Isotope Substitution and Polytype Control for Point Defects Identification: The Case of the Ultraviolet Color Center in Hexagonal Boron Nitride

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key, but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>h</a:mi></a:math>-BN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>h</c:mi></c:math>-BN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>h</e:mi></e:math>-BN matrix, we unveil different optical responses to hydrostatic pressure for the nonequivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>h</g:mi></g:math>-BN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.