Three issues related to the p-type layer of a light-emitting diode (LED) are presented. First, by increasing the Mg-doping level and hence the hole concentration in the p-AlGaN electron-blocking layer (EBL), particularly around the interface between the EBL and the top quantum barrier, of an LED, the polarization field in this layer can be screened for reducing the potential barrier of hole and hence enhancing the hole tunneling efficiency such that the overall LED emission efficiency is increased. The increase of Mg-doping level is implemented based on an Mg pre-flow growth technique, in which Mg source is supplied into the metalorganic chemical vapor deposition chamber for several minutes before the growth of p-AlGaN or p-GaN. It is found that by increasing the Mg doping level by ~20 times near the interface between the EBL and the top quantum barrier, LED emission intensity can be enhanced by ~9.4 times. Based on a simulation study, we observe that the energy difference between the valence band-edge and the quasi-Fermi level of hole in the EBL is reduced by increasing the Mg-doping level in this layer such that the total hole density in the quantum wells is increased for enhancing the LED emission efficiency. Second, based on the aforementioned technique, the high performance of an LED with the total p-type thickness as small as 38 nm is demonstrated. Then, the surface plasmon coupling effects, including the enhancement of internal quantum efficiency, increase of output intensity, reduction of efficiency droop, and increase of modulation bandwidth, among the thin p-type LED samples of different p-type thicknesses are compared. With a circular mesa size of 10 mm in radius, we achieve the record-high modulation bandwidth of 625.6 MHz among c-plane GaN-based LEDs. Third, p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.038 W-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. The Mg doping in a thin p-GaN layer can lead to a high Mg-doping concentration for supplying holes to the neighboring u-GaN layers. Simulations based on a one-dimensional drift diffusion charge control model and the Brooks-Herring theory of ionized impurity scattering are undertaken to first obtain the depth-dependent distributions of hole concentration, mobility, and hence resistivity. Then, weighted averaging processes are used for evaluating the effective hole concentration, mobility, and resistivity of a p-GaN/u-GaN alternating-layer nanostructure to give consistent results with the measured data.
Professor Yang received his BS and Ph.D. degrees, both in electrical engineering, from National Taiwan University and University of Illinois at Urbana-Champaign, in 1976 and 1984, respectively. After nine year service as a faculty member at the Pennsylvania State University, he returned to Taiwan in 1993 and became a faculty member in the Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, in which he is currently a distinguished professor. Professor Yang has published about 280 SCI journal papers and made more than 680 presentations at prestigious international conferences, including over 110 invited talks. His research areas include MBE and MOCVD growths of wide-band-gap semiconductor nanostructures, LED fabrication, plasmonics, and bio-photonics.