Showing papers on "Contact resistance published in 2009"

Thermal contact resistance between graphene and silicon dioxide

Abstract: The thermal contact resistance between graphene and silicon dioxide was measured using a differential 3ω method. The sample thicknesses were 1.2 (single-layer graphene), 1.5, 2.8, and 3.0 nm, as determined by atomic force microscopy. All samples exhibited approximately the same temperature trend from 42 to 310 K, with no clear thickness dependence. The contact resistance at room temperature ranges from 5.6×10−9 to 1.2×10−8 m2 K/W, which is significantly lower than previous measurements involving related carbon materials. These results underscore graphene’s potential for applications in microelectronics and thermal management structures.

Acoustic mismatch model for thermal contact resistance of van der Waals contacts

Abstract: Nanoparticles are typically in contact with another surface through weak van der Waals force. Thermal transport in these nanostructured systems is mainly limited by the contact resistance (Rc). Rc of nanoparticles have been typically calculated using the traditional acoustic or diffuse mismatch models, which assume very strong bond at the interface. In this paper, an analytical model of Rc that accounts for the strength of the interfacial bonding is presented. Conductance/area is proportional to the square of the adhesion energy of the interface for weak bonding and is the same as that given by traditional acoustic mismatch model for strong bonding.

Two-dimensional numerical simulation of radio frequency sputter amorphous In–Ga–Zn–O thin-film transistors

Abstract: We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Controlling Electron and Hole Charge Injection in Ambipolar Organic Field-Effect Transistors by Self-Assembled Monolayers

Abstract: Controlling contact resistance in organic field-effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ambipolar and/or light-emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) ambipolar OFETs using thiol-based self-assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM-induced work-function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM-induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top-gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.

Contact resistance between metal and carbon nanotube interconnects: Effect of work function and wettability

Abstract: The contact resistance of 14 different electrode metals with the work function between 3.9 and 5.7 eV has been investigated for carbon nanotube (CNT) interconnects. We observed that the contact resistance was mainly influenced by the two following parameters: the wettability and the work function difference of electrode metal to CNT. Ti, Cr, and Fe with good wettability showed lower resistance than other metals. Furthermore, no dependence of the contact resistance on the work function difference has been observed. However, the contact resistance of Au, Pd, and Pt with poor wettability increased as the work function difference became larger.

Metal/graphene contact as a performance Killer of ultra-high mobility graphene analysis of intrinsic mobility and contact resistance

Abstract: Graphene with a high carrier mobility of more than 10,000 cm2/Vs on SiO 2 has attracted much attention as a promising candidate of future high-speed transistor materials. The contact resistance (RC) between graphene and metal electrodes is crucially important for achieving potentially high performance of graphene from both physics and practical viewpoints. This paper discusses metal/graphene contact properties by separating from the intrinsic conduction of graphene.

Amorphous In–Ga–Zn–O coplanar homojunction thin-film transistor

Abstract: A fabrication process of coplanar homojunction thin-film transistors (TFTs) is proposed for amorphous In–Ga–Zn–O (a-IGZO), which employs highly doped contact regions naturally formed by deposition of upper protection layers made of hydrogenated silicon nitride (SiNX:H). The direct deposition of SiNX:H reduced the resistivity of the semiconductive a-IGZO layer down to 6.2×10−3 Ω cm and formed a nearly ideal Ohmic contact with a low parasitic source-to-drain resistance of 34 Ω cm. Simple evaluation of field-effect mobilities (μsat) overestimated their values especially for short-channel TFTs, while the channel resistance method proved that μsat was almost constant at 9.5 cm2 V−1 s−1.

Piezoelectric potential gated field-effect transistor based on a free-standing ZnO wire.

Abstract: We report an external force triggered field-effect transistor based on a free-standing piezoelectric fine wire (PFW). The device consists of an Ag source electrode and an Au drain electrode at two ends of a ZnO PFW, which were separated by an insulating polydimethylsiloxane (PDMS) thin layer. The working principle of the sensor is proposed based on the piezoelectric potential gating effect. Once subjected to a mechanical impact, the bent ZnO PFW cantilever creates a piezoelectric potential distribution across it width at its root and simultaneously produces a local reverse depletion layer with much higher donor concentration than normal, which can dramatically change the current flowing from the source electrode to drain electrode when the device is under a fixed voltage bias. Due to the free-standing structure of the sensor device, it has a prompt response time less than 20 ms and quite high and stable sensitivity of 2%/μN. The effect from contact resistance has been ruled out.

Contact resistance and shot noise in graphene transistors

Abstract: Potential steps naturally develop in graphene near metallic contacts. We investigate the influence of these steps on the transport in graphene field effect transistors. We give simple expressions to estimate the voltage-dependent contribution of the contacts to the total resistance and noise in the diffusive and ballistic regimes.

