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The energy band alignments at interfaces often determine the electrical functionality of a device. Along with the size reduction into the nanoscale, functional coatings become thinner than a nanometer. With the traditional analysis of the energy band alignment by in situ photoelectron spectroscopy, a critical film thickness is needed to determine the valence band offset. By making use of the Auger parameter, it becomes possible to determine the energy band alignment to coatings, which are only a few Angstrom thin. This is demonstrated with experimental data of Cu2O on different kinds of substrate materials.

We have investigated the electronic properties of the CuGaSe2∕In2S3 heterointerface by photoelectron spectroscopy. In2S3 was evaporated by physical vapor deposition onto contamination free polycrystalline CuGaSe2 surface prepared by the selenium decapping process. A valence band offset ΔEVB=0.78±0.1 has been determined.

The contact formation of thin-film epitaxial CuInSe₂(001) with a physical-vapor-deposited CdS layer is presented in this work. Synchrotron-excited photoelectron spectroscopy was used for this investigation. The epitaxial CuInSe₂ films contain a surface layer of reduced Cu stoichiometry similar to the ordered defect compound CuIn₃Se₅. A valence band offset of 0.79±0.15 eV has been determined for this heterojunction. The comparison to literature data indicates that neither surface orientation nor surface copper content have a major impact on the valence band offset of CuIn₃Se₅, respectively, CuInSe₂ with CdS.

The electronic structures and interface properties of the TiO₂/Cu₂S interface have been in situ studied after each growth step by x-ray and ultraviolet photoelectron spectroscopy. The p-doped Cu₂S films (BEVBM=0.1 eV) were grown on the highly n-doped chemical vapor deposition prepared TiO₂ (BEVBM=3.4 eV) films by thermal evaporation in a multistep growth procedure. The conduction band offset (0.7 eV), valence band offset (2.9 eV) and interface dipole (0.5 eV) were determined based on the quantitative examination of band bending occurring in the Cu2S films at higher coverage, leading to a staggered energy level configuration.

The band alignment at polycrystalline CdS/CdTe heterointerfaces for thin-film solar cells is determined by photoelectron spectroscopy from stepwise CdTe deposition on polycrystalline CdS substrates and from subsequent sputter depth profiling. Identical values of 0.94��0.05 eV for the valence band offset are obtained.

Surfaces of reactively evaporated In2O3 films were investigated in situ by synchrotron-excited photoemission. Work function, valence band maximum, and electronic states in the band gap were determined as a function of oxygen pressure. Surface and bulk electronic properties can only be explained consistently with the assumption of a surface depletion layer.

The growth of the layered chalcogenide GaSe on cleaved GaAs(110) surfaces was investigated with photoemission and low-energy electron diffraction (LEED). GaSe films grow with their c axis perpendicular to the GaAs(110) surface. LEED patterns after initial film growth are a superposition of rectangular GaAs:Se spots and two hexagonal domains rotated by ��5�� with respect to the GaAs ���001��� axis. At higher film thickness a hexagonal LEED pattern with GaSe���120��� ��� GaAs ���001��� is obtained.

The valence and conduction band structures of In2 O3 have been measured using a combination of valence band x-ray photoemission spectroscopy, O K -edge resonant x-ray emission spectroscopy, and O K -edge x-ray absorption spectroscopy. Excellent agreement is noted between the experimental spectra and O 2p partial density of states calculated within hybrid density functional theory. Our data are consistent with a direct band gap for In2 O3. © 2009 American Institute of Physics.

The formation of the ZnSe/CuGaSe₂ heterointerface was studied by x-ray photoelectron spectroscopy (XPS). ZnSe was sequentially grown on CuGaSe₂(001) epilayers. In situ photoemission spectra of the Ga 3d and Zn 3d core levels as well as XPS valence bands were acquired after each deposition step. The valence-band offset is determined to be ΔEV=0.6±0.1 eV. As a consequence, a nearly symmetric "type-I" band alignment for the ZnSe/CuGaSe₂ heterojunction with a conduction-band offset of ΔEC=0.4±0.1 eV is found. Concerning the band alignment ZnSe can, therefore, be expected to be a suitable buffer material for CuGaSe₂-based thin-film solar cells.

The interface formation between vacuum evaporated CdS and ultrahigh vacuum-cleaved CuInS2 single crystals has been studied by synchrotron excited photoelectron spectroscopy. The valence band discontinuity is determined directly from valence band difference spectra to be ΔEV=0.6 (±0.1) eV. This value is significantly smaller than for other preparation conditions given in the literature and evidently not suitable for solar cell applications. The similarity to observations at the CdS/CuInSe2 interfaces suggests that neutrality levels play a dominant role in establishing the band lineup at interfaces containing chalcopyrite semiconductors.

The interface between indium tin oxide and p-type silicon is studied by in situ X-ray photoelectron spectroscopy (XPS). This is done by performing XPS without breaking vacuum after deposition of ultrathin layers in sequences. Elemental tin and indium are shown to be present at the interface, both after 2 and 10 s of deposition. In addition, the silicon oxide layer at the interface is shown to be composed of mainly silicon suboxides rather than silicon dioxide.

Surface dipole modifications were identified for contamination-free In2O3-based transparent conducting oxides by ultraviolet photoelectron spectroscopy on both thin film and bulk ceramic specimens. In particular, heating in air was found to result in an increase in ionization potential and work function. The formation of surface dipoles may be related to the unique structure (crystal, defect) of bixbyite-based materials. These findings have important ramifications for the tuning of work functions in In2O3-based transparent conductors.