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Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition
Thin Solid Films 517 (2009) 3635–3638
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition P. Němec ⁎, M. Frumar Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Legions Sq. 565, 53210 Pardubice, Czech Republic
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 6 November 2008 Accepted 13 November 2008 Available online 24 November 2008
a b s t r a c t Thin amorphous As–S films were prepared using pulsed laser deposition. Raman scattering spectroscopy, variable angle spectroscopic ellipsometry, and optical transmittance spectra revealed irreversible photostructural effects, significant photoinduced changes of refractive index, and optical band gap energy in the films. Observed effects are discussed in terms of structural transformations of basic structural units. © 2008 Elsevier B.V. All rights reserved.
PACS: 68.55.-a 78.20.Ci 78.66.Jg 81.15.Fg Keywords: Amorphous materials Optical properties Raman scattering Pulsed laser deposition
1. Introduction Photoinduced phenomena in semiconducting thin films originating in amorphous chalcogens and chalcogenides (S-, Se-, and Te-based compounds) are widely studied in last time due to scientific as well as technological significance [1]. From a variety of irradiation induced effects, photocrystallization (photoamorphization), photopolymerization, photodecomposition, and photocontraction (photoexpansion) should be mentioned as examples. These processes are linked with the changes in band gap energy (photodarkening or photobleaching), absorption coefficient, and refractive index (photorefraction) [2]. Mentioned phenomena are not observed in crystalline materials nor in amorphous semiconductors from IV or V group of periodical system; they are characteristic for amorphous chalcogens and chalcogenides thanks to their structural flexibility and electronic lone-pair p states forming the top part of their valence band [2,3]. Irreversible photoinduced effects occur in as-deposited thin films, whereas reversible ones proceed in well-annealed (below but near by the glass transition temperature) films and bulk glasses, respectively. The reversibility of the process means the ability of the films to restore their initial properties after post-irradiation re-annealing [1].
⁎ Corresponding author. Tel.: +420466037265; fax: +420466037068. E-mail address: [email protected] (P. Němec). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.110
Different deposition techniques are useful for fabrication of thin films; vacuum thermal evaporation, DC/RF sputtering, chemical vapour deposition, and spin coating are of importance. One of prospective deposition methods is pulsed laser deposition (PLD) being favourable due to its simplicity, easy control of the process, often stoichiometric transfer of target material to the films, and possibility to fabricate films of unusual composition. Probably the most important actual merit of PLD over other thin film fabrication methods is its flexibility reflecting the fact that PLD can be applied to essentially any material, from metals, through binary/ternary compounds, to multicomponent materials [4]. Glasses and amorphous thin films from As–S system are probably ones of the most studied chalcogenide materials, because they possess superior glass forming tendency (up to 40 at.% of arsenic), ability to prepare thin films of good quality, and strong light- and annealinginduced changes of structure, optical properties, etc. [1] and references cited therein. The research of As–S thin films is of high importance due to their remarkable properties applicable in alloptical photonic circuits and their parts (waveguides, gratings, etc.) [1]. The motivation of this work is thus to connect the preparation of interesting inorganic thin films with perspective deposition method — PLD, to characterize, to describe and understand photoinduced phenomena in fabricated films. In previous work, we studied the preparation of As–S (As–Se) amorphous thin films using PLD and their structure using Raman
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spectroscopy [5,6]. In this paper, we report on irreversible photoinduced changes of structure and optical properties (optical band gap, refractive index) of PLD As–S layers caused by cw Ar+ laser exposure. 2. Experimental details For the preparation of amorphous thin films by PLD, targets of chalcogenide glass with nominal composition of As40S60 were used. The target glass was synthesized by conventional melt-quenching. A KrF excimer laser operating at 248 nm with constant output energy of 300 mJ per pulse (±1%), with pulse duration of 30 ns, and with repetition rate of 20 Hz was used for PLD of amorphous As–S thin films. The energy fluency on the target was constant (~1.6 J cm− 2). The laser beam was incident on the target under an angle of about 45°. Amorphous thin films were deposited in a vacuum chamber (background pressure ~6 x 10− 4 Pa). The substrates used for PLD (chemically cleaned microscope glass slides) were positioned parallel to the target surface. The target to substrate distance was 5 cm. The number of laser pulses used for the preparation of the films was 15,000. The off-axis PLD technique combining rotating substrates and scanning of the targets’ surface (~1 x 1.2 cm2 area) by the laser beam was used for the fabrication of appropriate quality films in terms of homogeneity in thickness. The composition of the films was controlled using X-ray fluorescence analyzer (EAGLE XPL μ-Probe) and energy-dispersive X-ray microanalyzer (IXRF Systems) attached to electron microscope (JEOL JSM-5500LV instrument). Room temperature Raman scattering spectra were measured with the Fourier transform Raman spectrophotometer Bruker IFS 55/FRA106 using a backscattering method with Nd:YAG laser (1064 nm) as excitation source. The resolution and number of scans were 2 cm− 1 and 300, respectively. Refractive indices and the thicknesses of the films were evaluated using variable angle spectroscopic ellipsometry (VASE, J.A. Woollam Co., Inc.) in spectral range 600–2300 nm measuring at 65, 70, and 75° angles of incidence. The values of optical band gap (Eopt g ) of the films were evaluated from the optical transmission spectra using α = 0 intersects of (αhν)1/ 2 vs. hν plots, where α is the absorption coefficient estimated according
Fig. 1. Raman scattering spectra of PLD thin As48S52 films: (1) as-deposited film, (2) film exposed by 488 nm laser light. Spectra were normalized to the amplitude of Raman band at ~ 360 cm− 1.
