Tunnel Barrier Formation Research Articles

Fabrication Techniques for a Tuneable Room Temperature Hybrid Single-electron Transistor and Field-effect Transistor

Hybrid room-temperature (RT) silicon single-electron – field effect transistors (SET-FETs) provide a means to switch between ‘classical’, high current FET, and low-power SET operation, using a gate voltage. While operating as a SET, charge on a silicon quantum dot (QD) within the current channel, can be controlled at the one-electron level using the Coulomb blockade effect. This paper investigates nanofabrication methods for sub-10 nm ‘fin’ channel hybrid RT SET-FETs, and their influence on the energy band diagram, and formation of tunnel barriers and QDs, along the channel. Devices are fabricated in heavily n-doped SOI material using electron beam lithography, with thermal oxidation to reduce the as-defined fin width. Effective channel dimensions, following oxidation and excluding Si/SiO2 interface dopant deactivation, are ∼2.4 nm × 32 nm × 20 nm. Dopant disorder, fin width variation at the nanometre scale, and quantum confinement effects are considered as mechanisms for the formation of tunnel barriers and QDs, with dopant disorder the most likely reason. Arrhenius plots of Ids vs. 1/T allow extraction of a potential barrier energy ∼0.2 eV along the fin channel. For 180devices fabricated on four chips, 37% show RT SET-FET operation, ∼3 times higher than the corresponding yield observed in previous work on point-contact silicon SETs.

Исследование тонких пленок для изготовления туннельных переходов Nb/AlN/NbN и линий передач NbTiN-SiO2-Al.

The surface of thin films of Nb, Al, NbTiN, SiO2, Al2O3 is investigated in this work. These films are necessary for the fabrication of high-sensitive devices of THz range. The fabrication processes of such devices are described briefly. All films were fabricated using a Kurt J. Lesker magnetron sputtering system. The study of the film surface roughness was carried out using a Bruker Ikon atomic force microscope. The surface quality of films is determined not only deposition mode, but plasma etching process also. The best values of the root-mean-square deviation of the surface profile Rq = 2 nm were obtained for the used NbTiN film with a thickness of 325 nm. Thin Al-layers that is used for tunnel barrier formation is studied. It is shown than Al films with a thickness of more than 6 nm are already continuous. The surface roughness of the single-layer and multilayer films has been studied.

Probing Proximity-Induced Superconductivity in InAs Nanowires Using Built-In Barriers

Bound states in superconductor-nanowire hybrid devices play a central role, carrying informationon the ground states properties (Shiba or Andreev states) or on the topological properties of thesystem (Majorana states). The spectroscopy of such bound states relies on the formation of well-defined tunnel barriers, usually defined by gate electrodes, which results in smooth tunnel barriers.Here we used thin InP segments embedded into InAs nanowire during the growth process to forma sharp built-in tunnel barrier. Gate dependence and thermal activation measurements have beenused to confirm the presence and estimate the height of this barrier. By coupling these wires tosuperconducting electrodes we have investigated the gate voltage dependence of the induced gap inthe nanowire segment, which we could understand using a simple model based on Andreev boundstates. Our results show that these built-in barriers are promising as future spectroscopic tools

Single electron transistors with e-beam evaporation of SiO2 tunnel barriers

Recent work on fabricating metal-insulator-metal (MIM) single electron transistors (SETs) using deposited dielectrics shows promise for becoming a manufacturable process due to compatibility with modern CMOS processes. This process, the “rib-SET” process [V. Joshi, A. O. Orlov, and G. L. Snider, J. Vac. Sci. Technol. B 26, 2587 (2008); G. Karbasian, A. O. Orlov, and G. L. Snider, J. Vac. Sci. Technol. B 33 (2015)], features a self-aligned island and should allow for scaling SETs below 10 nm. However, one of the biggest roadblocks in realizing a high-quality SET with this process has been difficulties in developing high-quality, low-noise, MIM tunnel junctions. In this work, the authors report Pt-SiO2-Pt MIM SETs with tunnel barriers deposited by e-beam evaporation as an alternative to atomic layer deposition. There are some challenges in the formation of tunnel barriers via e-beam evaporation that are addressed. It is expected that platinum has a negligible native oxide; however, there is a substantial resistance in as-deposited Pt-SiO2-Pt structures that can be reduced by over 5 orders of magnitude by subjecting the finished devices to an anneal in a hydrogen plasma, suggesting the presence of an interfacial platinum oxide. It is shown that this treatment not only increases the conductance through the SET, but that it is necessary for forming high conductance tunnel barriers that are desired for making low-noise SETs.

