Sebastian Ihle

Scanning transmission electron microscopy strain measurement from millisecond frames of a direct electron charge coupled device

A high-speed direct electron detection system is introduced to the field of transmission electron microscopy and applied to strain measurements in semiconductor nanostructures. In particular, a focused electron probe with a diameter of 0.5 nm was scanned over a fourfold quantum layer stack with alternating compressive and tensile strain and diffracted discs have been recorded on a scintillator-free direct electron detector with a frame time of 1 ms. We show that the applied algorithms can accurately detect Bragg beam positions despite a significant point spread each 300 kV electron causes during detection on the scintillator-free camera. For millisecond exposures, we find that strain can be measured with a precision of 1.3  × 10−3, enabling, e.g., strain mapping in a 100×100 nm2 region with 0.5 nm resolution in 40 s.

A pnCCD-based, fast direct single electron imaging camera for TEM and STEM

We report on a new camera that is based on a pnCCD sensor for applications in scanning transmission electron microscopy. Emerging new microscopy techniques demand improved detectors with regards to readout rate, sensitivity and radiation hardness, especially in scanning mode. The pnCCD is a 2D imaging sensor that meets these requirements. Its intrinsic radiation hardness permits direct detection of electrons. The pnCCD is read out at a rate of 1,150 frames per second with an image area of 264 x 264 pixel. In binning or windowing modes, the readout rate is increased almost linearly, for example to 4000 frames per second at 4× binning (264 x 66 pixel). Single electrons with energies from 300 keV down to 5 keV can be distinguished due to the high sensitivity of the detector. Three applications in scanning transmission electron microscopy are highlighted to demonstrate that the pnCCD satisfies experimental requirements, especially fast recording of 2D images. In the first application, 65536 2D diffraction patterns were recorded in 70 s. STEM images corresponding to intensities of various diffraction peaks were reconstructed. For the second application, the microscope was operated in a Lorentz-like mode. Magnetic domains were imaged in an area of 256 x 256 sample points in less than 37 seconds for a total of 65536 images each with 264 x 132 pixels. Due to information provided by the two-dimensional images, not only the amplitude but also the direction of the magnetic field could be determined. In the third application, millisecond images of a semiconductor nanostructure were recorded to determine the lattice strain in the sample. A speed-up in measurement time by a factor of 200 could be achieved compared to a previously used camera system.

Optical test results of fast pnCCDs

Measurements with a high-speed pn-charge coupled device (pnCCD) system have been performed in the optical and near infrared (NIR) wavelength range. Some of the key parameters of the system that were determined include the overall quantum efficiency (QE), the point spread function (PSF) and the photon transfer curve (PTC). The results of these measurements will be presented below. The measurements have been carried out at the optical test bench of ESO in Garching, Germany. There we also demonstrated for the first time the feasibility of a fast readout scheme that allows the system to be operated at a speed of up to 1000 frames per second (fps) for use with optical light. Additionally astronomical test measurements have been performed at the Skinakas telescope on Crete, Greece.

4D-STEM Imaging With the pnCCD (S)TEM-Camera

4D-STEM imaging describes a powerful imaging technique where a two-dimensional image is recorded for each probe position of a two-dimensional STEM image. For typical STEM images of 256x256 probe positions, a total of 65,536 2D images needs to be recorded. This amount of data can be recorded with the pnCCD (S)TEM camera in a practical timeframe. This camera uses a direct detecting, radiation hard pnCCD with a minimum readout speed of 1000 full frames per second (fps) [1]. With the pnCCD (S)TEM camera a 4D data cube consisting of 256x256 probe positions with a 264x264 pixel detector image for each probe position can be recorded in less than 70 s. Several measurements have been performed to prove the capability of the camera for 4D-STEM imaging, among them strain analysis, magnetic domain mapping and most recently electron ptychography.

STEM strain analysis at sub-nanometre scale using millisecond frames from a direct electron read-out CCD camera

We report on strain analysis by nano-beam electron diffraction with a spatial resolution of 0.5nm and a strain precision in the 4–710−4 range. Series of up to 160000 CBED patterns have been acquired in STEM mode with a semi-convergence angle of the incident probe of 2.6mrad, which enhances the spatial resolution by a factor of 5 compared to nearly parallel illumination. Firstly, we summarise 3 different algorithms to detect CBED disc positions accurately: selective edge detection and circle fitting, radial gradient maximisation and cross-correlation with masks. They yield equivalent strain profiles in growth direction for a stack of 5 InxGa1−xNyAs1−y/GaAs layers with tensile and compressive strain. Secondly, we use a direct electron read-out pnCCD detector with ultrafast readout hardware and a quantum efficiency close to 1 both to show that the same strain profiles are obtained at 200 times higher readout rates of 1kHz and to enhance strain precision to 3.510−4 by recording the weak 008 disc.

