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CW-laser-induced synthesis of Sb2Se3 thin films
Applied Surface Science 54 (1992) 358-361 North-Holland
applied surface science
CW-laser-induced synthesis of Sb2Se 3 thin films K. Kolev Central Laboratory of Photoprocesses, Bulgarian A«ademv of Sciences, 1040 Sofia, Bulgaria
and L.D. Laude Department of Materials, University of Mons-Hainaut, 7000 Mons, Belgium Received 29 May 1991, accepted for publication 13 June 1991
Sb2Se3 films are synthesized by CW-Ar+ laser irradiation of a sandwich stack of elemental layers. The atomic proportion in the stack is closest to the aimed stoichiometry. Time-resolved reflectivity measurements are performed to determine the kinetics of the phase formation at fixed laser power values. Two steps of transformation are observed at and above a given power value. At first, reflectivity decreases due to the interdiffusion of Sb and Se without interatomic reaction. Then, it increases and saturates following the synthesis and crystallization of a so-formed Sb2Se3 homogeneous film. At low power densities, the sandwich stack is not transformed homogeneously throughout the whole depth. All irradiated films are analyzed with optical and X-ray photoelectron spectroscopies. Given the good optical reflectivity contrast between virgin and synthesized material, possible application of Sb2Se3 in optical WORM recording is discussed.
1. lntroduction Recently the laser-induced synthesis of comp o u n d s e m i c o n d u c t i n g thin layers has f o u n d wide application in c o n t e m p o r a r y optoelectronics technologies, various solid-state devices, e.g. switching a n d memory, image converters a n d optical inform a t i o n recording. The present c o m m u n i c a t i o n discusses the experimental results of the CW-laser-induced synthesis of thin films of Sb2Se 3 which is a m o n g the attractive chalcogenide s e m i c o n d u c t o r s scarcely studied until now [1-4].
2. Experimental T h i n film stacks of a n t i m o n y a n d s e l e n i u m layers were alternately a n d successively deposited o n t o glass substrate by electron g u n e v a p o r a t i o n at a v a c u u m better than 10 -4 Pa. The thickness of
the layers was strictly controlled d u r i n g each e v a p o r a t i o n by a high-frequency quartz detector in order to m a i n t a i n a n overall atomic p r o p o r t i o n of 2 : 3 b e t w e e n the two elements. I n all cases the total thickness of the layer did not exceed 100 nm. T o realize the synthesis, the samples were irradiated with a c o n t i n u o u s argon ion laser (2~ = 514 nm). The process was evaluated by the optical changes occurring in the irradiated film, i.e. by the change of the reflectivity, R, at the center of the irradiated zone. R is m e a s u r e d by recording the o u t p u t signal of a photodiode, collecting the reflected light of a low-power He N e laser from the sample surface at a n incidence angle of a few degrees. This time-resolved reflectivity measurem e n t is used to d e t e r m i n e the kinetics of the phase f o r m a t i o n at fixed A r + laser power values. The t e m p e r a t u r e of the layer d u r i n g the synthesis was evaluated b y a n interferometric m e t h o d [5]. The changes in the studied system were m o n i t o r e d by
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Kolev, L.D. Laude / C W-laser-induced synthesis of Sb2Se J thin films
T
Se
"
...~Se
~:"iÜSbn"":i:~[
Fig, 1. Three different configurations of the stacks studied.
optical microscopy, optical spectroscopy in the range ?, = 400-1300 nm and by X-ray photoelectron spectroscopy.
