Measurement of defect densities in Ge films grown on Si
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The effect of Si+ and Ge+ source ions on surface morphology and strain relaxation of Si1−xGex (0.25≤x≤1.0) film on Si (001) substrate was investigated using potential-enhanced molecular beam epitaxy. The growth temperature ranged from 450 to 710 °C. The applied potential to the substrate was varied between −2.0 and 1.5 kV. The acceleration of a small fraction of source ions toward the substrate suppressed three-dimensional nucleation mode, and dramatically improved the surface morphology. Contrary to the negative potential, the application of a positive potential did not contribute to improving the surface smoothness of Ge on Si. For the growth of Si0.5Ge0.5 film, the surface morphology was degraded further by applying a positive potential of 1.5 kV. Si0.75Ge0.25 films grown with a negative potential of −1.6 kV relieved the strain at the much earlier stage of heteroepitaxial growth than that of conventional molecular-beam epitaxy.
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Rutherford backscattering spectrometry
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Raman microscopy has been used to study the carrier concentration and mobility in n‐doped 3C‐SiC epilayers grown on different silicon substrates, namely (100) Si and (111) Si on axis and off‐axis towards the [110] direction. By analyzing the longitudinal optical phonon‐plasmon coupled mode (LOPC), we were able to estimate the 3C‐SiC electron bulk mobility (μ) within the range between 5 and 500 cm2/Vs. The carrier concentration (n) was ranging from 2×1016 to 6×1018 cm−3. The observed trend shows a reduction in the electron mobility as the carrier concentration increases for films grown on any substrate considered, accordingly to the existent theory. For equal values of doping concentration, 3C‐SiC epitaxial films grown on (100) Si substrates show a higher mobility than films grown on (111) Si counterparts. This could be ascribed to a higher defect density in (111) Si samples. A deeper characterization by performing Raman maps shows a broadening and a splitting of the 3C‐SiC transverse optical (TO) peaks for the (111) Si samples, confirming a lower crystal quality than the (100) Si.
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High-quality Ge films were grown on Si substrates by solid-source molecular beam epitaxy using SiGe graded layer and Sb surfactant-mediation technique. Transmission electron microscopy measurements show that samples grown using this method have a lower threading dislocation density than those grown by other typical methods, such as grading at high temperature (700 °C) only, grading at intermediate temperature (510 °C) only, and the use of low temperature Si buffer. A relaxed Ge film on a 4-μm-thick graded buffer was grown and shown to have a threading dislocation density of 5.4×105 cm−2 and surface roughness of 35 Å. Ge p–i–n diodes were fabricated and tested. Under a reverse bias of 1 V, the p–i–n Ge mesa photodiodes exhibit a very low dark current density of 0.15 mA/cm2.
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Stress and strain in single-crystal Ge and GaAs/Ge films grown on SiO2-coated Si substrates have been investigated through the Raman microprobe analysis. The Ge film is found to be subject to a tensile stress of (3.4–6.1)×109 dyne/cm2 and a strain of (2.4–4.3)×10−3 without distinguished spatial variation within a SiO2 island. The stress agrees well with that evaluated from the GaAs electroluminescence spectral shift with the deformation-potential model. The strain can be attributed dominantly to the thermal expansion difference between the Ge and the substrate Si. A large Raman spectral shift to the lower frequency side and a large linewidth for Ge have been observed in the seeding opening region, which represents Ge lattice disorder in this region.
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In this work we performed measurements of photoluminescence (PL), scanning tunneling microscopy (STM), and Rutherford backscattering (RBS) at grazing angles of incidence in a set of samples grown by molecular beam epitaxy, in which a Ge layer was deposited on a Si (001) substrate covered with a thin SiO 2 layer. Three different thicknesses for either layer were deposited: 0.5, 0.75 or 1 monolayer (ML) of SiO 2 , and 0.3, 0.6 or 0.9 nm of Ge . The PL measurements for the samples with thicker layers show a broad band at ~ 0.85 eV superimposed on a dislocation related band at ~ 0.81 eV . The attribution of the high energy band to Ge islands in this sample is supported by STM and RBS measurements, as well as by PL measurements after hydrogen passivation of the sample surface. For the samples with thinner SiO 2 and Ge layers, no evidence for the formation of Ge islands was found.
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