Optical second harmonic generation from the twin boundary of ZnO thin films grown
on silicon
Kuang-Yao Lo, Shih-Chieh Lo, Chang-Feng Yu, Teddy Tite, Jung-Y. Huang, Yi-Jen Huang, Ren-Chuan Chang, and Sheng-Yuan Chu
Citation: Applied Physics Letters 92, 091909 (2008); doi: 10.1063/1.2891334
View online: http://dx.doi.org/10.1063/1.2891334
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/9?ver=pdfcov Published by the AIP Publishing
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Optical second harmonic generation from the twin boundary of ZnO thin
films grown on silicon
Kuang-Yao Lo,1,a兲Shih-Chieh Lo,1Chang-Feng Yu,1Teddy Tite,1Jung-Y. Huang,2 Yi-Jen Huang,3Ren-Chuan Chang,3and Sheng-Yuan Chu3
1
Department of Applied Physics, National Chia Yi University, ChiaYi 600, Taiwan 2
Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan 3
Department of Electrical Engineering, National Cheng Kung University, Tainan 700, Taiwan
共Received 27 January 2008; accepted 7 February 2008; published online 6 March 2008兲
The symmetry of the twin boundaries of ZnO epitaxial film was detected with reflective second harmonic generation 共RSHG兲. The twin boundaries exhibit mirror symmetry with a polar configuration across the boundary plane and yield a nonvanishing polar contribution to RSHG. The nonvanishing second-order susceptibility supports the notion that the measured RSHG originates from the planar defect, which depends on the residual stress in the thin film. We analyzed our RSHG result by correlating the macroscopic data from optic probe with the microscopic data from tunneling electron microscope. © 2008 American Institute of Physics.关DOI:10.1063/1.2891334兴
Zinc oxide 共ZnO兲 thin film has attracted considerable interest for its optical, electrical, and mechanical properties. Experimental and theoretical studies on ZnO crystals have revealed the presence of a giant permanent dipole moment, which yields a significant piezoelectric effect for a variety of micromechanical devices.1Zinc oxide is a wide direct band-gap共3.37 eV兲 II-VI semiconductor with large exciton bind-ing energy 共60 meV兲,2 which have been shown to be valu-able for short wavelength optoelectronic devices such as light emitting diodes and lasers. From the viewpoint of ma-terial synthesis, ZnO offers further advantages of low cost and no toxic chemicals involved in its synthesis.
ZnO films have been deposited with a variety of meth-ods such as magnetron sputtering, pulsed laser deposition, or metal organic chemical vapor deposition 共MOCVD兲.3,4 The resulting films with single-crystal quality possess very high piezoelectric coefficients. It was found that a proper selection of substrate materials is crucial for yielding a high-quality ZnO film.5,6Silicon crystal has been widely used as the sub-strate for thin film deposition because of the large resources available in the highly matured silicon-based industry. How-ever, the large discrepancy in the thermal expansion coeffi-cients and lattice mismatch between ZnO film and Si sub-strate often produce a fairly large stress in the film. Moreover, extended defects such as twin boundaries were also observed in the wurtzite structure of ZnO thin films7,8 with high-resolution Z-contrast transmission electron micros-copy共TEM兲. The twin boundaries observed have the head-to-tail polar configuration 兩, where the arrow indicates the polarity direction from the O to the Zn atoms along the O–Zn bonds parallel to the c axis and the兩 symbol denotes the twin-boundary plane.7The resulting twin boundaries ex-hibit mirror symmetry with the polar configuration providing a nonvanishing contribution to the second-order nonlinear optical susceptibility, which allows the polar structure to be probed with optical second harmonic generation共SHG兲 tech-nique.
Reflective SHG共RSHG兲 has been developed into a ver-satile probe for thin film, surface, and interfacial studies.9,10
It has been widely used for the process control of thin film deposition and the rapid thermal annealing of ion implantation.11,12Rotational anisotropy SHG共RA-SHG兲 pro-vided useful information about the strained layer,13,14 the quality of epitaxial films,15 and the surface reconstruction.12 It was found that misfit dislocation defects in an epitaxial film can affect the second-order nonlinear optical suscepti-bility 共2兲; therefore, their presence shall be revealed with RA-SHG.16
In our previous study of ZnO thin films grown on the c-plane of sapphire by MOCVD, we found that the RSHG signal originated from the mirror symmetry of twin bound-aries is mixed with the 3 mm symmetric RSHG pattern that is raised from the Zn–O bonding on the surface of a well-grown ZnO thin film.17However, the additional contribution with mirrorlike symmetry originated from the twin bound-aries deserves to be investigated further for its potential ap-plication as an indicator of the film quality.
