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ABSTRACT: In this Letter we present the electrical and electro-optical characterization of single crystalline germanium nanowires (NWs) under tensile strain conditions. The measurements were performed on vapor-liquid-solid (VLS) grown germanium (Ge) NWs, monolithically integrated into a micromechanical 3-point strain module. Uniaxial stress is applied along the ⟨111⟩ growth direction of individual, 100 nm thick Ge NWs while at the same time performing electrical and optical characterization at room temperature. Compared to bulk germanium, an anomalously high and negative-signed piezoresistive coefficient has been found. Spectrally resolved photocurrent characterization on strained NWs gives experimental evidence on the strain-induced modifications of the band structure. Particularly we are revealing a rapid decrease in resistivity and a red-shift in photocurrent spectra under high strain conditions. For a tensile strain of 1.8%, resistivity decreased by a factor of 30, and the photocurrent spectra shifted by 88 meV. Individual stressed NWs are recognized as an ideal platform for the exploration of strain-related electronic and optical effects and may contribute significantly to the realization of novel optoelectronic devices, strain-enhanced field-effect transistors (FETs), or highly sensitive strain gauges.
Nano Letters 11/2012; · 13.20 Impact Factor
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Applied Surface Science 01/2012; 258(8):3444-3449. · 2.10 Impact Factor
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Journal of Micromechanics and Microengineering 01/2012; 22(8). · 2.11 Impact Factor
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Thin Solid Films 01/2012; 520(19):6230-6232. · 1.89 Impact Factor
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ABSTRACT: Recently focused-electron-beam-induced etching of silicon using molecular chlorine (Cl(2)-FEBIE) has been developed as a reliable and reproducible process capable of damage-free, maskless and resistless removal of silicon. As any electron-beam-induced processing is considered non-destructive and implantation-free due to the absence of ion bombardment this approach is also a potential method for removing focused-ion-beam (FIB)-inflicted crystal damage and ion implantation. We show that Cl(2)-FEBIE is capable of removing FIB-induced amorphization and gallium ion implantation after processing of surfaces with a focused ion beam. TEM analysis proves that the method Cl(2)-FEBIE is non-destructive and therefore retains crystallinity. It is shown that Cl(2)-FEBIE of amorphous silicon when compared to crystalline silicon can be up to 25 times faster, depending on the degree of amorphization. Also, using this method it has become possible for the first time to directly investigate damage caused by FIB exposure in a top-down view utilizing a localized chemical reaction, i.e. without the need for TEM sample preparation. We show that gallium fluences above 4 × 10(15) cm(-2) result in altered material resulting from FIB-induced processes down to a depth of ∼ 250 nm. With increasing gallium fluences, due to a significant gallium concentration close beneath the surface, removal of the topmost layer by Cl(2)-FEBIE becomes difficult, indicating that gallium serves as an etch stop for Cl(2)-FEBIE.
Nanotechnology 06/2011; 22(23):235302. · 3.98 Impact Factor
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ABSTRACT: We demonstrate a reliable microfabrication process for a combined atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) measurement tool. Integrated cone-shaped sensors with boron doped diamond (BDD) or gold (Au) electrodes were fabricated from commercially available AFM probes. The sensor formation process is based on mature semiconductor processing techniques, including focused ion beam (FIB) machining, and highly selective reactive ion etching (RIE). The fabrication approach preserves the geometry of the original AFM tips resulting in well reproducible nanoscaled sensors. The feasibility and functionality of the fully featured tips are demonstrated by cyclic voltammetry, showing good agreement between the measured and calculated currents of the cone-shaped AFM-SECM electrodes.
Nanotechnology 04/2011; 22(14):145306. · 3.98 Impact Factor
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ABSTRACT: Schottky barrier SOI-MOSFETs incorporating a La(2)O(3)/ZrO(2) high-k dielectric stack deposited by atomic layer deposition are investigated. As the La precursor tris(N,N'-diisopropylformamidinato) lanthanum is used. As a mid-gap metal gate electrode TiN capped with W is applied. Processing parameters are optimized to issue a minimal overall thermal budget and an improved device performance. As a result, the overall thermal load was kept as low as 350, 400 or 500 °C. Excellent drive current properties, low interface trap densities of 1.9 × 10(11) eV(-1) cm(-2), a low subthreshold slope of 70-80 mV/decade, and an I(ON)/I(OFF) current ratio greater than 2 × 10(6) are obtained.
