Metals, semiconductors and insulators are fundamental components of modern electronics. However, efforts to improve device performance have largely focused on semiconductor quality, while metal crystallinity has received far less attention.
As transistor dimensions continue to shrink, though, structural disorder in metals and at metal-semiconductor interfaces increasingly impedes carrier injection and transport. Metals therefore need a level of structural order comparable to that of single-crystal semiconductors in order to push device performance toward its physical limits.
To solve this problem, a research team led by Prof. CHU Junhao at the Shanghai Institute of Technical Physics (SITP) of the Chinese Academy of Sciences (CAS) has developed an atomic-scale stepwise evaporation method called Step-Eva that enables the direct in situ growth of single-crystal metal films on semiconductors. This process greatly reduces contact resistance and potentially redefines how transistors are built at the nanoscale.
The findings were published in Science on August 27.
Step-Eva breaks down metal deposition into repeated cycles comprising an atomic-scale deposition dose followed by a prolonged stabilization pause. By temporally separating deposition from structural relaxation, the method creates a distinct kinetic growth window that suppresses disordered secondary nucleation, promotes atomic diffusion and facilitates lateral domain coalescence.
In this way, Step-Eva overcomes the competition between continuous nucleation and atomic diffusion inherent in conventional evaporation, thereby enabling high-quality van der Waals epitaxial growth of single-crystal metals with long-range order.
According to the researchers, Step-Eva is broadly applicable to a range of metals, enabling the growth of high-quality single-crystal bismuth (Bi), silver (Ag), indium (In), gold (Au), and palladium (Pd). The resulting single-crystal metals form clean, low-damage metal–semiconductor interfaces and exhibit well-defined, spatially uniform work functions with minimal local potential fluctuations.
The resulting single-crystal metal contacts for 2D semiconductors exhibit strongly suppressed Fermi-level pinning, near-ideal Schottky–Mott behavior, and ultralow contact resistance.
Specifically, both n-type and p-type 2D transistors exhibit on/off current ratios exceeding 1010, while devices with channel lengths scaled to 50 nm deliver on-state currents above 1.1 mA μm⁻¹. The contact resistances of n-type and p-type transistors are as low as 36 Ω·μm and 145 Ω·μm, respectively, demonstrating highly efficient carrier injection and transport.
In addition, single-crystal metals exhibit excellent dimensional scalability, maintain continuous electrical conduction at ultrathin thicknesses, and demonstrate enhanced thermal stability. These attributes could enable further device scaling and high-density integration, providing a promising materials foundation for next-generation energy-efficient electronic and optoelectronic systems.
