Rafael Fritz

Determination of the chemical composition of GaNAs using STEM HAADF imaging and STEM strain state analysis.

The nitrogen concentration of GaN(0.01≤x≤0.05)As(1-x) quantum wells was determined from high resolution scanning transmission electron microscopy (HRSTEM) images taken with a high-angle annular dark field (HAADF) detector. This was done by applying two independent methods: evaluation of the scattering intensity and strain state analysis. The HAADF scattering intensity was computed by multislice simulations taking into account the effect of static atomic displacements and thermal diffuse scattering. A comparison of the mean intensity per atom column on the experimental images with these simulations enabled us to generate composition maps with atomic scale resolution. STEM simulations of large supercells proved that local drops of the HAADF intensity observed close to embedded quantum wells are caused by surface strain relaxation. The same STEM images were evaluated by strain state analysis. We suggest a real space method which is not affected by fly-back errors in HRSTEM images. The results of both evaluation methods are in accordance with data obtained from X-ray diffraction measurements.

Strain measurement in semiconductor heterostructures by scanning transmission electron microscopy.

This article deals with the measurement of strain in semiconductor heterostructures from convergent beam electron diffraction patterns. In particular, three different algorithms in the field of (circular) pattern recognition are presented that are able to detect diffracted disc positions accurately, from which the strain in growth direction is calculated. Although the three approaches are very different as one is based on edge detection, one on rotational averages, and one on cross correlation with masks, it is found that identical strain profiles result for an In x Ga1-x N y As1-y /GaAs heterostructure consisting of five compressively and tensile strained layers. We achieve a precision of strain measurements of 7-9·10-4 and a spatial resolution of 0.5-0.7 nm over the whole width of the layer stack which was 350 nm. Being already very applicable to strain measurements in contemporary nanostructures, we additionally suggest future hardware and software designs optimized for fast and direct acquisition of strain distributions, motivated by the present studies.

Quantitative chemical evaluation of dilute GaNAs using ADF STEM: avoiding surface strain induced artifacts.

The high angle annular dark field intensity (HAADF) in scanning transmission electron microscopy (STEM) can be used for a quantitative evaluation of the chemical composition in dilute GaNAs quantum wells by comparison with simulated intensities. As the scattered intensity is highly sensitive to surface strain fields originating from the quantum wells embedded in GaAs, the HAADF intensity is difficult to evaluate in a quantitative way as long as strain contrast cannot be distinguished from chemical contrast. We present a method to achieve full 2D HAADF STEM compositional mapping of GaNAs/GaAs quantum well systems by making use of information from two different camera lengths.

Determination of Nitrogen Concentration in Dilute GaNAs by STEM HAADF Z-Contrast Imaging

We determine the nitrogen concentration of GaN0.01≤x≤0.05As1−x quantum wells by evaluation of high resolution scanning transmission electron microscopy (STEM) images using a high-angle annular dark field detector. Although nitrogen has a smaller atomic number than Ga the image intensity increases with the nitrogen content. This is explained by the influence of static atomic displacements by comparison with frozen lattice simulations. The resulting nitrogen concentrations agree with high resolution X-ray diffraction measurements and strain state analysis applied to STEM images.

Bismuth-containing III–V semiconductors: Epitaxial growth and physical properties

The growth, surface, and bulk properties of GaAsBi and related III-V alloys are examined and the potential benefits of these materials are explored in terms of device applications. The methods used include molecular beam epitaxy growth, scanning tunneling microscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, deep-level transient spectroscopy, dynamic modeling, and theoretical analysis. The results show that considerable progress has been made in alloying bismuth with GaAs and that the structural, optical, and electronic quality is very good for the alloys investigated.

Laser operation of the III/V compound material Ga(NAsP) grown lattice matched on (001) Si substrate

Nowadays silicon photonics gains more and more interest especially for the future architecture of Si based optoelectronic integrated circuits (OEI C). OEIC building blocks such a s modulators, detectors as well as light wave guides have been demonstrated in a down-scalable process technology. However, a commercial solution for the monolithic integration of long-term stable laser diodes has not been achieved yet, which is, however, the key device component to full y profit from silicon photonics.

