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Discussion on the mechanism of photodoping
Journal of Non-Crystalline Solids 20 (1976) 131 - 139 North-Holland Publishing Company
DISCUSSION ON THE MECHANISM OF PHOTODOPING H. KOKADO, I. SHIMIZU and E. INOUE Imaging Science and £ngineering Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama, Japan Received 10 February 1975
The photo-enhanced reaction between metallic silver and vitreous chalcogenides is known as "photodoping". Based on a series of expeiimental results, a model for photodoping was proposed. It was assumed in this model, that a junction barrier at the silverchalcogenide interface worked for separating photocarriers. Holes are captured by raetallic silver, and electrons are trapped by active or loosely bound chalcogen atoms after travel toward the interior of a glass layer. The Coulomb attraction field between ions thus formed is large enough to overcome the kinetic barrier in the process of silver diffusion. The square root dependence for the growth of the photodoped depth with exposure time has been explained in the light of this proposed model.
I. Introduction
Photodoping [ 1] is a photo-enhanced reaction of metals (e.g. silver) with chaicogenide glasses (e.g. As2S3). Since there is chemical affinity between metals and chalcogenides, the essential step in the process should be a photo-elimination of the kinetic barrier preventing the reaction in the dark. In a recent publication [2], deNeufville et al. suggested that localized electronic defects, including trapped charges and broken bonds, are possibly connected to this step. The present authors agree with their opinion, but feel that the driving force for the mass ~.~ansport has to be looked for since the photodoped depth sometimes exceeds 20/am [3]. This paper discusses the mechanism of photodoping with an emphasis on the electronic process at the metal-chalcogenide interface. A model is proposed, based upon the assumption of an interfacial barrier potential that was in,['erred in ref. [4], to be formed between silver and chalcogenide [4]. Some important experimental facts concerning photodoping are reviewed in ~ection 2. In section 3, it is mentioned that the same type of photoresponse as reported in ref. [4] was also observed in a heterojunc.tion of two chalcogenide glasses. In ~ection 4 the mechanism of photodoping is discussed. 131
H. Kokado et aL / Mechanism oj'photodoping
132
2, Observations concerning photodoping
2.1. Spectral sensitivity In order that a distinct amount of photodoping is observed, the band-gap excitation is required at the metal-chalcogenide interface. For a quantitative investigation on this point, the spectral sensitivity in a Ag-As2S 6 glass system was measured using a spectrograph equipped with a logarithmic slit. Results are shown in fig. I. When samples were illuminated from the As2S6 layer side, the shorter wavelength limit of sensitivity varied with the thickness of the As2S6 layer. At photon energies greater than the band-gap energy, the optical absorption coefficient a in As2S6 saris. ties a relation
[K(hv- Eg)2/hv],
(1)
which is varied for an indirect transition. Here K is a constant, hv is the energy of photons and Eg is the energy gap. Let a particular light intensity at the interface be lmin that yields a minimum amount of detectable photodoping. Assuming a photodoping efficiency independent of the wavelength of excitation, the sensitivity limit by* will be determined by the following expression:
/rain
=
!0 exp [-K(hv* -
(2)
Eg)2d/hv*],
where I 0 denotes the incident light intensity of the samples and d is the thickness of the As2S 6 layers. Since/min is considered to be constant in a series of measureu
d"
500A
]400 A" 2,100
3000/, ,A 1
5000 A -
*
.
•
t
._AkA
340 ,]40 WAVELENGTH (nm)
I
,,
540
Fig. 1. Spectral photosensitivity of the Ag-As2 $6 photodoping system, when illuminated through As2S6 layers of thickness d. Multiple peaks seen in thick layers are due to the inter° ference effect and are not essential.
1t. Kokado et al. / Mechanism of photodoping
133
O
;5 e..-,.i 11(
3 !
!
i,
3.0
2.8
i,
3.2
!
