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sapphire interface and formation of ZnO nanocrystalline by laser MBE
Applied Surface Science 159–160 Ž2000. 514–519 www.elsevier.nlrlocaterapsusc
Investigation of ZnOrsapphire interface and formation of ZnO nanocrystalline by laser MBE I. Ohkubo ),1, Y. Matsumoto, A. Ohtomo 2 , T. Ohnishi, A. Tsukazaki 3, M. Lippmaa, H. Koinuma 4 , M. Kawasaki Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8502, Japan
Abstract Epitaxial ZnO thin films were prepared on atomically flat sapphire Ž a-Al2O3. Ž0001. substrates at various substrate temperatures by laser molecular beam epitaxy. Crystal structure was analyzed by four-circle X-ray diffraction. Atomic force microscope ŽAFM. and reflection high energy electron diffraction ŽRHEED. were used to evaluate surface morphology. When ZnO was deposited on atomically flat sapphire Ž0001., an epitaxial ZnO film was grown with c-axis orientation, having two different in-plane orientation, ZnO w1010x I sapphire w1010x Ž400–4508C. and ZnO w1010x I sapphire w1120x Ž800–8358C., depending on deposition temperature. The detailed observation of the initial growth of ZnO film deposited at 8358C revealed that the growth mode followed Stranski–Krastanov growth mechanism. q 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10.y i; 81.10.y h; 81.15.Fg; 82.65.Dp Keywords: Zinc oxide; X-ray scattering, diffraction, reflection; Single crystal epitaxy
1. Introduction Blue laser diode is expected to be useful especially for applications in high density storage and )
Corresponding author. Tel.: 81-45-9245561; fax: q81-459245399. E-mail address: [email protected] ŽI. Ohkubo.. 1 Department of Innovative and Engineered Materials. 2 Materials and Structures Laboratory. 3 Also a member of CREST, Japan Science and Technology Corporation. 4 Current address : Ernest Orlando Lawrence, Berkeley National Laboratory, MS 2 – 300, 1 Cyclotron Road Berkeley, CA 94720, USA.
display devices. So far, the most promising known materials are GaN Žroom temperature band gap : Eg s 3.44 eV. and related compounds w1x. However, recently it was found that ZnO is also a candidate for ultraviolet ŽUV.-emitting devices. Room temperature UV-laser action due to excitonic process of ZnO nanocrystal thin film was observed by our group w2x. ZnO is not only a wide-gap semiconductor with room temperature band gap of 3.37 eV but also has the largest excitation binding energy Ž60 meV., which is higher than that of GaN Ž28 meV. and ZnSe Ž19 meV. Žroom temperature energy is 26 meV.. In addition, ZnO film is composed of hexagonal shaped nanocrystals and this excitonic stimulated emission
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 1 3 8 - 0
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
was enhanced only from specific nanocrystal size of 50 nm3 at low pumping intensities of about 24 kWrcm 2 . Furthermore, well-defined fabry-perot cavity mode was observed w3x. For a detailed study of confinement effect of exciton and practical applications, it is necessary to prepare uniform and controllable sized ZnO nanocrystals or dots. In this research, formation of uniform-sized ZnO nanocrystals or dots and in-plane orientation of ZnO thin films on atomically flat sapphire Ž0001. substrates were studied.
515
2. Experimental The Laser molecular beam epitaxy ŽLaser MBE. system we used in this experiment is similar to that we reported in earlierw4x. The back ground pressure was about 1 = 10 -9 Torr. Substrates were heated by an infrared lamp focused on the backside of the sample holder to 8508C in 1 = 10y6 Torr oxygen Ž99.9999% purity. for 20 min before depositing thin films to remove carbon contaminants from a-Al 2 O 3 Ž0001. surface. ZnO thin films were deposited in
Fig. 1. XRD pole figure for ZnO grown at Ža. 4008C, Žb. 5508C, Žc. 8358C. The pole figure of sapphire Ž0001. substrate is shown in Žd.. The poles are taken for ZnO 10104 and sapphire 11204 planes.
