Vibrations of the AFM tip relative to the mounted diamond sample are important to consider because the magnetic signal at a NV sensor from a sample on the tip varies rapidly with nanometer-scale variations in NV-tip separation. To quantify the total rms vibrations of the Magnetometer B setup with the TF-based AFM, we first engaged a sharp silicon tip-on-TF to the sample, as in Fig. 2.23(b), by the approach method described above. Once the tip is in stable tapping-mode feedback with the diamond surface, we set the DAC to output a sinusoidal signal (e.g., 1-mV ≡ 10-nm peak, 5 Hz) to the z axis of the 3-axis scanning stage. The height signal in feedback, that is the output of the PI loop, is acquired for several seconds. The Fourier transform of this signal gives a peak
at 5 Hz over a noise floor, where units are amplitude in V/√Hz. We repeat the same
measurement without the 5-Hz drive. We integrate the area under this peak, from it the noise floor under the peak, and compare the known piezo displacement (10-nm peak) to the measured voltage for a ratio r. We use r to convert the undriven measurement to a power spectrum of displacement. Integrating up to 500 Hz gives the spectrum plotted in Fig. 2.29, where we have taken a square-root to give the cumulative rms vibration from
1.0 0.8 0.6 0.4 0.2 0.0 Cumulative rms amplitude (nm) 2 3 4 5 6 7 10 2 3 4 5 6 7 100 2 3 4 5 Frequency (Hz)
Figure 2.29: Sample-tip vibration spectrum for AFM of Magnetometer B, where the y axis is total rms summed from low to high frequency. The jump at 105 Hz is due to internal feedback for position stabilization in the x, y, z scan stage. The measurement procedure is described in the main text.
dc to frequency f . The jump at 105 Hz in Fig. 2.29 is due to an closed-loop control of the x, y, z scanning stage that is internal to the commercial amplifier and unfortunately could not be tuned over a wide bandwidth range. Nevertheless, these results show that up to 60 Hz the rms vibration level is only 0.64 nm and beyond 105 Hz is 0.81 nm. Thus, over short times scales the variation in NV-sample distance due to uncontrolled vibrations is smaller than the size of the driven tuning fork oscillation at the typical drive voltage 1 mV.
Engineering near-surface
nitrogen-vacancy centers in diamond
We demonstrate nanometer-scale precision in depth control of nitrogen-vacancy centers created near the surface of diamond using a nitrogen delta-doping technique during chem- ical vapor deposition diamond growth. The delta-doped NV centers in these synthetic
diamond films display coherence times of T2 > 100 µs despite nanoscale distance to
the diamond surface. This delta-doping technique enables surface-proximate NVs for nanoscale magnetic imaging, such as of external target spins, and integration of the NVs into hybrid quantum systems. Furthermore, the ultrapure quality of the diamond films grown with isotopic carbon-12 enrichment allows the spin-decoherence-inducing effects of the surface to be distinguished from bulk effects, which is explored in subsequent chapters.
1Three figures and accompanying discussions in this chapter have been adapted from reference [43]:
K. Ohno, F. J. Heremans, L. C. Basset, B. A. Myers, D. M. Toyli, A. C. Bleszynski Jayich, C. J. Palmstrom, and D. D. Awschalom, Engineering shallow spins in diamond with nitrogen delta-doping, Appl. Phys. Lett. 101 (2012) 082413 c 2012 American Institute of Physics
3.1
Introduction
The nanoscale spatial resolution and single-spin sensitivity afforded by the diamond nitrogen-vacancy magnetometer rely upon bringing the sensing NV spin as close as prac- tically possible to the sample under study. Apart from sensing magnetic fields from spins within the diamond itself, this reduction of sensor-target separation requires the NV center to be formed just nanometers from the diamond surface. Near-surface NVs do not occur naturally in sufficient abundance, so it is necessary to employ some means of creating depth-localized spins.
The conventional method to create shallow NVs has been implantation of the dia- mond with few-keV nitrogen ions [181, 182, 40]. This process also produces vacancies within the ion range, and annealing the diamond causes the vacancies to migrate and combine with a fixed substitutional nitrogen atom. These synthetically formed NVs can be distinguished from naturally occurring ones through nuclear spin labeling with the less-abundant nitrogen-15 isotope [181]. Advantages of N implantation include the tun- ability of NV depth and concentration by ion energy and fluence and the capability to laterally localize the spins by implanting through nanofabricated apertures [182] or by focusing the incident ion beam [40].
Despite the benefits of implantation, studies of both deep and shallow (depth . 50 nm) implanted NV centers show that the coherence time of NVs vary over a wide range of a few microseconds to 10s of microseconds [182]. This reduction in coherence time,
in comparison to T2 of 100s of microseconds in bulk naturally occurring NVs [136], is
thought to be due to surface effects and the variance due to undesired vacancy-related paramagnetic defects that form and persist during implantation and annealing at certain temperatures [183]. These defects have more recently been studied in detail by correlating ensemble EPR measurements with implanted NV coherence times in the same diamond
[127]. In addition, the depth localization of nitrogen implantation suffers from increased depth variation [38] with increased incident energy as well as ion channeling that is sensi- tive to angle [184]. The issue of low yield of stable NV centers with long coherence times is a challenge for applications from ensemble-NV magnetometry to single-NV scanning probe imaging.
The drawbacks of nitrogen implantation motivated work at UCSB to develop an alter- native method to form shallow NV centers with the dual goals of maintaining consistently long coherence times and localizing the NVs to a few-nanometer-thin slice. This chapter first describes single-crystal epitaxial diamond growth using plasma-enhanced chemical vapor deposition (PE-CVD). We then describe the in situ nitrogen delta-doping technique developed to form near-surface depth-localized NVs. Finally, we summarize the results of depth localization and spin coherence properties for NVs created by delta-doping, for which complete details can be found in [43]. These engineered diamond materials mo- tivated both parts of this dissertation: 1) investigation into the nature of decoherence sources that affect near-surface NVs, and 2) practical application of near-surface NVs to nanoscale imaging.