Vibrational modes of AgIn3Te5 and effect of laser irradiation

Accepted Manuscript Title: Vibrational modes of AgIn3 Te5 and effect of laser irradiation Author: C. Rangasami PII: DOI: Reference:

S0924-2031(17)30088-7 https://doi.org/10.1016/j.vibspec.2017.08.010 VIBSPE 2738

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Please cite this article as: C.Rangasami, Vibrational modes of AgIn3Te5 and effect of laser irradiation, Vibrational Spectroscopy https://doi.org/10.1016/j.vibspec.2017.08.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Vibrational modes of AgIn3Te5 and effect of laser irradiation C. Rangasami Department of Physics, Kongu Engineering College, Erode-638052, Tamil Nadu, India Abstract Polycrystalline AgIn3Te5 has been synthesized by melt-quench technique. Phase homogeneity and crystal structure of AgIn3Te5 have been analyzed using X-ray diffraction (XRD). Vibrational modes and effect of laser irradiation on the surface of AgIn3Te5 have been studied by Raman spectroscopy. XRD analysis has shown that single phase AgIn3Te5 has crystallized in p-type tetragonal structure with P 42c space group. Raman spectrum of as-formed sample has shown a strong peak at 125.6 cm-1 which is attributed to A1 mode of chalcopyrite family and substantiates the results obtained from XRD analysis. Tentative mode assignment for the rest of the peaks observed has been done based on simplified Keating model and by comparison with modes reported as well as predicted theoretically for AgInTe2. Raman spectra obtained with laser beam power of 20 mW have revealed the formation of amorphous phase during acquisition. Interaction with 20 mW laser beam has resulted in the formation of In2O3 on the sample surface. The peak at 132.8 cm−1 that appeared after irradiation for 75s may be attributed to In2O3 or to the silent A2 mode, which probably has become active due to defect formation.

Keywords: AgIn3Te5, melt-quench method, X-ray diffraction, Raman spectra and laser induced surface changes.

1. Introduction I-III-VI2 based direct band gap semiconductors with large absorption coefficient have been studied extensively (Siebentritt and Rau, 2006; Yu and Cardona, 2010) and find potential application in photovoltaics and nonlinear optics. Efficiencies of more than 20 % have been demonstrated for thin film solar cells with Cu-III-VI2 based absorber layer (http://www.pv-tech.org). Ag-III-VI2 based chalcopyrites, especially AgGaSe2 for second harmonic generation, exhibit large nonlinear optical coefficient (~33 pm/V at 10.6 µm) and high transmission in the infrared region (Xue et. al., 2000; VI Nikogosian, 2005). A series of compounds with general formula AI BIII2n+1C3n+2 , where n

= 0, 1, 2, and with structure similar to that of chalcopyrites have been reported to stabilize when the occupancy of cations in the ideal chalcopyrite lattice is disturbed (Zhang et al., 1997; Lehmann et al., 2011; Ludwig et al., 2011). Existence of these off-stoichiometric phases, which are reported to present in the region I-III-VI2 - III2VI3 of ternary phase diagram, can be explained based on the ordering of defect pair . Stabilization of vacancy compounds is energetically favorable because the 2VI- + III2+ I formation energy associated with the charge-neutral defect pair is low (Zhang et al., 1997). These vacancy compounds have received much attention ever since the experimental observation concluded that the presence of a thin layer of CuIn3Se5 phase over CuInSe2 film surface can enhance significantly the efficiency of CuInSe2 absorber based thin film solar cells (Schmid et al., 1993). Moreover, efficiency of 6.92 % has been reported for CuIn3Te5 based narrow-band gap thin-film solar cell (Mise and Nakada, 2011). Cu-III3-VI5 based vacancy compounds have been studied by several authors (Honle et al., 1988; Tseng and Wert, 1989; Hanada et al., 1997), while Ag-III-VI based vacancy compounds have attracted much less attention. In fact, the reported works on Ag-based vacancy compounds mainly concern with Ag-III5VI8. In addition to the preliminary work (O΄Kane and Mason, 1964), the crystal

structure of AgIn3Te5 which is located in between AgInTe2 and AgIn5Te8 on the Ag2Te-In2Te3 tie line in the Ag-In-Te ternary phase diagram has been resolved as pchalcopyrite ( P42c ) recently by Rangasami et al (2011). Raman spectroscopy has been recognized as an invaluable tool to investigate the phase homogeneity and crystalline quality of solids. For example, phase segregation in Cu-III-VI2 based compounds due to variation in Cu content (Morell et al., 1996; Rudigier et al., 2003; Papadimitriou et al., 2005; Wang et al., 2008) has been investigated using Raman measurements. Raman spectroscopy has also been used successfully to probe the formation of multiphase such as Sb2Te3 and Sb2O3 in Ge2Sb2Te5 thin films due to annealing (Jang et al., 2009). In addition, Raman measurements have been employed in probing the phase transition, especially, in phase change materials (Zhang et al., 2005). In solids, phonons with well-defined wave vectors close to Brillouin zone participate in Raman scattering (Exarhos, 1995). Therefore, the width of Raman lines for a well ordered crystalline solid would be very sharp. On the other hand, amorphous solids lack long-range order and therefore momentum conservation no longer holds. Consequently, there is no restriction on phonon wave vectors and hence phonons from all over the Brillouin zone participate in Raman scattering (Brodsky, 1985; Grimsditch, 2003). As a result, the spectra of amorphous materials will be broad and will essentially reflect the phonon density of states. Thus, the crystalline quality of a specimen can be determined (Voustas et al., 1995) and in fact, it is possible to differentiate co-existence of amorphous and nanocrystalline phases (van der Voort et al., 2001).

