Session Z7: Transport and Optical Properties of Non-Carbon Nanotubes and Nanowires

Exchange induced electron transport in heavily n-doped Si nanowires

Silicon nanowires (Si NWs) have been used as building blocks in the ``bottom-up'' approach to nanoelectronics because of their excellent electrical properties. Despite the potential of Si NWs, unfortunately, there is little known about the electrical properties of heavily doped Si NWs. We have synthesized heavily doped $n$-type Si NWs and measured the electrical resistance using four-probe method. As we decrease the temperature, the resistivity of Si NWs decreases initially, shows a resistivity minimum around 60 K, and thereafter increases logarithmically. Below the resistivity minimum temperature (T$_{min})$, we have observed a dip around zero-bias in the differential conductance, and a negative magnetoresistance (MR) which depends on the angle between the applied magnetic field and current flow. These results are associated with the impurity band conduction and electron scattering by the localized spins at phosphorus donor states. The analysis on the MR reveals that the localized spins are coupled antiferromagnetically at low temperature via the exchange interaction.   

Low-field mobility in ultrathin silicon nanowire junctionless transistors

We theoretically investigate the phonon, surface roughness and ionized impurity limited low-field mobility of ultrathin silicon n-type nanowire junctionless transistors in the long channel approximation with wire radii ranging from $2$ to $5$ nm, as function of gate voltage. A few years ago, the junctionless nanowire transistor (JNT) or pinch-off nanowire was proposed by several research groups and was recently fabricated for the first time. The JNT is a uniformly doped nanowire with no junctions, i.e. source, channel and drain are doped with the same doping type. The main motivation for introducing this novel device concept are the absence of doping junctions which makes the fabrication easier, and the reduction of detrimental interactions occuring at the interface between the silicon body of the wire and the insulator (surface roughness). We investigate the case where due to quantum mechanical confinement the surface roughness scattering becomes again important and we report on the behavior of phonon, ionized impurity and surface roughness limited mobility as a function of radius and gate voltage.   

Photovoltaic properties of axial in-situ doped SiGe heteronanowires

We report on vapor-liquid-solid (VLS) growth and photovoltaic properties of axial in-situ doped Ge-Si heteronanowire (hetero-NW) solar cells. Modern VLS growth has recently demonstrated the growth of Si-Ge axial NWs with abrupt heterojunctions [1], and simultaneous control of both material composition and doping profiles. This advance allows for the integration of Ge with Si in a hetero-NW structure for broad-spectrum and high absorption efficiency solar cells. Our preliminary optical measurements on $p$-Ge/$i$-Si/$n-$Si wires provide important ingredients for a hetero-NW photovoltaic device. We achieved good rectification with $\sim $ 10$^{3}$ ratio between forward and reverse bias currents at moderate voltage. Under laser illumination (532 nm), we measured a large open circuit voltage $V_{OC} \quad \sim $ 0.54 V and a high short-circuit current density $J_{SC} \quad \sim $ 4x10$^{3}$A/cm$^{2}$, comparable to state-of-the-art reported single NW results [2]. Additional optimization of separate Ge and Si \textit{pin} NW structures and their integration in a combined Ge/Si tandem hetero-NW solar cell will be reported. \\[4pt] [1] D. E. Perea \textit{et al.}, \textit{Nano Lett.} \textbf{11}, 3117 (2011). \\[0pt] [2] B. Tian \textit{et al.}, \textit{Chem. Soc. Rev.} \textbf{38}, 16 (2009).   

Strong Tuning of Spin Orbit Interaction in an InAs Nanowire by an Electrolyte Gate

Manipulating electron spin in solid state devices has been the main target of spintronics. A key concept in many spintronic devices is to apply gate voltage or electric field ($E)$ to tune spin precession via the Rashba spin orbit interaction (SOI). Quasi-one dimensional (1D) indium arsenide (InAs) nanowires are promising platforms in this regard due to InAs's strong intrinsic SOI and more effective control of Rashba SOI in 1D system with uni-directional momentum. Here, we demonstrate efficient control of Rashba SOI where $E$ is created at the surface of InAs nanowire by solid electrolyte surrounding gate. Six-fold tuning of Rashba coefficient and 2.5 order of magnitude tuning of spin relaxation time are achieved within only 1 V of gate bias. Such a dramatic tuning of SOI paves a way towards quasi-1D nanowire spintronic devices with low power consumption.   

