ADF Detector Research Articles

Fast generation of calculated ADF-EDX scattering cross-sections under channelling conditions

Advanced materials often consist of multiple elements which are arranged in a complicated structure. Quantitative scanning transmission electron microscopy is useful to determine the composition and thickness of nanostructures at the atomic scale. However, significant difficulties remain to quantify mixed columns by comparing the resulting atomic resolution images and spectroscopy data with multislice simulations where dynamic scattering needs to be taken into account. The combination of the computationally intensive nature of these simulations and the enormous amount of possible mixed column configurations for a given composition indeed severely hamper the quantification process. To overcome these challenges, we here report the development of an incoherent non-linear method for the fast prediction of ADF-EDX scattering cross-sections of mixed columns under channelling conditions. We first explain the origin of the ADF and EDX incoherence from scattering physics suggesting a linear dependence between those two signals in the case of a high-angle ADF detector. Taking EDX as a perfect incoherent reference mode, we quantitatively examine the ADF longitudinal incoherence under different microscope conditions using multislice simulations. Based on incoherent imaging, the atomic lensing model previously developed for ADF is now expanded to EDX, which yields ADF-EDX scattering cross-section predictions in good agreement with multislice simulations for mixed columns in a core–shell nanoparticle and a high entropy alloy. The fast and accurate prediction of ADF-EDX scattering cross-sections opens up new opportunities to explore the wide range of ordering possibilities of heterogeneous materials with multiple elements.

Effects of small-angle mistilts on dopant visibility in ADF-STEM imaging of nanocrystals

Quantitative ADF-STEM imaging paired with image simulations has proven to be a powerful technique for determining the three dimensional location of substitutionally doped atoms in thin films. Expansion of this technique to lightly-doped nanocrystals requires an understanding of the influence of specimen mistilt on dopant visibility due to the difficulty of accurate orientation determination in such systems as well as crystal movement under the beam. In this study, the effects of specimen mistilt on ADF-STEM imaging are evaluated using germanium-doped silicon nanocrystals as model systems. It is shown that dopant visibility is a strong function of specimen mistilt, and the accuracy of specimen orientation is an important factor in the analysis of three-dimensional dopant location, but the sensitivity to mistilt can be weakened by increasing the STEM probe convergence angle and optimizing ADF detector inner angle.

Rapid Measurement of Nanoparticle Thickness Profiles

A method to measure the thickness of a single-crystal nanoparticle in the direction parallel to the incident beam from annular dark field scanning transmission electron microscope (ADF-STEM) images is reported, providing a map of thickness versus position across the nanoparticle--a 'thickness profile' image. The method is rapid and hence suitable for surveying large numbers of nanoparticles. The method measures the intensity scattered to a characterised ADF detector and compares this to the incident beam intensity, to obtain a normalized ADF image. The normalised intensity is then converted to thickness via dynamical ADF image simulations. The method is accurate within 10% and the precision is dominated primarily by 'shot noise'. Merits and limitations of this method are discussed. A method to calibrate the response function of the ADF detector without external equipment is also described, which is applicable to the entire range of gain and background settings.

An advanced study of the response of ADF detector

The detailed characteristics of the Annular Dark Field detector and acquisition system may play a strong role in quantitative analysis. This article discusses the role in high angle experiments of a non-uniform detection efficiency and of the saturation. The former can be neglected in most comparative experiment while the latter can produce hidden nonlinearities that reduce the reliability of quantitative analysis.

Study of strain fields at a-Si/c-Si interface

The contrast due to a strain field at an amorphous silicon/crystalline silicon (a-Si/c-Si) interface relative to the bulk crystal is studied with a scanning transmission electron microscope equipped with a low angle annular dark field (LAADF) detector and a high angle ADF (HAADF) detector. Experimental observations suggest that strain contrast depends closely on sample thickness and collection angle. For a thin sample (<100 Å) strain contrast is negative in both the LAADF and HAADF images. For a thick sample (>150 Å) strain contrast is positive in the LAADF image and negative in the HAADF image. Theoretical calculations of the effect of a random strain field are carried out. First, a simple model based on atomic scattering with an extra Debye–Waller factor is employed. It predicts a positive strain contrast in the LAADF image and no contrast in the HAADF image. The simple model fails for the HAADF contrast because it does not consider the propagation process of the electron beam inside the sample. Therefore multislice simulations including propagation details are carried out. The multislice simulation results agree well with the experimental observations. A measure of the strain amplitude is attained by comparing the experimental data with multislice simulation results. Quantitative measurements of strain contrast are only possible if the sample thickness is known.

Progress with the IBM Very High Resolution STEM

Abstract The IBM high resolution STEM project has now reached the point where integration of new subsystems into the instrument is the main activity. During the past three years, we have demonstrated a 50meV EELS resolution using a high brightness electron monochromator incorporated into the electron gun [1, 2, 3], and better than 0.14mn spatial resolution using spherical aberration correction at 120KeV. [4] Recent effort has been to identify and remove many sources of instabilities in the original instrument; to rebuild several existing subsidiary pieces of equipment such as the ADF detector, the EELS spectrometer electron optical coupling, and the EELS CCD detection system; and to set up software and hardware control of each system to allow routine operation. The new STEM column is summarized in Fig. 1. Briefly, the original VG gun flange has been lowered about 4cm, using a spacer, to allow insertion of the Fringe Field monochromator.

