Interfacial structure of a-plane ZnO grown on r-plane sapphire
by pulsed laser deposition
Chun-Yen Peng
n, Wei-Lin Wang, Yen-Teng Ho, Jr-Sheng Tian, Li Chang
nDepartment of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan
a r t i c l e
i n f o
Article history:
Received 17 October 2012 Accepted 1 December 2012 Available online 8 December 2012 Keywords: Nonpolar ZnO Epitaxial growth Interfaces HRTEM RSM
a b s t r a c t
In this study, interfacial structure of a-ZnO with misfit accommodation on r-sapphire at low and high growth temperatures (LT and HT) by pulsed laser deposition is presented. Along ½1100ZnOof large
lattice mismatch of ZnO with sapphire, TEM examinations show that a-type misfit dislocations are spaced 1.3–2.2 nm on HT-ZnO/sapphire interface, whereas dislocation pairs in spacing of 2.8–3.5 nm are observed for LT-ZnO/sapphire. For smaller lattice mismatch along the ZnO c-axis direction, reciprocal space maps of ð112 2ÞZnO and ð3030Þsapphire reflections show that HT-ZnO is nearly fully
strained without much relaxation and has a highly coherent interface with sapphire, in contrast with partial relaxation in LT-ZnO.
&2012 Elsevier B.V. All rights reserved.
1. Introduction
As a wide direct bandgap wurtzite semiconductor, zinc oxide (ZnO) is an attractive material for potential applications in optoelectronic devices [1–4]. Recently, growth of nonpolar epi-taxial films has attracted a lot of interest due to the lack of the red-shift caused by the quantum-confined Stark effect [5,6]. Because the spontaneous polarization of ZnO is nearly two times of GaN ( 0.057 to 0.05 C/m2 versus 0.029 to 0.022 C/m2)
[7,8], ZnO films without polarity along the growth direction are important for light emitting applications.
Sapphire is a commonly used substrate for growth of epitaxial thin films. Nonpolar ð1120Þ a-ZnO epitaxial films grown on ð101 2Þ r-sapphire substrates have been achieved by domain matching epitaxy methods [9] using pulsed laser deposition (PLD) [10], sputtering [11], plasma-assisted molecular beam epitaxy [12–13], and metal–organic chemical vapor deposi-tion [14]. From domain matching epitaxial relationships of ½0001ZnO==½1011sapphire and ½1100ZnO==½1210sapphire, anisotropic stresses resulted from anisotropic lattice and thermal mismatches may have significant effects on its crystallinity [15–17]. It has been shown that the misfit is relaxed by regularly spaced misfit dislocations along the large lattice mismatch direction of ½1100ZnO, whereas misfit dislocations are hardly seen along the ZnO c-direction due to its small lattice misfit [12,17–19]. In addition, the densities of threading dislocations (TDs) and stacking
faults are often observed in 1081010cm2 and 104–105cm1,
respectively[18–20].
It is known that growth temperature plays an important role on crystallinities and optical properties[11,21–22]. However, the variation of strains and misfit accommodations with different growth temperatures has been rarely studied. In our previous study, we have shown that the transition of the film growth mode may occur at 600 1C for a-ZnO grown on r-sapphire[22]. In this paper, we show that misfit accommodations along in-plane directions are indeed varied with growth temperature from the evidence of transmission electron microscopy (TEM) and x-ray reciprocal space mapping (RSM).
2. Experiments
ZnO films were grown on 8 8 mm2 r-sapphire wafers at 10 mTorr oxygen partial pressure in a Pascal laser-MBE system at 450 1C (LT-ZnO) and 750 1C (HT-ZnO). The KrF excimer laser irradiation with the energy density of about 1–3 J/cm2 and the repetition rate of 2 Hz was used to ablate a sintered ZnO target. Sapphire substrates were ultrasonically cleaned in acetone before loading into the vacuum chamber. Before PLD growth, 850 1C thermal cleaning of the substrate in vacuum at about 106Torr
was done for 30 min.
A Philips Tecnai 20 TEM operated at 200 kV was employed to investigate microstructures of ZnO on sapphire. The preparation of cross-sectional TEM (XTEM) specimens was carried out using mechanical grinding process and Arþ ion milling at 3.5–4.0 kV.
The RSM experiments were performed in a PANalytical X’Pert Pro Contents lists available atSciVerse ScienceDirect
journal homepage:www.elsevier.com/locate/matlet
Materials Letters
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.003
n
Corresponding authors. Tel.: þ 886 3 5731615; fax: þ 886 3 5724727. E-mail addresses: [email protected] (C.-Y. Peng), [email protected] (L. Chang).
(MRD) system employing a Ge (220) monochromator and Ge (220) channel cut analyzer.
