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Polarization properties of nonpolar ZnO films grown on R-sapphire substrates using high-temperature H2O generated by a catalytic reaction
Polarization properties of nonpolar ZnO films grown on R-sapphire substrates using high-temperature H 2 O generated by a catalytic reaction Ariyuki Kato, Shotaro Ono, Munenori Ikeda, Ryouichi Tajima, Yudai Adachi, Kanji Yasui PII: DOI: Reference:
S0040-6090(17)30655-7 doi:10.1016/j.tsf.2017.06.062 TSF 36198
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
24 March 2017 31 May 2017 4 June 2017
Please cite this article as: Ariyuki Kato, Shotaro Ono, Munenori Ikeda, Ryouichi Tajima, Yudai Adachi, Kanji Yasui, Polarization properties of nonpolar ZnO films grown on Rsapphire substrates using high-temperature H2 O generated by a catalytic reaction, Thin Solid Films (2017), doi:10.1016/j.tsf.2017.06.062
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Polarization properties of nonpolar ZnO films grown on R-sapphire substrates using
SC RI
PT
high-temperature H2O generated by a catalytic reaction
Ariyuki Kato, Shotaro Ono, Munenori Ikeda, Ryouichi Tajima, Yudai Adachi, Kanji Yasui * Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan,
MA
NU
* [email protected]
Abstract
ED
Nonpolar ZnO films were grown on r-plane sapphire substrates through the reaction
PT
between dimethylzinc (DMZn) and high-temperature H2O, the latter produced by the
CE
Pt-catalyzed reaction between H2 and O2. The resulting ZnO films were characterized using
AC
atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. They were found to have an anisotropic surface morphology with stripe arrays, and exhibited a diffraction peak associated with ZnO (11-20) index planes. The PL spectra indicated anisotropy in polarization between the directions parallel and perpendicular to the c-axis. The band-edge luminescence at 3.3 eV exhibited a maximum when the electric field vector E was perpendicular to the c-axis (parallel to the [1-100] direction) and another minimum when E was parallel to the c-axis. The angular dependence of the linear polarization of the band-edge luminescence was
ACCEPTED MANUSCRIPT large for ZnO films grown at low temperatures. The large degree of polarization observed for
AC
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PT
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Keywords: ZnO, nonpolar, catalytic reaction, polarization
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low-temperature growth is thought to be due to the film geometry.
ACCEPTED MANUSCRIPT 1. Introduction
PT
Zinc oxide (ZnO) has recently attracted considerable interest owing to its potential applications in the ultraviolet wavelength range [1]. In addition to its large direct band gap of
SC RI
3.37 eV at room temperature, excitons in ZnO offer significant advantages for optoelectronic
NU
applications, such as light-emitting diodes and ultraviolet laser diodes, because of their large binding energy of approximately 60 meV [2]. In order to fabricate efficient optical emission
MA
devices, a superlattice structure is often adopted using ZnO and magnesium zinc oxide (MgZnO)
ED
layers. The ZnO films are mainly grown along the <0001> direction of the ZnO hexagonal
PT
wurtzite structure on c-plane sapphire substrates [3]. However, a macroscopic electrostatic field
CE
is generated along the c-axis, which results in spontaneous piezoelectric polarization, which in
AC
turn induces an electric field [4]. This electric field has an adverse effect on device properties, such as a decrease in the overlap of the electron and hole wave functions in the ZnO quantum well layer, which reduces the internal quantum efficiency of optical emission devices [5]. Thus, the growth of nonpolar ZnO films is required to eliminate such polarization effects. There have been a number of reports on the growth of nonpolar ZnO (a-plane) on r-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) [6,7], molecular beam epitaxy (MBE) [8], and pulsed laser deposition [9]. We have developed a new CVD method using high-temperature H2O generated by a catalytic reaction between H2 and O2 on platinum (Pt)
ACCEPTED MANUSCRIPT nanoparticles. The CVD method using this catalytic reaction was demonstrated to be useful for
PT
the growth of metal oxide thin films under energy-saving conditions, and has also been found to produce c-plane ZnO films on a-plane (11-20) sapphire (a-Al2O3) substrates with excellent
SC RI
optical and electrical properties [10]. The investigation of nonpolar ZnO film growth using
NU
catalytically assisted CVD is motivated by the desire to fabricate optical emission devices with superior properties.
