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The effects of deposition conditions on the structural properties of ZnO sputtered films on sapphire substrates
Applied Surface Science 169±170 (2001) 512±516
The effects of deposition conditions on the structural properties of ZnO sputtered ®lms on sapphire substrates Yasuhiro Igasakia,*, Takashi Naitoa, Kenji Murakamia, Waichi Tomodab a
Research Institute of Electronics, Shizuoka University, Johoku 3-5-1, Hmamatsu 432-8011, Japan Center for Joint Research, Shizuoka University, Shin-Miyakoda 1-3-4, Hamamatsu 431-2103, Japan
b
Received 30 July 1999; accepted 8 November 1999
Abstract Zinc oxide (ZnO) ®lms were deposited on (1 1 2 0) or (0 0 0 1) oriented sapphire substrates heated up to 8008C with a radio frequency (rf) power ranging from 40 to 200 W at an argon gas pressure range 0.08±11.7 Pa by rf magnetron sputtering from a ZnO target, and the dependence of structural properties of these ®lms on the preparation conditions was studied by using XRD, RHEED, SEM and AFM. The results obtained by XRD and RHEED measurements showed that ®lms deposited on sapphire (1 1 2 0) plane were (0 0 0 1) oriented heteroepitaxially grown ®lms of mosaic structure independently of the deposition conditions and the crystallinity of ®lms was improved with increase in ®lm thickness and substrate temperature, and that most of the ®lms grown on sapphire (0 0 0 1) plane consisted of (0 0 0 1) oriented ®ber-texture crystallites, the degree of whose a-axis ordering was changed depending on the deposition conditions. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Zinc oxide ®lm; ZnO; Epitaxial growth; Crystal structure; Sputtering; rf magnetron sputtering
1. Introduction Zinc oxide is a wide band gap II±VI compound semiconductor with the wurtzite structure. In many applications, such as SAW devices, transparent conducting electrodes and optical waveguide, polycrystalline ®lms have been used. However, in the applications such as buffer layers for the epitaxial growth of III±V nitride on to sapphire substrate [1] and room temperature ultraviolet laser using selfassembled ZnO microcrystallite thin ®lm on sapphire substrate [2], highly oriented heteroepitaxial ®lms are needed and have been deposited by pulsed laser
* Corresponding author. Tel./fax: 81-53-478-1308. E-mail address: [email protected] (Y. Igasaki).
deposition and laser molecular beam epitaxy technique. On the other hand, we have attempted the use of sputtering method most commonly used as one of thin ®lm techniques to grow the highly oriented heteroepitaxial ZnO ®lms [3±5]. In this work, we report the study on the effects of deposition conditions such as substrate temperature, argon gas pressure and rf power on the crystallinity of ®lms deposited on sapphire (1 1 2 0) and (0 0 0 1) plane. 2. Experimental ZnO ®lms were prepared by rf magnetron sputtering. The apparatus used was a NEVA type FP-45 rf sputtering system modi®ed for magnetron sputtering, made up of a water-cooled stainless steel jar of inside
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 4 9 - 2
Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
radius 300 mm connected with a turbo molecular pump (Balzers, type TPU-520M). Magnets stored in an aluminum receptacle were installed on a watercooled cathode. A sintered disc of ZnO (purity, 99.99%), 125 mm in diameter, was used as a target. A substrate-heating device was mounted on a watercooled anode. The distance between target and substrate was about 50 or 70 mm. The residual gas and the sputtering gas were monitored with a B-A type ionization gauge (ANELVA, type MIG-430). Sapphire (0 0 0 1) or (1 1 2 0) plane was used as a substrate. The substrates were ultrasonically cleaned in a weak alkaline cleaning solution provided by Furuuchi Chemical Laboratory, in acetone and ®nally in methyl alcohol. Prior to pre-sputtering, the jar and the substrates were heated, respectively at several 10 degrees and at about 8008C for degassing the equipment. After the jar was