Local Interface Composition and Band Offset Tuning in ZnSe-GaAs(001) Heterostructures
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Laboratorio Tecnologie Avanzate Superfici e Catalisi del Consorzio Interuniversitario di Fisica della Materia, Area di Ricerca, Padriciano 99, 1-34012 Trieste, Italy. 2 Dipartimento di Science Fisiche dellrUniversita' di Cagliari, Via Ospedale 72,1-09124 Cagliari, Italy. 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455.
ABSTRACT ZnSe-GaAs(001) heterostructures have been grown by molecular beam epitaxy and characterized in situ by means of reflection high energy electron diffraction and x-ray photoemission spectroscopy, and ex-situ by near edge photoluminescence spectroscopy and by cross sectional transmission electron microscopy. By changing the Zn/Se flux intensity ratio (we explored the 0.1-10 range) we were able to control the Zn/Se relative concentration in the interface region, while maintaining a similar structure and high degree of long range order at the interface. Correspondingly, the valence band discontinuity is found to vary from 0.6eV (Se-rich interface) to 1.2eV (Zn-rich interface) in the interface composition range examined. INTRODUCTION ZnSe-GaAs heterostructures represent an important component of most recent blue-green light emitting devices [ 1,2]. The large valence band offset reported for this interface [3,4] hinders hole injection from the IU-V substrate into the II-VI active layers, and requires the fabrication of complex ternary graded structure to increase hole collection. However, recent theoretical developments indicate that because of the heterovalent character of the corresponding II-VI/III-V heterojunction the band alignment should depend on the detail of the interface atomic structure and composition [5-7], and might in principle be modified to yield a lower valence band offset. Experimentally, it was recently shown that the band offsets depend on the crystallographic orientation of the interface [8] and can be modified through the fabrication of pseudomorphic Ge layers in the interface region [4,9]. In this paper we examine the effect of the ZnSe-GaAs(001) local interface composition on the band alignment. Changes in the local interface composition were produced during heterostructure fabrication by means of molecular beam epitaxy (MBE). Large changes in the Zn/Se relative concentration at the interface were achieved by varying the Zn/Se beam pressure ratio (BPR) in the 0.1 to 10 range during the early stages of growth. Interface composition and band offsets were determined in-situ by means of monochromatic x-ray photoemission spectroscopy (XPS), while the atomic structure of the interface was examined in-situ by means of reflection high energy electron diffraction (RHEED) and ex-situ by cross-sectional transmission electron microscopy 3 Mat. Res. Soc. Symp. Proc. Vol. 326. @1994 Materials Research Society
(XTEM). The optical quality of the II-VI active layer was gauged by means of near-edge photoluminescence spectroscopy (PL). We found that large changes in band alignment (up to 0.6eV) can be achieved by controlling the Zn
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