Yue Wu

Ge/Si nanowire heterostructures as high- performance field-effect transistors

Semiconducting carbon nanotubes and nanowires are potential alternatives to planar metal-oxide-semiconductor field-effect transistors (MOSFETs) owing, for example, to their unique electronic structure and reduced carrier scattering caused by one-dimensional quantum confinement effects. Studies have demonstrated long carrier mean free paths at room temperature in both carbon nanotubes and Ge/Si core/shell nanowires. In the case of carbon nanotube FETs, devices have been fabricated that work close to the ballistic limit. Applications of high-performance carbon nanotube FETs have been hindered, however, by difficulties in producing uniform semiconducting nanotubes, a factor not limiting nanowires, which have been prepared with reproducible electronic properties in high yield as required for large-scale integrated systems. Yet whether nanowire field-effect transistors (NWFETs) can indeed outperform their planar counterparts is still unclear. Here we report studies on Ge/Si core/shell nanowire heterostructures configured as FETs using high-κ dielectrics in a top-gate geometry. The clean one-dimensional hole-gas in the Ge/Si nanowire heterostructures and enhanced gate coupling with high-κ dielectrics give high-performance FETs values of the scaled transconductance (3.3u2009mSu2009µm-1) and on-current (2.1u2009mAu2009µm-1) that are three to four times greater than state-of-the-art MOSFETs and are the highest obtained on NWFETs. Furthermore, comparison of the intrinsic switching delay, τ = CV/I, which represents a key metric for device applications, shows that the performance of Ge/Si NWFETs is comparable to similar length carbon nanotube FETs and substantially exceeds the length-dependent scaling of planar silicon MOSFETs.

Single-crystal metallic nanowires and metal/semiconductor nanowire heterostructures

Substantial effort has been placed on developing semiconducting carbon nanotubes1,2,3 and nanowires4 as building blocks for electronic devices—such as field-effect transistors—that could replace conventional silicon transistors in hybrid electronics or lead to stand-alone nanosystems4,5. Attaching electric contacts to individual devices is a first step towards integration, and this step has been addressed using lithographically defined metal electrodes1,2,3,4,6,7,8. Yet, these metal contacts define a size scale that is much larger than the nanometre-scale building blocks, thus limiting many potential advantages. Here we report an integrated contact and interconnection solution that overcomes this size constraint through selective transformation of silicon nanowires into metallic nickel silicide (NiSi) nanowires. Electrical measurements show that the single crystal nickel silicide nanowires have ideal resistivities of about 10u2009µΩu2009cm and remarkably high failure-current densities, >108u2009Au2009cm-2. In addition, we demonstrate the fabrication of nickel silicide/silicon (NiSi/Si) nanowire heterostructures with atomically sharp metal–semiconductor interfaces. We produce field-effect transistors based on those heterostructures in which the source–drain contacts are defined by the metallic NiSi nanowire regions. Our approach is fully compatible with conventional planar silicon electronics and extendable to the 10-nm scale using a crossed-nanowire architecture.Substantial effort has been placed on developing semiconducting carbon nanotubes and nanowires as building blocks for electronic devices—such as field-effect transistors—that could replace conventional silicon transistors in hybrid electronics or lead to stand-alone nanosystems. Attaching electric contacts to individual devices is a first step towards integration, and this step has been addressed using lithographically defined metal electrodes. Yet, these metal contacts define a size scale that is much larger than the nanometre-scale building blocks, thus limiting many potential advantages. Here we report an integrated contact and interconnection solution that overcomes this size constraint through selective transformation of silicon nanowires into metallic nickel silicide (NiSi) nanowires. Electrical measurements show that the single crystal nickel silicide nanowires have ideal resistivities of about 10u2009µΩu2009cm and remarkably high failure-current densities, >108u2009Au2009cm-2. In addition, we demonstrate the fabrication of nickel silicide/silicon (NiSi/Si) nanowire heterostructures with atomically sharp metal–semiconductor interfaces. We produce field-effect transistors based on those heterostructures in which the source–drain contacts are defined by the metallic NiSi nanowire regions. Our approach is fully compatible with conventional planar silicon electronics and extendable to the 10-nm scale using a crossed-nanowire architecture.

One-dimensional hole gas in germanium/silicon nanowire heterostructures

Two-dimensional electron and hole gas systems, enabled through band structure design and epitaxial growth on planar substrates, have served as key platforms for fundamental condensed matter research and high-performance devices. The analogous development of one-dimensional (1D) electron or hole gas systems through controlled growth on 1D nanostructure substrates, which could open up opportunities beyond existing carbon nanotube and nanowire systems, has not been realized. Here, we report the synthesis and transport studies of a 1D hole gas system based on a free-standing germanium/silicon (Ge/Si) core/shell nanowire heterostructure. Room temperature electrical transport measurements clearly show hole accumulation in undoped Ge/Si nanowire heterostructures, in contrast to control experiments on single-component nanowires. Low-temperature studies show well-controlled Coulomb blockade oscillations when the Si shell serves as a tunnel barrier to the hole gas in the Ge channel. Transparent contacts to the hole gas also have been reproducibly achieved by thermal annealing. In such devices, we observe conductance quantization at low temperatures, corresponding to ballistic transport through 1D subbands, where the measured subband energy spacings agree with calculations for a cylindrical confinement potential. In addition, we observe a 0.7 structure, which has been attributed to spontaneous spin polarization, suggesting the universality of this phenomenon in interacting 1D systems. Lastly, the conductance exhibits little temperature dependence, consistent with our calculation of reduced backscattering in this 1D system, and suggests that transport is ballistic even at room temperature.

Radial addressing of nanowires

We introduce radial encoding of nanowires (NWs), a new method of differentiating and controlling NWs by a small set of mesoscale wires for use in crossbar memories. We describe methods of controlling these NWs and give efficient manufacturing algorithms. These new encoding and decoding methods do not suffer from the misalignment characteristic of flow-aligned NWs. They achieve comparable effective pitch and resulting memory density with axially encoded NWs, while avoiding potential cases of address ambiguity and simplifying NW preparation. We also explore hybrid axial/radial encodings and show that they offer no net benefit over pure codes.