11 Apr 2003
Planar mid-IR LEDs contain massive temperature gradients that dramatically affect the device's performance.
The emitting structure of planar mid-IR LEDs could contain temperature gradients up to 3000°C/cm, according to the results of research being carried out in the Ukraine and France. The researchers say that these variations in temperature cause irreversible deformations within the LED's emitting structure and are one of the main reasons why devices fail. (Journal of Applied Physics to appear June 15)
"The most important aspect of our work is high-resolution mapping of heat and light patterns in semiconductor devices," Professor Malyutenko from the Institute of Semiconductor Physics, Ukraine told Optics.org. "We are the first to create a two-dimensional pictures of the heat in LEDs and detect these temperature gradients in tiny devices."
According to Malyutenko, the overheating happens in a small micron-sized volume. "The main danger here is not the temperature itself but the gradients in space and time," he said. "Heat build-up is becoming one of the major limitation to creating tomorrow's IR devices that run more efficiently and waste less energy."
Malyutenko and colleagues from Jean-Michel Ortega's group at the Labaratoire pour I'Utilisation du Rayonnement Electromagnetique in Orsay, France, use two techniques to produce high-resolution heat and light maps of uncooled planar devices emitting 4.3 microns at room temperature.
The first method uses a calibrated IR microscope and a scanning thermal camera. The second approach involves scanning an optical fiber over the device's surface. The latter allows the team to track micrometer-range changes in the radiative map when the drive current is varied. Using these techniques, the team can produce maps with a spatial resolution of several microns and a temporal resolution of 10 microseconds.
"Scientists have suspected that local overheating induces a large temperature gradient," said Malyutenko. "But to measure it you need both a high temporal and spatial resolution. We are the first team to see this."
One factor that leads to devices overheating is known as current crowding. Here, electrons gather in a defect or near the edge of a structure producing an uneven current density that leads to a hotspot. At low drive current this is not a problem. But as the drive current increases, the effects of overheating become obvious.
"When the current increases, the light pattern becomes drastically non-uniform and most of the emitted power concentrates around the LED's top point contact," report the researchers. Malyutenko adds that continuous wave operation of conventional uncooled mid-IR LEDs operating at room temperature is not possible above 150 mA. "Our next task is to double the bias current," he said.
Malyutenko's group in Ukraine and Ortega's team in France are now looking to collaborate with semiconductor scientists to develop efficient mid-IR devices.