Site-Dependent Properties of Quantum Emitters in Nanostructured Silicon Carbide

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Site-Dependent Properties of Quantum Emitters in Nanostructured Silicon Carbide

Tamanna Joshi and Pratibha Dev

PRX Quantum 3, 020325 – Published 5 May 2022

Abstract

Deep defects in silicon carbide ( $Si C$ ) possess atomlike electronic, spin, and optical properties, making them ideal for quantum computing and sensing applications. In these applications, deep defects are often placed within fabricated nanostructures that modify defect properties due to surface and quantum confinement effects. Thus far, theoretical studies exploring deep defects in $Si C$ have ignored these effects. Using density-functional theory, this work demonstrates site dependence of properties of bright negatively charged silicon monovacancies within a $Si C$ nanowire. It is shown that the optical properties of defects depend strongly on the hybridization of the defect states with the surface states and on the structural changes allowed by proximity to the surfaces. Additionally, analysis of the first-principles results indicates that the charge-state conversion and/or migration to thermodynamically favorable undercoordinated surface sites can deteriorate deep-defect properties. These results illustrate the importance of considering how finite-size effects tune defect properties and of creating mitigating protocols to ensure the charge-state stability of a defect within nanostructured hosts.

Received 13 September 2021
Revised 2 February 2022
Accepted 4 April 2022

DOI:https://doi.org/10.1103/PRXQuantum.3.020325

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Quantum information with solid state qubits

Electronic structure Nanowires Wide band gap systems

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Tamanna Joshi and Pratibha Dev ^*

Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA

^*pratibha.dev@howard.edu

Popular Summary

Spin-active color centers (deep-level defects) in wide bandgap semiconductors can be described as “artificial atoms”, owing to their atom-like behavior upon illumination. Since these defects are embedded within a solid-state matrix, they provide the much-needed solid state implementation of qubits (quantum bits) for use in quantum technologies such as quantum-computing and quantum- sensing. Silicon carbide is a desirable host for defect-based qubits as it is a technologically-mature semiconductor that allows for easy nanofabrication. In particular, deep defects in silicon carbide, including the negatively charged silicon vacancies, are noteworthy qubit- candidates and are being extensively studied. For most quantum applications, deep defects are placed in nanofabricated devices to improve either the sensing capabilities of the defects or to enhance the signal from an individual bright (i.e. photoluminescent), high spin defect. In nanostructured hosts, quantum confinement and surface effects come into play and modify defect properties. To date, however, theoretical studies exploring properties of deep defects in silicon carbide have ignored these effects.

In this paper, we focus on finite size effects by considering negatively-charged silicon vacancies in a silicon carbide nanowire. Using first-principles (quantum-mechanical) calculations, we demonstrate site-dependent modification of different properties of the silicon vacancies within the nanowire. Our theoretical work discovers and highlights different mechanisms that modify ground- and excited- state properties of the defects in nanostructured hosts.

In particular, we show that four factors — strain, larger structural changes upon photoexcitation as compared to the bulk, reduced symmetry and hybridization with the surface states — modify the frequencies of the quantum emission from the negatively charged silicon vacancies within the nanowire. With an improved understanding of the different mechanisms, one can use nanostructuring as a tool to tune optical properties of the defects themselves.

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Vol. 3, Iss. 2 — May - July 2022