evacuated to a pressure below 1 10ÿ4 Pa, presputtering of 10 min was carried out at an argon gas pressure of 0.65 or 0.13 Pa with an rf power of 100 W. A constant ¯ow of argon gas was maintained at 1.0, 3.0 or 5.0 ccm with a STEC type SEC-400 Mark 3 mass ¯ow controller. After the pre-sputtering, the jar was re-evacuated to a pressure below 5 10ÿ5 Pa. ZnO ®lms were deposited on the substrates heated to 200±8008C with an rf power ranging from 40 to 200 W at an argon pressure in the range 0.08±11.7 Pa. Deposition time was 5±230 min. A ®nal thickness and the deposition rate were 90±2580 nm and 4±24 nm/ min, respectively. Film thickness was determined with a surface roughness detector DEKTAK IIA. The crystal structure was studied by X-ray diffraction using Cu Ka line with a RIGAKU type RAD-IIA and by electron diffraction with a JEOL type JEM100U. The correction for measured values of 2y was made by using a diffraction line from sapphire substrate. A ®eld emission type scanning microscope (JEOL, type JSM-6320F) and an atomic force microscope (SII, type SPI-3700) were used for the observation of surface morphology. 3. Results and discussion In order to explain the dependence of structural properties on the deposition conditions and/or the orientation of sapphire substrates, we investigated
513
the changes in structural characteristics such as crystal structure and surface morphology. Re¯ection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) were employed to determine the crystal structure and epitaxial relationship between the ®lm and the substrate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used for observation of the surface morphology. In the ®lms grown on (1 1 2 0) and (0 0 0 1) sapphire substrates, no extra-peak other than (0 0 0 1) peak from ZnO and (1 1 2 0) or (0 0 0 1) peak from sapphire was found for the X-ray diffraction scans between 30 and 70 degrees. This indicates that these ®lms were composed of (0 0 0 1) oriented ZnO crystallites alone. For the ®lms grown on (1 1 2 0) oriented sapphire, we observed two kinds of RHEED patterns which correspond to the diffraction patterns of the two major zones perpendicular to basal plane. Each of them was found to repeat every 60 degrees, and the patterns alternated every 30 degrees as the ®lm was rotated in the beam. This is the type of behavior expected from a crystal with a sixfold symmetry. From these results, we concluded that ®lms deposited on (1 1 2 0) sapphire substrates were (0 0 0 1) oriented heteroepitaxially grown ZnO ®lms. On the other hand, the RHEED pattern from most of the ®lm grown on (0 0 0 1) oriented sapphire was a mixture of some patterns which could be observed as ZnO single crystal was rotated in the electron beam whose incidental azimuth was perpendicular to the c-axis. This indicates that the structure of these ®lms was (0 0 0 1) textured one. However, the degree of a-axis ordering in the substrate surface was changed depending on the preparation conditions. Fig. 1 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of ®lm thickness or substrate temperature. The ®lms were deposited on (1 1 2 0) sapphire substrate (10 mm 10 mm) heated to 2008C (*), 4008C (*), 7008C (&), and 8008C (^) with rf power of 100 W at Ar pressure of 0.13 Pa. The X-ray diffraction angle 2y of thicker ®lms than 500 nm is in the range from 34.39 to 34.43 degrees, and in agreement with the value of 34.44 degrees for bulk materials (solid line) within experimental errors. A degree of deviation of 2y for ®lm from the 2y for bulk materials re¯ects the degree of homogeneous distortion in ®lm which is mainly ascribed to the discrepancy of thermal expansion
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Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
Fig. 1. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of ®lm thickness or substrate temperature. The ®lms were deposited on (1 1 2 0) sapphire substrates heated to 2008C (*), 4008C (*), 7008C (&), or 8008C (^) with rf power of 100 W at Ar pressure of 0.13 Pa.
