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Growth kinetics and stoichiometry of ZnS films obtained by close-spaced vacuum sublimation technique
Solid State Sciences 13 (2011) 1068e1071
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Growth kinetics and stoichiometry of ZnS films obtained by close-spaced vacuum sublimation technique D. Kurbatov a, *, A. Opanasyuk a, S.M. Duvanov b, A.G. Balogh c, H. Khlyap d a
Sumy State University, Rimsky-Korsakov Str. 2, UA-40007 Sumy, Ukraine Applied Physics Institute NAS of Ukraine, Petropavlovskaya Str. 58, UA-40030 Sumy, Ukraine c Institute of Material Science, Technische Universität Darmstadt, Petersen Str. 23, D-64287, Darmstadt, Germany d TU Kaiserslautern, Distelstr. 11, D-67657, Kaiserslautern, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 October 2010 Received in revised form 5 January 2011 Accepted 26 January 2011 Available online 2 February 2011
Surface roughness, stoichiometry, deposition rates and activation energy of different growth mechanisms of zinc sulfide films are investigated. The films were deposited onto glass-ceramic substrates by closespaced vacuum sublimation technique (CSVS) at different deposition temperatures. Rutherford backscattering spectroscopy (RBS) using helium ions with 1.8 MeV energy and EDAX method were applied for determination of film thickness and stoichiometry. The effect of the deposition temperature on film properties has been studied. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: ZnS films Rutherford backscattering spectrometry Deposition rate Stoichiometry
1. Introduction Zinc sulfide films are widely used for manufacturing wide band gap semiconductor windows for solar cells [1], electroluminescent diodes [2] and other optoelectronic devices. Stable lifetime of these devices as basic layers requires the structurally perfect zinc sulfide films with well reproducible physical characteristics. The substrate temperature (Ts) is one of the most important parameters defining the growth kinetics of the films and, consequently, their structural and substructural characteristics. In Refs. [3e5] is described in detail the effect of the substrate temperature on the growth mechanisms and the thickness of CdSe and CdTe films deposited onto the mica substrates at constant evaporation temperature. It was shown that the processes of surface diffusion, adsorption and desorption of atoms condensed on the substrate are mainly defined by the deposition temperature. However, such examinations are not available for polycrystalline ZnS films. At the same time, the condensation rate which is directly connected to substrate temperature Ts, changes the structural and electrophysical properties of both polycrystalline and monocrystalline ZnS films [6]. Thus, the study of the deposition temperature effect on the growth rate and the growth mechanisms of the ZnS films, their * Corresponding author. E-mail address: [email protected] (D. Kurbatov). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.017
stoichiometry and chemical composition present not only fundamental interest, but have also practical applications. 2. Experimental The ZnS films were fabricated by the CSVS method [7]. As substrates ultrasonically cleaned polished glass-ceramic wafers (ST50-1, manufactured by “Sitall” Public Corporation, Ukraine) were used. Pure ZnS powder (produced from Koch Chemicals Ltd., England) was used to produce the layers. The evaporant (ZnS powder) was placed in a tungsten boat, which was used as an evaporation source. The evaporation temperature was Te ¼ 1273 K. The substrate temperature has been changed in the range of Ts ¼ (493e993) K. To avoid beam charging effects during the measurements on the semi-insulating ZnS films a thin Ag layer was deposited on surface by thermal evaporation in vacuum. RBS measurements with He ions have been performed using an electrostatic accelerator with a maximum energy of 1.8 MeV. The primary beam of particles was normal to the target. RBS spectra were collected in a vacuum chamber under the scattering angle of 170 using a semiconductor surface-barrier detector and standard electronic equipment. The spectra were evaluated using SIMNRA and DVBS programs. The thickness of ZnS films (d) was calculated from the RBS spectra according to the expression:
