Linear and nonlinear optical changes in amorphous As 2 Se 3 thin film upon UV exposure

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1 Linear and nonlinear optical changes in amorphous As 2 Se 3 thin film upon UV exposure Rashmi CHAUHAN 1, 4, Amit Kumar SRIVASTAVA 2, Arvind TRIPATHI 3, Krishna Kant SRIVASTAVA 4 1. Department of Physic, DAV College, CSJM University, Kanpur, , India; 2. Materials Science Programme, IIT Kanpur, , India; 3. Research Foundation for Education and Technology, Datia, , India; 4. Department of Physics, DBS College, CSJM University, Kanpur, , India Received 2 March 2011; accepted 26 May 2011 Abstract: Amorphous As 2 Se 3 chalcogenide thin film was exposed to UV light (i line and g line) using mercury lamp for 30 min. XRD, UV/VIS spectroscopy and thickness measurements were taken for unexposed and exposed thin film for structural and optical characterizations. Linear optical constants (linear refractive index, extinction coefficient and linear optical absorption coefficient) of the film were calculated from transmission spectra using Swanepoel method. Optical bandgap was determined using Tauc s relation of indirect bandgap material. Dispersion of the material was described by single-oscillator Wemple-DeDiDomenico model. Nonlinear optical constants (χ (3) and n 2 ) were determined from linear optical parameters using semi-empirical relations in long wavelength limit. Observed changes in linear optical parameters (increase in refractive index, and extinction coefficient; reduction in optical bandgap), nonlinear optical parameter (increase in χ (3) ) are proposed for various photonic applications. Key words: amorphous chalcogenide thin films; UV exposure; optical properties; swanepoel method; Wemple-DeDiDomenico model; semi-empirical relations 1 Introduction The change in optical, structural and surface properties of amorphous chalcogenide thin film upon photoexposure is a well-known phenomenon. Various research groups throughout the world are trying to determine the change in optical constants, structural configuration and surface morphology upon different photo treatments for amorphous chalcogenide thin films. The studies reported that, upon photo treatment, amorphous chalcogenide thin films show changes in linear optical constants (i.e. α, E g, n and k), nonlinear optical constants (i.e. χ (3) and n 2 ), thickness of the film (d), phase of the material (i.e. amorphous or crystalline), and their resistance to alkali solvents etc [1]. These properties are utilized for fabrication of micro-optical elements, optical waveguides, photonic bandgap structures, diffractive optical elements, ultra-fast optical switches etc, which are important parts of integrated optics/photonics [1, 2]. The changes in properties due to photo exposure can be reversible or non-reversible in nature. Generally, structure fabrication is achieved on the fresh-deposited films of compositions having nonreversible changes [1 3]. The compositions having reversible changes are used in the applications like optical switching etc [4]. As-Se based amorphous thin films can be used for micro/nano structure fabrications due to their wide range of photo-sensitivity and low-phonon energy [2]. IR transparency and high refractive index make them suitable material for integrated IR optics [1, 5, 6]. Moreover, due to the presence of high values of nonlinearities, these materials can be utilized for telecommunication and photonic applications [1, 7]. Lot of studies were based on broadband illumination [1, 8, 9] and specific wavelength illumination [1, 2, 10 12] on these thin films. Intensity, exposure time and chemical composition are the basis of changes in linear and non-linear optical properties of amorphous chalcogenide thin films. In most of the previous studies, based on the UV illuminations, high intensities were used to cause changes in optical/structural/surface properties. However, we have already investigated the linear optical changes Corresponding author: Rashmi CHAUHAN; Tel: ; chauhanrasmi@gmail.com

206 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) 205 210 caused by low intensity UV exposures for 10 min and 2 h (i line: λ=365 nm, intensity=3.

The present study reports changes in linear and nonlinear optical properties, caused upon 30 min low intensity UV exposures (i line: λ=365 nm, intensity=3.5 mw/cm 2 ; g line: λ=436 nm, intensity=12.

