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

In this letter, a GaN-based current aperture vertical electron transistor (CAVET) with a p-type gate layer and an implantation-based current blocking structure is presented. The devices measured showed a breakdown voltage of 450 V and no dispersion. The factors limiting higher breakdown voltages in these devices were carefully studied and discussed. The devices were grown on sapphire and relied on a box-shaped Mg implanted current blocking scheme. This is the first demonstration of an implantation-based CAVET, grown on sapphire blocking of 450 V with respectable on-state characteristics.

Dispersion Free 450-V p GaN-Gated CAVETs With Mg-ion Implanted Blocking Layer

A fully relaxed In0.1Ga0.9N layer was grown by plasma-assisted molecular beam epitaxy on c-plane GaN using a grading technique. The growth of the graded InGaN layer in the intermediate regime enabled a smooth surface without the accumulation of In droplets. Transmission electron microscopy images show that the relaxation occurs through the formation of a high density of threading dislocations (TDs). Despite the presence of these TDs, relaxed InGaN films were then successfully used as a pseudo-substrate for the growth of InGaN/GaN quantum wells which luminesced at room temperature.

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Relaxed c-plane InGaN layers for the growth of strain-reduced InGaN quantum wells

Light Emitting Diodes (LEDs) where the emission region, usually a (Al,In,Ga)N layer, is structured for efficient light extraction, are disclosed. The structuring is designed for light extraction from thin films, such as a photonic crystal acting as a diffraction grating. In addition, the structuring controls the in-plane emission and allows new modes into which light will be emitted. Various electrode designs are proposed, including ZnO structures which are known to lead to both excellent electrical properties, such as good carrier injection, and high transparency. Alternatively, the (Al,In,Ga)N layer can be replaced by structures with other materials compositions, in order to achieve efficient light extraction.

High light extraction efficiency light emitting diode (LED) with emitters within structured materials

The present invention allows the growth of InGaN with greater compositions of Indium than traditionally available now, which pushes LED and LD wavelengths into the yellow and red portions of the color spectrum. The ability to grow with Indium at higher temperatures leads to a higher quality AlInGaN. This also allows for novel polarization-based band structure designs to create more efficient devices. Additionally, it allows the fabrication of p-GaN layers with increased conductivity, which improves device performance.

LIGHT EMITTING DIODE AND LASER DIODE USING N-FACE GaN, InN, AND AlN AND THEIR ALLOYS

In this study, two methods to tune the emission wavelength of micro-LEDs fabricated on tile patterned compliant GaN-on-porous-GaN pseudo-substrates (PSs) are presented. The mechanical flexibility of porous GaN was utilized to relax the strain induced during the growth of light-emitting diode (LED) structures with n- and p-InGaN layers and enhance the indium incorporation via the composition pulling effect. The first approach involved only varying the size of the PS square tiles used for LED structure regrowth, from 20 × 20 μm2 to 8 × 8 μm2. Higher n-InGaN base layer relaxation with decreasing tile size resulted in a red shift of emission from 525 nm to 561 nm with no change in the growth conditions. The second method involved changing the mole fraction, x, of the n-InxGa1−xN base layer of the LED structure from 0.04 to 0.09 by reducing growth temperature, while maintaining the high temperature growth conditions of the multi-quantum well (MQW) and p-InGaN targeting 530 nm emission. The resulting wavelength shift was a remarkable 536–616 nm due to the stronger composition pulling effect, providing a pathway to enable high indium content MQW active regions to be grown at high temperature.

Stacia Keller

Papers

Dispersion Free 450-V p GaN-Gated CAVETs With Mg-ion Implanted Blocking Layer

Relaxed c-plane InGaN layers for the growth of strain-reduced InGaN quantum wells

High light extraction efficiency light emitting diode (LED) with emitters within structured materials

LIGHT EMITTING DIODE AND LASER DIODE USING N-FACE GaN, InN, AND AlN AND THEIR ALLOYS

Color-tunable <10 μm square InGaN micro-LEDs on compliant GaN-on-porous-GaN pseudo-substrates