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Dielectric properties of TiO2 thin films deposited by a DC magnetron sputtering system
Thin Solid Films 372 Ž2000. 246᎐249
Dielectric properties of TiO2 thin films deposited by a DC magnetron sputtering system Marius D. StamateU Bacau Uni¨ ersity, Calea Marasesti nr. 157, Bacau 5500, Romania Received 15 January 2000; received in revised form 10 April 2000; accepted 17 May 2000
Abstract DC magnetron thin films were used to build up metal᎐oxide᎐metal ŽMOM., aluminum᎐TiO2 ᎐aluminum structures. Aluminum was deposited through thermal vacuum evaporation in a dedicated installation. The electrical polarization effect for the electric capacitance and dielectric loss and the behavior of dielectric properties of MOM structures with signal frequency and temperature were analyzed. We found that the amorphous TiO2-based MOM structures have an unusual dependence of dielectric properties on the polarization and signal frequency, in accordance with the higher value of the TiO2 dielectric constant. We observed that near 270 K there is a transition that occurs for the dielectric constant. This transition is consistent with incorporation of water and OH-ions in the film. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous materials; Dielectric properties; Titanium oxide
1. Introduction Recently, there has been an increasing demand for high dielectric constant insulators to replace SiO2 for high-density dynamic-memory applications w1᎐3x. Among these insulators, titanium dioxide ŽTiO2 . films have attracted attention for use in fabricating capacitors in microelectronics devices due to their unusually high dielectric constants w4,5x. Many deposition methods can be used to prepare titanium oxide films: thermal w6x or anodic w7x oxidation of titanium, electron beam evaporation w8x, chemical vapor deposition w9x, plasma-enhanced chemical vapor deposition w10x, sol᎐gel method w11,12x and reactive sputtering methods w13᎐16x. Among these methods, DC reactive magnetron sputtering has made the fabrication of insulator films with reproducible and desired properties possible. In this paper we have studied the dielectric properties of U
Tel.: q40-34-134712; fax: q40-34-181080. E-mail address: [email protected] ŽM.D. Stamate..
DC magnetron sputtered TiO2 films for metal᎐insulator᎐metal ŽMIM. structures. 2. Experimental details The films were deposited in a home-built magnetron sputtering system w17x. The vacuum chamber was an 80-l-volume stainless steel chamber; a circular magnetron with a 60-mm-diameter erosion zone was used as the cathode. The discharge characteristics were controlled using a variable DC power supply Ž3 kV and 500 mA.. Pure titanium Ž99.95%. of 130 mm diameter and 3 mm thickness were used as a sputtering target. Pure argon Ž4 N. and oxygen were used as the sputtering and reactive gases, respectively. The gases were mixed prior to the admission in the sputtering chamber in the proportion 75% argon and 25% oxygen. The target᎐substrate distance was 35 mm. The sputtering pressure was maintained at 2 = 10y3 torr. Prior to the deposition the target was well cleaned in order to remove the surface oxide layer. The substrate tempera-
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 0 2 7 - 0
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
ture was held at 300⬚C using a quartz halogen lamp whose power was controlled by varying the input voltage. Titanium oxide films were deposited on vacuumevaporated aluminum-bottomed electrodes made on a well-cleaned microscope glass slide Ž75 = 25 = 1.. The deposition time was chosen in order to obtain films of several thicknesses and the sputtering power was approximately 110 W Ž200 mA= 550 V., that corresponds to a power density of 1.25 W cmy2 . The top electrode was vacuum-evaporated aluminum. The thickness of the films was determined using a multiple beam interferometry method to an accuracy of "10 nm. The area of the metal᎐TiO2 ᎐metal surface was approximately 10 mm2 , so we managed to obtain several structures on the same glass substrate. The structure of the films was examined by using X-ray diffraction with CuK␣ radiation in a standard X-ray diffractometer. The conductivity of the film was recorded with a four-probe method that is sensitive to resistivity up to 1011 ⍀ cm. Dielectric properties were recorded with an RLC bridge Tesla BM 439, that has a wide range for capacitance measurements Ž100 pF᎐100 F. at different applied voltage and signal frequencies. 3. Results and discussion TiO2 thin films deposited in the DC magnetron sputtering system were amorphous and X-ray diffraction analysis proved that the crystalline diffraction peaks corresponding to the anatase crystalline phase of titanium dioxide appeared only for films that were annealed at a temperature greater than 500⬚C. In Fig. 1 the X-ray diffraction for TiO2 films as deposited and after annealing is shown. We present the results for dielectric properties for two different thicknesses of aluminum᎐TiO2 ᎐aluminum structures for films with amorphous structure. In Fig. 2 the electrical polarization effects for capacitance and dielectric loss are shown. From Fig. 2 it is revealed that there is a significant variation of electric capacitance and dielectric loss with the intensity of the electric field
Fig. 1. The X-ray diffraction spectra for TiO2 as deposited and annealed at 500⬚C.
