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CW laser-induced transformation of thin Sb, Se and Sb2Se3 films in air
418
Applied
Surface
Sclence 46
(1990)418-421 North-Holland
CW laser-induced K. Kolev
*, M. Wautelet
Depurtnwwt of Materials. Uniwrsq Received
2X May 1990; accepted
transformation
of thin Sb, Se and Sb$e,
films in air
and L.D. Laude of Mom,
B-7000
for publication
Mom,
Belgium
6 July 1990
Thin films of Sb. Se and Sb,Se, are deposited onto glass and irradiated by a CW Ar’ laser beam. The variation of the optlcal reflectivity and temperature of the irradiated zone of the films are measured as a function of the laser beam power P and time r. It turns out that the main parameter, correlated with a given transformation, is the temperature T of the film. In all three cases, the transformation starts abruptly when T attains a critical temperature. T,. independently of the value of P. For Sb films. T, compares with the melting temperature of the oxide. For Sb,Se,, T,‘s are equal to the crystallization and oxidation temperatures measured by others under furnace processing
1. Introduction When films are irradiated in air by means of a CW-laser beam, some physical and chemical transformations are observed (crystallization, oxidation [l]. reaction between sandwich films [2]. etc.). In the case of an as-deposited amorphous compound film, some transformations may compete, like crystallization, oxidation and decomposition and the chronological order of these transformations is of great interest. It is the aim of this work to study the transformations occurring upon CW-laser processing of thin amorphous Sb,Se, films. Such a film is a potential candidate for optical data storage applications [3,4]. In order to understand more closely the physico-chemistry of the transformations, the companion elemental films (Sb and Se) are studied in parallel. Upon CW-laser-induced irradiation of thin elemental films (Cd, In [5], Te [6], V [7], Zn [5]), it has been demonstrated experimentally that the kinetics of oxidation is influenced by the existence of a threshold temperature rather than a threshold laser
* Permanent address: Bulgarian Academy
Central Laboratory of Photoprocesaes. of Sciences. Sofia 1040. Bulgaria.
0169.4332/90/$03.50
fi> 1990 - Elsevier Science Publishers
beam power. The same is true for laser-induced synthesis of CuTe compounds [8]. The reason of this fact is not well understood. So, it is necessary to know if the same conclusions hold for other transformations. like crystallization of elemental (Se, Sb) or compound films (Sb,Se,). and oxidation of Sb. Se and Sb?Se,.
2. Experimental
Films of Sb. Se and SbzSe, (respectively 150, 200 and 100 nm thick) are deposited onto glass substrates by vacuum evaporation, either by an electron gun (vacuum lo-’ Pa) or Joule effect (lop4 Pa). The substrates are ultrasonically and glow-discharge cleaned 1 mm thick microscope plates. The layer thickness is glass controlled by a vibrating quartz monitor. The films are irradiated in air by a CW-Ar’ laser (Spectra Physics Model 171) operating on all green lines (488-514 nm). The spatial profile of the beam is Gaussian, with a diameter (l/e’) at the sample of 1.6 mm. The change of reflectivity of the films at the centre of the irradiated zone is measured during irradiation by recording the output signal of a photodiode,
B.V. (North-Holland)
K. K&o et al. / Transformation of Sb, Se and Sb,Se,
collecting the reflected light of a He-Ne laser at an incidence angle of a few degrees to the centre of the irradiated zone. An interferometric method of time-resolved measurement of the sample temperature is applied, as described previously [6]. In this method a He-Ne laser probe beam is directed onto the back-side of the glass substrate at quasinormal incidence. The reflected HeNe beam intensity is measured by a photodiode connected to a servocorder. When the system is heated, both the refractive index and the thickness of the glass substrate increase. These changes give rise to intensity oscillations of the reflected probe beam. These oscillations are precalibrated in a furnace under identical geometrical conditions. It turns out that successive extrema of reflectivity (minimum and maximum) appear when the temperature increases by AT = 11 K around 300 K and by AT = 8 K around 750 K. However, a fit by a straight line characterized by AT = 9.4 K, between 300 and 750 K, gives a maximum absolute error of 5 K in the measured temperature, i.e. the same order as the experimental error bars. The final products are transparent and very adherent to the substrates. Some samples are characterized by electron diffraction or photoemission spectroscopy (XPS).
