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Optical properties and surface morphology of Li-doped ZnO thin films deposited on different substrates by DC magnetron sputtering method
Physica B 308–310 (2001) 949–953
Optical properties and surface morphology of Li-doped ZnO thin films deposited on different substrates by DC magnetron sputtering method Galal A. Mohamed, El-Maghraby Mohamed*, A. Abu El-Fadl Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
Abstract Thin films of zinc oxide doped with Zn1xLixO with x ¼ 0:2 (ZnO : Li), have been prepared on sapphire, MgO and quartz substrates by DC magnetron sputtering method at 5 mTorr. The substrate temperatures were fixed to about 573 K. We have measured the transmission and reflection spectra and determined the absorption coefficient, optical opt ), the high frequency dielectric constant e0N and the carrier concentration N for the as-prepared films at band-gap (Egd opt room temperature. The films show direct allowed optical transitions with Egd values of 3.38, 3.43 and 3.29 eV for films deposited on sapphire, MgO and quartz substrates, respectively. The dependence of the obtained results on the substrate type are discussed. r 2001 Elsevier Science B.V. All rights reserved. Keywords: ZnO : Li films; DC sputtering; Band gap; Optical properties
1. Introduction Among different oxides, ZnO thin films (undoped and doped) have in recent years been rediscovered as a subject of considerable research interest due to their very unique physical properties (piezoelectricity, conductivity, magnetic and optical) and a wide range of possible device applications. Special care is directed to optical and magnetic memory devices, laser systems, blue light diodes, solar cells (transparent conducting electrodes), displays, ultrasonic transducers and sensors [1–3]. Since ZnO has the hexagonal structure (Wurtzite type) with four-fold tetrahedral coordination [4] and lies in the border between ionic and covalent semiconductors (II– VI), it is both suitable and important for fabrication of a high-quality oriented or epitaxial thin film which shows typical n-type piezoelectric semiconductor character. This is besides the fact that ZnO-based semiconductors can cover the same wavelength as GaN with an excitonic band energy much larger than that of GaN with large *Corresponding author. Fax: +20-88-342708. E-mail address: [email protected] (El-Maghraby Mohamed).
direct band-gap (3.1–3.32 eV). Hence, ZnO-based thin films are increasingly being used as potential materials for highly coherent light generation and in optical devices in the blue to UV wavelength region. In order to develop ZnO films with high quality for devices with good performance, it is necessary to clarify the role and effect of so many additives, different conditions of growth and substrate types. This will result in different microstructures being suitable for different applications. Now, it is well established that doping ZnO with Al ions decreases its resistivity while doping with Li ions increases its resistivity [5–8] and induces a ferroelectric phase suitable for optical memory devices [5,9–10]. The study of optical constants with their variation with frequency enables to make correlation with the band structures derived by other methods. In this research, ZnO : Li thin films have been prepared on sapphire, quartz and MgO substrates at B573 K by DC magnetron sputtering method. Our goal is to investigate growth and microstructure of ZnO : Li thin films. Another goal is to measure the absorption, opt reflectance and determine the optical band-gap (Egd ), 0 the high frequency dielectric constant eN and the carrier
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 8 8 4 - 5
950
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
concentration N for the as-prepared films at room temperature and discuss its dependence on the type of substrate.
2. Experimental Using DC magnetron sputtering method, we prepared ZnO : Li thin films deposited on sapphire MgO and quartz substrates. The sputtering target was a mixture of Li powder and Zn powder, with atomic ratio of Zn/Li (80/20), pressed on a copper saucer with diameter of 100 mm. Li powder was mixed in 20% excess in order to compensate the loss of Li during deposition. It is necessary to continue sputtering for over 40 h to make the target surface Li-rich and the film composition was saturated to the value nearly equal to the target composition. The deposition time for this experiment is fixed to be about 3 h long. When the film samples were deposited, presputtering was carried out for 5 min before opening the shutter which covered the substrate. The discharge was a mixture of Ar and O2 with the ratio 70 : 30 and its pressure was about 5 mTorr. The voltage was fixed to 310 V and the current was about 110 mA. The substrate temperature was fixed at 573 K. After deposition, the samples were immediately cooled to room temperature. The identification of the crystalline structure and surface morphology were performed by Xray diffraction analysis (XRD) (Phlips Type: 1710) using ( and scanning electron Cu Ka radiation (l ¼ 1:5418 A) microscope (SEM Jeol type: JSM-5400 LV). The optical transmittance and reflectance of ZnO : Li films were recorded using Shimadzu UV–VIS 2101 PC dual beam spectrometer in the wavelength range from 190 to 900 nm. The measurements were carried out at room temperature with surrounding medium of dried air.
