The bulk electronic band structures of BeTe and BeSe have been studied by resonant inelastic soft x-ray scattering (RIXS) at the Be $1s$ edge. We derive direct and indirect bulk band gaps and observe the coexistence of core excitons and momentum conservation. A quantitative analysis of the coherent spectral fraction gives insight into the femtosecond time scale of the relevant dephasing processes. Experimental results agree very well with calculations based on the Kramers-Heisenberg and density-functional theories.

Synchrotron-based X-ray emission spectroscopy (XES) has been utilized to study the local chemical bonding of sulfur atoms in HgS-based mixed crystals in the zinc blende (sphalerite) structure. All investigated samples were bulk crystals grown by the Bridgman method. Complementary X-ray powder diffraction experiments reveal a variation in the lattice parameter of the different mixed crystals between 5.845 Angstrom for Hg1-xMnxS (x = 0.02) and 6.063 Angstrom for HgSe1-xSx (x = 0.10). In the XES spectra we find a substantial wave function overlap between S 2p core levels and Hg 5d-derived valence states, elucidating the chemical bond between sulfur and mercury and indicating a considerable covalent contribution.

We demonstrate how a combination of photoelectron spectroscopy and x-ray emission spectroscopy can be utilized to derive semi-quantitative information about the localization of impurities at buried interfaces. In the case of the CdS/Cu(In,Ga)Se2 (CIGS) thin-film solar cell heterojunction, segregated Na, which stems from the soda-lime glass substrate or is deliberately added, plays an important role. We find that almost all Na atoms are located at the external CIGS surface or at the CdS/CIGS interface, and that the Na concentration in the bulk of the CIGS film is <1 ppm. Moreover, we show that the Na surface coverage at internal CIGS surfaces is significantly lower than at the external CIGS surface or CdS/CIGS interface, which demonstrates that the internal surfaces may not be regarded merely as a special case of the external surface. Copyright © 2000 John Wiley & Sons, Ltd.

The surface termination and geometric structure of molecular beam epitaxially (MBE)-grown HgSe(001) has been studied by means of X-ray photoelectron spectroscopy (XPS) and high-resolution low-energy electron diffraction. The surface exhibit a c(2×2) reconstruction and is terminated by Hg. In addition, the valence band offset of the HgSe/CdSe(001) heterostructure has been investigated by k-resolved ultraviolet photoemission (UPS). Special care was taken to determine the true position of the VB maximum in the Brillouin zone (Γ-point) by using Ar–I excitation during the angle-dependent UPS measurements. Thus, the valence band discontinuity was determined as 0.58±0.05 eV. This value and recent results for the HgTe/CdTe heterojunction support the trend expected by theory predicting a larger ΔEVBO for selenides than for tellurides.

The localization of Na impurities at the buried heterojunction of CdS/Cu(In,Ga)Se2 thin film solar cells has been studied by photoelectron spectroscopy and X-ray emission spectroscopy. This combination of a surface- and a bulk-sensitive technique allows to identify the localization of impurities at a buried interface in a non-destructive, semi-quantitative, and element-specific way. We compare samples with increasing CdS-overlayer thickness on (a) a CIGS film with nominal Na content and (b) a Na-rich CIGS film. The data clearly indicate a self-limitation of the Na content at this interface. The consequences are discussed in view of the possibility to tailor the electronic structure of the buried heterojunction by controlling the nominal Na content in the CIGS film.

Stimulated by recent photoemission results which suggest a positive fundamental energy gap in HgSe, we have investigated the electronic structure of molecular beam epitaxially grown HgSe(001) c(2 x 2) layers by a combination of direct ultraviolet photoemission spectroscopy (UPS) and inverse (IPES) photoelectron spectroscopy. Our UPS results do not support the finding of additional peaks above the valence band maximum (VBM) of Gawlik et al. [Phys. Rev. Lett. 78, 3165 (1997)]. A comparison of angle-integrated UPS and IPES spectra and nb initio calculated density of states of HgSe and HgTe demonstrates dissimilar behavior of the two compounds in the dispersion of the conduction bands between 0 and 2 eV above the VBM. Our results are compatible with the common view that HgSe is a semimetal.

Scanning tunneling microscopy (STM) under laser illumination and microspot X-ray photoelectron spectroscopy (micro-ESCA) have been utilized to study the local variation of the photovoltaic properties and of the stoichiometry of Cu(In,Ga)Se2 (CIGS) thin film solar cell absorbers. The STM results clearly demonstrate that the photovoltaic quantities like the local surface photovoltage (SPV) and photoinduced tunneling current (PITC) vary significantly between different grains. This observation is in accordance with the local variation of the chemical composition of CIGS absorber films derived by micro-ESCA. In addition, the (local) photoelectric parameters vary with exposure time to red light, in agreement with recently reported metastability effects.