3. Results
419
films in air
R (%)
A q 633 nm
I,,,
20
60
100 140 180
3.
t (set)
Fig. 1. Change of reflectivity (a) and temperature (b) of the Sb films, at the centre of the irradiated zone, for 2.6, 2.8 and 3 W.
and many of its oxides (T, compares only with the melting temperature of Sb,O,, about 370 o C) [9]. In fig. 2 the variation of the time t,, at which R(t) begins to decrease abruptly, is plotted on a logarithmic scale as a function of Pp’. Two different regimes appear, as concluded from the two slopes of fig. 2. At low P, just after t,, optical microscopy shows the presence of a circular, nontransparent dark zone, expanding with increasing time. At high P, just above t,, one observes the formation of a circular transparent zone, surrounded by a narrow dark ring. So, the two slopes
3.1. Antimony t (set) Typical results of the time dependence of the reflectivity of thin Sb films are shown in fig. la. Some time after the start of laser irradiation, the reflectivity R decreases abruptly to a roughly constant value and the time scale of the transformation decreases with increasing incident laser beam power P. Obviously, this fact is related with the different temperature rise velocities at the different laser beam powers, as one can see from the variations of T with time t shown in fig. lb. It appears that the critical temperature, T,, corresponding to the beginning of the observed optical transformation, is independent of P. This T, is far below the melting point temperature of the metal
0
Sb
400
20
II
>
035 Fig. 2. Characteristic time, of the transformation
0.4
0.45
t,, corresponding
VP
(w-j
to the beginning of Sb films versus l/P.
420
AT PC)
Tc(“C)
0
t
Se 300.
,
Fig. ?. Change
t (set)
40
20
of the Se films temperature different P.
with
time
I at
in fig. 2 are associated with different transformations. The characteristic products obtained after laser irradiation are analysed by means of XPS studies. At high P they reveal unambiguously the presence of Sb oxides in the irradiated zone. 7 Seleniunl _7._.
The variations of T with r are shown in fig. 3. In contrast with Sb it turns out that the temperature goes through a maximum and that T, has a very low value (about 40 o C). Our optical microscope observations indicate a crystallization of initial amorphous Se film at that temperature. Further increase of T provokes sublimation of Se.
TL. of the uystalliatwn Fig. 5. C‘ritlcal temperatures, values)and oxidation of the ShzSe, films, a\ a function
(lower of I’.
steps of transformation: (1) An increase of the reflectivity. due to the crystallization of the amorphous Sb,Se, films (as confirmed by electron diffraction), followed by (2) A considerable decrease of R. due to the oxidation of the crystallized Sb,Se,. This is identified by means of XPS studies. The displacement of the photoelectron signals confirms the transformation of SblSe, into Sb,Se, and Sb205. In fig. 5 the characteristic temperatures, Tc.of the two processes are plotted as a function of P. It is evident that in both cases. T is independent of P. Furthermore, the T, valuea correspond well to the values obtained by differential thermal analyses of Sb2Seq amorphous films [IO].
3.3. Sh,Se: 4. Discussion The time dependence of R of Sb,Se, thin films obtained for various values of P are shown in fig. 4. It is interesting to note the existence of two
t 60
R(%)
nm
h-633
i
30 Fig. 4. Time dependence
60
90
of the reflectivity films.
t (set) of Sh,Se,
amorphow
The present results indicate that the kinetics of transformation of Sb. Se and Sb?Se, films are governed by the existence of threshold temperatures. As-deposited Sb films crystallize at the lowest temperatures (observation of non-transparent dark zone), similarly to what has been observed by others under thermal annealing conditions [ll]. Oxidation of Sb starts abruptly at T = 325 k 10 o C, to be compared with the melting temperature of Sb,O, (370 o C). This differs from previous excimer laser-assisted oxidation of Sh [12]. Under those conditions, photonic effects were evidenced during the synthesis of the first few atomic layers of oxides on Sb. at a temperature much lower than those reported here. Also. the
K. Koleu et al. / Transformation of Sb, Se and Sh_Se, films rn air
authors observe the synthesis of Sb,O, layers [12], while our results seem to indicate the formation of Sb,O, together with some Sb,O,, as seen from XPS studies. For Se, crystallization first takes place at low temperature and sublimation dominates at larger T.
