Silicon Quantum Wires Research Articles

Fully Quantum Mechanical Simulations of Gated Silicon Quantum Wire Structures: Investigating the Effects of Changing Wire Cross-Section on Transport

We present transport simulations of three-dimensional (3D) silicon quantum wires in which electron-phonon scattering is incorporated via local modifications to the potential encountered by the electron waves. Computing resistance as a function of wire length, we find that the effects of dissipation essentially take hold immediately, and are apparent right from the point at which the wire is long enough to act as a true waveguide.

Effects of nonparabolic bands in quantum wires

A nonparabolic band model has been implemented for a one-dimensional electron gas, using a modified Schrödinger equation which takes into account size quantization in the transverse cross section of a silicon quantum wire. The quantized states and the corresponding one-dimensional density of states have been analyzed when the nonparabolicity is present, to quantify the importance of the effect of realistic bands at higher energies.

Local Tunneling Spectroscopy of Silicon Nanostructures

The recharging of many-hole and few-electron quantum dots under the conditions of the ballistic transport of single charge carriers inside self-assembled quantum well structures on a Si (100) surface are studied using local tunneling spectroscopy at high temperatures (up to room temperature). On the basis of measurements of the tunneling current-voltage characteristics observed during the transit of single charge carriers through charged quantum dots, the modes of the Coulomb blockade, Coulomb conductivity oscillations, and electronic shell formation are identified. The tunneling current-voltage characteristics also show the effect of quantum confinement and electron-electron interaction on the characteristics of single-carrier transport through silicon quantum wires containing weakly and strongly coupled quantum dots.

Self-aligned silicon quantum wires on Ag(1 1 0)

Upon deposition of silicon onto the (1 1 0) surface of a silver crystal we have grown massively parallel one-dimensional Si nanowires. They are imaged in scanning tunnelling microscopy as straight, high aspect ratio, nanostructures, all with the same characteristic width of 16 Å, perfectly aligned along the atomic troughs of the bare surface. Low energy electron diffraction confirms the massively parallel assembly of these self-organized nanowires. Photoemission reveals striking quantized states dispersing only along the length of the nanowires, and extremely sharp, two-components, Si 2p core levels. This demonstrates that in the large ensemble each individual nanowire is a well-defined quantum object comprising only two distinct silicon atomic environments. We suggest that this self-assembled array of highly perfect Si nanowires provides a simple, atomically precise, novel template that may impact a wide range of applications.

Spin depolarization in quantum wires polarized spontaneously in zero magnetic field

The conditions for a spontaneous spin polarization in a quantum wire positioned in a zero magnetic field are analyzed under weak population of one-dimensional subbands that gives rise to the efficient quenching of the kinetic energy by the exchange energy of carriers. The critical linear concentration of carriers above which the quasi one-dimensional gas undergoes a complete spin depolarization is determined by the Hartree-Fock approximation. The dependence of the critical linear concentration on the concentration of carriers is defined to reveal the interplay of the spin depolarization with the evolution of the 0.7 (2e2/h) feature in the quantum conductance staircase from the e2/h to 3/2 (e2/h) values. This dependence is used to study the effect of the hole concentration on the 0.7 (2e2/h) feature in the quantum conductance staircase of the quantum wire prepared inside the p-type silicon quantum well using the split-gate technique. The 1D channel is demonstrated to be spin-polarized at the linear concentration of holes lower than the critical linear concentration, because the 0.7 (2e2/h) feature is close to the value of 0.5 (2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The 0.7 (2e2/h) feature is found to take however its normal magnitude when the linear concentration of holes attains the critical value corresponding to the spin depolarization. The variations in the height of the 0.7 (2e2/h) feature observed in the hole quantum conductance staircase that is revealed by the p-type silicon quantum wire seem to be related to the evidences of the quantum conductance staircase obtained by varying the concentration of electrons in the 1D channel prepared inside the GaAs-AlGaAs heterojunction.

