C. Pettenkofer

Band lineup of layered semiconductor heterointerfaces prepared by van der Waals epitaxy: Charge transfer correction term for the electron affinity rule

The occurrence of quantum dipoles at layered materials semiconductor heterointerfaces was investigated by photoemission spectroscopy (PES). Due to the unique properties of layered compounds the prepared interfaces are essentially free of the structural problems known from the usually investigated heterosystems composed of III–V, IV or II–VI materials allowing the detailed investigation of electronic phenomena at the interfaces. We investigated heterostructures composed of epitaxial layers of SnS2 and SnSe2 on different single crystalline layered chalcogenide substrates (WSe2, MoS2, MoTe2, and GaSe). The epilayers were grown by van der Waals epitaxy (vdWe) on the (0001) plane of the substrate crystals. For every system the valence band offset was determined by careful evaluation of the PES data as a function of the film thickness. Using published values for the band gaps and the experimentally determined work functions and surface potentials the band lineup for each system was determined. The band offsets ...

Formation and electronic properties of the CdS/CuInSe2 (011) heterointerface studied by synchrotron‐induced photoemission

The heterointerface p‐CuInSe2/CdS was investigated by soft x‐ray photoelectron spectroscopy. CdS was deposited sequentially in steps onto CuInSe2 (011) cleavage planes at room temperature (RT) and at elevated temperatures (≳120 °C). At RT a nonreactive interface to cubic CdS is formed. The valence band and conduction‐band discontinuities are determined to be 0.8 and 0.7 eV, respectively. A band bending of 0.9 eV is deduced for the p‐type substrate. Annealing to temperatures above 120 °C leads to the formation of a CuxS reactive layer at the interface. As a consequence the valence‐band offset and band bending is found to be considerably reduced. The experimentally determined band energy diagram is in agreement with heterojunctions of zincblende‐type semiconductors, and its consequences for solar cells are discussed.

Thin film growth and band lineup of In2O3 on the layered semiconductor InSe

Thin films of the transparent conducting oxide In2O3 have been prepared in ultrahigh vacuum by reactive evaporation of indium. X-ray diffraction, optical, and electrical measurements were used to characterize properties of films deposited on transparent insulating mica substrates under variation of the oxygen pressure. Photoelectron spectroscopy was used to investigate in situ the interface formation between In2O3 and the layered semiconductor InSe. For thick In2O3 films a work function of φ=4.3 eV and a surface Fermi level position of EF−EV=3.0 eV is determined, giving an ionization potential IP=7.3 eV and an electron affinity χ=3.7 eV. The interface exhibits a type I band alignment with ΔEV=2.05 eV, ΔEC=0.29 eV, and an interface dipole of δ=−0.55 eV.

Single crystalline GaSe/WSe2 heterointerfaces grown by van der Waals epitaxy. I. Growth conditions

Epitaxial GaSe films have been prepared on WSe2 (0001) substrates with 14% lattice mismatch and characterized by photoelectron spectroscopy, electron diffraction, and ex situ by tunneling microscopy. The films grow in the Frank–van der Merve growth mode. The best films with perfect azimuthal orientation are formed after an annealing step at 720 K. The basic mechanisms of this van der Waals epitaxy are qualitatively discussed in terms of thermodynamic and kinetic parameters.

Band lineup of lattice mismatched InSe/GaSe quantum well structures prepared by van der Waals epitaxy: Absence of interfacial dipoles

Epitaxial growth of the strongly lattice mismatched (6.5%) layered chalcogenides InSe and GaSe on each other is obtained with the concept of van der Waals epitaxy as proven by low‐energy electron diffraction and scanning tunnel microscope. InSe/GaSe/InSe and GaSe/InSe/GaSe quantum well structures were prepared by molecular beam epitaxy and their interface properties were characterized by soft x‐ray photoelectron spectroscopy. Valence and conduction band offsets are determined to be 0.1 and 0.9 eV, respectively, and do not depend on deposition sequence (commutativity). As determined from the measured work functions the interface dipole is 0.05 eV; the band lineup between the two materials is correctly predicted by the Anderson model (electron affinity rule).

Surface photovoltage measurements on pyrite (100) cleavage planes: Evidence for electronic bulk defects

Temperature dependent contactless surface photovoltage measurements by photoelectron spectroscopy have been performed on cleaved (100) surfaces of pyrite (FeS2) single crystals. The results have been fitted by thermionic emission, recombination, and tunneling models for the majority carrier transport to the surface. Neither of them is able to explain the small photovoltages consistently. By calculating electronic defect levels due to the sulfur deficiency of pyrite a high number of defect states in the band gap is obtained. As a consequence a nonuniform depletion layer is expected with a part of the band bending potential falling off at a very small distance near the surface. The small photovoltages can be explained by a tunneling of majority carriers through the narrow barrier and by recombination losses due to the defects.

Band lineup between CdS and ultra high vacuum-cleaved CuInS2 single crystals

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.

Thin pyrite (FeS2) films by molecular beam deposition

Abstract Polycrystalline pyrite films have been prepared by evaporation of Fe and S from separate molecular beam sources. It is shown by X-ray diffraction and by X-ray and ultraviolet photoelectron spectroscopy that at S pressures of 6–8 · 10 −5 , Pa pyrite is formed at a substrate temperature of 390 K. At higher temperatures, pyrrhotite (Fe 7 S 8 ) is present in the films.

Optical and photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy

Indium selenide thin films have been grown on p-type gallium selenide single crystal substrates by van der Waals epitaxy. The use of two crucibles in the growth process has resulted in indium selenide films with physical properties closer to these of bulk indium selenide than those prepared by other techniques. The optical properties of the films have been studied by electroabsorption measurements. The band gap and its temperature dependence are very close to those of indium selenide single crystals. The width of the fundamental transition, even if larger than that of the pure single crystal material, decreases monotonously with temperature. Exciton peaks are not observed even at low temperature, which reveals that these layers still contain a large defect concentration. The current–voltage characteristic of indium selenide thin film devices was measured under simulated AM2 conditions. The solar conversion efficiency of these devices is lower than 0.6%. The high concentration of defects reduces the diffusion length of minority carriers down to values round to 0.2 μm.

RDS, LEED and STM of the P-rich and Ga-rich surfaces of GaP(100)

Abstract Reflectance difference spectroscopy was measured in the metal organic chemical vapor deposition reactor and also in UHV at 20 K. It revealed a characteristic negative peak at the low energy side that was indicative of the specific surface reconstruction. This peak disappeared completely if the sample was kept within a narrow intermediate temperature range. At 20 K the negative peak appeared at 2.4 eV for the Ga-terminated (2×4)-reconstructed surface and at 2.6 eV for the P-terminated (2×1)/(2×2)-reconstructed surface. RDS for the two different surface reconstructions displayed strong structures also in the range of the bulk transitions. A characteristic zig-zag pattern was observed in the STM image of the P-terminated surface.