Showing papers on "Contact resistance published in 2000"

Silicon nanowire devices

Abstract: Transport measurements were carried out on 15–35 nm diameter silicon nanowires grown using SiH4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440°C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 10^4. Three terminal devices indicate that the doping of the wires is p type.

Formation of low-resistance ohmic contacts between carbon nanotube and metal electrodes by a rapid thermal annealing method

Abstract: The contact resistance between a carbon nanotube and metal electrodes decreases by several orders of magnitude and becomes long-term stable when the nanotube contacted by Ti-Au electrodes was annealed by a rapid thermal annealing method at 600-800 °C for 30 s. The contact resistances of the annealed samples are in the range 0.5-50 kΩ at room temperature, depending on the electrical properties of the nanotube. The short and relatively low-temperature annealing process enables us to make a surface Ti-nanotube contact suitable for electrical measurements. For the samples with relatively low contact resistances (0.5-5 kΩ) at room temperature, the contact resistance remained constant or decreased slightly as the temperature was lowered. Those with a relatively high contact resistance (5-50 kΩ), on the other hand, showed increasing contact resistance with a lowering of the temperature.

p-GaN surface treatments for metal contacts

Abstract: The chemical bonding and electronic properties of wet, chemically treated p-GaN surfaces were studied using synchrotron radiation photoemission spectroscopy. Chlorine-based chemical bonding was identified on the conventional HCl-treated p-GaN surface, which is associated with a shift of the surface Fermi level toward the conduction band edge by ∼0.9 eV with respect to the thermally cleaned surface. Compared to the HCl-treated surface, the surface Fermi level on the KOH-treated surface lies about ∼1.0 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-type materials than the HCl-treated surface. The smaller surface barrier height to p-GaN after KOH treatment can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

A unified simulation of Schottky and ohmic contacts

Abstract: The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated.

Long-term thermal stability of Ti/Al/Pt/Au Ohmic contacts to n-type GaN

Abstract: We present a study of the long-term thermal stability and low specific contact resistance of Ti/Al/Pt/Au multilayer contacts to n-type GaN. The Ohmic performance can be maintained up to 60 and 540 min for thermal annealing at 950 and 850 °C, respectively, and even longer than 600 min for thermal annealing at 750 °C. By comparing the specific contact resistances for Ti/Al/Pt/Au and Ti/Al/Au Ohmic contacts to n-type GaN, the long-term thermal stability may be deduced as due to the contribution of the Pt barrier for preventing Au penetration.

Tunable resistance of a carbon nanotube-graphite interface

Abstract: The transfer of electrons from one material to another is usually described in terms of energy conservation, with no attention being paid to momentum conservation. Here we present results on the junction resistance between a carbon nanotube and a graphite substrate and show that details of momentum conservation also can change the contact resistance. By changing the angular alignment of the atomic lattices, we found that contact resistance varied by more than an order of magnitude in a controlled and reproducible fashion, indicating that momentum conservation, in addition to energy conservation, can dictate the junction resistance in graphene systems such as carbon nanotube junctions and devices.

Low-resistance Ti/Au ohmic contacts to Al-doped ZnO layers

Abstract: We report on low-resistance ohmic contacts to the moderately doped n-type ZnO:Al(nd=2×1017 cm−3) obtained using Ti (30 nm)/Au (50 nm) metallization schemes. Annealed Ti/Au contacts exhibit linear current–voltage characteristics, showing that high-quality ohmic contacts are formed. The Ti/Au scheme produces a specific contact resistance of 2×10−4 Ω cm2 when annealed at 300 °C for 1 min in a N2 atmosphere.

Piezoresistivity in continuous carbon fiber polymer-matrix composite

Abstract: Piezoresistivity involving the volume resistivity of a continuous unidirectional carbon fiber epoxy-matrix composite in the fiber direction decreasing reversibly upon tension in the fiber direction was observed by the four-probe method, due to an increase in the degree of fiber alignment. Use of the two-probe method resulted in measurement of the contact resistance rather than the volume resistance. The contact resistance increased reversibly upon tension.

Various bonding configurations of transition-metal atoms on carbon nanotubes: Their effect on contact resistance

Abstract: Our investigations reveal that the bonding of the transition-metal atoms on a single-wall carbon nanotube (SWCN) depends on the detailed contact conditions. On the basis of our results, we suggest that the early 3-d elements (Sc, Ti, and V) can be expected to be good candidates for making metal–SWCN contacts of low resistance, while contacts employing the late 3-d elements (Fe, Co, and Ni) and Cu are expected to exhibit large contact resistance.

