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Oxidation temperature dependent properties of MgO thin film on alumina
Applied Surface Science 258 (2011) 1535–1540
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Oxidation temperature dependent properties of MgO thin film on alumina S. Patil, Vijaya Puri ∗ Thick and Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur, 416004, India
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 27 September 2011 Accepted 29 September 2011 Available online 6 October 2011 Keywords: Magnesium oxide thin film Oxidation temperature Morphology Resistivity Microwave transmittance Permittivity
a b s t r a c t The magnesium oxide thin films were prepared by thermal oxidation (in air) of vacuum evaporated magnesium thin film on alumina. It was found that oxidation temperature (623 K, 675 K and 723 K) and thickness (103 nm and 546 nm) dependent effects were prominently manifested in the surface morphology. Electrical and microwave properties (8–12 GHz) of the MgO thin films were also carried out. X-ray diffraction showed orientation along (2 0 0) and (2 2 0) directions. Flowerlike morphology was observed from SEM and flake like morphology for films of higher thickness oxidized at higher temperatures. The magnesium oxide thin film showed NTC behavior. Microwave transmittance was found to increase with increase in oxidation temperature but was lower than alumina. Frequency and oxidation temperature dependent microwave permittivity was obtained. The microwave dielectric constant varied in the range 8.3–15.3. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxides have been of great interest due to their tunable applications in the radio frequency range. Magnesium oxide has high dielectric constant which is very suitable for microwave integrated circuits and also used as a substrate [1–3]. Numbers of methods have been reported for deposition of MgO thin films such as vacuum arc deposition [4], magnetron sputtering [5,6], electron beam evaporation [7,8], spray pyrolysis [9] and ion beam assisted deposition [10]. The interesting properties of magnesium oxide thin film have attracted a number of applications [11]. To the authors knowledge there are no reports available on the studies of MgO thin films by thermal oxidation of vacuum evaporated Mg thin films on alumina, though MgO thin films on glass substrates are reported [12,13]. Compared to other techniques this is a more cost effective method, since oxidation is done in ambient air at relatively low temperatures. This paper reports the study of morphology, electrical and microwave properties of magnesium oxide thin films oxidized at different oxidation temperatures. Frequency and thickness dependence microwave properties are also presented in this paper. 2. Experimental Magnesium oxide thin films have been prepared by thermal oxidation (in air) of vacuum evaporated magnesium films, deposited
on alumina substrates under vacuum of 10−5 mbar. The pure (99.98%) metallic magnesium (Alfa Aesar) was used as the source material. After the deposition of magnesium by resistive heating, magnesium thin films were oxidized in air atmosphere at different temperatures; 623 K, 675 K and 723 K. Two thicknesses of 103 nm and 546 nm were deposited and measured by surface profiler. The onset of oxidation (formation of magnesium oxide) could be seen from the disappearance of magnesium and the film becoming transparent. X-ray diffraction analysis was performed by a Philips (PW 3710) ˚ The surdiffractometer (using Cu K␣ radiation = 1.541838 A). face morphological study was carried out by scanning electron microscopy (JSM-6360 JEOL, Japan). The DC electrical resistivity of magnesium oxide thin films was measured as a function of temperature in the range 300–600 K by using two-probe method. Microwave transmittance was studied in the X-band (8–12 GHz) using the waveguide reflectometer set-up [14]. The permittivity of magnesium oxide thin film at microwave frequency was measured by using Voltage Standing Wave Ratio (VSWR) slotted section method [15]. By measuring the displacements of maxima and minima, the permittivity of the medium has been determined. The real and imaginary parts of the permittivity were calculated by using Smith chart [15]. 3. Results and discussion 3.1. X-ray diffraction studies
∗ Corresponding author. Fax: +91 231 2691533. E-mail address: vrp [email protected] (V. Puri). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.126
The typical XRD patterns of magnesium oxide thin film on alumina substrate for all oxidation temperature are shown in Fig. 1.