247
Fig. 2. The polarization effects for capacitance and dielectric loss for metal᎐TiO2 ᎐metal structures.
that induces the polarization process. The measurements were carried out at a signal frequency of 1 kHz, and the serial capacitance of structures was recorded. In order to eliminate the errors in calculating the value of dielectric constant we considered the electric capacitance data recorded suitable only if the dielectric losses were under 0.3 w18x. For dielectric losses larger than 0.3, the serial method for capacitance and dielectric constant measurements gives higher errors and a suitable method in determining the electric capacitance and dielectric constant is the parallel method. We calculated the dielectric constant for TiO2 thin films for electric field intensity lower than 50 kV cmy1 Ždielectric loss was lower than 0.3. and we found the dielectric constant value was close to r s 60. From measurements of I᎐V characteristics on metal᎐TiO 2 ᎐ metal structures, made elsewhere w19x, we have also found that this value of the electric field intensity is the value where non-linear effects begin to occur. The value that we have found for the dielectric constant of TiO2 thin films is close to what is typical for anatasecrystalline phases of TiO2 . Figs. 3 and 4 show the variations observed in electric capacitance and dielectric loss for two different thicknesses of metal᎐TiO2 ᎐metal structures with the change in frequency of an external signal applied to the measurement bridge. From the figures it is revealed that both structures have a similar dependence in electric capacitance or dielectric loss, with the signal frequency. At lower frequencies both the electric capacitance and dielectric loss have larger values than the values corresponding to the high frequency domain. In the high frequency region we have observed that there is a saturation effect and the electric capacitance and dielectric constant are independent of the signal frequency. This variation with frequency that we have found for electric capacitance appears to be in contradiction with the normal behavior expected. The dielectric constant for polar dielectrics should be almost independent of the signal frequency. If the signal fre-
248
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
Fig. 5. The temperature dependence of electric capacitance for a TiO2 thin film.
Fig. 3. The variation of electric capacitance with the signal frequency for two different thicknesses of TiO2 structures.
quency becomes larger than a specific value that corresponds to an incomplete polarization, the dielectric constant should start to decrease with increase in signal frequency. We notice that dielectric loss smaller than 0.3 occurs only if the signal frequency is higher than 10 kHz and the thickness of the film is larger Že.g. 0.6 m.. For thinner TiO2 films, we find that only for signal frequency higher than 150 kHz, the dielectric losses are smaller than 0.5. If we consider that only for these values the electric capacitance is correctly measured for serial measurements, we conclude that we have been able to measure only the constant domain for the polar dielectric in accordance with the theoretical approach, and the highest frequency of our signal is lower than the limit frequency that corresponds to incomplete polarization. We have found that there is a weak thickness dependence of dielectric constant. TiO2 films with lower thickness have a larger value for dielectric constant r s 70 for ds 0.3 m, while for thickness ds 0.6 m we found r s 50. However, the
Fig. 4. The variation of dielectric loss with the signal frequency for two different thicknesses of TiO2 structures.
