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Under controlled ambient conditions it is possible to produce different types of structures onto WSe2 (0 0 0 1) surface with AFM by applying a voltage pulse between tip and sample. The structure size can be varied over three orders of magnitude from the nanometer-scale up to structures of several μm in diameter. Four different types of structures were observed as surface roughening, hole structures induced by etching, surface oxide layers and volcano-like structures depending on experimental parameters as humidity as well as polarity, height and duration of the voltage pulses. Models of cluster formation mechanism will be given based on systematic studies of the experimental dependencies.

A powder containing the CuAlO 2 phase has been prepared from α-LiAlO 2 by ion exchange with CuCl at temperatures significantly below 500 °C. The diameter of the particles in the powder was in the order of 1 μm. Spectral photovoltage (PV) measurements have been carried out on layers of CuAlO 2 particles deposited on glass substrates covered with conductive SnO 2 :F. The sign of the PV corresponds to a p-type semiconductor. The optical band gap and the parameter of the exponential absorption tail of the prepared CuAlO 2 amount to 3.54 eV and 40 meV, respectively.

The ternary system Cu–Sn–S was re-investigated and the phase diagram Cu2S–SnS2 studied in detail by differential thermal analysis and X-ray diffractometry. Three phases of composition Cu4SnS4, Cu2SnS3 and Cu2Sn3+xS7+2x(0≤×≤1) were found exhibiting melting points at 833, 856 and 803 °C, respectively. Ellipsometric and diffuse reflectance measurements revealed that the latter two sulfides possess a fundamental band gap of 0.93 eV followed by a higher transitions. For the first time it could be demonstrated that Cu2Sn3S7 has semiconducting properties and an absorption coefficients of the order 105 cm−1.

β-Ga2O3 is a transparent oxide and intrinsically an insulator. Doping allows the variation of the conductivity for both p- and n-type over a wide range. There are a number of potential applications in optoelectronics such as insulating or conductive window material, or as a substrate. Consequently, the influence of doping on the electrical and optical properties is an issue of crucial importance for pushing this material forward to applications. We report on the successful growth of undoped, Ge- and Ti-doped β-Ga2O3 single crystals by the floating zone technique. Both electrical and optical properties were characterized.

We report on the successful growth of β-Ga2O3 single crystals using the Czochralski method. Model calculations show that the gas phase consists of Ga2O, GaO or Ga independent of the ratio of oxygen and Ar or N2. We find that for growing single crystals the evaporation has to be suppressed by a finite amount of oxygen. A CO2/Ar gas atmosphere was found to meet this requirement.

Cs was adsorbed at room temperature onto the (0001) cleavage planes of 1T–TaSe2. The deposited Cs forms no metallic overlayer, but intercalates after an initial adsorption stage into the van der Waals gap of the layered crystal. A limit of the relative concentration ratio cCs/cTa of ca. 60% is observed. Due to the electron transfer associated with the intercalation the charge density wave induced p( 13 × 13 )R13.89° superstructure exhibits a phase transition to a c( 2 3 ×4 )rect. structure. As a further consequence of the intercalation a metal to semiconductor transition occurs. The change in the electronic structure could not be interpreted within the rigid band model, but is tentatively explained by an localization effect, caused by the charge density wave phase transition.

The electronic structure of heterojunctions between II-VI semiconductors (CdS, CdTe) and layered transition metal dichalcogenide semiconductors (MoTe2, WSe2) were investigated by soft x-ray photoelectron spectroscopy. The interfaces were formed sequentially by molecular beam epitaxy (MBE) of the II-VI semiconductors on van der Waals (0001)-surfaces of layered compound single crystals. The II-VI semiconductors grow with their polar hexagonal axis ([111] for CdTe, [0001] for CdS) parallel to the non-polar substrate [0001]-axis, without evidence for lattice distortions as prooved by TEM and LEED. Band offsets were determined from soft x-ray photoelectron spectra. Large substrate independent interface dipoles of 1.2 eV for CdS and 0.8 eV for CdTe were determined. The deviation of experimental band offsets from the electron affinity rule prediction is described by a simple model which takes the polarity of the grown interfaces into account.

Cu was locally deintercalated from intercalated 1T TaSe2 and 1T TiSe2 single crystals by applying a voltage pulse between the sample surface and an atomic force microscope (AFM) tip under ambient conditions. Using this method, clusters with diameters of approximately 60 nm were grown. This would allow the use of intercalated transition metal dichalcogenides as mass-storage devices. The intercalated samples were prepared in ultrahigh vacuum (UHV) and characterised by X-ray as well as ultraviolet photoelectron spectroscopy (XPS, UPS). It is shown that the preparation of metal-free surfaces requires prolonged heating steps.

