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Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films
Current Applied Physics 9 (2009) 667–672
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films L. Raja Mohan Reddy, P. Prathap, K.T. Ramakrishna Reddy * Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
a r t i c l e
i n f o
Article history: Received 14 January 2008 Received in revised form 7 May 2008 Accepted 11 June 2008 Available online 5 July 2008 PACS: 68.55 75.50.pp 73.61 Keywords: Dilute magnetic semiconductors Zn1xMnxO films Spray pyrolysis
a b s t r a c t Zn1xMnxO thin films have been synthesized by chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a manganese composition, x = 15%, on corning 7059 glass substrates. The as-grown layers were characterized to evaluate their chemical and physical behaviour with substrate temperature. The change of dopant level in ZnO films with substrate temperature was analysed using X-ray photoelectron spectroscope measurements. The X-ray diffraction studies revealed that all the films were strongly oriented along the (0 0 2) orientation that correspond to the hexagonal wurtzite structure. The crystalline quality of the layers increased with the increase of substrate temperature up to 400 °C and decreased thereafter. The crystallite size of the films varied in the range, 14–24 nm. The surface morphological studies were carried out using atomic force microscope and the layers showed a lower surface roughness of 4.1 nm. The optical band gap of the films was 3.35 eV and the electrical resistivity was found to be high, 104 X cm. The films deposited at higher temperatures (>350 °C) showed ferromagnetic behaviour at 10 K. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The physical behaviour of Zn1xMnxO films is highly dependent on the doping level of Mn in ZnO lattice. The theoretical prediction of Dietl et al. [1] showed that ZnO can exhibit ferromagnetism at room temperature with a manganese doping concentration of 5 at.% and this has increased the attention on oxide based magnetic semiconductors. Fukumura et al. reported that Mn can be incorporated into Zn1xMnxO upto x = 0.35 whereas the equilibrium solubility limit is x = 0.13 [2]. The doping level of ‘Mn’ in Zn1xMnxO films prepared by both physical and chemical methods is different and the observations showed that the magnetic properties of oxide based films are highly sensitive to the preparation conditions. The substrate temperature is one of the most significant parameters that influence the dopant position in the host lattice. ZnMnO films have been deposited using different techniques such as molecular beam epitaxy [3], rf magnetron sputtering [4], pulsed laser deposition [5], sol–gel process [6], spray pyrolysis [7] etc. To our knowledge there are no reports available on the preparation of ZnMnO films by chemical spray pyrolysis and their characterization. Hence in the present study, an attempt has been made for the first time to prepare Zn0.85Mn0.15O films using a simple and cost effective process, chemical spray pyrolysis at different substrate temperatures. * Corresponding author. Tel.: +91 8772249666x272; fax: +91 8772249611. E-mail address: [email protected] (K.T. Ramakrishna Reddy). 1567-1739/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.06.016
The compositional, structural, morphological, electrical, optical and photoluminescence behaviour of the grown layers were investigated as a function of substrate temperature and the results were discussed. 2. Experimental Thin films of Zn0.85Mn0.15O were prepared using chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a ‘Mn’ composition, x = 0.15. The precursor solutions were prepared by dissolving 4N pure Zn(CH3COO)2 and Mn(CH3COO)2 compounds in methanol to achieve a solution concentration of 0.1 M. The substrate temperature was maintained using Eurotherm temperature controller and the spray head was moved in the horizontal plane by means of a microprocessor controlled stepper motor system in order to get uniform films on the substrate. Compressed purified air was used as the carrier gas at a pre-determined flow rate of 8 l/min and the solution was sprayed at a flow rate of 6 ml/min onto ultrasonically cleaned corning 7059 glass substrates. The composition of the layers was determined using VG Microtech ESCA2000 X-ray photoelectron spectrometer. The structural and morphological studies of the films were carried out using Siemens X-ray diffractometer and Vecco atomic force microscope, respectively. The spectral transmittance of the films was recorded using Hitachi UV–vis-NIR spectrophotometer in order to determine the optical energy band gap,
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absorption coefficient, refractive index and extinction coefficient. The photoluminescence properties were studied using Fluorolog 3 fluorescence spectrophotometer. SQUID magnetometer was used to study ferromagnetic behaviour of the grown films. The electrical resistivity of the layers was measured by four-probe method using evaporated silver as electrodes. 