Translucent touts rare-earth layer for cheaper LEDs

15 Jul 2011

Gadolinium oxide on a silicon substrate is suitable for integration with conventional light–emitting nitride layers, says start-up.

The early-stage Californian materials company Translucent is touting a novel approach to gallium nitride (GaN) LED epiwafer design that it claims will offer a route to cheaper solid-state lighting (SSL) technology.

Although developers of blue and white LEDs have made huge strides in improving the efficiency of their emitter structures in recent years, and the technology is beginning to find some general lighting applications, it is acknowledged that the cost of SSL is still at least an order of magnitude too high for widespread adoption.

Like many others, Translucent’s idea is to produce the LEDs on silicon wafers, rather than sapphire, silicon carbide or GaN itself – all of which are relatively expensive. The big problem is that the crystal structures of GaN and silicon are pretty incompatible, meaning that a strain-balancing buffer layer of semiconductor material typically has to be grown between the two. Where the new approach is different is that it integrates layers of a rare-earth-oxide within the LED structure to form distributed Bragg reflector (DBR) mirrors.

According to the company, the cubic crystal structure of gadolinium oxide is similar to that of silicon, but still offers the strain mitigation that makes it compatible with a cap layer of GaN - upon which the light-emitting structure can subsequently be grown by MOCVD. Using the mirror structure also means that the substrate material does not need to be removed to allow more of the blue light to escape from the LED.

Cost trade-offs
Although the rare-earth elements are not actually that rare in terms of their global abundance, the restricted supply of rare-earth oxides from China – which controls more than 95% of the market – means that they can be expensive, and there have been wild fluctuations in price over the past 12 months.

Another drawback is that the engineered wafers developed by Translucent are produced using molecular beam epitaxy (MBE) – a high-precision but much slower process than MOCVD, the incumbent approach for LED manufacturing. Both of those factors will weigh against the proposed ability of the technology to reduce SSL costs, traded off against the potential cost reduction of growing on the (111) structure of silicon.

Translucent, which is owned by the Australian company Silex Systems and managed by former Optoelectronics Industry Development Association (OIDA) executive Mike Lebby, presented the novel technology in a post-deadline session at the International Conference on Nitride Semiconductors in Glasgow, UK, on July 15. So far, it has produced material on a 100 mm diameter wafer, and is offering samples to prospective customers. Any future commercial roll-out will be based on 150 mm and 200 mm sizes, as LED makers are likely to have moved to the larger formats when and if the novel materials approach is ready for volume production.

Other companies working on GaN-on-silicon LED formats include Azzurro Semiconductors and Bridgelux, while in the UK there is strong expertise at the universities of Cambridge and Bath.