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I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
1 = 10y6 Torr oxygen ŽO2. by impinging KrF excimer laser Ž l s 248 nm. pulses on ceramic ZnO target Ž99.9999 % purity. placed 4 cm away from the substrate. Typical laser fluence and repetition rate were 0.6 Jrcm2 and 10 Hz, respectively. Atomically flat sapphire Ž a-Al 2 O 3 Ž0001.. substrates were prepared by annealing as-polished sapphire Ž0001. substrates at 10008C for 3 h and then at 7508C for 3 h in the air. The surface of the annealed substrates showed evenly spaced steps with a height of 0.22 nm, equal to minimum charge neutral unit and atomically flat terraces w5x. Surface morphology of the films were evaluated by atomic force microscope ŽAFM: Seiko SPI-3700. and in-situ reflection high energy electron diffraction ŽRHEED.. Film crystallinity and the epitaxial relationship to the substrate were studied by four-circle X-ray diffraction ŽXRD; Philips, X’pert-MRD. using a 18 kW Cu target rotating-anode generator at 45 KV and 120 mA. A continuous He–Cd laser Ž325 nm. was used for the photoluminescence ŽPL. measurements.
in-plane orientation regardless of the deposition conditions as reported in our previous paper w7a,bx. The crystallinity of ZnO thin films deposited on atomically flat sapphire Ž0001. was found to improve as the growth temperature was increased, as confirmed by XRD and PL spectra. In fact the full width at half maximum ŽFWHM. of XRD rocking curve for the film deposited at 8358C is as small as 14 arcsec and is much better than that for single crystal Ž65 arcsec. w8x. PL spectra in Fig.2 show that the ZnO film deposited at highest temperature gives very narrow emission peak which shifts to lower energy along with broading with decreasing deposition temperature. As mentioned above, ZnO film has two kinds of in-plane orientations depending on deposition temperature. Lattice mismatch between sapphire and ZnO with aligned Ž800–8358C. and 308 twisted Ž400–4508C. in-plane orientations are is 18.3% and 31.8%, respectively. It should be pointed out that the highest crystallinity of ZnO films deposited at high temperature as shown in PL spectra is due to small lattice mismatch. From the results, it looks favorable for the investigation of initial growth to of ZnO films were carried
3. Results and discussion All ZnO films deposited on a-Al 2 O 3 Ž0001. showed only 000l 4 peaks in 2 u – u XRD patterns, indicating that ZnO films are grown with c-axis orientation. Fig. 1 shows the pole figure of films deposited at various deposition temperatures on atomically flat sapphire Ž0001. substrates. The poles are taken from ZnO 10104 and sapphire 11204 planes. In-plane relationship for the films deposited at low temperature Ž400–4508C. is only ZnO w1010x I sapphire w1010x Ž308 twisted. as shown in Fig. 1Ža.. Upon increasing the deposition temperature above 5008C, another ZnO film with different in-plane relationship, ZnO w1010x I sapphire w1120x Žaligned in-plane. also appears ŽFig.1Žb.. and becomes dominant above 8008C ŽFig.1Žc... Similar trend of in-plane orientation as a function of deposition temperature was also reported by Vispute et al. w6x, although authors did not mention any substrate surface treatment. It should be noted that the films deposited on as-polished sapphire substrates preferred aligning
Fig. 2. Photoluminescence spectra of ZnO films grown on atomically flat sapphire Ž0001. at Ža. 8358C, Žb. 5508C and Žc. 4008C.
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
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˚ Žb, f. Fig. 3. RHEED patterns Ža. – Žd. and AFM images Že. – Žh. of ZnO films at the initial stages of growth. Film thickness is Ža, e. 7.5 A, ˚ Žc, g. 225 A, ˚ Žd, h. 532 A. ˚ Growth temperature was 8358C. 15 A,
out at higher growth temperature because of uniformity of in-plane orientation and high crystallinity.
Fig. 3 shows RHEED patterns and AFM images of the initial growth of ZnO films with various thick-
˚ Žb. 50 A, ˚ Žc. 100 A˚ and Žd. 200 A. ˚ Growth Fig. 4. Ža. – Žd., AFM images of ZnO dot structures with various thicknesses, Ža. 25 A, temperature was 8358C. Histogram of the ZnO dot height Že. and diameter Žf. in this figure.
