Dark-field transmission electron microscopy for a tilt series of ordering alloys: toward electron tomography

Here we show a technique to obtain a tilt series of dark-field (DF) transmission electron microscopy (TEM) images in ordering alloys for tomographic three-dimensional (3D) observations. A tilt series of DF TEM images of D1a-ordered Ni4Mo precipitates in a Ni-Mo alloy was successfully obtained by adjusting a diffraction condition for a superlattice reflection from the Ni4Mo precipitates. Since the superlattice reflection usually has a long extinction distance, dynamic diffraction effects such as thickness fringes can be suppressed to some extent with precise realignment of the diffraction condition. By using the tilt series of the DF TEM images, we attempted a computed TEM tomography to visualize 3D shapes and positions of the precipitates.     

The structure of an Al3Pd-based modulated structure studied by atomic-scale electron microscopy observations

A modulated structure based on an Al3Pd structure in an Al70Co25Pd5 alloy has been studied by atomic-scale observations with high-resolution transmission electron microscopy (HREM) and high-angle annular detector dark-field (HAADF) technique using a scanning transmission electron microscope. From the electron microscopy observations, we can reveal an arrangement of atom columnar clusters, which produces peculiar shifts of diffraction spots in an electron diffraction pattern. Arrangements of transition-metal atoms in defect regions in the modulated structure are proposed from the observed HAADF image.     

Quantitative intensity measurement of equal thickness fringes in Si and MgO crystal images with an energy-filtering transmission electron microscope using an imaging plate

Quantitative measurement of intensity profiles of equal thickness fringes has been carried out in Si and MgO crystal images with an energy-filtering transmission electron microscope using an imaging plate. The crystals have a 90 degrees wedge-shape with [110] surfaces for Si and with [100] surfaces for MgO, and are observed under the exact axial incidence of a 200 keV electron beam along the [100] axis for Si and along the [110] axis for MgO. The intensities are measured in bright field and 022 and 040 dark field images for Si, and in bright field and 111, 002, 220, 113, 222, and 004 dark field images for MgO, with and without an energy slit having +/- 5 eV energy width for incident electrons. The intensity profiles obtained from the images are presented as standard experimental data for calculation of electron diffraction intensities. A few simulation programs for high-resolution transmission electron microscopy are checked by comparing the calculated diffraction intensities with the experimental data. The complex potential suitable for matching the data is discussed.     

Nano-area electron diffraction pattern reconstructed from three-dimensional Fourier spectrum.

A method to obtain a nano-area electron diffraction pattern in transmission electron microscopy (TEM) was proposed, based on three-dimensional (3D)) image formation theory. This method allows us to reconstruct an electron diffraction pattern from a 3D Fourier spectrum of high-resolution through-focus images. As a test case, an electron diffraction pattern from a tilted Si single crystal was reconstructed using the proposed method and compared with the conventional selected-area electron diffraction pattern. The intensity distribution of the reconstructed electron diffraction pattern was confirmed to be qualitatively equal to that of the selected-area electron diffraction pattern, though the degree of the equivalency between these patterns reduces at the high frequency region and the reproducibility of the intensity degrades when the number of images used in the image processing was decreased. By selecting areas in a reconstructed exit wave field, some electron diffraction patterns were obtained from the nano-areas without the influence of spherical aberration.     

葡萄石微结构的TEM研究

本文利用电子衍射、电子显微像和晶格像,研究了湖北大治铁山矿床中葡萄石的完整／不完整晶畴、位错、堆垛层镜、复杂界面和斑点状衍射衬度等微结构特征;并从地质温度压力特征、各种电镜现象和葡萄石、绿纤石和多硅白云母晶胞参数特征,探讨了三种矿物形成的纳米拓一向晶格共生的机理。     

A real-time quantitative assessment of self-assembled quantum dots by reflection high-energy electron diffraction

A method for in-situ quantitative characterization of quantum dots during growth is provided using the readily available reflection high-energy electron diffraction (RHEED). RHEED patterns of uncapped self-assembled InAs/GaAs quantum dots are investigated theoretically and experimentally. Previously predicted intensity fringes along chevron tails of quantum dot's RHEED diffraction spots are observed experimentally. Post-growth atomic force microscopic images and theoretical RHEED images of the same are obtained parallel to experimental data. The bounding facets of the quantum dots are determined using the angle between the chevrons. The size (height) of the quantum dots is determined using the periodicity of intensity fringes along the chevrons during growth.     