MA
In the present work, growth of nonpolar ZnO (a-plane) films was investigated on r-plane
ED
sapphire substrates using catalytically assisted CVD. The structure, surface morphology, and
PT
optical emission properties (in particular, polarization properties) of a-plane ZnO films were
AC
CE
characterized.
2. Experimental
The present CVD apparatus had the same features as that reported in a previous paper [10]. A gas mixture of H2 and O2 was introduced into a catalyst cell containing a Pt-dispersed ZrO2 catalyst, following which the temperature of the catalyst cell quickly (within 1 min) rose above 1000°C, owing to the exothermic reaction between H2 and O2 over the catalyst (Pt nanoparticles). Within 10 min of the introduction of the H2-O2 gas mixture, the catalytic cell temperature stabilized, at which time (i.e., precisely 10 min after the introduction of H2-O2) the shutter
ACCEPTED MANUSCRIPT located between the skimmer cone and the substrate holder was opened. The generated
PT
high-temperature H2O molecules were ejected from a de Laval nozzle into the reaction zone in the chamber and allowed to collide with DMZn gas ejected from another small nozzle (inner
SC RI
diameter: 1.0 mm). The H2 and O2 gas flow rates during growth were set at 400 and 130 sccm,
NU
respectively. The DMZn gas flow rate was adjusted to maintain a partial pressure of 4.6×10-3 Pa. The skimmer cone between the H2O nozzle and the substrate served the purpose of
MA
selectively directing only high-velocity H2O molecules to the substrate. Epitaxial ZnO films
ED
were grown on r-plane Al2O3 substrates at substrate temperatures from 400 to 600°C for 60 min
PT
without a buffer layer. The total reaction gas pressure in the chamber during deposition ranged
CE
from 0.4 to 0.7 Pa. The pressure was measured using an ionization vacuum gauge; however, the
AC
measured value was not multiplied by the correction factor for the specific gas species. The sapphire substrates were degreased by washing with methanol and acetone in an ultrasonic cleaner, etched with a H2SO4/H3PO4 solution, rinsed with ultrapure water, and finally, set on a substrate holder in the CVD chamber. The film thicknesses were 2.0-3.0 m. The crystallinity and crystal orientation for each ZnO film were characterized by X-ray diffraction (XRD, Bragg-Brentano configuration, -2 scan) using Cu-Kα1 radiation (Rigaku, SmartLab). The surface morphology was observed using atomic force microscopy (AFM, Shimadzu 9500J2 System). Photoluminescence (PL) spectra were measured at room temperature using a
ACCEPTED MANUSCRIPT spectrometer (HORIBA JOBIN YVON, LabRAM HR-PL) in conjunction with a CCD detector
PT
(HORIBA JOVAN YVON, Synapse). PL excitation was obtained using the 325 nm line of a He-Cd laser. Furthermore, the polarization of the band-edge emission from the ZnO films was
SC RI
determined by using a linear polarizer. The linear polarizer and a polarization canceling plate
NU
were inserted between the sample and spectrometer. By rotating the linear polarizer, the angular
MA
dependence of the polarization was recorded, as shown in Fig. 1.
ED
3. Results and discussion
PT
The XRD patterns for all ZnO films grown on r-plane sapphire substrates exhibited an
CE
intense (11-20) peak at 2=56.64°, associated with ZnO A-planes (see Fig. 2). No diffraction
AC
peaks associated with other planes, such as (10-10) planes at 2=31.8°, the (0002) plane at 2=34.4°, and (10-11) planes at 2=36.3°, were observed. This indicates that all films were composed of domains grown parallel to the a-axis. From the diffraction peak at 2=56.64°, the lattice constant of the a-axis was estimated to be 0.3248 nm, which is almost the same as that of bulk ZnO crystals (0.32496 nm) [11]. Thus, the residual compressive stress perpendicular to the film surface was very small irrespective of the growth temperature. The full width at half maximum (FWHM) values of the diffraction peaks of films grown at 450, 500, and 600°C were 0.100°, 0.0897°, and 0.0796°, respectively. Based on these FWHM values, the average crystal
ACCEPTED MANUSCRIPT sizes perpendicular to the film surface of the films grown at 450, 500 and 600°C were estimated
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to be 94, 105, and 118 nm, respectively, using Scherrer’s formula [12]. The surface morphology, as determined by AFM, was different from that of c-plane ZnO,
SC RI
which features hexagonal facets. Instead, striped patterns were observed, as shown in Fig. 3. The
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long axes of the stripes were parallel to the <1-102> axis of the sapphire substrate and parallel to the c-axis of the ZnO film. The domain size and surface roughness increased with increasing
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growth temperature. In particular, the widths of the stripes were less than 1 m for ZnO films
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grown at 450°C.
