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The high-power SiC devices require ohmic contact materials, which are prepared by annealing at temperatures lower than 800 °C. Recently, we demonstrated in our previous paper [J. Appl. Phys. 95, 2187 (2004)] that an addition of a small amount of Ge to the conventional binary Ti/Al contacts reduced the ohmic contact formation temperature by about 500 °C, and this ternary contacts yielded a specific contact resistance of approximately 1×10-4 Ω cm2 after annealing at a temperature as low as 600 °C. In this paper, the electrical properties and the microstructures of the Ge/Ti/Al contacts (where a slash “/” indicates the deposition sequence) were investigated by current-voltage measurements and transmission electron microscopy observations, respectively, in order to understand the ohmic contact formation mechanism. Ti3SiC2 compound layers (which were previously observed at the metal/SiC interface in the Ti/Al ohmic contacts after annealing at temperatures higher than 1000 °C) were observed to grow epitaxially on the SiC surface after annealing at temperatures as low as 600 °C. The Ti3SiC2 layers were believed to act as a p-type intermediate semiconductor layer, which played a key role to reduce the Schottky barrier height at the contacting metal/SiC interface. Further reduction of the contact resistances of the Ge/Ti/Al contacts would be achieved by increasing the coverage of the Ti3SiC2 layers on the SiC surface. [ABSTRACT FROM AUTHOR]

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 O[sub 2] and N[sub 2] mixed gas ambient had the specific contact resistance (ρ[sub c]) of high-10[sup -3] Ω cm[sup 2] and strong adhesion to the GaN substrates. The NiAu contact annealed in the partial O[sub 2] ambient had poor adhesion to the GaN, although the ρ[sub c] value of low-10[sup -3] Ω cm[sup 2] was obtained. The TaTi contact had the lowest ρ[sub c] values of less than 10[sup -4] Ω cm[sup 2]. After contact formation, the Pt and NiAu ohmic contacts prepared by annealing in the partial O[sub 2] 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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Part II. Presents transport measurements and materials studies carried out for molybdenum-germanium-indium-tungsten contacts. Importance of thermally stable ohmic contacts for integrated circuits; Role of indium in forming ohmic contacts; Development of a controllable fabrication process to yield thermally stable low-resistance contacts.

Studies the effects of interfacial microstructure on uniformity and thermal stability of gold-nickel-germanium ohmic contact to n-type gallium arsenide. Experimental procedures; Experimental results; Discussion of findings.

Reports on the development of thermally stable, low-resistance p-type ohmic contacts by depositing NiInW metals on gallium arsenide (GaAs) substrates in which beryllium (Be) and fluorine were coimplanted. Experimental procedures; Be distribution in GaAs; NiInW ohmic contacts.

Presents a study that analyzed the effect of silicon or germanium addition to nitrogen-indium-tungsten ohmic contacts on their electrical behavior. Analysis of the ohmic contacts prepared by two-step anneal; Difference between silicon and germanium addition to the ohmic contacts; Comparison between gold-germanium nitride and nitrogen-indium-tungsten ohmic contacts.

Presents a study that analyzed the electrical properties and microstructure of indium arsenide ohmic contacts to n-type gallium arsenide semiconductors. Measurement of the contact resistance; Comparison of contacts prepared by sputter-deposition and evaporation; Effect of intervening oxide layer on indium arsenide and gallium arsenide interaction.

Presents a study that examined electrical properties and thermal stability of indium/tungsten ohmic contacts in gallium arsenide semiconductors. Analysis of the interfacial microstructure of the semiconductor; Measurement of the contact resistances; Influence of layer thickness on the contact resistances of the semiconductor.

Part IV. Focuses on a study which examined the microstructure of the nickel indium tungsten (NiInW) ohmic contact material by cross-sectional transmission electron microscopy. Materials and procedure; Analysis of the role of nickel in the NiInW contacts; Factors that are important in preparing contacts with large fractional coverage of indium[subx] gallium[sub1-x] arsenide phases; Effect of nickel[sub2] gallium arsenide on native oxide sat the gallium arsenide substrate.

Part V. Focuses on a study which investigated the thermal stability of self-aligned refractory metal semiconductor field effect transistors with silver nickel germanium ohmic contact metallurgy or nickel indium tungsten ohmic contact metallurgy. Materials and procedure; Discussion on the performance of the fabricated devices; Observations on the deterioration of device performance; Cause of deterioration.

Presents information on a study which discussed improvement in thermally stable, low-resistance ohmic contacts to n-type gallium arsenide (GaAs) for germanium indium tungsten and nickel indium tungsten contact metals. Experimental procedures; Contact resistance measurements; Microstructural analysis; Correlation between the contact resistance and the indium(In)GaAs; Carrier transport mechanism for Indium-based ohmic contact.

Presents a study which investigated the formation of ohmic contacts to p-type diamond using carbide forming metals. Estimation of the acceptor concentrations; Features of a diamond semiconductor; Techniques used in the study of the applicability of semiconducting diamond films to the electronic devices.

