Engineering low resistance contacts on p-type hydrogenated diamond surfaces

Gold is expected to form a relatively low barrier on hydrogenated thin film diamond, and this metallisation has therefore been widely used as the 'ohm ic' contact for electronic devices fabricated using this material. However, gold contacts are not truly 'ohmic' and suffer from reliability problems a ssociated with poor adhesion to the diamond surface. Furthermore, the conta ct properties of this system have not been studied in any detail. For the first time we report the results of a study, carried out using the circular transmission line method, to explore the specific contact resistan ce (SCR) of differing metallisation schemes on hydrogenated p-type CVD diam ond. Gold, aluminium and reacted metal-carbide (Au/Ti, Al/Ti) contacts have been characterised. The effects of sample/contact pre- and post-treatments have been studied, including annealing to 600 degrees C, acid and plasma t reatments. Our measurements show that the simple gold contacts exhibit an S CR of 0.04 Omega cm(2) with an associated barrier height of 0.39-0.44 eV. T hese values were obtained for films displaying carrier concentrations of 10 (17)-10(19) cm(-3). Low specific contact resistance is important for effect ive device operation, especially in high power situations; previously, on b oron doped material, a reacted (carbide) interface has been shown to improv e contact resistance and mechanical integrity - typically Au/Ti is used. Ou r studies show that Au/Ti contacts can be formed on hydrogenated diamond bu t display rectifying characteristics up to annealing temperatures of 250 de grees C. However, a low resistance reacted ohmic contact (SCR similar to 0. 02 Omega cm(2)) can be formed if the anneal temperature is increased to 600 degrees C and is then followed by a careful rehydrogenation step to replac e the carriers which are lost during the annealing. (C) 2000 Elsevier Scien ce S.A. All rights reserved.