Defects Near c-Si(n ++ )/TiO 2 Interfaces Revealed By Persistent Charging Analysis in Modulated Surface Photovoltage Spectroscopy

Thin layers of semiconductors with wide band gaps (wide gap SCs), such as TiO 2 , are of great interest for the passivation and/or specific activation of semiconductor surfaces in photoelectrochemical (PEC) systems [1]. Defect states in the bulk of wide gap SCs and at related semiconductor hetero-junctions play an important role for the electronic properties of PEC systems. Due to high activation energies in wide gap SCs, electronic states can be charged for relatively long times. This persistent charging of wide gap SCs can have large impact on photogeneration and modulated charge separation in modulated surface photovoltage (SPV) spectroscopy and thus opens a new perspective for the investigation of electronic defect states at/near interfaces with wide gap SCs. In this work, persistent charging is applied to the investigation of defect states in c-Si(n ++ )/TiO 2 (ALD) systems before and after conversion from amorphous TiO 2 to anatase by post annealing. Thin layers of amorphous TiO 2 with thicknesses of up to 100nm were deposited onto highly doped silicon wafers (c-Si(n ++ )) and converted into anatase by annealing for 30 min at 450°C. Modulated SPV measurements were performed at room temperature in the fixed capacitor arrangement [2] with a quartz prism monochromator and a xenon arc lamp for excitation over a range from 0.8 to 4.5 eV. Incidentally, the in-phase (X) or phase-shifted by 90° (Y) signals are much faster or slower, respectively, than the modulation period (see for details also [3]). The X- and Y- signals of two modulated SPV spectra were measured subsequently within about 30 min. This kind of measurement regime opened the opportunity for the analysis of the influence of charging on electronic transitions of defect states in c-Si(n ++ )/TiO 2 (ALD) systems and it will be straight forward to compare those results with the optical properties of holes and electrons trapped in TiO 2 [4]. As an example, the figure shows modulated SPV spectra of an as prepared (a) and annealed (b) sample. The X-signals of the as-prepared sample where quenched with ongoing absorption in amorphous TiO 2 whereas the onset energy for quenching shifted by more than 0.1 eV towards lower energies for the second scan. In contrast, the Y-signals of the as-prepared sample became more negative between about 3 and 3.5 eV and changed to more positive and changed sign at higher photon energies. Electrons photogenerated in amorphous TiO 2 near the interface with c-Si were transf