Array Of Nanoscale Holes Research Articles

Optimization of Contact Resistance and DC Characteristics for AlGaN/GaN HEMTs Utilizing Sub-10 nm Nanohole Etching

In this paper, the contact resistance of AlGaN/GaN high electron mobility transistor (HEMT) was improved by introducing nanoscale hole arrays in ohmic regions, and the DC characteristics of the conventional structure and nanohole etching structure for HEMTs were measured for comparison. Sub-10 nm nanoholes were patterned on the ohmic area surface of AlGaN using electron beam lithography and a low-temperature short-time development. Various dwell times of e-beam exposure from 5 to 30 μs were investigated and the corresponding contact resistance of the nano hole etching structure and planar structure were compared by the transmission line model (TLM) method. We observed a reduced contact resistance from 1.82 to 0.47 Ω-mm by performing a dwell time of 5 μs of exposure for nanohole formation compared to the conventional structure. Furthermore, the DC characteristics demonstrate that the maximum drain current for HEMTs was enhanced from 319 to 496 mA/mm by utilizing this optimized ohmic contact. These results show that devices with sub-10 nm nanohole ohmic contacts exhibit an improved contact resistance over the conventional structure, optimizing device performance for HEMTs, including a lower on-resistance and higher maximum drain current.

Terahertz radiation enhancement in gallium arsenide nano-hole array under low power optical pump

Though accelerating photocarriers by the semiconductor surface electric field is the simplest way to generate broadband terahertz pulses, the weak THz power under low optical pump hinders its application in these compact systems. Here, we report a ⟨100⟩ semi-insulating gallium arsenide nano-hole array under above-the-bandgap excitation, which boosts terahertz emission power up to 5.75 folds of bare gallium arsenide with a 32 mW pump. The nano-structured array lifts the absorption of the optical pump and localizes the photocarriers near the surface of gallium arsenide, benefiting the transient photocurrents and thus the THz power. Interestingly, the enhancement is poorly related to the terahertz frequency, and the power difference of the emitted THz wave under the TE and TM pumps is greatly smoothed. In addition, the THz emission enhancement of the nanoscale hole arrays favors a low-power pump. The demonstration shown here provides a potential route for advancing the weak THz power of surface emission, which will promote the application of the surface emitting THz source in the practical THz systems employing compact femtosecond lasers.

Superconducting diode effect via conformal-mapped nanoholes

A superconducting diode is an electronic device that conducts supercurrent and exhibits zero resistance primarily for one direction of applied current. Such a dissipationless diode is a desirable unit for constructing electronic circuits with ultralow power consumption. However, realizing a superconducting diode is fundamentally and technologically challenging, as it usually requires a material structure without a centre of inversion, which is scarce among superconducting materials. Here, we demonstrate a superconducting diode achieved in a conventional superconducting film patterned with a conformal array of nanoscale holes, which breaks the spatial inversion symmetry. We showcase the superconducting diode effect through switchable and reversible rectification signals, which can be three orders of magnitude larger than that from a flux-quantum diode. The introduction of conformal potential landscapes for creating a superconducting diode is thereby proven as a convenient, tunable, yet vastly advantageous tool for superconducting electronics. This could be readily applicable to any superconducting materials, including cuprates and iron-based superconductors that have higher transition temperatures and are desirable in device applications.

Merging transformation optics with electron-driven photon sources

Relativistic electron beams create optical radiation when interacting with tailored nanostructures. This phenomenon has been so far used to design grating-based and holographic electron-driven photon sources. It has been proposed recently that such sources can be used for hybrid electron- and light-based spectroscopy techniques. However, this demands the design of a thin-film source suitable for electron-microscopy applications. Here, we present a mesoscopic structure composed of an array of nanoscale holes in a gold film which is designed using transformation optics and delivers ultrashort chirped electromagnetic wave packets upon 30–200 keV electron irradiation. The femtosecond photon bunches result from coherent scattering of surface plasmon polaritons with hyperbolic dispersion. They decay by radiation in a broad spectral band which is focused into a 1.5 micrometer beam waist. The focusing ability and broadband nature of this photon source will initiate applications in ultrafast spectral interferometry techniques.

Transferrable Plasmonic Au Thin Film Containing Sub-20 nm Nanohole Array Constructed via High-Resolution Polymer Self-Assembly and Nanotransfer Printing.