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As oxidation temperature increases, the (2 0 0) diffraction peak becomes progressively more dominant. At the higher temperature of 450 ◦ C, the film is strongly textured with preferential orientation along the (2 0 0) axis. The observed relationship between the degree of (2 0 0) preferred orientation and the oxidation temperature may be explained in terms of the migration of molecules onto the growing surface. MgO is expected to achieve a (2 0 0) preferential orientation that has a lower energy configuration during nucleating. The absorbed Mg and O atoms with a higher mobility enhanced at higher temperature could easily move to equilibrium atomic sites on the surface, i.e. the (2 0 0) plane of MgO which is energetically stable [16]. Thus, the increase in (2 0 0) preferred orientation occurs when the substrate temperature is high. 3.2. Surface morphology
Fig. 1. XRD patterns of magnesium oxide thin film on alumina for different oxidation temperatures.
Crystal structure does not show any variation with thickness of the thin film, so the film of thickness 546 nm was used for all X-ray diffraction studies. XRD patterns of magnesium oxide thin films show polycrystalline cubic structure with mainly (2 0 0) plane and (2 2 0) plane orientations. The peak intensity increases with decrease in the peak broadening due to increase in oxidation temperature. The increased intensity can be attributed to the improved crystallinity. As temperature increases the MgO thin films becomes more crystalline. Due to heating, mobility of adatoms as well as nucleation sites increases and elimination of trapped excess vacancies occur which results in increased thin films crystallinity. It was observed that, the crystallinity of MgO thin films showed improvement with increase in oxidation duration. The figures showed only (2 0 0) peaks as fundamental characteristic peaks of MgO which were confirmed by the standard JCPDS data file no. 78-0430 for copper target. The XRD patterns did not show the presence of magnesium metal, indicating complete oxidation of magnesium metal during magnesium oxide thin films preparation. For all oxidation temperatures (2 0 0) and (2 2 0) plane orientations and cubic structure was observed. No characteristic peaks of impurity and other phases were observed. The crystallite size of magnesium oxide thin film deposited on alumina was ∼53 nm calculated from Scherer’s formula. The crystallite size of magnesium oxide thin film increases to ∼55.09 nm with increase in oxidation temperature.
The surface morphology of deposited thin films depends on the thin film growth. In this paper metal thin films were deposited by vacuum evaporation technique and after deposition oxidation was done. So the oxidation temperature also plays a vital role in surface morphology modification. During the thin film growth there are two steps, the first step aims to form a seed layer on the substrate by the formation of nucleation centers in desired orientation and improve the quality of thin film as it grows thicker. Similarly, during oxidation of metal thin films, first atoms move due to increased substrate temperatures till it attains thermal equilibrium. During which the adatom movement tries to achieve lower energy state. The proper plane orientation depends on the force of attraction and bond formation between metal and oxygen atoms. SEM images of magnesium oxide thin film at different oxidation temperatures for the two thicknesses are shown in Fig. 2 along with the SEM image of the alumina substrate. It appears that the MgO grains have grown on granular alumina surface since, the shape of alumina grains is clearly visible. The magnesium oxide thin films for thickness 103 nm oxidized at temperature 623 K showed flowerlike morphology which contains small spherical grains. The film oxidized at 675 K shows that the morphology is disturbed and films become more compact because of agglomeration of the grains. For the higher oxidation temperature the spherical grains are separately formed with increase in porosity. This might be due to evaporation of magnesium at higher temperature. For film thickness of 546 nm, the same flowerlike morphology was maintained for lower oxidation temperature. This morphology gets transformed into the flakes-like morphology for oxidation temperature 675 K. At the higher oxidation temperature 723 K the flowerlike morphology is totally destroyed and films become more flaky and porous. The effect of substrate is evident here since the same MgO deposited on glass substrate show granular morphology [12,13]. The grain size varied due to oxidation temperature and due to thickness variations. The grain size increases with increase in thickness and oxidation temperature. The grain size of magnesium oxide thin film for lower oxidation temperature was 453 nm and for higher oxidation temperature the grain size of magnesium oxide thin film was 523 nm. 3.3. Electrical properties Fig. 3 shows the graph of log vs 1000/T which reveals that the resistivity of the sample decreases with increase in the temperature showing the semiconducting behavior of the samples. At the room temperature, electrical resistivity of magnesium oxide thin film is in the order of 104 cm. From the figure it is seen that, resistivity decreases with increase in temperature and shows NTC behavior. As the thickness as well as oxidation temperature increased, the
S. Patil, V. Puri / Applied Surface Science 258 (2011) 1535–1540
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Fig. 2. SEM images of magnesium oxide thin films at different oxidation temperatures.
