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Silicon carbide goes quantum

Carbon anti-site vacancy pairs in SiC are sufficiently bright to allow detection at the single-photon level enabling the generation of single photons at a high repetition rate, This makes them potentially useful qubits for quantum information processing and applications in photonics
Silicon carbide is a semiconductor that is now widely used in a variety of micro-electromechanical systems (MEMS), LEDs and high-power electronics.

Its technological appeal stems from the fact that it is amenable to mature, robust nanofabrication methodologies and possesses both a high Young’s modulus and excellent thermal conductivity.

To many, silicon carbide (SiC) is a material that offers few surprises. Nevertheless, the increasing need for novel materials for implementing quantum technologies and nanophotonic integrated circuits is forcing scientists to revisit several traditional materials - SiC is one of them.

The crucial step in manipulating a material in the quantum regime is the ability to modify and probe individual quantum states, which can be employed as qubits.

Until recently, only diamond has offered a solid-state platform for optically stable, room-temperature single quantum emitters.

The game has changed now, say scientists Stefania Castelletto and Hannes Kraus and their colleagues. who have isolated single emitters and identified microwave spin qubits in SiC.

The results were reported in the journal Nature Materials. The researchers, including those at the University of Sydney, identified individual defects, known as carbon anti-site vacancy pairs, in SiC that are sufficiently bright to allow detection at the single-photon level.

These emitters can generate single photons at a high repetition rate, which makes them potentially useful qubits for quantum information processing and applications in photonics. The defects have the practical benefit that they are optically active at room temperature.

Another notable advantage is their natural abundance in the host matrix. Consequently, there is no need for external ion implantation, as in the case of diamond, or for the epitaxial growth typical of III-V semiconductors.

Electron irradiation and annealing provide the material restructuring needed to optically activate the defects. It is thought that the defects are clusters comprising carbon atoms that reside in silicon sites adjacent to carbon vacancies.

In a parallel report in Nature Physics, Kraus and his colleagues describe optically induced population inversion of the spin states of another type of single defect in SiC - a silicon vacancy. This result could pave the way to SiC solid-state masers and extremely sensitive travelling-microwave amplifiers.

What's more, by exploiting the double radio optical resonance technique, the team proved that the silicon vacancy has a spin-3/2 ground state - a topic that has been under debate for many years. They are therefore able to detect an optical magnetic-resonance signature from a silicon-vacancy defect at room temperature.

The ability to focus exclusively on a known defect at room temperature in a forest of paramagnetic centres is crucial for practical and scalable engineering of sensors and solid-state devices for quantum information processing.

Missing silicon atoms in a SiC crystal can produce a steady flow of single photons when excited by a laser operating at a wavelength below the bandgap. This is shown in the figure below.

So, despite many years of intense research on SiC and its spectroscopic characterisation, why have single-photon emission and unambiguous identification of paramagnetic defects not been previously observed?

The answer to this question is rather simple. In traditional semiconductor physics, most spectroscopy is performed using above-bandgap laser excitation that predominantly excites near-gap emissions.

However, this approach cannot address deep-level states. Instead, individual defect states that reside within the forbidden energy gap must be optically accessed using sub-bandgap excitation. This is a critical issue for wide-bandgap semiconductors.

Indeed, this is the case for the infamous negatively charged nitrogen-vacancy centre in diamond, which is visible on green excitation but inaccessible by ultraviolet excitation.

SiC, not surprisingly, shows similar behaviour. Red and infrared excitation, as was used by Castelletto, Kraus and their colleagues, is not sufficiently energetic to promote electrons into the conduction band, but provides an excellent way to identify new quantum systems at room temperature.

Both works highlight the urgent need to revisit other wide-bandgap materials, including zinc oxide and AlN. Hopefully, it will not be long before more robust room-temperature quantum systems are unveiled.

The pivotal observation of single-photon sources and stimulated microwave emission from SiC complements the recent discovery of novel fluorescent SiC nanostructures (tetrapods) and ushers in a new era for this technologically important material.

Integration of single defects into nanomechanical systems will hopefully pave the way to the burgeoning field of optomechanics and enable unprecedented applications in sensing, quantum information processing and magnetometry.

At last, we may have a convincing candidate for scalable quantum devices at our fingertips.

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