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Ag diffusion in ZnS thin films prepared by spray pyrolysis
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Materials Letters 61 (2007) 5239 – 5242 www.elsevier.com/locate/matlet
Ag diffusion in ZnS thin films prepared by spray pyrolysis E. Bacaksiz a,⁎, O. Görür b , M. Tomakin b , E. Yanmaz a , M. Altunbaş a a
Department of Physics, Faculty of Arts and Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey b Department of Physics, Faculty of Arts and Sciences, Rize University, Rize, Turkey Received 30 January 2006; accepted 5 April 2007 Available online 19 April 2007
Abstract ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80–400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10− 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N2 atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnS thin films; Silver diffusion; Diffusion coefficient
1. Introduction ZnS thin films are promising materials for their use in various device applications. In the opto-electronics, it can be used as light emitting diode in the blue to ultraviolet spectral region due to its band gap (3.7 eV) at room temperature. It is well known that the electrical conductivity of ZnS films is too low to act as a substrate for transistors, however it can be used as light source for display screens and buffer layers for Cu(In,Ga) (S,Se)2 solar cells [1,2]. Furthermore, ZnS/Ag/ZnS multilayer films have been used as an important low emittance films such as heat mirrors [3]. It is known that the ZnS semiconductor has an n-type conductivity. Donor centers, which formed in ZnS during growth, were attributed to the native point defects caused by deviation of ZnS composition from the stoichiometry. It is very difficult to change film conductivity from the n-type to the ptype by conventional doping and diffusion processes. It was reported in the literature [4–6] that doping of ZnS by silver causes p-type ZnS layers having low resistivity under excess Zn ⁎ Corresponding author. E-mail address: [email protected] (E. Bacaksiz). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.038
atmosphere. The method of application of excess Zn pressure fills the Zn vacancies which are incompatible with utilization of the Zn vacancy related to the so-called blue emission for light emitting devices. Thus, the choice of proper ion impurity centers related to blue emission is quite important. Ag impurities occupying the Zn sites in ZnS are known to make the so-called blue centers [7]. It is well known that group I metals such as Ag and Cu are fast-diffusing impurities in II–VI compounds [8]. Therefore, the interdiffusion of components of Ag–ZnS bilayer structures and particularly diffusion penetrations of silver into ZnS can cause changes in physical properties of the near-interface region of ZnS and thereby in characteristics of structure. Generally, the mechanism of diffusion in thin films is different from that in the bulk sample. Diffusion in thin films can proceed through the grain or along the grain boundaries depending upon the microstructure of films. In large grained films, diffusion is generally via the grains and analysis gives the lattice diffusion coefficients. As the grain size decreases, atomic transport also occurs preferable along the grain boundary surfaces and analysis yields grain boundary diffusion parameters [9]. According to our knowledge, the thermal diffusion behavior of silver in ZnS films has not been reported. In this
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work, we have investigated the diffusion of silver into ZnS and its influence on the structural, electrical and optical properties of ZnS thin films. 2. Experimental ZnS thin films (d = 3 μm) are obtained by spray pyrolysis in air atmosphere on the glass substrate. The initial solution is prepared from zinc chloride ZnCl2 (98% purity) at 0.1 M concentration and 0.1 M concentration thioure (NH2)2CS (99% purity) in deionized water. The growth was performed with a spray rate of about 5 ml/min and growth rate of ∼50 nm/min on the glass substrates cleaned in ethanol dried in vacuum. During the growth, the substrates were rotated with a speed of 10 rev/ min at a temperature of 500 °C and in atmospheric pressure in order to produce homogenous films [10]. The obtained films had good adhesion to the substrate surfaces. The thickness of these films measured by the SEM method is found to be about 3 μm. Silver diffusion into ZnS thin films was carried out for 3 min using the vacuum evaporated layer of Ag on the open surface of the film. The diffusion annealing of the films with the deposited layer of silver was performed under a nitrogen atmosphere in the temperature range 80–400 °C with 80 °C steps. The undoped films (without silver layer) were annealed under the same conditions. After the annealing, the rest of the Ag layer on the upper ZnS surface was removed by using HNO3:H2SO4:H2O (5:5:90) solution and lateral sides of the ZnS films were cleaned by grinding. By measuring the surface electrical conductivity it was tested whether the whole remaining metallic Ag layer was removed from the ZnS film by the etching in a HNO3:H2SO4:H2O solution. The concentration distributions of Ag atoms in the ZnS thin films were analyzed by the successive removal of thin layers from the sample by using a HF:H2O (1:2) solution and by measuring the energy dispersive X-ray fluorescence (EDXRF) intensity of the Ag-Kα peak. The thickness of the film after the removal of each layer by etching in HF:H2O (1:2) was measured by forming a series of 10 equal etching steps and measuring the total decrease in the thickness by SEM and then finally calculating the
Fig. 1. The XRD pattern of undoped ZnS film.
