SiC quantum spintronics: towards quantum devices in a technological material

Silicon carbide (SiC) has recently shown to be a promising material that hosts colour centers with excellent optical and spin properties suitable for different applications in quantum technology. Among these, intrinsic defects, such as the Si vacancy and the divacancy, can be created irradiation of high-energy particles with well-controlled concentrations down to the levels that allow to isolate single emitters. However, controlling their charge states for achieving bright and stable single emitters is still a challenge. This requires controlling the charge compensation processes between residual impurities, such as the shallow N donor and shallow acceptors (Al and B), and other intrinsic defects induced by irradiation in order to tune the Fermi level. 

In this talk, an overview on defects created by electron irradiation in SiC and charge compensation processes in irradiated low-doped and undoped n-type and p-type epitaxial layers grown by chemical vapour deposition and commercial high-purity semi-insulating materials is given. Based on the data from electron paramagnetic resonance, deep level transient spectroscopy, photoluminescence and theoretical calculations, the charge state control of the Si vacancy and the divacancy and the role of the carbon vacancy in tuning the Fermi level are discussed. 

Colour centers in wide-bandgap semiconductors are attractive systems for quantum technologies since they can combine long-coherent electronic spin and bright optical properties. Several suitable centers have been identified, most famously the nitrogen-vacancy defect in diamond. However, integration in communication technology is hindered by the fact that their optical transitions lie outside telecom wavelength bands. Several transition-metal impurities in silicon carbide do emit at and near telecom wavelengths, but knowledge about their spin and optical properties is incomplete. We present all-optical identification and coherent control of molybdenum-impurity spins in silicon carbide with transitions at near-infrared wavelengths. Our results identify spin S=1/2 for both the electronic ground and excited state, with highly anisotropic spin properties that we apply for implementing optical control of ground-state spin coherence. Our results show optical lifetimes of about 60 ns and inhomogeneous spin dephasing times of about 1 microsecond, establishing relevance for quantum spin-photon interfacing.

Paramagnetic and luminescent point defects in silicon carbide (SiC) are very promising for integrating semiconductor technology and quantum technology into a single platform. In particular, divacancy defects akin to nitrogen-vacancy center in diamond show similar electronic structure but the former are more sensitive to strain and electric fields that enables to employ them for quantum sensing protocols. First principles calculations show that divacancies in SiC can be also used to polarize the surrounding nuclear spins at arbitrary direction by controlling a small, constant magnetic field. First principles calculations also revealed interesting features on the silicon-vacancy quantum bits. Silicon-vacancy quantum bits have a special electronic structure that makes the optical emission almost intact to electrical stray fields and makes it a prominent candidate for quantum communication in the near infrared region with conversion to the telecom wavelengths. Finally, a transition metal defect will be discussed, vanadium, that was originally used to compensate electrically active impurities in SiC. Vanadium has optical transition in the O-band of telecom communication and an electron spin that can be detected by traditional electron spin resonance techniques which implies favourable spin coherence times. First principles calculations indicate that the Debye-Waller factor of the optical transition is around 0.5 that makes this centre a promising candidate for quantum communication in the telecom wavelengths applied in the every-day classical communication.

Colour center defects in solids are attractive candidates for quantum information processing. Silicon carbide (SiC) has emerged as a technologically viable material for quantum technologies. It hosts defects that can be exploited both for spin-based and photonics applications. I will present our work on the electronic structure of the silicon vacancy defect in SiC and the intersystem crossing mechanism under optical drive. These results enable the design of spin-photon interfaces that can be used to generate highly entangled ‘graph’ states of photons, which are crucial for applications in quantum computation and communication.

