CoSi2 formation in contact systems based on Ti-Co alloy with low cobalt content

The process of integrated-circuit contact formation based on Ti–Co alloy with low content of cobalt has been investigated. By AES and XRD it is shown that nitrogen doping of the alloy film during magnetron deposition and its subsequent annealing at 800–850°C allow simultaneously to obtain a CoSi₂ contact layer and a diffusion barrier layer on the basis of TiN. Electrical characteristics of Schottky barrier and ohmic contacts have been studied.     

Measurements of the dynamic conductance on SS-and NS-point contacts

Mechanically point contacts made of Ta, Al, Ag (anvil) and of Ta, Ag (needle) were investigated in the temperature range between 4.2 K and 1.3 K and in magnetic fields up to 0.8 T. For point contacts consisting of two superconducting electrodes we measured the current voltage characteristics and also the dynamic conductance. The characteristics with a negative differential conductance and the occurrence of an excess current point to the appearance of Andreev-multiple reflection at an SNS-interface in the contact region.The investigated current voltage characteristics and the dynamic conductance of typical NS-point contacts (Ta–Ag and Ta–Al) point to a microconstriction with a barrier of arbitrary strength at the interface between the normalconductor and the superconductor. We performed theoretical calculations forT=0 and also for temperatures up to the critical temperature using the model of Blonder, Tinkham and Klapwijk (BTK model). In this model the Andreev-reflection is the main scattering mechanism at the NS-interface but also elastic scattering and transmission processes are taken into consideration. The comparison of our theoretical results with the experimental results and also the determined excess current at all investigated point contacts lead to the assumption that there exist metallic contacts with very small barrier strengths at the NS-interface where the Andreev-reflection is the dominant process and the other scattering mechanisms play a subdominant role.     

The effect of carrier concentration in the semiconductor on the voltage-current characteristics of Schottky barrier diodes at low temperatures

We report the results of a study of voltage-current characteristics of gallium arsenide metal-semiconductor Schottky barrier structures, in which palladium and W-Ni alloy were electrochemically deposited in SiO₂ windows to form the barrier. It is demonstrated that the voltage-current characteristics are significantly distorted at low temperatures. The observed behavior of the voltage-current characteristics at low temperatures —the appearance of excess current and the increase in the ideality factor with carrier concentration in the excess current regime —is explained by a model in which peripheral mechanical stresses in the contacts lower the potential barrier height on the periphery of the contacts.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No, 10, pp. 87–93, October, 1986.     

Analysis of thermionic emission from electrodeposited Ni–Si Schottky barriers

Ni–Si Schottky barriers are fabricated by electrodeposition using n on n+ Si substrates. I–V, C–V and low temperature I–V measurements are presented. A high-quality Schottky barrier with extremely low reverse leakage current is revealed. The results are shown to fit an inhomogeneous barrier model for thermionic emission over a Schottky barrier proposed by Werner and Guttler [J.H. Werner, H.H. Guttler, Barrier inhomogeneities at Schottky contacts, J. Appl. Phys. 69 (3) (1991) 1522–1533]. A mean value of 0.76 V and a standard deviation of 66 mV is obtained for the Schottky barrier height at room temperature with a linear bias dependence. X-ray diffraction and scanning electron microscopy measurements reveal a polycrystalline Ni film with grains that span from the Ni–Si interface to the top of the Ni layer. The variation in Ni orientation is suggested as a possible source of the spatial distribution of the Schottky barrier height.     

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%. PACS 75.47.-m; 75.70.-i; 72.25.-b     

Thickness dependence of the work function in double-layer metallic films

The work function of metallic thin films limited by symmetric surfaces is expected to be thickness dependent at a level of 0.1 eV and a thickness range of about 5 nm. Recent experiments, however, demonstrated that Cu films on glass or Ni substrates show a long ranging (10–20 nm) increase of the work function with increasing film thickness [1]. This effect was attributed to a violation of local charge neutrality in films with unlike surfaces. In this paper we show that the barrier height of thin film diodes like metal-insulator-metal (MIM)-, metal-semiconductor (Schottky contacts)-and metal-vacuum-metal (Kelvin capacitors) structures decreases with increasing thickness of one metal electrode. This metal electrode consists of a double layer whose single layer thicknesses are of the order of few tens of nm. The observed effect can be attributed to a decrease of the work function at the counter limiting interface not exposed to the evaporation beam. A possible explanation can be found again in the violation of the local charge neutrality in films with unlike surfaces.     

Self-oscillating mixing in a QW double barrier diode at W-band

We report the presence of self-oscillating mixing in an asymmetric quantum well double barrier diode at 86GHz. We have demonstrated that the QW device can be injection-locked to the RF signal, and that the intermediate frequency (IF) can exhibit both upper and sub harmonics. Conversion gain was measured and found to be typically –30 to –40dB, with a best measurement of –18.79dB. Under certain conditions, the IF could show wideband chaotic-like oscillations extending to 1500Mhz.     