Lattice dynamics of I-III-VI2 family of semiconductors have been investigated theoretically as well as experimentally by Parlak and Eryigit (2002). Neumann (1985) has proposed a model that can be used to obtain theoretical estimates of normal modes

of chalocpyrites. This model is actually a simplified version of the well-known Keating model (Keating, 1966) and it is possible to get simple analytical relations for the frequencies of some of the modes of the chalcopyrite lattice using this model. In the Keating model, the nearest neighbor interactions are described by two central force constants, while the interaction between the second nearest neighbors are accounted for by additional five non-central (bond bending) force constants. In I-IIIVI2 compounds, the bond-bending force constants are small when compared to the bond-stretching force constants. In the simplified Keating model, the bond-bending force constants are neglected. This assumption is equivalent to taking the ratio of lattice parameters (c/a) and the free position parameter “x” to be 2 and 0.25, the values of ideal chalcopyrite lattice. In the present work, the simplified Keating model has been used to calculate some of the modes of the system under investigation. It is inferred from the literature that there are no elaborate reports available on the vibrational modes of AgIn3Te5 and also the effect of laser irradiation on the modes.

The aim of the present work is to synthesis bulk AgIn3Te5 by melt-quench technique and to characterize the same using X-ray diffraction (XRD) and Raman spectroscopy. Moreover, an effort was taken to understand the effect of laser irradiation on the vibrational modes of AgIn3Te5. The details of the melt-quench technique can be found elsewhere (Malar et al., 2005).

2. Experimental details 2.1. Synthesis of AgIn3Te5 Polycrystalline AgIn3Te5 was formed by melt-quench method. Briefly, 5N purity Ag, In and Te pieces with compositional ratio of 1: 3:5 and total mass of ~5 gm were taken in a cleaned quartz ampoule. The ampoule was then evacuated to a pressure of ~5×10-6 mbar and sealed, with the charge inside. The sealed ampoule was

heated in a vertical tubular furnace, the temperature of which was controlled to an accuracy of 2 K. The furnace temperature was initially increased to 523 K at a rate of 4 K per min and was maintained at this temperature for 3 h in order to allow the exothermic reaction between In and Te to take place at a controlled rate (Champnes, 1999). Heating was continued at the same rate up to 1323 K, which is higher than the melting point of any of the constituent elements. The charge in the ampoule was allowed to react in this molten state for 60 h with intermittent shaking (2 to 5 times) in order to improve the homogeneity. Finally, the ampoule with the charge inside was quenched in water. The ingot thus obtained was black in color with shiny surface. 2.2. Characterization of AgIn3Te5 2.2.1. X-ray diffraction Analysis Powder X-ray diffraction data of AgIn3Te5 was recorded in the 2θ range 10 to 130º (step size: 0.0170º) using PANalytical (model X'Pert PRO) with Cu-Kα radiation at room temperature. The elemental ratio of the constituent elements was estimated using high resolution scanning electron microscope (HRSEM) of model FEI Quanta 200F facilitated with EDX analyser (detector type: SUTW-SAPPHIRE with resolution 133.22). The XRD and HRSEM analyses have shown that nearstoichiometric AgIn3Te5 is crystallized in P-chalcopyrite structure with space group P 42c (Rangasami et al., 2011).

2.2.2. Micro Raman Measurements Micro-Raman measurements were made in backscattering geometry using a Horiba Jobin Yvon, model HR800 UV Raman microscope (resolution = 0.3 cm-1) with a 1800 grooves/mm grating. An argon-ion laser with 488 nm line was used as the excitation source. The laser beam of power 0.5 mW/1mW was focused to a spot of diameter <2 m during the measurements to record Raman spectra. It has been reported that (Ananthan et al., 2009) depending on the absorption coefficient of the

sample and laser beam intensity, either reversible or irreversible changes may be effected on the sample surface by the probing laser beam. Since these changes are made by the probing beam itself, it is possible to study the changes while the changes take place (Ananthan, 2009). In the present case, 20 mW laser beam was used to alter the sample surface and to probe the changes simultaneously. The measurements are carried out in a specific sequence and are presented in the following section. 3. Result and discussion 3.1 Raman Spectroscopy A typical Raman spectrum of bulk AgIn3Te5 recorded at room temperature is shown in Fig. 1. In order to identify the peak positions of vibrational modes, the spectrum was deconvoluted using multiple Lorentzians. In Fig. 1, open circles, grey lines and the black line through the open circles denote respectively the experimental data, individual Lorentzians and the overall fit. The inset to Fig. 1 shows the data in the higher-shift range on an expanded scale. Deconvolution yielded ten peaks at 125.6, 132.0, 142.4, 150.9, 162.1, 170.8, 185.4, 214.4, 269.8 and 295.9 cm-1. Of these, the last three peaks (Fig. 1) have very large FWHMs and the FWHM is quite high (~237 cm-1), especially for the last peak (295.9 cm-1). Therefore, the peak at 295.9 is not considered for Raman analysis. The Raman spectrum of AgIn3Te5 is expected to show features similar to those of I-III-VI2 chalcopyrite compounds. This is because the space group P 4 2c has the same point group ( 4 2m) as that of I 4 2d. In addition, the anion sub-lattice remains same in both systems. Therefore, analysis of Raman modes of AgIn3Te5 can be done following the procedure suggested for I-III-VI2 compounds, in general. Group theoretical calculations for the chalcopyrite system are reported in the literature (Kaminow et al., 1970). The results have shown that at the Brillouin zone center (q = 0), the irreducible representation for optical modes can be written as 1A1 