Full Quantum Mechanical Simulation of Ultra-Small Silicon Devices in Three-Dimensions: Physics and Issues

The combination of the need for alternative devices and the improvement in process technology has led to the examination of silicon quantum wires for future MOS technology. However, in order to properly model these devices, a full three-dimensional quantum mechanical treatment is required. In this paper, we present the results of a three-dimensional, fully quantum mechanical, self-consistent simulation of a silicon quantum wire MOSFET (Metal Oxide Field Effect Transistor) with a narrow channel (8 nm). A quasi-standing wave is formed in the narrow channel at certain gate voltages as the electron density is trapped in narrow channel. These effects are the result of two competing effects: (1) the interaction of the propagating electrons with the channel dopants, as well as with the dopants in the source and drain of the device. (2) the reflections from the boundaries that form the narrow channel both on the source side and the drain side.

Exciton photoluminescence in doped quasi-1D structures based on silicon

The dependence of carrier density in silicon quantum wires sheathed with SiO2 on the wire diameter and the position of impurity atoms in respect to the wire center is analyzed theoretically. It is shown that, as the diameter of wires and nanocrystals decreases, the ionization energy of a dopant increases; therefore, the free carrier density decreases, and the screening of the Coulomb attraction becomes ineffective. As a result, the photoluminescence is defined by the radiative recombination of excitons even in the case of heavily doped Si. These conclusions are supported by the data of experimental study of spectral, excitation-power, and temperature dependences of photoluminescence in porous silicon structures fabricated on lightly and heavily doped Si substrates.

Interplay between quantum confinement and electron-electron interaction in deformed silicon quantum wires

In this paper we investigate quantum confinement and electron-electron correlation in silicon deformed quantum wires. Starting from the single-particle picture, which shows the possibility of localizing one electron inside the wire deformation, we build a variational ansatz for the two-electron ground state. We compare the localizing effect induced by the deformation with the Coulomb repulsion, pointing out the existence of a two-electron confined ground state, depending on the wire geometry. On varying the geometrical parameters characterizing the wire deformation, it is possible to switch from structures dominated by the localization to structures in which the increase of the available volume makes the Coulomb contribution very relevant, as for ordinary quantum dots. The effects induced by the dielectric mismatch between the wire and the surrounding medium are included and discussed.

Semiclassical and Quantum Transport in Si/SiO2Superlattices

In the last few years, many research groups have been trying to develop electroluminescent devices based on silicon. In particular, it has been shown that low-dimensional structures, such as silicon clusters, quantum wires and quantum wells, are suitable for this purpose. In this work we investigate transport properties of a particular superlattice using two approaches. The first method is a Monte Carlo simulation of electron transport in the biased superlattice. The band structure is calculated using the envelope function approximation, and the scattering mechanisms introduced in the simulator are confined optical phonons. Owing to the particularly flat band structure, drift velocities are very low, but it will be shown that a parallel component of the electric field can significantly increase the vertical drift velocity. Moreover, a superlattice based device is proposed in order to obtain high recombination efficiency. Finally, a quantum calculation is introduced, in order to describe with higher accuracy the high field transport regime.

Comparison Between Non-Equilibrium Green's Function and Monte Carlo Simulations for Transport in a Silicon Quantum Wire Structure

We present comparisons of simulations conducted with non-equilibrium Green's functions and Monte Carlo approaches. As prototype, we consider an idealized silicon quantum wire structure, consisting of a conduction channel of rectangular cross-section, terminated by two contacts. The Monte Carlo model treats the particles as semi-classical, but distributed over up to seven subbands and with scattering model similar to the one used for the Green's functions model. Results for drift velocity under various field conditions agree very closely using the two techniques, suggesting that particle simulation may continue to be a useful physical investigation tool at the nanoscale with an appropriate introduction of the most important quantum features of the transport.

