Far infrared properties of bulk sintered and thin film Zn2SnO4

Materials Science and Engineering B 138 (2007) 7–11

Far infrared properties of bulk sintered and thin film Zn2SnO4 M.V. Nikoli´c a,∗ , T. Iveti´c b , D.L. Young c , K.M. Paraskevopoulos d , T.T. Zorba d , V. Blagojevi´c e , P.M. Nikoli´c b , D. Vasiljevi´c-Radovi´c f , M.M. Risti´c b a

Center for Multidisciplinary Studies of the University of Belgrade, Kneza Viˇseslava 1, 11000 Belgrade, Serbia b Institute of Technical Sciences of SASA, Knez Mihailova 35/IV, 11000 Belgrade, Serbia c National Renewable Energy Laboratory, Golden, CO 80401, USA d Solid State Section, Physics Department, Aristotle University, 52124 Thessaloniki, Greece e Faculty of Electronic Engineering, University of Belgrade, Bulevar Kralja Aleksandra 72, 11000 Belgrade, Serbia f Institute of Microelectronics and Single Crystals, Njegoseva 12, 11000 Belgrade, Serbia Received 27 February 2006; received in revised form 1 November 2006; accepted 1 December 2006

Abstract Room temperature far infrared reflectivity spectra of single phase spinel zinc-stannate thin films, prepared by rf magnetron sputtering on a glass substrate, and bulk sintered samples were measured. The sintered samples were obtained by mechanical activation of starting ZnO and SnO2 powders for 10 min followed by sintering at 1300 ◦ C for 2 h. The reflectivity diagrams obtained for bulk samples were numerically analyzed using the four-parameter model of coupled oscillators. The optical parameters were determined for six observed ionic oscillators belonging to the spinel structure and two additional oscillators originating from the sintering procedure resulting from pores and grain boundaries. The reflectivity diagrams obtained for thin film samples were analyzed using the four-parameter model for optical phonons with a standard optical multi-layer technique. © 2006 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Thin films; Optical properties; Zinc-stannate

1. Introduction Zinc-stannate is a transparent high resistivity semiconductor whose basic properties are still not completely known. As it is a low cost and non-toxic transparent material it can be used for thin film devices [1,2]. It has been extensively studied for its gassensing properties [3]. Synthesis of polycrystalline zinc-stannate (ZTO) by a solid-state reaction and the influence of mechanical activation on its formation were recently studied [4]. Thin films of ZTO were recently grown on glass [2] and silica substrates [5] by rf magnetron sputtering. Synthesis of single-crystal ZTO nanobelts and nanorings using a thermal evaporation method was also studied [6]. One dimensional nanobelts and nanocones of ZTO were produced and studied via a reaction between Sn, ZnS and Fe (NO) element at 1350◦ C in a high temperature tube furnace [7]. In [8] a ZTO film was integrated into a CdS/CdTe solar cell as a buffer layer resulting in improved device performance.

∗

Corresponding author. Tel.: +381 11 637 367; fax: +381 11 3055289. E-mail address: [email protected] (M.V. Nikoli´c).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.12.005

The basic structural, optical and electron transport properties of rf magnetron sputter deposited ZTO films onto glass substrates were investigated in detail in [2]. Photoacoustic properties of these films were investigated in [9] enabling determination of thermal diffusivity and the mobility of minority free carriers. In this work, we have studied optical properties of rfmagnetron sputtered ZTO thin films and sintered ZTO in the mid and far infrared ranges. 2. Experimental Zn2 SnO4 films were prepared by rf magnetron sputtering on a Corning 7059 (barium–borosilicate) glass substrate using a two-inch planar magnetron in a sputter-up configuration and a phase-pure ceramic zinc-stannate target (Cerac. Inc. Milwaukee, WI) as the sputter target. A detailed description of the experimental conditions is given in [2]. The thin films obtained were amorphous by their XRD pattern [2,9]. Single-phase zincstannate films were obtained by annealing in Ar atmosphere with Ar flow of 2 l/min. The heating rate was 57.5 ◦ C/min and the samples were annealed at 600 ◦ C for 45 min. The cooling rate was 36.7 ◦ C/min.