3.4
3.6
h~ (eV)
Fig. 2. (h~,*/d) lr~ versus photon energy, at short-wavelength limit of photosensitivity from fig. 1.
ments,
(3)
by* = (hv*/d)l/2
Energies hv* were read from fig. 1. Relation (3) was confirmed to hold, as demonstrated in fig. 2. As photodoping proceeds, a layer of silver.doped chalcogenide grows, and the absorption by the whole layer turns out to be extended toward longer wavelengths [5]. It will be interesting to know whether or not photons absorbed by the resultant silver-doped chalcogenide are effective for further photodoping. In an experiment in which Ag-As2S 6 samples were illuminated from the As2S 6 layer side, the i
15 min
60 min [
340
440 WAVELENGTH(nm)
180 min
540
Fig. 3. Shift of photosensitivity maximt~m (arrow) in the Ag-As2S 6 system with exposure time. Sensitivity scale for the lower two spectrograms are reduced.
134
H. Kokado et al. ~Mechanismof photodoping
spectral sensitivity maximum was found to shift toward longer wavelengths with increasing exposure, as shown in fig. 3. The silver-doped chalcogenide is to be found mainly in the blackened areas in fig. 3. But photodoping also takes effect in the very outside of the contours to a visually undetectable degree. The observed extension of spectral sensitivity is presumably brought about by such a lightly doped part with an increased absorbance for longer wavelength light. It is suggested, therefore, that photons absorbed by silver-doped chalcogenide contribute at least partly to photodoping. Occasionally, photodoping is observed with light of a far longer wavelength than the absorption edge of the chalcogenide used. For instance, holograms could be recorded on Ag-As2S 3 samples by light from a He-Ne laser [6]. Since the absorption edge of As2S 3 is located at about 500 nm, the absorption at 632.8 nm must be associated with some localized levels in As2S 3. The rate of photodoping, however, was extremely slow compared to that under band-gap illumination.
2.2. Rate o f photodoping In an experiment with the Ag-As2S9.6 system [3], it was found that the photo. doping rate was very fast at the initial stage but then slowed down, suggesting a change in the rate-determining step° The initial fast rate was proportional to the light intensity and its activation energy was as i'ow as 0.033 eV in the examined temperature range ( - 2 0 0 - + 20°C). The fast process was taken over by the slow process after a definite thickness (~-,60 A) of silve~rwas doped in, irrespective of light intensity. The fast process probably implies a photo-enhanced surface reaction and the slow process a diffusion.controlled one. The fast surface reaction mea,l,ured in a series of As2Sn glasses (2.6 < n ~< 18) attained the highest rate at n ~ 10. Tsuchihashi and Kawamoto [7] have found that the concentration of S-S bonds in the glass network becomes highest in the composition around this, since the excessive sulfur tends to separate S8 rings. Weakly bound sulfur atoms such as those forming S-S bonds may provide a good base for stabilization of diffused silver and theref~re play a significant role in the mechanism of photodoping,
2.3. Depth and states o f photodoped silver According to an examination with an electron-proof X-ray microanalyzer (EMX), photodoped silver distributes along the depth in an almost constant density. For instance, a well-defined boundary was observed at 21.6 pm from the illuminated surface, in a bulk sample of AsI6SsoTe4 in which silver of 1500 A was photodoped [3]. The growth of the doping depth was proportional to the square root of the exposure time [8]. These results emphasize the importance of material diffusion through the photodoped part of the glass. Kostyshin and Romanenko [9] mentioned that silver can diffuse into the in-
H. Kokado et al. / Mechanism o f photodoping
135
terior through the crystalline c~-Ag2Sphase segregated during photodoping. We have investigated this possibility bat found no proof that it occurs. The examination was carried out on glasses with a variety of compositions. Only when silver photodoped samples were heated for a few minutes at 150-200°C, an unidentified but crystalline X-ray aiffraction pattern appeared. The detailed description will be published elsewhere.
