4H-SiC’s a-plane orientation simplifies silicon vacancy qubit control

Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg have demonstrated a new approach to controlling silicon vacancy (VSi) qubits by fabricating a lateral pin-diode structure within 4H-SiC, but using the a-plane orientation instead of the conventional c-plane. This alignment positions the VSi dipole moment parallel to the wafer surface, enabling direct optical excitation and facilitating scalable integration with quantum photonic integrated circuits (QPICs).

The work addresses a critical need for robust, scalable quantum technologies, with potential applications spanning high-precision quantum sensing to scalable quantum computing. This architecture allows for precise electrical Stark tuning of VSi emission frequencies, a core prerequisite for generating indistinguishable photons within a scalable CMOS architecture.

4H-SiC a-Plane Orientation Enables Lateral Pin-Diode Fabrication

Lateral pin-diodes fabricated on 4H-SiC wafers oriented along the a-plane demonstrate good blocking capabilities up to -150 volts, a critical threshold for minimizing charge noise during sensitive optical measurements. These devices, created using a scalable CMOS process, also exhibit reverse leakage currents below the noise floor of the measurement setup, less than 10 picoamperes, further enhancing signal clarity. The fabrication process successfully integrates silicon vacancy (VSi) defects, positioned as localized spin qubits with robust spin-optical properties, into the diode structure.

The a-plane orientation of the 4H-SiC substrate is key to enabling direct, out-of-plane resonant optical excitation of the VSi centers, a departure from conventional approaches using c-plane wafers. This alignment allows for the creation of lateral device architectures using established semiconductor manufacturing techniques, potentially streamlining the production of quantum components.

Two-dimensional cross-sectional doping profiles were simulated using Sentaurus TCAD software on an a-plane 4H-SiC substrate with a low n-type doped epitaxial layer, confirming the feasibility of the design. The simulations aligned with experimental results, validating the modeling approach. Cryogenic optical characterization of the V2 silicon vacancies, performed at 4 Kelvin, revealed narrow optical linewidths of 69.4 MHz and 75.6 MHz. The ability to apply highly controlled electric fields parallel to the c-axis, using the lateral pin-diode architecture, enables precise Stark tuning of the emission frequency of single VSi centers.

This work addresses a need for spectral control of individual VSi emitters, allowing for distinct centers to be shifted into or out of spectral resonance. The researchers at the Chair of Electron Devices, Friedrich-Alexander-Universität Erlangen-Nürnberg and the Institute of Applied Quantum Technologies, also at Friedrich-Alexander-Universität Erlangen-Nürnberg, successfully integrated V2 silicon vacancies into these lateral pin-diodes. The successful demonstration of these devices confirms the potential of a-plane 4H-SiC for advanced quantum technologies.

VSi as In Situ Quantum Sensors in Power Electronics

The fabrication of silicon vacancy (VSi) defects within lateral pin-diode structures on a 4H-SiC a-plane wafer demonstrates a scalable CMOS-compatible process for integrating quantum sensors directly into semiconductor devices. This approach moves beyond traditional external sensing methods by embedding quantum emitters within the material itself, allowing for localized measurements of electric fields. Applying a reverse bias to the lateral pin-diode structure enabled manipulation of the VSi emission frequencies, a capability important for applications in quantum computing and integrated quantum networks.

This ability to map local electric fields in three dimensions offers a new pathway for calibrating TCAD models and identifying failure mechanisms within power electronics. Researchers used a tunable diode laser at 916 nm, set to 20 nanowatts, for resonant excitation and a 728 nm laser for off-resonant excitation, collecting emitted photons through a high numerical aperture objective.

Drive frequencies for microwave signals were generated using an Arbitrary Waveform Generator (Windfreak SynthNV PRO), demonstrating a complete optical and electrical control setup. This architecture supports the development of advanced quantum sensors and scalable quantum computing systems directly within existing semiconductor manufacturing infrastructure. The work demonstrates a significant step toward realizing in situ monitoring and optimization of power electronic devices at the nanoscale.

Lateral Pin-Diode Architecture for Stark Tuning of VSi

These devices enable precise electrical control of VSi emission, a critical step toward generating indistinguishable photons for multi-qubit entanglement. The architecture positions the VSi defects within an intrinsic region designed for localized electric field manipulation, a feature confirmed through both experimental and simulated device characteristics.