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I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
nesses, deposited on atomically flat sapphire Ž0001. substrate at 8358C. The direction of the incident electron beam in the RHEED pattern was sapphire w1120x ŽZnO w1010x.. When the thickness of ZnO ˚ both diffraction streaks from film was about 7.5A, both sapphire and ZnO were observed. AFM image showed that the surface was still atomically flat originated from sapphire substrate. With the increase ˚ ., RHEED in thickness of ZnO film Žca. 15–225A pattern showed broader streaks of only ZnO film. AFM images showed that dots had started to grow on the terraces though a spot pattern attributed to ZnO dots was not observed in the RHEED. It is probably because the RHEED pattern itself was much broader and the intensity of spots was too weak to be observed. At last, ZnO film was thick enough to give a sharp RHEED pattern and coalesced ZnO islands and grain boundaries were observed in the AFM image. This observation indicates that the growth mode of ZnO films on sapphire Ž0001. follows Stranski–Krastanov ŽS–K. mechanism. Fig. 4 shows detailed growth process of ZnO dots. As the film thickness was increased, the height and density of ZnO dots were increased drastically, but their diameter was not changed much. The nucleus density should be y1011 cmy3 for the fabrication of 50 nm3-sized ZnO nanocrystals. However, under present growth condition, the maximum dots density was 4.0 = 10 9 cmy3 . In our previous study, ZnO nanocrystal were formed at 5508C on aspolished sapphire substrate and the growth rate of ZnO was 0.1–0.06 nmrpulse. In this study, however, the growth temperature was much higher and the growth rate was lowered to about 0.005 nmrpulse in order to investigate the initial growth of ZnO. The differences of growth conditions might be the reason why the nucleus density was much decreased as compared to that in the previous results.
4. Conclusion Epitaxial ZnO thin films were prepared on atomically flat sapphire Ž0001. substrates at various sub-
519
strate temperatures by a laser MBE technique. It was found that the in-plane orientation of ZnO films to the substrate could be controlled by changing deposition temperature. RHEED and AFM observations revealed that the growth mode of ZnO film is Stranski–Krastanov on atomically flat sapphire.
Acknowledgements This work was partly supported by JSPS Research for Future Program in the Area of Atomic-scale Surface and Interface Dynamics ŽRTFT96P00205.. A.O. and T.O. are supported by JSPS Research Fellowships for Young Scientists.
References w1x S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. w2x P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 Ž1997. 459. w3x M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, B.P. Zhang, Y. Segawa, Mater. Sci. Eng., B 56 Ž1998. 239. w4x H. Koinuma, N. Kanda, J. Nishino, A. Ohtomo, H. Kubota, M. Kawasaki, M. Yoshimoto, Appl. Surf. Sci. 109r110 Ž1997. 514. w5x M. Yoshimoto, T. Maeda, T. Ohnishi, H. Koinuma, O. Ishiyama, M. Shinohara, M. Kubo, R. Miura, A. Miyamoto, Appl. Phys. Lett. 67 Ž1995. 2615. w6x R.D. Vispute, V. Talyansky, Z. Trajanovic, S. Choopun, M. Downes, R.P. Sharma, T. Venkatesan, M.C. Woods, R.T. Lareau, K.A. Jones, A.A. IIiadis, Appl. Phys. Lett. 70 Ž1997. 2735. w7ax T. Ohnishi, A. Ohtomo, M. Kawasaki, K. Takahashi, M. Yoshimoto, H. Koinuma, Appl. Phys. Lett. 72 Ž1998. 824. w7bxT. Ohnishi, A. Ohtomo, I. Ohkubo, M. Kawasaki, M. Yoshimoto, H. Koinuma, Mat. Sci. Eng. B 56 Ž1998. 256. w8x I. Ohkubo, A. Ohtomo, T. Ohnishi, Y. Matsumoto, H. Koinuma, M. Kawasaki, Surf. Sci. 443 Ž1999. L1043.
