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Control of polarity of heteroepitaxial ZnO films by interface engineering
Applied Surface Science 190 (2002) 491±497
Control of polarity of heteroepitaxial ZnO ®lms by interface engineering Soon-Ku Honga,*, Takashi Hanadaa, Yefen Chena, Hang-Ju Koa, Takafumi Yaoa, Daisuke Imaib, Kiyoaki Arakib, Makoto Shinoharab a Institute for Materials Research, Tohoku University, Sendai 980-8677, Japan Surface Analysis and Semiconductor Equipment Division, Shimadzu Co., Kanagawa 259-1304, Japan
b
Abstract Control of polarity of heteroepitaxial ZnO ®lms has been examined by interface engineering. ZnO ®lms were grown by plasmaassisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO ®lms have successfully grown by Znand O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga2O3 interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO ®lms. The polarity of ZnO ®lms grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO ®lms on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. Highresolution transmission electron microscopy revealed atomically ¯at interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer. # 2002 Elsevier Science B.V. All rights reserved. PACS: 61.66.Fn; 68.35.Ct; 61.18.Bn; 81.05.Dz; 81.05.Ea; 81.15.Hi Keywords: Polarity; Polarity inversion; Interface; Interface engineering; ZnO; GaN
1. Introduction Polarity of crystal comes from lack of inversion center. Therefore, h0 0 01i direction and hence {0 0 0 1} planes of wurtzite-structure crystal have polar characteristic. The polarity of wurtzite-structure semiconductors, such as ZnO or GaN, has been receiving increasing interests because of its strong effects on properties of ®lms and applications for devices.
*
Corresponding author. Tel.: 81-22-215-2074; fax: 81-22-215-2073. E-mail address: [email protected] (S.-K. Hong).
Several groups reported the growth of GaN ®lms with a controlled polarity by employing surface modi®cations of substrate, buffer layer growth and change of growth process, where procedures for growing Gaand N-polar GaN ®lms are nominally different between metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). In some cases, the observed polarity is different for MOCVD and MBE grown GaN ®lms even the same buffer layers (low-temperature GaN buffer) and substrates (c-plane sapphire without a nitridation process) are used. In this paper, we report the effects of the modi®cations of interfaces on the polarity of heteroepitaxial
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 2 4 - 2
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S.-K. Hong et al. / Applied Surface Science 190 (2002) 491±497
ZnO ®lms grown by plasma-assisted molecular beam epitaxy (P-MBE) on GaN template and c-plane sapphire substrates. We have tried to modify the interfaces between ZnO and GaN to grow Zn-polar and O-polar ZnO ®lms on the same Ga-polar GaN templates. Furthermore, we have examined possibility of inverting the polarity of ZnO ®lms to the other by engineering the interfaces. 2. Experimental Substrates used in this study are Ga-polar (0 0 0 1) GaN epilayers (4 mm thick) grown on c-plane sapphire by MOCVD and c-plane sapphire. Degreasing of epi-GaN/Al2O3 and c-plane sapphire substrates was done by ultrasonic cleaning in acetone and methanol at RT followed by rinsing with deionized water and spin-drying. The degreased epi-GaN template substrate was thermally cleaned at 700±800 8C in ultrahigh vacuum for about 1 h. The degreased sapphire substrate was chemically etched using a solution of H2SO4 (96 vol.%):H3PO4 86 vol:% 3:1 at 160 8C for 15 min followed by rinsing with deionized water and spin-drying. Chemically etched substrates are thermally cleaned at 650 8C under O-plasma ambient for 30 min. ZnO and MgO ®lms were grown by P-MBE using a radio frequency oxygen-plasma source, a Zn solid source, and a Mg solid source. Zn- or O-plasma pre-exposure on the cleaned GaN surfaces was carried out at 700 8C for 3 min. The GaN template substrate and the cleaning procedures are the same for both Zn- and O-plasma pre-exposed samples. After these pre-exposures on the GaN surfaces, ZnO ®lms were grown at 550±700 8C and MgO ®lms were grown at 490±550 8C. The vacuum pressure was 2 10 10 Torr prior to deposition and 9 10 5 Torr during growth. The Zn ¯ux was set to 0.15±0.20 nm/s and the Mg ¯ux was set to 0.03±0.10 nm/s, which were monitored by quartz thickness monitor before and after growth. Oplasma power was set to 300±400 W with an oxygen gas ¯ow rate of 2.5±3.5 sccm. Polarity of the grown ®lms is determined by coaxial impact collision ion scattering spectroscopy (CAICISS). Primary He ions with energy of 2 KeV are used for measurements. Polar angle (PA) dependence of CAICISS signal intensities are measured along the