Results of a pnCCD Detector System for High-Speed Optical Imaging

We present the design and optical imaging performance of a pnCCD detector system for highest frame rates and excellent sensitivity over a wide wavelength range from the UV to near IR region. To achieve frame rates higher than one thousand frames per second with an exceptionally low noise level, the devices are based on proven technology with column parallel readout and operated in a split-frame transfer mode. The CCDs are back illuminated and coated with an Anti-Reflective- Coating. The sensitivity over their full thickness of 450 &mgr;m allows for a quantum efficiency near 100% over a wide spectral range. At an optical test bench we determined the photon transfer curve, quantum efficiency and point-spread function within a wavelength region between 300 nm to 1100 nm for various detector parameter. To demonstrate the ability of a pnCCD to perform high-speed optical differential photometry, the crab nebula with the crab pulsar as central object were observed at the 1.3m SKINAKAS telescope on crete. For these observations the pnCCD was operated at a speed of 2000 frames per second. The high speed, low noise and high quantum efficiency makes this detector an ideal instrument to be used as a wavefront sensor in adaptive optics systems.

Energy Dispersive X-Ray Diffraction Imaging

Position resolved structural information from polycrystalline materials is usually obtained via micro beam techniques illuminating only a single spot of the specimen. Multiplexing in reciprocal space is achieved either by the use of an area detector or an energy dispersive device. Alternatively spatial information may be obtained simultaneously from a large part of the sample by using an array of parallel collimators between the sample and a position sensitive detector which suppresses crossfire of radiation scattered at different positions in the sample. With the introduction of an X-ray camera based on an energy resolving area detector (pnCCD) we could combine this with multiplexing in reciprocal space.

pnCCDs for ultra-fast and ultra-sensitive optical and NIR imaging

We present the design, status of fabrication and testing, and expected performance characteristics of new CCDs for highest frame rates and excellent sensitivity over a wide wavelength range, from the near UV to the to near IR. To achieve frame rates in the kHz regime, the devices are based on proven technology with column parallel readout. The CCDs are back illuminated, sensitive over their full thickness of 450 μm, allowing a peak quantum efficiency near 100% at any chosen wavelength between 400 nm and 1,000 nm. Test results, including astronomical trials at Skinakas observatory, using available devices with an on‐chip JFET amplifier are presented. Based on the above CCD design we are currently producing two novel variants with single photon counting capabilities: They either use integrated avalanche diodes in the readout chain or a repetitive non‐destructive readout. Both readout schemes have already successfully proven their single‐photon sensitivity and we present the related results.

Direct measurement of the position accuracy for low energy X-ray photons with a pnCCD

We undertook a comparative study on optimizing the position accuracy of pnCCDs for single X-ray photon measurements. Various methods were analyzed by Monte Carlo simulations and related to experimental data obtained with a focused X-ray beam. Even with low energy photons of 1320 eV, a position accuracy much smaller than the actual pixel size of 48 μm × 48 μm can be achieved. This is possible since signal charges from a single photon interaction spread into more than one pixel, allowing a reconstruction of the original point of interaction. We found that a) making a decision on which pixels to use for the reconstruction and b) choosing a centroiding algorithm for carrying out the reconstruction were particularly crucial. For a) we introduce a new and superior method using a two step analysis with an adaptive pattern. It is compared to using a threshold or a fixed pattern. For b) we present a Center-of-Gravity method with a Gaussian correction taking into account the shape of the signal charge cloud. Both methods are also optimized for fast execution by implementing lookup tables rather than time consuming calculations. Our results show that with the appropriate analysis an uncertainty of the position measurement of better than 3.0 μm rms for 1320 eV photons is possible.

Analysis of Polymorphs Using Simultaneous X-ray Fluorescence and Diffraction with an Imaging Spectrometer

X-ray fluorescence (XRF) is a widely used method for materials characterization. However, for polymorphs such as Rutile and Anatase, both of which are TiO2, the fluorescence spectra will be nearly identical. Differentiating between these polymorphs requires a second measurement with an X-ray diffractometer. Using two instruments increases the cost of the measurement, and it introduces issues with image registration and sample preparation. Previously, simultaneous XRD-XRF has been done by attaching a silicon drift detector (SDD) to an X-ray diffraction (XRD) system. However, due to the typical geometry and source monochromation of an XRD system, the count rate in the SDD is typically low, making the measurement process slow [1]. Overcoming these limitations requires a detector that is both position sensitive and energy dispersive.