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Photo-induced absorption changes in selenium-based chalcogenide glass films
Thin Solid Films, 58 ( 1979) 403-406 0 Elsevier Sequoia S.A., Lausanne-Printed
PHOTO-INDUCED CHALCOGENIDE
403
in the Netherlands
ABSORPTION CHANGES IN SELENIUM-BASED GLASS FILMS*
MASAHIRO OKUDA AND TRAN TRI NANG College of Engineering, University of Osaka Prefecture, Mom, Sakai, Osaka (Japan) TATSUHIKO MATSUSHITA College of Engineering, Osaka Industrial University, Daito, Osaka (Japan) (Received July 26,1978; accepted September 13, 1978)
The random network model for amorphous alloys proposed by White has been developed for some selenium-based amorphous semiconductors (Ge-Se and As-Se) annealed or irradiated with light. The model suggests that the photo-bleaching and the photo-darkening observed in these systems may be due to changes in the numbers of bonds.
1. INTRODUCTION
Changes in optical transmission due to light irradiation and reversible photostructural changes caused by repeated cycles of band gap illumination and annealing have been observed in some glasses lv5 . A reversible optical transition has been found in thin films of Ge-Se 6 where heating causes the absorption edge to shift to shorter wavelengths and illumination moves it to longer wavelengths. In this work we studied the mechanism of the photostructural change in Ge-Se and As-Se films taking the influence of the numbers of bonds into consideration. Our calculated results are compared with previously measured experimental data. 2. RESULTS AND DISCUSSION The changes in the optical transmission of a GeSe, film on heating and light illumination are shown in Fig. 1. The GeSe, film of thickness l-2 pm was evaporated at a pressure of 10V6 Torr onto a Pyrex glass substrate at a deposition rate of about 10 A s- ’ and with a substrate temperature of 40 “C. Curve a is the transmission of the virgin film. Curve b is the transmission measured after the virgin film had been illuminated. When the illuminated sample (curve b) was heated to 180 “C for several minutes its transmission characteristic shifted to curve c. There was reversible interchange between curves b and c but the virgin state of the film could not be reproduced by any illumination or annealing treatment. In general, sputtered or evaporated films have a more random structure than quenched films because the formation process is so rapid that the structure of an ideal vitreous material (in the virgin state) can be regarded as a mixture of all kinds of *Paper presented at the Fourth International September 11-15, 1978; Paper 554.
Congress on Thin Films, Loughborough,
Gt. Britain,
404
M. OKUDA,
T. T. NANG, T. MATSUSHITA
bonds, i.e. the coordination environment can adjust to satisfy the valence requirement of each atom’. It is therefore assumed that the random covalent network is dominant over the entire composition, as shown in curve a of Fig. 2. On annealing, the bond energies tend to be lowered and a “strong chemical ordering” takes place. For a given R, _,X, system in the annealed state (R and X are components, x is the atomic concentration), in order to maximize unlike bonds R-X it is suggested that on the R-rich side of the system there are no X-X bonds while on the X-rich side there are no R-R bonds. This model is illustrated by curved in Fig. 2. When the vitreous material is illuminated it undergoes a structural change due to the accumulation of broken bonds (ordered states become disordered and bonds tend to attain the virgin states). The bond distribution of the svstem in this state is shown by curve bin Fig. 2.
Fig. 1. Changes in the optical transmission of GeSe, films on heating virgin state: curve b, after illumination; curve c, after annealing.
and with illumination:
curve a,
Fig. 2. Variation in the numbers of bonds with illumination and annealing as a function of the atomic concentration x in Ge, -Sex films: -, curve a, virgin state; --, curve b, after illumination; ---, curve c, after annealing; -- ‘, curve d, quenched glass.
