Michel Bockstedte

We propose three transition-metal adatom systems on SiC surfaces as a versatile platform to realize massless Dirac fermions and flat bands with strong electronic correlations. Using density functional theory combined with the constrained random phase approximation and dynamical mean-field theory, we investigate the electronic properties of Ti, V, and Cr adatoms. The triangular surface lattices exhibit narrow bandwidths and effective two-band Hubbard models near the Fermi level, originating from partially filled adatom d orbitals. For the undoped systems our calculations reveal two distinct Mott insulating ground states. While the V lattice is a paramagnetic textbook case with large local moments, the Cr lattice, in contrast, is on the edge of a phase transition toward a flat-band Fermi liquid. The Ti lattice realizes a heavy Dirac semimetal at zero doping.

Nanoelectrical and photonic integration of quantum optical components is crucial for scalable solid-state quantum technologies. Silicon carbide stands out as a material with mature quantum defects and a wide variety of applications in semiconductor industry. Here, we study the behaviour of single silicon vacancy (V2) colour centres in a metal-semiconductor (Au/Ti/4H-SiC) epitaxial wafer device, operating in a Schottky diode configuration. We explore the depletion of free carriers in the vicinity of the defect, as well as electrical tuning of the defect optical transition lines. By detecting single charge traps, we investigate their impact on V2 optical line width. Additionally, we investigate the charge-photon-dynamics of the V2 centre and find its dominating photon-ionisation processes characteristic rate and wavelength dependence. Finally, we probe the spin coherence properties of the V2 system in the junction and demonstrate several key protocols for quantum network applications. Our work shows the first demonstration of low temperature integration of a Schottky device with optical microstructures for quantum applications and paves the way towards fundamentally scalable and reproducible optical spin defect centres in solids.

When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence analyses on the single defect level. They reveal that the three occurring spectral signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose three different crystallographic orientations in the basal plane, which relate to the projections of the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies, when applying mechanical strain. This dependency is quantitatively calibrated. A complementary electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS defect at 0.6 eV above the valence band. For a future identification, this accurate characterization of its optical and electronic properties along with their response to mechanical strain is a milestone.

Color centers in the technologically mature material silicon carbide are candidates for the implementation of quantum applications. Chargestate control and electrical read-out of qubits includes spin-to-charge conversion via optical excitation and subsequent ionization. In this work we address the dominant photoionization mechanism and the photophysical properties of the ionized silicon vacancy in 4H SiC using ab initio theory. We find that its nominally dark chargestates are infrared emitters.

Color centers in 4H-SiC and diamond are candidates for single photon sources and qubits for the implementation of quantum applications. The CI-cRPA method is able to correctly reproduce the structure of the underlying highly correlated wave functions for low and high spin multiplets. Aiming for a more complete understanding of spin-orbit mediated intersystem crossing between such states, we have extended CI-cRPA by a first order treatment of the spin-orbit interactions. We present preliminary results for intercrossing matrix elements and contributions of spin-orbit coupling to the zero field splitting of the silicon vacancy and di-vacancy in 4H-SiC, as well as the NV center in diamond using this approach.

For a reliable MOSFET performance it is crucial to identify and reduce device performance limiting 4H-SiC/SiO₂interface defects. Previous studies have focused on the quantification of the interface states density, however, the atomic defect structure is still to be investigated. Here, we introduce a new approach based on opto-electrical measurements, which allow to determine the energetic position of interface defects. The measurement routine is performed at cryogenic temperatures to suppress the thermal emission of electrons, which were trapped at interface states during the cooldown processed (from 300 K to 15 K) while a positive gate voltage of 50 V was applied. At 15 K the photon-assisted emission of trapped electrons is measured in dependence on the photon’s energy (1.8 eV-3 eV). The reduction of the threshold voltage is taken as an indicator for the amount of released charges after each photon irradiation. We found an enhanced emission efficiency at certain photon energies, especially at 2.8 eV. Near-interface-traps (NITs), reported by Afanasevet al.are located close to the measuredtrap level with E_NIT= E_{C, SiO2}−2.77(5) eV. Recently, El-Sayedet al.suggested wide-angle O-Si-Obonds as defect configuration, that act as electron traps and have a similar energy as the measure traplevel.

4H-SiC/SiO2 interface states play a major role in the performance and reliability of modern 4H-SiC metal–oxide–semiconductor field effect transistors (MOSFETs). To gain new insights into these interface states, we developed a cryogenic measurement technique that uses photon-assisted electron depopulation to probe device performance limiting 4H-SiC/SiO2 interface states. This technique enables the characterization of shallow as well as deep states at the 4H-SiC/SiO2 interface of fully processed devices using a cryogenic probe station. Our method is performed on n-channel 4H-SiC MOSFET test structures with deposited oxide and postoxidation anneal. We identify conditions under which the electrons remain trapped at 4H-SiC/SiO2 interface states and trigger the controlled photon-assisted electron depopulation within a range of photon energies. This allows us to prove the presence of near interface traps, which have previously been found in thermally grown 4H-SiC MOS structures. Our results are supported by device simulations. Additionally, we study the impact of irradiation intensity and light exposure time on the photon-induced processes.

Switching a silicon carbide (SiC) metal–oxide–semiconductor field-effect transistor between inversion and accumulation with removed drain and grounded source terminals leads to defect-assisted carrier recombination and light emission. The energy spectrum of the emitted photons provides valuable information on the involved defects, located both at the 4H-SiC/SiO2 interface and in the 4H-SiC bulk. Here, we measured and analyzed the emitted light over a broad temperature range between 12 and 297 K. Our results reveal two local maxima in light intensity around 30 and 140 K. Most importantly, the local intensity maxima and the related temperatures correlate with both the overall recombination current and gate capacitance measurements. The spectral analysis allowed us to distinguish between recombinations occurring on 4H-SiC bulk defects and 4H-SiC/SiO2 interface-related defects. We explain an initial increase of light emission with decreasing temperature to competing non-radiative pathways with activation energies of 34 and 60 meV for SiC/SiO2 interface- and 4H-SiC bulk-related emissions, respectively. Based on an extensive literature review, we link the measured photon emission to donor–acceptor pair recombination, the EH6/7 or the Z1/2 defect centers. In addition to that, we could link a prominent peak at 2.915 eV to the L1 line of the D1-center. Most importantly, we conducted our own ab initio simulations revealing that recombination via PbC-centers, previously identified with carbon dangling bonds at the 4H-SiC/SiO2 interface [Cottom et al., J. Appl. Phys. 124, 045302 (2018)], could also provide an explanation for the photon emission around 1.75 eV. Finally, our simulation of an interface-related silicon vacancy VSi,I reveals a radiative transition around 2.8 eV.