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M.V. Nikoli´c et al. / Materials Science and Engineering B 138 (2007) 7–11

Bulk polycrystalline Zn2 SnO4 samples were prepared from commercially available zinc-oxide and tin-oxide powders (Aldrich) with a molar ratio ZnO:SnO2 = 2:1. They were mechanically activated in a planetary ball mill (Fritsch Pulversette) in a continuous grinding regime in air for 10 min as explained in detail in [10]. The obtained powder was sintered at 1300 ◦ C for 2 h. X-ray analysis of the obtained annealed films and bulk sintered samples was conducted on an X-ray diffractometer (Norelco-Philips PW-1050) with Cu K␣ radiation and a step scan mode of 0.02◦ /0.4 s. Atomic force (AFM) images of the thin film and bulk samples were made on a Thermo Microscopes Auto Probe CP Research device. Room temperature far infrared and infrared reflectivity measurements of the thin film and bulk ZTO samples were performed with near normal incidence light in the range between 50 and 2000 cm−1 using a Brucker 113 V FTIR spectrometer (the reflectivity was actually measured in two ranges 50–600 and 600–2000 cm−1 using two different beam splitters). Before measuring the bulk samples were highly polished first with silicon carbide P1000 and P1500 sand paper and then with 3 ␮m grade diamond paste. Reflectivity measurements were also performed in the range 50–2000 cm−1 of the Corning 7059 glass substrate used for thin films in order to be able to take into account the influence of the substrate used on the obtained thin film spectra. 3. Results and discussion X-ray diffraction analysis of both thin film and bulk Zn2 SnO4 samples showed that they were polycrystalline with peaks characteristic for spinel zinc-stannate [9,10]. The reflectivity of near normal incidence light in the range measured as a function of the wave number is given in Fig. 1 for two thin film sample thickness values and the glass substrate used. The Corning 7059 glass substrate (barium–borosilicate glass) has two expressed reflection bands at about 440 and 1060 cm−1 and smaller ones at about 670 and 1365 cm−1 in accordance with literature values for this type of glass. It is evident that these reflection bands have an influence on the mea-

Fig. 2. Measured (points) and calculated (dashed line) far infrared reflectivity curves for bulk sintered Zn2 SnO4 mechanically activated for 10 min.

sured thin film spectra. This influence is higher for the thinner film, where the peaks originating from the substrate are more expressed. Fig. 2 shows the reflectivity spectrum measured for a bulk ZTO sintered sample. The measured reflectance is much stronger for the bulk sample, compared to the thin film. One can see that peaks characteristic for zinc-stannate occur in the range between 100 and 700 cm−1 . AFM images of the analyzed thin films and bulk sintered samples are given in Fig. 3. One can see that the thin film has homogenous grains with an average diameter of about 100 nm, while the bulk sintered sample contains pores, crystalline grains and agglomerates as a result of the synthesis procedure (mechanical activation followed by sintering). Zn2 SnO4 has an inverse cubic spinel structure. The unit cell ¯ or O7 ) with lattice is face-centered cubic (space group Fd 3m h parameter a = 8.65 [11]. The tetrahedral voids are occupied by Zn2+ atoms and the octahedral voids are occupied randomly both by Zn2+ and Sn4+ atoms [12,13]. Using group theory analysis and taking into account the position of the atoms in the unit cell of zinc-stannate (Wyckoff sites, ICSD Code 28235) that are 8a and 16d for zinc atoms, 16d for tin atoms and 32e for oxygen atoms, one can calculate the total number of active and inactive IR and Raman modes. For zincstannate with a cubic structure we calculated the total number of infrared and Raman modes as: Γ = 1A1g + 1Eg + 3F2g + 7F1u

Fig. 1. Far infrared reflectivity curves obtained for Zn2 SnO4 thin films and the Corning 7059 glass substrate.

where 1A1g , 1Eg and represent the active Raman modes, while 7F1u represent infrared modes. As one IR mode is inactive there are 6 IR active infrared modes (F1u ) for zinc-stannate with a cubic inverse non-defective ZTO spinel structure. In order to perform a precise analysis of reflectivity diagrams of thin film zinc-stannate it was necessary to first analyze reflectivity diagrams obtained for bulk ZTO samples. Numerical analysis of the experimental results obtained was performed using a four-parameter model of coupled oscillators first introduced by Gervais and Piriou [14]. The factorized form

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Table 1 Parameter values obtained after fitting bulk sintered Zn2 SnO4 mechanically activated for 10 min (ε∞ = 3.2) ωTO (cm−1 )

γ TO (cm−1 )

ωLO (cm−1 )

γ LO (cm−1 )

