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

Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

AlGaN/GaN HEMTs-an overview of device operation and applications

More than one-fifth of US electricity is used to power artificial lighting. Light-emitting diodes based on group III/nitride semiconductors are bringing about a revolution in energy-efficient lighting.

Prospects for LED lighting

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

The impact of post-metallization annealing of N-polar AlSiO metal oxide semiconductor (MOS) capacitors was investigated. Annealing in air at 320 and 370 oC reduced the density of near-interface traps from 5.6x1012 to 2.8x1012 cm-2 and extended the region of flat-band voltage stability and low-leakage operation from 0-2.6 MV/cm to 0-4 MV/cm in the forward bias accumulation region. Moreover, annealing at 370oC fully suppressed the instabilities in the flat-band voltage within the test voltage range (-10 to -25V) of depletion operation. The robust dielectric results demonstrated in this paper are promising for further enhancements of gate-robustness in N-polar GaN-based MOS-based transistors.

Improved operation stability of in-situ AlSiO dielectric grown on (000-1) N-polar GaN by MOCVD

A method of maskless lateral epitaxial overgrowth (LEO) of aluminum nitride (AlN) and high aluminum composition aluminum gallium nitride (AlGaN) layers by crystal growth techniques, such as metalorganic chemical vapor deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), other vapor phase transport techniques such as sublimation, and Molecular Beam Epitaxy (MBE). The process etches periodic patterns into a suitable material, such AlN or high aluminum composition AlGaN base layers heteroepitaxially grown on a substrate or a substrate itself. A lateral epitaxial overgrowth is performed of the AlN or high aluminum composition AlGaN layers on the suitable material. Lateral epitaxial overgrowth of the AlN or high aluminum composition AlGaN layers may be enhanced by using low V/III ratios and fast growth rates. The process reduces the threading dislocation density (TDD) in high Al containing nitrides by several orders of magnitude.

Maskless lateral epitaxial overgrowth of aluminum nitride and high aluminum composition aluminum gallium nitride

Parasitic pre-reactions are known to play a role in the growth of aluminum nitride (AlN) via metal organic chemical vapor deposition, where they can deplete precursor molecules before reaching the substrate, leading to poor growth efficiency. Studies have shown that reducing the growth pressure and growth temperature results in improved growth efficiency of AlN; however, superior crystal quality and reduced impurity incorporation are generally best obtained when growing at high temperatures. This study shows that, with proper alkyl source dilution, parasitic pre-reactions can be suppressed while maintaining high growth temperatures. The results show an 18× increase in growth rate and efficiency of AlN films: from 0.04 μm h−1 to 0.73 μm h−1, and 26 μm mol−1 to 502 μm mol−1, respectively; under constant TMAl flow and a small change in total gas flow. This results in 6.8% of Al atoms from the injected TMAl being utilized for AlN layer growth for this reactor configuration. This is better than the standard GaN growth, where 6.0% of the Ga atoms injected from TMGa are utilized for GaN growth.

https://iopscience.iop.org/article/10.1088/0268-1242/31/8/085003/pdf

Improving source efficiency for aluminum nitride grown by metal organic chemical vapor deposition

Abrupt GaN/p-GaN:Mg junctions grown via metalorganic chemical vapor deposition

Ferromagnetic MnGa(111) layers were grown on GaN(0001) by molecular beam epitaxy. MnGa/GaN Schottky diodes with a doping level of around n = 7 × 1018 cm−3 were fabricated to achieve single step tunneling across the metal/semiconductor junction. Below the GaN layer, a thin InGaN quantum well served as optical spin detector (“spin-LED”). For electron spin injection from MnGa into GaN and subsequent spin transport through a 45 nm (70 nm) thick GaN layer, we observe a circular polarization of 0.3% (0.2%) in the electroluminescence at 80 K. Interface mixing, spin polarization losses during electrical transport in the GaN layer, and spin relaxation in the InGaN quantum well are discussed in relation with the low value of the optically detected spin polarization.

Stacia Keller

Papers

Improved operation stability of in-situ AlSiO dielectric grown on (000-1) N-polar GaN by MOCVD

Maskless lateral epitaxial overgrowth of aluminum nitride and high aluminum composition aluminum gallium nitride

Improving source efficiency for aluminum nitride grown by metal organic chemical vapor deposition

Abrupt GaN/p-GaN:Mg junctions grown via metalorganic chemical vapor deposition

Spin injection in epitaxial MnGa(111)/GaN(0001) heterostructures