Abstract and presentation provided by Professor Sophia Economou, Virginia Tech, USA

Optically active colour centres in silicon carbide and diamond have attracted considerable attention in the past few years as candidates for quantum applications, single photon sources, nanomagnetic resonance imaging and spintronics devices. The control of defects position and their placement at a desired location in a chip, necessary to integrate them within optical and electronic devices, is still a challenge. Recently, laser writing emerged as a new tool to generate vacancies in crystals as a starting point for the formation of colour centres. In this work, a laser writing method has been used to produce colour centres in 4H and 6H bulk silicon carbide and diamond by using a femtosecond laser with repetition rate of 200kHz. Array of colour centres were fabricated by different pulse laser energies in sites of square grids at varied depths (from surface level to 10µm below surface). We optically characterized the fabricated colour centres using confocal imaging and spectroscopy. We show that the technique can produce specifically the silicon vacancy colour centres with a relevant emission in the 900nm and other emitters in the visible range. This method can be adopted to engineer colour centres in silicon carbide for the above-mentioned applications in addition to display fabrication and light emitting diodes.

Epitaxial growth is required to obtain high quality and high purity layers of 4H-SiC with controlled doping concentration. Such layers are usually grown using horizontal hot-wall CVD reactor, developed at Linköping university in 90’s, which today is a standard reactor design being used worldwide. Off-cut substrates are used to replicate substrate’s polytype in the epilayer and smooth surface. High quality large area (150 mm diameter) substrates are commercially available and off-cut epitaxial growth is already mastered to produce thick layers suitable for electronic devices. However, epitaxial growth process is not matured enough to produce ultra-high purity epilayers with extremely low doping and complex layer structure with controlled n- and p-type doping which is one of the basic requirements for spintronics applications. This is mainly due to high temperature growth (>1600 C) which leads to the formation of high density intrinsic defects and also limits the control over residual doping and surface morphology. At Linköping University, Dr Ul-Hassan has unique possibility to grow epilayers at extremely fast growth rate (>100 m/h) which enables significantly low background doping, growth of isotopically enriched 4H-28Si12C epilayers with high thermal conductivity and no nuclear spin, and growth on on-axis substrates which leads to ultra-high purity and low defect density epilayers. In this talk, Dr Ul-Hassan will present the current status of SiC epitaxial growth and discuss different challenges and possible solutions to obtain ultra-high purity epilayers with controlled low doping using standard and non-standard growth processes on different off-cut substrates.

A revolution in power electronics is being brought about by the maturity of Silicon Carbide (SiC) as a wide bandgap semiconductor, which has been forecast to dominate the high voltage power electronics market by 2022. SiC possesses attractive qualities in addition to its high electric breakdown field, which allow it to be a material for high frequency, harsh environment and high temperature applications. When compared to Silicon, SiC power devices allow higher voltage and higher temperature operation at greatly improved efficiency.

SiC is the key to future power electronics for voltages above 600 V where Silicon undergoes severe efficiency degradation resulting in significant power losses. HVDC conversion requires bipolar transistors and diodes, PiN, BJT, GTO or IGBT devices, however, SiC unipolar SBDs and MOSFETs have already been commercialised as discrete devices and incorporated in inverter modules and so are an early indicator and true litmus test in the trend of future SiC technology. The use of SiC in Quantum applications can benefit from this massive body of work which is still progressing at an incredible pace. The different flavours of power device will be discussed in addition to how the material quality can affect the device performance. Best practices, conventional wisdoms and unknowns in the difficulty of processing such devices will also be touched on.

The growth of silicon carbide crystals is today made with three different technologies: sublimation, HTCVD and liquid phase. Sublimation is the dominating process and it produce high quality n-type conducting substrates. CVD is used for epitaxial growth and can be both n-type and p-type doped depending on need. The key properties in the n-type substrates is developed and specified to fit power device (diodes and transistors) application and for the semi-insulating substrates it is to fit for nitride epitaxy targeting high frequency HEMT devices. For applications in the quantum spintronics field it is new requirements on the substrate and the epitaxy. The talk will describe the different methods for growth of crystals and epitaxy together with the benefit or disadvantage they have for applications in the quantum spintronics. The possibility of post-growth processing to change the properties of the substrates and epitaxy will also be discussed.