3. Electronic potential at the interface In order to know whether photodoping occurs in a particular direction or not, a sample was prepared in a structure as demonstrated in fig. 4. Silver was evaporated on a substrate in stripes, each 100/~m wide and covered with ASl6S80Te 4 glass. The distribution of photodoped silver was surveyed by means of EMX along the direction perpendicular to the surface. For comparison, a thermally doped sample was examined in the same manner. An apparent difference in the doped silver distribution was found as represented in fig. 4. In photodoping, silver preferred to diffuse perpendicularly to the illuminated surface, while thermally the diffusion exhibited no particular directionality. One possible explanation for this directionality is to assume a participation of the directional field, such as an electrical field, in photodoping. In other words, the mechanism of photodoping presumably includes a photo-electrical effect at the surface. Another suggestion on the participation of the electronic process may be made from the following test. Separate films of silver and chalcogenide glass were
l
IO0~L ! AS~6$1oIe4 l-- /Ag .itOi \\\\\\\\\\ ,\'~ substrate as-deposited
T
photo-doped
v ,..I Oft
thermally doped '
2
'
4
'
6
Fig. 4. Distributions of diffused silver in photodoped and thermally doped samples. The structure of the sample is also shown (top).
136
H. Kokado et aL ] Mechanism of photodoping
brought into contact and the air in the gap was evacuated to insure that contact was as close as possible. No photodoping was possible in such a sample. However, when a chalcogenide layer with a very thin overlayer (50 A) of evaporated silver was in contact with another silver layer, then even the latter layer was lost by photodoping. For photodoping to occur, it seems that the contact has to be close enough to allow atomic interactions involving formation of a junction barrier. In ref. [4] which dealt with photo-electrical effects in Ag-As2S3-AI cells, a potential with a deep valley (see fig. 7) was proposed to be formed at the interface between silver and chalcogenide. Short-circuit photocurrents had a component of spiky polarization current which attributed to the interfacial junction. The junction can be looked upon as a sort of heterojunction between silverdoped and undoped parts of As2S 3, though it may be incomplete*. Therefore, a heterojunction was deliberately prepared with two different chalcogenide glasses and its photocurrents were compared with the responses in Ag-As2S 3 interfaces. A thin layer of As22S55Ge23 (/~g = 2.28 eV) was evaporated on a layer of As10Seg0Te23 (Eg = 1.77 eV). Then the binary layer was sandwiched with two evaporated gold elect:~'odes, l~llumination of the cell from the wider-gap glass side yielded short-circuit photocurrents greatly dependent on the excitation wavelength (fig. 5) as well as those observed in Ag-As2S 3 interfaces. The positive polarity here h v (eV)
Short-circuit Photocurrent .... i ~
a
1.55 - 1.77
b
1.84
c
1.91
d
1.98 - 2.76
e
3.10 -- 3.50
~off
J-
Fig. 5. Short-circuit photocurrents in a heterojunction As22SssGe~ 3 (Eg = 2.28 eV, illuminated) -AszoSesoTeto (Eg = 1.77 eV). * A few tens A of silver is doped in the chalcogenide layer, prior to the illumination, probably during the procedure of vac,uum evaporation.
137
H. Kokado et al. / Mechanism o f photodoping -9
lO
I
o
\
2,
T
1
10"'°
(J
v I-.-. =w
.Q
"~'d..o_
w Q:=
10. 10" 1.~2.0 -I!
c~ l-.c~
(2,,.
2.5
3.0
I 10.1 3.5 ] h v (eV)
t
l +
10
.Io
Fig. 6. Spectral dependences of short-circuit photocurrent parameters in the heterojunction As22SssGe23 (illuminated)-AsloSesoTelo.
represents a current flowing from the narrower.gap glass to the wider-gap one. Magnitudes (ipo, ido) and the relaxation times (rp, rd) of current spikes at the beginning and end of illumination, respectively, were plotted in fig. 6 as a function of the photon energy. When the sample was illuminated from the opposite side of the narrower-gap glass, short-circuit photocurrents had the same shape as curve (a) in fig. 5, irrespective of the photon energy. Therefore, photoresponses related to the heterojunction region should be looked for in curves (b)-(d). Though no typical polarization current was observed, curve (d) might be a combination of curve (e) and a poiartzation current with a negative spike at the start of illumination. The latter would be of the same origin as the spiky response in Ag-As 2 S3-AI cells reported in ref. [4].
