## 5 All-dielectric-DBR GaN-based VCSELs Fabricated by Epitaxial Lateral Overgrowth  
To mitigate some of the challenges associated with existing all-dielectric-DBR GaN-based VCSELs, the present author's group developed a GaN-based VCSEL with a novel structure [14; 15; 25]. This device incorporated dielectric DBRs at both ends of the cavity, with the DBR on one side embedded in n-type gallium nitride grown by the epitaxial lateral overgrowth (ELO) technique [26]. This type of GaN-based VCSEL has several advantages, including the ability to form highly reflective dielectric DBR mirrors at both ends of the cavity and precise control of short cavity lengths during the epitaxial growth process. Figure 2 illustrates the device introduced in the present section. This unit was fabricated by first forming DBR islands consisting of 14.5 SiN/SiO\({}_{2}\) bilayers on an n-GaN substrate, after which n-type GaN seed crystals were grown in the window regions between the DBR islands (where the GaN substrate was exposed) using MOCVD. As the MOCVD process progressed, these seeds grew in size, engulfing the DBR islands and finally forming planar surfaces above them. MOCVD was then used to grow four quantum wells a p-GaN layer and a contact layer, to a total thickness of approximately 100 nm. A 30-nm-thick indium tin oxide (ITO) layer and a p-side DBR with 11.5 Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) bilayers were subsequently deposited on the contact layer and a Ti/Pt/Au n-electrode was overlaid on the  
Figure 1: (**a**) Calculated coupling (diffraction) losses per round trip for beams having Gaussian lateral profiles with a beam waist from 1 to 10 μm. (**b**) Longitudinal mode spacing calculated for the parameters n = 2.45, \(\lambda\) = 450 nm and dn/d\(\lambda\) = \(-\)0.001 [16]. bottom of the substrate. A circular current injection region with a diameter of 8 \(\upmu\)m was electrically confined by boron ion implantation [15], where implanted boron (>1E19 cm\({}^{-3}\)) nullify electrical conductivity of GaN around the aperture. The cavity region was composed of the n-GaN layer grown over the DBR islands. The thickness of this n-GaN layer governed the cavity length of the device, which could be controlled to less than several microns by applying a predetermined growth rate. Figure 3 presents a cross-sectional SEM image of the ELO structure, in which an n-side DBR island was embedded inside an n-type GaN layer. This image demonstrates that the n-side DBR was completely embedded in the n-type GaN grown by the ELO technique. No cracks or voids are evident in or around the DBR and a planar surface that provides the basal plane for the p-side mirror is seen to have been formed above the n-side DBR. The distance from the surface of the epitaxial layer to the interface between the n-type GaN and n-side DBR was determined to be 4.5 \(\upmu\)m, which corresponds to the cavity length for this device. The reflectance spectra generated from the top and bottom DBRs exhibited peak reflectance values of more than 99.9% and stopband widths of 80-97 nm. These widths are 4-5 times greater than those reported for nitride semiconductor DBRs. Figure 3: Cross-sectional scanning electron microscopy (SEM) image of the epitaxial lateral overgrowth (ELO) structure, in which the n-side DBR is engulfed in n-GaN grown using the ELO process. The flat surface of the epitaxial layer forms a 4.5-mm-long cavity. Figure 2: Schematic of an all-dielectric-distributed Bragg reflectors (DBR) gallium nitride based (GaN)-based VCSEL in which the bottom side DBR is encased in n-GaN generated using an epitaxial lateral overgrowth technique. Figure 4 plots the current versus light output (I-L) and current versus voltage (I-V) curves for the device, for which the threshold current (Ith) and density (jth) were determined to be 18 mA and 35.8 kA cm\({}^{-2}\), respectively. The maximum output power of this device was 1.1 mW. Figure 5 shows the spectra obtained below and above Ith and demonstrates a single longitudinal mode at a wavelength of 453.9 nm above Ith.