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Thermal-induced gradually changes in the optical properties of amorphous GeSe2 film prepared by PLD
ARTICLE IN PRESS Physica B 404 (2009) 3397–3400
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Thermal-induced gradually changes in the optical properties of amorphous GeSe2 film prepared by PLD R.K. Pan a,b, H.Z. Tao a,, H.C. Zang a, T.J. Zhang b, X.J. Zhao a, a b
Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan 430070, PR China School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China
a r t i c l e in fo
abstract
Article history: Received 10 February 2009 Received in revised form 7 May 2009 Accepted 15 May 2009
Amorphous GeSe2 film was prepared by the pulsed laser deposition technique and annealed at different temperature from 473 to 623 K. Using the ‘non-direct transition’ model proposed by Tauc, the short wavelength absorption edges of the films were well fitted and the optical band gaps (Eopt g ) were determined. The Tauc slope of the as-deposited film is smaller than those of annealed films, which were proposed as an indicator of the degree of structural randomness of amorphous semiconductors. The refractive index and thickness of the films were calculated from the optical transmission spectra using the Swanepoel method. The index of refraction decreased while Eopt g increased gradually with increasing the annealing temperature. The thermal-bleaching and thermal-contraction effects were observed, which were interpreted as the reduction in the density of homopolar bonds according to the Raman spectra analysis and the diminution of porous structure in the fragments of the annealed films, respectively. & 2009 Elsevier B.V. All rights reserved.
Keywords: GeSe2 film Pulsed laser deposition Thermal-bleaching Optical properties
1. Introduction Amorphous germanium selenide chalcogenide semiconductors have attracted much attention due to their promising technological applications [1–3]. GeSe2 is a typical chalcogenide semiconductor, which can be easily prepared either in glass form or in crystalline form. In recent years, many researches on GeSe2 glass have been made for studying rigidity percolation, short and intermediate range order of the structure [4–6]. Wahab investigated the AC conduction mechanism and photodarkening phenomenon of amorphous GeSe2 films prepared by thermal evaporation technique [2]. Sleekx reported thermalinduced bleaching in amorphous GeSe2 films prepared by plasmaenhanced chemical vapour deposition [7]. The preparation of homogeneous thin films of multicomponent chalcogenide systems is a difficult task. Thermal vacuum evaporation results very often in non-homogeneous films and in films with changed composition due to different volatility of components [8]. Pulsed laser deposition (PLD) technique has several advantages for thin film preparation, such as the stoichiometric transfer from the original target material to a given substrate, the easy set-up, short process time [9–11]. It is necessary to research the optical properties of GeSe2 film prepared by PLD. As-deposited films
Corresponding authors. Tel./fax: +86 27 87669729.
E-mail addresses: [email protected] (H.Z. Tao), [email protected] (X.J. Zhao). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.021
prepared with high energy density of the laser beam are away from the equilibrium state. Annealing can lead to changes in the structure and optical properties of the pulsed laser deposited films, which is interesting to be investigated. In this paper, GeSe2 film was prepared by PLD and was annealed at different temperatures below the glass transition temperature (Tg). The index of refraction and optical band gaps of the films were calculated from the optical transmission spectra and absorption spectra, respectively. Annealing was accompanied by structural changes in the GeSe2 films, which was confirmed by Raman spectra analysis. The changes in the refractive index and the thickness of the films caused by thermal annealing were also discussed.