Investigation of the performance and band-to-band tunneling effect of a new double-halo-doping carbon nanotube field-effect transistor

Carbon nanotube field-effect transistors (CNTFETs) can be fabricated with Ohmic- or Schottky-type contacts. We focus here on Ohmic CNTFETs. The CNTFETs suffer from band-to-band tunneling which in turn causes the ambipolar conduction. In this paper, to suppress the ambipolar behavior of CNTFETs and improve the performance of these devices, we have proposed application of symmetric double-halo (DH)-doping in CNTFETs. In this new structure, the source-side halo doping reduces the drain-induced barrier lowering (DIBL) and the drain-side halo reduces the band-to-band tunneling effect. Simulation results show in the DH-CNTFET, subthreshold swing below the 60 mV/decade conventional limit can be achieved. Also it decreases significantly the leakage current and drain conductance and increases on–off current ratio and voltage gain as compared to conventional CNTFET.     

Effect of Ga and As sublayers on the structures and properties of Ni-GaAs contacts

Studies were made of heat-treated Ni-GaAs contacts, at which, prior to the electrochemical deposition of nickel, thin layers of gallium or arsenic were deposited. The physicochemical reactions at the interfaces were investigated, as well as the electrophysical characteristics of the diodes, the morphology of the metallic coatings, and the mechanical stresses. The phase composition of the contacts and the height of the barrier are seen to be practically independent of the introduction of excess Ga or As, whereas the thermal stability of the electrical parameters of the diodes varies considerably. This difference is associated with the effect of the Ga and As sublayers on the magnitude of the mechanical stresses arising at the metal-semiconductor interface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 46–51, February, 1987.     

Temperature and carbon source effects on methane–air flame synthesis of CNTs

We have conducted experimental and numerical studies on flame synthesis of carbon nanotubes (CNTs) to investigate the effects of three key parameters – selective catalyst, temperature and available carbon sources – on CNT growth. Two different substrates were used to synthesize CNTs: Ni-alloy wire substrates to obtain curved and entangled CNTs and Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates to grow well-aligned, self-assembled and size-controllable CNTs, each using two different types of laminar flames, co-flow and counter-flow methane–air diffusion flames. An appropriate temperature range in the synthesis region is essential for CNTs to grow on the substrates. Possible carbon sources for CNT growth were found to be the major species CO and those intermediate species C₂H₂, C₂H₄, C₂H₆, and methyl radical CH₃. The major species H₂, CO₂ and H₂O in the synthesis region are expected to activate the catalyst and help to promote catalyst reaction.     

Mapping of Two-Dimensional Contact Problems on a Problem with a One-Dimensional Parametrization

We discuss a possible generalization of the ideas of the method of dimensionality reduction (MDR) for the mapping of two-dimensional contact problems (line contacts). The conventional formulation of the MDR is based on the existence and uniqueness of a relation between indentation depth and contact radius. In two-dimensional contact problems, the indentation depth is not defined unambiguously, thus another parametrization is needed. We show here that the Mossakovskii-Jäger procedure of representing a contact as a series of incremental indentations by flat-ended indenters can be carried out in two-dimensions as well. The only available parameter of this process is, however, the normal load (instead of indentation depth as in the case of threedimensional contacts). Using this idea, a complete solution is obtained for arbitrary symmetric two-dimensional contacts with a compact contact area. The solution includes both the relations of force and half-width of the contact and the stress distribution in the contact area. The procedure is generalized for adhesive contacts and is illustrated by solutions of a series of contact problems.     

Electrical performance of gallium nitride nanocolumns

The electrical characterization of gallium nitride (GaN) nanocolumns with a length up to 1 μm and a diameter of about 30–80 nm grown on doped silicon is a challenge for nano analytics. To determine the conductivity of these nanocolumns, I–V characteristics were recorded by atomic force microscopy (AFM). To measure the conductivity of a single nanocolumn, a conductive AFM tip was placed at the top of the nanocolumn. The measured current/voltage characteristic of a single nanocolumn shows the typical performance of a Schottky contact, which is caused by the contact between the metallic AFM tip and the semiconductor material of the nanocolumn. The height of the Schottky barrier is dependent on the work function of the AFM tip metal used. The linear part of the curve was used to calculate the differential resistance, which was found to be about 13 Ω cm and slightly dependent on the diameter.     

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10¹⁸ cm⁻³). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10⁻⁴ (Al), 8.0×10⁻⁵ (Al/Au) and 1.2×10⁻⁵ Ω cm² (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.     