Fig. 2. Spectral dependencies of refractive index of PLD thin As48S52 films: (1) asdeposited film, (2) film exposed by 488 nm laser light.
to [7], and hν is the photon energy. Error in determination of Eopt g values is ± 0.01 eV. In order to study irreversible photoinduced effects, as prepared PLD films were exposed in nitrogen atmosphere (in order to avoid the possibility of oxidation of the films) by cw Ar+ ion laser (Melles-Griot, λ = 488 nm, P =85 mW, laser spot diameter of 2 mm) for 120 min. To confirm the irreversibility of photoinduced effects, exposed films were annealed in inert atmosphere (pure nitrogen) at 80 °C (~20 °C below but near by their glass transition temperature, which is ~100 °C) for 120 min and slowly cooled down to room temperature at 1 °C min− 1. 3. Results Using PLD, we have fabricated As–S thin amorphous films with thickness of ~800 nm, which we selected considering penetration depth of the laser light used for the study of photoinduced phenomena. Estimated thickness gives growth rate of the layers ~1.1 nm s− 1. Recognized composition of the films is As48S52 (±1 at.%). In comparison with the glassy target (As40S60), the films contain overstoichiometric content of arsenic (~8 at.%). Prepared thin films were amorphous and homogeneous as results from optical transmittance spectra and optical microscopy. The presence of droplets on the surface of the films is sporadic as observed by electron microscope. The roughness of the films is low (~1–2 nm) as determined by VASE. Raman scattering spectra of As–S thin films are shown in Fig. 1. Dominant band of Raman spectrum of as-deposited film is peaking near 360 cm− 1 having shoulders at ~330, 344, and 380 cm− 1. Other features of the Raman spectrum are bands with maxima observed at ~ 273, 236 (with shoulder at ~ 260 cm− 1), 222, 200, 187 (with shoulder at ~170 cm− 1), and 146 cm− 1 (with shoulders at ~ 154 and 134 cm− 1). There are changes in Raman spectra of exposed films, when compared with as-deposited films; the most prominent are increase of amplitude of band peaking at ~273 cm− 1 and increase of bands’ amplitudes in 130-210 cm− 1 region (Fig. 1). Raman band in 290410 cm− 1 region becomes narrower; peaks (~ 343, 359 cm− 1) and shoulders (~ 310, 330, 377 cm− 1) forming this band are emphasized due to exposure. Raman band in 210-265 cm− 1 region is narrower, the maxima of this band are more pronounced and the amplitudes are depressed. The annealing of exposed films has nearly no effect on the shape of Raman spectra. We note that the peaks in the Raman spectra are relatively sharp; this fact is probably connected with ”molecular“
P. Němec, M. Frumar / Thin Solid Films 517 (2009) 3635–3638
structure of the films implying a weak intermolecular coupling of vibrating units leading to observed Raman bands being sharper in comparison with broad features typical for amorphous materials. Refractive indices values and thicknesses of As–S layers were obtained from the analysis of VASE data. These data were analyzed by a model consisting of three layers (substrate, film, surface) optical constants (index of refraction (n), extinction coefficient (k)) description. Refractive indices of the As–S films were fitted using Cauchy dispersion formula, i.e. n = A + B/λ2 + C/λ4, where A, B, and C are constants, λ is the wavelength. Surface roughness layer was defined by effective medium approximation. Obtained spectral dependencies of refractive indices for as-prepared and exposed films are given in Fig. 2. Optical band gap value of as-prepared films determined from Tauc plots was estimated to be ~2.17 eV. After the exposure, large bleaching of the films was observed; optical band gap values increased to ~2.33 eV, when 488 nm laser line was used for the irradiation. The annealing of exposed films has nearly no effect on optical band gap values — they are saturated near 2.36 eV.
