Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

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Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Course Description This is an advanced course in which we explore the field of Statistical Optics. Topics covered include such subjects as the statistical properties of natural (thermal) and laser light, spatial and temporal coherence, effects of partial coherence on optical imaging instruments, effects on imaging due to randomly inhomogeneous media, and a statistical treatment of the detection of light. Development of this more comprehensive model of the behavior of light draws upon the use of tools traditionally available to the electrical engineer, such as linear system theory and the theory of stochastic processes.

http://ieeexplore.ieee.org/iel5/3/23094/01073023.pdf

Statistical optics

Selective non-covalent interactions have been widely exploited in solution-based chemistry to direct the assembly of molecules into nanometre-sized functional structures such as capsules, switches and prototype machines1,2,3,4,5. More recently, the concepts of supramolecular organization have also been applied to two-dimensional assemblies on surfaces6,7 stabilized by hydrogen bonding8,9,10,11,12,13,14, dipolar coupling15,16,17 or metal co-ordination18. Structures realized to date include isolated rows8,13,14,15, clusters9,10,18 and extended networks10,11,12,17, as well as more complex multi-component arrangements16. Another approach to controlling surface structures uses adsorbed molecular monolayers to create preferential binding sites that accommodate individual target molecules19,20. Here we combine these approaches, by using hydrogen bonding to guide the assembly of two types of molecules into a two-dimensional open honeycomb network that then controls and templates new surface phases formed by subsequently deposited fullerene molecules. We find that the open network acts as a two-dimensional array of large pores of sufficient capacity to accommodate several large guest molecules, with the network itself also serving as a template for the formation of a fullerene layer.

Controlling molecular deposition and layer structure with supramolecular surface assemblies

X-ray computer tomography (CT) is fast becoming an accepted tool within the materials science community for the acquisition of 3D images. Here the authors review the current state of the art as CT transforms from a qualitative diagnostic tool to a quantitative one. Our review considers first the image acquisition process, including the use of iterative reconstruction strategies suited to specific segmentation tasks and emerging methods that provide more insight (e.g. fast and high resolution imaging, crystallite (grain) imaging) than conventional attenuation based tomography. Methods and shortcomings of CT are examined for the quantification of 3D volumetric data to extract key topological parameters such as phase fractions, phase contiguity, and damage levels as well as density variations. As a non-destructive technique, CT is an ideal means of following structural development over time via time lapse sequences of 3D images (sometimes called 3D movies or 4D imaging). This includes information nee...

https://www.tandfonline.com/doi/pdf/10.1179/1743280413Y.0000000023

Quantitative X-ray tomography

IMAGING with hard X-rays is an important diagnostic tool in medicine, biology and materials science. Contact radiography and tomography using hard X-rays provide information on internal structures that cannot be obtained using other non-destructive methods. The image contrast results from variations in the X-ray absorption arising from density differences and variations in composition and thickness of the object. But although X-rays penetrate deeply into carbon-based compounds, such as soft biological tissue, polymers and carbon-fibre composites, there is little absorption and therefore poor image contrast. Here we describe a method for enhancing the contrast in hard X-ray images of weakly absorbing materials by resolving phase variations across the X-ray beam1–4. The phase gradients are detected using diffraction from perfect silicon crystals. The diffraction properties of the crystal determine the ultimate spatial resolution in the image; we can readily obtain a resolution of a fraction of a millimetre. Our method shows dramatic contrast enhancement for weakly absorbing biological and inorganic materials, compared with conventional radiography using the same X-ray energy. We present both bright-field and dark-field phase-contrast images, and show evidence of contrast reversal. The method should have the clinical advantage of good contrast for low absorbed X-ray dose.

Phase-contrast imaging of weakly absorbing materials using hard X-rays

The phonon density of states was measured by observing the nuclear resonant fluorescence of ${}^{57}$Fe versus the energy of incident x rays from a synchrotron radiation beam. An energy resolution of 6 meV was achieved by use of high-resolution crystal optics for the incident beam. Extremely low background levels were obtained via time discrimination of the nuclear fluorescent radiation.

http://csrri.iit.edu/~segre/phys570/15S/presentations/gao.pdf

Phonon density of states measured by inelastic nuclear resonant scattering

The phonon energy spectrum of a polycrystalline o. -Fe foil was observed via nuclear resonant scattering of synchrotron radiation for the first time. The measured spectrum is in good agreement with earlier neutron inelastic scattering data. One of the advantageous features of this method is that excitation of only a specific element is possible. Our results show that this method is applicable to the study of lattice dynamics and open a new field of nuclear resonant scattering spectroscopy.

Observation of Nuclear Resonant Scattering Accompanied by Phonon Excitation Using Synchrotron Radiation

The surface structure of Si(111) √3×√3-Ag has been analyzed from the X-ray intensity versus energy curves measured both for the integral- and fractional-order spots. A modified trimer model with three Ag atoms in the √3×√3 unit cell is proposed, in which triangles made of Ag atoms are chained in a honeycomb arrangement. The distances between the nearest neighbor Ag atoms are about twice the van der Waals radius. The height of the Ag layer measured from the ideal first Si layer is 2.9±0.1 A. The center of each triangle coincides with the position of the honeycomb protrusion observed by scanning tunneling microscopy.

https://iopscience.iop.org/article/10.1143/JJAP.27.L753/pdf

Study on the Si(111) √3×√3-Ag Surface Structure by X-Ray Diffraction

X-ray holograms of two- or three-dimensionally distributed objects are recorded and reconstructed. The in-line holograms of a line-like object of a chemical fiber and a point-like object of red blood cell are made using AlKα radiation (8.34A) from a two-stage fine focus X-ray generator. A magnified real image is reconstructed from the negative hologram using a He–Ne laser (6328A). A lateral resolution of about 4 µm and a longitudinal resolution of about 4 mm are obtained in the two-step imaging process.

https://iopscience.iop.org/article/10.1143/JJAP.13.1385/pdf

X-Ray Holographic Microscopy

A versatile precision diffractometer for nuclear Bragg scattering was designed and constructed at the Photon Factory. Standardized component goniometers can be arranged to address various requirements of incident beam conditioning for nuclear Bragg scattering, including a high‐energy resolution monochromator with wide angular acceptance as well as a circularly polarized incident beam.

Seishi Kikuta

Papers

Phonon density of states measured by inelastic nuclear resonant scattering

Observation of Nuclear Resonant Scattering Accompanied by Phonon Excitation Using Synchrotron Radiation

Study on the Si(111) √3×√3-Ag Surface Structure by X-Ray Diffraction

X-Ray Holographic Microscopy

Construction of a precision diffractometer for nuclear Bragg scattering at the Photon Factory