Formation of ZnSe/GaAs Heterovalent Heterostructures by Movpe
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ABSTRACT ZnSe/GaAs (001) heterovalent heterostructures are fabricated by metalorganic vapor phase epitaxy. During the growth, both GaAs and ZnSe surfaces are kept atomically flat to achieve precise control of the interface formation. Interface composition, Ga/As, are controlled by means of either Zn or Se treatment of a GaAs surface, and then ZnSe growth follows. Consequently, it is revealed by X-ray photoemission spectroscopy (XPS) that artificial control of Ga/As from 1.0 to 2.8 leads to the variation of valence band offsets from 0.6 to 1.1 eV. Based on the electron counting model and layer-attenuation model, it is proposed that the As plane just below the interface consists of As, anti-site Ga and As vacancy.
INTRODUCTION Heterovalent heterostructures, which are characterized by chemical valence mismatch at the interfaces, possess attractive feature of variable band offset. The band offset at a heterojunction interface plays an important role to determine carrier transport and confinement properties, and it has been recognized to be constant for a given heterostructure. However, recent considerable works reveal that it may be changed by an interfacial atomic structure[1-6]. Theoretically calculated changes of the offsets in several heterovalent heterostructures are, for example, 1.0, 0.6 and 0.8 eV for ZnSe/GaAs[1,2], Ge/GaAs[3,4] and Si/GaP[3], respectively. The large variation in band offsets at the heterovalent interfaces is originated from the existence of nonoctet bonds which form donor and acceptor states. Due to the charge transfer from the donor to the acceptor states electronic dipoles which determine the offsets are induced. The strength and the direction of the dipoles depend sensitively on the microscopic interface atomic configuration and thus, so do the band offsets. Therefore, to achieve tunable band offsets, it is crucially important to control formation processes of an interface in an atomic order. Experimentally, X-ray photoemission spectroscopy (XPS) has chiefly been utilized to study contributions of interface structures to the band offsets[1,5,6]. It has been reported that interface composition can be controlled by the Zn/Se flux ratio employed during molecular beam epitaxy (MBE) of ZnSe on GaAs (001), resulting in the variation of valence band offsets from 0.58 eV (Se-rich condition) to 1.20 eV (Zn-rich condition)[1]. In the study, however, since the interface composition is modified by changing the epitaxial growth condition of beam pressure ratio, bulk properties as well as interface properties would be influenced. Further, the interface formation and the epitaxial growth occur at the same time, and thus, the strict control of the formation processes of the interfaces has not been achieved yet. In this study, we demonstrate the tunability of the band offset in the ZnSe/GaAs (001) heterostructure by changing compositional ratio, Ga/As, at the interface. The local composition is modified by treatments of GaAs surfaces by Zn or Se precursors. The conditions of the following ZnSe growth are
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