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Quantum effects in Si nanowires
RESEARCH NEWS
Quantum effects in Si nanowires NANOTECHNOLOGY
STM image and schematic of a Si nanowire with Si(001) facets.(© 2003 AAAS.)
Shuit-Tong Lee and coworkers at the City University of Hong Kong claim to have fabricated the smallest Si nanowires ever [Science (2003), 299, 1874]. Si nanowires can be fabricated in various ways, such as by the laser-ablation metal-catalytic vapor-liquid-solid method, oxideassisted catalyst-free method, or solution techniques, but until now the smallest wire diameter reported was 3-5 nm. Using an oxide-assisted growth method, in which SiO powders are heated to 1200°C in an alumina tube under a gas flow of 4% H2 in Ar, Si nanowires with diameters from a few to a few tens of nanometers were produced. The nanowires have a single
crystalline core surrounded by an oxide sheath making up about a third of the diameter. The researchers used HF to remove the surface oxide, resulting in H-terminated, 1.3-7 nm nanowires with (111) or (001) facets. Scanning tunneling microscopy (STM) in ultrahigh vacuum reveals atomic resolution images of the surface (as shown), which appears to be more stable and oxygen-resistant than standard Si wafer surfaces. Scanning tunneling spectroscopy (STS) indicates that the bandgap of the nanowires increases with decreasing diameter, from 1.1 eV for a 7 nm nanowire to 3.5 eV for a 1.3 nm wire. This trend is in agreement with theoretical predictions and provides evidence for quantum confinement effects in nanowires with diameters less than 3 nm. The researchers also confirmed other theoretical predictions, including the insensitivity of the bandgap to nanowire orientation. Lee and his team suggest that the Si nanowires could be used for UV light-emitting diodes and lasers if the wide bandgap is a direct one, which is believed to be the case for small wires. The stability of the H-terminated surface of the nanowires and the achievement of atomic resolution in STM and STS also open up exciting opportunities for the exploration of Si nanowire properties, say the researchers.
Making nanowire lattices in a snap NANOTECHNOLOGY Researchers from the California NanoSystems Institute (CNSI) at the University of California at Santa Barbara (UCSB) and Los Angeles (UCLA) have invented a new method for producing ultrahigh density arrays of nanowires [Sciencexpress (13 March 2003), 1081940]. Dubbed ‘SNAP’ for superlattice nanowire pattern transfer, the concept is based on the use of a GaAs/AlGaAs superlattice as a stamp. In effect, the technique enables translation of the atomic-scale vertical thickness control possible in thin film growth to a lateral spatial pattern. The method involves taking a GaAs/AlGaAs superlattice grown by molecular beam epitaxy (MBE), etching the AlGaAs with HF to create 20-30 nm voids between the GaAs, and then evaporating the metal nanowires. In this way, the researchers
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May 2003
were able to fabricate Au, Cr, Al, Ti, Ni, Pt, and Ni nanowires with diameters down to 5 nm and a pitch (wire separation) of 15 nm. “The SNAP process has demonstrated the smallest metal lines with the closest spacing that have ever been made,” says Pierre Petroff of UCSB. “That is an achievement in itself.” Unlike other printing and stamping techniques, the SNAP method does not require lithography to fabricate the stamp or a lift-off step. The nanowires can be simply transferred to a Si wafer by contacting the superlattice to a heat-curable epoxy film on the wafer. Curing, suspension in HPO3/H2O2/H2O or KI/I2/H2O, and an O2 plasma etch complete the transfer. The process can be repeated multiple times to create simple circuits of crossed nanowires. Spatial alignment is relatively easy
Schematic of the SNAP process. (a) GaAs-AlGaAs superlattice; (b) after selective etching of the AlGaAs; (c) metal deposition at a tilt of 36°; (d) contacting superlattice onto adhesive Si layer; (e) release of metal wires by etching GaAs oxide; (f) optional O2 plasma etch to remove adhesive layer. (Courtesy of CNSI.)
because the arrays are so long (in the order of millimeters), but only simple structures can be created. Alternatively, the nanowires can be used as masks to transfer the pattern to an underlying thin film such as Si on SiO2. As an example, the researchers created Si nanowires 20 nm wide at 30 nm and 60 nm pitches. By
suspending the nanowire arrays over a trench, they fabricated high frequency, nanomechanical resonators. “Now the question is how do we affix a contact to one wire without touching another,” says Petroff. This is one of the outstanding problems in nanotechnology, but he is confident that a way will be found.