Fig. 2. Concentration profiles of Ag in ZnS films T = 320 °C for 180 s (the full curve is calculated according to (2)).
decrease in the thickness for each etching step. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. Intensity measurements of the Ag peaks were detected with a Si (Li) solid-state dedector [11]. The X-ray diffraction (XRD) data of ZnS and ZnS/Ag films were taken using a Rigaku D/Max-IIIC diffractometer with CuKα radiation over the range 2θ = 3–70° at room temperature. The surface morphology was studied by using JEOL JST-6400 scanning electron microscope. The absorption spectra of undoped and Ag-doped ZnS films were measured by PerkinElmer Lambda 2SUV/Vis Spectrometer with 190–1100 nm wavelength range using non-polarised ligth. The resistivity of ZnS and Ag-doped ZnS films was determined by Van Der Pauw measurements at room temperature. The carrier concentration was determined by Hall effect measurements. 3. Results and discussion The values of resistivity and charge carrier concentration for the asgrown n-type ZnS films obtained on the glass substrate with a thickness of about 3 μm were found to be 2.8 × 105 Ω cm and n = 9.5 × 1014 cm− 3 respectively. Fig. 1 shows the X-ray diffraction pattern of ZnS film grown by spray pyrolysis technique. The diffraction pattern arising from the film consisted of a single intense peak at ∼28.96° due to the fcc (111) reflection. The lattice constant a is calculated from the peak
Fig. 3. The temperature dependence of Ag diffusion coefficient in ZnS films.
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the diffusion of Ag into ZnS can be determined. Fig. 2 shows a typical concentration profile for a sample annealed at 320 °C for a period of 180 s. The experimental data of Fig. 2 are in agreement with the theoretical curve and by using the experimental data of Fig. 2, the diffusion coefficient of Ag atom for the film annealed at 320 °C was determined to be D = 1 × 10− 12 cm2/s. The temperature dependence of the silver diffusion coefficient in the temperature range of 80–400 °C obtained from the EDXRF data for ZnS is described by the following relation (Fig. 3): 0:10eV D ¼ 8 109 exp ð4Þ kT
Fig. 4. The SEM micrographs of (a) undoped and (b) Ag-doped ZnS films.