3. Results and discussion Three different configurations of the initial samples were studied (fig. 1). In the first one, the Sb layer was in contact with the substrate and the
X = 633 nm
R (%) 38"
Se layer on the external face. In the second configuration, the sequence of evaporation was inverted and for the third, a multilayer sandwich of consecutive, very thin layers of Sb or Se was evaporated. Fig. 2 shows the changes of the reflectivity, R, as a function of irradiation time for a sample with multilayer configuration at three different powers of the Ar + laser. It is seen that in all cases the coarse of the curves is the same. The increase of the Ar + laser power leads to an increase of the rate of the optical changes. The steep drop of the reflectivity is connected with the shift of the absorption edge of the system to longer wavelength, i.e. it corresponds to the Sb exhaustion in massive metal state. This is logically explained by the interdiffusion of Sb and Se. The same phenomenon is also observed in fig. 3 showing the curves of R-change with time during laser
R(%)
/
R(O/o) J
4845
43: 38-
33
38 0"5W
1W
J
2W
28 ¸ 0 20 40
0 10 20
0
4
8
t (s)
Fig. 2. Time-resolved reflectivity measurements for a multilayer sample at different Ar + laser power values.
R(%)
R(%)
x = 633
75 75.
45 45. (b) 10
30
50
70
(s)
359
10 20 30
t (s)
Fig. 3. Time-resolved reflectivity measurements for a bilayer sample, irradiated from Sb (a) and Se (b) face at 2 W laser power.
K. Koleu, L.D. Laude / C W-laser-induced synthesis of SbzSe « thin films
360
T (°C)
200"
100
10
20
sb
46
5ò
6b
70 ~(s)
Fig. 4. Time dependence of the temperature of a bilayer sample (see fig. 3a) at the center of the irradiated zone.
treatment of a bilayer configuration irradiated either from the Sb (fig. 3a) or Se (fig. 3b) face. A proof for the diffusion of Sb in Se is the previously observed abrupt increase of electroresistance of an Sb layer covered with selenium, after irradiation with a powerful H e - N e laser [6]. Further evidence for the interdiffusion of Sb and Se is the quite lower initial reflectivity of the multilayer samples, containing absolutely equal quantities of the elements like the bilayers samples. It is seen from the figures that after passing through a minimum the reflectivity starts to increase. This change of R is connected with the interaction of Sb and Se, i.e. with the compound formation. This is confirmed by the observed chemical shift of the antimony photoelectron signal to higher binding energy, only at this phase of the process, thus corresponding to the Sb2Se 3 formation. The temperatures at which the process of compound formation stops are in all cases below the melting temperatures of Sb and Se, respectively. This would mean that the process is a solid-state reaction. In principle, the chemical reactions in solid state are exothermal. Therefore it should be expected that the laser-induced synthesis in this case also would be exothermal. As a confirmation, fig. 4 gives the thermograph of the synthesis shown in fig. 3a. It is clearly seen that the abrupt increase of the rate of temperature rise coincides with the initial increase in R and
terminates immediately after R passes through its maximum. Along with a further laser treatment (temperature increases to - 300°C) one observes the appearance of a circular transparent zone on the layer, at around the irradiated spot due to its oxidation. Such laser-induced oxidation of thin Sb2Se 3 layers has been already reported [7]. Comparing the curves in figs. 3a and 3b it should be stressed that the rates of interdiffusion of the two elements differ substantially when the irradiation is performed on either the Se or Sb side. This is explained by the higher optical absorption of Se in the visible spectrum, i.e. with the greater efficiency of irradiation of the Ar + laser and the rapid temperature rise of the system. However, in both cases the kinetics of formation and oxidation of Sb2Se 3 is the same. Finally, it should be noted that the rate of the synthesis process depends considerably on the intensity of the laser light on the sample surface. This is clearly expressed by the curves in fig. 2. Moreover, the observation of the irradiated samples in the optical microscope shows that for bilayer configurations, low power irradiation (below 0.7 W) does not lead to sufficiently homogeneous optical changes corresponding to a single phase material regardless of the duration of laser treatment. Probably, the interelement diffusion through the reaction product is hindered or there exists a threshold temperature above which rapid (liquid state) mixing and interaction of Sb and Se begins. An essential characteristic of the laser-synthesized Sb2Se 3 films is their specular reflectivity in the near-infrared and the whole visible part of the
R(%) virg
8O 60 40. 20 400
600
800
1000
1200
~ (nm)
Fig. 5. Optical reflectivity spectra of a bilayer sample before and after CW Ar + laser treatment.
