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Compositional dependence of photodissolution kinetics in amorphous AsS films
Journal of Non-Crystalline Solids 97&98 (1987) 1127-1130 North-Holland, Amsterdam.
1127
COMPOSITIONAL DEPI~DI~CE OF PHOTODISSOLUTICN KINETICS IN AMORPHOUS AS-S FILMS
P.J.S. EWH~, A. ZAKERY, A.P. FIRTH and A.E. OW~q
Department of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 3JL, U.K.
The kinetics of the photedissolution of Ag into amorphous As-S films of various• compositions in the range As tit) .~S.~ to. As~^S^^ has been investigated . t) Z U • . . by monltorlng the changes that eecur in ~helr re~l~tlvlty during the photodissolution process. It was found that as the S content was increased above 60 at.% the rate of photedissolution increased to a maximum around A~s~St_ and then decreased, the maximum rate being approximately J ~/ dou6]e that for As~^S^^. This is attributed to the fact that only ZU 15 . composltlons wlthxn a ~ew atomic percent of As33S67 yleld a homogeneous material when photodoped. .
.
.
.
i. INTRODUCTION The metal photedissolution effect that occurs in chalcogenide glasses is currently of interest because of its potential applications in holography and VLSI lithography I.
A knowledge of the kinetics of the effect is essential to
understanding the basic mechanism and also to exploiting its technological potential.
This paper is specifically concerned with the photodissolution of
Ag into As-S films and describes how the kinetics, measured using an optical reflectivity technique, depend on the As-S composition.
2. EXP~IMENTAL Samples were prepared by evaporating onto cleaned glass substrates first a layer of Ag and then a layer of the desired As-S cGmposition, there being no break in the vacuu~ (2x10 -5 tort) between evaporations.
The As-S
evaporation sources were powdered melt-quenched glasses and electron microprobe analysis indicated that the stated film compositions are correct to +0.5 at.%. To isolate the effect of ecfnposition on the kinetics, film thicknesses were kept constant from sample to sample at 4000 ~ and 1500 ~ for the As-S and Ag layers respectively. The photodissolution rate was measured using a technique, described elsewhere 2 , based on meni£oring the changes that occur in the reflectivity of the sample as photodissolution proceeds.
Figure 1 shows schematically the
multi-layer structure of the sample during photodissolution: there is a well-defined boundary between the undoped As-S and the photodoped material (which is generally highly absorbing) and this propagates towards the top
0022-3093/87/$03.50 © E l s e v i e r S c i e n c e Publishers B . V . (North-Holland Physics Publishing Division)
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
1128
surface, so that the thickness of the V /
undoped As-S decreases with time. REFLECII~
ILII~INATION
LIGHT Mow~
A
~
Because the reflectivity of a thin, weakly absorbing film varies
~-As-S GLASS periodically with its thickness (as a result of interference between light reflected from the top and bottom - SUBS~,A'I'E
FI~
1. schematic c r o s s - s e c t i o n
through a sample during the photodis sel ution process
surfaces) oscillations are observed in the reflectivity (see Figure 3) and these can easily he analysed to obtain the photodoped layer thickness as a function of time.
The rate is
determined from a plot of thickness against time or square root of t/me, depending on whether the process is reaction- or diffusion-controlled respectively. In these experiments broadband illuatination from a tungsten lamp was focussed through a microscope objective onto the sample and the reflected light was collected, passed through a monochranator, and detected by a photodiode coupled to a phase-sensitive detection system.
The incident
intensity was approximately 75 mW/cm 2 . Under these conditions the photodoped layer thickness varied linearly with the square root of time.
3. RESULTS AND DISCUSSION The variation of photodissolution rate with As-S composition is shown in Figure 2 (filled circles), the error bars corresponding to a measured spread of +5% in the rates. As the S content is increased above 60 at.% the rate increases to a maximum around As33S67 and then falls off, the m a x i m a and m i n i m a rates differing by a factor of about 2. Recent IDQAFS measurements we have made confirm the conclusion of an earlier Raman study 3
that only As-S
compositions within about +3 at.% of As33S67 yield a hcmogeneous material when photodeped with Ag, compositions outside this range yielding a phase-separated structure.