position and is determined to be a = 5.28 Å. This indicated that the crystallites in the film have a single preferred (111) orientation that the fcc (111) plane was parallel to the substrate surface. This was expected since the cubic (111) lattice plane has the lowest surface energy [12,13]. To calculate the diffusion coefficient of Ag into ZnS thin films, Fick's diffusion equations were used. Eq. (1) below is known as Fick's law of diffusion. AN ð x; tÞ A2 N ð x; tÞ ¼D At Ax2
ð1Þ
The relatively small value of the activation energy (0.10 eV) and large values of diffusion coefficients of silver in polycrystalline ZnS films can be caused by Ag migration via grain boundaries, vacancies, etc., which are channels for rapid impurity migration in ZnS films. We believe that atomic migration of silver in ZnS films takes place by means of (a) fast migration along grain boundaries and (b) relatively slow migration into grains [15]. The grain boundary diffusion is usually obtained for the low temperature region, whereas the diffusion into grains is obtained for the relatively high temperature region. We suppose that the temperature range from 80 °C to 400 °C is in low temperature region with comparing ZnS semiconductor melting point of 1800 °C, so, the grain boundary diffusion is dominant in our work. The group I impurities in II–VI semiconductors diffuse by a dissociative mechanism, which is connected with their rapid migration along the interstitials as positive ions being subsequently situated in vacancies, where they exhibit acceptor properties [16,17]. It was found that Ag-doped ZnS films annealed at 400 °C for 3 min in the N2 atmosphere have changed their n-type conductivity to the p-type. The films annealed at temperatures less than 400 °C showed n-type conductivity. From the Hall coefficient measurement and hot probe analysis, it was determined that the resistivity and carrier concentration of Ag-doped ZnS films annealed at 400 °C for 3 min were found to be 5.3 × 105 Ω cm, p = 1.0 × 1014 cm− 3 respectively. The changes in electrical properties of ZnS films can be explained by acceptor behavior of Ag or its complexes (silver-vacancy type) in ZnS. The reason of the change in the conductivity from n-type to the p-type can be attributed to the increasing annealing temperature, a part of Ag atoms in grain boundaries can diffuse into the grains and some of the silver atoms in the grain are thermally ionized and inhabit the Zn site of the ZnS lattice. The same type of conductivity transition with the same reason was found by Magnea et al. and Poolton for the diffusion of Ag in ZnTe and Ag in ZnS
D is the diffusion coefficient, x is the coordinate in the direction of flow, t is the diffusion time and N is the concentration that is assumed to be a function of x and t. Since the layer of Ag can be assumed as a source of silver atoms, the solution of Eq. (1) can be simplified by incorporating the initial and boundary conditions. The solution for Eq. (1) can be written as [14]: x N ð x; tÞ ¼ N0 1 erf pffiffiffiffiffi 2 Dt
ð2Þ
Where N0 is the surface concentration of impurity and erf is the error function. The diffusion coefficient determined experimentally over a range of diffusion temperatures can often be expressed as E : ð3Þ D ¼ D0 exp kT Where D0 is the frequency factor, E is the activation energy, T is temperature and k is the Boltzmann constant. From the EDXRF profile,
Fig. 5. The absorption spectra of (a) undoped and (b) Ag-doped ZnS films.
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respectively [18,19]. Magnea et al. have reported that the intensity of photoluminescence transitions was enhanced by the diffusion of Ag into ZnTe crystals and assigned the observed acceptor level to the Ag substitutions on a Zn-lattice site AgZn. In addition, Poolton have reported that the simple silver substitution in ZnS film behaves as an acceptor which is indicated as the complex of [AgZn]. It was observed that XRD patterns of undoped and Ag-doped ZnS films have almost similar patterns with a slight displacement of (111) peak shown in Fig. 1. The calculated lattice parameter for the ZnS(Ag) film is found to be a = 5.33 Å which is 0.9% larger than for the undoped ZnS film (a = 5.28 Å). The SEM micrographs of undoped and Agdoped ZnS films are shown in Fig. 4 respectively. It was observed that ZnS film without Ag indicates a clear and nonhomogeneous grain distributions (Fig. 4 (a)). It is thought that the connectivity between grains seems poor which affect the physical properties. Comparing this micrograph with that of Ag-doped ZnS, slightly better connectivity between grains is observed (Fig. 4 (b)) and the substructure seems to consist of smaller grains and be more disordered. Fig. 5 shows the absorption spectra of the undoped and Ag-doped ZnS films. The optical band gap values were determined from the intercept of the straight-line portion of the (αhν)2 against on the hν axis. It can be seen that the band gap decreases from 3.52 eV to 3.43 eV due to silver diffusion. The decrease in the band gap of ZnS doped by Ag can be explained by the influence of near band levels, arising on the introduction of silver and related with silver and silver-vacancy complexes. A small fraction of the silver incorporated into ZnS lattice as an acceptor could give rise to localized level near the valance band [20]. In conclusion, the silver diffussion in polycrystalline ZnS films is characterised in the temperature range 80–400 °C by a rather fast Ag migration with a diffusion coefficient changing from 3× 10− 13 cm2/s to 5× 10− 12 cm2/s. The silver migration mechanism in ZnS films is attributed to both fast diffusion along grain boundaries and the simultaneous penetration into grain. The band gaps of films are decreased with Ag doping. Ag-doped ZnS films annealed at about 400 °C changed their n-type conductivity to the p-type. These effects are tentatively attributed to the formation of the energy levels related to silver and its complexes.