3. Results and discussion Typical XRD measurements of the as -deposited films are shown in Fig. 1(a). It is clear that the type of substrate plays an important role in the crystallinity of the ZnO : Li films. The films deposited onto quartz substrates showed a large peak at 2yD341; which was attributed to (0 0 2) ZnO. This indicates that the deposited film has a good orientation along the c-axis. For films deposited on MgO and Sapphire substrates, caxis ordering is degraded as shown in Fig. 1(a) but they still give the Wurtzite hexagonal structure. SEM images revealed smooth surface morphology without cracks and hillocks, see Fig. 1(b). The grain distribution is uniform but dense microstructure is relevant in films deposited onto quartz substrate. The optical transmission and reflection spectra are given in Figs. 2 and 3 in the wavelength from 300 to
800 nm. The observed high transmitivity of these films suggests that ZnO : Li films have good transparency, especially above 700 nm wavelength. The total absorption coefficient was calculated using the relation [11] " 0:5 # 1 ð1 RÞ2 ð1 RÞ4 2 a ¼ ln þ R ; ð1Þ d 2T 4T 2 where d is the thickness, R is the reflectance and T is the transmittance. Generally, the equation (written here in a simplified form) used to determine the band-gap nature and the value of Eg (the gap energy) is [12] ðahnÞ ¼ Aðhn E 0 Þn :
ð2Þ
Here, A is a constant that depends on the transition, hn opt is the photon energy and E 0 the transition energy (Egd opt for direct transitions, Egin 7Ep for indirect transitions [13], i.e. Ep being the energy of the associated phonon) and the value of the exponent n depends on the nature of the optical transition type (n ¼ 12; 13; 2; 23 for allowed and forbidden indirect transitions, and for allowed and forbidden direct ones, respectively). An analysis of the absorption spectrum of Fig. 2 shows that the absorption coefficient can be reasonably well fitted by Eq. (2) with n ¼ 12: This means that (ZnO : Li ) films deposited on sapphire, MgO and quartz substrates have an allowed direct interband transition which corresponds to the fundamental absorption edge of these films. Fig. 4 shows the plot of ðahnÞ2 versus photon energy near the absorption edge of ZnO : Li. Values of the optical energy gap for films deposited on sapphire, MgO or quartz substrates are obtained by extrapolating the linear regions to ðahnÞ2 ¼ 0: Values of the room opt temperature optical energy gap Egd and the constant A are listed in Table 1. It is clear that the obtained opt values of Egd depend on the substrate type. Also, the admixture of ZnO with Li ions generally increases the opt value of Egd over that of pure ZnO films. We can say that Li doping converts the shallow donor Zn sublevels to deep sublevels below the conduction band. This coincides with the results of Sbrveglieri et al. [14]. They conclude that Li ions lead to the formation of two deeplevel donors in ZnO thin films which are, respectively, 0.38 and 0.81 eV below the conduction band, while the undoped films show two levels at 0.16 and 0.48 eV. In opt the case of the quartz substrate, the value of Egd is near to that of pure ZnO films which may be attributed to the very good c-axis orientation in this case (see, Fig. 1 (a)). For a better understanding of the physical properties of ZnO : Li films, it is interesting to study some optical constants used to describe the optical properties, e.g. the high-frequency dielectric constant e0N and the relative carrier concentration N/m*.
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G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
Fig. 1. (a) X-ray diffraction patterns and (b) SEM images of the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K. Table 1 Optical constants for as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates Physical quantity
Films deposited on sapphire substrates
Films deposited on MgO substrates
Films deposited on quartz substrates
opt Direct allowed energy gap Egd (eV) 12 The constant (A 10 ) High-frequency dielectric constant (e0N ) N=m (1021 cm3)
3.38470.05 2.45 8.3370.05 8.0770.05
3.42570.05 3.61 9.0670.05 8.6570.05
3.28570.05 3.93 4.1370.05 3.3970.05
Accordingly [15], the real component of relative permittivity (e0 ) and the square of the wavelength (l2 ) are related by
¼ e0 e2 =pc2 N=mn l2 ;
e0 ¼ n2 ¼ ð1 þ
pffiffiffiffi pffiffiffiffi R=lð1 RÞÞ2 ;
ð3Þ
where n is the refractive index, e is the electronic charge and c the velocity of light. From this equation, the high-frequency component of the relative permittivity (e0N ) and the ratio of the carrier
952
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
Fig. 2. Spectral distribution of the optical transmittance for the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K.
