Mechanical strength and band alignment of BAs/GaN heterojunction polar interfaces: A first-principles calculation study

Recently, GaN-on-BAs has been synthesized and demonstrated as a promising architecture for efficient thermal management in GaN based high-power electronic devices with remarkably reduced thermal boundary resistance compared to that of GaN-on-diamond. In this paper, we report studies on ideal strengths and band alignments for polar BAs/GaN heterojunctions and superlattices using first-principles calculations. The results show that under normal compression, all BAs/GaN interface configurations show much higher compressive stiffness compared to that in bulk GaN [0001] direction, with the GaN softening during its structural transformation under compression markedly suppressed, which improves protection of the electronic properties under external impacts. The natural band alignments of the mismatched BAs/GaN heterojunctions are calculated by a three-step approach. Most of the heterojunction and all the superlattice interfaces show type-II staggered band offsets. Large polarization built-in electric fields are predicated in superlattice with repeatedly positive- and negative-charged N-As and Ga-B interfaces, producing a saw-tooth like dipole potential, which can effectively separate electrons and holes to different interfaces, desirable for the photocatalytic processes. Our research shows that BAs/GaN heterojunction can not only provide a much-needed alternative to GaN-on-diamond heat dissipation system in future designing of high-power electronic devices, but also offers possibilities of applications in photovoltaic and photocatalytic devices as a type-II semiconductor heterojunction or superlattice.

Figure 1

The relaxed structures of (a) wurtzite GaN, (b) zinc-blende BAs bulk crystals, respectively. The species of atoms are indicated in the inset, where the darkened atoms represent those forming the bonding interface of the heterojunction.

Figure 2

The atomic structures of the polar BAs (111)/GaN (0001) heterojunction supercells, with an inserted 40 $\AA$ vacuum space (not fully shown), for (a) B-N (interface 1), (b) B-Ga (interface 2), (c) As-Ga (interface 3) and (d) As-N interface (interface 4). The species of atoms are indicated in the inset. The layer atoms with their z coordinates fixed during each step of the incrementally tensile or compressive deformations are indicated in (a).

Figure 3

(a) Tensile and compression stress–strain curves of BAs/GaN heterojunction for different interface configurations along [0001] direction normal to the interfaces. Also shown are the stress-strain curves for bulk GaN and BAs along [0001] and [111] directions, respectively. (b) Tensile and compression stress-strain curves of slab-GaN (8L), slab-BAs (9L) and SL-1 along [0001] direction. The other three curves are the same as those in (a), plotted for an easy comparison.

Figure 4

[(a)–(d)] Variations of bond lengths in different atomic layers, as labeled in Fig. 2, at selected tensile strains for BAs/GaN heterojunctions with the interface 1-4 configurations.

Figure 5

[(a)–(c)] Snapshots of bulk GaN compressed in [0001] direction at (a) $\epsilon =0$ (equilibrium structure), (b) $\epsilon =-0.14$ (peak stress) and (c) $\epsilon =-0.2$ (structural phase transformation from P63mc (No.186) to P63/mmc (No.194)). [(d)–(f)] Snapshots of BAs/GaN heterojunction with a B-N interface compressed in [0001] direction at the same compression strains as those in (a)–(c).

Figure 6

[(a)–(d)] Variations of bond angles in different atomic layers, as labeled in Fig. 2, at selected compression strains for BAs/GaN heterojunctions with the interface 1-4 configurations before the heterojunctions collapse.

Figure 7

[(a),(b)] The orthorhombic supercells of BAs and GaN bulk crystals. [(c),(d)] The homojunction structure of BAs used to calculate the deformation potential in $a_1$ and $a_2$ directions by the three-step method, with the calculated averaged electrostatic potentials given in (e) and (f), respectively.

Figure 8

[(a)–(d)] Planar average $\overline{V}\left(z\right)$ and macroscopic average (MAEP) $\tilde{V}\left(z\right)$ of electrostatic potentials of BAs/GaN heterojunctions for interface 1-4 configurations.

Figure 9

[(a)–(d)] The band alignments without the deformation potential energies and [(e)–(h)] the band alignments with deformation potential energies for different BAs/GaN heterojunction interface 1-4 configurations.

Figure 10

(a) The superlattice structure of SL-1 with B-N and As-Ga interfaces. (b) The superlattice structure of SL-2 with B-Ga and As-N interfaces.

Figure 11

[(a),(b)] Planar average $\overline{V}\left(z\right)$ and macroscopic average (MAEP) $\tilde{V}\left(z\right)$ of electrostatic potential of the SL-1 and SL-2 superlattices. [(c),(d)] The band alignment diagrams of SL-1 and SL-2. (e) Electron density difference $\mathrm{\Delta }n$ (in unit of ${10}^{-2}/a_B^3$ with $a_B$ the Bohr radius) for SL-2 with B-Ga and As-N interface on a ($\overline{1}2\overline{1}0$) crystallographic plane. (f) The illustration of electron–hole separations in SL-2.