Ik Jae Lee

Strain effects in ZnO thin films and nanoparticles

We grew Stranski-Krastanow-type ZnO thin film and Volmer-Weber-type self-assembled ZnO nanocrystals using magnetron sputtering methods. The evolution of surface roughness and strain effects in thin ZnO films on Al2O3(0001) substrate and ZnO nanocrystals on Pt(111) surface studied by synchrotron x-ray scattering. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage in the thin films. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. The accumulated strain energy in the thin film grown at low temperature slowly relaxed while the strain energy in the high temperature system rapidly relaxed. When the three-dimensional islands on the 2D surface of thin ZnO film grown at the low and high temperatures were quickly developed by strain relaxation, the critical exponent β were roughly 0.693 and 1.579, respectively. The thickn...

Synchrotron x-ray scattering study on the evolution of surface morphology of the InN/Al2O3(0001) system

Dynamic scaling behavior was studied for InN films grown on sapphire(0001) substrates using high-resolution synchrotron x-ray reflectivity and atomic force microscopy measurements. In the early stage of growth, highly strained planar InN films were grown. As the film thickness approaches an effective critical thickness, the growth gradually crosses over to the island growth. Concurrently, the relaxation of the lattice strain begins and the growth front becomes rougher. The roughness increases mostly during the intermediate crossover regime where the strain is relieved. In this regime, the dynamic scaling exponent, β, is estimated as 1.754±0.071. The evolution of the surface roughness in the final-stage growth can be described by the dynamic scaling exponent of 0.236±0.022.

Near-edge x-ray absorption fine structure and x-ray photoemission spectroscopy study of the InN epilayers on sapphire (0001) substrate

A polarization-dependent near-edge x-ray absorption fine structure (NEXAFS) study was performed on InN films epitaxially grown on sapphire (0001) substrates, in order to investigate the change in structure and crystallographic orientation of the film as a function of film thickness. For thin films, the N K-edge NEXAFS spectra showed a strong polarization-dependent spectral feature. The polarization dependence decreased with increasing film thickness and disappeared at a thickness of about 3000 A. Chemical configuration in InN films was investigated using high-resolution x-ray photoemission spectroscopy (XPS). XPS analysis on the In 3d peak and the N 1s main peak at 396.4 eV suggested that indium and nitrogen are bound in the form of InN in all of the samples. An additional peak observed at 397.4 eV in the N 1s photoelectrons is believed to originate from the formation of oxynitrides at the topmost region of the film.

Synchrotron x-ray scattering study of lattice relaxation in inn epitaxial layers on sapphire(0001) during dc sputter growth

We present the results of a high-resolution synchrotron x-ray scattering study of the lattice relaxation in an InN (0001) epitaxial layer grown on sapphire (0001) by a dc faced magnetron sputtering deposition method. X-ray powder diffraction, rocking curve, x-ray reflectivity, and atomic force microscopy surface morphology studies consistently suggest that films thinner than ∼170 A should be highly strained, and be grown as two-dimensional epitaxial layers, in spite of the large mismatch of ∼29%. We deduced the condition for extended domain matching. The strain was relieved as the film thickness increased, while columnar seeds started to nucleate on parts of the film.

Morphological and structural characterization of epitaxial α-Fe2O3 (0001) deposited on Al2O3 (0001) by dc sputter deposition

We investigated α-Fe2O3 (0001) film growth on α-Al2O3 (0001) using a laboratory-built dc faced magnetron sputtering system with x-ray diffraction, x-ray reflectivity, and atomic force microscopy (AFM). The films were deposited from iron targets at a growth rate of ∼6.3nm∕min in an ambient argon and oxygen mixture. The structural properties and quality of the α-Fe2O3 thin films were characterized using high-resolution synchrotron x-ray scattering. Films thinner than 16nm were to be found highly strained due to the large lattice mismatch between film and substrate of 5.8%. This strain was found to decrease as the film thickness increased, and most of the strain was released through surface modulation once the film thickness reached 20nm. The epitaxial relationships were found to be α-Fe2O3 [0001]‖α-Al2O3 [0001] in the out-of-plane direction. That the film and substrate display the in-plane alignment anticipated for the epitaxy of isomorphous materials was established by verifying that the [112¯0] directions of film and substrate were coincident. X-ray reflectivity and AFM were used to show that layer-by-layer growth of α-Fe2O3 (0001) occurs up to a thickness of 11nm despite the large lattice mismatch.We investigated α-Fe2O3 (0001) film growth on α-Al2O3 (0001) using a laboratory-built dc faced magnetron sputtering system with x-ray diffraction, x-ray reflectivity, and atomic force microscopy (AFM). The films were deposited from iron targets at a growth rate of ∼6.3nm∕min in an ambient argon and oxygen mixture. The structural properties and quality of the α-Fe2O3 thin films were characterized using high-resolution synchrotron x-ray scattering. Films thinner than 16nm were to be found highly strained due to the large lattice mismatch between film and substrate of 5.8%. This strain was found to decrease as the film thickness increased, and most of the strain was released through surface modulation once the film thickness reached 20nm. The epitaxial relationships were found to be α-Fe2O3 [0001]‖α-Al2O3 [0001] in the out-of-plane direction. That the film and substrate display the in-plane alignment anticipated for the epitaxy of isomorphous materials was established by verifying that the [112¯0] directions...

Study of the strain in InN thin films using synchrotron X-ray scattering

Semiconductor InN films epitaxially grown by the radio frequency magnetron sputtering deposition method were studied in terms of the strain evolution as a function of the film thickness and growth temperature. The structure and surface morphology of the InN films were analyzed using synchrotron X-ray scattering and atomic force microscopy (AFM) experiments, respectively. The lattice strain of the InN films grown at 300°C was larger than that of the films grown at 490°C. As the film thickness increases, the lattice strain is reduced and completely relaxes when the thickness is larger than 350 A. The average roughness on the surface of the InN film increased with the growth temperature.

Identification of hexagonal polycrystal in epitaxially grown InN by synchrotron x-ray diffraction and near-edge x-ray absorption fine structure spectroscopy

The structures and crystallographic orientations of indium nitride films of varying thicknesses on sapphire(0001) were investigated using high-resolution synchrotron x-ray scattering and angle-dependent near-edge x-ray absorption fine structure (NEXAFS) spectroscopy with linearly polarized x rays. The x-ray scattering data showed that epitaxially grown InN films have a polycrystalline structure when their thickness is greater than 3000 A. The N 1s NEXAFS spectra of thin films have a strong polarization-dependent spectral feature resulting from the preferred c-axis orientation. This polarization dependence decreases as the film thickness increases and is not present in the spectra of films that are more than 3000 A thick. These results indicate that the c axis has a preferred orientation in thin films, but that this orientation is random in thick films, which have a polycrystalline hexagonal structure.

Effect of oxygen pressure on the structural and optical properties of ZnO/Si(100) thin films

ZnO thin films (thickness ∼ 400 nm) were prepared at different oxygen content (O2) by radio frequency (rf) sputtering method. Crystal structure and optical properties of these films were investigated by x-ray diffraction (XRD) and UV-VIS-NIR spectrophotometer, respectively. XRD measurement suggests that all films have hexagonal wurtzite structure. The transparency in the visible region of all the film is more than 90%. The optical absorption edge was described using the direct transition model proposed by Tauc and the optical band gap was calculated from the absorption coefficient by Tauc’s extrapolation procedure. The value of optical band gap was decreased with increase in the O2 content; due to Burstein-Moss shift.