InGaN/GaN Quantum Well LED Based on Floating Microdisk Cavity

Although microcavity lasers with different structures have been proposed, the on-chip laser is still a critical bottleneck restricting the development of integrated optoelectronic systems. III-V semiconductors Light Emitting Diodes (LEDs) and Laser Diodes (LDs) on Si substrates, featured with suitable for large-scale and large-wafer-size manufacturing, are a convenient option for on-chip light sources. With the development of material fabrication, the quality of GaN wafers is high enough, optically pumped lasing has been realized in Si-based GaN microcavity, and more effort has been put into studying electrically pumped lasers. The reported electrically pumped lasing devices could he classified as Vertical. Cavity Surface-Emitting Lasers (VCSELs) and waveguide lasers according to the type of structures. Until now, the electrically pumped GaN lasing has also been realized in the Fabry-Perot (F-P) cavity and Whispering. Gallery Modes (WGM) cavity. Many issues, such as the optical loss between the gain materials and the substrate, the improvement of the cavity quality, and the photon-electron coupling in the cavity region, can highly influence the optical performance of the device. In general, the floating process of the device will reduce the optical loss significantly, and improving the cavity quality is a critical issue in realizing high-quality lasing. Hence, the structure design will be essential in achieving a high-quality GaN laser. In this paper, we designed and fabricated three types of electrically pumped InGaN/GaN Quantum (QW) microdisk devices to analyze and optimize their optical gain and loss and balance the coupling of the gain region and optical resonant region. The samples are fabricated using a standard microfabrication process, including photolithography, ICP etching, and wet etching based on InGaN/GaIN epitaxial wafer on Si substrate. All the devices show well-circular structures. The device I, with a planar structure, was designed with a cylindrical p-type GaN region on the inner side of the microdisk. The inner and outer radius is 95 tint and 200 tan for Device I. Device II is a floating device with the same planar structure as that of device I. Device ill was designed with a ring-shaped p-GaN region on the outside of the microdisk, and it is also designed as a floating structure. Device HI has an inner and out radius of 65 tail and 95,mu m. We define a gap between the n-GaN and p-GaN area to avoid device short circuits. For Devices II and III, through the isotropic wet etching of Si substrate, the whole LED is suspended for several micrometers. This strategy can ensure the reduction of optical loss of the cavity. The IV curves, EL spectra, and luminous images are recorded during the experiment. IV curves indicate that the turn-on voltage of device I about 18 V, and the wetting etching process will increase the turn on voltage of the device; the turn-on voltage of device III is over 21 V. Driven current-dependent EL spectra of different devices indicates that peak wavelength are located at about 408.5 nm, 408.2 nm, and 406.3 nm for device I, II and III, respectively. The EL, intensity of the microdisk device gradually increases with the increase of injection currents. FWHM of device I, and device III is in the region of 12 similar to 14 nm. What struck us was that the EL spectra are also related to the electrode region. CCD images of samples under fixed driven current indicate that the light emission mainly occurs near the electrode, but the light will be transmitted in the microcavity. Compared with others, device II can ensure that the luminous and resonance microcavity regions overlap owing to the better surface current distribution. In addition, the floating structure of the microdisk reduces the optical loss of the microdisk laser in the vertical direction and favors better light confinement. Finally, device II realizes EL emission with resonant mode under an injection current of about 0.7 mA. Considering the resonant spectra, the spectra show resonant mode at a peak wavelength of 408.2 nm and a Full Width at Half Maximum (FWHM) of 2.62 nm. The novel design of floating electrically pumped InGaN/GaN QW microdisk is significant for electrically pumped microdisk or microring laser.