K. Pantzas

Nanometer-scale, quantitative composition mappings of InGaN layers from a combination of scanning transmission electron microscopy and energy dispersive x-ray spectroscopy

Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.

Distributed Bragg reflectors based on diluted boron-based BAlN alloys for deep ultraviolet optoelectronic applications

Highly reflective deep UV distributed Bragg reflectors (DBRs) based on the BAlN material system have been grown by metalorganic vapour phase epitaxy on AlN template substrates. These structures make use of the transparency of BAlN in the deep UV and the high refractive index contrast between BAlN and AlN, which has been demonstrated to exceed 0.27 at 280 nm. 18-pair BAlN/AlN DBRs showed experimental peak reflectivity of 82% at 311 nm and a stop-bandwidth of 20 nm. At 282 nm, a 24-pair BAlN/AlN DBR structure is demonstrated with experimental peak reflectivity of 60% and stop-bandwidth of 16 nm.

Characteristics of the surface microstructures in thick InGaN layers on GaN

This paper focuses on a comparative study of optical, morphological, microstructural and microcompositional properties of typical InGaN samples which exhibit V-defects but also two additional surface defects features, referred to as inclusion#1 (Ic1) and inclusion#2 (Ic2). HR-XRD, AFM, SEM, STEM and EDX are used to characterize such defects. Furthermore, hyperspectral mapping, spot mode and depth-resolved CL measurements provided useful informations on the optical emission properties and microstructure. The main characteristic of Ic1 luminescence peak is a decrease in intensity and no obvious shift in the CL peak position when going from the outside to the middle of such defect. More interesting was Ic2 which is shown to be local 3D top surface In-rich InGaN domains embedded in an homogeneous InGaN matrix. In fact, this study pointed out that close to the interface GaN/InGaN, it exists a 30 nm thick fully strained InGaN layer with constant indium incorporation. As the growth proceeds spatial fluctuation of the In content is observed and local In-rich 3D domains are shown to emerge systematically around threading dislocations terminations.

Sharpening the Interfaces of Axial Heterostructures in Self-Catalyzed AlGaAs Nanowires: Experiment and Theory.

The growth of III-III-V axial heterostructures in nanowires via the vapor-liquid-solid method is deemed to be unfavorable because of the high solubility of group III elements in the catalyst droplet. In this work, we study the formation by molecular beam epitaxy of self-catalyzed GaAs nanowires with AlxGa1-xAs insertions. The composition profiles are extracted and analyzed with monolayer resolution using high-angle annular dark-field scanning transmission electron microscopy. We test successfully several growth procedures to sharpen the heterointerfaces. For a given nanowire geometry, prefilling the droplet with Al atoms is shown to be the most efficient way to reduce the width of the GaAs/AlxGa1-xAs interface. Using the thermodynamic data available in the literature, we develop numerical and analytical models of the composition profiles, showing very good agreement with experiments. These models suggest that atomically sharp interfaces are attainable for catalyst droplets of small volumes.

Nanoscale selective area growth of thick, dense, uniform, In-rich, InGaN nanostructure arrays on GaN/sapphire template

Uniform, dense, single-phase, 150 nm thick indium gallium nitride (InGaN) nanostructure (nanorods and nanostripes) arrays have been obtained on gallium nitride templates, by metal organic chemical vapor deposition and nanoscale selective area growth on silicon dioxide patterned masks. The 150 nm thick InGaN nanorods have a perfect hexagonal pyramid shape with relatively homogenous indium concentration up to 22%, which is almost twice as high as in planar InGaN grown in the same condition, and luminesce at 535 nm. InGaN nanostripes feature c-axis oriented InGaN in the core which is covered by InGaN grown along semi-polar facets with higher In content. Transmission electron microscope and sub micron beam X-rays diffraction investigations confirm that both InGaN nanostructures are mostly defect free and monocrystalline. The ability to grow defect-free thick InGaN nanostructures with reduced polarization and high indium incorporation offers a solution to develop high efficiency InGaN-based solar cells.

Evaluation of the surface bonding energy of an InP membrane bonded oxide-free to Si using instrumented nanoindentation

Instrumented nanoindentation is used in conjunction with scanning transmission electron microscopy to evaluate the mechanical resistance at the bonding interface of a 450 nm thick InP membrane bonded oxide-free to Si. Indentation using a Berkovich tip is shown to cause the planes in InP to rotate by as much as 16°. The shear stress resulting from this rotation causes the InP membrane to buckle, forming a debonded blister around the indented zone. The geometry of this blister is used to compute the surface bond energy of InP bonded oxide-free to Si. An average surface bonding energy of 585 mJ m−2 is reported.

Role of compositional fluctuations and their suppression on the strain and luminescence of InGaN alloys

Advanced electron microscopy techniques are combined for the first time to measure the composition, strain, and optical luminescence, of InGaN/GaN multi-layered structures down to the nanometer scale. Compositional fluctuations observed in InGaN epilayers are suppressed in these multi-layered structures up to a thickness of 100 nm and for an indium composition of 16%. The multi-layered structures remain pseudomorphically accommodated on the GaN substrate and exhibit single-peak, homogeneous luminescence so long as the composition is homogeneous.

Nanoselective area growth and characterization of dislocation-free InGaN nanopyramids on AlN buffered Si(111) templates

We report the metal organic chemical vapor deposition growth of dislocation-free 100 nm thick hexagonal InGaN nanopyramid arrays with up to 33% of indium content by nano-selective area growth on patterned AlN/Si (111) substrates. InGaN grown on SiO2 patterned templates exhibit high selectivity. Their single crystal structure is confirmed by scanning transmission electron microscope combined with an energy dispersive X-ray analysis, which also reveals the absence of threading dislocations in the InGaN nanopyramids due to elastic strain relaxation mechanisms. Cathodoluminescence measurements on a single InGaN nanopyramid clearly show an improvement of the optical properties when compared to planar InGaN grown under the same conditions. The good structural, morphological, and optical quality of the InGaN nanostructures grown on AlN/Si indicates that the nano-selective area growth technology is attractive for the realization of site-controlled indium-rich InGaN nanostructure-based devices and can also be tran...

Polarization-Induced Electric Fields Make Robust n-GaN/i-InGaN/p-GaN Solar Cells

We study n-GaN/i-InGaN/p-GaN solar cells on Ga-face substrates. We find that polarization charges at the heterointerfaces pin the value of VOC, JSC, and the fill factor to nearly optimal levels when the InGaN layer thickness exceeds a value dmin, even as the p- and n-layer thicknesses and doping concentrations vary widely. We verify an analytical approximation for dmin. We report nearly undiminished performance when an i-GaN setback layer is added between a p-doped layer and the i-InGaN layer - an addition that may be necessary to obtain good quality heterointerfaces. Additionally, the 2-D electron gas formed at the n-GaN/i-InGaN interface facilitates thinner n-GaN window layers for improved external quantum efficiency.

Void-free direct bonding of InP to Si: Advantages of low H-content and ozone activation

Oxide-mediated direct bonding of InP to silicon has been investigated using a variety of oxidation and oxide-activation techniques to achieve void-free bonding without resorting to complex outgassing schemes. Void formation is shown to be related to the H content of the SiO2 layer. Transmission Fourier transformed infrared spectroscopy measurements corroborate this hypothesis. Finally, the use of ozone instead of oxygen plasma activation is also revealed to be beneficial for void-free bonding.