2. Experimental The target used for PLD was bulk glass with stoichiometry GeSe2. Glass was synthesized from high purity elements (Ge and Se, all of 5 N purity) in evacuated (102 Pa) and flame-sealed silica ampoule which was loaded in a rocking furnace. The mixture was melted at about 950 1C for 12 h. After that, the ampoule was quenched in cold water then annealed near the corresponding glass transition temperature (Tg) for 2 h. The bulk sample was taken out from the ampoule and cut into 2 mm-thick-targets for depositing. The differential thermal analysis (DTA) measurement of bulk glass powder (40–50 mg) was carried out by a differential thermal analyzer (PE, DTA-1700) with an accuracy of 72 1C at a
ARTICLE IN PRESS 3398
R.K. Pan et al. / Physica B 404 (2009) 3397–3400
heating rate of 10 1C/min. Pure Al2O3 powder was used as reference material. The Tg of the glass was determined by the slope intercept method from measured DTA curves. Thin film was prepared from the target by the PLD technique using a KrF excimer laser operated at the wavelength of 248 nm. The film was deposited on the chemically cleaned microscope glass slides. The target was ablated in a vacuum chamber at a background pressure of 2 104 Pa with the laser energy 200 mJ/ pulse. The incident laser beam was focused to the average flux of about 5 J/cm2. The distance between the target and the substrate is about 8 cm in an off/axis approach geometry [12,13]. GeSe2 film was deposited at room temperature. The pulse repetition rate was set at 4 Hz and the incidence angle was 451. The as-deposited film was cut into five samples and four of them were annealed at different temperatures (473, 523, 573, 623 K) for 1 h in the vacuum chamber at pressure of 2 104 Pa, respectively. And the heating rates and cooling rates were about 10 and 1.25 1C/min, respectively. The homogeneity of the samples was confirmed by the electron probe X-ray microanalyzer (EPMA) (JEOL, JCXA-733). X-ray fluorescence spectrophotometer (Shimadzu, XRF-1800) was used to determinate the composition of the films. The transmission spectra and absorption spectra of the studied films were measured using the spectrophotometer (Shimadzu, UV-1601). For comparison, the thickness of the films were measured by a surface profiler (Taylor Honson, S4C-3D) and calculated using the Swanepoel method based on the transmission spectra of thin films [14–16]. The Raman spectra were measured at the room temperature by using a Raman spectrometer (Renishaw, RM-1000) in back (1801) scattering configuration. Laser irradiation at the wavelength of 514 nm was used for the excitation. The laser power was properly under an approximate level of 2.0 mW to avoid the laser damage on the samples. The resolution of the Raman spectra was 1 cm1.
thermal crystallization temperature of the evaporated GeSe2 film was between 360 and 390 1C [17]. It can be deduced that Tg of the evaporated GeSe2 film is lower than that of pulsed laser deposited films. The difference of Tg shows smaller structural fluctuation in the deposited films than in the evaporated films. Fig. 2 shows the optical transmission spectra of the asdeposited and annealed films. The absorption edges of films shift to shorter wavelength after annealing (i.e. thermalbleaching). The observed thermally induced bleaching effect cannot be correlated to oxygen-assisted bond reconstruction since the preparation and annealing of the films had proceeded in the vacuum chamber. In the strong absorption region (the absorption coefficient a4104 cm1) of the absorption spectra, the absorption coefficient is given by the following quadratic equation according to the ‘nondirect transition’ model proposed by Tauc [18],
aðhvÞ ¼ B
2 ðhv Eopt g Þ
hv
(1)
The Tg of GeSe2 bulk sample is about 414 1C. The EPMA and XRF measurements show that the chemical compositions of the films are homogenous and close to the target. The atomic ratio of the target is maintained within 1% in the film deposited by PLD. The XRD patterns confirm the amorphous character of the asdeposited and annealed films (see Fig. 1). Matsuda reported the
is the Tauc where B is a constant, hv is the photon energy and Eopt g can be calculated from the optical band gap. The values of Eopt g a ¼ 0 intersects of ðahnÞ1=2 vs. hn plots (see Fig. 3). Fig. 3 indicates that non-direct transition is certainly the mechanism responsible for the optical absorption in this spectral region of the GeSe2 films. The Tauc slopes B1/2 in Fig. 3 are different (see Table 1). The B1/2 value of the as-deposited film is smaller than those of annealed films. And the higher annealing temperature, the bigger Tauc slopes B1/2. The parameter B in Eq. (1) is assumed to be an indicator of the degree of structural randomness of amorphous semiconductors. The smaller B value, the higher the structural disorder [19]. It can be deduced that the film deposited at room temperature has higher disorder than the annealed films. This can also be seen from the next discussion about Raman spectra, which confirms as-deposited film has more homopolar bonds than annealed films. Table 1 shows annealing of the GeSe2 film increases Eopt g monotonously. The reduction in the density of homopolar bonds has been proposed to contribute to such increase of the optical band gap in amorphous chalcogenide films [19]. Se–Se homopolar bond defects are related to the localized states in the band structure. These unsatruated defects are gradually annealed out, which increases the optical band gap. Nemec et al. reported that PLD technique can produce amorphous thin films with different structure (in terms of different amounts of Ge–Ge or Se–Se
Fig. 1. XRD patterns of GeSe2 films as-deposited and annealed at 623 K.