The fabrication and characterization of nanoscale hole arrays (NHA) have been extensively performed for a variety of unique characteristics including extraordinary optical transmission phenomenon observed for plasmonic NHAs. Although the size miniaturization and hole densification are strongly required for enhancement of high-frequency optical responses, from a practical point-of-view, it is still not straightforward to manufacture NHA using conventional lithography techniques. Herein, a facile, cost-effective, and transferrable fabrication route for high-resolution and high-density NHA with sub-50 nm periodicity is demonstrated. Solvent-assisted nanotransfer printing with ultrahigh-resolution combined with block copolymer self-assembly is used to fabricate well-defined Si nanomesh master template with 4-fold symmetry. An Au NHA film on quartz substrate is then obtained by thermal-evaporation on the Si master and subsequent transfer of the sample, resulting in NHA structure having a hole with a diameter of 18 nm and a density over 400 holes/μm2. A resonance peak at the wavelength of 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film. Finite-difference time-domain (FDTD) simulation results propose that the unexpected peak appears because of plasmonic surface guiding mode. The position of the resonance peak shows the sensitivity toward the change of the refractive index of surrounding medium, suggesting it as a promising label-free sensor application. In addition, other types of Au nanostructure arrays such as geometry-controlled NHA and nanoparticle arrays (NPAs) shows the outstanding versatility of our approach.

Polymer Periodic Nanostructures on Curved Substrates by UV-Curable Hybrid Soft Nanoimprint Lithography

With rapidly developing nonplanar flexible electronics and photonics devices, high-throughput and low-cost fabrication of polymer periodic nanostructures on curved substrates has become a significant research area. Herein, we report fabrication of different polymer periodic nanostructures directly on cylinder substrate by using hybrid soft nanoimprint lithography (NIL) at room environment. The composite stamp was made up of hybrid soft nanoimprint mold that includes a thin rigid features layer and thick elastomeric poly(dimethylsiloxanes) (PDMS) support layer. Ultraviolet (UV)-curable thiol-ene material has good properties, such as low viscosity, high resolution, rapid reaction rate in benign ambient, and is particularly suitable as nanoimprint resist. Periodic nanopillar arrays, nanoscale hole arrays, and nanograting were fabricated on large-area curved substrate based on this new method. The nanostructures had high resolution and were precisely distributed uniformly onto the curved surfaces.

Bilayer self-assembly on a hydrophilic, deterministically nanopatterned surface

We present measurements of the in situ, microscopic architecture of a self-assembled bilayer at the interface between a regularly nanopatterned surface and an aqueous sub-phase using neutron reflectometry. The substrate is patterned with a rectangular array of nanoscale holes. Because of the high quality of the pattern, using neutron reflectometry, we are able to map the surface-normal density distribution of the patterned silicon, the penetration of water into the pattern, and the distribution of a deposited film inside and outside of the etched holes. In this study, 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) single bilayers were deposited on the hydrophilic patterned surface. For bilayers deposited either by vesicle fusion (VF) or by the Langmuir-Schaefer (L-S) technique, the most consistent model found to fit the data shows that the lipids form bilayer coatings on top of the substrate as well as the bottoms of the holes in an essentially conformal fashion. However, while there is a single bilayer on the unetched silicon surface, the lipids coating the bottoms of the holes form a complex bimodal structure consistent with a rough surface produced by the etching process. This study provides insight into film transfer both outside and inside regular nanopatterned features. Open image in new window

Enhancing the critical current of a superconducting film in a wide range of magnetic fields with a conformal array of nanoscale holes

The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have non-uniform density while preserving the local ordering. We find that the conformal array of nanoscle holes provides a more significant critical current enhancement at high magnetic fields. The better performance can be attributed to its arching effect that not only gives rise to the gradient in hole-density for pinning vortices with a wide range of densities but also prevent vortex channeling occurring in samples with a regular lattice of holes.

Anisotropy of the critical temperature of a superconducting niobium thin film with an array of nanoscale holes in an external magnetic field

We report on magnetotransport experiments investigating the effect of a regular array of nanoscale holes on the anisotropic response in the transition temperature of a superconducting niobium thin film. We find that the angle dependence of the critical temperature exhibits two strong anisotropic effects: Little-Parks oscillations whose period varies with field direction and a smooth background arising from one-dimensional confinement by the finite lateral space between neighboring holes. The two components of the anisotropy are intrinsically linked and appear in concert with one superimposed on top of the other.

Mechanical and Thermal Properties of Graphene Nanomeshes

ABSTRACTGraphene possesses excellent mechanical, thermal and electronic properties, not to mention its optical transparency and chemical stability. Much effort has been made towards the control of its physical properties for technological applications. One way to achieve this control is by modifying graphene size and structure. Recently, in a search for the development of semiconducting graphene structures to be produced at large scale, a new structure called graphene nanomesh was synthesized by means of block copolymer lithography and other methods. Basically, a graphene nanomesh is a graphene structure made to possess a periodic array of nanoscale holes whose sizes and hole-to-hole (or neck) distances are considered as control parameters for its overall electronic properties. Although the electronic properties of graphene nanomeshes are being intensively studied, their mechanical properties are still to be investigated. This work, then, presents the first study of mechanical, structural and thermal properties of graphene nanomeshes as a function of hole and neck sizes, through atomistic molecular dynamics simulations. The dependence of the Young’s modulus and coefficient of thermal expansion of graphene nanomeshes on the hole and neck sizes will be shown.