4. The mechanism of photodoping On tile basis of the above experimental indications, a model for photodoping has been speculated. Even in darkness, a chemical reaction between silver and chalcogenide produces localized electronic states of silver and a resultant potential bamer at the interface. The potential is assumed to possess a minimum at the
138
H. Kokado et oL / Mechanism of photodoping
~
~ f
Ag
V.B
chatcogenide
~
v
Fig. 7. Illustrationof proposed model: (a) photocarrier separation, (b) diffusion of silver ions attracted by trapped electrons.
vicinity of the interface (fig. 7). When photons are absorbed by chalcogenide at the interface, photocarriers are generated. Holes will be captured by silver and electrons will be driven off by the junction potential into the interior tu be trapped, if electron traps are associated with some active or loosely bound chalcogen atom:; they will greatly increase the affinity for silver ions. The attraction ~eld between the negative chalcogen ions and the positive silver ions may be larg~:~enough to overcome the kinetic barrier of the reaction. As an appreciable amo~nt of silver enters into the glass layer, a layer of new composition, rich in silver, is iorn~ed there. Then the interfacial barrier will move to the inner place and gradually forms a heterojunction. The sensitivity limit moves to longer wavelengths (fig. 3), reflecting the narrower energy gap of the new composition. Since the distance between the silver layer and the junction region becomes larger with the progress of photodoping, the rate-determining step must be taken over by the diffusion process of silver through the silver-rich composition. It has been well established that silver ions are fairly movable in silver-doped chalcogenides [ 1]. The rate equation for the growth of doped depth x with time t is expressed by
dx/dt = k/ab s +(k'/X)labs,
(4)
where lab s is the number of absorbed photons per unit time and k, k' are constants. The first term comes from the photo-enhanced chemical reactivity with a short-range influence. The second term implies the Coulomb attraction between photogenerated ions which drives the diffusion of silver. At the very beginning of photodoping (for example, until a 60 A thick silver layer has been doped in), the second term must be
1t. Kokado et al. / Mechanism o f photodoping
139
negligible, but in heavy doping experiments the first term can be neglected because of early consumption of active sites for the reaction in the neighborhood of the interface. In this case, the square root dependence
x =(2k'labst)l/2
(5)
results from eq. (4), consistent with the experiment [8]. This model, though it is still speculative, explains most of the experimental observations described.
5. Conclusion A hypothetical model for photodoping has been proposed. A potential barrier at the Ag-chalcogenide interface is assumed to drive photo-electrons into the interior, while holes are left at the surface of the silver layer. The Coulomb attraction field between trapped electrons and hole-captured silver may provide a driving force for the diffusion-controlled photodoping. The model explains the experimental observations to a satisfying extent.
References [ 1] H. Sakuma, I. Shi; lizu, H. Kokado and E. lnoue, Proc. Third Conf. on Solid State Devices (Tokyo, 1971); suppl, to Oyo Buturi 41 (1972) 76. [2] J.P. deNeufville, S.C. Moss and S.R. Obshinsky, J. Non-Crystalline Solids 13 (1974) 191. [3] E. Inoue, H. Kokado and 1. Shimizu, Proc. Fifth Conf. on Solid State Devices (Tokyo, 1973); suppl, to J. Jap. Soc. Appl. Phys. 43 (1974) 101. [4] H. Kokado, I. Shimizu, T. Tatsuno and E. lnoue, submitted to J. Non-Crystalline Solids. [5] I. Shimizu, H. Sakuma, H. Kokado and E. Inoue, Bull. Chem. Soc. Jap. 46 (1973) 1291. [6] 1. Shimizu, H. Sakuma, H. Kokado and E. inoue, Bull. Chem. Soc. Jap. 44 (1971) 1173. [7] S. Tsuchihashi and Y. Kawamoto, Yogyo-Kyokai-Shi 77 (1969) 328. [8] T. Shirakawa, I. Shimizu, H. Kokado and E. Inoue, Photogr. Sei. Eng. to be published. [9] M.T. Kostyshin and R.F. Romanenko, Ukr. Fiz. Zh. 17 (1972) 230.
