Shadow Deposition Research Articles

Phenomenological studies of optical properties of Cu nanowires

We report on the experimental observation of surface plasmon resonance in Cu nanowires fabricated by shadow deposition method. When the incident light is polarized perpendicular to the wire axes, plasmon maxima appeared at about 2.3 eV in the absorption spectra. Plasmon resonance appeared at lower photon energy when the incident light is polarized parallel to the wire axes. Resonance peaks move to lower energy when the nanowire widths are increased. We have found that finite-difference time-domain (FDTD) simulation gives better results than Maxwell–Garnett model in explaining the relation between the light polarization and the energies of the observed absorption maxima.

FABRICATION OF PERIODIC STANDING ROD ARRAYS BY THE SHADOW CONE METHOD

A simple method for fabricating periodic arrays of high aspect ratio (1:20) standing nanorods on silicon substrates is described. It is based on shadow deposition onto periodically arranged arrays of mini-rods on a rotating sample stage. Consequently, such nanostructures can be prepared on relatively large areas and at low cost, making the method suitable for industrial applications.

Fabrication of Nano-Gap Electrode Pairs Using Atomic-Layer-Deposited Sacrificial Layer and Shadow Deposition

We propose a new fabrication process of nano-gap electrode pairs using an atomic-layer-deposited (ALD) sacrificial layer and the shadow deposition technique. In this process, gap width can be precisely controlled by the number of deposition cycles of the ALD process, whereas junction area is defined by the deposition angle of the second electrode material through an overhanging shadow mask on top of the first electrode. In comparison with our previous method, process reliability has been highly improved because the unintentional deposition of the second electrode material on the sidewall of the first electrode is completely prevented. We have fabricated 10 ×10 arrays of n-type polycrystalline silicon (n-poly-Si)/Au nano-gap electrode pairs with gap widths of 6 and 9 nm, which show good insulating properties at room temperature.

Fabrication of Large Number Density Platinum Nanowire Arrays by Size Reduction Lithography and Nanoimprint Lithography

Large number density Pt nanowires with typical dimensions of 12 microm x 20 nm x 5 nm (length x width x height) are fabricated on planar oxide supports. First sub-20 nm single crystalline silicon nanowires are fabricated by size reduction lithography, and then the Si nanowire pattern is replicated to produce a large number of Pt nanowires by nanoimprint lithography. The width and height of the Pt nanowires are uniform and are controlled with nanometer precision. The nanowire number density is 4 x 10(4) cm(-1), resulting in a Pt surface area larger than 2 cm(2) on a 5 x 5 cm(2) oxide substrate. Bimodal nanowires with different width have been generated by using a Pt shadow deposition technique. Using this technique, alternating 10 and 19 nm wide nanowires are produced.

Selected area growth of II–VI nanostructures using shadow masks

AbstractWe investigate molecular beam epitaxy of II–VI materials through GaAs/AlGaAs epitaxial shadow masks on GaAs(001) substrate. Scanning electron and atomic force microscopy are employed to investigate the effects of surface diffusion, faceting, partial shadow (minimized in the experimental set‐up), and material deposition on the mask (closing of apertures) on the selected area growth of ZnSe and CdSe. The results illustrate that selected area growth of II–VI nanostructures is controlled by the growth geometry. In contrast to the case of III–V materials, we found no macroscopic features (>10 nm) which could be attributed to surface diffusion. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanodimensional effects in the structure of epitaxial YBa2Cu3O7−y films

We have studied the structure of YBa2Cu3O7−y (YBCO) high-T c superconductor films obtained by deposition for various times under otherwise identical conditions. Thin YBCO films prepared by the nonaxial laser shadow deposition technique are epitaxial and have microstrains below 1×10−3. The main factor determining variations of the unit cell parameter c in the films is the difference of the crystal block size 〈D〉 in the direction normal to the film surface, which reflects the nanodimensional effects known in oxides.

Magnetic nanowires on faceted sapphire surfaces

We have grown magnetic stripes on faceted Al 2O 3 (101̄0) substrates using molecular beam epitaxial methods. The planar sapphire substrate of this orientation is unstable, and spontaneously forms stripe-like facets upon annealing at high temperatures. Shadow deposition at a controlled small incident angle onto the faceted sapphire leads to stripes of the evaporated material. We have used this method as an easy way for producing iron nanowires. The magnetization reversal process of the iron nanowires, as revealed by magneto-optical Kerr effect measurements, is controlled by a coherent rotation of the magnetization vector.

Anisotropic optical second-harmonic generation from the Au nanowire array on the NaCl(1 1 0) template

We have measured azimuthal angle dependence of the optical second-harmonic (SH) intensity from Au nanowire arrays prepared by shadow deposition on the self-organized NaCl(1 1 0) template at the fundamental photon energy of ℏ ω=1.17 eV. The observed SH intensity patterns as a function of the sample rotation angle around its surface normal are quite anisotropic and are characterized by the effective nonlinear susceptibility arising from the quasi-C 2v symmetry of the Au nanowire array/NaCl(1 1 0) system.