Fig. 4. Relation between ðahnÞ2 and photon energy (hn) for the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K.
with those in Fig. 3, most of the changes in the N/m* ratio are regarded as corresponding to the change in N:
4. Summary
Fig. 3. Spectral distribution of the optical reflectance for the asprepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K. The inset is the real dielectric permittivity (e0 ) versus l2 :
concentration to the effective mass N/m* could be determined. The inset of Fig. 3 shows the relation between relative permittivity (e0 ) and l2 for (ZnO : Li ) films deposited on sapphire, MgO and quartz substrates. It can be noticed that relative permittivity decreases exponentially with increasing l2 : To obtain the high-frequency dielectric constant, Eq. (4) was applied on the linear parts of these curves. The values of e0N and N/m* are given in Table 1. In general, it can be concluded that both the the highfrequency dielectric constant and the ratio N/m* are related to the internal microstructure, as can be emphasized by considering the results in Table 1 together
Zn1xLixO films with x ¼ 0:2 onto sapphire, MgO and quartz substrates are prepared by DC sputtering method at about 573 K. The films are highly c-axisoriented, normal to the surface with high transmittance in the case of quartz substrate, while non-uniform orientation with reduced transmittance is obtained for both sapphire and MgO substrates. The films show opt direct allowed optical transitions with Egd values of 3.38, 3.43 and 3.29 eV for films deposited on sapphire, MgO and quartz substrates, respectively. Values of e0N and N/m* are dependent on the substrate kind. It is important to study the effect of temperature on the optical properties of ZnO : Li thin films. These films are good to examine the ferroelectric phase transition at 613 K by optical measurements at high temperatures above ambient, this is now under study. In this case, the present samples will be very suitable for applications in optical memory devices.
Acknowledgements We do thank Prof. T. Yamazaki of Toyama university, Japan, for the facilities provided during the preperation of the specimens.
References [1] F.S. Hickernell, IEEE Trans. MTT 17 (1969) 957.
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953 [2] C. Campbell, Surface Acoustic Wave Devices and Their Signal Processing Applications, San Diego, Academic Press, 1989. [3] T. Shiozaki. M. Ooishi, S. Ohmishi, A. Kawabata, Proceedings of the First Meeting on Ferroelectric Materials and Their Applications, Kyoto, 1977, p. 43. [4] H.D. Megaw, Crystal Structures, A Working Approach, W.B. Sounders Company, Philadelphia, 1973, p. 88. [5] E.D. Kolb, R.A. Laudise, J. Am. Ceram. Soc. 49 (1966) 302. [6] S. Takada, J. Appl. Phys. 73 (10) (1993) 4739. [7] F.S. Mahmood, R.D. Gould, Thin Solid Films 253 (1994) 529. [8] K. Tominaga, M. Kataoka, T. Ueda, M. Chong, Y. Shintani, I. Mori, Thin Solid Films 9 (1994) 253.
953
[9] Z.C. Jin, I. Hamberg, C.G. Granqvist, J. Appl. Phys. 64 (1988) 5117. [10] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (19) (1999) 2534. [11] Pankove, J.I., Optical Processes in Semiconductors, Dover, New York, 1971, p. 103. [12] T.S. Moss, Semiconductor Opto-electronics, Butterworths, London, 1973, p. 48 (Chapter 3). [13] I. Watanbe, T. Okumura, Jpn. J. Appl. Phys. 25 (1986) 1851. [14] G. Sberveglieri, S. Groppelli, P. Nelli, F. Quaranta, A. Valentini, L. Valentini, Sensors Actuators B 7 (1992) 747. [15] M. Di Giulio, D. Manano, R. Rella, P. Siciliano, A. Tepore, Solar Energy Mater. 15 (1987) 209.