In this work, we studied RSHG of ZnO thin film grown on Si共111兲 substrate by sputtering. The resulting surfaces of ZnO thin films were not perfectly smooth to eliminate the RSHG contribution of 3 mm symmetry.17 Since s-polarized RSHG excited by s-polarized fundamental light 共ss-RSHG兲 is sensitive to the anisotropic contribution dominated by the symmetrical structure,18 we therefore employed the ss-RSHG to probe the nonvanishing polarity of twin bound-aries in a ZnO film. We analyzed our RSHG result by corre-lating the macroscopic data from optic probe with the micro-scopic data from TEM.
ZnO films were deposited from a ZnO target of 99.9% purity by using a magnetron sputtering system with a rf power of 60 W. Substrates used were p-type Si共111兲 共Boron dopant, 4 – 10⍀ cm兲, cleaned thoroughly with organic sol-vents and dried before loading in the sputtering system. The chamber was pumped down to 6⫻10−6torr before
introduc-ing premixed Ar and O2 gases into the chamber through a
precision leak valve. The volume ratio of argon and oxygen was controlled to 1:1 by an electronic mass flow controller. The working pressure was kept at 3 mtorr. The ZnO films were deposited at the substrate temperatures of 100, 200, and 300 ° C, respectively, for 3 h and naturally cooled down to room temperature. The x-ray diffraction共XRD兲 patterns of
a兲Electronic mail: kuanglo@mail.ncyu.edu.tw.
APPLIED PHYSICS LETTERS 92, 091909共2008兲
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the resulting ZnO films on the Si共111兲 with different sub-strate temperatures were presented in the Fig. 1. Only one peak corresponding to the共0002兲 plane of ZnO crystal ap-pears, suggesting these films to be preferentially deposited at a c-axis orientation. In some reported literatures, the strain of ZnO film was calculated by measuring the diffraction peak shift relative to that of bulk ZnO.19,20 The inset of Fig. 1
shows the embedding stresses estimated by the above-mentioned method. The diffraction peak shift from the 共0002兲 plane to the higher 2 side indicates that the tensile strain in the film decreases with increasing substrate tem-perature.
The setup of SHG measurement was detailed in Ref.12. During an azimuthal scan of the sample, the surface normal was aligned along the rotation axis and the excitation laser hit the rotational center to ensure the RSHG beam striking the cathode of photomultiplier tube at the same position. The ss-RSHG pattern of ZnO film grown on Si共111兲 substrate was shown in the Fig.2.