D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
d ¼
DE ½3ZnS nZnS
1069
(1)
where ΔE is an energy peak width, nZnS is the concentration of the compound atoms; ½3ZnS is a decelerating cross-section of the charged particles in the material calculated by the Bragg law using the decelerating cross-section of the constituent elements [8]:
½3ZnS ¼ ½3Zn c1 þ ½3S c2 nZnS ¼
rZn c1 þ rS c2 AZn c1 þ AS c2
(2) (3)
here c1 and c2 are weight factors. The ratio of the atomic concentrations in the elements of the compound averaged over the film thickness had been calculated by the position of the partial peaks maxima using the expression:
g ¼
s H DE CZn ¼ S Zn Zn sZn HS DES CS
(4)
where sZn , sS are scattering cross-sections of the charged particles by the atoms of sulfur and zinc; HZn , HS are the values of the corresponding signals in the RBS spectra. Additional investigation of stoichiometry and elemental composition of the films was carried out with the scanning electron microscope PEM-103-01 using the EDAX method. According to the technique of measurements the chemical composition of the deposited layers were carried out in five different points of the film surface. These results gave a possibility to estimate the homogeneity of the samples. The activation energy of the ZnS film growth mechanisms was determined by the function ln(R) ¼ f(1/Ts) using the Arrhenius equation which describes the growth rate (R) depending on the condensation temperature [3e5,9]:
R ¼ A expðEa =kB Ts Þ
(5)
where A is a constant; Ea is the activation energy; kB is the Boltzmann constant. The surface morphology of the samples was studied by 3D-laser scanning microscope VK-9700. According to the surface profiles the roughness of the surface was calculated in correspondence with the ISO/R 468 standard [10]. The profile arithmetic mean departure Ra from the centerline S was determined as follows:
Ra ¼
n 1X ½y n i¼1 i
Fig. 1. Typical optical micrograph (M ¼ 5 103) and the profile sections of the ZnS films: Ts ¼ 573 K.
inhomogeneous film relief and the increase of the surface roughness may be explained by the increase of the crystallite size under elevating deposition temperature. The crystallite size in the films increases under elevating deposition temperature. The fastest increment of the profile arithmetic mean departure Ra of the layers surface is observed in the region of their columnar growth, which probability also depends on the deposition temperature [12]. 3.2. Kinetics of the films growth The RBS spectra of ZnS films for different deposition temperatures are shown in Fig. 2. It is obvious that in case of thinner ZnS films (Ts ¼ (828e993) K) the peaks from zinc and sulfur are completely separated. The peaks from the thicker layers (Ts ¼ (493e573) K) overlap make the evaluation of the spectra more difficult. RBS spectra verify the dependence of film thickness in the range of d ¼ (240e900) nm on the condensation temperature. The average growth rate R of the chalcogenide films was calculated as the ratio of their as-grown thicknesses d and the time
where yi are departures of the film surface profile from the centerline, n is a number of the profile peaks. 3. Results and discussion According to our previous studies, the ZnS films prepared by the CSVS technique are polycrystalline, homogeneous over their area and have a good adhesion to the substrate [11]. The effect of the substrate temperature on structural, substructural and optical characteristics of the ZnS films has been discussed in [12]. 3.1. Investigation of the Surface roughness Fig. 1 shows the typical optical micrographs of the surface and profilograms of the ZnS films. The analysis of the samples surface demonstrated that the surface roughness increases from Ra ¼ 0.074 mm to 0.147 mm as the substrate temperature elevates from Ts ¼ 523 K to 883 K, Appearing
Fig. 2. RBS spectra of the multilayer Ag/ZnS/glass-ceramic structure. Arrows indicate kinematical edges of different elements.