and/or lattice mismatch between the ®lm and the substrate. Therefore, no detectable homogeneous distortion is considered to exist in these ®lms. Half widths (FWHM) of diffraction peaks decrease with increase in ®lm thickness and/or substrate temperature. The values less than 0.09 degrees correspond to those of the diffraction peaks for Cu Ka1 line. This means that thicker ®lms than 500 nm deposited at 8008C had so good crystallinity that could monochromize the incident Cu Ka doublet into Cu Ka1 and Cu Ka2. In the same manner as FWHM, the standard deviation s of the rocking curve on (0 0 0 2) diffraction decreases with increase in ®lm thickness and/or substrate temperature. s for thicker ®lms than 500 nm deposited at 8008C is very small but relatively larger than the value of diffraction peak from sapphire substrate (0.06 degrees). From this, we can conclude that ®lms deposited on (1 1 2 0) oriented sapphire substrates do not consist of one heteroepitaxial ®lm but mosaic of heteroepitaxial crystallites whose c-axis is slightly tilted from substrate normal. These results indicate that the crystallinity of epitaxially grown ZnO ®lm was improved with increasing ®lm thickness and/ or growth temperature.
Fig. 2. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of Ar pressure. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power of 100 W. Solid marks represent ®lms on 10 mm 10 mm sapphire and open marks ®lms on 5 mm 5 mm sapphire.
Fig. 2 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of Ar pressure during deposition. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power of 100 W at Ar pressure range 0.07±11.7 Pa. Solid and open marks represent the ®lms deposited on 10 mm 10 mm and 5 mm 5 mm substrates, respectively. The ®lm thickness was in the range 350±670 nm. 2y increases as Ar pressure is increased regardless of substrate used but 2y of ®lm on (0 0 0 1) plane is larger than one of ®lm on (1 1 2 0) plane. The difference between 2y of ®lm on (1 1 2 0) sapphire and 2y of ®lm on (0 0 0 1) sapphire is mainly ascribed to the discrepancy of thermal expansion and/or lattice mismatch between the ®lm and the substrate. However, thermal expansion perpendicular to c-axis a?c of ZnO in temperature range 300±800 K is nearly equal to a?c and a==c of sapphire [6,7]. Therefore, the difference between the 2y in ®gure may be caused by lattice mismatch and/or experimental errors. On the other hand, the results for FWHM, s and I show that the crystallinity of ®lms on
Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
515
crystallinity of ®lms on both substrates is almost independent of rf power, and that the crystallinity of ®lms on (1 1 2 0) sapphire are generally better than that of ®lms on (0 0 0 1) sapphire. The RHEED pattern showed that ®lm on (0 0 0 1) sapphire deposited with rf power range 160±200 W was a heteroepitaxially grown ®lm but other ®lm on (0 0 0 1) sapphire deposited with lower power was a ®ber textured ®lm. 4. Conclusions
Fig. 3. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of rf power. The ®lms were deposited at 8008C at Ar pressure of 0.13 Pa. Marks correspond to those in Fig. 2.
sapphire (1 1 2 0) plane is almost independent of Ar pressure but one of ®lms on (0 0 0 1) sapphire improves as Ar pressure is increased, and that the former is generally better than the latter. The RHEED pattern showed that ®lm on (0 0 0 1) sapphire deposited at Ar pressure of 0.13, 0.9 or 11.7 Pa was a heteroepitaxially grown ®lm but other ®lms on (0 0 0 1) sapphire were ®ber textured ®lms. Fig. 3 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of rf power. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power range 40±200 W at Ar pressure of 0.13 Pa. Solid and open marks represent the ®lms deposited on 10 mm 10 mm and 5 mm 5 mm sapphire substrates, respectively. rf power is closely connected with the deposition rate of ®lm. In the present case, rf power ranging from 40 to 200 W corresponds to deposition rate range 5±21 nm/min. Final thickness of these ®lms was in the range from 440 to 2000 nm. 2y of ®lms is almost constant regardless of rf power and substrate used. On the other hand, the results for FWHM, s and I indicate that the