Optical properties and surface morphology of Li-doped ZnO thin films deposited on different substrates by DC magnetron sputtering method Galal A. Mohamed, El-Maghraby Mohamed*, A. Abu El-Fadl Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
Abstract Thin films of zinc oxide doped with Zn1xLixO with x ¼ 0:2 (ZnO : Li), have been prepared on sapphire, MgO and quartz substrates by DC magnetron sputtering method at 5 mTorr. The substrate temperatures were fixed to about 573 K. We have measured the transmission and reflection spectra and determined the absorption coefficient, optical opt ), the high frequency dielectric constant e0N and the carrier concentration N for the as-prepared films at band-gap (Egd opt room temperature. The films show direct allowed optical transitions with Egd values of 3.38, 3.43 and 3.29 eV for films deposited on sapphire, MgO and quartz substrates, respectively. The dependence of the obtained results on the substrate type are discussed. r 2001 Elsevier Science B.V. All rights reserved. Keywords: ZnO : Li films; DC sputtering; Band gap; Optical properties
1. Introduction Among different oxides, ZnO thin films (undoped and doped) have in recent years been rediscovered as a subject of considerable research interest due to their very unique physical properties (piezoelectricity, conductivity, magnetic and optical) and a wide range of possible device applications. Special care is directed to optical and magnetic memory devices, laser systems, blue light diodes, solar cells (transparent conducting electrodes), displays, ultrasonic transducers and sensors [1–3]. Since ZnO has the hexagonal structure (Wurtzite type) with four-fold tetrahedral coordination [4] and lies in the border between ionic and covalent semiconductors (II– VI), it is both suitable and important for fabrication of a high-quality oriented or epitaxial thin film which shows typical n-type piezoelectric semiconductor character. This is besides the fact that ZnO-based semiconductors can cover the same wavelength as GaN with an excitonic band energy much larger than that of GaN with large *Corresponding author. Fax: +20-88-342708. E-mail address: [email protected] (El-Maghraby Mohamed).
direct band-gap (3.1–3.32 eV). Hence, ZnO-based thin films are increasingly being used as potential materials for highly coherent light generation and in optical devices in the blue to UV wavelength region. In order to develop ZnO films with high quality for devices with good performance, it is necessary to clarify the role and effect of so many additives, different conditions of growth and substrate types. This will result in different microstructures being suitable for different applications. Now, it is well established that doping ZnO with Al ions decreases its resistivity while doping with Li ions increases its resistivity [5–8] and induces a ferroelectric phase suitable for optical memory devices [5,9–10]. The study of optical constants with their variation with frequency enables to make correlation with the band structures derived by other methods. In this research, ZnO : Li thin films have been prepared on sapphire, quartz and MgO substrates at B573 K by DC magnetron sputtering method. Our goal is to investigate growth and microstructure of ZnO : Li thin films. Another goal is to measure the absorption, opt reflectance and determine the optical band-gap (Egd ), 0 the high frequency dielectric constant eN and the carrier
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 8 8 4 - 5
950
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
concentration N for the as-prepared films at room temperature and discuss its dependence on the type of substrate.
2. Experimental Using DC magnetron sputtering method, we prepared ZnO : Li thin films deposited on sapphire MgO and quartz substrates. The sputtering target was a mixture of Li powder and Zn powder, with atomic ratio of Zn/Li (80/20), pressed on a copper saucer with diameter of 100 mm. Li powder was mixed in 20% excess in order to compensate the loss of Li during deposition. It is necessary to continue sputtering for over 40 h to make the target surface Li-rich and the film composition was saturated to the value nearly equal to the target composition. The deposition time for this experiment is fixed to be about 3 h long. When the film samples were deposited, presputtering was carried out for 5 min before opening the shutter which covered the substrate. The discharge was a mixture of Ar and O2 with the ratio 70 : 30 and its pressure was about 5 mTorr. The voltage was fixed to 310 V and the current was about 110 mA. The substrate temperature was fixed at 573 K. After deposition, the samples were immediately cooled to room temperature. The identification of the crystalline structure and surface morphology were performed by Xray diffraction analysis (XRD) (Phlips Type: 1710) using ( and scanning electron Cu Ka radiation (l ¼ 1:5418 A) microscope (SEM Jeol type: JSM-5400 LV). The optical transmittance and reflectance of ZnO : Li films were recorded using Shimadzu UV–VIS 2101 PC dual beam spectrometer in the wavelength range from 190 to 900 nm. The measurements were carried out at room temperature with surrounding medium of dried air.
