Superhigh-Resolution Structural Image of InP Along [110] Zone Axis

Abstract

During the past decade , K.IZUI and S.FURUNO [1] reported the structural image of Si along [110] direction with the bright spots representing the columns of atoms. This result remain somewhat controversial. J.L.HUTCHISON and W.G.WADDINGTON [2] elucidated the image as the lattice image, not the structural image , for the computer simulation showed that this just is the artifice due to electron optics. Mallard et.al [3] reported HREM image of GaInAs/InP interface along [110] direction and consider that it is not a true structure image because the spots in pairs on the picture were not representative of accurate atomic positions. Here we report the superhigh resolution image of InP along [110] direction. Combining with computer simulation technique, it is confirmed that the observed image is the structural image of InP along [110] direction. The black spots represent to the atomic columns. The nearest distance of In and P is 1.47Å as shown in the picture, which is smaller than the point-point resolution 1.9Å of JEOL-4000EX electromicroscope. Fig.1 is the superhigh resolution image of InP along [110] direction. The thickness of the specimen is about 350Å, and defocus is about ΔF=-470Å. The bigger bright spots correspond to the atomic channels. The different size black spots represent the columns of In and P atoms.Fig.2 shows the simulated structural image of InP along [110] direction obtained by multi-slice method. Fig.3 show that the thickness of the specimen depends on the amplitude of electron waves. When the amplitudes of transmitted beams reach nadirs and the amplitudes of diffracted 111, 002,220,004 beams are relatively strong, the contrast of the structural image is better. It is indicated that the amplitudes of transmitted beam interacts with diffracted beams according to the dynamical theory of electron diffraction and the electron waves passing through the sample have nearly the same amplitude, so that both In and P atomic columns may imaged. These results are in good agreement with the results on another paper about the explanation of structural image of InP along [001] direction[4]. The phases of the electron waves were investigated as shown in Fig.4. The transmitted beam has, the same value of phases as diffraced beams at the thickness of 350Å. This work is very similar to that of Au atomic image along [110] direction reported by prof. H.Hashimoto.[5] It is considered that the atomic images can be formed when obseved specimen is rather thin or near optimum thickness at which the phase of the image-forming waves the same values. This is very important for the explanation of the superhigh resolution image. The curve of CTF of JEOL--4000ex operated at 400kv with different defocuses were calculated. The CTF of sherzer condition is ΔF=-406Å. In our experiment, the superhigh resolution structural images were obtained with ΔF=-465Å, which is close to the scherzer condition. It is obvious that the electron beams with lower indexes were nearly included in the plateau of CTF curve where sinx ≈ 1, as shown in Fig.5, which may carries a lot of information about the structure, thus the atomic images of InP can be formed.