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.
Over the last few years it has been shown that information about the bulk band structure can be obtained by resonant inelastic X -ray scattering (RIXS) in the soft X -ray regime. However, conceptual differences exist about the underlying pictures involving keywords such as "resonant Xray emission" processes, "electronic resonant Raman scattering", and "core exciton formation". To understand and clarify the applicability of the different concepts, we have studied the properties of a new class of materials -beryllium chalcogenides (i.e., BeS, BeSe, and BeTe) -with soft X-ray K α emission following a resonant Be 1s core hole formation. Be chalcogenides constitute interesting candidates for several electronic applications of II-VIsemiconductors, e.g., in ZnSe-based blue light-emitting devices or Mn-containing spin aligner systems. We will show that even for such large band gap materials with an indirect gap, the observed spectral variations can be nicely correlated to the band structure derived from accompanying LDA-, EXX-, and GW-calculations taking the involved matrix elements into account. Furthermore, we observe Raman-like inelastic scattering features below the absorption threshold. Finally, "core excitonic" features above the valence band maximum can be observed for sufficiently high excitation energies. In the case of BeS, we will demonstrate that such information can even be obtained from insulating powder samples. The obtained results will be discussed in correlation with the above mentioned theoretical band structures, in view of the different conceptual approaches, and with a discussion of the underlying physical processes contributing to a loss of k-conservation and the appearance of a major non-coherent fraction in the observed spectra.The experiments were performed at beamline 8.0
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.
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