Research results 05｜Research Center for Materials Nanoarchitectonics

Large-scale first-principles calculations and experiments for the design of nanoscale devices

T. Miyazaki , D. R. Bowler , N. Fukata

To enable first-principles electronic structure calculations using density functional theory (DFT) to be performed on systems that correspond to practical nanoscale devices and materials, we developed a world-leading linear-scaling DFT code: CONQUEST. Using CONQUEST, we conducted collaborative theory-experimental research on Si/Ge core-shell nanowires.

Fig. 1(a) (upper) Optimized structure of a Ge nanoisland on Si(001) substrate calculated using CONQUEST and experimental structure. (lower) Atomic models of Si/Ge core-shell nanowire, along with transmission electron microscopic and scanning electron microscopic measurements and schematic representation of how nanowires can be used in transistors. (b) Example of the distribution of a hole carrier in a Ge/Si core-shell nanowire.

The control and growth of semiconductor microstructures and nanostructures have driven the modern electronics industry. As device sizes shrink, an atomistic description of the structure of surfaces and interfaces in semiconductor nanostructures is becoming increasingly valuable.

First-principles calculations based on DFT are a powerful tool that can provide reliable information on the atomic positions and electronic structures of materials independently from experiments. However, since the cost of DFT calculations is expensive and increases rapidly with the cube of the number of atoms N, it is almost impossible to treat systems containing more than a few thousand atoms using standard DFT implementations. Thus, it was very difficult to model practical nanoscale devices by DFT methods. To overcome this problem, we developed a linear-scaling DFT code, CONQUEST, for which the computational cost is only proportional to N. With CONQUEST, we can perform robust, accurate electronic structure calculations, including structural relaxations and molecular dynamics, on very large systems containing more than one million atoms.

Using CONQUEST, we performed DFT studies of three-dimensional Ge nanoislands on Si(001) substrates and Ge/Si core-shell nanowires (Fig. 1(a)). For the nanowires, which are a promising material for next-generation vertical transistors, we performed collaborative theory-experimental research. Experimentally, we can control the radius of the core and thickness of the shell of the nanowires with high crystallinity. Properties of the core-shell nanowires are expected to depend strongly on the size, interface between Si and Ge, impurity distribution, and other structural factors, which could not be modeled before. Using CONQUEST, we succeeded in calculating the strain distribution in the nanowires and electronic structure near the Fermi level (Fig. 1(b)). Based on those calculated results, we synthesized Ge/Si core-shell nanowires and found conclusive evidence of hole gas accumulation in the core-shell nanowires experimentally.