Self-organized semiconductor surfaces as templates for nanostructured magnetic thin films

Spontaneous pattern formation during epitaxial growth or ion erosion of semiconductor wafers offers an elegant route towards large-area nanostructured surfaces. In homoepitaxy, kinetics may result in rather uniform three-dimensional islands. In the case of semiconductor heteroepitaxy, strain relief leads to the formation of nanofaceted three-dimensional crystallites, which may self-organize into quasiperiodic arrays. By tuning substrate miscut and film thickness, or growing superlattices, a variety of patterns with different symmetries can be obtained, as will be summarized for the model system of SiGe on Si(001). Since these self-organized nanostructure arrays cover the entire wafer on which they are grown, they can serve as large-area nanopatterned substrates for subsequent deposition of magnetic thin films. It will be demonstrated that such templates allow the study of correlations between magnetic and chemical interfacial roughness, as well as the influence of pattern symmetry on the magnetic anisotropy of thin Co films. Furthermore, shadow deposition of magnetic material onto specially faceted nanostructure arrays allows the fabrication of nanomagnet arrays and the study of their magnetic properties.

Titanium single-electron transistor fabricated by electron-beam lithography

A method to fabricate non-superconducting mesoscopic tunnel junctions by oxidation of Ti is presented. The fabrication process uses conventional electron-beam lithography and shadow deposition through an organic resist mask. Superconductivity in Ti is suppressed by performing the deposition under a suitable background pressure. We demonstrate the method by making a single-electron transistor which operated at T<0.4 K and had a moderate charge noise of 2.5×10 −3 e/ Hz at 10 Hz . Based on non-linearities in the current–voltage characteristics at higher voltages, we deduce the oxide barrier height of approximately 110 mV . The non-superconducting Ti junctions can be useful in several applications.

Quasi-one-dimensional cobalt particle arrays embedded in 5 nm-wide gold nanowires

An array of Co-Au heterogeneous nanowires lying in the film plane have been grown by shadow deposition onto stepped NaCl[110] surfaces prepared by homoepitaxial growth. The quasi-one-dimensional Co dot arrays are embedded in Au nanowires of 5 nm in minimum width. The magnetization process is dominated by dipole coupling and the easy axis is parallel to the lines. No significant difference in magnetization reversal is observed between bare Co particle arrays and Au-overcoated arrays.

Shadowed Off-Axis Pulsed Laser Deposition of YBa2Cu3O7−x Thin Films

Using a shadowed pulsed laser deposition allows us to reduce the number of droplets on the YBa2Cu3O7−x film surface obtained by pulsed laser deposition method. In this study, we present a new modification of pulsed laser deposition method: a shadowed off-axis pulsed laser deposition. The substrate surface lies in the plain defined by the laser beam. A rectangular shadow mask located between target and substrate reduce the quantity of large particles deposited from the diffusion flow. Such geometry allows us to carry out the laser plume scanning on the target surface for simultaneous one-step double-sided deposition films with a droplet-free surface. Both films had good superconducting properties.

Fully in plane aligned SmCo based films prepared by pulsed laser deposition

SmCo based films, ≈1–3 μm thick, have been directly crystallized onto moderately heated polycrystalline susbstrates, 375 °C, by high pulse energy pulsed laser deposition. Films have been deposited to compare shadow and direct deposition. Only for shadow deposition conditions has it been possible to deposit CaCu5-type structure films that exhibit hysteresis loops characteristic of single phase materials with high coercivities. The shadow deposited films are mirror-like, very fined grained (less that 0.2 μm), and with the crystallite c axes completely aligned onto the substrate plane. For optimally deposited films, room temperature hysteresis loops measured perpendicular to the film plane were closed with approximately zero coercivity. For such films, hysteresis loops measured in plane with fields to ±90 kOe at room temperature exhibited intrinsic coercivities of at least 22.5 kOe. Measurement fields of ±19 kOe yielded only minor loops that did not enclose the origin in some cases. The fully textured SmCo based films in the present study were deposited using λ=248 nm and a pulse energy of 1200 mJ. The magnetic properties are a function of the laser pulse rate and this has been studied from 5 to 50 Hz repetition rates.

Fabrication of nanomagnet arrays by shadow deposition on self-organized semiconductor substrates

It is demonstrated how large-scale arrays of nanomagnets can be efficiently fabricated by shadow deposition onto faceted surfaces of self-organized Si1−xGex films. By pulsed laser deposition of Co in a grazing incidence geometry, we succeeded to cover just one selected type of facets resulting in isolated Co patches with an areal density of about 0.25×1012/in.2. These uniformly oriented nanomagnets have a parallelogram-shaped base with about 25 nm×35 nm edge lengths. Magneto-optic Kerr effect measurements reveal a clear in-plane anisotropy of the nanomagnets.