Experimental Observation of Quantum Confinement Effect in and Silicon Nanowire Field-Effect Transistors and Single-Electron/Hole Transistors Operating at Room Temperature

The quantum confinement effect (QCE) in ultranarrow silicon nanowire channel field-effect transistors (FETs) as well as single-electron/hole transistors (SET/SHTs) operating at room temperature is intensively investigated for the optimization of device design and fabrication. By adopting a “shared channel” structure with the directions of <110> and <100>, a carrier-dependent QCE is systematically examined. It is found that <110> nanowire pFETs exhibit a smaller threshold voltage (Vth) variability due to a weaker QCE, while <110> nFETs and <100> n/pFETs show comparable Vth variabilities coming from the QCE. It is also found that only SETs exhibit clear Coulomb oscillations in the case of the <110> channel, suggesting the formation of higher tunnel barriers than SHTs. On the other hand, <100> SHTs show undesirable multidot behavior in spite of their comparable QCEs for electrons and holes. It is concluded that <110>-directed nanowire channel SETs and n/pFETs are suitable for the integration of CMOS and SETs.

Subgap Leakage in $\hbox{Nb/Al}$– $\hbox{AlO}_{\rm x}\hbox{/Nb}$ Josephson Junctions and Run-to-Run Reproducibility: Effects of Oxidation Chamber and Film Stress

Many applications of Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junctions (JJs) in superconducting electronics require high-quality tunnel barriers with low subgap leakage that is usually characterized by figure of merit V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> = I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sg</sub> , where I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is the critical current and R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sg</sub> is the subgap resistance at 2 mV and 4.2 K. It is widely believed, and there is considerable literature suggesting, that quality and reproducibility of JJs critically depend on the intrinsic stress in Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb trilayers, and the stress therefore should be carefully minimized and controlled. Contrary to this belief, we show that JJ quality V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and reproducibility do not depend on the stress in the trilayer, at least in the studied range from -300 to 300 MPa. In this range, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> depends neither on the stress in a Nb/Al base electrode nor in a Nb counter electrode. We have found, however, that V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> crucially depends on the way the tunnel barrier formation by thermal oxidation of Al is done. For instance, room-temperature dynamic oxidation (in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow at low pressures) in a cryopumped chamber leads to poor run-to-run reproducibility of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and reduced V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> values, whereas dynamic oxidation at the same parameters but in a chamber with a turbomolecular pump results in high V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> values and excellent run-to-run reproducibility.

Nonlocal spin transport in single-walled carbon nanotube networks

Spin transport in carbon-based materials has stimulated much interest due to their ballistic conductance and a long phase coherence length. While much research has been conducted on individual carbon nanotubes, current growth and placement techniques are incompatible with large-scale fabrication. Here we report nonlocal spin injection and detection in single wall carbon nanotube networks. We observe spin transport over a distance of 1 um, and extract a spin diffusion length of 1.6 - 2.4 um with an injected spin polarization from CoFe into nanotube network of 18 - 41%. Our observations demonstrate that spin transport is possible in carbon nanotube networks due to the formation of natural tunnel barriers between nanotubes and metallic contacts.

Character and fabrication of Al/Al2O3/Al tunnel junctions for qubit application

The superconductive Josephson junction is the key device for superconducting quantum computation. We have fabricated Al/ Al2O3/Al tunnel junctions using a double angle evaporation method based on a suspended shadow mask. The Al2O3 junction barrier has been formed by introducing pure oxygen into the chamber during the fabrication process. We have adjusted exposure conditions by changing either the oxygen pressure or the oxidizing time during the formation of tunnel barriers to control the critical current density Jc and the junction specific resistance Rc . Measurements of the leakage in Al/Al2O3/Al tunnel junctions show that the devices are suitable for qubit applications.