ZnO ®lms were deposited on sapphire (0 0 0 1) and (1 1 2 0) substrates by rf magnetron sputtering from a ZnO target and the dependence of structural properties on preparation conditions, such as substrate temperature, Ar gas pressure and rf power, was studied. The results obtained from XRD and RHEED measurements are as follows: 1. ZnO ®lms deposited on (1 1 2 0) sapphire substrates were (0 0 0 1) oriented heteroepitaxial ®lms with mosaic structure regardless of deposition conditions, but the degree of c-axis ordering of mosaics was improved with increase in substrate temperature and/or ®lm thickness. 2. The structure of ZnO ®lm deposited on (0 0 0 1) sapphire heated to 8008C was mostly a ®ber texture structure but heteroepitaxial ®lm was accidentally obtained when ®lm was deposited at Ar pressure of 0.13, 0.9 or 11.7 Pa with rf power of 100 W, and at Ar pressure of 0.13 Pa with rf power of 160, 180 or 200 W. These results indicate that if we can carefully control the deposition conditions, especially at early stage of ®lm growth, heteroepitaxial ®lm can be grown on (0 0 0 1) sapphire, too. Last, in this study, the growth of ®lms composed of self-assembled microcrystallites could not be recognized through SEM and/or AFM observation. References [1] R.D. Vispute, V. Talyansky, Z. Trajanovic, S. Chooopun, M. Downes, R.P. Sharma, T. Venkatesan, M.C. Woods, R.T. Lareau, K.A. Jones, A.A. Iliadis, Appl. Phys. Lett. 70 (1997) 2735.
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Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
[2] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [3] Y. Igasaki, H. Saito, J. Appl. Phys. 69 (1991) 2190. [4] Y. Igasaki, H. Saito, J. Appl. Phys. 70 (1991) 3613. [5] Y. Igasaki, J. Cryst. Growth 116 (1992) 357.
[6] B.H. Billings et al. (Eds.), American Institute of Physics Hand Book, 3rd Edition, McGraw Hill, New York, 1972, pp. 4±136. [7] O. Madelung (Ed.), Neumerical Data and Functional Relationships in Science and Technology, Vol. 17b, Springer, New York, 1982, p. 44.
The effects of deposition conditions on the structural properties of ZnO sputtered ®lms on sapphire substrates Yasuhiro Igasakia,*, Takashi Naitoa, Kenji Murakamia, Waichi Tomodab a
Research Institute of Electronics, Shizuoka University, Johoku 3-5-1, Hmamatsu 432-8011, Japan Center for Joint Research, Shizuoka University, Shin-Miyakoda 1-3-4, Hamamatsu 431-2103, Japan
b
Received 30 July 1999; accepted 8 November 1999
Abstract Zinc oxide (ZnO) ®lms were deposited on (1 1 2 0) or (0 0 0 1) oriented sapphire substrates heated up to 8008C with a radio frequency (rf) power ranging from 40 to 200 W at an argon gas pressure range 0.08±11.7 Pa by rf magnetron sputtering from a ZnO target, and the dependence of structural properties of these ®lms on the preparation conditions was studied by using XRD, RHEED, SEM and AFM. The results obtained by XRD and RHEED measurements showed that ®lms deposited on sapphire (1 1 2 0) plane were (0 0 0 1) oriented heteroepitaxially grown ®lms of mosaic structure independently of the deposition conditions and the crystallinity of ®lms was improved with increase in ®lm thickness and substrate temperature, and that most of the ®lms grown on sapphire (0 0 0 1) plane consisted of (0 0 0 1) oriented ®ber-texture crystallites, the degree of whose a-axis ordering was changed depending on the deposition conditions. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Zinc oxide ®lm; ZnO; Epitaxial growth; Crystal structure; Sputtering; rf magnetron sputtering
1. Introduction Zinc oxide is a wide band gap II±VI compound semiconductor with the wurtzite structure. In many applications, such as SAW devices, transparent conducting electrodes and optical waveguide, polycrystalline ®lms have been used. However, in the applications such as buffer layers for the epitaxial growth of III±V nitride on to sapphire substrate [1] and room temperature ultraviolet laser using selfassembled ZnO microcrystallite thin ®lm on sapphire substrate [2], highly oriented heteroepitaxial ®lms are needed and have been deposited by pulsed laser
* Corresponding author. Tel./fax: 81-53-478-1308. E-mail address: [email protected] (Y. Igasaki).