Investigation of ZnOrsapphire interface and formation of ZnO nanocrystalline by laser MBE I. Ohkubo ),1, Y. Matsumoto, A. Ohtomo 2 , T. Ohnishi, A. Tsukazaki 3, M. Lippmaa, H. Koinuma 4 , M. Kawasaki Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8502, Japan
Abstract Epitaxial ZnO thin films were prepared on atomically flat sapphire Ž a-Al2O3. Ž0001. substrates at various substrate temperatures by laser molecular beam epitaxy. Crystal structure was analyzed by four-circle X-ray diffraction. Atomic force microscope ŽAFM. and reflection high energy electron diffraction ŽRHEED. were used to evaluate surface morphology. When ZnO was deposited on atomically flat sapphire Ž0001., an epitaxial ZnO film was grown with c-axis orientation, having two different in-plane orientation, ZnO w1010x I sapphire w1010x Ž400–4508C. and ZnO w1010x I sapphire w1120x Ž800–8358C., depending on deposition temperature. The detailed observation of the initial growth of ZnO film deposited at 8358C revealed that the growth mode followed Stranski–Krastanov growth mechanism. q 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10.y i; 81.10.y h; 81.15.Fg; 82.65.Dp Keywords: Zinc oxide; X-ray scattering, diffraction, reflection; Single crystal epitaxy
1. Introduction Blue laser diode is expected to be useful especially for applications in high density storage and )
Corresponding author. Tel.: 81-45-9245561; fax: q81-459245399. E-mail address: [email protected] ŽI. Ohkubo.. 1 Department of Innovative and Engineered Materials. 2 Materials and Structures Laboratory. 3 Also a member of CREST, Japan Science and Technology Corporation. 4 Current address : Ernest Orlando Lawrence, Berkeley National Laboratory, MS 2 – 300, 1 Cyclotron Road Berkeley, CA 94720, USA.
display devices. So far, the most promising known materials are GaN Žroom temperature band gap : Eg s 3.44 eV. and related compounds w1x. However, recently it was found that ZnO is also a candidate for ultraviolet ŽUV.-emitting devices. Room temperature UV-laser action due to excitonic process of ZnO nanocrystal thin film was observed by our group w2x. ZnO is not only a wide-gap semiconductor with room temperature band gap of 3.37 eV but also has the largest excitation binding energy Ž60 meV., which is higher than that of GaN Ž28 meV. and ZnSe Ž19 meV. Žroom temperature energy is 26 meV.. In addition, ZnO film is composed of hexagonal shaped nanocrystals and this excitonic stimulated emission
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 1 3 8 - 0
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
was enhanced only from specific nanocrystal size of 50 nm3 at low pumping intensities of about 24 kWrcm 2 . Furthermore, well-defined fabry-perot cavity mode was observed w3x. For a detailed study of confinement effect of exciton and practical applications, it is necessary to prepare uniform and controllable sized ZnO nanocrystals or dots. In this research, formation of uniform-sized ZnO nanocrystals or dots and in-plane orientation of ZnO thin films on atomically flat sapphire Ž0001. substrates were studied.
515
2. Experimental The Laser molecular beam epitaxy ŽLaser MBE. system we used in this experiment is similar to that we reported in earlierw4x. The back ground pressure was about 1 = 10 -9 Torr. Substrates were heated by an infrared lamp focused on the backside of the sample holder to 8508C in 1 = 10y6 Torr oxygen Ž99.9999% purity. for 20 min before depositing thin films to remove carbon contaminants from a-Al 2 O 3 Ž0001. surface. ZnO thin films were deposited in
Fig. 1. XRD pole figure for ZnO grown at Ža. 4008C, Žb. 5508C, Žc. 8358C. The pole figure of sapphire Ž0001. substrate is shown in Žd.. The poles are taken for ZnO 10104 and sapphire 11204 planes.
