Showing papers on "Contact resistance published in 2004"

Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti∕Pr0.7Ca0.3MnO3 interface

Abstract: We have characterized the vertical transport properties of epitaxial layered structures composed of Pr0.7Ca0.3MnO3(PCMO) sandwiched between SrRuO3(SRO) bottom electrode and several kinds of top electrodes such as SRO, Pt, Au, Ag, and Ti. Among the layered structures, Ti∕PCMO∕SRO is distinct due to a rectifying I–V characteristic with a large hysteresis. Corresponding to the hysteresis of the I–V characteristics, the contact resistance of the Ti∕PCMO interface reversibly switches between two stable states by applying pulsed voltage stress. We propose a model for the resistance switching at the Ti∕PCMO interface, in which the width and/or height of a Schottky-like barrier are altered by trapped charge carriers in the interface states.

Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance.

Abstract: Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current−voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, β, was found to be approximately 1.1 per carbon atom (C-1) or 0.88 A-1 and was independent of applied bias (over a voltage range of ±1.5 V) and electrode work function. In contrast, the contact resistance, R0, extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same...

Gated four-probe measurements on pentacene thin-film transistors: Contact resistance as a function of gate voltage and temperature

Abstract: We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs) The devices consisted of metal source and drain electrodes contacting a 300-A-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature The contact-corrected linear hole mobilities were as high as 175cm2∕Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ∕◻ and 13kΩ-cm, respectively, at high gate voltages In the temperature range of 50–200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30meV Importa

Mobility measurement and degradation mechanisms of MOSFETs made with ultrathin high-k dielectrics

Abstract: Accurate measurements and degradation mechanisms of the channel mobility for MOSFETs with HfO/sub 2/ as the gate dielectric have been systematically studied in this paper. The error in mobility extraction caused by a high density of interface traps for a MOSFET with high-k gate dielectric has been analyzed, and a new method to correct this error has been proposed. Other sources of error in mobility extraction, including channel resistance, gate leakage current, and contact resistance for a MOSFET with ultrathin high-k dielectric have also been investigated and reported in this paper. Based on the accurately measured channel mobility, we have analyzed the degradation mechanisms of channel mobility for a MOSFET with HfO/sub 2/ as the gate dielectric. The mobility degradation due to Coulomb scattering arising from interface trapped charges, and that due to remote soft optical phonon scattering are discussed.

Variable temperature film and contact resistance measurements on operating n-channel organic thin film transistors

Abstract: We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activa...

Contact resistance in organic thin film transistors

Abstract: This letter reports on the unexpected dependence of contact resistance on the dielectric layer for pentacene thin film transistors with printed organic conducting electrodes. While the intrinsic mobility is weakly reliant on the dielectric, the contact resistance does vary considerably with dielectric layer. We show that while morphological changes are not apparent, contact resistances vary by an order of magnitude. This result suggests that the barrier to charge injection may depend not only on interactions at the complex triple interface but also on the details of the electronic structure at the semiconductor/dielectric interface.

Temperature-dependent contact resistances in high-quality polymer field-effect transistors

Abstract: Contact resistances between organic semiconductors and metals can dominate the transport properties of electronic devices incorporating such materials. We report measurements of the parasitic contact resistance and the true channel resistance in bottom contact poly(3-hexylthiophene) field-effect transistors with channel lengths from 400 nm up to 40 μm, from room temperature down to 77 K. For fixed gate voltage, the ratio of contact to channel resistance decreases with decreasing temperature. We compare this result with a recent model for organic semiconductor-metal contacts. Mobilities corrected for this contact resistance can approach 1 cm2/V s at room temperature and high gate voltages.