The peak in the photodissolution rate may be due to
this change in the morphology of the p/~otedoped material since the photodissolution process involves the transport of electrons as well as ions and this may be slower through phase-separated regions due to the presence of phase boundaries and additional defects. It is relevant to note that the amount of Ag that can be. incorporated into an As-S glass is a minimt~n at As33S674 and this c a n a l s o be explained by the change in morphology, since in the phase-separated material additional silver can be a c ~ a t e d
at the phase
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
.\ \
/ .
\
300
1129
'T
\
/ /
r'r"
2
20O
c)
\
oD
/ \
100
C3
/ \ /
(D z Q_ 0
50
60
70
80
90
GLRSS COMPOSITION (RT. X $) FIGURE 2. Photodissolution rate (.) and optical density (m) v. As-S composition.
boundaries.
The open squares in Figure 2 are taken from Reference 4 and show
as a function of composition the variation in the optical density, D, of an As-S film dipped in AgNO 3 relative to the minimu~ value observed, Dmi n, which was for As33S6T The optical density is a measure of the amount of Ag that can be taken up by the As-S film. We have also examined how the compositional dependence of the photodissolution rate is affected by the wavelength of illumination.
In addition to
using a tungsten lamp, which is predominantly a red/IR source, we have also obtained
results using a mercury lamp (providing broadband illumination out to
the UV) and the 5145 and 4880 ~ Ar laser lines.
These results snow a peak at
As34S66 together with additional structure that is wavelength dependent. The rate will be influenced by variations in the optical constants of the materials with composition and at short wavelengths these variations may be more pronounced or the photodissolution process may be more sensitive to th~n. In addition, the nature of the time-dependence of the photodissolution process may also be influenced by the optical constants: in the case of the mercury source, which provided light of wavelength down to 380 nm (allowing for absorption in the components in the optical system), the photodoped layer thickness did not vary linearly with time or square root of time but had a super-linear dependence. Figure 3 shows a typical reflectivity curve (obtained for As34S66) from these e x p e r ~ t s
and the corresponding plot of photo-
doped layer thickness as a function of time. clearly, the rate increases with
I 13 0
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
O. 35
,
,
,
~
~^ 0C
>. 4J > O 0
~-g
.-4
q@
C
2
0. I 0
5
10
TIME
15
20
(seconds)
FIGURE 3. Reflectivity and photodoped layer thickness v. time (mercury source)
time, which would be expected if absorption of the actinic radiation occurs at the interface between the undoped and photodoped material5: as the [IV component of the mercury illumination will be strongly absorbed by the undoped As-S layer, the amount reaching this interface will be very sensitive to the thickness of the As-S layer and so will increase as the AS-S layer thickness decreases. As more light reaches the interface the process will speed up. Once the optical constants of the doped and undoped materials are known and it is definitely established where the actinic radiation is being absorbed it will be possible to correct the measured rates for the composition-dependent changes in the optical constants of the doped and undoped layers. REFERENCES i) K.L. Tai, E. Ong and R.G. Vadimsky, Proc. Electroch~n. Soc. 82-89 (1982) 9. 2) A.P. Firth, P.J.S. Ewen and A.E. Owen, J. Non-Cryst. Sol. 77-78 (1985) 1153. 3) A.P. Firth, P.J.S. E~en and A.E. Owen, in: The Structure of Non-Crystalline Materials, ]982, eds. P.H. Gaskell, J.M. Parker and E.A. Davis (Taylor and Francis, London, 1983) pp. 286-293. 4) S. Petrova, P. Simidchieva and A. Buroff, in: Proceedings of the Conference "Amorphous Semiconductors - 84", eds. E. Fahri-Vateva and A. Buroff (Bulgarian Academy of Sciences, Sofia, 1984) pp. 256-258. 5) A.E. Owen, A.P. Firth and P.J.S. Ewen, Phil. ~ g . B52 (1985) 347.
