- Email: [email protected]
Uniaxial locked growth of high-quality epitaxial ZnO films on (112̄0)α -Al2O3
Journal of Crystal Growth 209 (2000) 532}536
Uniaxial locked growth of high-quality epitaxial ZnO "lms on (1 1 21 0)a-Al O 2 3 P. Fons*, K. Iwata, S. Niki, A. Yamada, K. Matsubara, M. Watanabe Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract ZnO epitaxial "lms have been grown on sapphire substrates using molecular beam epitaxy (MBE). Elemental sources of Zn and O were used with a RF radical source being employed to increase the reactivity of the oxygen source gas. High-sensitivity pole "gure measurements indicated that the "lms were uniquely (0 0 0 1) oriented with no trace of secondary orientations. The unique orientation is a consequence of the coincidental near zero lattice mismatch of the ZnO a lattice constant of 0.3250 nm and the sapphire c lattice constant over four or 0.3248 nm leading to the term uniaxial locked epitaxy. Atomic force microscopy of as-grown samples indicated that the "lms were #at with a RMS roughness of less than 0.4 nm. Two dimensional X-ray reciprocal space mapping of the ZnO(1 0 11 4) asymmetric re#ection using a triple axis con"guration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0 0 0 1)ZnO grown on sapphire(0 0 0 1) substrates to about 0.5 lm for growth on (1 11 2 0)sapphire substrates. This is interpreted as being a consequence of less in-plane twisting of domains due to stress from lattice mismatch. Preliminary photoluminescence measurements indicate a dramatic increase in intensity with bound exciton features less than 0.7 meV. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10; 61.72; 68.55; 68.65 Keywords: ZnO; Heteroepitaxy; (1 1 21 0)sapphire; Reciprocal space mapping; X-ray di!raction
As is evident from the large volume of GaN research, there is a great deal of interest in wide band gap semiconductors for use in optical devices in the near ultra-violet (UV) region. Besides GaN, the 3.37 eV room-temperature direct band gap material ZnO (space group P6 mc"C4 ) holds 67 3 promise for near UV optoelectronic applications as well. In addition, unlike GaN, bulk ZnO substrates
* Corresponding author. Tel.: #81-298-54-5636; fax: #81298-54-5615. E-mail address: [email protected] (P. Fons)
are available, making homoepitaxial growth of ZnO optical devices a possibility. The large excitonic binding energy (&60 meV) of ZnO also raises the interesting possibility of utilizing excitonic e!ects in room-temperature devices. In this work, we have used sapphire as a substrate material due to its low-cost and high crystalline perfection. ZnO can be grown on sapphire epitaxially and with a high degree of surface #atness, an essential attribute for fabrication of devices on the epilayer material. Previous reports of ZnO heteroepitaxial growth on sapphire(0 0 0 1) have used a variety of growth techniques including laser
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 1 4 - 4
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
ablation, metalorganic chemical vapor deposition, RF sputtering, and molecular beam epitaxy (MBE) [1}3]. Due to the large lattice mismatch between the ZnO epilayer (a"3.250 As , c"5.213 As ) and the underlying sapphire (a"4.754 As , c"12.99 As ) substrate, most as-grown ZnO "lms have displayed large crystal mosaics making their use for optoelectronic applications less feasible. In addition, ZnO to date has predominantly been grown on (0 0 0 1) (c-face) sapphire substrates with the resulting "lm substrate orientation taking the form ZnO(0 0 0 1)E sapphire(0 0 0 1) and ZnO[1 0 11 0]Esapphire[1 1 21 0]. In reports of ZnO growth on the (1 1 21 0) (a-face) sapphire surface using RF sputtering, ZnO has been reported to grow in the con"guration ZnO(1 1 21 0)Esapphire (0 1 11 2) [4]. In this work, ZnO "lms were deposited using MBE from an elemental Knudsen cell of Zn (7N) and oxygen (6N) supplied via a RF radical (Eiko Engineering) source. The RF radical source was also "tted with an electrostatic ion trap which was operated at &300 V during growth. After degreasing in organic solvents, (1 1 21 0)sapphire single side polished substrates were mounted using clips to Inconel substrate blocks. Prior to growth, substrate surfaces were cleaned by heating the samples to &6003C in ultra-high vacuum for 15 min. In situ Auger electron spectroscopy observation of the surface demonstrated the presence