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D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
t of the layer deposition. The calculated growth rate of the CSVSZnS on glass-ceramic R ¼ (0.5e4.5 nm/s) at identical evaporation temperatures was similar to CVD-ZnS films (3.3 nm/s) [13,14] and partially higher than for the CBD-ZnS films e R ¼ (0.04e0.45) nm/s [9] and CSE-ZnS R ¼ (0.1e0.6) nm/s [6]. It was found that the growth rate decreases with increasing substrate temperature. The analysis of these results indicates that under low condensation temperatures Ts ¼ (500e550) K the growth rate is controlled by the evaporator temperature. Almost all atoms, which have reached the substrate surface, are adsorbed on it, e.g. the dominant process of the growth mechanism is the surface diffusion. Decreasing growth rate with increasing condensation temperature up to Ts ¼ (600e650) K is explained by the desorption of the atoms from the substrate surface. Studies for ZnS [15] and CdTe [3,4,16] films gave similar results. The activation energy of the film growth can be defined from the dependence of Ln R on (1/Ts) (Fig. 3). As it can be seen from Fig. 3, two lines with different slopes can be selected corresponding to the different growth mechanisms of ZnS films discussed above. From the slopes one can determine the activation energies of the two different mechanisms as Ea1 ¼ 0.40 and Ea2 ¼ 0.09 eV, respectively. The values of the activation energy obtained for the high-temperature range are in good agreement with the results in the literature for films prepared by CBD and MOCVD methods: Ea ¼ 0.33 eV [9] and Ea ¼ (0.25e0.64) eV [17], respectively. In [3] a more complicated dependence of ln R on 1/Ts for CdTe films was proposed. It explains by the peculiarities of the processes taking place in a closed space under condensation. However, these processes are probable only at low supersaturation of the compound vapor and are coming to reality at small differences between the temperatures of the evaporator and substrate: Δ ¼ Te Ts ¼ (50e100) K. For zinc sulfide, having the sublimation temperature w(1273e1293) K [18], it is technologically very difficult to achieve such a value of Δ. 3.3. Stoichiometry and chemical compositions The stoichiometry and the elemental composition of the films were determined from RBS spectra. In ZnS layers besides the main elements impurities as Si, Ca and Na have been found and these atoms have diffused to the chalcogenide from the glass-ceramics substrate under high deposition temperatures Ts. Oxygen peaks observed in the spectra belong to the substrate. As it was demonstrated earlier for ZnS films deposited on oxygen-free substrates (carbon) [19], oxygen peak from the system ZnS/carbon is not fixed at the energetic place of its possible appearance; this fact is valid for
Fig. 4. Typical RBS spectra of the ZnS/carbon (oxygen-free substrate) structure. The vertical dashed line in the graph shows an energy position of probable oxygen peak [17].
Table 1 Dependence of atomic concentrations of Zn and S and stoichiometric ratio g in ZnS films on Ts. Ts, K
493 573 828 883 993
RBS
EDAX
CZn, at.%
CS, at.%
g
Error, %
CZn, at.%
CS, at.%
g
Error, %
39.50 39.39 53.10 50.30 49.50
49.00 46.96 51.90 47.20 48.90
0.81 0.84 1.02 1.07 1.01
w 10 w10 2e6 2e6 2e6
53.31 e e 50.37 50.38
46.69 e e 49.63 49.62
1.14 e e 1.02 1.01
5 e e 5 5
investigations of the films immediately deposited as well as for the layers annealed on air at 475 K (Fig. 4). Consequently, the main volume of the layers does not contain these impurities within the limits of the accuracy of the method. One should note that in ZnS films fabricated by pyrolysis and from chemical solutions the concentration of oxygen dissolved in the chalcogenide can achieve (3e8) at. %, and such films can be considered rather as solid solutions ZnSxO1x [20]. Averaged values of film thickness of ZnS films stoichiometry, as calculated from the experimental RBS spectra, are shown in Table 1. The average relative error of the chemical composition for ZnS films by this method does not exceed (2e4)%. At the same time, the film deposited at 193 K has a relatively larger thickness (w900 nm) which leads to the sufficient overlapping of peaks from Zn and S in RBS spectrum (Fig. 2). The inaccuracy of the stoichiometric measurements is somewhat increasing [8]. Table 1 illustrates that the ZnS film stoichiometry becomes better if Ts is increasing. Similar results are reported in [21] for films deposited by the CSE method; here the ratio g changed from 0.88 to 1.19 with increasing Ts between 473 and 623 K. For verification the data obtained from the RBS results also EDAX measurements have been carried out (Table 1). As it can be seen, the ZnS films have some excess of Zn compare to the sulfur concentration in the whole range of Ts, in agreement with the reference data. At the same time, the film stoichiometry becomes better with increasing deposition temperature Ts, i.e. a good correlation with the RBS data can be achieved if the condensation conditions become closer to the thermodynamically equilibrium.
4. Conclusions
Fig. 3. Dependence of ln R on 1/T for determination of the activation energy.