3. Results and discussion The visual appearance of all Zn0.85Mn0.15O layers grown in the substrate temperature range, 250–450 °C, were pinhole free, strongly adherent to the substrate surface and whitish in appearance. The composition and chemical bond configuration of the elements present in the films deposited at different substrate temperatures were estimated using XPS measurements. The reflections in the XPS spectra were calibrated by taking the carbon C 1s peak (284.6 eV) as reference. Fig. 1 shows the XPS spectrum of the experimental films deposited at 400 °C recorded in the binding energy range 0–1100 eV. The XPS spectrum showed two strong peaks at the binding energies 1021.7 eV and 1047.4 eV that respectively correspond to the Zn 2p3/2 and Zn 2p1/2 in addition to other normal and Auger reflections. The O 1s peak located at an energy of 529.79 eV corresponds to Zn–O bonding (lattice oxygen). Reflections corresponding to Mn 2p1/2 and 2p3/2 states were also observed at 654.4 eV and 641.6 eV respectively, which confirms the presence of manganese in Mn2+ state. The Auger spectral distribution of Zn (LMM) peak observed at 497.5 eV indicates the bond of Zinc with oxygen in ZnO. The evaluated elemental composition of the layers grown at 400 °C is Zn = 35.49 at.%, Mn = 14.57 at.%, O = 49.94 at.%. Fig. 2 shows the growth rate of Zn0.85Mn0.15O thin films as a function of the deposition temperature. The monotonous increase in the growth rate at deposition temperatures below 400 °C would probably indicate an activation energy limited process as reported by Suntola [8]. The lower thermal energy available for the molecules in the solution at lower deposition temperatures can also cause incomplete reactions between the precursors on the surface of the substrate. At higher deposition temperatures, the growth rate was non-linear. This might be due to the vaporisation of methanol from the precursor solution prior to the sprayed droplet reaching the substrate surface owing to its high vapour pressure, resulting in the formation of film with powdery nature. As the physical properties depend on the crystalline quality of the grown films, the films deposited at different substrate temperatures, Ts that varied in the range, 250–450 °C were analysed to
Fig. 2. Growth rate as a function of deposition temperature.
evaluate the various structural parameters of the layers. Fig. 3 shows the X-ray diffraction spectra of Zn0.85Mn0.15O films recorded in the 2h range, 30°–70°. All the deposited layers were polycrystalline in nature and the diffraction peaks, as indexed in the spectrum, have originated from the hexagonal structure of ZnO with the lattice constant, c = 5.206 Å. The layers formed at a temperature of 400 °C were preferably oriented along the (0 0 2) plane in addition to the appearance of other peaks that correspond to the (1 0 0), (1 0 1), (1 0 2), and (1 1 0) orientations of ZnO. The intensity of the (0 0 2) reflection increased with the increase of substrate temperature. The observed higher intensity of the (0 0 2) reflection for the layers grown at 400 °C might be due to the enhancement of horizontal mobility of ad-atoms and the condensation coefficient during the cluster formation with the increase of substrate temperature that led to an improvement in the crystallinity, which is the usual phenomenon observed in the growth of thin films. For deposition temperatures higher than 400 °C, a decrease of intensity of the (0 0 2) peak was noticed. This might be due to the vaporisation of methanol from the precursor solution prior to the sprayed droplet reaching the substrate surface due to its higher vapour pressure [9], leading to an incomplete reaction between the constituent elements in the precursor solution because of the insufficient temperature that can not initiate the chemical reaction in the droplets.
Fig. 1. XPS spectrum of Zn0.85Mn0.15O films deposited at 400 °C.
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Fig. 3. XRD spectra of Zn0.85Mn0.15O films.
Fig. 4. The variation of lattice constant with deposition temperature.
Fig. 4 shows the variation of lattice constant, c with the deposition temperature. The c-axis lattice constant of Zn0.85Mn0.15O films deposited at different substrate temperatures was found to be
greater than the reported value of 5.206 Å for the hexagonal ZnO. The lattice constant of the films decreased from 5.222 Å to 5.207 Å as the substrate temperature increased to 400 °C. This indicated an effective substitutional incorporation of Mn2+ ions in the ZnO lattice with the increase of substrate temperature that results in a decrease of lattice constant. However, for the films grown at temperatures >400 °C, the lattice constant increased to 5.210 Å that might be due to incomplete reaction of the precursors causing to an interstitial incorporation of Mn atoms in the ZnO lattice for the films grown at such temperatures. The crystallite size was calculated using the Scherrer formula [10]. The variation of crystallite size as a function of substrate temperature is shown in Fig. 5. The crystallite size increased initially with the increase of substrate temperature and reached a maximum value of 23.8 nm at 400 °C and became more or less constant afterwards. The increase in grain size with the growth temperature could be attributed to the enhanced reaction kinetics among the sprayed droplets as well as improvement in the ad-atom mobility on the substrate surface. Ghosh et al. [11] explained the decreasing trend of grain size with the increase of strain due to the retarded crystal growth as the stretched lattice increases the lattice energy and diminishes the driving force for the grain growth in sol–gel grown ZnO films. The analysis of structural data for preferred orientation in Zn0.85Mn0.15O films, synthesized at different substrate tempera-
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tures was also carried out by Harrison texture analysis [12,13]. The texture coefficient (Ci), gives a measure of the orientation of each reflection compared to a randomly oriented sample. The texture coefficients of Zn0.85Mn0.15O thin films along the (0 0 2) and (1 0 1) directions were calculated as a function of substrate temperature using the relation [14],Texture coefficient,
Ci ¼
Fig. 5. The variation of grain size with substrate temperature.