of only Al and O from the clean sapphire surface. Re#ection highenergy electron di!raction (RHEED) observations of the surface showed streaky patterns with no signs of surface reconstruction. After lowering the substrate temperature to the range 375}5003C, growth was initiated. Samples were grown for 2 h with a growth rate of 0.4 lm/h. RHEED patterns along the [1 21 1 0] changed from a sharp, streaky pattern at the onset of growth, subsequently disappeared, and then gradually transformed into a sharp streaky pattern indicative of (0 0 0 1)ZnO growth after several minutes of growth. The pattern remained sharp and streaky until the end of growth. No surface reconstruction was observed for either the sapphire substrate or the ZnO epilayer. RHEED observations indicated that the ZnOS1 1 21 0TEsapphire [0 0 0 1] direction as was con"rmed by high-sensitivity pole "gure measurements elaborated upon below. Un-
533
like the case of GaN grown on sapphire, in which it is necessary to grow a low-temperature bu!er layer which is subsequently recrystallized, for the case of ZnO on sapphire an amorphous bu!er layer forms. The presence of the amorphous bu!er layer is evidenced by the disappearance of the RHEED pattern upon the initiation of ZnO growth continuing for a thickness of 5}10 nm after which a broad streaky epitaxial pattern reappears and gradually sharpens with continued growth. This layer is referred to here as an amorphous pattern based upon the absence of any discernible RHEED pattern, but the underlying structure may have a strong c-orientated texture based upon analogous low temperature growth on glass experiments in which all samples were c-oriented. Pole "gure measurements were carried out with a point source focus rotating anode generator running at 16 000 W using a 2]2 mm beam and a Soller slit/graphite analyzer con"guration. Volume fractions of on the order of ppm are observable using this con"guration. Previous high-sensitivity pole "gure measurements using the ZnO(1 0 11 1) planes of the in-plane orientation of ZnO(0 0 0 1)E sapphire(0 0 0 1), ZnO[1 0 11 0]Esapphire [1 1 21 0] ZnO samples have typically indicated small amounts of rotation domains are present in which ZnO[1 0 11 0]Esapphire[1 0 11 0]. This is most likely a consequence of the large lattice mismatch present and the metastability of orientation domains in which the ZnO and sapphire a axes are parallel. These orientational domains, although constituting a small volume fraction, are present even in thin "lms with (0 0 0 2) mosaics of less than 10 arcsec as they do not contribute to tilt in the overlayer. The domains, however, most likely propagate from the substrate/"lm interface to the surface giving rise to additional scattering centers as well as potential deep level defects. In order to eliminate these orientational domains, growth was carried out on (1 1 21 0) sapphire surface. A typical pole "gure measurement of a MBE grown ZnO layer on (1 1 21 0) is shown in Fig. 1. The pole "gure was performed using the ZnO(1 0 11 1) planes (36.283). These poles are indicated in black text. As the sapphire(1 0 11 4) is located nearby at 35.183, it is possible to determine uniquely the relative orientation of the ZnO/(1 1 21 0)sapphire system
534
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
Fig. 1. X-ray pole "gure of ZnO 1 0 11 1 poles.
as ZnO(0 0 0 1)Esapphire(1 1 21 0), ZnO[1 1 21 0]E sapphire(0 0 0 1). As underlying symmetry of the sapphire(1 1 21 0) surface is only two-fold as opposed to the six-fold symmetric (0 0 0 1)ZnO surface, it is speculated that the near perfect alignment of the two layers along the sapphire[0 0 0 1]/ ZnO[1 1 21 0] directions is responsible for the unique relative orientation observed. No alternative ZnO orientations or change in relative orientation were observed over the entire range of growth temperatures employed, although at higher temperature the emergence of ZnOM1 0 11 3N growth twins was observed. To the best knowledge of the authors, this is the "rst time the occurrence of ZnOM1 0 11 3N growth twins have been reported for hexagonal material in the literature. Fig. 2 shows atomic force microscopy measurements made using a Nanoscope DI3000 operating in tapping mode of an as-grown ZnO "lm. These measurements con"rmed in situ RHEED observations that the "lms are #at to within the resolution of the AFM with a RMS roughness of approximately 0.4 nm.
