Mike Engelhardt

Characterization of the native oxide of CuInSe2 using synchrotron radiation photoemission

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to investigate the native oxide of n‐type single‐crystal CuInSe2. Photoemission measurements were acquired on the oxide surface before and after removal using sputter etching. Observed changes in the valence‐band electronic structure as well as changes in the In 4d and Se 3d core lines were correlated with the interface chemistry at the oxide/CuInSe2 interface. These results show the native oxide to be composed of an In2O3 outer layer (no SeO2) with an additional Cu2Se interface layer.

Valence‐band electronic structure of Zn3P2 as a function of annealing as studied by synchrotron radiation photoemission

Ultraviolet photoemission (UPS) utilizing synchrotron radiation has been used to characterize changes in the valence‐band electronic structure of crystalline Zn3P2 as a function of annealing temperature. The Zn3P2 crystal was etched in bromine‐methanol prior to analysis and annealing was performed in vacuum at 300 and 350 °C after sputter cleaning. The UPS spectra for the virgin material are qualitatively similar to the photoemission results for various II‐VI Zn compound semiconductors and a comparison of the Zn 3d binding energies with respect to the valence band maximum is presented. The results for the virgin material and the 300 °C anneal are further compared with the theoretically predicted band structure of Zn3P2 as determined by a pseudopotential energy band calculation. Loss of phosphorus from the surface and the presence of elemental zinc on the surface after the 350 °C anneal is evident.

Phonons in epitaxially grown α‐Sn1−xGex alloys

We present a Raman scattering study of optical phonons in α‐Sn1−xGex alloys (x≤0.08) grown by molecular beam epitaxy on CdTe (100) substrates. The Raman spectra provide strong evidence for the growth of high quality films in the diamond‐structure phase. The composition dependence of the Raman modes shows some qualitative differences with results from the isomorphic Ge1−xSix system. We show that these differences can be understood in terms of a simple model that considers confinement and strain effects.

Synchrotron radiation assisted metalorganic layer epitaxy

Experiments of the photodissociation of tetramethyltin by synchrotron radiation and the epitaxial atomic layer growth of diamond structured α‐Sn films are reported. We used surface sensitive core and valence band photoemission spectroscopy to study the temperature dependence of the condensation and desorption behavior of tetramethyltin on CdTe(100). A quantitative analysis of the deposition and photodissociation of tetramethyltin including the work function change and the evolution of the substrate Te 4d and overlayer Sn 4d intensities during the overlayer deposition suggests that individual monolayers can be grown by the technique of synchrotron radiation assisted metal organic layer epitaxy. The photofragmentation process does not cause a significant carbon build up in the overlayer film. The photodissociated hydrocarbons seem to recombine and desorb from the surface.

Formation and Schottky barrier height of Au contacts to CuInSe2

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to investigate the development of the electronic structure at the Au/CuInSe2 interface. Au overlayers were deposited in steps on single‐crystal p and n‐type CuInSe2 at ambient temperature. Reflection high‐energy electron diffraction analysis before and during growth of the Au overlayers indicated that the Au overlayer was amorphous. Photoemission measurements were acquired after each growth in order to observe changes in the valence band electronic structure as well as changes in the In 4d and Se 3d core lines. The results were used to correlate the interface chemistry with the electronic structure at these interfaces and to directly determine the Au/CuInSe2 Schottky barrier height.

Synchrotron radiation photoemission study of the electronic structure of the filled tetrahedral semiconductors LiZnAs and LiZnP

Abstract Ultraviolet photoemission utilizing synchrotron radiation has been used to characterize the electronic band structure of the Nowotny-Juza A I B II C v type interstitially filled tetrahedral semiconductors LiZnAs and LiZnP. This is the first photoemission investigation performed on these materials. The LiZnAs and LiZnP crystals were grown by direct fusion of the constituent elements and are p-type semiconductors with direct optical band gaps of 1.25 eV and 2.1 eV, respectively. The experimentally observed Zn 3d binding energies with respect to the valence band maximum for these materials are compared with the photoemission results for various II–V and II–VI Zn compounds in order to qualify the relative ionicity as well as the proposed p-d repulsion and hybridization effects. The results are further compared with the theorectically predicted band structure of LiZnAs and LiZnP calculated using self-consistent first principles theory and physically interpreted by an interstitial insertion rule.

