Anas Mouti

Polarization field mapping of Al0.85In0.15N/AlN/GaN heterostructure

Off-axis electron holography has been used to measure the built-in potential profile across an Al0.85In0.15N/AlN/GaN high electron mobility transistor heterostructure. Profile measurements indicated a polarization-induced electric field of 6.9 MV/cm within the AlN layer. A two-dimensional electron gas with a density of ∼2.1×1013 cm−2 was located in the GaN layer at ∼0.8 nm away from the AlN/GaN interface in reasonable agreement with the reported simulations. Electron microscopy confirmed that the Al0.85In0.15N layer was uniform and that Al0.85In0.15N/AlN and AlN/GaN interfaces were abrupt and well defined.

Observation of dodecagon-shape V-defects in GaN/AlInN multiple quantum wells

The microstructure of GaN(Si)/AlInN multiple quantum wells grown on GaN/Al2O3 (0001) templates by metalorganic vapor-phase epitaxy has been investigated using transmission electron microscopy and associated techniques. Dodecagon-shape V-defects with hexagonal apexes, which nucleate on screw-component threading dislocations, are observed at the film surface. The hexagonal apexes are bounded by {1121} planes, whereas the dodecagons are bounded by {1011} and {1121} planes, where the {1011} facets are generated from the edges between adjacent {1121} planes. Indium segregation is observed along these edges. A possible reason for formation of these defects is briefly discussed.

High-Mobility AlGaN/GaN Two-Dimensional Electron Gas Heterostructure Grown on (111) Single Crystal Diamond Substrate

High mobility Al0.28Ga0.72N/GaN two-dimensional electron gas (2DEG) is achieved on (111) oriented single crystal diamond substrate. The surface morphology of the epilayer is free of cracks thanks to the use of an AlN interlayer for strain relaxation. The rms roughness of the sample surface deduced from atomic force microscopy is 0.6 nm for a 2 ×2 µm2 scan area, which indicates an excellent surface morphology. Hall effect measurements reveal a 2DEG with room temperature mobility and sheet carrier density of 750 cm2 V-1 s-1 and 1.4 ×1013 cm-2, respectively. These results compare fairly well with AlGaN/GaN 2DEG characteristics obtained on other substrates like silicon and demonstrate that high power electronics can be developed on diamond substrates with high power dissipation capabilities.

Measuring strain on HR-STEM images: application to threading dislocations in Al0.8In0.2N

The Geometrical Phase Analysis (GPA) is an efficient method to measure strain from High Resolution Transmission Electron Microscopy (HRTEM) images. Here we show that it can also be applied efficiently to High Resolution Scanning Transmission Electron Microscopy (HR-STEM) images, which have several advantages: (i) patterns in HR-STEM do not change with thickness and chemical composition (ii) thicker samples can be analysed and (iii) strain and composition can be simultaneously determined. In many situations, the distortions due to the scanning of the beam can be corrected. The strain fields around different threading dislocations in an AlInN layer have been determined from plan view samples prepared by focus ion beam (FIB). Experimental strain maps were compared to analytical calculations that take into account the strain field of dislocations and of the In segregation. Mixed type dislocations are always terminated by an inverse hexagonal pyramidal pit, at the sample surface. The edges of the inverse pyramid are indium rich. The dislocation core is not situated at the centre of the inverse pyramid, which is indium-rich, but slightly shifted.

Biexciton emission and crystalline quality of ZnO nano-objects

The design of cost-effective standards for the quality of nano-objects is currently a key issue toward their massive use for optoelectronic applications. The observation by photoluminescence of narrow excitonic and biexcitonic emission lines in semiconductor nanowires is usually accepted as evidence for high structural quality. Here, we perform time-resolved cathodoluminescence experiments on isolated ZnO nanobelts grown by chemical vapor deposition. We observe narrow emission lines at low temperature, together with a clear biexciton line. Still, drastic alterations in both the CL intensity and lifetime are observed locally along the nano-object. We attribute these to non-radiative recombinations at edge dislocations, closing basal plane stacking faults, inhomogeneously distributed along the NB length. This leads us to the conclusion that the observation of narrow excitonic and biexcitonic emission lines is far from sufficient to grade the quality of a nano-object.

Pushing the Limits of Cathodoluminescence Signal Detection: Analyzing 2D Materials

We report in this communication the analysis of hexa-BN atomic layers by cathodoluminescence in a scanning transmission electron microscope (STEM-CL). Both polychromatic imaging and light spectroscopy were performed varying thicknesses of BN, down to a few monolayers. Previous studies have reported spatially resolved structural and optical properties of BN flakes [1] ; our ultimate goal in this study is to collect useful data from a single monolayer, which would be the thinnest possible sample. We are conducting the research on a custom CL system that we have built and integrated onto a VG 601 STEM, equipped with an aberration corrector to increase probe current at higher magnifications. System characteristics include 2 str collection angle, 360-1000nm wavelength spectroscopy range, and 180900nm photomultiplier detection range. We report success in collecting spectra and polychromatic images from samples as thin as 5-6 monolayers. In Fig.1, (a) is the ADF picture of the edge of a BN flake, and A its thinnest region, (b) is the corresponding polychromatic CL image, showing that signal is clearly collected from A. Fig. 1(c) is a high resolution image of A taken with a Nion UltraSTEM 100, from which it can be deducted that the thickness is about 5 monolayers. Finally, (c) is a CL spectrum acquired from the 5 monolayers.