Sb,Se, crystallizes at T, = 165 + lO”C, in agreement with thermal analysis data (170°C) [lo]. Then, oxidation starts abruptly at 300 * lO”C, again in agreement with thermal analysis (315°C). The coincidence between thermal data and the present results indicates that the mechanisms involved in CW-Ar+ laser-assisted transformation of Sb, Se and Sb,Se, films may be described by a purely thermal approach. This seems to be a common point to many CW-Ar+ laser-assisted transformation of thin films in air, when compared with previous works [5-81. In these works, Tc’s have been associated with the melting points of some elements (Cd, In [5]) or decomposition of oxides (Te [6]). Here we note a coincidence with the melting points of oxides (Sb) and the crystallization temperature of a compound semiconductor (Sb,Se,). The fact that oxidation of Sb is accelerated drastically when one attains the melting temperature of Sb,O, may be qualitatively understood. Indeed, if one assumes that below T,, a native oxide Sb,O, is formed, this native oxide, being in the solid phase, may block further oxidation of Sb, by inhibiting the diffusion of oxygen through it. When the oxide melts, oxygen is easily transported to the Sb/Sb,O, interface, where a rapid reaction takes place.
421
5. Conclusions The present work shows that CW-Ar+ laser-assisted transformations of thin Sb. Se and Sb,Se, films, in air, occur via a thermal mechanism. Transformations are drastically accelerated when the temperature attains critical values, T,, independent of the incident laser power density P.
Acknowledgements The authors are thankful to M.-C. Joliet and E. Petit for their help in performing, respectively, TEM diffraction and XPS studies.
References 111 M. Wautelet, Appl. Phys. A 50 (1990) 131. 14 L.D. Laude, M. Wautelet and R. Andrew. Appl. Phys. A 40 (1986) 133. [31 K. Watanabe, T. Oyama, Y. Aoki. N. Sato and S. Wiyaoka. SPIE Proc. 383 (1983) 191. [41 K. Kolev. Proc. 3rd Int. Technical Science Conf.. Varna, 1989. [51 M. Wautelet, V. Miroir and Z.H. Huang, Appl. Phys. A 50 (1990) 311. [61 M. Wautelet, J. Appl. Phys. 65 (1989) 4033. and M. Wautelet, SPIE Proc. 171 P. Quenon, 0. Toubeau 1022 (1988) 141. I81 F. Harms and M. Wautelet. Appl. Surf. Sci. 43 (1989) 271. New [91 G.V. Samsonov, The Oxide Handbook (IFI-Plenum, York. 1973). and M. Mikoda, JRECT 16 Semicond. IlO1 M. Takenaga Technol. Dev. (1984) 266. H. Sugibuchi and K. Kambe, Thin Solid [ill M. Hashimoto, Films 98 (1982) 197. 1121 E.J. Petit. J. Riga, R. Caudano and J. Verbist. Appl. Surf. Sci. 43 (1989) 285.
Applied
Surface
Sclence 46
(1990)418-421 North-Holland
CW laser-induced K. Kolev
*, M. Wautelet
Depurtnwwt of Materials. Uniwrsq Received
2X May 1990; accepted
transformation
of thin Sb, Se and Sb$e,
films in air
and L.D. Laude of Mom,
B-7000
for publication
Mom,
Belgium
6 July 1990
Thin films of Sb. Se and Sb,Se, are deposited onto glass and irradiated by a CW Ar’ laser beam. The variation of the optlcal reflectivity and temperature of the irradiated zone of the films are measured as a function of the laser beam power P and time r. It turns out that the main parameter, correlated with a given transformation, is the temperature T of the film. In all three cases, the transformation starts abruptly when T attains a critical temperature. T,. independently of the value of P. For Sb films. T, compares with the melting temperature of the oxide. For Sb,Se,, T,‘s are equal to the crystallization and oxidation temperatures measured by others under furnace processing
1. Introduction When films are irradiated in air by means of a CW-laser beam, some physical and chemical transformations are observed (crystallization, oxidation [l]. reaction between sandwich films [2]. etc.). In the case of an as-deposited amorphous compound film, some transformations may compete, like crystallization, oxidation and decomposition and the chronological order of these transformations is of great interest. It is the aim of this work to study the transformations occurring upon CW-laser processing of thin amorphous Sb,Se, films. Such a film is a potential candidate for optical data storage applications [3,4]. In order to understand more closely the physico-chemistry of the transformations, the companion elemental films (Sb and Se) are studied in parallel. Upon CW-laser-induced irradiation of thin elemental films (Cd, In [5], Te [6], V [7], Zn [5]), it has been demonstrated experimentally that the kinetics of oxidation is influenced by the existence of a threshold temperature rather than a threshold laser
* Permanent address: Bulgarian Academy
Central Laboratory of Photoprocesaes. of Sciences. Sofia 1040. Bulgaria.
0169.4332/90/$03.50
fi> 1990 - Elsevier Science Publishers
beam power. The same is true for laser-induced synthesis of CuTe compounds [8]. The reason of this fact is not well understood. So, it is necessary to know if the same conclusions hold for other transformations. like crystallization of elemental (Se, Sb) or compound films (Sb,Se,). and oxidation of Sb. Se and Sb?Se,.
