A potential path to realize GaN-VCSELs using epitaxial lateral overgrowth

A new approach to solve n-DBR mirror issue of the III-nitride VCSEL

GaN-based light-emitting devices are widely used in different consumer applications like displays and automotive lighting. While the fabrication technology of LEDs and edge-emitting lasers (EELs) is well developed, the fabrication technology of vertical-cavity surface-emitting lasers (VCSELs) is still under development owing to its complexities associated with the manufacturing processes. Generally, a VCSEL constitutes a pair of high-reflectivity distributed Bragg reflector (DBR) mirrors, also called p-DBR and n-DBR, for confining the photons on the resonant cavity. The two main reflectors used are dielectric or epitaxial DBRs, which have enabled the development of VCSELs for various applications. Unfortunately, the challenges associated with preparing suitable DBRs for mass production, especially the n-DBR, hinder the mass production of GaN-based VCSELs.

Although various versions of DBR mirrors have been developed, the low growth rate and complexity of the lattice-matched epitaxial DBR bilayers remains a great concern in both academia and industrial fields. Most of the existing methods for preparing DBRs can be broadly classified into two: those that require substrate removal, such as photoelectrochemical (PEC) etching and laser-induced liftoff (LLO), and those that do not. All the substrate removal methods, save for PEC etching, require smooth surface morphology to eliminate optical scattering loss, which is a challenging task. Even though PEC etching allows accurate control of cavity thickness and maintaining the surface morphology smoothness, but the etchant used in the PEC limits the crystal orientation of substrate growth. Therefore, efficient substrate removal techniques are highly desirable for improved fabrication of GaN-based VCSELs.

Recently, the feasibility of epitaxial lateral overgrowth (ELO)-based removal methods was demonstrated for non-polar and semi-polar crystal orientation epitaxial layers of GaN substrate by Gandrothula et al. (APEX, 13, 041003 (2020)). Motivated by these results, the same group from the University of California at Santa Barbara: Dr. Srinivas Gandrothula, Mr. Takeshi Kamikawa, Professor James S. Speck, Professor Shuji Nakamura and Professor Steven P. DenBaars proposed the use of low defect density wing region emanating from ELO to develop Group III-nitride flip-chip VCSELs. The original research article is currently published in the journal, Applied Physics Express.

In their approach, the research team started by studying the surface roughness of the lifted-off ELO GaN for DBR mirrors. Two types of ELO masks were fabricated. The ELO wing was very smooth because it was incorporated within the VCSEL cavity to support the n-side of the DBR. The patterned substrates were grown in a metal-organic chemical vapor deposition (MOCVD) reactor. The surface morphology of the ELO surface materials was measured after it was separated from the growth substrate.

Results showed that the interface roughness of the GaN ELO layer surface changed with the thickness and composition of the ELO mask. Specifically, masks layers terminating in Si₃N₄ or via sputtered SiO₂ with a thickness of 300 nm exhibited suitable sub-nanometer surface roughness for placing the DRB mirror. Precise control of the surface roughness was successfully achieved. SEM images illustrated the formation of very smooth DBR layers indicated the enhancement of the quality of the defect-free epitaxial material. A reflectivity of approximately 98% is achieved as a measure of smoothness on the lifted ELO wing. Furthermore, the process eliminates complicated surface smoothening steps and substrate thinning without affecting the crystal plane orientation.

In summary, a simple, reliable and robust technique for fabricating n-side DBR mirrors used to prepare VCSELs was reported. The process is advantageous because it eliminates the complex surface smoothing steps and allows for precise control of the surface morphology. In a statement to Advances in Engineering, the authors said the study will advance the VCSEL fabrication technology for mass production.