E. Fred Schubert

Light Emitting Diodes

Inorganic semiconductor light-emitting diodes (LEDs) are environmentally benign and have found widespread use as indicator lights, mobile displays, large-area displays, signage applications, and lighting applications. The entire visible spectrum can be covered by light-emitting semiconductors: AlGaInP and AlGaInN compound semiconductors are capable of emission in the red-to-yellow wavelength range and violet-to-green wavelength range, respectively. For white light sources based on LEDs, the most common approach is the combination of a blue LED chip with a yellow phosphor. Alternatively, a group of red, green, and blue (RGB) LEDs can be used; such source allows for color tunability. White LEDs are currently used to replace incandescent and fluorescent sources. In this review, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current-flow patterns. Structures providing high internal quantum efficiency, namely heterostructures and multiple quantum well structures, will be discussed. Advanced techniques enhancing the external quantum efficiency will be reviewed, including die shaping (chip shaping) and surface roughening. Different approaches to white LEDs will be presented and figures-of-merit such as the color rendering index and luminous efficacy will be explained. Besides visible LEDs, the technical challenges of newly evolving deep ultraviolet (UV) LEDs will be introduced. Finally, the packaging of low-power and high-power LED dies will be discussed.

Origin of efficiency droop in GaN-based light-emitting diodes

The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Polarization-matched GaInN∕AlGaInN multi-quantum-well light-emitting diodes with reduced efficiency droop

Blue multi-quantum-well light-emitting diodes (LEDs) with GaInN quantum wells and polarization-matched AlGaInN barriers are grown by metal-organic chemical vapor deposition. The use of quaternary alloys enables an independent control over interface polarization charges and bandgap and has been suggested as a method to reduce electron leakage from the active region, a carrier loss mechanism that can reduce efficiency at high injection currents—an effect known as the efficiency droop. The GaInN∕AlGaInN LEDs show reduced forward voltage, reduced efficiency droop, and improved light-output power at large currents compared to conventional GaInN∕GaN LEDs.

Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes

Measurements of light-output power versus current are performed for GaInN∕GaN light-emitting diodes grown on GaN-on-sapphire templates with different threading dislocation densities. Low-defect-density devices exhibit a pronounced efficiency peak followed by droop as current increases, whereas high-defect-density devices show low peak efficiencies and little droop. The experimental data are analyzed with a rate equation model to explain this effect. Analysis reveals that dislocations do not strongly impact high-current performance; instead they contribute to increased nonradiative recombination at lower currents and a suppression of peak efficiency. The characteristics of the dominant recombination mechanism at high currents are consistent with processes involving carrier leakage.

Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics

Design, fabrication, and characterization of a broadband, omnidirectional, graded-index antireflection (AR) coating made using nanostructured low-refractive-index (n=1.05–1.40) silica deposited by oblique-angle deposition are reported. Averaged over wavelength range from 400 to 1100 nm and 0°–90° angle of incidence, polished Si reflects ∼37% of incident radiation. The reflection losses are reduced to only 5.9% by applying a three-layer graded-index AR coating to Si. Our experimental results are in excellent agreement with theoretical calculations. The AR coatings reported here can be optimized for photovoltaic cells made of any type of material.

Analysis of high-power packages for phosphor-based white-light-emitting diodes

An optimized packaging configuration for high-power white-light-emitting diode (LED) lamps that employs a diffuse reflector cup, a large separation between the primary emitter (the LED chip) and the wavelength converter (the phosphor) and a hemispherically shaped encapsulation is presented. Ray tracing simulations for this configuration show that the phosphor efficiency can be enhanced by up to 50% over conventional packages. Dichromatic LED lamps with phosphor layers on the top of a diffuse reflector cup were fabricated and studied experimentally. The experimental enhancement of phosphor efficiency is 15.4% for blue-pumped yellow phosphor and 27% for ultraviolet-pumped blue phosphor. Those improvements are attributed to reduced absorption of the phosphorescence by the LED chip and the reduction of deterministic optical modes trapped inside the encapsulant.

Realization of a near-perfect antireflection coating for silicon solar energy utilization

To harness the full spectrum of solar energy, Fresnel reflection at the surface of a solar cell must be eliminated over the entire solar spectrum and at all angles. Here, we show that a multilayer nanostructure having a graded-index profile, as predicted by theory [J. Opt. Soc. Am. 66, 515 (1976); Appl. Opt. 46, 6533 (2007)], can accomplish a near-perfect transmission of all-color of sunlight. An ultralow total reflectance of 1%-6% has been achieved over a broad spectrum, lambda = 400 to 1600 nm, and a wide range of angles of incidence, theta = 0 degrees-60 degrees . The measured angle- and wavelength-averaged total reflectance of 3.79% is the smallest ever reported in the literature, to our knowledge.

Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes

Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.

Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm

Designs of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials are optimized using a genetic algorithm. Co-sputtered and low-refractive-index materials allow the fine-tuning of refractive index, which is required to achieve optimum anti-reflection characteristics. The algorithm minimizes reflection over a wide range of wavelengths and incident angles, and includes material dispersion. Designs of antireflection coatings for silicon-based image sensors and solar cells, as well as triple-junction GaInP/GaAs/Ge solar cells are presented, and are shown to have significant performance advantages over conventional coatings. Nano-porous low-refractive-index layers are found to comprise generally half of the layers in an optimized antireflection coating, which underscores the importance of nano-porous layers for high-performance broadband and omnidirectional antireflection coatings.

GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer

Enhancement of light extraction in a GaInN light-emitting diode (LED) employing a conductive omnidirectional reflector (ODR) consisting of GaN, an indium-tin oxide (ITO) nanorod low-refractive-index layer, and an Ag layer is presented. An array of ITO nanorods is deposited on p-type GaN by oblique-angle electron-beam deposition. The refractive index of the nanorod ITO layer is 1.34 at 461nm, significantly lower than that of dense ITO layer, which is n=2.06. The GaInN LEDs with GaN∕low-n ITO/Ag ODR show a lower forward voltage and a 31.6% higher light-extraction efficiency than LEDs with Ag reflector. This is attributed to enhanced reflectivity of the ODR that employs the low-n ITO layer.