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The PL spectra of ZnO films grown at various temperatures were measured at room
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temperature using a He-Cd laser and a linear polarizer. Figure 4 shows the PL spectra obtained
AC
for the case where the electric field vector E of the PL was parallel to the [1-100] direction (0°) (solid lines) and for the case where E was parallel to the c-axis (90°) (dotted lines) at room temperature. Two emission bands were observed. The band-edge luminescence, with a peak position at about 3.3 eV, was strong (weak) in the former (latter) case, i.e., the a-ZnO films emitted stronger band-edge luminescence when the polarization was perpendicular to the c-axis. On the other hand, the peak intensity of the green band with a peak at about 2.3-2.4 eV, commonly referred to as the deep-level emission caused by defects [1], was larger for 90° than for 0°, i.e., the a-ZnO films emitted stronger green luminescence when the polarization was
ACCEPTED MANUSCRIPT along the c-axis. Band-edge luminescence originates from the electron transitions between a
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conduction band and three valence bands, viz., the A band, B band, and C band, in wurtzite-type semiconductors such as ZnO and GaN. Electron transitions between the conduction band and the
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A and B valence bands are allowed when E is perpendicular to the c-axis. On the other hand,
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transitions related to the C-band are allowed for E parallel to the c-axis [13]. The hole concentration in the A and B bands is greater than that in the C band because the energy levels of
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the A and B bands are higher than that of the C band. Therefore, the band-edge emission
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intensity perpendicular to the c-axis is greater than that parallel to the c-axis.
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Figure 5 shows the polarization angle dependence of the peak intensity of the band-edge
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luminescence. Although the angular dependence was similar for all ZnO films, the variation in
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emission intensity was different. The ZnO films grown at high temperature (600°C) showed little variation, while those grown at low temperatures (450-500°C) showed a large variation. Figure 6 shows the degree of polarization defined as [(I[0001]-I//[0001])/(I[0001] +I//[0001])×100 (%)] calculated from the peak intensities of the band-edge luminescence perpendicular to the c-axis (I[0001]) and parallel to the c-axis (I//[0001]). The ZnO films grown at 450°C exhibited the highest degree of polarization (63%), while those grown at 600°C exhibited a small intensity ratio (22%). The maximum value of 63% was greater than the 16% reported for ZnO nanorods by Hsu et al. [14]. The high degree of polarization for low-temperature growth is considered to
ACCEPTED MANUSCRIPT be due to the film geometry, namely, a small striped domain structure with a large aspect ratio.
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This large polarization is a unique property and could lead to applications of
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polarization-sensitive optoelectronic devices in the ultraviolet wavelength region.
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4. Conclusion
Nonpolar ZnO films were grown on r-plane sapphire substrates by a new CVD method using
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a catalytic reaction between H2 and O2. The films exhibited an anisotropic surface morphology
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with arrays of stripes. PL spectra indicated anisotropy in polarization between the directions
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parallel and perpendicular to the c-axis. The band-edge luminescence was large when the electric
CE
field vector E was parallel to the [1-100] direction and weak when E was parallel to the c-axis.
AC
The angular dependence of the linear polarization of the band-edge luminescence was large for ZnO films grown at low temperatures. The degree of polarization determined from the peak intensities of the band-edge luminescence perpendicular to the c-axis and the band-edge luminescence parallel to the c-axis was 63%, which is much greater than the value reported for ZnO nanorods. This high degree of polarization for low-temperature growth is considered to be due to the small striped domain structure in the film.
Acknowledgements
ACCEPTED MANUSCRIPT This work was supported in part by a Grant-in-Aid for Scientific Research (No. 16H03869) from
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the Japan Society for the Promotion of Science.
ACCEPTED MANUSCRIPT References
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[1] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005)
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041301.
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[2] B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Christein, A. Hoffmann, M. Straßburg, M. Dworzak, U. Haboeck, A.V. Rodina, Bound exciton and
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donor-acceptor pair recombinations in ZnO, Phys. Stat. Sol. (b) 241 (2004) 231-260.