resistivity of magnesium oxide thin film decreased. The results obtained from the DC resistivity measurements are in good agreement with thin films surface morphology. It was found that; resistivity was high due to small grain size, high porosity and less defect inside the crystallites [17]. In general, the electrical properties of thin films usually differ from those of bulk material because of the essential differences in the microstructures and the geometrical limitations. Resistivity is actually the result of the combined factors such as grain size, crystal structure, defects and microstructure homogeneity. The low room temperature resistivity obtained in this work can be attributed to
larger grain size of the samples. Larger grains imply small number of insulating grain boundaries and hence less energy barrier to electron conduction resulting thereby in lower resistivity. The obtained value of electrical resistivity is lower than the reported value [18]. Vacuum evaporation of Mg and further thermal oxidation in air gives larger imperfections and defects than that observed in spray pyrolysis, which may be one of the reasons for decrease in the value of electrical resistivity. It can be clearly seen that the resistivities of film surface are strongly dependent on the substrate temperature. The oxidation temperatures used in our work are also lower than those reported [18]. The grain
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Fig. 3. Variation in DC electrical resistivity of magnesium oxide thin film on alumina substrate oxidized at different temperatures [T1 = 623 K, T2 = 675 K and T3 = 723 K].
size obtained in this work is larger than that obtained by other workers. The activation energy represents the location of trap levels below the conduction band. The activation energy of magnesium
oxide thin film changes with change in thickness and oxidation temperature. As thickness and oxidation temperature of film increased, the activation energy also increased. When the grain boundaries scattering is predominant there is less activation energy
Fig. 4. Microwave transmittance of magnesium oxide thin film.
S. Patil, V. Puri / Applied Surface Science 258 (2011) 1535–1540
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Fig. 5. Real (ε ) and imaginary (ε ) parts of the permittivity of magnesium oxide thin film [T1 = 623 K, T2 = 675 K and T3 = 723 K].
and at higher temperature the activation energy is high which indicates the position of trap levels in the semiconductor forbidden band towards the lower limit of the conduction band. The activation energy of magnesium oxide thin film was 0.013 eV for low oxidation temperature whereas for higher oxidation temperature it was 0.087 eV. 3.4. Microwave transmittance Microwave transmittance of magnesium oxide thin film oxidized at different temperature is shown in Fig. 4. Effect of thickness and oxidation temperature dependent microwave transmittance of magnesium oxide thin film was observed. As thickness and oxidation temperature increased, the transmittance of magnesium oxide thin films also increased. For high oxidation temperature (723 K) the film of thickness 546 nm shows more transmission. In case of magnesium oxide, for higher oxidation temperature the high dielectric loss was obtained resulting high transmittance and also for higher oxidation temperature high grains size with high porosity was observed from SEM images. For both thickness and oxidation temperatures the magnesium oxide thin film showed lower transmittance than the alumina substrate. The transmittance of bare substrate has been given for comparison. 3.5. Microwave permittivity Fig. 5 shows the microwave permittivity of magnesium oxide thin film as a function of frequency. From the figure it is seen
that, as oxidation temperature increased the dielectric constant ε increased and dielectric loss ε decreased. As thickness of film increased the dielectric constant also increased but dielectric loss decreased. The microwave permittivity decreased with increase in frequency in the X band. The dielectric behavior of the sample can be explained by the mechanism of polarization and also the losses may be mainly caused by grain size and porosity. Imaginary part indicates the extent of dielectric loss in the film. The larger transmittance for the film of higher thickness and higher oxidation temperature can be related to the higher dielectric constant and lower loss. The dielectric loss of magnesium oxide thin film varied from 1.2 to 4.5. MgO thin films showed high transmittance with increase in oxidation temperature can be explained on the phenomenon