Smoothing Effects of MOCVD Grown GaAs/Al x Ga 1-x As Superlattices
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1 SMOOTHING EFFECTS OF MOCVD GROWN GaAs/AlxGal-xAs SUPERLATTICES
Xian-gang XU, Bai-biao HUANG, Shi-wen LIU, Hong-wen REN, and Min-hua JIANG. Institute of Crystal Materials, Shandong Univ., Jinan, P.R. China, 250100.
ABSTRACT GaAs/AlxGai-xAs (x=0.5,0.6,1.0) superlattices used as buffer layers in HEMT devices have been grown by Metalorganic Chemical Vapor Deposition (MOCVD) at atmospheric pressure, and characterized by cross-sectional transmission electron microscopy (XTEM). The initial stage of nucleation on the substrates has been clearly verified by examining the undulations of a 30nm GaAs layer sandwiched between the substrate and the superlattice. Both Alo.sGao.sAs/GaAs and AlAs/GaAs superlattices can smooth out interface roughness caused by contaminations and dislocations on the substrate surface. The mechanism of smoothing effect has been discussed in detail.
INTRODUCTION Metalorganic Chemical Vapor Deposition(MOCVD) has received a great deal of attention as a promising epitaxial growth technique of semiconductors[l]. In addition to its versatility, The MOCVD growth technique offers the capability of providing large area uniform quantum heterostructure epilayers (quantum well(QW), superlattice(SL), etc.) with sharp interfaces, suitable for both optoelectronic and microelectronic device applications. The research of III-V semiconductor quantum heterostructures is advanced from the viewpoint of both the applications to electronic devices and the theory of solid state physics[2]. Quantum heterostructures permit the adjustment of electron and hole subband energy levels[2], which lead to enhancements of device performances and appearances of new devices with unique properties, such as semiconductor quantum well laser diodes(QWLDs)[3], high electron mobility transistors (HEMTs)[4], self electro-optic effect devices (SEEDs)[5], heterojunction bipolar transistors (HBTs)[61, quantum well infrared detectors (QWIDs)[7]. In addition to their unique electronic and optical properties, superlattices have also been used as buffer layers to improve the properties of epilayers [8,9] for microelectronic and optoelectronic device applications, which can smooth out interface roughness, bend substrate dislocations propagating into epilayers, and block impurities diffusing from substrates to epilayers. Stimulated by these potential applications, great efforts have been made to the growth of superlattices with a high level of structure and chemical composition controls[l]. Cross-sectional transmission electron microscopy(XTEM) of GaAs/AlxGai-xAs epilayers has been used widely to assess the abruptness of heterojunction interfaces[lO]. The spatial variations in composition can be imaged directly by using (002) dark-field (DF) diffraction. The intensity of AlxGai-xAs layer is proportional to x2 . 1.
Work supported by the National Natural Science Foundation of China. Mat. Res. Soc. Symp. Proc. Vol. 237. @1992 Materials Research Society
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In this paper, we present MOCVD growth and XTEM characterization of GaAs/AlxGai-xAs superlat
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