Papers by Keyword: TLM

Abstract: Ohmic CoSi₂ contacts to n-type 4H-SiC showing low contact resistance have been made by sputter depositing sequential layers of Si and Co on 4H-SiC substrates followed by a two-step rapid thermal anneal at 600 °C and 950 °C. The contacts formed have been characterized at temperatures ranging from-40 °C to 500 °C with a specific contact resistance of 3.80∙10^-5 Ωcm² at 25 °C and a minimum of 6.0∙10^-6 Ωcm² at 500 °C.

Abstract: The mechanism of Ohmic contacts formation to p-type SiC is a fundamental and technological concern continuously under debate. Typically, Ti/Al-based contacts are a good choice for Ohmic contacts to p-type SiC, even though some aspects strictly related to the specific nature of Al (susceptibility to oxidation, low melting temperature, etc.) remain to be optimized. In this work, the evolution of the electrical properties of a Ti/Al/W multiple-layer contact has been studied by TLM characterization and correlated to the changes in the morphology and microstructure upon thermal annealing. The formation of an Ohmic contact has been observed after a thermal annealing at 1100°C, discussing the possible reasons determining the transition to an Ohmic behavior.

Abstract: Lateral JFET transistors have been fabricated with N and P-type channels tentatively integrated monolithically on the same SiC wafer. Buried P⁺ SiC layers grown by Vapor-Liquid-Solid (VLS) selective epitaxy were utilized as source and drain for the P-JFET and as gate for the N-JFET. The ohmicity of the contacts, both on VLS grown P⁺ and ion implanted N⁺ layers, has been confirmed by Transfer Length Method (TLM) measurements. A premature leakage current is observed on the P/N junction created directly by the P⁺ VLS gate layer, probably due to imperfect VLS (P⁺) / CVD (N⁺) SiC interface.

Abstract: Extreme temperature measurements of Ni/Ti/Al contacts to p-type SiC (N_a = 1∙10¹⁸cm^-3), with a specific contact resistivity ρ_c = 6.75∙10^-4 Ωcm² at 25 °C, showed a five time increase of the specific contact resistivity at -40 °C (ρ_c = 3.16∙10^-3 Ωcm²), and a reduction by almost a factor 10 at 500 °C (ρ_c = 7.49∙10^-5 Ωcm²). The same response of ρ_c to temperature was seen for contacts on lower doped epitaxial layer. Also N-type nickel contacts improved with higher operational temperature but with a considerably smaller variation over the same temperature interval. No degradation of the performance was seen to either the Ni/Ti/Al or the Ni contacts due to the high temperature measurements.

Abstract: Al-Free Nanolayered Metallizations Based on the Transition Metals Ti, Mo, Ni and Pd, with Varied Ti Content, Have Been Developed as an Alternative of the Al-Based Contacts for Sub-Micron Hemts. the Electrical, Morphological and Thermal Properties of the Metallization Schemes Have Been Studied with the Aim of Obtaining the Most Suitable Combination of Low Resistivity, a Smooth Surface and an Acute Edge. the Lowest Resistivity of 8.8x 10-6 Ω.cm2 Has Been Determined with the Ti/Mo/Ti/Au Contact, while the Lowest Surface Roughness of 6 Nm Has Been Measured for the Ti/Ni/Ti/Au Metallization. these Contact Schemes Have Shown much Better Edge Acuity in Comparison to the Al-Based Metallizations.

Abstract: Transfer Length Method (TLM) based-structures were fabricated on 0.8 µm-thick epitaxial p-type Silicon Carbide (4H-SiC) layers. TLM mesas were defined by a 2 µm height using an SF₆/O₂ reactive ion etching. TLM metal patterns were obtained by a lift-off procedure and electron beam deposition of Ni, Ti, Al and Pt. The patterned samples were annealed in Argon ambient at temperature ranging from 700°C up to 1000°C in a RTA furnace with a rapid heating ramp (up to 50°C/s) to complete the ohmic contact with the p-type SiC layer. Specific contact resistances were extracted from current/voltage measurements. To identify and follow the profile evolution of constituting element in the contacts and at the SiC/contact interface, the ohmic contacts were characterized using Secondary Ion Mass Spectrometry and Energy-Dispersive X-Ray spectroscopy before and after annealing. Ohmic contacts are obtained only for the Ni/Ti/Al and Ni/Ti/Al/Ni stacking layers and not for the Ti/Al/Ti/Ni and Ti/Al/Ti/Pt/Ni compositions. The specific contact resistance of Ni/Ti/Al/Ni stacking layers was observed to decrease from 2.7×10^-4 Ω.cm² at 700°C and 6.3×10^-5 Ω.cm² at 750°C to a minimal value of 1.5×10^-5 Ω.cm² at 800°C. Ohmic contacts are obtained with a reproducibility of 80 %.

