Browsing by Author "Liu, W. -R."

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Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epi-films
(2009)
Kuo, C. C.
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Hsu, H. C.
Crystal symmetry breaking of wurtzite C(6V) to orthorhombic C(2V) due to in-plane anisotropic strain was investigated for nonpolar a-ZnO epi-films grown on R-sapphire.
Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epifilms
(2009-07-06)
Kuo, C. C.
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Hsu, H. C.
Crystal symmetry breaking of wurtzite C-6V to orthorhombic C-2V due to in-plane anisotropic strain was investigated for nonpolar (11 (2) over bar0) ZnO epifilms grown on the R-sapphire. X-ray diffraction results reveal the epilayer is subjected to a compressive strain along the polar c-axis and tensile strains along both a-[11 (2) over bar0] surface normal and in-plane p-[1 (1) over bar 00] axis. The polarized Raman spectra of E-2 modes reveal violation of the C-6V selection rules. Oppositely, the C-2V configuration satisfies the selection rules for the Raman modes. The observed E-1 and E-2 bands in polarized optical reflection and photoluminescence spectra confirm the anisotropic strain causes the structure change to the orthorhombic one. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3159470]
Domain Matching Epitaxial Growth of High-Quality ZnO Film Using a Y(2)O(3) Buffer Layer on Si (111)
(2009-01-01)
Liu, W. -R.
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Li, Y. -H.
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Hsieh, W. F.
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Hsu, C. -H.
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Lee, W. C.
High-quality ZnO epitaxial films have been grown by pulsed-laser deposition on Si (111) substrates using a nanothick high-k oxide Y(2)O(3) buffer layer. Determined by X-ray diffraction and transmission electron microscopy, the epitaxial relationship between ZnO and Y(2)O(3) follows (0001)< 2 (1) over bar(1) over bar0 >(ZnO)parallel to(111)< 10 (1) over bar > Y(2)O(3). ZnO lattice aligns with the hexagonal O sublattice in Y(2)O(3) and the interfacial structure can be well described by domain matching epitaxy with 7 or 8 ZnO {11 (2) over bar0} planes matching 6 or 7 {4 (4) over bar0} planes Of Y(2)O(3) and lead to a significant reduction of residual strain. Superior optical properties were obtained even for ZnO films as thin as 0.21 mu m from photoluminescence results.
Domain Matching Epitaxial Growth of High-Quality ZnO Film Using a Y2O3 Buffer Layer on Si (111)
(2009-01-01)
Liu, W. -R.
;
Li, Y. -H.
;
Hsieh, W. F.
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Hsu, C. -H.
;
Lee, W. C.
High-quality ZnO epitaxial films have been grown by pulsed-laser deposition on Si (111) substrates using a nanothick high-k oxide Y2O3 buffer layer. Determined by X-ray diffraction and transmission electron microscopy, the epitaxial relationship between ZnO and Y2O3 follows (0001)< 2 (1) over bar(1) over bar0 >(ZnO)parallel to(111)< 10 (1) over bar > Y2O3. ZnO lattice aligns with the hexagonal O sublattice in Y2O3 and the interfacial structure can be well described by domain matching epitaxy with 7 or 8 ZnO {11 (2) over bar0} planes matching 6 or 7 {4 (4) over bar0} planes Of Y2O3 and lead to a significant reduction of residual strain. Superior optical properties were obtained even for ZnO films as thin as 0.21 mu m from photoluminescence results.
Effect of Threading Dislocations on Local Contacts in Epitaxial ZnO Films
(2010)
Lin, C. Y.
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Liu, W. -R.
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Chang, C. S.
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Hsu, C. -H.
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Hsieh, W. F.
Local conductance of a ZnO epifilm with a columnar-grain structure was studied by conductive-mode atomic force microscopy. The probe-ZnO junction at the grain boundary with high density edge threading dislocations (TDs) behaves as a Schottky contact while the junction at the epitaxial core behaves as an ohmic contact, resulting in the nonuniformity of conductance throughout the film. The calculated Schottky barrier is 0.4 +/- 0.025 eV. The point defects of doubly charged Zn vacancies accumulated at the edge TDs induce local band bending of ZnO, thus contributing to the Schottky nature at the grain boundary.
Effects of ultraviolet treatment on the contact resistivity and electronic transport at the Ti/ZnO interfaces
(2009-07-01)
Lin, Yow-Jon
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Tsai, Chia-Lung
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
We report on the effect of ultraviolet (UV) treatment on the specific contact resistance (rho) and electronic transport at the Ti/ZnO interfaces. The experimental results show the same barrier height of Ti/ZnO samples without UV treatment as Ti/ZnO samples with UV treatment and the higher rho of Ti/ZnO samples with UV treatment than Ti/ZnO samples without UV treatment, suggesting the barrier-height independence of rho. Based on the thermionic-emission model and x-ray photoelectron spectroscopy results, we found that the induced decrease in the number of the hydroxides at the surface region of ZnO by UV treatment resulted in decreases in the electron concentration near the surface region and the excess current component related to tunneling, increasing in rho of Ti/ZnO samples.
Influence of the threading dislocations on the electrical properties in epitaxial ZnO thin films
(2006-12-29)
Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Liang, K. S.
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Chien, F. S. -S.
The structural and electrical properties of epitaxial ZnO films grown by pulsed-laser deposition on sapphire (0001) were investigated by high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), and transmission electron microscopy (TEM). The results of XRD and AFM revealed that the ZnO films have a columnar-grain structure consisting of epitaxial cores surrounded by annular boundaries. The core and boundary regions exhibited significantly different capacitive responses and field emission current. The results of TEM indicated that the annular boundaries have high-density edge threading dislocations. The shift of flatband voltage and the raise of potential barrier at the boundaries observed by SCM and C-AFM were attributed to the interface trap densities caused by the existence of high-density edge threading dislocations. (c) 2006 Elsevier B.V. All rights reserved.
