Exhaustive characterization of modified Si vacancies in 4H-SiC

Abstract

The negatively charged silicon vacancy $\left(V_{Si}^{-}\right)$ in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon antisite (C_Si) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of $V_{Si}^{-}+C_{Si}$ up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.

Keywords: SiC; high-throughput; photoluminescence; point defects; silicon vacancy.

Figure 1:

Possible sites of the carbon antisites for the h silicon vacancy. The silicon atoms are colored grey and the carbons are colored brown. The equivalent atoms in C_3v symmetry are colored with the same color in each subplot. The grey isosurface is the silicon vacancy spin density. (a) The three symmetrically non-equivalent locations of the carbon antisite for the three closest configurations: $h_{3.1}^a$ (magenta), $h_{3.1}^p$ (red) and $h_{3.1}^b$ (blue); (b) a top view of the non-equivalent locations of the carbon antisite in the plane (p) with the silicon vacancy; (c) the layers above (a) and below (b) of the silicon vacancy. Additional layers above and below are denoted by primes (’) to indicate the layer distance. The color scheme is retained in subsequent plots to denote the distance between carbon antisite and silicon vacancy.

Figure 2:

Energetics and the site dependent behavior of modified silicon vacancies. (a) The formation energy (HSE results) for the closest modified vacancy configurations (colored lines) compared to the isolated silicon vacancy and carbon antisite (black and grey lines); (b) the binding energy as a function of distance between the silicon vacancy and the carbon antisite. Notice the guideline to show the oscillating behavior of the binding energy. Here, the PBE results for the two different supercell sizes are shown. (c) The spin density overlap (PBE results) for the carbon antisite on the isolated silicon vacancy defect states. Sites with large spin density overlap in (c) correspond to the local maxima in binding energy in (b).

Figure 3:

The optical data for the modified silicon vacancies. (a) The ZPL (both PBE and HSE results, the PBE results are shifted so that the isolated vacancies are align with the HSE results) and TDM for all the configurations of the modified silicon vacancies where the isolated silicon vacancies are marked with vertical dashed lines. The six closest configurations are marked with red arrows. (b) And (c) zoomed in versions close to the k and h isolated silicon vacancy, respectively.

Figure 4:

The Kohn–Sham eigenvalues (HSE results) for the modified vacancy for the (h) and (k) configurations compared with the isolated silicon defects. On the x-axis are the different configurations, where the subscript is the distance between the carbon antisite and the silicon vacancy. The dashed lines denote the eigenvalues for the occupied defect states of isolated silicon vacancy for both h and k configurations. The red and blue filled dots represent occupied states in each spin channel. Unfilled dots represent unoccupied states.

Figure 5:

ZFS for the modified silicon vacancies (HSE results). (a) The D value and (b) the E value for the six closest configurations of the modified silicon vacancies compared with the isolated silicon vacancy; (c) the D-tensor and (d) the E-tensor for the remaining configurations. The data is grouped by distinct colors to easily identify the carbon antisite distance from the silicon vacancy.

Figure 6:

PL for the high-purity semi-insulating 4H-SiC samples. (a) Show the range of 860–885 nm containing the V1 and V1′ signals and (b) show the range of 910–935 nm containing the V2 signal. The figures show there are several small additional lines next to the V1 and V2 lines. These additional lines appear in as-irradiated samples and only some faint signals remain after annealing at 800 °C. The polarization is also shown for the 400 °C annealed sample.