Session V28: Focus Session: Charge Transport in Nanostructures I

Nanoelectronics and quantum transport based on semiconductor nanowires

Semiconductor nanowires represent a uniquely powerful platform for exploring a diverse range of physical phenomena at the nanoscale due to the demonstrated capabilities of rational design and precise control of diameter, composition, morphology electronic properties during synthesis. In this talk, we will review advances of nanowires as high performance transistor and quantum devices with a focus on the prototypical Ge/Si core/shell nanowire heterostructure model system. First, a clean one-dimensional hole gas is formed due to band structure design, which sustains ballistic transport up to room temperature. Large subband spacing indicates a truly one-dimensional channel. Second, field-effect transistors utilizing Ge/Si nanowire heterostructure as the active channel are discussed with results demonstrating that these devices can outperform state-of-the-art Si MOSFETs. Third, the scaling of transistors with sub-100 nm channels are discussed with respect to pushing performance limit. Measurements and analyses show that devices with channel lengths down to 40 nm operate close to the ballistic limit and provide an intrinsic speed of 2 THz. Finally, advances in top-gate defined multi-quantum dot devices are reviewed, where control of contact transparency is used to enable studies in different quantum regimes. A fully tunable double quantum dot with integrated charge sensor is demonstrated. The characterization of charge transport and spin states, as well as its promise as a long coherence time spin qubit will be discussed.   

Effect of Surface-optical Phonons on the Charge Transport in Wrap-gated Semiconducting Nanowire Field-effect Transistors

Surface phonons (SO-phonons) arise at the boundary of two different dielectric mediums. Though the effect of electron-surface phonon scattering on low-filed charge transport has been studied extensively for thin Si-MOSFET [1] and graphene [2], its effect on the 1D nanowire devices has not studied so far. Vibrating diploes in polar gate-dielectric induces a time-varying potential inside the nanowires. The frequencies of these time-varying fields have been calculated by implementing electrostatic boundary conditions at different interfaces of nanowire-dielectric-metal system. Our calculation shows that the electron-SO phonon interaction strength decays exponentially from the gate-nanowire interface towards the nanowire axis. Electron-SO phonon scattering rate has been calculated using Boltzmann transport equation under relaxation time approximation. We find that for thin nanowires (radius 1-20 nm), electron-SO phonon scattering rate is comparable to other dominant scattering mechanisms (such as impurity and bulk optical phonon scatterings) and reduces carrier mobility significantly. Calculating surface-phonon limited mobility of Si nanowires on various available common dielectrics, we have predicted the optimum choice of gate-dielectrics for nanowire-based electronic devices. \\[4pt] [1] M. V. Fischetti et. al J. Appl. Phys. 90 4581 (2001). \\[0pt] [2] A. Konar \textit{et. al}. arXiv: 0902.0819.   

Study of electrical conductivity and Schottky contacts in single gallium nitride nanowire by Atomic Force Microscope

We report the electrical conductivity of individual n-type gallium nitride (GaN) nanowires grown by the vapor-liquid-solid process. Longitudinal and transverse conductivity of bare and gold-decorated GaN nanowires were studied using conductive atomic force microscope (C-AFM) at room temperature. The devices were manufactured by photo-lithography technique in the nanotechnology cleanroom. We explored the nanodevices by scanning electron microscope (SEM) and identified the nanowires with good electrical contacts. The same nanowires were investigated under C-AFM for conductivity measurements. We observed a decrease in conductivity in terms of magnitude on gold-decorated nanowires compared to bare GaN nanowires due to the formation of depletion region. Schottky contacts employing individual n-type GaN nanowires were realized using Pt/Ir coated silicon AFM tip.   

Theoretical study of conductance in kinked nanowires

Controlled growth of single-crystalline kinked semiconductor nanowires has recently been observed experimentally. The wires could be key in the integration of nano-scale devices. The conductance of a perfect nanowire is an integer multiple of the quantum unit of conductance. Using first-principles transport calculations we have studied how the conductance properties of nanowires change due to kinks and turns. We used mono-atomic chains as well as 1-4 nm diameter Si nanowires as prototypical examples. We have found that the transmission strongly depends on the geometry of the kinks, especially for thin nanowires.   