The determination of nonlinear optical properties (χ (3) and n 2 ) requires some complicated experimental techniques [13, 14], which are not generally available in most of the laboratories, hence,

2 206 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) caused by low intensity UV exposures for 10 min and 2 h (i line: λ=365 nm, intensity=3.5 mw/cm 2 ; g line: λ=436 nm, intensity=12.5 mw/cm 2 ) on amorphous As-Se based chalcogenide thin films [11, 12]. The present study reports changes in linear and nonlinear optical properties, caused upon 30 min low intensity UV exposures (i line: λ=365 nm, intensity=3.5 mw/cm 2 ; g line: λ=436 nm, intensity=12.5 mw/cm 2 ) on amorphous As 2 Se 3 thin films in order to propose their usability for more compact and efficient photonic devices. The determination of nonlinear optical properties (χ (3) and n 2 ) requires some complicated experimental techniques [13, 14], which are not generally available in most of the laboratories, hence, the present study reports the determination of nonlinear optical parameters (χ (3) and n 2 ) from linear optical parameters (χ (1), n 0 and E g ) using semi-empirical relations [15 17] in long wavelength limit. 2 Experimental UV/VIS computerized spectrophotometer (Hitachi, Model U 3310) in wavelength range of nm. 3 Results and discussion 3.1 Structural analysis Figure 1 shows XRD patterns of bulk sample and thin films (unexposed and exposed). The absence of any prominent peak verifies their amorphous nature. The change in XRD patterns shows that structural changes were taking place during thin film preparation process. Moreover, UV exposure caused local structural modifications in thin film, and thus, change in XRD pattern of exposed film was observed. Amorphous As 2 Se 3 bulk sample was prepared by conventional melt quenching technique. Consequently, materials (99.999% pure As and Se) were weighted according to their atomic percentages and be sealed in quartz ampoules (length 8 cm, internal diameter 8 mm) in the vacuum of Pa (10 5 Torr). The sealed ampoules were kept inside a furnace, where the temperature was raised to 950 C at a heating rate of 3 4 C/min. The ampoules were frequently rocked for 10 h at a maximum temperature to make the melt homogenous, and the quenching was done in ice water. Thin films of glassy alloy were prepared using thermal evaporation technique upon cleaned glass substrates at room temperature inside a coating system (HIND-HIVAC 12A 4D-T) at a pressure of ~ Pa (10 6 Torr). During evaporation, the substrate was kept rotating with a frequency of 5 Hz, so that the uniformity of the films can be maintained. These films were kept inside the deposition chamber for 24 h to achieve the metastable equilibrium as suggested by ABKOWITZ et al [18]. These thin films were exposed to UV light (i line: λ=365 nm, intensity=3.5 mw/cm 2 ; g line: λ=436 nm, intensity= 12.5 mw/cm 2 ) for 30 min. The thicknesses of the unexposed (~1225 nm) and exposed films (~1400 nm) were measured using stylus based surface profilometer (Tencore Instrument, Model Alpha Step 100). Amorphous nature of bulk sample and film was verified using X-ray diffractometer (Thermo Electron Corporation, Model ARL X TRA) at scan rate of 3( )/min. The homogeneity and correct compositions of all the alloys and thin films were determined by means of scanning electron microscope (Zeiss, Model EVO 40). The optical transmissions of unexposed and exposed As 2 Se 3 thin films were measured by double beam Fig. 1 XRD patterns of As 2 Se 3 bulk sample and thin films (unexposed [11, 12] and exposed) EDX analysis of amorphous As 2 Se 3 bulk sample and thin films were discussed in previous studies [11, 12], which showed that the observed chemical composition of thin film prepared using thermal evaporation process remains close (inaccuracy ~ 0.4%) to bulk sample. 3.2 Linear optical analysis Transmission plot of amorphous As 2 Se 3 thin films (unexposed and exposed) are shown in Fig. 2. Transmission curves are oscillatory in nature, which is the resultant of multiple interference of three interfaces (air film, film substrate, substrate air). Linear optical constants of unexposed and exposed As 2 Se 3 thin film were evaluated from transmission spectra using Swanepoel method [19]. Using this method, upper envelope (T M ), lower envelope (T m ) and fringe-free transmission (T α ) are drawn on transmission spectra with the help of MATLAB, which are shown in Fig. 2 along with the transmission of reference (T R ). These envelopes were used to determine theoretical thickness of the film (d) and various linear optical constants (n, n 0, k, α, E g, χ (1) ), where some of the linear optical constants (n 0, χ (1)

