Electrical properties of rolled-up p-type Si/SiGe heterostructures

We report a theoretical study of the hole density and the low-field mobility in modulation p-doped rolled-up Si/SiGe heterostructures. Solving coupled Poisson and Schrödinger equations, we show that the total hole density is strongly affected by charged surface states and can reach value of 1011 cm−2 for available doping level at room and low temperature. The simulation of the hole transport along the structure axis based on a Monte Carlo method reveals that the interface roughness scattering is a main mechanism limiting the mobility magnitude, which reaches the value of 104 cm2/V s

Amplification of a terahertz field in a semiconductor superlattice via phase-locked-k-space bunches of Bloch oscillating electrons

Applying a Monte Carlo technique for the simulation of the dynamics of electrons in a semiconductor superlattice, we studied the action of a terahertz field on the motion of electrons by performing Bloch oscillations subject to inelastic scattering. We found that, due to polar optic phonon emission, the Bloch oscillating electrons can form k-space bunches phase locked to the terahertz field. For frequencies smaller than the Bloch frequency, this gives rise to the amplification of the terahertz field. Our study may contribute to the development of a Bloch laser for terahertz radiation

Features of electron gas in InAs nanowires imposed by interplay between nanowire geometry, doping and surface states

We present a study of electron gas properties in InAs nanowires determined by interaction between nanowire geometry, doping and surface states. The electron gas density and space distribution are calculated via self-consistent solution of coupled Schroedinger and Poisson equations in the nanowires with a hexagonal cross-section. We show that the density of surface states and the nanowire width define the spatial distribution of the electrons. Three configurations can be distinguished, namely the electrons are localized in the center of the wire, or they are arranged in a uniform tubular distribution, or finally in a tubular distribution with additional electron accumulation at the corners of the nanowire. The latter one is dominating for most experimentally obtained nanowires. N-type doping partly suppresses electron accumulation at the nanowire corners. The electron density calculated for both, various nanowire widths and different positions of the Fermi level at the nanowire surface, is compared with the experimental data for intrinsic InAs nanowires. Suitable agreement is obtained by assuming a Fermi level pinning at 60 to 100 meV above the conduction band edge, leading to a tubular electron distribution with accumulation along the corners of the nanowire

Electric field distribution in biased GaAs microstructures with field-pinning layers

Field-pinning layers are an approach to improve the homogeneity of the electric field in a biased semiconductor structure of length above the Kroemer criterion. Building a THz Bloch oscillator with such a structure requires superlattice regions. Nevertheless, GaAs layers are investigated here. We compare different periodic structures (alternating transit and field-pinning layers) via simulating the field distribution. It is shown that the development of propagating Gunn domains is suppressed when field-pinning layers are included, but the homogeneity of the field is still not satisfying for the purpose of building a Bloch gain THz source. Depending on the temperature, intra- and inter-period inhomogeneities occur

Flux periodic magnetoconductance oscillations in GaAs/InAs core/shell nanowires

Magnetotransport experiments on epitaxial GaAs/InAs core/shell nanowires are performed in which the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. The core/shell nanowires are grown by molecular beam epitaxy. It is found that the nanowire conductance oscillates with the magnetic field oriented parallel to its axis, with a period of the magnetic flux quantum ϕ0=h/e. Related to that, it is shown that the electronic transport is mediated by closed loop quantum states encircling the wire axis rather than by electron interference of partial waves. By means of a gate voltage the conductance at zero magnetic field can be changed between an oscillation minimum and maximum. The experimental findings are supported by numerical calculations

Angle-dependent magnetotransport in GaAs/InAs core/shell nanowires

We study the impact of the direction of magnetic flux on the electron motion in GaAs/InAs core/shell nanowires. At small tilt angles, when the magnetic field is aligned nearly parallel to the nanowire axis, we observe Aharonov–Bohm type h/e flux periodic magnetoconductance oscillations. These are attributed to transport via angular momentum states, formed by electron waves within the InAs shell. With increasing tilt of the nanowire in the magnetic field, the flux periodic magnetoconductance oscillations disappear. Universal conductance fluctuations are observed for all tilt angles, however with increasing amplitudes for large tilt angles. We record this evolution of the electron propagation from a circling motion around the core to a diffusive transport through scattering loops and give explanations for the observed different transport regimes separated by the magnetic field orientation

Resolving ambiguities in nanowire field-effect transistor characterization

We have modeled InAs nanowires using finite element methods considering the actual device geometry, the semiconducting nature of the channel and surface states, providing a comprehensive picture of charge distribution and gate action. The effective electrostatic gate width and screening effects are taken into account. A pivotal aspect is that the gate coupling to the nanowire is compromised by the concurrent coupling of the gate electrode to the surface/interface states, which provide the vast majority of carriers for undoped nanowires. In conjunction with field-effect transistor (FET) measurements using two gates with distinctly dissimilar couplings, the study reveals the density of surface states that gives rise to a shallow quantum well at the surface. Both gates yield identical results for the electron concentration and mobility only at the actual surface state density. Our method remedies the flaws of conventional FET analysis and provides a straightforward alternative to intricate Hall effect measurements on nanowires