A new process of growing gallium nitride on an etched sapphire substrate, called cantilever epitaxy, may help light up the world with brighter green, blue, and even white semiconductor light emitting diodes (LEDs). The process was developed at the US Department of Energy’s Sandia National Laboratories.
Coloured LEDs are of interest for displays and higher-powered lamps like ones used in traffic lights. An initiative in the US is beginning to develop solid-state sources for high-efficiency white lighting.
‘Our new process eliminates many of the problems that have limited the optical and electronic performances of LEDs previously grown on sapphire/gallium nitride substrates,’ says Sandia researcher Carol Ashby.
Over the past several years LEDs have been grown with various combinations of gallium nitride alloys on sapphire substrates. However, the atoms of the two materials do not line up perfectly due to differences in the natural lengths of the bonds in their respective crystal lattices. Regions of imperfections, called dislocations, accommodate this lattice mismatch. These dislocations are said to limit LEDs’ brightness and performance.
The new cantilever epitaxy process developed by a team of Sandia researchers reduces the numbers of dislocations, giving the potential for longer-lived and better performing LEDs. It also means that LEDs grown on the patterned sapphire/gallium nitride substrates can produce brighter, more efficient, green, blue, and white lights than previously accomplished.
The cantilever epitaxy process, done at the micron level, involves two major steps.
First, etching the sapphire substrate using plasma-assisted etching forms narrow supports. A multiple-layer photoresist mask is used to define the features to develop a post/trench-striped pattern on the substrate. A gallium nitride nucleation layer is then grown on the sapphire posts at a temperature of 500-600 Degrees Centigrade. This nucleation layer helps bridge the crystal-lattice difference between the gallium nitride and the sapphire.
The growth then proceeds with steps at 1050 Degrees Centigrade and then decreased to 950 Degrees Centigrade.
‘At this point a very key thing happens,’ says Daniel Koleske, who is involved in the gallium nitride growth process. ‘The gallium nitride grows mostly upward, forming natural pyramids that reflect the crystal symmetry of gallium nitride.’
The next step is the coalescence. The temperature is increased to 1100 Degrees Centigrade, and the pyramids grow out laterally at a rate faster than they grow vertically. This produces free-hanging cantilevers over the trenches between adjacent posts.
The cantilevers first grow from adjacent posts and meet over the middle of the trench. They then grow together, producing a continuous smooth surface held up by the narrow supports. The areas over the supports have very few dislocations when complete pyramids are formed during the 950 Degrees Centigrade growth step. When dislocations growing up from the post surface encounter the angled walls of the pyramids, they are turned from vertical to horizontal so they don’t reach the surface as the material continues to grow thicker.
There are some dislocations where two cantilevers grow together (the coalescence front), but almost no dislocations in the cantilever regions between the posts and the coalescence front.
The final result is a continuous smooth surface area with greatly reduced numbers of dislocations. This surface can then be used like a regular gallium nitride substrate to grow LEDs and other devices on top.
Ashby, Follstaedt, Sandia researcher Christine Mitchell and Jung Han (a former Sandian) have recently been awarded a patent for the cantilever epitaxy process and cantilever epitaxy substrates have been supplied to LED manufacturers for testing.