A Study of the Effect of Oxide Structure on the Synthesis of Nanocrystalline Ge from Si 1 - x Ge x O 2
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The high pressure system used for this study was designed to run either dry (pure 02) or wet oxidation experiments. The dry (ultra high purity oxygen) method is described in detail elsewhere [7] and the wet (deionized water) is described below. For dry high pressure oxidation (DHPO), a temperature of 550'C and a pressure of 70 MPa was employed for 5 hours. Wet high pressure experiments involved shorter processing times of 2-3 hours, lower temperatures of 450500'C, and lower pressures of 30-40 MPa. The focus of this study was the use of the hydrothermal method. However, dry high pressure runs were also made in order to determine the influence of the oxidizing ambient on the incorporation of hydroxyl groups before and after H 2 reduction. The hydrothermal high pressure oxidation (HHPO) system consists of a Rene 41 pressure vessel fitted with an air-driven hydrostatic pump. The HHPO vessel is externally heated in a standard tube furnace mounted on rollers to allow rapid cooling. The high pressure seal is a water cooled cone-type and the internal temperature of the vessel was calibrated using a thermocouple with a high pressure feedthrough. Several precautionary measures were taken to minimize contamination during oxidation. Samples were stacked between layers of silicon wafers and then loaded into a clean quartz tube for isolation. This tube and its contents were then sealed inside the hydrothermal vessel. Following oxidation, samples were annealed at 800°C, 850°C, and 900'C in flowing forming gas for 10, 15, and 30 minutes. A novel application of the AFM technique was developed to obtain cross-sectional images of the annealed and unannealed oxides. A Digital Instruments Dimension 5000 Scanning Probe instrument was used to determine particle size distribution in the oxides in cross-section and to provide an accurate measure of the oxide thickness. Successful implementation of this approach requires a high quality surface for AFM imaging. For this reason samples were polished in crosssection and a smooth, clean surface was produced using a TEM tripod polisher in conjunction with a series of diamond lapping papers on a flat glass polishing wheel with water. Edge rounding effects were avoided by sputtering approximately one micron of silicon onto the top surface of the sample prior to cleaving. RESULTS Cross-sectional field emission SEM images were acquired from H 2 annealed hydrothermally processed samples. A representative micrograph from a 15% Ge sample annealed at 800°C for 30 minutes is presented in Fig. 1, which shows that the Ge crystallites are distributed non-uniformly through the oxide layer with larger particles clearly visible at the oxide/Si interface and a higher density of smaller particles near the top surface of the oxide. Images were also
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Fig. 1: HRSEM crosssectional image of hydrothermally processed SiO.8 5Geo.1502 (after a 30 min. anneal at 800'C) revealing a large density of small particles at the oxide surface and a smaller density of larger particles closer to the interface.
•oxide/Si
620
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