First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

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Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By using the laser-lift-off technique followed by an anisotropic etching process to roughen the surface, an n-side-up GaN-based LED with a hexagonal “conelike” surface has been fabricated. The enhancement of the LED output power depends on the surface conditions. The output power of an optimally roughened surface LED shows a twofold to threefold increase compared to that of an LED before surface roughening.

Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening

High-power light-emitting diodes (LEDs) have begun to differentiate themselves from their more common cousins the indicator LED. Today these LEDs are designed to generate 10-100 lm per LED with efficiencies that surpass incandescent and halogen bulbs. After a summary of the motivation for the development of the high-power LED and a look at the future markets, we describe the current state of high-power LED technology and the challenges that lay ahead for development of a true "solid state lamp." We demonstrate record performance and reliability for high-power colored and white LEDs and show results from the worlds first 100-plus lumen white LED lamp, the solid state equivalent of Thomas Edison's 20-W incandescent lightbulb approximately one century later.

https://www.lightingassociates.org/i/u/2127806/f/tech_sheets/Illumination_With_Solid_State_Lighting_Technology.pdf

Illumination with solid state lighting technology

Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. This feature article provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop.

Illustration of LED efficiency droop (details in Fig. 13).

Efficiency droop in nitride-based light-emitting diodes

Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is “flipped-over” or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion (“wall-plug”) efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A.

High-power AlGaInN flip-chip light-emitting diodes

Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2

Variable-temperature Hall-effect measurements were employed to optimize doping for GaN layers utilized in blue, blue-green and green light emitting diodes (LEDs). N-type doping was accomplished by doping with Si, Ge, and O, and the electronic properties of these donors were studied. Si and Ge, which substitute for Ga, are shallow donors with almost identical activation energies for ionization (ca. 17 and ca. 19 meV, respectively, for a donor concentration of ca. 3×10 17 cm −3 ). O substitutes for N and introduces a slightly deeper donor level into the bandgap of GaN having an activation energy of ca. 29 meV (for a donor concentration of ca. 1×10 18 cm −3 ). Mg doping was employed to achieve p-type conductivity for GaN device layers. Mg substitutes for Ga introducing a relatively deep acceptor level. For the analysis of the variable-temperature Hall-effect data, it was found important to take the coulomb interaction between ionized acceptors into account, leading to lower activation energy with increasing degree of ionization (increasing temperature). The activation energy for ionization of Mg acceptors in GaN was thus estimated to be (208±6) meV for very low acceptor concentrations. Using optimized nitride layers, LEDs with typical external quantum efficiencies of ca. 10% in the blue and blue-green, and ca. 8% in the green wavelength range were achieved. Due to optimized doping, the forward voltages for these diodes were as low as 3.2 V at 20 mA drive current.

Hall-effect characterization of III–V nitride semiconductors for high efficiency light emitting diodes

Publisher Summary This chapter discusses high-brightness nitride-based visible-light-emitting diodes (LEDs). There are numerous immediate applications for the long-awaited short wavelength LEDs. These applications—for example, large-screen full-color video displays, red-amber-green traffic signal lights, and interior and exterior automotive lighting—are hindered by the lack of blue and green devices with light output performance comparable to the yellow to red aluminum gallium indium phosphide (AlGaInP) and aluminum gallium arsenide (AlGaAs) devices that have been available for 10 years. Highly efficient LEDs possess many beneficial characteristics relative to incandescent filament bulbs—namely, low power consumption and long lifetime. In addition, the maintenance costs associated with bulb replacement are greatly reduced, because an LED traffic signal head is expected to last 5 to 10 years.

W. Götz

Papers

High-power AlGaInN flip-chip light-emitting diodes

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2

Hall-effect characterization of III–V nitride semiconductors for high efficiency light emitting diodes

Chapter 3 High-Brightness Nitride-Based Visible-Light-Emitting Diodes