average value for the dielectric constant was found to be approximately r s 60, a value that correlates with the results obtained from measurement of dielectric constant at various electric field intensities. Fig. 5 shows the temperature dependence of electric capacitance measured for a wide range of temperatures. The thickness of TiO2 thin films in the MOM structures were approximately 1.44 m. Measurements were carried out at 1 kHz, and the dielectric losses were temperature independent and smaller than 0.3. In the figure we notice that there are two distinct temperature regions for the electric capacitance. At lower temperatures we have found that the electric capacitance and dielectric constant are not dependent upon the temperature and have small values, 12᎐18 pF, for electric capacitance and r s 10᎐12. For higher temperatures we also have found that the electric capacitance and the dielectric constant are not dependent on the temperature and have higher values: 70 pF and r s 60, respectively. The transition from the lower to the higher values for the electric capacitance, respectively, the dielectric constant occurs at a temperature near 273 K. This transition occurs in a small temperature range, approximately 50 K, the medium slope being: ⌬Cr⌬T s 1 pF Ky1 and ⌬rr⌬Ts 1 Ky1 , respectively. Generally, the dielectric constant for polar dielectrics is weakly or not dependent upon the temperature. An abrupt variation in dielectric constant occurs only with a structural phase change. This transition could not be due to a phase transition in TiO2 films, very few experimental data being published. It is, we think, more likely due to the incorporation of water in TiO2 thin films. Water is revealed by the IR absorption spectra carried out on TiO2 thin films deposited on KBr substrates ŽFig. 6. w20,21x. From the IR transmission spectrum we found that the features from 1300᎐1700 cmy1 correspond to the water incorporated in TiO2 thin film. Water that is incorporated in TiO2 films changes the aggregation state near 273 K. It is possible that ice gives a contribution at the dielectric
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
249
the water and OH-ions incorporated in films, near the melting point of ice, may be used for the development of sensors Že.g. temperature, humidity, etc... Acknowledgements This work was partially supported by the Romanian CNCSIS organization. References
Fig. 6. The IR-transmission spectrum for a TiO2 thin film.
constant of TiO2 thin films. Over the melting temperature, water changes the aggregation state Žwith a larger value for dielectric constant. that gives a different contribution to the dielectric constant of TiO2 thin films. However, considering that the capacitance increase is four to five times, it would require a large water content in the film. One would expect a much more narrow and distinct absorption peak at approximately 3000 cmy1 , that is not consistent with the IR transmittance spectrum. Therefore, in order to give a sustainable explanation, we have to consider also the contribution to film capacitance of other species that are incorporated in film, such as OH-ions. 4. Conclusion We have studied the dielectric properties of DC magnetron TiO2 thin films in MOM structures. We have found that it is possible to obtain structures with reproducible and controllable dielectric properties for microelectronic devices. We have observed a transition in temperature dependence of dielectric constant for metal᎐TiO2 ᎐metal structures near 273 K, which we think is due to the incorporation of water and OH-ions in TiO2 thin films. The capacitance increase of four to five times which was observed for TiO2 thin film, due to
w1x M. Gurwitch, L. Manchanda, J.M. Gibson, Appl. Phys. Lett. 51 Ž1987. 919. w2x Y. Nishioka, H. Shinriki, K. Mukai, J. Appl. Phys. 61 Ž1987. 2335. w3x G.S. Oehrlein, J. Appl. Phys. 59 Ž1986. 1587. w4x C.N. Wilmsen, Physics and Chemistry of Compound III-V Semiconductor Interfaces, Plenum Press, New York, 1985. w5x L. Messick, J. Appl. Phys. 47 Ž1976. 4949. w6x B. Morris Henry, US Patent No. 4200474, 1978. w7x M.R. Kozlowski, P.S. Tyler, W.H. Smyrl, R.T. Atanasoki, J. Electrochem. Soc. 136 Ž1989. 442. w8x M. Lottiaux, C. Boulesteix, G. Nihoul et al., Thin Solid Films 170 Ž1989. 107. w9x K.S. Yeung, Y.W. Lam, Thin Solid Films 109 Ž1983. 405. w10x L.M. Williams, D.W. Hess, J. Vac. Sci. Technol. A 1 Ž1983. 1810. w11x K.A. Vorotilov, E.V. Orlova, V.I. Petrovsky, Thin Solid Films 207 Ž1992. 180. w12x M. Gartner, C. Parlog, P. Osiceanu, Thin Solid Films 234 Ž1993. 561. w13x M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 Ž3. Ž1992. 1421. w14x L.-J. Meng, M. Andritschky, M.P. dos Santos, Thin Solid Films 223 Ž1993. 242. w15x L.-J. Meng, M.P. dos Santos, Thin Solid Films 226 Ž1993. 22. w16x H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 Ž4. Ž1994. 2042. w17x M. Stamate, E. Iordachescu, A. Stamate, I. Vascan, Analele Stiint. Univ. ‘Al. I. Cuza’ Iasi ŽSer. Noua. IB Fiz. XXXVIIIrXXXIX Ž1993. 548. w18x B. Tareev, Physics of dielectric materials, Mir, Moscow, 1975. w19x M. Stamate, Ph.D Thesis, ‘Al. I. Cuza’ University, Iasi, Romania, 1999. w20x M. Stamate, G.I. Rusu, I. Vascan, Proc. SPIE 3405 Ž1998. 951. w21x G. Lazar, I. Vascan, Rom. J. Optoelectron. 5 Ž2. Ž1997. 44.