azimuth direction of [11 20]. CAICISS spectra are obtained from time-of-¯ight spectra (Shimadzu Co., TALIS 9700). Polarity of the ZnO ®lms was determined by comparison of CAICISS PA dependence with the simulated one. High-resolution transmission electron microscopy (HRTEM) on ZnO/GaN interfaces was carried out using a JEOL JEM-ARM1250 high voltage electron microscope at 1250 kV. Cross-sectional TEM specimens are prepared by conventional techniques of mechanical polishing, dimpling and ion milling. Zone axis of h2 1 10iGaN and ZnO is selected for the HRTEM observations. 3. Results and discussion 3.1. Growth of Zn- and O-polar ZnO ®lms on Ga-polar GaN templates In order to investigate interfaces at ZnO/GaN with different pre-exposures, HRTEM study was carried out. Fig. 1(a) and (b) shows HRTEM micrographs for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on Ga-polar GaN templates, respectively. The interface structure showed a drastic difference as shown in Fig. 1. In the case of O-plasma pre-exposure, a continuous interface layer with a thickness of about 3.5 nm was formed at the ZnO/GaN interface, while no interface layer was observed in a Zn pre-exposed sample. The orientation relationship between ZnO and GaN was found to be [0 0 0 1]ZnO//[0 0 0 1]GaN, and [01 10]ZnO//[01 10]GaN for both types of pre-exposures. The interface layer was identi®ed as single crystalline monoclinic Ga2O3 with a zone axis of [0 1 0]. Detailed structure of the interface and model are reported elsewhere [1]. The different interface structures for Zn- and Oplasma pre-exposures would open the possibility of controlling the lattice polarity of ZnO epilayers. In order to check and con®rm the polarities of ZnO ®lms on Ga-polar GaN templates with different interface structures, the polarity of ZnO ®lms was determined by CAICISS measurements. Fig. 2 shows experimental and simulated PA dependence of Zn signals from (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms. The spectra are signi®cantly different between the two samples. Experimental spectra agree well with the
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Fig. 1. HRTEM micrographs for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on GaN templates. Single-crystalline monoclinic Ga2O3 interface layer was formed by O-plasma pre-exposure, while no interface layer was formed by Zn pre-exposure. Zone axis is parallel to h2 1 10iGaN and ZnO.
simulated spectra for Zn- and O-polar ZnO, respectively. Here, it should be noted that our results agreed well with reported data on the Zn-face and O-face of bulk ZnO [2]. Therefore, we conclude that Zn pre-exposed ZnO ®lms have a characteristic of Zn polarity, while O-plasma pre-exposed ZnO ®lms have a characteristic of O-polarity. The results mean we have successfully grown Zn-polar (cation polar) and O-polar (anion polar) ZnO ®lms, selectively, even on the same Ga-polar (cation polar) epilayer by engineering the interface. A feature of the experimental CAICISS spectrum from the O-polar ZnO ®lm shows broader character than that from the Zn-polar ZnO ®lm. We note, however, that Zn-polar domains are not observed from TEM observations. Surface morphologies of Zn-polar and O-polar ZnO ®lms grown by three-dimensional growth mode show a slight difference in RMS values from atomic force microscope observations (23 and
26 nm in 5 mm 5 mm regions for Zn-polar and Opolar ZnO ®lms indicating a slight larger value for the O-polar ZnO ®lm). Therefore, we think that the broader character results from multiple scattering and/or inelastic interactions [2], which seems to be severe for O-polar ZnO ®lm. The same reasons can be applied to Figs. 3 and 4 will be shown later. However, more systematic investigations for effects of the surface roughness on the broadening of CAICISS spectra are needed. Here, we would like to comment that the surface morphologies of ZnO ®lms in present study are mainly affected by growth conditions not by polarities although the RMS values for O-polar ZnO ®lms are slightly larger than those for Zn-polar ZnO ®lms. Crystal quality evaluated by high-resolution X-ray diffraction is revealed that larger full widths at half maximum (FWHMs) for O-polar ZnO ®lms than Znpolar ZnO ®lms. Typical FWHMs for two types of
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Fig. 2. PA dependence of CAICISS intensity for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on GaN, which showing typical Zn- and Opolar characters, respectively. Simulated dependencies for Zn- and O-polar ZnO are also shown. The curves are plotted as a function of incident angle, which is equal to 908 minus PA.
