Steve A. Stockman

High-power AlGaInN flip-chip 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 truncated-inverted-pyramid (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency

A truncated-inverted-pyramid (TIP) chip geometry provides substantial improvement in light extraction efficiency over conventional AlGaInP/GaP chips of the same active junction area (∼0.25 mm2). The TIP geometry decreases the mean photon path-length within the crystal, and thus reduces the effects of internal loss mechanisms. By combining this improved device geometry with high-efficiency multiwell active layers, record-level performance for visible-spectrum light-emitting diodes is achieved. Peak efficiencies exceeding 100 lm/W are demonstrated (100 mA dc, 300 K) for orange-emitting (λp∼610 nm) devices, with a peak luminous flux of 60 lumens (350 mA dc, 300 K). In the red wavelength regime (λp∼650 nm), peak external quantum efficiencies of 55% and 60.9% are measured under direct current and pulsed operation, respectively (100 mA, 300 K).

High Power LEDs – Technology Status and Market Applications

High power light emitting diodes (LEDs) continue to increase in output flux with the best III-nitride based devices today emitting over 150 lm of white, cyan, or green light. The key design features of such products will be covered with special emphasis on power packaging, flip-chip device design, and phosphor coating technology. The high-flux performance of these devices is enabling many new applications for LEDs. Two of the most interesting of these applications are LCD display backlighting and vehicle forward lighting. The advantages of LEDs over competing lighting technologies will be covered in detail.

Optical cavity effects in InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes

Optical cavity effects have a significant influence on the extraction efficiency of InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes (FCLEDs). Light emitted from the quantum well (QW) self-interferes due to reflection from a closely placed reflective metallic mirror. The interference patterns couple into the escape cone for light extraction from the FCLED. This effect causes significant changes in the extraction efficiency as the distance between the QW and the metallic mirror varies. In addition, the radiative lifetime of the QW also changes as a function of the distance between the QW and the mirror surface. Experimental results from packaged FCLEDs, supported by optical modeling, show that a QW placed at an optimum distance from the mirror provides a ∼2.3× increase in total light output as compared to a QW placed at a neighboring position corresponding to a minimum in overall light extraction.

Performance of High‐Power AlInGaN Light Emitting Diodes

The performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output-current-voltage (L-I-V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm. The highest external quantum efficiency (EQE) is measured for short wavelength LEDs (428 nm) at 29%. EQE decreases with increasing wavelength, reaching 13% at 527 nm. With low forward voltages ranging from 3.3 to 2.9 V at a drive current density of 50 A/cm 2 , these LEDs exhibit power conversion efficiencies ranging from 26% (428 nm) to 10% (527 nm).

High‐Power III‐Nitride Emitters for Solid‐State Lighting

High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or flip-chip, configuration are described. These devices are mounted in specially designed high-power (1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 x 1 mm 2 device) and 1 W (2 x 2 mm 2 device) are delivered at standard operating current densities (50 A/cm 2 ), corresponding to wall-plug efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.

GaN-Based Light Emitting Diodes with Tunnel Junctions

We have demonstrated hole injection through a tunnel junction embedded in the GaN-based light emitting diode structure. The tunnel junction consists of 30 nm GaN:Si++ and 15 nm InGaN:Mg++ grown on a GaN–InGaN quantum well heterostructure. The forward voltage of the light emitting diode, including the voltage drop across the reverse-biased tunnel junction, is 4.1 V at 50 A/cm2, while that of a standard light emitting diode with a conventional contact structure is 3.5 V. The light output of the diode with the tunnel junction is comparable to that of the standard device. This tunnel junction eliminates the need for a highly resistive p-AlGaN cladding layer in short-wavelength laser diodes and the semi-transparent electrode required for current spreading in conventional GaN-based light emitting diodes.

1.4× efficiency improvement in transparent-substrate (AlxGa1−x)0.5In0.5P light-emitting diodes with thin (⩽2000 Å) active regions

Improvement of 1.4× in the external quantum efficiency and luminous efficiency (lm/W) of transparent-substrate (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes is demonstrated. The improvement is accomplished by reducing the thickness of the active layer to ⩽2000 A and increasing the internal quantum efficiency by using multiple thin (⩽500 A) active layers. The maximum luminous efficiency achieved is 73.7 lm/W at λp∼615 nm and the maximum external quantum efficiency is 32.0% at λp∼632 nm.

Current Dependence of In-Plane Electroluminescence Distribution of InxGa1-xN/GaN Multiple Quantum Well Light Emitting Diodes

We have studied the radiative recombination mechanism of InxGa1-xN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) by measuring the in-plane electroluminescence (EL) distribution with a near-field scanning optical microscope (NSOM) as a function of current over a range of 20 µA to 300 mA. We discuss the relationship between peak intensity and peak wavelength of local EL spectra with changing current. The region of strong EL intensity exhibits a long emission wavelength. Only a part of the wavelength distribution exhibits blue shift and the range of peak wavelength and intensity distribution becomes narrow with increasing current up to 10 mA. In the high-current region, the entire distribution shifts to shorter wavelength with increasing current. These findings suggest that localized carriers play a dominant role in the radiative recombination and those localized states saturate with increasing current in the low-current region, which leads to the blue shift of the longer wavelength part of the distributions due to state filling. In the high-current region the screening of the piezoelectric field by injected carriers is thought to be a dominant mechanism of the blue shift.

High-power truncated-inverted-pyramid (AlxGa1-x)0.5In0.5P light-emitting diodes

High power light emitting diodes (LEDs) are of interest for many lighting applications. Flux improvements can be achieved by scaling conventional chips to larger dimensions. However this scaling results in a decrease in extraction efficiency. These penalties can be offset by modifying the chip geometry such that the number of internal reflections is reduced, thereby increasing the probability of photon escape. LEDs with a truncated-inverted-pyramid (TIP) geometry have been fabricated and packaged. Peak efficiencies exceeding 100 lm/W have been measured (100 mA dc, 300 K) for orange ((lambda) p approximately 610 m) devices. In the red wavelength regime ((lambda) p approximately 650 nm), peak external quantum efficiencies of 55% (100 mA dc, 300 K) have been achieved. Flux exceeding 65 lumens from a single 594 nm device has also been demonstrated. These characteristics match and/or exceed the performance of many conventional lighting sources.