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This thesis is supported by the fact that VASE results did not evidence changes in the thickness of the films (photoexpansion/photocontraction) accompanying the photorefraction. According to “Moss rule” [29], negative irreversible photorefraction should be followed by the increase of optical band gap energy (i.e. blueshift of the fundamental absorption edge, photobleaching). We found that optical band gap value of PLD As48S52 films is increasing from initial value of ~2.17 to ~2.33 eV due to the exposure, thus significant growth of 7 % is observed. The value of Eopt of as-deposited PLD As40S60 films is ~ 2.23 eV as g reported in [30]. Considering optical band gap energy of amorphous arsenic, which varies from ~ 1.1 eV to ~ 1.4 eV depending on the method of preparation and the form (bulk or film) [31], we assume that higher content of arsenic is responsible for observed lower Eopt g values of PLD As48S52 films when compared with As40S60 films. One can speculate that the reduction of arsenic clusters content during the irradiation, assuming their photoinduced reactions with As4S5 molecules forming α-, β-, γ-As4S4 and As4S3 molecules (as can be expected from the Raman spectra discussed above), is the main reason for observed photobleaching.
4. Discussion 5. Conclusion In agreement with our previous report [5], using literature data cited therein [8–24], and on the basis of presented Raman spectra (Fig. 1) we can predict that the structure of as-prepared As–S thin films is formed by (i) γ-As4S4 (pararealgar-like) and χ-As4S4 molecules, (ii) β-As4S4 molecules, (iii) clusters of amorphous arsenic (probably formed by As4 structural units), (iv) S2As–AsS2 clusters, and (v) As4S5 (uzonite-like) entities. The presence of α-As4S4 (realgar), As4S3 (dimorphite) molecules, and AsS3 pyramids is confirmed as well. Large content of arsenic-rich molecular entities in the films is due to high arsenic content in the layers (8 at. % overstoichiometry in comparison with As40S60 stoichiometric compound). Due to the exposure of as-deposited As–S films, content of As4S3 entities in the films is increasing, as we conclude from the increase of amplitudes of Raman bands peaking at 273 and (partly) 200 cm− 1, which can be assigned to As4S3 molecules [25]. From the rising of Raman bands amplitudes at 146, 187, and ~ 360 cm− 1, we deduce that the content of α- and β-As4S4 is growing after the irradiation [26,27]. Shift of the Raman band peaking at 222 cm− 1 to 217 cm− 1 due to the exposure as well as the decrease of the amplitude of the band at 222 and 235 cm− 1 can be connected with the decrease of the content of χAs4S4 molecules and arsenic clusters [28] but not γ-As4S4 (having in crystalline form its characteristic strongest Raman doublet at 230 and 235 cm−1 [26]), because other Raman active bands of γ-As4S4 (~ 154, 170 cm− 1) are emphasized due to irradiation. According to Lucovsky [8], we can assume that α- and β-As4S4 molecules are formed from S2As-AsS2 clusters; this is again accompanied with the decrease of Raman intensity in 220-230 cm− 1 region. Significant intensity decrease in 290–350 cm − 1 region has probably its origin in photoinduced transformations of As4S5 molecules. The content of AsS3 pyramids in exposed films slightly increases in comparison with as-prepared films as we can judge from small increase of 134 cm− 1 band intensity and pronouncing of the 343 cm− 1 band [28]. Spectral dependencies of refractive index of PLD As–S films obtained from VASE measurements show clearly that refractive index values are significantly decreasing due to exposure of the films (Fig. 2). As an example, refractive index value drops from initial value of 2.32 to 2.23 at telecommunication wavelength of 1550 nm; this represents 4% decrease of original value belonging to as-deposited film. Annealing of exposed films below but near the glass transition temperature has only marginal effect on refractive index values confirming the irreversibility of photorefraction effect. Observed irreversible photorefraction has probably its origin in pure photostructural phenomena leading in changes in polarizability due to different content of basic structural motifs (having different polarizabilities) present in the films before and after exposure.