ZnO ®lms are 399 (678) arcsec from Zn-polar (Opolar) ZnO ®lm for (0 0 0 2) O scan and 748 (1 0 2 3) arcsec from Zn-polar (O-polar) ZnO ®lm for (10 11) F scan. Whether the degraded crystal quality of Opolar ZnO ®lm comes from the different polarity or the Ga2O3 interface layer, which results in an increase of the lattice mis®t [1], is not clear yet. 3.2. Polarity of ZnO ®lms on c-Al2O3 substrates with MgO buffer We have recently succeeded in two-dimensional layer-by-layer growth of ZnO ®lms on c-plane sapphire by using novel MgO buffer [3]. Similarly to the cases of GaN-based ®lms to control the polarity by employing
the buffer layers, con®rmation of the polarity of ZnO ®lms with a MgO buffer layer seems to be meaningful. Growth procedure for this sample is ``MgO buffer growth ! ZnO buffer growth ! annealing of the ZnO buffer layer under the O-plasma ambient ! Zn shutter open for ZnO growth again on the annealed ZnO layer''. Here, the chamber is maintained under oxygen ambient and thus O-polar ZnO ®lm can be expected similarly to ZnO on Al2O3 without a MgO buffer [2] because (1) termination of Al2O3 would be oxygen reminding that the sapphire is thermally cleaned by Oplasma prior to growth and (2) MgO buffer under the oxygen ambient most likely has O-stabilized MgO surface. Consequently, further growth of ZnO would
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Fig. 3. PA dependence of CAICISS intensity for ZnO ®lm on c-plane sapphire with MgO buffer. Polarity of the ZnO ®lm showed a typical Opolar character. Simulated dependence for O-polar ZnO is also shown. The curves are plotted as a function of incident angle. Details of growth procedure are described in text.
form ``Mg±O±Zn±O'' bonding sequence at the MgO/ ZnO interface. An experimental CAICISS spectrum shown in Fig. 3 from this sample is revealed as one from the O-polarity. The samples without the annealing
process also revealed as O-polar. For comparison, simulated CAICISS spectrum for O-polar ZnO is also shown in Fig. 3. We note structure of MgO is revealed as rocksalt and bonding sequence described
Fig. 4. PA dependence of CAICISS intensity for ZnO ®lm on Ga-polar GaN template, where a MgO layer was inserted between lower and upper ZnO layers. Polarity of ZnO ®lm shows a typical O-polar character. Simulated dependence for O-polar ZnO is also shown. The curves are plotted as a function of incident angle. Details of growth procedure are described in text.
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here results in growth of O-polar ZnO ®lm on Zn-polar ZnO ®lm by using a MgO intermediate layer will be discussed in Section 3.3. Here, it should be noted that the polarity of ZnO ®lms on O-stabilized sapphire substrates has been reported as O-polar [2]. 3.3. Growth of a polarity-inverted ZnO structure We have examined the possibility of inversion of the Zn-polar ZnO ®lm grown on Ga-polar GaN template to O-polar ZnO ®lm, i.e. fabrication of polarityinverted ZnO heterostructure. Growth procedure for this sample is ``Zn pre-exposure ! low-temperature (LT) ZnO buffer growth ! Zn shutter close and hightemperature (HT) annealing of the LT ZnO buffer under the O-plasma ambient ! Zn shutter open for growth of a lower ZnO layer ! Zn shutter close and
Mg shutter open for growth of a thin MgO intermediate layer 15 ML ! Mg shutter close and Zn shutter open for growth of an upper ZnO layer on the MgO layer''. We have employed LT ZnO buffer growth followed by HT annealing to obtain very smooth ZnO surface prior to MgO growth. Consequently, RHEED oscillation was observed during the growths of the lower ZnO layer [4], MgO intermediate layer, and upper ZnO layer. It means the growth procedure was done two-dimensionally across the ZnO/MgO/ZnO interfaces, which implies the formation of very ¯at and abrupt interfaces. Note that the LT ZnO buffer and lower ZnO layers are Zn-polar because Zn pre-exposure is employed. We expected the upper ZnO layer would be grown as O-polar, i.e. polarity inversion would occur because a MgO intermediate layer under the oxygen ambient
Fig. 5. HRTEM micrograph for a ZnO/MgO/ZnO/GaN heterostructure of the sample for Fig. 4. Interfaces at lower and upper ZnO/MgO heterointerfaces are atomically ¯at and no inversion domain boundaries are observed in the upper ZnO layer. Zone axis is parallel to h2, 1, 1iGaN and ZnO.
