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Ceramic foundation boosts brightness

25th March 2014
Superior thermal conductivity enables an output power hike for high-voltage LEDs



Epistar is one of the pioneers of high-voltage LEDs, which feature an array of cells connected in series

Engineers fromEpistar and National Central University in Taiwan have increased the output power of a blue  LED by turning to a ceramic substrate.

The ceramic-based emitter, which operates at a high voltage and features 16 LEDs connected in series, can deliver an unsaturated output of 1800 W cm-2 at a current density of 450 A cm-2.

This chip could help to increase the uptake of solid-state lighting, which needs to be more competitive in terms of its performance-to-cost ratio. The team’s work offers a route to making bigger LED chips operating at higher powers, and this could lead to cheaper solid-state lighting products, thanks to material cost savings associated with fabrication and packaging.

Producing larger chips while maintaining performance is not easy, because it requires addressing thermal degradation and current crowding, two contributors to LED droop. This pair of impediments to high performance is particularly difficult to address when LEDs are formed on sapphire – despite its poor thermal conductivity, it is the most common platform for the device.
Several groups have addressed thermal issues and current crowding with vertical devices formed by wafer-bonding and laser lift-off. These LEDs feature an input current flowing through a conductive substrate, and benefit from superior current spreading within the device.

Another way to minimise thermal issues and current crowding is to turn to high-voltage devices, which feature several LEDs connected in series on the same chip. This combination of a high bias voltage and low driving current not only improves current spreading, thanks to the reduced area of the individual LEDs, but also simplify demands on the transformer that converts the supply from the mains into a voltage for powering the bulb.
Epistar is one of the pioneers of the high-voltage LED, and it late 2011 it unveiled chipsets with blue and red emitters. In this latest work, the company has built on that previous success and partnered with National Central University, Taiwan, to demonstrate the benefits of high-voltage chips on ceramic substrates.

This recent study involved a comparison between a thin-film LED wafer-bonded to a silicon substrate – which is said to be the most popular device type for the thin-film process – and high-voltage  LEDs with foundations of sapphire and ceramic AlN.
All three devices had an identical chip size of 1.14 mm by 1.14 mm, emit at around 450 nm, and featured an InGaN/GaN multi-quantum well structure and a magnesium-doped AlGaN electron-blocking layer.

Light extraction from the LED with a ceramic foundation is enhanced by forming a reflective mirror on p-type GaN via deposition of 1 nm of nickel, followed by 200 nm of silver. The growth of additional metallic layers aids bonding to an AlN ceramic substrate, which is supplied by Maruwa and has a thermal conductivity of 230 W m-1 K-1. Laser lift-off removes the sapphire, before photolithography and dry-etching cut the mesa area into a 4 x 4 array, and a potassium hydroxide etch roughens the n-type GaN to increase light extraction. Finally, the walls are passivated with Al2O3 before a combination of chromium and aluminium forms metal bridges and electrodes.
The silicon-based LED is fabricated in a similar manner. Isolating individual LEDs is very difficult on this foundation, due to the high conductivity of silicon that leads to undesired leakage paths, so the team decided to not attempt to form a high-voltage structure.
Sapphire provided the foundation for the third type of LED: a high-voltage device featuring an array of 4 x 4 cells formed by dry etching. Reflective layers deposited on the backside of the sapphire increased reflectivity by 15 percent.

The high-voltage LED with the ceramic foundation produced the highest level of uniformity, in terms of optical intensity, followed by its high-voltage sapphire cousin and then the low-voltage silicon device.

This order reflects current spreading uniformity, which is lowest in the silicon device, due to the long lateral current path. The ceramic-based LED produces the most uniform emission, thanks to the low sheet resistance of its window layer.

Driven towards output power saturation, the high-voltage sapphire and low-voltage silicon peaked at 200 A cm-2 and 250 A cm-2, respectively, while the ceramic-based device continued to emit more power up to 450 A cm-2.

“Such a high operation current density of 450 A cm-2 is rarely found in the literature,” wrote the team in its paper.

M. –L. Tsai et. al.

Appl. Phys. Express 7 022103 (2014)

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