This model can also be applied to the alloy system when it is first illuminated by the exciting light. Thus, with exciting light, all bonds (like bonds and unlike bonds) are broken and some rearrangement of the bonds in short range order is expected. The bonding moves towards the equilibrium state with much greater numbers of unlike bonds. In Fig. 2 the transitions between curves a and b (the first illumination) and between curves b and c correspond respectively to the non-reversible process between curves a and band to the reversible process-between curves b and c in Fig. 1. A similar model can be applied to the As-Se system. Let us consider a random network of two components R and X where
PHOTO-INDUCED
CHANGES
IN Se-BASED
CHALCOGENIDE
405
GLASS FILMS
component R is always bonded to Z, neighbours while component X has Zx neighbours. If x is the atomic concentration of component X, then in the virgin state the numbers of bonds are related to the composition as follows: number of R-X bonds = Zxx number of X-X bonds = iZxx
(1-x)-G Z,x+(l
-x)Z,
Zxx zxx + (1 - x)Z,
number of R-R bonds = k Z,( 1 -x) -
(1)
Z,Z,( 1- x) Z,x+(l -x)Z, 1
In the equilibrium state, in order to maximize the number of R-X bonds the distribution of bonds is number of R-X bonds = Zxx number of R-R bonds = $(Z, -(Z,
+ Zx)x>
o
ZR Z,+Zx
(2)
number of X-X bonds = 0 number of R-X bonds = Z,( 1 - x) number of R-R bonds = 0
Z, ____ Z,+Zx
< x < 1
(3)
number of X-X bonds = $((ZR + Zx)x -Z,} Using the method proposed by White* for calculating the limits to the tight binding energies for amorphous alloys, we can determine the limits of the bonding and antibonding bands in the Ge-Se and As-Se systems. Using eqns. (l)-(3) the numbers of bonds as a function of the atomic concentration x for the system can be calculated and are shown in Fig. 2. The solid curves indicate the numbers of bonds in the virgin state and the double-chain curves the numbers of bonds in the equilibrium state. Also shown in this figure are the numbers of bonds for intermediate distributions which correspond to the illuminated state (chain curve) or the annealed state (broken curve). The difference in the numbers of Ge-Ge, Ge-Se and Se-Se bonds is significant at about x = 0.66. Taking the optical gaps of germanium, selenium and GeSe, to be 0.7,2.1 and 1.6 eV respectively with the overlap integrals SGcGe = 0.653, &-se = 0.549 and See-se = 0.588, we calculated the matrix element V, involving a given bond and that V, involving two different bonds for the Ge-Se system. The limits to the tight binding energies for Ge-Se alloys as a function of composition x were obtained exactly. When these values were compared with the limits for the system in the equilibrium state, we found that the bonding band and the lone-pair width did not change much but the antibonding band shifted to higher energies by 0.3 eV at x = 0.66 (Fig. 3). The experimental results for GeSe, films gave shifts of about 0.07 and 0.13 eV from the virgin state when the sample was illuminated and annealed respectively; these correspond to the intermediate distributions at a = 0.25 and
406
M. OKUDA, T. T. NANG, T. MATSUSHITA
a = 0.5 (also included in the figure). Here a = 0.25 means that the number of bonds is calculated as 0.75 x (number of bonds in the virgin state)+0.25 x (number of bonds in the equilibrium state). The reversible process between illumination and annealing is expected to occur somewhere in the region between a = 0.25 and a = 0.5.
0.2
0.4
&
0.6
OB
:e
Fig. 3. The antibonding band of the Ge, _,Se, system is shifted to higher energies with illumination or annealing: curve a, virgin state; curve b, after illumination; curve c, after annealing; curve d, quenched glass.
The model was also applied to the As-Se system and gives an explanation for the photo-darkening observed by Toth6. 3. CONCLUSION We have presented a model for the photo-induced absorption edge shift in GeSe and As-St films based on experimental results. With this model we are able to explain the photo-bleaching in Ge-Se films and the photo-darkening in As-Se films. REFERENCES 1 T. Igo and Y. Toyoshima, J. Non-Cryst. Solids, 1 I (1973) 304. J. P. DeNeufville, S. C. Moss and S. R. Ovshinsky, J. Non-Cryst. So/ids, 13 (1974) 191.. 3 K. Tanaka, Solid State Commun., 15 (1974) 1521. 4 K. Tanaka, Appl. Phys. Lett., 26 (1975) 243. 5 H. Hamanaka, K. Tanaka and S. Izima, Solid State Commun., 23 (1977) 63. 6 L. Toth, J. Hajto and G. Zentai, SolidState Commun., 23 (1977) 185. 7 K. Tanaka, Res. Rep. Electrotechnical Laboratory No. 799, 1978, p. 41 (in Japanese). 8 R. M. White, J. Non-Cryst. Solids, 16 (1974) 387. 2
PHOTO-INDUCED CHALCOGENIDE
403
in the Netherlands
ABSORPTION CHANGES IN SELENIUM-BASED GLASS FILMS*
MASAHIRO OKUDA AND TRAN TRI NANG College of Engineering, University of Osaka Prefecture, Mom, Sakai, Osaka (Japan) TATSUHIKO MATSUSHITA College of Engineering, Osaka Industrial University, Daito, Osaka (Japan) (Received July 26,1978; accepted September 13, 1978)
The random network model for amorphous alloys proposed by White has been developed for some selenium-based amorphous semiconductors (Ge-Se and As-Se) annealed or irradiated with light. The model suggests that the photo-bleaching and the photo-darkening observed in these systems may be due to changes in the numbers of bonds.