Fig. 2. Optical transmission spectra of the GeSe2 films of as-deposited and annealed at different temperature.
3. Results and discussion
ARTICLE IN PRESS R.K. Pan et al. / Physica B 404 (2009) 3397–3400
Fig. 3. The determination of the optical band gaps in terms of Tauc’s law as linear extrapolation of the strong absorption data.
3399
Fig. 4. Raman spectra of GeSe2 films as-deposited and annealed at 623 K.
Table 1 Values of Tauc slope (B1/2), optical band gap (Eopt g ), and thickness (d) of the asdeposited and annealed GeSe2 films. GeSe2 film
B1/2 (cm eV)1/2
Eopt (eV) g
d (nm)
As-deposited 473 K Annealed 523 K Annealed 573 K Annealed 623 K Annealed
1004 1042 1048 1070 1104
1.9570.01 2.0270.01 2.0670.01 2.1070.01 2.1370.01
1830710 1630710 1350710 1310710 1290710
homopolar bonds) [20]. Annealing decreased the amounts of Se–Se homopolar bonds, which caused increase of the optical band gap. The discussion about Raman spectra confirms that the density of homopolar bonds decreased in as-deposited films after annealing. Fig. 4 shows the Raman spectra of the as-deposited GeSe2 film and the film annealed at 623 K. For the as-deposited film, a broad plateau less than 140 cm1 is related to unresolved lattice modes [21]. The weak shoulder near 175 cm1 can be attributed to Se3Ge–GeSe3 units. The main band near 197 cm1 is assigned to the A1 vibration modes of the corner-sharing GeSe4/2 tetrahedrons [22]. The companion peak near 213 cm1 can probably be assigned to the Ac1 breathing vibrations of edgeshared Ge2Se8/2 bi-tetrahedrons [20,22]. Due to higher disorder, laser ablated films contain higher density of Se–Se bonds (formation of ‘wrong bonds’) and large amounts of molecular fragments (including Sen chains) compared with bulk glasses. The second band of the Raman spectra of studied samples is a broad one (235–330 cm1), of moderate intensity, with three maxima near 250, 267 and 310 cm1. They can be assigned to chalcogen–chalcogen stretching vibration mode at the edges of large clusters, the A1 vibration modes of Se chains, F2 antisymmetric vibration modes of GeSe4/2 based structural units, respectively [20]. Annealing leads to two major changes in the Raman spectrum of as-deposited GeSe2 film. Firstly, the broad band from 235 to 330 cm1 drops after annealing, which means the density of Se–Se bonds deceased. Secondly, the weak shoulder near 175 cm1 and companion peak near 213 cm1 are enhanced slightly, which means more Se3Ge–GeSe3 units and edge-shared Ge2Se8/2 bi-tetrahedra formed at the expense of Se–Se bonds and cornersharing GeSe4/2 tetrahedrons. The relative enhanced companion
Fig. 5. Spectral dependences of refractive index of GeSe2 films as-deposited and annealed at different temperature.
peak near 213 cm1 can be related to the relative increase of the population of the medium-range structure which contain the Ge2Se8/2 bi-tetrahedra [17]. The Raman spectral evolution is consistent with the gradually increasing Tauc slopes after annealing. Spectral dependencies of the refractive index were calculated from the optical transmission spectra of as-deposited and annealed thin films using the Swanepoel method. Annealing of the as-deposited films induces the descending of refractive indices (see Fig. 5). Kincl et al. explained thermal-induced decrease in refractive index by the changes of structure and bonding arrangement, especially the decrease of homopolar bonds [23]. In the studied GeSe2 films, the bonding rearrangement can be confirmed by Raman spectral evolution. On the other hand, changes of refractive index induced by the density changing is a small part, which has been demonstrated by Kincl et al. [23]. The film thickness values calculated based on the transmission spectra are well consistent with those measured by the surface profiler. Table 1 shows the thickness of the as-deposited film decreased after annealing. The decreasing of thickness in the annealed GeSe2 films can be mainly explained by the diminution of porous structure in the fragments, which have some free spaces in the as-deposited films [8]. The as-deposited films prepared
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with high energy density of the laser beam, which are away from the equilibrium state, have some fragments of the ablated materials. Fragments interaction and diminution of porous structure in the fragments occured during annealing, which results in the decrease of the film thickness.
