Microscopic-scale lateral inhomogeneities of the GaSe-Ge heterojunction energy lineup

Abstract

Scanning-photoemission-spectromicroscopy data revealed substantial inhomogeneities in the lineup of the electronic states at the interface between the two semiconductors GaSe and Ge. These inhomogeneities would lead to valence-band discontinuity changes from place to place, whose magnitude is approximately 0.4 eV.Scanning-photoemission-spectromicroscopy experiments with undulator synchrotron radiation revealed a lateral modification of the energy lineup of the electronic states between two semiconductors.We performed this test on Ge deposited on GaSe; the equivalent valenceband discontinuity, AE"reproduced previously published values' for a visually defect-free region, but was smaller by =0.4 eV, well beyond the experimental uncertainty, in another region. This result shows that the theory of heterojunctions must take into account the possibility of the coexistence of different energy lineups and, therefore, of different band discontinuities at the same interface.For many years, semiconductor heterojunction band lineups have been one of the central problems of condensed-matter physics, because of the technological importance of the resulting band discontinuities, but also because of their fundamental importance. The theoretical understanding of the lineups is indeed a complicated conceptual problem, which requires, in turn, a good understanding of the absolute energy scale for the two band structures, plus an equally good understanding of the interface electronic structure. This problem has been extensively investigated both with theory and with experiments. ' Most of the experiments have been based on photoemission spectroscopy.Conventional photoemission spectroscopy, however, cannot clarify the important issue of the possible lateral dependence of the band lineup:Are the band-edge discontinuities different from place to place?We decided to explore the possible existence of lineup inhomogeneities by using the technique of scanning photoemission spectromicroscopy, by means of the instrument MAXIMUM at the Wisconsin Synchrotron Radiation Center in Madison. MAXIMUM is a scanning soft-x-ray photoemission microscope that focuses undulator radiation using a multilayer coated Schwarzschild objective. This instrument can take both energy-resolved photoelectron intensity two-dimensional images and photoemission spectra on microscopic areas. The images are formed by scanning the sample and collecting the photoelectrons. The size of the microscopic areas from which the photoemission spectra are taken depends on the experimental setup; in our experiment, it was 5 X 5 pm . The focusing concentrates approximately 35% of the total photon-beam intensity in the spot, and the rest is in the diffuse background. The density of photons in the primary beam is of the order of 10' photons/cm /s, whereas that of the scattered light is nearly five orders of magnitude smaller. The focus of the analyzer (a doublepass cylindrical mirror analyzer) is on the primary spot and only 4% of the total signal produced by the scattered light reach...

Charge Density and Band Offsets at Heterovalent Semiconductor Interfaces

“…The calculated VBO of the heterovalent interface, shown in Table for a few representative heterojunctions, with comprehensive results given in Supporting Information Section 3 (see Supporting Information Table 2), varies significantly with the atomic structure of the interface for all orientations of the heterojunctions. This suggests that the range of BOs from CC to AC interfaces can be spanned continuously by varying the relative proportions of the two structures at the interface and that the VBO at actual heterovalent interfaces could vary locally with structure, either in a controlled manner or unintentionally. The results of Figure were found to be representative throughout.…”

Soft X-Ray Spectromicroscopy and its Application to Semiconductor Microstructure Characterization

“…Lateral variations of approximately 0.1 eV in band bending were found by Gozzo et al in a GaSe/Au interface [20]. For a GaSe/Ge interface, the observed lateral variations of the band lineup were approximately 0.4 eV [21].…”

“…Figure 4 shows a layout of MAXIMUM [12]. One of the first applications of MAXIMUM to the study of semiconductor surfaces and interfaces was the search for possible lateral variations of semiconductor interface parameters in both metal/semiconductor and semiconductor/semiconductor interfaces [20,21]. Lateral variations of approximately 0.1 eV in band bending were found by Gozzo et al in a GaSe/Au interface [20].…”

“…5 was obtained by focusing the photon beam on a cleaved GaSe surface covered with a 0.5 monolayer of Ge and scanning the sample while detecting the photoemission signal from the Ge 3d states [21]. The image revealed substantial inhomogeneities in the signal, and in particular a line-shaped …”

Interface states at semiconductor junctions

“…In recent years, however, a new problem has emerged (Gozzo et al 1993(Gozzo et al , 1995, for which a complete clarification has not yet been achieved. Virtually all theoretical models of semiconductor interfaces-such as metal-semiconductor and heterojunction systemsassume that the interface electronic structure and the interface parameters are constant over the entire plane of the interface.…”