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[3] T. Gruber, C. Kirchner, K. Thonke, R. Sauer, A. Waag, MOCVD Growth of ZnO for
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Optoelectronic Applications, Phys. Stat. Sol. (a) 192 (2002) 166-170.
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[4] C. Morhain, T. Bretagnon, P. Lefebvre, X. Tang, P. Valvin, T. Guillet, B. Gil, T. Taliercio, M. B.
Vinter,
C.
Deparis,
Internal
electric
field
in
wurtzite
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Teisseire-Doninelli,
ZnO/Zn0.78Mg0.22O quantum wells, Phys. Rev. B 72 (2005) 241305. [5] F. Bernardini, V. Fiorentini, D. Vandervilt, Spontaneous polarization and piezoelectric constants of III-V nitrides, Phys. Rev. B 56 (1997) R10024-R10027. [6] C.R. Gorla, N.W. Emanetoğlu, S. Liang, W. E. Mayo, Y. Lu, M. Wraback, H. Shen, Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (1999) 2595-2602. [7] Y. Kashiwaba, T. Abe, S. Onodera, F. Masuoka, A. Nakagawa, H. Endo, I. Niikura, Y.
ACCEPTED MANUSCRIPT Kashiwaba, Comparison of non-polar ZnO(1120) films deposited on single crystal ZnO
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(11-20) and sapphire (01-12 ) substrates, J. Cryst. Growth 298 (2007) 477-480. [8] J.-M. Chauveau, D.A. Buell, M. Laügt, P. Vennegues, M. Teisseire-Doninelli, S.
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Berard-Bergery, C. Deparis, B. Lo, B. Vinter, C. Morhain, Growth of non-polar
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ZnO/(Zn,Mg)O quantum well structures on R-sapphire by plasma-assisted molecular beam epitaxy, J. Cryst. Growth 301-302 (2007) 366-369.
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[9] P. Pant, J.D. Budai, R. Aggarwal, R.J. Narayan, J. Narayan, Thin film epitaxy and structure
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property correlations for non-polar ZnO films, Acta Mater. 57 (2009) 4426-4431.
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[10] K. Yasui, M. Miura, H. Nishiyama, Deposition of zinc oxide thin films using a surface
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reaction on platinum nanoparticles, Mater. Res. Soc. Symp. Proc., 1315 (2011) 21-26.
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[11] H. Karzel, W. Potzel, M. Köfferlein, W. Schiessl, M. Steiner, U. Hiller, G. M. Kalvius, D. W. Mitchell, T. P. Das, P. Blaha. K. Schwarz, M. P. Pasternak, Lattice dynamics and hyperfine interactions in ZnO and ZnSe at high external pressures, Phys. Rev. B 53 (1996) 11425-11438. [12] T. Pandiyarajan, B. Karthikeyan, Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles, J. Nanopart. Res., 14 (2012) 647. [13] S.K. Han, S.K. Hong, J.W. Lee, J.Y. Lee, J.H. Song, Y.S. Nam, S.K. Chang, T. Minegishi, T. Yao, Structural and optical properties of non-polar A-plane ZnO films grown on R-plane
ACCEPTED MANUSCRIPT sapphire substrates by plasma-assisted molecular-beam epitaxy, J. Cryst. Growth, 309 (2007)
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121-127. [14] N.E. Hsu, W.K. Hung, Y.F. Chen, Origin of defect emission identified by polarized
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luminescence from aligned ZnO nanorods, J. Appl. Phys. 96 (2004) 4671-4673.
ACCEPTED MANUSCRIPT Figure captions
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Fig. 1. Experimental setup for polarization of the optical emission measurement. Fig. 2. X-ray diffraction patterns for ZnO films grown on r-plane sapphire substrates at various
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temperatures.
Fig. 3. Atomic force microscopy images of ZnO films grown on r-plane sapphire substrates
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(20×20 m).
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Fig. 4. Polarization dependence of PL spectra for a-plane ZnO films grown at various
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temperatures.
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Fig. 5. Variation in the band-edge luminescence intensity for ZnO films grown at various
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temperatures as a function of polarization angle.
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Fig. 6. Degree of polarization defined by [(I⊥[0001]-I//[0001])/(I⊥[0001] +I//[0001])×100(%)] calculated from the peak intensities of the band-edge luminescence perpendicular and parallel to the c-axis.