4. Conclusion Amorphous GeSe2 film has been successfully deposited at room temperature by PLD method. Analysis with Raman spectra shows that annealing decreased the refractive index of asdue to the reduction in the deposited films and increased Eopt g density of homopolar bonds. Bigger Tauc slope indicates lower degree of structural randomness of annealed films than that of asdeposited film. The as-deposited film thickness decreased after annealing, which was caused by the diminution of porous structure in the fragments of the films.
Acknowledgements This work has been supported by the National Natural Science Foundation of PR China (no. 60808024) and the Fund of Key Lab of Silicate Materials Science and Engineering, Ministry of Education, PR China (no. SYSJJ2007-07). References [1] N. Thoge, Y. Yamamto, T. Minami, M. Tanaka, Appl. Phys. Lett. 34 (1979) 640.
[2] L.A. Wahab, A. Adam, M.R. Balboul, N. Makram, A.A. El-Ela, K. Sedeek, Physica B 387 (2007) 81. [3] K. Sedeek, A. Adam, M.R. Balboul, L.A. Wahab, N. Makram, Mater. Res. Bull. 43 (2008) 1355. [4] S. Sugai, Phys. Rev. B 35 (1987) 1345. [5] M.F. Thorpe, J. Non-Cryst. Solids 182 (1995) 355. [6] C. Massobrio, A. Pasquarello, Phys. Rev. B 77 (2008) 144207. [7] E. Sleekx, L. Tichy, P. Nagels, R. Callnaerts, J. Non-Cryst. Solids 198–200 (1996) 723. [8] P. Neˇmec, M. Frumar, B. Frumarov, M. Jelinek, J. Lancok, J. Jedelsky, Opt. Mater. 15 (2000) 191 and papers cited therein. [9] M. Martino, A.P. Caricato, M. Fernandez, G. Leggieri, A. Jha, M. Ferrari, M. Mattarelli, Thin Solid Films 433 (2003) 39. [10] J.P. Kloock, L. Moreno, A. Bratov, S. Huachupoma, J. Xu, T. Wagner, T. Yoshinobu, Y. Ermolenko, Y.G. Vlasov, M.J. Scho¨ning, Sens. Actuators B 118 (2006) 149. [11] R.A. Jarvis, R.P. Wang, A.V. Rode, C. Zha, B.L. Davies, J. Non-Cryst. Solids 353 (2007) 947. [12] M. Frumar, B. Frumarova, P. Nemec, T. Wagner, J. Jedelsky, M. Hrdlicka, J. NonCryst. Solids 352 (2006) 544. [13] P. Neˇmec, M. Frumar, J. Optoelectron. Adv. Mater. 5 (2003) 1047. [14] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [15] A.M. Bernal-Oliva, E. Marquez, J.M. Gonzalez-Leal, A.J. Gamez, R. Prieto-Alcon, R. Jimenez-Garay, J. Mater. Sci. Lett. 16 (1997) 665. [16] E. Marquez, J.B. Ramirez-Malo, P. Villares, R. Jimenez-Garay, R. Swanepoel, Thin Solid Films 254 (1995) 83. [17] O. Matsuda, H. Takeuchi, Y. Wang, K. Inoue, K. Murase, J. Non-Cryst. Solids 232–234 (1998) 554. [18] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974, p. 171. [19] R.A. Street, R.J. Nemanich, G.A.N. Connell, Physica B 18 (1978) 690. [20] P. Neˇmec, J. Jedelsky, M. Frumar, M. Munzar, M. Jelinek, J. Lanccok, J. NonCryst. Solids 326–327 (2003) 53 and papers cited therein. [21] E. Mytilineou, B.S. Chao, D. Papadimitriou, J. Non-Cryst. Solids 195 (1996) 279. [22] T. Ikari, T. Tanaka, K. Ura, K. Maeda, K. Futagami, Phys. Rev. B 47 (1993) 4984. [23] M. Kincl, L. Tichy, Mater. Chem. Phys. 110 (2008) 322.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Thermal-induced gradually changes in the optical properties of amorphous GeSe2 film prepared by PLD R.K. Pan a,b, H.Z. Tao a,, H.C. Zang a, T.J. Zhang b, X.J. Zhao a, a b
Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan 430070, PR China School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China
a r t i c l e in fo
abstract
Article history: Received 10 February 2009 Received in revised form 7 May 2009 Accepted 15 May 2009
Amorphous GeSe2 film was prepared by the pulsed laser deposition technique and annealed at different temperature from 473 to 623 K. Using the ‘non-direct transition’ model proposed by Tauc, the short wavelength absorption edges of the films were well fitted and the optical band gaps (Eopt g ) were determined. The Tauc slope of the as-deposited film is smaller than those of annealed films, which were proposed as an indicator of the degree of structural randomness of amorphous semiconductors. The refractive index and thickness of the films were calculated from the optical transmission spectra using the Swanepoel method. The index of refraction decreased while Eopt g increased gradually with increasing the annealing temperature. The thermal-bleaching and thermal-contraction effects were observed, which were interpreted as the reduction in the density of homopolar bonds according to the Raman spectra analysis and the diminution of porous structure in the fragments of the annealed films, respectively. & 2009 Elsevier B.V. All rights reserved.
