/"/f"
/-"
0
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V/ 7
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IRRADIATION
1'2"1 ; o ' TIME
6b
(mi n.)
Fig. 1 Sulphur amount in irradiated and etched samples. a m o u n t s o f A g a r e - - + - - 5 . ~ , - - - o - - - 1 1 a n d -....7--.22 ) z g / c m 2 .
Vacuum deposited
327
J. Pfocharski et aL / Photodiffusion kinetic studies
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4
6
a
lb
i'2
IRRADIATION
TIME
30
60
(min.)
Fig. 2 Arsenic amount in irradiated and etched samples.
V a c u u m deposited
a m o u n t s of Ag a r e - - + - - 5 . g , .... o---11 and ..-.,-..~7---.22 u g / c m 2.
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IRRADIATION TIME (min.) Fig.3 Silver amount in irradiated and etched samples. Lines , ....... and ............correspond .. to v a c u u m deposited amounts of A g and points ~ , o and 7 m a r k the analytical results, r e s p e c t i v e l y . i) The m o l a r r a t i o of As to S in the p h o t o r e a c t i o n p r o d u c t seems to be the same as in p r i s t i n e A s 2 S 3, at l e a s t for t h i c k e r l a y e r s of the p r o d u c t . F i g u r e 4 shows the v a l u e s of this r a t i o c a l c u l a t e d f r o m the r e s u l t s p r e s e n t e d in Figs. 1 a n d 2. ii) The a m o u n t s of S and As c o m b i n e d w i t h the same a m o u n t of silver r e m a i n s c o n s t a n t for long i r r a d i a t i o n times. The Ag c o n c e n t r a t i o n in the s a t u r a t e d s a m p l e s d o e s n o t d e p e n d on the i n i t i a l t h i c k ness of the s i l v e r f i l m and e q u a l s ca. 53 wt%. iii)
As
can
be seen
in Fig,
3, there
is no c o r r e l a t i o n
between
the
J. PIocharski et aL / Photodiffusion kinetic studies
328
Ag content in the samples and the irradiation time. No deposited silver dissolves in the etching solution. This strongly indicates that the silver c o n c e n t r a t i o n in p h o t o d i f f u s e d samples exhibits a step-like depth d i s t r i b u t i o n (see the Appendix). //
//
+
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o
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7
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i
i
4,
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. .8 . . 10 . .
12 /~t
IRRADIATION TIME
30
'
60
:
(rain.)
Fig.~ Sulphur to arsenic molar ratio in irradiated and etched samples. V a c u u m deposited amounts of A s w e r e + 5.4, o ii and V 22}lg/cm z.
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.
.
.
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.
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.4
.6
.8
1.0
1.2
IRRADIATION TIME
1.4
Fig.5 Change in silver film thickness c a l c u l a t e d surements d u r i n g
1.6
1.8
(rain.)
from resistance mea-
irradiation.
Summarizing, it can be stated that the product of the p h o t o r e a c t i o n of metallic Ag with amorphous As2S3 contains ca. 53 wt.% Ag and the As to S molar ratio is 1:1.5. Such a substance has a formula of Ag2.6As2S3. This is in good agreement with a suggestion by Janai (7) who assumed that a relatively stable amorphous phase is being created during the p h o t o d i f f u s i o n process. If one accepts this idea, the rate of m e t a l l i c silver consumption resulting from its c o n v e r s i o n into Ag2.6As2S3 can be estimated from the curves in Figs. l a n d 2. These values, calculated from the m a x i m u m slope of curves (i) and (2) from Figs. 1 and 2, are listed in Table i.
I Pl~ocharski et al. / Photodiffusion kinetic studies
329
It is w o r t h w h i l e to compare the silver c o n s u m p t i o n rate estimates based on the e l e m e n t a l analysis and s u p p l e m e n t e d by the above mentioned a s s u m p t i o n s for analogous values d e r i v e d from another m e t h o d of p h o t o d i f f u s i o n k i n e t i c testing. The m e t h o d d e s c r i b e d by G o l d s c h m i d t and Rudman (6) seems to be the most adequate for this purpose. The continuous m e a s u r e m e n t (during irradiation) of the sheet resistance of the m e t a l l i c silver film is the crucial p o i n t of this method; the r e s i s t i v i t y of m e t a l l i c Ag is several orders of m a g n i t u d e lower than that of both pure and A g - d o p e d As2S 3. The values o b t a i n e d are compared with the e x p e r i m e n t a l l y d e t e r m i n e d resistance vs Ag thickness curve. Figure 5 shows the effect of such a t r e a t m e n t for three A s 2 S 3 / A g samples d e p o s i t e d on glass substrates and i r r a d i a t e d through the c h a l c o g e n i d e layer. The rates of metallic Ag c o n s u m p t i o n c a l c u l a t e d from the m a x i m u m slope of the curves from Fig. 5 are listed on Table i. Table
1.
The
comparison
of the silver
Ag c o n s u m p t i o n rates calculated from elemental analysis (nm/min.)
consumption
rates.