Contact mechanisms and design principles for alloyed ohmic contacts to n-GaN

Abstract: Contact mechanisms and design principles of alloyed ohmic contacts to n-GaN are investigated. For the investigation, both tunnel contacts and thermionic contacts are considered. While the tunnel contacts include the Ti/Al/Ti/Au, Ti/Al/Ni/Au, Ti/Al/Pd/Au, (Ta,Ti)/Ni/Au, Ti/Al/Mo/Au, and Ti/Au/Pd/Au contacts, the thermionic contacts include the Ni/Au contacts. The proposed design principles correctly dictate the characteristics of all these contacts. At present, tunneling is believed to be the primary mechanism for low resistivity of the tunnel contacts. The present study demonstrates that both tunneling and thermionic emission are equally important for the low resistivity of these contacts. Band-gap narrowing and/or image force lowering due to heavy doping also contribute to the resistivity reduction of these contacts. An exciting feature of the present study is the observation of a very low work function intermetallic alloy formed during annealing of an appropriate combination of large work function metal...

Simultaneous formation of multiwall carbon nanotubes and their end-bonded ohmic contacts to Ti electrodes for future ULSI interconnects

Abstract: We have succeeded in growing multiwall carbon nanotubes (MWNTs) with low-resistance ohmic contacts to titanium electrodes by hot-filament chemical vapor deposition (HF-CVD) using a nickel catalyst layer on a titanium electrode. The contact resistance of the sample with nickel/titanium electrodes was two orders of magnitude smaller than that of the sample with nickel catalyst electrodes without titanium. We assumed that the low-resistance ohmic contact was achieved by forming titanium carbide (TiC) during the growth at the MWNT/titanium electrode interface. Moreover, we have demonstrated the growth of vertically aligned bundles of MWNTs, which were end-bonded to the lower titanium electrodes, selectively in via holes. We believe this is the first report of such simultaneous formation of MWNTs and their end-bonded low-resistance ohmic contacts, and its first trial application to carbon nanotube (CNT) vias for future ULSI interconnects.

Selecting metal alloy electric contact materials for MEMS switches

Abstract: This paper presents a method for selecting metal alloys as the electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This procedure consists of reviewing macro-switch lessons learned, utilizing equilibrium binary alloy phase diagrams, obtaining thin film material properties and, based on a suitable model, predicting contact resistance performance. After determining a candidate alloy material, MEMS switches were designed, fabricated and tested to validate the alloy selection methodology. Minimum average contact resistance values of 1.17 and 1.87 Ω were measured for micro-switches with gold (Au) and gold–platinum (Au–(6.3%)Pt) alloy electric contacts, respectively. In addition, 'hot-switched' life cycle test results of 1.02 × 108 and 2.70 × 108 cycles were collected for micro-switches with Au and Au–(6.3%)Pt contacts, respectively. These results indicate increased wear with a small increase in contact resistance for MEMS switches with metal alloy electric contacts.

Electrical contact resistance theory for conductive rough surfaces separated by a thin insulating film

Abstract: An electrical contact resistance (ECR) theory is presented for conductive and rough (fractal) surfaces separated by a thin insulating film, which is treated as an energy barrier that impedes current flow due to the electric-tunnel effect. The analysis yields insight into the effects of film properties, current flow, surface topography, mechanical properties, and contact load on the ECR. It is shown that the variation of ECR with the contact load is less pronounced than that observed in the absence of an insulating layer due to the intrinsic voltage dependence of the tunnel resistance and the associated voltage compensation mechanism. The effect of nonohmic behavior on the relationships of the ECR with the contact load and the real contact area is discussed and results are compared with approximate analytical relationships developed herein. The relationship between the real contact area and the ECR depends on the current intensity and film properties and is independent of the surface topography and mechani...

Single Wall Carbon Nanotubes for p-Type Ohmic Contacts to GaN Light-Emitting Diodes

Abstract: Homogeneous films of pure single wall carbon nanotubes (SWNTs) sufficiently thin to be optically transparent in the visible range of the spectrum were employed as p-Ohmic contacts on GaN-InGaN quantum-well light-emitting diodes. The specific contact resistance of the SWNT films on the p-GaN was 1.1 x 10 -2 Ω cm 2 after annealing at 700°C for 60 s under N 2 , which was a factor of 3 lower than standard Ni/Au contacts on the same p-GaN. The SWNT-contacted LEDs showed bright blue emission centered at 434 nm and demonstrate that the SWNT films provide a new class of electrically conducting, p-type, transparent electrode for use with photonic devices.