Fig. 2. AFM measurements of the surface of an as-grown ZnO epilayer grown on a-sapphire.
High-resolution X-ray reciprocal space maps were taken using a four crystal Bartels-type monochromator using 153 asymmetric cut Ge(2 2 0) re#ections along with a three bounce Ge(2 2 0)
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
535
Fig. 3. (1 0 11 4) X-ray reciprocal space maps of ZnO epilayers grown on (a) (0 0 0 1) and (b) (1 1 21 0)sapphire.
analyzer. The goniometer was a Philips MRD which has an absolute precision of approximately 0.00013 along the u and 2h axes. The angular resolution function was approximately square with du and dh&0.0063. Fig. 3 shows a region of reciprocal space about the ZnO(1 0 11 4) obtained used an asymmetric re#ection geometry for the case of ZnO grown on (0 0 0 1) and (1 1 21 0)sapphire, respectively. As the lateral and temporal resolution of the incident monochromatized X-ray beam are large with respect to the coherence lengths of the sample, the lateral coherence of the di!racted X-ray beam may be determined. Details of the procedure have been described by Fewster [5]. Typical lateral coherence lengths of ZnO "lms grown on (0 0 0 1)sapphire are of the order of 50 nm, while those of ZnO "lms grown on (1 1 21 0)sapphire are typical of the order of 0.5 lM. This dramatic increase in coherence length is most likely due to a change in the "lm/substrate relaxation mechanism. In the case of ZnO grown on (0 0 0 1)sapphire, ZnO islands exhibit a distribution in relative orientation about the (0 0 0 1) axis which is referred to here as twist. Reduction in this twist, results in the increase in coherence length of the ZnO "lms grown on (1 1 21 0)sapphire. An attempt was made to quantify the decrease in twist by measuring the symmetric (1 0 11 1)ZnO re#ection which is located approximately 653 from the surface normal, hence giving a good indication of the in-plane twist. Taking the projection of the
Fig. 4. 1.5 K photoluminescence from a typical ZnO sample grown on a (1 1 21 0)sapphire substrate.
relative u half-width into the (0 0 0 1)ZnO plane showed a decrease in twist by approximately a factor of two, from 0.21 to 0.133. Preliminary photoluminescence measurements as can be seen in Fig. 4 have been performed using a Bomem DA-8 FTIR and a HeCd laser. Emission was measured from 11 000 to 30 000 cm~1 at a resolution of 1 cm~1 (0.1 As ). Sharp bandedge excitonrelated emissions along with a free exciton peak were con"rmed via excitation power dependence. Unlike, the sharp mosaic ZnO "lms reported earlier [4], there was no detectable visible emission in the visible (yellow) region. Emission half-widths on the order of 0.7 meV were noted indicating the presence of a highly uniform matrix over the 3 mm diameter laser probe area. In addition, overall bandedge emission intensities of ZnO grown on
536
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
(1 1 21 0)sapphire were observed to dramatically increase over those of "lms grown on c-sapphire substrates indicating a substantial decrease in nonradiative recombination center concentration. Intensity dependence data indicates that all peaks are exciton related. In summary, we have grown heteroepitaxially, high-quality ZnO epilayers with on sapphire(1 1 21 0) substrates. X-ray pole "gure measurements indicate that there are no rotational domains present in the as-grown "lms and photoluminescence measurements indicate high optical quality.
References [1] M.A.L. Johnson, Shizuo Fujita, W.H. Rowland Jr., W.C. Hughes, J.W. Cook Jr., J.F. Schetzina, J. Electron. Mater. 25 (1996) 855. [2] Yefan Chen, D.M. Bagnall, Ziqiang Zhu, Takashi Sekiuchi, Kitae Park, Kenji Hiraga, Takafumi Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [3] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, J. Cryst. Growth 201}202 (1999) 627. [4] Tadashi Shiosaki, Takashi Yamamoto, Mitsuo Yagi, Akira Kawabata, Appl. Phys. Lett. 39 (1981) 399. [5] P.F. Fewster, in: AndreH Authier, Stefano Lagomarsion, Brian K. Tanner (Eds.), X-ray and Neutron Dynamical Di!raction, Plenum Press, New York, 1996, p. 283.
