Low Contact Resistance Values Research Articles

Low Resistance Ohmic Contact on Epitaxial MOVPE Grown β-Ga2O3 and β-(AlxGa1−x)2O3 Films

We report on the realization of record low resistance Ohmic contacts to MOVPE-grown heavily Si-doped $\beta$-Ga$_2$O$_3$ and $\beta$-(Al$_x$Ga$_1-x$)$_2$ O$_3$ epitaxial films. Transfer length measurement (TLM) patterns were fabricated on the heavily Si-doped homoepitaxial $\beta$-Ga$_2$O$_3$ films with electron concentration (n) ranging from 1.77 to 3.23e20 cm^-3. Record low specific contact resistance and total contact resistance (Rc) of 1.62e-7 Ohm.cm^2 and 0.023 Ohm.mm were realized for $\beta$-Ga$_2$O$_3$: Si films with n > 3e20 cm^-3. TLM structures were also fabricated on heavily Si doped coherently strained $\beta$-(Al$_x$Ga$_1-x$)$_2$ O$_3$ (x=12%, 17% and 22%) films. The film with 12% Al composition (n=1.23e20 cm^-3) showed \r{ho}c of 5.85e-6 Ohm.cm^2, but it increased to 2.19e-4 Ohm.cm^2 for a layer with a 22% Al composition. Annealing the samples post metal deposition has generally led to a decrease in contact resistance, but for high Al content $\beta$-(Al$_x$Ga$_1-x$)$_2$ O$_3$, the contact resistance did not change significantly after the annealing process. The low contact resistance values measured in this work are very promising for the fabrication of high frequency power devices.

Lowering the Schottky Barrier Height by Titanium Contact for High-Drain Current in Mono-layer MoS2 Transistor

This paper investigates quantum mechanical effects on surface potential and threshold voltage in a two-dimensional molybdenum disulfide (MoS $$_{2}$$ ) based transistor. Drain current improvement occurs as a result of the low work function for metal Ti and electron affinity of MoS $$_{2}$$ , thereby reducing the Schottky barrier height (SBH). The titanium (Ti) contact introduced into the mono-layer MoS $$_{2}$$ helps in obtaining lower contact resistance and improved drain current due to the lowering of SBH. An analytical model is proposed for evaluating various device parameters such as surface potential, threshold voltage, contact resistance, mobility, and drain current. This model provides useful aspects for understanding the physics of quantum mechanical effects. Compared to other conventional devices, this transistor incorporates Ti, a low work function metal, in the contact region, producing better current drive and lower contact resistance values. A positive Schottky barrier is noticed between the mono-layer MoS $$_{2}$$ and the Ti metal. Compared to other high function metals, the proposed device displayed about 11% improvement in current drive and 8% less contact resistance. By attaining an enhanced carrier injection and decreased contact resistance, this device serves as a suitable prospect for low-power applications.

A Study on the Charge Carrier Transport of Passivating Contacts

Recently, the charge carrier transport mechanism of passivating contacts, which feature an ultra-thin oxide layer, has been investigated by studying temperature-dependent current–voltage ( I–V ) characteristics. The measurement revealed that tunneling is the dominant transport path for tunnel oxide passivated contact (TOPCon) with wet chemically grown oxide layer. Furthermore, higher annealing temperatures led to the deterioration of the surface passivation most likely because of excessive pinhole formation. In this contribution, we are going to extend the previous study by analyzing other interfacial oxides as well. We will show that extremely low recombination current densities and low contact resistivity values can be achieved by differently processed TOPCon structures, which are characterized by a predominant tunnel transport as well as one where current flow via pinholes likely predominates. Furthermore, an I–V ( T ) study on solar cells with passivating rear contact reveals that fill factor transitions from a nonlinear to linear behavior when the Si layer turns partially crystalline.

Atomic-layer deposited Nb2O5 as transparent passivating electron contact for c-Si solar cells

Passivating contacts based on metal oxides have proven to enable high energy conversion efficiencies for crystalline silicon (c-Si) solar cells at low processing complexity. In this work, the potential of atomic-layer deposited (ALD) Nb2O5 as novel electron-selective passivating contact is explored in terms of recombination parameter J0 and contact resistivity ρc. It is shown that after forming gas annealing, ALD Nb2O5 can provide adequate surface passivation with J0 values down to 25–30 fA/cm2. On HF-treated c-Si surfaces a minimum film thickness of ~ 3 nm is required to achieve this high level of passivation, whereas on surfaces with a wet-chemical SiO2 interlayer the high passivation level is persistent down to film thicknesses of only 1 nm. Ohmic n-type contacts have been achieved using Al as contacting metal, where annealing the samples after Al contacting proved crucial for obtaining good contact properties. Low contact resistivity values of 70 and 124 mΩ cm2 for 1 and 2 nm Nb2O5 films, respectively, have been achieved on c-Si substrates that received an HF treatment prior to Nb2O5 deposition. Transmission electron microscopy imaging shows that on such surfaces the annealing treatment leads to the formation of a (1.7 ± 0.2) nm interfacial oxide in between the c-Si substrate and the Nb2O5 film. The presented results demonstrate the potential of ALD Nb2O5 as electron-selective passivating contact and directions for future research are outlined.