deposition and laser molecular beam epitaxy technique. On the other hand, we have attempted the use of sputtering method most commonly used as one of thin ®lm techniques to grow the highly oriented heteroepitaxial ZnO ®lms [3±5]. In this work, we report the study on the effects of deposition conditions such as substrate temperature, argon gas pressure and rf power on the crystallinity of ®lms deposited on sapphire (1 1 2 0) and (0 0 0 1) plane. 2. Experimental ZnO ®lms were prepared by rf magnetron sputtering. The apparatus used was a NEVA type FP-45 rf sputtering system modi®ed for magnetron sputtering, made up of a water-cooled stainless steel jar of inside
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 4 9 - 2
Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
radius 300 mm connected with a turbo molecular pump (Balzers, type TPU-520M). Magnets stored in an aluminum receptacle were installed on a watercooled cathode. A sintered disc of ZnO (purity, 99.99%), 125 mm in diameter, was used as a target. A substrate-heating device was mounted on a watercooled anode. The distance between target and substrate was about 50 or 70 mm. The residual gas and the sputtering gas were monitored with a B-A type ionization gauge (ANELVA, type MIG-430). Sapphire (0 0 0 1) or (1 1 2 0) plane was used as a substrate. The substrates were ultrasonically cleaned in a weak alkaline cleaning solution provided by Furuuchi Chemical Laboratory, in acetone and ®nally in methyl alcohol. Prior to pre-sputtering, the jar and the substrates were heated, respectively at several 10 degrees and at about 8008C for degassing the equipment. After the jar was evacuated to a pressure below 1 10ÿ4 Pa, presputtering of 10 min was carried out at an argon gas pressure of 0.65 or 0.13 Pa with an rf power of 100 W. A constant ¯ow of argon gas was maintained at 1.0, 3.0 or 5.0 ccm with a STEC type SEC-400 Mark 3 mass ¯ow controller. After the pre-sputtering, the jar was re-evacuated to a pressure below 5 10ÿ5 Pa. ZnO ®lms were deposited on the substrates heated to 200±8008C with an rf power ranging from 40 to 200 W at an argon pressure in the range 0.08±11.7 Pa. Deposition time was 5±230 min. A ®nal thickness and the deposition rate were 90±2580 nm and 4±24 nm/ min, respectively. Film thickness was determined with a surface roughness detector DEKTAK IIA. The crystal structure was studied by X-ray diffraction using Cu Ka line with a RIGAKU type RAD-IIA and by electron diffraction with a JEOL type JEM100U. The correction for measured values of 2y was made by using a diffraction line from sapphire substrate. A ®eld emission type scanning microscope (JEOL, type JSM-6320F) and an atomic force microscope (SII, type SPI-3700) were used for the observation of surface morphology. 3. Results and discussion In order to explain the dependence of structural properties on the deposition conditions and/or the orientation of sapphire substrates, we investigated
513
the changes in structural characteristics such as crystal structure and surface morphology. Re¯ection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) were employed to determine the crystal structure and epitaxial relationship between the ®lm and the substrate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used for observation of the surface morphology. In the ®lms grown on (1 1 2 0) and (0 0 0 1) sapphire substrates, no extra-peak other than (0 0 0 1) peak from ZnO and (1 1 2 0) or (0 0 0 1) peak from sapphire was found for the X-ray diffraction scans between 30 and 70 degrees. This indicates that these ®lms were composed of (0 0 0 1) oriented ZnO crystallites alone. For the ®lms grown on (1 1 2 0) oriented sapphire, we observed two kinds of RHEED patterns which correspond to the diffraction patterns of the two major zones perpendicular to basal plane. Each of them was found to repeat every 60 degrees, and the patterns alternated every 30 degrees as the ®lm was rotated in the beam. This is the type of behavior expected from a crystal with a sixfold symmetry. From these results, we concluded that ®lms deposited on (1 1 2 0) sapphire substrates were (0 0 0 1) oriented heteroepitaxially grown ZnO ®lms. On the other hand, the RHEED pattern from most of the ®lm grown on (0 0 0 1) oriented sapphire was a mixture of some patterns which could be observed as ZnO single crystal was rotated in the electron beam whose incidental azimuth was