3B1  3B2  6E. Among these, B2 and E are doubly degenerate and they are both Raman and infrared active. Further, B2 and E modes can be subdivided into longitudinal and transverse optical modes due to their polar character. Thus, there are twelve E modes, six B2 modes, three B1 modes, and a single A1 mode, leading to a total of twenty two modes.

3.1.2 Mode Assignment First principle lattice dynamical calculations have not been performed for chalcopyrites. However, two main simplified models viz., tetra-atomic linear chain (Holah et al., 1981) and simplified Keating (Neumann, 1985) are available, using which Raman shifts of certain modes can be calculated. The expression for A1 mode is same in both the models, though there is some ambiguity in the mode assignment in tetra-atomic linear chain model. In the present work, the simplified Keating model has been followed to estimate the Raman shits of A1 and E modes. The Keating model (Keating, 1966) considers two parameters. The first is the force constant that accounts for the stretching of bond between an atom and its first nearest neighbor and the second is the force constant that accounts for the bond-bending motion. The latter is a three-body interaction and is essentially an angular interaction. Further, the tetragonal distortion is neglected in the Keating model. In the simplified Keating model, the bond-bending force constants are not considered, in addition. This allows simplification of the dynamical matrix to obtain expressions for few of the modes so that estimation of the same can be made. It is important at this stage to note here that in the simplified Keating model, the expressions for certain modes come out to be same and also the Raman shift of some of the modes will become zero (Neumann, 1985).

The basic tetrahedral unit of AgIn3Te5 lattice consists of Te atom surrounded by one Ag atom and three In atoms, with vacancies in Ag as well as in one of the three In positions. This is in contrast to the chalcopyrite lattice in which the basic tetrahedral unit consists of one group VI atom surrounded by two group I atoms and two group III atoms. This difference is brought out in Fig. 2, which compares the basic tetrahedral arrangement in Ag-In-Te based chalcopyrite and ordered vacancy compound having P 4 2c space group. In Fig. 2, the notations In1, In2 and In3 represent three indium atoms with different Wyckoff positions and site occupancies. The Te-Ag, Te-In1, Te-In2 and TeIn3 bond lengths are unequal. Due to the change in the site occupancies in the basic tetrahedral structure (Fig. 2), the expression given by Neumann (1985) cannot be used directly. The modified expression for A1 mode, after taking into account of the site occupancies in AgIn3Te5, can be written as

 2 ( A1 )  [(0.8 A  0.21  0.4 B  0.6 2  3  4 ) / MTe ]

(3.1)

where M Te is mass of Te atom.  A and  B are respectively the force constants of Ag-Te and In-Te bonds as in chalcopyrite AgInTe2. They are related to the respective bond

lengths

l1

and

l2 such

that

 A  0.0401(19.67 / l1 )3.255 and

 B  0.145(15.91/ l2 )3.105 (Kumar and Chandra, 1999). The reported values for  A and  B are 23.19 and 33.48 N/m respectively.

The force constant 1 can be

calculated by substituting the bond length Ag-Te obtained for AgIn3Te5 unit cell (Rangasami et al., 2011) and the remaining  2 , 3 and  4 can be obtained by substituting the bond lengths of In1-Te, In2-Te and In3-Te (Rangasami et al., 2011), respectively in the expression for  B . It is to be noted here that the average force constants are obtained, since the calculation of force constant of vacancy-Te bond

from Rietveld analysis is not possible. The Raman shift of A1 mode for AgIn3Te5 was calculated to be 130.1 cm-1, which is close to the observed value of 125.6 cm-1 at which the peak with highest intensity is present. In chalcopyrites, in-plane motion of anions with cations at rest is the origin for A1 mode. It is the highest intensity mode (Artus et al., 1990; Garcia et al., 2005) and has been considered as the signature of chalcopyrite structure as well. In fact, the crystalline quality of thin films of several chalcopyrites, including the widely studied CuInSe2, has been assessed using this mode (pl. see for example, Wang et al., 2008). Similarly, the expressions for different E modes can be modified as follows.  1 1    , where, i = 1, 2 and 3  mIn mTe 

1  2 ( ETO )   2 ( B21 )  4 xi 

(3.2)

 2  2 1  1     B     m   mIn mTe   Ag mTe 

 2 ( ETO2 )   2 ( B11 )   A 

1/ 2

2     2 2 1  1   4 A B          B    A     mTe2   mIn mTe      mAg mTe  

 1 1    m  Ag mTe 

3  2 ( ETO )   2 ( B22 )  4 A 

(3.3)

(3.4)

