Growth orientations of semipolar ZnO on GaN(1 1 2̄ 2)
Yi-Sen Shih
n,1, Pei-Yin Lin, Jr-Yu Chen, Li Chang
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
a r t i c l e i n f o
Article history: Received 18 July 2014 Accepted 24 August 2014 Available online 4 September 2014 Keywords:
Electron microscopy X-ray techniques Thinfilms
a b s t r a c t
On semipolar GaNð1 1 2 2Þ, epitaxial ZnO grown by chemical vapor deposition can form in two different semipolar orientations as evidenced by transmission electron microscopy and X-ray diffraction. One orientation relationship is shown to be ZnOð1 1 2 2Þ//GaNð1 1 2 2Þ and ½1 1 0 0ZnO//½1 1 0 0GaN
which is expected for ZnO growth on GaNð1 1 2 2Þ, while the other is a newly found relationship of ZnOð1 0 1 1Þ//GaN(0 0 0 2) and ½5 7 2 3ZnO//½1 1 0 0GaN.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
Wurtzite semiconductors are of great interest in optoelectronic applications, in particular, GaN and ZnO of wide bandgap. Due to the asymmetry of wurtzite structure the presence of spontaneous polarizationfield in the c-axis results in attenuation of electron– hole recombination rate which is known as quantum-confined Stark effect. The effect crucially lowers the luminous efficiency and the performance of light-emitting devices[1]. Though most of the wurtzite semiconductors are grown in c-orientation, nonpolar and semipolar orientations can provide better performance by redu-cing the Stark effect. It has been shown that semipolar GaN growth may have some advantages over nonpolar GaN such as a larger growth window and better surface morphologies[2].
ZnO (a¼3.249 Å, c¼5.206 Å) has a band gap of 3.37 eV with large free exciton binding energy which is expected to exhibit outstanding optical properties. Similar to GaN, nonpolar and semipolar ZnO can enhance the quantum efficiency [1,3,4]. Compared with intensive studies on semipolar GaN, semipolar ZnO has received less attention so far. Though growth of semipolar ð1 1 2 2Þ, ð1 0 1 2Þ and ð1 0 1 1Þ ZnO have been recently demonstrated[5–7], it still lacks of good understanding. As ZnO and GaN have similar lattice para-meters (GaN: a¼3.189 Å, c¼5.185 Å), the GaN substrate provides an excellent template for growth of epitaxial ZnO [8]. In this study ð1 1 2 2Þ semipolar GaN is used for the growth of semipolar ZnO. Structural characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows that semipolar ZnO grown on GaN exhibits two different orientations.
2. Experimental
GaNð1 1 2 2Þ was obtained from a faceted a-plane GaN template grown on 2 inches r-plane sapphire by metal–organic chemical vapor deposition, as shown inFig. 1a. The GaNð1 1 2 2Þ facets on the surface were identified with using scanning electron microscopy (SEM) and XRD.
ZnO was grown by chemical vapor deposition in a vertical tube furnace at 5001C using zinc acetylacetonate as the precursor with N2
carrier gas. Also, O2gas was separatelyflowed into the furnace. The
flow rates of N2and O2were respectively 30 and 120 sccm. The N2/O2
ratio was maintained at 0.25. The crystallinity of thin films was examined with high-resolution XRD. The surface morphologies were observed by using SEM. The orientation relationships of ZnO on the faceted GaN template were also investigated by using cross-sectional TEM (XTEM). A dual-beam focused ion beam system was used to prepare cross-sectional specimens for SEM and TEM observations.
3. Results and discussion
All the crystallographic directions in SEM were verified with XRD using sapphire and GaN reflections based on sapphire wafer flat (//sapphire a-plane and GaN m-plane). On the GaN template, the topflat surface is of a-plane with other faceted planes on the inclined sides (Fig. 1a). Here we only focus on GaNð1 1 2 2Þ facets which are inclined 31.51 with a-plane and have an average area about 2 4 μm2
.Fig. 1b is the top-view SEM image of ZnO grown on a-plane and GaNð1 1 2 2Þ, showing the island-like surface morphology for ZnO on theð1 1 2 2Þ faceted GaN.Fig. 1c shows the cross-sectional view of the sample after cutting by focused ion beam to reveal ZnO on the inclinedð1 1 2 2Þ and flat a-plane GaN surfaces.Fig. 2a shows a XRDω/2θ pattern of ZnO on GaN/sapphire in which ZnO exhibits only theð1 1 2 0Þ peak, implying that it Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/matlet
Materials Letters
http://dx.doi.org/10.1016/j.matlet.2014.08.132
0167-577X/& 2014 Elsevier B.V. All rights reserved.
nCorresponding author. Tel.:þ886 3 5712121x55373; fax: þ886 3 5724727.
