Ultra-smooth and lattice relaxed ZnO thin films (original) (raw)
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Ultra-smooth and Lattice relaxed ZnO thin films N. Fouda, El Shazly M. Duraia, E.A. Eid
The crystal structure and quality of ZnO thin films were enhanced by high temperature vacuum annealing. High quality ZnO thin films have been grown on a-plane sapphire substrate by radio frequency (rf) magnetron sputtering method at a substrate temperature of 600 ° C. A remarkable improvement in the epilayer quality were established by in situ high temperature annealing. The film quality, smoothness, the in plane stress, and the degree of epitaxyof the films have been evaluated. The crystalline quality was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy analyses. An extremely smooth ZnO films were achieved at higher annealing temperatures with root mean square roughness of 0.3nm. The transverse optical mode A1(TO) observed in all the samples and the longitudinal optical mode A1(LO) appeared only at higher annealing temperatures over 800 ° C in the mico-Raman scattering measurements. The strain of c-axis were relaxed and the lattice parameter was comparable to that of bulk ZnO at high annealing temperature of 900 ° C. Keywords: Zinc oxide thin films; vacuum annealing; lattice relaxation; Raman spectroscopy
Improvement in the crystallinity of ZnO thin films by introduction of a buffer layer
Thin Solid Films, 2002
The influence of pre-deposition of homo-buffer layers on film quality is studied as functions of temperature and duration of pre-deposition, for zinc oxide (ZnO) crystalline films prepared by pulsed laser deposition on sapphire (0 0 0 1) substrates. This preparation technique is necessary to prepare high quality films suitable for the development of ZnO devices. Crystallinity and surface morphology were characterized by X-ray diffraction (XRD), reflection high energy electron diffraction and scanning electron microscopy. The line width of the rocking curve observed for ZnO (0 0 0 2) XRD of ZnO films decreases (to 0.098 from 0.2-0.38) upon introduction of a buffer layer of ZnO itself at a low temperature approximately 500 8C, indicating the formation of high quality films. The surface morphology and flatness were also improved. The film prepared under optimal conditions shows a high optical transmittance of ;90% with a steep falloff at 380 nm and a fairly small carrier concentration (1.8=10 17 cm ). These results imply that the buffer layer relaxes the strain due to lattice mismatch between ZnO and sapphire (by 18%) y3 and improves the film crystallinity. ᮊ
Improved optical and structural properties of ZnO thin films by rapid thermal annealing
Solid State Communications, 2007
The influence of rapid thermal annealing (RTA) on the optical and structural properties of ZnO thin films grown on Si substrate has been investigated by X-ray diffraction (XRD), photoluminescence (PL), and Raman scattering (RS) measurements. The relaxation of the residual stress by increasing the annealing temperature during the RTA process was observed by the measured shift of (002) XRD diffraction peak towards 34.40 • and the shift of RS E 2 (high) mode closer to 437 cm −1. The process also resulted in a reduction of the measured full-width at half maximum (FWHM) of the PL emission line and that of the asymmetrical broadening of RS E 2 (high) mode. The observed changes have demonstrated that RTA is a viable technique for improving the crystalline quality of ZnO/Si films.
Dependence of film thickness on the structural and optical properties of ZnO thin films
Applied Surface Science, 2009
ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers–Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet–visible–near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10−4 Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.
Crystallinity Improvement of ZnO Thin Film on Different Buffer Layers Grown by MBE
Journal of Nanomaterials
The material and optical properties of ZnO thin film samples grown on different buffer layers on sapphire substrates through a two-step temperature variation growth by molecular beam epitaxy were investigated. The thin buffer layer between the ZnO layer and the sapphire substrate decreased the lattice mismatch to achieve higher quality ZnO thin film growth. A GaN buffer layer slightly increased the quality of the ZnO thin film, but the threading dislocations still stretched along the c -axis of the GaN layer. The use of MgO as the buffer layer decreased the surface roughness of the ZnO thin film by 58.8% due to the suppression of surface cracks through strain transfer of the sample. From deep level emission and rocking curve measurements it was found that the threading dislocations play a more important role than oxygen vacancies for high-quality ZnO thin film growth.
Nuclear Instruments and …, 2008
Highly oriented and transparent ZnO thin films have been fabricated on ultrasonically cleaned quartz substrates by the sol-gel technique. X-ray diffraction, UV-VIS, FTIR, photoluminescence and SEM are used to characterize ZnO thin films. X-ray diffraction study show that all the films prepared in this work have hexagonal wurtzite structure, with lattice constants a = b = 3.260 Å, c = 5.214 Å. The optical band gap energy of the thin films is found to be direct allowed transition ~3.24 eV. The FTIR spectrum of the film has the characteristics ZnO absorption band at 482 cm −1. The photoluminescence spectrum of the samples has an UV emission peak centred at 383 nm with broad band visible emission centred in the range of 500-600 nm.
Nano and micro structural studies of thin films of ZnO
Journal of Materials Science, 2006
Zinc oxide thin films grown by sol-gel and RF sputtering methods have been characterized. The characterization techniques used involve ellipsometry, optical absorption, scanning tunneling microscopy, scanning and transmission electron microscopy. The films grown by sol-gel spin method which followed zinc acetate route exhibited a smoother texture than the films, which were deposited by using zinc nitrate route. The later type of films showed a dendritic character. Nano-structured fine grains of size ranging from 20 to 60 nm were observed with zinc nitrate precursor film. Individual grains show a sharp contrast with different facets and boundaries. Crystal planes and lattice parameters calculated by electron diffraction and X-ray diffraction are quite close and in agreement with the reported values in literature. Scanning tunneling microscopy has been used for measuring the average roughness of the surface and estimating the lattice constants. The STM studies of RF sputtered films, although showing a ZnO structure, exhibited a disturbed lattice. This was presumably due to the fact that after deposition the films were not annealed. Nanographs of 2D and 3D view of atomic positions of ZnO have been presented by using scanning tunneling microscopy.
Tem Characterization of ZnO and AIN/ZnO Thin Films Grown on Sapphire
MRS Proceedings, 1998
Thin (~ 250 nm) films of ZnO grown by pulsed laser deposition on basal plane of sapphire were studied by transmission electron microscopy (TEM). Plan-view TEM study proved the films to be single crystal with the following epitaxial relationship with the substrate: (0001)znO || (0001)sap with the 30 30° in-plane rotation - [0110]ZnO || [1210]sap. Dislocations lying mostly in basal plane of ZnO and aligned along both and <1120> directions having b=1/3[1120] were found. ZnO films were found to have layered growth morphology contrary to columnar morphology of III-nitrides. Consequently, the threading dislocation density in ZnO films (opposing to the AIN and GaN) drops very fast with the thickness: down to 107cm-2 at ~ 250 nm. The effect of post-annealing (which caused significant improvement in electrical and optical properties) on the microstructure of ZnO films was also studied. Contrary to the atomically sharp and clean interface in the as-deposited films, the post-annealed ZnO...