We perform first-principles density functional calculations to find the migration pathway and barrier for B diffusion at the Si/SiO2 interface. For various interface models, in which crystalline α-quartz or amorphous silica (a-SiO2) is placed on Si, we examine stable and metastable configurations of B-related defects which play a role in B diffusion. While a substitutional B alone is immobile in Si, it tends to diffuse to the interface via an interstitialcy mechanism in the presence of a self-interstitial and then changes into an interstitial B in oxide via a kick-out mechanism, leaving the self-interstitial at the interface. At the defect-free interface, where bridging O atoms are inserted to remove interface dangling bonds, an interstitial B prefers to intervene between the interface Si and bridging O atoms and subsequently diffuses through the hollow space or along the network of the Si-O-Si bonds in oxide. The overall migration barriers are calculated to be 2.02–2.12 eV at the Si/α-quartz interface, while they lie in the range of 2.04 ± 0.44 eV at the Si/a-SiO2 interface, similar to that in α-quartz. The migration pathway and barrier are not significantly affected by interface defects such as suboxide bond and O protrusion, while dangling bonds in the suboxide region can increase the migration barrier by about 1.5 eV. The result that the interface generally does not hinder the B diffusion from Si to SiO2 assists in understanding the underlying mechanism for B segregation which commonly occurs at the Si/SiO2 interface.
Silicene has a two-dimensional buckled honeycomb lattice and is chemically reactive because of its mixed sp 2 −sp 3 bonding character unlike graphene. Despite recent advances in epitaxial growth, it remains a great challenge to synthesize a stable silicene layer. Here, we propose an encapsulation method, in which silicene is self-encapsulated between Si(110) layers in the cubic diamond lattice and effectively protected from reaction with environmental gases. Although Si atoms are all fourfold coordinated, selfencapsulated silicene exhibits the band topology of Dirac semimetals. In a superlattice structure, in which silicene is periodically encapsulated between Si(110) layers, we also find a topological transition from a normal semiconductor to a topological nodal line semimetal as the number of Si(110) layers increases. Our results provide insights into the design of a stable silicene layer that retains the nontrivial band topology and is useful for applications of Si-based devices.
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