DISCUSSION ON THE MECHANISM OF PHOTODOPING H. KOKADO, I. SHIMIZU and E. INOUE Imaging Science and £ngineering Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama, Japan Received 10 February 1975
The photo-enhanced reaction between metallic silver and vitreous chalcogenides is known as "photodoping". Based on a series of expeiimental results, a model for photodoping was proposed. It was assumed in this model, that a junction barrier at the silverchalcogenide interface worked for separating photocarriers. Holes are captured by raetallic silver, and electrons are trapped by active or loosely bound chalcogen atoms after travel toward the interior of a glass layer. The Coulomb attraction field between ions thus formed is large enough to overcome the kinetic barrier in the process of silver diffusion. The square root dependence for the growth of the photodoped depth with exposure time has been explained in the light of this proposed model.
I. Introduction
Photodoping [ 1] is a photo-enhanced reaction of metals (e.g. silver) with chaicogenide glasses (e.g. As2S3). Since there is chemical affinity between metals and chalcogenides, the essential step in the process should be a photo-elimination of the kinetic barrier preventing the reaction in the dark. In a recent publication [2], deNeufville et al. suggested that localized electronic defects, including trapped charges and broken bonds, are possibly connected to this step. The present authors agree with their opinion, but feel that the driving force for the mass ~.~ansport has to be looked for since the photodoped depth sometimes exceeds 20/am [3]. This paper discusses the mechanism of photodoping with an emphasis on the electronic process at the metal-chalcogenide interface. A model is proposed, based upon the assumption of an interfacial barrier potential that was in,['erred in ref. [4], to be formed between silver and chalcogenide [4]. Some important experimental facts concerning photodoping are reviewed in ~ection 2. In section 3, it is mentioned that the same type of photoresponse as reported in ref. [4] was also observed in a heterojunc.tion of two chalcogenide glasses. In ~ection 4 the mechanism of photodoping is discussed. 131
H. Kokado et aL / Mechanism oj'photodoping
132
2, Observations concerning photodoping
2.1. Spectral sensitivity In order that a distinct amount of photodoping is observed, the band-gap excitation is required at the metal-chalcogenide interface. For a quantitative investigation on this point, the spectral sensitivity in a Ag-As2S 6 glass system was measured using a spectrograph equipped with a logarithmic slit. Results are shown in fig. I. When samples were illuminated from the As2S6 layer side, the shorter wavelength limit of sensitivity varied with the thickness of the As2S6 layer. At photon energies greater than the band-gap energy, the optical absorption coefficient a in As2S6 saris. ties a relation
[K(hv- Eg)2/hv],
(1)
which is varied for an indirect transition. Here K is a constant, hv is the energy of photons and Eg is the energy gap. Let a particular light intensity at the interface be lmin that yields a minimum amount of detectable photodoping. Assuming a photodoping efficiency independent of the wavelength of excitation, the sensitivity limit by* will be determined by the following expression:
/rain
=
!0 exp [-K(hv* -
(2)
Eg)2d/hv*],
where I 0 denotes the incident light intensity of the samples and d is the thickness of the As2S 6 layers. Since/min is considered to be constant in a series of measureu
d"
500A
]400 A" 2,100
3000/, ,A 1
5000 A -
*
.
•
t
._AkA
340 ,]40 WAVELENGTH (nm)
I
,,
540
Fig. 1. Spectral photosensitivity of the Ag-As2 $6 photodoping system, when illuminated through As2S6 layers of thickness d. Multiple peaks seen in thick layers are due to the inter° ference effect and are not essential.
1t. Kokado et al. / Mechanism of photodoping
133
O
;5 e..-,.i 11(
3 !
!
i,
3.0
2.8
i,
3.2
!