Acknowledgement This work was supported by the research fund of Karadeniz Technical University, Trabzon, Turkey, under contract no. 2003.111.001.1.
References [1] E. Monroy, F. Omnes, Semicond. Sci. Technol. 18 (2003) R33. [2] M. Rusu, W. Eisele, R. Würz, A. Emnaoui, M.Chi. Lux-Steiner, T.P. Niesen, F. Karg, J. Phys. Chem. Solids 64 (2003) 2037. [3] G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, Sol. Energy Mater. Sol. Cells 61 (2000) 107. [4] T. Butkhuzi, T.G. Tchelidze, E.G. Chikkoidze, N.P. Kekelidze, Phys. Status Solidi, B Basic Res. 229 (2002) 365. [5] T. Yamamoto, S. Kishimoto, S. Ida, Phys. Status Solidi, B Basic Res. 229 (2002) 371. [6] T. Yamamoto, A. Kato, A. Naito, Y. Sakamoto, S. Ida, Phys. Status Solidi, B Basic Res. 229 (2002) 391. [7] S. Kishimoto, T. Haseqawa, H. Kinto, O. Matsumoto, S. Ida, J. Cryst. Growth 214–215 (2000) 556. [8] I.D. Desnica-Frankovic, U.V. Desnica, A. Stötzler, M. Deicher, Physica, B + C 273–274 (1999) 887. [9] D. Gupta, D.R. Cample, P.S. Ho, Interdiffusion and Reactions, Wiley, New York, 1978. [10] B. Elidrissi, M. Addou, M. Regragui, A. Bougrine, A. Kachouane, J.C. Bernede, Mater. Chem. Phys. 68 (2001) 175. [11] O. Dogan, M. Ertugrul, U. Cevik, E. Bacaksız, E. Tirasoglu, A.I. Kopya, H. Erdogan, X-Ray Spectrom. 32 (2003) 363. [12] E. Bacaksız, T.D. Dzhafarov, V.D. Novruzov, K. Öztürk, M. Tomakin, T. Küçükömeroğlu, B. Abay, E. Yanmaz, M. Altunbaş, Phys. Status Solidi, A Appl. Res. 201 (2004) 2948. [13] K. Wright, G.W. Watson, S.C. Parker, D.J. Vaughan, J. Am. Mineral. 83 (1998) 14. [14] G.B. Abdullayev, T.D. Dzhafarov, Atomic Diffusion in Semiconductor Structures, Harwood Academic, New York, 1987. [15] T.D. Dzhafarov, Phys. Status Solidi, A Appl. Res. 158 (1996) 335. [16] D.W. Lane, K.D. Rogers, J.D. Painter, D.A. Wood, M.E. Ozsan, Thin Solid Films 361 (2000) 1. [17] H. Bidadi, M. Kalafi, H. Tajalli, A. Bairamov, T.D. Dzhafarov, Opt. Mater. 6 (1996) 27. [18] N. Magnea, D. Bensahel, J.L. Pautrat, J.C. Pfister, Phys. Status Solidi, B Basic Res. 94 (1979) 627. [19] N.R.J. Poolton, J. Phys. C. Solid State Phys. 20 (1987) 5867. [20] E. Hao, Y.P. Sun, B. Yang, X. Zhang, J.M. Liu, J.C. Shen, J. Colloid Interface Sci. 204 (1998) 369.
