Nanoelectronic Device Metrology
Research Activities
- Spin-polarized Inelastic Electron Tunneling Spectroscopy of a Molecular Spintronic Device Investigated by EEEL Researchers. Molecular-monolayer magnetic tunnel devices have been fabricated and characterized via sophisticated tunneling spectroscopy. Molecular spintronic systems were fabricated by sandwiching a self-assembled monolayer of octanethiol between two ferromagnetic electrodes in a nanopore, demonstrating that single molecules can be used as the ultimate building blocks for spintronic devices. By using inelastic electron tunneling spectroscopy (IETS), the first unambiguous experimental evidence of the existence of molecular species in such magnetic tunnel junctions was obtained. Tunneling spectroscopy was also utilized to investigate the spin-polarized inelastic electron tunneling processes in the molecular devices. The measurements revealed that inelastic scattering due to molecular vibrations is likely the main cause of an observed junction magnetoresistance bias-dependence. These results illustrate that such inelastic scattering events must be accounted for when predicting the performance of practical molecular spintronic devices. Molecular electronic devices with spin-dependent tunneling transport behavior offer an innovative and extremely enticing direction towards spin electronics, both from fundamental and technological points of view. Due to the weak spin-orbital and hyperfine interactions in molecules, the spin coherence over time and distance could be preserved much longer in molecular nanosystems than in traditional semiconductors, which makes them a suitable playground for spin manipulations.
- Novel Approach to Investigate Buried Metal-Organic Interfaces for Molecular Electronics Developed and Utilized. In collaboration with CSTL, researchers in the NEDM have co-developed a novel characterization approach based upon backside-incident FTIR spectroscopy to investigate the top metal contact in metal-organic monolayer devices. In the emerging arena of molecular electronics, detailed characterization of organic monolayers encapsulated between two electrodes is necessary to correlate the electrical responses of molecular devices with the fundamental physical properties of the monolayers. The technique developed at NIST takes advantage of the natural infrared transparency of Si wafers to enable vibrational characterization of monolayer films after deposition of a technologically relevant metal electrode. The samples, as prepared, encapsulate the organic layer in exactly the same manner as fully fabricated devices. IR and electrical samples were fabricated simultaneously, allowing direct comparison of the spectroscopic results with electrical device performance.
- Characterization of Interfaces in Molecular-Monolayer/SiO2 Based Molecular Junctions. The results of dc-current-voltage (IV) and ac-capacitance-voltage (CV) measurements were correlated with vibrational spectroscopy of Au/monolayer/SiO2/Si structures to establish an improved understanding of the interactions at the buried metal/monolayer and dielectric/silicon interfaces. Towards the goal of observing and characterizing electrical device behavior based upon intrinsic molecular effects, the role of test structures and their interfaces must be understood and eventually effectively controlled. The novel backside-incidence Fourier-transform infrared-spectroscopy technique previously developed by members of the NIST research team was used to characterize the buried metal/molecule interface to probe the interaction of the top-metallization with the organic monolayers. Both the spectroscopic and electrical results indicate that Au has a minimal interaction with alkane monolayers deposited on SiO2 via silane chemistry. An intriguing negative-differential-resistance and hysteresis was observed in the IV measurements of Au/alkane/SiO2/Si devices. It is unlikely that this behavior is intrinsic to the simple alkane monolayers in these structures. Based on the results of extensive electrical characterization, the observed IV features are attributed to charge trapping and detrapping at both the alkane/SiO2 and the Si/SiO2 interfaces. These data illustrate that the dielectrics and other materials used when fabricating molecular devices must be made at the highest level of control to avoid impurities and defects which are likely to lead to spurious device behavior.
- Process Developed to Align Single Nanowires and Form Electrical Test Structures. A system to precisely maneuver and align individual nanowires has been developed and successfully experimentally demonstrated. This system, in which a single nanowire can be picked up and transferred to a pre-defined location by electrostatic force, overcomes a significant research challenge: the integration of semiconductor nanowires into electrical test structures. Compatible fabrication processes have been developed to simultaneously pattern multiple aligned nanowires by using one level of photo-lithography to fully integrate the nanowires into test devices for electrical characterization. Representative devices and test structures including bottom-gated silicon nanowire field-effect transistors and both transfer-length-method and Kelvin test structures have been fabricated and characterized. In addition, novel Si-nanowire-based nanoelectromechanical (NEMS) switches were made and observed to have large on/off current ratios. All these devices were fabricated from Si-nanowires grown by NEDM staff in-house at NIST by using a catalytically controlled chemical-vapor process. The fabrication of various test structures and devices by using one regular photolithographic step indicates that this novel single nanowire manipulation approach is an attractive strategy to integrate nanowires for research device applications.