Investigations of the surface morphology and nondestructive elemental analysis on multilayer structures of Ag/ZnS/glass were carried out. ZnS films are deposited by CSVS technique under
D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
different deposition temperatures Ts. A decreasing rate of film condensation R from 4.5 to 0.5 nm/s with increasing Ts from 493 to 993 K was found. For the first time the activation energies Ea of the ZnS film growth mechanisms on glass-ceramic substrates were determined as 0.09 and 0.40 eV, respectively. RBS and EDAX techniques allowed determining the stoichiometry and the elemental composition of the condensates. It is also shown that the stoichiometry of the ZnS films improves with increasing deposition temperature. Acknowledgements This work was supported by the Project No 0110U001151 of the Ministry of Science and Education of the Ukraine and Bilateral Cooperation Project UKR05/003 of the Ministry of Science and Education of the Ukraine and International Büro of BMBF at DLR, Germany. The authors wish to thank Prof. V. Kulikauskas from the Research Institute for Nuclear Physics at Moscow State University for the RBS analysis of some ZnS thin film samples. References [1] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480 (2005) 99. [2] A.O. Yoshimasa, Electroluminescent Displays. World Scientific, New York, 1995. [3] P. _.Kalinkin, V.B. Aleskovsky, A.V. Simashkevich, Epitaxial Films of A2B6 Compounds. Leningrad University, St. Petersburg, 1978, [in Russian].
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[4] K.K. Muravyeva, I.P. Kalinkin, V.B. Aleskovsky, et al., Thin Solid Films 10 (3) (1972) 355. [5] I.P. Kalinkin, N.S. Bogomolov, V.A. Sanitarov, et al., Thin Solid Films 66 (1) (1980) 25. [6] P. Prathap, Y. Subbaiah, K. Ramakrishna Reddy, R. Miles, J. Phys. D Appl. Phys. 40 (2007) 5275. [7] V. Kosyak, A. Opanasyuk, P. Bukivskij, Yu. Gnatenko, J. Cryst. Growth 312 (2010) 1726. [8] J.R. Tesma, M. Nastasi, J.C. Barbow, Handbook of Modern Ion Beam Materials Analysis. Materials Research Society, Pittsburgh, 1995. [9] Q. Liu, M. Guobing, A. Jianping, Appl. Surface Sci. 254 (2008) 5711. [10] ISO Recommendations on Roughness Measurements. ISO/R 468; 1878. 1880, TC 57. [11] D. Kurbatov, V. Kosyak, A. Opanasyuk, Integr. Ferroelectr. 103 (2008) 32. [12] D. Kurbatov, A. Opanasyuk, H. Khlyap, Phys. Status Solidi. A 206 (7) (2009) 1549. [13] E. Lee, N.H. Tran, R.N. Lamb, Appl. Surf. Sci. 241 (2005) 493. [14] D. Barreca, E. Tondello, D. Lydon, T.R. Spalding, M. Fabrizio, Chem. Vapor Depos. 9 (2003) 93. [15] C.H. Lee, C.Y. Pueng, J. Mater. Sci. 28 (1993) 811. [16] J. Luschitz, K. Lakus-Wollny, A. Klein, W. Jaegermann, Thin Solid Films 515 (2007) 5814. [17] L.F. Zharovsky, L.V. Zavyalova, G.S. Svechnikov, Thin Solid Films 128 (1985) 241. [18] S. Kasap, Springer Handbook of Electronic and Photonic Materials. Springer, Berlin, 2007. [19] A.B. Kramchenkov, D.I. Kurbatov, M.I. Zaharets, A.S. Opanasyuk, Poverkhnost, Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya 3 (2010) 45 [in Russian]. [20] A.O. Pudov, J.R. Sites, T. Nakada, Jpn. J. Appl. Phys. 41 (2) (2002) 672. [21] Y. Subbaiah, P. Prathap, K. Ramakrishna Reddy, Appl. Surf. Sci. 253 (2006) 2409.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Growth kinetics and stoichiometry of ZnS films obtained by close-spaced vacuum sublimation technique D. Kurbatov a, *, A. Opanasyuk a, S.M. Duvanov b, A.G. Balogh c, H. Khlyap d a
Sumy State University, Rimsky-Korsakov Str. 2, UA-40007 Sumy, Ukraine Applied Physics Institute NAS of Ukraine, Petropavlovskaya Str. 58, UA-40030 Sumy, Ukraine c Institute of Material Science, Technische Universität Darmstadt, Petersen Str. 23, D-64287, Darmstadt, Germany d TU Kaiserslautern, Distelstr. 11, D-67657, Kaiserslautern, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 October 2010 Received in revised form 5 January 2011 Accepted 26 January 2011 Available online 2 February 2011
Surface roughness, stoichiometry, deposition rates and activation energy of different growth mechanisms of zinc sulfide films are investigated. The films were deposited onto glass-ceramic substrates by closespaced vacuum sublimation technique (CSVS) at different deposition temperatures. Rutherford backscattering spectroscopy (RBS) using helium ions with 1.8 MeV energy and EDAX method were applied for determination of film thickness and stoichiometry. The effect of the deposition temperature on film properties has been studied. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: ZnS films Rutherford backscattering spectrometry Deposition rate Stoichiometry
