UHV-CVD growth of Ge/Si nanostructures
Sammanfattning: This thesis is based on the results concerning the epitaxial growth and characterization of silicon (Si) and germanium (Ge) nanostructures. The growth technique was the Ultra High Vacuum Chemical Vapor Deposition (UHV-CVD) that works in relatively low temperatures and low growth pressures. The UHV-CVD chamber was also used for the reduction experiments. The work presented here involves three different projects: (i) Ge islands as crystal seeds for the growth of a pure Ge layer on Si(100); (ii) Positioning of Ge/Si islands using surface patterning and (iii); Growth of SiGe nanostructures for realization of Esaki Diodes. The goal of the first project (i) was to use self-assembled Ge islands as crystal seeds for growing a thick layer of pure Ge on Si substrate which is of a great technological importance. A two-step growth process was used to achieve an almost unimodal size distribution of Ge islands. The study of the surface included oxidation under different conditions (temperature, atmosphere) and reduction in hydrogen at different temperatures. Conditions were found where the islands were partially oxidized, leaving part of the islands intact. The reduction of the surface in hydrogen did not result in the complete recovery of the Ge islands as intended. Instead crystalline germanium segregations that were epitaxially oriented to the substrate were observed. Finally, the native oxide of the surface structure was used for the overgrowth with Ge. Single crystalline Ge could be grown over the islands. This was achieved by a oxidation/reduction step, before the growth on top of the Ge islands. The epitaxial information was transformed from the island through the reduced oxide layer to the deposited Ge. Atomic Force Microscope (AFM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were used for characterization of the samples. The aim of the second project (ii) was the controlled spatial positioning of Ge islands that could be useful for different applications. The surface patterning was realized through the deposition of nanometer-sized carbon masks by an electron beam. Partial overgrowth of the masked surface with Si resulted in pits over the C depositions. Ge deposition on these structures occurred preferentially in this pits. By changing the size of the carbon growth mask, and thereby by changing the lateral extension of the pit, the number of the dots around the pit was altered. Quadruples of Ge islands could be grown. grown AFM was used for characterization while Electron Beam Lithography (EBL) for patterning of Si substrate. In the third project, a combination of UHV-CVD epitaxial growth of SiGe hetero-structures with Rapid Thermal Diffusion (RTD) process of Phosphorous was used to fabricate SiGe Esaki tunnel diodes. Heavily doped Boron SiGe layers followed by intrinsic SiGe layers were grown on p-doped Si substrates by UHV-CVD. Phosphorous was diffused through the intrinsic layer realizing the p-n junction. This combined approach is suitable for the integration of tunnel diodes with the mainstream SiGe-technology. The diodes show a peak current density of 0.18 kA/cm2 and a peak to valley ratio of 2.6 at room temperature.
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