Fig. 6. Change of texture coefficient with substrate temperature.
ð1=nÞ
I1 =Ioi ; PN i¼1 ðI1 =I oi Þ
ð1Þ
where Ii is the intensity of a generic peak in the spectrum, Ioi the intensity of a generic peak for a completely random sample (JCPDS) and N is the number of reflections considered in the analysis. From Fig. 6, the TC(0 0 2) was found to increase with the increase of substrate temperature and has its highest value for the samples grown at 400 °C and its value decreased thereafter. The TC(1 0 1) was less than unity, implying that the films were highly textured along the (0 0 2) direction. This might be due to a reduction in the lattice strain, dislocation density and an improvement in the grain size with substrate temperature. The surface morphological studies were carried out using atomic force microscope that revealed the decrease of average surface roughness of the layers from 6.3 nm to 4.5 nm with the increase of substrate temperature from 250 °C to 400 °C, which then increased to 5.2 nm with further increase of substrate temperature to 450 °C. Fig. 7 shows the AFM images of Zn0.85Mn0.15O films prepared at substrate temperatures, 250 °C and 400 °C. The evaluated grain size is nearly equal to the crystallite size determined from the XRD data. Fig. 8 shows the transmittance spectra of Zn0.85Mn0.15O films deposited at four different substrate temperatures in the wavelength range, 300–1500 nm. It can be observed that the optical transmittance of the films was highly influenced by the deposition temperature. The optical transmittance of the films changed significantly from 22% to 65% with the increase of deposition temperature from 250 °C to 400 °C. The films prepared at 450 °C showed a decrease of optical transmittance. The higher transmittance observed in the films at Ts < 400 °C was attributed to less scattering effects, structural homogeneity and better crystallinity whereas the observed low transmittance in the layers grown at Ts 450 °C might be due to the less crystallinity leading to more light scattering [15]. The optical energy band gap of the Zn0.85Mn0.15O films was estimated from the relation,
ðahmÞ2 ¼ Aðhm Eg Þ;
Fig. 7. AFM pictures of Zn0.85Mn0.15O films prepared at (a) 250 °C and (b) 400 °C.
ð2Þ
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Fig. 10. Photoluminescence spectra of Zn0.85Mn0.15O films. Fig. 8. Transmittance vs. wavelength spectra of Zn0.85Mn0.15O films.
Fig. 9. (ahm)2 vs. hm plots of Zn0.85Mn0.15O films.
increase of green band intensity compared to UV band is higher. This shows the effective incorporation Mn atoms into ZnO lattice that increase the intensity of green band at the expense of near band edge emission. A similar decrease of band edge emission with the increase of Mn- concentration was reported by Roy et al. for Mn-doped ZnO tetrapods where Mn is considered as a strong quencher of the band edge luminescence in ZnO [18]. Also the increase of luminescence intensity with substrate temperature might be due to the improvement in the crystalline quality of the layers as observed in the structural analysis, which caused to a decrease of non-radiative recombination centers. Lee et al. [19] also reported a similar increase of luminescence intensity with substrate temperature in pulsed laser deposited ZnMnO films. Further, a shift in both the bands towards the blue region was observed with the increase of substrate temperature, which could be due to the widening of energy band gap with the increase of substrate temperature as observed in the present study. The magnetic properties of Zn1xMnxO films were measured at a temperature of 10 K with magnetic field applied parallel to the plane of the films. Fig. 11 shows the magnetization vs. magnetic field curve of the films deposited at Ts = 400 °C, which clearly shows the ferromagnetic loop with remanent magnetization and
where Eg is the optical band gap, hm is the photon energy and A is a constant. Fig. 9 shows the change of (ahm)2 with the photon energy, hm, which indicated a direct electronic transition across the band gap of the films. The extrapolation of the linear part of the curves on to the energy axis would give the optical band gap that varied in the range, 3.12–3.35 eV with the increase of deposition temperature from 250 °C to 400 °C. This increase of band gap with Ts is probably due to the reduction of defects at the grain boundaries and a decrease of structural disorder in the films [16]. Moreover, the decrease of lattice strain in the films with growth temperature might also have contributed to the increase of energy band gap in the present study. Fig. 10 shows the room temperature photoluminescence spectra of Zn0.85Mn0.15O layers prepared at three different substrate temperatures when excited using a light of wavelength, 335 nm. The films showed two emission bands, one in the UV region and the other in the visible region. The observed UV band can be assigned to the near band edge emission in ZnO host material and the band that appeared in the green region to the emission from oxygen defect states [17]. The intensity of these two bands increased with the increase of substrate temperature and the
Fig. 11. Magnetization curve measured at 10 K with the magnetic field applied along the film plane.
