C. Bostedt

Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction

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,u200aGa)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.

Strong quantum-confinement effects in the conduction band of germanium nanocrystals

Quantum-confinement effects in the conduction band of deposited germanium nanocrystals are measured to be greater than in similar-sized silicon nanocrystals. The germanium particles are condensed out of the gas phase and their electronic properties are determined with x-ray absorption spectroscopy. The conduction band edge shifts range from 0.2 eV for 2.7 nm particles up to 1.1 eV for 1.2 nm particles.

X‐Ray Emission Spectroscopy of Cu(In,Ga)(S,Se)2‐Based Thin Film Solar Cells: Electronic Structure, Surface Oxidation, and Buried Interfaces

The electronic and chemical structure of Cu(In,Ga)(S,Se) 2 (CIGSSe) thin film surfaces and of relevant interfaces in CIGSSe-based thin film solar cells is investigated with a combination of X-ray emission spectroscopy (XES) and photoelectron spectroscopy. Examples of sulfur L 2,3 XES spectra of CdS and CIGSSe are discussed in view of resonant excitation, surface oxidation, and chemical bonding. The combination of the two techniques proves to be a powerful tool to identify spectral features correlated to certain chemical states or bonds. By monitoring these features in interface formation sequences, chemical and electronic information about buried interfaces can be obtained, which will be discussed in detail for the ZnO/CIGSSe interface. The experimental results provide valuable information on the CIGSSe surface and the ZnO/CIGSSe interface and, in general, demonstrate some of the spectroscopic advantages of X-ray emission spectroscopy.

Damp-heat induced sulfate formation in Cu(In,Ga)(S,Se)2-based thin film solar cells

To investigate the impact of damp heat treatments on the electronic and chemical structure of Cu(In,Ga)(S,Se)2-based thin film solar cells, we have performed a detailed soft x-ray emission study of the ZnO/CuIn(S,Se)2 and ZnO/CdS/CuIn(S,Se)2 interfaces. By comparing the sulfur L2,3 emission spectra of pristine and damp-heat treated samples, we find a sulfate formation at the ZnO/CuIn(S,Se)2 and the ZnO/CdS interface. The intensity behavior as a function of ZnO film thickness further reveals a diffusion of sulfur atoms into the ZnO film, leading to the formation of zinc sulfate in the ZnO window layer of damp-heat-treated Cu(In,Ga)(S,Se)2-based solar cells.

LOCALIZATION OF NA IMPURITIES AT THE BURIED CDS/CU(IN, GA)SE2 HETEROJUNCTION

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,u200aGa)Se2 thin film solar cell absorber has been investigated, showing that Na impurities are localized at the buried CdS/Cu(In,u200aGa)Se2 heterojunction.

Evidence for cubic phase in deposited germanium nanocrystals

Germanium nanocrystals with sizes ranging from 1 to 5 nm are condensed out of the gas phase in helium or argon buffer-gas atmospheres and subsequently deposited. The generated particle sizes are found to depend on the buffer gas, with helium yielding a narrower size distribution than argon and argon exhibiting a stronger pressure dependence of the produced particle sizes. Structural analysis of nanoparticles with average sizes around 5 nm reveals the bulklike cubic (diamond) phase—in contrast to recent experiments which suggest the tetragonal phase for similar-sized particles. These results are explained in terms of particle formation dynamics.

X-ray Absorption Analysis of KDP Optics

Abstract Potassium dihydrogen phosphate (KDP) is a non-linear optical material used for laser frequency conversion and optical switches. Unfortunately, when KDP crystals are coated with a porous silica anti-reflection coating (Thomas, Appl. Opt. 1986;25(9):1481) and then exposed to ambient humidity, they develop dissolution pits (Wheeler et al., SPIE 1999;3902:451–459; Wheeler et al., Mater. Res. Soc. Conf., San Francisco, 2000). Previous investigations (Wheeler et al., SPIE 1999;3902:451–459) have shown that thermal annealing renders KDP optics less susceptible to pitting, suggesting that a modification of surface chemistry has occurred. X-ray absorption was used to characterize changes in the composition and structure of KDP optics as a function of process parameters. KDP native crystals were also analyzed to provide a standard basis for interpretation. Surface sensitive total electron yield and bulk sensitive fluorescence yield from the K2p, P2p (L 2,3 -edge) and O1s (K-edge) absorption edges were measured at each process step. Results indicate that annealing at 160°C dehydrates the surface of KDP resulting in a metaphosphate surface composition with K:P:O=1:1:3.

Self-limitation of Na content at the CdS/Cu(In,Ga)Se2 solar cell heterojunction

Abstract 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.

Controlling the electronic structure of nanocrystal assemblies by variation of the particle-particle interaction

The change in the electronic structure of germanium nanocrystals is investigated as their concentration is increased from noninteracting, individual particles to assembled arrays of particles. The electronic structure of the individual nanoclusters shows clear effects due to quantum confinement which are lost in the concentrated assemblies of bare particles. When the surface of the individual particles is passivated, they retain their quantum confinement properties also upon assembly. These effects are interpreted in terms of a particle-particle interaction model.

Photoemission spectroscopy of germanium nanocrystal films

Abstract Photoemission spectroscopy measurements have been performed on films of individual germanium nanocrystals. The Ge 3d core-level photoemission reveals mainly bulk-like coordinated atoms and structural disorder on the nanocrystal surface. The Ge 3d plasmon excitation exhibits a reduced bulk- and an increased surface-plasmon contribution underlining the importance of the surface region to the overall particle electronic structure. In the Ge nanocrystal valence-band photoemission features similar to amorphous-Ge are found, further indicating structural disorder in the surface region of the nanocrystals.