Ultrathin AlN‐Based HEMTs Grown on Silicon Substrate by NH3‐MBE

AlN‐based HEMTs grown on silicon by ammonia‐assisted molecular beam epitaxy (NH3‐MBE) are demonstrated and studied. As shown by photoluminescence, the very thin GaN channel (35–55 nm‐thick) is compressively strained on the 250 nm‐thick relaxed AlN buffer layer grown on silicon substrate. The structure is then completed by an 8 nm‐thick AlN barrier and 2 nm‐thick GaN cap. Despite an ultrathin total epilayer (only ≈300 nm‐thick), a high 2DEG density (≈2.7 × 1013 cm−2) is measured by Hall effect. Room temperature mobility values, as high as 636 cm2 V−1 s−1, are measured. They are higher than those reported on similar structures grown on sapphire, SiC and bulk AlN substrates, showing the interest of growing such structures on silicon substrate. Also, low contact resistance values are obtained, as low as 0.18 Ω mm, by using a basic fabrication process.

Contact resistance properties of electron-beam-cured anisotropic conductive films

An anisotropic conductive film (ACF) is a thin adhesive epoxy layer that is widely used to connect circuits in highly-integrated electronic devices. To bind two electric circuits, an ACF is pasted between the circuits and pressed with an optimum pressure so that the joints can be electronically connected. For the curing of the resin, electron-beam (E-beam) irradiation is known to be an environmentally friendly technique and exhibits many technical advantages over conventional thermal curing in terms of low temperature and reduced curing time. In this method, the applied pressure and electron-beam dose are critical parameters to maximize the conductivity without a failing of the connection between the circuits. For the minimization of the contact resistance after curing by using E-beam irradiation, curable epoxy resin was mixed with conductive particles made of Ni/Au-plated polymer spheres with a mean particle size of 10 µm in a ratio of 5: 1 by weight. The mixed resin was pasted on rigid boards having circuits of 100-µm pitch and was irradiated by using an E-beam. The optimum dose for E-beam irradiation for a reasonable curing, the contact resistance values, and the reliability of the cured film were systematically investigated as functions of E-beam dose, applied pressure and temperature. The lowest contact resistance values were obtained when the specimens were irradiated by an 80-kGy E-beam under a constant pressure of 5 kgf/cm2. At this condition, the minimum contact resistance was 80 ± 0.015 mΩ, which is significantly lower than the values obtained from commercial products, and the values were maintained within a 5% increase when the samples were kept at 100 °C for 294 hours. Our results demonstrate that an E-beam irradiation technique can be applied for curing ACFs for commercial electronic devices.

Fabrication of giant magneto resistance sensing devices with vertically grown <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Co</mi><mo>∕</mo><mi mathvariant="normal">Cu</mi></mrow></math></inline-formula> nanowires on a substrate

We describe the fabrication and analysis of a giant magneto resistance (GMR) sensing device with multilayered magnetic nanowires containing alternating ferromagnetic and nonmagnetic layers on a silicon substrate. Multilayered nanowires with Co and Cu layers were vertically grown using pulsed electrochemical deposition to control the periodic structure of nanolayers in a nanopore template. Lithography-assisted template bonding (LATB) was applied to attach a polymer nanopore membrane and grow nanowires on silicon and polyimide substrates. For magnetic field sensing, the multilayered nanowires were electrically connected to top and bottom electrodes by overgrowing nanowires, and low contact resistance values of GMR sensing devices were achieved. The GMR effect was measured with magnetic field changes perpendicular and parallel to the axis of the nanowires. This simple fabrication process is highly favorable for the development of nanoscale electronics with magnetic nanowires at low cost and would offer large design flexibility.

Metallic thin films heated by pulsed lasers. Numerical simulation of the thermal field and the melting kinetics

This modeling is especially applied to the pulsed laser induced heating and melting of a metallic film deposited on a substrate. Study of the thermal field over a surface is usually performed by considering the assumption of ‘semi-infinite medium’. However, a thin film deposited on a rough substrate surface induces bad thermal contacts commonly known as ‘thermal contact resistance’. This interfacial thermal resistance affects the melting kinetics mainly when the film thickness (Z) is small comparatively to the heat diffusion length (ZT). In this work the heat conduction equation and related boundary conditions are resolved by using the implicit finite differences method. The heat source (i.e. the laser intensity) is treated as a surface boundary layer. The thermal contact resistance is introduced in the computation procedure when the heat wave propagation reaches the thin film/substrate interface. It is then possible to calculate the critical temperatures and the melting threshold fluence for high and low contact resistance values. Under these conditions, the temperature profile and melting depth are plotted considering different thickness.. Finally, for 750 mJ/cm² excimer laser fluence and 0.1 cm²/s thin film apparent diffusivity results show that for Z/ZT higher than 0.5, there is no sensitive effect of the thermal contact resistance on the melting kinetics.

