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

Significance 

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.

About the author

S. Gandrothula is currently visiting researcher at Solid State Lighting & Energy Electronics Center (SSLEEC), UCSB, USA. He received Ph.D  degree in 2013 form the University of Electro-Communications, Japan. His research currently focuses on GaN optical devices. Major interests include, low defect density epitaxial growth, fabrication, and characterization of group III-nitride materials and devices, including nonpolar/semipolar orientations and visible region edge emitting laser diodes, micro-LEDs and VCSELs. He has published ten refereed articles in leading journals and applied 19 US and worldwide patents.

About the author

T. Kamikawa was a visiting researcher at Solid State Lighting & Energy Electronics Center (SSLEEC), UCSB, USA. He has been working on GaN laser devices for optical storage and GaN LED for lighting for 25years. He developed, facet coating technique of high-power lasers, stress management technique. He has been focused on low defect density epitaxial growth, fabrication, and characterization of group III-nitride materials and devices, including nonpolar/semi-polar/polar orientations and visible region edge emitting laser diodes, micro-LEDs and VCSELs.

About the author

Prof. Speck’s early work focused on epitaxial oxide films on semiconductors, ferroelectric thin films, and strain relaxation in highly misfitting epitaxial systems. He has worked extensively on the materials science of GaN and related alloys. Major aspects of his work on nitrides include elucidating basic growth modes and defect generation, the development of MBE growth of GaN, and the development of nonpolar and semipolar GaN. In 2008, Speck and his longtime collaborators founded Soraa. Speck has over 725 publications in the referred archival literature.

About the author

Prof. Shuji Nakamura, Co-Director of the Solid State Lighting & Energy Electronics Center. Professor of Materials and The Cree Professor in Solid State Lighting and Display. His research includes MOCVD, HVPE, and growth and device fabrication of light-emitters based on the wide-bandgap semiconductor indium gallium nitride (InGaN). He is the recipient of the 2006 Millennium Technology Prize for his invention of revolutionary new energy-saving light sources and the 2014 Nobel Laureate in Physics for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources. He is named the 2015 Global Energy Prize recipient for the invention, commercialization and development of energy-efficient white LED lighting technology.

About the author

Prof. Steven P DenBaars, Co-Director of the Solid State Lighting & Energy Electronics Center. Professor of Materials and Electrical & Computer Engineering and The Mitsubishi Chemical Professor in Solid State Lighting & Display. His research areas are in MOCVD growth of wide-bandgap semiconductors (GaN-based) and their application to blue LEDs and laser and high power electronic devices. Amongst other prestigious awards and recognitions, Professor DenBaars is the recipient of the National Scientist Foundation Young Investigator Award in 1994 and received the Institute of Electrical and Electronics Engineering Fellow Award in 2005 and the  IEEE Aron Kressel Award in 2010. Steven DenBaars is a fellow of the National Academy of Engineering (NAE) and the National Academy of Inventors (NAI). He has over 800+ publications and over 63 patents filed.

Reference

Gandrothula, S., Kamikawa, T., Speck, J., Nakamura, S., & DenBaars, S. (2021). Study of surface roughness of lifted-off epitaxial lateral overgrown GaN layers for the n-DBR mirror of a III-nitride vertical-cavity surface emitting laserApplied Physics Express, 14(3), 031002.

Go To Applied Physics Express

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