Solid phase epitaxy of Germanium on Silicon substrates
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Solid phase epitaxy of Germanium on Silicon substrates R.R. Lieten1,2,3 , Q.-B. Ma1,3, J. Guzman2,4, J.W. Ager III2, E.E. Haller2,4, J.-P. Locquet1 1
Department of Physics and Astronomy, K.U. Leuven, 3001 Leuven, Belgium Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA USA 3 IMEC, 3001 Leuven, Belgium 4 Department of Materials Science and Engineering, UC Berkeley, Berkeley, CA USA 2
ABSTRACT We demonstrate the possibilities of plasma enhanced chemical vapor deposition (PECVD) and solid phase epitaxy to obtain germanium on silicon with excellent crystalline properties, even for very thin layers (< 100 nm). Amorphous germanium layers are deposited by PECVD on silicon substrates. Deposition of an amorphous layer, without the presence of crystalline seeds, is critical. Crystalline inclusions must be avoided to obtain high crystal quality and a smooth surface after crystallization. PECVD is well suited for deposition of amorphous layers because low temperature deposition and high growth rates are possible. Additional experiments with molecular beam epitaxy show that it is not mandatory to have hydrogen present inside the germanium layer to obtain highly crystalline germanium. Atomic hydrogen plays, however, an important role during deposition by lowering the surface adatom mobility and consequently increasing the disorder of the deposited layer. Synchrotron X-ray diffraction shows no germanium diffraction, indicating that the layer does not contain crystalline seeds. Crystallization can be performed at limited temperatures: Raman measurements show crystallization between 400 and 425 °C. Another important advantage of the proposed method is the scalability: germanium layers of larger diameter can be obtained by simply using larger silicon substrates. INTRODUCTION Epitaxial germanium on silicon is useful for many applications: high performance complementary metal-oxide-semiconductor (CMOS) circuits, germanium photo detectors and modulators for optical interconnections, integrating III-V devices on Si, etc. [1,2] Using bulk germanium substrates for high volume applications is not feasible, as there is insufficient material available to cover the market needs [3]. However, by employing only a thin layer of germanium, deposited on a suitable substrate, this problem can be avoided. An important advantage of using epitaxial growth of Ge on Si substrates is the scalability: germanium layers of larger diameter can be obtained by simply using larger Si substrates. Epitaxial growth is mostly utilized to obtain a crystalline layer on top of another crystalline material. However, heteroepitaxial growth of germanium on silicon is rather difficult because of the large mismatch of 4 % between the two lattice constants [4]. This difference in lattice dimensions leads to island growth, causing high surface roughness and high density of
threading dislocations in the Ge layer. Obtaining high quality and smooth crystalline germanium, directly on silicon, is therefore challenging. Recently we have demonstra
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