National Synchrotron Light Source Lunch Time Seminar

Synchrotron-based x-ray radiation at Cornell High Energy Synchrotron Source (CHESS - A2 beamline) was used to excite luminescence spectra in InGaN/GaN multiple quantum well (MQW) structures for optoelectronic devices. Both, cw and time-resolved techniques have been performed in detecting the x-ray excited optical luminescence (XEOL) signal. The peak of XEOL for GaN layers coincides with that for conventional PL obtained with a laser excitation, while the XEOL peak for the InGaN active region has a 50 nm shift compared to spectra measured with laser-based PL and cathode-luminescence. Time-resolved measurements were done on XEOL spectra using a streak camera. The temporal structure of the x-ray synchrotron beam at CHESS permits exciton life-time measurements in a broad range from 0.5 ns up to a few microseconds. We determined that GaN exciton lifetime varies between 1.3 and 4.5 ns in different structures, while the InGaN exciton lifetime is on the microsecond scale due to higher defect density. Our experimental approach has been extended to the use of x-ray micro-beams. Using this approach a micron-size spot can probe InGaN/GaN device structures simultaneously measuring: (i) x-ray diffraction, (ii) photoluminescence spectra, and (iii) exciton life-times.
Work is in collaboration with S.M. O'Malley (Rutgers University - Camden), P. Revesz and A. Kazimirov
(CHESS)