Contact resistance | EPFL Graph Search

The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is described by the term electrical contact resistance (ECR) and arises as the result of the limited areas of true contact at an interface and the presence of resistive surface films or oxide layers. ECR may vary with time, most often decreasing, in a process known as resistance creep. The idea of potential drop on the injection electrode was introduced by William Shockley to explain the difference between the experimental results and the model of gradual channel approximation. In addition to the term ECR, interface resistance, transitional resistance, or just simply correction term are also used. The term parasitic resistance is used as a more general term, of which it is usually assumed that contact resistance is a major compo

Edge-contacted graphene with slope engineering by stencil lithography

Farzad Rezaeianaran

One approach to overcome the high contact resistance between 3D metals and 2D materials/heterostructures is employing edge contacts [1]. A low-resistance edge contact on boron nitride/graphene/boron nitride heterostructure using photolithography has been demonstrated in 2013[1]. The aim of this project is therefore to simplify the process by replacing the photolithography step with stencil lithography. In the first part of the project, the focus is on the transferring of chemical vapor grown graphene to SiO2/Si wafers. We were able to make two graphene transfers; however, in both cases the graphene had many cracks and tears which would distort any electrical measurements. Therefore, we used these samples for experimenting with etching of HfO2 and slope engineering and showed that ion beam etching is suitable for both purposes. Later, graphene on SiO2/Si chips were purchased which were accordingly processed and used for electrical characterization. Some of the (transfer length measurement) patterns showed conductive behavior; however, the measured contact resistance was considerably high (8kΩ). Further studies are required to explain the high contact resistance observed. A good starting point would be to obtain a cross-section sample from the contact area using focused ion beam and study the said area using transmission electron microscopy.

2018

Contact Resistivity Measurements and their Applicability for Accurate Series Resistance Breakdown in Heterojunction Solar Cell

Luca Massimiliano Antognini, Christophe Ballif, Mathieu Gérard Boccard, Matthieu Despeisse, Julie Amandine Dreon, Lison Sylou Marthey, Bertrand Yves Paul Paviet-Salomon, Laurie-Lou Senaud, Deniz Türkay

In this work, we use several approaches to perform accurate Series Resistance (R-S) breakdown of a state of the art 2 cm x 2 cm screen-printed solar cell reaching 82.5% FF. On the one hand, Haschke et al.'s model for the lateral transport through the cell, coupling the TCO and wafer sheet resistances through the contact resistivity (rho(C)), predicts a reduction of R-S with increasing injection (Delta n) through the enhanced wafer conductivity [Haschke et al., J. Appl. Phys., 2020]. In contrast, we observe that the R-S of the solar cell, obtained from the difference between the J-V and Jsc-Voc curves, increases with Delta n. Similarly, Senaud et al. observed increasing rho(C) with Delta n using TLM measurements under illumination [Senaud etal., EU-PVSEC, 2020]. To investigate the discrepancy between these experimental observations and theoretical expectations, we extract rho(C) values either from dark TLM, illuminated TLM or illuminated symmetrical sample measurements and incorporate them into Haschke et al.'s model to reconstruct the R-S of the solar cell. Detailed series resistance breakdown using rho(C) values from all three tested methods show accurate R-S predictions within at MPP +/- 0.1 Omega cm(2), showing that the different approaches have sufficient accuracy to estimate the resistive losses in the solar cell under study. Regarding dependence upon injection, it was only possible to predict an increasing series-resistance trend using contact resistivity from illuminated TLM, therefore matching closer the measured linear increase of the R-S of the solar cell from MPP to open circuit conditions, while the other methods predicted a decreasing trend. We discuss practical differences between the methods and propose possible improvements.

AMER INST PHYSICS2022

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

We present a theoretical study of the physical characteristics of metal/semiconductor junctions. Using first principle pseudopotential calculations, we have investigated the nature of electronic states with energies within the semiconductor band gap of representative abrupt, defect-free, anion-terminated metal/III-V interfaces. Namely, we focused on Al contacts to GaAs(001), AlAs(001) and cubic GaN(001) as well as on Al, Au and Cu junctions to cubic GaN(001). Recent advances in Schottky barrier concepts emphasize the possible relationship between interface states and the formation of the Schottky barrier. We aim at understanding the atomic-scale mechanisms responsible for interface states as well as their role in the Schottky barrier formation process. At As-terminated Al/GaAs(001) and Al/AlAs(001) junctions, resonant and localized interface states occur at the J point of the interface 2D Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between the outermost cation atoms of the semiconductor and the interfacial Al atoms of the metal. These interface states derive from an interaction between localized states of the Al(001) surface and semiconductor conduction band states, mediated by localized states of the unreconstructed, As-terminated semiconductor (001) surface. Our results indicate that interface states of the intermetallic, bonding-like kind could play an important role in the transport properties of metal/AlxGa1-xAs junctions. We have also investigated the electronic structure of Al, Au and Cu junctions to cubic, N-terminated GaN(001). The localized interface state reported for As-terminated Al/GaAs(001) and Al/AlAs(001) junctions occurs also at metal/GaN interfaces under the condition that atoms on the outermost atomic plane of the metal are placed in front of the outermost semiconductor cation. This indicates that the formation mechanism of this state is a very general one. In contrast to Al/AlxGa1-xAs junctions, these states occur at energy much larger than EF for the contacts to GaN. Thus, they are not expected to contribute significantly to the electronic transport of the latter interfaces. However, a large number of interface states attributed to d-type orbitals occur over a wide energy range including EF at contacts of noble metals to GaN.

EPFL2003

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

EPFL2003

Contact Resistivity Measurements and their Applicability for Accurate Series Resistance Breakdown in Heterojunction Solar Cell

AMER INST PHYSICS2022