Keywords: GeSe2 film Pulsed laser deposition Thermal-bleaching Optical properties
1. Introduction Amorphous germanium selenide chalcogenide semiconductors have attracted much attention due to their promising technological applications [1–3]. GeSe2 is a typical chalcogenide semiconductor, which can be easily prepared either in glass form or in crystalline form. In recent years, many researches on GeSe2 glass have been made for studying rigidity percolation, short and intermediate range order of the structure [4–6]. Wahab investigated the AC conduction mechanism and photodarkening phenomenon of amorphous GeSe2 films prepared by thermal evaporation technique [2]. Sleekx reported thermalinduced bleaching in amorphous GeSe2 films prepared by plasmaenhanced chemical vapour deposition [7]. The preparation of homogeneous thin films of multicomponent chalcogenide systems is a difficult task. Thermal vacuum evaporation results very often in non-homogeneous films and in films with changed composition due to different volatility of components [8]. Pulsed laser deposition (PLD) technique has several advantages for thin film preparation, such as the stoichiometric transfer from the original target material to a given substrate, the easy set-up, short process time [9–11]. It is necessary to research the optical properties of GeSe2 film prepared by PLD. As-deposited films
Corresponding authors. Tel./fax: +86 27 87669729.
E-mail addresses: [email protected] (H.Z. Tao), [email protected] (X.J. Zhao). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.021
prepared with high energy density of the laser beam are away from the equilibrium state. Annealing can lead to changes in the structure and optical properties of the pulsed laser deposited films, which is interesting to be investigated. In this paper, GeSe2 film was prepared by PLD and was annealed at different temperatures below the glass transition temperature (Tg). The index of refraction and optical band gaps of the films were calculated from the optical transmission spectra and absorption spectra, respectively. Annealing was accompanied by structural changes in the GeSe2 films, which was confirmed by Raman spectra analysis. The changes in the refractive index and the thickness of the films caused by thermal annealing were also discussed.
2. Experimental The target used for PLD was bulk glass with stoichiometry GeSe2. Glass was synthesized from high purity elements (Ge and Se, all of 5 N purity) in evacuated (102 Pa) and flame-sealed silica ampoule which was loaded in a rocking furnace. The mixture was melted at about 950 1C for 12 h. After that, the ampoule was quenched in cold water then annealed near the corresponding glass transition temperature (Tg) for 2 h. The bulk sample was taken out from the ampoule and cut into 2 mm-thick-targets for depositing. The differential thermal analysis (DTA) measurement of bulk glass powder (40–50 mg) was carried out by a differential thermal analyzer (PE, DTA-1700) with an accuracy of 72 1C at a
ARTICLE IN PRESS 3398
R.K. Pan et al. / Physica B 404 (2009) 3397–3400
heating rate of 10 1C/min. Pure Al2O3 powder was used as reference material. The Tg of the glass was determined by the slope intercept method from measured DTA curves. Thin film was prepared from the target by the PLD technique using a KrF excimer laser operated at the wavelength of 248 nm. The film was deposited on the chemically cleaned microscope glass slides. The target was ablated in a vacuum chamber at a background pressure of 2 104 Pa with the laser energy 200 mJ/ pulse. The incident laser beam was focused to the average flux of about 5 J/cm2. The distance between the target and the substrate is about 8 cm in an off/axis approach geometry [12,13]. GeSe2 film was deposited at room temperature. The pulse repetition rate was set at 4 Hz and the incidence angle was 451. The as-deposited film was cut into five samples and four of them were annealed at different temperatures (473, 523, 573, 623 K) for 1 h in the vacuum chamber at pressure of 2 104 Pa, respectively. And the heating rates and cooling rates were about 10 and 1.25 1C/min, respectively. The homogeneity of the samples was confirmed by the electron probe X-ray microanalyzer (EPMA) (JEOL, JCXA-733). X-ray fluorescence spectrophotometer (Shimadzu, XRF-1800) was used to determinate the composition of the films. The transmission spectra and absorption spectra of the studied films were measured using the spectrophotometer (Shimadzu, UV-1601). For comparison, the thickness of the films were measured by a surface profiler (Taylor Honson, S4C-3D) and calculated using the Swanepoel method based on the transmission spectra of thin films [14–16]. The Raman spectra were measured at the room temperature by using a Raman spectrometer (Renishaw, RM-1000) in back (1801) scattering configuration. Laser irradiation at the wavelength of 514 nm was used for the excitation. The laser power was properly under an approximate level of 2.0 mW to avoid the laser damage on the samples. The resolution of the Raman spectra was 1 cm1.