2. Experimental
Films of Sb. Se and SbzSe, (respectively 150, 200 and 100 nm thick) are deposited onto glass substrates by vacuum evaporation, either by an electron gun (vacuum lo-’ Pa) or Joule effect (lop4 Pa). The substrates are ultrasonically and glow-discharge cleaned 1 mm thick microscope plates. The layer thickness is glass controlled by a vibrating quartz monitor. The films are irradiated in air by a CW-Ar’ laser (Spectra Physics Model 171) operating on all green lines (488-514 nm). The spatial profile of the beam is Gaussian, with a diameter (l/e’) at the sample of 1.6 mm. The change of reflectivity of the films at the centre of the irradiated zone is measured during irradiation by recording the output signal of a photodiode,
B.V. (North-Holland)
K. K&o et al. / Transformation of Sb, Se and Sb,Se,
collecting the reflected light of a He-Ne laser at an incidence angle of a few degrees to the centre of the irradiated zone. An interferometric method of time-resolved measurement of the sample temperature is applied, as described previously [6]. In this method a He-Ne laser probe beam is directed onto the back-side of the glass substrate at quasinormal incidence. The reflected HeNe beam intensity is measured by a photodiode connected to a servocorder. When the system is heated, both the refractive index and the thickness of the glass substrate increase. These changes give rise to intensity oscillations of the reflected probe beam. These oscillations are precalibrated in a furnace under identical geometrical conditions. It turns out that successive extrema of reflectivity (minimum and maximum) appear when the temperature increases by AT = 11 K around 300 K and by AT = 8 K around 750 K. However, a fit by a straight line characterized by AT = 9.4 K, between 300 and 750 K, gives a maximum absolute error of 5 K in the measured temperature, i.e. the same order as the experimental error bars. The final products are transparent and very adherent to the substrates. Some samples are characterized by electron diffraction or photoemission spectroscopy (XPS).
3. Results
419
films in air
R (%)
A q 633 nm
I,,,
20
60
100 140 180
3.
t (set)
Fig. 1. Change of reflectivity (a) and temperature (b) of the Sb films, at the centre of the irradiated zone, for 2.6, 2.8 and 3 W.
and many of its oxides (T, compares only with the melting temperature of Sb,O,, about 370 o C) [9]. In fig. 2 the variation of the time t,, at which R(t) begins to decrease abruptly, is plotted on a logarithmic scale as a function of Pp’. Two different regimes appear, as concluded from the two slopes of fig. 2. At low P, just after t,, optical microscopy shows the presence of a circular, nontransparent dark zone, expanding with increasing time. At high P, just above t,, one observes the formation of a circular transparent zone, surrounded by a narrow dark ring. So, the two slopes
3.1. Antimony t (set) Typical results of the time dependence of the reflectivity of thin Sb films are shown in fig. la. Some time after the start of laser irradiation, the reflectivity R decreases abruptly to a roughly constant value and the time scale of the transformation decreases with increasing incident laser beam power P. Obviously, this fact is related with the different temperature rise velocities at the different laser beam powers, as one can see from the variations of T with time t shown in fig. lb. It appears that the critical temperature, T,, corresponding to the beginning of the observed optical transformation, is independent of P. This T, is far below the melting point temperature of the metal
0
Sb
400
20
II
>
035 Fig. 2. Characteristic time, of the transformation
0.4
0.45
t,, corresponding
VP
(w-j
to the beginning of Sb films versus l/P.
420
AT PC)
Tc(“C)
0
t
Se 300.
,
Fig. ?. Change
t (set)
40
20
of the Se films temperature different P.
with
time
I at
in fig. 2 are associated with different transformations. The characteristic products obtained after laser irradiation are analysed by means of XPS studies. At high P they reveal unambiguously the presence of Sb oxides in the irradiated zone. 7 Seleniunl _7._.
The variations of T with r are shown in fig. 3. In contrast with Sb it turns out that the temperature goes through a maximum and that T, has a very low value (about 40 o C). Our optical microscope observations indicate a crystallization of initial amorphous Se film at that temperature. Further increase of T provokes sublimation of Se.
TL. of the uystalliatwn Fig. 5. C‘ritlcal temperatures, values)and oxidation of the ShzSe, films, a\ a function
(lower of I’.
steps of transformation: (1) An increase of the reflectivity. due to the crystallization of the amorphous Sb,Se, films (as confirmed by electron diffraction), followed by (2) A considerable decrease of R. due to the oxidation of the crystallized Sb,Se,. This is identified by means of XPS studies. The displacement of the photoelectron signals confirms the transformation of SblSe, into Sb,Se, and Sb205. In fig. 5 the characteristic temperatures, Tc.of the two processes are plotted as a function of P. It is evident that in both cases. T is independent of P. Furthermore, the T, valuea correspond well to the values obtained by differential thermal analyses of Sb2Seq amorphous films [IO].
