Valley-Free Silicon Fins Caused by Shear Strain

Electron spins confined in silicon quantum dots are promising candidates for large-scale quantum computers. However, the degeneracy of the conduction band of bulk silicon introduces additional levels dangerously close to the window of computational energies, where the quantum information can leak. The energy of the valley states-typically 0.1 meV-depends on hardly controllable atomistic disorder and still constitutes a fundamental limit to the scalability of these architectures. In this work, we introduce designs of complementary metal-oxide-semiconductor (CMOS)-compatible silicon fin field-effect transistors that enhance the energy gap to noncomputational states by more than one order of magnitude. Our devices comprise realistic silicon-germanium nanostructures with a large shear strain, where troublesome valley degrees of freedom are completely removed. The energy of noncomputational states is therefore not affected by unavoidable atomistic disorder and can further be tuned in situ by applied electric fields. Our design ideas are directly applicable to a variety of setups and will offer a blueprint toward silicon-based large-scale quantum processors.

Metallization of diamond

Experimental discovery of ultralarge elastic deformation in nanoscale diamond and machine learning of its electronic and phonon structures have created opportunities to address new scientific questions. Can diamond, with an ultrawide bandgap of 5.6 eV, be completely metallized, solely under mechanical strain without phonon instability, so that its electronic bandgap fully vanishes? Through first-principles calculations, finite-element simulations validated by experiments, and neural network learning, we show here that metallization/demetallization as well as indirect-to-direct bandgap transitions can be achieved reversibly in diamond below threshold strain levels for phonon instability. We identify the pathway to metallization within six-dimensional strain space for different sample geometries. We also explore phonon-instability conditions that promote phase transition to graphite. These findings offer opportunities for tailoring properties of diamond via strain engineering for electronic, photonic, and quantum applications.

Six novel carbon and silicon allotropes with their potential application in photovoltaic field

By stacking up five novel cagelike structures, three novel three-dimensional (3D)sp3bonding networks, namedhP24,hP30 andhP36, were predicted in this work for the first time. These three newly discovered structures have trigonal unit cell with the space groups ofP-3m1,P-3m1 andP3m1, respectively. Using first-principle calculations, the physical properties, including structural, mechanical, electronic and optical properties of C and Si inhP24,hP30 andhP36 phases were systematically studied. All these newly discovered carbon and silicon allotropes were proven to be thermodynamically and mechanically stable. The wide indirect bandgap value in range of 3.89-4.03 eV suggests that C inhP24,hP30 andhP36 phases have the potential to be applied in high frequency and high power electronic devices. The direct bandgap value in range of 0.60-1.16 eV, the smaller electron and hole effective mass than diamond-Si, and the significantly better photon absorption characteristics than diamond-Si suggest thathP24-Si,hP30-Si andhP36-Si are likely to have better performance in photovoltaic applications than diamond-Si.hP24-Si also has the potential to be applied in infrared detectors.

Physical properties of Si-Ge alloys in C2/m phase: a comprehensive investigation

A new phase of C2/m Ge₁₆ is first proposed in this paper. The structures and mechanical, anisotropic, electronic, transport and optical properties of Si-Ge alloys in the C2/m phase are studied using first principles calculations. All Ge₁₆ and Si_16-x Ge _x alloys in the C2/m phase are proven to have mechanical and dynamic stability. By analyzing the three-dimensional (3D) perspective of the effective mass and Young's modulus, obvious anisotropies of transport and mechanical properties are found. Higher-resolution full band structures are obtained to determine the positions of the valence band maximum (VBM) and conduction band minimum (CBM). All materials have a higher photoelectron absorption than that of diamond Si. A high electronic mobility (16 527 cm² V^-1 s^-1) and hole mobility (3033 cm² V^-1 s^-1) are found in C2/m Si₈Ge₈ and Si₄Ge₁₂, respectively. Based on the large mobility and photoelectron absorption, the Si-Ge alloys in the C2/m phase are promising materials for electronics and optoelectronics applications.

Subband engineering in n-type silicon nanowires using strain and confinement

We present a model based on k · p theory which is able to capture the subband structure effects present in ultra-thin strained silicon nanowires. For electrons, the effective mass and valley minima are calculated for different crystal orientations, thicknesses, and strains. The actual enhancement of the transport properties depends highly on the crystal orientation of the nanowire axis; for certain orientations strain and confinement can play together to give a significant increase of the electron mobility. We also show that the effects of both strain and confinement on mobility are generally more pronounced in nanowires than in thin films. We show that optimal transport properties can be expected to be achieved through a mix of confinement and strain. Our results are in good agreement with recent experimental findings.

Spin-orbit symmetries of conduction electrons in silicon

We derive a spin-dependent Hamiltonian that captures the symmetry of the zone edge states in silicon. We present analytical expressions of the spin-dependent states and of spin relaxation due to electron-phonon interactions in the multivalley conduction band. We find excellent agreement with experimental results. Similar to the usage of the Kane Hamiltonian in direct band-gap semiconductors, the new Hamiltonian can be used to study spin properties of electrons in silicon.

Review: Semiconductor Piezoresistance for Microsystems

Piezoresistive sensors are among the earliest micromachined silicon devices. The need for smaller, less expensive, higher performance sensors helped drive early micromachining technology, a precursor to microsystems or microelectromechanical systems (MEMS). The effect of stress on doped silicon and germanium has been known since the work of Smith at Bell Laboratories in 1954. Since then, researchers have extensively reported on microscale, piezoresistive strain gauges, pressure sensors, accelerometers, and cantilever force/displacement sensors, including many commercially successful devices. In this paper, we review the history of piezoresistance, its physics and related fabrication techniques. We also discuss electrical noise in piezoresistors, device examples and design considerations, and alternative materials. This paper provides a comprehensive overview of integrated piezoresistor technology with an introduction to the physics of piezoresistivity, process and material selection and design guidance useful to researchers and device engineers.