Electrochemical Property of Boron-Doped Heteroepitaxial Diamond Treated By Different Oxidation Method

Abstract

Boron-doped diamond electrode is expected to realize high sensitive electrochemical sensors because of its unique properties such as wide potential window and low background current. Many studies of diamond electrochemistry have been already reported but most of these works were carried out using polycrystalline diamond. Because several crystal faces and many grain boundary exists, electrochemical reactions at the surface of polycrystalline diamond becomes complex. On the other hand, epitaxial diamond (nearly single crystal) is structurally uniform, we believe that boron-doped epitaxial diamond will be more ideal material in order to realize highly-sensitive and highly-reliable sensor. Furthermore, it is useful to clarify basic electrochemical reactions at the surface of diamond. Several studies have been already reported using boron-doped homoepitaxial diamond. In the case of homoepitaxial growth, HPHT (high pressure high temperature) diamond is used for substrate but available size of HPHT diamond is small and also costs much. On the other hand, heteroepitaxial growth is much easier to fabricate diamond in large size at low cost. From the viewpoint of practical use, studies on boron-doped heteroepitaxial diamond will be important. In this study, fundamental electrochemical property of hydrogen-terminated and oxygen-terminated boron-doped heteroepitaxial diamond was evaluated. There are several methods to change the surface termination of diamond from hydrogen to oxygen such as heat mixed acid, oxygen plasma, UV irradiation, anodic oxidation treatment. The treatment conditions were optimized at each method and the oxidation peak shift of potassium ferrocyanide was compared. First, non-doped epitaxial diamond(100) was grown on Ir(100)/MgO(100) substrate in thickness of 100 μm by direct current plasma enhanced chemical vapor deposition (PE-CVD) method. After the growth, Ir/MgO substrate was removed by acid treatment in order to fabricate free-standing diamond platelet. Finally, boron-doped epitaxial diamond was grown on non-doped epitaxial diamond at boron concentration of 5000 ppm. Oxidation peak potential of potassium ferrocyanide (5mM in 0.1 M sodium sulfate) was measured by cyclic voltammetry after oxidation treatment under defined condition. To optimize oxidation treatment condition, this operation was repeated until the peak potential becomes constant. For heat mixed acid treatment, sample was dipped in mixed acid (60% nitric acid:95% sulfuric acid = 3:1) at 523 K for 15 min. For anodic oxidation treatment, 0.5 M sulfuric acid was used for electrolyte and the potential was kept at 3.0 V for 5 s. From the X-ray diffraction measurement, single sharp peak at around 120 degrees which is due to diamond(004) was detected from 2theta-omega spectrum and four-fold symmetry peaks were detected from diamond{111} pole figure which indicates that the diamond was epitaxially grown on Ir/MgO substrate. When the sample was terminated by hydrogen, sharp oxidation and reduction peaks were observed from the cyclic voltammogram and its peak separation was around 70 mV which reflects a quasi-reversible reaction (very close to reversible reaction). Shift of oxidation peak was observed after both heat mixed acid and anodic oxidation treatment. By repeating heat mixed acid treatment for four times (total 60 min treatment), oxidation peak became constant and its potential was 0.42 V vs Ag/AgCl. On the other hand, repeating anodic oxidation treatment for six times (total 30 s treatment), the peak became constant at potential of 1.52 V. Heat mixed acid treatment is a well-known method to remove surface conductive layer of diamond but the peak potential at optimized treatment condition was extremely different between two methods which indicates that termination structure differs. Termination structures analyzed by X-ray photoelectron spectroscopy will be discussed. Furthermore, the results of other methods such as oxygen plasma and UV irradiation treatment will be also discussed.