155.7 252.8* 257.9 321.9 391.5 484.2* 564.6 654.1

14.3 74.7 33.5 60.3 60.9 98.9 58.1 29.8

160.4 253.7* 285.1 375.1 451.2 619.2* 621.2 655.6

11.7 28.5 46.2 60.4 36.0 19.8 24.2 12.1

In the case of bulk sintered ZTO eight coupled oscillators of varied strength were observed. The values of all parameters calculated for the bulk ZTO sample activated for 10 min are given in Table 1 and the obtained fitted curve is shown as a dashed line on Fig. 2. In the case of the thin film samples the theoretical reflection spectra can be calculated using a standard optical multi-layer technique [15,16]. The incident wave on striking the top of the film generates at each layer interface a superposition of incident, reflected and transmitted waves [15]. In this case we must take into account the contribution from two layers — the zincstannate film and the Corning glass substrate, i.e. reflection from the vacuum-film and film-substrate interfaces. Each layer is characterized by its thickness and complex dielectric function. The analysis is simplified by ignoring the contribution of IR reflection from the bottom of the substrate, as it is negligible in the range of interest. The reflectivity can then be calculated as:    rvf + rfs e2ikf df 2  R =  (2) 1 + rvf rfs e2ikf df  Fig. 3. AFM images of (a) thin film and (b) bulk sintered samples of Zn2 SnO4 .

of the dielectric function is defined as: ε = ε1 ± jε2 = ε∞

2  ωjLO − ω2 + iγjLO ω j

2 ωjTO − ω2 + iγjTO ω

(1)

where ωjTO and ωjLO are transverse (TO) and longitudinal (LO), frequencies, γ jTO and γ jLO are transverse and longitudinal damping factors, respectively, while ε∞ is the high frequency dielectric permittivity contribution. In order to obtain starting values for the four-parameter model the reflectivity diagram of the bulk sintered sample shown in Fig. 2 was first analyzed using the Kramers–Kronig method. Determination of the refractive index, n and extinction coefficient, k enabled calculation of the change of the complex dielectric permittivity and dielectric loss function. The peaks of the imaginary part of the complex dielectric function (ε2 ) practically corresponded to positions of transversal optical modes, while maximums of the dielectric loss function are at positions of longitudinal modes. These values were then used as starting values for numerical analysis using the four-parameter model. The numerical analysis was conducted using a program package developed in FORTRAN 95, enabling separate or simultaneous fitting of all parameters.

where rvf is the reflection coefficient from vacuum to film and rfs is the reflection coefficient from film to substrate, expressed for normal incidence as √ √ √ εf + εs 1 − εf rvf = rfs = √ (3) √ , √ 1 + εf εf + εs √ kf = 2πω εf , df is the film thickness and εf , εs are the dielectric function for the thin film and substrate, respectively. The dielectric functions for the thin film and substrate are modeled using the factorized form of the dielectric function (Eq. (1)). Thus, the measured spectra of the glass substrate was analyzed first using the four-parameter model of coupled oscillators in order to determine parameter values that will be used for analysis of the measured thin film spectra. Good agreement between measured and calculated spectra was obtained for five oscillators (Table 2; Fig. 4). The most expressed peaks for Corning 7059 glass at ωTO , 453 and 1044 cm−1 originate from Si O Si rocking and Si O asymmetric stretching, respectively [17]. The oscillator parameters obtained for the glass substrate were then incorporated as fixed parameters for the substrate layer in subsequent fitting of the measured Zn2 SnO4 thin film spectra. Good agreement between the measured and calculated thin film on substrate spectra for both film thickness values was obtained in the frequency range 100–800 cm−1 (Fig. 5) where optical

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Table 2 Parameters values obtained after fitting Corning 7059 glass substrate (ε∞ = 2.4) ωTO (cm−1 )

γ TO (cm−1 )

ωLO (cm−1 )

γ LO (cm−1 )

453.0 676.6 785.8 1043.8 1392.2

90.2 27.2 209.9 172.9 183.1

497.8 677.0 791.9 1176.2 1439.6

97.4 29.3 168.4 149.0 160.6

Table 3 Parameters values obtained for the thin film Zn2 SnO4 layer for two film thickness values (d) d = 316 nm (ε∞ = 3.2)

d = 600 nm (ε∞ = 3.2)

ωTO

γ TO

ωLO

γ LO

ωTO

γ TO

ωLO

γ LO

154.0 246.0 323.9 421.8 548.6 654.4

69.0 165.7 51.3 83.6 110.2 47.4

164.2 312.5 385.8 452.2 605.2 659.4

77.3 66.6 632.3 52.5 92.1 58.7

155.5 272.9 315.8 424.0 574.5 654.3

114.1 161.4 88.4 114.1 123.4 74.2

160.0 298.9 377.9 457.1 597.0 659.4

141.8 88.0 404.6 202.3 120.9 99.8

Transverse and longitudinal frequencies and damping factors are given in cm−1 .

