Temperature Dependence Of Contact Resistance Research Articles

Experiment and Analysis on Temperature-Dependent Electric Contact Resistivity of an NI HTS Coil

This paper reports experimental and numerical analysis results of a no-insulation (NI) high-temperature superconductor (HTS) coil in terms of temperature-dependent electric contact resistivity. A test coil was designed, constructed, and operated. The coil is divided into four sections according to the radial locations of inserted voltage taps to measure local voltages. A preliminary experiment was performed to evaluate the electromagnetic properties of the coil in liquid nitrogen. Then, the temperature-dependent contact resistance of the coil was evaluated from 10K to 80K at every 10K increment in a cooling conduction facility. To investigate the impact of the contact resistivity on the charge/discharge dynamics of the coil, we selected two arbitrary current ramp rates values, namely fast (0.5) and slow (0.01) charge/discharge. In this study, the experimental results were analyzed with equivalent circuit and finite element models. The analysis results suggest the following conclusions. First, the contact resistivity of the test coil calculated from the experiment at the coil terminals level increases by a factor 2 with respect to the temperature, ranging between 1.16 to 2.59. Second, although a constant winding tension was applied during the coil fabrication, the contact resistivity was found to increase within different sections (inner to outer radius). Third, the impact of the ramp rate on temperature dependency is negligible.

Rhenium-based low resistivity and low annealing temperature ohmic contacts to n-GaN

We report an ohmic contact metallization scheme for n-GaN based on a Re–Al–Ni–Au multilayer stack that offers low-resistivity and provides better edge sharpness and lower surface roughness at reduced annealing temperature compared to standard Ti–Al-based metallization. We studied three sets of samples with Re thicknesses of 10, 30, and 60 nm and measured specific contact resistances using the circular transmission line method. We obtained reliable ohmic contacts with specific contact resistance of the order of 10−6 Ω cm2 at 550 °C temperature with the contact stack having a large annealing temperature window of >300 °C for resistivity below 5 × 10−6 Ω cm2. The lowest contact resistivity, below 10−7 Ω cm2, is achieved at annealing temperature around 650–700 °C. A reduction in surface roughness by a factor of 4, with excellent edge definition for Re >30 nm thickness is observed as compared to Ti–Al contacts. Grazing incidence x-ray diffraction and electron dispersive x-ray spectroscopy (EDS) show intermetallic phases of RexNy, Re–Al–Ni, Al–Re, and Al–Au being formed. Atomic force microscope and EDS measurements show the formation of crystalline Re–Al–Ni agglomerates, surrounded by phases of AlAu2 and Al2Au5. The presence of Re seems to suppress the formation of viscous AlAu4 phase, thus minimizing the lateral flow of the metals and providing better edge acuity. The temperature dependence of contact resistivity suggests a field-emission mechanism for current transport across the contact. Our results show that Re-based ohmic contacts, with their lower annealing temperature and excellent edge definition, may offer a promising alternative to Ti–Al contacts.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Mechanisms of charge carrier transport in polycrystalline silicon passivating contacts

We use temperature-dependent contact resistivity ( ρ c ) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature ( T ann ), (ii) the oxide thickness ( t ox ), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiO x layer for increasing T ann . This transition occurs for T ann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiO x with t ox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for T ann = 1050 °C, whereas no electric current through the SiO x layer could be detected for lower T ann . Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρ c were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing T ann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality. • Electron transport mechanisms through the oxide are investigated by temperature-dependent contact resistivity measurements. • A transition from tunneling to pinhole dominant transport was observed at T ann = 900–950°C for a 1.5 nm thick thermal oxide. • For a 1.3 nm thick wet-chemical oxide, the transition was observed at T ann = 700–750°C. • Good passivation quality is preserved even in the pinhole regime, although it degrades at extremely high T ann .

Temperature-dependent contact resistance of carrier selective Poly-Si on oxide junctions

Carrier selective junctions using a poly-silicon/ silicon oxide stack on crystalline silicon feature low recombination currents J0 whilst allowing for low contact resistivity ρC. We describe the limiting current transport mechanism as a combination of homogeneous tunneling through the interfacial silicon oxide layer and transport through pinholes where the interfacial silicon oxide layer is locally disrupted. We present an experimental method and its theoretical basis to discriminate between homogenous tunneling and local pinhole transport mechanisms on n + /n or p + /p junctions by measuring the temperature-dependent contact resistance. Theory predicts opposing trends for the temperature dependencies of tunneling and pinhole transport. This allows identifying the dominant transport path. For the contact resistance of two differently prepared poly-Si/ silicon oxide/ c-Si junctions we either find clear pinhole-type or clear tunneling-type temperature dependence. Pinhole transport contributes more than 94% to the total current for the sample with a 2.1 nm-thick interfacial silicon oxide that we anneal at a temperature of 1050 °C to achieve highest selectivity. In contrast pinhole transport contributes less than 35 % to the total current for the sample with a 1.7 nm-thick silicon oxide that we annealed at only 700 °C in order to avoid pinholes.