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Figure 1
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ACCEPTED MANUSCRIPT Highlights
ZnO films were grown on r-plane sapphire substrates by CVD using a catalytic reaction. Crystallinity, morphology, PL properties were measured for a-plane ZnO films.
The polarization of the luminescence was shown to highly anisotropic.
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S0040-6090(17)30655-7 doi:10.1016/j.tsf.2017.06.062 TSF 36198
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
24 March 2017 31 May 2017 4 June 2017
Please cite this article as: Ariyuki Kato, Shotaro Ono, Munenori Ikeda, Ryouichi Tajima, Yudai Adachi, Kanji Yasui, Polarization properties of nonpolar ZnO films grown on Rsapphire substrates using high-temperature H2 O generated by a catalytic reaction, Thin Solid Films (2017), doi:10.1016/j.tsf.2017.06.062
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Polarization properties of nonpolar ZnO films grown on R-sapphire substrates using
SC RI
PT
high-temperature H2O generated by a catalytic reaction
Ariyuki Kato, Shotaro Ono, Munenori Ikeda, Ryouichi Tajima, Yudai Adachi, Kanji Yasui * Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan,
MA
NU
* [email protected]
Abstract
ED
Nonpolar ZnO films were grown on r-plane sapphire substrates through the reaction
PT
between dimethylzinc (DMZn) and high-temperature H2O, the latter produced by the
CE
Pt-catalyzed reaction between H2 and O2. The resulting ZnO films were characterized using
AC
atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. They were found to have an anisotropic surface morphology with stripe arrays, and exhibited a diffraction peak associated with ZnO (11-20) index planes. The PL spectra indicated anisotropy in polarization between the directions parallel and perpendicular to the c-axis. The band-edge luminescence at 3.3 eV exhibited a maximum when the electric field vector E was perpendicular to the c-axis (parallel to the [1-100] direction) and another minimum when E was parallel to the c-axis. The angular dependence of the linear polarization of the band-edge luminescence was
ACCEPTED MANUSCRIPT large for ZnO films grown at low temperatures. The large degree of polarization observed for
AC
CE
PT
ED
MA
NU
SC RI
Keywords: ZnO, nonpolar, catalytic reaction, polarization
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low-temperature growth is thought to be due to the film geometry.
ACCEPTED MANUSCRIPT 1. Introduction
PT
Zinc oxide (ZnO) has recently attracted considerable interest owing to its potential applications in the ultraviolet wavelength range [1]. In addition to its large direct band gap of
SC RI
3.37 eV at room temperature, excitons in ZnO offer significant advantages for optoelectronic
NU
applications, such as light-emitting diodes and ultraviolet laser diodes, because of their large binding energy of approximately 60 meV [2]. In order to fabricate efficient optical emission
MA
devices, a superlattice structure is often adopted using ZnO and magnesium zinc oxide (MgZnO)
ED
layers. The ZnO films are mainly grown along the <0001> direction of the ZnO hexagonal
PT
wurtzite structure on c-plane sapphire substrates [3]. However, a macroscopic electrostatic field
CE
is generated along the c-axis, which results in spontaneous piezoelectric polarization, which in
AC
turn induces an electric field [4]. This electric field has an adverse effect on device properties, such as a decrease in the overlap of the electron and hole wave functions in the ZnO quantum well layer, which reduces the internal quantum efficiency of optical emission devices [5]. Thus, the growth of nonpolar ZnO films is required to eliminate such polarization effects. There have been a number of reports on the growth of nonpolar ZnO (a-plane) on r-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) [6,7], molecular beam epitaxy (MBE) [8], and pulsed laser deposition [9]. We have developed a new CVD method using high-temperature H2O generated by a catalytic reaction between H2 and O2 on platinum (Pt)
ACCEPTED MANUSCRIPT nanoparticles. The CVD method using this catalytic reaction was demonstrated to be useful for
PT
the growth of metal oxide thin films under energy-saving conditions, and has also been found to produce c-plane ZnO films on a-plane (11-20) sapphire (a-Al2O3) substrates with excellent
SC RI
optical and electrical properties [10]. The investigation of nonpolar ZnO film growth using
NU
catalytically assisted CVD is motivated by the desire to fabricate optical emission devices with superior properties.
MA
In the present work, growth of nonpolar ZnO (a-plane) films was investigated on r-plane
ED
sapphire substrates using catalytically assisted CVD. The structure, surface morphology, and
PT
optical emission properties (in particular, polarization properties) of a-plane ZnO films were
AC
CE
characterized.