of complex permittivity and also on the microstructures of the materials. The obtained dielectric loss was lower for higher oxidation temperature; resulting in high transmittance. For higher oxidation temperature high band gap was observed which might be also responsible for higher transmittance. Also surface morphological studies reveals that larger grain size with more porosity formed for magnesium oxide results in high surface area and hence, high transmittance at higher oxidation temperature. Oxides are reported to exhibit high polarisability and high dielectric constant, which extend into microwave range. The temperature dependent permittivity is dominated by reorientation of molecular dipoles. At higher temperature the orientational polarization vanishes and the polarisability is determined by the
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localized electronic states, the relative permittivity decreases. The polarization is also affected by some factors such as structural homogeneity, density, grain size and porosity of the sample. The higher density implies decrease in the porosity and higher number of polarizing species per unit volume, both contributing to the observed increase in polarization. Here for higher oxidation temperature low dielectric loss was observed. 4. Conclusion Magnesium oxide thin films were successfully prepared by thermal oxidation of vacuum evaporated magnesium thin films on alumina. The surface morphology changes due to thickness of MgO thin films. The grain size of the magnesium oxide thin film deposited on alumina substrate increased with increase in oxidation temperature. Due to increase in oxidation temperature and thickness; the transmittance of magnesium oxide thin film increases. As oxidation temperature increased, dielectric constant also increased whereas dielectric loss decreased. Since these thin films show higher dielectric constant than the alumina substrate (9.6) they can be a possible choice for substrates for miniaturized microwave components. Acknowledgments Vijaya Puri gratefully acknowledges the University Grants Commission (UGC), India for Research Scientist ‘C’ award. The authors
gratefully acknowledge the Department of Science and technology (DST), India for the financial support References [1] S. Watanabe, K. Hamanaka, T. Tachiki, T. Uchida, Phys. C: Supercond. 469 (2009) 818. [2] D.C. Kim, B.H. Kong, C.H. Ahn, H.K. Cho, Thin Solid Films 518 (2009) 1185. [3] E. Mariani, J. Mecklenburgh, J. Wheeler, D.J. Prior, F. Heidelbach, Acta Mater. 57 (2009) 1886. [4] J. Kim, E. Lee, D.H. Kim, D.G. Kim, Thin Solid Films 447–448 (2004) 95. [5] C. Park, J. Choi, M. Choi, Y. Kim, H. Lee, Surf. Coat. Technol. 197 (2005) 223. [6] K. Nam, M. Jung, J. Han, T. Kopte, U. Hartung, C. Peters, Vacuum 75 (2004) 1. [7] H. Nomura, S. Murakami, A. Ide-ektessabi, Y. Tanaka, Y. Tsukuda, Appl. Surf. Sci. 238 (2004) 113. [8] S. Moon, T. Heo, S. Park, J. Kim, H. Kim, J. Electrochem. Soc. 154 (2007) J408. [9] A. Raj, L.C. Nehru, M. Jayachandran, C. Sanjeeviraja, Cryst. Res. Technol. 42 (2007) 867. [10] A. Ide-ektessabi, H. Nomura, N. Yasui, Y. Tsukuda, Thin Solid Films 447–448 (2004) 383. [11] D. Caceres, I. Colera, I. Vergara, R. Gonzalez, E. Roman, Vacuum 67 (2002) 577. [12] S.H. Tamboli, V. Puri, R.K. Puri, Appl. Surf. Sci. 256 (2010) 4582. [13] S.H. Tamboli, V. Puri, R.K. Puri, J. Alloys Compd. 477 (2009) 855. [14] B.B. Vhankhande, S.V. Jadhav, D.C. Kulkarni, V. Puri, Microw. Opt. Technol. Lett. 50 (2008) 761. [15] S.F. Adam, Microwave Theory and Applications, vol. 392, Prentice hall, New Jersey, 1969. [16] L. Rayleigh, Philos. Mag. 5 (1992) 184. [17] V.V. Killedar, C.H. Bhosale, C.D. Lokhande, Turk. J. Phys. 22 (1998) 825. [18] J.M. Bian, X.M. Li, T.L. Chen, X.D. Gao, W.D. Yu, Appl. Surf. Sci. 228 (2004) 297.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Oxidation temperature dependent properties of MgO thin film on alumina S. Patil, Vijaya Puri ∗ Thick and Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur, 416004, India
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 27 September 2011 Accepted 29 September 2011 Available online 6 October 2011 Keywords: Magnesium oxide thin film Oxidation temperature Morphology Resistivity Microwave transmittance Permittivity