 2  2 1  1       B   m  mIn mTe   Ag mTe 

4  2 ( ETO )   2 ( B12 )   A 

1/ 2

2     2 2 1  1   4 A B                A   B  mTe2   mIn mTe      mAg mTe  

Where,  A  (0.8 A  0.21 ) 

 B  xi 

3 eL2 ( In Te ) 16  0 di3

3 eL2 ( Ag Te ) 16  0 d 3

(3.5)

(3.6)

(3.7)

In Eqns. 3.2 to 3.7, x1  (0.4 B  0.6 2 ) , x2  3 and x3   4 . mAg and mIn are the masses of Ag and In atoms,  0 is the permittivity of free space, eL ( Ag Te ) (= 0.66e) and eL ( In Te ) (=0.669e) are the effective bond charge of Ag-Te and In-Te bonds respectively in AgInTe2. d  0.8l1  0.2d Ag Te and di  0.4l2  0.6d Ini Te , where l1 and

l2 , are the Ag-Te and In-Te bond lengths as in AgInTe2 (Xue et al., 2000). d Ag Te and d Ini Te (i = 1, 2, 3) are respectively the bond lengths of Ag-Te, In1-Te, In2-Te and In3Te bonds in AgIn3Te5.

The modes, E1 , E 2 and E 4 appear due to the in-phase and anti-phase vibrations of both cations. In chalcopyrite structure, Ag and In atoms have unique Wyckoff position. Hence, the modes, E1 , E 2 and E 4 will have unique values. On the other hand, in the unit cell of AgIn3Te5 (Rangasami et al., 2011) indium atoms have three different Wyckoff positions with different occupancies and bond lengths. In other words, the tetrahedral structures with In as central atom will have three different configurations: In1 surrounded by four Te, In2 by four Te and In3 by four Te. Consequently, each of the expressions that involve  B (Eqns. 3.2, 3.3 and 3.5, essentially expressions for E1 , E 2 and E 4 , respectively) yield three different frequencies corresponding to the three bond lengths, d Ini Te (i = 1, 2, 3). The values obtained, for instance, for E1 mode are 212.2, 187.2 and 191.3 cm-1, corresponding to In1-Te, In2-Te and In3-Te bond lengths respectively. Similarly, the values obtained for E 2 mode are 174.5, 157.1 and 159.8 while those obtained for E 4 mode are 112.7, 109.8 and 110.4 cm-1. The Raman shift obtained for E 3 mode was 140.7 cm-1. The observed modes, apart from A1 can now be compared with the calculated transverse optical E modes, for mode assignment. The peak at 142.4 cm-1 can be assigned to the transverse optical E 3 mode as it falls close to the calculated value of 140.7 cm-1. The

peaks observed at 162.1 and 170.8 cm-1 may be assigned to E2 modes with the difference occurring due to In3-Te and In1-Te bonds. Similarly, the peaks at 185.4 and 214.4 cm-1 may be attributed to E1 mode with the “splitting” due to In2-Te and In1-Te bonds. The peaks at 132.0, 150.9, 269.8 cm-1 cannot be assigned to any specific modes at this stage. However, it is worth to discuss the results of Raman studies on I-III3-VI5 compounds, in general, as a discussion of this nature may help assign modes to the remaining peaks in the spectra.

There has been no report on Raman studies of AgIn3Te5 in the literature and in fact, the vibrational spectroscopy of only few I-III3-VI5 compounds has been investigated. The compounds studied include, CuIn3Se5, CuIn3Te5, CuGa3Se5 and the Cu(In1-xGax)3Se5 series. Of these, CuIn3Se5 has been studied by several groups. The very first report of Raman measurements (Nelson et al., 1994) was on thin films of CuIn3Se5 grown on 100 GaAs substrates from Cu2Se and In2Se3 sources. The A1 mode was observed at 152 cm-1. For CuInSe2, A1 mode appears at ~173 cm-1 and the difference was attributed to weakening of bond strengths due to the presence of vacancies. Tiwari et al. (1994) have studied the vibrational modes of epitaxial films of CuIn3Se5 grown on Si (111) substrates. The films were grown using three-source evaporation of the elements from Knudsen cells. The Raman shift of A1 mode was found to be 153 cm-1. Xu et al. (2004) studied thin films of several off-stoichiometric compounds, including CuIn3Se5 in the Cu-In-Se system. The films were formed by selenization of sequentially evaporated Cu and In bi layers. The reported value of A1 mode for CuIn3Se5 films was 153 cm-1. Similar results were obtained on bulk samples as well. Nomura et al. (1997) reported 153 cm-1 for A1 mode, while Rincon et al. (1998) found A1 mode to occur at 153 cm-1. On the other hand, Nomura and Endo (2002) reported A1 mode at 160 cm-1 for bulk crystals of Cu(In1-xGax)3Se5 grown by normal freezing. Interestingly, Raman shift of A1 was found to remain same in the

composition range investigated. In all these studies, for peaks observed at other frequencies the mode assignment was made following those reported for the chalcopyrite counterpart, CuInSe2.

