N. Nakanishi

Formation and structure of inverted hexagonal pyramid defects in multiple quantum wells InGaN/GaN

We have determined the structure of inverted hexagonal pyramid defects (IHPs) in multiple quantum wells InGaN/GaN by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). HAADF STEM images reveal definitely that the IHP nucleates at a threading dislocation and grows in the form of a thin six-walled structure with InGaN/GaN {1011} layers. It has been found that IHPs start even at In-rich dots under adverse growth conditions.

Atomic-scale strain field and In atom distribution in multiple quantum wells InGaN/GaN

We present an atomic-scale structural and compositional analysis of ultrathin layers in multiple quantum well InGaN/GaN, by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). A high-quality HAADF STEM image processed by two-dimensional smoothing and deconvolution provides precise atomic-column positions and clear contrast, thereby allowing us to map the strain field and In atom distribution in successive GaN and InGaN layers. We conclude from these maps that there is a local fluctuation of In atoms in the InGaN layers and the In-rich regions, considered as quantum dots, cause large expansion only along the [0001] direction.

Lattice imaging in low-angle and high-angle bright-field scanning transmission electron microscopy

Atomic resolution low-angle bright-field (LABF) scanning transmission electron-microscope (STEM) images and high-angle bright-field (HABF) STEM images of [011]-orientated Si have been experimentally obtained together with high-angle annular dark-field (HAADF) STEM images. The contrast formation mechanisms of the LABF STEM and HABF STEM images are examined in comparison with HAADF STEM images. The HABF STEM images independent of defocus and thickness have spatial resolution comparable with HAADF STEM images, and are shown to be given as a simple convolution under the non-dispersion approximation of localized Bloch waves.

Microstructures formed in recrystallized Si

Our study using systematic transmission electron microscopy observation and simulation shows that microstructures formed in recrystallized Si are characterized as microtwin or lamellar microtwin. Detailed analysis leads to their atomic structures. The discovery of exceptional diffraction spots offers direct evidence of long-periodic-order structures and antiphase boundaries, due to the ordering of projected lamellar microtwins.

Measurement of 3rd Order Spherical Aberration Coefficient for Scanning Transmission Electron Microscopy

Recently, as the technique for improvement of the spatial resolution of HRTEM and highresolution scanning transmission electron microscopy (STEM), the developments of an spherical aberration corrector (Cs corrector) have been positively performed and succeeded by many researchers in the whole world over. In this case, it is necessary to exactly measure the uncorrected and corrected Cs. Conventionally, the Cs has been determined by analyzing a power spectrum image of high resolution image of an amorphous film [1, 2]. However, use of the power spectrum image becomes problematic when the spherical aberration coefficient is low (< 0.5 mm) [3]. Furthermore, it is very difficult to obtain the high qualitative BF STEM image of an amorphous structure because of the low level of probe intensities and the low signal-to-noise ratio. In fact, the estimated values include a very large percentage error. On the other hand, a ronchigram has been expected as another promising technique. In this paper, the measurement taken using crystalline material along lowerindex zone axis is suggested by expanding the method for Cs measurement suggested by Liu and Cowley [4]. Czochralski-grown [100]-orientated p-type Si wafer as a specimen for ronchigram observations was prepared because it is easy to make the thin specimen for TEM observation. TEM observations were performed with a JEM-2010F TEM/STEM, operated at 200 keV, equipped with a pole piece designed for Cs = 1.0 mm. The ronchigrams were obtained by entirely removing the objective aperture. In order to present how to measure Cs, the simulated ronchigram at Cs = 1.0 mm, ∆f = -450 nm and t = 20 nm is displayed in Fig. 1(a). It should be noted that there are characteristic fringes along Kikuchi bands. The corresponding intensity line profile between two white arrow heads shown in Fig. 1(a) is plotted in Fig. 1(b) by solid line. In order to find the origin of this fringe, the ronchigram simulation was performed using three beam condition with 000, 111, and -1-1-1 reflections. The corresponding line profile is shown in Fig. 1(b) by dashed line. The both period and shape under three beam condition are almost the same as those of many beam condition. Therefore, it is concluded that these characteristic fringes are formed by the interactions between 000, 111 and -1-11 reflections. The turning points of intensity line profiles along Ky direction are labeled by n = 1, 2, ..., as shown in Fig. 1(b), so that we can get the Ky-n plot. Under three beam condition constituted by systematic reflections, the relationship between n and Ky is given by n = λCsgKy +λg∆f + (1/2)λCsg + D, (1) where λ is the wave length, D the constant value. As a result, the Cs is determined from the sloop of the straight line. Figure 1(c) shows the Ky-n plots obtained from the dynamical simulation of ronchigrams using many beam and three beam conditions, respectively. Both results show a good DOI: 10.1017/S1431927605503404 Copyright 2005 Microscopy Society of America Microsc Microanal 11(Suppl 2), 2005 2166