516
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
1 = 10y6 Torr oxygen ŽO2. by impinging KrF excimer laser Ž l s 248 nm. pulses on ceramic ZnO target Ž99.9999 % purity. placed 4 cm away from the substrate. Typical laser fluence and repetition rate were 0.6 Jrcm2 and 10 Hz, respectively. Atomically flat sapphire Ž a-Al 2 O 3 Ž0001.. substrates were prepared by annealing as-polished sapphire Ž0001. substrates at 10008C for 3 h and then at 7508C for 3 h in the air. The surface of the annealed substrates showed evenly spaced steps with a height of 0.22 nm, equal to minimum charge neutral unit and atomically flat terraces w5x. Surface morphology of the films were evaluated by atomic force microscope ŽAFM: Seiko SPI-3700. and in-situ reflection high energy electron diffraction ŽRHEED.. Film crystallinity and the epitaxial relationship to the substrate were studied by four-circle X-ray diffraction ŽXRD; Philips, X’pert-MRD. using a 18 kW Cu target rotating-anode generator at 45 KV and 120 mA. A continuous He–Cd laser Ž325 nm. was used for the photoluminescence ŽPL. measurements.
in-plane orientation regardless of the deposition conditions as reported in our previous paper w7a,bx. The crystallinity of ZnO thin films deposited on atomically flat sapphire Ž0001. was found to improve as the growth temperature was increased, as confirmed by XRD and PL spectra. In fact the full width at half maximum ŽFWHM. of XRD rocking curve for the film deposited at 8358C is as small as 14 arcsec and is much better than that for single crystal Ž65 arcsec. w8x. PL spectra in Fig.2 show that the ZnO film deposited at highest temperature gives very narrow emission peak which shifts to lower energy along with broading with decreasing deposition temperature. As mentioned above, ZnO film has two kinds of in-plane orientations depending on deposition temperature. Lattice mismatch between sapphire and ZnO with aligned Ž800–8358C. and 308 twisted Ž400–4508C. in-plane orientations are is 18.3% and 31.8%, respectively. It should be pointed out that the highest crystallinity of ZnO films deposited at high temperature as shown in PL spectra is due to small lattice mismatch. From the results, it looks favorable for the investigation of initial growth to of ZnO films were carried
3. Results and discussion All ZnO films deposited on a-Al 2 O 3 Ž0001. showed only 000l 4 peaks in 2 u – u XRD patterns, indicating that ZnO films are grown with c-axis orientation. Fig. 1 shows the pole figure of films deposited at various deposition temperatures on atomically flat sapphire Ž0001. substrates. The poles are taken from ZnO 10104 and sapphire 11204 planes. In-plane relationship for the films deposited at low temperature Ž400–4508C. is only ZnO w1010x I sapphire w1010x Ž308 twisted. as shown in Fig. 1Ža.. Upon increasing the deposition temperature above 5008C, another ZnO film with different in-plane relationship, ZnO w1010x I sapphire w1120x Žaligned in-plane. also appears ŽFig.1Žb.. and becomes dominant above 8008C ŽFig.1Žc... Similar trend of in-plane orientation as a function of deposition temperature was also reported by Vispute et al. w6x, although authors did not mention any substrate surface treatment. It should be noted that the films deposited on as-polished sapphire substrates preferred aligning
Fig. 2. Photoluminescence spectra of ZnO films grown on atomically flat sapphire Ž0001. at Ža. 8358C, Žb. 5508C and Žc. 4008C.
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
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˚ Žb, f. Fig. 3. RHEED patterns Ža. – Žd. and AFM images Že. – Žh. of ZnO films at the initial stages of growth. Film thickness is Ža, e. 7.5 A, ˚ Žc, g. 225 A, ˚ Žd, h. 532 A. ˚ Growth temperature was 8358C. 15 A,
out at higher growth temperature because of uniformity of in-plane orientation and high crystallinity.
Fig. 3 shows RHEED patterns and AFM images of the initial growth of ZnO films with various thick-
˚ Žb. 50 A, ˚ Žc. 100 A˚ and Žd. 200 A. ˚ Growth Fig. 4. Ža. – Žd., AFM images of ZnO dot structures with various thicknesses, Ža. 25 A, temperature was 8358C. Histogram of the ZnO dot height Že. and diameter Žf. in this figure.