1127
COMPOSITIONAL DEPI~DI~CE OF PHOTODISSOLUTICN KINETICS IN AMORPHOUS AS-S FILMS
P.J.S. EWH~, A. ZAKERY, A.P. FIRTH and A.E. OW~q
Department of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 3JL, U.K.
The kinetics of the photedissolution of Ag into amorphous As-S films of various• compositions in the range As tit) .~S.~ to. As~^S^^ has been investigated . t) Z U • . . by monltorlng the changes that eecur in ~helr re~l~tlvlty during the photodissolution process. It was found that as the S content was increased above 60 at.% the rate of photedissolution increased to a maximum around A~s~St_ and then decreased, the maximum rate being approximately J ~/ dou6]e that for As~^S^^. This is attributed to the fact that only ZU 15 . composltlons wlthxn a ~ew atomic percent of As33S67 yleld a homogeneous material when photodoped. .
.
.
.
i. INTRODUCTION The metal photedissolution effect that occurs in chalcogenide glasses is currently of interest because of its potential applications in holography and VLSI lithography I.
A knowledge of the kinetics of the effect is essential to
understanding the basic mechanism and also to exploiting its technological potential.
This paper is specifically concerned with the photodissolution of
Ag into As-S films and describes how the kinetics, measured using an optical reflectivity technique, depend on the As-S composition.
2. EXP~IMENTAL Samples were prepared by evaporating onto cleaned glass substrates first a layer of Ag and then a layer of the desired As-S cGmposition, there being no break in the vacuu~ (2x10 -5 tort) between evaporations.
The As-S
evaporation sources were powdered melt-quenched glasses and electron microprobe analysis indicated that the stated film compositions are correct to +0.5 at.%. To isolate the effect of ecfnposition on the kinetics, film thicknesses were kept constant from sample to sample at 4000 ~ and 1500 ~ for the As-S and Ag layers respectively. The photodissolution rate was measured using a technique, described elsewhere 2 , based on meni£oring the changes that occur in the reflectivity of the sample as photodissolution proceeds.
Figure 1 shows schematically the
multi-layer structure of the sample during photodissolution: there is a well-defined boundary between the undoped As-S and the photodoped material (which is generally highly absorbing) and this propagates towards the top
0022-3093/87/$03.50 © E l s e v i e r S c i e n c e Publishers B . V . (North-Holland Physics Publishing Division)
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
1128
surface, so that the thickness of the V /
undoped As-S decreases with time. REFLECII~
ILII~INATION
LIGHT Mow~
A
~
Because the reflectivity of a thin, weakly absorbing film varies
~-As-S GLASS periodically with its thickness (as a result of interference between light reflected from the top and bottom - SUBS~,A'I'E
FI~
1. schematic c r o s s - s e c t i o n
through a sample during the photodis sel ution process
surfaces) oscillations are observed in the reflectivity (see Figure 3) and these can easily he analysed to obtain the photodoped layer thickness as a function of time.
The rate is
determined from a plot of thickness against time or square root of t/me, depending on whether the process is reaction- or diffusion-controlled respectively. In these experiments broadband illuatination from a tungsten lamp was focussed through a microscope objective onto the sample and the reflected light was collected, passed through a monochranator, and detected by a photodiode coupled to a phase-sensitive detection system.
The incident
intensity was approximately 75 mW/cm 2 . Under these conditions the photodoped layer thickness varied linearly with the square root of time.
3. RESULTS AND DISCUSSION The variation of photodissolution rate with As-S composition is shown in Figure 2 (filled circles), the error bars corresponding to a measured spread of +5% in the rates. As the S content is increased above 60 at.% the rate increases to a maximum around As33S67 and then falls off, the m a x i m a and m i n i m a rates differing by a factor of about 2. Recent IDQAFS measurements we have made confirm the conclusion of an earlier Raman study 3
that only As-S
compositions within about +3 at.% of As33S67 yield a hcmogeneous material when photodeped with Ag, compositions outside this range yielding a phase-separated structure.
