University of Hawaii

Electrical Engineering

Growth and Characterization of GaN-based LEDs by Nanoscale Epitaxial Lateral Overgrowth Technique

Date: 2010-06-23
Time: 11:00 -- 12:00 noon
Location: Holmes 389
Speaker: Professor Charles Surya Associate Dean of Engineering and Professor in the Department of Electronic and Information Engineering The Hong Kong Polytechnic University


Nanoscale epitaxial lateral overgrowth (NELO) of GaN is performed using PECVD-grown SiO2 with nanoscale windows as the growth mask.  It is found that the overgrown GaN epilayers exhibit significant reduction in the threading dislocations (TDs) as characterized by atomic force microscopy.  Low and uniform TDs density were observed over the whole 2-inch wafer, which is different from ELO GaN layers grown on conventional SiO2 stripes in which the coherent GaN layers grown in the window regions still suffer from high dislocation density.  LED structures with active regions consisting of five-period multiple quantum wells (MQWs) were fabricated.  Detailed characterizations of the optoelectronic properties of the devices were performed by I-V and electroluminescence (EL) measurements.  Significant improvements in the EL intensity are observed among the devices grown on the NELO GaN layers (type N) compared to the control devices which have a conventional structure (type C).  It is interesting to note that a 15 nm blue shift in the EL peak is observed among the type N devices.  Detailed high resolution X-ray diffraction and reciprocal space mapping characterizations were performed.  The experimental data demonstrate substantial stress relaxation in the type N MQWs resulting in significant lowering in the quantum confinement Stark effect in the MQWs which is believed to underlie the observed blue shift in the EL peak.   The reduction in the quantum confinement Stark effect is also partly responsible for the observed improvement in the quantum efficiency of the device.  Detailed investigations in the degradation of the devices due to hot-electron injection indicate that stress relaxation in the MQWs utilizing NELO growth technique results in improvements in the hot-electron hardness of the LEDs.

Speaker Bio: 
Professor Charles Surya graduated from the Electrical Engineering Department of the University of Hawaii at Manoa in 1981.  He did his graduates studies at the University of Rochester, New York where he received his MSc and PhD degrees in Electrical Engineering in 1983 and 1987.  He was with the Northeastern University from 1987 to 1994.  In 1994 he joined the Electronic and Information Engineering Department of the Hong Kong Polytechnic University where he is now a professor and Associate Dean for academic affairs.  His research interest is in semiconductor optoelectronic devices.  He published over 100 papers and has two patents on GaN-based UV detectors.