In case of CuGa3Se5, the first report on Raman measurements was by Rincon et al. (1998) and it was followed by Nomura and Endo (2002). The studies were on bulk samples and the reported values for A1 mode were 166 and 160 cm-1 respectively and were lower compared to that reported for CuGaSe2 (184 cm-1). Grossberg et al. (2009) formed thin films of Cu-Ga-Se system deposited on GaAs substrates. They observed the A1 mode for CuGa3Se5 films at 166 cm-1. First Raman measurements on CuIn3Te5 were reported by Rincon et al. (2000) who studied the Raman modes of several compounds in Cu-In-Te system. The A1 mode for CuIn3Te5 was reported at 126 cm-1. Further, they observed that the Raman shifts of all the modes observed for CuIn3Te5 were close to those of CuInTe2. On the other hand, Diaz et al. (2005) reported the A1 mode at 112 cm-1 based on micro-Raman investigations of single crystals of CuIn3Te5, grown by the vertical Bridgman method. A more recent study of CuIn3Te5 thin films reported a value of 112 cm-1 for A1 mode (Mise and Nakada, 2010).

The following may be concluded from the results of Raman investigations on some of the I-III3-VI5 compounds reported in the literature. The A1 mode, which has its origin on the symmetric vibration of anions, has been identified unambiguously in the reported data. The A1 mode is also the most intense mode. In Cu-(In, Ga)-Se system, the Raman shifts of Cu(In,Ga)3Se5 are much lower than those of their chalcopyrite counter parts Cu(In,Ga)Se2. On the other hand, in the case of Cu-In-Te system, the Raman shifts observed for CuIn3Te5 are close to those of its chalcopyrite counterpart. Rincon et al. (2000) have attributed this to formation of local positive potential around a vacancy and the consequent attraction of the valance electrons of

group VI (Te) atoms. Peaks observed beyond 250 cm-1 were attributed to the combination of B and E modes, due to the upper limit on frequencies (Lazewski et al., 1999). Mode assignment for other peaks, in general, was done closely following the modes observed for the chalcopyrite counterparts.

Following the inferences made above, the mode at 132.0 cm-1 may be assigned to one of the B2 modes as Raman studies of AgInTe2 (the chalcopyrite counterpart of AgIn3Te5) have reported B2 mode at ~131 cm-1 (Ohrendorf and Haeuseler, 1999; Jagomagi et al., 2005). A recent ab initio calculation (Kopytov and Kosobutsky, 2009) of Raman modes of AgInTe2 has predicted a E longitudinal optical mode at 151 cm-1 and hence the peak at 150.9 cm-1 may be assigned to this mode. The peak that appears at 269.8 cm-1 may be due to the combination of B2 and E 3 modes as their sum is close to the observed value. The observed, calculated and the possible mode assignment are given in table 1. 3.2.2 Interaction of Laser Beam with AgIn3Te5 It was found during Raman measurements that, increasing the power of the probing laser beam beyond a certain level resulted in permanent changes on the surface of the samples. The changes were investigated in detail by recording Raman spectra with two different laser powers, viz., 0.5 mW and 20 mW. These laser powers were chosen following a similar study on bulk and thin films of CuInTe2, wherein it was shown that 0.5 mW laser beam had not affected the sample surface while 20 mW laser beam altered the sample surface (Ananthan et al., 2009; Ananthan, 2009). A constant integration time of 200 s was used for probing the as-formed sample surface, while three different integration times of 25, 50 and 75 s were used when the beam power was 20 mW. These measurements were performed sequentially on a given spot. The very first measurement performed with 0.5 mW laser beam power yielded

information about as formed samples. This was followed by measurement with 20 mW beam and 25 s integration time, which allowed probing the surface while it was undergoing changes. Subsequently, measurement was made with 0.5 mW laser beam and integration time of 200 s which allowed the study of permanent changes, if any, made on the sample during the previous measurement with 20 mW laser beam. These steps were repeated, but with 50 and 75 s integration time when the laser beam power used was 20 mW, which essentially helped study the laser-sample interaction for different duration. The results of the experiments mentioned above are displayed in Fig. 3. The sequence of acquisition of spectra displayed in Fig. 3 is a-a1-b-b1-c-c1-d. The spectrum of as-formed sample (panel-a) is reproduced here for the purpose of comparison. The spectra obtained with 20 mW laser beam with 25, 50 and 75 s are displayed respectively in panels (a1), (b1) and (c1). The spectra displayed in panels (b), (c) and (d) are obtained after acquiring the spectra shown in panels (a1), (b1) and (c1) respectively. All these spectra are acquired at a given spot on the sample surface.

To summarize, the spectra shown in panels (a1), (b1) and (c1) represent the state of the sample during laser-sample interaction for different durations. The spectra displayed in (b), (c) and (d) show respectively the permanent changes formed after those shown in (a1), (b1) and (c1) were recorded. The spectrum displayed in panel (a) is acquired on the as-formed sample and is included here for the sake of comparison. The spectra shown in panels (b) to (d) were deconvoluted as discussed earlier and the continuous line shows the resulting fit. It is important to note here that, hereafter, the measurements made with

20 mW laser beam may interchangeably be referred to as

laser-sample interaction, for the sample undergoes changes during these acquisitions. The results are given in Table 1

The following may be concluded from Fig. 3 and Table 3.3. All the spectra acquired with 20 mW laser beam exhibit broad featureless pattern, typical of noncrystalline solids. This indicates that the sample is in amorphous state locally. There is no significant and systematic change in the position of Raman modes except the mode at 132.0 cm-1, which shifts to 129.1 for spectra acquired after 75 s. The relative intensity of the mode at 142.4 cm-1 (E3 ) increases and its intensity eventually becomes comparable to that at 125.6 cm-1(A1 mode). Further, the FWHM of peaks appearing at 150.9 cm-1 and above, in general, increases considerably. Further, a broad peak appears at 302.2 cm-1 in the spectra acquired after the one showed in Fig. 3 (b1) was obtained (20 mW, 50 s). The most striking changes are the appearance of four additional peaks at 132.8, 307.9, 367.7 and 497.3 cm-1 and the missing of the peak at ~270 cm-1, in the spectra (Fig. 3d) obtained after the measurements with 20 mW laser beam and 75 s integration time.