Off-axis pulsed laser deposition combining rotating substrates and scanning of the targets’ surface by the laser beam was employed for the preparation of thin As–S amorphous films. Irreversible changes of the structure caused by Ar+ laser beam and observed by Raman spectroscopy were interpreted in terms of photoinduced transformations of amorphous arsenic, S2As–AsS2 clusters, χ-As4S4, and As4S5 molecules mainly to As4S3, α- and β-As4S4 entities. Significant negative photorefraction (~4 % decrease of refractive index) and photobleaching (~7 % increase of optical band gap energy) were tentatively assigned to photostructural effects. Observed irreversible photoinduced phenomena are of opposite behaviour in comparison with As–S films prepared by thermal evaporation technique (where positive photorefraction and photodarkening are usually observed), probably originating from differences in chemical composition of the layers and different structure of as-deposited films prepared by various techniques. Acknowledgements It is a pleasure for authors to acknowledge financial support of the Ministry of Education, Youth and Sports of the Czech Republic (projects MSM 0021627501 and LC523, respectively) and Czech Science Foundation (project No.104/08/0229). We are grateful to Dr. Z. Černošek for the measurements of Raman scattering spectra. References [1] A.V. Kolobov, Photo-Induced Metastability in Amorphous Semiconductors, WileyWCH, Weinheim, 2003, p. 23. [2] A. Zakery, S.R. Elliott, J. Non-Cryst. Solids 330 (2003) 1. [3] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. Non-Cryst. Solids 299-302 (2002) 917. [4] M. Frumar, B. Frumarova, P. Nemec, T. Wagner, J. Jedelsky, M. Hrdlicka, J. Non-Cryst. Solids 352 (2006) 544. [5] P. Nemec, J. Jedelsky, M. Frumar, M. Vlcek, J. Optoelectron. Adv. Mater. 7 (2005) 2635. [6] P. Nemec, M. Frumar, Thin Solid Films 516 (2008) 8277. [7] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [8] G. Lucovsky, F.L. Galeener, R.H. Geils, R.C. Keezer, Proc. Conf. Structure of NonCrystalline Materials, Taylor and Francis, London, 1977, p. 127. [9] G. Lucovsky, R.M. Martin, J. Non-Cryst. Solids 8-10 (1972) 185. [10] R.J. Kobliska, S.A. Solin, J. Non-Cryst. Solids 8-10 (1972) 191. [11] S.A. Solin, G.V. Papatheodorou, Phys. Rev. B 15 (1977) 2087. [12] R. Forneris, Am. Mineral. 54 (1969) 1062. [13] W. Bues, M. Somer, W. Brockner, Z. Anorg. Allg. Chem. 499 (1983) 7. [14] B.H. Christian, R.J. Gillespie, J.F. Sawyer, Inorg. Chem. 20 (1981) 3410. [15] T. Chattopadhyay, C. Carlone, A. Jayaraman, H.G. Vonschnering, J. Phys. Chem. Solids 43 (1982) 277. [16] Z. Polak, Ph.D. Thesis, University of Pardubice, Czech Republic, 1998. [17] M.L. Slade, R. Zallen, Solid State Commun. 30 (1979) 357. [18] J.S. Lannin, Phys. Rev. B 15 (1977) 3863.
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[19] A. Rogstad, J. Mol. Struct. 14 (1972) 421. [20] V.I. Karataev, V.M. Lyubin , B.A. Mamyrin, Zh. Tekh. Fiz. 58 (1988) 1767. [21] D.J. Treacy, U. Strom, P.B. Klein, P.C. Taylor, T.P. Martin, J. Non-Cryst. Solids 35-36 (1980) 1035. [22] G. Lucovsky, in: J. Stuke, W. Bremig (Eds.), Amorphous and Liquid Semiconductors, Taylor and Francis, New York, 1974, p. 1099. [23] A.P. De Fonzo, J. Tauc, Phys. Rev. B 18 (1978) 6957. [24] T. Mori, K. Matsuishi, T. Arai, J. Non-Cryst. Solids 65 (1984) 269. [25] W. Bues, M. Somer, W. Brockner, Z. Naturforsch. 35 (1980) 1063. [26] M. Muniz-Miranda, G. Sbrana, P. Bonazzi, S. Menchetti, G. Pratesi, Spectrochim. Acta A 52 (1996) 1391.