InAs/InSb axially heterostructured nanowires for small junction area semiconductor diodes

Semiconductor nanowires are low-dimensional systems which appear as ideal candidates to enable ultra-fast electronic technology. Even if the realization of complex [1], nanowire-based electronic devices such as transistors and diodes has been demonstrated, the ultra-small axial electronic capacitance, related to the semiconductor tiny cross-section, is rarely exploited, due to the difficulty in creating homo/hetero-junctions within the nanowire itself. We propose a solution to produce axial, majority-carrier diodes, by employing growth interruption to create InAs/InSb heterojunction along the nanowire axis [2]. Despite being both InAs and InSb n-type materials, their broken gap band alignment produces strong asymmetry in the I-V characteristic, similarly to standard Schottky-barrier diodes. The band line-up determines a strong nonlinear current response at positive source-drain bias: when an additional InP barrier layer is inserted in-between the heterojunction, the maximum nonlinearity is drawn towards zero bias and the leakage current reduced, making these devices promising candidates for high cut-off frequency rectifying detectors/photomixers. \\[4pt] [1] N. Wang et al., Mat. Sci. Eng. R 60, 1 (2008).\\[0pt] [2] A. Pitanti et al. Phys. Rev. X 1, 011006 (2011).   

Growth and electrical properties of vertically oriented organic single-crystalline nanostructures

We present our growth of vertically oriented organic single crystalline nanowires of copper tetra-tertbutylphthalocyanine (t-butyl-CuPc) on highly ordered pyrolytic graphite (HOPG). The nanowires are grown by using physical vapor transport (PVT) method, with directed crystallization on ordered substrates. Single crystal nanostructures are free of grain boundaries and this promotes charge transport through the material. The pi-pi interactions between adjacent molecules of the single crystal provide a direct path for charge transfer, leading to high carrier mobility. Vertically oriented single-crystal nanostructures have applications as high surface area solar cells. We are studying the electrical conductivity of these nanostructures using conductive atomic force microscopy (CAFM). The AFM tip makes an electrical contact to an individual wire, and current is measured as a function of applied voltage.   

Rectification in an idealized model junction of strongly correlated electrons

Junctions of oppositely doped Mott insulators offer the possibility of rectification at extremely high frequencies. To simulate an idealized junction we use a model of spinless electrons moving in one dimension, the t-V chain, and control the chemical potential on the two halves of the chain to create a p-n junction. For short chains the many-body Schrodinger equation can be integrated forward in time numerically exactly, and we find that when subjected to an oscillating electric field the system rectifies by transferring charge in a preferred direction. Dissipation can be included in a phenomenological way by rotating time slightly off the real axis. The time dependent density-matrix renormalization-group may be used to extend the simulation to longer chains with spinning electrons.   

Temperature effects on the Raman lineshape of cupric oxide thin films and nanowires

The Raman spectrum can be used as a fingerprint for a given material but the details of the Raman spectrum (e.g. the peak lineshape) can also impart considerable information about various environmental effects. In this study, we will describe the effects of heating on the Raman lineshape using a temperature-dependent model. Homogeneous heating (i.e. spatially uniform temperature) leads to symmetrically broadened lineshapes while asymmetric lineshapes can be associated with electron-phonon coupling in or phonon confinement effects. Recently, several groups have found that asymmetric lineshapes from nanoparticles and nanowires require an additional term that describes the inhomogeneous heating - temperature gradients within the sample - due to a gaussian laser beam. In our samples, phonon confinement does not play a role at all as nanowire dimensions are well above 20 nm. Hence, we have invoked a purely temperature-dependent model. This model has considerable flexibility as it can explain both the symmetrical and asymmetrical broadening of the lineshape. We have used it to describe and compare a thin film of cupric oxide (which mostly exhibits homogeneous heating effects) and a vertical forest of cupric oxide nanowires (which exhibit effects of inhomogeneous heating).   

