Submerged electrodes boost high-brightness LED output
One way to judge an LED’s performance is to measure its efficiency at a particular drive current. High efficiencies are relatively easy to realize at low current densities, but efficiency tends to decline as the current is cranked up. However, high efficiencies at high currents hold the key to cutting the electricity bill for lighting, and ultimately spurring the deployment of this light source in more applications.
The pocket projector is one application that is emerging as a beneficiary of higher-performance LEDs. If this product is to project high-brightness images, then LEDs must deliver outstanding luminance at a high drive current and a high efficiency from a small form factor design operating within a limited power budget.
Another opportunity for LEDs is automotive headlights, which require die that operate at high current densities and deliver a scalable light output. Last, but by no means least, is the lucrative general lighting market. This demands new, low-cost device structures with very high efficacy, excellent scalability and exceptional stability over many years of operation. At Osram Opto Semiconductors we have developed an innovative LED design that fulfills all of these goals. At the heart of this groundbreaking chip is a new approach to electrode design.
Our latest LED draws on our ThinGaN technology, employing a soldering method to add a reflecting metallic film prior to removal of the sapphire substrate by a laser lift-off technique. This process, which we introduced in 2003, is superior to conventional flip-chip technologies that attach LEDs upside down on a pre-mounted board. This unites entire epiwafers to carrier wafers. By carefully selecting the solder, it is possible to introduce a highly reflective layer between it and the epitaxial film.
ThinGaN technology produces LEDs with highly reflective mirrors and well defined scattering surfaces (figure 1). Waveguiding and absorption are potential problems in this type of structure, but they are mitigated by introducing some degree of surface roughness. In principle, all of the light could be coupled out of the chip if there is no internal absorption because every photon has numerous chances to find an escape cone. However, in practice out-coupling of 75–80% is realized, due to internal absorption.
These LEDs have a proven set of attributes that empowers them to serve backlighting, solid-state lighting and automotive applications. However, there is still room for improvement because the metallic n-contact grid that distributes the current on this side of the device hinders the total light extraction. The reduction in light output stems from absorption by the metal film and a reduction in the active emitting area.
Improving the design of the metallic contact is not easy though. Increasing the separation of the metallic line contacts that form the grid diminishes absorption in the LED, but also leads to inferior current spreading.
Low current spreading is a major problem in high-power chips due to the low lateral conductivity in the n-type GaN layer, which is less than 5 μm thick. The other obvious option for reducing light absorption is to decrease the width of the metallic lines that form the grid, but this again pays the penalty of inferior current spreading.
A far better approach is to completely eliminate the metallic grid contact from the top of the chip and bury it under the p-contact. This is the option that we have selected and it exploits a novel reflecting n-contact. This is created by forming a grid of vias that penetrate through the p-type material and into the n-type layers.
We use a highly reflective, silver-based layer for the p-contact, which is ideal for injecting electrons into the device. The p-contact is then covered with an insulating layer that prevents electrical shorting of the two electrodes through the reflecting layer.
This type of LED is manufactured by growing the epistructure on sapphire, bonding the wafer to a carrier and then removing the original substrate by laser lift-off. By carefully designing and controlling the mechanical carrier and the interconnect it is possible to build a mechanically stable device that features homogenous heat spreading. These attributes have enabled our LEDs to pass reliability assessments set by the automotive industry, including temperature cycling and stress tests at elevated temperatures and high humidity levels.
Our latest generation of LED chips combines increased wall-plug efficiency with a higher quantum efficiency that stems from reduced light absorption by the n-contact. This improvement, which is greater at high current densities, results from a better epitaxial structure that reduces the serial resistance. Thanks to lower ohmic losses, our 1 × 1 mm devices can operate at drive currents of up to 3 A.
Combating droop
Any LED that is driven hard suffers from droop – a reduction in LED efficiency as the current is cranked up. The origin of droop is controversial, but our research efforts have led us to conclude that it is caused by a form of Auger recombination – a nonradiative process that involves three or more carriers, including at least one electron and one hole.
It’s not possible to eliminate Auger recombination in LEDs – electrons and holes have to be brought together so that the device can emit light. But the extent of Auger recombination can be diminished significantly by increasing the volume of the active region, because this form of non-radiative recombination is proportional to the cube of the carrier density. We have adopted this approach and minimized the current density in a multiquantum-well device.
Our new design is paying dividends. 1 × 1 mm chips emitting at 440 nm have a peak external quantum efficiency and a wall-plug efficiency of 68% (figure 2). An output of 640 mW is produced at 350 mA, rising to 3.2 W at 3 A. White-emitting chips are fabricated by adding a phosphor. These deliver a peak efficacy of 136 lm/W and a maximum output of 830 lm at 3 A.
The emission pattern from these LEDs is independent of drive current, even at high densities, which makes this design ideal for deployment in automotive headlamp systems, and projection and illumination applications. Because this light source is a genuine surface emitter, the technology is not only fully scalable, it is also capable of forming homogenously emitting, high-luminance multichip arrays.
We have also produced green LEDs that emit at 523 nm and deliver a peak efficacy of more than 200 lm/W. This source, which is a promising candidate for forming high-performance projection systems based on red, green and blue sources, produces 117 lm (100 lm/W) at 350 mA and 224 lm at 1 A.
To aid and simplify customer transition to our latest generation of LED chip technology (UX:3), we produced a design with dimensions and primary optical characteristics that are compatible with our first generation of ThinGaN devices. Upgrading existing products is then possible without changes to the optical design parameters of the secondary optics.
Compatible LED packages are used to fully exploit the strengths of our latest chips. This type of housing enables a high degree of linearity between the light output and the drive current, and offers the opportunity to produce sophisticated optical designs. A good thermal package is incorporated in this product, which employs stable materials to ensure a long lifetime for the chip. Other features include a small footprint, a versatile optical interface and decoupling of the electrical and thermal paths, which means that changes to chip polarity do not affect the customer’s PCB layout.
An example of this type of package was unveiled in May – the Oslon SSL. Thanks to the strong overlap with the ThinGaN family, it has been relatively easy to manufacture this product in high volumes.
One of the merits of the Oslon SSL is its thermal resistance of just 7 K/W, which simplifies thermal management and allows the product to serve highpower- density applications. The combination of a ceramic and silicone package ensures high reliability and allows decoupling of electrical and thermal interfaces. Lighting designers will also appreciate the flexibility of this package. It has a footprint of just 3 × 3 mm and enables the emission of a well defined beam that is easy to couple to secondary optics. If particularly bright illumination or color mixing is required, it is easy to combine several light sources into clusters.
Lighting, projection and automotive applications will benefit from our UX:3 chip technology because it provides a high-efficiency, high-current platform that draws on proven ThinGaN processes. Its small size and compatibility with a thermally optimized package creates a cost-effective, highly flexible light source that operates at very high efficiencies.