1. INTRODUCTION
Changes in optical transmission due to light irradiation and reversible photostructural changes caused by repeated cycles of band gap illumination and annealing have been observed in some glasses lv5 . A reversible optical transition has been found in thin films of Ge-Se 6 where heating causes the absorption edge to shift to shorter wavelengths and illumination moves it to longer wavelengths. In this work we studied the mechanism of the photostructural change in Ge-Se and As-Se films taking the influence of the numbers of bonds into consideration. Our calculated results are compared with previously measured experimental data. 2. RESULTS AND DISCUSSION The changes in the optical transmission of a GeSe, film on heating and light illumination are shown in Fig. 1. The GeSe, film of thickness l-2 pm was evaporated at a pressure of 10V6 Torr onto a Pyrex glass substrate at a deposition rate of about 10 A s- ’ and with a substrate temperature of 40 “C. Curve a is the transmission of the virgin film. Curve b is the transmission measured after the virgin film had been illuminated. When the illuminated sample (curve b) was heated to 180 “C for several minutes its transmission characteristic shifted to curve c. There was reversible interchange between curves b and c but the virgin state of the film could not be reproduced by any illumination or annealing treatment. In general, sputtered or evaporated films have a more random structure than quenched films because the formation process is so rapid that the structure of an ideal vitreous material (in the virgin state) can be regarded as a mixture of all kinds of *Paper presented at the Fourth International September 11-15, 1978; Paper 554.
Congress on Thin Films, Loughborough,
Gt. Britain,
404
M. OKUDA,
T. T. NANG, T. MATSUSHITA
bonds, i.e. the coordination environment can adjust to satisfy the valence requirement of each atom’. It is therefore assumed that the random covalent network is dominant over the entire composition, as shown in curve a of Fig. 2. On annealing, the bond energies tend to be lowered and a “strong chemical ordering” takes place. For a given R, _,X, system in the annealed state (R and X are components, x is the atomic concentration), in order to maximize unlike bonds R-X it is suggested that on the R-rich side of the system there are no X-X bonds while on the X-rich side there are no R-R bonds. This model is illustrated by curved in Fig. 2. When the vitreous material is illuminated it undergoes a structural change due to the accumulation of broken bonds (ordered states become disordered and bonds tend to attain the virgin states). The bond distribution of the svstem in this state is shown by curve bin Fig. 2.
Fig. 1. Changes in the optical transmission of GeSe, films on heating virgin state: curve b, after illumination; curve c, after annealing.
and with illumination:
curve a,
Fig. 2. Variation in the numbers of bonds with illumination and annealing as a function of the atomic concentration x in Ge, -Sex films: -, curve a, virgin state; --, curve b, after illumination; ---, curve c, after annealing; -- ‘, curve d, quenched glass.