3 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) formula Fig. 2 Transmission spectra for As 2 Se 3 unexposed [11, 12] (a) and exposed (b) thin films and E g ) were used to calculate nonlinear constants (n 2 and χ (3) ) using semiempirical methods [15 17]. Figure 3 shows the comparison of fringe-free transmission for unexposed and exposed As 2 Se 3 thin films. It is clear that fringe-free transmission (T α ) reduces due to UV exposure (i line, g line), which is an indication of photodarkening in these thin films. It is also supported by previous studies made on this sample for 10 min and 2 h UV exposures (i line, g line) [11 12]. The same trend is available for this sample for other types of broadband illumination [1, 8 9] and laser exposures [1, 2, 10], where the intensity of photoexposure is large compared to the present study. The reduction in transmission, even for low intensity exposure, is a consequence of highly metastable state and flexible structure of this sample. Hence, it is easy to alter its local structure and optical transmission by photoexposure. Experimental determination of refractive index is done from transmission spectra using Swanepoel method [19] and its theoretical fitting is done using Cauchy n=a+b/λ 2 (1) MATLAB was used for theoretical fitting of Cauchy formula on discrete values of refractive index, which gives n= /λ 2 for the unexposed film, and n= /λ 2 for the exposed film. The goodness of fit is determined using R-square value, which is ~0.99 for both the cases. Figure 4 shows the variation of refractive index with wavelength. It was observed that the value of refractive index decreased with the wavelength. It is evidence of normal dispersion behavior of the material. It was also observed that the value of refractive index increased with UV exposure (for i line, g line), which was supported by previous studies on this sample for 10 min UV exposure (i line, g line). Various other types of high intensity photoexposures [1, 2, 8 10] showed same trends. This is an indication of photorefraction (optical densification) caused by UV exposure. Increase in refractive index upon UV exposure is a consequence of local structural modification, which brings the local structure of the film close to bulk glass [1]. It can also be seen in XRD patterns (Fig. 1). Fig. 4 Plot of refractive index for unexposed [11] and exposed As 2 Se 3 thin films Fig. 3 Fringe-free transmission for unexposed and exposed As 2 Se 3 thin films Investigation of dispersion in the material is important for the photonic and telecommunication applications. In present study, the dispersion of the refractive index is described by Wemple-DiDomenico single oscillator relationship [20, 21]: n 2 (hν)=1+(e d E 0 )/(E 0 2 (hν) 2 ) (2) where hν is the photon energy, E 0 is the single oscillator energy and E d is the dispersive energy. This relation is valid only for photon energies less than optical bandgap. Linear least square fitting of 1/(n 2 1) with (hν) 2 was obtained using MATLAB as shown in Fig. 5, where the goodness of fit is determined using R-square value,

