Development of Crystal Structure Analysis due to Convergent-Beam Electron Diffraction

点击翻译按钮获取中文摘要

This imaging plate (IP) has the following advantages for the detection of the election beam : (1) High sensitivity to small electron dose, (2) wide dynamic range, (3) linearity of sensitivity, (4) high precision digital images and (5) dry system. We have applied the IP to the convergent-beam electron diffraction (CBED) and obtained the following results.(a) It has been shown that the IP is effective to detect the symmetry of CBED patterns. As an example, very weak 200 and equivalent reflections of GaAs were recorded by the IP at the 001 incidence. These reflections all very important for the symmetry study of the material, and had not been recorded clearly by conventional photo films due to the back ground noise. The IP recording and successive image processing of the reflections enable us to reveal the symmetry formed by the 200 and equivalent reflections.(b) The symmetry of CBED patterns has been quantitatively examined for the first time. The symmetry of CBED pattern has been judged … More by human eyes but has not ever been studied quantitatively. Two reflections which are connected by a symmetry operation were compared. Intensity of one reflection is subtracted from that of the other and the value of R=ROO<SIGMA(I_1-I_2)^2/SIGMAI_1^2> is calculated. It was revealed that a CBED pattern of <111> Si, which is a typical example of good symmetry, showed a value of R<similar or equal>10 %. Human eyes recognize the breakdown of symmetry when CBED patterns which have a value of R beyond 20 %.(c) The displacement vector of a stacking fault can be determined by analyzing the intensity profiles of reflections appearing in CBED patterns. The IP is quite useful to record the intensities of profiles accurately. The displacement vectors of stacking faults in Si and TiO_2 could be reliably determined by applying IP.(d) One of the final goal of CBED is the crystal structure analysis, which requires precise intensity measurements. The IP is indispensable to develop the crystal structure analysis using CBED. Firstly, we tried to solve the crystal structure of the low temperature phase of SrTiO_3, or the rotation angle of the oxygen octahedron and the Debye-Waller factor. The computer analysis program using a nonlinear least-square method (SALS) was developed. Using the intensity data recorded by IP and the analysis program, the rotation angle of the oxygen octahedron was determined to be 1.12 <plus-minus> 0.04ﾟ. The value shows a good agreement with that obtained by the electron-spin resonance method. Secondary, the crystal structure of the intermediate phase of the hexagonal BaTiO_3, which had been remained unsolved, was determined under the guide of the result of the soft-mode analysis. These results strongly promise the success of the crystal structure analysis of a small specimen area using CBED. Less