Solid-state quantum electronics Silicon carbide as a platform for quantum communication, computing, and sensing

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Isotope-optimized silicon carbide could become an important component for future quantum computers: In addition to commercial availability on a wafer scale and very good compatibility with established CMOS technology, it can store quantum information.

Quantum technology will change the world. Based on groundbreaking research findings, numerous players from science and industry are already working on a variety of completely novel applications today. Politics has also recognized the disruptive potential of quantum technology and its societal dimension and has launched major funding programs. Thus, revolutionary technological developments in the fields of quantum optics, quantum communication, quantum sensing, and especially quantum computing are to be expected soon.

However, the quantum revolution will only take place as long as there is a practical platform technology for quantum components and quantum systems. The quantum computers that exist today are made of complicated opto-electronic setups and are very sensitive to the slightest external influences. The quantum registers used there operate only at temperatures close to absolute zero and require extremely elaborate cooling.

Another challenge is the integration with existing information and communication technology. Consequently, such large-scale solutions are of interest primarily for research purposes or for commercial cloud computing companies. In contrast, there is a high demand in industry and among small and medium-sized enterprises for their own supercomputers for a variety of complex simulation tasks and optimization problems, which could be perfectly solved by quantum computing.

Quantum bits from color centers

A possible way out of this dilemma might be provided by quantum components whose quantum bits consist of so-called color centers. Quantum bits, or qubits, represent the smallest unit of quantum information. A color center is a special atomic defect in the crystal structure where a single lattice atom is missing. Alternatively, a color center can also be a complex of a few defects in the material, where foreign atoms replace the lattice atoms. Since the defect can absorb and emit light, it is referred to as a color center. Quantum information can then be stored in the electron spin of these color centers.

Currently, diamond, a wide-bandgap (WBG) semiconductor material, is very prominent and well-researched. Diamond has excellent quantum properties, but this material is technologically difficult to handle, and connecting it to established electronics technologies is complex.

There are also efforts to realize qubits using classical silicon technology, in part in combination with germanium or graphene. The advantage: for quantum components based on silicon, the complete range of proven semiconductor processes would be available, and integration into the well-known silicon electronics would be relatively easy. However, silicon is not a wide bandgap (WBG) semiconductor and thus does not offer optimal conditions as a base material for quantum components.

Solid-state devices as game changers?

Under these circumstances, solid-state devices based on the wide-bandgap (WBG) semiconductor material silicon carbide (SiC) could pave the way for quantum technology into broad application fields. Semiconductor components based on SiC are now mass-produced. Currently, SiC devices are primarily demonstrating their qualities in the field of power electronics in practical use.

Just like in diamond, quantum information can be stored in the spins of color centers in SiC. In contrast to diamond, however, SiC combines the highly attractive quantum properties with a mature material platform. For example, the SiC platform offers commercial availability on a wafer scale, very good compatibility with established CMOS technology (CMOS: Complementary Metal-Oxide-Semiconductor), or the ability to manufacture hybrid photonic, electrical, and mechanical components.

Optimized isotope-controlled silicon carbide as a quantum memory

Color centers in SiC can be utilized for quantum information processing, quantum sensing, or quantum communication. Such applications, however, require comparatively long lifetimes of the quantum states, for instance, for performing complex computational tasks. In SiC, this can be achieved by transferring quantum information from a color center to the nuclear spins of neighboring silicon (Si) or carbon (C) atoms.

Not every Si or C atom in SiC is suitable as a quantum memory. Only certain isotopes like Si-29 and C-13, which occur in different concentrations in SiC, can be used for this purpose. In current quantum research, only isotope concentrations that naturally occur in SiC have been studied, or SiC material that does not contain such storage isotopes.

In contrast, the Fraunhofer IISB specifically focuses on optimized material quality tailored to the respective quantum application. With its long-standing expertise in the field of SiC epitaxy and highly developed process technology, the institute is capable of producing epitaxial SiC layers on SiC substrates with precisely defined isotopic concentrations. The IISB is one of the few institutions worldwide that can produce SiC material with properties specified exactly for the respective quantum application.

Placing isotope atoms: The dose matters

A key criterion in using SiC as a quantum material is the optimal number and placement of the special isotope atoms relative to the central color center. Too low an isotope concentration leads to insufficient coherence of the qubits, while too high a concentration causes overlapping signals, making the states of the qubits difficult to distinguish.

The distribution of the isotopic atoms is directly related to the isotopic concentrations in SiC. Using elaborate computer simulations, the IISB investigates which isotopic content in SiC is best suited for applications in quantum communication or quantum computing. Through a self-developed numerical algorithm, the signal quality of the nuclear spins can be determined depending on their position relative to the respective central color center.

