Defect Engineering in Diamond-Based Semiconductors: Exploring the Role of Lithium Vacancy Defects

Understanding the properties of lithium vacancy, Li_CV, defects in diamond is crucial for optimizing diamond-based semiconductor performance and unlocking its full potential in advanced electronic applications. In this study, we present a comprehensive theoretical investigation of the neutral Li_CV defect, resulting from the interaction between a lithium substitutional and a vacancy, in diamond. Employing periodic supercell calculations, we utilize global (B3LYP and PBE0) and range-separated (HSE06) hybrid functionals, as well as a local basis set, within the CRYSTAL₁₇ code to analyze the relaxed lattice structure, formation energy, magnetic properties, and electronic structures of Li_CV and other related isolated defects (Li_C V and 2V). Our results reveal that Li_CV exhibits three distinct spin states (doublet, quartet, and sextet) with significant differences in spin density distributions on first-neighbor carbon atoms. The quartet state is energetically favored by 0.09 eV and 0.88 eV (B3LYP) compared to the doublet and sextet states, respectively. The Li atom is positioned midway between the two vacancies in the Li_CV defect, exhibiting a local D_3d symmetry. Our findings reveal that the Li_CV defect exhibits remarkable stability within the diamond matrix, with its dissociation into Li_C and a vacancy (V) demanding a substantial energy barrier of 6.07 eV. In environments rich in vacancies, Li_C shows a strong preference for binding with these vacancies, thereby facilitating the formation of Li_CV complexes. Raman scattering calculations offer intriguing insights into the Li_CV defect, revealing distinctive spectral features that significantly aid in distinguishing this particular defect among isolated lithium Li_C and vacancy (V) defects. This observation is crucial, especially considering the challenges in fabricating shallow donors involving interstitial lithium in diamond, where preexisting vacancies can significantly influence the process. The comprehensive analysis provided in this study not only deepens our understanding of the Li_CV defect and its physical properties but also enhances our ability to detect this defect during the synthetic process, and to precisely control the formation conditions. This capability is vital for optimizing the electronic properties of diamond for advanced technological applications.