K. Kolev, L.D. Laude / C W-laser-induced synthesis of Sb2Se 3 thin films
spectrum. Fig. 5 shows the spectrum of R for this material. The presence of a sufficiently good contrast between the virgin and the CW-Ar + laserirradiated layer in the wavelength range 500 to 900 nm should be noted. This means that the laser-induced synthesis of Sb2Se 3 studied here could be applied also to non-ablative optical data storage.
361
tions (laser-power density). The possibility for using this synthesis as a method for digital laser recording will depend on the rate of the process at short laser pulse irradiation (of the order of nanoseconds) which will be the object of another investigätion. References
4. Conclusion The present work reveals the possibility to synthesize a thin Sb2Se 3 layer by CW-Ar + laser irradiation of a stack of the two elements. It is shown that the synthesis proceeds in two phases: initial interdiffusion of the elements at lower temperatures followed by a solid-state reaction between Sb and Se with heat liberation leading to the Sb2Se 3 formation. The optical homogeneRy of the end-layer depends on the irradiation condi-
[1] Y. Nakane, Optical Mass Data Storage, Proc. SPIE 529 (1985) p. 76. [2] K.K. Shwartz, F.V. Pirogov, Yu.N. Shunin and J.A. Teteris, Cryst. Lattice Defects Amorph. Mater. 17 (1987) 133. [3] K.K. Shwartz, J.A. Teteris, V.I. Gerbreder, Izv. Akad. Nauk Latv. SSR No. 5 (490) (1988) 132. [4] K. Kolev, M. Wautelet and L.D. Laude, Appl. Surf. Sci. 46 (1990) 418. [5] M. Wautelet, J. Appl. Phys. 65 (1989) 4033. [6] V.I. Gerbreder, K. Gerbreder, J. Teteris and A. Cvetkov, Izv. Akad. Nauk Latv. SSR Ser. Fiz. Tekh. Nauk No. 3 (1989) 28. [7] K. Kolev and M. Wautelet, Appl. Phys. A 52 (1991) 192.
applied surface science
CW-laser-induced synthesis of Sb2Se 3 thin films K. Kolev Central Laboratory of Photoprocesses, Bulgarian A«ademv of Sciences, 1040 Sofia, Bulgaria
and L.D. Laude Department of Materials, University of Mons-Hainaut, 7000 Mons, Belgium Received 29 May 1991, accepted for publication 13 June 1991
Sb2Se3 films are synthesized by CW-Ar+ laser irradiation of a sandwich stack of elemental layers. The atomic proportion in the stack is closest to the aimed stoichiometry. Time-resolved reflectivity measurements are performed to determine the kinetics of the phase formation at fixed laser power values. Two steps of transformation are observed at and above a given power value. At first, reflectivity decreases due to the interdiffusion of Sb and Se without interatomic reaction. Then, it increases and saturates following the synthesis and crystallization of a so-formed Sb2Se3 homogeneous film. At low power densities, the sandwich stack is not transformed homogeneously throughout the whole depth. All irradiated films are analyzed with optical and X-ray photoelectron spectroscopies. Given the good optical reflectivity contrast between virgin and synthesized material, possible application of Sb2Se3 in optical WORM recording is discussed.
1. lntroduction Recently the laser-induced synthesis of comp o u n d s e m i c o n d u c t i n g thin layers has f o u n d wide application in c o n t e m p o r a r y optoelectronics technologies, various solid-state devices, e.g. switching a n d memory, image converters a n d optical inform a t i o n recording. The present c o m m u n i c a t i o n discusses the experimental results of the CW-laser-induced synthesis of thin films of Sb2Se 3 which is a m o n g the attractive chalcogenide s e m i c o n d u c t o r s scarcely studied until now [1-4].