3.4
3.6
h~ (eV)
Fig. 2. (h~,*/d) lr~ versus photon energy, at short-wavelength limit of photosensitivity from fig. 1.
ments,
(3)
by* = (hv*/d)l/2
Energies hv* were read from fig. 1. Relation (3) was confirmed to hold, as demonstrated in fig. 2. As photodoping proceeds, a layer of silver.doped chalcogenide grows, and the absorption by the whole layer turns out to be extended toward longer wavelengths [5]. It will be interesting to know whether or not photons absorbed by the resultant silver-doped chalcogenide are effective for further photodoping. In an experiment in which Ag-As2S 6 samples were illuminated from the As2S 6 layer side, the i
15 min
60 min [
340
440 WAVELENGTH(nm)
180 min
540
Fig. 3. Shift of photosensitivity maximt~m (arrow) in the Ag-As2S 6 system with exposure time. Sensitivity scale for the lower two spectrograms are reduced.
134
H. Kokado et al. ~Mechanismof photodoping
spectral sensitivity maximum was found to shift toward longer wavelengths with increasing exposure, as shown in fig. 3. The silver-doped chalcogenide is to be found mainly in the blackened areas in fig. 3. But photodoping also takes effect in the very outside of the contours to a visually undetectable degree. The observed extension of spectral sensitivity is presumably brought about by such a lightly doped part with an increased absorbance for longer wavelength light. It is suggested, therefore, that photons absorbed by silver-doped chalcogenide contribute at least partly to photodoping. Occasionally, photodoping is observed with light of a far longer wavelength than the absorption edge of the chalcogenide used. For instance, holograms could be recorded on Ag-As2S 3 samples by light from a He-Ne laser [6]. Since the absorption edge of As2S 3 is located at about 500 nm, the absorption at 632.8 nm must be associated with some localized levels in As2S 3. The rate of photodoping, however, was extremely slow compared to that under band-gap illumination.
2.2. Rate o f photodoping In an experiment with the Ag-As2S9.6 system [3], it was found that the photo. doping rate was very fast at the initial stage but then slowed down, suggesting a change in the rate-determining step° The initial fast rate was proportional to the light intensity and its activation energy was as i'ow as 0.033 eV in the examined temperature range ( - 2 0 0 - + 20°C). The fast process was taken over by the slow process after a definite thickness (~-,60 A) of silve~rwas doped in, irrespective of light intensity. The fast process probably implies a photo-enhanced surface reaction and the slow process a diffusion.controlled one. The fast surface reaction mea,l,ured in a series of As2Sn glasses (2.6 < n ~< 18) attained the highest rate at n ~ 10. Tsuchihashi and Kawamoto [7] have found that the concentration of S-S bonds in the glass network becomes highest in the composition around this, since the excessive sulfur tends to separate S8 rings. Weakly bound sulfur atoms such as those forming S-S bonds may provide a good base for stabilization of diffused silver and theref~re play a significant role in the mechanism of photodoping,
2.3. Depth and states o f photodoped silver According to an examination with an electron-proof X-ray microanalyzer (EMX), photodoped silver distributes along the depth in an almost constant density. For instance, a well-defined boundary was observed at 21.6 pm from the illuminated surface, in a bulk sample of AsI6SsoTe4 in which silver of 1500 A was photodoped [3]. The growth of the doping depth was proportional to the square root of the exposure time [8]. These results emphasize the importance of material diffusion through the photodoped part of the glass. Kostyshin and Romanenko [9] mentioned that silver can diffuse into the in-
H. Kokado et al. / Mechanism o f photodoping
135
terior through the crystalline c~-Ag2Sphase segregated during photodoping. We have investigated this possibility bat found no proof that it occurs. The examination was carried out on glasses with a variety of compositions. Only when silver photodoped samples were heated for a few minutes at 150-200°C, an unidentified but crystalline X-ray aiffraction pattern appeared. The detailed description will be published elsewhere.
