Impact of the H2 anneal on the structural and optical properties of thin and thick Ge layers on Si; Low temperature surface passivation of Ge by Si

Using a low temperature/high temperature strategy, we have grown thin (0.27 mu m) and thick (2.45 mu m) Ge layers on Si(0 0 1) substrates that we have submitted to various constant temperature (750 degrees C) or cyclic (750 degrees C/890 degrees C) H-2 anneals, the objective being to identify those yielding the smoothest surfaces, the lowest threading dislocations densities (TDDs) and the highest near infra-red optical absorptions. The best trade-off for thin layers was 750 degrees C, 60 min H-2 anneals. Using longer duration 750 degrees C anneals and especially 750 degrees C/890 degrees C cyclic anneals indeed yielded rougher surfaces and vastly degraded optical absorption (deleterious formation of GeSi alloys). By contrast, short 750 degrees C/890 degrees C thermal cyclings yielded the best metrics in thick Ge layers (while being at the same time the best in terms of throughput): root mean square surface toughness around 0.8 nm, TDD around 10(7) cm(-2), Slightly tensily-strained layers (which a plus for optical absorption as the absorption edge is shifted to higher wavelengths), a limited penetration of Si into Ge (and thus absorption coefficients at 1.3 and 1.55 mu m almost equal to those of as-grown layers), etc. We have also described the low temperature (450 degrees C/525 degrees C) process that we have developed to passivate Ge surfaces thanks to SiH4 prior to gate stack deposition. Si layer thickness should be below 20 angstrom in order to have conformal deposition. A transition of the growth front to 3 dimensions has indeed been evidenced for 20 angstrom and higher. (C) 2009 Elsevier B.V. All rights reserved.