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coercivity of 5.6 105 emu and 122 Oe respectively. Ferromagnetism in these magnetic semiconductors arises from the exchange interaction between the free delocalized carriers and the localized d-spin on the Mn ions. It was reported that the magnetic properties in these films arise due to the formation of secondary phases of the dopant atoms and their clusters [20]. However, in the present study there were no such indications of secondary phase formation in the XRD spectrum of the corresponding sample. Therefore, it can be concluded that the observed ferromagnetism in the films is entirely derived from the substitution of Mn2+ ions in Zn2+ ions in ZnO lattice without affecting the wurtzite structure. For the films deposited at lower temperatures (<400 °C), the ferromagnetic loop could not be observed. This might be due to the poor crystallinity of the films grown at such temperatures as revealed by the structural studies. Similar observations were also reported by Lee et al. [21] for sputter deposited Zn0.93Mn0.07O films. The present study indicated that magnetic properties depend on the microstructural features of the grown layers. All the layers synthesized at different temperatures showed ntype electrical conductivity. The electrical resistivity of as-grown films decreased from 5.1 106 to 2.3 104 X cm when the deposition temperature increased from 250 °C to 400 °C. The decrease of electrical resistivity of the layers with deposition temperature can be attributed to the improvement in the crystallinity and the decrease of strain in the films [11]. Also the films prepared at substrate temperatures >400 °C, showed an increased resistivity of 8.4 104 X cm, which might be due to higher thermal energy provided to the substrate that caused to disturb the crystallinity of the films grown at such temperatures, as supported by the XRD analysis, which led to the formation of defects and scattering centers. 4. Conclusions Zn0.85Mn0.15O films were successfully grown by spray pyrolysis at different substrate temperatures in the range, 250–450 °C. The films prepared at 400 °C showed near stoichiometric composition and highly oriented along the (0 0 2) crystal plane. These layers
had a crystallite size of 23.8 nm along with a surface roughness of 4.5 nm compared to the layers grown at other temperatures. These highly oriented films showed a less distorted lattice constant, 5.207 Å that nearly matched with the bulk value. The layers prepared at 400 °C showed a transmittance of >65% in the visible region with an energy band gap of 3.35 eV. The layers exhibited an intense visible band in addition to the appearance of a UV band and a small blue shift in the luminescence peak was observed with the increase of substrate temperature. The films deposited at 400 °C showed ferromagnetic behaviour at 10 K. The near stoichiometric layers showed a lower electrical resistivity of 2.3 104 X cm. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [2] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kaw asaki, Appl. Phys. Lett. 78 (1999) 3366. [3] Z.W. Jin, Y.Z. Yoo, T. Sekigushi, T. Chikyoww, H. Ofuchi, H. Fujioka, M. Oshima, H. Koinuma, Appl. Phys. Lett. 83 (2003) 39. [4] K.J. Kim, Y.R. Park, J. Appl. Phys. 94 (2003) 867. [5] S.W. Jung, S.J. An, G.C. Yi, Appl. Phys. Lett. 80 (2002) 4561. [6] U.N. Maiti, P.K. Ghosh, S. Nandy, K.K. Chattopadhyay, Phys. B 387 (2007) 103. [7] I. Akyuz, S. Kose, F. Atay, V. Bilgin, Semicond. Sci. Technol. 21 (2006) 1620. [8] T. Suntola, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, vol. 3, Elsevier, Amsterdam, 1994, p. 601. [9] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [10] B.E. Warren, X-ray Diffraction, Dover, New York, 1990. p. 253. [11] R. Ghosh, D. Basak, S. Fujihara, J. Appl. Phys. 96 (2004) 2689. [12] C. Barret, T.B. Massalaski, Structure of Metals, Pergmon, Oxford, 1980. p. 204. [13] K.H. Kim, J.S. Chun, Thin Solid Films 141 (1986) 287. [14] H.R. Moutinho, F.S. Hasoon, F. Abulfotuh, L.L. Kazmerski, J. Vac. Sci. Technol. A 13 (1995) 2877. [15] D.S.D. Amma, V.K. Vaidyan, P.K. Manoj, Mater. Chem. Phys. 93 (2005) 194. [16] J.D. Dow, D. Redfield, Phys. Rev. B 5 (1972) 594. [17] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [18] V.A.L. Roy, A.B. Djuris, H. Liu, X.X. Zhang, Y.H. Leung, M.H. Xie, J. Gao, H.F. Lui, C. Surya, Appl. Phys. Lett. 84 (2004) 756. [19] S. Lee, T.W. Kang, D.Y. Kim, J. Cryst. Growth 284 (2005) 6. [20] S-J. Han, T.-H. Jang, Y.B. Kim, B.-G. Park, J.-H. Park, Y.H. Jeong, Appl. Phys. Lett. 83 (2003) 920. [21] S. Lee, H.S. Lee, S.J. Hwang, Y. Shon, D.Y. Kim, E.K. Kim, J. Cryst. Growth 286 (2006) 223.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films L. Raja Mohan Reddy, P. Prathap, K.T. Ramakrishna Reddy * Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
a r t i c l e
i n f o
Article history: Received 14 January 2008 Received in revised form 7 May 2008 Accepted 11 June 2008 Available online 5 July 2008 PACS: 68.55 75.50.pp 73.61 Keywords: Dilute magnetic semiconductors Zn1xMnxO films Spray pyrolysis