E-mail address:eason.mse95g@nctu.edu.tw(Y.-S. Shih).
1
Postal address: 1001 University Road, Engineering Building 6, Room EF707A, Hsinchu 30010, Taiwan.
may be in epitaxy with nonpolar and semipolar GaN facets.Fig. 2b shows aχ-compensated ω/2θ scan pattern (χ-tilt about 301 with GaN c-axis) in which onlyð1 1 2 2Þ peaks of both ZnO and GaN can be seen, suggesting that ZnOð1 1 2 2Þ epitaxially grows on the GaNð1 1 2 2Þ facets with the same orientation. To ensure that the ZnOð1 1 2 2Þ reflection in the XRD pattern is only generated from the ZnO thinfilm formed on GaNð1 1 2 2Þ facet, we used XTEM to examine the orientation of the ZnO film with GaN. A typical bright field XTEM image in Fig. 3a (beam//½1 1 0 0GaN)
shows that the ZnOfilm thickness is about 160 nm, and the ZnO film exhibits bright and dark diffraction contrast which may arise from the difference in orientations.Fig. 3b shows the correspond-ing selected-area diffraction (SAD) pattern in which two coexisted patterns are clearly revealed. One of the patterns can be identified with the½1 1 0 0GaNzone axis pattern, whereas the other one is
½5 7 2 3ZnO, indicating ½1 1 0 0GaN//½5 7 2 3ZnO. Surprisingly,
no ZnO a-plane diffraction spots can be seen in this zone-axis pattern with GaN a-plane diffraction spots. Instead, it can be found that ZnOð1 0 1 1Þ is almost parallel to GaN(0 0 0 2) and ZnOð1 1 0 4Þ is slightly tilted about 71 away from GaNð1 1 2 2Þ. From the SAD pattern and high-resolution TEM (HRTEM) shown later, it can be shown that the ZnO grains in bright contrast in Fig. 3a are oriented along½5 7 2 3ZnO-axis, while those in dark
contrast are actually oriented with ½1 1 0 0GaN in the
cross-section view. Thus, it is reasonable that the ZnOð1 1 2 2Þ reflec-tion in the XRD pattern is resulted from the ZnO in dark contrast. For the ZnO in bright contrast, it can be designated as
ð1 1 0 4Þ-oriented ZnO with respect to the ZnO/GaN interface. To further verify the orientation relationships, we have prepared another thin TEM specimen for HRTEM observations from the same ZnO/GaN sample. The HRTEM images inFig. 3c–e taken from GaN and different ZnO grains with bright and dark contrast around the same ZnO/GaN interface region (i.e. on the same GaN facet) with the beam along the same½1 1 0 0GaN direction show the
lattice fringes of GaN and ZnO in½1 1 0 0 zone (Fig. 3c and e), and a distinctly different lattice fringes of ZnO inFig. 3d, ensuring that two orientation types of ZnO grains form on GaNð1 1 2 2Þ. Also the fast Fourier transform (FFT) patterns of the GaN and ZnO lattice images in the insets of Fig. 3c–e clearly confirm that the ZnO grains are aligned with½1 1 0 0ZnOand½5 7 2 3ZnO
direc-tions. Consequently, the orientation relationships between ZnO and GaN can be deduced as ZnOð1 1 2 2Þ//GaNð1 1 2 2Þ and ½1 1 0 0ZnO//½1 1 0 0GaN, and ZnOð1 0 1 1Þ//GaN(0 0 0 2) and
½5 7 2 3ZnO//½1 1 0 0GaN. As a result, the interpretation of the
SAD pattern inFig. 3b should be taken into consideration that the ZnO reflections in ½1 1 0 0 zone axis pattern actually coincide nearly with GaN ones as they have a small mismatch. To verify the formation of theð1 1 0 4Þ-oriented ZnO on the GaN facets of the template is a general case, we used the asymmetrical grazing exit method to obtain the reciprocal space map of ZnOð1 1 0 4Þ shown in Fig. 4as it is absent in ω/2θ scans because of the its scattering angle.