2. Experimental
The present CVD apparatus had the same features as that reported in a previous paper [10]. A gas mixture of H2 and O2 was introduced into a catalyst cell containing a Pt-dispersed ZrO2 catalyst, following which the temperature of the catalyst cell quickly (within 1 min) rose above 1000°C, owing to the exothermic reaction between H2 and O2 over the catalyst (Pt nanoparticles). Within 10 min of the introduction of the H2-O2 gas mixture, the catalytic cell temperature stabilized, at which time (i.e., precisely 10 min after the introduction of H2-O2) the shutter
ACCEPTED MANUSCRIPT located between the skimmer cone and the substrate holder was opened. The generated
PT
high-temperature H2O molecules were ejected from a de Laval nozzle into the reaction zone in the chamber and allowed to collide with DMZn gas ejected from another small nozzle (inner
SC RI
diameter: 1.0 mm). The H2 and O2 gas flow rates during growth were set at 400 and 130 sccm,
NU
respectively. The DMZn gas flow rate was adjusted to maintain a partial pressure of 4.6×10-3 Pa. The skimmer cone between the H2O nozzle and the substrate served the purpose of
MA
selectively directing only high-velocity H2O molecules to the substrate. Epitaxial ZnO films
ED
were grown on r-plane Al2O3 substrates at substrate temperatures from 400 to 600°C for 60 min
PT
without a buffer layer. The total reaction gas pressure in the chamber during deposition ranged
CE
from 0.4 to 0.7 Pa. The pressure was measured using an ionization vacuum gauge; however, the
AC
measured value was not multiplied by the correction factor for the specific gas species. The sapphire substrates were degreased by washing with methanol and acetone in an ultrasonic cleaner, etched with a H2SO4/H3PO4 solution, rinsed with ultrapure water, and finally, set on a substrate holder in the CVD chamber. The film thicknesses were 2.0-3.0 m. The crystallinity and crystal orientation for each ZnO film were characterized by X-ray diffraction (XRD, Bragg-Brentano configuration, -2 scan) using Cu-Kα1 radiation (Rigaku, SmartLab). The surface morphology was observed using atomic force microscopy (AFM, Shimadzu 9500J2 System). Photoluminescence (PL) spectra were measured at room temperature using a
ACCEPTED MANUSCRIPT spectrometer (HORIBA JOBIN YVON, LabRAM HR-PL) in conjunction with a CCD detector
PT
(HORIBA JOVAN YVON, Synapse). PL excitation was obtained using the 325 nm line of a He-Cd laser. Furthermore, the polarization of the band-edge emission from the ZnO films was
SC RI
determined by using a linear polarizer. The linear polarizer and a polarization canceling plate
NU
were inserted between the sample and spectrometer. By rotating the linear polarizer, the angular
MA
dependence of the polarization was recorded, as shown in Fig. 1.
ED
3. Results and discussion
PT
The XRD patterns for all ZnO films grown on r-plane sapphire substrates exhibited an
CE
intense (11-20) peak at 2=56.64°, associated with ZnO A-planes (see Fig. 2). No diffraction
AC
peaks associated with other planes, such as (10-10) planes at 2=31.8°, the (0002) plane at 2=34.4°, and (10-11) planes at 2=36.3°, were observed. This indicates that all films were composed of domains grown parallel to the a-axis. From the diffraction peak at 2=56.64°, the lattice constant of the a-axis was estimated to be 0.3248 nm, which is almost the same as that of bulk ZnO crystals (0.32496 nm) [11]. Thus, the residual compressive stress perpendicular to the film surface was very small irrespective of the growth temperature. The full width at half maximum (FWHM) values of the diffraction peaks of films grown at 450, 500, and 600°C were 0.100°, 0.0897°, and 0.0796°, respectively. Based on these FWHM values, the average crystal
ACCEPTED MANUSCRIPT sizes perpendicular to the film surface of the films grown at 450, 500 and 600°C were estimated
PT
to be 94, 105, and 118 nm, respectively, using Scherrer’s formula [12]. The surface morphology, as determined by AFM, was different from that of c-plane ZnO,
SC RI
which features hexagonal facets. Instead, striped patterns were observed, as shown in Fig. 3. The
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long axes of the stripes were parallel to the <1-102> axis of the sapphire substrate and parallel to the c-axis of the ZnO film. The domain size and surface roughness increased with increasing
MA
growth temperature. In particular, the widths of the stripes were less than 1 m for ZnO films
ED
grown at 450°C.