a b s t r a c t Zn1xMnxO thin films have been synthesized by chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a manganese composition, x = 15%, on corning 7059 glass substrates. The as-grown layers were characterized to evaluate their chemical and physical behaviour with substrate temperature. The change of dopant level in ZnO films with substrate temperature was analysed using X-ray photoelectron spectroscope measurements. The X-ray diffraction studies revealed that all the films were strongly oriented along the (0 0 2) orientation that correspond to the hexagonal wurtzite structure. The crystalline quality of the layers increased with the increase of substrate temperature up to 400 °C and decreased thereafter. The crystallite size of the films varied in the range, 14–24 nm. The surface morphological studies were carried out using atomic force microscope and the layers showed a lower surface roughness of 4.1 nm. The optical band gap of the films was 3.35 eV and the electrical resistivity was found to be high, 104 X cm. The films deposited at higher temperatures (>350 °C) showed ferromagnetic behaviour at 10 K. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The physical behaviour of Zn1xMnxO films is highly dependent on the doping level of Mn in ZnO lattice. The theoretical prediction of Dietl et al. [1] showed that ZnO can exhibit ferromagnetism at room temperature with a manganese doping concentration of 5 at.% and this has increased the attention on oxide based magnetic semiconductors. Fukumura et al. reported that Mn can be incorporated into Zn1xMnxO upto x = 0.35 whereas the equilibrium solubility limit is x = 0.13 [2]. The doping level of ‘Mn’ in Zn1xMnxO films prepared by both physical and chemical methods is different and the observations showed that the magnetic properties of oxide based films are highly sensitive to the preparation conditions. The substrate temperature is one of the most significant parameters that influence the dopant position in the host lattice. ZnMnO films have been deposited using different techniques such as molecular beam epitaxy [3], rf magnetron sputtering [4], pulsed laser deposition [5], sol–gel process [6], spray pyrolysis [7] etc. To our knowledge there are no reports available on the preparation of ZnMnO films by chemical spray pyrolysis and their characterization. Hence in the present study, an attempt has been made for the first time to prepare Zn0.85Mn0.15O films using a simple and cost effective process, chemical spray pyrolysis at different substrate temperatures. * Corresponding author. Tel.: +91 8772249666x272; fax: +91 8772249611. E-mail address: [email protected] (K.T. Ramakrishna Reddy). 1567-1739/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.06.016
The compositional, structural, morphological, electrical, optical and photoluminescence behaviour of the grown layers were investigated as a function of substrate temperature and the results were discussed. 2. Experimental Thin films of Zn0.85Mn0.15O were prepared using chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a ‘Mn’ composition, x = 0.15. The precursor solutions were prepared by dissolving 4N pure Zn(CH3COO)2 and Mn(CH3COO)2 compounds in methanol to achieve a solution concentration of 0.1 M. The substrate temperature was maintained using Eurotherm temperature controller and the spray head was moved in the horizontal plane by means of a microprocessor controlled stepper motor system in order to get uniform films on the substrate. Compressed purified air was used as the carrier gas at a pre-determined flow rate of 8 l/min and the solution was sprayed at a flow rate of 6 ml/min onto ultrasonically cleaned corning 7059 glass substrates. The composition of the layers was determined using VG Microtech ESCA2000 X-ray photoelectron spectrometer. The structural and morphological studies of the films were carried out using Siemens X-ray diffractometer and Vecco atomic force microscope, respectively. The spectral transmittance of the films was recorded using Hitachi UV–vis-NIR spectrophotometer in order to determine the optical energy band gap,
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absorption coefficient, refractive index and extinction coefficient. The photoluminescence properties were studied using Fluorolog 3 fluorescence spectrophotometer. SQUID magnetometer was used to study ferromagnetic behaviour of the grown films. The electrical resistivity of the layers was measured by four-probe method using evaporated silver as electrodes. 3. Results and discussion The visual appearance of all Zn0.85Mn0.15O layers grown in the substrate temperature range, 250–450 °C, were pinhole free, strongly adherent to the substrate surface and whitish in appearance. The composition and chemical bond configuration of the elements present in the films deposited at different substrate temperatures were estimated using XPS measurements. The reflections in the XPS spectra were calibrated by taking the carbon C 1s peak (284.6 eV) as reference. Fig. 1 shows the XPS spectrum of the experimental films deposited at 400 °C recorded in the binding energy range 0–1100 eV. The XPS spectrum showed two strong peaks at the binding energies 1021.7 eV and 1047.4 eV that respectively correspond to the Zn 2p3/2 and Zn 2p1/2 in addition to other normal and Auger reflections. The O 1s peak located at an energy of 529.79 eV corresponds to Zn–O bonding (lattice oxygen). Reflections corresponding to Mn 2p1/2 and 2p3/2 states were also observed at 654.4 eV and 641.6 eV respectively, which confirms the presence of manganese in Mn2+ state. The Auger spectral distribution of Zn (LMM) peak observed at 497.5 eV indicates the bond of Zinc with oxygen in ZnO. The evaluated elemental composition of the layers grown at 400 °C is Zn = 35.49 at.%, Mn = 14.57 at.%, O = 49.94 at.%. Fig. 2 shows the growth rate of Zn0.85Mn0.15O thin films as a function of the deposition temperature. The monotonous increase in the growth rate at deposition temperatures below 400 °C would probably indicate an activation energy limited process as reported by Suntola [8]. The lower thermal energy available for the molecules in the solution at lower deposition temperatures can also cause incomplete reactions between the precursors on the surface of the substrate. At higher deposition temperatures, the growth rate was non-linear. This might be due to the vaporisation of methanol from the precursor solution prior to the sprayed droplet reaching the substrate surface owing to its high vapour pressure, resulting in the formation of film with powdery nature. As the physical properties depend on the crystalline quality of the grown films, the films deposited at different substrate temperatures, Ts that varied in the range, 250–450 °C were analysed to
Fig. 2. Growth rate as a function of deposition temperature.