Fig. 4. Measured (points) and calculated (dashed line) infrared reflectivity curves for barium-borosilicate glass (Corning 7059).

phonons of zinc-stannate only exist, according to the analysis performed of reflection spectra of bulk ZTO. The optical phonon parameters obtained for the thin film layer are given in Table 3. For both film thickness values six oscillators were determined. A comparison of the values obtained for the thin film and bulk ZTO sample shows that six oscillators (as predicted by group theory analysis) are at similar positions, while the bulk sintered sample has two extra oscillators (marked with (*) in Table 1). These two extra oscillators are weak and we believe they are the consequence of mechanical activation of the starting powder mixture and sintering. The AFM image of the thin film (Fig. 3a) shows a relatively homogenous structure, compared to the AFM image obtained for the bulk sintered ZTO sample (Fig. 3b) where the presence of pores and agglomerates besides crystalline grains

Fig. 5. Measured (points) and calculated (dashed line) far infrared reflectivity curves for Zn2 SnO4 thin films on glass substrate.

are noted. Mechanical activation can lead to formation of a defect spinel structure and after sintering the resulting structure contains pores, aggregates and intergranular material besides crystalline grains. Thus, additional infrared modes originate from the defective nature of the spinel lattice [18]. Disordering could also modify the local lattice symmetry to a non-stoichiometric spinel. A more detailed investigation of the influence of mechanical activation and sintering on changes in the IR spectra of ZTO is given in [10]. Thus, we can conclude that the polycrystalline structure of the thin film sample is much closer to the single crystal structure than the bulk sintered sample. In available literature data Porotnikov et al. [19] determined seven IR (159, 266, 380, 430, 535, 575 and 650 cm−1 ) modes using infrared transmission spectroscopy and six Raman active (148, 226, 375, 435, 526 and 667 cm−1 ) modes for two forms of Zn2 SnO4 , Coutts et al. [1] determined one strong Raman mode at 666.6 cm−1 for thin film ZTO, while Wang et al. [6] observed four Raman modes (at 107, 224, 528 and 668 cm−1 ) for ZTO nanobelts. The six IR modes predicted by group theory analysis and the six IR modes we determined for both ZTO bulk samples and films are in accordance with the values determined by Porotnikov et al. [19]. Porotnikov et al. [19] assigned the Zn2 SnO4 modes between 150 and 420 cm−1 to be characteristic for Zn O bonds as they noted a shifting of the observed infrared bands for the two forms of ZTO, while the higher frequency modes between 550 and 650 cm−1 are characteristic for Sn O bonds. However, according to [20–22] all allowed IR modes are typical lattice vibrations with contributions of all atoms and all bonds of the spinel structure, though Lutz et al. state in [22] that some bands can be more affected by the nature of the octahedrally coordinated metals or the metals on the tetrahedral sites. Segev and Wei [12] studied the cation distribution in eighteen closed-shell spinel oxides including zinc-stannate in detail. The stability of spinels was correctly identified using the atomistic method enabling calculation of the anion displacement parameter u. The inverse spinel structure was modeled using a small special quasirandom structure (SQS) with the same lattice vectors as normal spinel. For zinc-stannate the cubic lattice constant was determined to be a = 8.658, anion displacement parameter u = 0.3833, cation inversion parameter x = 1, with inverse energy E = −0.64 eV/molecule and inverse band-gap reduction of Eg = 0.6 eV [12] proving that ZTO is stable in the inverse spinel structure. Wei and Zhang [13] calculated the

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tetrahedral Rtetra and octahedral Rocta bond lengths between the cations and their oxygen nearest neighbours for zinc-stannate as (bond: Sn O, Rtetra = 1.97, Rocta = 2.067, Zn O, Rtetra = 1.975, Rocta = 2.082). The calculated bond lengths for both Zn O and Sn O bonds are relativelly similar, though, as in the inverse spinel structure Zn cations are on both octahedral and tetrahedral sites and the Sn cations are only on octahedral sites the lowest calculated bond lengths were obtained for Zn on tetrahedral sites. Segev and Wei [12] also determined that for Zn2 SnO4 in the spinel structure with the Oh space group, the local oxygen site has C3v symmetry, while the tetrahedral and octahedral sites have the local Td and D3d symmetry, respectively. Zn has shallow occupied d states and prefers to occupy tetrahedral sites [12]. Having all this in mind, we can assume that of the six oscillators determined for both bulk and thin film samples the higher frequency modes are more affected by the nature of Sn cations on octahedral sites and the lower frequency modes are more affected by Zn cations on both octahedral and tetrahedral sites. 4. Conclusion Room temperature far infrared spectra were measured for thin film and bulk sintered samples of Zn2 SnO4 . Application of a multi-layer technique where the dielectric functions for the thin film and substrate were modeled using the four-parameter model of coupled oscillators enabled determination of six ionic oscillators for the thin film layer in the range between 100 and 700 cm−1 . They were in reasonable agreement with the same oscillators determined for the bulk sintered samples. The ionic oscillators for bulk sintered samples had a higher intensity. The bulk sintered samples also had two additional weak oscillators that are the result of the applied synthesis procedure (mechanical activation followed by sintering). The number of six active ionic oscillators for Zn2 SnO4 was theoretically confirmed by group theory analysis and known Wyckoff sites for Zn2 SnO4 atoms. Acknowledgements We would like to thank Dr. S. Djuric for X-ray measurements. This work was performed as part of projects 142011G and

