Spectroscopy of single photon emitting defects in Gallium Nitride and Diamond
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A single photon is among the few quantum mechanical systems that are finding applications in myriad fields. The applications include serving as building blocks for the ongoing endeavour to realise faster computers and secure communication technologies. As a result, a variety of platforms are being inspected to generate single photons on-demand. Point defects and complexes in wide bandgap semiconductors such as nitrogen-vacancy (NV) and silicon-vacancy (SiV) centres in diamond, carbon antisite in Silicon Carbide (SiC), etcetera, are shown to be reliable room temperature (RT), single photon emitters (SPEs). Despite reports of several defect based SPEs in diamond and other semiconductors, the exploration continues to find ideal sources for applications. The central part of this work also focuses on the discovery and characterisation of novel SPE in the device fabrication friendly material- Gallium Nitride (GaN). The other important aspect in the study of SPEs is the method by which emitters are excited. While optical technique via laser excitation is the standard approach, electrically excited single photon generation is highly desirable for large-scale nanophotonic applications. The second part of the work investigates electrically driven fluorescence from SiV ensemble in diamond, whose properties so far, were only investigated using optical excitations. Therefore, the thesis consists of two main parts. First, the discovery as well as study of a new family of SPEs in GaN via optical excitation is covered. The second part features electrically driven characterisation of SiV centre in diamond. The RT stable, SPEs are discovered in GaN films using a confocal microscope. The emitters are off-resonantly excited using a continuous wave (cw) laser of wavelength 532 nm. The centre of wavelength in the emission spectra spans a wide range of from around 600 nm to 780 nm. Also, a significant portion of the emission comes from the characteristic, narrow zero-phonon lines (ZPLs) with the mean cryogenic and RT Full Width at Half Maximum (FWHM) of around 0.3 nm and 5 nm, respectively. The nature of the defect responsible for the emission is studied experimentally via temperature resolved spectroscopy as well as numerical modelling giving a strong indication that the emitter is a defect localised near cubic inclusions. Absorption and emission polarisation properties from the SPEs in GaN is studied in detail via polarization-resolved spectroscopy. High degree of linear, emission polarisation is observed with an average visibility of more than 90 %. The absorption polarisation measurement shows that individual emitters may have different dipole orientation. In addition, brightness measurements from several of the SPEs in GaN show the average maximum intensity of around 427 kCounts/s placing the emitters among the brightest reported so far. A three-level model describes the transition kinetics of the SPEs successfully which explains some of the observed properties of the emitters such as photon statistics. A small number of the SPEs in GaN show unusual photo-induced blinking. This blinking is shown to be due to a permanent change in the transition kinetics of the emitters when exposed to a laser power above a certain threshold. This is evidenced by the change in the transition kinetics observed before and after blinking of SPEs. Combining long-time autocorrelation measurement and photon statistics analysis, numerical values for power-dependent blinking behaviours are determined. The second major result in this work is the first electrically driven luminescence from the negative charge state of Silicon-Vacancy (SiV⁻). The result was directly obtained by measuring photoluminescence (PL) and electroluminescence (EL) spectra from SiV⁻ ensemble located in PIN diamond diode. The defect was incorporated into the diode via ion implantation. Further characterisation shows that the saturation behaviour under excess carrier injection yields similar results with when the defect is pumped optically by lasers. Finally, charge state switching between the negative and neutral states of the defect was also attempted by using reverse-biased PL elucidating transition dynamics of SiV centres in diamond. This work, therefore, reports new findings in the spectroscopic studies of defect based single photon emission. Furthermore, it provides detailed photophysical studies which may serve as a benchmark for future investigation of SPEs in GaN for multiple applications. The results provide new platform as well as alternative excitation approach for the application of defect based SPEs in nanophotonics.
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