Defects Near c-Si(n++)/TiO2 Interfaces Revealed By Persistent Charging Analysis in Modulated Surface Photovoltage Spectroscopy
Authors: Fengler Steffen , Kriegel Herman , Schieda Mauricio , Gutzmann Henning , Klassen Thomas , Dittrich Thomas 
Published: May 2020

Thin layers of semiconductors with wide band gaps (wide gap SCs), such as TiO2, 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++)/TiO2(ALD) systems before and after conversion from amorphous TiO2 to anatase by post annealing.
Thin layers of amorphous TiO2 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++)/TiO2(ALD) systems and it will be straight forward to compare those results with the optical properties of holes and electrons trapped in TiO2 [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 TiO2 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 TiO2 near the interface with c-Si were transferred into c-Si and partially trapped near the interface resulting in an increase of the SPV signals related to photogeneration in c-Si(n++) and in quenching of charge transfer from amorphous TiO2 into c-Si(n++) in the second scan.
For the annealed sample, the signals were much larger and modulated charge separation with opposite direction at the c-Si and TiO2 surfaces became dominant. The Y-signals for the virgin scan were larger than for the second scan at photon energies up to about 2.5 eV, but smaller for higher photon energies, i.e. photogeneration by defects in anatase caused an increase of positive charge at the interface with c-Si(n++) and of negative charge at the external surface whereas the signature of the hole traps nearly disappeared in the second scan shown in the figure.
Furthermore, the influence of the specific spectrum of the photon flux could be widely eliminated by analyzing the spectra of the ratio of the amplitudes before and after charging (not shown) which allowed for the extraction of defect peaks induced by charging.
Acknowledgement: The authors are grateful to A. Schwartzberg, D. Olynick and S. Cabrini for their insight and support for the preparation and coating of the samples by atomic layer deposition. The authors furthermore gratefully acknowledge financial support by the Federal Ministry of Education and Research of Germany, in the framework of the project "FocusH2" (No. 03SF0479A). The transient SPV spectroscopy instrumentation was developed at HZG with support from the Helmholtz Association of German Research Centres, within the "HEMF" platform (Helmholtz Energy Materials Foundry). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
[1] S. Hu et al., Science 344 (2014) 1005.
[2] V. Duzhko et al., Phys. Rev. B 64 (2001) 075204.
[3] Th. Dittrich, S. Fengler, Surface photovoltage analysis of photoactive materials, World Scientific. ISBN No. 978-1786347657 (2019).
[4] D. Bahnemann et al., J. Phys. Chem. 88 (1984) 709; Bahnemann et al., J. Phys. Chem. B 101 (1997) 4265; R. Memming, Semiconductor Electrochemistry, Wiley-VCH (2001 2nd ed. 2015); J. Schneider and D. Bahnemann, J. Phys. Chem. C 122 (2018) 13979.

Figure 1