We report on a PE study of the electronic structure of pristine TiSe2. The experimental data are compared with results of band-structure calculations. It is confirmed that TiSe2 reveals semimetallic properties. For the first time rare-earth metals were attempted to be incorporated in situ into TiSe2. It is found that divalent Eu can be easy intercalated, whereas deposition of trivalent Gd leads to strong interfacial reactions. The electronic structure of the Eu-intercalation compound cannot be simply described within a rigid-band approach assuming charge transfer from Eu to unoccupied Ti d states. In a more elaborated approach hybridization phenomena have to be taken into consideration.

The analysis of mobility in Cr-doped p-type β-FeSi 2 single crystals is performed taking into account acoustic, non-polar and polar optical phonon scattering, scattering by ionised impurities and space charge scattering. The dominant scattering mechanisms are determined. The value of the valence band deformation potential is estimated. The temperature dependence of the Hall coefficient is explained in the limit of a two acceptor-one donor model. The value of the activation energy of the deep acceptors, the concentration of the shallow and deep acceptors as well as the concentration of the compensating donors were estimated.

Using EPR we have investigated bulk single crystals of \ensuremath{\mathrm{B}}-${\mathrm{FeSi}}_{2}$ grown by chemical vapor transport. By an interpretation of the hyperfine structure combined with intentional doping during crystal growth, we prove that most of the paramagnetic centers observed arise from isolated iron group impurities. The 3d transition metals Cr, Mn, and Co, and with less certainty Ni and Cu, have been identified. The angular variations of the corresponding spectra show orthorhombic symmetry and can be explained with an electronic spin S=. The substructure of the EPR lines due to the ligand hyperfine interaction indicates an eightfold Si coordination. The simple center model of an impurity atom substituting for iron is consistent with the observed spin, symmetry, and coordination. Whereas Cr and Mn can occupy either of both crystallographically inequivalent Fe sites Co, Ni, and Cu prefer one site. For Cu, however, an interstitial incorporation also seems possible.

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.

Undoped and doped β-FeSi2 single crystals were prepared by chemical transport reaction using iodine as transporting agent. The doping elements used were Co, Mn, Cr and Ni. The effect of impurities was studied by EPR and electrical measurements. The impurities substitute Fe on its two inequivalent lattice sites. Electrical measurements indicated p-type behavior. For Cr-doped samples an activation energy of 75 meV was determined. Optical studies on (Fe1−xCox)Si2(x ⩽ 0.15) layers prepared by ion implantation into Si(100) showed a monotonous decrease of the gap.

The intercalation of Li into UHV cleaved layered chalcogenides TiS2, TiSe2, and InSe was investigated mainly by soft x-ray photoelectron spectroscopy and related to electrochemical results by comparing work function changes to the battery voltage. The reactivity as well as the changes in electronic structure were additionally investigated.

FeS2 (pyrite) crystals of defined stoichiometry, high structural perfection and controlled dopant concentration were grown by chemical vapor transport. For Au-doping it has been found that the growth conditions at the surface of the crystal are changed leading to larger crystals of up to 8 mm edge length. Au-doped crystals show n-type conductivity and a uniform Au-distribution of about 0.2 at% in the bulk. The carrier concentration varied from 6 × 1015 to 9 × 1017 cm−3, the mobility ranged from 100–320 cm2/V · s (at room temperature). In comparison undoped crystals have about one order of magnitude lower carrier concentrations (7 × 1014–5 × 1016 cm−3). By using Co as doping agent carrier concentrations in the range 2 × 1017–3 × 1019 cm−3 were adjusted.

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.

Epitaxial thin film ${\mathrm{CuGaSe}}_{2}$ and ${\mathrm{CuInSe}}_{2}$ samples grown on GaAs substrates with varying [Cu]/[Ga,In] ratios were studied using positron annihilation Doppler-broadening spectroscopy and were compared to bulk crystals. We find both Cu monovacancies and Cu-Se divacancies in ${\mathrm{CuInSe}}_{2}$, whereas, in ${\mathrm{CuGaSe}}_{2}$, the only observed vacancy defect is the Cu-Se divacancy.

A model for the electronic structure of the (100) surface of the d-band semiconductor pyrite (FeS2) is presented based on ligand field theory. The existence of a new type of surface states at the valence band edge resulting from a symmetry reduction of the Fe coordination sphere is deduced. The model is able to explain the inversion layer observed on n-type crystals as well as surface core level shifts in S 2p photoemission spectra.

The junction properties of GaSe/WSe2 heterointerfaces grown by van der Waals epitaxy have been characterized by photoelectron spectroscopy and surface photovoltage measurements (SPV). The surfaces of p‐WSe2 substrates doped with Se excess convert to n doping during annealing at T≥720 K, leading to a SPV of 330 mV. Deposited p GaSe forms an n‐p heterodiode opposing the p‐n homodiode within the substrate. Promising results are obtained for n‐WSe2/p‐GaSe heterointerfaces with SPV of at least 0.3 eV. The valence band and conduction band offsets are 0.6 and 0.2±0.1 eV, respectively, in accordance with the Anderson model of heterojunction formation.

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.