S.-K. Hong et al. / Applied Surface Science 190 (2002) 491±497
most likely has O-stabilized MgO surface. Consequently, the upper ZnO layer would be grown as Opolar by forming O±Zn bondings at the MgO/ZnO interface. Fig. 4 shows an experimental CAICISS spectrum from this sample, which indicates the perfect O-polar character. In order to comparison, simulated CAICISS spectrum for O-polar ZnO is also shown in Fig. 4. Here it should be noted again that the lower ZnO layer is grown as Zn-polar by employing the Zn pre-exposure. Therefore, the result means we have succeeded in growing the polarity-inverted heterostructure by employing a MgO intermediate layer between the lower (Zn-polar) and upper (O-polar) ZnO layers. In order to con®rm the ¯atness at the interfaces and the formation of inversion domain boundary, HRTEM was carried out. Fig. 5 shows an HRTEM micrograph of ZnO/MgO/ZnO/GaN heterostructure. As shown in Fig. 5, the interfaces at lower and upper ZnO/MgO are atomically ¯at. Moreover, no inversion boundary, which is inevitably observed for inversion of Ga-polar GaN to N-polar [5,6], is observed in the upper ZnO layer. Note also that there is no interface layer between ZnO and GaN, which directly indicate that lower ZnO layer bellow the MgO intermediate layer is inevitably Zn-polar. Finally, we note that the polarity of the upper ZnO ®lm on a very thin (2 ML thick with the same growth conditions) MgO intermediate layer is revealed as mixed polarity not unipolar. This is not observed from O-polar ZnO ®lm on sapphire substrate (non-polar substrate) with a MgO buffer layer. We tentatively think the origin of the observed thickness dependence is caused by the polarity inversion, which would form high-energetic interfaces. Detailed interface structure is now in investigation and will be reported elsewhere [7]. 4. Summary Controlling methods for the polarity of heteroepitaxial ZnO ®lms have been studied by engineering the
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interface. Zn-polar and O-polar ZnO ®lms on Ga-polar GaN templates have successfully grown by forming the ZnO/GaN interfaces with and without an interface layer, respectively. The interface layer, single-crystalline monoclinic Ga2O3, was formed by O-plasma preexposure, while its formation was prevented by Zn pre-exposure prior to ZnO growth. The polarity of ZnO ®lms grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. By inserting a MgO layer between two ZnO layers, the Zn-polarity (lower ZnO layer) is changed to the Opolarity (upper ZnO layer), which indicating successful fabrication of ZnO heterostructure with an inverted polar. No inversion domain boundaries were observed in the upper ZnO layer. The result would be useful in devices where a fabrication of inverted polar structures is needed. Acknowledgements S.-K. Hong would like to thank International Communications Foundation (ICF) for its ®nancial support, ``Fellowship for Researchers and Graduate Students from Abroad''. References [1] S.K. Hong, H.J. Ko, Y. Chen, T. Hanada, T. Yao, J. Vac. Sci. Technol. B 18 (2000) 2313. [2] T. Ohnishi, A. Ohtomo, M. Kawasaki, K. Takahashi, M. Yoshimoto, H. Koinuma, Appl. Phys. Lett. 72 (1999) 824. [3] Y. Chen, H.J. Ko, S.K. Hong, T. Yao, Appl. Phys. Lett. 73 (2000) 559. [4] H.J. Ko, Y. Chen, S.K. Hong, T. Yao, J. Cryst. Growth 209 (2000) 816. [5] V. Ramachandran, R.M. Feenstra, W.L. Samey, L. SalamancaRiba, J.E. Northrup, L.T. Romano, D.W. Greve, Appl. Phys. Lett. 75 (1999) 808. [6] L.T. Romano, J.E. Northrup, A.J. Ptak, T.H. Myers, Appl. Phys. Lett. 77 (2000) 2479. [7] S.K. Hong, Y. Chen, T. Hanada, H.J. Ko, T. Yao, D. Imai, K. Araki, M. Shiohara, Unpublished.
