Release Process Could Broaden Uses of GaN SCs

Nitride semiconductors grow only on certain surfaces, and their utility is limited by the substrate on which they are fabricated. But a new release process not only makes the method cheaper and easier, but also expands the potential uses of the materials.

Yasuyuki Kobayashi and colleagues at Nippon Telegraph and Telephone Corp. (NTT) demonstrated the process using a technique called mechanical transfer using a release layer (MeTRe). They grew a very thin hexagonal layer of boron nitride (h-BN) between a sapphire substrate and a gallium nitride (GaN)-based semiconductor. Sandwiched in the middle, the h-BN works as a release layer, allowing the investigators to easily detach the semiconductor and transfer it to other substrates without using expensive laser beam machining or chemical treatment.

Comparison between the traditional methods of fabricating GaN semiconductors compared with that of Yasuyuki Kobayashi and colleagues.(Images: NTT Science and Core Technology Laboratory Group)

GaN-based semiconductors have a wide range of applications in high-power electronic devices and light sources, but their utility is hampered by the thickness of the substrates on which they are grown. The substrates are hard to separate from the GaN and can be 100 times the size of the film. They also must be stable at high temperatures because the GaN-based film's growth temperature is 1000 °C. Using the MeTRe fabrication process, GaN-based thin-film devices can be separated easily from their substrate and transferred onto other devices. They can be grown on nearly any single crystal substrate, both of which greatly increase their utility.

Prototype of a thin LED produced using nitride semiconductors.

Boron nitride also is difficult to grow on a single-crystal sapphire substrate because of a very different crystal structure. However, the researchers optimized its growth using metallorganic chemical vapor deposition (MOCVD), which uses the constituent gases to encourage single-crystal, thin-film growth on the substrate's surface. They also found that the GaN layers could be grown on top of the BN film if they used a buffer layer of Al1-xGaxN, an aluminum/gallium nitride alloy.

A prospective use for flexible nitride semiconductors is in window-based solar panels.

The MeTRe method of semiconductor fabrication is cheaper, faster and easier than conventional methods, and there has been a worldwide movement to develop such an efficient technique. The process also allows thin, flexible semiconductors with a large surface area (up to 2 cm) to be made. One key application of such a semiconductor is in the development of flexible solar panels that are sensitive to UV wavelengths only and can be put in windows to filter out the harmful rays while also collecting and storing solar energy. This can be accomplished easily by attaching GaN-based solar cells to pre-existing Si-based ones.

Other potential applications include thin LEDs, highly functional hybrid CMOS and flexible devices.

The research appeared in the April 12 issue of Nature.

For more information, visit: www.ntt.co.jp