Kazuo UCHIDA (内田 和男)

We fabricated an AlGaN light emitting diode (LED) with a heavily Al-doped n-type ZnO layer on a p-type contact layer as a tunnel junction (TJ) to improve carrier injection into the LED. We characterized its electrical and optical properties and compared them to those of an AlGaN LED without ZnO. From the I–V characteristic of the LED with ZnO, we observed a threshold voltage of circa 2 V, which could be due to Zener breakdown of the type II heterostructure of n-ZnO/p-GaN as a TJ. From the electroluminescence measurement, we observed a similar emission peak in both AlGaN LEDs at ultraviolet (UV) wavelengths, but a broad emission band around 365 nm in the LED with ZnO. This emission could be originating from ZnO photoexcited by the UV LED emission. The dependence of these peak intensities on input currents shows that there is a monotonic increase in the light emission intensity for the UV LED emission, but a saturation behavior after the threshold voltage for the emission from the ZnO. This saturation behavior is attributed to an overflow of photoexcited electron–hole pairs into p-GaN, strongly suggesting that n-ZnO/p-GaN works as a TJ. Electroluminescence data also show that the presence of the ZnO film facilitates current spreading, which enables device operation at large input currents. Therefore, ZnO can work as a current spreading TJ layer and improve the performance of the AlGaN LED.

Abstract

GaN micro-disk cavities undercut by laser-assisted photo-electrochemical (PEC) etching are fabricated and optically characterized. The laser source used in the PEC etching is tuned to be absorbed by the InGaN/GaN superlattice beneath the GaN disk to selectively etch the superlattice. Whispering gallery modes (WGMs) in fabricated GaN micro-disk cavities are evaluated by micro-photoluminescence spectroscopy of light emission from the embedded InGaN quantum wells. Quality factors estimated for the WGMs reach approximately 6700 at blue-violet wavelengths. Detailed analysis suggests that the high-Q WGMs are the fundamental WGMs. These results indicate the high applicability of laser-assisted PEC etching to the fabrication of air-clad GaN micro-cavities.

Abstract

GaN two-dimensional (2D) photonic crystal nanocavities with a single embedded InGaN quantum well are undercut by photo-electrochemical (PEC) etching and optically characterized to investigate the fundamental mode. The PEC etching selectively removes an InGaN-based sacrificial layer to form air-suspended GaN photonic crystal cavity slabs. We investigated the resonant modes of the photonic crystal nanocavities by micro-photoluminescence spectroscopy measurement at room temperature. The wavelengths of the measured resonant peaks and their dependence on the photonic crystal period agreed well with numerical analysis, allowing us to determine the fundamental mode in the measured spectra. The highest quality factor for the fundamental mode reached 3400 at blue wavelengths. This work would contribute to the improvement of GaN 2D photonic crystal nanocavities using PEC etching as well as their applications towards integrated light sources in visible wavelengths.