S.-H. Lim

Epitaxial Growth of ZnO Films on (0001) Sapphire at Low Temperatures by Electron Cyclotron Resonance-assisted Molecular Beam Epitaxy and Their Microstructural Characterizations.

The details of epitaxial growth and microstructural characteristics of ZnO films grown on (0001) sapphire were investigated using X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED), and plan-view and cross-sectional transmission electron microscopy (TEM). The films were grown by electron cyclotron resonance-assisted molecular beam epitaxy (ECR-assisted MBE). The ECR-assisted MBE was found to be capable of growing high quality epitaxial ZnO films at low temperatures in comparison with chemical vapor deposition (CVD) and RF sputtering. The films exhibited a high degree of epitaxy, a sharp interface between film and substrate, and a smooth surface morphology. The TEM observations revealed that the films were epitaxial with an orientational relationship of (0001)ZnO ∥(0001)Al_2O_3 and [20]ZnO ∥[100] Al_2O_3. This is equivalent to a 30° rotation of ZnO relative to sapphire in (0001) plane.

Defect structure of epitaxial ZnO films on (0001) sapphire studied by transmission electron microscopy

Defects and structural characteristics (threading dislocations and angles of rotational disorder in the subgrain) of wurtzite-type ZnO films grown by electron cyclotron resonance-assisted molecular beam epitaxy on (0001) c-plane sapphire have been investigated extensively using conventional transmission electron microscopy and high-resolution electron microscopy (HREM). Through the cross-sectional and plan-view observations, the existence of threading dislocations in the ZnO film and the basic crystallographic orientation relationship of (0001)ZnO∥(0001)sapphire and [2110]ZnO∥[1100]sapphire were clarified. The line directions of most threading dislocations were found to be normal to the interface. The density of the threading dislocations in ZnO film was estimated to be 1.9×1011 cm−2 and the subgrains being accompanied by the threading dislocations and Burgers vectors of 1/3〈1120〉 were clearly observed. It was found that the size of the subgrains ranges from 15 to 150 nm and the subgrains are rotated ...

Study of defects and interfaces in epitaxial ZnO films on (112̄0) Al2O3 grown by electron cyclotron resonance-assisted molecular beam epitaxy

The detailed defects and interface in the ZnO films on (1 120) a-plane of sapphire have been characterized using transmission electron microscopy. The single crystal ZnO films are grown by electron cyclotron resonance-assisted molecular beam epitaxy. The orientation relationship between ZnO films and sapphire is (0001) ZnO ∥(11 120) sapphire and [211 0] ZnO ∥[000 1] sapphire . A majority of the threading dislocations was found to be screw or mixed. When the interfaces are observed in [0001] sapphire direction, the interfaces appear structurally semicoherent with a comparative regular array of misfit dislocations at an interface accommodating a mismatch of about 2.45%. Good matches between simulated and experimental images of the ZnO/a-plane sapphire were obtained. The structural model for the atomic arrangements of the interface was proposed.

Structure Modulation of Al0.5In0.5P Studied by Energy-Filtered Electron Diffraction and High-Resolution Electron Microscopy

Structure modulation of a III–V alloy semiconductor Al0.5In0.5P was investigated by electron diffraction and high-resolution electron microscopy (HREM). By utilizing an energy filter, the background of the electron diffraction patterns was markedly reduced and two types of the diffuse scattering were clearly revealed. One type of diffuse scattering is situated at the midpoint of the fundamental reflections while the other type is situated around the fundamental reflections. From the analysis of the HREM image, the structure modulation is interpreted to result from the ordering and the concentration modulation of Al and In. Furthermore, a structure model of Al0.5In0.5P is derived from the HREM image, and diffraction intensity calculated based on the structure model shows good agreement with the observed intensity.

Digital Electron Microscopy on Advanced Materials

Abstract Digital electron microscopy has been developed and applied to the structure analysis of advanced materials such as semiconductors and alloys. First of all, quantitative high-resolution electron microscopy was carried out on a Z-type faulted dipole in GaAs with the through-focus method. Through the quantitative analysis of the high-resolution images, the atomic displacement around the stacking fault was accurately evaluated. In the analysis of electron diffraction patterns of a Cu 0.725 Pd 0.275 alloy, an energy filter was utilized to obtain electron diffraction patterns with a small background removing the inelastically scattered electrons. From the analysis of diffuse scattering of the Cu 0.725 Pd 0.275 alloy, the short-range order parameters were quantitatively evaluated. Finally, it is pointed out that, based on the digital data of electron microscope images, the construction of the data base such as “EMILIA” (Electron Microscope Image Library and Archive: http://asma7.iamp.tohoku.ac.jp/EMILIA ) is quite important for the future research of advanced materials characterization.

Structure and Climb of Faulted Dipoles in GaAs

Z-shape faulted dipoles in deformed GaAs were investigated by using high resolution electron microscopy. There is no difference in the core structure of the α and β dislocations of stair-rod and 90° Shockley partials, nor in the dissociated stacking fault width. The central stacking fault is found to generate large local atomic displacements and exhibits a different structure from that of the intrinsic stacking fault of a dissociated dislocation considered so far. 90° Shockley partials of Z-shape faulted dipole climbed through absorption of interstitials during electron irradiation.