Raman Scattering in α-Sn1_xGex Alloys

The Si-Ge system provides a prototypical example of a substitutional semiconductor alloy. Because of its potential technological applications, it has been intensively studied during the past thirty years. In particular, light scattering techniques have been used to investigate the compositional dependence of phonon modes. Most of this work has concentrated on the three main Raman active optical vibrations, assigned to “Si-Si”, “Si-Ge”, and “Ge-Ge” modes.1,2,3,4,5,6,7,8 In spite of this effort, however, the compositional dependence of Raman modes is not well understood. A powerful experimental approach to this problem would be the comparison of Raman spectra for different isovalent alloys such as Si1-xCx, Ge1-xSi, and α-Sn1-xGex. Unfortunately, only α-Sn1-xSix shows solid solubility for the entire compositional range 0 ≤ x ≤ 1. Non-equilibrium growth techniques such as Molecular Beam Epitaxy, however, open up a new alternative for the fabrication of these alloys. Recently, the successful MBE growth of α-Sn1-xGex has been reported.9 In this paper, we summarize recent Raman results on this system and discuss the implications of the results for the understanding of phonons in IV-IV alloys.

Characterisation of MBE-grown α-Sn films and α-Sn1-xGex alloys

The electronic structure and interface properties of epitaxial alpha -Sn and alpha -Sn1-xGex alloy films grown by molecular beam epitaxy on GaSb, CdTe, InSb and Ge substrates were studied by synchrotron radiation photoemission spectroscopy and X-ray diffraction. Deposition at temperatures below approximately 150 degrees C results in amorphous alloy films. At higher substrate temperatures GaSb and CdTe partially react with the overlayer to form precipitates containing either GeSb or SnTe in the interface region. Homogeneous single-crystal alloy films of 200-300 AA thickness were grown on (100)Ge at approximately 400 degrees C. Angle-resolved valence band photoemission spectra show the opening of the band gap by alloying Ge to alpha -Sn. For an alloy with x=0.52 the Gamma 8 point is shifted by approximately 0.16 eV below EF.

Soft X-ray photoemission investigation of the CdS/CuInSe2 heterojunction interface

Abstract Synchrotron radiation soft X-ray photoemission spectroscopy was used to investigate the development of the electronic structure at theCdS/CuInSe 2 heterojunction interface. CdS overlayers were deposited sequentially in steps on steps on single-crystal p- and n-type CuInSe 2 at 250°C. Results indicate that the CdS grows in registry with the substrate, initially in a two-dimensional growth mode followed by three-dimensional island growth as is corroborated by RHEED analysis. Photoemission measurements were acquired after each growth in order to observe changes in the valence band electronic structure as well as changes in the In4d, Se3d, Cd4d and S2p core lines. The results were used to correlate the interface chemistry with the electronic structure at these interfaces and to directly determine theCdS/CuInSe 2 heterojunction valence band discontinuity and the consequent heterojunction band diagram.

Adsorption And Photodissociation Studies Of Tetramethyltin On CdTe(100)

First experiments of the epitaxial growth of diamond structured a-Sn films by synchrotron radiation assisted photodissociation are reported. Surface sensitive core and valence band photoemission spectroscopy is used to study the adsorption behavior and photodissociation of tetramethyltin on clean and highly ordered CdTe(100) surfaces. A quantitative analysis including the work function change and the evolution of substrate and adsorbate intensities during the overlayer deposition suggests that individual monolayers can be grown by the technique of synchrotron radiation assisted metalorganic layer epitaxy (SRMOLE).