Kalyan Kumar Das
Department of Electrical Engineering
Electrically and Optically Active Defects in GaN-based LEDs
Location: EB1 Room 1011
Friday, December 6th 2013 - 11:00 am
Information pertaining to electrically and optically active defects in Blue and UV emitting GaN based LEDs were extracted from their I-V characteristics and electro-luminescence spectra obtained nominally at room temperature. These diodes exhibited an ohmic regime at low forward biases; then the current increased sharply as bias increased. Several changes in the slope of logarithmic I-V plots indicated that, I~Vx. These changes in the slope were interpreted as single-carrier space-charge-limited (SCL) transport in the presence of deep-level states across the diode active region. As bias increased the deep states were filled and for 400- and 468-nm diodes ideality factors of ~ 2 were obtained. This indicated that, as bias increased the transport mechanism changed from SCL conduction to current due to recombination of injected carriers in the space-charge region. For 380- and 430-nm diodes, ideality factors >>2 were obtained, although the observed electroluminescence spectrum indicated substantial radiative recombination. For the diode emitting at 430 nm, several peaks including the major peak at ~ 424 nm appeared to have resulted from transitions between the conduction-band edge and deep states, identified from the I-V characteristics, likely to be associated with Zn doping of the InGaN active region. Deep states in the other diodes appeared to be ineffective in the radiative recombination process. This study indicates that an analysis of the room temperature I-V characteristics provide an effective technique for identifying defects states in the wide bandgap LEDs.
Kumar Das received a B. S. and an M. S. in Physics at Jadavpur University, Calcutta, M. S. in Solid State Device Physics from London and a Ph. D. in Microelectronics from Middlesex University, London. He held Post-doc positions at Middlesex and UC San Diego from 1982-85. In 1986 he moved to on NCSU as a Visiting/Adjunct Associate Professor in ECE and MS&E. Subsequently, he was employed as a Senior Research Scientist at the Kobe Steel Materials Research Center in RTP. While at Kobe, Kumar continued as an Adjunct Professor of ECE and MS&E at NCSU, advising Ph. D. dissertation research. In 1993 he joined the Electrical Engineering Faculty at Tuskegee University from where he retired recently. At Tuskegee University, he established a Microelectronics Fabrication Lab and developed a broad research program on semiconductor devices and materials including SIMOX, SiC, Si/Ge, AlGaN/GaN, semiconducting diamond, organic semiconductor P3HT and thin film solar cell materials, such as CIS and CGS. Das holds ten US patents on diamond based devices.