a b s t r a c t The magnesium oxide thin films were prepared by thermal oxidation (in air) of vacuum evaporated magnesium thin film on alumina. It was found that oxidation temperature (623 K, 675 K and 723 K) and thickness (103 nm and 546 nm) dependent effects were prominently manifested in the surface morphology. Electrical and microwave properties (8–12 GHz) of the MgO thin films were also carried out. X-ray diffraction showed orientation along (2 0 0) and (2 2 0) directions. Flowerlike morphology was observed from SEM and flake like morphology for films of higher thickness oxidized at higher temperatures. The magnesium oxide thin film showed NTC behavior. Microwave transmittance was found to increase with increase in oxidation temperature but was lower than alumina. Frequency and oxidation temperature dependent microwave permittivity was obtained. The microwave dielectric constant varied in the range 8.3–15.3. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxides have been of great interest due to their tunable applications in the radio frequency range. Magnesium oxide has high dielectric constant which is very suitable for microwave integrated circuits and also used as a substrate [1–3]. Numbers of methods have been reported for deposition of MgO thin films such as vacuum arc deposition [4], magnetron sputtering [5,6], electron beam evaporation [7,8], spray pyrolysis [9] and ion beam assisted deposition [10]. The interesting properties of magnesium oxide thin film have attracted a number of applications [11]. To the authors knowledge there are no reports available on the studies of MgO thin films by thermal oxidation of vacuum evaporated Mg thin films on alumina, though MgO thin films on glass substrates are reported [12,13]. Compared to other techniques this is a more cost effective method, since oxidation is done in ambient air at relatively low temperatures. This paper reports the study of morphology, electrical and microwave properties of magnesium oxide thin films oxidized at different oxidation temperatures. Frequency and thickness dependence microwave properties are also presented in this paper. 2. Experimental Magnesium oxide thin films have been prepared by thermal oxidation (in air) of vacuum evaporated magnesium films, deposited
on alumina substrates under vacuum of 10−5 mbar. The pure (99.98%) metallic magnesium (Alfa Aesar) was used as the source material. After the deposition of magnesium by resistive heating, magnesium thin films were oxidized in air atmosphere at different temperatures; 623 K, 675 K and 723 K. Two thicknesses of 103 nm and 546 nm were deposited and measured by surface profiler. The onset of oxidation (formation of magnesium oxide) could be seen from the disappearance of magnesium and the film becoming transparent. X-ray diffraction analysis was performed by a Philips (PW 3710) ˚ The surdiffractometer (using Cu K␣ radiation = 1.541838 A). face morphological study was carried out by scanning electron microscopy (JSM-6360 JEOL, Japan). The DC electrical resistivity of magnesium oxide thin films was measured as a function of temperature in the range 300–600 K by using two-probe method. Microwave transmittance was studied in the X-band (8–12 GHz) using the waveguide reflectometer set-up [14]. The permittivity of magnesium oxide thin film at microwave frequency was measured by using Voltage Standing Wave Ratio (VSWR) slotted section method [15]. By measuring the displacements of maxima and minima, the permittivity of the medium has been determined. The real and imaginary parts of the permittivity were calculated by using Smith chart [15]. 3. Results and discussion 3.1. X-ray diffraction studies
∗ Corresponding author. Fax: +91 231 2691533. E-mail address: vrp [email protected] (V. Puri). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.126
The typical XRD patterns of magnesium oxide thin film on alumina substrate for all oxidation temperature are shown in Fig. 1.
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As oxidation temperature increases, the (2 0 0) diffraction peak becomes progressively more dominant. At the higher temperature of 450 ◦ C, the film is strongly textured with preferential orientation along the (2 0 0) axis. The observed relationship between the degree of (2 0 0) preferred orientation and the oxidation temperature may be explained in terms of the migration of molecules onto the growing surface. MgO is expected to achieve a (2 0 0) preferential orientation that has a lower energy configuration during nucleating. The absorbed Mg and O atoms with a higher mobility enhanced at higher temperature could easily move to equilibrium atomic sites on the surface, i.e. the (2 0 0) plane of MgO which is energetically stable [16]. Thus, the increase in (2 0 0) preferred orientation occurs when the substrate temperature is high. 3.2. Surface morphology
Fig. 1. XRD patterns of magnesium oxide thin film on alumina for different oxidation temperatures.
