The aim of this paper is to review almost a decade of direct-bonding activities at Philips Research including the diversity and feasibility of direct bonding. The bondability of a material is determined by its geometrical shape and mechanical, physical, and chemical surface states. Physically direct bonding provides a vacuumtight bond, which is jointless and glueless, and it permits engineering of the interfaces to be bonded. Layers can be buried, and reflective-lossless bonds between optical elements can be created. A variety of materials are investigated: (refractory) metals, a semimetal, boron, diamond, a carbide, fluorides, nitrides, oxides, and a chalcogenide. The applications that we describe relate to interface engineering, waveguiding, and the direct bonding of a fiber plate.
The interface quality of Si/Si1−xGex (0.08≤x≤0.33) interfaces grown by molecular beam epitaxy has been studied by means of secondary-ion mass spectrometry. Ge segregation occurs into the Si capping layers. The segregation is characterized by a 17 nm/dec slope; the total amount of segregated Ge corresponds to a few tenths of a monolayer. The phenomenon is independent of the Ge fraction and does not depend on temperature as long as crystal growth is perfect. A possible explanation is given in terms of a Ge adlayer that is formed during growth as a result of site exchange between subsurface Ge and surface Si atoms. This adlayer is incorporated slowly during further Si growth. The Ge segregation can be suppressed by having an adlayer of Ga on the surface of the growing structure.
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