Ohmic contact mechanism for RF superimposed DC sputtered-ITO transparent p-electrodes with a variety of Sn2O3 content for GaN-based light-emitting diodes

The dependence of the electrical and optical properties of radio frequency (RF) superimposed direct current (DC) sputtered–indium tin oxide (ITO) on the tin oxide (Sn2O3) content of the ITO is investigated, in order to elucidate an ohmic contact mechanism for the sputtered-ITO transparent electrodes on p-type gallium nitride (p-GaN). Contact resistivity of the RF superimposed DC sputtered-ITO on p-GaN in LEDs decreased when Sn2O3 content was increased from 3wt% to 7wt% because of the reduced sheet resistance of the sputtered-ITO with the increasing Sn2O3 content. Further increases in Sn2O3 content from 7wt% to 15wt% resulted in deterioration of the contact resistivity, which can be attributed to reduction of the work function of the ITO with increasing Sn2O3 content, followed by increasing Schottky barrier height at the sputtered ITO/p-GaN interface. Temperature-dependent contact resistivity of the sputtered-ITO on p-GaN also revealed that the ITO contacts with 7wt% Sn2O3 yielded the lowest effective barrier height of 0.039eV. Based on these results, we devised sputtered-ITO transparent p-electrodes having dual compositions of Sn2O3 content (7/10wt%). The radiant intensity of LEDs having sputtered-ITO transparent p-electrodes with the dual compositions (7/10wt%) was enhanced by 13% compared to LEDs having ITO with Sn2O3 content of 7wt% only.

Low-Temperature Ohmic Contact Formation in AlN/GaN HEMT Using Microwave Annealing

In this brief, a low-temperature microwave annealing (MWA) method is demonstrated for the formation of ohmic contact to AlN/GaN high electron mobility transistors (HEMTs) for the first time. Compared with the traditional rapid thermal annealing (RTA) technique, MWA-HEMT can achieve a comparable low ohmic contact resistance with much smoother surface of ohmic contacts. Transmission electron microscopy results show that no direct current path connecting the 2-D electron gas and the metal is formed. Temperature-dependent contact resistance measurement indicates that field emission tunneling dominates the current transport mechanism in ohmic contact formation. The maximum dc output current density of 1.4 A/mm and the peak extrinsic transconductance ( ${G}_{\textsf {m}}$ ) of 270 mS/mm are measured on ${2} \times {8} \,\,{\sf \mu } $ $m^2$ gate MWA-HEMTs. Besides, MWA-HEMTs have higher ${I}_{ \mathrm{ ON}}/{I}_{ \mathrm{ OFF}}$ ratio due to the lower gate leakage current than that of RTA-HEMTs.

Thermal-assisted band to band tunneling at the electron donor/acceptor energy harvesting assembly

Understanding interfacial electronic properties at the electron donor/acceptor energy harvesting assembly in photovoltaic devices is important in that it can provide insights into revealing charge carrier recombination mechanism at the interface. From temperature dependent contact resistance measurements at the aluminum-doped zinc oxide (AZO)/poly-hexylthiophene (P3HT)/aluminum-doped zinc oxide (AZO) structure in which AZO (electron acceptor) acts as highly conductive source and drain electrodes, charge transfer mechanism at the electron donor/acceptor interface is elucidated. In the mechanism, charge transfer process is initiated by band to band tunneling (BTBT) of hole carriers from AZO to P3HT through localized electronic states populated near the highest occupied molecular orbital (HOMO) level of the P3HT close to the AZO favored by energetic alignment between the conduction band edge of AZO and the HOMO of P3HT. Following BTBT, charge transfer process is completed by thermal activated hopping in the P3HT (electron donor) close to the AZO. It is found that thermally activated transport in the P3HT close to the AZO dominates the magnitude of the contact resistance, providing insights into the origin of carrier recombination mechanism at the donor/acceptor interface in photovoltaic devices.

