This thesis presents research on improving spin coherence times of bulk and single NV centres in CVD diamond via material engineering, alongside electrical readout of the NV electronic ground state. A chemical-mechanical polishing (CMP) technique is investigated to remove sub-surface damage on (001) surfaces caused by scaife polishing. 13C-limited coherence times of shallow implanted NV centres were compared across samples prepared with ICP-RIE or CMP, with CMP samples showing an improvement of 340µs. The effect of co-doping with phosphorus and sulphur on charge stability, creation yield, spin coherence times, and AC sensitivity of shallow NV centres was studied. 10 CVD diamond samples were characterised using above-bandgap imaging, photoluminescent, and Raman spectroscopy. Coherence times in four samples were measured with a home-built confocal microscope. A sample implanted with 110keV32S ions at 1011cm-2 showed the best shot noise-limit to the AC sensitivity. Preferentially-oriented NV centres were produced on pseudo-(111) surfaces. (001) diamonds were nickel-etched to produce (111) trenches overgrown with nitrogen-containing diamond. Preferential orientation was evidenced via optically detected magnetic resonance under an external ⟨111⟩ magnetic field. High-density nitrogen overgrowth in trenches gave short T2 times (<2µs), while lower-density samples showed mean spin coherence times of 160µs, closer to the unetched surface. Finally, a home-built device for electrical spin-state readout is described, including a confocal microscope, metallic contacts, and a readout PCB. Photoelectrically detected magnetic resonance contrast of 63% (compared to 5% optically) is demonstrated , with initial RF device development.
This thesis presents research on improving spin coherence times of bulk and single NV centres in CVD diamond via material engineering, alongside electrical readout of the NV electronic ground state. A chemical-mechanical polishing (CMP) technique is investigated to remove sub-surface damage on (001) surfaces caused by scaife polishing. 13C-limited coherence times of shallow implanted NV centres were compared across samples prepared with ICP-RIE or CMP, with CMP samples showing an improvement of 340µs. The effect of co-doping with phosphorus and sulphur on charge stability, creation yield, spin coherence times, and AC sensitivity of shallow NV centres was studied. 10 CVD diamond samples were characterised using above-bandgap imaging, photoluminescent, and Raman spectroscopy. Coherence times in four samples were measured with a home-built confocal microscope. A sample implanted with 110keV32S ions at 1011cm-2 showed the best shot noise-limit to the AC sensitivity. Preferentially-oriented NV centres were produced on pseudo-(111) surfaces. (001) diamonds were nickel-etched to produce (111) trenches overgrown with nitrogen-containing diamond. Preferential orientation was evidenced via optically detected magnetic resonance under an external ⟨111⟩ magnetic field. High-density nitrogen overgrowth in trenches gave short T2 times (<2µs), while lower-density samples showed mean spin coherence times of 160µs, closer to the unetched surface. Finally, a home-built device for electrical spin-state readout is described, including a confocal microscope, metallic contacts, and a readout PCB. Photoelectrically detected magnetic resonance contrast of 63% (compared to 5% optically) is demonstrated , with initial RF device development.