PT
The PL spectra of ZnO films grown at various temperatures were measured at room
CE
temperature using a He-Cd laser and a linear polarizer. Figure 4 shows the PL spectra obtained
AC
for the case where the electric field vector E of the PL was parallel to the [1-100] direction (0°) (solid lines) and for the case where E was parallel to the c-axis (90°) (dotted lines) at room temperature. Two emission bands were observed. The band-edge luminescence, with a peak position at about 3.3 eV, was strong (weak) in the former (latter) case, i.e., the a-ZnO films emitted stronger band-edge luminescence when the polarization was perpendicular to the c-axis. On the other hand, the peak intensity of the green band with a peak at about 2.3-2.4 eV, commonly referred to as the deep-level emission caused by defects [1], was larger for 90° than for 0°, i.e., the a-ZnO films emitted stronger green luminescence when the polarization was
ACCEPTED MANUSCRIPT along the c-axis. Band-edge luminescence originates from the electron transitions between a
PT
conduction band and three valence bands, viz., the A band, B band, and C band, in wurtzite-type semiconductors such as ZnO and GaN. Electron transitions between the conduction band and the
SC RI
A and B valence bands are allowed when E is perpendicular to the c-axis. On the other hand,
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transitions related to the C-band are allowed for E parallel to the c-axis [13]. The hole concentration in the A and B bands is greater than that in the C band because the energy levels of
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the A and B bands are higher than that of the C band. Therefore, the band-edge emission
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intensity perpendicular to the c-axis is greater than that parallel to the c-axis.
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Figure 5 shows the polarization angle dependence of the peak intensity of the band-edge
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luminescence. Although the angular dependence was similar for all ZnO films, the variation in
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emission intensity was different. The ZnO films grown at high temperature (600°C) showed little variation, while those grown at low temperatures (450-500°C) showed a large variation. Figure 6 shows the degree of polarization defined as [(I[0001]-I//[0001])/(I[0001] +I//[0001])×100 (%)] calculated from the peak intensities of the band-edge luminescence perpendicular to the c-axis (I[0001]) and parallel to the c-axis (I//[0001]). The ZnO films grown at 450°C exhibited the highest degree of polarization (63%), while those grown at 600°C exhibited a small intensity ratio (22%). The maximum value of 63% was greater than the 16% reported for ZnO nanorods by Hsu et al. [14]. The high degree of polarization for low-temperature growth is considered to
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This large polarization is a unique property and could lead to applications of
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polarization-sensitive optoelectronic devices in the ultraviolet wavelength region.
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4. Conclusion
Nonpolar ZnO films were grown on r-plane sapphire substrates by a new CVD method using
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a catalytic reaction between H2 and O2. The films exhibited an anisotropic surface morphology
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with arrays of stripes. PL spectra indicated anisotropy in polarization between the directions
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parallel and perpendicular to the c-axis. The band-edge luminescence was large when the electric
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field vector E was parallel to the [1-100] direction and weak when E was parallel to the c-axis.
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The angular dependence of the linear polarization of the band-edge luminescence was large for ZnO films grown at low temperatures. The degree of polarization determined from the peak intensities of the band-edge luminescence perpendicular to the c-axis and the band-edge luminescence parallel to the c-axis was 63%, which is much greater than the value reported for ZnO nanorods. This high degree of polarization for low-temperature growth is considered to be due to the small striped domain structure in the film.
Acknowledgements
ACCEPTED MANUSCRIPT This work was supported in part by a Grant-in-Aid for Scientific Research (No. 16H03869) from
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the Japan Society for the Promotion of Science.
ACCEPTED MANUSCRIPT References
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[1] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005)
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041301.
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[2] B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Christein, A. Hoffmann, M. Straßburg, M. Dworzak, U. Haboeck, A.V. Rodina, Bound exciton and
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donor-acceptor pair recombinations in ZnO, Phys. Stat. Sol. (b) 241 (2004) 231-260.
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[3] T. Gruber, C. Kirchner, K. Thonke, R. Sauer, A. Waag, MOCVD Growth of ZnO for
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Optoelectronic Applications, Phys. Stat. Sol. (a) 192 (2002) 166-170.
