Closer to Flexible LED Displays

Nanoetching product. Photo courtesy of Professor Jung Han.

LED lights are the future: compared to fluorescent lights, LEDs use energy more efficiently, shine more brightly, and last ten times longer. Unfortunately, cost impedes the revolutionary switch to LEDs: LEDs are currently three to four times more expensive than fluorescent lights. Gallium nitride (GaN), the semiconductor in LEDs, is difficult to shape because of its inertness and its physical nature, thus making GaN more expensive to manufacture. Yale Professor of Electrical Engineering Jung Han is researching novel nanoetching techniques in order to more easily manipulate GaN – techniques that could potentially make LEDs more commercially available.

Like silicon, GaN is a semiconductor, so it has an electrical conductivity between that of a conductor and an insulator. The semiconductor’s pattern is a key factor in how it is used in electronics, which is why Han’s research focuses on techniques that would make GaN easier to manipulate.

Han’s nanoetching technique essentially drilled pores through a block of GaN to increase its elasticity, ultimately producing a thin, flexible film of GaN. He created these films through an electrochemical procedure in which a sample of GaN lay in a container of electrolyte solution with an electrode on each end. As the current flowed through the GaN, its surface was etched with pores that grew deeper and deeper into the GaN block, gradually reducing its thickness. The key discovery in Han’s nanoetching technique was that the drilling action was concentrated at the bottom of each pore.

Switching from low to high voltage causes aggressive drilling strong enough to cause a film of GaN to peel off. Photo courtesy of Professor Jung Han.

As the voltage of the current running through the sample increased, the drilling became more aggressive. Han’s nanoetching method took advantage of this property: the initial drilling occurred at a lower voltage and once these pores reached the desired depth, the voltage was increased to induce uniform, aggressive drilling at that depth. Such drilling caused a break across the surface of the GaN sample and formed a porous GaN film. With Han’s revolutionary nanoetching technique, a free-standing film of 1-micrometer thickness was created in 20 minutes. Previous complex methods could only create a GaN film of about 6 to 7 micrometers thick.

This electrochemical procedure was used in the past to study the properties of silicon and aluminum, but this is the first time it was used with GaN. The success of the experiment is very encouraging and Han’s lab plans to apply this technique for various uses. A porous medium changes the optical properties of GaN using the air introduced through the pores. Porous GaN is also a potential candidate for solar harvesting due to its high surface area and visible bandgap. Most notably, Han’s lab recently attached a GaN film to a transparent film, printed on it, and used a UV laser to optically excite the GaN so that it would glow green. While this was not a true LED, it was a step in the right direction. Ultimately, with flexible GaN films, it may be possible to create of what we could once only dream: flexible, thin, roll-up LED monitors for TVs and computers. When this happens, Han’s revolutionary nanoetching research will undoubtedly be one of the many steps that would have made it possible.

A photo of Professor Jung Han in his office. Photo courtesy of Professor Jung Han.