Single-electron transistors fabricated with sidewall spacer patterning

We have implemented a sidewall spacer patterning method for novel dual-gate single-electron transistor (DGSET) and metal–oxide–semiconductor-based SET (MOSET) based on the uniform SOI wire, using conventional lithography and processing technology. A 30 nm wide silicon quantum wire is defined by a sidewall spacer patterning method, and depletion gates for two tunnel junctions of the DGSET are formed by the doped polycrystalline silicon sidewall. The fabricated DGSET and MOSET show clear single-electron tunneling phenomena at liquid nitrogen temperature and insensitivity of the Coulomb oscillation period to gate bias conditions. On the basis of the phase control capability of the sidewall depletion gates, we have proposed a complementary self-biasing method, which enables the SET/CMOS hybrid multi-valued logic (MVL) to operate perfectly well at high temperature, where the peak-to-valley current ratio of Coulomb oscillation severely decreases. The suggested scheme is evaluated by SPICE simulation with an analytical DGSET model, and it is confirmed that even DGSETs with a large Si island can be utilized efficiently in the multi-valued logic.

Unified compact model for the ballistic quantum wire and quantum well metal-oxide-semiconductor field-effect-transistor

We present a compact model based on the Landauer transmission theory for the silicon quantum wire and quantum well metal-oxide-semiconductor field effect transistor (MOSFET) working in the ballistic limit. This model captures the static current-voltage characteristics in all the operation regimes, below and above threshold voltage. The model provides a basic framework to account for the electronic transport in MOSFETs, being easily adaptable to gate structures such as the double-gate or gate-all-around. Numerical simulations based on the proposed model have been compared with experiments and quantum mechanical self-consistent simulations, with good agreement.

Characteristics of confined exciton states in silicon quantum wires

Characteristics of confined exciton states in silicon quantum wires

The features of photoluminescence from nanograins of gallium-doped cubic silicon carbide

The photoluminescence (PL) and photoexcitation spectra of carbonized porous silicon (por-Si) doped with gallium in the course of a high-temperature annealing were studied. It is shown that carbonization leads to the formation of a heterojunction between 3C-SiC nanograins and silicon quantum wires. The spectrum of PL from gallium-doped silicon carbide nanograins is shifted by 0.35 eV toward higher energies relative to the spectrum of bulk por-Si and exhibits several features related to the radiative annihilation processes involving phonons and donor-acceptor (N-Ga) pairs. The PL excitation spectra of carbonized por-Si display two resonance bands with the energies E1=2.8–3.1 eV and E2=3.2–3.7 eV.

Quantized conductance in silicon quantum wires

The results of studying the quantum-mechanical staircase for the electron and hole conductance of one-dimensional channels obtained by the split-gate method inside self-assembled silicon quantum wells are reported. The characteristics of quantum wells formed spontaneously between the heavily doped δ-shaped barriers at the Si(100) surface as a result of nonequilibrium boron diffusion are analyzed first. To this end, secondary-ion mass spectrometry, and also the detection of angular dependences of the cyclotron resonance and ESR, is used; these methods make it possible to identify both the crystallographic orientation of the self-assembled quantum wells and the ferroelectric properties of heavily doped δ-shaped barriers. Since the obtained silicon quantum wells are ultrathin (∼2 nm) and the confining δ-shaped barriers feature ferroelectric properties, the quantized conductance of one-dimensional channels is first observed at relatively high temperatures (T≥77 K). Further, the current-voltage characteristic of the quantum-mechanical conductance staircase is studied in relation to the kinetic energy of electrons and holes, their concentration in the quantum wells, and the crystallographic orientation and modulation depth of electrostatically induced quantum wires. The results show that the magnitude of quantum steps in electron conductance of crystallographically oriented n-type wires is governed by anisotropy of the Si conduction band and is completely consistent with the valence-valley factor for the [001] (G0=4e2/h and gv=2) and [011] (G0=8e2/h and gv=4) axes in the Si(100) plane. In turn, the quantum staircase of the hole conductance of p-Si quantum wires is caused by independent contributions of the one-dimensional (1D) subbands of the heavy and light holes; these contributions manifest themselves in the study of square-section quantum wires in the doubling of the quantum-step height (G0=4e2/h), except for the first step (G0=2e2/h) due to the absence of degeneracy of the lower 1D subband. An analysis of the heights of the first and second quantum steps indicates that there is a spontaneous spin polarization of the heavy and light holes, which emphasizes the very important role of exchange interaction in the processes of 1D transport of individual charge carriers. In addition, the temperature-and field-related inhibition of the quantum conductance staircase is demonstrated in the situation when kT and the energy of the field-induced heating of the carriers become comparable to the energy gap between the 1D subbands. The use of the split-gate method made it possible to detect the effect of a drastic increase in the height of the quantum conductance steps when the kinetic energy of electrons is increased; this effect is most profound for quantum wires of finite length, which are not described under conditions of a quantum point contact. It is shown in the concluding section of this paper that detection of the quantum-mechanical conductance under the conditions of sweeping the kinetic energy of the charge carriers can act as an experimental test aiding in separating the effects of quantum interference in modulated quantum wires against the background of Coulomb oscillations as a result of the formation of QDs between the delta-shaped barriers.