- Silicon nanowires as enhancement-mode Schottky-barrier field-effect transistors. We have shown that SiNWs with Schottky contacts can be used as enhancement-mode FETs with an excellent on/off current ratio. The process does not require any source and drain doping or silicide formation, thereby allowing for a simple process without thermal annealing. Silicon nanowire field-effect transistors (SiNW FETs) were fabricated with a highly simplified integration scheme to function as Schottky barrier transistors with excellent enhancement-mode characteristics and a high on/off current ratio ~107. SiNW FETs show significant improvement in the thermal emission leakage (~6×10-13 A/μm) compared to reference FETs with a larger channel width (~7×10-10 A/μm). The drain current level depends substantially on the contact metal work function as determined by examining devices with different source-/drain- contacts of Ti (≈4.33 eV) and Cr (≈4.50 eV). The different conduction mechanisms for accumulation- and inversion-mode operation were determined and confirmed by comparison with two-dimensional numerical simulation results. Schottky barrier FETs are of great interest in their own respect as an alternative to traditional doped source and drain device structure, because sub-100 nm range scaling encounters fundamental problems including high leakage current and parasitic resistance. Schottky barrier FETs have a number of advantages including simple and low-temperature processing, good suppression of short-channel effects, and the elimination of doping and subsequent activation steps. These features are particularly desirable for SiNW devices because they can circumvent difficult fabrication issues such as an accurate control of the doping type/level and the formation of reliable ohmic contacts.
- Two-dimensional Electrostatics Enhance Channel Modulation in Dual-gated Silicon Nanowire Devices. Enhanced channel modulation in dual-gated SiNW FETs has been experimentally observed. SiNW FETs were fabricated by using electron-beam lithography to investigate the electrostatic control of current in semiconductor nanowire devices. These novel top- and bottom- gated FETs are based upon simple top-down test structures that rely upon self-aligned Schottky-contacts to enable the electronic properties of SiNWs to be readily studied. Improved device performance is observed for the dual-gated SiNW FETs when compared to simultaneously fabricated large area control FETs. The SiNW devices (with widths down to approximately 60 nm) exhibit an on/off current ratio greater than 106, which is more than 3 orders of magnitude higher than that of control devices prepared simultaneously having a large channel width (5 μm). In addition, the top gate is found to suppress ambipolar conduction effectively, which is one of the factors limiting the use of nanotube or nanowire FETs for complimentary logic applications. Two-dimensional numerical simulations have confirmed an important physical insight illustrated by this work: due to the reduced dimensionality of SiNWs, electrostatic control is enhanced when compared to larger channel width devices.
- Enhanced Inversion Mobility in Silicon Nanowire Field Effect Transistors Demonstrated. Dr. Sang-Mo Koo and colleagues have demon-strated that SiNW FETs fabricated by a standard ‘top-down’ approach exhibit substantially en-hanced transport performance. A systematic study on the inversion electron transport properties of SiNWs with different channel geometries has shown that a SiNW device exhibits enhanced inversion channel current density: the extracted electron inversion mobility of the 20 nm width nanowire channel (1000 cm2/Vs) is found to be 2 times higher than that of the reference MOSFET of large dimension (W >1 µm). The enhancement is attributed to the possible suppression of inter-valley phonon scattering due to strain in SiNW caused by the oxidation process. As the feature sizes of FETs are scaled downward, the semiconductor industry is working to meet the increasing challenges of nanoscale devices that are smaller and yet can be manufactured with minimal deviation from today’s standard manufacturing processes. These results strongly suggest that lithographically fabricated SiNW FETs, which are compatible with Si ULSI technology, can bring about significant performance benefits in nanoscale electronics, preserving the basic silicon technology infrastructure upon which current industry relies.
- Joint NIST/HP Research Progresses Toward Critical Molecular Electronics Measurements. Research at the NEDM and Hewlett Packard (HP) Laboratories is progressing toward reliable methods for measuring the electrical behavior of molecular electronic devices, an emerging nanotechnology eyed for future integrated circuits. By using a crossbar test structure consisting of a molecular monolayer sandwiched between a series of perpendicular metal wires, collaborators at separate facilities recorded nearly identical electrical measurements. This step, along with others taken to eliminate potential sources of error, ensures that the measured behavior is directly attributable to the device and not the experimental set up. Electrical (current-voltage, or IV) measurements of crossbar devices containing eicosanoic acid exhibit a controllable, two-state switching behavior that is due to the presence of the molecular layer. However, the molecular monolayer is not the sole cause. Rather, the switch-like behavior most likely arises from the interaction of the molecules with the electrodes. This example illustrates that the properties of molecular electronic devices are often determined not by the molecule alone, but by the entire device that consists of both the molecules and the attachment electrodes. This two-state behavior was independently measured in two separate laboratories, indicating that it is not a measurement artifact and illustrating that these devices are robust enough to ship via conventional methods and remain active. In addition to IV measurements, what well may be the first capacitance-voltage (CV) measurements of molecular monolayer-based devices were taken at NIST. These CV curves also show two-state behavior.
- Improved Methods to Attach Long-Chain Aliphatic Molecules to Silicon Developed. Dr. Christina Hacker and colleagues have developed an improved solution-based method for the direct attachment of long-chain aliphatic molecules to Si. In this method, ultraviolet (UV) radiation is used to assist the attachment of alcohols to the hydrogen-terminated Si(111) surface to form molecular monolayers successfully. To investi-gate the quality of these organic monolayers, they were physically and chemically characterized with infrared spectroscopy, spectroscopic ellipsometry, and contact angle measurements. The electrical properties of these organic films were probed by using IV and CV measurements obtained from a metal-organic-silicon test structure fabricated by post-monolayer metal deposition.