Energy gaps in graphene nanomeshes

We report on the band gap opening and electronic structures of graphene nanomeshes (GNMs), the defected graphene containing a high-density array of nanoscale holes, from first-principles calculations. As expected, quantum confinement at the GNM necks leads to a sizable band gap; however, surprisingly, the appearance of a gap depends sensitively on the hole arrangement and periodicity. For the simplest hexagonal zigzag-edged holes passivated by hydrogen, two-thirds of GNMs remain semimetallic while the rest are semiconductors. Furthermore, we show that the energy gap opening in GNM results from the combination of quantum confinement and the periodic perturbation potential due to perforation.

Vortex interaction enhanced saturation number and caging effect in a superconducting film with a honeycomb array of nanoscale holes

The electrical transport properties of a MoGe thin film with a honeycomb array of nanoscale holes are investigated. The critical current of the system shows nonmatching anomalies as a function of applied magnetic field, enabling us to distinguish between multiquanta vortices trapped in the holes and interstitial vortices located between the holes. The number of vortices trapped in each hole is found to be larger than the saturation number predicted for an isolated hole and shows a nonlinear field dependence, leading to the caging effect as predicted from the Ginzburg-Landau (GL) theory. Our experimental results are supplemented by numerical simulations based on the GL theory.

CHARACTERIZATION OF CONDUCTION MECHANISMS RELEVANT TO DEVICE PERFORMANCE IN NANOPERFORATED GRAPHENE

We have recently reported on the synthesis and characterization of a new form of nanostructured graphene that we call "nanoperforated graphene". Nanoperforated graphene is fabricated by etching a periodic array of nanoscale holes into atomic membranes of graphene to create an ultrathin superlattice-like structure. Nanoperforated graphene demonstrates semiconductor-like behavior and we have realized room-temperature field-induced conductance modulation as high as 450 (compared with < 10 for unpatterned graphene) with field-effect mobilities of ~ 1 cm2V-1s-1. Here, we discuss the conduction mechanisms in nanoperforated graphene and the relevance of this new material for field-effect transistor devices. In nanoperforated graphene with 15 nm nanoconstrictions, we observe that the low-bias mobility is independent of temperature, consistent with elastic scattering-limited conduction. At low temperatures, a transport gap limits conduction in the sub-threshold regime and affects the threshold voltage for band conduction. We show that the high-bias electrical characteristics of nanoperforated graphene are similar to "artificial solids," a class of materials made of 2D arrays of Coulomb islands, consistent with observed Coulomb Blockade features in the sub-threshold regime. Currently, the device characteristics of the nanopatterned graphene material are found to be suitable for large-area, thin-film transistor applications. Future higher-performance applications are expected.

Matching effect and dynamic phases of vortex matter in Bi2Sr2CaCu2O8 nanoribbon with a periodic array of holes

We report investigations on the dynamics of vortex matter with periodic pinning in crystalline Bi2Sr2CaCu2O8 nanoribbons containing an array of nanoscale holes. We found that the matching effect is enhanced near the melting field and persists to higher fields beyond the melting line. We attribute this enhancement to the existence of a soft-solid phase and a mixture of solid-liquid phases near the melting line, enabling the vortices to pin more effectively. We observed distinct regions in the voltage-current curves attributed to transitions of various dynamic phases which also account for the driving current dependent appearance of the matching effect.

Direct Evidence for Surface Plasmon-Mediated Enhanced Light Transmission through Metallic Nanohole Arrays

This paper provides direct evidence for the role of surface plasmons in the enhanced optical transmission of light through metallic nanoscale hole arrays. Near-field optical images directly confirmed the presence of surface plasmons on gold nanohole arrays with interhole spacings larger than the surface plasmon wavelength. A simple interference model provides an intuitive explanation of the two types of fringe wavelengths observed in the near-field optical images. Far-field spectroscopy revealed a surface plasmon band that contributed a factor > 8 to the transmission enhancement. Furthermore, silicon nanohole arrays did not exhibit any features in the near-field, which demonstrates that metallic materials are necessary for enhanced light transmission through nanohole arrays.

Magnetic-flux periodic response of nanoperforated ultrathin superconducting films

We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} < $5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of $R(T)$ dominated by amplitude fluctuations is larger in lower $T_{co}$ films and thus, grows with proximity to the superconductor to insulator transition.

Vortex pinning in Nb films patterned with nano-scale hole-arrays

Abstract Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide as substrate. Pronounced matching effects in the magnetization are seen in fields up to 9 kOe . Flux pinning in the patterned samples is enhanced by two orders of magnitude as compared to unpatterned reference samples in applied fields exceeding 5 kOe . The matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the nano-scale spacing of the holes.

Superconducting transition and vortex pinning in Nb films patterned with nanoscale hole arrays

Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide (AAO) as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is enhanced by two orders of magnitude as compared to unpatterned reference samples in applied fields exceeding 5 kOe. Matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the extremely small spacing of the holes.