[27] P. Bonazzi, S. Menchetti, G. Pratesi, M. Muniz-Miranda, G. Sbrana, Am. Mineral. 81 (1996) 874. [28] P. Nemec, J. Jedelsky, M. Frumar, Z. Cernosek, M. Vlcek, J. Non-Cryst. Solids 351 (2005) 3497. [29] T.S. Moss, Proc. Phys. Soc. London 63 B (1950) 167. [30] T. Wagner, M. Krbal, J. Jedelsky, M. Vlcek, B. Frumarova, M. Frumar, J. Optoelectron. Adv. Mater. 7 (2005) 153. [31] G.N. Greaves, S.R. Elliott, E.A. Davis, Adv. Phys. 28 (1979) 49.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition P. Němec ⁎, M. Frumar Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Legions Sq. 565, 53210 Pardubice, Czech Republic
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 6 November 2008 Accepted 13 November 2008 Available online 24 November 2008
a b s t r a c t Thin amorphous As–S films were prepared using pulsed laser deposition. Raman scattering spectroscopy, variable angle spectroscopic ellipsometry, and optical transmittance spectra revealed irreversible photostructural effects, significant photoinduced changes of refractive index, and optical band gap energy in the films. Observed effects are discussed in terms of structural transformations of basic structural units. © 2008 Elsevier B.V. All rights reserved.
PACS: 68.55.-a 78.20.Ci 78.66.Jg 81.15.Fg Keywords: Amorphous materials Optical properties Raman scattering Pulsed laser deposition
1. Introduction Photoinduced phenomena in semiconducting thin films originating in amorphous chalcogens and chalcogenides (S-, Se-, and Te-based compounds) are widely studied in last time due to scientific as well as technological significance [1]. From a variety of irradiation induced effects, photocrystallization (photoamorphization), photopolymerization, photodecomposition, and photocontraction (photoexpansion) should be mentioned as examples. These processes are linked with the changes in band gap energy (photodarkening or photobleaching), absorption coefficient, and refractive index (photorefraction) [2]. Mentioned phenomena are not observed in crystalline materials nor in amorphous semiconductors from IV or V group of periodical system; they are characteristic for amorphous chalcogens and chalcogenides thanks to their structural flexibility and electronic lone-pair p states forming the top part of their valence band [2,3]. Irreversible photoinduced effects occur in as-deposited thin films, whereas reversible ones proceed in well-annealed (below but near by the glass transition temperature) films and bulk glasses, respectively. The reversibility of the process means the ability of the films to restore their initial properties after post-irradiation re-annealing [1].
⁎ Corresponding author. Tel.: +420466037265; fax: +420466037068. E-mail address: [email protected] (P. Němec). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.110
Different deposition techniques are useful for fabrication of thin films; vacuum thermal evaporation, DC/RF sputtering, chemical vapour deposition, and spin coating are of importance. One of prospective deposition methods is pulsed laser deposition (PLD) being favourable due to its simplicity, easy control of the process, often stoichiometric transfer of target material to the films, and possibility to fabricate films of unusual composition. Probably the most important actual merit of PLD over other thin film fabrication methods is its flexibility reflecting the fact that PLD can be applied to essentially any material, from metals, through binary/ternary compounds, to multicomponent materials [4]. Glasses and amorphous thin films from As–S system are probably ones of the most studied chalcogenide materials, because they possess superior glass forming tendency (up to 40 at.% of arsenic), ability to prepare thin films of good quality, and strong light- and annealinginduced changes of structure, optical properties, etc. [1] and references cited therein. The research of As–S thin films is of high importance due to their remarkable properties applicable in alloptical photonic circuits and their parts (waveguides, gratings, etc.) [1]. The motivation of this work is thus to connect the preparation of interesting inorganic thin films with perspective deposition method — PLD, to characterize, to describe and understand photoinduced phenomena in fabricated films. In previous work, we studied the preparation of As–S (As–Se) amorphous thin films using PLD and their structure using Raman