Direct Observation of Plasmonic Enhancement of Emission in Ag-nanoparticle-decorated ZnO nanostructures

1. Vanderbilt University, Department of Physics and Astronomy, Nashville, TN USA 2. Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN USA 3. Vanderbilt University, Interdisciplinary Materials Science Program, Nashville TN, USA 4. Fisk University, Department of Physics and Astronomy, Nashville, TN USA 5. National University of Singapore, Department of Materials Science and Engineering, Singapore 6. Vanderbilt University, Department of Electrical Engineering and Computer Science, Nashville, TN USA

Probing Plasmons in Three Dimensions within Random Morphology Nanostructures

1. Vanderbilt University, Department of Physics and Astronomy, Nashville, TN USA 2. Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN USA 3. Vanderbilt University, Interdisciplinary Materials Science Program, Nashville TN, USA 4. Fisk University, Department of Physics and Astronomy, Nashville, TN USA 5. National University of Singapore, Department of Materials Science and Engineering, Singapore 6. Vanderbilt University, Department of Electrical Engineering and Computer Science, Nashville, TN USA

Stress Distribution in Multiple Quantum Well Stacks and its Effect on Optical Emission Energy Using Cathodoluminescence in a STEM

We present a broad-range experimental study of vertical emission energy shifts in GaN/AlGaN multiple quantum wells (QW), with respect to structure. Multiple techniques were utilized, such a cathodoluminescence (CL) in a scanning transmission electron microscope (STEM) at low accelerating energies (60-80-kV), electron energy loss spectroscopy (EELS), probe corrected zcontrast for structure imaging, and high resolution TEM (HRTEM) coupled with geometric phase analysis (GPA) for strain mapping. The study comprises room and liquid nitrogen temperatures studies of the relationship between stress and optical emission energy, at different thicknesses, simultaneously measured with EELS. Because of the mismatch between the GaN buffer and the AlGaN QW barriers, the QW stack emission is red-shifted at the interface with the buffer (i.e. QWs at the interface are exposed to a lower compressive strain from the AlGaN barriers than the ideal pseudomorphic one), then their emission shifts vertically to the blue and tends to the fully strained emission energy. This is illustrated in Fig.1, which features CL measurement in a STEM at 80kV, (a) is the bright field survey image of the 67 GaN/AlGaN QWs c-grown on a GaN buffer layer, with the line-scan trace (along the c-axis) drawn on it, (b) is the line-scan crude spectrum image at a temperature of 90K (scan step: 1.5 nm) dispersed in wavelength, in which we clearly see the continuous strain-induced blue-shift with decreasing distance to the free surface, and (c) a high resolution z-contrast image of the QW structure. We can see a comparison between room temperature (300K) and liquid nitrogen temperature (90K) spectra obtained in similar experimental conditions (probe size, position on sample, mirror alignment, spectrometer dispersion, acquisition time), in (d): signal at 90k is obviously higher (the room temperature spectrum is multiplied by 5 for better graphical presentation), and narrower. Furthermore, background noise is higher in the room temperature spectrum in comparison with the emission peaks. Fig1. (e-f) quantify the amplitude of the spectral energy shifts observed in (b): the continuous blue shifts observed in (b) have an overall amplitude of about 4 meV for both bulk GaN and the GaN quantum wells. The light was dispersed with a 1200line/mm grating, which, as shown, is sufficient for detecting sub-meV spectral features. We demonstrate that STEM-CL can bring insight into nanoscale sub-meV emission fluctuations and explain optical phenomena observed by non-local techniques such as photoluminescence. Further analysis involving finite elements strain simulations and k.p calculations of optical emission are undergoing.

Mapping Polarization Fields in Al0.85In0.15N/AlN/GaN Heterostructures

Materials based on Al1−xInxN offer much potential for the fabrication of high electron mobility transistors (HEMT) because the spontaneous polarization difference between InAlN and GaN should give rise to positive polarization charge at the AlInN/GaN interface [1]. Furthermore, electrons in nearby regions should compensate for this polarization charge, leading to the formation of twodimensional electron gas (2DEG). AlInN/GaN HEMT heterostructures grown on sapphire substrates have been reported with a 2DEG density of ~2.6x10cm [2]. The position of the 2DEG layer has yet to be determined and structural analysis is lacking. In this study, we have investigated the microstructure and electrostatic potential profiles across Al0.85In0.15N/AlN/GaN HEMT heterostructures. These materials were grown in an AIXTRON 200/4 RF-S metalorganic vaporphase epitaxy (MOVPE) system on 2-in. c-plane sapphire substrates. A JEOL 4000EX was used for microstructural analysis, a JEOL 2010 was used for small-probe microanalysis, and a Philips-FEI CM200 was used for holographic studies.