The fluorescence of the nitrogen-vacancy (NV) centre in artificial diamond has been widely studied for applications such as quantum communication protocols, quantum computation, fluorescence-based biolabelling, and optical sensing. Optical sensing by itself encompasses a wide range of possible applications: from magnetic and electric field sensing to temperature sensing and enabling, for example, the detection of paramagnetic ions and the identification of molecules by nuclear magnetic resonance. Compared to many other systems capable of optical sensing, the NV centre works at room temperature, and its spin-dependent fluorescence emission is the key property to achieve a high sensitivity to the surrounding environment. Additionally, the NV centre emission is highly photostable, has a relatively short lifetime (~ 12ns) and spans a broad wavelength range, from the red to the near-infrared region of the electromagnetic spectrum (600 - 800nm). While the common denominator of all mentioned applications is the NV centre, the properties of the artificial diamond surrounding the NV centre have a significant impact on its sensing performance. At present, the processing of artificial diamond has not been fully optimised, and research and development towards the realisation of the `best¿ diamond sample for optical sensing is far from over. Specifically, the role of other defects in diamond and their effects on the photo-physical properties of the NV centres are not entirely understood. In this thesis, I will discuss the potential benefits of a novel processing technique to create ensembles of NV centres, and the importance that surrounding nitrogen defects has on the achievable magnetic field sensitivity. Additionally, I will show both theoretical and experimental investigations of the NV centres photo-physical processes, with focus on magnetic field sensing. I will initially explore a protocol for microwave-free magnetometry. This protocol predicts a theoretical magnetic field sensitivity <10µT/√Hz in the range 0.3 - 30mT and I will demonstrate it experimentally with a maximum sensitivity of 3.6mT/Hz. Following, I will focus on exploring techniques to increase the magnetic field sensitivity by improving the material properties of the diamond samples. I will demonstrate that the simultaneous electron irradiation and annealing improves the creation efficiency of an ensemble of NV centres by up to 117%. Finally, I will investigate how substitutional nitrogen defects in the surrounding lattice affects the charge state of the NV centre, its emission intensity, its lifetime and its magnetic field sensitivity. I will study a sample characterised by multiple regions across its volume, with nitrogen concentration changing between 10ppm and 100ppm in each region. There si an optimal region with ~ 40ppm of substitutional nitrogen which demonstrates the highest fluorescence emission, mainly from the negatively-charged NV centre, and the highest magnetic field sensitivity of 1.6 ± 0.2µT/√Hz. However, both the regions with lower and higher substitutional nitrogen concentration than the optimal region show lower emission intensity and worse magnetic field sensitivity. Following is a summary of the content in each chapter: Chapter 1 will present an introduction to the diamond material and the classification of artificial diamond samples. It will introduce the NV centre, its fluorescence properties and the process to create NV centres in diamond. The chapter will also introduce the concepts around optical magnetometry and the working principle that allows the NV centre to sense magnetic fields as well as other external quantities, such as temperature or pressure. Finally, it will include a brief overview of the defects that are found in artificial diamond or are created during the sample processing. Chapter 2 will describe the working principle of the instruments utilised in this work. I will list the details of each instrument and describe the methods used for the analysis of the data. Chapter 3 will introduce the background of modelling the fluorescence emission from NV centres. I will discuss the implementation of a computational model based on the Lindblad master equation to simulate the photo-physical dynamics of the NV centre. I will demonstrate the successful simulation of the saturation emission, fluorescence lifetime and optically detected magnetic resonance using the theoretical model. Finally, I will explore the theoretically predicted sensitivity of microwave-free magnetometry, based on the NV centre emission intensity change under an external magnetic field. Chapter 4 will demonstrate experimentally the microwave-free magnetometry technique. In relation to the study of microwave-free magnetometry, this chapter will explore an unexpected increase in emission intensity at high magnetic fields > 100mT. Combining the experimental observations and the theoretical simulation developed in the previous chapter, I will identify and discuss a phenomenological decrease in the NV centre transition rate from the triplet state to the singlet state. The chapter will conclude comparing the experimental and theoretical sensitivities associated with the microwave-free magnetometry technique. Chapter 5 will show the characterisation of a diamond sample at each step of the process to create NV centres. I will study the electron irradiation followed by separate annealing on a high-pressure high-temperature diamond sample. I will then characterise the sample employing visible and infrared absorption spectroscopy, as well as measure the fluorescence spectra and lifetime of the defects identified at each step. Chapter 6 will examine an alternative method to create NV centres in diamond. I will compare diamond samples irradiated and annealed separately with samples simultaneously irradiated and annealed (referred to as 'annealing in situ'). To compare the two processes, I will estimate the concentration of nitrogen defects and NV centres and calculate their N-to-NV conversion efficiency. Chapter 7 will discuss the effects of substitutional nitrogen on the properties of the NV centres in diamond. For this discussion, I will use a single-crystal diamond that shows the presence of 4 growth sectors. Each growth sector is characterised by a different nitrogen concentration, as I will demonstrate at the beginning of the chapter. I will focus on 4 main properties affected by the concentration of nitrogen in the different sectors: the balance between the charge states of the NV centre, the overall fluorescence intensity from each sector, the fluorescence lifetime and its relation to non-radiative processes, and the magnetic field sensitivity obtained with the optically detected magnetic resonance technique. Chapter 8 will gather the results and conclusions of the entire body of work. It will also discuss future avenues of research in the topic of diamond processing and characterisations, to improve the existing optical sensing applications.
