Dependence of the Electrochemical Behavior of p-WSe2 Single Crystals on Ohmic Contacts

Abstract

With the development of a wide variety of semiconductor devices (for instance injection lasers, light emitting diodes, microwave Schottky barriers, ®eld effect transistors, photovoltaic cells etc), the need for low resistance, reproducible, and stable ohmic contacts is increasingly felt. In fact, gradual degradation in many devices seems, to a large extent, to be related to the contact problems. A number of detailed investigations and reviews on the realization of ohmic contacts on different semiconductors have appeared [1]. However, experimental ®ndings indicate that achieving a good, reliable, low resistance ohmic contact is still an art. In the last two decades, electrochemical behavior of layered structure semiconductor=liquid junctions have been intensively investigated both in the dark and under illumination. Among the crystals that have a layered structure, semiconductor transition metal dichalcogenides (TMDC) electrodes used in solar cells, particularly MoSe2 and WSe2 have attracted much interest. Because the back ohmic contacts signi®cantly affect the photoresponse of a photoelectrochemical (PEC) solar cell, various investigators have tried several ohmic contact materials in the preparation of their photoelectrodes. In most of the cases, the electrical back ohmic contacts were made with Ag paint and Ag epoxy. In the present study, the authors tried various metal contacts to determine an appropriate ohmic contact for p-WSe2 layered single crystals. The effect of the metallic materials providing the best ohmic contact on photoconversion has then been compared with PEC solar cells prepared with usual silver paint back contacts. Single crystals used in the present work were grown by a direct vapor transport (DVT) technique. To prepare the ohmic contacts to p-WSe2 single crystals, the following procedure was adopted. Before starting the contact preparation, crystals were thoroughly cleaned: ®rst in trichloroethylene, then in acetone, and ®nally in methanol solution. Contact metals, usually Ag, Au, Ti-Ag, and In, were then coated individually at four different points on the crystal surface, maintaining a linear geometry in the vacuum coating unit at a pressure better than 10y3 Pa. To provide contacts to the deposited metal on the crystal specimen, a specially prepared alloy of silver was used. Copper wires were then ®xed to metal contacts using this alloy, with the help of a soldering gun. Current and voltage measurements were carried out using a Keithley electrometer. To evaluate speci®c contact resistance for different contacting materials, the current-voltage (I-V) characteristics obtained for p-WSe2 single crystals with usual Ag-paste and different metal contacts, In, Au, Ti-Ag, and Ag-metal, are given in Figs 1 to 4, respectively. The values of speci®c contact resistance obtained from these graphs for each of the metal contacts are listed in Table I. In each case, the value of contact resistance was determined by taking the slope near the origin.