Characterization Of Ohmic Contacts Research Articles

Extension of Shockley’s transmission line method (TLM) to characterize ohmic contacts

A method to extract the resistance of the metallic pad of ohmic contacts is presented. It utilizes the transmission line method array of ohmic contacts and measurements required to obtain their specific contact resistance plus just an additional current voltage measure. The method was applied to fully characterize, included their reliability, standard ohmic contacts on p-GaAs and n-GaInP thin epitaxial layers whose measured sheet resistances were 96 and 6.86 (Ohms Sq–1) respectively. Our extended method found contact pad sheet resistances of RshMP = 0.9 and 0.09 (Ohms Sq–1) for those on p-GaAs and n-InGaP, respectively, giving a ratio of 1%–2%, in this case. For their reliability it was found that after 430 °C for a 30 min thermal anneal, those on n-InGaP strongly degrade. The degradation initiates increasing the metallic pad sheet resistance and then the contact resistance, while those on p-GaAs remain unchanged. This method should contribute to a better characterization of ohmic contacts.

Characterization of ohmic contacts in polymer organic field-effect transistors

It is well known that contact resistance Rc limits the performance of organic field-effect transistors (OFETs) that have high field-effect mobilities (μFET ≳ 0.3 cm2 V−1 s−1) and short channel lengths (Lch ≲ 30 μm). The usual transfer-line method (TLM) to analyze Rc calls for extrapolation of total resistance to zero Lch at constant drain and gate voltages. This requires an unrealistic assumption that Rc does not vary with source−drain current Isd (nor with channel carrier density σ). Here we describe a self-consistent TLM analysis that instead imposes the condition of constant Isd and σ. The results explicitly reveal the dependence of Rc on Isd and σ. We further describe how this Rc(Isd, σ) surface can be modelled to yield the specific contact resistivity ρc of the metal/organic semiconductor (OSC) interface, a key parameter that has so far been neglected in OFETs. We illustrate the application of these analyses to high-performance staggered top-gate bottom-contact poly(2,5-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole-3,6-diyl-terthiophene-2,5″-diyl) (DPPT2-T) OFETs fabricated on bottom Au source–drain electrode arrays, with high contact-corrected μFET of 0.5 cm2 V−1 s−1. We show that when these electrodes are modified to impose weak, and then strong hole-doping of the DPPT2-T interface, Rc diminishes and its dispersion, i.e. dependence on Isd and σ, weakens. The ultimate ρc attained for the strongly hole-doped contact is ca. 1 Ω cm2, broadly independent of Isd and σ, which we propose is a hallmark of a true metal/OSC ohmic contact. For comparison, the bare Au/DPPT2-T contact gives ρc of the order of 10 Ω cm2 with a marked σ dependence. The lowest ρc reached here shortens the current transfer length down to ca. 5 μm, enabling short electrode lengths to be advantageously employed in technology.

Ohmic contact on single ZnO nanowires grown by MOCVD

AbstractWe present work on the fabrication and the characterization of ohmic contacts on single zinc oxide nanowires grown by metalorganic chemical vapour deposition. We demonstrate pre‐positionning of single nanowires before electronic‐beam lithography for 2 point metallic contacts. We characterize single nanowires by microphotoluminescence and by current‐voltage measurements. I–V results present a linear curve down to 1 mV with resistivities from 0.23 to 2.4 Ω ċ cm are measured, consistent with previous observation on ZnO nanowires. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Ohmic contacts on GaN/AlGaN heterostructures

The surface morphology and electrical resistance of the contacts on semiconductor devices are strongly influenced by metallization scheme and annealing conditions. In this work is presented an investigation of Ohmic contacts formed by metal–semiconductor alloying on epitaxial GaN/AlGaN heterostructures. After the deposition of metallic multi-layers (Ti, Al, Au and Pt), the process of rapid thermal annealing was carried out in nitrogen, argon and forming gas atmosphere. A series of the samples with different sequences of metallic layers and diverse thicknesses was prepared by employing electron beam evaporation and lift-off deposition techniques. The chemical composition of the samples before and after annealing was studied by means of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques combined with low energy Ar + ion sputtering. The sputtering has been carried out in two different modes: by using constant (square) or gradually narrowed sputtered area. The changes in the chemical state of constituent elements and compositional profiles of the contacts after thermal annealing were revealed from the obtained results. Among the technological problems, influencing on the quality of the contact, were found to be the oxidation and nitridation of the contact surface during thermal annealing, as well as the intermediate sub-layers of Al and Ti oxides, formed during the deposition of metallic multi-layered structure.

Characterization of ohmic contacts on n- and p-type GaSb

Ohmic contacts on GaSb were achieved using ZnAu multilayers for p-type GaSb and TeAu multilayers for n-type GaSb. Contacts were electrically characterized by their specific resistance, which could be obtained currently as low as 10 −5 Ω cm 2 for p-type GaSb and 10 −6 Ω cm 2 for n-type GaSb. The surface composition was studied by means of secondary ion mass spectrometry, which showed that interface alloying gives the best results.

Formation and characterization of ohmic contacts to n–AlGaAs using Pd/AuGe/Ag/Au

Sequential deposition of Pd/AuGe/Ag/Au and rapid thermal annealing at 450 to 550 °C were employed to form a shallow ohmic contact to n–Alx Ga1−x As. Contact resistivity was in the range of (0.7–1) × 10−5 at x = 0.3 to (2–4) × 10−5 ohm-cm2 at x = 0.55. Limited interfacial reaction was verified by ion backscattering and x-ray diffraction techniques and attributed to the stability of Au–Ag solid solutions against underlying substrates. Contacts form at 450 to 500 °C, possibly via solid-state reaction, whereas liquid phase reaction may take place at 550 °C.

Formation and Characterization of Ohmic Contacts on Diamond

ABSTRACTLow resistance ohmic contacts have been fabricated on a naturally occurring lib diamond crystal and on polycrystalline diamond films by B ion-implantation and subsequent Ti/Au bilayer metallization. A high B concentration was obtained at the surface by ion implantation, a postimplant anneal and a subsequent chemical removal of the graphite layer. A bilayer metallization of Ti followed by Au, annealed at 850°C, yielded specific contact resistance values (as measured using a standard transmission line model (TLM) pattern) of the order of 10-5Ω cm2for chemical vapor deposition (CVD) grown polycrystalline films and the natural lib crystal. Specific contact resistance values have also been determined from circular TLM measurements on CVD films and the values compared to those from standard TLM measurements. These contacts were stable to a measurement temperature of ∼400°C and no degradation due to temperature cycling was observed.