A First-Principles Study to Understand the Role of Defects at Diamond/c-BN Interface

Diamond and cubic boron nitride (c-BN) are ultrawide band gap materials with potential applications in high-power and radio frequency (RF) electronics due to their high thermal conductivities, high breakdown fields, and high mobilities. We have developed a protocol for preparing high-quality heteroepitaxial c-BN films on diamond substrates for next-generation power and RF field-effect transistors (FETs). However, developing practical electronic devices based on these heterojunctions remains challenging due to a limited understanding of the electronic structure and intrinsic interfacial defects at the diamond/c-BN interface and their impacts on dopant activation and free carrier density. In this work, we present results from a first-principles study of various diamond/c-BN interface types, including B- and N-terminated (100) and (111) interfaces, as well as the non-polar (110) interface. Our findings show that interface states cross the Fermi level for both C–B and C–N interfacial bonds, suggesting strong p-type (type-II band alignment) and n-type (type-I band alignment) doping, respectively. Notably, no interface states were observed at the (110) interfaces. We further analyze the formation energies of potential substitutional, vacancy, and interstitial defects at these low-index interfaces, which can be beneficial or detrimental to the carrier density. For instance, we show that B_C and C_N defects at the (110) interface induce p-type doping, forming a two-dimensional hole gas (2DHG) in the diamond substrate. The intrinsic electron-deficient nature of these defects and the type-II band alignment are key factors in forming a 2DHG at the interface. However, we have observed that these B_C and C_N defects can be compensated by external dopants such as O and F, which are commonly introduced unintentionally in c-BN epitaxial growth processes. This leads to more stable configurations under B-rich conditions, viz. B_CO_C, B_CO_N, C_NF_N, and C_NO_N. Moreover, we find that substitutional defects and their combination are detrimental to n-type carrier density at the C-N (100) interface. Therefore, it is essential to determine the ideal interface types, dopants, and growth conditions conducive to the formation of high-density 2D electron and hole gases. Our comprehensive analysis provides insights into the structure and properties of diamond/c-BN heterointerfaces and offers a detailed roadmap for engineering next-generation power and RF FETs.