Imaging Threading Dislocations and Surface Steps in Nitride Thin Films Using Electron Backscatter Diffraction.

Extended defects, like threading dislocations, are detrimental to the performance of optoelectronic devices. In the scanning electron microscope, dislocations are traditionally imaged using diodes to monitor changes in backscattered electron intensity as the electron beam is scanned over the sample, with the sample positioned so the electron beam is at, or close to the Bragg angle for a crystal plane/planes. Here, we use a pixelated detector instead of single diodes, specifically an electron backscatter diffraction (EBSD) detector. We present postprocessing techniques to extract images of dislocations and surface steps, for a nitride thin film, from measurements of backscattered electron intensities and intensity distributions in unprocessed EBSD patterns. In virtual diode (VD) imaging, the backscattered electron intensity is monitored for a selected segment of the unprocessed EBSD patterns. In center of mass (COM) imaging, the position of the center of the backscattered electron intensity distribution is monitored. Additionally, both methods can be combined (VDCOM). Using both VD and VDCOM, images of only threading dislocations, or dislocations and surface steps can be produced, with VDCOM images exhibiting better signal-to-noise. The applicability of VDCOM imaging is demonstrated across a range of nitride semiconductor thin films, with varying surface step and dislocation densities.

Kikuchi pattern simulations of backscattered and transmitted electrons.

We discuss a refined simulation approach which treats Kikuchi diffraction patterns in electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD). The model considers the result of two combined mechanisms: (a) the dynamical diffraction of electrons emitted coherently from point sources in a crystal and (b) diffraction effects on incoherent diffuse intensity distributions. Using suitable parameter settings, the refined simulation model allows to reproduce various thickness- and energy-dependent features which are observed in experimental Kikuchi diffraction patterns. Excess-deficiency features are treated by the effect of gradients in the incoherent background intensity. Based on the analytical two-beam approximation to dynamical electron diffraction, a phenomenological model of excess-deficiency features is derived, which can be used for pattern matching applications. The model allows to approximate the effect of the incident beam geometry as a correction signal for template patterns which can be reprojected from pre-calculated reference data. As an application, we find that the accuracy of fitted projection centre coordinates in EBSD and TKD can be affected by changes in the order of 10 - 3 - 10 - 2 if excess-deficiency features are not considered in the theoretical model underlying a best-fit pattern matching approach. Correspondingly, the absolute accuracy of simulation-based EBSD strain determination can suffer from biases of a similar order of magnitude if excess-deficiency effects are neglected in the simulation model.

Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN.

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black-white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

Energy-weighted dynamical scattering simulations of electron diffraction modalities in the scanning electron microscope.

Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD [1] and ECP [2] to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.

Many-beam dynamical simulation of electron backscatter diffraction patterns.

We present an approach for the simulation of complete electron backscatter diffraction (EBSD) patterns where the relative intensity distributions in the patterns are accurately reproduced. The Bloch wave theory is applied to describe the electron diffraction process. For the simulation of experimental patterns with a large field of view, a large number of reflecting planes has to be taken into account. This is made possible by the Bethe perturbation of weak reflections. Very good agreement is obtained for simulated and experimental patterns of gallium nitride GaN{0001} at 20kV electron energy. Experimental features like zone-axis fine structure and higher-order Laue zone rings are accurately reproduced. We discuss the influence of the diffraction of the incident beam in our experiment.

Spatially-resolved optical and structural properties of semi-polar [Formula: see text] Al x Ga1-x N with x up to 0.56.

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar [Formula: see text] Al x Ga1-x N epilayers with AlN contents up to x = 0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

Activity of trabectedin (ET-743, Yondelis) in metastatic uterine leiomyosarcoma.

BACKGROUND: The ongoing search for more effective agents in the treatment of uterine leiomyosarcomas is warranted because of the poor prognosis related to these tumors. CASE: A case of advanced, recurrent and refractory uterine leiomyosarcoma is presented that responded to trabectedin (ET-743) 1.2 mg/m2 intravenously over 24 h every 3 weeks after failing four prior regimens. A durable objective response lasting at least 8 months was documented. CONCLUSION: Trabectedin (ET-743) has activity in uterine leiomyosarcoma and warrants further investigation.

Salvage bevacizumab (rhuMAB VEGF)-based therapy after multiple prior cytotoxic regimens in advanced refractory epithelial ovarian cancer.

OBJECTIVES: Bevacizumab (BEV) is a humanized monoclonal antibody against vascular endothelial growth factor. We reviewed our experience with BEV in patients with recurrent advanced epithelial ovarian cancer who had failed multiple prior chemotherapeutic regimens. METHODS: Thirty-two patients not participating in an ongoing clinical trial were treated with BEV (15 mg/kg every 3 weeks IV). Demographic and clinicopathologic data, clinical outcomes, and adverse events were extracted from patient charts. RECIST and CA-125 Rustin criteria were retrospectively applied to evaluate response and progression. Median progression-free survival (PFS) and overall survival (OS) were determined using Kaplan-Meier methods. Adverse events were retrospectively categorized using the common terminology criteria for adverse events version 3. RESULTS: The median patient age was 57 years (range 35-80) with 84% being Caucasian and 50% having a GOG performance status of 2. FIGO stages included 80% stage III and 10% stage IV. The tumors were mostly grades 2 (29%) and 3 (64%) and serous histological subtype (69%). All patients had failed multiple prior cytotoxic chemotherapies (median of 5 (range 2-10)) prior to BEV. The median duration of follow-up was 4.8 months (range 0.4-16.3). Twenty-three patients were treated with BEV alone, 2 received BEV with another chemotherapy regimen (5-FU/lecovorin plus oxaliplatin, cyclophosphamide), and 8 initially received BEV alone, followed by BEV with capcitabine, cyclophosphamide, docetaxel, carboplatin, or weekly paclitaxel. A median of 6 cycles (range 1-20) with 196 total doses of BEV was administered. One patient was lost to follow-up after cycle 1. We observed a 16% response rate (all in those treated with BEV alone) with 62.5% of patients demonstrating stable disease. Median OS was 6.9 months, and the median PFS was 5.5 months. Three grade 3 and no grade 4 adverse events were observed. Grade 3 toxicities included hypertension, proteinuria, and enterocutaneous fistula. The fistula occurred after 5 cycles of BEV in a patient who had undergone 7 debulking surgeries prior to BEV. CONCLUSIONS: BEV is generally well tolerated after multiple prior cytotoxic regimens and results in significant clinical benefit among women with recurrent ovarian cancer.