Lateral diffusion effects in AuGe based source-drain contacts to AllnAs/InGaAs/InP doped channel MODFETs

We have investigated the formation of source-drain AuGe/Au and Ni/AuGe/Ni/Au alloyed ohmic contacts to AlInAs/InGaAs/InP doped channel MODFETs, and observed lateral diffusion of the contact system after the standard annealing procedure at the temperature range of 185 to 400°C. Auger depth profiling of contacts annealed at 250°C, revealed that Au(Ge) diffused through the top InGaAs and AlInAs layers into the active InGaAs layer, but had reduced penetration into the AlInAs buffer layer. This reduction in diffusion along the depth axis at the AlInAs buffer layer boundary is believed to result in enhanced lateral diffusion and the observed lateral encroachment of the contacts. Both Au and Ni containing contact systems showed similar behavior in terms of lateral diffusion with encroachment extending between 0.25 and 0.5 µm at the periphery of the contacts for annealing temperatures between 300 and 400°C. A controlled ramp-to-peak temperature annealing procedure is developed to suppress such lateral diffusion effects. Low temperature annealing (250°C) using this procedure resulted in equally low contact resistance values (∼0.1Θ-mm) and no lateral diffusion. It is concluded that in thin multilayered structures the modified annealing procedure presented here, is necessary for optimal ohmic contact formation.

Reliability of commercial relays during life tests at low electrical contact load

The influence of the electrical stress on the contact resistance behavior of several signal and power relays has been investigated during life tests comprising two million operations. The results obtained with the aid of a specially developed test device showed characteristic contact resistance patterns for different electrical loads, contact materials, and designs. Under certain load conditions the contact resistance was increased by carbon deposits, while both lower and higher loads caused lower contact resistance values. >

Electron beam annealed Ge-WSi-Au and Ge-Ni-WSi-Au high temperature stable ohmic contacts on n-GaAs

Abstract Detailed analysis of electron beam annealed Ge-WSi-Au and Ge-Ni-WSi-Au ohmic contacts is reported. Lower contact resistance values with better reproducibility and much lower standard deviation are achieved with Ni doped TLM devices. Ar+ ion X-ray photoelectron depth profiling was used to relate the distribution of Ge, Ni, W, Si, Au, Ga and O to the electrical properties of contacts. Ohmic behaviour was found probably to be dependent upon the Ge and Ni compound formation on the GaAs surface.

The Switching Performance of Refractory Carbide-Silver Contacts

Eight refractory carbide-silver contacts have been evaluated for switching performance: WC-Ag (WC 40 volume percent); WC-AG-Co ((a)WC 45 volume percent, Co 7 volume percent, (b) WC 35 volume percent, Co 7 volume percent); TiC-Ag ((a)TiC 80 volume percent, b) TiC 60 volume percent, c) TiC 48 volume percent); and NbC-Ag ((a)NbC 72 volume percent b) NbC 48 volume percent). The contacts were placed in an experimental switch and were operated to make and break a 20-A 110-V resistive circuit. The contact resistance was monitored at 0, 1000, 2000, and 3000 switching operations by measuring the temperature rise at the fixed contact terminal after 20 A had been flowing for ½ h. The contact surfaces were examined using a scanning electron microscope. The performance of the WC-Ag was similar to that which was previously observed with W-Ag contacts; high contact resistances were measured after only 1000 operations. The WC-Ag-Co contacts showed consistently lower contact resistance values than were observed for the WC-Ag contacts. The TiC-Ag and NbC-Ag contacts performed about as well as the WC-Ag-Co contacts did. These data showed that the presence of carbon in the WC-Ag contacts did little to alleviate the problem of high contact resistance values resulting from insulating oxides of W and Ag forming on the contact surfaces. The presence of Co, on the other hand, resulted in conditions which were not favorable to the formation of the silver tungstate. The TiC-Ag data showed that Ag was always present on the contact surfaces together with TiC and with oxides of titanium; there was no evidence of the formation of silver titanate. The NbC-Ag data were similar to the TiC-Ag data. After continued switching, however, the TiC and NbC contacts began to form a surface skin of refractory material on the contact surface. The relative merits of these refractory carbide-silver contacts are discussed.

Silver-Ruthenium Dioxide Contacts

This paper describes the development of a novel electrical contact material, silver-ruthenium oxide, for use in light duty switching applications where low contact resistance values must be maintained. Material fabrication and contact resistance behaviour after exposure to a tarnishing atmosphere are described. An explanation for the low contact resistance values obtained for silver-ruthenium dioxide in comparison with pure silver is offered.