Temperature dependence of contact resistance at metal/MWNT interface

Although contact resistance of carbon nanotube (CNT) is one of the most important factors for practical application of electronic devices, a study regarding temperature dependence on contact resistance of CNTs with metal electrodes has not been found. Here, we report an investigation of contact resistance at multiwalled nanotube (MWNT)/Ag interface as a function of temperature, using MWNT/polydimethylsiloxane (PDMS) composite. Electrical resistance of MWNT/PDMS composite revealed negative temperature coefficient (NTC). Excluding the contact resistance with Ag electrode, the NTC effect became less pronounced, showing lower intrinsic resistivity with the activation energy of 0.019 eV. Activation energy of the contact resistance of MWNT/Ag interface was determined to be 0.04 eV, two times larger than that of MWNT-MWNT network. The increase in the thermal fluctuation assisted electron tunneling is attributed to conductivity enhancement at both MWNT/MWNT and MWNT/Ag interfaces with increasing temperature.

Temperature Dependence and High-Temperature Stability of the Annealed Ni/Au Ohmic Contact to p-Type GaN in Air

We report on the temperature-dependent contact resistivity and high-temperature stability of the annealed Ni/Au ohmic contacts to p-type GaN in air. As the measure temperature increases from 25°C to 390°C, both the specific contact resistivity (ρc) and sheet resistance (Rsh) decrease by factors ∼10, contributing to the 10-fold increase in current at 390°C compared with that at 25°C. It was also observed that the ρc was further reduced by 36%, i.e., from 2.2 × 10−3 Ω cm2 to 1.4 × 10−3 Ω cm2, during the 48-h high-temperature stability test at 450°C in air, showing excellent stability of the contacts. An increase in ρc was observed after the contacts were subjected to 500°C in air. Higher temperature stress led to a significant increase in ρc. The contacts show rectifying I–V characteristics after being subjected to 700°C for 1 h. The degradation mechanics were analyzed with the assistance of transmission electron microscopy and energy dispersive x-ray spectroscopy.

Mechanism of Ti/Al/Ti/W Au-free ohmic contacts to AlGaN/GaN heterostructures via pre-ohmic recess etching and low temperature annealing

The physical mechanism of low-thermal-budget Au-free ohmic contacts to AlGaN/GaN heterostructures is systematically investigated with current-voltage, high-resolution transmission electron microscopy, and temperature-dependent contact resistivity characterizations. With a low annealing temperature of 600 °C, pre-ohmic recess etching of the AlGaN barrier down to several nanometers is demonstrated to be an effective method to reduce the contact resistance between Ti/Al/Ti/W ohmic metals and AlGaN/GaN heterostructures. However, further over recess of the AlGaN barrier leads to only sidewall contact to 2D electron gas channel and thus degraded contact performance. It is verified by temperature-dependent contact resistivity measurements that field emission (tunneling) dominates the current transport mechanism in Au-free ohmic contacts with AlGaN barrier partially and over recessed, while both field emission and thermionic emission contribute to traditional Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures that annealed at high temperature (850 °C).

Modeling the Electrical Contact Resistance at Steel–Carbon Interfaces

In the aluminum smelting industry, electrical contact resistance at the stub–carbon (steel–carbon) interface has been recurrently reported to be of magnitudes that legitimately necessitate concern. Mitigating this via finite element modeling has been the focus of a number of investigations, with the pressure- and temperature-dependent contact resistance relation frequently cited as a factor that limits the accuracy of such models. In this study, pressure- and temperature-dependent relations are derived from the most extensively cited works that have experimentally characterized the electrical contact resistance at these contacts. These relations are applied in a validated thermo-electro-mechanical finite element model used to estimate the voltage drop across a steel–carbon laboratory setup. By comparing the models’ estimate of the contact electrical resistance with experimental measurements, we deduce the applicability of the different relations over a range of temperatures. The ultimate goal of this study is to apply mathematical modeling in providing pressure- and temperature-dependent relations that best describe the steel–carbon electrical contact resistance and identify the best fit relation at specific thermodynamic conditions.

Ni(Ge1−x−ySixSny) Ohmic Contact Formation on p-type Ge0.86Si0.07Sn0.07

A single-crystalline Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.86</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> alloy was grown on double Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Ge buffers on Si (100) using magnetron sputtering. The temperature-dependent contact resistivity (ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of Ni/p-type Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.86</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> structure was investigated in detail. Good ohmic contacts with ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of ~1.96 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> Ω · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> have been demonstrated on p-type Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.86</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> after annealing at 350 °C. The low contact resistivity is attributed to the formation of Ni(Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x-y</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) complex, and the hole Schottky barrier height of Ni(Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x-y</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> )/p-type Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.86</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.07</sub> is measured.

