Papers by Keyword: Ohmic Contact

Abstract: An ohmic contact process by using tri-layer materials for a source contact of a silicon carbide (SiC) metal oxide semiconductor field effect transistor (MOSFET) is proposed. The authors validate its extremely low contact resistance for both n-type and p-type SiC by a simple process. The characteristics of Ti/Al/Si ohmic contacts were measured by using the transfer length method (TLM). We examined the dependence of the contact resistance on the thickness of each layer of Ti/Al/Si. Then, it is found that Ti/Al/Si contacts with an appropriate thickness show excellent ohmic properties for both n-type and p-type SiC. N-type specific contact resistance (ρ_n) of 3.7 × 10^-6 Ω cm² and p-type specific contact resistance (ρ_p) of 1.7 × 10^-4 Ω cm² are obtained with Ti (20 nm) /Al (30 nm) /Si (30 nm).

Abstract: Epitaxial graphene fabricated by thermal decomposition of the Si-face of silicon carbide (SiC) forms a defined interface to the SiC substrate. As-grown monolayer graphene with buffer layer establishes an ohmic interface even to low-doped (e. g. [N] ≈ 10¹⁵ cm^-3) SiC, and a specific contact resistance as low as ρ_C = 5.9×10^-6 Ωcm² can be achieved on highly n-doped SiC layers. After hydrogen intercalation of monolayer graphene, the so-called quasi-freestanding graphene forms a Schottky contact to n-type SiC with a Schottky barrier height of 1.5 eV as determined from C-V analysis and core level photoelectron spectroscopy (XPS). This value, however, strongly deviates from the respective value of less than 1 eV determined from I-V measurements. It was found from conductive atomic force microscopy (C-AFM) that the Schottky barrier is locally lowered on other crystal facets located at substrate step edges. For very small Schottky contacts, the barrier height extracted from I-V curves approaches the value of 1.5 eV from C-V and XPS.

Abstract: This work deals with two applications of the Selective Epitaxial Growth of highly p-type doped buried 4H-SiC in Vapor-Liquid-Solid configuration (SEG-VLS). The first application is the improvement of the Specific Contact Resistance (SCR) of contacts made on such p-type material. As a result of the extremely high doping level, SCR values as low as 1.3x10^-6 Ω.cm² have been demonstrated. Additionally, the high Al concentration of the SEG-VLS 4H-SiC material induces a lowering of the Al acceptor ionization energy down to 40 meV. The second application is the fabrication of PiN diodes with SEG-VLS emitter and guard-rings peripheral protection. Influence of some process parameters and crystal orientation on the forward and reverse characteristics of the PiN diodes is discussed.

Abstract: In this study, Ohmic contact were fabricated on AlGaAs HEMTs structure. A good metal-semiconductor interface are essentially for achieving lower specific contact resistance. An AlGaAs epi wafer was supply by the vendor. AlGaAs substrate was cleaned using wet chemical etching. Electrodes were fabricated through a sequenced of lithography, cleaning, sputtering and lift-off processes. The electrodes were made with metal layers of Ge, Au and Ni. Parameters such as metal thickness, annealing temperatures (from 300°C to 400°C) and annealing time were varies during fabrication process. Electrical characterizations after annealing are carried out using transmission line method (TLM) to obtain the specific contact resistance. Annealing temperature between 340°C to 360°C produced contact resistance below 5 x 10ˉ³Ω/cm^-2.

Abstract: This paper reports on the development of InP transferred-electron-device sources in mainland of China for operation at around 100 GHz. Using n+-n-n+ structure with graded doping profiles, the oscillations were obtained at 101.8 GHz from a 1 μm structure with an n-doped drift zone and the doping concentration linearly increases from 1.0×1016 to 3.0×1016cm-3. Its continuous wave radio frequency (CWRF) output power was evaluated to be several milliwatt and these results are believed to correspond to a fundamental mode operation. This result is attributed to a processing technique based on the use of etch-stop layers, removal of substrate and the formation of good ohmic contacts.

Abstract: The p-type doping of high purity semi-insulating 4H-SiC by Al⁺ ion implantation and a conventional thermal annealing of 1950 °C/5 min has been studied for implanted Al concentration in the range of 1 x10¹⁹ - 8 x 10²⁰ cm^-3 (0.36 μm implanted thickness). Sheet resistance in the range of 1.6 x 10⁴ to 8.9 x10² Ω, corresponding to a resistivity in the range of 4.7 x 10^-1 to 2.7 x 10^-2 Ωcm for increasing implanted Al concentration have been obtained. Hall carrier density and mobility data in the temperature range of 140–600 K feature the transition from a valence band to an intra-band conduction for increasing implanted Al concentration. The specific contact resistance of Ti/Al contacts on the 5 x10¹⁹ cm^-3 Al implanted specimen features a thermionic field effect conduction with a specific contact resistance in the 10^-6 Ωcm2 decade.

Abstract: The near-SiC-interfaces of annealed Ni/SiC contacts were observed directly by high-resolution transmission electron microscopy (HRTEM). 1 nm native oxide layer was observed in the as-deposited contact interface. The native oxide layer cannot be removed at 650°C through rapid thermal annealing (RTA) and it was completely removed at 1000°C RTA. The residue of native oxide layer resulted in the Schottky characters. High temperature annealing (>950°C) not only removes the oxide layer in the near-SiC-interface, but also forms a well arranged flat Ni2Si/SiC interface, which contribute to the formation of ohmic behavior.

Abstract: The front electrode is usually made by the screen printing thick-film silver pastes and the high-temperature firing process in industrial production of silicon solar cells. This paper analyzed the ohmic contact mechanism of thick-film front silver electrodes and studied the microstructure of Ag-Si interface by SEM. The paste samples, used to form front silver electrodes of silicon solar cells, were prepared. Thick-film silver electrodes were printed on silicon wafers with different sheet resistances, and the relationships between the sheet resistances and the contact properties were investigated by changing the firing temperature. By adding right amount of phosphorus compounds to the silver paste, the effects of the donor-doping (N-doping) concentrations on the series resistance of cells were studied. The experimental results show that firing temperature is critical to the Ag-Si ohmic contact, particularly when the silver pastes are designed for the wafers with high sheet resistance and the right amount of N-doping addition in the paste may decrease the series resistances of solar cells.

Abstract: In this work the electrical properties of Ni and Ni₂Si contacts on n-type 4H-SiC were correlated to the strong structural changes at the contact/SiC interface upon annealing. We can conclude that only δ-Ni₂Si grains play a main role in determining electrical transport properties of the Ni-based ohmic contacts to n-SiC. It is presumed that a recrystallization and texturization of δ-Ni₂Si phase on (0001)SiC-surface during high temperature annealing (> 900°C) contributes to the change of barrier heights, as well as specific contact resistance of contacts.

Abstract: In this paper the structural and electrical evolution of Au/Ni contacts to p-type gallium nitride (GaN) upon annealing in different atmospheres was monitored. Rapid annealing of the contacts in an oxidizing atmosphere (N2/O2) resulted into a lower specific contact resistance (ρc) with respect to annealing in non-reacting ambient (Ar). The formation of a nickel oxide (NiO) layer was observed on the surface of the sample annealed in N2/O2 ,while was not present at the interface with p-GaN. The improvement of the contacts was associated with a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing), as determined by the temperature dependence of the ρc. Local electrical measurements demonstrated the formation of inhomogeneous barriers. The electrical measurements were correlated with the different microstructure of the annealed contacts.