Intrinsic and doped coupled quantum dots created by local modulation of implantation in a silicon nanowire

We present a systematic study of various ways (top gates, local doping, substrate bias) to fabricate and tune multi-dot structures in silicon nanowire multigate metal-oxide-semiconductor field-effect transistors. The carrier concentration profile of the silicon nanowire is a key parameter to control the formation of tunnel barriers and single-electron islands. It is determined both by the doping profile of the nanowire and by the voltages applied to the top gates and to the substrate. Local doping is achieved with the realization of up to two arsenic implantation steps in combination with gates and nitride spacers acting as a mask. We compare nominally identical devices with different implantations and different voltages applied to the substrate, leading to the realization of both intrinsic and doped coupled dot structures. We demonstrate devices in which all the tunnel resistances toward the electrodes and between the dots can be independently tuned with the control top gates wrapping the silicon nanowire.

Spin precession in lateral all-metal spin valves: Experimental observation and theoretical description

Mesoscopic all-metal spin-valve devices containing two ferromagnetic permalloy electrodes and an interconnecting aluminum strip have been studied experimentally and theoretically in nonlocal geometry. Spin-valve devices have been fabricated by thermal evaporation of permalloy electrodes, formation of tunnel barriers by natural oxidation in air or in pure oxygen, and subsequent deposition of an aluminum strip on top of the electrodes. Transport measurements at temperatures of liquid helium have been performed. Spin-dependent phenomena, namely the nonlocal spin-valve effect and spin precession, are observed. A theoretical description of spin-dependent transport is presented including spin diffusion, spin relaxation, spin precession, and tunnel barriers at the interfaces between the electrodes and the aluminum strip. From the comparison of the experimentally observed spin precession to the theoretical description, we obtain a spin-relaxation time of 111 ps and a spin-relaxation length of 1034 nm in aluminum.

Characteristics of Zr-based single- and multiple-barrier superconducting tunnel junctions

Using Zr oxide for the tunnel barrier formation, it is shown experimentally that the effect of pinholes is suppressed in the double-barrier versus single-barrier junction configuration. It is found that Josephson current in Nb∕Zr double-barrier junctions is much lower than in Nb∕Al junctions with comparable specific tunneling resistance and middle layer thickness. Multiple oxidation process is used to improve the quality of superconductor-insulator-superconductor Zr-based junctions.

Aluminum Nitride Tunnel Barrier Formation with Low-Energy Nitrogen Ion Beams

We report the use of low-energy nitrogen ion beams to form ultra-thin (&lt;2 nm) layers of AlNx to act as tunnel barriers in Nb/Al–AlNx/Nb Josephson junctions. We fabricated reproducible, high-quality devices with independent control of the ion energy and dose, enabling exploration of a wide parameter space. Critical current density Jc ranged from 550 to 9400 A/cm2 with subgap-to-normal resistance ratios from 50 to 12.6. The spatial variation of ion-current density was roughly correlated with Jc over a large-area on a Si substrate. The junctions were stable on annealing up to temperatures of at least 200 °C. This technique could be applied to form other metal nitrides at room temperature for device applications where a high degree of control is desired.

Formation of tunnel barrier using a pseudo-atomic layer deposition method and its application to spin-dependent tunneling junction

The tunneling barrier is crucial to the overall performance in magnetic tunnel junctions. We have suggested a new formation method for the tunnel barrier, which has utilized pseudo-atomic layer deposition with sputtering. As is well known, all metallic thin films oxidize more or less under atmospheric conditions. Using this phenomenon, an ultra-thin metallic layer was prepared and exposed to the oxygen ambient repeatedly to reach a desired thickness for the tunnel barrier. From transmission electron microscopy, the tunnel barrier has been confirmed to have a clear and smooth interface between magnetic layers and the tunnel barrier. From atomic force microscopy, it has also been confirmed to have a low surface roughness. The fabricated magnetic tunnel junction has been shown to exhibit tunnel resistivities from 60 to 92 kΩ μm2 and a maximum tunneling magnetoresistance ratio of 40%.