Temperature-dependent contact resistivity of radio frequency superimposed direct current sputtered indium tin oxide ohmic contact to p-type gallium nitride

The temperature-dependent contact resistivity of radio frequency (RF) superimposed direct current (DC) sputtered indium tin oxide (ITO) contacts to p-type gallium nitride (p-GaN) films as well as the temperature-dependent sheet resistivity of p-GaN films were investigated to understand the carrier transport mechanism of the sputtered ITO ohmic contacts to p-GaN. As the measurement temperature was decreased from 400 to 200K, the contact resistivity increased by three orders of magnitude. Furthermore, the sheet resistivity of the p-GaN increased linearly with exp (1/Temperature(T))1/4 from 200 to 340K, indicating variable-range hopping (VRH) conduction via the Mg-related deep level defect (DLD) band. Based on the VRH conduction model, the effective barrier height between the sputtered ITO and the DLD band were calculated to be 0.12eV, which is sufficiently low to explain the formation of the low contact resistivity of the RF superimposed DC sputtered ITO contacts to p-GaN (~10−2Ω-cm2).

Fabrication and characterization of nickel contacts for magnesium silicide based thermoelectric generators

Magnesium silicide based solid solutions are highly attractive materials for thermoelectric energy harvesting due to their abundance and excellent thermoelectric properties. Identification and testing of suitable contacts is – besides material optimization – the major challenge in the development of thermoelectric modules. We have applied Ni contacts on doped Mg2Si samples using a simple one-step sintering technique. These contacts were analyzed by combining microstructural analysis with spatially resolved and temperature dependent contact resistance measurements. We observe very good adhesion, homogeneous and low contact resistances <10μΩcm2. as well as good stability with temperature. Three different approaches for determining the contact resistances are compared and the respective errors are discussed.

On a feature of temperature dependence of contact resistivity for ohmic contacts to n-Si with an n + -n doping step

We present both theoretical and experimental temperature dependences of contact resistivity ρс(Т) for ohmic contacts to the silicon n + -n-structures whose n + -layer was formed using phosphorus diffusion or ion implantation. The ρс(Т) dependence was measured in the 125-375 K temperature range with the transmission line method, with allowance made for conduction in both the n + -layer and n + -n doping step.

Mechanism of current flow and temperature dependence of contact resistivity in Au-Pd-Ti-Pd-n+-GaN ohmic contacts

Mechanism of current flow and temperature dependence of contact resistivity in Au-Pd-Ti-Pd-n+-GaN ohmic contacts

Carrier transport mechanism of low resistance Ti/Al/Au ohmic contacts to AlInN/GaN heterostructures

The carrier transport mechanism of a low resistance Ti/Al/Au Ohmic contact to AlInN/GaN heterostructures was investigated. The Ohmic contact produced upon thermal annealing was due to the generation of TiN contact inclusions with a density of 2.8 × 108 cm−2, i.e., spike mechanism. The sheet resistance of channel layer was found to follow power law, yielding the power index of −1.57. Temperature dependent contact resistance could be understood based on the parallel network model consisting of distributed resistance components of TiN contact inclusion (predominant) and the rest sound region (negligible), giving the barrier height of 0.65 eV and the carrier density of 2.3 × 1019 cm−3 at the TiN/GaN interfaces.

The effect of a copper interfacial layer on spin injection from ferromagnet to graphene

Aiming at enhancing the spin-injection efficiency from ferromagnet to graphene, graphene-based spin valve devices with ferromagnet–Cu–graphene contacts have been fabricated and studied. The ferromagnet–Cu–graphene contact has attracted our attention because of the spin-preserving characteristic of ferromagnet–Cu interface and possible presence of moderate potential barrier at the Cu–graphene interface. The latter may help to alleviate the conductance mismatch issue in spin injection from ferromagnet to graphene. Devices with or without Cu interfacial layers have been fabricated by both evaporation and sputtering on mechanically exfoliated graphene sheets. A potential barrier of 33 meV was derived for the Cu–graphene interface in evaporation-deposited devices from the temperature-dependence of contact resistance. On the other hand, pure ferromagnet–graphene contact exhibits a much lower contact resistance due to its chemisorption interface with graphene. Nonlocal magnetoresistance measurements showed a moderate enhancement of spin injection efficiency with the aid of the Cu interfacial layer.

Features of temperature dependence of contact resistivity in ohmic contacts on lapped n-Si

The temperature dependence of contact resistivity ρc in lapped silicon specimens with donor concentrations of 5 × 1016, 3 × 1017, and 8 × 1017 cm−3 was studied experimentally. We found that, after decreasing part of the ρc(T) curve in the low temperature range, an increasing part is registered with increasing temperature T. It is demonstrated that the formation of contact to a lapped Si wafer results in the generation of high dislocation density in the near-surface region of the semiconductor and also in ohmic contact behavior. In this case, current flows through the metal shunts associated with dislocations. The theory developed is in good agreement with experimental results.