Low resistivity contacts to YBa2Cu3O7−y superconductors using Ag2O powder
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Ohmic contact
Electrical contacts
Contact resistance
A technique is described to estimate the contact resistance of an ohmic contact, under dynamic conditions, applied to a high resistivity photoconductor. It has been used to estimate the contact resistance of an ohmic contact (Au–Ge) applied to a semiinsulating Cr-doped GaAs (p≊108 W cm).
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The feasibility of replacing the Au layer in a conventional ohmic contact of Ti/Al/Ni/Au for AlGaN/GaN structure with Cu has been investigated. While the thicknesses of Ti, Al and Cu layers were fixed throughout the study, the thickness of Ni layer was varied to examine its influence on the Cu-based ohmic contacts. It is showed that the Ni layer thickness played an important role in the Ti/Al/Ni/Cu metallization for achieving low contact resistance and smooth surface morphology. With a 50-Å Ni layer, a low specific contact resistance (r c ) of 1.35x10 -6 ohm-cm 2 has been realized. This result is comparable with the conventional ohmic contact (r c = 1.19x10 -6 ohm-cm 2 ). High electron mobility transistor (HEMT) devices fabricated using both Au- and Cu-based ohmic contact exhibited similar DC characteristics, suggesting that the electrical performance did not degrade with the used of Cu in the ohmic contact. Furthermore, in the absence of Au, the surface roughening caused by Au-Al alloy in the conventional ohmic contact was also prevented. As a result, the root-mean-square roughness of the optimized Cu-based contact was only 7.62 nm as compared to 134 nm for the conventional structure.
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Using contacts to silicon carbide as an example, a method of ohmic contact characterization from heat stability and contact resistance is proposed. The method is based on the known empirical dependences of (i) contact resistivity on the semiconductor doping level and (ii) ohmic contact degradation on time and temperature.
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Ti/Al ohmic contact with an extremely low specific contact resistance has been formed by the deposition of Ti and Al films on Si+ lanted GaN. The ohmic contact formed by annealing at 600 o C of Ti film with a thickness of 50 nm and Al film with a thickness of 200 nm reveals the good smooth surface and uniform structure as compare to those of contacts formed above 700 °C, which is correlated to whether the Al-Ti alloy is melted during the annealing of ohmic contact or not. The specific contact resistance of 2 × 10−6Ω−cm2 is obtained for Si+ implanted GaN with a dose of 5 × 1013 cm−2. As Si ion dose increases to 5 × 1014 /cm2, the specific contact resistance is reduced to 2 × 10−8 Ω−cm2. It is revealed that the selective doping at high impurity concentration in the surface region by Si+ implantation is useful to reduce the contact resistance for Ti/Al contact to GaN.
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The authors study the effect of etch chemistry and metallization scheme on recessed Au-free Ohmic contacts to AlGaN/GaN heterostructures on silicon. The effect of variation in the recess etch chemistry on the uniformity of Ohmic contact resistance has been studied using two different etch chemistries (BCl3/O2 and BCl3/Cl2). Experiments to determine the optimum recess etch depth for obtaining a low value of contact resistance have been carried out, and it is shown that near-complete etching of the AlGaN barrier layer before metallization leads to the lowest value of contact resistance. Furthermore, two metal schemes, namely, Ti/Al and Ti/Al/Ti/W, are investigated, and it is found that the Ti/W cap layer on Ti/Al leads to low contact resistance with a smooth contact surface morphology. The effect of maintaining unequal mesa and contact pad widths on the extracted values of contact resistance and sheet resistance using the linear transfer length method (LTLM) has been studied. This is important as LTLM structures are used as monitors for process control during various steps of fabrication. It is shown that the extracted contact resistance and sheet resistance values are reliable when the mesa width is equal to the contact pad width. Finally, a possible mechanism for carrier transport in the Ohmic contacts formed using this process has been discussed, based on temperature dependent electrical characterization, and the field emission mechanism is found to be the dominant mechanism of carrier transport. A low Ohmic contact resistance of 0.56 Ω mm, which is one of the lowest reported values for identical metal schemes, and good contact surface morphology has been obtained with moderate post-metal annealing conditions of 600°C.
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The stabilities of the electrical properties and microstructures of Pt, PtAu, NiAu, and TaTi ohmic contacts after contact formation were studied. The Pt and PtAu contacts annealed in an O2 and N2 mixed gas ambient had the specific contact resistance (ρc) of high-10−3 Ω cm2 and strong adhesion to the GaN substrates. The NiAu contact annealed in the partial O2 ambient had poor adhesion to the GaN, although the ρc value of low-10−3 Ω cm2 was obtained. The TaTi contact had the lowest ρc values of less than 10−4 Ω cm2. After contact formation, the Pt and NiAu ohmic contacts prepared by annealing in the partial O2 ambient showed the excellent electrical and microstructural stabilities during room temperature storage and current injection. However, the contact resistance of the TaTi contact prepared by annealing at 800 °C increased during room temperature storage, and the mechanical failure of the contact was also observed after injecting current as low as 4 kA/cm2. From the present experiments, it was concluded that the Pt and PtAu contacts were the most reliable ohmic contact materials for p-GaN among four contact metals.
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In this article, a scheme for fabricating low resistance Ohmic contacts to n-GaN was developed. This approach takes advantage of Ar plasma treatment and thermal annealing in forming gas ambient. As a result, the adjustment of Ar flow rate was very effective in improving the contact resistance. After proper Ar plasma treatment, the contact resistance and specific contact resistance of as-deposited Ohmic contacts were reduced to 0.362 Ω mm and 3.9×10−5 Ω cm2, respectively. Low contact resistance (0.103 Ω mm) and specific contact resistance (3.2×10−6 Ω cm2) were obtained after annealing in N2 gas ambient. By performing thermal annealing in forming gas ambient, even lower contact resistance (0.093 Ω mm) and specific contact resistance (2.6×10−6 Ω cm2) were successfully achieved, indicating that the electrical characteristics of Ohmic contacts would not be affected by the effect of hydrogen passivation of dopants in n-GaN.
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We show that ε1-Cu3Ge forms a low-resistance ohmic contact to n-type GaAs. The ε1-Cu3Ge contact exhibits a planar and abrupt interface and contact resistivity of 6.5×10−7 Ω cm2 which is considerably lower than that reported for Ge/Pd and AuGeNi contacts on n-type GaAs with similar doping concentrations (∼1×1017 cm−3). The contact is electrically stable during annealing at temperatures up to 450 °C. We also show that in the Ge/Cu/n-type GaAs system, the contact remains ohmic over a wide range of Ge concentration that extends from 15 to 40 at. %. n-channel GaAs metal–semiconductor field-effect transistors using the ε1-Cu3Ge ohmic contacts demonstrate a higher transconductance compared to devices with Ge/Pd and AuGeNi contacts.
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Ohmic contacts with low contact resistances were fabricated on n + 6H-SiC layers grown by liquid phase epitaxy. Ni was deposited on the layers for the contact metal Ohmic characteristics were obtained on as-deposited n + samples with carrier concentrations higher than about 5×10 19 cm -3 . The contact resistance was lowered by thermal annealing at 1000° C for 5 minutes. A specific contact resistance as low as 1×10 -6 Ω· cm 2 was fabricated on an n + layer with a carrier concentration of 4.5×10 20 cm -3 . It was found that Ti/Al, which is widely used as a p-type ohmic contact metal, also made a good ohmic contact on the n + layer.
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Ohmic contact
Contact resistance
Electron Mobility
Tungsten disulfide
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