Overcoming the stacking constraints of hexagonal boron nitride via metal organic chemical vapor deposition.

Researchers at Pohan University of Science and Technology (Postech) and the University of Montpellier have synthesized wafer-scale hexagonal boron nitride (HBN) that exhibits an AA stacking structure, a crystal structure previously considered impossible.

This achievement is achieved through metal-organic chemical vapor deposition (MOCVD) on gallium nitride (GAN) substrates, introducing a new route for accurate stacking control of van der Waals materials, affecting the potential applications of quantum photonics, deep traviolet (DUV) optics, and neighboring generation electronic devices.

The study, led by Professors John Kyu Kim, Shi Yong Choi (Postech) and Guillaume Cassabois (Montpellier University), provides important insights into factors that influence unconventional stacking composition.

This finding, published in Nature Materials, challenges previous assumptions regarding HBN stacking constraints and shows that stepwise induced growth and charge uptake are essential in stabilizing thermodynamically unfavourable AA stacking configurations.

HBN has long been considered an important insulation for 2D electron, photonic and quantum applications. Typically, HBN adopts an AA’ stacking configuration. In this configuration, boron and nitrogen atoms are employed vertically and alternately between layers. In contrast, AA stacking configurations in which the same atoms are vertically aligned were traditionally considered unstable due to strong interlayer electrostatic repulsion.

Through detailed investigation, the researchers found that step edges of the reverse cancer substrate act as nucleation sites, facilitating unidirectional alignment of the HBN layer and minimizing rotational disturbances. This step edge guided growth mechanism allows the formation of high-quality wafer-scale AA stack HBN films, ensuring both the structural uniformity and crystallinity required for practical electronic and photonic applications.

Furthermore, this study highlights the important role of electron doping through carbon uptake during the MOCVD process. The presence of carbon introduces excess charge carriers, alters the interaction between layers, effectively reducing the repulsion forces normally associated with AA stacking. Together, this charge-mediated stabilization and stepwise alignment constitutes a previously undiscovered mechanism for engineering tailored stacking sequences of van der Waals materials.

“Our research shows that stacked configurations of van der Waals materials are not purely governed by thermodynamic considerations, but can be stabilized instead by substrate properties and charge uptake,” said Professor Junggi Kim, who led the study. “This insight greatly expands the possibilities for customized 2D material architectures with different electronic and optical properties.”

The optical properties of the synthesized AA stacked HBN reveal the enhanced formula second generation (SHG), a feature of the non-central crystal structure. Furthermore, this material exhibits sharp band edge emission in the DUV region, suggesting the potential for highly efficient optoelectronic devices operating in the DUV spectrum.

“Achieving wafer-scale control of stacking orders is a key milestone for scalable, high-performance 2D electronic and photonic systems,” said Soho Moon, a postdoctoral researcher in Professor John Kim’s lab and a leading author of the study.

“This work highlights the versatility of MOCVD as a platform for precisely designed van der Waals materials.”