The peak in the photodissolution rate may be due to
this change in the morphology of the p/~otedoped material since the photodissolution process involves the transport of electrons as well as ions and this may be slower through phase-separated regions due to the presence of phase boundaries and additional defects. It is relevant to note that the amount of Ag that can be. incorporated into an As-S glass is a minimt~n at As33S674 and this c a n a l s o be explained by the change in morphology, since in the phase-separated material additional silver can be a c ~ a t e d
at the phase
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
.\ \
/ .
\
300
1129
'T
\
/ /
r'r"
2
20O
c)
\
oD
/ \
100
C3
/ \ /
(D z Q_ 0
50
60
70
80
90
GLRSS COMPOSITION (RT. X $) FIGURE 2. Photodissolution rate (.) and optical density (m) v. As-S composition.
boundaries.
The open squares in Figure 2 are taken from Reference 4 and show
as a function of composition the variation in the optical density, D, of an As-S film dipped in AgNO 3 relative to the minimu~ value observed, Dmi n, which was for As33S6T The optical density is a measure of the amount of Ag that can be taken up by the As-S film. We have also examined how the compositional dependence of the photodissolution rate is affected by the wavelength of illumination.
In addition to
using a tungsten lamp, which is predominantly a red/IR source, we have also obtained
results using a mercury lamp (providing broadband illumination out to
the UV) and the 5145 and 4880 ~ Ar laser lines.
These results snow a peak at
As34S66 together with additional structure that is wavelength dependent. The rate will be influenced by variations in the optical constants of the materials with composition and at short wavelengths these variations may be more pronounced or the photodissolution process may be more sensitive to th~n. In addition, the nature of the time-dependence of the photodissolution process may also be influenced by the optical constants: in the case of the mercury source, which provided light of wavelength down to 380 nm (allowing for absorption in the components in the optical system), the photodoped layer thickness did not vary linearly with time or square root of time but had a super-linear dependence. Figure 3 shows a typical reflectivity curve (obtained for As34S66) from these e x p e r ~ t s
and the corresponding plot of photo-
doped layer thickness as a function of time. clearly, the rate increases with
I 13 0
P.J.S. Ewen et al. / Compositional dependence of photodissolution kinetics
O. 35
,
,
,
~
~^ 0C
>. 4J > O 0
~-g
.-4
q@
C
2
0. I 0
5
10
TIME
15
20
(seconds)
FIGURE 3. Reflectivity and photodoped layer thickness v. time (mercury source)
time, which would be expected if absorption of the actinic radiation occurs at the interface between the undoped and photodoped material5: as the [IV component of the mercury illumination will be strongly absorbed by the undoped As-S layer, the amount reaching this interface will be very sensitive to the thickness of the As-S layer and so will increase as the AS-S layer thickness decreases. As more light reaches the interface the process will speed up. Once the optical constants of the doped and undoped materials are known and it is definitely established where the actinic radiation is being absorbed it will be possible to correct the measured rates for the composition-dependent changes in the optical constants of the doped and undoped layers. REFERENCES i) K.L. Tai, E. Ong and R.G. Vadimsky, Proc. Electroch~n. Soc. 82-89 (1982) 9. 2) A.P. Firth, P.J.S. Ewen and A.E. Owen, J. Non-Cryst. Sol. 77-78 (1985) 1153. 3) A.P. Firth, P.J.S. E~en and A.E. Owen, in: The Structure of Non-Crystalline Materials, ]982, eds. P.H. Gaskell, J.M. Parker and E.A. Davis (Taylor and Francis, London, 1983) pp. 286-293. 4) S. Petrova, P. Simidchieva and A. Buroff, in: Proceedings of the Conference "Amorphous Semiconductors - 84", eds. E. Fahri-Vateva and A. Buroff (Bulgarian Academy of Sciences, Sofia, 1984) pp. 256-258. 5) A.E. Owen, A.P. Firth and P.J.S. Ewen, Phil. ~ g . B52 (1985) 347.