Doctor of Philosophy

dissertationLocalized electronic states, in particular defect states and charge carrier transitions into and between these states, are the microscopic origin for major efficiency limitations of semiconductor materials and devices. Investigating these defects and their physical properties, including their chemical identity, their energy and spatial distribution, and also their paramagnetic behavior (spin relaxation times, spin-dependent transitions, spin coupling parameters), can strongly improve the understanding of how defects interact with macroscopic materials' properties and thus, it can help to find and create better semiconductor materials and devices. The focus of the study presented in the following chapters was to open up experimental access to the detection of single electronic defects in condensed matter with sub-nanometer spatial resolution that simultaneously also allows for the identification of a defect's chemical identity and magneto-electronic properties like spin. First, a brief overview about a theoretically described single-spin magnetic resonance tunneling force microscopy concept is presented, for which it is proposed to observe the spin-manifold of individual defects through detection of random telegraph noise produced by spin-dependent tunneling into and out of the probe state. Then, several key requirements for the implementation of this concept are implemented and verified by utilization of a low-temperature ultra--high--vacuum scanning probe microscope. In particular, it is demonstrated that it is possible to (i) prepare a silicondioxide layer on crystalline silicon with very high (>5x10^{18} cm^{-3}) densities of silicon dangling bonds (so-called E' centers) that possess a spin-dynamics that is suitable for their utilization as spin--readout probes for the investigated spin-microscopy concept; (ii) implement a set of magnetic field field coils into the given low-temperature ultra-high-vacuum scanning-probe setup that allow for the excitation of magnetic resonance at low magnetic fields (<20mT); (iii) use this setup for the detection and imaging of individual phosphorus donor atoms, individual surface defect states, as well as charge currents that percolate through these states under appropriate bias conditions, and (iv) observe random telegraph noise of the Coulomb forces caused by individual electrons that randomly tunnel into and out of the observed highly localized surface states

An efficient and low-cost method to create high-density nitrogen-vacancy centers in CVD diamond for sensing applications

The negatively charged Nitrogen-Vacancy (NV-) center in diamond is one of the most versatile and robust quantum sensors suitable for quantum technologies, including magnetic field and temperature sensors. For precision sensing applications, densely packed NV- centers within a small volume are preferable due to benefiting from 1/N^1/2 sensitivity enhancement (N is the number of sensing NV centers) and efficient excitation of NV centers. However, methods for quickly and efficiently forming high concentrations of NV- centers are in development stage. We report an efficient, low-cost method for creating high-density NV- centers production from a relatively low nitrogen concentration based on high-energy photons from Ar+ plasma. This study was done on type-IIa, single crystal, CVD-grown diamond substrates with an as-grown nitrogen concentration of 1 ppm. We estimate an NV- density of ~ 0.57 ppm (57%) distributed homogeneously over 200 um deep from the diamond surface facing the plasma source based on optically detected magnetic resonance and fluorescence confocal microscopy measurements. The created NV-s have a spin-lattice relaxation time (T1) of 5 ms and a spin-spin coherence time (T2) of 4 us. We measure a DC magnetic field sensitivity of ~ 104 nT Hz^-1/2, an AC magnetic field sensitivity of ~ 0.12 pT Hz^-1/2, and demonstrate real-time magnetic field sensing at a rate over 10 mT s-1 using an active sample volume of 0.2 um3

Separating hyperfine from spin-orbit interactions in organic semiconductors by multi-octave magnetic resonance using coplanar waveguide microresonators

Separating the influence of hyperfine from spin-orbit interactions in spin-dependent carrier recombination and dissociation processes necessitates magnetic resonance spectroscopy over a wide range of frequencies. We have designed compact and versatile coplanar waveguide resonators for continuous-wave electrically detected magnetic resonance, and tested these on organic light-emitting diodes. By exploiting both the fundamental and higher-harmonic modes of the resonators we cover almost five octaves in resonance frequency within a single setup. The measurements with a common pi-conjugated polymer as the active material reveal small but non-negligible effects of spin-orbit interactions, which give rise to a broadening of the magnetic resonance spectrum with increasing frequency

Detection of Iron in Nanoclustered Cytochrome C Proteins Using Nitrogen-Vacancy Magnetic Relaxometry

Nitrogen-vacancy (NV) magnetometry offers an alternative tool to detect iron levels in neurons and cells with a favorable combination of magnetic sensitivity and spatial resolution. Here we employ NV-T1 relaxometry to detect Fe in cytochrome C (Cyt-C) nanoclusters. Cyt-C is a water-soluble protein that contains a single heme group and plays a vital role in the electron transport chain of mitochondria. Under ambient conditions, the heme group remains in the Fe+3 paramagnetic state. We perform NV-T1 relaxometry on a functionalized diamond chip and vary the concentration of Cyt-C from 6 uM to 54 uM, resulting in a decrease of T1 from 1.2 ms to 150 us, respectively. This reduction is attributed to spin-noise originating from the Fe spins present within the Cyt-C. We perform relaxometry imaging of Cyt-C proteins on a nanostructured diamond chip by varying the density of adsorbed iron from 1.44 x 10^6 to 1.7 x 10^7 per um^2