More information
Description
The project addresses the unsolved question about production technology of single-frequency semiconductor lasers emitting in visible range λ=380-530 nm. Stable single wavelength operation
with high side mode suppression ratio is required for such applications as: high-speed, last-mile communication based on plastic optical fibers, precise time measurements by atomic clocks or
advanced sensors based on interferometry. The III-nitride material family offers a high promise to fill the niche existing on the market. The reason why the nitride DFB LDs are not yet available
on the market is because of severe limitations related to inherent material properties of (In,Al,Ga)N alloys, namely: low refractive index contrast and high lattice mismatch. A few concepts to
address these issue have been reported, utilizing a photonic grating on top of LD structure. Project PI, Marta Sawicka, based on her earlier work on electrochemical etching of GaN:Si, proposes
a novel approach, namely introduction of photonic grating buried inside the LD for high coupling with optical mode. A periodic array of nanometer size air-channels inside GaN will be formed in
order to locally obtain a very high refractive index contrast. The goal of the project is to develop a combination of two technologies: ion implantation and electrochemical etching in order to
fabricate buried photonic structure (air-GaN grating) that could be located below the active region of the device. Such air-GaN grating will be integrated in a blue LD structure grown by plasmaassisted
molecular beam epitaxy in order to demonstrate novel design nitride DFB LD. Performance of such development will be investigated theoretically and characterized experimentally in
order to verify the applicability of the proposed invention.
Summary of project results
The project addresses the unsolved question about production technology of single-frequency semiconductor lasers emitting in visible range λ=380-530 nm. Stable single wavelength operation with high side mode suppression ratio is required for such applications as: high-speed, last-mile communication based on plastic optical fibers, precise time measurements by atomic clocks or advanced sensors based on interferometry. The III-nitride material family offers a high promise to fill the niche existing on the market. The reason why the nitride DFB LDs are not yet available on the market is because of severe limitations related to inherent material properties of (In,Al,Ga)N alloys, namely: low refractive index contrast and high lattice mismatch. A few concepts to address these issue have been reported, utilizing a photonic grating on top of LD structure. The project team proposed proposes a novel approach, namely introduction of photonic grating buried inside the LD for high coupling with optical mode. A periodic array of nanometer size air-channels inside GaN was formed in order to locally obtain a very high refractive index contrast. The goal of the project was to develop a combination of two technologies: ion implantation and electrochemical etching in order to fabricate buried photonic structure (air-GaN grating) that could be located below the active region of the device
In this project, a novel approach for high refractive index-contrast diffraction gratings that can be integrated within LD structures below the active region was developed. This flexible grating positioning allows for high coupling to the laser optical field. We demonstrated a fabrication process for periodic air-channels in GaN using selective area ion implantation doping, electrochemical etching (ECE), and plasma-assisted molecular beam epitaxy. Ultra-high-pressure annealing enabled efficient electrical activation of implanted Si, allowing material removal by ECE while preserving surface morphology. This new solution has been submitted for intellectual property protection. We successfully integrated submicron period air/GaN diffraction gratings within blue laser structures. The devices, compared with standard ones, showed promising characteristics. LDs with embedded air/GaN diffraction gratings operated in pulse mode and emitted at 446.0 nm. Efficient suppression of Fabry-Perot resonator modes was observed, encouraging for undertaking further development to achieve pure single-mode emission. The project results demonstrated the potential of air-GaN embedded diffraction gratings for innovative DFB LD designs.
Knowing the challenges in the fabrication of compact nitride distributed feedback laser diodes we proposed an alternative concept to the ones already reported in the literature. The idea was based on the selective area implantation doping and electrochemical etching to form air-channels, that could be positioned beneath the active region of a laser diode and when arranged in an array of specially designed periodicity, act as a diffraction grating.
We have successfully proven that the idea works and can provide new quality to the field. The technology of embedded photonic structures fabrication based on high-refractive index contrast materials: air and GaN, is in our opinion one of the most important result of the project. The proposed technology involves several steps: mask formation e.g. by e-beam lithography, ion implantation, regrowth and high-pressure annealing for efficient donor activation, electrochemical etching, LD structure growth by plasma-assisted molecular beam epitaxy and processing. Some optimization and revisions of initial plan were implemented to finally demonstrate working devices with embedded air/GaN gratings.