Control of polarity of heteroepitaxial ZnO ®lms by interface engineering Soon-Ku Honga,*, Takashi Hanadaa, Yefen Chena, Hang-Ju Koa, Takafumi Yaoa, Daisuke Imaib, Kiyoaki Arakib, Makoto Shinoharab a Institute for Materials Research, Tohoku University, Sendai 980-8677, Japan Surface Analysis and Semiconductor Equipment Division, Shimadzu Co., Kanagawa 259-1304, Japan
b
Abstract Control of polarity of heteroepitaxial ZnO ®lms has been examined by interface engineering. ZnO ®lms were grown by plasmaassisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO ®lms have successfully grown by Znand O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga2O3 interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO ®lms. The polarity of ZnO ®lms grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO ®lms on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. Highresolution transmission electron microscopy revealed atomically ¯at interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer. # 2002 Elsevier Science B.V. All rights reserved. PACS: 61.66.Fn; 68.35.Ct; 61.18.Bn; 81.05.Dz; 81.05.Ea; 81.15.Hi Keywords: Polarity; Polarity inversion; Interface; Interface engineering; ZnO; GaN
1. Introduction Polarity of crystal comes from lack of inversion center. Therefore, h0 0 01i direction and hence {0 0 0 1} planes of wurtzite-structure crystal have polar characteristic. The polarity of wurtzite-structure semiconductors, such as ZnO or GaN, has been receiving increasing interests because of its strong effects on properties of ®lms and applications for devices.
*
Corresponding author. Tel.: 81-22-215-2074; fax: 81-22-215-2073. E-mail address: [email protected] (S.-K. Hong).
Several groups reported the growth of GaN ®lms with a controlled polarity by employing surface modi®cations of substrate, buffer layer growth and change of growth process, where procedures for growing Gaand N-polar GaN ®lms are nominally different between metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). In some cases, the observed polarity is different for MOCVD and MBE grown GaN ®lms even the same buffer layers (low-temperature GaN buffer) and substrates (c-plane sapphire without a nitridation process) are used. In this paper, we report the effects of the modi®cations of interfaces on the polarity of heteroepitaxial
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 2 4 - 2
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S.-K. Hong et al. / Applied Surface Science 190 (2002) 491±497
ZnO ®lms grown by plasma-assisted molecular beam epitaxy (P-MBE) on GaN template and c-plane sapphire substrates. We have tried to modify the interfaces between ZnO and GaN to grow Zn-polar and O-polar ZnO ®lms on the same Ga-polar GaN templates. Furthermore, we have examined possibility of inverting the polarity of ZnO ®lms to the other by engineering the interfaces. 2. Experimental Substrates used in this study are Ga-polar (0 0 0 1) GaN epilayers (4 mm thick) grown on c-plane sapphire by MOCVD and c-plane sapphire. Degreasing of epi-GaN/Al2O3 and c-plane sapphire substrates was done by ultrasonic cleaning in acetone and methanol at RT followed by rinsing with deionized water and spin-drying. The degreased epi-GaN template substrate was thermally cleaned at 700±800 8C in ultrahigh vacuum for about 1 h. The degreased sapphire substrate was chemically etched using a solution of H2SO4 (96 vol.%):H3PO4 86 vol:% 3:1 at 160 8C for 15 min followed by rinsing with deionized water and spin-drying. Chemically etched substrates are thermally cleaned at 650 8C under O-plasma ambient for 30 min. ZnO and MgO ®lms were grown by P-MBE using a radio frequency oxygen-plasma source, a Zn solid source, and a Mg solid source. Zn- or O-plasma pre-exposure on the cleaned GaN surfaces was carried out at 700 8C for 3 min. The GaN template substrate and the cleaning procedures are the same for both Zn- and O-plasma pre-exposed samples. After these pre-exposures on the GaN surfaces, ZnO ®lms were grown at 550±700 8C and MgO ®lms were grown at 490±550 8C. The vacuum pressure was 2 10 10 Torr prior to deposition and 9 10 5 Torr during growth. The Zn ¯ux was set to 0.15±0.20 nm/s and the Mg ¯ux was set to 0.03±0.10 nm/s, which were monitored by quartz thickness monitor before and after growth. Oplasma power was set to 300±400 W with an oxygen gas ¯ow rate of 2.5±3.5 sccm. Polarity of the grown ®lms is determined by coaxial impact collision ion scattering spectroscopy (CAICISS). Primary He ions with energy of 2 KeV are used for measurements. Polar angle (PA) dependence of CAICISS signal intensities are measured along the