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I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
I. Ohkubo et al.r Applied Surface Science 159–160 (2000) 514–519
nesses, deposited on atomically flat sapphire Ž0001. substrate at 8358C. The direction of the incident electron beam in the RHEED pattern was sapphire w1120x ŽZnO w1010x.. When the thickness of ZnO ˚ both diffraction streaks from film was about 7.5A, both sapphire and ZnO were observed. AFM image showed that the surface was still atomically flat originated from sapphire substrate. With the increase ˚ ., RHEED in thickness of ZnO film Žca. 15–225A pattern showed broader streaks of only ZnO film. AFM images showed that dots had started to grow on the terraces though a spot pattern attributed to ZnO dots was not observed in the RHEED. It is probably because the RHEED pattern itself was much broader and the intensity of spots was too weak to be observed. At last, ZnO film was thick enough to give a sharp RHEED pattern and coalesced ZnO islands and grain boundaries were observed in the AFM image. This observation indicates that the growth mode of ZnO films on sapphire Ž0001. follows Stranski–Krastanov ŽS–K. mechanism. Fig. 4 shows detailed growth process of ZnO dots. As the film thickness was increased, the height and density of ZnO dots were increased drastically, but their diameter was not changed much. The nucleus density should be y1011 cmy3 for the fabrication of 50 nm3-sized ZnO nanocrystals. However, under present growth condition, the maximum dots density was 4.0 = 10 9 cmy3 . In our previous study, ZnO nanocrystal were formed at 5508C on aspolished sapphire substrate and the growth rate of ZnO was 0.1–0.06 nmrpulse. In this study, however, the growth temperature was much higher and the growth rate was lowered to about 0.005 nmrpulse in order to investigate the initial growth of ZnO. The differences of growth conditions might be the reason why the nucleus density was much decreased as compared to that in the previous results.
4. Conclusion Epitaxial ZnO thin films were prepared on atomically flat sapphire Ž0001. substrates at various sub-
519
strate temperatures by a laser MBE technique. It was found that the in-plane orientation of ZnO films to the substrate could be controlled by changing deposition temperature. RHEED and AFM observations revealed that the growth mode of ZnO film is Stranski–Krastanov on atomically flat sapphire.
Acknowledgements This work was partly supported by JSPS Research for Future Program in the Area of Atomic-scale Surface and Interface Dynamics ŽRTFT96P00205.. A.O. and T.O. are supported by JSPS Research Fellowships for Young Scientists.
References w1x S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. w2x P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 Ž1997. 459. w3x M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, B.P. Zhang, Y. Segawa, Mater. Sci. Eng., B 56 Ž1998. 239. w4x H. Koinuma, N. Kanda, J. Nishino, A. Ohtomo, H. Kubota, M. Kawasaki, M. Yoshimoto, Appl. Surf. Sci. 109r110 Ž1997. 514. w5x M. Yoshimoto, T. Maeda, T. Ohnishi, H. Koinuma, O. Ishiyama, M. Shinohara, M. Kubo, R. Miura, A. Miyamoto, Appl. Phys. Lett. 67 Ž1995. 2615. w6x R.D. Vispute, V. Talyansky, Z. Trajanovic, S. Choopun, M. Downes, R.P. Sharma, T. Venkatesan, M.C. Woods, R.T. Lareau, K.A. Jones, A.A. IIiadis, Appl. Phys. Lett. 70 Ž1997. 2735. w7ax T. Ohnishi, A. Ohtomo, M. Kawasaki, K. Takahashi, M. Yoshimoto, H. Koinuma, Appl. Phys. Lett. 72 Ž1998. 824. w7bxT. Ohnishi, A. Ohtomo, I. Ohkubo, M. Kawasaki, M. Yoshimoto, H. Koinuma, Mat. Sci. Eng. B 56 Ž1998. 256. w8x I. Ohkubo, A. Ohtomo, T. Ohnishi, Y. Matsumoto, H. Koinuma, M. Kawasaki, Surf. Sci. 443 Ž1999. L1043.