Proposes a model for current transport of nickel germanium (NiGe)-based ohmic contacts based on the present electrical measurements and microstructural analysis. Substrates used in the experiments; Atomic distribution normal to gallium arsenide substrate after annealing at elevated temperatures; Improvement of contact resistance and process window of the present NiGe.

Presents information on a study which developed non-gold NiGe Ohmic contacts by a lift-off and annealing technique that was extensively used to fabricate the conventional AuGeNi contacts. Experimental procedures; Ohmic contact formation; Isochronal annealing for contact formation; Correlation between electrical properties and microstructures.

Presents information on a study which located process parameters to develop low resistance and thermally stable NiSiW Ohmic contacts, where the top W layers were used to reduce the contact metal sheet resistance. Experimental procedure; Correlation between electrical properties and microstructural analysis; Comparison between NiSiW contacts and NiGe contacts.

Reduction of annealing temperature to prepare low resistance ohmic contact materials for p-type 4H-SiC was achieved by adding Ge to the conventional TiAl contacts. Although the binary TiAl contact is required to anneal at temperature as high as 1000 °C to convert Schottky to ohmic behavior after deposition of the Ti and Al layers on the SiC substrate, the GeTiAl contacts provided the specific contact resistance of about 1×10[sup -4] Ω cm[sup 2] by annealing at temperature as low as 600 °C. This low annealing temperature is desirable to reduce the gate leakage current of the SiC devices. The GeTiAl ohmic contacts were thermally stable during isothermal annealing at 400 °C subsequently after preparing the ohmic contacts by annealing at 600 °C, which is also required by the device packaging process. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: To realize high-performance silicon carbide (SiC) power devices, low-resistance ohmic contacts to p-type SiC must be developed. To reduce the ohmic contact resistance, reduction of the barrier height at metal/SiC interfaces or increase in the doping concentration in the SiC substrates is needed. Since the reduction of barrier height is extremely difficult, the increase in the Al doping concentration in 4H-SiC by an ion-implantation technique was challenged. The Ti/Al and Ni/Ti/Al metals (where a slash “/” sign indicates the deposition sequence) were deposited on the Al ion-implanted SiC substrates. By comparing the experimental and theoretical contact resistances, the current transport mechanism through the metal/SiC interfaces was concluded to be thermionic field emission and the barrier height was determined to be ∼0.4eV. Although the hole concentration increased with increasing the Al doping concentration in 4H-SiC, the barrier height at metal/SiC interfaces increased due to high density of dislocation loops observed in the implanted SiC layers by transmission electron microscopy. The present experiments suggested that the low-resistance ohmic contacts would be formed when a technique to eliminate the crystal defects formed in the 4H-SiC substrates after ion implantation was developed. [Copyright &y& Elsevier]

In order to understand a mechanism of TiAl-based ohmic contact formation for p-type 4H-SiC, the electrical properties and microstructures of Ti/A1 and Ni/Ti/Al contacts, which provided the specific contact resistances of approximately 2 x 10-5 Ω-cm² and 7 x 10-5 Ω-cm² annealing at 1000°C and 800°C, respectively, were investigated using x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Ternary Ti3SiC2 carbide layers were observed to grow on the SiC surfaces in both the Ti/Al and the Ni/Ti/Al contacts when the contacts yielded low resistance. The Ti3SiC2 carbide layers with hexagonal structures had an epitaxial orientation relationship with the 4H-SiC substrates. The (0001)-oriented terraces were observed periodically at the interfaces between the carbide layers and the SiC, and the terraces were atomically flat. We believed the Ti3SiC2 carbide layers primarily reduced the high Schottky barrier height at the contact metal/p-SiC interface down to about 0.3 eV, and, thus, low contact resistances were obtained for p-type TiAl-based ohmic contacts. [ABSTRACT FROM AUTHOR]