thermal crystallization temperature of the evaporated GeSe2 film was between 360 and 390 1C [17]. It can be deduced that Tg of the evaporated GeSe2 film is lower than that of pulsed laser deposited films. The difference of Tg shows smaller structural fluctuation in the deposited films than in the evaporated films. Fig. 2 shows the optical transmission spectra of the asdeposited and annealed films. The absorption edges of films shift to shorter wavelength after annealing (i.e. thermalbleaching). The observed thermally induced bleaching effect cannot be correlated to oxygen-assisted bond reconstruction since the preparation and annealing of the films had proceeded in the vacuum chamber. In the strong absorption region (the absorption coefficient a4104 cm1) of the absorption spectra, the absorption coefficient is given by the following quadratic equation according to the ‘nondirect transition’ model proposed by Tauc [18],
aðhvÞ ¼ B
2 ðhv Eopt g Þ
hv
(1)
The Tg of GeSe2 bulk sample is about 414 1C. The EPMA and XRF measurements show that the chemical compositions of the films are homogenous and close to the target. The atomic ratio of the target is maintained within 1% in the film deposited by PLD. The XRD patterns confirm the amorphous character of the asdeposited and annealed films (see Fig. 1). Matsuda reported the
is the Tauc where B is a constant, hv is the photon energy and Eopt g can be calculated from the optical band gap. The values of Eopt g a ¼ 0 intersects of ðahnÞ1=2 vs. hn plots (see Fig. 3). Fig. 3 indicates that non-direct transition is certainly the mechanism responsible for the optical absorption in this spectral region of the GeSe2 films. The Tauc slopes B1/2 in Fig. 3 are different (see Table 1). The B1/2 value of the as-deposited film is smaller than those of annealed films. And the higher annealing temperature, the bigger Tauc slopes B1/2. The parameter B in Eq. (1) is assumed to be an indicator of the degree of structural randomness of amorphous semiconductors. The smaller B value, the higher the structural disorder [19]. It can be deduced that the film deposited at room temperature has higher disorder than the annealed films. This can also be seen from the next discussion about Raman spectra, which confirms as-deposited film has more homopolar bonds than annealed films. Table 1 shows annealing of the GeSe2 film increases Eopt g monotonously. The reduction in the density of homopolar bonds has been proposed to contribute to such increase of the optical band gap in amorphous chalcogenide films [19]. Se–Se homopolar bond defects are related to the localized states in the band structure. These unsatruated defects are gradually annealed out, which increases the optical band gap. Nemec et al. reported that PLD technique can produce amorphous thin films with different structure (in terms of different amounts of Ge–Ge or Se–Se
Fig. 1. XRD patterns of GeSe2 films as-deposited and annealed at 623 K.
Fig. 2. Optical transmission spectra of the GeSe2 films of as-deposited and annealed at different temperature.
3. Results and discussion
ARTICLE IN PRESS R.K. Pan et al. / Physica B 404 (2009) 3397–3400
Fig. 3. The determination of the optical band gaps in terms of Tauc’s law as linear extrapolation of the strong absorption data.
3399
Fig. 4. Raman spectra of GeSe2 films as-deposited and annealed at 623 K.