Uniaxial locked growth of high-quality epitaxial ZnO "lms on (1 1 21 0)a-Al O 2 3 P. Fons*, K. Iwata, S. Niki, A. Yamada, K. Matsubara, M. Watanabe Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract ZnO epitaxial "lms have been grown on sapphire substrates using molecular beam epitaxy (MBE). Elemental sources of Zn and O were used with a RF radical source being employed to increase the reactivity of the oxygen source gas. High-sensitivity pole "gure measurements indicated that the "lms were uniquely (0 0 0 1) oriented with no trace of secondary orientations. The unique orientation is a consequence of the coincidental near zero lattice mismatch of the ZnO a lattice constant of 0.3250 nm and the sapphire c lattice constant over four or 0.3248 nm leading to the term uniaxial locked epitaxy. Atomic force microscopy of as-grown samples indicated that the "lms were #at with a RMS roughness of less than 0.4 nm. Two dimensional X-ray reciprocal space mapping of the ZnO(1 0 11 4) asymmetric re#ection using a triple axis con"guration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0 0 0 1)ZnO grown on sapphire(0 0 0 1) substrates to about 0.5 lm for growth on (1 11 2 0)sapphire substrates. This is interpreted as being a consequence of less in-plane twisting of domains due to stress from lattice mismatch. Preliminary photoluminescence measurements indicate a dramatic increase in intensity with bound exciton features less than 0.7 meV. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10; 61.72; 68.55; 68.65 Keywords: ZnO; Heteroepitaxy; (1 1 21 0)sapphire; Reciprocal space mapping; X-ray di!raction
As is evident from the large volume of GaN research, there is a great deal of interest in wide band gap semiconductors for use in optical devices in the near ultra-violet (UV) region. Besides GaN, the 3.37 eV room-temperature direct band gap material ZnO (space group P6 mc"C4 ) holds 67 3 promise for near UV optoelectronic applications as well. In addition, unlike GaN, bulk ZnO substrates
* Corresponding author. Tel.: #81-298-54-5636; fax: #81298-54-5615. E-mail address: [email protected] (P. Fons)
are available, making homoepitaxial growth of ZnO optical devices a possibility. The large excitonic binding energy (&60 meV) of ZnO also raises the interesting possibility of utilizing excitonic e!ects in room-temperature devices. In this work, we have used sapphire as a substrate material due to its low-cost and high crystalline perfection. ZnO can be grown on sapphire epitaxially and with a high degree of surface #atness, an essential attribute for fabrication of devices on the epilayer material. Previous reports of ZnO heteroepitaxial growth on sapphire(0 0 0 1) have used a variety of growth techniques including laser
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 1 4 - 4
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
ablation, metalorganic chemical vapor deposition, RF sputtering, and molecular beam epitaxy (MBE) [1}3]. Due to the large lattice mismatch between the ZnO epilayer (a"3.250 As , c"5.213 As ) and the underlying sapphire (a"4.754 As , c"12.99 As ) substrate, most as-grown ZnO "lms have displayed large crystal mosaics making their use for optoelectronic applications less feasible. In addition, ZnO to date has predominantly been grown on (0 0 0 1) (c-face) sapphire substrates with the resulting "lm substrate orientation taking the form ZnO(0 0 0 1)E sapphire(0 0 0 1) and ZnO[1 0 11 0]Esapphire[1 1 21 0]. In reports of ZnO growth on the (1 1 21 0) (a-face) sapphire surface using RF sputtering, ZnO has been reported to grow in the con"guration ZnO(1 1 21 0)Esapphire (0 1 11 2) [4]. In this work, ZnO "lms were deposited using MBE from an elemental Knudsen cell of Zn (7N) and oxygen (6N) supplied via a RF radical (Eiko Engineering) source. The RF radical source was also "tted with an electrostatic ion trap which was operated at &300 V during growth. After degreasing in organic solvents, (1 1 21 0)sapphire single side polished substrates were mounted using clips to Inconel substrate blocks. Prior to growth, substrate surfaces were cleaned by heating the samples to &6003C in ultra-high vacuum for 15 min. In situ Auger electron spectroscopy observation of the surface demonstrated the presence of only Al and O from the clean sapphire surface. Re#ection highenergy