Electronic properties of graphene (carbon) nanoribbons are studied and compared to those of carbon nanotubes. The nanoribbons are found to have qualitatively similar electron band structure which depends on chirality but with a significantly narrower band gap. The low- and high-field mobilities of the nanoribbons are evaluated and found to be higher than those of carbon nanotubes for the same unit cell but lower at matched band gap or carrier concentration. Due to the inverse relationship between mobility and band gap, it is concluded that graphene nanoribbons operated as field-effect transistors must have band gaps <0.5eV to achieve mobilities significantly higher than those of silicon and thus may be better suited for low power applications.

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The technologically important question of whether the reduced density of electron states (DOS) for scattering in one-dimensional (1D) wire transport devices gives an advantage over the planar metal–oxide–semiconductor field-effect-transistor (MOSFET) for electron mobility is assessed by simulations. We self-consistently solve the Schrödinger–Poisson equations to calculate phonon-limited electron mobility in a multisubband cylindrical Si gated wire. We find that the benefit of reduced 1D DOS is offset by an increased phonon scattering rate due to increased electron–phonon wave function overlap and results in a degraded mobility in narrow wires. The applied gate bias voltage and the wire size control the transition from wire geometry to surface field-dominated confinement. The size scale for this 1D to two-dimensional (2D) transition is also found to be surprisingly small: A wire with a 75 A radius has an essentially 2D DOS and has a 2D mobility that is degraded from the planar (100) MOSFET due to the anisotropy of the inversion mobility in different Si crystallographic planes.

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Full-scale quantum computers require the integration of millions of qubits, and the potential of using industrial semiconductor manufacturing to meet this need has driven the development of quantum computing in silicon quantum dots. However, fabrication has so far relied on electron-beam lithography and, with a few exceptions, conventional lift-off processes that suffer from low yield and poor uniformity. Here we report quantum dots that are hosted at a 28Si/28SiO2 interface and fabricated in a 300 mm semiconductor manufacturing facility using all-optical lithography and fully industrial processing. With this approach, we achieve nanoscale gate patterns with excellent yield. In the multi-electron regime, the quantum dots allow good tunnel barrier control—a crucial feature for fault-tolerant two-qubit gates. Single-spin qubit operation using magnetic resonance in the few-electron regime reveals relaxation times of over 1 s at 1 T and coherence times of over 3 ms.

We present results for relaxation of the spin of a hole in a cylindrical quantum dot due to acoustic phonon assisted spin flips at low temperatures with an applied magnetic field. The hole dispersion is calculated by numerical diagonalization of the Luttinger Hamiltonian and applying perturbation theory with respect to the magnetic field, and the hole-phonon coupling is described by the Bir-Pikus Hamiltonian. We find that the decoherence time for hole spins for dots Շ20 nm is on the order of 10 Ϫ8 s. This is several orders smaller than the decoherence time due to phonon assisted processes for electron spins in similar dots and is comparable to the total decoherence time of an electron spin in a quantum dot, which is controlled by the hyperfine interaction with nuclei. We obtain the dependence of the relaxation rate of the hole spin on dot size and hole mass.