Electrochemical Properties of Polycrystalline Phosphorus-Doped Diamond

Abstract

Introduction Since a p-type boron-doped diamond (BDD) has excellent electrochemical properties, such as wide potential window, low background current, and so on, it has been investigated for both of fundamentals and electrochemical applications1. On the other hand, few studies have reported on the electrochemical properties of n-type diamond. The electrons in the conduction band of the diamond have the quite high reducing ability because it is situated at much higher energy level than that of other semiconductors. Hence, the n-type diamond electrode may present new electrochemical properties. In this work, we study on the preparation and the characterization of polycrystalline phosphorus-doped diamond (PDD), which is the most promising candidate for n-type diamond, and investigate the electrochemical properties of PDD. Experimental The polycrystalline PDD films were deposited on Si wafer substrates by a microwave plasma-assisted chemical vapor deposition system in following steps. Firstly, undoped diamond layer was deposited in CH4/H2 plasma for 4 hours. Subsequently, red phosphorus sublimed at 400ºC was introduced into CH4/H2 plasma for 1 hour in order to deposit PDD layer. The surface morphology was characterized by scanning electron microscopy (SEM), and structure was investigated by Raman spectroscopy and X-ray diffraction (XRD). Phosphorus doping level was evaluated by secondary ion mass spectroscopy (SIMS). The hall effect measurement was performed using van der Pauw method. Electrical contacts were made by sputtering Ti and Au on the PDD surface, followed by annealing in vacuum at 450ºC for 45 min to form the Ti-C layer. Electrochemical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in an aqueous solution of 0.1 M H2SO4. Mott-Schottky plots were obtained by EIS measurements. Moreover, these results were compared with electrochemical properties of BDD electrode. Results and discussion The SEM images showed a polycrystalline nature with grain size of ca. 2 μm. Raman spectrum showed a sharp peak from center zone optical phonon of diamond at 1332 cm−1 and a wide band from sp2-bonded carbon at around 1500 cm−1. Phosphorus concentration in the synthesized film was 1.36×1018 cm−3. From Hall effect measurement, it was confirmed that the PDD film was n-type semiconductor because hall coefficient was negative. From the CV measurement, the cathodic current was much larger than the anodic current, which is a characteristic of the interface between n-type semiconductor and solution (Fig. 1). Since p-type BDD showed the reverse behavior, this characteristic is ascribed to the influence of phosphorus doping. In order to estimate the position of band edges of PDD and BDD, the space charge capacitance C was measured by EIS measurement. The obtained C as a function of potential E was fitted with Mott-Schottky equation (1): 1/C 2 = (±2/eε0εSCN)(E−Efb −kT/e) (1) where the plus and minus signs represent the type of semiconductors, ε0 and εSC are the permittivities of free space and diamond, respectively, e is the electron charge, and N is the donor or acceptor density. The Mott-Schottky plots obtained by EIS measurement (Fig. 2) revealed that the synthesized PDD film is n-type semiconductor because a positive slope was obtained. Moreover, from the intercept of the potential axis, flat band potential (Efb ) of the PDD and BDD was estimated because kT/e is almost zero at room temperature (298 K). These obtained Efb values were used to determine the band edge positions. The difference between Efb and EVB of BDD was assumed to be ~0.4 eV from the acceptor level of boron. On the other hand, the difference between Efb and ECB of PDD was assumed to be ~0.6 eV from the donor level of phosphorus. From the result of comparing PDD and BDD, the band edges of PDD were found to agree well with the ones of BDD and also the single-crystalline PDD reported on the previous research2. References (1) Y. Einaga, J. Appl. Electrochem. 40. 1807 (2010). (2) Y. Mukuda, T. Watanabe, A. Ueda, Y. Nishibayashi, and Y. Einaga, Electrochim. Acta 179. 599 (2015). Figure 1