Structural properties of heteroepitaxial Ge films on a Si(100)-2×1 surface

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Abstract
Structural and electrical properties of epitaxial Ge films grown on a Si(100)-2×1 surface have been studied by means of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), x-ray diffraction, and Raman scattering. The attempt has been made to find the growth conditions, such as the substrate temperature TSi and Ge thickness dGe, for the single-crystalline Ge (100) on the Si surface. The sharp (2×1) LEED pattern of Ge (100) accompanied by the absence of any AES signals from the Si substrate is unambiguous evidence for the growth of single-crystalline Ge films. The structural quality of the Ge films was examined by the peak position and width of the Ge(400) Bragg line in x-ray diffraction. From these results, we found that more than 1000-Å Ge thickness is required to grow the high-quality epitaxial Ge films on the Si substrate at elevated temperatures. This is due to island formation at the initial stage of Ge deposition. Therefore, we also studied the role of thin Ge buffer layers fabricated at low temperatures below TSi=200 °C to grow thinner epitaxial Ge films without island formation. It was found that the amorphous buffer layers are poorly crystallized during the rapid increase in TSi up to 400–500 °C, prior to subsequent Ge deposition on the buffer layer. Consequently, single-crystalline Ge films are grown at relatively thin dGe in such a way that the underlying buffer layers accommodate the strains or misfit dislocations near the Si-Ge interface. The single-crystalline Ge films with and without the buffer layers thus grown on the Si substrates were also characterized by the line shape of the phonon Raman scattering spectra. Depending on the details of sample preparation, the spectra exhibited a characteristic change in shape including peak position and width. Having estimated the probing depth of the Raman scattering, we were able to gain deep insight into not only the Ge–Ge bond strength, but also the inhomogeneity of the Ge films and the extent of the strains into the Ge films, which were not accessible by LEED or x-ray diffraction.