ABSTRACT
We report on optimization of electron transparent GaN based specimens for transmission electron microscopy (TEM) and scanning TEM (STEM) studies by combining focused ion beam thinning and low-energy (≤500 eV) Ar-ion milling. Energy dependent ion milling effects on GaN based structures are investigated and the quality of ion milled samples is compared with that of specimens prepared by wet chemical etching. Defects formed during ion milling lead to amorphization of the specimen. The experimental results are compared with Monte-Carlo simulations using the SRIM (stopping and range of ions in matter) software. Specimen thickness was deduced from high-angle annular dark field STEM images by normalization of measured intensities with respect to the intensity of the scanning electron probe and comparison with multislice simulations in the frozen lattice approach. The results show that the thickness of the amorphous surface layer can be successfully reduced below 1 nm by low energy ion milling, leading to a homogeneous image contrast in TEM and STEM, so that good conditions for quantitative analysis can be achieved. For an ion energy of 400 eV the thickness measurements resulted in an etching rate of about 6-8 nm/min.
ABSTRACT
We suggest a method for chemical mapping that is based on scanning transmission electron microscopy (STEM) imaging with a high-angle annular dark field (HAADF) detector. The analysis method uses a comparison of intensity normalized with respect to the incident electron beam with intensity calculated employing the frozen lattice approximation. This procedure is validated with an In(0.07)Ga(0.93)N layer with homogeneous In concentration, where the STEM results were compared with energy filtered imaging, strain state analysis and energy dispersive X-ray analysis. Good agreement was obtained, if the frozen lattice simulations took into account static atomic displacements, caused by the different covalent radii of In and Ga atoms. Using a sample with higher In concentration and series of 32 images taken within 42 min scan time, we did not find any indication for formation of In rich regions due to electron beam irradiation, which is reported in literature to occur for the parallel illumination mode. Image simulation of an In(0.15)Ga(0.85)N layer that was elastically relaxed with empirical Stillinger-Weber potentials did not reveal significant impact of lattice plane bending on STEM images as well as on the evaluated In concentration profiles for specimen thicknesses of 5, 15 and 50 nm. Image simulation of an abrupt interface between GaN and In(0.15)Ga(0.85)N for specimen thicknesses up to 200 nm showed that artificial blurring of interfaces is significantly smaller than expected from a simple geometrical model that is based on the beam convergence only. As an application of the method, we give evidence for the existence of In rich regions in an InGaN layer which shows signatures of quantum dot emission in microphotoluminescence spectroscopy experiments.
ABSTRACT
OBJECTIVE: Diagnoses of adjustment disorders are frequent in clinical settings, although diagnostic criteria are not very reliable or valid. This has led to a neglection of this topic in research. This article presents and applies a new concept (Maercker et al., 2007) for diagnosing adjustment disorders. METHODS: 96 patients of a psychosomatic policlinic were examined regarding their emotional well-being with a new questionnaire (Adjustment Disorder--New Module | ADNM). Furthermore, according to the new concept, adjustment disorders were assessed with a structured interview. Coping behaviour (CISS) as well as general self efficacy (SWE) in patients with scores within a normal range and patients with elevated scores in ADNM, respectively, were examined. The following statistical measures were used: cluster analysis, cross tables, non-metrical and metrical test procedures. RESULTS: The theory-guided evaluation of the ADNM questionnaire divided the whole sample into patients with elevated (49 %) and with normal (51 %) scores. A cluster analysis of patients with elevated scores showed two groups, so that a total of three groups were compared: patients with scores in a normal range; patients with elevated scores and a low number of symptoms; patients with elevated scores and a high number of symptoms. Patients with elevated scores showed significantly more emotion-oriented coping strategies (t = - 6.338; p = 0.000) and reported, referring to present stress, on being less able to cope with it (Z = - 4.445; p = 0.000). Patients with elevated scores and a high number of symptoms named more stress factors (Z = - 2.634; p = 0.008) and reported on being more impulsive and irritable (Z = - 3.406; p = 0.003). The comparison of the ADNM questionnaire and the clinical main diagnoses shows a separation between emotional and non-emotional disturbances according to ICD-10 (chi2(2, n = 96) = 6.928; p = 0.031). DISCUSSION: The results show that the ADNM questionnaire according to the new diagnostic concept of adjustment disorders (Maercker et al., 2007) identifies patients who experience a high degree of stress caused by adverse life events of life conditions. The ADNM reliably differentiates non-emotional from emotional ICD-10 diagnoses. Further studies should examine, whether the new concept as well as the questionnaire are able to assess adjustment disorders in a valid and reliable way.
