Tunnel Barrier Formation Research Articles

Tunnel Barrier Formation in Silicon Nanowires

Multiple tunnel junctions have been realized in silicon using highly doped nanowires in silicon-on-insulator material by making use of pattern-dependent local oxidation. The electrical characteristics show clear Coulomb blockade oscillations of a single dot. On depleting the nanowires with side gates, peak pairing is observed in the oscillations. This characteristic of double dot structures can be explained by the creation of additional tunnel barriers in the nanowire due to the potential distribution caused by the dopants.

Conductance oscillations near bonded interface in the ultra thin silicon-on-insulator layers at room temperature

The conductance oscillations have been observed at room temperature in the ultra thin silicon layers of the silicon-on-insulator structures prepared with the bonding and hydrogen slicing technology. An origin of the oscillations is attributed to a formation of tunnel barriers for one type of carriers due to the interface charge fluctuation in combination with the surface relief and the dopant segregation. The increase in the interface charge was observed with the thinning of the top silicon layer. This phenomenon is most likely connected with the transformation of the hydrogen incorporating complexes during the oxidation process used for the thinning of the top silicon layer.

Characterization of the fabrication process of Nb/Al–AlNx/Nb tunnel junctions with low RnA values up to 1 Ω μm2

We discuss and characterize the fabrication process of superconductor–insulator–superconductor (SIS) junctions based on a Nb/Al–AlNx/Nb tri-layer. Utilization of the AlNx tunnel barrier, produced by Al nitridation in a nitrogen glow discharge, enables us to produce high-quality SIS junctions with low RnA values (a product of junction resistance and area). We characterize the tunnel barrier formation and investigate the correlation of plasma characteristics and junction properties. The experiment shows that an increase in nitridation time and applied power results in an increase in junction resistance, while variation in nitrogen pressure has almost no influence on the junction characteristics. Analysing the correlation of junction resistance and plasma properties, it is concluded that the mechanism of tunnel barrier formation is based on nitrogen implantation into the Al layer with subsequent diffusion of nitrogen, stimulated by plasma heating.

Ultrathin aluminum oxide tunnel barriers.

Ballistic electron emission microscopy is used to study the formation of ultrathin tunnel barriers by the oxidization of aluminum. An O2 exposure, approximately 30 mTorr sec, forms a uniform tunnel barrier with a barrier height straight phi(b) of 1.2 eV. Greater O2 exposure does not alter straight phi(b) or the ballistic transmissivity of the oxide conduction band. Tunneling spectroscopy indicates a broad energy distribution of electronic states in the oxide. With increasing O2 dose the states below 1.2 eV gradually become localized, but until this localization is complete these states can provide low-energy single-electron channels through the oxide.

The role of surface roughness in the fabrication of stacked Nb/Al–AlOx/Nb tunnel junctions

The surface roughness of sputtered Nb films was determined with high precision using x-ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70-nm-thick film to 1.8 nm for a 210-nm-thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.

Characteristics of Y-Ba-Cu-O/Nb and Bi-Sr-Ca-Cu-O/Nb tunnel-type Josephson junctions

The authors fabricated Y-Ba-Cu-O/Nb and Bi-Sr-Ca-Cu-O/Nb tunnel-type junctions and observed the DC and AC Josephson effects. Gold thin films were deposited for an interlayer to protect the reaction between the base electrode and the tunnel oxide before the tunnel barrier formation. In Y-Ba-Cu-O/Au/MgO/sub x//Nb junctions, superconducting current, hysteresis of current-voltage characteristics, and RF-induced voltage steps as high as 0.13 mV in the current-voltage characteristics were observed. Moreover, the superconducting current was modulated by the magnetic field. In Bi-Sr-Ca-Cu-O/Au/MgO/sub x//Nb junctions, superconducting current and RF-induced voltage steps as high as 0.15 mV were also observed.

Fabrication and dc characteristics of small-area tantalum and niobium superconducting tunnel junctions

We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.

Characterization of Nb/Nb oxide structures in Josephson tunnel junctions

The subgap conductance of Nb, Nb oxide/Pb-alloy Josephson tunnel junctions was found to strongly depend on the rf plasma processing used to form the Nb oxide tunnel barrier. We have therefore studied the Nb/Nb oxide structures after rf plasma processing using x-ray photoelectron spectroscopy (XPS) and transmission electron microscropy (TEM) data. A crystalline Nb-C-O layer was formed as a transition region between the polycrystalline Nb base electrode and the Nb 2 O 5 tunnel barrier. The thickness of this transition region is sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. An unexpected relation between junction electrical properties and the Nb/Nb oxide structure exists. The lowest subgap conductances are those obtained for tunnel junctions with transition regions about 30A thick. An abrupt interface between Nb and Nb 2 O 5 leads to junctions with high subgap conductances.

Structure of a Nb oxide tunnel barrier in a Josephson junction

Oxides grown on the surface of a polycrystalline Nb thin film using rf plasma cleaning and plasma oxidation processes were examined by transmission electron microscopy. The oxides were prepared on the Nb surface using the same rf plasma processes developed for fabricating the tunnel barriers of Nb/Nb oxide/Pb alloy Josephson tunnel junctions. Transmission electron diffraction patterns obtained from the tunnel barrier formed on the Nb surface indicated the presence of a crystalline NbC0.25O0.75 layer between the amorphous Nb2O5 tunnel barrier layer and the Nb base electrode. The thickness of this transition layer was found to be sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. The formation of this NbC0.25O0.75 layer of proper thickness is correlated to good junction quality. An epitaxial relationship between the NbC0.25O0.75 layer and the underlying Nb crystal was also observed.

Fabrication Process for Josephson Integrated Circuits

A process for fabricating experimental Josephson integrated circuits is described that is based primarily on the use of vacuum-deposited Pb-alloy and SiO films patterned by photoresist stencil lift-off. The process has evolved from one previously reported, with changes having occurred in junction electrodes, tunnel barrier formation, layer patterning, device geometry, and minimum linewidths. Films of Pb-In(12 wt%)-Au(4 wt%) alloy (200-800 nm thick) are used for forming junction base electrodes, interferometer controls, and interconnection lines. Tunnel barriers are formed on the base electrode films by thermal oxidation and subsequent sputter-etching in an rf-oxygen plasma. Junction counter electrodes are formed from 400-nm-thick Pb-Bi(29 wt%) alloy films. Ground planes are formed from 300-nm-thick Nb films patterned by subtractive etching and insulated in part by a Nb2O5 layer formed by liquid anodization. Films of the intermetallic compound AuIn2(30-43 nm thick) are used for forming terminating, load, and damping resistors. The Si0 films are used for interlayer insulation, for defining junction areas in interferometers, and as protective coatings. Layer patterning is achieved mainly by means of photoresist lift-off stencils. By utilizing this process, experimental logic and memory circuits containing ≅100 interferometers with lines as small as 2.5 µm in width have been successfully fabricated.