The ZnO crystal prepared exhibits a wurtzite structure with 6 mm共C6v兲 symmetry. The second-order nonlinear
op-tical susceptibility tensor is characterized by the following nonvanishing components xzx共2兲=yyz共2兲=15, zxx共2兲=zyy共2兲=31, andzzz共2兲=33. There shall be no ss-RSHG signals from the
ZnO bulk. Actually, the mirrorlike symmetrical patterns are shown in Fig.2, in which there is no additional 3 mm sym-metrical contribution raised from Zn–O bonding on the surface.17
ZnO 共0002兲 layer with hexagonal wurtzite structure 共a=3.249 Å, c=5.207 Å兲 can be epitaxially grown on silicon 共111兲 substrate by matching the four silicon 共220兲 planes with the five 共112¯0兲 planes of ZnO. The spacing of ZnO 共112¯0兲 planes 共a/2=1.62 Å兲 has about 40.1% strain with 共220兲 planes of silicon.21
As the deposited film exceeds the critical thickness, planar defects appear at the strained inter-face in order to relax the excess stress.22 The planar defects could be a discontinuity of a perfect crystal structure across a plane, which contains grain boundaries, stacking faults, and twin boundaries.23In particular, a twin boundary is the regu-lar growing together of crystals of the same sort, sharing some of the same crystal lattice with a mirror symmetry op-eration, which often occurs with crystal growth or through mechanical stress.23Yan et al. presented the atomic structure and the electronic effects of the关11¯00兴/共112¯2兲 twin bound-aries in wurtzite ZnO by using high-resolution Z-contrast TEM. The twin boundary was found to have the head-to-tail polarity configuration to avoid dangling bonds and yields fairly low twin-boundary energy of 0.040 J/m2. The polarity
in the twin boundary exhibits a mirror symmetry across the boundary plane.7
The overall effect of these twin boundaries on the strained interface is a C1v symmetry with a vertical mirror
plane v perpendicular to the x axis, which is the 关112¯0兴
direction, as shown in Fig.3.17Therefore, the nonvanishing second-order nonlinear optical susceptibility tensor compo-nents shall possess an even number of y subscript. The second-order susceptibility perturbed by the twin boundary leads to a nonvanishing ss-RSHG intensity,24 which can be expressed as
Is共2兲,m→s ⬀ 兩cos共cos2共2兲,mxxx + 2 sin2xyy共2兲,m兲兩2, 共1兲
whereijk共2兲,m is the component of the second-order suscepti-bility tensor with C1v symmetry. In view that the ss-SHG
FIG. 1. The x-ray patterns of ZnO films deposited on Si共111兲 at different substrate temperature. The inset shows the stress calculated by the XRD profiles and peak shifts of the ZnO共0002兲 peak.
FIG. 2. The s-polarized RSHG pattern excited with s-polarized light ac-quired with a ZnO film on Si共111兲 substrate deposited at a temperature of 100, 200, and 300 ° C.
FIG. 3.共Color online兲 The arrow indicates the direction of the nonvanishing polarity from the O to the Zn atoms on the twin boundary. The dashed line denotes the关11¯00兴/共1102兲 twin boundary in ZnO along the 关11¯00兴 zone axis共Ref.7兲.
091909-2 Lo et al. Appl. Phys. Lett. 92, 091909共2008兲
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intensity from the bulk of ZnO 共0002兲 film is vanishing, ss-SHG can be used to reveal sensitively the planar defects developed in the film. However, it is difficult to deduce qualitatively how ijk共2兲,m varies with the generation of twin boundaries. Notice that the stress of ZnO 共0002兲 thin film estimated from XRD decreases with increasing substrate temperature. The stress appearing at the ZnO film and sub-strate shall be the major contribution to the amplitude of
ijk 共2兲,m.
Figure 2 exhibits an asymmetric two-lobed pattern, which does not agree with the symmetrical two-lobed pattern described by Eq. 共1兲. One plausible reason to generate the asymmetrical two-lobed pattern is that the twin boundary does not have a mirror symmetry instead, the two planes relatively shift along the twin boundary.7The mirror struc-ture with forward-backward symmetry is therefore broken. The second-order susceptibility can be modified by the stress gradient, which is properly described with a parameter a. Equation共1兲then becomes
Is共2兲,m→s ⬀ 兩cos共cos2共2兲,mxxx + 2 sin2xyy共2兲,m兲 + a兩2. 共2兲
The theoretical curves 共solid lines兲 based on Eq. 共2兲 agree well with the experimental measured data shown in Fig. 2. From the fit, we found that xxx共2兲,m reveals a mirror plane with nonlinear optical polarization perpendicular to 关112¯0兴, supporting the notion that the measured ss-RHG is caused by a planar defect.
The measured results with SHG are in fact averaged over the irradiated spot共10 mm2兲 and shall cover many
do-mains. It is therefore surprising to discover that the measured azimuthal patterns of RSHG shown in Fig.2exhibit twofold symmetry, a result that has not been observed before in other studies. The nonzero polarity produced by the appearance of the plane defects between the neighboring twinned ZnO crystal domains related to the residual stress. This leads to the observed relation between ss-SHG and the residual stress in the films.
This work was supported from the National Science Council of the Republic of China under Contract Nos. NSC95-2112-M-415-002- and NSC 96-2112-M-415-004-.
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