Materials Letters 61 (2007) 5239 – 5242 www.elsevier.com/locate/matlet
Ag diffusion in ZnS thin films prepared by spray pyrolysis E. Bacaksiz a,⁎, O. Görür b , M. Tomakin b , E. Yanmaz a , M. Altunbaş a a
Department of Physics, Faculty of Arts and Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey b Department of Physics, Faculty of Arts and Sciences, Rize University, Rize, Turkey Received 30 January 2006; accepted 5 April 2007 Available online 19 April 2007
Abstract ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80–400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10− 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N2 atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnS thin films; Silver diffusion; Diffusion coefficient
1. Introduction ZnS thin films are promising materials for their use in various device applications. In the opto-electronics, it can be used as light emitting diode in the blue to ultraviolet spectral region due to its band gap (3.7 eV) at room temperature. It is well known that the electrical conductivity of ZnS films is too low to act as a substrate for transistors, however it can be used as light source for display screens and buffer layers for Cu(In,Ga) (S,Se)2 solar cells [1,2]. Furthermore, ZnS/Ag/ZnS multilayer films have been used as an important low emittance films such as heat mirrors [3]. It is known that the ZnS semiconductor has an n-type conductivity. Donor centers, which formed in ZnS during growth, were attributed to the native point defects caused by deviation of ZnS composition from the stoichiometry. It is very difficult to change film conductivity from the n-type to the ptype by conventional doping and diffusion processes. It was reported in the literature [4–6] that doping of ZnS by silver causes p-type ZnS layers having low resistivity under excess Zn ⁎ Corresponding author. E-mail address: [email protected] (E. Bacaksiz). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.038
atmosphere. The method of application of excess Zn pressure fills the Zn vacancies which are incompatible with utilization of the Zn vacancy related to the so-called blue emission for light emitting devices. Thus, the choice of proper ion impurity centers related to blue emission is quite important. Ag impurities occupying the Zn sites in ZnS are known to make the so-called blue centers [7]. It is well known that group I metals such as Ag and Cu are fast-diffusing impurities in II–VI compounds [8]. Therefore, the interdiffusion of components of Ag–ZnS bilayer structures and particularly diffusion penetrations of silver into ZnS can cause changes in physical properties of the near-interface region of ZnS and thereby in characteristics of structure. Generally, the mechanism of diffusion in thin films is different from that in the bulk sample. Diffusion in thin films can proceed through the grain or along the grain boundaries depending upon the microstructure of films. In large grained films, diffusion is generally via the grains and analysis gives the lattice diffusion coefficients. As the grain size decreases, atomic transport also occurs preferable along the grain boundary surfaces and analysis yields grain boundary diffusion parameters [9]. According to our knowledge, the thermal diffusion behavior of silver in ZnS films has not been reported. In this
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work, we have investigated the diffusion of silver into ZnS and its influence on the structural, electrical and optical properties of ZnS thin films. 2. Experimental ZnS thin films (d = 3 μm) are obtained by spray pyrolysis in air atmosphere on the glass substrate. The initial solution is prepared from zinc chloride ZnCl2 (98% purity) at 0.1 M concentration and 0.1 M concentration thioure (NH2)2CS (99% purity) in deionized water. The growth was performed with a spray rate of about 5 ml/min and growth rate of ∼50 nm/min on the glass substrates cleaned in ethanol dried in vacuum. During the growth, the substrates were rotated with a speed of 10 rev/ min at a temperature of 500 °C and in atmospheric pressure in order to produce homogenous films [10]. The obtained films had good adhesion to the substrate surfaces. The thickness of these films measured by the SEM method is found to be about 3 μm. Silver diffusion into ZnS thin films was carried out for 3 min using the vacuum evaporated layer of Ag on the open surface of the film. The diffusion annealing of the films with the deposited layer of silver was performed under a nitrogen atmosphere in the temperature range 80–400 °C with 80 °C steps. The undoped films (without silver layer) were annealed under the same conditions. After the annealing, the rest of the Ag layer on the upper ZnS surface was removed by using HNO3:H2SO4:H2O (5:5:90) solution and lateral sides of the ZnS films were cleaned by grinding. By measuring the surface electrical conductivity it was tested whether the whole remaining metallic Ag layer was removed from the ZnS film by the etching in a HNO3:H2SO4:H2O solution. The concentration distributions of Ag atoms in the ZnS thin films were analyzed by the successive removal of thin layers from the sample by using a HF:H2O (1:2) solution and by measuring the energy dispersive X-ray fluorescence (EDXRF) intensity of the Ag-Kα peak. The thickness of the film after the removal of each layer by etching in HF:H2O (1:2) was measured by forming a series of 10 equal etching steps and measuring the total decrease in the thickness by SEM and then finally calculating the
Fig. 1. The XRD pattern of undoped ZnS film.