Nonlocal Response of the Plasmonic Nanowire Metamaterials

Nanowire metamaterials are a class of composite photonic media formed by an array of aligned plasmonic nanowires embedded in a dielectric matrix. Depending on exact composition, geometry, and excitation wavelength, nanowire structures are known to exhibit elliptical, hyperbolic, or epsilon-near-zero (ENZ) responses. In the ENZ regime optical response of the composite becomes strongly nonlocal. Excitation of an additional wave, caused by nonlocality, has been experimentally demonstrated in nanowire-based metamaterials. Here we present numerical and analytical studies of the nonlocal optical response of plasmonic nanowire metamaterials. Dispersion of photonic modes of plasmonic metamaterials has been studied in finite-element-method (FEM) simulations as a function of wavelength, geometry, and material parameters. Analytical description of nonlocal effective permittivity tensor has been developed. These analytical results are in agreement with FEM simulations and experimental data.   

Equilibration of Luttinger liquid and conductance of quantum wires

Luttinger liquid theory describes one-dimensional electron systems in terms of non-interacting bosonic excitations. In this approximation thermal excitations are decoupled from the current flowing through a quantum wire, and the conductance is quantized. We show that relaxation processes not captured by the Luttinger liquid theory lead to equilibration of the excitations with the current and give rise to a temperature-dependent correction to the conductance. In long wires, the magnitude of the correction is expressed in terms of the velocities of bosonic excitations. In shorter wires it is controlled by the relaxation rate.   

Rectification in Y-junctions of Luttinger liquid wires

We investigate rectification of a low-frequency ac bias in Y-junctions of one-channel Luttinger liquid wires with repulsive electron interaction. Rectification emerges due to three scatterers in the wires. We find that it is possible to achieve a higher rectification current in a Y-junction than in a single wire with an asymmetric scatterer at the same interaction strength and voltage bias. The rectification effect is the strongest in the absence of the time-reversal symmetry. In that case, the maximal rectification current can be comparable with the total current $\sim e^2V/h$ even for low voltages, weak scatterers and modest interaction strength. In a certain range of low voltages, the rectification current can grow as the voltage decreases. This leads to a bump in the $I$-$V$ curve.\\[4pt] [1] Chenjie Wang and D. E. Feldman, Phys. Rev. B {\bf 83}, 045302 (2011).   

Phonon scattering in a strongly-driven 1D electron system

We consider phonon relaxation kinetics of a non-equilibrium, one-dimensional electron system driven by a strong, high frequency electric field. For a single-band system, and assuming that the phonon scattering rate is small on the scale of the driving frequency $\omega$, we derive and solve the coupled equations for the Keldysh Green's functions and self energies. In the presence of the periodic driving field, the electrons' energy is replaced by the Floquet quasienergy; in the single-band case it just equals the energy averaged over the period (modulo $\hbar\omega$). The energies of the phonons emitted or absorbed by the system correspond to transitions between the Floquet energy bands, and are strongly dependent on both the amplitude and the frequency of the driving field. Of note is a system in the regime of dynamical localization, where the average electron kinetic energy vanishes. Here, the phonon energies must be in resonance with harmonics of the driving frequency, and the stationary electron distribution function reduces to a constant, with infinite effective temperature.   

Nonequilibrium transport through a spinful quantum dot with superconducting leads

We study the nonlinear cotunneling current through a spinful quantum dot contacted by two superconducting leads. Applying a general nonequilibrium Green function formalism to an effective Kondo model, we study the rich variation in the IV-characteristics with varying asymmetry in the tunnel coupling to source and drain electrodes. The current is found to be carried respectively by multiple Andreev reflections in the symmetric limit, and by spin-induced Yu-Shiba-Russinov bound states in the strongly asymmetric limit. The interplay betweeen these two mechanisms leads to qualitatively different IV-characteristics in the cross-over regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and re-positioned conductance peaks in sub-gap cotunneling spectroscopy.