IAS Affiliate Member Steven DenBaars and his research group has recently designed novel chips to increase the efficiency of both c-plane (polar) and non-polar (a-plane, m-plane) LEDs, including blue LEDs with a high external quantum efficiency and corresponding white LEDs with luminous efficacies of 152 lm/W at low current densities. Using Semi-polar GaN crystals, true direct semiconductor green laser diodes operating at 516nm has been achieved.
Abstract
Recently, we have used novel chip designs to increase the efficiency of both c-plane (polar) and non-polar (a-plane, m-plane) LEDs. Current commercially available wurtzite nitrides based visible optoelectronic devices are grown along the c-axis.^1 These devices are characterized by the presence of polarization-induced electric fields in the multi-quantum wells (MQWs). These electric fields, caused by both discontinuities in spontaneous and piezoelectric polarization at heterointerfaces, lead to the Quantum Confined Stark Effect (QCSE) and lowered radiative recombination probability.^2,3) Additionally, the peak emission wavelength of LEDs grown on c-plane GaN blue-shift with increasing drive current due to screening of the internal fields.^4 Using novel semiconductor fabrication schemes we have achieved blue LEDs with a high external quantum efficiency of 65%, 32.4mW at 20mA, and corresponding white LEDs with luminous efficacies of 152 lm/W at low current densities. Further research is needed to achieve these remarkable efficiencies at the high power levels needed for general lighting. Toward this goal we have also recently developed non-polar LED devices which exhibit efficiencies as high as 31mW at 20mA. The efficiency of non-polar LEDs now approaches that of the best c-plane LEDs with the added benefit of less efficiency roll-off at high current densities. Using Semi-polar GaN crystals we have also recently achieved true direct semiconductor green laser diodes operating at 516nm.
About the speaker
Dr. DenBaars is a Professor of Materials and Co-Director of the Solid-State Lighting Center at UC Santa Barbara. Professor DenBaars joined UCSB in 1991 and currently holds the Mitsubishi Chemical Chair in Solid State Lighting and Displays. From 1988-1991 Prof. DenBaars was a member of the technical staff at Hewlett- Packard's Optoelectronics Division involved in the growth and fabrication of visible LEDs. He received his PhD in Electrical Engineering from the University of Southern California in 1988. Specific research interests include growth of wide-band gap semiconductors (GaN based), and their application to Blue LEDs and lasers and energy efficient solid state lighting. This research has lead to the first US university demonstration of a Blue GaN laser diode and over 17 patents on GaN growth and processing. He has been awarded a NSF Young Investigator award, Young Scientist Award of the ISCS, is an IEEE Fellow, and received the Distinguished Alumni Award in Academia from USC in 2007.
