Identification of the carbon antisite-vacancy pair in 4H-SiC

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

Quantum Information and Algorithms for Correlated Quantum Matter

Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from correlations. It is the promise of quantum materials for quantum information science superimposed with the potential of new computational quantum algorithms to discover new quantum materials that inspires this Review. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, and computing. Simultaneously, efforts toward developing new quantum algorithmic approaches for quantum simulation and advanced calculation methods for many-body quantum systems enable major advances toward functional quantum materials and their deployment. The advent of quantum computing brings new possibilities for eliminating the exponential complexity that has stymied simulation of correlated quantum systems on high-performance classical computers. Here, we review new algorithms and computational approaches to predict and understand the behavior of correlated quantum matter. The strongly interdisciplinary nature of the topics covered necessitates a common language to integrate ideas from these fields. We aim to provide this common language while weaving together fields across electronic structure theory, quantum electrodynamics, algorithm design, and open quantum systems. Our Review is timely in presenting the state-of-the-art in the field toward algorithms with nonexponential complexity for correlated quantum matter with applications in grand-challenge problems. Looking to the future, at the intersection of quantum information science and algorithms for correlated quantum matter, we envision seminal advances in predicting many-body quantum states and describing excitonic quantum matter and large-scale entangled states, a better understanding of high-temperature superconductivity, and quantifying open quantum system dynamics.

Ligand hyperfine interactions at silicon vacancies in 4H-SiC

The negative silicon vacancy ([Formula: see text]) in SiC has recently emerged as a promising defect for quantum communication and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H-SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to [Formula: see text] in the past: the so-called isolated no-zero-field splitting (ZFS) [Formula: see text] center and another four axial configurations with small ZFS: T _V1a, T _V2a, T _V1b, and T _V2b. Due to overlapping with ²⁹Si hyperfine (hf) structures in EPR spectra of natural 4H-SiC, hf parameters of T _V1a have not been determined. Using isotopically enriched 4H-²⁸SiC, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S  =  3/2 T _V1a and T _V2a centers. The obtained EPR data support the conclusion that only T _V1a and T _V2a are related to [Formula: see text] and the two configurations of the so-called isolated no-ZFS [Formula: see text] center, [Formula: see text] (I) and [Formula: see text] (II), are actually the central lines corresponding to the transition |-1/2〉  ↔ |+1/2〉  of the T _V2a and T _V1a centers, respectively.

Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation

Defect-based color centers in wide-bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite-vacancy pairs (CAV centers) in 4H-SiC, which serve as single-photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect-based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.

Optical switching of defect charge states in 4H-SiC

We demonstrate optically induced switching between bright and dark charged divacancy defects in 4H-SiC. Photoluminescence excitation and time-resolved photoluminescence measurements reveal the excitation conditions for such charge conversion. For an energy below 1.3 eV (above 950 nm), the PL is suppressed by more than two orders of magnitude. The PL is recovered in the presence of a higher energy repump laser with a time-averaged intensity less than 0.1% that of the excitation field. Under a repump of 2.33 eV (532 nm), the PL increases rapidly, with a time constant 30 μs. By contrast, when the repump is switched off, the PL decreases first within 100–200 μs, followed by a much slower decay of a few seconds. We attribute these effect to the conversion between two different charge states. Under an excitation at energy levels below 1.3 eV, V_SiV_C⁰ are converted into a dark charge state. A repump laser with an energy above 1.3 eV can excite this charged state and recover the bright neutral state. This optically induced charge switching can lead to charge-state fluctuations but can be exploited for long-term data storage or nuclear-spin-based quantum memory.

Room temperature quantum emission from cubic silicon carbide nanoparticles

The photoluminescence (PL) arising from silicon carbide nanoparticles has so far been associated with the quantum confinement effect or to radiative transitions between electronically active surface states. In this work we show that cubic phase silicon carbide nanoparticles with diameters in the range 45-500 nm can host other point defects responsible for photoinduced intrabandgap PL. We demonstrate that these nanoparticles exhibit single photon emission at room temperature with record saturation count rates of 7 × 10(6) counts/s. The realization of nonclassical emission from SiC nanoparticles extends their potential use from fluorescence biomarker beads to optically active quantum elements for next generation quantum sensing and nanophotonics. The single photon emission is related to single isolated SiC defects that give rise to states within the bandgap.

A silicon carbide room-temperature single-photon source

Over the past few years, single-photon generation has been realized in numerous systems: single molecules, quantum dots, diamond colour centres and others. The generation and detection of single photons play a central role in the experimental foundation of quantum mechanics and measurement theory. An efficient and high-quality single-photon source is needed to implement quantum key distribution, quantum repeaters and photonic quantum information processing. Here we report the identification and formation of ultrabright, room-temperature, photostable single-photon sources in a device-friendly material, silicon carbide (SiC). The source is composed of an intrinsic defect, known as the carbon antisite-vacancy pair, created by carefully optimized electron irradiation and annealing of ultrapure SiC. An extreme brightness (2×10(6) counts s(-1)) resulting from polarization rules and a high quantum efficiency is obtained in the bulk without resorting to the use of a cavity or plasmonic structure. This may benefit future integrated quantum photonic devices.