The data marked with the symbol* are not present in the as-formed sample.

It may be inferred from the results presented in the previous paragraphs dealing with laser-sample interaction that when the probing laser beam power is 20 mW, there is significant rise in the temperature of the sample on a local scale. In order to obtain further insight, it is essential to obtain an estimate of the rise in temperature due to the heating of the sample by the laser beam. For the case of a thick sample with infinitely large coefficient of absorption, the instantaneous rise (t) in temperature on the sample surface can be calculated following Andrikopoulos et al. (2006). Since real samples have finite absorption at a given wavelength, tmax can be modified as,

 1    r0 PL (1  R)  exp  14 x 2 dx   2 r0  r0  x    0





(3.8)

where, PL and r0 are the power and radius of the laser beam, which in the present case are, 1 mW/20 mW and 0.9 m respectively. , R and  are respectively the thermal conductivity, reflectivity and the absorption coefficient of the sample. The term (1-R) accounts for the reduction in incident power due to sample reflectivity. The integral within the bracket is valid for the case of Gaussian beam and accounts for the finite absorption coefficient (Lax, 1977). Since AgIn3Te5 has not been studied well, parameters like thermal conductivity are not available. Therefore, the relevant parameters of its chalcopyrite counterpart, AgInTe2 has been used (Sanchez et al., 2005; Charoenphakdee et al., 2009) to estimate the temperature rise. The assumed values are 0.25 for reflectivity at 488 nm, 107 m-1 for the absorption coefficient, , at 488 nm and 2.05 W/mK, for thermal conductivity. The change in temperature was computed to be ~1152 K and hence temperature at the probing spot on the surface sample is ~1452 K, which is quite high. This estimate may be on the higher side as other losses are not taken into account while calculating the temperature. Further, the temperature will depend on time as well. In principle, it requires solving heat conduction equation with a finite source term under suitable boundary conditions (Pazionis et al., 1989). The calculated temperature value will be reduced by a factor of 20, when the probing laser beam power is 1 mW. Therefore, the temperature at local region will be around 373. Although the actual values of the temperature may be different, the following conclusions may be arrived at. When the probing laser beam power is 20 mW, the temperature can reach to values that the beam can alter the sample at a local spot on the surface. On the other hand, when the probing beam power is 1 mW the temperature increase will not be sufficient to cause any permanent damage.

Interaction of laser beam with chalcopyrite semiconductors has principally been utilized for ablation-growth of thin films. The other studies related to laser-sample interaction are laser annealing and processing of CuInSe2 films, CIGS films and solar cells based on these films (Wang, 2005; Wang et al., 2005; Ahmed et al., 2006; Uchida et al., 2007; Jost et al., 2008; Nakada and Shirakat, 2011). It is important to note the following point at this stage. As for as laser-sample interaction is concerned, a bulk sample with high absorption coefficient is quite different from its thin film counterpart, in the sense that it can be considered as an infinitely thick solid. On the other hand, for a thin film substrate effects cannot be neglected while formulating the dynamics of interaction.

Joliet et al. (1985) used CW Ar+ ion laser (457.9 – 514.5 nm) to irradiate sequentially deposited Cu, In and Se films covered by a ~15 nm thick top SiO protective film. They observed that irradiation by 3 W beam for 0.7 s resulted in the formation of CuInSe2 film. Tanino et al. (1993b) were the first to carry out a detailed study of interaction of laser beam (514 nm) with chalcopyrite semiconductors. The samples were thin films of several Cu and Ag based selenides and tellurides. The laser beam power density used was similar to what is used in the present experiments. Raman measurements were made after subjecting the films to laser irradiation for different duration. For CuIn(S,Se,Te)2 and AgInSe2 films, the spectra showed the presence of additional set of peaks, irrespective of the compound. These peaks were attributed to In based oxides, although the actual phase was not identified. Wang et al. (2005) irradiated CIGS and CdS/CIGS bilayer films with KrF excime laser (248 nm, 25 ns pulse width). They observed that solar cells fabricated using CdS/CIGS films, which were subjected to 30 mJ/cm2 (5 pulses) laser beam irradiation showed ~5% increase in efficiency. Ahmed et al. (2006) noted that flash evaporated CuInSe2 thin films exposed to 5 shots of 25 ns pulses with energy 170 mJ from excimer laser,

showed preferred growth of (112) plane. Jost et al. (2008) investigated the growth of CuInSe2 films using laser (Nd YAG, 1064nm) annealing of precursors that were formed by simultaneous electrodeposition of Cu, In and Se. They found that repetitive exposure to 1.5 laser beam resulted in the formation of 112 oriented CuInSe2 films along with small fraction of In2O3. Uchida et al. (2007) reported that interaction of 10 shots of 200 mJ/cm2 KrF excimer laser beam (248 nm, 20 ns pulse width and 1 Hz repetition rate) with CIGS thin films resulted in void formation due to local melting of the film. More recently, Nakada and Shirakata (2011) studied laser assisted growth of CIGS thin films. They showed irradiation of the substrates during the growth of CIGS films by 30 mJ/cm2, 100 Hz laser beam (248 nm) resulted in considerable reduction in surface roughness of the films along with significant enhancement in the preferred orientation along [112] direction.