1. Introduction Zinc sulfide films are widely used for manufacturing wide band gap semiconductor windows for solar cells [1], electroluminescent diodes [2] and other optoelectronic devices. Stable lifetime of these devices as basic layers requires the structurally perfect zinc sulfide films with well reproducible physical characteristics. The substrate temperature (Ts) is one of the most important parameters defining the growth kinetics of the films and, consequently, their structural and substructural characteristics. In Refs. [3e5] is described in detail the effect of the substrate temperature on the growth mechanisms and the thickness of CdSe and CdTe films deposited onto the mica substrates at constant evaporation temperature. It was shown that the processes of surface diffusion, adsorption and desorption of atoms condensed on the substrate are mainly defined by the deposition temperature. However, such examinations are not available for polycrystalline ZnS films. At the same time, the condensation rate which is directly connected to substrate temperature Ts, changes the structural and electrophysical properties of both polycrystalline and monocrystalline ZnS films [6]. Thus, the study of the deposition temperature effect on the growth rate and the growth mechanisms of the ZnS films, their * Corresponding author. E-mail address: [email protected] (D. Kurbatov). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.017
stoichiometry and chemical composition present not only fundamental interest, but have also practical applications. 2. Experimental The ZnS films were fabricated by the CSVS method [7]. As substrates ultrasonically cleaned polished glass-ceramic wafers (ST50-1, manufactured by “Sitall” Public Corporation, Ukraine) were used. Pure ZnS powder (produced from Koch Chemicals Ltd., England) was used to produce the layers. The evaporant (ZnS powder) was placed in a tungsten boat, which was used as an evaporation source. The evaporation temperature was Te ¼ 1273 K. The substrate temperature has been changed in the range of Ts ¼ (493e993) K. To avoid beam charging effects during the measurements on the semi-insulating ZnS films a thin Ag layer was deposited on surface by thermal evaporation in vacuum. RBS measurements with He ions have been performed using an electrostatic accelerator with a maximum energy of 1.8 MeV. The primary beam of particles was normal to the target. RBS spectra were collected in a vacuum chamber under the scattering angle of 170 using a semiconductor surface-barrier detector and standard electronic equipment. The spectra were evaluated using SIMNRA and DVBS programs. The thickness of ZnS films (d) was calculated from the RBS spectra according to the expression:
D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
d ¼
DE ½3ZnS nZnS
1069
(1)
where ΔE is an energy peak width, nZnS is the concentration of the compound atoms; ½3ZnS is a decelerating cross-section of the charged particles in the material calculated by the Bragg law using the decelerating cross-section of the constituent elements [8]:
½3ZnS ¼ ½3Zn c1 þ ½3S c2 nZnS ¼
rZn c1 þ rS c2 AZn c1 þ AS c2
(2) (3)
here c1 and c2 are weight factors. The ratio of the atomic concentrations in the elements of the compound averaged over the film thickness had been calculated by the position of the partial peaks maxima using the expression:
g ¼
s H DE CZn ¼ S Zn Zn sZn HS DES CS
(4)
where sZn , sS are scattering cross-sections of the charged particles by the atoms of sulfur and zinc; HZn , HS are the values of the corresponding signals in the RBS spectra. Additional investigation of stoichiometry and elemental composition of the films was carried out with the scanning electron microscope PEM-103-01 using the EDAX method. According to the technique of measurements the chemical composition of the deposited layers were carried out in five different points of the film surface. These results gave a possibility to estimate the homogeneity of the samples. The activation energy of the ZnS film growth mechanisms was determined by the function ln(R) ¼ f(1/Ts) using the Arrhenius equation which describes the growth rate (R) depending on the condensation temperature [3e5,9]:
R ¼ A expðEa =kB Ts Þ
(5)
where A is a constant; Ea is the activation energy; kB is the Boltzmann constant. The surface morphology of the samples was studied by 3D-laser scanning microscope VK-9700. According to the surface profiles the roughness of the surface was calculated in correspondence with the ISO/R 468 standard [10]. The profile arithmetic mean departure Ra from the centerline S was determined as follows:
Ra ¼
n 1X ½y n i¼1 i
Fig. 1. Typical optical micrograph (M ¼ 5 103) and the profile sections of the ZnS films: Ts ¼ 573 K.