evaluate the various structural parameters of the layers. Fig. 3 shows the X-ray diffraction spectra of Zn0.85Mn0.15O films recorded in the 2h range, 30°–70°. All the deposited layers were polycrystalline in nature and the diffraction peaks, as indexed in the spectrum, have originated from the hexagonal structure of ZnO with the lattice constant, c = 5.206 Å. The layers formed at a temperature of 400 °C were preferably oriented along the (0 0 2) plane in addition to the appearance of other peaks that correspond to the (1 0 0), (1 0 1), (1 0 2), and (1 1 0) orientations of ZnO. The intensity of the (0 0 2) reflection increased with the increase of substrate temperature. The observed higher intensity of the (0 0 2) reflection for the layers grown at 400 °C might be due to the enhancement of horizontal mobility of ad-atoms and the condensation coefficient during the cluster formation with the increase of substrate temperature that led to an improvement in the crystallinity, which is the usual phenomenon observed in the growth of thin films. For deposition temperatures higher than 400 °C, a decrease of intensity of the (0 0 2) peak was noticed. This might be due to the vaporisation of methanol from the precursor solution prior to the sprayed droplet reaching the substrate surface due to its higher vapour pressure [9], leading to an incomplete reaction between the constituent elements in the precursor solution because of the insufficient temperature that can not initiate the chemical reaction in the droplets.
Fig. 1. XPS spectrum of Zn0.85Mn0.15O films deposited at 400 °C.
L. Raja Mohan Reddy et al. / Current Applied Physics 9 (2009) 667–672
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Fig. 3. XRD spectra of Zn0.85Mn0.15O films.
Fig. 4. The variation of lattice constant with deposition temperature.
Fig. 4 shows the variation of lattice constant, c with the deposition temperature. The c-axis lattice constant of Zn0.85Mn0.15O films deposited at different substrate temperatures was found to be
greater than the reported value of 5.206 Å for the hexagonal ZnO. The lattice constant of the films decreased from 5.222 Å to 5.207 Å as the substrate temperature increased to 400 °C. This indicated an effective substitutional incorporation of Mn2+ ions in the ZnO lattice with the increase of substrate temperature that results in a decrease of lattice constant. However, for the films grown at temperatures >400 °C, the lattice constant increased to 5.210 Å that might be due to incomplete reaction of the precursors causing to an interstitial incorporation of Mn atoms in the ZnO lattice for the films grown at such temperatures. The crystallite size was calculated using the Scherrer formula [10]. The variation of crystallite size as a function of substrate temperature is shown in Fig. 5. The crystallite size increased initially with the increase of substrate temperature and reached a maximum value of 23.8 nm at 400 °C and became more or less constant afterwards. The increase in grain size with the growth temperature could be attributed to the enhanced reaction kinetics among the sprayed droplets as well as improvement in the ad-atom mobility on the substrate surface. Ghosh et al. [11] explained the decreasing trend of grain size with the increase of strain due to the retarded crystal growth as the stretched lattice increases the lattice energy and diminishes the driving force for the grain growth in sol–gel grown ZnO films. The analysis of structural data for preferred orientation in Zn0.85Mn0.15O films, synthesized at different substrate tempera-
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tures was also carried out by Harrison texture analysis [12,13]. The texture coefficient (Ci), gives a measure of the orientation of each reflection compared to a randomly oriented sample. The texture coefficients of Zn0.85Mn0.15O thin films along the (0 0 2) and (1 0 1) directions were calculated as a function of substrate temperature using the relation [14],Texture coefficient,
Ci ¼
Fig. 5. The variation of grain size with substrate temperature.