azimuth direction of [11 20]. CAICISS spectra are obtained from time-of-¯ight spectra (Shimadzu Co., TALIS 9700). Polarity of the ZnO ®lms was determined by comparison of CAICISS PA dependence with the simulated one. High-resolution transmission electron microscopy (HRTEM) on ZnO/GaN interfaces was carried out using a JEOL JEM-ARM1250 high voltage electron microscope at 1250 kV. Cross-sectional TEM specimens are prepared by conventional techniques of mechanical polishing, dimpling and ion milling. Zone axis of h2 1 10iGaN and ZnO is selected for the HRTEM observations. 3. Results and discussion 3.1. Growth of Zn- and O-polar ZnO ®lms on Ga-polar GaN templates In order to investigate interfaces at ZnO/GaN with different pre-exposures, HRTEM study was carried out. Fig. 1(a) and (b) shows HRTEM micrographs for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on Ga-polar GaN templates, respectively. The interface structure showed a drastic difference as shown in Fig. 1. In the case of O-plasma pre-exposure, a continuous interface layer with a thickness of about 3.5 nm was formed at the ZnO/GaN interface, while no interface layer was observed in a Zn pre-exposed sample. The orientation relationship between ZnO and GaN was found to be [0 0 0 1]ZnO//[0 0 0 1]GaN, and [01 10]ZnO//[01 10]GaN for both types of pre-exposures. The interface layer was identi®ed as single crystalline monoclinic Ga2O3 with a zone axis of [0 1 0]. Detailed structure of the interface and model are reported elsewhere [1]. The different interface structures for Zn- and Oplasma pre-exposures would open the possibility of controlling the lattice polarity of ZnO epilayers. In order to check and con®rm the polarities of ZnO ®lms on Ga-polar GaN templates with different interface structures, the polarity of ZnO ®lms was determined by CAICISS measurements. Fig. 2 shows experimental and simulated PA dependence of Zn signals from (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms. The spectra are signi®cantly different between the two samples. Experimental spectra agree well with the
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Fig. 1. HRTEM micrographs for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on GaN templates. Single-crystalline monoclinic Ga2O3 interface layer was formed by O-plasma pre-exposure, while no interface layer was formed by Zn pre-exposure. Zone axis is parallel to h2 1 10iGaN and ZnO.
simulated spectra for Zn- and O-polar ZnO, respectively. Here, it should be noted that our results agreed well with reported data on the Zn-face and O-face of bulk ZnO [2]. Therefore, we conclude that Zn pre-exposed ZnO ®lms have a characteristic of Zn polarity, while O-plasma pre-exposed ZnO ®lms have a characteristic of O-polarity. The results mean we have successfully grown Zn-polar (cation polar) and O-polar (anion polar) ZnO ®lms, selectively, even on the same Ga-polar (cation polar) epilayer by engineering the interface. A feature of the experimental CAICISS spectrum from the O-polar ZnO ®lm shows broader character than that from the Zn-polar ZnO ®lm. We note, however, that Zn-polar domains are not observed from TEM observations. Surface morphologies of Zn-polar and O-polar ZnO ®lms grown by three-dimensional growth mode show a slight difference in RMS values from atomic force microscope observations (23 and
26 nm in 5 mm 5 mm regions for Zn-polar and Opolar ZnO ®lms indicating a slight larger value for the O-polar ZnO ®lm). Therefore, we think that the broader character results from multiple scattering and/or inelastic interactions [2], which seems to be severe for O-polar ZnO ®lm. The same reasons can be applied to Figs. 3 and 4 will be shown later. However, more systematic investigations for effects of the surface roughness on the broadening of CAICISS spectra are needed. Here, we would like to comment that the surface morphologies of ZnO ®lms in present study are mainly affected by growth conditions not by polarities although the RMS values for O-polar ZnO ®lms are slightly larger than those for Zn-polar ZnO ®lms. Crystal quality evaluated by high-resolution X-ray diffraction is revealed that larger full widths at half maximum (FWHMs) for O-polar ZnO ®lms than Znpolar ZnO ®lms. Typical FWHMs for two types of
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Fig. 2. PA dependence of CAICISS intensity for (a) Zn- and (b) O-plasma pre-exposed ZnO ®lms on GaN, which showing typical Zn- and Opolar characters, respectively. Simulated dependencies for Zn- and O-polar ZnO are also shown. The curves are plotted as a function of incident angle, which is equal to 908 minus PA.