Direct observation of contact and channel resistance in pentacene four-terminal thin-film transistor patterned by laser ablation method

Abstract: We established a dry-etching patterning process for the channel formation of pentacene thin-film transistor, and fabricated a four-terminal device equipped with a gate electrode. The four-terminal device enabled us to divide two-terminal source–drain resistance into two components of contact resistance and pentacene channel resistance. We obtained direct evidence of a gate-voltagedependent contact resistance change: the gate-induced charge significantly reduced the contact resistance and increased source–drain current. Furthermore, the temperature dependence of the device clearly indicated that the contact resistance was much higher than the channel resistance and was dominated in the two-terminal total resistance of the device below 120 K. An observed activation energy of 80 meV for contact resistance was higher than that of 42 meV for pentacene channel resistance.

Low contact resistance PEM fuel cell

Abstract: A PEM fuel cell having a current collector comprising a polymer composite and a diffusion media engaging said polymer composite. The polymer composite has a hyperconductive surface layer engaging the diffusion media to reduce the contact resistance therebetween. The hyperconductive surface layer is formed by depositing or smearing an electrically-conductive material on the surface of the polymer composite.

Probing intrinsic transport properties of single metal nanowires: Direct-write contact formation using a focused ion beam

Abstract: The transport characteristics of 70-nm-diameter platinum nanowires (NWs), fabricated using a pore-templated electrodeposition process and individually contacted using a focused ion beam (FIB) method, are reported. This approach yields nanowire devices with low contact resistances (∼400Ω) and linear current–voltage characteristics for current densities up to 65kA∕cm2. The intrinsic nanowire resistivity (33±5μΩcm) indicates significant contributions from surface- and grain-boundary scattering mechanisms. Fits to the temperature dependence of the intrinsic NW resistance confirm that grain-boundary scattering dominates surface scattering (by more than a factor of 2) at all temperatures. Our results demonstrate that FIB presents a rapid and flexible method for the formation of low-resistance ohmic contacts to individual metal nanowires, allowing intrinsic nanowire transport properties to be probed.

Modeling Thermal Contact Resistance: A Scale Analysis Approach

Abstract: A compact analytical model is developed for predicting thermal contact resistance (TCR) of nonconforming rough contacts of bare solids in a vacuum. Instead of using probability relationships to model the size and number of microcontacts of Gaussian surfaces, a novel approach is taken by employing the scale analysis method. It is demonstrated that the geometry of heat sources on a half-space for microcontacts is justifiable for an applicable range of contact pressure

Mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN

Abstract: The mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN was investigated using synchrotron photoemission spectroscopy. A low contact resistivity of 6.6×10−5Ωcm2 was obtained from Ni(50A)∕Ag(1200A) contact after annealing at 500°C in O2 ambient. Ni out-diffused to form a NiO and Ag in-diffused into the contact interface during the oxidation annealing. Out-diffused Ga atoms from GaN could dissolve in the Ag layer to form Ag–Ga solid solutions, leaving Ga vacancies below the contact. Ga vacancies could increase the net hole concentration and reduce the surface band bending, resulting in the ohmic contact formation.

Short channel effects in regioregular poly(thiophene) thin film transistors

Abstract: The effects of the physical channel length on the current-voltage characteristics of thin film transistors (TFTs) made with poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] were examined. Coplanar transistors with fully patterned electrodes on insulating substrates and with a common gate structure on thermal oxide were fabricated. The output characteristics of TFTs with channel lengths shorter than 10μm showed the presence of a parasitic contact resistance and the lack of current saturation. The origin of these nonidealities was examined by the application of models that included self-heating effects and breakdown of the channel region at high applied biases. The analysis suggests that carriers can break away from the channel at high bias voltages and flow through a bulk region of the semiconducting film leading to higher currents than otherwise expected.

