Fluorescence spectroscopy of nitrogen vacancy centers in HPHT and CVD diamonds

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Title: Fluorescence spectroscopy of nitrogen vacancy centers in HPHT and CVD diamonds
Authors: Tamang, Rajesh
Thesis advisor: Prof. Dr. Michael Reichling
Thesis referee: Prof. Dr. Wolfgang Harneit
Abstract: Diamond is a wide band gap material with many optically active defect centers. Among all, the most interesting negatively charged nitrogen vacancy (NV-) defect center in diamond has been investigated for almost two decades, often in relation to applications in quantum computing and quantum sensing. Nitrogen vacancy centers are formed by a substitutional nitrogen atom next to a vacancy trapped at an adjacent lattice position. Usually, these centers are prepared in synthetic diamond, where single substitutional nitrogen impurities are in the ideal case homogenously dispersed. To obtain bright luminescence from a sample, additional vacancies are created by electron or neutron irradiation and allowing them to diffuse to nitrogen atoms by annealing at temperature above 600 0C. However, already untreated synthetic diamond samples provide a concentration of NV centers well suited for the study of ensembles. Therefore, to investigate ensemble luminescence centers in diamond crystals, the untreated samples are sufficient. The spectral analysis allowed to clearly identify NVs by fluorescence spectroscopy in such samples. Even at room temperature, the zero-phonon line (ZPL) at 638 nm (NV-) is clearly visible and an additional photon contribution results in the characteristic shape with an overall width of about 120 nm and a maximum at ~685 nm. The broad spectral emission is one of the few drawbacks of NV fluorescence. In this thesis, I developed a conventional fluorescence detection technique, with a homebuilt sample stage which can be precisely positioned in x- and y- direction on a sub-micrometer scale. The sample is excited by laser light focused into a spot size of < 500 µm, and the fluorescence sampling is acquired within a sampling distance of 0.25 µm. Taking advantage of this, it is possible to take fluorescence sampling of an ensemble of NVs from the whole of the crystal, or from a desired section applying a fluorescence matrix methodology. Using this technique, a wide variety of CVD and HPHT synthesized diamond samples were investigated giving first-hand experience of omnipresent NV centers in diamond samples containing a nitrogen impurity concentration of less than 1 ppm (or <200 ppm). This study provides a good base for further work aiming at artificially creating near-surface NVs, which is the basis of many applications with the requirement for better sensitivity and strong coupling to the external spins. To ensure that the fluorescence detected is reliable and repeatable, extensive fluorescence measurements were performed within different matrix regions of the sample for several days, and it turned out that the fluorescence emission is identical when the excitation laser is excited at the middle of the sample. The outcome of the experiments evolved in setting a reference sample for other fluorescence measurements. This reference sample was fluorescence measured over several months, and performed identical spectrum characteristic with less than 3-5% difference in absolute fluorescence intensity. In the spectrum, the often mixed Raman line at 573 nm and the NV0 centers were resolved using higher spectrometer grating. A series of annealing studies in HPHT diamond samples was performed at UHV ambience with a base pressure at ~1 x 10-11 mbar on a sample with [N] < 200 ppm. The fluorescence examined on the sample annealed at temperature 500 0C revealed an increased fluorescence intensity, and remained at constant intensity on consecutive annealing cycles at the same temperature under the same conditions. However, at an increased temperature, the fluorescence emission increased, increasing NVs concentration in the crystal. The untreated HPHT diamond crystals varied in fluorescence characteristic feature, but the sample showed the presence of NVs. The differences in spectroscopic features were identified as due to nitrogen content and possibilities of different nitrogen defect complexes present in the crystal, and they were modified when the sample was annealed at temperatures above 500 0C. The most effective defect formation within the crystal takes place at two temperature ranges 650 –750 0C and 800– 850 0C. The calculated activaction energy at 0.22 eV and 1.26 eV are the energy of mobile interstitial atoms and that of substitutional nitrogen atoms respectively. In the process of annealing, the desorbtion of nitrogen atoms from the surface crystal has been identified by a mass spectrometer. The study contributes to the fundemental understanding of anneling effects in diamond crystals, without being bombarded by high energy electron or neutron radiation. For the creation of a high density of NV centers, annealing in UHV could be sufficient, or even controlled NVs in ultra-pure diamond. The CVD diamond crystals with [N] < 1ppm were observed to contain a high density of NVs, and had no significant change when the additional creations of NVs were attempted. Prolonged X-ray radiation followed by annealing of ultra-pure diamond ([N] <5ppb) during the XPS measurements, showed a significant impact in fluorescence intensity at the surface region confirmed by confocal measurements. However, the sensitivity of the fluorescence spectroscopy setup was not enough to observe the ZPL of the NV centers, though significant changes have been observed in the spectra. Finally, the shallow NV- creation with nitrogen ion implantation at energy of 1 keV has been confirmed by an ODMR experiment and confocal imaging.
URL: https://repositorium.ub.uni-osnabrueck.de/handle/urn:nbn:de:gbv:700-2016052614492
Subject Keywords: Diamond; nitrogen vacancy centers
Issue Date: 26-May-2016
License name: Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported
License url: http://creativecommons.org/licenses/by-sa/3.0/
Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB04 - E-Dissertationen

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