electron di!raction (RHEED) observations of the surface showed streaky patterns with no signs of surface reconstruction. After lowering the substrate temperature to the range 375}5003C, growth was initiated. Samples were grown for 2 h with a growth rate of 0.4 lm/h. RHEED patterns along the [1 21 1 0] changed from a sharp, streaky pattern at the onset of growth, subsequently disappeared, and then gradually transformed into a sharp streaky pattern indicative of (0 0 0 1)ZnO growth after several minutes of growth. The pattern remained sharp and streaky until the end of growth. No surface reconstruction was observed for either the sapphire substrate or the ZnO epilayer. RHEED observations indicated that the ZnOS1 1 21 0TEsapphire [0 0 0 1] direction as was con"rmed by high-sensitivity pole "gure measurements elaborated upon below. Un-
533
like the case of GaN grown on sapphire, in which it is necessary to grow a low-temperature bu!er layer which is subsequently recrystallized, for the case of ZnO on sapphire an amorphous bu!er layer forms. The presence of the amorphous bu!er layer is evidenced by the disappearance of the RHEED pattern upon the initiation of ZnO growth continuing for a thickness of 5}10 nm after which a broad streaky epitaxial pattern reappears and gradually sharpens with continued growth. This layer is referred to here as an amorphous pattern based upon the absence of any discernible RHEED pattern, but the underlying structure may have a strong c-orientated texture based upon analogous low temperature growth on glass experiments in which all samples were c-oriented. Pole "gure measurements were carried out with a point source focus rotating anode generator running at 16 000 W using a 2]2 mm beam and a Soller slit/graphite analyzer con"guration. Volume fractions of on the order of ppm are observable using this con"guration. Previous high-sensitivity pole "gure measurements using the ZnO(1 0 11 1) planes of the in-plane orientation of ZnO(0 0 0 1)E sapphire(0 0 0 1), ZnO[1 0 11 0]Esapphire [1 1 21 0] ZnO samples have typically indicated small amounts of rotation domains are present in which ZnO[1 0 11 0]Esapphire[1 0 11 0]. This is most likely a consequence of the large lattice mismatch present and the metastability of orientation domains in which the ZnO and sapphire a axes are parallel. These orientational domains, although constituting a small volume fraction, are present even in thin "lms with (0 0 0 2) mosaics of less than 10 arcsec as they do not contribute to tilt in the overlayer. The domains, however, most likely propagate from the substrate/"lm interface to the surface giving rise to additional scattering centers as well as potential deep level defects. In order to eliminate these orientational domains, growth was carried out on (1 1 21 0) sapphire surface. A typical pole "gure measurement of a MBE grown ZnO layer on (1 1 21 0) is shown in Fig. 1. The pole "gure was performed using the ZnO(1 0 11 1) planes (36.283). These poles are indicated in black text. As the sapphire(1 0 11 4) is located nearby at 35.183, it is possible to determine uniquely the relative orientation of the ZnO/(1 1 21 0)sapphire system
534
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
Fig. 1. X-ray pole "gure of ZnO 1 0 11 1 poles.
as ZnO(0 0 0 1)Esapphire(1 1 21 0), ZnO[1 1 21 0]E sapphire(0 0 0 1). As underlying symmetry of the sapphire(1 1 21 0) surface is only two-fold as opposed to the six-fold symmetric (0 0 0 1)ZnO surface, it is speculated that the near perfect alignment of the two layers along the sapphire[0 0 0 1]/ ZnO[1 1 21 0] directions is responsible for the unique relative orientation observed. No alternative ZnO orientations or change in relative orientation were observed over the entire range of growth temperatures employed, although at higher temperature the emergence of ZnOM1 0 11 3N growth twins was observed. To the best knowledge of the authors, this is the "rst time the occurrence of ZnOM1 0 11 3N growth twins have been reported for hexagonal material in the literature. Fig. 2 shows atomic force microscopy measurements made using a Nanoscope DI3000 operating in tapping mode of an as-grown ZnO "lm. These measurements con"rmed in situ RHEED observations that the "lms are #at to within the resolution of the AFM with a RMS roughness of approximately 0.4 nm.
