Boron arsenide heterostructures: lattice-matched heterointerfaces and strain effects on band alignments and mobility

www.nature.com/npjcompumats ARTICLE OPEN Boron arsenide heterostructures: lattice-matched heterointerfaces and strain effects on band alignments and mobility 1234567890():,; Kyle Bushick 1,2 , Sieun Chae1,2, Zihao Deng1, John T. Heron1 and Emmanouil Kioupakis1* BAs is a III–V semiconductor with ultra-high thermal conductivity, but many of its electronic properties are unknown. This work applies predictive atomistic calculations to investigate the properties of BAs heterostructures, such as strain effects on band alignments and carrier mobility, considering BAs as both a thin film and a substrate for lattice-matched materials. The results show that isotropic biaxial in-plane strain decreases the band gap independent of sign or direction. In addition, 1% biaxial tensile strain increases the in-plane electron and hole mobilities at 300 K by >60% compared to the unstrained values due to a reduction of the electron effective mass and of hole interband scattering. Moreover, BAs is shown to be nearly lattice-matched with InGaN and ZnSnN2, two important optoelectronic semiconductors with tunable band gaps by alloying and cation disorder, respectively. The results predict type-II band alignments and determine the absolute band offsets of these two materials with BAs. The combination of the ultra-high thermal conductivity and intrinsic p-type character of BAs, with its high electron and hole mobilities that can be further increased by tensile strain, as well as the lattice-match and the type-II band alignment with intrinsically n-type InGaN and ZnSnN2 demonstrate the potential of BAs heterostructures for electronic and optoelectronic devices. npj Computational Materials (2020)6:3 ; https://doi.org/10.1038/s41524-019-0270-4 INTRODUCTION Boron arsenide (BAs) is an attractive electronic material due to its ultra-high thermal conductivity (~1300 W m–1 K–1),1–3 native ptype dopability,4 and the availability of millimeter-size single crystals as substrates for thin-film growth.3,5,6 Following the experimental validation1–3 of the theoretical prediction7,8 of its ultra-high thermal conductivity, research efforts have focused on its growth and fundamental characterization.9 These results include studies of the thermal, optical, electronic, and structural properties of BAs in both bulk and 2D forms.4,9–16 However, to fully evaluate its potential in electronic and optoelectronic applications, the properties of BAs heterostructures with other semiconductor materials, either as a thin film or as a substrate, must be investigated. Two important degrees of freedom for thin-film engineering in device architectures are mechanical strain and band alignment. Strain arising from epitaxial mismatch strongly affects the electronic properties of materials, including the band gap,17 band alignments,18 effective masses,19 and mobility.20,21 Though the epitaxial growth of BAs thin films has not been demonstrated yet, it should be feasible under growth conditions that replicate the established chemical vapor transport growth procedure. Candidate tetrahedrally bonded substrate materials for thin-film growth on the (111) plane of BAs are ZnO (3.249 Å22), GaN (3.189 Å23), or GaAs (3.997 Å24), while for (001)-oriented BAs thin films, candidate substrates are (001) oriented rutile TiO2 (4.59 Å25) or MgF2 (4.64 Å26), which result in epitaxial strains ranging from −6% to +15% (Fig. 1a). However, the effects of mechanical strain on the electronic properties of BAs are unknown. On the other hand, BAs can also be utilized as a high-thermalconductivity substrate for the epitaxial growth of other semiconductors (Fig. 1b). Specifically, the lattice constant of the (111) plane of BAs (3.380 Å1) is well matched with InGaN alloys (3.189–3.533 Å23,27,28) and disordered ZnSnN2 (~3.383 Å).29 These materials are prominent optoelectronic materials due to their direct band gap in the visible range (and the resulting strong optical absorption and emission), which can be tuned by adjusting the alloy composition (InGaN) or the degree of cation disorder (ZnSnN2),30 but they both face challenges in device applications. On the one hand, the amount of In that can be incorporated into InGaN is limited in part due to the large mismatch with the underlying substrate, typically GaN or sapphire. On the other, ZnSnN2 films are unintentionally heavily n-type in nature and require a partner p-type semiconductor in devices such as lightemitting diodes (LEDs) or solar cells. Both materials could benefit from p-type substrates with minimal lattice misfit for high-quality thin-film growth. BAs has intrinsic p-type conductivity owing to native defects such as BAs and VB as well as unintentional impurities such as C and Si,4,12 and can thus be a potential junction partner to these materials with good lattice match. However, band offsets for these hybrid structures and the assessment of their potential for device applications have not yet been determined. In this work, we aim to understand the effect of biaxial strain on the band structure, mobility, and absolute band alignments of BAs with density functional theory (DFT) calculations and explore its possible applications in semiconductor heterostructures, either as a thin film, where biaxial strain may enhance its electronic properties, or as a substrate, where it may form advantageous heterojunctions with other materials. We show that, when epitaxially grown with 1% biaxial tensile strain, both electron and hole mobilities are increased by >60%, showing promise for fast-switching and energy-efficient transistors. These results motivate BAs as a promising thin-film semiconductor. We also 1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA. 2These authors contributed equally: Kyle Bushick, Sieun Chae. *email: Published in partnership with the Shanghai Institute of Ceramics of the Chinese Academy of Sciences K. Bushick et al. 2 Fig. 1 Potential configurations of BAs in semiconductor devices. a Epitaxially grown BAs thin film as part of a transistor. Epitaxially straining BAs (inset) increases both the electron and the hole mobility. b Schematic of an optoelectronic device utilizing the junction between a thermally conducting BAs substrate for efficient heat extraction in conjunction with nearly lattice-matched direct-band-gap semiconductor films for efficient light emission and absorption. (a) (b) Tensile Compressive -2 % -1 % 0% 1% 2% (c) (100) plane (111) plane 1234567890():,; CB Band gap (eV) CB 1.15 1.30 1.44 1.58 1.78 1.50 1.29 1.01 0.89 1.32 1.51 1.57 1.65 1.66 1.63 1.58 1.55 VB VB Fig. 2 Strain effects on the band structure and absolute band positions. a Calculated band structure of unstrained BAs along X to Γ and Γ to Z using HSE hybrid functional considering spin–orbit effects. The X and Z directions are equivalent due to the cubic symmetry. b The effect of biaxial strain within the (100) plane on the band structure of BAs. T (...truncated)