Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S3: Nanoscale Metal-Semiconductor Schottky Contacts |
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Sponsoring Units: DCMP Chair: Jonathan Pelz, Ohio State University Room: LACC 515B |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S3.00001: Enhanced tunneling across nanometer-scale semiconductor interfaces Invited Speaker: A Schottky diode is one of the simplest semiconductor devices, which makes it a suitable test-case to study the performance of a nanometer- scale semiconductor-device. Experimental systems range from an STM tip pressed onto a semiconductor substrate to semiconductor nanowires in contact with a metal. All show deviations from bulk behavior which is usually attributed to a lowered Schottky barrier. We have investigated systematically the transport properties of nanometer-scale metal-semiconductor contacts, formed by self-assembled epitaxial CoSi2-islands on Si, contacted by an STM. Our measurements indicate that tunneling plays an increasingly important role for smaller devices due to a size effect in the Schottky barrier. For metal-semiconductor contacts with a diameter smaller than the Debye length, the barrier thickness decreases with the contact size. This leads to an increase in tunneling current which explains the measured transport properties without the need to assume a Schottky barrier height deviating from the bulk value. Discreteness of doping atoms can no longer be neglected and its influence on the transport properties has been investigated both experimentally and theoretically. Statistical analysis of the conductance shows that the electronic properties of small diodes are dominated by a single dopant atom close to the metal-semiconductor interface, causing local barrier lowering even at room temperature. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S3.00002: Direct Measurement of Quantum Confinement and Environmental Pinning Effects on Metal/Nanostructure Schottky Contacts Invited Speaker: I will discuss direct nm-resolution measurements of metal/quantum well (QW) Schottky contacts made using Cross- sectional Ballistic Electron Emission Microscopy (XBEEM), in order to quantify the influence of small-size effects on hot- carrier injection into semiconductor nanostructures. Molecular Beam Epitaxy was used to grow a sequence of GaAs QWs with width varying from 1nm to 15 nm, separated by thick Al$_{0.3}$Ga$_ {0.7}$As barrier layers. The samples were cleaved ex-situ and polycrystalline Au contacts were electron-beam evaporated on the cleaved edge using shadow mask or photo-lithography. Samples were studied in ultra-high vacuum using Scanning Tunneling Microscopy and XBEEM. The Schottky barrier height over the QWs was found to systematically increase with decreasing QW width, by up to $\sim$140 meV for the 1 nm QW. This is mostly due to a large quantum-confinement increase ( up to $\sim$200 meV) of the QW conduction band minimum (CBM), as estimated by a simple 1D QW model. We also did finite element electrostatic modeling to estimate the ``environmental" effects of the surrounding metal/Al$_{0.3}$Ga$_{0.7}$As interface on the QW CBM. Excellent quantitative agreement over the full QW width range is obtained when both quantum confinement and electrostatic effects are considered.I will also discuss on-going measurements to use the metal/QW nanocontacts as unique ``nano-apertures" to directly image and quantify the lateral hot-electron spreading profile in the metal film. This profile is surprisingly large, with a FWHM of $\sim$15nm ($\sim$21nm) for a 4nm (7nm) thick Au film. XBEEM images directly show that hot-electron spreading is strongly modified by the grain structure in the metal film. In collaboration with J.P. Pelz, M.K. Hudait, and S.A. Ringel. Work supported by NSF and ONR [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S3.00003: Schottky contacts for quantum dots in SiGe modulation-doped heterostructures Invited Speaker: Silicon has certain unusual properties, including a spin-0 nuclear isotope, that make quantum dots in this material excellent candidates for quantum information processing. Schottky contacts have many advantages for fabrication of such quantum dots, including large dot-gate capacitance and excellent screening of interface states. We discuss recent progress in the fabrication of silicon quantum dots using Schottky contacts to SiGe modulation-doped heterostructures. In addition to understanding the physics of electron confinement in silicon quantum dots, it is also important to understand the Hamiltonian for electrons in silicon modulation-doped quantum wells. We present measurements of the anisotropy of the spin relaxation time in silicon quantum wells, and we show results of a new spectroscopic measurement of the energy gap between the two lowest-lying valleys in silicon quantum wells. Both results have important implications for the application of Schottky-gated silicon quantum dots to quantum computation. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S3.00004: High Performance Schottky Barrier MOSFETs Invited Speaker: Schottky barrier metal-oxide-semiconductor field-effect transistor (SB MOSFETs) has been recognized as one of the potential candidates for nano-scale device applications. Nevertheless, the inherent high off-state leakage and poor on/off current ratio represent major showstoppers for nano-scale device operation. We have recently reported the fabrication of a novel SB SOI MOS transistor featuring electrical source/drain junctions induced by a sub-gate overlying the passivation oxide. Pt or Co salicide process was adopted to form the Schottky barrier source/drain (S/D). The new device is extremely simple in fabrication and requires no implantation or associated annealing steps. We have shown that, a proper sub-gate bias could not only increase the on-state current but also significantly suppress the off-state current. In addition, the device is capable of bi-channel operation, which is unique, interesting, and greatly simplifies CMOS process integration. Ambipolar device characteristics with excellent on/off current ratios ($>$10$^{8})$ have been demonstrated on a same single device with CoSi$_{2}$ S/D. Moreover, near-ideal sub-threshold swing ($\sim $ 60 mV/decade) for both n- and p-channel modes of operation could be realized on a same single device, as the planar channel length is scaled to less than 100 nm. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S3.00005: Carbon Nanotubes as Schottky Barrier Transistors Invited Speaker: Field-effect transistors (FETs) made with carbon nanotubes have many attractive features, and are being widely studied as a potential nanoscale successor to silicon FETs. Remarkably, we found that nanotube FETs generally operate by a completely different principle than ordinary Si FETs. Rather than modulate the conductance of the channel, the gate field acts to modulate the tunneling conductance of a Schottky barrier at the contact [1]. As a result, the device performance is determined by completely different factors than in familiar FETs [2-4]. In particular, the nanoscale electric field distribution near the contacts plays a crucial role. As a result, the geometry and workfunction of the contact become as important as more familiar factors like gate-oxide thickness. In addition, there are fundamental differences in the role of Fermi-level pinning at the metal-nanotube contact, compared to ordinary semiconductor interfaces [5]. \\ 1. S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, and Ph. Avouris, Phys. Rev. Lett. 89, 106801 (2002). \\ 2. S. Heinze, M. Radosavljevic, J. Tersoff, and Ph. Avouris, Phys. Rev. B 68, 235418 (2003). \\ 3. M. Radosavljevic, S. Heinze, J. Tersoff, and Ph. Avouris, Appl. Phys. Lett. 83, 2435 (2003). \\ 4. S. Heinze, J. Tersoff, and Ph. Avouris, Appl. Phys. Lett. 83, 5038 (2003). \\ 5. F. Leonard and J. Tersoff, Phys. Rev. Lett. 84, 4693 (2000). [Preview Abstract] |
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