Example from current research
Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films
The surface composition of Cu(In,Ga)(S,Se)2 (“CIGSSe”) thin films intrinsically deviates from the corresponding bulk composition, which also modifies the electronic structure and thus the optical properties. We used a combination of photon and electron spectroscopies with different information depths to gain depth-resolved information on the band gap energy (Eg) in CIG(S)Se thin films. We find an increasing Eg with decreasing information depth, indicating the formation of a surface region with significantly higher Eg. This Eg-widened surface region seems to extend further into the bulk of the sulfur-free CIGSe thin film compared to the CIGSSe thin film.
Left: optical reflection spectra of CIGSe (top) and CIGSSe (bottom) thin films, together with the respective envelopes of the interference extrema and the constructed mean values (red dotted lines). Insets: Eg determination (±0.05 eV) using an approximated absorption coefficient for the CIGSe (top) and CIGSSe (bottom) samples.
Center: Cu L3 XES (left) and XAS (right) spectra of the investigated CIGSe (middle spectra) and CIGSSe (bottom spectra) samples and of a metallic Cu reference (top). The band gap energies “Eg” (i.e., lower bounds for the true band gap values) are determined by linear extrapolation of the leading edges (±0.20 eV), indicated by the solid green and red lines.
Right: UPS (left) and IPES (right) of the CIGSe (top) and CIGSSe (bottom) samples. Spectra are shown for the as-introduced (thin solid lines) and for the cleaned samples (dots). The solid green and red lines indicate the linear extrapolation of the leading edges to determine Eg (±0.15 eV).
Schematic presentation of the determined CIGSe (top) and CIGSSe (bottom) band gap energies using the three different spectroscopic approaches (optical reflection, left; XES and XAS, center; UPS and IPES, right). The respective information depths of the applied spectroscopies are also shown. The gray area represents the measurement error. CBM (VBM) denotes the conduction band minimum (valence band maximum).
For more details see:
M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, O. Fuchs, M. Blum, W. Yang, J.D. Denlinger, W.N. Shafarman, and C. Heske, Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films, Appl. Phys. Lett. 93, 244103 (2008).