inhomogeneous film relief and the increase of the surface roughness may be explained by the increase of the crystallite size under elevating deposition temperature. The crystallite size in the films increases under elevating deposition temperature. The fastest increment of the profile arithmetic mean departure Ra of the layers surface is observed in the region of their columnar growth, which probability also depends on the deposition temperature [12]. 3.2. Kinetics of the films growth The RBS spectra of ZnS films for different deposition temperatures are shown in Fig. 2. It is obvious that in case of thinner ZnS films (Ts ¼ (828e993) K) the peaks from zinc and sulfur are completely separated. The peaks from the thicker layers (Ts ¼ (493e573) K) overlap make the evaluation of the spectra more difficult. RBS spectra verify the dependence of film thickness in the range of d ¼ (240e900) nm on the condensation temperature. The average growth rate R of the chalcogenide films was calculated as the ratio of their as-grown thicknesses d and the time
where yi are departures of the film surface profile from the centerline, n is a number of the profile peaks. 3. Results and discussion According to our previous studies, the ZnS films prepared by the CSVS technique are polycrystalline, homogeneous over their area and have a good adhesion to the substrate [11]. The effect of the substrate temperature on structural, substructural and optical characteristics of the ZnS films has been discussed in [12]. 3.1. Investigation of the Surface roughness Fig. 1 shows the typical optical micrographs of the surface and profilograms of the ZnS films. The analysis of the samples surface demonstrated that the surface roughness increases from Ra ¼ 0.074 mm to 0.147 mm as the substrate temperature elevates from Ts ¼ 523 K to 883 K, Appearing
Fig. 2. RBS spectra of the multilayer Ag/ZnS/glass-ceramic structure. Arrows indicate kinematical edges of different elements.
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D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
t of the layer deposition. The calculated growth rate of the CSVSZnS on glass-ceramic R ¼ (0.5e4.5 nm/s) at identical evaporation temperatures was similar to CVD-ZnS films (3.3 nm/s) [13,14] and partially higher than for the CBD-ZnS films e R ¼ (0.04e0.45) nm/s [9] and CSE-ZnS R ¼ (0.1e0.6) nm/s [6]. It was found that the growth rate decreases with increasing substrate temperature. The analysis of these results indicates that under low condensation temperatures Ts ¼ (500e550) K the growth rate is controlled by the evaporator temperature. Almost all atoms, which have reached the substrate surface, are adsorbed on it, e.g. the dominant process of the growth mechanism is the surface diffusion. Decreasing growth rate with increasing condensation temperature up to Ts ¼ (600e650) K is explained by the desorption of the atoms from the substrate surface. Studies for ZnS [15] and CdTe [3,4,16] films gave similar results. The activation energy of the film growth can be defined from the dependence of Ln R on (1/Ts) (Fig. 3). As it can be seen from Fig. 3, two lines with different slopes can be selected corresponding to the different growth mechanisms of ZnS films discussed above. From the slopes one can determine the activation energies of the two different mechanisms as Ea1 ¼ 0.40 and Ea2 ¼ 0.09 eV, respectively. The values of the activation energy obtained for the high-temperature range are in good agreement with the results in the literature for films prepared by CBD and MOCVD methods: Ea ¼ 0.33 eV [9] and Ea ¼ (0.25e0.64) eV [17], respectively. In [3] a more complicated dependence of ln R on 1/Ts for CdTe films was proposed. It explains by the peculiarities of the processes taking place in a closed space under condensation. However, these processes are probable only at low supersaturation of the compound vapor and are coming to reality at small differences between the temperatures of the evaporator and substrate: Δ ¼ Te Ts ¼ (50e100) K. For zinc sulfide, having the sublimation temperature w(1273e1293) K [18], it is technologically very difficult to achieve such a value of Δ. 3.3. Stoichiometry and chemical compositions The stoichiometry and the elemental composition of the films were determined from RBS spectra. In ZnS layers besides the main elements impurities as Si, Ca and Na have been found and these atoms have diffused to the chalcogenide from the glass-ceramics substrate under high deposition temperatures Ts. Oxygen peaks observed in the spectra belong to the substrate. As it was demonstrated earlier for ZnS films deposited on oxygen-free substrates (carbon) [19], oxygen peak from the system ZnS/carbon is not fixed at the energetic place of its possible appearance; this fact is valid for
Fig. 4. Typical RBS spectra of the ZnS/carbon (oxygen-free substrate) structure. The vertical dashed line in the graph shows an energy position of probable oxygen peak [17].