Crystal structure does not show any variation with thickness of the thin film, so the film of thickness 546 nm was used for all X-ray diffraction studies. XRD patterns of magnesium oxide thin films show polycrystalline cubic structure with mainly (2 0 0) plane and (2 2 0) plane orientations. The peak intensity increases with decrease in the peak broadening due to increase in oxidation temperature. The increased intensity can be attributed to the improved crystallinity. As temperature increases the MgO thin films becomes more crystalline. Due to heating, mobility of adatoms as well as nucleation sites increases and elimination of trapped excess vacancies occur which results in increased thin films crystallinity. It was observed that, the crystallinity of MgO thin films showed improvement with increase in oxidation duration. The figures showed only (2 0 0) peaks as fundamental characteristic peaks of MgO which were confirmed by the standard JCPDS data file no. 78-0430 for copper target. The XRD patterns did not show the presence of magnesium metal, indicating complete oxidation of magnesium metal during magnesium oxide thin films preparation. For all oxidation temperatures (2 0 0) and (2 2 0) plane orientations and cubic structure was observed. No characteristic peaks of impurity and other phases were observed. The crystallite size of magnesium oxide thin film deposited on alumina was ∼53 nm calculated from Scherer’s formula. The crystallite size of magnesium oxide thin film increases to ∼55.09 nm with increase in oxidation temperature.
The surface morphology of deposited thin films depends on the thin film growth. In this paper metal thin films were deposited by vacuum evaporation technique and after deposition oxidation was done. So the oxidation temperature also plays a vital role in surface morphology modification. During the thin film growth there are two steps, the first step aims to form a seed layer on the substrate by the formation of nucleation centers in desired orientation and improve the quality of thin film as it grows thicker. Similarly, during oxidation of metal thin films, first atoms move due to increased substrate temperatures till it attains thermal equilibrium. During which the adatom movement tries to achieve lower energy state. The proper plane orientation depends on the force of attraction and bond formation between metal and oxygen atoms. SEM images of magnesium oxide thin film at different oxidation temperatures for the two thicknesses are shown in Fig. 2 along with the SEM image of the alumina substrate. It appears that the MgO grains have grown on granular alumina surface since, the shape of alumina grains is clearly visible. The magnesium oxide thin films for thickness 103 nm oxidized at temperature 623 K showed flowerlike morphology which contains small spherical grains. The film oxidized at 675 K shows that the morphology is disturbed and films become more compact because of agglomeration of the grains. For the higher oxidation temperature the spherical grains are separately formed with increase in porosity. This might be due to evaporation of magnesium at higher temperature. For film thickness of 546 nm, the same flowerlike morphology was maintained for lower oxidation temperature. This morphology gets transformed into the flakes-like morphology for oxidation temperature 675 K. At the higher oxidation temperature 723 K the flowerlike morphology is totally destroyed and films become more flaky and porous. The effect of substrate is evident here since the same MgO deposited on glass substrate show granular morphology [12,13]. The grain size varied due to oxidation temperature and due to thickness variations. The grain size increases with increase in thickness and oxidation temperature. The grain size of magnesium oxide thin film for lower oxidation temperature was 453 nm and for higher oxidation temperature the grain size of magnesium oxide thin film was 523 nm. 3.3. Electrical properties Fig. 3 shows the graph of log vs 1000/T which reveals that the resistivity of the sample decreases with increase in the temperature showing the semiconducting behavior of the samples. At the room temperature, electrical resistivity of magnesium oxide thin film is in the order of 104 cm. From the figure it is seen that, resistivity decreases with increase in temperature and shows NTC behavior. As the thickness as well as oxidation temperature increased, the
S. Patil, V. Puri / Applied Surface Science 258 (2011) 1535–1540
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Fig. 2. SEM images of magnesium oxide thin films at different oxidation temperatures.
resistivity of magnesium oxide thin film decreased. The results obtained from the DC resistivity measurements are in good agreement with thin films surface morphology. It was found that; resistivity was high due to small grain size, high porosity and less defect inside the crystallites [17]. In general, the electrical properties of thin films usually differ from those of bulk material because of the essential differences in the microstructures and the geometrical limitations. Resistivity is actually the result of the combined factors such as grain size, crystal structure, defects and microstructure homogeneity. The low room temperature resistivity obtained in this work can be attributed to
larger grain size of the samples. Larger grains imply small number of insulating grain boundaries and hence less energy barrier to electron conduction resulting thereby in lower resistivity. The obtained value of electrical resistivity is lower than the reported value [18]. Vacuum evaporation of Mg and further thermal oxidation in air gives larger imperfections and defects than that observed in spray pyrolysis, which may be one of the reasons for decrease in the value of electrical resistivity. It can be clearly seen that the resistivities of film surface are strongly dependent on the substrate temperature. The oxidation temperatures used in our work are also lower than those reported [18]. The grain
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S. Patil, V. Puri / Applied Surface Science 258 (2011) 1535–1540
Fig. 3. Variation in DC electrical resistivity of magnesium oxide thin film on alumina substrate oxidized at different temperatures [T1 = 623 K, T2 = 675 K and T3 = 723 K].
size obtained in this work is larger than that obtained by other workers. The activation energy represents the location of trap levels below the conduction band. The activation energy of magnesium
oxide thin film changes with change in thickness and oxidation temperature. As thickness and oxidation temperature of film increased, the activation energy also increased. When the grain boundaries scattering is predominant there is less activation energy
Fig. 4. Microwave transmittance of magnesium oxide thin film.