3D polysilicon diode with low contact resistance and method for forming same

Abstract: A semiconductor p-i-n diode and method for forming the same are described herein. In one aspect, a SiGe region is formed between a region doped to have one conductivity (either p+ or n+) and an electrical contact to the p-i-n diode. The SiGe region may serve to lower the contact resistance, which may increase the forward bias current. The doped region extends below the SiGe region such that it is between the SiGe region and an intrinsic region of the diode. The p-i-n diode may be formed from silicon. The doped region below the SiGe region may serve to keep the reverse bias current from increasing as result of the added SiGe region. In one embodiment, the SiGe is formed such that the forward bias current of an up-pointing p-i-n diode in a memory array substantially matches the forward bias current of a down-pointing p-i-n diode which may achieve better switching results when these diodes are used with the R/W material in a 3D memory array.

Solution processed invisible all-oxide thin film transistors

Abstract: We report on a fully transparent solution processed thin-film transistor (TFT) device with oxide semiconductor and oxide electrode. Selective doping into the sol–gel derived ZnO materials tailors the electrical properties to range from metallic to semiconducting characteristics. Integration of a spin-coated zinc tin oxide (ZTO) semiconductor with an ink-jet-printed zinc indium oxide (ZIO) electrode creates a transparent TFT with high performance and good transparency (∼90%). Use of the same ZnO-based oxide materials in a TFT allows for the formation of good electrical contacts characterized by low contact resistance, comparable to those with a vacuum-deposited Al electrode. Our results suggest that the solution-processed ZnO-based TFT has great potential to work as a building block for future printed transparent electronics.

Comprehensive analysis of advanced solar cell contacts consisting of printed fine‐line seed layers thickened by silver plating

Abstract: This work presents a detailed analysis of a new two-layer process to contact industrial solar cells. However, most of the results seem to be transferable to standard screen print paste contacts. The seed layer was created by a pad or screen printer and thickened by light-induced plating (LIP) of silver. These contact structures were investigated microscopically to gain a better understanding of the observed electrical parameters. A review of the present microscopic contact formation model for flat surfaces is presented. This model was extended and applied to surfaces textured with random pyramids. This analysis has revealed two new types of silver crystallites which can be described by a crystallographic model. The dependence of the silver crystallite density on the surface doping concentration was investigated. Next, the dependence of the contact resistance on the width of the seed layer was measured showing that the contact resistivity increases with a reduction of the seed layer width. These results have been further approved by an analysis of SEM images of wet-chemically etched contacts examining the density of crystallites and the fraction of removed SiNx layer. Contact resistance RC measurements before and after LIP of silver showed surprisingly a positive influence of the plating process on RC. A detailed microscopical analysis revealed four new possible current flow paths due to the LIP of a conventional contact or a seed layer. The results led to an extension of the existing model for a screen-printed contact. Copyright © 2008 John Wiley & Sons, Ltd.

First-principles investigation on bonding formation and electronic structure of metal-graphene contacts

Abstract: Metal-graphene contacts play a critical role in graphene-based electronics. It is found through first-principles calculation of contacts between graphene and 12 different metals that there exist two types of contacts depending on the strength of interaction between d-orbitals in metals and pz-orbitals in graphene. Fermi level shift in the contacted graphene from the freestanding one is investigated, and the electronic structure and electrostatic potential are calculated. The carrier transport through these contacts is calculated using the extended Huckel theory-based non-equilibrium Green’s function formalism, and one type of contact is shown to have less contact resistance than the other.

Characterization of heavily doped polysilicon films for CMOS-MEMS thermoelectric power generators

Abstract: This paper presents the material characterization of boron- and phosphorus-doped LPCVD polysilicon films for the application of thermoelectric power generators. Electrical resistivity, Seebeck coefficient and thermal conductivity of polysilicon films doped with doses from 4 × 1015 to 10 × 1015 at cm−2 have been measured at room temperature. Specific contact resistance between polysilicon and aluminum is studied and nickel silicidation is formed to reduce the contact resistance. The overall thermoelectric properties, as characterized by the figure of merit, are reported for polysilicon doped with different doping concentrations. For the most heavily doping dose of 10 × 1015 at cm−2, figure of merit for p- and n-type polysilicon is found as 0.012 and 0.014, respectively. Based on the characterization results, a CMOS compatible thermoelectric power generator composed of boron- and phosphorus-doped polysilicon thermopiles is fabricated. When 5 K temperature difference is maintained across two sides of a device of size of 1 cm2, the output power is 1.3 µW under a matched electrical resistance load.

A high-precision apparatus for the characterization of thermal interface materials.

