Optical microprism cavities based on dislocation-free GaN

Filip Hjort,Marcus Bengths,Åsa Haglund,Johan Gustavsson,Anders Gustafsson,Lars Samuelson,Jörgen Bengtsson,Maryam Khalilian

doi:10.1063/5.0032967

Abstract

Three-dimensional growth of nanostructures can be used to reduce the threading dislocation density that degrades III-nitride laser performance. Here, nanowire-based hexagonal GaN microprisms with flat top and bottom c-facets are embedded between two dielectric distributed Bragg reflectors to create dislocation-free vertical optical cavities. The cavities are electron beam pumped, and the quality (Q) factor is deduced from the cavity-filtered yellow luminescence. The Q factor is ∼500 for a 1000 nm wide prism cavity and only ∼60 for a 600 nm wide cavity, showing the strong decrease in Q factor when diffraction losses become dominant. Measured Q factors are in good agreement with those obtained from quasi-3D finite element frequency-domain method and 3D beam propagation method simulations. Simulations further predict that a prism cavity with a 1000 nm width will have a Q factor of around 2000 in the blue spectral regime, which would be the target regime for real devices. These results demonstrate the potential of GaN prisms as a scalable platform for realizing small footprint lasers with low threshold currents.

Highlights

GaN-based lasers are grown on free-standing GaN substrates with a dislocation density of approximately 106 cm–2 or below.[3,7] This is true for state-of-the-art blue-emitting GaN-based vertical-cavity surface-emitting lasers (VCSELs).[8,9] free-standing GaN substrates are limited in size and significantly more expensive than GaN templates on sapphire, which despite their high dislocation densities of $107–108 cm–2 are the standard substrates for commercial LEDs
These results demonstrate the potential of GaN prisms as a scalable platform for realizing small footprint lasers with low threshold currents
Three-dimensional growth of GaN nanowires is useful to reduce threading dislocations, it normally results in structures with pyramid-shaped tips[10,11,26] that are unsuitable for use in vertical cavities

Summary

Introduction

GaN-based lasers are grown on free-standing GaN substrates with a dislocation density of approximately 106 cm–2 or below.[3,7] This is true for state-of-the-art blue-emitting GaN-based vertical-cavity surface-emitting lasers (VCSELs).[8,9] free-standing GaN substrates are limited in size and significantly more expensive than GaN templates on sapphire, which despite their high dislocation densities of $107–108 cm–2 are the standard substrates for commercial LEDs. Scitation.org/journal/apl micrometer-sized hexagonal GaN prisms that instead of pyramidal tips have atomically flat top c-facets, achieved by an in situ reformation step.[27] In this paper, we demonstrate the potential of these dislocation-free hexagonal prisms to form the central building block of vertical microcavity lasers.

Results

Conclusion