Dielectric properties of TiO2 thin films deposited by a DC magnetron sputtering system Marius D. StamateU Bacau Uni¨ ersity, Calea Marasesti nr. 157, Bacau 5500, Romania Received 15 January 2000; received in revised form 10 April 2000; accepted 17 May 2000
Abstract DC magnetron thin films were used to build up metal᎐oxide᎐metal ŽMOM., aluminum᎐TiO2 ᎐aluminum structures. Aluminum was deposited through thermal vacuum evaporation in a dedicated installation. The electrical polarization effect for the electric capacitance and dielectric loss and the behavior of dielectric properties of MOM structures with signal frequency and temperature were analyzed. We found that the amorphous TiO2-based MOM structures have an unusual dependence of dielectric properties on the polarization and signal frequency, in accordance with the higher value of the TiO2 dielectric constant. We observed that near 270 K there is a transition that occurs for the dielectric constant. This transition is consistent with incorporation of water and OH-ions in the film. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous materials; Dielectric properties; Titanium oxide
1. Introduction Recently, there has been an increasing demand for high dielectric constant insulators to replace SiO2 for high-density dynamic-memory applications w1᎐3x. Among these insulators, titanium dioxide ŽTiO2 . films have attracted attention for use in fabricating capacitors in microelectronics devices due to their unusually high dielectric constants w4,5x. Many deposition methods can be used to prepare titanium oxide films: thermal w6x or anodic w7x oxidation of titanium, electron beam evaporation w8x, chemical vapor deposition w9x, plasma-enhanced chemical vapor deposition w10x, sol᎐gel method w11,12x and reactive sputtering methods w13᎐16x. Among these methods, DC reactive magnetron sputtering has made the fabrication of insulator films with reproducible and desired properties possible. In this paper we have studied the dielectric properties of U
Tel.: q40-34-134712; fax: q40-34-181080. E-mail address: [email protected] ŽM.D. Stamate..
DC magnetron sputtered TiO2 films for metal᎐insulator᎐metal ŽMIM. structures. 2. Experimental details The films were deposited in a home-built magnetron sputtering system w17x. The vacuum chamber was an 80-l-volume stainless steel chamber; a circular magnetron with a 60-mm-diameter erosion zone was used as the cathode. The discharge characteristics were controlled using a variable DC power supply Ž3 kV and 500 mA.. Pure titanium Ž99.95%. of 130 mm diameter and 3 mm thickness were used as a sputtering target. Pure argon Ž4 N. and oxygen were used as the sputtering and reactive gases, respectively. The gases were mixed prior to the admission in the sputtering chamber in the proportion 75% argon and 25% oxygen. The target᎐substrate distance was 35 mm. The sputtering pressure was maintained at 2 = 10y3 torr. Prior to the deposition the target was well cleaned in order to remove the surface oxide layer. The substrate tempera-
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 0 2 7 - 0
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
ture was held at 300⬚C using a quartz halogen lamp whose power was controlled by varying the input voltage. Titanium oxide films were deposited on vacuumevaporated aluminum-bottomed electrodes made on a well-cleaned microscope glass slide Ž75 = 25 = 1.. The deposition time was chosen in order to obtain films of several thicknesses and the sputtering power was approximately 110 W Ž200 mA= 550 V., that corresponds to a power density of 1.25 W cmy2 . The top electrode was vacuum-evaporated aluminum. The thickness of the films was determined using a multiple beam interferometry method to an accuracy of "10 nm. The area of the metal᎐TiO2 ᎐metal surface was approximately 10 mm2 , so we managed to obtain several structures on the same glass substrate. The structure of the films was examined by using X-ray diffraction with CuK␣ radiation in a standard X-ray diffractometer. The conductivity of the film was recorded with a four-probe method that is sensitive to resistivity up to 1011 ⍀ cm. Dielectric properties were recorded with an RLC bridge Tesla BM 439, that has a wide range for capacitance measurements Ž100 pF᎐100 F. at different applied voltage and signal frequencies. 3. Results and discussion TiO2 thin films deposited in the DC magnetron sputtering system were amorphous and X-ray diffraction analysis proved that the crystalline diffraction peaks corresponding to the anatase crystalline phase of titanium dioxide appeared only for films that were annealed at a temperature greater than 500⬚C. In Fig. 1 the X-ray diffraction for TiO2 films as deposited and after annealing is shown. We present the results for dielectric properties for two different thicknesses of aluminum᎐TiO2 ᎐aluminum structures for films with amorphous structure. In Fig. 2 the electrical polarization effects for capacitance and dielectric loss are shown. From Fig. 2 it is revealed that there is a significant variation of electric capacitance and dielectric loss with the intensity of the electric field
Fig. 1. The X-ray diffraction spectra for TiO2 as deposited and annealed at 500⬚C.