High registry semiconducting nanowire growth and fabrication of out-of-plane nanowire device arrays

We present results on the fabrication of out-of-plane nanowire devices from metal nanoparticle seeds, assembled with high registry using a combination of e-beam lithography and chemical recognition assembly. The registry of the nanoparticle seeds allows the growth of nanowires at predefined locations followed by fabrication of top electrodes using predesigned lithographic masks. This approach enables large scale device array fabrication, which has not yet been demonstrated in vertical device geometry. The nanowires were grown from Au seeds using the vapor-liquid-solid (VLS) method in a cold-wall low pressure CVD system with good control over epitaxy, morphology and doping. Out-of-plane devices were fabricated from the nanowires using post-growth processes including planarization after SiO$_{2}$ layer deposition. Due to the precision of location of the nanowires, top metal contacts on the nanowires were then deposited using lithographic techniques, introducing the possibility of addressing single or a group of nanowires. The electrical characterization of the devices indicated integrity of the nanowires during rigorous device fabrication process. We will discuss the device fabrication process along with results from the device characterization.   

Photocurrent Spectroscopy of single ZB, WZ InP Nanowires at Low Temperature

We use photocurrent spectroscopy to study single InP nanowire devices having either zinc-blende(ZB) or wurtzite(WZ) crystal structures as a function of temperature. Photolithography techniques are used to fabricate the Ti/Al metal contact pads separated by 3 microns on several ZB or WZ InP nanowires. Using a tunable (1.3 to 1.55 eV) CW laser, we obtain current-voltage (I-V) photocurrent curves by broad illumination of the InP nanodevice. At room temperature, we find that the photocurrent drops exponentially for photon energies below the fundamental band edge, showing evidence for an Urbach tail. The WZ energy gap (1.408eV) lies 70meV above the ZB energy gap (1.338eV) at room temperature, consistent with previous photoluminescence measurements. The ZB sample at 10K shows strong evidence for a broadened excitonic resonance peak in the photocurrent which lies on the top of the Urbach tail for the ZB device. Analysis of this low temperature photocurrent spectrum is consistent with an exciton absorption peak 5 meV below the fundamental band edge, consistent with the known exciton binding energy. Support provided by the NSF ({\#}0701703, {\#}0806700, {\#}0806572 and {\#}0804199) and the Australian Research Council.   

Electron dynamics in nanostructures subjected to a laser field

Recent experiments (Zhu et al., J. Appl. Phys. 102, 114302 (2007); Gabor et al., Science, 325, 1367 (2009)) have shown that application of a laser field can significantly influence the electron dynamics in nanostructures. The study of such phenomena is vital both for fundamental understanding as well as for technological applications. We use time-dependent density functional theory to study how laser fields affect electron dynamics in nanostructures. Examples include the enhancement of field emission from carbon nanotubes (CNT) and effects on transport properties of a CNT-based nanowire.   

Electronic Transport in Molecular Diode Heterojunctions

Anode-donor, donor-acceptor, and acceptor-cathode interfaces dominate the performance of organic solar cells. However, within thin-film, bulk-heterojunction, or nanostructured morphologies, interfacial transport affects are not well understood. In order to better understand these interfaces, a simplified system consisting of a single, small, diode molecule covalently bound to electrodes (anode-end group-donor-bridge-acceptor-end group-cathode) is considered. The end groups and bridge moities can be interchanged using chemical synthesis technique to understand how these parameters affect electronic transport. Here, we report our findings on single-molecule diode measurements using a conducting atomic force microscope on four newly synthesized molecules consisting of bithiophene donors and naphthalene diimide acceptors with systematic interchange of two end groups and two bridge moities. We explain the electronic structure of these molecules using absorption and fluorescence spectrometry, cyclic voltammetry, and transition voltage spectrometry in conjunction with newly developed theory.   

Effect of a Coulombic dot-lead coupling on the dynamics of a quantum dot

We investigate the effect of a Coulombic coupling on the dynamics of a quantum dot coupled to leads. Two cases are studied: a dot coupled to a Luttinger liquid and a dot coupled to a three-dimensional metallic lead. The leading divergences arising from the long-ranged Coulomb interaction are found to cancel, resulting in a slow decay of electronic correlations, controlled by subleading divergences. Explicit results are given for the short-time dynamics.   

Capture and emission of an electron from a single trap in a InAs semiconductor nanowire and influence on its conductance

Experiments on random telegraph noise in InAs nanowires demonstrate regimes of gate voltage where their conductance is appreciable but also intriguingly sensitive to trapping of a single electron. When a defect is charged repulsively/discharged, we observe transitions from a conductive to an insulating state, and vice versa. The phenomena is tied directly to the screened potential produced by the repulsive defect. We discuss the dependence of the conductance switching amplitude on electron density, nanowire diameter, and dielectric constant of the nanowire's surroundings. We also discuss electronic properties of traps we've encountered as well as trapping and de-trapping mechanisms. The results are of significance for fundamental studies of defects in one dimensional conductors, as well as device-oriented applications of nanowires such as nano-electronic memories and sensing devices whose performance is tied to their sensitivity to charge.