Vanadium photoluminescence in 3C-SiC

Early investigations on vanadium in the cubic 3C-SiC polytype concluded that the luminescence of the center is not possible because the excited state is degenerate with the conduction band. However, later work refuted this notion, demonstrating a doublet in photoluminescence (PL) at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>∼</a:mo> <a:mn>1493</a:mn> <a:mo>–</a:mo> <a:mn>1495</a:mn> <a:mspace width="0.28em"/> <a:mi>nm</a:mi> </a:mrow> </a:math> , apparently associated with vanadium (V). In this work, we undertake a detailed experimental and theoretical study of the V center in 3C-SiC. We show that the vanadium PL can be seen as a recombination of an exciton bound to V, in contrast to the other two common hexagonal polytypes (4H- and 6H-SiC), where the PL is due to an interatomic transition of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>3</c:mn> <c:mi>d</c:mi> </c:mrow> </c:math> electron within the V center, according to a well-established model. We also show that the splitting observed in the PL lines is likely not inherent to the center and is due to stress present in the samples. Ideally, in a strain-free 3C-SiC material, the vanadium PL will comprise a single line, which makes the V center in this polytype attractive for applications as a qubit at moderate temperatures. This contrasts with the hexagonal polytypes where the splitting of the zero-phonon line is inherent to the center due to the crystal structure, which has a negative effect on the spin coherence at liquid helium temperature, and millikelvin temperatures are needed to improve the spin coherence.