The best isotopic positions can be found in a cloverleaf-shaped area around the color center. The "cloverleaf" must be populated by an optimal number of nuclear spins, which can be achieved by setting a specific isotopic concentration. Therefore, the key to producing SiC optimized for quantum applications lies in the precise control of isotopic concentrations.

Cooperation between research and industry: Better together!

In manufacturing optimized SiC, the Fraunhofer IISB can draw on many years of expertise in the field of epitaxy on both Si and SiC substrates. For instance, the institute in Erlangen has the world's first planetary epitaxy reactor operated by a research institution for 150- and 200-mm SiC wafers. For several years, the Fraunhofer IISB has maintained a strategic cooperation with the company Aixtron in the development of SiC facilities and processes. Aixtron is a global leading provider of equipment for SiC epitaxy using the CVD method (Chemical Vapor Deposition).

In a clean room at the IISB, both partners maintain a joint “Joint Lab” to further develop epitaxy equipment and processes together. The "Joint Labs" model allows for the realization of intensive synergies between industry and science. For the institute, the cooperation with Aixtron represents an ideal opportunity to expand its activities in the field of industrial SiC epitaxy development like few other research institutions. Aixtron, in turn, benefits from the direct integration into the IISB's clean room technology line and the extensive characterization and analysis capabilities available on-site.

Moreover, both partners can share the high technological and personnel effort required to meet the extreme quality demands on the material. Through direct collaboration with a renowned industrial company "in-house," the IISB can produce very specific epitaxial layers for "high-end" demonstrator components, which commercial providers cannot offer in the same way.

The Fraunhofer IISB expressly does not want to produce solely for its own needs, but also aims to provide other organizations access to high-quality SiC substrates and offer the research community optimal base materials for quantum applications. For this purpose, development also includes SiC substrates with alternative crystal orientations, such as the so-called "a-plane" material.

From material to system

The extensive epitaxy activities of the IISB are embedded in the institute's strategy to offer research services along the entire value chain—from semiconductor base material to power electronic systems. The technological foundation for this is a continuous and industry-compatible CMOS process line for Si wafers up to 200 mm and SiC wafers up to 150 mm in diameter.

As part of the joint initiative "Research Fab Microelectronics Germany" (FMD), this CMOS line is currently being qualified for 200-mm SiC wafers. Within the FMD, the Fraunhofer IISB has positioned itself as a center of competence for SiC and is consistently expanding its activities in this area. With the process line, the IISB can also access advanced technologies for heterointegration and structuring at the nanometer scale. The work of the IISB's Hybrid Integration department on packaging and interconnect technology, for example for extreme environmental conditions such as cryogenic environments, complements the technological portfolio.

Through its holistic approach, the IISB is able to apply its expertise and process technology from the field of power electronic SiC components to solid-state quantum electronics. Accordingly, the focus is not only on the optimized base material but also on developing technological processes for the fabrication of defined point defects or color centers in SiC.

Reinforcement for the theory and design of quantum devices in solids

Moreover, special attention is paid to the usability of the technological processes for various quantum devices and further applications in quantum technology. In this context, the long-standing cooperation with the Chair of Electronic Devices (LEB) at FAU Erlangen-Nürnberg is particularly noteworthy.

The chair of LEB, Prof. Jörg Schulze, is also the head of the Fraunhofer IISB; and recently, Prof. Roland Nagy was recruited for the chair as an expert in the theory and design of solid-state quantum components. In recognition of his research project on the realization of a SiC-based quantum computer network, Prof. Nagy recently received funding from the BMBF. The funding measure supports the establishment of an independent junior research group, with which Prof. Nagy at LEB and IISB is implementing new research approaches to SiC quantum components.

A universal technology platform for quantum electronics

The declared goal of the Fraunhofer IISB is to establish SiC as an essential platform for quantum communication, quantum computing, and quantum sensing. The advantages are manifold: SiC solid-state quantum components are compatible with the manufacturing processes of classical microelectronics based on Si, and the entire range of electronic peripherals would be available for SiC-integrated quantum components. Through a direct connection to existing technologies, quantum electronics could be seamlessly integrated into existing information systems. Since the working temperature for SiC quantum electronics is at least 1,000 times higher than that of current large-scale solutions, compact desktop setups with miniaturized cooling devices are becoming a tangible possibility in the area of quantum computing.

According to researchers, SiC as a material platform offers a realistic prospect for marketable quantum devices and for their integration into established microelectronics technologies. By linking quantum properties with electronic components, isotope-pure SiC is expected to unlock tremendous value creation potential for quantum electronics. (me)