Joint use of XPS and STS methods for MoSx van der Waals face cleaned by removal of a few layers in air have allowed to determine the connection between change of electronic properties and variation in Mo content for MoSx surface layers. STS results (I–V curves with electron transport across MoSx layers) show the p-type conductivity for MoS2; p- and n- regions for MoS1.6. The surface state density is higher for p- MoS1.6 than for p-MoS2. From I–V curve with electron transport along layers for MoS1.6 surface the bandgap above 1.1 eV and |Ef - Ec|≌ 0.4 eV are determined. For results explanation we assume that: surface states creation occurs due to top layer defects formation under top layer breaks ; n- type regions creation is caused by growth of concentration of donor centers associated with excess Mo atoms.

Li was adsorbed at room temperature onto UHV cleaved TiSe2 (0001) van der Waals surfaces and investigated by soft X-ray photoelectron spectroscopy. The adsorbed Li is readily intercalated into the substrate without interface decomposition. The electronic structure of the intercalated phase is analyzed from the valence band spectra giving clear evidence that the intercalation follows the rigid band model. Shifts in the Fermi level and related changes in the work function are analyzed in terms of the electronic contribution of the free energy of intercalation.

By means of alternating (AC) and direct (DC) photocurrent measurements, the photoelectric dichroism of thin CdS films on glass is demonstrated at 300 K. The samples are formed by pulsed-laser deposition, and defined orientations, i.e. the c-axis perpendicular (CdS⊥) or parallel (CdS∥) to the glass substrate, are achieved by variation of the laser fluence. The AC peaks of the photocurrent spectra of CdS⊥ and CdS∥ are separated by 55 meV. The absorbance spectra prove that the peak shift is caused by absorption dichroism, which is comparable with that of single crystal CdS. The separation of the DC spectra (140 meV) clearly exceeds the value of the absorption dichroism due to long-term trapping of the photocarriers at the surface of the films.

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 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.

High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.

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).

High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.

The intercalation of Cu adsorbed onto IT TaSe(2) and TiSe(2) (0001) van der Waals planes as well its deintercalation by adsorbed Br(2) is studied by synchrotron induced photoelectron spectroscopy and low energy electron diffraction. Cu intercalation into 1T TaSe(2) leads to a change in lattice distortion (charge density waves) as is evident from a transition of a commensurate root 13x root 13 to a 3x3 superstructure and changes in Ta 4f core line and valence band spectra. For 1T TiSe(2) intercalation follows closely the rigid band model. After adsorption of Br(2) at 100 K and annealing to room temperature a CuBr overlayer is detected on both samples. The substrate spectra indicate the deintercalation of near surface Cu. The experimental results suggest that the diffusion of Cu proceeds normal to the van der Waals plane. Ionics

Ellipsometric and reflectance spectroscopy in the range 1.4-5.0 eV as well as reflectance measurements in the infrared (10–80 meV) were performed on small single crystals of β FeSi2. The results show a clear anisotropy in the optical response which has been compared with theoretical calculations and data obtained on thin epitaxial films. Such a comparison gives evidence of the dependence of the optical response on the crystalline orientation of the film with respect to the surface.

Polycrystalline ß-FeSi2 layers prepared by codeposition of Si and Fe on cold and hot Si substrates and ß-FeSi2, crystals grown by chemical vapor transport were investigated. Resistivity and Hall effect measurements revealed the p-type conductivity of undoped material and the influence of some dopants of the iron group. The activation energy of a Cr-related acceptor was determined to about 85 meV. The mobility data were found to depend significantly on the purity of the preparation process.

GaSe layers were grown on the van der Waals (0001) planes of WSe2 (van der Waals epitaxy). The substrate (0001) plane was cleaned in UHV by heating to 400°C. GaSe was deposited from resistively heated Knudsen cells at T=300° C. After annealing at 450°C an epitaxial GaSe overlayer is formed as evidenced by X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction and photoelectron spectroscopy.

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material.

{100} surfaces, produced from sublimation grown PbS single crystals by cutting, mechanical and chemical polishing, were characterized with regard to topography, structure and chemical composition. For this purpose, some methods independent from another were used, for instance profile measuring, dislocation etching, X-ray rocking-curves, RHEED, Rutherford backscattering (RBS), and AES. The results allow to describe the change of structural perfection with the depth (damage depth), the concentration profile of the main components (half-quantitatively), and the significant decreasing of contamination. Following from this study, the model of the damage layer near the surface discussed earlier for PbTe is modified slightly.

The II-VI semiconductor CdS was grown by molecular beam epitaxy on the van der Waals faces of the layered semiconductor InSe. Chemical, electronic and morphological properties of the deposited films were investigated by means of low energy electron diffraction (LEED), soft x-ray photoelectron spectroscopy (SXPS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). In contrast to observations made on other layered crystals, nucleation of CdS on InSe is possible also at elevated temperatures which is attributed to the small lattice mismatch. The growth front of CdS corresponds to the sulfur terminated polar (111)-B surface which has a strong tendency to form facets.