Contrast Effect of Strain Fields in ADF-STEM Imaging

STEM is an important tool for the study of material by forming Z-contrast ADF images. Studies of strain fields suggest that strain can cause extra contrast in ADF images. Since strain fields exist in many TEM samples, especially interfaces of two different materials, it is necessary to study the contrast effect of strain fields in more detail. in this paper, we study the contrast effect of strain fields by STEM experiments and multislice simulations.Sample thickness and the dimensions of ADF detector can both affect strain contrast. We first study the influence of sample thickness. A cross-section TEM sample of an interface of crystal silicon and amorphous silicon was prepared by typical tripod polishing and was studied with the 100 kV Cornell VG 5.0 UHV STEM. The sample was tilted so that the crystal was on the (110) zone axis and ADF images of the interface were recorded.

On the origin of transverse incoherence in Z-contrast STEM.

We use a Bloch wave approach to further investigate the origins of the incoherent nature of Z-contrast imaging using an ADF detector in a STEM. We discuss how, although at high angles the collected electrons will be mostly thermally scattered in addition to the elastic scattering, it is not the thermal scattering that destroys the coherence, rather the combination of the large detector with the high-angle elastic scattering. This incoherent nature of the elastic scattering arises through the filtering of the 1s-type Bloch states by the detector geometry. We show that it is this filtering that renders an atomic column an independent scatterer insensitive to the configuration of neighbouring columns. It also makes the image contrast insensitive to the effects of beam spreading onto neighbouring columns as the probe propagates through the crystal. We also discuss the implications of this for previous calculations of the intensity of Z-contrast images.

Detector geometry, thermal diffuse scattering and strain effects in ADF STEM imaging

Intensities of atomic-resolution Annular Dark Field Scanning Transmission Electron Microscopy (ADF STEM) images of zone-axis-oriented specimens change with defocus at rates that depend on lattice spacing. Thickness and strain effects on the intensities have been demonstrated. In this paper, image simulations (with some experimental basis) are presented that consider the dimensions of the ADF detector. Changing the inner radius of the detector seems to have relatively small effect on the image except to lower the detected intensity. Probe size was explored and a case identified where multiple scattering was important in the image. Thermal diffuse scattering (TDS) is important in high-angle scattering at room temperatures but it does not seem to alter the image appearance markedly. Finally, the image arising from the strain field around a single boron atom has been simulated and the results suggest increased scattering in agreement with observations. This mechanism may be adequate for single impurity atom detection at low temperatures and with special detector angles.

High-resolution scanning transmission electron microscopy

High-resolution imaging in scanning transmission electron microscopy (STEM) instruments is achieved with a variety of detector configurations including bright-field/dark-field (BF/DF) imaging by coherent phase contrast, high-angle annular dark-field (HAADF) imaging by incoherent atomic number contrast and many other imaging modes. With the use of post-specimen lenses to expand/compress the whole diffraction pattern, the collection angles of both the BF and the ADF detector can be varied continuously and different detector configurations may be realized. Image resolution is improved by increasing the collection angle of the BF detector. The phase and diffraction contrast in a large-angle bright-field (LABF) image is reduced and LABF images have less dependence on the change of beam defocus and the variations of sample thickness. Applications of these high-resolution STEM imaging modes to the study of interfaces are discussed.

Imaging dislocations with an annular dark-field detector

High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has been recently the subject of active research because of its successful applications to the characterization of supported catalysts, interface problems in MBE grown semiconductors, superconductors and X-ray multilayers. The characteristics of HAADF images are different from those of the TEM images. For perfect single crystals the HAADF signal is mainly generated from thermal diffuse scattering. HAADF technique has also been used to study dopant contrast effects in semiconductors and dislocations have also been observed with an ADF detector. In this paper we report a study of dislocation contrasts and their dependence on the inner collection angle of the ADF detector.The STEM instrument used for the observations was the HB5 from VG Microscopes, Ld., modified by the addition of an ultra-high resolution pole piece (Cs = 0.8 mm) and a two-dimensional detector system. Post specimen lenses and various beam stops were used to change the inner (α) and outer (β) collection angles of the ADF detector.

Complexities in STEM analyses of polymer blend thin films

Polystyrene (PS)-polyether sulphone (PES) polymer blend thin films were prepared for examination in a scanning transmission electron microscope. The microstructures observed in 75 wt % PS-25 wt % PES films consisted of spherical inclusions, ranging from ≈0.2 to ≈1.2μm in diameter. X-ray spectrometric analysis in the microscope revealed that the inclusions were PES-rich, while the matrix contained only PS. Attention in this paper is paid to the contrast in the annular dark-field detector (ADF) images from these thin films. This image contrast has a complicated dependence on both the angular range subtended by the dark-field detector and “mass-thickness” variations within the films. On microscopes with appropriate lens controls which permit the acceptance angle of the ADF detector to be varied, it becomes possible actually to reverse the contrast between the two phases.