perpendicular to the c-axis. This indicates that the structure of these ®lms was (0 0 0 1) textured one. However, the degree of a-axis ordering in the substrate surface was changed depending on the preparation conditions. Fig. 1 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of ®lm thickness or substrate temperature. The ®lms were deposited on (1 1 2 0) sapphire substrate (10 mm 10 mm) heated to 2008C (*), 4008C (*), 7008C (&), and 8008C (^) with rf power of 100 W at Ar pressure of 0.13 Pa. The X-ray diffraction angle 2y of thicker ®lms than 500 nm is in the range from 34.39 to 34.43 degrees, and in agreement with the value of 34.44 degrees for bulk materials (solid line) within experimental errors. A degree of deviation of 2y for ®lm from the 2y for bulk materials re¯ects the degree of homogeneous distortion in ®lm which is mainly ascribed to the discrepancy of thermal expansion
514
Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
Fig. 1. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of ®lm thickness or substrate temperature. The ®lms were deposited on (1 1 2 0) sapphire substrates heated to 2008C (*), 4008C (*), 7008C (&), or 8008C (^) with rf power of 100 W at Ar pressure of 0.13 Pa.
and/or lattice mismatch between the ®lm and the substrate. Therefore, no detectable homogeneous distortion is considered to exist in these ®lms. Half widths (FWHM) of diffraction peaks decrease with increase in ®lm thickness and/or substrate temperature. The values less than 0.09 degrees correspond to those of the diffraction peaks for Cu Ka1 line. This means that thicker ®lms than 500 nm deposited at 8008C had so good crystallinity that could monochromize the incident Cu Ka doublet into Cu Ka1 and Cu Ka2. In the same manner as FWHM, the standard deviation s of the rocking curve on (0 0 0 2) diffraction decreases with increase in ®lm thickness and/or substrate temperature. s for thicker ®lms than 500 nm deposited at 8008C is very small but relatively larger than the value of diffraction peak from sapphire substrate (0.06 degrees). From this, we can conclude that ®lms deposited on (1 1 2 0) oriented sapphire substrates do not consist of one heteroepitaxial ®lm but mosaic of heteroepitaxial crystallites whose c-axis is slightly tilted from substrate normal. These results indicate that the crystallinity of epitaxially grown ZnO ®lm was improved with increasing ®lm thickness and/ or growth temperature.
Fig. 2. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of Ar pressure. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power of 100 W. Solid marks represent ®lms on 10 mm 10 mm sapphire and open marks ®lms on 5 mm 5 mm sapphire.
Fig. 2 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of Ar pressure during deposition. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power of 100 W at Ar pressure range 0.07±11.7 Pa. Solid and open marks represent the ®lms deposited on 10 mm 10 mm and 5 mm 5 mm substrates, respectively. The ®lm thickness was in the range 350±670 nm. 2y increases as Ar pressure is increased regardless of substrate used but 2y of ®lm on (0 0 0 1) plane is larger than one of ®lm on (1 1 2 0) plane. The difference between 2y of ®lm on (1 1 2 0) sapphire and 2y of ®lm on (0 0 0 1) sapphire is mainly ascribed to the discrepancy of thermal expansion and/or lattice mismatch between the ®lm and the substrate. However, thermal expansion perpendicular to c-axis a?c of ZnO in temperature range 300±800 K is nearly equal to a?c and a==c of sapphire [6,7]. Therefore, the difference between the 2y in ®gure may be caused by lattice mismatch and/or experimental errors. On the other hand, the results for FWHM, s and I show that the crystallinity of ®lms on
Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
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crystallinity of ®lms on both substrates is almost independent of rf power, and that the crystallinity of ®lms on (1 1 2 0) sapphire are generally better than that of ®lms on (0 0 0 1) sapphire. The RHEED pattern showed that ®lm on (0 0 0 1) sapphire deposited with rf power range 160±200 W was a heteroepitaxially grown ®lm but other ®lm on (0 0 0 1) sapphire deposited with lower power was a ®ber textured ®lm. 4. Conclusions
Fig. 3. Some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of rf power. The ®lms were deposited at 8008C at Ar pressure of 0.13 Pa. Marks correspond to those in Fig. 2.