Abstract: An apparatus has been designed and constructed to characterize thermal interface materials with unprecedented precision and sensitivity. The design of the apparatus is based upon a popular implementation of ASTM D5470 where well-characterized meter bars are used to extrapolate surface temperatures and measure heat flux through the sample under test. Measurements of thermal resistance, effective thermal conductivity, and electrical resistance can be made simultaneously as functions of pressure or sample thickness. This apparatus is unique in that it takes advantage of small, well-calibrated thermistors for precise temperature measurements (+/-0.001 K) and incorporates simultaneous measurement of electrical resistance of the sample. By employing precision thermometry, low heater powers and minimal temperature gradients are maintained through the meter bars, thereby reducing uncertainties due to heat leakage and changes in meter-bar thermal conductivity. Careful implementation of instrumentation to measure thickness and force also contributes to a low overall uncertainty. Finally, a robust error analysis provides uncertainties for all measured and calculated quantities. Baseline tests were performed to demonstrate the sensitivity and precision of the apparatus by measuring the contact resistance of the meter bars in contact with each other as representative low specific thermal resistance cases. A minimum specific thermal resistance of 4.68x10(-6) m(2) K/W was measured with an uncertainty of 2.7% using a heat transfer rate of 16.8 W. Additionally, example measurements performed on a commercially available graphite thermal interface material demonstrate the relationship between thermal and electrical contact resistance. These measurements further demonstrate repeatability in measured effective thermal conductivity of approximately 1%.

Multiphysics Simulation of Thermoelectric Systems for Comparison with Experimental Device Performance

Abstract: The efficiency of thermoelectric devices is determined not only by the quality of the thermoelectric material but also by the geometrical design of the legs and the properties and design of the contacts with the corresponding soldering process. These influences on the performance of a thermoelectric generator are studied by multiphysics finite element modeling. The simulated data are compared with experimental results for modules manufactured from Bi2Te3 compounds with ZT values >0.8. A decrease of the ZT value for the module by a factor of about four can be traced back to the high contact resistance. The thermal losses at the contact interfaces are negligible for these devices.

Comparison of Au and Au–Ni Alloys as Contact Materials for MEMS Switches

Abstract: This paper reports on a comparison of gold and gold-nickel alloys as contact materials for microelectromechanical systems (MEMS) switches. Pure gold is commonly used as the contact material in low-force metal-contact MEMS switches. The top two failure mechanisms of these switches are wear and stiction, which may be related to the material softness and the relatively high surface adhesion, respectively. Alloying gold with another metal introduces new processing options to strengthen the material against wear and reduce surface adhesion. In this paper, the properties of Au-Ni alloys were investigated as the lower contact electrode was controlled by adjusting the nickel content and thermal processing conditions. A unique and efficient switching degradation test was conducted on the alloy samples, using pure gold upper microcontacts. Solid-solution Au-Ni samples showed reduced wear rate but increased contact resistance, while two-phase Au-Ni (20 at.% Ni) showed a substantial improvement of switching reliability with only a small increase of contact resistance. Discussion of the effects of phase separation, surface topography, hardness, and electrical resistivity on contact resistance and switch degradation is also included.

Detailed Characterization of Contact Resistance, Gate-Bias-Dependent Field-Effect Mobility, and Short-Channel Effects with Microscale Elastomeric Single-Crystal Field-Effect Transistors

Abstract: The organic field-effect transistor (OFET) has proven itself invaluable as both the fundamental element in organic circuits and the primary tool for the characterization of novel organic electronic materials. Crucial to the success of the OFET in each of these venues is a working understanding of the device physics that manifest themselves in the form of electrical characteristics. As commercial applications shift to smaller device dimensions and structure/property relationships become more refined, the understanding of these phenomena become increasingly critical. Here, we employ high-performance, elastomeric, photolithographically patterned single-crystal field-effect transistors as tools for the characterization of short-channel effects and bias-dependent parasitic contact resistance and field-effect mobility. Redundant characterization of devices at multiple channel lengths under a single crystal allow the morphology-free analysis of these effects, which is carried out in the context of a device model previously reported. The data show remarkable consistency with our model, yielding fresh insight into each of these phenomena, as well as confirming the utility of our FET design.

Specific contact resistance at metal/carbon nanotube interfaces

Abstract: In this report, the specific contact resistance between a thin film single wall carbon nanotube electrode and a deposited silver contact was measured. The specific contact resistance was found to be 20 mΩ cm2, which is an order of magnitude higher than typically observed in standard Si photovoltaic technology. We demonstrate that when utilized as the transparent anode in organic photovoltaics, the specific contact resistance has the potential to induce non-negligible resistive power losses. Thus, specific contact resistance will adversely affect the performance of these systems and should therefore be addressed.