Energy-filtered electron diffraction and high-resolution electron microscopy on short-range ordered structure in GaAs(0.5)Sb(0.5)

Characteristic intensity distribution of diffuse scattering in III-V alloy semiconductor GaAs(0.5)Sb(0.5) epitaxially grown was observed by the energy-filtered electron diffraction method with [110] incidence. The diffuse scattering situates at the one-third positions between the fundamental reflections extending parallel to the q002 direction in the reciprocal space. A high-resolution electron microscope image shows weak contrast modulation corresponding to the diffuse scattering. The image processed with the Fourier transform by selecting the diffuse scattering and a fundamental reflection shows small regions consisting of bright dots being elongated along the (111) planes and aligning on the (002) planes, which are considered to result from the ordering of As and Sb during the growth process. The effect of including the fundamental reflection for imaging the ordered regions in the image processing method is also discussed. Finally, based on the results obtained by energy-filtered electron diffraction and high-resolution electron microscopy, a simple structure model for the short-range ordered structure in GaAs(0.5)Sb0.5 is proposed.     

Threading Screw Dislocations in 4H-SiC Wafer Observed by the Weak-Beam Method in Bragg-Case X-ray Topography

Dislocations in 4H-SiC have been observed by x-ray topography in the Bragg-case (reflection) geometry by means of a weak-beam technique. Using an x-ray beam with an angular divergence of 0.87 μrad, collimated by an asymmetric Si 331 reflection, a nearly intrinsic rocking curve of the SiC 0008 reflection was obtained. High-resolution contrast of threading screw dislocations by kinematical diffraction was observed under diffraction conditions that deviated from the rocking curve peak by 24 μrad. Threading screw dislocations running inside the wafer were projected onto the topograph. In addition, the sense of each screw dislocation was determined directly from the weak-beam image in consideration of lattice displacement around the dislocation.     

Inhomogeneous substitution of polyhalogenated copper-phthalocyanine studied by high-resolution imaging and electron crystallography

The present study was aimed at distinguishing halogen atoms substituted in a molecule of copper-phthalocyanine (CuPc) by high-resolution transmission electron microscopy (HRTEM). The subject was carried out on octabromooctachloro-CuPc and obtained HRTEM images suggested inhomogeneous distribution of bromine and chlorine atoms at 1-, 2-, 3- and 4-substitution positions of CuPc in image contrast. As a result of electron crystallography, the occupancy of bromine atoms at the 1-position (and equally at the 4-position) was found to be 52%, which is slightly higher than for random substitution. This small deviation causes apparent inhomogeneous image contrast of halogen substitution observed in the high-resolution images, as concluded from computer simulation of images. Such potential to detect inhomogeneity in substitution by TEM should be important for the chemical understanding of substitution reactions at the atomic level.     

Anomalies in annealed LT-GaAs samples

The annealing behavior of low temperature (LT)-GaAs layers was investigated using transmission electron microscopy, x-ray rocking curves, and H⁺ ion channeling. These data were compared to the Hall-effect and conductivity data obtained earlier on the same samples. An expansion of the lattice parameter above those observed for as-grown LT-GaAs layers was observed for the layers annealed at 300 and 350°C. No precipitation was observed in transmission electron micrographs for these annealing temperatures. Based on ion-channeling results, the As atoms (split interstitials) appear to be in the same position as found for the as-grown layers. A special arrangement of As split interstitials or out-annealing of gallium vacancies would be consistent with a decrease of the dominant acceptor in these layers and an increase in the lattice parameter. For annealing above 400°C, the lattice parameter decreased and in fact was found to achieve the substrate value at annealing temperatures of 500°C and above. The decrease in the lattice parameter above 400°C is related to the decrease of excess As antisite defects and As split interstitials in the formation of As precipitates.     

The reaction characteristics of ultra-thin Ni films on undoped and doped Si (100)

Reaction characteristics of ultra-thin Ni films (5 nm and 10 nm) on undoped and highly doped (As-doped and B-doped) Si (100) substrates are investigated in this work. The sheet resistance (Rs) measurements confirm the existence of a NiSi salicidation process window with low Rs values within a certain annealing temperature range for all the samples except the one of Ni(5 nm) on P⁺-Si(100) substrate (abnormal sample). The experimental results also show that the transition reaction to low resistivity phase NiSi is retarded on highly doped Si substrates regardless of the initial Ni film thickness. Micro-Raman and x-ray diffraction (XRD) measurement show that NiSi forms in the process window and NiSi₂ forms in a higher temperature annealing process for all normal substrates. Auger electron spectroscopy (AES) results for the abnormal sample show that the high resistivity of the formation film is due to the formation of NiSi₂.     

Strain analysis of arsenic-doped silicon using CBED rocking curves of low-order reflections

Local lattice strains of semiconductor devices have been so far examined using higher order Laue zone (HOLZ) line patterns of convergent-beam electron diffraction (CBED). Recently, strain analyses in highly strained regions near interfaces have been reported using split HOLZ line patterns. In the present paper, it is demonstrated for arsenic-doped silicon that the use of CBED rocking curves of low-order reflections provides a promising new tool for the determination of strain distributions of highly strained specimen areas. That is, the anomalous intensity increase in the CBED rocking curves of low-order reflections is explained using a model structure with a strain gradient in the electron beam direction, which is similar to the models used for the split HOLZ line patterns.     