Fig. 6. Change of texture coefficient with substrate temperature.
ð1=nÞ
I1 =Ioi ; PN i¼1 ðI1 =I oi Þ
ð1Þ
where Ii is the intensity of a generic peak in the spectrum, Ioi the intensity of a generic peak for a completely random sample (JCPDS) and N is the number of reflections considered in the analysis. From Fig. 6, the TC(0 0 2) was found to increase with the increase of substrate temperature and has its highest value for the samples grown at 400 °C and its value decreased thereafter. The TC(1 0 1) was less than unity, implying that the films were highly textured along the (0 0 2) direction. This might be due to a reduction in the lattice strain, dislocation density and an improvement in the grain size with substrate temperature. The surface morphological studies were carried out using atomic force microscope that revealed the decrease of average surface roughness of the layers from 6.3 nm to 4.5 nm with the increase of substrate temperature from 250 °C to 400 °C, which then increased to 5.2 nm with further increase of substrate temperature to 450 °C. Fig. 7 shows the AFM images of Zn0.85Mn0.15O films prepared at substrate temperatures, 250 °C and 400 °C. The evaluated grain size is nearly equal to the crystallite size determined from the XRD data. Fig. 8 shows the transmittance spectra of Zn0.85Mn0.15O films deposited at four different substrate temperatures in the wavelength range, 300–1500 nm. It can be observed that the optical transmittance of the films was highly influenced by the deposition temperature. The optical transmittance of the films changed significantly from 22% to 65% with the increase of deposition temperature from 250 °C to 400 °C. The films prepared at 450 °C showed a decrease of optical transmittance. The higher transmittance observed in the films at Ts < 400 °C was attributed to less scattering effects, structural homogeneity and better crystallinity whereas the observed low transmittance in the layers grown at Ts 450 °C might be due to the less crystallinity leading to more light scattering [15]. The optical energy band gap of the Zn0.85Mn0.15O films was estimated from the relation,
ðahmÞ2 ¼ Aðhm Eg Þ;
Fig. 7. AFM pictures of Zn0.85Mn0.15O films prepared at (a) 250 °C and (b) 400 °C.
ð2Þ
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Fig. 10. Photoluminescence spectra of Zn0.85Mn0.15O films. Fig. 8. Transmittance vs. wavelength spectra of Zn0.85Mn0.15O films.
Fig. 9. (ahm)2 vs. hm plots of Zn0.85Mn0.15O films.
increase of green band intensity compared to UV band is higher. This shows the effective incorporation Mn atoms into ZnO lattice that increase the intensity of green band at the expense of near band edge emission. A similar decrease of band edge emission with the increase of Mn- concentration was reported by Roy et al. for Mn-doped ZnO tetrapods where Mn is considered as a strong quencher of the band edge luminescence in ZnO [18]. Also the increase of luminescence intensity with substrate temperature might be due to the improvement in the crystalline quality of the layers as observed in the structural analysis, which caused to a decrease of non-radiative recombination centers. Lee et al. [19] also reported a similar increase of luminescence intensity with substrate temperature in pulsed laser deposited ZnMnO films. Further, a shift in both the bands towards the blue region was observed with the increase of substrate temperature, which could be due to the widening of energy band gap with the increase of substrate temperature as observed in the present study. The magnetic properties of Zn1xMnxO films were measured at a temperature of 10 K with magnetic field applied parallel to the plane of the films. Fig. 11 shows the magnetization vs. magnetic field curve of the films deposited at Ts = 400 °C, which clearly shows the ferromagnetic loop with remanent magnetization and
where Eg is the optical band gap, hm is the photon energy and A is a constant. Fig. 9 shows the change of (ahm)2 with the photon energy, hm, which indicated a direct electronic transition across the band gap of the films. The extrapolation of the linear part of the curves on to the energy axis would give the optical band gap that varied in the range, 3.12–3.35 eV with the increase of deposition temperature from 250 °C to 400 °C. This increase of band gap with Ts is probably due to the reduction of defects at the grain boundaries and a decrease of structural disorder in the films [16]. Moreover, the decrease of lattice strain in the films with growth temperature might also have contributed to the increase of energy band gap in the present study. Fig. 10 shows the room temperature photoluminescence spectra of Zn0.85Mn0.15O layers prepared at three different substrate temperatures when excited using a light of wavelength, 335 nm. The films showed two emission bands, one in the UV region and the other in the visible region. The observed UV band can be assigned to the near band edge emission in ZnO host material and the band that appeared in the green region to the emission from oxygen defect states [17]. The intensity of these two bands increased with the increase of substrate temperature and the
Fig. 11. Magnetization curve measured at 10 K with the magnetic field applied along the film plane.
