M. Y. Simmons

This review describes recent groundbreaking results in Si, $\mathrm{Si}/\mathrm{SiGe}$, and dopant-based quantum dots, and it highlights the remarkable advances in Si-based quantum physics that have occurred in the past few years. This progress has been possible thanks to materials development of Si quantum devices, and the physical understanding of quantum effects in silicon. Recent critical steps include the isolation of single electrons, the observation of spin blockade, and single-shot readout of individual electron spins in both dopants and gated quantum dots in Si. Each of these results has come with physics that was not anticipated from previous work in other material systems. These advances underline the significant progress toward the realization of spin quantum bits in a material with a long spin coherence time, crucial for quantum computation and spintronics.

In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show up to 26 quantized ballistic plateaus, as well as a structure close to $0.7({2e}^{2}/h)$. In an in-plane magnetic field all the 1D subbands show linear Zeeman splitting, and in the wide channel limit the $g$ factor is $\ensuremath{\mid}g\ensuremath{\mid}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.4$, close to that of bulk GaAs. For the last subband, spin splitting originates from the structure at $0.7({2e}^{2}/h)$, indicating spin polarization at $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. The measured enhancement of the $g$ factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions.

We demonstrate the controlled incorporation of P dopant atoms in Si(001), presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si(001) and show that it is possible to thermally incorporate P atoms into Si(001) below the H-desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H lithography. We demonstrate the positioning of single P atoms in Si with approximately 1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms.

As silicon electronics approaches the atomic scale, interconnects and circuitry become comparable in size to the active device components. Maintaining low electrical resistivity at this scale is challenging because of the presence of confining surfaces and interfaces. We report on the fabrication of wires in silicon--only one atom tall and four atoms wide--with exceptionally low resistivity (~0.3 milliohm-centimeters) and the current-carrying capabilities of copper. By embedding phosphorus atoms within a silicon crystal with an average spacing of less than 1 nanometer, we achieved a diameter-independent resistivity, which demonstrates ohmic scaling to the atomic limit. Atomistic tight-binding calculations confirm the metallicity of these atomic-scale wires, which pave the way for single-atom device architectures for both classical and quantum information processing.