S. Patil, V. Puri / Applied Surface Science 258 (2011) 1535–1540
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Fig. 5. Real (ε ) and imaginary (ε ) parts of the permittivity of magnesium oxide thin film [T1 = 623 K, T2 = 675 K and T3 = 723 K].
and at higher temperature the activation energy is high which indicates the position of trap levels in the semiconductor forbidden band towards the lower limit of the conduction band. The activation energy of magnesium oxide thin film was 0.013 eV for low oxidation temperature whereas for higher oxidation temperature it was 0.087 eV. 3.4. Microwave transmittance Microwave transmittance of magnesium oxide thin film oxidized at different temperature is shown in Fig. 4. Effect of thickness and oxidation temperature dependent microwave transmittance of magnesium oxide thin film was observed. As thickness and oxidation temperature increased, the transmittance of magnesium oxide thin films also increased. For high oxidation temperature (723 K) the film of thickness 546 nm shows more transmission. In case of magnesium oxide, for higher oxidation temperature the high dielectric loss was obtained resulting high transmittance and also for higher oxidation temperature high grains size with high porosity was observed from SEM images. For both thickness and oxidation temperatures the magnesium oxide thin film showed lower transmittance than the alumina substrate. The transmittance of bare substrate has been given for comparison. 3.5. Microwave permittivity Fig. 5 shows the microwave permittivity of magnesium oxide thin film as a function of frequency. From the figure it is seen
that, as oxidation temperature increased the dielectric constant ε increased and dielectric loss ε decreased. As thickness of film increased the dielectric constant also increased but dielectric loss decreased. The microwave permittivity decreased with increase in frequency in the X band. The dielectric behavior of the sample can be explained by the mechanism of polarization and also the losses may be mainly caused by grain size and porosity. Imaginary part indicates the extent of dielectric loss in the film. The larger transmittance for the film of higher thickness and higher oxidation temperature can be related to the higher dielectric constant and lower loss. The dielectric loss of magnesium oxide thin film varied from 1.2 to 4.5. MgO thin films showed high transmittance with increase in oxidation temperature can be explained on the phenomenon of complex permittivity and also on the microstructures of the materials. The obtained dielectric loss was lower for higher oxidation temperature; resulting in high transmittance. For higher oxidation temperature high band gap was observed which might be also responsible for higher transmittance. Also surface morphological studies reveals that larger grain size with more porosity formed for magnesium oxide results in high surface area and hence, high transmittance at higher oxidation temperature. Oxides are reported to exhibit high polarisability and high dielectric constant, which extend into microwave range. The temperature dependent permittivity is dominated by reorientation of molecular dipoles. At higher temperature the orientational polarization vanishes and the polarisability is determined by the
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localized electronic states, the relative permittivity decreases. The polarization is also affected by some factors such as structural homogeneity, density, grain size and porosity of the sample. The higher density implies decrease in the porosity and higher number of polarizing species per unit volume, both contributing to the observed increase in polarization. Here for higher oxidation temperature low dielectric loss was observed. 4. Conclusion Magnesium oxide thin films were successfully prepared by thermal oxidation of vacuum evaporated magnesium thin films on alumina. The surface morphology changes due to thickness of MgO thin films. The grain size of the magnesium oxide thin film deposited on alumina substrate increased with increase in oxidation temperature. Due to increase in oxidation temperature and thickness; the transmittance of magnesium oxide thin film increases. As oxidation temperature increased, dielectric constant also increased whereas dielectric loss decreased. Since these thin films show higher dielectric constant than the alumina substrate (9.6) they can be a possible choice for substrates for miniaturized microwave components. Acknowledgments Vijaya Puri gratefully acknowledges the University Grants Commission (UGC), India for Research Scientist ‘C’ award. The authors
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