2. Experimental T h i n film stacks of a n t i m o n y a n d s e l e n i u m layers were alternately a n d successively deposited o n t o glass substrate by electron g u n e v a p o r a t i o n at a v a c u u m better than 10 -4 Pa. The thickness of
the layers was strictly controlled d u r i n g each e v a p o r a t i o n by a high-frequency quartz detector in order to m a i n t a i n a n overall atomic p r o p o r t i o n of 2 : 3 b e t w e e n the two elements. I n all cases the total thickness of the layer did not exceed 100 nm. T o realize the synthesis, the samples were irradiated with a c o n t i n u o u s argon ion laser (2~ = 514 nm). The process was evaluated by the optical changes occurring in the irradiated film, i.e. by the change of the reflectivity, R, at the center of the irradiated zone. R is m e a s u r e d by recording the o u t p u t signal of a photodiode, collecting the reflected light of a low-power He N e laser from the sample surface at a n incidence angle of a few degrees. This time-resolved reflectivity measurem e n t is used to d e t e r m i n e the kinetics of the phase f o r m a t i o n at fixed A r + laser power values. The t e m p e r a t u r e of the layer d u r i n g the synthesis was evaluated b y a n interferometric m e t h o d [5]. The changes in the studied system were m o n i t o r e d by
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Kolev, L.D. Laude / C W-laser-induced synthesis of Sb2Se J thin films
T
Se
"
...~Se
~:"iÜSbn"":i:~[
Fig, 1. Three different configurations of the stacks studied.
optical microscopy, optical spectroscopy in the range ?, = 400-1300 nm and by X-ray photoelectron spectroscopy.
3. Results and discussion Three different configurations of the initial samples were studied (fig. 1). In the first one, the Sb layer was in contact with the substrate and the
X = 633 nm
R (%) 38"
Se layer on the external face. In the second configuration, the sequence of evaporation was inverted and for the third, a multilayer sandwich of consecutive, very thin layers of Sb or Se was evaporated. Fig. 2 shows the changes of the reflectivity, R, as a function of irradiation time for a sample with multilayer configuration at three different powers of the Ar + laser. It is seen that in all cases the coarse of the curves is the same. The increase of the Ar + laser power leads to an increase of the rate of the optical changes. The steep drop of the reflectivity is connected with the shift of the absorption edge of the system to longer wavelength, i.e. it corresponds to the Sb exhaustion in massive metal state. This is logically explained by the interdiffusion of Sb and Se. The same phenomenon is also observed in fig. 3 showing the curves of R-change with time during laser
R(%)
/
R(O/o) J
4845
43: 38-
33
38 0"5W
1W
J
2W
28 ¸ 0 20 40
0 10 20
0
4
8
t (s)
Fig. 2. Time-resolved reflectivity measurements for a multilayer sample at different Ar + laser power values.
R(%)
R(%)
x = 633
75 75.
45 45. (b) 10
30
50
70
(s)
359
10 20 30
t (s)
Fig. 3. Time-resolved reflectivity measurements for a bilayer sample, irradiated from Sb (a) and Se (b) face at 2 W laser power.
K. Koleu, L.D. Laude / C W-laser-induced synthesis of SbzSe « thin films
360
T (°C)
200"
100
10
20
sb
46
5ò
6b
70 ~(s)
Fig. 4. Time dependence of the temperature of a bilayer sample (see fig. 3a) at the center of the irradiated zone.