3.3. Sh,Se: 4. Discussion The time dependence of R of Sb,Se, thin films obtained for various values of P are shown in fig. 4. It is interesting to note the existence of two
t 60
R(%)
nm
h-633
i
30 Fig. 4. Time dependence
60
90
of the reflectivity films.
t (set) of Sh,Se,
amorphow
The present results indicate that the kinetics of transformation of Sb. Se and Sb?Se, films are governed by the existence of threshold temperatures. As-deposited Sb films crystallize at the lowest temperatures (observation of non-transparent dark zone), similarly to what has been observed by others under thermal annealing conditions [ll]. Oxidation of Sb starts abruptly at T = 325 k 10 o C, to be compared with the melting temperature of Sb,O, (370 o C). This differs from previous excimer laser-assisted oxidation of Sh [12]. Under those conditions, photonic effects were evidenced during the synthesis of the first few atomic layers of oxides on Sb. at a temperature much lower than those reported here. Also. the
K. Koleu et al. / Transformation of Sb, Se and Sh_Se, films rn air
authors observe the synthesis of Sb,O, layers [12], while our results seem to indicate the formation of Sb,O, together with some Sb,O,, as seen from XPS studies. For Se, crystallization first takes place at low temperature and sublimation dominates at larger T.
Sb,Se, crystallizes at T, = 165 + lO”C, in agreement with thermal analysis data (170°C) [lo]. Then, oxidation starts abruptly at 300 * lO”C, again in agreement with thermal analysis (315°C). The coincidence between thermal data and the present results indicates that the mechanisms involved in CW-Ar+ laser-assisted transformation of Sb, Se and Sb,Se, films may be described by a purely thermal approach. This seems to be a common point to many CW-Ar+ laser-assisted transformation of thin films in air, when compared with previous works [5-81. In these works, Tc’s have been associated with the melting points of some elements (Cd, In [5]) or decomposition of oxides (Te [6]). Here we note a coincidence with the melting points of oxides (Sb) and the crystallization temperature of a compound semiconductor (Sb,Se,). The fact that oxidation of Sb is accelerated drastically when one attains the melting temperature of Sb,O, may be qualitatively understood. Indeed, if one assumes that below T,, a native oxide Sb,O, is formed, this native oxide, being in the solid phase, may block further oxidation of Sb, by inhibiting the diffusion of oxygen through it. When the oxide melts, oxygen is easily transported to the Sb/Sb,O, interface, where a rapid reaction takes place.
421
5. Conclusions The present work shows that CW-Ar+ laser-assisted transformations of thin Sb. Se and Sb,Se, films, in air, occur via a thermal mechanism. Transformations are drastically accelerated when the temperature attains critical values, T,, independent of the incident laser power density P.
Acknowledgements The authors are thankful to M.-C. Joliet and E. Petit for their help in performing, respectively, TEM diffraction and XPS studies.
References 111 M. Wautelet, Appl. Phys. A 50 (1990) 131. 14 L.D. Laude, M. Wautelet and R. Andrew. Appl. Phys. A 40 (1986) 133. [31 K. Watanabe, T. Oyama, Y. Aoki. N. Sato and S. Wiyaoka. SPIE Proc. 383 (1983) 191. [41 K. Kolev. Proc. 3rd Int. Technical Science Conf.. Varna, 1989. [51 M. Wautelet, V. Miroir and Z.H. Huang, Appl. Phys. A 50 (1990) 311. [61 M. Wautelet, J. Appl. Phys. 65 (1989) 4033. and M. Wautelet, SPIE Proc. 171 P. Quenon, 0. Toubeau 1022 (1988) 141. I81 F. Harms and M. Wautelet. Appl. Surf. Sci. 43 (1989) 271. New [91 G.V. Samsonov, The Oxide Handbook (IFI-Plenum, York. 1973). and M. Mikoda, JRECT 16 Semicond. IlO1 M. Takenaga Technol. Dev. (1984) 266. H. Sugibuchi and K. Kambe, Thin Solid [ill M. Hashimoto, Films 98 (1982) 197. 1121 E.J. Petit. J. Riga, R. Caudano and J. Verbist. Appl. Surf. Sci. 43 (1989) 285.