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Exciton understanding aids semiconductor devices

22 Aug 2007

Semiconductor optoelectronic devices could benefit from a better understanding of exciton behavior.

Studying the behavior of excitons within semiconductors is helping researchers in the US to improve their understanding of devices such as laser diodes and LEDs. When an electron and hole bind together in a semiconductor device, they form an exciton. (PNAS 10.1073 0701273104)

"This research tests the fundamental theories of how electrons interact with each other in a semiconductor optoelectronic device," Steven Cundiff, a researcher at JILA, a joint institute of the University of Colorada and NIST, told optics.org. "Such interactions significantly influence the operating characteristics of these devices."

For example, Cundiff says that if you try to predict the wavelength of a laser diode based on the crystal structure alone, you could be out by up to 5 %. This is because of the many-body interactions among the electrons in the device.

"The theory can also be used to predict the operating characteristics of lasers by merely looking at the luminescence from the as-grown wafer, without the need to process it into laser devices," added Cundiff.

Many-body interactions in semiconductors have been studied extensively in the last two decades using traditional ultrafast optical spectroscopic techniques. Although these have yielded lots of information, Cundiff believes that many questions remain unanswered. The team turned to optical 2D Fourier Transform Spectroscopy (2DFTS) to see what extra information it could discover.

"Optical 2DFTS is a powerful method for studying many-body interactions among optically created excitations in semiconductors," commented Cundiff. "It is able to separate out specific pathways and phase resolve the signal."

The team fired pulses from a mode-locked Ti:Sapphire laser emitting at 800 nm and a repetition rate of 76 MHz at a gallium arsenide sample. By altering the frequency and orientation of the laser's electric field, the researchers identified coupling between pairs of excitons with different energy levels. The experimental data matched the theoretical calculations.

The next step in the research is to study the relationship between disorder and many-body interactions, which tends to localize electrons and excitons and reduce their interaction. "We also plan to look at semiconductor quantum dots both coupled to the macroscopic world and to other quantum dots and systems," concluded Cundiff. "It will be a challenge to get to the point where significant signal strength can be obtained from quantum dots."

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