ZnO ®lms are 399 (678) arcsec from Zn-polar (Opolar) ZnO ®lm for (0 0 0 2) O scan and 748 (1 0 2 3) arcsec from Zn-polar (O-polar) ZnO ®lm for (10 11) F scan. Whether the degraded crystal quality of Opolar ZnO ®lm comes from the different polarity or the Ga2O3 interface layer, which results in an increase of the lattice mis®t [1], is not clear yet. 3.2. Polarity of ZnO ®lms on c-Al2O3 substrates with MgO buffer We have recently succeeded in two-dimensional layer-by-layer growth of ZnO ®lms on c-plane sapphire by using novel MgO buffer [3]. Similarly to the cases of GaN-based ®lms to control the polarity by employing
the buffer layers, con®rmation of the polarity of ZnO ®lms with a MgO buffer layer seems to be meaningful. Growth procedure for this sample is ``MgO buffer growth ! ZnO buffer growth ! annealing of the ZnO buffer layer under the O-plasma ambient ! Zn shutter open for ZnO growth again on the annealed ZnO layer''. Here, the chamber is maintained under oxygen ambient and thus O-polar ZnO ®lm can be expected similarly to ZnO on Al2O3 without a MgO buffer [2] because (1) termination of Al2O3 would be oxygen reminding that the sapphire is thermally cleaned by Oplasma prior to growth and (2) MgO buffer under the oxygen ambient most likely has O-stabilized MgO surface. Consequently, further growth of ZnO would
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Fig. 3. PA dependence of CAICISS intensity for ZnO ®lm on c-plane sapphire with MgO buffer. Polarity of the ZnO ®lm showed a typical Opolar character. Simulated dependence for O-polar ZnO is also shown. The curves are plotted as a function of incident angle. Details of growth procedure are described in text.
form ``Mg±O±Zn±O'' bonding sequence at the MgO/ ZnO interface. An experimental CAICISS spectrum shown in Fig. 3 from this sample is revealed as one from the O-polarity. The samples without the annealing
process also revealed as O-polar. For comparison, simulated CAICISS spectrum for O-polar ZnO is also shown in Fig. 3. We note structure of MgO is revealed as rocksalt and bonding sequence described
Fig. 4. PA dependence of CAICISS intensity for ZnO ®lm on Ga-polar GaN template, where a MgO layer was inserted between lower and upper ZnO layers. Polarity of ZnO ®lm shows a typical O-polar character. Simulated dependence for O-polar ZnO is also shown. The curves are plotted as a function of incident angle. Details of growth procedure are described in text.
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here results in growth of O-polar ZnO ®lm on Zn-polar ZnO ®lm by using a MgO intermediate layer will be discussed in Section 3.3. Here, it should be noted that the polarity of ZnO ®lms on O-stabilized sapphire substrates has been reported as O-polar [2]. 3.3. Growth of a polarity-inverted ZnO structure We have examined the possibility of inversion of the Zn-polar ZnO ®lm grown on Ga-polar GaN template to O-polar ZnO ®lm, i.e. fabrication of polarityinverted ZnO heterostructure. Growth procedure for this sample is ``Zn pre-exposure ! low-temperature (LT) ZnO buffer growth ! Zn shutter close and hightemperature (HT) annealing of the LT ZnO buffer under the O-plasma ambient ! Zn shutter open for growth of a lower ZnO layer ! Zn shutter close and
Mg shutter open for growth of a thin MgO intermediate layer 15 ML ! Mg shutter close and Zn shutter open for growth of an upper ZnO layer on the MgO layer''. We have employed LT ZnO buffer growth followed by HT annealing to obtain very smooth ZnO surface prior to MgO growth. Consequently, RHEED oscillation was observed during the growths of the lower ZnO layer [4], MgO intermediate layer, and upper ZnO layer. It means the growth procedure was done two-dimensionally across the ZnO/MgO/ZnO interfaces, which implies the formation of very ¯at and abrupt interfaces. Note that the LT ZnO buffer and lower ZnO layers are Zn-polar because Zn pre-exposure is employed. We expected the upper ZnO layer would be grown as O-polar, i.e. polarity inversion would occur because a MgO intermediate layer under the oxygen ambient
Fig. 5. HRTEM micrograph for a ZnO/MgO/ZnO/GaN heterostructure of the sample for Fig. 4. Interfaces at lower and upper ZnO/MgO heterointerfaces are atomically ¯at and no inversion domain boundaries are observed in the upper ZnO layer. Zone axis is parallel to h2, 1, 1iGaN and ZnO.