Fig. 2. Concentration profiles of Ag in ZnS films T = 320 °C for 180 s (the full curve is calculated according to (2)).
decrease in the thickness for each etching step. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. Intensity measurements of the Ag peaks were detected with a Si (Li) solid-state dedector [11]. The X-ray diffraction (XRD) data of ZnS and ZnS/Ag films were taken using a Rigaku D/Max-IIIC diffractometer with CuKα radiation over the range 2θ = 3–70° at room temperature. The surface morphology was studied by using JEOL JST-6400 scanning electron microscope. The absorption spectra of undoped and Ag-doped ZnS films were measured by PerkinElmer Lambda 2SUV/Vis Spectrometer with 190–1100 nm wavelength range using non-polarised ligth. The resistivity of ZnS and Ag-doped ZnS films was determined by Van Der Pauw measurements at room temperature. The carrier concentration was determined by Hall effect measurements. 3. Results and discussion The values of resistivity and charge carrier concentration for the asgrown n-type ZnS films obtained on the glass substrate with a thickness of about 3 μm were found to be 2.8 × 105 Ω cm and n = 9.5 × 1014 cm− 3 respectively. Fig. 1 shows the X-ray diffraction pattern of ZnS film grown by spray pyrolysis technique. The diffraction pattern arising from the film consisted of a single intense peak at ∼28.96° due to the fcc (111) reflection. The lattice constant a is calculated from the peak
Fig. 3. The temperature dependence of Ag diffusion coefficient in ZnS films.
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the diffusion of Ag into ZnS can be determined. Fig. 2 shows a typical concentration profile for a sample annealed at 320 °C for a period of 180 s. The experimental data of Fig. 2 are in agreement with the theoretical curve and by using the experimental data of Fig. 2, the diffusion coefficient of Ag atom for the film annealed at 320 °C was determined to be D = 1 × 10− 12 cm2/s. The temperature dependence of the silver diffusion coefficient in the temperature range of 80–400 °C obtained from the EDXRF data for ZnS is described by the following relation (Fig. 3): 0:10eV D ¼ 8 109 exp ð4Þ kT
Fig. 4. The SEM micrographs of (a) undoped and (b) Ag-doped ZnS films.
position and is determined to be a = 5.28 Å. This indicated that the crystallites in the film have a single preferred (111) orientation that the fcc (111) plane was parallel to the substrate surface. This was expected since the cubic (111) lattice plane has the lowest surface energy [12,13]. To calculate the diffusion coefficient of Ag into ZnS thin films, Fick's diffusion equations were used. Eq. (1) below is known as Fick's law of diffusion. AN ð x; tÞ A2 N ð x; tÞ ¼D At Ax2
ð1Þ
The relatively small value of the activation energy (0.10 eV) and large values of diffusion coefficients of silver in polycrystalline ZnS films can be caused by Ag migration via grain boundaries, vacancies, etc., which are channels for rapid impurity migration in ZnS films. We believe that atomic migration of silver in ZnS films takes place by means of (a) fast migration along grain boundaries and (b) relatively slow migration into grains [15]. The grain boundary diffusion is usually obtained for the low temperature region, whereas the diffusion into grains is obtained for the relatively high temperature region. We suppose that the temperature range from 80 °C to 400 °C is in low temperature region with comparing ZnS semiconductor melting point of 1800 °C, so, the grain boundary diffusion is dominant in our work. The group I impurities in II–VI semiconductors diffuse by a dissociative mechanism, which is connected with their rapid migration along the interstitials as positive ions being subsequently situated in vacancies, where they exhibit acceptor properties [16,17]. It was found that Ag-doped ZnS films annealed at 400 °C for 3 min in the N2 atmosphere have changed their n-type conductivity to the p-type. The films annealed at temperatures less than 400 °C showed n-type conductivity. From the Hall coefficient measurement and hot probe analysis, it was determined that the resistivity and carrier concentration of Ag-doped ZnS films annealed at 400 °C for 3 min were found