Tuning the room temperature nonlinearI−Vcharacteristics of a single-electron silicon quantum dot transistor by split gates: A simple model

We propose an experiment that potentially allows a single-electron silicon quantum dot transistor to operate at room temperature. The emitter and collector of the device consist of silicon quantum wires and the base contains a single silicon dot buried in silicon dioxide. We suggest that split gates are added to the usual experimental situation, to provide additional and variable confinement perpendicular to the transport direction in the emitter and collector regions. The current-voltage curve is calculated using the Bardeen transfer Hamiltonian method. The potential defined by the gates is approximated to a harmonic form. We predict the nonlinear structure in the current-voltage curve, will survive to room temperature for systems with an emitter and collector with dimensions of the order 20-40 nm, and where the harmonic potentials have subband level spacing of the order 4-8.5 meV. Furthermore, we predict that the peak positions and peak to valley ratios in the current-voltage curve can be tuned by changing the split gate voltage.

Directional dependence of the spontaneous emission of Si quantum wires

The electron states in silicon quantum wires obtained within an effective approximation are used to study the spontaneous emission rate for direct and phonon-assisted transitions. It is found that light emission from the wires has strong direct character if the wires are directed along the main crystallographic directions or along [110]. The intensity of phonon-assisted transitions is in this case three orders of magnitude smaller.

Single-electron transistor based on a silicon-on-insulator quantum wire fabricated by a side-wall patterning method

We propose and implement a promising fabrication technology for geometrically well-defined single-electron transistors based on a silicon-on-insulator quantum wire and side-wall depletion gates. The 30-nm-wide silicon quantum wire is defined by a combination of conventional photolithography and process technology, called a side-wall patterning method, and depletion gates for two tunnel junctions are formed by the doped polycrystalline silicon sidewall. The good uniformity of the wire suppresses unexpected potential barriers. The fabricated device shows clear single-electron tunneling phenomena by an electrostatically defined single island at liquid nitrogen temperature and insensitivity of the Coulomb oscillation period to gate bias conditions.

A silicon quantum wire transistor with one-dimensional subband effects

A silicon quantum wire transistor, in which electrons are transported through a very narrow wire, has been fabricated using silicon-on-insulator technology, electron beam lithography, anisotropic dry etching, and thermal oxidation. We have obtained the quantum wire with a width of 65 nm, which is fully embedded in silicon dioxide. This narrow dimension of the wire and large potential barrier between silicon and silicon dioxide make the electrons moving through the wire experience one-dimensional confinement. The step-like structure in the conductance versus gate voltage curve, which is a typical evidence of one-dimensional conductance, has been observed at temperatures below 4.2 K. A period of step appearance and a step size have been analyzed to compare experimental characteristics with theoretical calculation.

Ballistic conductance of a quantum wire at finite temperatures

The temperature-dependent ballistic conductance of a quantum wire is calculated without consideration of carrier scattering. The contribution to the conductance (G) from size-quantization subbands with E j - μ ≫ kT is described by the Landauer-Buttiker formula G=2e 2/h, where e is the elementary charge, h is Planck’s constant, μ(T) is the electrochemical potential, E j is the j-th subband bottom, T is temperature, and k is the Boltzmann constant. The contribution from other subbands decreases as their number increases, being exponentially small for higher subbands. The quantum staircase disappears when kT approaches the energy spacing between the size-quantization levels. Such temperature quenching of the quantum staircase at gate potentials corresponding to a stepwise change in the ballistic conductance is observed in studies of the quantized conductance of a silicon quantum wire.