Overcoming stacking constraints in hexagonal boron nitride via metal-organic chemical vapor deposition

Researchers from Pohang University of Science and Technology (POSTECH) and the University of Montpellier have successfully synthesized wafer-scale hexagonal boron nitride (hBN) exhibiting an AA-stacking configuration, a crystal structure previously considered unattainable. This achievement, accomplished via metal-organic chemical vapour deposition (MOCVD) on a gallium nitride (GaN) substrate, introduces a novel route for precise stacking control in van der Waals materials, impacting potential applications in quantum photonics, deep-ultraviolet (DUV) optoelectronics, and next-generation electronic devices.

The study, led by Professors Jong Kyu Kim and Si-Young Choi (POSTECH) and Guillaume Cassabois (University of Montpellier), provides key insights into the factors influencing unconventional stacking configurations. Published in Nature Materials, the findings challenge previous assumptions about stacking constraints in hBN, demonstrating that step-edge guided growth and charge incorporation are essential in stabilizing the thermodynamically unfavorable AA stacking configuration.

hBN has long been regarded as a key insulating material for 2D electronic, photonic, and quantum applications. Typically, hBN adopts an AA' stacking configuration, in which boron and nitrogen atoms alternate vertically between layers. In contrast, the AA stacking configuration―where identical atoms align vertically―has traditionally been considered unstable due to strong interlayer electrostatic repulsion.

Through detailed investigation, the research team discovered that step-edges on vicinal GaN substrates serve as nucleation sites, promoting the unidirectional alignment of hBN layers and minimizing rotational disorder. This step-edge guided growth mechanism enabled the formation of high-quality, wafer-scale AA-stacked hBN films, ensuring both structural uniformity and crystallinity required for practical electronic and photonic applications.

Furthermore, the study highlights the critical role of electronic doping through carbon incorporation during the MOCVD process. The presence of carbon introduces excess charge carriers, altering interlayer interactions and effectively mitigating the repulsive forces typically associated with AA stacking. Together, this charge-mediated stabilization and step-edge alignment constitute a previously unexplored mechanism for engineering tailored stacking sequences in van der Waals materials.

"Our research demonstrates that stacking configurations in van der Waals materials are not purely governed by thermodynamic considerations, but can instead be stabilized through substrate characteristics and charge incorporation," remarked Professor Jong Kyu Kim who led the study. "This insight significantly expands the potential for customized 2D material architectures with distinct electronic and optical properties.“

Optical characterization of the synthesized AA-stacked hBN revealed enhanced second-harmonic generation (SHG)—a hallmark of non-centrosymmetric crystal structures—indicating promising applications in nonlinear optics. Additionally, the material exhibited sharp band-edge emission in the DUV region, suggesting its potential for high-efficiency optoelectronic devices operating in the DUV spectrum.

"Achieving wafer-scale control of stacking order is an important milestone for scalable, high-performance 2D electronic and photonic systems," said Seokho Moon, a postdoctoral researcher in Professor Jong Kyu Kim’s lab and the lead author of the study. "This work highlights the versatility of MOCVD as a platform for precisely engineered van der Waals materials.“

The research was supported by the Global Ph.D. Fellowship Program and the Basic Science Research Capacity Enhancement Program (Materials Imaging & Analysis Research Center) by the Ministry of Education, the Mid-Career Researcher Program and the Nano and Materials Technology Development Program of the Ministry of Science and ICT, the Electronic Components Industry Technology Development Program of the Ministry of Trade, Industry & Energy, and Samsung Electronics.