Transmission Electron Microscopy Studies of Electrical Active GaAs/GaN Interface Obtained by Wafer Bonding
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Transmission Electron Microscopy Studies of Electrical Active GaAs/GaN Interface Obtained by Wafer Bonding J. Jasinski, Z. Liliental-Weber, S. Estrada1, E.Hu1 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 1 Materials Department, University of California, Santa Barbara, CA 93106 ABSTRACT Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) studies of GaAs/GaN interfaces, obtained by direct wafer bonding, are presented. TEM observations show that most of the interface area was well bonded. A thin oxide layer, confirmed by EDX, was present at the interface in the well-bonded regions. Plan-view TEM studies showed the presence of two dislocation networks in such regions. They formed to accommodate: (1) tilt between bonded crystals and (2) strain, which appeared during sample cooling due to mismatch in thermal expansion coefficients. Asymmetrical, often elongated, cavities, formed on the GaAs side, were present at the interface between the well-bonded regions. It was shown by EDX that the walls of these cavities are covered with native oxide. INTRODUCTION Wafer bonding is a well-recognized method for semiconductor integration. It has been successfully applied to Si on Si integration [1] and to a number of lattice mismatched material systems such as: GaAs/Si [2], InP/GaAs [3-5], InP/GaN [6] and GaAs/sapphire [7,8]. Rapid development in research and application of GaN and related materials has resulted in increased interest in the possible integration of GaN with other semiconductors, among which GaAs plays an important role. Successful wafer bonding of these two materials would allow integration of GaAs and GaN based optoelectronic devices. A GaAs/GaN interface combines also the high breakdown voltage of GaN with the high mobility of GaAs, which are ideal characteristics for transistors and other electronic devices. Promising attempts to apply direct wafer bonding, to create an electrically active GaAs/GaN interface have recently been reported [9,10]. In this paper we present transmission electron microscopy studies of these same interfaces. EXPERIMENTAL RESULTS GaN layers with a nominal thickness of 2 µm were grown on c-plane (0001) sapphire substrates by metal organic chemical vapor deposition (MOCVD). They were directly bonded with 1 µm-thick GaAs layers grown by molecular beam epitaxy (MBE) on 0.3 µm-thick MBE AlAs etch-stop layers grown on (001) GaAs substrates. Atomic force microscopy revealed the rms roughness of both GaAs and GaN surfaces to be about 0.4 nm. Prior to bonding, “escape channels” were etched into GaAs, to prevent liquid and gas from being trapped at the interface when GaAs and GaN were later brought together. These channels were 10 µm wide, in a rectangular grid of 150 µm x 400 µm. Before bonding the wafers were cleaved into 1 cm x 1cm pieces, cleaned, joint together in solvent and kept at 750oC for up to1 hour in a nitrogen ambient under uniaxial pressure of 2 MPa. In the next step, when the pressure was released and the bo
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