Table 1 Dependence of atomic concentrations of Zn and S and stoichiometric ratio g in ZnS films on Ts. Ts, K
493 573 828 883 993
RBS
EDAX
CZn, at.%
CS, at.%
g
Error, %
CZn, at.%
CS, at.%
g
Error, %
39.50 39.39 53.10 50.30 49.50
49.00 46.96 51.90 47.20 48.90
0.81 0.84 1.02 1.07 1.01
w 10 w10 2e6 2e6 2e6
53.31 e e 50.37 50.38
46.69 e e 49.63 49.62
1.14 e e 1.02 1.01
5 e e 5 5
investigations of the films immediately deposited as well as for the layers annealed on air at 475 K (Fig. 4). Consequently, the main volume of the layers does not contain these impurities within the limits of the accuracy of the method. One should note that in ZnS films fabricated by pyrolysis and from chemical solutions the concentration of oxygen dissolved in the chalcogenide can achieve (3e8) at. %, and such films can be considered rather as solid solutions ZnSxO1x [20]. Averaged values of film thickness of ZnS films stoichiometry, as calculated from the experimental RBS spectra, are shown in Table 1. The average relative error of the chemical composition for ZnS films by this method does not exceed (2e4)%. At the same time, the film deposited at 193 K has a relatively larger thickness (w900 nm) which leads to the sufficient overlapping of peaks from Zn and S in RBS spectrum (Fig. 2). The inaccuracy of the stoichiometric measurements is somewhat increasing [8]. Table 1 illustrates that the ZnS film stoichiometry becomes better if Ts is increasing. Similar results are reported in [21] for films deposited by the CSE method; here the ratio g changed from 0.88 to 1.19 with increasing Ts between 473 and 623 K. For verification the data obtained from the RBS results also EDAX measurements have been carried out (Table 1). As it can be seen, the ZnS films have some excess of Zn compare to the sulfur concentration in the whole range of Ts, in agreement with the reference data. At the same time, the film stoichiometry becomes better with increasing deposition temperature Ts, i.e. a good correlation with the RBS data can be achieved if the condensation conditions become closer to the thermodynamically equilibrium.
4. Conclusions
Fig. 3. Dependence of ln R on 1/T for determination of the activation energy.
Investigations of the surface morphology and nondestructive elemental analysis on multilayer structures of Ag/ZnS/glass were carried out. ZnS films are deposited by CSVS technique under
D. Kurbatov et al. / Solid State Sciences 13 (2011) 1068e1071
different deposition temperatures Ts. A decreasing rate of film condensation R from 4.5 to 0.5 nm/s with increasing Ts from 493 to 993 K was found. For the first time the activation energies Ea of the ZnS film growth mechanisms on glass-ceramic substrates were determined as 0.09 and 0.40 eV, respectively. RBS and EDAX techniques allowed determining the stoichiometry and the elemental composition of the condensates. It is also shown that the stoichiometry of the ZnS films improves with increasing deposition temperature. Acknowledgements This work was supported by the Project No 0110U001151 of the Ministry of Science and Education of the Ukraine and Bilateral Cooperation Project UKR05/003 of the Ministry of Science and Education of the Ukraine and International Büro of BMBF at DLR, Germany. The authors wish to thank Prof. V. Kulikauskas from the Research Institute for Nuclear Physics at Moscow State University for the RBS analysis of some ZnS thin film samples. References [1] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480 (2005) 99. [2] A.O. Yoshimasa, Electroluminescent Displays. World Scientific, New York, 1995. [3] P. _.Kalinkin, V.B. Aleskovsky, A.V. Simashkevich, Epitaxial Films of A2B6 Compounds. Leningrad University, St. Petersburg, 1978, [in Russian].
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