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6150B financed by the Ministry for Science and Environmental Protection of the Republic of Serbia. References [1] T.J. Coutts, D.L. Young, X. Li, W.P. Mullingan, X. Wu, J. Vac. Sci. Technol. A 18 (6) (2000) 2646–2660. [2] D.L. Young, H. Mountinho, Y. Yan, T.J. Coutts, J. Appl. Phys. 92 (2002) 310–319. [3] J.H. Yu, G.M. Choi, J. Electroceram. 8 (2003) 249–255. [4] N. Nikoli´c, T. Sre´ckovi´c, M.M. Risti´c, J. Eur. Ceram. Soc. 21 (2001) 2071–2074. [5] K. Satoh, Y. Kokehi, A. Okamoto, S. Murakami, Jpn. J. Appl. Phys. 44 (2005) L34–L37. [6] J.X. Wang, S.S. Xie, H.J. Yuan, X.Q. Dan, D.F. Liu, Y. Gao, Z.P. Zhou, L. Song, L.F. Liu, X.W. Zhao, X.Y. Dou, W.Y. Zhou, G. Wang, Solid State Commun. 131 (2004) 435–440. [7] Y. Li, X.L. Ma, Phys. Stat. Sol. 202 (2005) 435–440. [8] X. Wu, S. Asher, D.H. Levi, D.E. King, Y. Yan, T.A. Gessert, P. Sheldon, J. Appl. Phys. 89 (2001) 4564–4569. [9] T. Ivetic, M.V. Nikolic, D.L. Young, D. Vasiljevic-Radovic, D. Urosevic, Mater. Sci. Forum 518 (2006) 465–470. [10] M.V. Nikolic, T. Ivetic, K.M. Paraskevopoulos, K.T. Zorbas, V. Blagojevic, D. Vasiljevic-Radovic, Electroceramics, Toledo, Spain, June, 2006, 18–22, 175. [11] J. Choiznet, A. Deschanvers, B. Raveau, Comptes Rendes Serie C Sciences Chimiques 266 (1968) 543–548 (ICSD 28235). [12] D. Segev, S.-H. Wei, Phys. Rev. B 71 (2005) 125129. [13] W. Su-Huai, S.B. Zhang, Phys. Rev. B 63 (2001) 045112. [14] F. Gervais, B. Piriou, Phys. Rev. B 10 (1974) 1642–1654. [15] B.G. Almeida, A. Pietka, P. Caldelas, J.A. Mendes, J.L. Ribeiro, Thin Solid Films 513 (2006) 275–282. [16] G. Yu, L.F. Jiang, W.Z. Shen, H.Z. Wu, J. Phys. D: Appl. Phys. 35 (2002) 157–161. [17] C.T. Kirk, Phys. Rev. B 38 (1988) 1255–1273. [18] P. Thibaudeau, F. Gervais, J. Phys.: Condens. Matter 14 (2002) 3543– 3552. [19] N.V. Porotnikov, V.G. Savenko, O.V. Sidorova, Zh. Neorg. Khim. 28 (1983) 1653–1655; N.V. Porotnikov, V.G. Savenko, O.V. Sidorova, Rus. J. Inorg. Chem. 28 (1983) 983. [20] H.D. Lutz, H. Haeusler, J. Mol. Struct. 511–512 (1999) 69–75. [21] C. Kringe, B. Oft, V. Schellenschlaeger, H.D. Lutz, J. Mol. Struct. 596 (2001) 25–32. [22] H.D. Lutz, B. Mueller, H.J. Steiner, J.Solid State Chem. 90 (1991) 54–60.