Microelectronic Engineering 03/2011; 88(3):262-267. · 1.56 Impact Factor
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ABSTRACT: Dielectric thin films of La<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> deposited by atomic layer deposition (ALD) are investigated to be employed in Ge Schottky barrier p-MOSFETs. La<sub>2</sub>O<sub>3</sub> is used as a thin passivation layer and is capped by atomic-layer-deposited ZrO<sub>2</sub> as a gate dielectric. As the gate contact TiN capped by W is applied, midgap-level trap densities of ~ 3-4 × 10<sup>12</sup> eV<sup>-1</sup> cm<sup>-2</sup> and subtreshold slopes down to 115-120 mV/dec are achieved. The devices show negative threshold voltages of -0.5 to -0.6 V, as well as peak hole mobility values of ~ 50-75 cm<sup>2</sup>/V · s. Equivalent oxide thickness (EOT) is reduced to 0.96 nm upon postmetallization annealing without degrading the interface properties. The results show the scaling potential of the ALD La<sub>2</sub>O<sub>3</sub> interlayer capped with ZrO<sub>2</sub> gate dielectrics for the integration into sub-1-nm EOT Ge p-MOSFET devices.
IEEE Transactions on Electron Devices 01/2011; · 2.32 Impact Factor
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ABSTRACT: In this work, we demonstrate an approach to tune the electrical behavior of our Ω-gated germanium-nanowire (Ge-NW) MOSFETs by focused ion beam (FIB) implantation. For the MOSFETs, 35 nm thick Ge-NWs are covered by atomic layer deposition (ALD) of a high-κ gate dielectric. With the Ω-shaped metal gate acting as implantation mask, highly doped source/drain (S/D) contacts are formed in a self-aligned process by FIB implantation. Notably, without any dopant activation by annealing, the devices exhibit more than three orders of magnitude higher I(ON) currents, an improved I(ON)/I(OFF) ratio, a higher mobility and a reduced subthreshold slope of 140 mV/decade compared to identical Ge-NW MOSFETs without FIB implantation.
Nanotechnology 01/2011; 22(3):035201. · 3.98 Impact Factor
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Microelectronic Engineering 01/2011; 88(3):262-267. · 1.56 Impact Factor
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Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures. 01/2011; 29(1):01A8061-01A8067.
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ABSTRACT: For the first time focused electron beam induced etching of silicon using molecular chlorine has been developed as reliable and reproducible process. Around the etched pits the etching process was found to be accompanied by carbonaceous deposition from hydrocarbon contamination of the residual gas typically present in a scanning electron microscope (SEM). This work will focus on the effect of residual gas deposition on this silicon etch process using chlorine. The process mechanisms for residual gas deposition (in the absence of chlorine) as well as the silicon etch process in the presence of chlorine are discussed in terms of precursor replenishment of the sample surface and precursor delivery to the processed area. The formation of carbonaceous deposits from undesired contaminants can block the surface from an etch process. Especially for low beam currents this problem was encountered. Results suggest that the presence of chlorine when processed with a high electron beam current dominates over residual gas deposition. On the other hand, the etch-inhibiting effect of an increased level of SEM chamber contamination (resulting in increased residual gas deposition rates) is shown.
Journal of Applied Physics 12/2010; 108(12):124316-124316-8. · 2.17 Impact Factor
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ABSTRACT: The applicability of a novel silicon precursor with respect to reasonable nanowire (NW) growth rates, feasibility of epitaxial NW growth and versatility with respect to diverse catalysts was investigated. Epitaxial growth of Si-NWs was achieved using octochlorotrisilane (OCTS) as Si precursor and Au as catalyst. In contrast to the synthesis approach with SiCl(4) as precursor, OCTS provides Si without the addition of H(2). By optimizing the growth conditions, effective NW synthesis is shown for alternative catalysts, in particular, Cu, Ag, Ni, and Pt with the latter two being compatible to complementary metal-oxide-semiconductor technology. As for these catalysts, the growth temperatures are lower than the lowest liquid eutectic; we suggest that the catalyst particle is in the solid state during NW growth and that a solid-phase diffusion process, either in the bulk, on the surface, or both, must be responsible for NW nucleation.
