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Si and Ge nanostructures epitaxy on a crystalline insulating LaAlO3(001) substrate

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Abstract

We present a comparative structural study of the growth of Si and Ge deposited by molecular beam epitaxy (MBE) on a c(2 × 2) reconstructed LaAlO3(001) substrate. Our findings are based on complementary experimental techniques such as in situ X-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED), low energy electron diffraction (LEED) and ex situ atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). While the layers are amorphous and wet uniformly the substrate in a low deposition temperature range, above 500 °C both Si and Ge growths proceed in a Volmer–Weber (VW) mode leading to the formation of nanocrystals (NCs). The islands are found to be composed of pure Si and Ge and to have abrupt interfaces with the substrate.

Both semiconductors (SCs) crystallize in their own diamond structure leading to relaxed NCs. No facets could be observed on the crystalline islands. An epitaxial relationship is established for which the (001) planes of Si and Ge are parallel to the LaAlO3(001) surface but are rotated by 45° around the [001] growth axis. The Ge lattice undergoes a second rotation of 6° with respect to the (001) growth axis. This 6° tilt is an original mechanism to partially compensate the strain in the Ge islands induced by the large misfit. Whereas a unique epitaxial relationship is pointed out for Si NCs, many Ge NCs are randomly orientated on the surface. This is interpreted by the fact that the Ge islands are less anchored to the substrate due to a large misfit and to the fact that the Ge[BOND]O bonds are weaker than the Si[BOND]O ones.

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