Localising color centers in silicon carbide towards quantum technologies
Creators: name: Almutairi, Ahmed Fahad M
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)
classificationCode: 401799
subject: Mechanical engineering not elsewhere classified

description: Silicon vacancy (VSi) and divacancy (VSiVC) color centers in silicon carbide (SiC) are prominent candidates for many quantum technologies due to their attractive properties, including optical interfacing, long spin coherence times, near-infrared emission as well as the potential for scalability. Realisation of these color centers-based quantum applications requires a fabrication technique with high positional accuracy and high fabrication yield. Several methods have been employed to create these color centers, including electron and neutron irradiation. They create the color centers at random locations, making it challenging to integrate them into microstructures such as waveguides, nanobeams, and photonic crystals. Using a mask with holes and ion implantation, color centers can be created in a controlled manner. However, the lithography process of the mask causes inconvenience, and the thickness of the mask restricts the depth of implantation. Recently, femtosecond laser writing techniques has been shown to create color centers in wide bandgap semiconductors. This mask-less method offers three-dimensional positioning accuracies of the order of hundreds of nanometers has been successfully used to create nitrogen vacancy (NV) color centers in diamond while maintaining excellent optical and spin properties. Therefore, this PhD research explores the use of this controllable fabrication method for forming color centers in the highly promising quantum device material SiC and, in particular, the near-infrared VSi and VSiVC color centers formed in this material. The thesis reports on work done to optimise the optical properties of these two color centres, including the investigation of thermal annealing conditions.

The first objective of this thesis is to create VSi color centers in a SiC substrate using a femtosecond laser writing tool as a controllable fabrication technique. A variety of laser pulse energies were employed to create square array grids with VSi color centers positioned at varying depths (from the surface to 10 μm below the surface) for different array positions. VSi color centers created exhibit bright and stable emission depending on the pulse energy of the femtosecond laser. The fluorescence lifetime measurements from the created color centers revealed that the longest lifetime was observed at the depth of 10 μm with the lowest laser energy, while the shortest lifetime was observed at the surface with the highest laser energy.

Following the successful creation of VSi color centers, the author of this thesis then explores the creation of the complex color center with double vacancy (VSiVC color centers) using the femtosecond laser writing method. In this stage, the creation of VSiVC color centers required the additional step of thermal annealing following the laser writing process.  A bright and stable emission from the created VSiVC color centers is observed depending on the pulse energy of the femtosecond laser. Localisation accuracy was achieved within approximately 208 nm of the desired position in the transverse plane and a depth of about 700 nm below the surface of the sample.

In the final stage of this thesis, the author extends his investigation on the effect of the thermal annealing temperature on the VSi and VSiVC color centers created by femtosecond laser writing. SiC substrates have been thermally annealed at temperatures in the range from 500 °C to 1000 °C, following the laser writing process. The annealing study demonstrated that the maximum PL intensity of VSi color centers was measured after annealing at 600 °C and that of the VSiVC color centers after annealing at 800 °C.

Overall, the author has succeeded in establishing several important results in the course of this PhD research, including the creation of the VSi and VSiVC color centers in SiC at high positioning accuracy. Detailed investigations of the thermal annealing of VSi and VSiVC color centers in SiC will contribute to a better understanding of how to optimize their optical properties for quantum applications. It is expected that the results of this PhD research will contribute to the development of SiC color centers-based quantum technology applications.