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spectroscopy [5,6]. In this paper, we report on irreversible photoinduced changes of structure and optical properties (optical band gap, refractive index) of PLD As–S layers caused by cw Ar+ laser exposure. 2. Experimental details For the preparation of amorphous thin films by PLD, targets of chalcogenide glass with nominal composition of As40S60 were used. The target glass was synthesized by conventional melt-quenching. A KrF excimer laser operating at 248 nm with constant output energy of 300 mJ per pulse (±1%), with pulse duration of 30 ns, and with repetition rate of 20 Hz was used for PLD of amorphous As–S thin films. The energy fluency on the target was constant (~1.6 J cm− 2). The laser beam was incident on the target under an angle of about 45°. Amorphous thin films were deposited in a vacuum chamber (background pressure ~6 x 10− 4 Pa). The substrates used for PLD (chemically cleaned microscope glass slides) were positioned parallel to the target surface. The target to substrate distance was 5 cm. The number of laser pulses used for the preparation of the films was 15,000. The off-axis PLD technique combining rotating substrates and scanning of the targets’ surface (~1 x 1.2 cm2 area) by the laser beam was used for the fabrication of appropriate quality films in terms of homogeneity in thickness. The composition of the films was controlled using X-ray fluorescence analyzer (EAGLE XPL μ-Probe) and energy-dispersive X-ray microanalyzer (IXRF Systems) attached to electron microscope (JEOL JSM-5500LV instrument). Room temperature Raman scattering spectra were measured with the Fourier transform Raman spectrophotometer Bruker IFS 55/FRA106 using a backscattering method with Nd:YAG laser (1064 nm) as excitation source. The resolution and number of scans were 2 cm− 1 and 300, respectively. Refractive indices and the thicknesses of the films were evaluated using variable angle spectroscopic ellipsometry (VASE, J.A. Woollam Co., Inc.) in spectral range 600–2300 nm measuring at 65, 70, and 75° angles of incidence. The values of optical band gap (Eopt g ) of the films were evaluated from the optical transmission spectra using α = 0 intersects of (αhν)1/ 2 vs. hν plots, where α is the absorption coefficient estimated according
Fig. 1. Raman scattering spectra of PLD thin As48S52 films: (1) as-deposited film, (2) film exposed by 488 nm laser light. Spectra were normalized to the amplitude of Raman band at ~ 360 cm− 1.
Fig. 2. Spectral dependencies of refractive index of PLD thin As48S52 films: (1) asdeposited film, (2) film exposed by 488 nm laser light.
to [7], and hν is the photon energy. Error in determination of Eopt g values is ± 0.01 eV. In order to study irreversible photoinduced effects, as prepared PLD films were exposed in nitrogen atmosphere (in order to avoid the possibility of oxidation of the films) by cw Ar+ ion laser (Melles-Griot, λ = 488 nm, P =85 mW, laser spot diameter of 2 mm) for 120 min. To confirm the irreversibility of photoinduced effects, exposed films were annealed in inert atmosphere (pure nitrogen) at 80 °C (~20 °C below but near by their glass transition temperature, which is ~100 °C) for 120 min and slowly cooled down to room temperature at 1 °C min− 1. 3. Results Using PLD, we have fabricated As–S thin amorphous films with thickness of ~800 nm, which we selected considering penetration depth of the laser light used for the study of photoinduced phenomena. Estimated thickness gives growth rate of the layers ~1.1 nm s− 1. Recognized composition of the films is As48S52 (±1 at.%). In comparison with the glassy target (As40S60), the films contain overstoichiometric content of arsenic (~8 at.%). Prepared thin films were amorphous and homogeneous as results from optical transmittance spectra and optical microscopy. The presence of droplets on the surface of the films is sporadic as observed by electron microscope. The roughness of the films is low (~1–2 nm) as determined by VASE. Raman scattering spectra of As–S thin films are shown in Fig. 1. Dominant band of Raman spectrum of as-deposited film is peaking near 360 cm− 1 having shoulders at ~330, 344, and 380 cm− 1. Other features of the Raman spectrum are bands with maxima observed at ~ 273, 236 (with shoulder at ~ 260 cm− 1), 222, 200, 187 (with shoulder at ~170 cm− 1), and 146 cm− 1 (with shoulders at ~ 154 and 134 cm− 1). There are changes in Raman spectra of exposed films, when compared with as-deposited films; the most prominent are increase of amplitude of band peaking at ~273 cm− 1 and increase of bands’ amplitudes in 130-210 cm− 1 region (Fig. 1). Raman band in 290410 cm− 1 region becomes narrower; peaks (~ 343, 359 cm− 1) and shoulders (~ 310, 330, 377 cm− 1) forming this band are emphasized due to exposure. Raman band in 210-265 cm− 1 region is narrower, the maxima of this band are more pronounced and the amplitudes are depressed. The annealing of exposed films has nearly no effect on the shape of Raman spectra. We note that the peaks in the Raman spectra are relatively sharp; this fact is probably connected with ”molecular“
P. Němec, M. Frumar / Thin Solid Films 517 (2009) 3635–3638