treatment of a bilayer configuration irradiated either from the Sb (fig. 3a) or Se (fig. 3b) face. A proof for the diffusion of Sb in Se is the previously observed abrupt increase of electroresistance of an Sb layer covered with selenium, after irradiation with a powerful H e - N e laser [6]. Further evidence for the interdiffusion of Sb and Se is the quite lower initial reflectivity of the multilayer samples, containing absolutely equal quantities of the elements like the bilayers samples. It is seen from the figures that after passing through a minimum the reflectivity starts to increase. This change of R is connected with the interaction of Sb and Se, i.e. with the compound formation. This is confirmed by the observed chemical shift of the antimony photoelectron signal to higher binding energy, only at this phase of the process, thus corresponding to the Sb2Se 3 formation. The temperatures at which the process of compound formation stops are in all cases below the melting temperatures of Sb and Se, respectively. This would mean that the process is a solid-state reaction. In principle, the chemical reactions in solid state are exothermal. Therefore it should be expected that the laser-induced synthesis in this case also would be exothermal. As a confirmation, fig. 4 gives the thermograph of the synthesis shown in fig. 3a. It is clearly seen that the abrupt increase of the rate of temperature rise coincides with the initial increase in R and
terminates immediately after R passes through its maximum. Along with a further laser treatment (temperature increases to - 300°C) one observes the appearance of a circular transparent zone on the layer, at around the irradiated spot due to its oxidation. Such laser-induced oxidation of thin Sb2Se 3 layers has been already reported [7]. Comparing the curves in figs. 3a and 3b it should be stressed that the rates of interdiffusion of the two elements differ substantially when the irradiation is performed on either the Se or Sb side. This is explained by the higher optical absorption of Se in the visible spectrum, i.e. with the greater efficiency of irradiation of the Ar + laser and the rapid temperature rise of the system. However, in both cases the kinetics of formation and oxidation of Sb2Se 3 is the same. Finally, it should be noted that the rate of the synthesis process depends considerably on the intensity of the laser light on the sample surface. This is clearly expressed by the curves in fig. 2. Moreover, the observation of the irradiated samples in the optical microscope shows that for bilayer configurations, low power irradiation (below 0.7 W) does not lead to sufficiently homogeneous optical changes corresponding to a single phase material regardless of the duration of laser treatment. Probably, the interelement diffusion through the reaction product is hindered or there exists a threshold temperature above which rapid (liquid state) mixing and interaction of Sb and Se begins. An essential characteristic of the laser-synthesized Sb2Se 3 films is their specular reflectivity in the near-infrared and the whole visible part of the
R(%) virg
8O 60 40. 20 400
600
800
1000
1200
~ (nm)
Fig. 5. Optical reflectivity spectra of a bilayer sample before and after CW Ar + laser treatment.
K. Kolev, L.D. Laude / C W-laser-induced synthesis of Sb2Se 3 thin films
spectrum. Fig. 5 shows the spectrum of R for this material. The presence of a sufficiently good contrast between the virgin and the CW-Ar + laserirradiated layer in the wavelength range 500 to 900 nm should be noted. This means that the laser-induced synthesis of Sb2Se 3 studied here could be applied also to non-ablative optical data storage.
361
tions (laser-power density). The possibility for using this synthesis as a method for digital laser recording will depend on the rate of the process at short laser pulse irradiation (of the order of nanoseconds) which will be the object of another investigätion. References
4. Conclusion The present work reveals the possibility to synthesize a thin Sb2Se 3 layer by CW-Ar + laser irradiation of a stack of the two elements. It is shown that the synthesis proceeds in two phases: initial interdiffusion of the elements at lower temperatures followed by a solid-state reaction between Sb and Se with heat liberation leading to the Sb2Se 3 formation. The optical homogeneRy of the end-layer depends on the irradiation condi-
[1] Y. Nakane, Optical Mass Data Storage, Proc. SPIE 529 (1985) p. 76. [2] K.K. Shwartz, F.V. Pirogov, Yu.N. Shunin and J.A. Teteris, Cryst. Lattice Defects Amorph. Mater. 17 (1987) 133. [3] K.K. Shwartz, J.A. Teteris, V.I. Gerbreder, Izv. Akad. Nauk Latv. SSR No. 5 (490) (1988) 132. [4] K. Kolev, M. Wautelet and L.D. Laude, Appl. Surf. Sci. 46 (1990) 418. [5] M. Wautelet, J. Appl. Phys. 65 (1989) 4033. [6] V.I. Gerbreder, K. Gerbreder, J. Teteris and A. Cvetkov, Izv. Akad. Nauk Latv. SSR Ser. Fiz. Tekh. Nauk No. 3 (1989) 28. [7] K. Kolev and M. Wautelet, Appl. Phys. A 52 (1991) 192.