3. Results and discussion Typical XRD measurements of the as -deposited films are shown in Fig. 1(a). It is clear that the type of substrate plays an important role in the crystallinity of the ZnO : Li films. The films deposited onto quartz substrates showed a large peak at 2yD341; which was attributed to (0 0 2) ZnO. This indicates that the deposited film has a good orientation along the c-axis. For films deposited on MgO and Sapphire substrates, caxis ordering is degraded as shown in Fig. 1(a) but they still give the Wurtzite hexagonal structure. SEM images revealed smooth surface morphology without cracks and hillocks, see Fig. 1(b). The grain distribution is uniform but dense microstructure is relevant in films deposited onto quartz substrate. The optical transmission and reflection spectra are given in Figs. 2 and 3 in the wavelength from 300 to
800 nm. The observed high transmitivity of these films suggests that ZnO : Li films have good transparency, especially above 700 nm wavelength. The total absorption coefficient was calculated using the relation [11] " 0:5 # 1 ð1 RÞ2 ð1 RÞ4 2 a ¼ ln þ R ; ð1Þ d 2T 4T 2 where d is the thickness, R is the reflectance and T is the transmittance. Generally, the equation (written here in a simplified form) used to determine the band-gap nature and the value of Eg (the gap energy) is [12] ðahnÞ ¼ Aðhn E 0 Þn :
ð2Þ
Here, A is a constant that depends on the transition, hn opt is the photon energy and E 0 the transition energy (Egd opt for direct transitions, Egin 7Ep for indirect transitions [13], i.e. Ep being the energy of the associated phonon) and the value of the exponent n depends on the nature of the optical transition type (n ¼ 12; 13; 2; 23 for allowed and forbidden indirect transitions, and for allowed and forbidden direct ones, respectively). An analysis of the absorption spectrum of Fig. 2 shows that the absorption coefficient can be reasonably well fitted by Eq. (2) with n ¼ 12: This means that (ZnO : Li ) films deposited on sapphire, MgO and quartz substrates have an allowed direct interband transition which corresponds to the fundamental absorption edge of these films. Fig. 4 shows the plot of ðahnÞ2 versus photon energy near the absorption edge of ZnO : Li. Values of the optical energy gap for films deposited on sapphire, MgO or quartz substrates are obtained by extrapolating the linear regions to ðahnÞ2 ¼ 0: Values of the room opt temperature optical energy gap Egd and the constant A are listed in Table 1. It is clear that the obtained opt values of Egd depend on the substrate type. Also, the admixture of ZnO with Li ions generally increases the opt value of Egd over that of pure ZnO films. We can say that Li doping converts the shallow donor Zn sublevels to deep sublevels below the conduction band. This coincides with the results of Sbrveglieri et al. [14]. They conclude that Li ions lead to the formation of two deeplevel donors in ZnO thin films which are, respectively, 0.38 and 0.81 eV below the conduction band, while the undoped films show two levels at 0.16 and 0.48 eV. In opt the case of the quartz substrate, the value of Egd is near to that of pure ZnO films which may be attributed to the very good c-axis orientation in this case (see, Fig. 1 (a)). For a better understanding of the physical properties of ZnO : Li films, it is interesting to study some optical constants used to describe the optical properties, e.g. the high-frequency dielectric constant e0N and the relative carrier concentration N/m*.
951
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
Fig. 1. (a) X-ray diffraction patterns and (b) SEM images of the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K. Table 1 Optical constants for as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates Physical quantity
Films deposited on sapphire substrates
Films deposited on MgO substrates
Films deposited on quartz substrates
opt Direct allowed energy gap Egd (eV) 12 The constant (A 10 ) High-frequency dielectric constant (e0N ) N=m (1021 cm3)
3.38470.05 2.45 8.3370.05 8.0770.05
3.42570.05 3.61 9.0670.05 8.6570.05
3.28570.05 3.93 4.1370.05 3.3970.05
Accordingly [15], the real component of relative permittivity (e0 ) and the square of the wavelength (l2 ) are related by
¼ e0 e2 =pc2 N=mn l2 ;
e0 ¼ n2 ¼ ð1 þ
pffiffiffiffi pffiffiffiffi R=lð1 RÞÞ2 ;
ð3Þ
where n is the refractive index, e is the electronic charge and c the velocity of light. From this equation, the high-frequency component of the relative permittivity (e0N ) and the ratio of the carrier
952
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953
Fig. 2. Spectral distribution of the optical transmittance for the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K.
