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A number of thin silicon films are prepared through ultra-high-vacuum evaporation on optical quality fused quartz substrates with different growth temperatures. Through an analysis of grazing incidence X-ray diffraction results, a phase transition, from amorphous-to-crystalline, is found corresponding to increases in the growth temperature. The corresponding Raman spectra are also observed to change their form as the films go through this phase transition. Using a Raman peak decomposition process, this phase transition is then quantitatively characterized through the determination of the amount of intermediate-range order and the crystalline volume fraction for the various growth temperatures considered in this analysis. The possible device consequences of these results are then commented upon.
We present comprehensive quantitative analysis of Raman spectra in two-(Si/SiGe superlattices) and three-(Si/SiGe cluster multilayers) dimensional nanostructures. We find that the Raman spectra baseline is due to the sample surface imperfection and instrumental response associated with the stray light. The Raman signal intensity is analyzed, and Ge composition is calculated and compared with the experimental data. The local sample temperature and thermal conductivity are calculated, and the spectrum of longitudinal acoustic phonons is explained.
We report the degradation of low temperature photoluminescence (PL) from Si/SiGe three-dimensional cluster morphology nanostructures under continuous photoexcitation. The PL intensity initially decreases slowly for about 15 minutes, and then decreases rapidly, until only ∼ 10% of the original PL intensity remains. A complete recovery of the PL requires restoring the sample temperature to ∼ 300K. We propose that a slow accumulation of charge in SiGe clusters enhances the rate of Auger recombination and results in the observed PL degradation.
Strain engineering in composition-controlled Si-Si/Ge nanocluster multilayers with high germanium content (~ 50%) is achieved by varying thicknesses of Si/SiGe layers and studied by low temperature photoluminescence (PL) measurements. The PL spectra show reduction in strained silicon energy bandgap and a splitting presumably associated with partial removal of heavy hole-light hole degeneracy in SiGe valence band. Time-resolved PL measurements performed under different excitation wavelengths show dramatically different PL lifetimes, ranging from ~ 2 μs to 10 ns and an unusually high PL quantum efficiency. The results are explained by using the Si/SiGe interface recombination model, which is supported by ultra-high resolution transmission and analytical electron microscopy measurements.
Infrared vibrational spectroscopy in an attenuated total reflection geometry has been employed to investigate the presence of organic and inorganic thin layers on Si-wafer surfaces. Three different processes were compared for surface contaminant removal; microwave plasma, UV-ozone, and a piranha solution cleaning. The CH vibrations at 2928 and 2856 cm-1 characteristic of organic contaminants were monitored before and after each cleaning procedure to determine how well it removed surface contaminants. We found that native oxide removal from the Si surface should only be carried out after a cleaning essay. We observed that surface oxide removal exposed a hydrophobic bare Si surface, attracting organic molecules present in solution or the ambient. A large increase of the CH vibrational signature was observed for a Si wafer after an HF dip. A combination of plasma cleaning followed by UV-Ozone treatment was found the most effective one for Si wafer cleaning. We were able to evaluate the effectiveness of the cleaning methods, hydrogen surface passivation and oxide removal/regrowth.
Reliable fabrication of high-speed, delta-doped transistors and better understanding of two-dimensional metal-insulator transitions can be achieved using silicon molecular beam epitaxy (MBE). However, this fabrication technique should be performed with care, avoiding dopant segregation on epitaxial Si surfaces and improving the doping efficiency. Here we report comprehensive structural and optical investigations of MBE-grown Si/delta-doped Si:B multilayer structures. Measurements of Auger electron spectroscopy, Raman scattering, optical reflection and photoluminescence are performed. Our results indicate nearly metallic conductivity at room temperature with metal-insulator phase transition near T ∼100 K. In contrast to recently reported data, no enhancement of photoluminescence at room temperature is found. Occasionally, a few samples in specific areas exhibit strong photoluminescence at 1.4-1.6 micron attributed to structural defects, most likely due to B segregation.
We find that in SiGe clusters grown on Si using Stranski-Krastanov (S-K) growth mode, (i) photoluminescence (PL) spectra, (ii) PL lifetime and (iii) PL thermal quench activation energies exhibit strong dependence on the excitation intensity. Under PL excitation intensity increasing from 1 to 104 W/cm2, the PL spectra exhibit blue shift from below Ge bandgap up to ∼970 meV. The PL lifetime shows strong dependence on the excitation wavelength, decreasing from 20 μs at ∼0.8 eV to 200 ns at ∼ 0.9 eV. The process of PL thermal quench has two clearly distinguished activation energies. At low temperature, small (∼15 meV) and excitation-independent activation energy is attributed to exciton thermal dissociation. At higher temperature, excitation-dependent PL thermal quench activation energy (increasing from ∼ 120 to 340 meV as excitation intensity increases) is found, and it is attributed to hole redistribution via tunneling and/or thermal ionization over the Si/SiGe valence band energy barrier.
We examine the nanostructural properties of Si/Si1−xGex island superlattices with 0.37 < x < 0.56 grown at 620-640 °C by molecular beam epitaxy. Analytical transmission electron microscopy (TEM) shows that during growth Ge atoms migrate towards the center of the large islands to maintain epitaxial growth and that the most regular structures are obtained at higher Ge composition when the built-in strain is also higher. High-resolution x-ray reciprocal space mapping shows that these heterostructures remain pseudomorphic and that the undulations are aligned along  directions and exhibit a long-range coherence and vertical correlation as revealed by the presence of strong satellites in a wave vector direction parallel to the surface. Raman spectra of these samples exhibit the usual Ge-Ge, Si-Ge and Si-Si vibrational modes. When compared with planar Si/Si1−xGex superlattices the Ge-Ge and Si-Ge Raman peaks are shifted to lower frequencies indicating an average alloy composition that is approximately x = 0.1 less than the nominal values, which is in general agreement with analytical TEM.
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