Photoluminescence associated with basal stacking faults in c-plane ZnO epitaxial film grown by atomic layer deposition
(2012-03-05)
Yang, S.
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Kuo, C. C.
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Liu, W. -R.
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Lin, B. H.
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Hsu, H. -C.
Basal plane stacking faults (BSFs) with density of similar to 1 x 10(6) cm(-1) are identified as the dominant defect in the annealed ZnO thin films grown on c-plane sapphire by atomic layer deposition. The dominant peak centered at 3.321 eV in low-temperature photoluminescence measurements is attributed to the emission from the BSFs. The emission mechanism is considered to be the confined indirect excitons in the region of quantum-well-like structure formed by the BSFs. The observed energy shift of 19 meV with respect to the BSF-bounded exciton at low temperature may be caused by the localization effect associated with the coupling between BSF quantum wells. (C) 2012 American Institute of Physics. [http://dx.doi. org/10.1063/1.3692730]
Structural Characteristics and Annealing Effect of ZnO Epitaxial Films Grown by Atomic Layer Deposition
(2009-12-01)
Yang, S.
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Lin, B. H.
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Liu, W. -R.
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Lin, J. -H.
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Chang, C. -S.
Structural characteristics of the ZnO epitaxial films grown on c-plane sapphire by the atomic layer deposition method were thoroughly studied. The in-plane axes of the e-plane oriented ZnO layers are predominantly aligned with that of the sapphire substrate, yielding the relationship of {10 (1) over bar0}(ZnO) parallel to {10 (1) over bar0}(Al2O3). The minor orientation with a 30 degrees in-plane twist configuration, that is, {10 (1) over bar0}(ZnO) parallel to {11 (2) over bar0}(Al2O3),which is more commonly observed in ZnO films grown by metal organic chemical vapor deposition, pulsed laser deposition, and other methods, only amounts to less than 3% and can be eliminated by thermal annealing. The structure of the ZnO, epi-films exhibits significantly improvement upon thermal annealing, and intrinsic types of basal plane stacking faults are the predominant Structural defects in the ZnO after thermal treatment. The effect of post growth thermal treatment is reported in this work.
The influence of dislocations on optical and electrical properties of epitaxial ZnO on Si (111) using a gamma-Al2O3 buffer layer
(2012)
Liu, W. -R.
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Lin, B. H.
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Yang, S.
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Kuo, C. C.
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Li, Y. -H.
The structural, optical and electrical properties of the c-plane ZnO epitaxial films grown by pulsed laser deposition on a Si(111) substrate buffered with a thin layer of gamma-Al2O3 were investigated by X-ray diffraction, transmission electron microscopy, photoluminescence (PL) and Hall measurements. Detailed structural investigation showed that the dominant structural defects in the ZnO films are threading dislocations (TDs). Experimental results manifest the edge-and screw-type of TDs influence the optical and electric properties differently; the intensity ratio between the PL yellow-green band to near band edge emission and the carrier concentration are affected mainly by the edge TD, and the FWHM of the near band edge emission is dominantly influenced by the screw TD.
Thickness-dependent lattice relaxation and the associated optical properties of ZnO epitaxial films grown on Si (111)
(2012)
Liu, W. -R.
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Lin, B. H.
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Kuo, C. C.
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Lee, W. C.
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Hong, M.
The evolution of the strain state as a function of layer thickness of (0001) oriented ZnO epitaxial films grown by pulsed-laser deposition on Si (111) substrates with a thin oxide Y2O3 buffer layer was investigated by high resolution X-ray diffraction (XRD). The ZnO layers experience a tensile strain, which gradually diminishes with increasing layer thickness. Regions with a nearly strain-free lattice develop as the layer thickness exceeds a critical value and are correlated with the emergence of the < 11 (2) over bar0 > oriented crack channels. The influence of the biaxial strain to the vibrational and optical properties of the ZnO layers were also studied by micro-Raman, optical reflectance, and photoluminescence. The deformation-potential parameters, a(lambda) and b(lambda), of the E-2(high) phonon mode are determined to be -740.8 +/- 8.4 and -818.5 +/- 14.8 cm(-1), respectively. The excitonic transitions associated with the FXA, FXB, and D degrees X-A emissions and the A-exciton binding energy all show linear dependence on the in-plane strain with a negative slope.
Vertical-cavity and randomly scattered lasing from different thicknesses of epitaxial ZnO films grown on Y2O3-buffered Si (111)
(2013-01-28)
Kuo, C. C.
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Liu, W. -R.
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Lin, B. H.
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Hsieh, W. F.
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Hsu, C. -H.
Two different types of lasing modes, vertical Fabry-Perot cavity and random lasing, were observed in ZnO epi-films of different thicknesses grown on Si (111) substrates. Under optical excitation at room temperature by a frequency tripled Nd:YVO4 laser with wavelength of 355 nm, the lasing thresholds are low due to high crystalline quality of the ZnO epitaxial films, which act as microresonators. For the thick ZnO layer (1,200 nm), its lasing action is originated from the random scattering due to the high density of crack networks developed in the thick ZnO film. However, the low crack density of the thin film (555 nm) fails to provide feedback loops essential for random scattering. Nevertheless, even the lower threshold lasing is achieved by the Fabry-Perot cavity formed by two interfaces of the thin ZnO film. The associated lasing modes of the thin ZnO film can be characterized as the transverse Gaussian modes attributed to the smooth curved surfaces. (C) 2013 Optical Society of America