Nitrogen-Doping Induced Optical Bandgap Widening of P -Type Cu 2 O Flms
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1217-Y03-38
Nitrogen-Doping Induced Optical Bandgap Widening of P-Type Cu2O Flms Yoshitaka Nakano, Shu Saeki, and Takeshi Morikawa TOYOTA Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ABSTRACT We have investigated the effect of N doping into Cu2O films deposited by reactive magnetron sputtering. With increasing N-doping concentration up to 3 at.%, the optical bandgap energy is enlarged from ~2.1 to ~2.5 eV with retaining p-type conductivity as determined by optical absorption and Hall-effect measurements. Additionally, photoelectron spectroscopy in air measurements shows an increase in the valence and conduction band shifts with N doping. These experimental results demonstrate possible optical bandgap widening of p-type N-doped Cu2O films, which is a phenomenon that is probably associated with significant structural changes induced by N doping, as suggested from x-ray diffraction measurements. INTRODUCTION Cuprous oxide (Cu2O) is a direct-gap semiconductor with a bandgap energy of ~2.1 eV and spontaneously shows p-type conductivity due to the presence of negatively charged copper vacancies and probably interstitial oxygen [1-3]. Cu2O has been regarded as one of the most promising materials for application to photovoltaic cells because of its high absorption coefficient, nontoxicity, and low-cost producibility [4,5]. In this case, the general strategy relies on the formation of an electron-hole pair upon absorption of a photon by visible-light sensitive Cu2O. From the same viewpoint of solar energy conversion, Cu2O can be one of good p-type candidates for semiconductor-based photocatalysis and/or photoelectrolysis [6-8]. Particularly for the application of solar hydrogen production from water, the energy band structure of p-type Cu2O needs to be modified to position the conduction and valence band edges on optimal levels where the conduction band must be above 0 V vs. normal hydrogen electrode (NHE) to produce H2 with high efficiency, and the valence band must be below +1.2 V vs. NHE to produce O2 [6]. Thus, the modification of the Cu2O band structure will count for facilitating photoredox processes after the electron-hole formation. In general, most oxides exhibit poor mobility of holes in the valence band, because the O 2p states of the upper valence band are localized. Exceptionally, in Cu2O, the top of the valence band states are derived from fully occupied Cu 3d states close to the O 2p states and are more mobile when converted into holes [3]. Interestingly, p-type transparent conductive oxides (TCOs) based on Cu2O such as CuAlO2, CuGaO2, and SrCu2O2 are modified their energy band structures to reduce oxygen mediated d-d coupling between the Cu atoms, resulting in enlarging their optical bandgaps [9-11]. Conversely, the top of the valence band of Cu2O can be narrowed by reducing Cu d-d interactions, widening the bandgap. Furthermore, from another standpoint of bandgap engineering, our group has recently reported that the introduction of N as a dopant into substitutional O sites in TiO2 was eff
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