Electrical Properties Of Metal Contacts Research Articles

Low-resistivity Ohmic contacts of Ti/Al on few-layered 1T'-MoTe2/2H-MoTe2 heterojunctions grown by chemical vapor deposition.

This study explores the phase-controlled growth of few-layered 2H-MoTe2, 1T'-MoTe2, and 2H-/1T'-MoTe2 heterostructures and their impacts on metal contact properties. Cold-wall chemical vapor deposition (CW-CVD) with varying growth rates of MoOx and reaction temperatures with Te vapors enabled the growth of continuous thin films of either 1T'-MoTe2 or 2H-MoTe2 phases on two-inch sapphire substrates. This methodology facilitates the meticulous optimization of chemical vapor deposition (CVD) parameters, enabling the realization of phase-controlled growth of few-layered MoTe2 thin films and their subsequent heterostructures. The study further investigates the influence of a 1T'-MoTe2 intermediate layer on the electrical properties of metal contacts on few-layered 2H-MoTe2. Bi-layer Ti/Al contacts directly deposited on 2H-MoTe2 exhibited Schottky behavior, indicating inefficient carrier transport. However, introducing a few-layered 1T'-MoTe2 intermediate layer between the metal and 2H-MoTe2 layers improved the contact characteristics significantly. The resulting Al/Ti/1T'-MoTe2/2H-MoTe2 contact scheme demonstrates Ohmic behavior with a specific contact resistance of around 1.7 × 10-4 Ω cm2. This substantial improvement is attributed to the high carrier concentration of the 1T'-MoTe2 intermediate layer which could be attributed tentatively to the increased tunneling events across the van der Waals gap and enhancing carrier transport between the metal and 2H-MoTe2.

Geometry- and size-dependence of electrical properties of metal contacts on semiconducting nanowires

We report on a theoretical study of the geometry-and size-dependent effects of metal-semiconductor contacts to understand the origins of significant contact effects on nanowire (NW) devices. The difference in metal-semiconductor barrier width and height for a planar and NW device is calculated based on the differences in depletion-layer width and image-force barrier-lowering effects. We calculate the ratio of the specific contact resistance of a NW contact to that of a planar contact assuming the two systems have been made of the same material set. Using a modified cylindrical transmission line model, we compare the effects of contacts in the two systems and discuss the reasons for larger contact effects in NW devices. The study suggests that the formation of a higher doping concentration in the contact regions is essential for better NW transistors.

Structural and electrical properties of metal contacts on n-type ZnO thin film deposited by vacuum coating technique

The authors report on fabrication and characterization of Al and Pt metal contacts on ZnO thin films grown on ITO coated glass substrates using thermal evaporation technique. The structural and surface properties of ZnO thin film were studied by using x-ray diffraction and atomic force microscopy techniques. Atomic force micrographs revealed that ZnO microparticles have perfect pyramidal shape with small surface roughness (average rms value of 3nm). The current voltage characteristics of Pt∕ZnO and Al∕ZnO contacts were studied by scanning tunneling microscopy. The Pt contact on ZnO thin films behaves as a rectifying contact with a barrier height of 0.72eV, while Al contact on ZnO thin film turns out to be Ohmic in nature. The band gap of ZnO thin film was estimated to be 3.10eV from absorption spectroscopic measurements.

Effects of KrF excimer laser irradiation on metal contacts to n-type and p-type GaN

Electrical properties of metal contacts on laser-irradiated n-type and p-type GaN surfaces were investigated using current–voltage, capacitance–voltage, and synchrotron radiation photoemission spectroscopy. After the irradiation of a KrF excimer laser pulse (600 mJ/cm2 at 248 nm for 38 ns) onto Si-doped GaN, a nonalloyed Ti/Al metallization formed an ohmic contact with the specific contact resistivity of 1.7×10−6 Ω cm2. The laser irradiation decomposed GaN into metallic Ga and nitrogen gas. The decomposed metallic Ga reacted with oxygen in air to form a Ga oxide layer with the thickness of ∼40 Å, producing a large number of N vacancies near the surface. The formation of a degenerated n-type GaN layer resulted in the low contact resistivity. For Mg-doped GaN, the laser irradiation increased the effective acceptor concentration. Simultaneously, the activation efficiency of Mg dopants was enhanced by the photon-assisted breaking of Mg–H bonds and/or the removal of hydrogen atoms in the presence of oxygen, producing the p-type GaN with an increased hole concentration. As a result, the contact resistivity of an oxidized Ni/Au contact could be reduced from 1.3×10−3 to 3.6×10−4 Ω cm2.