247
Fig. 2. The polarization effects for capacitance and dielectric loss for metal᎐TiO2 ᎐metal structures.
that induces the polarization process. The measurements were carried out at a signal frequency of 1 kHz, and the serial capacitance of structures was recorded. In order to eliminate the errors in calculating the value of dielectric constant we considered the electric capacitance data recorded suitable only if the dielectric losses were under 0.3 w18x. For dielectric losses larger than 0.3, the serial method for capacitance and dielectric constant measurements gives higher errors and a suitable method in determining the electric capacitance and dielectric constant is the parallel method. We calculated the dielectric constant for TiO2 thin films for electric field intensity lower than 50 kV cmy1 Ždielectric loss was lower than 0.3. and we found the dielectric constant value was close to r s 60. From measurements of I᎐V characteristics on metal᎐TiO 2 ᎐ metal structures, made elsewhere w19x, we have also found that this value of the electric field intensity is the value where non-linear effects begin to occur. The value that we have found for the dielectric constant of TiO2 thin films is close to what is typical for anatasecrystalline phases of TiO2 . Figs. 3 and 4 show the variations observed in electric capacitance and dielectric loss for two different thicknesses of metal᎐TiO2 ᎐metal structures with the change in frequency of an external signal applied to the measurement bridge. From the figures it is revealed that both structures have a similar dependence in electric capacitance or dielectric loss, with the signal frequency. At lower frequencies both the electric capacitance and dielectric loss have larger values than the values corresponding to the high frequency domain. In the high frequency region we have observed that there is a saturation effect and the electric capacitance and dielectric constant are independent of the signal frequency. This variation with frequency that we have found for electric capacitance appears to be in contradiction with the normal behavior expected. The dielectric constant for polar dielectrics should be almost independent of the signal frequency. If the signal fre-
248
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
Fig. 5. The temperature dependence of electric capacitance for a TiO2 thin film.
Fig. 3. The variation of electric capacitance with the signal frequency for two different thicknesses of TiO2 structures.
quency becomes larger than a specific value that corresponds to an incomplete polarization, the dielectric constant should start to decrease with increase in signal frequency. We notice that dielectric loss smaller than 0.3 occurs only if the signal frequency is higher than 10 kHz and the thickness of the film is larger Že.g. 0.6 m.. For thinner TiO2 films, we find that only for signal frequency higher than 150 kHz, the dielectric losses are smaller than 0.5. If we consider that only for these values the electric capacitance is correctly measured for serial measurements, we conclude that we have been able to measure only the constant domain for the polar dielectric in accordance with the theoretical approach, and the highest frequency of our signal is lower than the limit frequency that corresponds to incomplete polarization. We have found that there is a weak thickness dependence of dielectric constant. TiO2 films with lower thickness have a larger value for dielectric constant r s 70 for ds 0.3 m, while for thickness ds 0.6 m we found r s 50. However, the
Fig. 4. The variation of dielectric loss with the signal frequency for two different thicknesses of TiO2 structures.
average value for the dielectric constant was found to be approximately r s 60, a value that correlates with the results obtained from measurement of dielectric constant at various electric field intensities. Fig. 5 shows the temperature dependence of electric capacitance measured for a wide range of temperatures. The thickness of TiO2 thin films in the MOM structures were approximately 1.44 m. Measurements were carried out at 1 kHz, and the dielectric losses were temperature independent and smaller than 0.3. In the figure we notice that there are two distinct temperature regions for the electric capacitance. At lower temperatures we have found that the electric capacitance and dielectric constant are not dependent upon the temperature and have small values, 12᎐18 pF, for electric capacitance and r s 10᎐12. For higher temperatures we also have found that the electric capacitance and the dielectric constant are not dependent on the temperature and have higher values: 70 pF and r s 60, respectively. The transition from the lower to the higher values for the electric capacitance, respectively, the dielectric constant occurs at a temperature near 273 K. This transition occurs in a small temperature range, approximately 50 K, the medium slope being: ⌬Cr⌬T s 1 pF Ky1 and ⌬rr⌬Ts 1 Ky1 , respectively. Generally, the dielectric constant for polar dielectrics is weakly or not dependent upon the temperature. An abrupt variation in dielectric constant occurs only with a structural phase change. This transition could not be due to a phase transition in TiO2 films, very few experimental data being published. It is, we think, more likely due to the incorporation of water in TiO2 thin films. Water is revealed by the IR absorption spectra carried out on TiO2 thin films deposited on KBr substrates ŽFig. 6. w20,21x. From the IR transmission spectrum we found that the features from 1300᎐1700 cmy1 correspond to the water incorporated in TiO2 thin film. Water that is incorporated in TiO2 films changes the aggregation state near 273 K. It is possible that ice gives a contribution at the dielectric
M.D. Stamate r Thin Solid Films 372 (2000) 246᎐249
249
the water and OH-ions incorporated in films, near the melting point of ice, may be used for the development of sensors Že.g. temperature, humidity, etc... Acknowledgements This work was partially supported by the Romanian CNCSIS organization. References
Fig. 6. The IR-transmission spectrum for a TiO2 thin film.