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most likely has O-stabilized MgO surface. Consequently, the upper ZnO layer would be grown as Opolar by forming O±Zn bondings at the MgO/ZnO interface. Fig. 4 shows an experimental CAICISS spectrum from this sample, which indicates the perfect O-polar character. In order to comparison, simulated CAICISS spectrum for O-polar ZnO is also shown in Fig. 4. Here it should be noted again that the lower ZnO layer is grown as Zn-polar by employing the Zn pre-exposure. Therefore, the result means we have succeeded in growing the polarity-inverted heterostructure by employing a MgO intermediate layer between the lower (Zn-polar) and upper (O-polar) ZnO layers. In order to con®rm the ¯atness at the interfaces and the formation of inversion domain boundary, HRTEM was carried out. Fig. 5 shows an HRTEM micrograph of ZnO/MgO/ZnO/GaN heterostructure. As shown in Fig. 5, the interfaces at lower and upper ZnO/MgO are atomically ¯at. Moreover, no inversion boundary, which is inevitably observed for inversion of Ga-polar GaN to N-polar [5,6], is observed in the upper ZnO layer. Note also that there is no interface layer between ZnO and GaN, which directly indicate that lower ZnO layer bellow the MgO intermediate layer is inevitably Zn-polar. Finally, we note that the polarity of the upper ZnO ®lm on a very thin (2 ML thick with the same growth conditions) MgO intermediate layer is revealed as mixed polarity not unipolar. This is not observed from O-polar ZnO ®lm on sapphire substrate (non-polar substrate) with a MgO buffer layer. We tentatively think the origin of the observed thickness dependence is caused by the polarity inversion, which would form high-energetic interfaces. Detailed interface structure is now in investigation and will be reported elsewhere [7]. 4. Summary Controlling methods for the polarity of heteroepitaxial ZnO ®lms have been studied by engineering the
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interface. Zn-polar and O-polar ZnO ®lms on Ga-polar GaN templates have successfully grown by forming the ZnO/GaN interfaces with and without an interface layer, respectively. The interface layer, single-crystalline monoclinic Ga2O3, was formed by O-plasma preexposure, while its formation was prevented by Zn pre-exposure prior to ZnO growth. The polarity of ZnO ®lms grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. By inserting a MgO layer between two ZnO layers, the Zn-polarity (lower ZnO layer) is changed to the Opolarity (upper ZnO layer), which indicating successful fabrication of ZnO heterostructure with an inverted polar. No inversion domain boundaries were observed in the upper ZnO layer. The result would be useful in devices where a fabrication of inverted polar structures is needed. Acknowledgements S.-K. Hong would like to thank International Communications Foundation (ICF) for its ®nancial support, ``Fellowship for Researchers and Graduate Students from Abroad''. References [1] S.K. Hong, H.J. Ko, Y. Chen, T. Hanada, T. Yao, J. Vac. Sci. Technol. B 18 (2000) 2313. [2] T. Ohnishi, A. Ohtomo, M. Kawasaki, K. Takahashi, M. Yoshimoto, H. Koinuma, Appl. Phys. Lett. 72 (1999) 824. [3] Y. Chen, H.J. Ko, S.K. Hong, T. Yao, Appl. Phys. Lett. 73 (2000) 559. [4] H.J. Ko, Y. Chen, S.K. Hong, T. Yao, J. Cryst. Growth 209 (2000) 816. [5] V. Ramachandran, R.M. Feenstra, W.L. Samey, L. SalamancaRiba, J.E. Northrup, L.T. Romano, D.W. Greve, Appl. Phys. Lett. 75 (1999) 808. [6] L.T. Romano, J.E. Northrup, A.J. Ptak, T.H. Myers, Appl. Phys. Lett. 77 (2000) 2479. [7] S.K. Hong, Y. Chen, T. Hanada, H.J. Ko, T. Yao, D. Imai, K. Araki, M. Shiohara, Unpublished.