Table 1 Values of Tauc slope (B1/2), optical band gap (Eopt g ), and thickness (d) of the asdeposited and annealed GeSe2 films. GeSe2 film
B1/2 (cm eV)1/2
Eopt (eV) g
d (nm)
As-deposited 473 K Annealed 523 K Annealed 573 K Annealed 623 K Annealed
1004 1042 1048 1070 1104
1.9570.01 2.0270.01 2.0670.01 2.1070.01 2.1370.01
1830710 1630710 1350710 1310710 1290710
homopolar bonds) [20]. Annealing decreased the amounts of Se–Se homopolar bonds, which caused increase of the optical band gap. The discussion about Raman spectra confirms that the density of homopolar bonds decreased in as-deposited films after annealing. Fig. 4 shows the Raman spectra of the as-deposited GeSe2 film and the film annealed at 623 K. For the as-deposited film, a broad plateau less than 140 cm1 is related to unresolved lattice modes [21]. The weak shoulder near 175 cm1 can be attributed to Se3Ge–GeSe3 units. The main band near 197 cm1 is assigned to the A1 vibration modes of the corner-sharing GeSe4/2 tetrahedrons [22]. The companion peak near 213 cm1 can probably be assigned to the Ac1 breathing vibrations of edgeshared Ge2Se8/2 bi-tetrahedrons [20,22]. Due to higher disorder, laser ablated films contain higher density of Se–Se bonds (formation of ‘wrong bonds’) and large amounts of molecular fragments (including Sen chains) compared with bulk glasses. The second band of the Raman spectra of studied samples is a broad one (235–330 cm1), of moderate intensity, with three maxima near 250, 267 and 310 cm1. They can be assigned to chalcogen–chalcogen stretching vibration mode at the edges of large clusters, the A1 vibration modes of Se chains, F2 antisymmetric vibration modes of GeSe4/2 based structural units, respectively [20]. Annealing leads to two major changes in the Raman spectrum of as-deposited GeSe2 film. Firstly, the broad band from 235 to 330 cm1 drops after annealing, which means the density of Se–Se bonds deceased. Secondly, the weak shoulder near 175 cm1 and companion peak near 213 cm1 are enhanced slightly, which means more Se3Ge–GeSe3 units and edge-shared Ge2Se8/2 bi-tetrahedra formed at the expense of Se–Se bonds and cornersharing GeSe4/2 tetrahedrons. The relative enhanced companion
Fig. 5. Spectral dependences of refractive index of GeSe2 films as-deposited and annealed at different temperature.
peak near 213 cm1 can be related to the relative increase of the population of the medium-range structure which contain the Ge2Se8/2 bi-tetrahedra [17]. The Raman spectral evolution is consistent with the gradually increasing Tauc slopes after annealing. Spectral dependencies of the refractive index were calculated from the optical transmission spectra of as-deposited and annealed thin films using the Swanepoel method. Annealing of the as-deposited films induces the descending of refractive indices (see Fig. 5). Kincl et al. explained thermal-induced decrease in refractive index by the changes of structure and bonding arrangement, especially the decrease of homopolar bonds [23]. In the studied GeSe2 films, the bonding rearrangement can be confirmed by Raman spectral evolution. On the other hand, changes of refractive index induced by the density changing is a small part, which has been demonstrated by Kincl et al. [23]. The film thickness values calculated based on the transmission spectra are well consistent with those measured by the surface profiler. Table 1 shows the thickness of the as-deposited film decreased after annealing. The decreasing of thickness in the annealed GeSe2 films can be mainly explained by the diminution of porous structure in the fragments, which have some free spaces in the as-deposited films [8]. The as-deposited films prepared
ARTICLE IN PRESS 3400
R.K. Pan et al. / Physica B 404 (2009) 3397–3400
with high energy density of the laser beam, which are away from the equilibrium state, have some fragments of the ablated materials. Fragments interaction and diminution of porous structure in the fragments occured during annealing, which results in the decrease of the film thickness.
4. Conclusion Amorphous GeSe2 film has been successfully deposited at room temperature by PLD method. Analysis with Raman spectra shows that annealing decreased the refractive index of asdue to the reduction in the deposited films and increased Eopt g density of homopolar bonds. Bigger Tauc slope indicates lower degree of structural randomness of annealed films than that of asdeposited film. The as-deposited film thickness decreased after annealing, which was caused by the diminution of porous structure in the fragments of the films.
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