Low-resistance ohmic contacts to p-type GaN

Abstract: The specific contact resistance of two types of ohmic contacts to p-type GaN is analyzed. First, an ohmic contact formed by a metal electrode deposited on a highly doped p-type GaN layer. Second, an ohmic contact formed by a metal electrode deposited on a thin GaN layer with an internal electric field caused by polarization effects. It is shown that contacts mediated by polarization effects can result, for typical materials parameters, in low contact resistances comparable or better than contacts mediated by dopant-induced surface fields. A type of contact is proposed and demonstrated. These contacts employ polarization charges to enhance tunneling transport as well as high doping. Experimental results on Ni contacts to p-type AlxGa1−xN/GaN doped superlattices are presented. The contacts have linear current–voltage characteristics with contact resistances of 9.3×10−4 Ω cm2, as inferred from linear transmission-line method measurements. The influence of annealing at temperatures ranging from 400 to 500 °C ...

Low resistance Ti'Pt'Au ohmic contacts to p-type GaN

Abstract: Electrical properties of Ti (15 nm)/Pt (50 nm)/Au (80 nm) contacts on moderately doped p-GaN (NA=3.0×1017 cm−3) are reported. Linear current–voltage characteristics were observed after annealing the contacts for 1 min at temperatures above 700 °C. The best ohmic contacts were obtained after annealing in a N2 ambient at 800 °C for 2 min. These contacts exhibited a specific contact resistance Rc of 4.2×10−5 Ω cm2 and contact resistivity ρc of 21 Ω mm. Possible mechanisms for the lower contact resistivity of Ti/Pt/Au contacts are discussed. The processing for the Ti/Pt/Au ohmic contacts is compatible with routine fabrication steps for GaN devices.

An electrostatically-actuated MEMS switch for power applications

Abstract: This paper presents the design, analysis, fabrication, and testing of an electrostatically-actuated MEMS power switch. The device can be switched electrostatically (20 V), pneumatically (1200 Pa), or through combined actuation. Prototype switches carry currents in excess of 400 mA in either current direction with a contact resistance as low as 14 m/spl Omega/. Their off-state resistance is higher than the 30 M/spl Omega/ limit of the test equipment. Breakdown voltages of 300 V have been achieved across their small air gaps. Their nominal switching time is 20 ms. Extended lifetime testing has not been carried out but our tests to date show that the prototype switches operate more than 4000 cycles without significant degradation in their contact resistance. Finally, a protective switching scheme is proposed to minimize contact wear due to arcing during switch opening and closing.

Oxidized Ni/Pt and Ni/Au ohmic contacts to p-type GaN

Abstract: The Ni/Pt and Ni/Au contacts to p-GaN heat treated in air are investigated to study the effect of the metallic phase of high work function on specific contact resistance (ρc). The current–voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum ρc of 4×10−6 Ω cm2. Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high ρc of about 2–5×10−2 Ω cm2 are obtained for the oxidized Ni/Pt contacts, which is attributed to the different microstructure from that of the oxidized Ni/Pt contact. The NiO phase of the oxidized Ni/Pt contact is separated from p-GaN by a continuous face-centered-cubic Pt–Ni–Ga film. Therefore, the oxidized Ni/Pt contact behaves just like a common metal contact, but not a metal/NiO composite contact. Comparison of the two oxidized contacts implies that the previously proposed model based on metal/p-NiO/p-GaN heterojunction is reliable to explain the low ρc to ...

Electronic transport mechanisms of nonalloyed Pt Ohmic contacts to p-GaN

Abstract: We report on the electronic transport mechanisms for nonalloyed Pt Ohmic contacts to p-GaN which were surface treated using a buffered oxide etch solution and (NH4)2Sx. Measurements show that the value of the effective Richardson constant (A**) is 12 A cm−2 K−2, which is considerably smaller than the theoretical value of 103.8 A cm−2 K−2. Based on Hall-effect results, the two-step surface-treated contact is modeled to consist of a Pt/p+-/p-GaN structure, and the conventionally treated contact consists of a Pt/p-GaN structure. The theoretical results obtained using these models are compared with the experimental data. It is shown that for the conventionally treated contact thermionic emission dominates the current flow, whereas for the two-step surface-treated contact, field emission is dominant.