For these two orientation relationships of ZnO with GaN, ZnOð1 1 2 2Þ grown on GaNð1 1 2 2Þ is as expected to follow
Fig. 1. SEM images showing (a) the top-view surface morphology of GaN, (b) the surface morphologies of ZnO on the GaN facets in top-view, and (c) the ZnO/GaN cross-section after focused ion beam milling.
Fig. 2. (a) XRDω/2θ pattern of ZnO on GaN/sapphire, and (b) χ-compensated (301) ω/2θ XRD scan pattern to show both GaN and ZnOð1 1 2 2Þ reflections.
the GaN lattice because they have a lattice mismatch of 2.9% in ½1 1 0 0GaNdirection and 1.6% in½1 1 2 3GaNdirection. However, it
is unexpected for the other orientation relationship of ZnO with GaNð1 1 2 2Þ from which it can be deduced that ZnO also has a small lattice mismatch of 0.4% with GaN in ½1 1 0 0GaN
(//½5 7 2 3ZnO) direction, and 4.5% in ½0 0 0 2GaN (3.571
deviated from½2 0 2 1ZnO).
4. Conclusions
Semipolar ZnO can epitaxially grow in two different orientation types on GaNð1 1 2 2Þ. XRD, SAD, and HRTEM results provide clear evidence that one of the orientation relationships between ZnO
and GaN is ZnOð1 1 2 0Þ//GaNð1 1 2 0Þ and ½1 1 0 0ZnO//
½1 1 0 0GaN, and the other one is ZnOð1 0 1 1Þ//GaN(0 0 0 2) and
½5 7 2 3ZnO//½1 1 0 0GaN.
Acknowledgements
This work was supported by National Science Council, Taiwan (NSC101-2221-E-009-050-MY3).
References
[1]Chemla DS, Damen TC, Miller DAB, Gossard AC, Wiegmann W. Electro-absorption by Stark-effect on room-temperature excitons in GaAs/GaAlAs multiple quantum well structures. Appl Phys Lett 1983;42:864–6.
[2]Morawiec S, Sarzaa RP, Nakwaski W. A method used to overcome polarization effects in semi-polar structures of nitride light-emitting diodes emitting green radiation. Appl Phys A 2013;113:801–9.
[3]Miller DAB, Chemla DS, Damen TC, Gossard AC, Wiegmann W, Wood TH, et al. Band-edge electroabsorption in quantum well structures—the quantum-confined Stark-effect. Phys Rev Lett 1984;53:2173–6.
[4]Waltereit P, Brandt O, Trampert A, Grahn HT, Menniger J, Ramsteiner M, et al. Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes. Nature 2000;406:865–8.
[5]Tian JS, Peng CY, Wang WL, Wu YH, Shih YS, Chiu KA, et al. Semipolar ð1 1 2 2Þ ZnO thin films grown on LaAlO3-buffered LSAT (1 1 2) single crystals by pulsed laser deposition. Phys Status Solidi RRL 2013;7:293–6.
[6]Chauveau J-M, Xia Y, Ben Taazaet-Belgacem I, Teisseire M, Roland B, Nemoz M, et al. Built-in electricfield in ZnO based semipolar quantum wells grown on ð1 0 1 2Þ ZnO substrates. Appl Phys Lett 2013;103:262104.
[7]Richardson JJ, Koslow I, Pan CC, Zhao Y, Ha JS, DenBaars SP. Semipolar single-crystal ZnO films deposited by low-temperature aqueous solution phase epitaxy on GaN light-emitting diodes. Appl Phys Express 2011;4:126502. [8]Zhou HL, Chua SJ, Pan H, Zhu YW, Osipowicz T, Liu W, et al. Morphology
controllable ZnO growth on facet-controlled epitaxial lateral overgrown GaN/ sapphire templates. J Phys Chem C 2007;111:6405–10.
Fig. 3. (a) Brightfield XTEM micrograph and (b) SAD pattern. (c)–(e) HRTEM images along GaN½1 1 0 0 showing lattice fringes of (c) GaN in ½1 1 0 0GaN, (d) ZnO in
½5 7 2 3ZnO, and (e) ZnO in½1 1 0 0ZnOwith FFT patterns in the insets.
Fig. 4. RSM of ZnOð1 1 0 4Þ.
Y.-S. Shih et al. / Materials Letters 137 (2014) 96–98 98
