New LED Format Is the Pits

Hank Hogan

The latest LED innovation literally is the pits. Researchers in Takashi Egawa’s group at Nagoya Institute of Technology in Japan have increased the efficiency of gallium nitride LEDs by fabricating them with densely packed V-shaped craters that measure 2 to 3 μm wide and deep. Because of the pits, the light generated in the device has an easier time escaping and, therefore, more is emitted.

This image depicts the surface morphology of a pitted GaN-based LED. Reused with permission from American Institute of Physics.

In a standard GaN LED, only ~4 percent of the light makes it out of the device’s flat surface. The problem is only going to get worse, noted Maosheng Hao, a guest associate professor at the institute. “With the increase of the power and the size of the LED, the aspect ratio of the device will decrease and more light will be trapped in the LED structure,” he said.

To develop the micropit approach, Hao and his colleagues exploited aspects of device fabrication. These LEDs typically are grown via metall-organic chemical vapor deposition on sapphire substrates. During a high-temperature phase, GaN islands form and then coalesce into a continuous film.

When the growth time is too short or the temperature too low, the resulting films have V-shaped micropits with sidewalls that lie along a crystal plane of the material. Because of their geometry, the pits allow additional light to escape from the sidewalls — if the distance between the pits is sufficiently small. The investigators developed a recipe compatible with standard LED fabrication for producing devices that have a high density of micropits.

This image shows the difference in emissions from the flat and pitted portions of an LED. Courtesy of Maosheng Hao.

They measured the emission power of a pitted LED and compared it with a standard one, finding the former to be more powerful than the latter by an amount that varied with drive current. For example, the difference between the standard and pitted LEDs was about 50 percent at 20 mA. Given that the pitted device has a smaller emitting surface than the nonpitted one, the researchers attributed the boost to increased light extraction and, possibly, higher internal quantum efficiency.

A schematic drawing illustrates the light extraction scenarios from the sidewall of V-shaped micropits in a GaN-based LED. The pits permit more light to escape the device, increasing the LED’s efficiency. Reused with permission from American Institute of Physics.

They found that the emission of the pitted LED was redshifted by ~10 nm from the standard one, resulting from the different growth characteristics of the sidewalls.

Unfortunately, they also found a problem that must be solved before the technique can be used commercially. The group had coated the pits with silicon dioxide to reduce current leakage. However, when subjected to a reverse voltage that should have shut off the device, the pitted LEDs instead leaked, permitting, for example, 1 mA of current to flow at 8-V reverse bias.

Finding a way to correct this is part of ongoing research. Various techniques are being explored, but Hao indicated that a trivial reduction is not required. “The leakage current must be reduced at least three orders of magnitude,” he said.

Applied Physics Letters, Dec. 11, 2006, 241907.