Comparative study of Ti∕Al∕Mo∕Au, Mo∕Al∕Mo∕Au, and V∕Al∕Mo∕Au ohmic contacts to AlGaN∕GaN heterostructures

Abstract: Ohmic contact formation by Ti∕Al∕Mo∕Au, Mo∕Al∕Mo∕Au, and V∕Al∕Mo∕Au on AlGaN∕GaN heterostructure field effect transistor layers have been studied and compared. Mo∕Al∕Mo∕Au ohmic contacts exhibited the lowest contact resistance of 0.22±0.02Ωmm over a range of anneal temperatures from 650to800°C. The minimum contact resistances of Ti∕Al∕Mo∕Au and V∕Al∕Mo∕Au ohmic contacts were measured to be 0.38±0.04Ωmm at an anneal temperature of 800°C and 0.35±0.07Ωmm at an anneal temperature of 700°C, respectively. Long-term thermal stability measurements were performed at 500, 600, and 700°C. Of the three metallization schemes, Ti∕Al∕Mo∕Au exhibited the best thermal stability (up to 8.5h at 700°C). Intermetallic reactions were investigated using Auger electron spectroscopy and x-ray diffraction measurements.

Calculating interface resistance

Abstract: The exposed surface area of many of today's high powered electronic packages is no longer sufficient for the removal of the heat generated during normal operation. Heat sinks are a commonly-used, low cost means of increasing the effective surface area for dissipating heat by means of convective air cooling. While the use of a heat sink lowers the fluid-side thermal resistance, it also introduces an interface resistance across the contact formed between itself and the package case. Under some circumstances, this contact resistance can be substantial, impeding heat flow and reducing the overall effectiveness of the heat sink. Figure 1 depicts an electronic package heat sink assembly which would typically be joined by plastic or metal spring clips around the perimeter of the assembly.

Resistive limitations to spatially inhomogeneous electronic losses in solar cells

Abstract: Spatial variations of the electronic properties of solar cells are simulated with the help of a multi-diode model. Increasing the local series resistance limits the degradation of the overall performance by reducing losses of the open circuit voltage originating from cell areas with minor electronic quality. At the same time, the fill factor of the device decreases such that an optimum local series resistance for maximum power conversion efficiency is found. The value of the optimum series resistance depends on the degree of electronic inhomogeneity. Thus, optimization rules for spatially uniform solar cells are different from those for nonuniform cells.

Spin-dependent boundary resistance in the lateral spin-valve structure

Abstract: We report the detection of clear spin-valve signal without any spurious magnetoresistive signal in a lateral spin-valve structure consisting of Cu∕Ni−Fe ohmic junction using local current injection. The obtained spin-valve signal is much larger than that of the nonlocal spin-valve configuration because of the efficient spin accumulation. The local current injection experiments with different probe configurations proved that the spin-valve signal is caused by the spin-dependent boundary resistance at the interface between the ferromagnetic voltage probe and the spin-polarized nonmagnetic wire.

Elastic contact stiffness and contact resistance for the Weierstrass profile

Abstract: The Weierstrass series comprises a system of superposed self-affine sine waves that can be used to define a simple idealization of a two-dimensional fractal rough surface profile. The load–compliance relation for the contact of this profile with a rigid plane is here estimated using Westergaard's solution for the contact of a single sine wave with a plane and various approximations concerning the interaction of the different terms in the series. These approximations are compared with a numerical solution for the contact of the profile defined by the first few terms of the series. Once the load–compliance relation is established, the electrical contact resistance can be determined, using an analogy between the conduction and incremental elastic contact problems. The results show that these simple estimates give quite good predictions of the relations between load, compliance and contact resistance. They also confirm that these relations are largely determined by the coarse scale features of the surface profile, in contrast to the predictions of classical asperity model theories.

Thermal contact resistance of cured gel polymeric thermal interface material

Abstract: This paper reports the experimental results on the contact resistance of curable polymer gel thermal interface materials (TIMs) that have different mechanical properties due to difference in the rheology of the polymers. A semi-analytical model for the prediction of the thermal contact resistance of cured gel TIMs is also introduced in this paper. A novel method of finding the transition from grease type behavior to gel type behavior, which is very important for post reliability stress performance, based on G' (storage shear modulus) and G'' (loss shear modulus) measurements is reported. Further, post thermal cycling thermal resistance degradation rate of gel TIMs are related to the ratio of G and G''. Finally, design guidelines for gel TIMs for use in flip-chip packages comprising heat spreaders are proposed.