X-ray in-situ study of copper electrodeposition on UHVprepared GaAs(001) surfaces

The physics and chemistry of semiconductor/electrolyte interfaces are of major interest for technological applications and for the understanding of the charge transfer process in fundamental research. For the metalization process in semiconductor industry electrochemical deposition is an attractive alternative beside the classical (and expensive) method of vacuum deposition. Often a better control of the deposition process is possible. Surface X-Ray diffraction (SXRD) is ideal for the investigation of electrochemical metal deposition on the atomic scale, as also buried interfaces are accessible. X-ray in-situ studies can give detailed structural information of the surface and the region close to it. For this work a unique setup for in-situ electrochemical studies was employed and improved. This setup permits UHV preparation of the GaAs(001) surface with a defined surface termination (arsenic-rich or gallium-rich) and its characterization by SXRD in UHV, under ambient pressure in inert gas and in electrolyte under potential control without passing through air. The GaAs(001) surfaces were prepared by MBE and capped by amorphous arsenic at the University of Trondheim by the group of B.O. Fimland. This permitted to ship them through ambient air. Afterwards smooth well defined GaAs(001) surfaces could be recovered by thermal annealing in UHV. A first investigation of the arsenic capped sample was done by atomic force microscopy (AFM) and Surface X-Ray Diffraction (SXRD). The non bulk like termination of the arsenic buried GaAs(001) surface was revealed. For the electrochemical metal deposition, arsenic terminated (2×4) reconstructed and gallium terminated (4×2) reconstructed GaAs(001) surfaces were employed. These surfaces were characterized by STM, LEED and a first time by SXRD. The surfaces are smooth, however, a higher degree of disorder than for MBE prepared reconstructed GaAs(001) is found. After exposure of the sample to nitrogen, the surfaces were then again studied by SXRD. These two steps characterizing the bare GaAs(001) surfaces permitted us to get a better knowledge of the starting surface and its influence on the later electrodeposited copper. At ambient pressure both reconstructions are lifted, but the surface is not bulk-like terminated as can be deduced from the crystal truncation rods. Epitaxial copper clusters grow upon electrodeposition on the UHV prepared GaAs(001) surface. The copper lattice is rotated and inclined with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains. The influence of the surface termination as well as the nucleation potential on the structure of the electrodeposited copper were investigated. The tilt and rotation angles do not depend on the deposition potential but a small influence of the surface termination (Ga-rich or As-rich) could be found. The quality dependence of the copper epitaxy depends on the nucleation potential and on the surface termination was investigated. The variation of the copper lattice constant and the out-of-plane mosaicity of the copper were found to depend on the nucleation potential. Further investigation of the Cu/GaAs interface revealed interdiffusion processes at the interface. Comparative studies of copper deposited on chemical etched (resulting rough) surfaces by SXRD revealed a less well defined epitaxy. In addition the surfaces obtained by potential controlled stripping from an arsenic capped wafer was investigated by AFM. The copper deposition on a such obtained surface was studied ex-situ by AFM.