Isotropic conductive adhesives filled with low-melting-point alloy fillers

Abstract: Conventional isotropic conductive adhesives (ICAs) are composed of a polymeric matrix and silver (Ag) flakes. As an alternative to lead-bearing solder, ICAs offer a number of benefits, but limitations do exist for ICA technology. ICAs filled with silver flakes generally show higher initial contact resistance, unstable contact resistance, and inferior impact strength. In this study, a new class of isotropic conductive adhesives was developed by using two different fillers, silver flakes and a low-melting-point-alloy filler, into the ICA formulations. After curing, the metallurgical connections between silver particles, and between silver particles and nickel (Ni) substrate were observed using scanning electron microscopy (SEM). Electrical properties including bulk resistance, initial contact resistance, and contact resistance shifts of the ICA were investigated and compared to those of a commercial ICA, an in-house ICA filled with only the silver flake, and a eutectic Sn/Pb solder. It was found that: (1) the low-melting-point alloy filler could wet the silver flakes and nickel substrate to form metallurgical connections, (2) this ICA had much lower bulk resistance than the commercial ICA and the in-house ICA filled with only the silver flake, and (3) this ICA showed especially low initial contact resistance and more stable contact resistance during aging on nickel metal compared to the ICA filled only with silver flakes.

Metallization scheme for highly low-resistance, transparent, and thermally stable Ohmic contacts to p-GaN

Abstract: We report on a promising metallization scheme for high-quality Ohmic contacts to surface-treated p-GaN:Mg (2–3×1017 cm−3). It is shown that the as-deposited Pt/Ru contact produces a specific contact resistance of 7.8(±2.2)×10−4 Ω cm2. However, annealing of the contact at 600 °C for 2 min results in a resistance of 2.2(±2.0)×10−6 Ω cm2. It is also shown that the light transmittance of the annealed contact is 87.3% at 470 nm. Furthermore, the surface of the contact annealed at 600 °C for 30 min is found to be very smooth with a rms roughness of 0.8 nm. These results strongly indicate that the Pt/Ru can be a suitable scheme for the fabrication of high-performance laser diodes or other devices.

Investigation of surface treatments for nonalloyed ohmic contact formation in Ti/Al contacts to n-type GaN

Abstract: To investigate the possibility of preparing a nonalloyed ohmic contact to n-type GaN, the effects of GaN surface treatments on the metal/GaN interface were studied using x-ray photoelectron spectroscopy. A specific contact resistance of 5.0×10−5 Ω cm2 for the Ti/Al nonalloyed ohmic contacts to (NH4)2Sx-treated n-type GaN can be obtained. The nonalloyed ohmic contact formation would be impeded by the native oxide and the hydroxyl induced from the surface treatment of chemical solutions.

Improved Ni based composite Ohmic contact to n-SiC for high temperature and high power device applications

Abstract: Ni/WSi/Ti/Pt Ohmic contacts to n-SiC were investigated as a function of annealing temperatures up to 1000 °C. Annealing at temperatures between 950 and 1000 °C yielded excellent Ohmic behavior. At these temperatures the contact–SiC interface was smooth, defect free, and characterized by a narrow Ni2Si reaction region. The annealed contacts possessed atomically smooth surface morphologies and exhibited minimal contact expansion. The residual carbon, resultant from SiC decompositon and reaction with Ni to form Ni2Si, was constrained by reaction with the WSi and Ti layers forming carbide phases of W and Ti spatially distant from the metal semiconductor interface. Our results demonstrate that the Ni/WSi/Ti/Pt composite Ohmic contact maintains the desirable electrical properties associated with Ni contacts and possesses excellent interfacial, compositional, and surface properties which are required for reliable high power and high temperature device operation.

Textured MoS2 thin films obtained on tungsten: Electrical properties of the W/MoS2 contact

Abstract: Textured films of molybdenum disulfide have been obtained by solid state reaction between the constituents in thin films form when a (200) oriented tungsten sheet is used as substrate. The crystallites have their c axis perpendicular to the plane of the substrate. The annealing conditions are T=1073 K and t=30 min. The films are stoichoimetric and p type. Such highly textured films are achieved without foreign atom addition (Ni, Co…). It appears, as shown by x-ray photoelectron spectroscopy, that a thin WS2 layer is present at the interface W/MoS2. The crystallization process is discussed by a van der Waals texturation (pseudoepitaxy) onto dangling bond sulfur terminated surfaces, these surfaces being ordered. After characterization of the W/MoS2 structure by x-ray diffraction and x-ray photoelectron spectroscopy, an upper electrode of tungsten was deposited by sputtering. The electrical properties of these W/MoS2/W structures have been investigated by analyzing the behavior of the current–voltage characteristics as a function of the measuring temperature. It is shown that an ohmic contact is obtained with a contact resistance smaller than the resistance of the MoS2 film.

