The Effect of Stress in the PdGe Mediated Solid Phase Epitaxial Growth of Ge on GaAs
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The Effect of Stress in the PdGe Mediated Solid Phase Epitaxial Growth of Ge on GaAs
Fabian Radulescu and John M. McCarthy Department of Materials Science and Engineering, Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, Oregon, 97291 ABSTRACT The residual stress and the microstructure associated with it were studied in connection with the Pd-Ge ohmic contact formation on GaAs. Evaporated Pd (20 nm) / Ge (150 nm) / Pd (50 nm) thin film stacks on GaAs were annealed at various temperatures and the resulting microstructures were investigated by transmission electron microscopy (TEM). Micro-cantilever beam structures were fabricated with a focused ion beam (FIB) workstation and the residual stress present was calculated from the deflection magnitude. It was found that Ge solid phase epitaxial (SPE) growth on GaAs is associated with a stress relaxation of the thin film system. A new model that suggests the tensile stress induced by the intermediate layer may play an important role in the SPE growth mechanism is proposed. Other cases of solid phase heteroepitaxial growth with an intermediate medium, such as Ge/Au/Si, Co/Ti/Si (the TIME method) and Co/SiOx/Si (the OME method) are discussed in light of this newly proposed model. Also, the possibility of using controlled stress to engineer new methods for growing SPE based heterostructures will be presented. INTRODUCTION Solid phase epitaxial (SPE) growth with a transport medium is a fascinating yet not completely understood phenomenon with many potential applications in the semiconductor industry. Good reviews [1,2] treating the SPE growth in general were written in the past, which, mainly, described homoepitaxial growth, with or without the transport medium. However, only a few cases of technologically important solid phase heteroepitaxy with transport medium were reported in the literature. Ma et al. [3] demonstrated SiGe SPE growth on Si through a Au layer. More recently, two systems relating to CoSi2 epitaxial growth on Si through different transport media were reported. Dass et al. [4] and Hsia et al. [5] showed CoSi2 SPE growth through a Ti layer by the titanium mediated epitaxy (TIME) method whereas Tung [6] demonstrated CoSi2 SPE growth through an oxide layer, the oxide mediated epitaxy (OME) method. But the first case of solid phase heteroepitaxial growth with transport medium and probably the most studied such system was, to the best of our knowledge, reported by Marshall et al. [7], in connection with the Pd-Ge ohmic contact formation on GaAs. Their studies revealed that Ge in excess of the amount needed to form the PdGe compound is necessary in order to obtain a low resistance ohmic contact on GaAs. They have also demonstrated that during annealing of a Pd:Ge bylayer on GaAs, the Ge is transported to the GaAs interface where it grows by a SPE mechanism.
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If the source material is deposited directly on the substrate, without intermediate medium, and then annealed, the quality of the epitaxialy-grown layer suffers or, in s
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