As discussed earlier, Raman spectra acquired after the samples were exposed to 20 mW beam for 75 s showed four additional peaks at 132.8, 307.9, 367.7 and 497.3 cm-1, apart from missing of the peak at ~270 cm-1. Among the new peaks, the one at 132.8 cm-1 was quite sharp and in addition, its intensity was comparable to the A1 mode of AgIn3Te5. It is to be noted that since all the measurements were performed on the same spot, the total duration for which the spot was exposed to 20 mW laser beams is 150 s.

In samples with high absorption coefficient at a given wavelength, the intensity of the probing laser beam will be attenuated as it travels across the sample. The penetration depth of the laser beam can be readily estimated from the absorption coefficient and it is ~100 nm, in the present case. This shows that the laser sampleinteraction should be confined to a very thin region on the sample surface. Chiang et al. (1967) investigated the phase diagram of Ag2Te-In2Te3 system in detail and

showed that AgIn3Te5 melts congruently at 972 K with large solid miscibility. This means that during Raman measurements with 20 mW laser beam, a small region on the surface should be certainly beyond its molten stage. Therefore, the broad featureless pattern in Fig. 3 (a1), (b1) and (c1) can be confidently associated with the amorphous nature of the local region, rather than with experimental artifact arising due to the temperature induced broadening of the line width. In a crystalline solid, due to the requirement of momentum conservation not all phonons, but certain phonons near the zone center alone can contribute. On the other hand, in an amorphous solid there is no long-range order. Therefore, all vibrational modes (phonons near the zone center) can contribute to first order Raman scattering. As a result, the Raman spectrum of amorphous solids will exhibit a broad spectrum, which may reflect the density of vibrational states.

In order to identify the origin of the additional peaks, the Raman shifts of the peaks are compared with those expected for the binary phases as well as oxides of the constituent elements of AgIn3Te5. The results have shown that the four peaks may be attributed to In2O3. Three different crystal structures have been proposed for In2O3 and the stable form under ambient conditions is cubic bixbyite with space group Ia 3 . The irreducible representation for the Raman active modes of In2O3 has the form,

4 Ag  4Eg  14Tg , which indicates that there are twenty two Raman active modes. Experimentally, six Raman modes have been observed at frequencies 109, 135, 307, 366, 495, 517 and 630 cm−1 for In2O3 (Zhang et al., 2007; Liu et al., 2008; Berengue et al., 2010). The four new peaks, 132.8, 307.9, 367.7 and 497.3 cm−1 in present observations are close to those at 135, 307, 366 and 495 cm−1 and hence it may be concluded that interaction of laser radiation with AgIn3Te5 results in oxidation of the sample on a local scale. Formation of In2O3 due to laser irradiation has been reported

in an earlier study on laser annealing of CuInSe2 thin films (Jost et al., 2008). It is interesting to compare the results of similar experiments that have been performed on bulk and thin films of CuInTe2 (Ananthan et al., 2009; Ananthan, 2009). In particular, it is observed that Te segregates on the surface when Raman measurements are performed with probing laser beam power of 20 mW. In the present case, though the atomic percentage of Te is the highest among the constituent elements, oxidation of In is favoured.

The following observation on one of the additional four peaks, viz., 132.8 cm−1 appears to be in order, before this section is concluded. The intensity of 132.8 cm−1 peak is quite high and its width is narrow as well. It was taken to be that of In2O3, based on the peak at ~135 cm-1 found in the Raman spectra available in the literature for In2O3. However, in all the reported spectra, the relative intensity of the peak at ~135 cm-1 is not large and moreover, its width is comparable to those of other peaks in the spectra (Liu et al., 2008). Therefore, the possibility of 132.8 cm−1 peak being intrinsic to AgIn3Te5 cannot be ruled out. In the reported experimental spectra for AgInTe2, there is no peak around this frequency. However, a recent theoretical study has shown the A2 mode, one of the Raman inactive modes for AgInTe2, at 136 cm−1 (Kopytov and Kosobutsky, 2009). For well-ordered crystalline solids, modes that are forbidden will be absent in the experimental Raman spectra. However, when there is disorder induced by defects or structural imperfections, the translational symmetry gets broken. Consequently, there will be additional scattering and some of the forbidden modes will cease to be silent. In the present case, the 132.8 cm−1 peak that has appeared along with three more after the possibly is the A2 mode.

laser-sample interaction for 75s,

There have been some examples of silent modes (A2) being active in chalcopyrite compounds which include, CuGaTe2 (Rincon et al., 2001), ZnSnP2 (Mintairov et al., 1999) and Li based chalcopyrite compounds such as LiAlTe2, LiGaTe2 and LiInTe2 (Kopytov and Kosobutsky, 2009). Further, in ZnO and ZnO:N thin films, some of the peaks which were interpreted as anomalous modes have been identified with forbidden modes, which have become active due to defects. In fact, Kopytov and Kosobutsky (2009) have observed that the magnitude of intensity of the silent modes, which should otherwise be absent, can be a useful measure of the “structural imperfection” of the given solid. In the present case, the structural imperfections may be created as the sample surface is subjected to fast heatingcooling-heating cycles. Such thermal shocks can give rise to strain, which in turn can give rise to defects. Further, there should be loss of In from the lattice as the formation of In2O3 should occur at the cost of In atoms from the lattice. This is because, the surface was stoichiometric to begin with and oxidation will contribute to defect formation.

