Nanowires show promise for the construction of next-generation electronics because their electronic properties can be readily tuned by changing their size, shape and chemical composition. Silicon and germanium nanowires, in particular, are very good at transporting charge (high mobility) and are compatible with existing fabrication techniques. Nanowires with cores of germanium and shells of silicon have been shown to have even higher mobilities than single-element nanowires, and have been used to build a variety of devices.

Fig. 1: A schematic illustration of an electronic wavefunction confined in the core region of a doped nanowire with a silicon core and germanium shell. The dopant is a single phosphorus atom.

The versatility of such core–shell nanowires can be extended by introducing other atoms to change the density of charge carriers in the nanowires. However, such atomic ‘dopants’ may also reduce charge-carrier mobilities, but the exact effects have remained unclear. Hyoung Joon Choi and Hyungjun Lee from Yonsei University in Korea1 have now constructed an accurate model of the effect of dopants on charge transport in core–shell nanowires from first principles.

The researchers modeled the charges in a core–shell nanowire using density functional methods, which describe electrons in quantum mechanical terms. Their method also took into account the redistribution of charge caused by the dopants. In agreement with previous results, they found that positive and negative charge carriers (‘holes’ and electrons, respectively) were confined to the core or the shell, or were unconfined, depending on the core and shell compositions. They also found, however, that silicon-core nanowires with phosphorus-doped (negative) germanium shells have electron mobilities that are as high as the hole mobilities in the more usual germanium-core nanowires with boron-doped (positive) shells. Configurations in which the nanowire core was doped rather than the shell had much lower mobilities.

These results indicate that the silicon and germanium band energies are sufficiently different from each other for holes confined in the germanium core (or electrons confined in the silicon core) to propagate almost unimpeded down these core–shell nanowires if the dopants are placed in the shell region. The results are important because both electron-carrying and hole-carrying devices are necessary for various applications in digital electronics. “For this reason, we expect that our work will stimulate experimental research into synthesizing doped silicon-core and germanium-core nanowires, and also electronic nano-devices based on them,” says Choi.