Berechnung von Spektren der energie- und winkelaufgelösten Photoemission an III/V-Halbleiteroberflächen

Strasser, Thomas

In this thesis, the software package KSAP (Kieler Software for Angular Resolved Photoemission) is used to calculate within the one-step model the spectra of the angle resolved photoemission spectroscopy for different III/V semiconductor surfaces. The numerical results are not only used to compare the experimental and theoretical spectra qualitatively. Moreover, the information from of a full calculation such as the matrix elements, complex band structure and density of states allow to analyse the microscopic details of the measured photocurrent. Gallium nitride has attracted considerable interest because of its application for blue light-emitting diodes and lasers. This thesis presents the photoemission spectra calculated for different surfaces of the cubic and the wurtzit phases of GaN. The direct comparison between experimental and theoretical spectra shows a better agreement, as could be expected by a comparison between an experimental band mapping and calculated band structures. In particular, for the case of GaN(0001) the numerical freedom is used to calculate spectra series in normal emission based on different theoretical band structures. The systematic comparison with experimental results allows the accurate determination of the valence band width, which permits the assessment of the accuracy of different ab initio band structure calculations for GaN. Furthermore, a shift in the binding energy of about 1.0 eV between the corresponding theoretical and experimental structures was observed. This difference could be related to the uncertainty in the experimental determination of the valence band maximum. Total energy calculations predict for GaN a stable 1x1 reconstruction of the (001) and (0001) surfaces respectively, which are stabilized through the metallic Ga bonds, a property rather different compared to semiconductors like GaAs. Within the scope of the photoemission calculations, some features of the measured spectra are clearly related to emission from these characteristic gallium layers, which underline the reliability of the used surface model. With GaAs(001)-c(4x4) surfaces and (GaAs)_n(AlAs)_m Superlattices the calculations are extended to systems with large unit cells parallel and longer periodicity perpendicular to the surface, respectively. For the GaAs(001)-c(4x4) surfaces, it is shown that in experiment the expected periodicity of a surface state is lost, which is related to the interaction between the contributions of the bulk and the surface to the photocurrent. It was demonstrated through the calculated photoemission spectra of (GaAs)_n(AlAs)_m Superlattices that photoemission, usually expected to be surface sensitive, can detect the change of the bulk periodicity. The features of the spectra associated to the change of the bulk periodicity are a lower dispersion of emission which are related to direct transitions, the occurrence of minigaps, which are detectable in the spectra, and the energetical position of surface states within these minigaps, which is strongly affected by the surface termination. Furthermore, the theoretical analysis allows to determine the contribution of the atomic layers below the surface to the photocurrent. It is shown that for a single emission, small differences in the photon energy changes drastically the depth of these layers. The latest effect is analysed in more detail for InAs. Experimental techniques allow to prepare samples with a well defined initial state at the upper conduction band edge. The photoemission spectra, which was measured in constant initial state mode, can be related to escape depth by theory. This shows that for low excitation energy, as used in the ultraviolet photoelectron spectroscopy, the band structure effects in the final states strongly affect the escape depth. Therefore, the well known universal curve of escape depth should be used carefully in this energy region.



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Strasser, Thomas: Berechnung von Spektren der energie- und winkelaufgelösten Photoemission an III/V-Halbleiteroberflächen. 2005.


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