Measurement of Composition and Strain by STEM

In this contribution we demonstrate methods for measurement of composition and strain by scanning transmission electron microscopy (STEM). On the one hand, STEM combined with a high angle annular dark field detector allows Z-contrast imaging with high material contrast that can be used for quantification of composition. The suggested method is based on comparison of experiment with image simulation [1], facilitated by normalizing the intensity with respect to the incident electron beam [2,1]. As an example, Fig. 1a shows a part of a high-resolution STEM image of an InGaN quantum well buried in GaN. First, we apply a Wiener filter for noise reduction, being followed by detecting the positions of the atomic columns (Fig. 1b). Then the original image is segmented into Voronoi cells (Fig. 1c), for which we compute the mean normalized intensity (Fig. 1d). Comparison with image simulations yields the local specimen thickness (Fig. 1e) and the indium concentration for each atomic column (Fig. 1f). Image simulations were performed with the STEMsim program [3] in the frozen lattice approximation and taking into account static atomic displacements (SAD), which occur due to the different covalent radii of In and Ga atoms. For the calculation of SADs we used the LAMMPS code [4] and Stillinger-Weber empirical potentials with parameters published by Lei et al. [5]. As a critical issue we also address a possible structural degradation of InGaN layers during illumination with electrons. As an example, Smeeton et al. [6] observed a clustering of indium after a few minutes irradiation time if parallel beam illumination was applied. In contrast, we could not detect any significant change of image contrast or composition during acquisition of 32 images with 80 s exposure time corresponding to a total irradiation time of 42 minutes. The high stability of the InGaN layers can be explained by the significantly smaller electron dose in our STEM experiments compared to that given in reference [6]. Acquisition of a series of several hundreds of diffraction patterns along a STEM line scan, on the other hand, can be used for evaluation of strain profiles. We tested three different algorithms in the field of (circular) pattern recognition that are able to detect diffracted disc positions accurately, from which the strain in growth direction is calculated. Although the three approaches are very different as one is based on edge detection, one on rotational averages and one on cross-correlation with masks, it is found that identical strain profiles (Fig. 2b) result for the InxGa1 xNyAs1 y/GaAs heterostructure shown in Fig. 2(a) consisting of five compressively and tensile strained layers. We achieve a precision of strain measurements of 7-9 · 10 4 and a spatial resolution of 0.5 0.7 nm over the whole width of the layer stack which was 350 nm.

TEM 3-beam study of annealing effects in InGaNAs using ab-initio structure factors for strain-relaxed supercells

We report on a Transmission Electron Microscopy 3-beam technique based on the interference of 000, 200 and 220. Nonlinear imaging artefacts are eliminated by Fourier filtering, yielding 200 and 220 lattice fringe images, from which chemically sensitive contrast and strain are measured, respectively. In this way, In and N composition can be mapped at atomic scale in quaternary InGaNAs by comparison with simulated reference data. Our Bloch wave simulations are based on structure factors derived from supercells with 106 atoms, which have been strain-relaxed by valence force field methods. Additionally, the influence of electron redistributions due to chemical bonding is accounted for by modified atomic scattering amplitudes derived from density functional theory. By comparing local compositions in an annealed In0.28Ga0.72N0.025As0.975 sample with its as-grown counterpart, we find homogenisation of InGaNAs layer thickness and –stoichiometry upon annealing.

Investigation of optical and concentration profile changes of InGaNAs/GaAs heterostructures induced by thermal annealing

In this contribution we compare optical and structural properties of In0.2Ga0.8N0.024As0.976 quantum wells before and after annealing at 550 °C and 600 °C in an N2 atmosphere. We measure strain and chemically sensitive contrast to determine local indium and nitrogen concentrations using a TEM 3-beam image formed by the 000, 220 and 200 beams. For quantification of the concentrations Bloch wave simulations are used, which include bonding and static atomic displacements. The samples were grown by metal-organic vapour phase epitaxy, prepared with focused ion beams (FIB), thinned with low energy ion milling and investigated in a Cs-corrected Titan 80/300 microscope using an L-shaped objective aperture. Imaging conditions with the Laue circle centre at (0 4.2 0) are used as they show a weak dependence on the lamella thickness. Absorption measurements show a blueshift of the band gap of 19 ± 7meV (550 °C) and 36 ± 7 meV (600 °C) after annealing. The average nitrogen concentration was found to be 2±1% and is unaffected by the annealing temperature. In contrast, the mean indium concentration appears to decrease from 18.5 ± 2% before to 15 ± 1% after annealing. Together with the blueshift, this observation is discussed in terms of a modification of electron structure factors, caused by preferred coordination of N to In atoms.

Lasing of the III/V compound semiconductor Ga(NAsP) integrated lattice-matched to Si substrate

Ga(NAsP) multi quantum well heterostructures were grown pseudomorphically on exactly oriented (001) silicon substrates without the formation of misfit dislocations. Optical pumped lasing operation was observed at temperatures up to 125 K.