Electrical properties of metal contacts on laser-irradiated n-type GaN

The electrical properties of metal contacts on laser-irradiated n-type GaN were investigated using synchrotron radiation photoemission spectroscopy. A KrF excimer laser pulse of 600 mJ/cm2 onto GaN led to a decrease in the Ni Schottky barrier height from 0.91 to 0.47 eV, resulting in the formation of a nonalloyed Ohmic contact with a specific contact resistivity of 1.7×10−6 Ω cm2. Metallic Ga decomposed from GaN by laser irradiation was transformed into GaOx, playing a role in promoting outdiffusion of N atoms. A large number of N vacancies were produced, forming a degenerated GaN layer near the surface, resulting in the good Ohmic contact.

Cathodic localization of metal coatings on silicon carbide crystals

Experimental data on the electrical properties of metal contacts fabricated by depositing copper from a porous electrode onto the surface of silicon carbide crystals are reported.

The electrical properties of junctions between aluminium and doped polypyrrole

Polypyrrole (PPy) doped with small anions like or large polymeric anions such as poly(styrenesulphonate) forms a conducting organic solid. The electrical properties of metal contacts to this material were evaluated by current - voltage characteristics and impedance spectroscopy measurements. The I - V characteristics of the -doped device was symmetrical but non-ohmic, while that of the -doped diode was rectifying. The complex impedance spectra of -doped structures showed two partially overlapping semi-circles, which revealed the existence of two distinct regions at the metal/doped PPy junctions. They were modelled by an equivalent circuit consisting of two parallel RC circuits in series representing a thin insulating interfacial layer and a depletion region. The impedance spectra of -doped devices manifested a single semi-circle whose diameter depended on the bias voltage. In this case, the thin interfacial non-conducting layer was absent.

Mechanical and electrical properties of metallic contacts at the nanometer scale

In the first part we present computer simulations of the influence of a layer of foreign atoms on the force of adhesion between an asperity and substrate of the same metal. Regardless of whether or not wetting of the foreign layer by the tip occurs, it is found that the force of adhesion is always reduced relative to the case where no foreign layer is present. A simple liquid drop model is presented to explain this result. We then consider the electronic conductance of the system, in the absence of a foreign layer, as the asperity is brought into contact with the substrate and subsequently removed. The conductance is calculated by a quantum mechanical scheme using atomic positions generated by molecular dynamics. The conductance is found to change in jumps during contact formation and fracture, but the conductance per atom is not quantized. The jumps coincide with gross structural rearrangements resulting from mechanical instabilities in the system. Relevant experimental observations are mentioned.

Correlation of interface chemistry to electrical properties of metal contacts on diamond

Three different metals (Au, Al and Ti) were deposited on polycrystalline diamond films. The effects of post-deposition annealing and pre-deposition Ar + sputtering were examined. Au and Al did not chemically interact with diamond and formed rectifying contacts on the as-grown surface. The non-reactive interface is an important factor in forming rectifying contacts on diamond. On the contrary, Ti formed a carbide at the interface upon annealing. The resulting contact showed ohmic characteristics. Au was non-reactive even on an Ar +-sputtered diamond surface, while Ti and Al formed carbides on this surface upon annealing at temperatures as low as 430°C. Ti and Al on the sputtered surface resulted in ohmic contacts. Based on these observations, a model of the metal-diamond interface band structure is presented. Combining experimental results with bulk thermodynamic data of carbides has allowed an understanding of the interface chemistry between diamond and various metals.

Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface. II. Titanium contacts: A carbide-forming metal

The I-V characteristics of titanium contacts on polycrystalline diamond have been correlated with x-ray-photoelectron-spectroscopy (XPS) and Auger-electron-spectroscopy (AES) characterizations of the interface. As-deposited titanium contacts were rectifying in nature because of minimal interaction between as-deposited titanium and diamond as confirmed via XPS and AES. Once annealed, however, these contacts became Ohmic. The change was related to the formation of a carbide at the interface as observed by XPS. The Schottky-barrier height of the titanium contacts, which was determined by valence-band XPS, decreased from 1.3 to 0.8 eV as a result of the postdeposition annealing. It is believed that the carbide formation at the interface creates a diamond surface layer rich in electrically active defects which lower the barrier height of the metal and increase the leakage current. The interface between titanium and an argon-sputtered diamond surface was also characterized. Titanium formed as-deposited Ohmic contacts on the sputtered surface. A high density of ion-radiation-induced defects and a formation of a carbide during deposition both contributed to the Ohmic-contact formation. These contacts remained Ohmic after postdeposition annealing despite the fact that the annealing did not increase the carbide formed at the interface. It is believed that the carbide formed by the deposition of titanium behaved as a diffusion barrier to prevent the damaged layer from being annealed out into the titanium overlayer. It is concluded that most materials will yield rectifying contacts on a clean diamond surface. Ohmic contacts can be obtained by modifying the interface in some way (i.e., carbide formation, sputtering, etc.).

Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface. I. Gold contacts: A non-carbide-forming metal.

The I-V characteristics of titanium contacts on polycrystalline diamond have been correlated with x-ray-photoelectron-spectroscopy (XPS) and Auger-electron-spectroscopy (AES) characterizations of the interface. As-deposited titanium contacts were rectifying in nature because of minimal interaction between as-deposited titanium and diamond as confirmed via XPS and AES. Once annealed, however, these contacts became Ohmic. The change was related to the formation of a carbide at the interface as observed by XPS. The Schottky-barrier height of the titanium contacts, which was determined by valence-band XPS, decreased from 1.3 to 0.8 eV as a result of the postdeposition annealing. It is believed that the carbide formation at the interface creates a diamond surface layer rich in electrically active defects which lower the barrier height of the metal and increase the leakage current. The interface between titanium and an argon-sputtered diamond surface was also characterized. Titanium formed as-deposited Ohmic contacts on the sputtered surface. A high density of ion-radiation-induced defects and a formation of a carbide during deposition both contributed to the Ohmic-contact formation. These contacts remained Ohmic after postdeposition annealing despite the fact that the annealing did not increase the carbide formed at the interface. It is believed that the carbide formed by the deposition of titanium behaved as a diffusion barrier to prevent the damaged layer from being annealed out into the titanium overlayer. It is concluded that most materials will yield rectifying contacts on a clean diamond surface. Ohmic contacts can be obtained by modifying the interface in some way (i.e., carbide formation, sputtering, etc.).

The effect of surface treatment on the electrical properties of metal contacts to boron-doped homoepitaxial diamond film

Both doped and undoped homoepitaxial diamond films were fabricated using microwave plasma-enhanced chemical vapor deposition (CVD). The conductivity of the diamond film is strongly affected by the surface treatment. In particular, exposure of film surface to a hydrogen plasma results in the formation of a conductive layer which can be used to obtain linear (ohmic) I-V characteristics of the Au/diamond contacts, regardless of the doping level. It is shown how the proper chemical cleaning of the boron-doped homoepitaxial diamond surface allows the fabrication of Au-gate Schottky diodes with excellent rectifying characteristics at temperatures of at least 400 degrees C. >

The structure and electrical properties of metal contacts on GaAs

This paper reports a study of the changes in the structural and chemical properties of Au and Al contacts on GaAs at annealing temperatures where significant changes in the Schottky barrier height are found. The annealing of Au contacts above 290 °C caused the Schottky barrier height to decrease. In contrast the Schottky barrier height of Al diodes increases upon annealing. An increase in As/Ga ratio in the GaAs under the as-deposited metal was consistently found in both cases. Upon annealing the As/Ga ratio is found to decrease for Al contacts, while it is found to increase for Au contacts. The orientation relationship of the Au contacts changed upon annealing. This was not observed with the annealed Al. The orientation relationship depended strongly on the surface preparation before the metal deposition and on the annealing environment. It was proposed that the presence of As at the interface can determine the orientation relationship between metals and GaAs.

Electrical characteristics of metal/Si-Ge contacts

Measurements and theoretical calculations have been made of the electrical properties of metal contacts to a polycrystalline Si-Ge alloy (80 at.% Si) for a wide variety of conditions. Thin-film contacts of W, Pb, and In have been formed on Si-Ge phosphorus doped in the range 2×1018−1×1020 cm−3. For P concentrations near 2×1019 cm−3 we have produced samples having several interfacial oxide thicknesses. We have heat treated W contacts to this material to simulate elevated-temperature aging effects of W thin-film contacts on thermopiles. The differential conductance for all these various contacts has been measured as a function of bias voltage for temperatures from 1.1 to 300 K. Calculations of the differential conductance of these contacts have been performed using a metal-insulator-semiconductor model that accounts for tunneling, thermionic field emission, and thermionic emission. The calculationg generally cover the ranges of temperature, bias voltage, doping level, and oxide thickness used in the experiments. Graphical summaries of these calculations are presented which include doping dependence of the zero-bias conductance ratio (300−4 K), the conductance values at several temperatures, and the location of the differential conductance minimum. Comparison of theory with experiment leads to the conclusion that the general features of the conductance characteristics can be predicted from the theory for certain ranges of oxide thickness and dopant concentration. Agreement appears to be best for thin oxides and P concentrations near 2×1019 cm−3, with both higher- and lower-doped contacts behaving qualitatively as if their effective doping levels were nearer this value. This effect has interpreted in terms of phosphorus depletion or enhancement mechanisms occurring in the Si-Ge depletion region during junction fabrication. When W contacts are hear treated after fabrication, agreement with theory improves. This believed to exhibit typical superconducting conductances at 1.1 K, thus showing that tunneling is, in fact, a major transport mechanism in Si-Ge/metal junctions.