T. Löher

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.

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.

Chemical interaction of Na with cleaved (011) surfaces of CuInSe2

To study the beneficial effect of sodium‐containing substrate material on the photovoltaic properties of thin‐film CuInSe2 solar cells the chemical interaction of Na with CuInSe2 has been investigated by synchrotron excited photoelectron spectroscopy. A clean CuInSe2 (011) surface was prepared by cleaving an oriented single crystal in UHV. The cleaved surface exhibits an electron affinity of χ≊4.6 eV. The Se 3d level shows a surface core level shift of −0.4 eV. Na was sequentially deposited in UHV in small steps from a commercial dispenser source. Initially ionized Na adsorbs on the surface leading to a shift of the surface Fermi level by 0.3 eV closer to the conduction band. Removal of Cu from the surface is observed. For higher Na deposition a chemical reaction of Na with the CuInSe2 surface occurs leading to the formation of metallic indium and Na2Se.To study the beneficial effect of sodium‐containing substrate material on the photovoltaic properties of thin‐film CuInSe2 solar cells the chemical interaction of Na with CuInSe2 has been investigated by synchrotron excited photoelectron spectroscopy. A clean CuInSe2 (011) surface was prepared by cleaving an oriented single crystal in UHV. The cleaved surface exhibits an electron affinity of χ≊4.6 eV. The Se 3d level shows a surface core level shift of −0.4 eV. Na was sequentially deposited in UHV in small steps from a commercial dispenser source. Initially ionized Na adsorbs on the surface leading to a shift of the surface Fermi level by 0.3 eV closer to the conduction band. Removal of Cu from the surface is observed. For higher Na deposition a chemical reaction of Na with the CuInSe2 surface occurs leading to the formation of metallic indium and Na2Se.

Van der Waals epitaxy of three‐dimensional CdS on the two‐dimensional layered substrate MoTe2(0001)

Epitaxial films of Wurtzite CdS in (0001) orientation have been grown on MoTe2 (0001) substrates in spite of a lattice mismatch of 15%. The film growth was in situ monitored by soft x‐ray photoelectron spectroscopy and low energy electron diffraction. The interface is found to be nonreactive and atomically abrupt. The CdS overlayer tends to grow in three‐dimensional clusters (Volmer–Weber growth mode).

Low temperature adsorption of water on cleaved GaAs(110) surfaces

Abstract The adsorption and desorption of water on UHV-cleaved GaAs(110) surfaces was studied using synchrotron-excited photoelectron spectroscopy. Water was adsorbed at T = 100 K. Desorption was studied during heating to room temperature. At low coverages, dissociated species are observed followed by molecular adsorption. Molecular water is desorbed at T = 160 K. The dissociated species are also mainly desorbed after heating to room temperature. The chemical changes are accompanied by substrate binding-energy shifts, reflecting the movement of the Fermi level at the surface.

Partial density of states in the CuInSe2 valence bands

The valence band spectra of a vacuum cleaved CuInSe2 (011) surface were measured with synchrotron radiation at photon energies between 16 and 95 eV. The strong dependence of the photoionization cross section of atomic levels between 28 and 60 eV is used to divide the valence band emissions into contributions from Se 4p and Cu 3d states in order to map the respective partial density of states. The derived partial density of Cu 3d states to the total valence band density of states is around 50% in the upper part of the valence band and about 75% at its maximum corresponding to non-bonding Cu d states.

Highly oriented layers of the three‐dimensional semiconductor CdTe on the two‐dimensional layered semiconductors MoTe2 and WSe2

The II–VI semiconductor CdTe was sequentially deposited onto the (0001) van der Waals surfaces of the layered compound semiconductors MoTe2 and WSe2 by molecular beam epitaxy. Growth could only be achieved after deposition of a nucleation layer of CdTe at room temperature. After nucleation subsequent deposition steps followed at increased substrate temperatures (T=170–370 °C) in order to increase the crystalline quality of the films. The deposited films were investigated after each growth step by low energy electron diffraction and photoelectron spectroscopy. The diffraction pattern indicates a facetting of the (111) oriented film surfaces. From photoemission data we exclude interface reactions between substrate and film material. Transmission electron microscopy was used to examine the film morphology after the UHV experiments. The mean diameter of the film crystallites is 200–400 A.The II–VI semiconductor CdTe was sequentially deposited onto the (0001) van der Waals surfaces of the layered compound semiconductors MoTe2 and WSe2 by molecular beam epitaxy. Growth could only be achieved after deposition of a nucleation layer of CdTe at room temperature. After nucleation subsequent deposition steps followed at increased substrate temperatures (T=170–370 °C) in order to increase the crystalline quality of the films. The deposited films were investigated after each growth step by low energy electron diffraction and photoelectron spectroscopy. The diffraction pattern indicates a facetting of the (111) oriented film surfaces. From photoemission data we exclude interface reactions between substrate and film material. Transmission electron microscopy was used to examine the film morphology after the UHV experiments. The mean diameter of the film crystallites is 200–400 A.

InSeGaSe heterointerfaces prepared by Van der Waals epitaxy

Abstract Epitaxial films of layered substrates can be prepared onto layered substrates even for large lattice mismatch, when the growth is attempted with the Van der Waals surfaces opposing each other (Van der Waals epitaxy). Thin epitaxial InSe(GaSe) films are prepared onto ultrahigh vacuum (UHV) cleaved GaSe(InSe) Van der Waals (0001) surfaces. The films and the heterointerface are characterized by photoelectron spectroscopy, electron diffraction and scanning tunneling microscopy (STM). High quality and stoichiometric films are obtained by direct InSe(GaSe) evaporation from a Knudsen cell at sample temperatures between 520 and 620 K. Despite a 6% lattice mismatch the deposited films are oriented with their c - and α -axis to the hexagonal substrate. The growth mostly follows the Frank-Van der Merwe mode. This rather ideal growth behaviour is related to the specific properties of the Van der Waals plane which contains no dangling bonds.

Epitaxial films of the 3D semiconductor CdS on the 2D layered substrate MX2 prepared by Van der Waals epitaxy

Abstract The growth of epitaxial CdS (wurtzite-type) films on the Van der Waals (0001) surface of the layered compound semiconductors WSe2 and MoTe2 is presented. The film and the heterointerfaces are investigated by electron diffraction, photoelectron spectroscopy, and transmission electron microscopy (TEM). At room temperature and above, the CdS sticking coefficient on Van der Waals surfaces is very low. Film growth could only be achieved by decreasing the substrate temperatures to 170 K for the initial deposition steps. On WSe2, textured films of CdS are grown with the c-axis parallel to the substrate c-axis. The crystallites of mean diameter 200 A have random azimuthal orientation. On MoTe2 the CdS films are aligned with their c- and a-axis parallel to the respective axes of the substrate.

Band Lineup of Van Der Waals-Epitaxy Interfaces

Epitaxial lattice mismatched heterointerfaces between layered semiconductors and themselves and II-VI semiconductors (CdS, CdTe), respectively, have been prepared and their band lineup determined by photoemission. Different physical mechanisms, which govern the heterointerface formation, can be discriminated due to the specific properties of the van der Waals (vdW) surface. The interfaces between layered semiconductors mostly follow the electron affinity rule with a small but systematic deviation, which is assigned to the influence of interfacial quantum dipoles. However, the band lineup to the II-VI semiconductors shows a large interface dipole, which is related to a structural dipole from the polar, Cd terminated, face of the (111)- in case of Zinkblende CdTe- and the (0001)- in case of Wurtzite CdS- oriented overlayer film.