Ag c o n s u m p t i o n rates calculated from resistance m e a s u r e m e n t s (nm/min.)
S analysis Fig. 1
curve (i) 5.8
curve (2) 3.3
curve Fig.
As analysis Fig. 2
curve (i) 5.9
curve (2) 4.3
3.7
(i) 5
curve Fig. 4.4
(2) 5
curve Fig.
(3 5
3.9
It can be seen that the rates of the p h o t o d i f f u s i o n process estimated by two s u b s t a n t i a l l y d i f f e r e n t m e t h o d s give fairly similar values and the o b s e r v e d deviations are within the limit of the e x p e r i m e n t a l error. All o b s e r v a t i o n s p r e s e n t e d in this paper agree with the p h o t o d i f f u s i o n models p r o p o s e d by Janai (7) and Yamaguchi, et al. (8). Janai believes that the a b s o r p t i o n of light in the m e t a l layer is the d r i v i n g force of photodiffusion. Silver atoms e x c i t e d in this process react w i t h As2S 3 and create a new relatively stable phase A g 5 A s 4 S 6. In o p p o s i t i o n to this idea, Yamaguchi et al. suggest that the a b s o r p t i o n of light in the c h a l c o g e n i d e layer plays the m a i n role, but only 20% of the total quantity of Ag atoms would be mobile under illumination. The remaining 80% are tightly bonded inside the c h a l c o g e n i d e lattice. The exact v e r i f i c a t i o n of the p h o t o d i f f u s i o n m o d e l s needs more work. We believe that the i n v e s t i g a t i o n of the t e m p e r a t u r e d e p e n d e n c e of the p h o t o d i f f u s i o n kinetics and the relation between the induction period and other p h o t o d i f f u s i o n p a r a m e t e r s w o u l d be fairly fruitful. 4.
CONCLUSIONS
(i) Chemical analysis indicates that the As:S ratio in the photor e a c t i o n p r o d u c t remains the same as in pristine As2S 3. (2) The o v e r a l l formula c o r r e s p o n d i n g to the reaction p r o d u c t can be e x p r e s s e d as A g 2 . 6 A s 2 S 3 and agrees w i t h the p o s s i b l e formation of an amorphous phase of the same stoichiometry. (3) The results p r e s e n t e d are consistent w i t h at least two different p h o t o d i f f u s i o n models (7,8), but more e x p e r i m e n t a l work is needed to e l u c i d a t e the detailed m e c h a n i s m of this phenomenon.
Z P2Ocharski et al. / Photodiffusion kinetic studies
330
Appendix: Etching rate of A g - d o p e d As2S 3 glass. It should be noted that the etching rate varies with the Ag content and its precise d e t e r m i n a t i o n could be helpful for i n t e r p r e t i n g some of the results p r e s e n t e d in this work. In order to estimate the etching rate, a set of bulk Ag-As2S 3 alloy samples was prepared. Known amounts of Ag and As2S 3 of 4N purity were melted and h o m o g e n i z e d in evacuated quartz ampoules and then quenched in air. Ingots were cut and polished and then part of the surface was masked with picien wax. The samples were etched in 1 N NaOH at %20°C and the height of each step created by the selective etching was m e a s u r e d by the stylus method. These results formed the basis of the etching rate calculations. The r e l a t i o n s h i p obtained is depicted in Fig. 6. All the prepared ingots were analyzed, by x-ray diffraction, also. Unfortunately, the samples whose Ag c o n c e n t r a t i o n exceeded ca. 20 wt.% were at least partially crystalline. Therefore, these results should be applied with caution to usually amorphous products of photodiffusion. However, one can assume that the chemical properties of amorphous phases do not differ s i g n i f i c a n t l y from those of crystalline phases.
~, l.Of° Fig. 6 Etching rate of A s 5 - A S bulk alloys in 1 ~3aOH solution at ~20oc.
LU
< ¢v
%
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,e~.....~,
.2
e-¢
•
,
U #.U
0
20
40
60
80
Ag CONCENTRATION (wt.%) References: i. 2. 3. 4. 5. 6. 7. 8.
Kostyshin, M.T., Mikhailovskaya, E.V., and Romanenko, P.F., Fiz. Tverd. Tela. 8/1966/571. Chang, M.S. and Chen, J.T., Appl. Phys. Lett., 33/1978/892 Kolwicz, K.D. and Chang, M.S., J. Electrochem. Soc., 127/1980/ 135. Yoshikawa, A., Ochi, 0., and Mizushima, Y., Appl. Phys. Lett., 36/1980/107. Yoshikawa, A., Hirota, S., Ochi, 0., Takeda, A., and Mizushima, Y., Jap. J. Appl. Phys., 20/1981/L81. Goldschmidt, D. and Rudman, P.S., J. Non-Cryst. Solids, 22/1976/ 229. Janai, M., Phys. Rev. Lett., 47/1981/726. Yamaguchi, M., Shimizu, I., and Inoue, E., J. Non-Cryst. Solids, 47/1982/341.