3. Electronic potential at the interface In order to know whether photodoping occurs in a particular direction or not, a sample was prepared in a structure as demonstrated in fig. 4. Silver was evaporated on a substrate in stripes, each 100/~m wide and covered with ASl6S80Te 4 glass. The distribution of photodoped silver was surveyed by means of EMX along the direction perpendicular to the surface. For comparison, a thermally doped sample was examined in the same manner. An apparent difference in the doped silver distribution was found as represented in fig. 4. In photodoping, silver preferred to diffuse perpendicularly to the illuminated surface, while thermally the diffusion exhibited no particular directionality. One possible explanation for this directionality is to assume a participation of the directional field, such as an electrical field, in photodoping. In other words, the mechanism of photodoping presumably includes a photo-electrical effect at the surface. Another suggestion on the participation of the electronic process may be made from the following test. Separate films of silver and chalcogenide glass were
l
IO0~L ! AS~6$1oIe4 l-- /Ag .itOi \\\\\\\\\\ ,\'~ substrate as-deposited
T
photo-doped
v ,..I Oft
thermally doped '
2
'
4
'
6
Fig. 4. Distributions of diffused silver in photodoped and thermally doped samples. The structure of the sample is also shown (top).
136
H. Kokado et aL ] Mechanism of photodoping
brought into contact and the air in the gap was evacuated to insure that contact was as close as possible. No photodoping was possible in such a sample. However, when a chalcogenide layer with a very thin overlayer (50 A) of evaporated silver was in contact with another silver layer, then even the latter layer was lost by photodoping. For photodoping to occur, it seems that the contact has to be close enough to allow atomic interactions involving formation of a junction barrier. In ref. [4] which dealt with photo-electrical effects in Ag-As2S3-AI cells, a potential with a deep valley (see fig. 7) was proposed to be formed at the interface between silver and chalcogenide. Short-circuit photocurrents had a component of spiky polarization current which attributed to the interfacial junction. The junction can be looked upon as a sort of heterojunction between silverdoped and undoped parts of As2S 3, though it may be incomplete*. Therefore, a heterojunction was deliberately prepared with two different chalcogenide glasses and its photocurrents were compared with the responses in Ag-As2S 3 interfaces. A thin layer of As22S55Ge23 (/~g = 2.28 eV) was evaporated on a layer of As10Seg0Te23 (Eg = 1.77 eV). Then the binary layer was sandwiched with two evaporated gold elect:~'odes, l~llumination of the cell from the wider-gap glass side yielded short-circuit photocurrents greatly dependent on the excitation wavelength (fig. 5) as well as those observed in Ag-As2S 3 interfaces. The positive polarity here h v (eV)
Short-circuit Photocurrent .... i ~
a
1.55 - 1.77
b
1.84
c
1.91
d
1.98 - 2.76
e
3.10 -- 3.50
~off
J-
Fig. 5. Short-circuit photocurrents in a heterojunction As22SssGe~ 3 (Eg = 2.28 eV, illuminated) -AszoSesoTeto (Eg = 1.77 eV). * A few tens A of silver is doped in the chalcogenide layer, prior to the illumination, probably during the procedure of vac,uum evaporation.
137
H. Kokado et al. / Mechanism o f photodoping -9
lO
I
o
\
2,
T
1
10"'°
(J
v I-.-. =w
.Q
"~'d..o_
w Q:=
10. 10" 1.~2.0 -I!
c~ l-.c~
(2,,.
2.5
3.0
I 10.1 3.5 ] h v (eV)
t
l +
10
.Io
Fig. 6. Spectral dependences of short-circuit photocurrent parameters in the heterojunction As22SssGe23 (illuminated)-AsloSesoTelo.
represents a current flowing from the narrower.gap glass to the wider-gap one. Magnitudes (ipo, ido) and the relaxation times (rp, rd) of current spikes at the beginning and end of illumination, respectively, were plotted in fig. 6 as a function of the photon energy. When the sample was illuminated from the opposite side of the narrower-gap glass, short-circuit photocurrents had the same shape as curve (a) in fig. 5, irrespective of the photon energy. Therefore, photoresponses related to the heterojunction region should be looked for in curves (b)-(d). Though no typical polarization current was observed, curve (d) might be a combination of curve (e) and a poiartzation current with a negative spike at the start of illumination. The latter would be of the same origin as the spiky response in Ag-As 2 S3-AI cells reported in ref. [4].
