Solid-state lighting in homes and offices could be transformed by bringing down the cost of light emitting diodes (LEDs) by using silicon rather than sapphire wafers. Researchers at Purdue University, West Lafayette, USA, claim to have overcome the difficulty in metallising silicon with a reflective layer.

Light emitting diodes, which are composed of layers of semiconducting materials that emit light when electricity is applied, are the subject of intense research. They are expected to be longer lasting, and more energy efficient and environmentally friendly than incandescent light bulbs and neon lights. However, their use has been restricted to lasers, indicator lamps and toys because the sapphire substrate is too expensive for widespread domestic and office lighting. The light-emitting component in LEDs is gallium nitride (GaN).

Professor Timothy Sands at Purdue says, ‘Sapphire is a relatively poor conductor of heat. For high performance LEDs, it may be necessary to remove the LED semiconductor layers from the substrate and bond them to a heat sink – a step that adds cost’. Sapphire-based LEDs also require a costly mirror-like collector to reflect lost light.

‘Silicon has been recognised as a desirable substrate from the standpoints of cost, availability and thermal conductivity,’ explains Sands. ‘Unfortunately, it absorbs visible light, and epitaxial growth of oriented crystalline layers of GaN on silicon is not trivial – although it is a fledgling commercial process.’

Engineers at Purdue have solved these difficulties by metallising silicon with a built-in reflective layer of zirconium nitride. Ordinarily, this material is unstable in the presence of silicon at high temperatures, but Sands’ team has deposited an insulating film of aluminium nitride between the two, enabling conventional growth temperatures for GaN over 1,000ºC.

‘The aluminium nitride does not react with silicon or zirconium nitride. It is also a good thermal conductor and electrical insulator. This simplifies the design of chips that include multiple LEDs on the same chips,’ says Sands.

Epitaxial layers of aluminium nitride and zirconium nitride are transferred onto the silicon wafer using sputter deposition, followed by chemical vapour deposition of GaN.

Steps forward

 

‘Moving away from sapphire, in the end, has to be an industry aim,’ comments Gareth Jones, Chief Technology Officer at Enfis, a designer and manufacturer of LED light engines for solid-state lighting, based in Swansea, UK. ‘A reduction in cost at comparable performance is of great interest.’

He adds, ‘To meet the aspirations of general lighting using LEDs, we are seeking to get one-millimetre sized LED chips emitting around 500 lumens of white light when driven at three watts input power, with lifetimes in excess of 30,000 hours at junction temperatures over 85ºC. The chips should be made available for less than US$0.50.

‘To achieve the cost and performance targets, it is necessary to move to larger and cheaper substrates [of] eight to 12 inches. It is also important to simplify the manufacturing process steps [and] reduce the drop in efficiency of the chip at higher drive currents’.

Sands’ team now aims to test the performance of its LED design, as well as overcome GaN cracking caused by the differing thermal expansion coefficient rates of the silicon and GaN as they cool from epitaxy temperature. Researchers are also working on nanostrucured GaN-based materials to expand the range of wavelengths (colours) available, and move towards phosphor-free white LEDs.

Author : Rupal MehtaMaterials World Magazine, 01 Sep 2008