2, Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo, , Japan
3, RIKEN, Saitama, , Japan
4, Global Solar Plus Initiative, The University of Tokyo, Tokyo, , Japan
In semiconductor photoelectrochemistry, surface states behave as both recombination centers and the site of charge transfer to a redox system [1, 2], and their characterization is vital. Impedance spectroscopy is often employed for this purpose but it can modify the surface due to reaction current. We therefore try to characterize surface states by observing open-circuit potential (OCP) as a function of irradiation light intensity. This method, which we call as illuminative OCP spectroscopy, will provides us the information on the density of surface states (DoSS) with the minimum impact of reaction current.
As a light source, He-Cd laser (325 nm, within an absorption band of GaN) was employed. In contrast to our previous study with Xe-lamp illumination , light intensity absorbed by GaN was controlled precisely in a wide range from 1 μW/cm2 to 175 mW/cm2. The OCP of n-GaN photoelectrode, corresponding to electron fermi level, almost reached the flat-band potential (FBP) at a light intensity of 100 mW/cm2, a similar behavior as reported in the literature .
The precise correlation between light intensity and OCP clarified a tendency that OCP is bound to a specific value at low light intensity range. The energy to which OCP was bound would correspond to the energy peak of DoSS. In order to confirm this hypothesis, we tried to increase DoSS intentionally by the exposure to Ar plasma, and characterized DoSS for “damaged” and “non-damaged” epitaxial n-GaN photoelectrodes by impedance spectroscopy, in order to explain the energy level at which OCP was bound.
The OCP of “non-damaged” n-GaN electrode was bound at -0.8 V v.s. Ag/AgCl under the light intensity range from 10-3 to 10-2 mW/cm2. After impedance measurement, this value moved to -0.5 V v.s. Ag/AgCl, indicating that even a small reaction current during impedance measurement modifies the surface of GaN. Conversely, the proposed method of illuminative OCP spectroscopy can detect such a subtle change in the surface states. For the “damaged” n-GaN electrode, OCP stayed at -0.2 V v.s. Ag/AgCl in a much wider intensity range from 10-3 to 10 mW/cm2 than the case of “non-damaged” GaN, suggesting that DoSS in the damaged GaN is larger than the non-damaged GaN. For both samples, the energy values to which OCP was bound agreed with the peak energy of DoSS obtained by impedance spectroscopy.
In summary, it is probable that illuminative OCP spectroscopy provides information corresponding to DoSS with the minimum surface damage during measurement. Combining it with impedance analysis, which is more established though destructive, we can analyze DoSS for a variety of semiconductor photoelectrodes in a more comprehensive manner.
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