4. The mechanism of photodoping On tile basis of the above experimental indications, a model for photodoping has been speculated. Even in darkness, a chemical reaction between silver and chalcogenide produces localized electronic states of silver and a resultant potential bamer at the interface. The potential is assumed to possess a minimum at the
138
H. Kokado et oL / Mechanism of photodoping
~
~ f
Ag
V.B
chatcogenide
~
v
Fig. 7. Illustrationof proposed model: (a) photocarrier separation, (b) diffusion of silver ions attracted by trapped electrons.
vicinity of the interface (fig. 7). When photons are absorbed by chalcogenide at the interface, photocarriers are generated. Holes will be captured by silver and electrons will be driven off by the junction potential into the interior tu be trapped, if electron traps are associated with some active or loosely bound chalcogen atom:; they will greatly increase the affinity for silver ions. The attraction ~eld between the negative chalcogen ions and the positive silver ions may be larg~:~enough to overcome the kinetic barrier of the reaction. As an appreciable amo~nt of silver enters into the glass layer, a layer of new composition, rich in silver, is iorn~ed there. Then the interfacial barrier will move to the inner place and gradually forms a heterojunction. The sensitivity limit moves to longer wavelengths (fig. 3), reflecting the narrower energy gap of the new composition. Since the distance between the silver layer and the junction region becomes larger with the progress of photodoping, the rate-determining step must be taken over by the diffusion process of silver through the silver-rich composition. It has been well established that silver ions are fairly movable in silver-doped chalcogenides [ 1]. The rate equation for the growth of doped depth x with time t is expressed by
dx/dt = k/ab s +(k'/X)labs,
(4)
where lab s is the number of absorbed photons per unit time and k, k' are constants. The first term comes from the photo-enhanced chemical reactivity with a short-range influence. The second term implies the Coulomb attraction between photogenerated ions which drives the diffusion of silver. At the very beginning of photodoping (for example, until a 60 A thick silver layer has been doped in), the second term must be
1t. Kokado et al. / Mechanism o f photodoping
139
negligible, but in heavy doping experiments the first term can be neglected because of early consumption of active sites for the reaction in the neighborhood of the interface. In this case, the square root dependence
x =(2k'labst)l/2
(5)
results from eq. (4), consistent with the experiment [8]. This model, though it is still speculative, explains most of the experimental observations described.
5. Conclusion A hypothetical model for photodoping has been proposed. A potential barrier at the Ag-chalcogenide interface is assumed to drive photo-electrons into the interior, while holes are left at the surface of the silver layer. The Coulomb attraction field between trapped electrons and hole-captured silver may provide a driving force for the diffusion-controlled photodoping. The model explains the experimental observations to a satisfying extent.
References [ 1] H. Sakuma, I. Shi; lizu, H. Kokado and E. lnoue, Proc. Third Conf. on Solid State Devices (Tokyo, 1971); suppl, to Oyo Buturi 41 (1972) 76. [2] J.P. deNeufville, S.C. Moss and S.R. Obshinsky, J. Non-Crystalline Solids 13 (1974) 191. [3] E. Inoue, H. Kokado and 1. Shimizu, Proc. Fifth Conf. on Solid State Devices (Tokyo, 1973); suppl, to J. Jap. Soc. Appl. Phys. 43 (1974) 101. [4] H. Kokado, I. Shimizu, T. Tatsuno and E. lnoue, submitted to J. Non-Crystalline Solids. [5] I. Shimizu, H. Sakuma, H. Kokado and E. Inoue, Bull. Chem. Soc. Jap. 46 (1973) 1291. [6] 1. Shimizu, H. Sakuma, H. Kokado and E. inoue, Bull. Chem. Soc. Jap. 44 (1971) 1173. [7] S. Tsuchihashi and Y. Kawamoto, Yogyo-Kyokai-Shi 77 (1969) 328. [8] T. Shirakawa, I. Shimizu, H. Kokado and E. Inoue, Photogr. Sei. Eng. to be published. [9] M.T. Kostyshin and R.F. Romanenko, Ukr. Fiz. Zh. 17 (1972) 230.