3. Results and discussion
High-resolution XTEM micrographs of 180 nm HT-ZnO and LT-ZnO on r-sapphire viewed along [0001]ZnO are shown
inFig. 1(a) and (b), respectively. Also, the Fourier-filtered images inFig. 1(a) and (b) using ð1100ÞZnO and ð1210Þsapphire reflections are shown inFig. 1(c) and (d), respectively. From ð1100ÞZnO and ð1210Þsapphire lattice fringes, the extra-half planes of misfit dis-locations can be clearly observed. For the HT-ZnO/sapphire inter-face in Fig. 1(c), the extra-half planes have a spacing of 1.3– 2.2 nm as often seen in literature [12,18,19,23], whereas 2.8– 3.5 nm spacing between adjacent misfit dislocations is observed for the LT-ZnO/sapphire interface in Fig. 1(d). In the in-plane direction of ½1100ZnO, the translation period of ZnO and sapphire lattices are 5.63 ˚A and 4.76 ˚A, respectively. Due to large lattice mismatch of 18.3%, introduction of misfit dislocations with edge component of Burgers vector (b) in ½1100ZnO is required. In our previously study, the difference in initial growth behavior between HT- and LT-ZnO had been observed[24]. It may closely correlate with different misfit accommodation mechanisms.
As the Burgers circuit failure in Fig. 1(e) gives b¼1=3/2110iZnO in a-type, misfit dislocations in the interface between HT-ZnO and sapphire can have a component (0.28 nm) of 1=2/1100iZnO in the interface. For the LT-ZnO/sapphire
interface in Fig. 1(f), the MDs have b¼/1100SZnO. Even with 18.3% misfit it is only possible for a-ZnO to grow on r-sapphire with domain matching epitaxy, where the misfit can be accom-modated by matching of 5 ½1100ZnO with 6 13½1210sapphire (domain misfit¼55:6364:76
64:76 ¼ 1.4%) or 6 ½1100ZnO with 7 1
3½1210sapphire (domain misfit¼65:6374:7674:76 ¼1.4%). Since the translation periods of ZnO and sapphire lattices in this direction are 5.63 and 4.76 ˚A, respectively, the spacing of MDs of b¼½1100ZnOis 2.8–3.4 nm. However, those dislocations with b¼½1100ZnO are unstable due to b2energy criterion where b¼ 9b9. Therefore, it may
decompose to two a-type dislocations as ½1100 ¼ 1=3½2110 þ 1=3½1210 in a more stable configuration[25].
Because the horizontal component along the interface is 1=2½1100ZnO, the spacing of a-type misfit dislocations is reduced to 1.4–1.7 nm. Moreover, a ½1100 misfit dislocation has two extra-half planes [25], whereas only one extra-half plane is present with an a-type misfit dislocation. For LT-ZnO, most of the MDs shown in Fig. 1(d) appear either in pairs of a-type dislocations or ½1100 MDs as seen inFig. 1(f) with two extra-half planes. In contrast, the MDs observed inFig. 1(c) for HT-ZnO are pure a-type MDs as the Fourier-filtered image inFig. 1(e) shows only one half plane associated with each MD.
Fig. 2shows high-resolution XTEM images of 180 nm HT-ZnO/
sapphire and LT-ZnO/sapphire viewed along ½1100ZnO. The corre-sponding Fourier-filtered images using ð0002ÞZnO and ð1014Þsapphirereflections are shown inFig. 2(c) and (d). Different
Fig. 1. HRTEM micrographs of (a) HT-ZnO/sapphire and (b) LT-ZnO/sapphire interface viewed along [0001]ZnO. (c,d) Fourier-filtered images of (a,b) using ð1100ÞZnOand
ð1210Þsapphirereflections. (e,f) Enlarged images of (a,b), points and arrows are Burgers circuits and vectors respectively.
C.-Y. Peng et al. / Materials Letters 94 (2013) 165–168 166
from those observed inFig. 1, MDs are rarely observed in this view due to small lattice misfit in ZnO c-direction similar to the previous reports[11,16–18]. For HT-ZnO, however, an extra-half plane marked by arrow shown inFig. 2(c) may be occasionally observed in the TEM examinations across a wide range of a few micrometers. In contrast, more extra-half planes can be found along the LT-ZnO/sapphire interface in this direction, implying that more strain relaxation occurs.
Fig. 3shows ð1122ÞZnOand ð3030ÞsapphireRSMs for HT- and LT-ZnO. For HT-ZnO, Fig. 3(a) shows nearly vertically aligned ð1122ÞZnOand ð3030Þsapphirereflections indicating that HT-ZnO is almost fully strained with sapphire in this direction, while 0.39% residual misfit (
x
c¼(½0001ZnO1=3½1011Sapphire)/1=3½1011Sapphire) is present for LT-ZnO as shown inFig. 3(b). It is known that misfit strain can be relaxed by introducing MDs which have an edge component along the interface. More amount of MDs observed in LT-ZnO/sapphire are helpful to relax the misfit between ½0001ZnO and ½1011Sapphire, while rare MDs for HT-ZnO in this direction are resulted from nearly fully strained lattice.4. Conclusions
In summary, distinctly different misfit accommodations for HT-ZnO on r-sapphire from that for LT-ZnO are observed by the HRTEM observations in cross section. MD pairs are spaced with
2.8–3.5 nm at the interface for LT-ZnO along ½1100ZnO, whereas HT-ZnO exhibits individual a-type MDs spaced with 1.3–2.2 nm. Furthermore, there are a few MDs along the ZnO c-direction for LT-ZnO, but MDs are rarely observed for HT-ZnO.
Acknowledgments
This research was partially supported by the National Science Council, Taiwan, ROC. under Grant no. NSC98-2221-E-009-042-MY3.
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