structure of the films implying a weak intermolecular coupling of vibrating units leading to observed Raman bands being sharper in comparison with broad features typical for amorphous materials. Refractive indices values and thicknesses of As–S layers were obtained from the analysis of VASE data. These data were analyzed by a model consisting of three layers (substrate, film, surface) optical constants (index of refraction (n), extinction coefficient (k)) description. Refractive indices of the As–S films were fitted using Cauchy dispersion formula, i.e. n = A + B/λ2 + C/λ4, where A, B, and C are constants, λ is the wavelength. Surface roughness layer was defined by effective medium approximation. Obtained spectral dependencies of refractive indices for as-prepared and exposed films are given in Fig. 2. Optical band gap value of as-prepared films determined from Tauc plots was estimated to be ~2.17 eV. After the exposure, large bleaching of the films was observed; optical band gap values increased to ~2.33 eV, when 488 nm laser line was used for the irradiation. The annealing of exposed films has nearly no effect on optical band gap values — they are saturated near 2.36 eV.
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This thesis is supported by the fact that VASE results did not evidence changes in the thickness of the films (photoexpansion/photocontraction) accompanying the photorefraction. According to “Moss rule” [29], negative irreversible photorefraction should be followed by the increase of optical band gap energy (i.e. blueshift of the fundamental absorption edge, photobleaching). We found that optical band gap value of PLD As48S52 films is increasing from initial value of ~2.17 to ~2.33 eV due to the exposure, thus significant growth of 7 % is observed. The value of Eopt of as-deposited PLD As40S60 films is ~ 2.23 eV as g reported in [30]. Considering optical band gap energy of amorphous arsenic, which varies from ~ 1.1 eV to ~ 1.4 eV depending on the method of preparation and the form (bulk or film) [31], we assume that higher content of arsenic is responsible for observed lower Eopt g values of PLD As48S52 films when compared with As40S60 films. One can speculate that the reduction of arsenic clusters content during the irradiation, assuming their photoinduced reactions with As4S5 molecules forming α-, β-, γ-As4S4 and As4S3 molecules (as can be expected from the Raman spectra discussed above), is the main reason for observed photobleaching.
4. Discussion 5. Conclusion In agreement with our previous report [5], using literature data cited therein [8–24], and on the basis of presented Raman spectra (Fig. 1) we can predict that the structure of as-prepared As–S thin films is formed by (i) γ-As4S4 (pararealgar-like) and χ-As4S4 molecules, (ii) β-As4S4 molecules, (iii) clusters of amorphous arsenic (probably formed by As4 structural units), (iv) S2As–AsS2 clusters, and (v) As4S5 (uzonite-like) entities. The presence of α-As4S4 (realgar), As4S3 (dimorphite) molecules, and AsS3 pyramids is confirmed as well. Large content of arsenic-rich molecular entities in the films is due to high arsenic content in the layers (8 at. % overstoichiometry in comparison with As40S60 stoichiometric compound). Due to the exposure of as-deposited As–S films, content of As4S3 entities in the films is increasing, as we conclude from the increase of amplitudes of Raman bands peaking at 273 and (partly) 200 cm− 1, which can be assigned to As4S3 molecules [25]. From the rising of Raman bands amplitudes at 146, 187, and ~ 360 cm− 1, we deduce that the content of α- and β-As4S4 is growing after the irradiation [26,27]. Shift of the Raman band peaking at 222 cm− 1 to 217 cm− 1 due to the exposure as well as the decrease of the amplitude of the band at 222 and 235 cm− 1 can be connected with the decrease of the content of χAs4S4 molecules and arsenic clusters [28] but not γ-As4S4 (having in crystalline form its characteristic strongest Raman doublet at 230 and 235 cm−1 [26]), because other Raman active bands of γ-As4S4 (~ 154, 170 cm− 1) are emphasized due to irradiation. According to Lucovsky [8], we can assume that α- and β-As4S4 molecules are formed from S2As-AsS2 clusters; this is again accompanied with the decrease of Raman intensity in 220-230 cm− 1 region. Significant intensity decrease in 290–350 cm − 1 region has probably its origin in photoinduced transformations of As4S5 molecules. The content of AsS3 pyramids in exposed films slightly increases in comparison with as-prepared films as we can judge from small increase of 134 cm− 1 band intensity and pronouncing of the 343 cm− 1 band [28]. Spectral dependencies of refractive index of PLD As–S films obtained from VASE measurements show clearly that refractive index values are significantly decreasing due to exposure of the films (Fig. 2). As an example, refractive index value drops from initial value of 2.32 to 2.23 at telecommunication wavelength of 1550 nm; this represents 4% decrease of original value belonging to as-deposited film. Annealing of exposed films below but near the glass transition temperature has only marginal effect on refractive index values confirming the irreversibility of photorefraction effect. Observed irreversible photorefraction has probably its origin in pure photostructural phenomena leading in changes in polarizability due to different content of basic structural motifs (having different polarizabilities) present in the films before and after exposure.