We report on a comprehensive study of the ZnSe(100)-c(2 x 2)-Na interface using x-ray and UV photoemission, and x-ray-induced Auger spectroscopy. Spectra were taken after stepwise Na deposition onto a clean c(2 x 2)-reconstructed ZnSe(100! surface at room temperature up to a saturation coverage of about 1 ML of Na and after annealing. Based on the analysis of Auger parameters and of the relative intensity evolution of various Na, Zn, and Se species, we present the following model for the ZnSe(100)-c(2 x 2)-Na interface: below a coverage of 0.5 ML, Na is adsorbed on Zn vacancy sites; above 0.5 ML, a cation exchange reaction occurs between Na and Zn atoms; Zn atoms segregate on top of the Na overlayer forming metallic Zn. In addition, band-bending, surface dipole, valence-band and surface states will be discussed.

We have investigated the valence band offset (Delta E-VBO) of the molecular beam epitaxially grown heterostructure HgTe/CdTe(001) by k-resolved ultraviolet (UPS) and X-ray (XPS) photoemission. Our angle-dependent UPS measurements at two different photon energies demonstrate that the dispersion of the valence band (VB) must be checked carefully in order to find the true position of the VB maximum in the Brillouin zone. With this information the valence band discontinuity was determined as (0.53 +/- 0.03) eV, which is different from previously found photoemission values but now agrees well with magneto-optical investigations. Tn addition, we have determined the energy gap of CdTe(001) and HgTe(001) by a combination of UV and inverse photoemission. The energy gap of CdTe is determined to (1.57 +/- 0.06) eV and that for HgTe as (0.0 +/- 0.06) eV. We therefore conclude that HgTe is a semi-metal with inverted band structure in agreement with other results.

A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

Direct and detailed information about the surface of differently sized CdS nanoparticles has been obtained from high-resolution X-ray photoelectron spectroscopy using tunable synchrotron radiation. We identify up to four distinct components in the S 2p core level spectra. Two of them are assigned to bulk and surface S-species of the nanoparticles, the latter arising from a surface core level shift due to their S-termination. The remaining S 2p components stem from S-atoms of the stabilizer thiol group bound to Cd and to S–S-bonds formed by thiol groups which are attached to surface S-atoms. In the latter case, the oxidation of the nanoparticle surface is drastically reduced.

We demonstrate a general approach to identify and locate minority species at buried interfaces which are of fundamental interest in many fields of solid state research. The approach combines soft x-ray emission for bulk and photoelectron spectroscopy for surface sensitivity. In the present study, the interface between a thin CdS layer and a Cu(In, Ga)Se2 thin film solar cell absorber has been investigated, showing that Na impurities are localized at the buried CdS/Cu(In, Ga)Se2 heterojunction.

The thermal behaviour of CdS nanoparticles and especially of their surfaces has been investigated by high resolution photoelectron spectroscopy. Different annealing characteristics were observed for particles of different sizes. From the changes of surface core-level shifts in the S 2p and Cd 3d(5/2) spectra we derive that the nanoparticle surface, which is S-terminated at room temperature, changes after annealing by removal of S atoms and segregation of Cd atoms. Furthermore, we observe that the temperature at which this process takes place depends on the particle size.

The surface structure of molecular beam epitaxially grown HgTe(001) has been studied under in situ ultrahigh vacuum conditions. The as-grown samples were investigated by means of x-ray photoelectron spectroscopy (XPS), spot profile analysis of low-energy electron diffraction, reflection of high-energy electron diffraction and scanning tunneling microscopy (STM). They exhibited a c(2×2) surface reconstruction with an additional weak (2×1) component in both diffraction experiments. The surface was shown by XPS to be terminated with Hg atoms. In addition, by means of STM experiments, we have been able to resolve the atomic structure of the reconstructed surface and to detect domain boundaries along the [11̄0] direction whose local symmetry is twofold and, therefore, the probable cause of the weak (2×1) reconstruction.

The interface formation of Zn/CdTe(100) has been investigated using synchrotron and Mg K alpha x-ray photoelectron spectroscopy. We identify five distinct phases of the interface formation process, including the passivation of surface defects by small amounts of adsorbed Zn (less than or equal to 0.1 Angstrom), diffusion of Zn into Cd vacancies and lattice-site defects, and a Cd-Zn exchange, thus forming the ternary compound Cd1-xZnxTe in a near-surface region. In the final stage of the indiffusion, we find significant Cd segregation and also, to a smaller extent, Te segregation. The formation of a metallic Zn overlayer for high Zn coverages is associated with a surface photovoltage effect at room temperature. The results derived from the investigation of Zn-induced band bending, surface core-level shifts, and peak area evaluations are discussed, and a model based on the variation of the photoemission information depth is given. We also present a simple method to determine the onset of Cd segregation in order to identify the ternary Cd1-xZnxTe surface-alloy with maximum in content.