Quantum effects in Si nanowires NANOTECHNOLOGY
STM image and schematic of a Si nanowire with Si(001) facets.(© 2003 AAAS.)
Shuit-Tong Lee and coworkers at the City University of Hong Kong claim to have fabricated the smallest Si nanowires ever [Science (2003), 299, 1874]. Si nanowires can be fabricated in various ways, such as by the laser-ablation metal-catalytic vapor-liquid-solid method, oxideassisted catalyst-free method, or solution techniques, but until now the smallest wire diameter reported was 3-5 nm. Using an oxide-assisted growth method, in which SiO powders are heated to 1200°C in an alumina tube under a gas flow of 4% H2 in Ar, Si nanowires with diameters from a few to a few tens of nanometers were produced. The nanowires have a single
crystalline core surrounded by an oxide sheath making up about a third of the diameter. The researchers used HF to remove the surface oxide, resulting in H-terminated, 1.3-7 nm nanowires with (111) or (001) facets. Scanning tunneling microscopy (STM) in ultrahigh vacuum reveals atomic resolution images of the surface (as shown), which appears to be more stable and oxygen-resistant than standard Si wafer surfaces. Scanning tunneling spectroscopy (STS) indicates that the bandgap of the nanowires increases with decreasing diameter, from 1.1 eV for a 7 nm nanowire to 3.5 eV for a 1.3 nm wire. This trend is in agreement with theoretical predictions and provides evidence for quantum confinement effects in nanowires with diameters less than 3 nm. The researchers also confirmed other theoretical predictions, including the insensitivity of the bandgap to nanowire orientation. Lee and his team suggest that the Si nanowires could be used for UV light-emitting diodes and lasers if the wide bandgap is a direct one, which is believed to be the case for small wires. The stability of the H-terminated surface of the nanowires and the achievement of atomic resolution in STM and STS also open up exciting opportunities for the exploration of Si nanowire properties, say the researchers.
Making nanowire lattices in a snap NANOTECHNOLOGY Researchers from the California NanoSystems Institute (CNSI) at the University of California at Santa Barbara (UCSB) and Los Angeles (UCLA) have invented a new method for producing ultrahigh density arrays of nanowires [Sciencexpress (13 March 2003), 1081940]. Dubbed ‘SNAP’ for superlattice nanowire pattern transfer, the concept is based on the use of a GaAs/AlGaAs superlattice as a stamp. In effect, the technique enables translation of the atomic-scale vertical thickness control possible in thin film growth to a lateral spatial pattern. The method involves taking a GaAs/AlGaAs superlattice grown by molecular beam epitaxy (MBE), etching the AlGaAs with HF to create 20-30 nm voids between the GaAs, and then evaporating the metal nanowires. In this way, the researchers
6
May 2003
were able to fabricate Au, Cr, Al, Ti, Ni, Pt, and Ni nanowires with diameters down to 5 nm and a pitch (wire separation) of 15 nm. “The SNAP process has demonstrated the smallest metal lines with the closest spacing that have ever been made,” says Pierre Petroff of UCSB. “That is an achievement in itself.” Unlike other printing and stamping techniques, the SNAP method does not require lithography to fabricate the stamp or a lift-off step. The nanowires can be simply transferred to a Si wafer by contacting the superlattice to a heat-curable epoxy film on the wafer. Curing, suspension in HPO3/H2O2/H2O or KI/I2/H2O, and an O2 plasma etch complete the transfer. The process can be repeated multiple times to create simple circuits of crossed nanowires. Spatial alignment is relatively easy
Schematic of the SNAP process. (a) GaAs-AlGaAs superlattice; (b) after selective etching of the AlGaAs; (c) metal deposition at a tilt of 36°; (d) contacting superlattice onto adhesive Si layer; (e) release of metal wires by etching GaAs oxide; (f) optional O2 plasma etch to remove adhesive layer. (Courtesy of CNSI.)
because the arrays are so long (in the order of millimeters), but only simple structures can be created. Alternatively, the nanowires can be used as masks to transfer the pattern to an underlying thin film such as Si on SiO2. As an example, the researchers created Si nanowires 20 nm wide at 30 nm and 60 nm pitches. By
suspending the nanowire arrays over a trench, they fabricated high frequency, nanomechanical resonators. “Now the question is how do we affix a contact to one wire without touching another,” says Petroff. This is one of the outstanding problems in nanotechnology, but he is confident that a way will be found.