Session W28: Focus Session: Charge Transport in Nanostructures II

Charge Transport in Silicon Nanomembranes

Charge transport in very thin semiconductor sheets, ribbons, or nanowires is dominated by surface and interface effects as a consequence of the absence of an extended bulk. In silicon, a model system for exploring these effects, factors can include interface states and fixed oxide charges if the Si nanomembrane is oxidized, surface states in chemically modified surfaces, reconstruction if the surface is clean, or a combination of these factors if the surfaces are not equivalent (e.g., one oxidized, the other clean). Additionally, in membranes or wires thin enough that quantum size effects are observable, surface roughness may influence conduction. For conventionally doped Si, effects become noticeable at nanomembrane thicknesses below $\sim $200 nm (depending on doping). We describe experiments on a platform based on (001) oriented silicon-on-insulator (SOI), using van der Pauw, Hall effect, and I-V measurements, along with scanning tunneling microscopy and diffraction, and theoretical analysis of several situations that shed light on the interplay of these factors. Measurements are compared on oxidixed membranes, clean and chemically modified surfaces on membranes, and attached and freestanding nanowires with well-defined surfaces, patterned from SOI. Most importantly, large changes in conductivity are possible with small changes in surface condition, making nanomembranes (well defined in surface orientation, thickness, and surface quality) an ideal vehicle for establishing a framework for understanding charge transport in nanostructured semiconductors. With W. Peng, S. Scott, F. Chen, J. Endres, I, Knezevic, D. Savage, M. Eriksson, C.-H. Lee, C. Ritz, M.-H. Huang, M. Ziwisky, and R. Blise \\[4pt] [1] P.P. Zhang et al., \textit{Nature} \textbf{439} 703 2006 \\[0pt] [2] S. Scott et al., \textit{ACS Nano} \textbf{3} 1683 2009 \\[0pt] [3] C-H. Lee et al., submitted   

Calculation of transmission in low symmetry lattices and its application

Conductance calculations using first-principles plane-wave method have been performed to study scattering in high symmetry lattices [1]. The Original implementation of above method in the code Quantum Espresso [2] has limitation that it only allows the transport direction along a lattice vector perpendicular to the basal plane formed by two other lattice vectors, e.g., the c-axis of a tetragonal lattice. We have generalized this method to non-orthogonal lattices with transport direction not necessarily aligned with any lattice vector. With the generalization, we have calculated transmission, reflection coefficients in transport direction and velocity of Bloch's states along various directions in the lattice. With first-principles results as input for Boltzmann's transport equations, we have obtained resistance of grain boundaries in Cu such as twin(111), $\Sigma $5(100) and $\Sigma $7(111). \\[4pt] [1] Hyoung Joon Choi and Jisoon Ihm, Phys. Rev. B 59, 2267 (1999) \\[0pt] [2] http://www.quantum-espresso.org/   

Resistance Measurements of Thin Silicon Nanomembranes in Ultra-high Vacuum

Transport properties of thin silicon nanomembranes are very sensitive to surface conditions [1, 2]. Here we report van der Pauw measurements of the sheet resistance of thin silicon nanomembranes in ultra-high vacuum (UHV) conditions (base pressure 1.2 $\times $10$^{-10}$ torr). The sample is cleaned in situ and the Si (100)-(2$\times $1) reconstruction is verified with low energy electron diffraction (LEED). The sheet resistance is then measured as a function of back gate voltage, enabling determination of the sign of the charge carriers and the influence of electric field. Simulations enable an understanding of the interaction among the silicon nanomembrane body, the empty surface $\pi $* band, and the back bonded Si-SiO$_{2}$ interface. Because of the large density of empty states in the $\pi $* band, a strong influence from the surface on the silicon nanomembrane conductance is observed. \\[4pt] [1] Zhang P. et al., \textit{Nature} \textbf{439} 703 (2006) \\[0pt] [2] Scott S. et al., \textit{ACS Nano} \textbf{3} 1683 (2009)   

Two-photon induced photocurrent imaging in CdS nanosheets

We study the photocurrent from photoexcitation of charged carriers in CdS nanosheet (NS) structures at room temperature. Schottky type contact pads separated by $\sim $ 4 $\mu $m across a $\sim $ 4 $\mu $m wide single NS were fabricated using photolithography followed by Ti/Al (20 nm/200 nm) metal evaporation and lift-off. Ar$^{+}$ bombardment before the metal deposition was used to create Ohmic contacts. For Schottky type contacts, spatial imaging of the photocurrent exhibits peak photocurrents near the reversed bias contact confirming the confinement of the electric field within the space charge region due to the applied bias voltage. For devices with the Ar$^{+}$ bombarded contacts, we observe the peak photocurrent distinctly away from the contacts. We find a nearly quadratic power dependence of photocurrent in the laser power range $\sim $0.1-10 GW/cm$^{2}$ for sub band gap excitation ($\lambda $ = 800 nm), while linear power dependence of photocurrent for above band gap excitation ($\lambda $ = 488 nm). Simple model calculations are used to obtain a two-photon absorption coefficient $\beta \quad \sim $100 cm/GW in these CdS nanosheets.   