Synthesis of carbon nanotubes on Ni-alloy and Si-substrates using counterflow methane–air diffusion flames

We have conducted an experimental study to investigate the synthesis of multi-walled carbon nanotubes (CNTs) in counterflow methane–air diffusion flames, with emphasis on effects of catalyst, temperature, and the air-side strain rate of the flow on CNTs growth. The counterflow flame was formed by fuel (CH₄ or CH₄ + N₂) and air streams impinging on each other. Two types of substrates were used to deposit CNTs. Ni-alloy (60% Ni + 26% Cr + 14% Fe) wire substrates synthesized curved and entangled CNTs, which have both straight and bamboo-like structures; Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates synthesized well-aligned, self-assembled CNTs. These CNTs grown inside nanopores had a uniform geometry with controllable length and diameter. The axial temperature profiles of the flow were measured by a 125 μm diameter Pt/10% Rh–Pt thermocouple with a 0.3 mm bead junction. It was found that temperature could affect not only the success of CNTs synthesis, but also the morphology of synthesized CNTs. It was also found, against previous general belief, that there was a common temperature region (1023–1073 K) in chemical vapor deposition (CVD) and counterflow diffusion flames where CNTs could be produced. CNTs synthesized in counterflow flames were significantly affected by air-side strain rate not through the residence time, but through carbon sources available for CNTs growth. Off-symmetric counterflow flames could synthesize high-quality CNTs because with this configuration carbon sources at the fuel side could easily diffuse across the stagnation surface to support CNTs growth. These results show the feasibility of using counterflow flames to synthesize CNTs for particular applications such as fabricating nanoscale electronic devices.     

Phase interaction in GaAs contacts with group I metals and its relationship to the degradation of structures with a Schottky barrier

The methods of Rutherford back scattering of helium ions and x-ray diffraction and electron microscope analysis are used to study phase interaction in GaAs contacts with layers of group I metals (Cu, Ag, Au) with annealings in a hydrogen atmosphere. The nature of the interaction and the mechanisms of degradation of the volt-ampere characteristics of Schottky barrier diodes are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 16–22, September, 1985.     

Hybrid stable-chaotic states in coupled quantum well stadia

We use tunnel current spectroscopy to investigate the quantum states of two GaAs quantum wells coupled by a low (100 meV) (AlGa)As tunnel barrier. A high tilted magnetic field is used to generate strongly chaotic electron motion in the two wells which act as coupled chaotic ‘stadia'. The effect of the tunnel barrier on the dynamics of the system depends on the magnitude of the applied bias voltage V. For V375 mV, the central potential barrier acts as a perturbation which modifies the trajectories of selected periodic orbits in the quantum well. Scattering off the central barrier also generates new periodic orbits involving multiple collisions on all three barriers. These orbits ‘scar' distinct sets of eigenstates which generate periodic resonant peaks in the current–voltage characteristics of the device. When the device is biased such that the injected electrons just surmount the central barrier, our calculations reveal novel hybrid scarred states with both stable and chaotic characteristics.     

Physicochemical interactions in thin film Pb-GaAs structures upon thermal annealing

Morphology and elemental and phase composition of Pb-GaAs contacts prepared by electroprecipitation of the metal and annealed in a hydrogen atmosphere are studied. It is shown that the interaction of the lead with the gallium arsenide occurs with participation of a liquid phase which appears at 100–300°C due to dissociation of the semiconductor and gallium diffusion into the metallic coating. The absence of complex ions of the Pb_nGa_m(As_m)⁺ type in the secondary ion mass spectra of the contacts analyzed indicates that chemical interaction of the lead with gallium and arsenic does not occur.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 12–15, September, 1987.     

Low switching power 2 × 2 thermo-optic switch using direct ultraviolet photolithography process

A Multimode Interferometers–Mach Zehnder Interference (MMI–MZI) polymer 2 × 2 thermo-optic switch is proposed using the thermo-optical effect of polymer materials and fabricated by simple direct ultraviolet photolithography process. We utilize a cross-linkable negative photoresist as core material. Once exposed to ultraviolet light through a photomask, the waveguide strips can be obtained upon development. The experiment results show that the switch has a low switching power of less than 7.5 mW.     

Effect of cryogenic temperature deposition on Au contacts to bulk, single-crystal n-type ZnO

Au contacts were deposited on bulk, n-type single-crystal ZnO at either 77 K or 300 K.The room temperature deposition produced contacts with ohmic characteristics. By sharp contrast, the cryogenic deposition produced rectifying characteristics with barrier heights around 0.4 eV. The differences in contact behavior were stable to anneal temperatures of ∼300 °C. There were no differences in near-surface stoichiometry for the different deposition temperatures, while the low temperature contacts showed a more uniform appearance. With further optimization of the pre-deposition cleaning process, this may be a useful method for engineering barrier heights on ZnO.     

Tunneling through the space charge region in a metal-amorphous silicon contact

By solving for the potential barrier profile in metal-amorphous silicon contacts using an exponential distribution of the density of states in the energy spectrum mobility gap, we compute the voltage-current characteristics of such contacts taking into account tunneling through the space charge region. We demonstrate that as the density of states in the middle of the mobility gap increases, such tunneling leads to significant changes in the rectifying properties of a metal-amorphous silicon contact.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 3–7, November, 1986.