Abstract: There is still little consensus regarding why low contact resistivity is achieved when Ni on n-type 4H- and 6H-SiC is annealed at temperatures of more than above 950°C. The objective of this paper is to provide an answer concerning to this question. It is has been reported that even Ni-based contacts formed in the n++ region exhibited a steep reduction of contact resistivity in an annealing temperature range > 900°C. This effect reduction cannot be explained by the carbon vacancy induced donor model (Vc model) proposed by Han and his coworkers [Appl. Phys. Lett., Vol. 79, p. 1816 (2001)]. And, it is clarified that It was observed that the surface of substrates annealed at 1000°C was not covered with not Ni2Si but with a thin layer of NiSi. Finally, a plausible model is proposed that as the result of annealing at higher temperatures, results in the formation of the a NiSi/SiC system is builtat the substrate interface, resulting in significant reduction in low causing contact resistivity to be reduced significantly.

Abstract: The dependence of the structure and composition of nanolayered Au/Ti/Al ohmic contacts to p-type 4H-SiC on the initial Ti:Al ratio has been investigated. Two contact compositions, Au/Ti(70%)/Al(30%) and Au/Ti(30%)/Al(70%), have been studied regarding the electrical properties, structure, composition and annealing temperature in the interval 850 – 1000o C. The correlation between the electrical behaviour and structure of the annealed contacts is discussed. Very low resistivity of 1.42x10-5 .cm2 after annealing at 900o C has been obtained for the contact having an initial composition Ti:Al (30:70), while the lowest resistivity of 1.21x10-5 .cm2 has been measured for the contact with a composition Ti:Al (70:30) after annealing at 1000o C. Strong dependence of the contact structure on the Ti:Al ratio and annealing temperature, respectively, has been found out. A presence of two phases, Au2Ti and Al3Ti, in all contacts has been determined after annealing, despite the temperature value and Ti:Al ratio. The TEM analysis reveals that titanium and aluminum silicides and carbides are formed after annealing as the Ti:Al ratio affects the kind of silicides and carbides created. It is obtained that the initial composition of the deposited metal layers influences only the phase composition of the annealed contact but not the grain sizes of the dominant phases formed. The origin of the ohmic properties improvement is explained by the formation of Ti3SiC2 compound and/or enhanced carrier transport by the presence of metal spikes into SiC depending on the initial contact composition and as consequence the optimal annealing temperature.

Abstract: Electrical, thermal and chemical properties of Ti/Al/Ti/Au ohmic contacts with different former Ti-Al ratio are investigated for application in GaN HEMTs. Lowest resistivity of 4.22x10-5 Ω.cm2 has been obtained to the channel of the HEMT structure. It is found out that the initial Ti/Al ratio influences the optimal annealing temperature at which the lowest resistivity is obtained and the element distribution and interface chemistry of the annealed contacts. XPS analysis revealed two compounds contributing to ohmic properties: an intermetal compound AlAu2 in the contact layer and a semimetal TiN at the interface with GaN.

Abstract: N+ 4H-SiC commercial substrates with n-type epilayers have been used to realize bipolar diodes and TLM structures. The p-type emitter of diodes was realized by Al implantations followed by a post-implantation annealing with or without a graphite capping layer. Ohmic contacts were formed by depositing Ti/Ni on the backside and Ni/Al on the topside of the wafer. It appears that capping the sample during the annealing reduces considerably the surface roughness and the specific contact resistance. Sheet resistance and specific contact resistance as low as 2kΩ/□ and respectively 1.75×10-4 Ωcm² at 300 K have been obtained. I-V measurements as a function of temperature have been performed from ~100 to ~500 K. The variations of the series resistance vs. temperature can be explained by the freeze-out of carriers and by the variation of carrier mobility.