Observation of discrete quantum levels in multi-wall carbon nanotube quantum dots

Quantum dots with a size less than 100 nm have been fabricated in multi-wall carbon nanotubes (MWNTs) by using the local Ar beam irradiation technique for tunnel barrier formation. The electrical transport measurement has been carried out at liquid helium temperatures, and the typical Coulomb blockade characteristics have been observed. The most striking observation is the clear indication of excited states, which is not usual for MWNTs with diffusive nature. The present result originates from the shorter dot size than the mean free path in the MWNT.

Observation of discrete quantum levels in multi-wall carbon nanotube quantum dots

Quantum dots with a size less than 100 nm have been fabricated in multi-wall carbon nanotubes (MWNTs) by using the local Ar beam irradiation technique for tunnel barrier formation. The electrical transport measurement has been carried out at liquid helium temperatures, and the typical Coulomb blockade characteristics have been observed. The most striking observation is the clear indication of excited states, which is not usual for MWNTs with diffusive nature. The present result originates from the shorter dot size than the mean free path in the MWNT.

Coulomb oscillations based on band-to-band tunneling in a degenerately doped silicon metal-oxide-semiconductor field-effect transistor

We report Coulomb oscillations based on band-to-band tunneling through a valence band in silicon metal-oxide-semiconductor field-effect transistors. Degenerately p+-doped channel and n+-doped source/drain enables band-to-band tunneling, which can play a major role in the transport between the channel and source/drain. The formation of tunnel barriers and a quantum dot in a single-electron transistor structure originates from two p+–n+ tunnel junctions and a p+-doped channel with mesoscopic dimension, respectively. Coulomb-blockade oscillations with multiple peaks were clearly observed at liquid nitrogen temperature. Using the electrical and thermal characterization of the quantum dot, single-electron charging effect based on band-to-band tunneling is confirmed.

Local Ar beam irradiation for making tunneling barriers and its application to single electron inverter in multi-wall carbon nanotubes

The local Ar beam irradiation technique has been developed to form reliable tunnel barriers in multi-wall carbon nanotubes (MWNTs). The technique was applied to fabricate single electron transistor (SET) by adjusting the irradiation dose for the tunnel barrier formation. The CMOS-like single electron inverter structure was also fabricated with two SETs connected in series. The inverter-like transfer curve was obtained. However, the gain and the voltage swing were not large enough for the useful inverter, which may be due to the non-uniform characteristics of each SET.

Modification of silicon-on-insulator structures under nano-scale device fabrication

Based on the characterization of the ultrathin silicon-on-insulator (SOI) test structures, the effects of charge at the interface between the top silicon layer and the buried oxide (bonded interface) are studied during different technology steps (thinning of the top silicon layer and plasma etching). SOI structures were fabricated by wafer bonding and hydrogen slicing technology. An increase in the charge with the thinning of the top silicon layer and introduction of the mobile charge or the slow interface states after the plasma etching were found for the SOI structures. The conductance oscillations have been observed at room temperature in the ultrathin silicon layers due to formation of tunnel barriers for one type of carriers caused by charge fluctuation.

Analysis of the fabrication process of Nb/Al-AlN/sub x//Nb tunnel junctions with low R/sub n/A values for SIS mixers

We characterize the fabrication process of superconductor-insulator-superconductor junctions (SIS) based on a Nb/Al-AlN/sub x//Nb tri-layer. Utilization of the AlN/sub x/ tunnel barrier, produced by Al nitridation in a nitrogen glow discharge, enables us to produce high quality SIS junctions with low R/sub n/A values (the product of junction resistance and area). Analyzing 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. The latter process plays a dominant role since R/sub n/A values are well correlated with the power dissipated on the substrate surface. An SIS mixer using this technology and electron-beam lithography has been successfully fabricated.

Fabrication of a single-electron inverter in multiwall carbon nanotubes

A single-electron inverter based on the Coulomb blockade effects has been fabricated in multiwalled carbon nanotubes, by using the Ar irradiation method for the tunnel barrier formation in the quantum dots. The device characteristics were measured at 2.6 K, and an indication of inverter-like transfer characteristics has been obtained. The performance of the device was not completely satisfactory for the inverter operation in terms of the voltage gain and the voltage swing, but the present technique turns out to be useful for the fabrication of single-electron logic devices in carbon nanotubes.