A research team led by Prof. REN Wencai from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences has achieved the wafer-scale epitaxial growth of monolayer MoSi₂N₄ single crystals, a high-performance p-type two-dimensional (2D) semiconductor.

Their findings, published on July 8 in Nature Materials, mark a significant step toward realizing complementary metal-oxide-semiconductor (CMOS) integrated circuits based on 2D materials.

As electronic devices continue to shrink, conventional semiconductors such as silicon face fundamental physical bottlenecks including short-channel effects. Van der Waals layered 2D semiconductors, with their atomically thin thickness and dangling-bond-free surfaces, offer compelling advantages for sub-5-nanometer technology nodes. While various n-type 2D semiconductors including MoS₂ and WS₂ have been successfully prepared as wafer-scale single crystals, p-type counterparts that combine high mobility and excellent stability remain extremely scarce, and their single-crystal wafer growth has proven even more challenging.

In 2020, the researchers pioneered the creation of the p-type 2D semiconductor MoSi₂N₄ by introducing silicon into a non-layered molybdenum nitride growth system. This established the MA₂Z₄ family of van der Waals layered materials. Monolayer MoSi₂N₄ exhibits a bandgap comparable to that of MoS₂, but surpasses it in theoretical carrier mobility, thermal conductivity, Young's modulus, and fracture strength, with exceptional stability. Until now, however, only polycrystalline MoSi₂N₄ films had been experimentally attainable. Grain boundaries severely degrade electrical transport performance and compromise film integrity during transfer.

To overcome this challenge, the researchers developed a chemical vapor deposition approach using Cu(111) single crystals enriched with Mo and Si atoms as the growth substrate. They found that the <110> steps on the Cu(111) substrate promote the oriented nucleation of monolayer MoSi₂N₄ domains in a single orientation, enabling seamless stitching into a continuous single-crystal film.

The resulting monolayer MoSi₂N₄ exhibits outstanding crystalline quality, with an intrinsic carrier mobility reaching 154 cm² V^-1 s^-1. Field-effect transistor arrays fabricated from this material deliver excellent electrical performance, including an on/off ratio of approximately 3.8 ± 1.4 × 10⁶ and an on-state current density of up to 17.96 μA μm^-1 at a channel length of 1 μm. These arrays also demonstrate superior stability compared to monolayer WSe₂-based devices.

Furthermore, this method is versatile and can be used to grow monolayer WSi₂N₄ single crystals on a wafer scale.

This work establishes a highly promising p-type 2D semiconductor platform for future integrated circuits and provides a general strategy for wafer-scale, single-crystal growth of other 2D materials. This advances their applications in electronics and information technology.

Growth of monolayer MoSi₂N₄ single-crystal wafers. (Image by IMR)

Single-orientation nucleation mechanism of monolayer MoSi₂N₄ induced by surface steps on Cu(111) single crystals. (Image by IMR)

Optical characterization of monolayer MoSi₂N₄ single crystals. (Image by IMR)