4 208 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) which is ~0.99 for both the unexposed and exposed films. wavelength. It is clear from this plot that k decreases with wavelength and increases upon UV exposure, which is in accordance with the previous studies [1, 2, 8 12]. The reduction of k with wavelength shows that material is getting transparent with wavelength, which makes it useful optical material in high wavelength range. Increment in k upon UV exposure is a consequence of structural and surface defects caused by UV exposure [1, 11, 12]. Fig. 5 Linear least square fitting of 1/(n 2 1) with (hν) 2 From Fig. 5, the values of E d and E 0 can be determined from the slope (E d E 0 ) 1 and intercept (E 0 /E d ) on the vertical axis. The value of static refractive index n 0 can be obtained by extrapolating Wemple- DiDomenico relation in low energy limit (hν 0). The values of E d, E 0 and n 0 are shown in Table 1. It is clear from Table 1 that E 0 2E g, which is in accordance with the findings reported by Tanaka [22]. E d represents oscillator strength, which increases upon low intensity photoexposure as shown in Table 1. Same trends are available for other high intensity photoexposures [1]. This increment in E d is a consequence of increment in coordination number of cation to nearest neighbor anions [1]. The value of n 0 matches with the literature and increases upon low intensity UV exposure (as shown in Table 1), which also follow the same trends available in the literature for high intensity photoexposures [1]. It is also a resultant of structural transformation caused by UV exposure. Whenever any electromagnetic wave propagates inside the medium, its loss by absorption or scattering process is represented by extinction coefficient. It can be determined using the relation of k=αλ/(4π), where α is the optical absorption coefficient, which can be determined using Swanepoel s method [19]. Figure 6 shows the variation of extinction coefficient with Fig. 6 Plot of extinction coefficient for unexposed [11] and exposed As 2 Se 3 thin films The determination of the optical bandgap was done by extrapolating Tauc s plot [23] for indirect bandgap material: (αhν) 1/2 =B 1/2 (hν E g ) (3) where B is the band tailing parameter, which is a measure of disorderness of the material. Tauc s plot of the unexposed and exposed films are shown in Fig. 7. It is clear from this figure and Table 1 that the value of E g matches with the literature [1, 8 12] and it decreases from 1.79 ev to 1.74 ev upon 30 min UV exposure (i line, g line). Previous studies [11, 12] on the sample showed that same trend was achieved for 10 min and 2 h UV exposures (i line, g line). Other kinds of high intensity exposures (broadband illumination and laser exposure) showed similar trends [1, 8 10]. It is also clear from Table 1 that the achieved values of E g for the unexposed and exposed films follow Tanaka s rule, Table 1 Linear and nonlinear optical constants of unexposed and exposed As 2 Se 3 thin film Film n 0 (hν 0) E d /ev E 0 /ev E g /ev χ (1) (hν 0) χ (3) [esu] (hν 0) n 2 [esu] As 2 Se 3 unexposed (Literature) [1, 2, 8 12] As 2 Se 3 unexposed (Present study) As 2 Se 3 exposed (Present study)