Ho arrangement in the Zn6Mg3Ho icosahedral quasicrystal studied by atomic-resolution Z-contrast STEM

The atomic structure of the Zn6Mg3Ho icosahedral quasicrystal has been studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast (Z: atomic number). We demonstrate that in particular Z-contrast imaging is quite powerful for specifying heavy atom positions in the quasicrystalline compound, as shown by a comparison with high-resolution phase-contrast imaging. It is confirmed that the observed Z-contrast images are fairly well explained by the projected potential of only the Ho atomic arrangement, which was recently proposed by X-ray diffraction analysis; Ho occupies an even-body-center site of the 3-dimensional Penrose lattice. Consequently, the present direct structural observation strongly supports the validity of the proposed Ho site.     

InP衬底上InAlAs异变缓冲层和高铟组分InGaAs的生长温度优化

利用气态源分子束外延技术在InP衬底上生长了包含InAlAs异变缓冲层的In0.83Ga0.17As外延层.使用不同生长温度方案生长的高铟InGaAs和InAlAs异变缓冲层的特性分别通过高分辨X射线衍射倒易空间图、原子力显微镜、光致发光和霍尔等测量手段进行了表征.结果表明, InAlAs异变缓冲层的生长温度越低, X射线衍射倒易空间图 (004) 反射面沿Qx方向的衍射峰半峰宽就越宽, 外延层和衬底之间的倾角就越大, 同时样品表面粗糙度越高.这意味着材料的缺陷增加, 弛豫不充分.对于生长在具有相同生长温度的InAlAs异变缓冲层上的In0.83Ga0.17As外延层, 采用较高的生长温度时, X射线衍射倒易空间图 (004) 反射面沿Qx方向的衍射峰半峰宽较小, 77K下有更强的光致发光, 但是表面粗糙度会有所增加.这说明生长温度提高后, 材料中的缺陷得到抑制.     

4H-SiC(004)面双晶衍射摇摆曲线的分析

用X射线双晶衍射法(XRD)对4H-SiC衬底和在该衬底上外延生长的4H-SiC单晶样品(004)面进行测试,在所有样品的测试结果中发现,摇摆曲线主峰左侧160″附近均出现傍肩,并且主峰衍射强度也较低.依据X射线衍射理论对系统射线在测试样品(004)面的衍射峰位展宽进行计算,并对4H-SiC原子结构进行了分析.研究结果证实,摇摆曲线中傍肩的存在与射线源中的Ka2射线参与衍射有关,而相对较低的衍射强度受其(004)面的特殊双层原子结构的影响.     

TEM investigation of defects in arsenic doped layers grown in-situ by MBE

A study concerning the effect of growth condition on As incorporation and formation of defects using transmission electron microscopy (TEM) is presented. It is well known that devices in a narrow bandgap HgCdTe material system could suffer from tunneling currents and generation recombination processes, especially at cryogenic temperatures, due to material defects. For in-situ As doped p-on-n device structures grown by molecular beam epitaxy (MBE), extended defects and in particular twinning in a p-type layer grown under Hg-rich conditions is believed to reduce the zero bias dynamic impedance of devices and significantly impact recombination of carriers in the space charge region. Using TEM we have studied defects formed in As-doped layers grown under Hg- and Te-rich conditions. Samples grown under high II/VI flux ratio at growth temperature of 170 C have a high density of columnar twin defects, whereas no twin defects were seen for layers grown under optimal growth conditions at 190 C. A very high flux of As, however, was required to incorporate As into the layers at growth temperature of 190 C.     

GaN薄膜电子声成像和电子背散射衍射研究

采用扫描电声显微镜(SEAM)和电子背散射衍射(EBSD)对异质外延在Al2O3衬底上GaN界面区域成像测试分析。异质外延失配应力导致在Al2O3和GaN界面附近的微区晶格畸变在SEAM的声成像中可以清楚看到,而且受应力影响集中区域的微区衬度差异明显。利用EBSD色带图及质量参数分析了失配应力变化,晶格应变和弹性形变在200nm内可以得到释放。     

Quantitative annular dark-field STEM images of a silicon crystal using a large-angle convergent electron probe with a 300-kV cold field-emission gun

Annular dark-field scanning transmission electron microscope (ADF-STEM) images of an Si (001) crystal were obtained by using an aberration-corrected electron microscope, at 30-mrad convergent probe and cold field-emission gun at 300?kV. The intensity of ADF-STEM images, that is, the number of scattered electrons relative to the incident electrons, obtained for specimen thickness from 10 to 50?nm was compared quantitatively with absorptive multi-slice simulation. The column and background intensities were analyzed by column-by-column two-dimensional Gaussian fitting. These intensities were found to increase linearly with the sample thicknesses. However, the simulated image gave higher column intensity and lower background intensity for all the sample thickness. We found that experimental images were reproduced by the simulation with Gaussian convolution of 70?pm full-width at half-maximum for all the sample thicknesses from 10 to 50?nm. The possible factors accounted for this Gaussian convolution is discussed.