Off-axis pulsed laser deposition combining rotating substrates and scanning of the targets’ surface by the laser beam was employed for the preparation of thin As–S amorphous films. Irreversible changes of the structure caused by Ar+ laser beam and observed by Raman spectroscopy were interpreted in terms of photoinduced transformations of amorphous arsenic, S2As–AsS2 clusters, χ-As4S4, and As4S5 molecules mainly to As4S3, α- and β-As4S4 entities. Significant negative photorefraction (~4 % decrease of refractive index) and photobleaching (~7 % increase of optical band gap energy) were tentatively assigned to photostructural effects. Observed irreversible photoinduced phenomena are of opposite behaviour in comparison with As–S films prepared by thermal evaporation technique (where positive photorefraction and photodarkening are usually observed), probably originating from differences in chemical composition of the layers and different structure of as-deposited films prepared by various techniques. Acknowledgements It is a pleasure for authors to acknowledge financial support of the Ministry of Education, Youth and Sports of the Czech Republic (projects MSM 0021627501 and LC523, respectively) and Czech Science Foundation (project No.104/08/0229). We are grateful to Dr. Z. Černošek for the measurements of Raman scattering spectra. References [1] A.V. Kolobov, Photo-Induced Metastability in Amorphous Semiconductors, WileyWCH, Weinheim, 2003, p. 23. [2] A. Zakery, S.R. Elliott, J. Non-Cryst. Solids 330 (2003) 1. [3] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. Non-Cryst. Solids 299-302 (2002) 917. [4] M. Frumar, B. Frumarova, P. Nemec, T. Wagner, J. Jedelsky, M. Hrdlicka, J. Non-Cryst. Solids 352 (2006) 544. [5] P. Nemec, J. Jedelsky, M. Frumar, M. Vlcek, J. Optoelectron. Adv. Mater. 7 (2005) 2635. [6] P. Nemec, M. Frumar, Thin Solid Films 516 (2008) 8277. [7] J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [8] G. Lucovsky, F.L. Galeener, R.H. Geils, R.C. Keezer, Proc. Conf. Structure of NonCrystalline Materials, Taylor and Francis, London, 1977, p. 127. [9] G. Lucovsky, R.M. Martin, J. Non-Cryst. Solids 8-10 (1972) 185. [10] R.J. Kobliska, S.A. Solin, J. Non-Cryst. Solids 8-10 (1972) 191. [11] S.A. Solin, G.V. Papatheodorou, Phys. Rev. B 15 (1977) 2087. [12] R. Forneris, Am. Mineral. 54 (1969) 1062. [13] W. Bues, M. Somer, W. Brockner, Z. Anorg. Allg. Chem. 499 (1983) 7. [14] B.H. Christian, R.J. Gillespie, J.F. Sawyer, Inorg. Chem. 20 (1981) 3410. [15] T. Chattopadhyay, C. Carlone, A. Jayaraman, H.G. Vonschnering, J. Phys. Chem. Solids 43 (1982) 277. [16] Z. Polak, Ph.D. Thesis, University of Pardubice, Czech Republic, 1998. [17] M.L. Slade, R. Zallen, Solid State Commun. 30 (1979) 357. [18] J.S. Lannin, Phys. Rev. B 15 (1977) 3863.
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