sapphire (1 1 2 0) plane is almost independent of Ar pressure but one of ®lms on (0 0 0 1) sapphire improves as Ar pressure is increased, and that the former is generally better than the latter. The RHEED pattern showed that ®lm on (0 0 0 1) sapphire deposited at Ar pressure of 0.13, 0.9 or 11.7 Pa was a heteroepitaxially grown ®lm but other ®lms on (0 0 0 1) sapphire were ®ber textured ®lms. Fig. 3 shows some characteristic parameters given by XRD from ZnO (0 0 0 2) plane as a function of rf power. The ®lms were deposited on (1 1 2 0) (*, *) and (0 0 0 1) (^, ^) sapphire substrates heated to 8008C with rf power range 40±200 W at Ar pressure of 0.13 Pa. Solid and open marks represent the ®lms deposited on 10 mm 10 mm and 5 mm 5 mm sapphire substrates, respectively. rf power is closely connected with the deposition rate of ®lm. In the present case, rf power ranging from 40 to 200 W corresponds to deposition rate range 5±21 nm/min. Final thickness of these ®lms was in the range from 440 to 2000 nm. 2y of ®lms is almost constant regardless of rf power and substrate used. On the other hand, the results for FWHM, s and I indicate that the
ZnO ®lms were deposited on sapphire (0 0 0 1) and (1 1 2 0) substrates by rf magnetron sputtering from a ZnO target and the dependence of structural properties on preparation conditions, such as substrate temperature, Ar gas pressure and rf power, was studied. The results obtained from XRD and RHEED measurements are as follows: 1. ZnO ®lms deposited on (1 1 2 0) sapphire substrates were (0 0 0 1) oriented heteroepitaxial ®lms with mosaic structure regardless of deposition conditions, but the degree of c-axis ordering of mosaics was improved with increase in substrate temperature and/or ®lm thickness. 2. The structure of ZnO ®lm deposited on (0 0 0 1) sapphire heated to 8008C was mostly a ®ber texture structure but heteroepitaxial ®lm was accidentally obtained when ®lm was deposited at Ar pressure of 0.13, 0.9 or 11.7 Pa with rf power of 100 W, and at Ar pressure of 0.13 Pa with rf power of 160, 180 or 200 W. These results indicate that if we can carefully control the deposition conditions, especially at early stage of ®lm growth, heteroepitaxial ®lm can be grown on (0 0 0 1) sapphire, too. Last, in this study, the growth of ®lms composed of self-assembled microcrystallites could not be recognized through SEM and/or AFM observation. References [1] R.D. Vispute, V. Talyansky, Z. Trajanovic, S. Chooopun, M. Downes, R.P. Sharma, T. Venkatesan, M.C. Woods, R.T. Lareau, K.A. Jones, A.A. Iliadis, Appl. Phys. Lett. 70 (1997) 2735.
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Y. Igasaki et al. / Applied Surface Science 169±170 (2001) 512±516
[2] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [3] Y. Igasaki, H. Saito, J. Appl. Phys. 69 (1991) 2190. [4] Y. Igasaki, H. Saito, J. Appl. Phys. 70 (1991) 3613. [5] Y. Igasaki, J. Cryst. Growth 116 (1992) 357.
[6] B.H. Billings et al. (Eds.), American Institute of Physics Hand Book, 3rd Edition, McGraw Hill, New York, 1972, pp. 4±136. [7] O. Madelung (Ed.), Neumerical Data and Functional Relationships in Science and Technology, Vol. 17b, Springer, New York, 1982, p. 44.