Nano Letters 10/2010; 10(10):3957-61. · 13.20 Impact Factor
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ABSTRACT: Ge nanowires (NWs) about 2 µm long and 35 nm in diameter are grown heteroepitaxially on Si(111) substrates in a hot wall low-pressure chemical vapor deposition (LP-CVD) system using Au as a catalyst and GeH(4) as precursor. Individual NWs are contacted to Cu pads via e-beam lithography, thermal evaporation and lift-off techniques. Self-aligned and atomically sharp quasi-metallic copper-germanide source/drain contacts are achieved by a thermal activated phase formation process. The Cu(3)Ge segments emerge from the Cu contact pads through axial diffusion of Cu which was controlled in situ by SEM, thus the active channel length of the MOSFET is adjusted without any restrictions from a lithographic process. Finally the conductivity of the channel is enhanced by Ga(+) implantation leading to a high performance Ω-gated Ge-NW MOSFET with saturation currents of a few microamperes.
Nanotechnology 09/2010; 21(43):435704. · 3.98 Impact Factor
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ABSTRACT: In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.
Nano Letters 08/2010; 10(8):3204-8. · 13.20 Impact Factor
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ABSTRACT: A new beam-assisted process for removing silicon from a surface in the nanometer scale in a conventional scanning electron microscope is presented. This approach is based on focused electron beam induced etching with pure chlorine gas being used as the precursor. In contrast to the established etching process using a focused ion beam (with or without the addition of a precursor), no amorphization and gallium implanting of the substrate takes place. The observed low etch rates facilitate removal with sub-nanometer precision. No spontaneous etching of silicon as in the case of xenon difluoride was observed. Etch rates of up to 4 nm min( - 1) could be achieved as well as a minimum feature size of below 80 nm. The effect of etching parameters like electron beam energy, electron beam accelerating voltage or pixel spacing were systematically examined. Finally, the underlying etching mechanism in terms of secondary electron interactions and precursor replenishment is discussed.
Nanotechnology 07/2010; 21(28):285306. · 3.98 Impact Factor
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ABSTRACT: In this work, an unusual low frequency behavior observed in scanning capacitance microscopy/spectroscopy on GaAs / Al <sub>2</sub> O <sub>3</sub> junctions is investigated. Using a two-dimensional simulation, we show that the frequency behavior of the capacitance—voltage curves can be explained through an increased minority carrier concentration at to the GaAs – Al <sub>2</sub> O <sub>3</sub> interface and tip geometry effects on the nanoscale. An analytic approach to estimate the transition frequency between the low frequency and high frequency regime is also given.
Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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ABSTRACT: We demonstrate the simultaneous vertical integration of self contacting and highly oriented nanowires (NWs) into airbridge structures. With the use of conventional semiconductor processing techniques and vapour-liquid-solid (VLS) growth a suspended vertical NW architecture is formed on a silicon on insulator (SOI) substrate where the nanodevice will later be fabricated on. The VLS grown Si-NWs are contacted to prepatterned airbridges by a self aligned process and there is no need for post-growth NW assembly or alignment. To demonstrate the potential of this method surround gated vertical MOSFETs have been fabricated with a highly simplified integration scheme combining top-down and bottom-up approaches.
Nanoelectronics Conference (INEC), 2010 3rd International; 02/2010
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Applied Surface Science 01/2010; 256(16):5031-5034. · 2.10 Impact Factor
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ABSTRACT: The impact of the ZrO2/La2O3 film thickness ratio and the post deposition annealing in the temperature range between 400 °C and 600 °C on the electrical properties of ultrathin ZrO2/La2O3 high-k dielectrics grown by atomic layer deposition on (1 0 0) germanium is investigated. As-deposited stacks have a relative dielectric constant of 24 which is increased to a value of 35 after annealing at 500 °C due to the stabilization of tetragonal/cubic ZrO2 phases. This effect depends on the absolute thickness of ZrO2 within the dielectric stack and is limited due to possible interfacial reactions at the oxide/Ge interface. We show that adequate processing leads to very high-k dielectrics with EOT values below 1 nm, leakage current densities in the range of 0.01 A/cm2, and interface trap densities in the range of 2-5 × 1012 eV-1 cm-2. © 2010 Elsevier B.V. All rights reserved.
Applied Surface Science. 01/2010; 256(16):5031-5034.