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coercivity of 5.6 105 emu and 122 Oe respectively. Ferromagnetism in these magnetic semiconductors arises from the exchange interaction between the free delocalized carriers and the localized d-spin on the Mn ions. It was reported that the magnetic properties in these films arise due to the formation of secondary phases of the dopant atoms and their clusters [20]. However, in the present study there were no such indications of secondary phase formation in the XRD spectrum of the corresponding sample. Therefore, it can be concluded that the observed ferromagnetism in the films is entirely derived from the substitution of Mn2+ ions in Zn2+ ions in ZnO lattice without affecting the wurtzite structure. For the films deposited at lower temperatures (<400 °C), the ferromagnetic loop could not be observed. This might be due to the poor crystallinity of the films grown at such temperatures as revealed by the structural studies. Similar observations were also reported by Lee et al. [21] for sputter deposited Zn0.93Mn0.07O films. The present study indicated that magnetic properties depend on the microstructural features of the grown layers. All the layers synthesized at different temperatures showed ntype electrical conductivity. The electrical resistivity of as-grown films decreased from 5.1 106 to 2.3 104 X cm when the deposition temperature increased from 250 °C to 400 °C. The decrease of electrical resistivity of the layers with deposition temperature can be attributed to the improvement in the crystallinity and the decrease of strain in the films [11]. Also the films prepared at substrate temperatures >400 °C, showed an increased resistivity of 8.4 104 X cm, which might be due to higher thermal energy provided to the substrate that caused to disturb the crystallinity of the films grown at such temperatures, as supported by the XRD analysis, which led to the formation of defects and scattering centers. 4. Conclusions Zn0.85Mn0.15O films were successfully grown by spray pyrolysis at different substrate temperatures in the range, 250–450 °C. The films prepared at 400 °C showed near stoichiometric composition and highly oriented along the (0 0 2) crystal plane. These layers
had a crystallite size of 23.8 nm along with a surface roughness of 4.5 nm compared to the layers grown at other temperatures. These highly oriented films showed a less distorted lattice constant, 5.207 Å that nearly matched with the bulk value. The layers prepared at 400 °C showed a transmittance of >65% in the visible region with an energy band gap of 3.35 eV. The layers exhibited an intense visible band in addition to the appearance of a UV band and a small blue shift in the luminescence peak was observed with the increase of substrate temperature. The films deposited at 400 °C showed ferromagnetic behaviour at 10 K. The near stoichiometric layers showed a lower electrical resistivity of 2.3 104 X cm. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [2] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kaw asaki, Appl. Phys. Lett. 78 (1999) 3366. [3] Z.W. Jin, Y.Z. Yoo, T. Sekigushi, T. Chikyoww, H. Ofuchi, H. Fujioka, M. Oshima, H. Koinuma, Appl. Phys. Lett. 83 (2003) 39. [4] K.J. Kim, Y.R. Park, J. Appl. Phys. 94 (2003) 867. [5] S.W. Jung, S.J. An, G.C. Yi, Appl. Phys. Lett. 80 (2002) 4561. [6] U.N. Maiti, P.K. Ghosh, S. Nandy, K.K. Chattopadhyay, Phys. B 387 (2007) 103. [7] I. Akyuz, S. Kose, F. Atay, V. Bilgin, Semicond. Sci. Technol. 21 (2006) 1620. [8] T. Suntola, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, vol. 3, Elsevier, Amsterdam, 1994, p. 601. [9] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [10] B.E. Warren, X-ray Diffraction, Dover, New York, 1990. p. 253. [11] R. Ghosh, D. Basak, S. Fujihara, J. Appl. Phys. 96 (2004) 2689. [12] C. Barret, T.B. Massalaski, Structure of Metals, Pergmon, Oxford, 1980. p. 204. [13] K.H. Kim, J.S. Chun, Thin Solid Films 141 (1986) 287. [14] H.R. Moutinho, F.S. Hasoon, F. Abulfotuh, L.L. Kazmerski, J. Vac. Sci. Technol. A 13 (1995) 2877. [15] D.S.D. Amma, V.K. Vaidyan, P.K. Manoj, Mater. Chem. Phys. 93 (2005) 194. [16] J.D. Dow, D. Redfield, Phys. Rev. B 5 (1972) 594. [17] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [18] V.A.L. Roy, A.B. Djuris, H. Liu, X.X. Zhang, Y.H. Leung, M.H. Xie, J. Gao, H.F. Lui, C. Surya, Appl. Phys. Lett. 84 (2004) 756. [19] S. Lee, T.W. Kang, D.Y. Kim, J. Cryst. Growth 284 (2005) 6. [20] S-J. Han, T.-H. Jang, Y.B. Kim, B.-G. Park, J.-H. Park, Y.H. Jeong, Appl. Phys. Lett. 83 (2003) 920. [21] S. Lee, H.S. Lee, S.J. Hwang, Y. Shon, D.Y. Kim, E.K. Kim, J. Cryst. Growth 286 (2006) 223.