4. Conclusion Results on the vibrational modes and the effect of laser irradiation on the vibrational modes of bulk AgIn3Te5 synthesized by melt-quench method have been presented in the present work. Combined analyses of XRD and HRSEM data have shown that near-stoichiometric AgIn3Te5 is crystallized in P-chalcopyrite with space group P 42c . Raman measurements on as-formed samples have shown a strong peak at 125.6 cm-1, which is attributed to A1 mode. Tentative mode assignment for the rest of the peaks has been done based on simplified Keating model and by comparison with modes observed as well as predicted theoretically for AgInTe2. Raman spectra obtained with 20 mW laser beam have revealed the formation of amorphous phase during acquisition. Interaction with 20 mW laser beam has resulted in the formation

of In2O3 on the sample surface. The peak at 132.8 cm−1 that appeared after irradiation for 75s may be attributed to In2O3 or to the silent A2 mode, which probably has become active due to defect formation.

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Raman spectrum of AgIn3Te5. The symbols denote the experimental data points, gray lines show the individual Lorentzians and the black line through the symbols shows the overall fit.

Intensity (a.u.)

Fig. 1

Fig. 2

100

300

400

500

600

Basic tetrahedral in (a) chalcopyrite (AgInTe2) and (b) AgIn3Te5. The 150 200 250 300 differenceRaman in siteshift occupancies results in four unequal bonds: Te-Ag, Te-In1, -1 (cm ) Te-In2 and Te-In3, as opposed to two different bonds (Te-Ag and Te-In) in AgInTe2.

d 300

400

500

600

b

c1

Intensity (a.u.)

Intensity (a.u.)

c

b1

a

100

150

200

250 -1

Raman shift (cm ) Fig. 3

300

a1

100

150

200

250

300

-1

Raman shift (cm )

Raman spectra showing the effect of probing laser beam power on AgIn3Te5. All the spectra were acquired at the same spot. The spectrum shown in (a) is for the as-formed sample and is reproduced here for the sake of comparison. The spectra displayed in (a1), (b1) and (c1) were acquired with 25, 50 and 75 s integration time respectively and with 20 mW laser beam. The spectra displayed in (b), (c) and (d) were acquired (laser beam power 500 µW and integration time 500 s) respectively after those shown in (a1), (b1) and (c1) were recorded. Thus, the sequence of acquisition is (a)-(a1)-(b)-(b1)-(c)(c1)-(d). Inset (d) shows the data in the range 275 to 550 cm-1. The dark line through the data points in panels (a) – (d) denotes the overall fit.

Table 1 Results of Raman analysis of AgIn3Te5. Sl.

Raman shift (cm-1)

FWHM

Mode

No

Observed

(cm-1)

assigned

Calculated

1

125.6

130.1

7.0

A1

2

132.0

131*

11.0

B2

3

142.4

140.7

5.3

E3

4

150.9

151*

17.3

ELO

5

162.1

159.8

9.8

E2 (In3-Te)

6

170.8

174.5

13.9

E2 (In1-Te)

7

185.4

187.2

18.1

E1 (In2-Te)

8

214.4

212.6

51.8

E1 (In1-Te)

9

269.8

271.7

37.7

B2  E 3

*Values are taken from Ohrendorf and Haeuseler (1999); Kopytov and Kosobutsky (2009). The terms in the brackets in the last column denote the particular bond, the length of which was used in the calculations

Table 2

Raman shifts obtained after laser-sample interaction for different duration (integration time). The data obtained before interaction is given for comparison.

As-formed

Laser-sample interaction time (s)

sample

25

Raman

50

Raman FWHM

shift

Raman FWHM

shift (cm-1)

-1

Raman FWHM

shift (cm-1)

-1

(cm )

75

shift (cm-1)

-1

(cm )

FWHM (cm-1) -1

(cm )

(cm )

125.6

7.0

124.6

5.6

124.5

5.7

124.8

7.1

132.0

11.0

129.4

12.2

130.5

10.9

129.1

3.6

---

---

---

---

---

---

132.8*

4.0

142.4

5.3

142.3

4.7

142.3

3.5

141.9

9.4

150.9

17.3

149.8

16.2

150.2

14.5

152.0

11.3

162.1

9.8

162.3

13.1

161.2

13.6

162.2

17.7

170.8

13.9

172.4

12.7

171.4

13.8

172.9

13.7

185.4

18.1

185.5

19.7

185.1

23.9

185.6

25.6

214.4

51.8

213.9

44.1

215.9

67.9

214.3

51.8

269.8

37.7

272.0

19.8

271.2

19.4

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302.2

63.4

307.9* 10.8

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367.7*

7.3

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497.3*

8.6