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

Compositionally graded InGaN layers grown on vicinal N-face GaN substrates by plasma-assisted molecular beam epitaxy

https://iopscience.iop.org/article/10.7567/APEX.10.111001/pdf

Digital growth of thick N-polar InGaN films on relaxed InGaN pseudosubstrates

Systematic stress tests that help to evaluate the stability and dielectric performance of Al2O3 films under DC bias conditions are reported. Capacitance-voltage (C-V) curves were monitored for changes after subjecting the dielectric film to constant forward and reverse bias stress. Stress tests, along with C-V analysis, are used to evaluate the effect of post deposition annealing on Metal-Organic Chemical Vapor Deposition) Al2O3 films deposited on GaN. The individual benefits and drawbacks of each film and anneal condition were identified. These suggest that the anneals can be tailored to the unannealed film characteristics to achieve desired improvements in performance. It is found that post deposition annealing in forming gas improves performance under reverse bias stress by reducing the fixed charge and the field in the oxide but does not improve performance under forward bias.

Dielectric stress tests and capacitance-voltage analysis to evaluate the effect of post deposition annealing on Al2O3 films deposited on GaN

Femtosecond transient transmission pump–probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100–250 femtoseconds was observed by the time-resolved pump–probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

GaN lateral transistors (HEMTs) continue to penetrate the power electronics market demonstrating excellent performance in the medium power applications. However, for power applications 10kW and higher, vertical GaN devices are preferred over lateral one, since the former offers higher current and power densities. To date, several different vertical transistor structures have been proposed and reported, such as in-situ oxide based vertical trench MOSFET with an undoped GaN interlayer as a channel (OGFET) [1, 2], current aperture vertical electron transistors (CAVETs) [3, 4], junction field effect transistors (JFETs) [5, 6] and MOSFETs [7, 8]. Gupta et al. have demonstrated the high performance OGFET with low specific on-state resistance (Ron, sp) recently [1]. This study presents the large device scaling of the OGFET to realize high output current.

Stacia Keller

Papers

Compositionally graded InGaN layers grown on vicinal N-face GaN substrates by plasma-assisted molecular beam epitaxy

Digital growth of thick N-polar InGaN films on relaxed InGaN pseudosubstrates

Dielectric stress tests and capacitance-voltage analysis to evaluate the effect of post deposition annealing on Al2O3 films deposited on GaN

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

First report of scaling a normally-off in-situ oxide, GaN interlayer based vertical trench MOSFET (OG-FET)