5 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) Fig. 7 Tauc s plot for unexposed [11] and exposed As 2 Se 3 thin films which is E g E 0 /2 [22]. The reduction in bandgap (photodarkening) can be understood in terms of local structural changes caused by UV exposure. The average coordination number of As 2 Se 3 is 2.4, hence, this system is in floppy phase [24]. The structural flexibility is observed in this phase. Hence, small intensity UV exposure also causes structural transformations in such a way that it increases the interaction in lone pair of electrons forming the top of the valance band, which causes upward shift in top of the valance band and reduction in optical bandgap [25, 26]. In the long wavelength limit (hν 0), this equation reduces to χ (1) =(n 0 2 1)/4π (6) By putting this value of χ (1) in Eq. (4), the following is obtained χ (3) =A(n 0 2 1) 4 /(4π) 4 (7) The value of n 0 for the unexposed and exposed amorphous As 2 Se 3 thin films can be calculated using Wemple-DiDomenico relation [20, 21] as shown in Table 1. Hence, nonlinear optical susceptibility χ (3) can be calculated using Eq. (7), which is also shown in Table 1 and compared with the literature for the unexposed sample. The calculated value and literature value show good agreement with each other. Table 1 also shows that the value of χ (3) increases by 1.9 times upon UV exposure. Local structural modification may alter the polarization of the medium, hence, the third order susceptibility may be changed due to UV exposure. Determination of nonlinear refractive index can be done according to the semi-empirical relation of Tichy et al [15]: n 2 ~B/E g 4 (8) where B= [esu (ev) 4 ]. Putting the value of E g, we obtain n 2 for both the unexposed and exposed thin films as shown in Table 1, showing good agreement with literature for the unexposed sample and remains almost unchanged upon UV exposure. 3.3 Nonlinear optical analysis Nonlinear optics deals with nonlinear polarization response of the material for the incident radiation. Determination of nonlinear optical parameters of the materials is helpful for the fabrication of frequency conversion and optical switching devices, which are the important part of integrated photonics. The third order nonlinear susceptibility (χ (3) ) and nonlinear refractive index (n 2 ) of chalcogenide glasses are up to thousand times larger than those of the silica based glasses. Hence, determination of these parameters is very important for various photonic applications. Approximate determination of χ (3) can be done using generalized Miller s rule [16, 17]: χ (3) =A(χ (1) ) 4 (4) where χ (1) is the linear optical susceptibility and A is a constant having estimated value of (for χ (3) in [esu]) [15]. For amorphous As 2 Se 3 chalcogenide thin film, which is considered as isotropic medium, the first order approximation of the first order linear susceptibility (χ (1) ) is given by χ (1) =(n 2 1)/4π (5) 4 Conclusions Following points can be concluded for 30 min UV exposure (i line: λ=365 nm, intensity=3.5 mw/cm 2 ; g line: λ=436 nm, intensity=12.5 mw/cm 2 ) on amorphous As 2 Se 3 chalcogenide thin films. 1) Amorphous nature of As 2 Se 3 thin film is retained, however, some local structural changes are taking place. 2) Linear refractive index (n), static refractive index (n 0 ), dispersive energy (E d ) and extinction coefficient (k) increase, however, fringe free transmission (T α ), single oscillator energy (E 0 ) and optical bandgap (E g ) decrease. 3) The obtained values of χ (3) and n 2 from semi-empirical relations show increment in χ (3), however, changes in n 2 are negligible. 4) All these changes in linear/nonlinear optical parameters are a consequence of local structural modifications made by UV exposure. 5) Flexible structure and highly metastable state of given sample are responsible for local structural changes made by low intensity UV exposure. This phenomena can be used to optimize linear/nonlinear optical parameters according to desired photonic application.