The Nitrogen-Vacancy (NV) color center is a defect of diamond which behaves as an artificial atom hosted in a solid-state matrix. Due to its electron spin properties which can be read-out and manipulated as an elementary quantum system, the NV center has found a wide panel of applications as a qubit for quantum information and as a magnetic field sensor. However these applications require to control the properties of the NV centers and their localization. This doctoral thesis investigates methods allowing us to tailor the properties of NV centers by combining techniques for the implantation of nitrogen atoms and the plasma-assisted (CVD) synthesis of diamond.The manuscript is divided into six chapters. The first chapter summarizes the properties of the NV center which will set our objectives for the NV engineering. The second chapter will describe how arrays of NV centers can be created using Focused Ion Beam implantation. The results open a wide range of applications for the targeted creation of NV centers in diamond structures such as photonic crystals and tips. However the low kinetic energy which is required for achieving implantation within a spot of 10-nm diameter leads to shallow defects which properties are strongly affected by the sample surface. The third chapter investigates how the overgrowth of a diamond layer over implanted NV centers can remove the detrimental influence of the surface. The fourth and fifth chapters describe effective methods for NV center fabrication through the control of the CVD growth conditions of the hosting crystal. Thin layers with high NV doping can be grown and almost perfect control of the orientation of the NV axis can be achieved. With the goal to optimize the spin coherence properties, the sixth chapter investigates how the electron spin of the NV center can be protected from decoherence effects induced by magnetic noise due to the unpolarized spins in the diamond lattice.
When the word diamond comes up, the majority of people thinks about engagement rings or gemstones. In fact, it is additionally, one of the most promising host materials for the implementation of advanced technology, exploiting the nature of quantum physics. Such quantum technologies have disruptive potential, ones the quantum effects can be applied in macroscopic, commercial products. The nitrogen-vacancy (NV) centre in diamond is a promising candidate to perform such tasks in the fields of quantum computation, quantum cryptography or metrology. Especially in the field of quantum sensing, the NV sensor impresses with nano-scale spatial resolution paired with high sensitivity. The electronic spin of the atomic-sized point defect can be initialised, manipulated and read-out at ambient conditions and coherence lifetimes are comparably long. Diamond magnetometers based on negatively charged NV centres have shown to be very attractive, and the sensitivity goes down to the single-atom level. In order to exploit such a high sensitivity level for sensing prospects, in this thesis, the NV centres are fabricated well confined in a certain distance to the diamond surface, since the detectable field usually decays with the third power of the distance between sensor and target. The surface depicts a fracture of the spin wise protecting diamond lattice and charge fluctuations affect the stability of NV centres, which are overcome by diamond surface termination. In order to improve the sensitivity further, the sensor density, measurement contrast and interaction time are increased. The nitrogen doping technique, which is applied during a plasma-enhanced chemical vapour deposition (PECVD) diamond growth process is used to fabricate NV centre ensembles aligned along one diamond crystal axis, thereby improving the measurement contrast by a factor up to four. The interaction time limiting spin bath is analysed using the sensor capabilities of the NVs, which leads to enhanced growth strategies and the fabrication of sensitivity optimised NV centre ensembles for magnetometry applications.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
Mechanical polishing of diamond is known to be detrimental to the spin coherence time and strain environment of near-surface defects via intrinsic introduction of subsurface damage: this damage is typically removed by inductively coupled plasma reactive ion etching (ICP-RIE). By utilizing a chemical mechanical polishing (CMP) process to prepare ⟨001⟩ diamond surfaces, we demonstrate that we can achieve 13C-limited spin lifetimes of shallow implanted (≤34 nm) nitrogen vacancy (NV) centers in an industrially scalable process. We compare spin lifetimes (T2) of three diamonds processed with CMP with one processed by ICP-RIE and observe an increased median T2 of 340 μs in the CMP-processed samples for 15NV centers implanted and annealed under identical conditions.
C-enriched diamond aligned along the [111] crystal axis. The preferentially-aligned NV centres are fabricated by a Plasma Enhanced Chemical Vapour Deposition (PECVD) process and their concentration is quantitatively determined by analysing the confocal microscopy images. We further observe that annealing the samples at high temperature (1500 °C) in vacuum leads to a conversion of substitutional nitrogen into NV centres. This treatment also increases the coherence time of the NV centres electron spins up to 40 μs, which corresponds to enhancement of the sensitivity by a factor of three. However, this procedure also leads to a loss of the preferential alignment by 34%.