Fig. 2. AFM measurements of the surface of an as-grown ZnO epilayer grown on a-sapphire.
High-resolution X-ray reciprocal space maps were taken using a four crystal Bartels-type monochromator using 153 asymmetric cut Ge(2 2 0) re#ections along with a three bounce Ge(2 2 0)
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
535
Fig. 3. (1 0 11 4) X-ray reciprocal space maps of ZnO epilayers grown on (a) (0 0 0 1) and (b) (1 1 21 0)sapphire.
analyzer. The goniometer was a Philips MRD which has an absolute precision of approximately 0.00013 along the u and 2h axes. The angular resolution function was approximately square with du and dh&0.0063. Fig. 3 shows a region of reciprocal space about the ZnO(1 0 11 4) obtained used an asymmetric re#ection geometry for the case of ZnO grown on (0 0 0 1) and (1 1 21 0)sapphire, respectively. As the lateral and temporal resolution of the incident monochromatized X-ray beam are large with respect to the coherence lengths of the sample, the lateral coherence of the di!racted X-ray beam may be determined. Details of the procedure have been described by Fewster [5]. Typical lateral coherence lengths of ZnO "lms grown on (0 0 0 1)sapphire are of the order of 50 nm, while those of ZnO "lms grown on (1 1 21 0)sapphire are typical of the order of 0.5 lM. This dramatic increase in coherence length is most likely due to a change in the "lm/substrate relaxation mechanism. In the case of ZnO grown on (0 0 0 1)sapphire, ZnO islands exhibit a distribution in relative orientation about the (0 0 0 1) axis which is referred to here as twist. Reduction in this twist, results in the increase in coherence length of the ZnO "lms grown on (1 1 21 0)sapphire. An attempt was made to quantify the decrease in twist by measuring the symmetric (1 0 11 1)ZnO re#ection which is located approximately 653 from the surface normal, hence giving a good indication of the in-plane twist. Taking the projection of the
Fig. 4. 1.5 K photoluminescence from a typical ZnO sample grown on a (1 1 21 0)sapphire substrate.
relative u half-width into the (0 0 0 1)ZnO plane showed a decrease in twist by approximately a factor of two, from 0.21 to 0.133. Preliminary photoluminescence measurements as can be seen in Fig. 4 have been performed using a Bomem DA-8 FTIR and a HeCd laser. Emission was measured from 11 000 to 30 000 cm~1 at a resolution of 1 cm~1 (0.1 As ). Sharp bandedge excitonrelated emissions along with a free exciton peak were con"rmed via excitation power dependence. Unlike, the sharp mosaic ZnO "lms reported earlier [4], there was no detectable visible emission in the visible (yellow) region. Emission half-widths on the order of 0.7 meV were noted indicating the presence of a highly uniform matrix over the 3 mm diameter laser probe area. In addition, overall bandedge emission intensities of ZnO grown on
536
P. Fons et al. / Journal of Crystal Growth 209 (2000) 532}536
(1 1 21 0)sapphire were observed to dramatically increase over those of "lms grown on c-sapphire substrates indicating a substantial decrease in nonradiative recombination center concentration. Intensity dependence data indicates that all peaks are exciton related. In summary, we have grown heteroepitaxially, high-quality ZnO epilayers with on sapphire(1 1 21 0) substrates. X-ray pole "gure measurements indicate that there are no rotational domains present in the as-grown "lms and photoluminescence measurements indicate high optical quality.
References [1] M.A.L. Johnson, Shizuo Fujita, W.H. Rowland Jr., W.C. Hughes, J.W. Cook Jr., J.F. Schetzina, J. Electron. Mater. 25 (1996) 855. [2] Yefan Chen, D.M. Bagnall, Ziqiang Zhu, Takashi Sekiuchi, Kitae Park, Kenji Hiraga, Takafumi Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [3] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, J. Cryst. Growth 201}202 (1999) 627. [4] Tadashi Shiosaki, Takashi Yamamoto, Mitsuo Yagi, Akira Kawabata, Appl. Phys. Lett. 39 (1981) 399. [5] P.F. Fewster, in: AndreH Authier, Stefano Lagomarsion, Brian K. Tanner (Eds.), X-ray and Neutron Dynamical Di!raction, Plenum Press, New York, 1996, p. 283.