High coercivity SmCo based films made by pulsed laser deposition

Films of SmCo based materials exhibiting high intrinsic coercivities and smooth hysteresis loops have been prepared by pulsed laser deposition (PLD) onto moderately heated substrates. Films directly crystallized from SmCo5 targets onto 375 °C substrates exhibited a maximum Hc=11.3 kOei at a pulse repetition rate of 10 Hz with lower coercivities for both lower and higher pulse repetition rates. In the present case the films were deposited onto polycrystalline substrates. The films exhibited a very small grain size of less than 1 μm diameter, were mirrorlike, and shadow deposited films were relatively particulate free under scanning electron microscope examination. Shadowed PLD deposition was used for the best films. Laser wavelengths of 193 and 248 nm were used with pulse repetition rates from 5 to 50 Hz. Films grown without shadowing exhibited a great deal of particulate contamination. The hysteresis loops of such nonshadowed films were constricted and exhibited drops in the 4πM values upon demagnetization. To our knowledge this is the first reporting of high coercive force SmCo based films deposited by PLD exhibiting single phase type hysteresis loops.

Polycrystalline and laminated La0.7Sr0.3MnO3 films made by pulsed laser deposition

Highly smooth and particulate free relatively thick polycrystalline La0.7Sr0.3MnO3 films out to 6 μm thick have been grown by shadowed pulse laser deposition at 30 Hz (248 nm, 500 mJ). Such films exhibited a low field and high field magnetoresistance as previously reported by a number of groups. Multilayer films made with alternating 8-nm-thick barium ferrite and 16-nm-thick La0.7Sr0.3MnO3 laminations exhibited the same high field response, but did not exhibit the low field magnetoresistance component. This is consistent with the low field component arising from intergrain scattering. Patterned films have been simultaneously deposited onto R-plane, C-plane, and polycrystalline alumina substrates. The films were postannealed at 750 K in 200 Torr O2 for 45 min. The low field magnetoresistance was most pronounced for the single layer films made onto C-plane sapphire and then polycrystalline alumina substrates. The films exhibited a broad maximum in resistivity versus temperature at 162, 210, and 218 K for films grown onto R-plane sapphire, C-plane sapphire, and alumina substrates, respectively. Restricted grain size films made with La0.7Sr0.3MnO3 and barium ferrite laminations exhibited a single linear magnetoresistance response of −0.43%/kOe for in plane applied fields of ±10 kOe.

Stoichiometry and thickness variation of YBa2Cu3O7−x in pulsed laser deposition with a shadow mask

The deposition of particulate species, typically seen in pulsed laser deposition can be eliminated by the deployment of a line-of-sight shadow technique. Since the technique uses the discrimination between ballistic and diffusive species, it is important to understand the lateral stoichiometry and thickness dependence under various deposition conditions. We present in this study the dependence of the lateral thickness and stoichiometry of YBa2Cu3O7−x deposited by a shadow mask pulsed laser technique on the background oxygen pressure and the shadow mask position from the target. We determined Y atoms to be limiting the lateral stoichiometric uniformity. Due to its diffusive nature, shadowed pulsed laser deposition also allowed us to improve the thickness uniformity across the wafer.

Self-organized Fe nanowire arrays prepared by shadow deposition on NaCl(110) templates

Iron nanowire arrays have been grown by shadow deposition on a self-organized grating template produced by annealing the sodium chloride (110) surface. The typical wire size as measured using transmission electron microscopy is 45 nm×13 nm×10 μm. The typical wire array period is 90 nm. The magnetic properties were dominated by a strong in-plane shape anisotropy. The hysteresis loops examined by magneto-optical Kerr effect measurements indicated coherent switching, even though the individual wires were isolated from one another.

Preparation and characterization of molecule-based transistors with a 50-nanometer source-drain separation with use of shadow deposition techniques. Toward faster, more sensitive molecule-based devices

Abstract : Preparation of Au microelectrode arrays having spacings of 50-100 nm and available electrode areas of 10 to the -7th power sq cm is described. The dimensions are qualitatively smaller than previously reported and lead directly to faster switching and smaller switching energy for molecule-based transistors. Such microelectrodes can be functionalized with redox active polymers by oxidation of aniline to give polyaniline or by electrochemically assisted deposition of N,N'-bis(p-trimethoxysilylbenzyl)-4,4'-bipyridinium promoting Si-OMe hydrolysis to give the redox polymer (BPQ2+/+)n. The fabrication procedure begins with a microelectrode array consisting of eight, individually addressable AU microelectrodes each approx. 50 micron long x approx 2.5 micron wide x approx. 0.1 micron thick with a spacing of approx. 1.5 micron between the microelectrodes.

An oblique shadow deposition technique for altering the profile of grating relief patterns on surfaces

An oblique shadow deposition technique for altering the profile of grating relief patterns on surfaces is described. The technique essentially represents a method whereby a series of grating masks may be superimposed in precise registration with each other. By appropriate choice of deposition angle, deep grooves may be generated from shallow profiles, rounded or sinusoidal profiles may be transformed to structures with planar vertical walls, and symmetric profiles may be made asymmetric. In conjunction with ion-beam milling, the technique has been used to increase the depth of fine period gratings on GaAs and to produce grooves ∼4000 Å deep on the surface of YAG, 10 times deeper than could be obtained by ion-beam milling of photoresist.