Fig. 4. Relation between ðahnÞ2 and photon energy (hn) for the as-prepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K.
with those in Fig. 3, most of the changes in the N/m* ratio are regarded as corresponding to the change in N:
4. Summary
Fig. 3. Spectral distribution of the optical reflectance for the asprepared ZnO : Li films deposited on sapphire, MgO and quartz substrates at 573 K. The inset is the real dielectric permittivity (e0 ) versus l2 :
concentration to the effective mass N/m* could be determined. The inset of Fig. 3 shows the relation between relative permittivity (e0 ) and l2 for (ZnO : Li ) films deposited on sapphire, MgO and quartz substrates. It can be noticed that relative permittivity decreases exponentially with increasing l2 : To obtain the high-frequency dielectric constant, Eq. (4) was applied on the linear parts of these curves. The values of e0N and N/m* are given in Table 1. In general, it can be concluded that both the the highfrequency dielectric constant and the ratio N/m* are related to the internal microstructure, as can be emphasized by considering the results in Table 1 together
Zn1xLixO films with x ¼ 0:2 onto sapphire, MgO and quartz substrates are prepared by DC sputtering method at about 573 K. The films are highly c-axisoriented, normal to the surface with high transmittance in the case of quartz substrate, while non-uniform orientation with reduced transmittance is obtained for both sapphire and MgO substrates. The films show opt direct allowed optical transitions with Egd values of 3.38, 3.43 and 3.29 eV for films deposited on sapphire, MgO and quartz substrates, respectively. Values of e0N and N/m* are dependent on the substrate kind. It is important to study the effect of temperature on the optical properties of ZnO : Li thin films. These films are good to examine the ferroelectric phase transition at 613 K by optical measurements at high temperatures above ambient, this is now under study. In this case, the present samples will be very suitable for applications in optical memory devices.
Acknowledgements We do thank Prof. T. Yamazaki of Toyama university, Japan, for the facilities provided during the preperation of the specimens.
References [1] F.S. Hickernell, IEEE Trans. MTT 17 (1969) 957.
G.A. Mohamed et al. / Physica B 308–310 (2001) 949–953 [2] C. Campbell, Surface Acoustic Wave Devices and Their Signal Processing Applications, San Diego, Academic Press, 1989. [3] T. Shiozaki. M. Ooishi, S. Ohmishi, A. Kawabata, Proceedings of the First Meeting on Ferroelectric Materials and Their Applications, Kyoto, 1977, p. 43. [4] H.D. Megaw, Crystal Structures, A Working Approach, W.B. Sounders Company, Philadelphia, 1973, p. 88. [5] E.D. Kolb, R.A. Laudise, J. Am. Ceram. Soc. 49 (1966) 302. [6] S. Takada, J. Appl. Phys. 73 (10) (1993) 4739. [7] F.S. Mahmood, R.D. Gould, Thin Solid Films 253 (1994) 529. [8] K. Tominaga, M. Kataoka, T. Ueda, M. Chong, Y. Shintani, I. Mori, Thin Solid Films 9 (1994) 253.
953
[9] Z.C. Jin, I. Hamberg, C.G. Granqvist, J. Appl. Phys. 64 (1988) 5117. [10] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (19) (1999) 2534. [11] Pankove, J.I., Optical Processes in Semiconductors, Dover, New York, 1971, p. 103. [12] T.S. Moss, Semiconductor Opto-electronics, Butterworths, London, 1973, p. 48 (Chapter 3). [13] I. Watanbe, T. Okumura, Jpn. J. Appl. Phys. 25 (1986) 1851. [14] G. Sberveglieri, S. Groppelli, P. Nelli, F. Quaranta, A. Valentini, L. Valentini, Sensors Actuators B 7 (1992) 747. [15] M. Di Giulio, D. Manano, R. Rella, P. Siciliano, A. Tepore, Solar Energy Mater. 15 (1987) 209.