Conduction Mechanisms in Zinc Oxide Nanowires

Due to the wide bandgap and large exciton binding energy, ZnO is recognized as a versatile material with promising applications in transparent electronics, sensors, UV photodetectors and emitters. It naturally exhibits $n$-type semiconducting behavior originating from native defects, mainly consisting of Zn interstitials or oxygen vacancies. Nanowires with diameters ranging from 20 to 100 nanometers have been synthesized via pulsed laser assisted chemical vapor deposition. The as-grown nanowires are configured into field effect transistor (FET) devices and measured in a helium cryostat. The conductivity of ZnO FET as a function of temperature shows Arrhenius behavior. For T $>$ 50 K, thermally activated conduction can be expressed as $\sigma \quad \sim $ \textit{exp}(-$E_{a}$/$k_{B}$T), where $E_{a}$ is the activation energy attributed to the shallow donor levels below the conduction band edge. For T $<$ 50 K, 3D Mott variable range hopping governs charge transport, with conductivity expressed as $\sigma \quad \sim $ \textit{exp}(-AT$^{-1/4})$. Furthermore, it has been recently observed that heavily-doped ZnO nanowires behave as a quasi-1D disordered system. The doped nanowires are measured at low temperatures in magnetic fields with directions both perpendicular and parallel to the wire axes. At low temperatures (T $<$ 20 K), the conductivity follows a power law dependence of T$^{-1/2}$. Negative magnetoresistance is characterized, with conductivity change proportional to B$^{2}$. The quadratic slope of the $\Delta \sigma -B^2$ curves for the magnetic field perpendicular to the wire axis is found to be about twice as large as for that in the parallel case. These experimental results are modeled with weak localization theory in the quasi-1D regime.   

Spatially-Resolved Photoluminescence and Photocurrent Spectroscopy of CdS Nanosheet Devices

We present a comparison of spatially-resolved photocurrent(PC) and photoluminescence(PL) images from single CdS nanosheet(NS) devices. Electrical Ti/Al contacts are fabricated across a single CdS NS using photolithography and liftoff. Dark I-V characterization shows back-to-back Schottky barriers with an unintentional doping of $\sim $10$^{16}$ cm$^{-3}$. Spatially-resolved above-gap PC imaging at room temperature shows that PC is most efficient for light absorbed near the reverse biased contact. Separately, a set of devices were fabricated using Ar$^{+}$ bombardment to create donor sulfur vacancies at the contacts. Imaging of the implanted NS devices shows the most efficient PC occurring away from the contacts. We investigate the electric field profiles of these same devices via the Stark effect by using spatially-resolved PL as a function of temperature.   

Emerging Functionality in Complex Oxides Driven by Spatial Confinement

The exotic properties displayed by correlated electronic materials (CEMs) such as the cuprates, manganites, ruthenates, Fe-based pnictides, and heavy-fermion compounds are intimately related to the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom--charge, lattice, orbital, and spin states. The striking phenomenon in these materials is due in large part to spatial electronic inhomogeneities, or nanoscale phase separation. The functionality in these CEMs is almost always associated with a phase transition, metal-to-insulator, magnetic-to-nonmagnetic, normal metal to superconductor, etc. Spatial confinement on the length scale of the inherent phase separation can probe the basic physics and reveal new emergent behavior. Several examples of the manifestation of spatial confinement will be discussed [1,2], focusing on the observed fluctuations between the competing phases [2]. Work done in collaboration with Jian Shen and Zac Ward at ORNL. \\[4pt] [1] T. Z. Ward, S. H. Liang, K. Fuchigami, L. F. Yin, E. Daggotto, E. W. Plummer, and J. Shen, ``Reemergent Metal-Insulator Transitions in Manganites Exposed with Spatial Confinement,'' Phys. Rev. Lett. 100, 247204 (2008)\\[0pt] [2] T. Z. Ward, X. G. Zhang, L. F. Yin, X. Q. Zhang, Ming Liu, P. C. Snijders, S. Jesse, E. W. Plummer, Z. H. Cheng, and J. Shen, ``Time-Resolved Electronic Phase Transitions in Manganites,'' Phys. Rev. Letters, 102, 087201 (2009).   

Spectroscopic Characterization of 1-D Excitons in Semiconductor Quantum Wires

Single-molecule microscopy studies have been performed in the frequency- and time-domains on semiconductor quantum wires (QWs) and quantum belts (QBs) to investigate the 1-D nature of excitons prepared in them. Photoluminescence (PL) intensity blinking that spans the entire lengths of QWs has been observed with continuous illumination. This blinking, which spans QWs as long as 20 microns, suggests that delocalized excitons can be formed when trap sites are filled. Multiple laser experiments have been performed to further probe the delocalization of excitons within QWs and QBs. Emission from multiple exciton states has been detected from single CdSe QWs at room temperature when using high excitation power densities. The measured PL lifetimes from the different exciton states indicate a systematic decrease with increasing population.   

Conduction via Spatially Extended Electronic States in Insulating Protein Fragments

We explore the electronic structure and transport properties stemming from the one-dimensional periodicity inherent in the backbone of all proteins. Using a combination of ab inito and semi-empirical techniques, we show that, while protein molecules are normally insulators, this periodicity gives rise to extended states resistant to disorder and able to support strong electron transport. We predict that these extended states can be accessed experimentally by electrochemical gating, resulting in colossal enhancement of the conductances of nanoscopic protein fragments bridging metal electrodes and transforming them from insulators into one-dimensional conductors.   

Charge transfer along DNA molecule within Peyrard-Bishop-Holstein model

Charge transport through DNA molecule is important in many areas ranging from DNA damage repair to molecular nanowires. It is now widely accepted that a phonon mediated hopping of a charge carrier plays a major role in charge transport through DNA. In the present study we investigate system dynamics within Peyrard-Bishop-Holstein model for the charge transfer between donor and acceptor sites. We found that an escape time of a charge, trapped at the donor state of the DNA strand, is very sensitive to the initial value of H-bond stretching. This suggests importance of ensemble averaging. Moreover sharp phase transitions were observed for escape time in parameter space of transfer integrals and phonon-charge coupling constant.