Interface Structure and Electronic Properties in Cubic Boron Nitride - Diamond Heterostructures

Abstract

Heterointerfaces of cubic boron nitride (cBN) with diamond have garnered significant interest due to their ultra-wide bandgaps and small lattice mismatch ($\sim1.5$\%), offering promising advancements in high-power and high-frequency electronic devices. However, the realization of this heterointerface has been limited by challenging growth conditions and insufficient understanding of interfacial properties. In this work, we employ density-functional theory to investigate the structural and electronic properties of diamond/cBN heterostructures as a function of interfacial stoichiometry, cBN thickness, and surface termination and passivation. Formation energies and interfacial bond lengths reveal that boron-terminated heterojunctions are the most stable while abrupt nitrogen-terminated heterojunctions are least stable, but can be stabilized by carbon-mixing. Bandstructures are computed for the heterostructures using hybrid functionals, where we find the abrupt boron-terminated and nitrogen-terminated heterojunctions exhibit $p$-type and $n$-type conductivity, respectively, while carbon-mixed heterojunctions retain wide insulating bandgaps ($4.2-4.4$ eV). The effective masses of the abrupt interfaces are found to vary strongly with stoichiometry. Intriguingly, charge analysis reveals two-dimensional electron or hole gas regions with ultra-high densities on the order of $10^{14}$ cm$^{-2}$, with distinct spatial localization on either side of the interface. Band alignments show type-I and type-II band offsets tunable by interfacial composition. Further analysis of the band alignments reveals that the diamond valence bands consistently lie above the cBN valence bands by $0.25-2.1$ eV. Interestingly, the interface termination type switches the relative conduction band position of diamond relative to the cBN conduction band, exhibiting a type-I to type-II band alignment transition...

Figures (6)

• Figure 1: a) Diamond/cBN heterostructure models for the nitrogen-terminated heterostructures of 11L, 19L, 23L, 25L, and 31L cBN slab thicknesses. b) A magnified interface with carbon intermixing in the NT(50%) heterostructure is shown. Two of the interfacial nitrogen atoms have been replaced by carbon, giving a carbon-mixing of 50%. c) Nitrogen-terminated heterostructure with the bulk-like (monomer) surface termination. d) Nitrogen-terminated heterostructure with the dimer surface termination. Hydrogen, carbon, boron, and nitrogen atoms are shown in pink, brown, green, and gray circles, respectively.
• Figure 2: Formation energies of each heterostructure are shown in eV/atom, calculated with respect to each elemental bulk material as in \ref{['eq:formation_energies_cBN_diamond']} as a function of interfacial carbon composition. a) Shows the formation energies of the carbon-mixed NT heterostructures while b) shows the formation energies of the carbon-mixed BT heterostructures. The formation energies of the heterostructures of 11, 19, 23, 25, and 31L cBN slab thicknesses are indicated by blue, orange, green, red, and purple colored bars, respectively. Darker colored bars show formation energies for heterostructures with dimer cBN surface reconstructions.
• Figure 3: Strain maps and charge density isosurfaces (isosurface level 0.3 e/Å$^3$ are shown for a) NT(0%), b) NT(25%), c) NT(50%), d) NT(75%), e) BT(0%), f) BT(25%), g) BT(50%), and f) BT(75%). Atoms and bonds far from the interface are shown as translucent for clarity. The heterostructures with 23L of cBN are used.
• Figure 4: Electronic bandstructures in the $\Gamma - X - M - \Gamma$$k$-path for NT and BT heterostructures. The total bandstructure of each heterostructure is shown with thin black lines while the contribution from the interfacial layers is proportional to the opacity of the blue colored lines. a) NT(0%)-11d, b) NT(25%)-11d, c) NT(50%)-11d, and d) NT(75%)-11d bandstructures are shown. e) BT(0%)-11d, f) BT(25%)-11d, g) BT(50%)-11d, and h) BT(75%)-11d bandstructures are shown. The red dashed line depicts the Fermi level for each heterostructure. Blue and green dotted lines indicate the VBM and CBM of the interfacial bands respectively. The dimer surface reconstructions are passivated with pseudo-hydrogen adsorbates with valencies 0.5 and 1.5 for the nitrogen and boron dimer surfaces respectivelyGong2022Jia2023 to eliminate the surface states. The colorbar reflects the contribution from the interfacial layers to the total bandstructure.
• Figure 5: Shows the electron (hole) sheet charge density isosurface due to the interfacial occupied conduction (unoccupied valence) states in yellow (blue). The isosurface level is 0.004 e/Å$^3$. The first column shows the charge density isosurface for a) NT(0%), d) NT(25%), h) BT(0%), and k) BT(25%) in the $[100]$ orientation. The second column shows the charge density isosurface for b) NT(0%), e) NT(25%), i) BT(0%), and l) BT(25%) in the $[010]$ orientation. Brown, green, and gray bars represent carbon, boron, and nitrogen atoms, respectively. The third column shows the planar-averaged charge density profiles in $e/$Å for c) NT(0%), f) NT(25%), j) BT(0%), and m) BT(25%), where brown, green, and gray lines represent the carbon, boron, and nitrogen interfacial layers, respectively.