Simulations reveal a near-linear relationship between reverse bias voltage and electric field strength below -5V, simplifying the process of Stark tuning; the simulations also indicate that any built-in electric field can be neglected when considering the Stark shift, allowing for a focus on the gradient of the electric field and its impact on photoluminescence excitation (PLE) shift. Confocal photoluminescence microscopy was used to locate integrated V2 silicon vacancies within the active region of the pin-diode, identifying bright spots corresponding to color centers against the background luminescence of the p-type region.

Precise spectral linewidth measurements, conducted at 0V bias, revealed a full width at half maximum (FWHM) of 69.4 MHz for the optical A1 transition and 75.6 MHz for the A2 transition of color center No. 2. To address variations between individual emitters, the Stark effect induced by a reverse bias on the pin-diode was employed as a tuning mechanism.

The ability to precisely control the emission wavelength of individual VSi centers through an applied voltage offers a pathway toward on-chip entanglement and scalable quantum computing. The work demonstrates a significant advance in the ability to manipulate and calibrate individual qubits within a solid-state device.

TCAD Modeling Calibrated by Diode Forward Current

Two-dimensional numerical simulations within Synopsys Sentaurus TCAD were used to evaluate internal electrostatics and electric field profiles at 300 K, incorporating crystallographic anisotropy, incomplete dopant ionization and an imperfect p-type Schottky-tunneling contact. To achieve this, a parameter calibration routine was executed using an in-house Python tool employing numerical optimization algorithms, refining five interdependent physical and geometric parameters.

These included effective carrier lifetime, p-contact Schottky barrier height, effective hole tunneling mass, external parasitic series resistance and effective out-ofplane conduction width, ultimately aligning the model with observed performance. The physical structure was constructed within the process module to accurately reproduce the fabricated lateral diode geometry, with the device coordinate system aligned parallel to the crystallographic c-axis. Analysis of the Stark shift for color centers No. 1 and No. Color center No.

Optical Characterization of VSi Centers at Cryogenic Temperatures

Cryogenic optical characterization of silicon vacancy (VSi) centers within 4H-SiC a-plane substrates used a custom confocal microscope operating at 4 Kelvin. This setup employed 728 nm off-resonant and 916 nm resonant excitation, coupled with superconducting nanowire single-photon detectors to capture phonon-sideband emission exceeding 950 nm, establishing a precise methodology for analyzing defect properties. Detailed experimental parameters are available in Supplementary Information Section SI 3, outlining the rigorous approach to data acquisition and analysis.

These variations suggest limitations in current modeling capabilities to accurately capture fine-scale field variations within the semiconductor material. Application of a reverse bias successfully compensated for natural variations in the local environment surrounding individual defects, demonstrating a method for fine-tuning qubit characteristics.

Electrical Performance of Lateral Pin-Diodes for Low-Noise Operation

Lateral pin-diode devices fabricated on a-plane 4H-SiC exhibited reverse leakage currents below 10 pA, a threshold critical for minimizing charge noise during optical measurements of integrated quantum emitters. This low-noise performance stems from the device architecture, which allows for good blocking capabilities up to -150V, and is a key element in enabling precise control of silicon vacancy qubits.

Simulations of the pin-diode structure revealed a localized enhancement of the electric field at the corner of the p-type implanted region, a phenomenon termed This lateral geometry creates a complex, position-dependent electric field unlike that found in vertical pin-diode designs, with the electric field vector possessing a significant component along the crystallographic a-axis near the p-type region. However, inherent spectral variations among individual color centers, caused by their local environment and proximity to dopant atoms, initially prevented the generation of indistinguishable photons needed for multi-qubit optical interference.

Application of a DC Stark effect, facilitated by the lateral pin-diode integration, effectively compensated for these variations. Narrow optical linewidths of 69. This preservation of crystal quality is vital for maintaining the coherence of the silicon vacancy qubits, a key metric for their performance in quantum information processing. The a-plane orientation not only enabled efficient optical excitation but also positioned the VSi dipole moment for direct integration with TE-polarized planar circuits.

This work demonstrates a pathway toward controlling and reading out qubit states with integrated color centers, offering a potential solution for building scalable quantum technologies. The results suggest that a-plane 4H-SiC is a promising material for future quantum photonic integrated circuits.

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