Optical tuning of the diamond Fermi level measured by correlated scanning probe microscopy and quantum defect spectroscopy

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Optical tuning of the diamond Fermi level measured by correlated scanning probe microscopy and quantum defect spectroscopy

Christian Pederson, Rajiv Giridharagopal, Fang Zhao, Scott T. Dunham, Yevgeny Raitses, David S. Ginger, and Kai-Mei C. Fu

Phys. Rev. Materials 8, 036201 – Published 14 March 2024

Abstract

Quantum technologies based on quantum point defects in crystals require control over the defect charge state. Here we tune the charge state of shallow nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a hydrogenated surface with moderate optical excitation and simultaneous spectral monitoring. The loss of conductivity and change in work function due to oxidation are measured in atmosphere using conductive atomic force microscopy and Kelvin probe force microscopy (KPFM). We correlate these scanning probe measurements with optical spectroscopy of the nitrogen-vacancy and silicon-vacancy centers created via implantation 15–25 nm beneath the diamond surface and annealing. The observed charge state of the defects as a function of optical exposure demonstrates that laser oxidation provides a way to precisely tune the Fermi level over a range of at least 2.00 eV. We also observe a significantly larger oxidation rate for implanted surfaces compared to unimplanted surfaces under ambient conditions. Combined with knowledge of the electron affinity of a surface, these results suggest KPFM is a powerful, high-spatial-resolution technique to advance surface Fermi level engineering for charge stabilization of quantum defects.

Received 29 September 2023
Accepted 12 February 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.036201

Physics Subject Headings (PhySH)

Color centers

Diamond Nitrogen vacancy centers in diamond Surfaces

Fluorescence spectroscopy Scanning probe microscopy

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Christian Pederson ^1,*, Rajiv Giridharagopal², Fang Zhao³, Scott T. Dunham⁴, Yevgeny Raitses⁵, David S. Ginger ^2,6, and Kai-Mei C. Fu^1,4,6

¹Department of Physics, University of Washington, Seattle, Washington, 98105, USA
²Department of Chemistry,University of Washington, Seattle, Washington, 98105, USA
³Department of Physics, Princeton University, Princeton, New Jersey, 08544, USA
⁴Department of Electrical and Computer Engineering, University of Washington, Seattle, Washington, 98105, USA
⁵Princeton Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, New Jersey 08543, USA
⁶Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

^*cpederso@uw.edu.

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Issue

Vol. 8, Iss. 3 — March 2024

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Images

Figure 1
(a) Schematic of implantation geometry. The diamond substrate is implanted through a TEM grid resulting in squares of implanted nitrogen and silicon. The squares are $28 µ m$ ( $90 µ m$ ) wide in sample A (B). (b) Schematic of the sample holder used in the plasma reactor. A large circular window on the diamond surface is exposed to the hydrogen radicals, while the edges are masked (c) Schematic of cold plasma reactor. (d) AFM topography measurements before and after the hydrogenation showing similar surface roughness. (e) Water wetting angle measurements before and after hydrogenation show the expected increase in hydrophobicity.
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Figure 2
(a) Schematic of sample with electrode. (b) C-AFM image of sample A (15-nm depth) with a 5-V sample bias. (c) Stitched FM-KPFM image of sample A taken with a 10-nm lift height. A constant $- 70$ -mV offset is applied to the right scan to match the CPD in the overlapping region. (d) Confocal photoluminescence image of sample A obtained using 1 mW of 532-nm excitation. Spectra taken at five locations across the boundary are shown below using 0.8 mW at 532 nm. (e) Total PL intensity for the three defect charge states for the spots indicated in (d).
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Figure 3
(a) Confocal PL image of sample A (15 nm) after laser-assisted oxidation. Laser-assisted oxidation is performed with 20 mW ( $1.0 \times 10^{10} {mW/cm}^{2}$ ) of 532 nm, while confocal imaging utilizes 1 mW ( $5.1 \times 10^{8} {mW/cm}^{2}$ ) of 532 nm. The entire time of exposure was 10 h. (b) AM-KPFM image of sample A after laser-assisted oxidation. (c) Time dependence of the ${SiV}^{-}$ PL intensity on sample B (25 nm) using 30 mW ( $1.5 \times 10^{10} {mW/cm}^{2}$ ) of 532 nm excitation. Colored data points correspond to the ${SiV}^{-}$ intensity from the three laser-assisted exposed regions. Inset: Representative PL spectra taken at the start and end of the laser exposure used to determine total PL intensity. (d) Confocal image of three laser-assisted exposed squares on sample B using 1 mW of 532-nm excitation. The dashed white line denotes the implantation square boundary. PL background from deeper native NV is observed throughout the implantation square (bottom half). (e) FM-KPFM image of the laser-assisted oxidized squares on sample B. (f) Photoluminescence spectra corresponding to the four marked regions in (d) and the colored data points in (c) obtained using 1 mW of 532-nm excitation.
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Figure 4
(a) Band diagram of the top 50 nm of hydrogenated and oxidized diamond surfaces using formation energies calculated in Ref. [30]. The Fermi level for hydrogenated diamond is located at approximately the valence band maximum where the 2D hole gas (2DHG) forms. The oxidized surface is shown pinned 2.4 eV above the valence band due to the presence of surface defects. (b) Left: FM-KPFM image depicting the boundary between the masked oxidized and hydrogenated diamond surface of sample A (15 nm). Right: Line scan across the unimplanted region, averaged over $2.5 µ m$ . (c) Left: FM-KPFM image depicting the square created by the longest exposure to the laser on sample B (25 nm). Right: FM-KPFM line scan across the square's boundary, averaged over $1.5 µ m$ . (d) Left: FM-KPFM image of two implantation squares of sample A after hydrogen passivation. Right: Line scan across the two implantation squares, averaged over $10 µ m$ . The implantation squares are indicated by the shaded regions.
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