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[4] C. Morhain, T. Bretagnon, P. Lefebvre, X. Tang, P. Valvin, T. Guillet, B. Gil, T. Taliercio, M. B.
Vinter,
C.
Deparis,
Internal
electric
field
in
wurtzite
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Teisseire-Doninelli,
ZnO/Zn0.78Mg0.22O quantum wells, Phys. Rev. B 72 (2005) 241305. [5] F. Bernardini, V. Fiorentini, D. Vandervilt, Spontaneous polarization and piezoelectric constants of III-V nitrides, Phys. Rev. B 56 (1997) R10024-R10027. [6] C.R. Gorla, N.W. Emanetoğlu, S. Liang, W. E. Mayo, Y. Lu, M. Wraback, H. Shen, Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys. 85 (1999) 2595-2602. [7] Y. Kashiwaba, T. Abe, S. Onodera, F. Masuoka, A. Nakagawa, H. Endo, I. Niikura, Y.
ACCEPTED MANUSCRIPT Kashiwaba, Comparison of non-polar ZnO(1120) films deposited on single crystal ZnO
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(11-20) and sapphire (01-12 ) substrates, J. Cryst. Growth 298 (2007) 477-480. [8] J.-M. Chauveau, D.A. Buell, M. Laügt, P. Vennegues, M. Teisseire-Doninelli, S.
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Berard-Bergery, C. Deparis, B. Lo, B. Vinter, C. Morhain, Growth of non-polar
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ZnO/(Zn,Mg)O quantum well structures on R-sapphire by plasma-assisted molecular beam epitaxy, J. Cryst. Growth 301-302 (2007) 366-369.
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[9] P. Pant, J.D. Budai, R. Aggarwal, R.J. Narayan, J. Narayan, Thin film epitaxy and structure
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property correlations for non-polar ZnO films, Acta Mater. 57 (2009) 4426-4431.
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[10] K. Yasui, M. Miura, H. Nishiyama, Deposition of zinc oxide thin films using a surface
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reaction on platinum nanoparticles, Mater. Res. Soc. Symp. Proc., 1315 (2011) 21-26.
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[11] H. Karzel, W. Potzel, M. Köfferlein, W. Schiessl, M. Steiner, U. Hiller, G. M. Kalvius, D. W. Mitchell, T. P. Das, P. Blaha. K. Schwarz, M. P. Pasternak, Lattice dynamics and hyperfine interactions in ZnO and ZnSe at high external pressures, Phys. Rev. B 53 (1996) 11425-11438. [12] T. Pandiyarajan, B. Karthikeyan, Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles, J. Nanopart. Res., 14 (2012) 647. [13] S.K. Han, S.K. Hong, J.W. Lee, J.Y. Lee, J.H. Song, Y.S. Nam, S.K. Chang, T. Minegishi, T. Yao, Structural and optical properties of non-polar A-plane ZnO films grown on R-plane
ACCEPTED MANUSCRIPT sapphire substrates by plasma-assisted molecular-beam epitaxy, J. Cryst. Growth, 309 (2007)
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121-127. [14] N.E. Hsu, W.K. Hung, Y.F. Chen, Origin of defect emission identified by polarized
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luminescence from aligned ZnO nanorods, J. Appl. Phys. 96 (2004) 4671-4673.
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Fig. 1. Experimental setup for polarization of the optical emission measurement. Fig. 2. X-ray diffraction patterns for ZnO films grown on r-plane sapphire substrates at various
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temperatures.
Fig. 3. Atomic force microscopy images of ZnO films grown on r-plane sapphire substrates
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(20×20 m).
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Fig. 4. Polarization dependence of PL spectra for a-plane ZnO films grown at various
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temperatures.
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Fig. 5. Variation in the band-edge luminescence intensity for ZnO films grown at various
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temperatures as a function of polarization angle.
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Fig. 6. Degree of polarization defined by [(I⊥[0001]-I//[0001])/(I⊥[0001] +I//[0001])×100(%)] calculated from the peak intensities of the band-edge luminescence perpendicular and parallel to the c-axis.
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Figure 1
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Figure 5
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Figure 6
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ZnO films were grown on r-plane sapphire substrates by CVD using a catalytic reaction. Crystallinity, morphology, PL properties were measured for a-plane ZnO films.
The polarization of the luminescence was shown to highly anisotropic.
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