Engineering low resistance contacts on p-type hydrogenated diamond surfaces

Abstract

Abstract Gold is expected to form a relatively low barrier on hydrogenated thin film diamond, and this metallisation has therefore been widely used as the ‘ohmic’ contact for electronic devices fabricated using this material. However, gold contacts are not truly ‘ohmic’ and suffer from reliability problems associated with poor adhesion to the diamond surface. Furthermore, the contact 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 resistance (SCR) of differing metallisation schemes on hydrogenated p-type CVD diamond. 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°C, acid and plasma treatments. Our measurements show that the simple gold contacts exhibit an SCR of 0.04 Ω cm 2 with an associated barrier height of 0.39–0.44 eV. These values were obtained for films displaying carrier concentrations of 10 17 –10 19 cm −3 . Low specific contact resistance is important for effective device operation, especially in high power situations; previously, on boron doped material, a reacted (carbide) interface has been shown to improve contact resistance and mechanical integrity — typically Au/Ti is used. Our studies show that Au/Ti contacts can be formed on hydrogenated diamond but display rectifying characteristics up to annealing temperatures of 250°C. However, a low resistance reacted ohmic contact (SCR ∼0.02 Ω cm 2 ) can be formed if the anneal temperature is increased to 600°C and is then followed by a careful rehydrogenation step to replace the carriers which are lost during the annealing.