Recent strong demands for optoelectronic communication and portable telephones have encouraged engineers to develop optoelectronic devices, microwave devices, and high-speed devices using hetero-structural GaAs-based compound semiconductors. Although the GaAs crystal growth techniques had reached a level to control the compositional stoichiometry and crystal defects on a nearly atomic scale by the advanced techniques such as molecular beam epitaxy and metal organic chemical vapor deposition techniques, development of ohmic contact materials (which play a key role to inject external electric current from the metals to the semiconductors) was still on a trial-and-error basis.Our research efforts have been focused to develop low resistance, refractory ohmic contact materials to n-type GaAs using the deposition and annealing techniques, and it was found the growth of homo-or hetero-epitaxial intermediate semiconductor layers (ISL) on the GaAs surface was essential for the low resistance ohmic contact formation. In this paper, two typical examples of ohmic contact materials developed by forming ISL were given. The one was refractory NiGe-based ohmic contact material, which was developed by forming the homo-epitaxial ISL doped heavily with donors. This heavily doped ISL was discovered to be formed through the regrowth mechanism of GaAs layers at the NiGe/GaAs interfaces during annealing at elevated temperatures. To reduce the contact resistance further down to a value required by the device designers, an addition of small amounts of third elements to NiGe, which have strong binding energy with Ga, was found to be essential. These third elements contributed to increase the carrier concentration in ISL. The low resistance ohmic contact materials developed by forming homo-epitaxial ISL were Ni/M/Ge where a slash ‘/’ denotes the deposition sequence and M is an extremely thin (∼5 nm) layer of Au, Ag, Pd, Pt or In. The other was refractory InxGa1−xAs-based ohmic contact materials which were developed by forming the hetero-epitaxial ISL with low Schottky barrier to the contacting metals by growing the InxGa1−xAs layers on the GaAs substrate by sputter-depositing InxGa1−xAs targets and subsequently annealing at elevated temperatures. To reduce the contact resistance, it was found that this InxGa1−xAs (ISL) layer had to have In compositional gradient normal to the GaAs surface: the In concentration being rich at the metal/InxGa1−xAs interface and poor close to the InxGa1−xAs/GaAs interface. This concentration graded ISL reduced both the barrier heights at the metal/ISL and ISL/GaAs interfaces and reduced the contact resistance. The ohmic contact materials developed by forming hetero-epitaxial ISL was In0.7Ga0.3As/Ni/WN2/W. These contact materials formed refractory compounds at the interfaces, which was also found to be essential to improve thermal stability of ohmic contacts used in the GaAs devices. [Copyright &y& Elsevier]

Since the Schottky barrier height at the metal/p-ZnSe and p-GaN interfaces is very high, and heavy p-type doping is difficult, it is extremely difficult to obtain low-resistance ohmic contact material for p-ZnSe and p-GaN. However, a key to the development of low-resistance ohmic contact material is to find an element that forms an intermediate semiconductor layer to reduce the Schottky barrier or to increase the doping concentration. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(10): 43–47, 1999 [ABSTRACT FROM AUTHOR]

Studies the effects of a nickel monoxide (NiO) layer on the electrical properties of NiAu-based ohmic contacts for p-gallium arsenide (GaN) by depositing a p-type NiO layer on the p-GaN using a sputter-deposition technique. P-type conduction of NiO layers doped with Li[NiO(Li); Sheet resistivity; Variety of the NiAu-based contacts with the NiO layers.

Develops a thermally stable, low-resistance indium-based NiInW ohmic contact to n-type gallium arsenide by sputter deposition. Composition of the contacts; Observation on the contact resistance and contact morphology; Incorporation of n-type dopants into the metallization.

Reports on thermally stable low-resistance ohmic contacts to n-type gallium arsenide (GaAs) incorporating a very thin layer of gold (Au) in conjunction with a layered nickel/germanium/tungsten (Ni/Ge/W) structure. Need for the beta-AuGa in the formation of low eutectic-based AuGeNi contacts; Resulting contact resistance; Contact stability under thermal stress.

A new thermally stable, low-resistance NiInW contact metal to n-type GaAs has been developed by depositing a thin In layer with Ni and W layers and annealing at elevated temperatures for a short time. Low resistances of ∼0.3 Ω mm were obtained at annealing temperatures in the range of 800 to 1000 °C. The contact resistances were stable during subsequent annealings at 400 °C for 100 h and 500 °C for 10 h. The thermal stability of the contact resistance and the surface morphology of this contact are superior to those of the conventionally used AuNiGe contacts and this new contact is suitable for various device applications. Further reduction of the contact resistance can be achieved simply by reducing the sheet resistance of the contact metals. [ABSTRACT FROM AUTHOR]

New Ni/Al and Ni/Ti/Al ohmic contact materials (a slash ‘/’ indicates the deposition sequence) were developed by depositing on p-type 4H-SiC substrates with doping concentrations (NA) of 3.0–9.0×1018 cm−3 and subsequently annealing at 800 °C in an ultra high vacuum chamber. In contrast with conventional Ti/Al ohmic contacts, which must be annealed at temperatures above 1000 °C, both the Ni/Al contacts and Ni/Ti/Al contacts showed ohmic behavior after annealing at 800 °C. The lowest specific contact resistances of the Ni/Al contacts and the Ni/Ti/Al contacts (prepared by annealing at 800 °C) were 5×10−3 and 6.6×10−5 Ω cm2, respectively. The later value is the lowest contact resistance ever obtained by annealing at such a low temperature. A theory indicates that a contact resistance less than 1.0×10−5 Ω cm2 would be obtained for the Ni/Ti/Al contacts when the hole concentration in the SiC is greater than 1.0×1019 cm−3. In addition, the Ni/Ti/Al contacts showed excellent thermal stability during isothermal annealing at 400 °C for 10 h in Ar atmosphere. The microstructural analysis at the metal/SiC interfaces showed that the Ni/Al contacts and the Ni/Ti/Al contacts reacted with the SiC substrates after annealing at 800 °C; the Ni/Al contacts formed Ni2Si and NiAl3 compounds, and the Ni/Ti/Al contacts formed ternary Ti3SiC2 compounds in addition to these binary compounds. A correlation between the electrical properties and the microstructure is discussed. [Copyright &y& Elsevier]