Understanding the Role of Defects at Diamond/c-BN Interface: A First-Principles Study

In the realms of high-power and radio frequency electronics, diamond and cubic boron nitride (c-BN) stand out for their wide bandgaps, high thermal conductivities, high breakdown fields, and high mobilities. We have recently 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, the development of 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. Here, we present results of a first-principles study of diamond/c-BN epitaxial heterostructures. We first explore the electronic structures of different interface types, including both B and N-terminated (100) and (111) interfaces as well as the non-polar (110) interface. We analyze the band alignments for these low-index interfaces and calculate formation energies of potential substitutional, vacancy, and interstitial defects at these interfaces. Further, we analyze the stabilities of potential complexes of both multiple intrinsic defects and intrinsic defects combined with extrinsic dopants, such as Si, Mg, S, and Be, to assess the feasibility of achieving high carrier densities of both p- and n-type at the diamond/c-BN interface. We finally combine these results to determine ideal interface types, dopants, and growth conditions conducive to the formation of high-density two-dimensional electron and hole gases useful for RF FET devices. For instance, we show that B_C and C_N defects at the (110) interface induce p-type doping and lead to the formation of 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, our calculated results for the diamond/cBN(100) interface show that interface states cross the Fermi level for both C–B and C–N interfacial bonds, indicating strong p-type (type-II band alignment) and n-type (type-I band alignment) doping, respectively. Therefore, high carrier densities on the diamond surface can be achieved through proper interface engineering with c-BN. Our comprehensive analysis reveals new insights into the structure and properties of diamond/c-BN heterointerfaces and provides a detailed roadmap for engineering next-generation power and RF FETs based on diamond/c-BN heterojunctions.