Integrating Spin Defects with Thin-Film Silicon Carbide Devices

Abstract

Thin film silicon carbide (SiC) is a material used in commercial power electronics, and finding application in quantum information processing, microelectromechanical systems, photonics, and harsh environment sensors. Desirable in these applications is obtaining thin film-suspended device layers of undoped silicon carbide, which could be used as the basis of SiC-on-insulator (SiCOI) or for suspended photonics. It is an open challenge to obtain scalable thin films of single crystal silicon carbide, a challenge addressed in detail in this dissertation by implementing a dopant free photoelectrochemical (PEC) etching of SiC. In this dissertation we examine several incarnations of suspended SiC devices, and examine their potential for use in spin defect control, mechanical oscillators, and optical cavities. In initial experiments, the photoluminescence and spin properties of ensembles of color centers in silicon carbide are enhanced by fabricating optically isolated slab waveguide structures using a doped PEC process and carefully controlling annealing and cooling conditions. We find that the photoluminescence signal of an ensemble of implanted defects is enhanced in slab waveguides by an order of magnitude over identically implanted bulk defects. The slab waveguide-enhanced photoluminescence of several defect species is used to study recombination and diffusion in the presence of thermal annealing with both rapid quench cooling and a longer return to ambient conditions. The confined mechanical geometry of a thin film is exploited to measure the spin-strain coupling of the negatively charged silicon monovacancy. The methods in this work can be used to exercise greater control on near-surface emitters in silicon carbide and better understand and control the effects of strain on spin measurements of silicon carbide based color centers. On the basis of these results we conclude that the properties of defects and devices are likely strongly degraded by the use of heavily doped epilayers. Consequently, we reexamine in detail and then redesign the photoelectrochemical etching process used to suspend the devices to be compatible with a fully undoped device layer of SiC. PEC etching is a simple, rapid means of wet processing SiC, including the use of dopant selective etch stops that take advantage of mature SiC homoepitaxy. However, dopant selective photoelectrochemical etching typically relies on highly doped material, which poses challenges for device applications such as quantum defects and photonics that benefit from low doping to produce robust emitter properties and high optical transparency. In this work, we develop a selective photoelectrochemical etching process that relies not on high doping but on the electrical depletion of a fabricated diode structure, allowing the selective etching of an n-doped substrate wafer versus an undoped epitaxial (carrier density of $1(10)^{14}\unit{\cm}^{-3}$) device layer. We characterize the photo-response and photoelectrochemical etching behavior of the diode under bias and use those insights to suspend large ($100\unit{\um}\times100\unit{\um}$) undoped membranes of SiC. We further characterize the compatibility of membranes with quantum emitters, performing comparative spin spectroscopy between undoped and highly doped membrane structures, finding the use of undoped material improves ensemble spin lifetime by $>5\times$. Furthermore, we examine these use of these films in the initial incarnations of photonic crystal cavities and mechanical "trampoline" type oscillators. This work enables the fabrication of high-purity suspended thin films suitable for scalable photonics, mechanics, and quantum technologies in SiC. Next, we consider electrically driven mechanical oscillators fabricated using a front-side back-side dry etch, known as Lateral Overtone Bulk Acoustic Resonators (LOBARs) and the coupling of their dynamic strain to defect centers in the SiC. Here we report acoustic spin control of naturally occurring negatively charged silicon monovacancies in a lateral overtone bulk acoustic resonator that is based on 4H silicon carbide. We show that acoustic driving can be used at room temperature to induce coherent population oscillations. Spin acoustic resonance is shown to be useful as a frequency-tuneable probe of bulk acoustic wave resonances, highlighting the dynamical strain distribution inside a bulk acoustic wave resonator at ambient operating conditions. Our approach could be applied to the characterization of other high quality-factor micro-electro-mechanical systems and has the potential to be used in mechanically addressable quantum memory. Finally, we consider an unconventional means of introducing defects into already suspended material through the process of energetic laser irradiation (referred to here as "laser-writing"). High-yield engineering and characterization of cavity-emitter coupling is an outstanding challenge in developing scalable quantum network nodes. Ex-situ defect formation systems prevent real-time analysis, and previous in-situ methods are limited to bulk substrates or require further processing to improve emitter properties. Here, we demonstrate direct laser-writing of cavity-integrated spin defects using a nanosecond-pulsed above-bandgap laser. Photonic crystal cavities in 4H-silicon carbide serve as a nanoscope monitoring silicon monovacancy defect formation within the approximately $200\unit{\nm}^3$ cavity mode volume. We observe spin resonance, cavity-integrated photoluminescence, and excited-state lifetimes consistent with conventional defect formation methods, without need for post-irradiation thermal annealing. We further find an exponential reduction in excited-state lifetime at fluences approaching the cavity amorphization threshold and show single-shot annealing of intrinsic background defects at silicon monovacancy formation sites. This real-time in-situ method of localized defect formation, paired with cavity-integrated defect spins, is necessary towards engineering cavity-emitter coupling for quantum networking. The proceeding work establishes a basis for the fabrication of integrated quantum systems using silicon carbide and the defects therein, from the incorporation of defects through laser writing, to the creating of nanofabricated geometries using PEC etching, to the control and read-out of their ground state spins using mechanical resonators. Though the experiments herein often investigate different devices, defects, and techniques, the underlying theme is same: improving the integration of spin defects in SiC with realistic devices that could lead to their use in quantum sensing and information processing.