M. Saif Islam

Ultrasmooth silver thin films deposited with a germanium nucleation layer.

We demonstrate an effective method for depositing smooth silver (Ag) films on SiO(2)/Si(100) substrates using a thin seed layer of evaporated germanium (Ge). The deposited Ag films exhibit smaller root-mean-square surface roughness, narrower peak-to-valley surface topological height distribution, smaller grain-size distribution, and smaller sheet resistance in comparison to those of Ag films directly deposited on SiO(2)/Si(100) substrates. Optically thin ( approximately 10-20 nm) Ag films deposited with approximately 1-2 nm Ge nucleation layers show more than an order of magnitude improvement in the surface roughness. The presence of the thin layer of Ge changes the growth kinetics (nucleation and evolution) of the electron-beam-evaporated Ag, leading to Ag films with smooth surface morphology and high electrical conductivity. The demonstrated Ag thin films are very promising for large-scale applications as molecular anchors, optical metamaterials, plasmonic devices, and several areas of nanophotonics.

A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc.) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties. Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices. The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges. In particular, we present an ultrafast surface-illuminated PD with 11.4-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication. Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application.

Mechanical properties of self-welded silicon nanobridges

Mechanical properties of self-welded [111] single-crystal silicon nanowire bridges grown between two silicon posts using metal-catalyzed chemical vapor deposition were determined using both dynamic and static measurements. The static tests were carried out using atomic force microscopy (AFM) to measure the nanowires’ Young’s modulus and the strength of the self-welded junctions. The AFM-measured Young’s modulus ranged from 93 to 250 GPa (compared to 185 GPa for bulk silicon in the [111] direction) depending on the nanowire diameter, which ranged from 140 to 200 nm. The self-welded wire could withstand a maximum bending stress in the range of 210–830 MPa (larger than bulk silicon), which also depended on the nanowire diameter and loading conditions. The beam broke close to the loading point, rather than at the self-welded junction, indicating the excellent bond strength of the self-welded junction. The vibration spectra measured with a network analyzer and a dc magnetic field indicated a dynamic Young’s mo...

A smooth optical superlens

We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at subnanometer scales. Our measurement of the resolved lines of 30 nm half-pitch shows a full-width at half-maximum better than 37 nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens leads promise to parallel imaging and nanofabrication in a single snapshot.

Ultralow-voltage field-ionization discharge on whiskered silicon nanowires for gas-sensing applications

Several hundred million volts per centimetre of electric-field strength are required to field-ionize gas species. Such fields are produced on sharp metallic tips under a bias of a few kilovolts. Here, we show that field ionization is possible at dramatically lower fields on semiconductor nanomaterials containing surface states, particularly with metal-catalysed whiskers grown on silicon nanowires. The low-voltage field-ionization phenomena observed here cannot be explained solely on the basis of the large field-amplification effect of suspended gold nanoparticles present on the whisker tips. We postulate that field penetration causes upward band-bending at the surface of exposed silicon containing surface states in the vicinity of the catalyst. Band-bending enables the valence electron to tunnel into the surface states at reduced fields. This work provides a basis for development of low-voltage ionization sensors. Although demonstrated on silicon, low-voltage field ionization can be detected on any sharp semiconductor tip containing proper surface states.

Surface depletion thickness of p-doped silicon nanowires grown using metal-catalysed chemical vapour deposition

An accurate evaluation of the radial dopant profile in a nanowire is crucial for designing future nanoscale devices synthesized using bottom-up techniques. We developed a very slow wet chemical etchant for gradually reducing the diameters of metal-catalysed, boron-doped silicon nanowires with varying diameters and lengths. Particular care has been taken to perform the experiment at room temperature to prevent dopant segregation, which is common in high temperature processes. By ensuring identical surface conditions subsequent to diameter reduction, the resistance of the nanowires was measured and, as anticipated, was found to increase with decreasing diameter. As the diameters were shrunk using wet-chemical etching, nanowires exhibited a non-linear increase of the resistance when the diameter was reduced to ~50 nm. This is an indication of near-complete depletion in the nanowires caused by nanowire surface charges. The dopant concentration of the nanowires was found to be 2.1 × 1018 cm−3 and the corresponding surface charge density was around 2.6 × 1012 cm−2.

InP nanobridges epitaxially formed between two vertical Si surfaces by metal-catalyzed chemical vapor deposition

The heteroepitaxial growth of III-V compound semiconductors on Si would enable the integration of high-performance III-V materials with Si technology. We report the epitaxial growth on (111)-oriented Si surfaces of highly aligned, single-crystalline InP nanowires by chemical vapor deposition catalyzed by Au. We demonstrate laterally oriented InP nanowires bridging between vertical (111) Si surfaces formed by anisotropically etching a [110]-oriented Si substrate. This method of connecting nanowires offers a facile way of integrating III-V nanoelectronic and photonic devices with Si.

Nanoscale materials and devices for future communication networks

New discoveries in materials on the nanometer- length scale are expected to play an important role in addressing ongoing and future challenges in the field of communication. Devices and systems for ultra-high-speed short- and long-range communication links, portable and power-efficient computing devices, high-density memory and logics, ultra-fast interconnects, and autonomous and robust energy scavenging devices for accessing ambient intelligence and needed information will critically depend on the success of next-generation emerging nanomaterials and devices. This article presents some exciting recent developments in nanomaterials that have the potential to play a critical role in the development and transformation of future intelligent communication networks.

Harvesting and Transferring Vertical Pillar Arrays of Single-Crystal Semiconductor Devices to Arbitrary Substrates

Development of devices that can be fabricated on amorphous substrates using multiple single-crystal semiconductors with different physical, electrical, and optical characteristics is important for highly efficient portable and flexible electronics, optoelectronics, and energy conversion devices. Reducing the use of single-crystal substrates can contribute to low-cost and environmentally benign devices covering a large area. We demonstrate a technique to harvest and transfer vertically aligned single-crystal semiconductor micro- and nanopillars from a single-crystal substrate to a low-cost carrier substrate while simultaneously preserving the integrity, order, shape, and fidelity of the transferred pillar arrays. The transfer technique facilitates multilayer process integration by exploiting a vertical embossing and lateral fracturing method using a spin-coated polymer layer on a carrier substrate. Electrical contacts are formed using a bilayer of metal and conducting polymer such as gold (Au) and polyaniline (PAni). In this method, the original single-crystal substrate can be repeatedly used for generating more devices and is minimally consumed, whereas in conventional fabrication methods, the substrate is employed solely as a mechanical support. This heterogeneous integration technique potentially offers devices with low physical fill factor contributing to lower leakage current and noise, reduced parasitic capacitance, and enhanced photon-semiconductor interactions, and enables heterogeneous multimaterial integration such as silicon with compound semiconductors for rapidly expanding large-scale applications, including low-cost and flexible electronics, displays, tactile sensors, and energy conversion systems.

High Purcell Factor Due To Coupling of a Single Emitter to a Dielectric Slot Waveguide

We demonstrate an all-dielectric quantum electrodynamical nanowire-slab system with a single emitter that concentrates the extremely intense light at the scale of 10 × 75 nm(2). The quantum dot exhibits a record high 31-fold spontaneous decay rate enhancement, its optical saturation and blinking are strongly suppressed, and 80% of emission couples into a waveguide mode.