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

This review will cover recent work on InN quantum dots (QDs), specifically focusing on advances in metalorganic chemical vapor deposition (MOCVD) of metal-polar InN QDs for applications in optoelectronic devices. The ability to use InN in optoelectronic devices would expand the nitrides system from current visible and ultraviolet devices into the near infrared. Although there was a significant surge in InN research after the discovery that its bandgap provided potential infrared communication band emission, those studies failed to produce an electroluminescent InN device in part due to difficulties in achieving p-type InN films. Devices utilizing InN QDs, on the other hand, were hampered by the inability to cap the InN without causing intermixing with the capping material. The recent work on InN QDs has proven that it is possible to use capping methods to bury the QDs without significantly affecting their composition or photoluminescence. Herein, we will discuss the current state of metal-polar InN QD growth by MOCVD, focusing on density and size control, composition, relaxation, capping, and photoluminescence. The outstanding challenges which remain to be solved in order to achieve InN infrared devices will be discussed.

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InN Quantum Dots by Metalorganic Chemical Vapor Deposition for Optoelectronic Applications

N-polar grown III-nitrides are very interesting materials for the fabrication of heterostructures devices such as transistors, photodetectors, solar cells or optoelectronic devices. In GaN/(In,Ga)N/GaN heterostructures with thin (In,Ga)N layers, polarization engineering allows to achieve interband tunneling. Thereby the tunneling probability is proportional to the indium concentration in the InGaN layers. Indium incorporation is higher for N-polar InGaN films than for the typically grown Ga-polar ones. N-polar III nitride films are often grown on misoriented substrates enabling the growth of smooth, high quality layers [1]. The crystal misorientation leads to the formation of surface steps, and misorientation angles of 4°-5° can result in up to 3-4 unit cell high steps. The growth of N-polar InGaN films is further complicated by the necessary reduced growth temperatures and the required absence of hydrogen in the growth ambient. In quantum well structures, however, hydrogen, which acts as surfactant and promotes the growth of smooth layers, can be introduced during GaN barrier growth, allowing the deposition of thick multiple quantum well (MQW) stacks. Ga-polar InGaN films have been extensively studied and are known for their local fluctuations in the indium composition [2]. Not much is known about the uniformity of N-polar InGaN layers.

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High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III–V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors. This review article describes the current state of InN nanostructure and quantum dot growth by metalorganic chemical vapor deposition and the use in optoelectronic applications.

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Metalorganic chemical vapor deposition of InN quantum dots and nanostructures.

In this letter, the first four-finger (4 <inline-formula> <tex-math notation="LaTeX">$\times25\,\,\mu \text{m}$ </tex-math></inline-formula>) N-polar GaN high-electron-mobility transistor (HEMT) with an outstanding large signal performance of 712-mW (7.1 W/mm) with 31.7% power-added efficiency (PAE) is demonstrated at 94 GHz. To the best of our knowledge, this is a record output power from a single device cell in any semiconductor technology at <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band. An equivalent two-finger device (2 <inline-formula> <tex-math notation="LaTeX">$\times37.5\,\,\mu \text{m}$ </tex-math></inline-formula>) exhibits 6.9 W/mm with 30.6% PAE, demonstrating no degradation of either power density or efficiency with increased number of fingers and gate width. Additionally, the design considerations for multifinger devices are presented with the simulations of airbridge parasitics using COMSOL Multiphysics and Ansys high-frequency structure simulator (HFSS), along with the exploration of the design space in number and width of fingers for the best gain performance for >1 W. Simultaneous high-output power with high efficiency (>30%) shows the great potential of multifinger N-polar GaN HEMT technology for the state-of-the-art <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band power amplifiers.

First Demonstration of Four-Finger N-polar GaN HEMT Exhibiting Record 712-mW Output Power With 31.7% PAE at 94 GHz

In this work, we report on a GaN/AlGaN superlattice based normally-off hole channel FinFET devices. A combination of Schottky gate and 60 nm wide fins led to enhancement mode operation. The device had an on-current of 13 mA/mm and an on-resistance of <inline-formula> <tex-math notation="LaTeX">$300~\Omega $ </tex-math></inline-formula>.mm. simultaneously, a large Ion/Ioff > 107 and a current modulation of more than two orders of magnitude in the enhancement mode regime was also achieved.

Stacia Keller

Papers

InN Quantum Dots by Metalorganic Chemical Vapor Deposition for Optoelectronic Applications

High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures.

First Demonstration of Four-Finger N-polar GaN HEMT Exhibiting Record 712-mW Output Power With 31.7% PAE at 94 GHz

GaN/AlGaN Superlattice Based E-Mode Hole Channel FinFET With Schottky Gate