Development of conductive adhesives for solder replacement

Abstract: Develops conductive adhesives with stable contact resistance and desirable impact performance. Effects of purity of the resins and moisture absorption on contact resistance are investigated. Several different additives (oxygen scavengers and corrosion inhibitors) on contact resistance stability during elevated temperature and humidity aging are studied, and effective additives are identified. Then, several rubber-modified epoxy resins and two synthesized epoxide-terminated polyurethane resins are introduced into ECA formulations to determine their effects on impact strength. The loss factor, tan /spl delta/, of each formulation is measured using a dynamic mechanical analyzer (DMA) and impact strength is evaluated using the National Center for Manufacturing Science (NCMS) standard drop test procedure. Finally, high performance conductive adhesives are formulated by combining the modified resins and the effective additives. It is found that 1) purity of the resins and moisture absorption of the formulation affect the contact resistance stability of an ECA; 2) the oxygen scavengers and corrosion inhibitors can delay contact resistance shift; 3) one of the corrosion inhibitors is very effective in stabilizing the contact resistance; 4) some rubber-modified epoxy resins and the epoxide-terminated polyurethane resins can provide the conductive adhesives with superior impact performance; and 5) conductive adhesives with stable contact resistance and desirable impact performance are developed.

Nonalloy Ohmic contact fabrication in a hydrothermally grown n-ZnO (0001) substrate by KrF excimer laser irradiation

Abstract: A nonalloy Ohmic electrode is formed on a hydrothermally grown n-type ZnO substrate by KrF excimer laser irradiation and metal deposition without any impurity doping. Laser irradiation changes the ZnO surface layer to a Zn-rich one with lower sheet resistance than the bulk. The Zn-rich premetallized layer (ZnOx layer, x=0–1) exhibits n+-type conduction, which enables current conduction into the ZnO substrate with Ohmic characteristics. An In/ZnOx contact fabricated by single-pulse laser irradiation with 0.3 J/cm2 on the ZnO substrate has a contact resistance of 7×10−1 Ω cm2. A postheat treatment for 5 min at 300 °C increases the sheet resistance in the premetallized layer, which causes an increase of the contact resistance of the fabricated contact.

Point contact current-voltage measurements on individual organic semiconductor grains by conducting probe atomic force microscopy

Abstract: Conducting probe atomic force microscopy (CP-AFM) was used to make point contact current–voltage (I–V) measurements on individual microscopic grains of the organic semiconductor sexithiophene (6T). The 6T grains ranged from 1 to 6 molecules (2–14 nm) in thickness, 1–2 μm in length and width, and were deposited by thermal evaporation onto SiO2 substrates previously patterned with 200 nm wide Au wires. Au-coated AFM probes were used to image the substrates in air to identify individual 6T grains which grew in contact with a wire. The same probes were used to record the I–V characteristics of single grains. Analysis of the differential resistance as a function of probe wire separation yielded typical grain resistivities of 100 Ω cm and contact resistances of ∼100 MΩ. Over the 0–3 V range probed, the shape of the I–V curves can be attributed to a combination of the nonlinear I–V characteristics of the Au-6T junctions and the ohmic response of the grain. In general, we have shown that CP-AFM is a reliable meth...

Improvement of EDM efficiency of silicon single crystal through ohmic contact

Abstract: This paper describes the improvement of the machining rate of electrical discharge machining (EDM) for silicon single crystals by reducing the contact resistance between the silicon single crystal and metal electric feeder. To decrease the resistance of the rectifying contact between a p-type silicon wafer and the metal feeder, attempts to achieve ohmic contact were made by plating the contact surface of the silicon wafer with aluminum by vacuum evaporation, followed by the diffusion process. To accomplish an ohmic contact between n-type silicon and metal, gold–antimony alloy was used in place of aluminum. The influence of polarity on the machining rate is also discussed from the viewpoint of the rectifying nature of the interface between the arc plasma and silicon single crystal.

Nitride semiconductor element and its manufacturing method

Abstract: PROBLEM TO BE SOLVED: To provide a nitride semiconductor element whose operation voltage can be reduced by reducing contact resistance with electrodes, and its manufacturing method. SOLUTION: In a semiconductor laser element 100, a buffer layer 2, an undoped GaN layer 3, an N-GaN contact layer 4, an N-InGaN crack preventing layer 5, an N-AlGaN clad layer 6, an MQW light emitting layer 7, a P-AlGaN clad layer 8, and a P-GaN contact layer 9 are laminated in order on a (0001) face of a sapphire substrate 1. A ridge is formed on the P-GaN contact layer 9 and the P-AlGaN clad layer 8, and unevenness is formed on the upper surface of the ridge. Unevenness is formed on the surface of a prescribed region of the N-GaN contact layer 4 which is exposed by etching. A P-electrode 10 and an N-electrode 11 are formed on the N-GaN contact layer 4 wherein unevenness is formed.