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Images

Figure 1
The 2H- $Si C$ nanowire. (a) Side and top views of pristine and/or defect-free NW, periodic along the [0001] direction. (b) The spin-resolved density-of-states (DOS) plot for the pristine NW, showing that it is nonmagnetic. (c) The three distinct silicon sites [circled in (a)] at which the defects are created: the tetrahedrally bonded interior site (S1), the fourfold-coordinated surface site (S2), and the undercoordinated site on the surface, bonded to only three carbon atoms (S3). (d) The configuration coordinate diagram, giving the total energy as a function of the ground- (bottom) and excited-state (top) configurations within the Franck-Condon picture. It is used to emulate the photoexcitation process, which is calculated using the constrained-occupation DFT (CDFT) method.
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Figure 2
A charged silicon monovacancy in bulk 2H- $Si C$ . (a) The $C_{3 v}$ -point group symmetry of $V_{Si}^{- 1}$ . Only the nearest-neighbor (NN) carbon atoms are shown for clarity. (b) The spin-density (difference in charge densities within the two spin channels) isosurface plot (in blue), showing the spatially localized nature of the defect-induced spins. (c) Charge-density plots for the optically active defect states (minority spin) that are involved in the photoexcitation process. From left to right, the lowest-energy filled singlet state ( $a_{1}$ symmetry), an unoccupied singlet, ${\tilde{a}}_{1}$ , and one of the doubly degenerate unoccupied $e$ states. The pink (yellow) color corresponds to positive (negative) isovalues. The defect states are labeled by their respective group-theoretic representations. The black (gray) arrows in the superscripts of the labels are used to emphasize that the states are filled (empty). (d) A schematic single-particle-level diagram (not to scale), showing the defect states introduced by $V_{Si}^{- 1}$ in the ground state and the two excited states.
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Figure 3
A negatively charged silicon vacancy, $V_{Si}^{- 1}$ . (a) The site-dependent spin, spin polarization energy ( $E_{pol} = E_{nonmagnetic} - E_{magnetic}$ ), and formation energy, $Δ E^{form}$ , of the defects at different sites in the NW. The defect-formation energy at the interior of the wire is used as the reference. (b) The equilibrium geometry for $V_{Si}^{- 1}$ at the threefold-coordinated surface site, showing dimerization of carbon atoms. The lower figure is a plot of the spin-density difference (in blue), showing the distribution of the spin of the defect on atoms around $V_{Si}^{- 1}$ . (c) Band structures (from left to right) for $V_{Si}^{- 1}$ created in bulk 2H- $Si C$ , at the NW center, and the fourfold-coordinated site at the NW surface, respectively. The optically active (spin-down channel) defect states are highlighted. The Fermi energy, $E_{f}$ , is used as the reference. Charge-density plots for the optically active defect states (energy ordered) for the defect at (d) the NW center and (e) the fourfold-coordinated site on the NW surface. Also shown are donorlike surface states (SSs) that are just below the filled defect states and that show a hybrid defect-surface-state character. The pink (yellow) color corresponds to positive (negative) isovalues.
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Figure 4
The optical excitation and charge conversion of $V_{Si}^{- 1}$ . (a) A schematic diagram (not to scale) showing the photoexcitation [ $V_{Si}^{- 1} \to (V_{Si}^{- 1})^{*}$ ] and deexcitation [ $(V_{Si}^{- 1})^{*} \to$ $V_{Si}^{- 1}$ ], resulting in photoluminescence from the $V_{Si}^{- 1}$ defect in the NW. Only the filled and empty minority-spin defect states (all belonging to the $a$ representation) are shown. $(V_{Si}^{- 1})^{*}$ refers to the excited state of the defect. The donor- and acceptor-type surface states (SSs) are indicated. (b) A schematic diagram showing the excitation of an excited defect [i.e., $◯ i$ ; followed by $◯ ii$ ] in a two-photon absorption process. Scattered into the localized acceptorlike surface state, the electron can either undergo phosphorescence [marked as $◯ iii$ ], resulting in blinking, or it can migrate away from the defect, resulting in a $V_{Si}^{- 1} \to V_{Si}^{0}$ charge conversion, with the latter being a dark state. (c) A schematic diagram showing another possible route to charge conversion to the neutral state, involving a two-photon absorption process [ $◯ i$ followed by $◯ ii$ ] followed by an Auger process [marked by $◯ iii$ and $◯ iv$ ] that ionizes the defect. Here, the filled majority spin states are also shown because the spin-up electron is ionized from the defect. VB and CB stands for valence and conduction bands, respectively.
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