This model can also be applied to the alloy system when it is first illuminated by the exciting light. Thus, with exciting light, all bonds (like bonds and unlike bonds) are broken and some rearrangement of the bonds in short range order is expected. The bonding moves towards the equilibrium state with much greater numbers of unlike bonds. In Fig. 2 the transitions between curves a and b (the first illumination) and between curves b and c correspond respectively to the non-reversible process between curves a and band to the reversible process-between curves b and c in Fig. 1. A similar model can be applied to the As-Se system. Let us consider a random network of two components R and X where
PHOTO-INDUCED
CHANGES
IN Se-BASED
CHALCOGENIDE
405
GLASS FILMS
component R is always bonded to Z, neighbours while component X has Zx neighbours. If x is the atomic concentration of component X, then in the virgin state the numbers of bonds are related to the composition as follows: number of R-X bonds = Zxx number of X-X bonds = iZxx
(1-x)-G Z,x+(l
-x)Z,
Zxx zxx + (1 - x)Z,
number of R-R bonds = k Z,( 1 -x) -
(1)
Z,Z,( 1- x) Z,x+(l -x)Z, 1
In the equilibrium state, in order to maximize the number of R-X bonds the distribution of bonds is number of R-X bonds = Zxx number of R-R bonds = $(Z, -(Z,
+ Zx)x>
o
ZR Z,+Zx
(2)
number of X-X bonds = 0 number of R-X bonds = Z,( 1 - x) number of R-R bonds = 0
Z, ____ Z,+Zx
< x < 1
(3)
number of X-X bonds = $((ZR + Zx)x -Z,} Using the method proposed by White* for calculating the limits to the tight binding energies for amorphous alloys, we can determine the limits of the bonding and antibonding bands in the Ge-Se and As-Se systems. Using eqns. (l)-(3) the numbers of bonds as a function of the atomic concentration x for the system can be calculated and are shown in Fig. 2. The solid curves indicate the numbers of bonds in the virgin state and the double-chain curves the numbers of bonds in the equilibrium state. Also shown in this figure are the numbers of bonds for intermediate distributions which correspond to the illuminated state (chain curve) or the annealed state (broken curve). The difference in the numbers of Ge-Ge, Ge-Se and Se-Se bonds is significant at about x = 0.66. Taking the optical gaps of germanium, selenium and GeSe, to be 0.7,2.1 and 1.6 eV respectively with the overlap integrals SGcGe = 0.653, &-se = 0.549 and See-se = 0.588, we calculated the matrix element V, involving a given bond and that V, involving two different bonds for the Ge-Se system. The limits to the tight binding energies for Ge-Se alloys as a function of composition x were obtained exactly. When these values were compared with the limits for the system in the equilibrium state, we found that the bonding band and the lone-pair width did not change much but the antibonding band shifted to higher energies by 0.3 eV at x = 0.66 (Fig. 3). The experimental results for GeSe, films gave shifts of about 0.07 and 0.13 eV from the virgin state when the sample was illuminated and annealed respectively; these correspond to the intermediate distributions at a = 0.25 and
406
M. OKUDA, T. T. NANG, T. MATSUSHITA
a = 0.5 (also included in the figure). Here a = 0.25 means that the number of bonds is calculated as 0.75 x (number of bonds in the virgin state)+0.25 x (number of bonds in the equilibrium state). The reversible process between illumination and annealing is expected to occur somewhere in the region between a = 0.25 and a = 0.5.
0.2
0.4
&
0.6
OB
:e
Fig. 3. The antibonding band of the Ge, _,Se, system is shifted to higher energies with illumination or annealing: curve a, virgin state; curve b, after illumination; curve c, after annealing; curve d, quenched glass.
The model was also applied to the As-Se system and gives an explanation for the photo-darkening observed by Toth6. 3. CONCLUSION We have presented a model for the photo-induced absorption edge shift in GeSe and As-St films based on experimental results. With this model we are able to explain the photo-bleaching in Ge-Se films and the photo-darkening in As-Se films. REFERENCES 1 T. Igo and Y. Toyoshima, J. Non-Cryst. Solids, 1 I (1973) 304. J. P. DeNeufville, S. C. Moss and S. R. Ovshinsky, J. Non-Cryst. So/ids, 13 (1974) 191.. 3 K. Tanaka, Solid State Commun., 15 (1974) 1521. 4 K. Tanaka, Appl. Phys. Lett., 26 (1975) 243. 5 H. Hamanaka, K. Tanaka and S. Izima, Solid State Commun., 23 (1977) 63. 6 L. Toth, J. Hajto and G. Zentai, SolidState Commun., 23 (1977) 185. 7 K. Tanaka, Res. Rep. Electrotechnical Laboratory No. 799, 1978, p. 41 (in Japanese). 8 R. M. White, J. Non-Cryst. Solids, 16 (1974) 387. 2