Silicon Nanowire Structure Displays Enhanced Optical Birefringence
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which are made of silicon dioxide only about 1.5-nm thick (about four atomic layers of SiO2). Further decreases in the thickness of the SiO 2 would result in unacceptably high leakage current that would drain batteries and overheat chips. To continue the advancement of microelectronics, materials with higher dielectric constant (k) and higher thickness will be needed to replace the SiO2. This is the main research focus of Roy G. Gordon and co-workers at Harvard University. In the August issue of Chemistry of Materials, the research team reported the use of chemical vapor deposition (CVD) and atomic layer deposition (ALD) techniques to deposit metal silicates and metal oxides. The researchers use reactions between the vapors of metal alkylamides and tris(tert-alkoxy)silanols. For CVD, a reaction chamber is supplied with continuous flows of the vapors of tetrakis(dimethylamino)hafnium and tris(tert-butoxy)silanol, which were dissolved separately in tetradecane solutions and then flash-vaporized. With substrate temperatures ~250-350°C, the deposited product is a hafnia-silica glass, HfO2(SiO2)x. For ALD, pulses of each vapor are dosed alternately and separately into the reaction chamber, producing the same composition with very uniform thickness, even inside small holes. ALD showed high growth rates of 0.3–0.4 nm per deposition cycle, a level higher than traditional ALD reactions. This ALD procedure can also be modified to deposit metal oxides by replacing the pulses of tris(tert-butoxyl)silanol vapor with water vapor. By varying the ratio of silanol pulses to water pulses, the ratio of silicon to hafnium can be controlled from x ~ 0 to 3. Rutherford backscattering measurements showed impurity levels of nitrogen and carbon to be very low (
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