6 210 Rashmi CHAUHAN, et al/progress in Natural Science: Materials International 21(2011) Acknowledgements The authors wish to thank to UGC, India for financial assistance, Dr. Jitendra KUMAR (Prof., Material Science Programme, I.I.T. Kanpur, India) for optical measurements, Mr. U. S. SINGH, Mr. THAPA, Mr. R. S. SHARMA (Technical Staff, ACMS, IIT Kanpur, India) for XRD, EDX and thickness measurements, Dr. S. K. SHARMA (Prof., AIF-USIC, JNU, New Delhi, India) for EDAX measurements and Dr. Prabhat DWIVEDI (Research Scientist, DST Unit on Nanosciences, IIT Kanpur, India) for fruitful discussions. References [1] KOLOBOV A V. Photo-induced metastability in amorphous semiconductors [M]. Weinheim: Wiley-VCH, [2] MUSGRAVES J D, CARLIE N, GUERY G, et al. Chalcogenide glasses and their photosensitivity: Engineered materials for device applications [C]// Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides. OSA Technical Digest (CD). Optical Society of America, 2010, paper BWD1. [3] DEVAUX E, LALUET J Y, STEIN B, et al. Refractive micro-optical elements for surface plasmons: From classical to gradient index optics [J]. Opt Express, 2010, 18: [4] TA EED V G, BAKER N J, FU L, et al. Ultrafast all-optical chalcogenide glass photonic circuits [J]. Opt Express, 2007, 15(15): [5] CHAUHAN R, SRIVASTAVA A K, MISHRA M, et al. Ge-As-Se chalcogenide thin films for integrated optics [C]//Proceedings of 12th International Symposium on Microwave and Optical Technology (ISMOT 2009). New Delhi, India, 2009: [6] ZAKERY A, ELLIOT S R. Optical properties and applications of chalcogenide glasses: A review [J]. J Non-Cryst Solids, 2003, 330: [7] TANAKA K. Optical nonlinearity in photonic glasses [J]. J Mater Sci: Mater Electron, 2005, 16: [8] ANDRIESH A M, IOVU M S. Optical properties of As-Se amorphous composites [J]. J Optoelectron Adv M, 2006, 8: [9] TICHY L, TICHA H, NAGELS P, et al. Photoinduced changes in the short wavelength optical absorption edge of a-as 2 S 3 and a-as 2 Se 3 thin amorphous film [J]. Mater Lett, 1998, 36: [10] ANTOINE K, JAIN H, VLCEK M. Optical spectroscopy of a-as 2 Se 3 under in situ laser irradiation [J]. J Non-Cryst Solids, 2006, 352: [11] CHAUHAN R, SRIVASTAVA A K, MISHRA M, et al. Effect of UV exposure on some optical properties of As-Se based chalcogenide glasses [J]. Integr Ferroelectr, 2009, 119: [12] CHAUHAN R, SRIVASTAVA A K, TRIPATHI A, et al. Photo-induced optical changes in Ge x As 40 Se 60 x thin films [J]. Prog Nat Sci: Materials International, 2010, 20: [13] SHEIK-BAHAE M, SAID A A, van STRYLAND E W. High-sensitivity, single-beam n 2 measurements [J]. Opt Letters, 1989, 14(17): [14] NASU H, IBARA Y, KUBODERA K. Optical third-harmonic generation from some high-index glasses [J]. J Non-Cryst Solids, 1989, 110(2 3): [15] TICHÁ H, TICHÝ L. Semiempirical relation between non-linear susceptibility (refractive index), linear refractive index and optical gap and its application to amorphous chalcogenides [J]. J Optoelectron Adv M, 2002, 4(2): [16] WANG C C. Empirical relation between the linear and the third-order nonlinear optical susceptibilities [J]. Phys Rev B, 1970, 2(6): [17] WYNNE J J. Optical third-order mixing in GaAs, Ge, Si, and InAs [J]. Phys Rev, 1969, 178: [18] ABKOWITZ M, FOLEY G M T, MORKOVICS J M, et al. Optical effects in amorphous semiconductors [J]. AIP Conference Proceedings, 1984, 120: [19] SWANEPOEL R. Determination of thickness and optical constants of amorphous silicon [J]. J Phys E: Sci Instrum, 1983, 16: [20] WEMPLE S H, DIDOMENICO M. Behavior of the electronic dielectric constant in covalent and ionic materials [J]. Phys Rev B, 1971, 3: [21] WEMPLE S H. Refractive-index behavior of amorphous semiconductors and glasses [J]. Phys Rev B, 1973, 7: [22] TANAKA K. Optical properties and photoinduced changes in amorphous As-S films [J]. Thin Solid Films, 1980, 66(3): [23] TAUC J. Amorphous and liquid semiconductors [M]. New York, USA: Plenum Press, [24] QU T, GEORGIEV D G, BOOLCHAND P, et al. The intermediate phase in ternary Ge x As x Se 1 2x glasses [J]. Mat Res Soc Symp Proc, 2003, 754: CC8.1.1 CC [25] PFEIFFER G, PAESLER M A, AGGRAWAL S C. Reversible photodarkening of amorphous chalcogens [J]. J Non-Cryst Solids, 1991, 130: [26] KOLOBOV A V, OYANAGI H, ROY A, et al. A nonometer scale mechanism for reversible photostructural change in amorphous chalcogenides [J]. J Non-Cryst Solids, 1998, : [27] SKORDEVA E R. The dispersion energies Wemple-DiDomenico in chalcogenide films based on Ge-As(Sb)-S(Se) [J]. J Optoelectron Adv M, 1999, 1(1):

Studies on In x (As 2 Se 3 ) 1 x thin films using variable angle spectroscopic ellipsometry (VASE)

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