to be 5.3 × 105 Ω cm, p = 1.0 × 1014 cm− 3 respectively. The changes in electrical properties of ZnS films can be explained by acceptor behavior of Ag or its complexes (silver-vacancy type) in ZnS. The reason of the change in the conductivity from n-type to the p-type can be attributed to the increasing annealing temperature, a part of Ag atoms in grain boundaries can diffuse into the grains and some of the silver atoms in the grain are thermally ionized and inhabit the Zn site of the ZnS lattice. The same type of conductivity transition with the same reason was found by Magnea et al. and Poolton for the diffusion of Ag in ZnTe and Ag in ZnS
D is the diffusion coefficient, x is the coordinate in the direction of flow, t is the diffusion time and N is the concentration that is assumed to be a function of x and t. Since the layer of Ag can be assumed as a source of silver atoms, the solution of Eq. (1) can be simplified by incorporating the initial and boundary conditions. The solution for Eq. (1) can be written as [14]: x N ð x; tÞ ¼ N0 1 erf pffiffiffiffiffi 2 Dt
ð2Þ
Where N0 is the surface concentration of impurity and erf is the error function. The diffusion coefficient determined experimentally over a range of diffusion temperatures can often be expressed as E : ð3Þ D ¼ D0 exp kT Where D0 is the frequency factor, E is the activation energy, T is temperature and k is the Boltzmann constant. From the EDXRF profile,
Fig. 5. The absorption spectra of (a) undoped and (b) Ag-doped ZnS films.
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respectively [18,19]. Magnea et al. have reported that the intensity of photoluminescence transitions was enhanced by the diffusion of Ag into ZnTe crystals and assigned the observed acceptor level to the Ag substitutions on a Zn-lattice site AgZn. In addition, Poolton have reported that the simple silver substitution in ZnS film behaves as an acceptor which is indicated as the complex of [AgZn]. It was observed that XRD patterns of undoped and Ag-doped ZnS films have almost similar patterns with a slight displacement of (111) peak shown in Fig. 1. The calculated lattice parameter for the ZnS(Ag) film is found to be a = 5.33 Å which is 0.9% larger than for the undoped ZnS film (a = 5.28 Å). The SEM micrographs of undoped and Agdoped ZnS films are shown in Fig. 4 respectively. It was observed that ZnS film without Ag indicates a clear and nonhomogeneous grain distributions (Fig. 4 (a)). It is thought that the connectivity between grains seems poor which affect the physical properties. Comparing this micrograph with that of Ag-doped ZnS, slightly better connectivity between grains is observed (Fig. 4 (b)) and the substructure seems to consist of smaller grains and be more disordered. Fig. 5 shows the absorption spectra of the undoped and Ag-doped ZnS films. The optical band gap values were determined from the intercept of the straight-line portion of the (αhν)2 against on the hν axis. It can be seen that the band gap decreases from 3.52 eV to 3.43 eV due to silver diffusion. The decrease in the band gap of ZnS doped by Ag can be explained by the influence of near band levels, arising on the introduction of silver and related with silver and silver-vacancy complexes. A small fraction of the silver incorporated into ZnS lattice as an acceptor could give rise to localized level near the valance band [20]. In conclusion, the silver diffussion in polycrystalline ZnS films is characterised in the temperature range 80–400 °C by a rather fast Ag migration with a diffusion coefficient changing from 3× 10− 13 cm2/s to 5× 10− 12 cm2/s. The silver migration mechanism in ZnS films is attributed to both fast diffusion along grain boundaries and the simultaneous penetration into grain. The band gaps of films are decreased with Ag doping. Ag-doped ZnS films annealed at about 400 °C changed their n-type conductivity to the p-type. These effects are tentatively attributed to the formation of the energy levels related to silver and its complexes.
Acknowledgement This work was supported by the research fund of Karadeniz Technical University, Trabzon, Turkey, under contract no. 2003.111.001.1.
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