constant of TiO2 thin films. Over the melting temperature, water changes the aggregation state Žwith a larger value for dielectric constant. that gives a different contribution to the dielectric constant of TiO2 thin films. However, considering that the capacitance increase is four to five times, it would require a large water content in the film. One would expect a much more narrow and distinct absorption peak at approximately 3000 cmy1 , that is not consistent with the IR transmittance spectrum. Therefore, in order to give a sustainable explanation, we have to consider also the contribution to film capacitance of other species that are incorporated in film, such as OH-ions. 4. Conclusion We have studied the dielectric properties of DC magnetron TiO2 thin films in MOM structures. We have found that it is possible to obtain structures with reproducible and controllable dielectric properties for microelectronic devices. We have observed a transition in temperature dependence of dielectric constant for metal᎐TiO2 ᎐metal structures near 273 K, which we think is due to the incorporation of water and OH-ions in TiO2 thin films. The capacitance increase of four to five times which was observed for TiO2 thin film, due to
w1x M. Gurwitch, L. Manchanda, J.M. Gibson, Appl. Phys. Lett. 51 Ž1987. 919. w2x Y. Nishioka, H. Shinriki, K. Mukai, J. Appl. Phys. 61 Ž1987. 2335. w3x G.S. Oehrlein, J. Appl. Phys. 59 Ž1986. 1587. w4x C.N. Wilmsen, Physics and Chemistry of Compound III-V Semiconductor Interfaces, Plenum Press, New York, 1985. w5x L. Messick, J. Appl. Phys. 47 Ž1976. 4949. w6x B. Morris Henry, US Patent No. 4200474, 1978. w7x M.R. Kozlowski, P.S. Tyler, W.H. Smyrl, R.T. Atanasoki, J. Electrochem. Soc. 136 Ž1989. 442. w8x M. Lottiaux, C. Boulesteix, G. Nihoul et al., Thin Solid Films 170 Ž1989. 107. w9x K.S. Yeung, Y.W. Lam, Thin Solid Films 109 Ž1983. 405. w10x L.M. Williams, D.W. Hess, J. Vac. Sci. Technol. A 1 Ž1983. 1810. w11x K.A. Vorotilov, E.V. Orlova, V.I. Petrovsky, Thin Solid Films 207 Ž1992. 180. w12x M. Gartner, C. Parlog, P. Osiceanu, Thin Solid Films 234 Ž1993. 561. w13x M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 Ž3. Ž1992. 1421. w14x L.-J. Meng, M. Andritschky, M.P. dos Santos, Thin Solid Films 223 Ž1993. 242. w15x L.-J. Meng, M.P. dos Santos, Thin Solid Films 226 Ž1993. 22. w16x H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 Ž4. Ž1994. 2042. w17x M. Stamate, E. Iordachescu, A. Stamate, I. Vascan, Analele Stiint. Univ. ‘Al. I. Cuza’ Iasi ŽSer. Noua. IB Fiz. XXXVIIIrXXXIX Ž1993. 548. w18x B. Tareev, Physics of dielectric materials, Mir, Moscow, 1975. w19x M. Stamate, Ph.D Thesis, ‘Al. I. Cuza’ University, Iasi, Romania, 1999. w20x M. Stamate, G.I. Rusu, I. Vascan, Proc. SPIE 3405 Ž1998. 951. w21x G. Lazar, I. Vascan, Rom. J. Optoelectron. 5 Ž2. Ž1997. 44.