Electrical Properties of Alkyl-Trichlorosilane Monolayers Grafted on Silicon Substrate
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gate) with a leakage current density as low as 10-8 A/cm 2 at 6 MV/cm, high dielectric breakdown field (12 MV/cm), electrically active defect density lower than 1011 cm -2, and low field dc conductivity as low as 10-16-10-15 Scm" 1. We demonstrate that these insulating films are thermally stable up to 450'C in oxygen free atmosphere, and technological processes are described in order to obtain such excellent insulating properties. These electrical properties are correlated with the structural properties of these OML films. Highly compact monolayers have been obtained with a surface density of about 5x10 14 molecules/cm 2 . Such a density corresponds to full monolayer coverage since each molecule should occupy 20 A2 at close packing [4]. Atomic force microscope measurements have shown that these OML are free of pinholes [5]. SAMPLE PREPARATION We have used oriented silicon substrates, boron doped to 1015 cm- 3 (p-type) and phospohorous doped to 1017 cm-3 (n-type), naturally covered with an ultra-thin (- I - 1.5 nm) layer of silicon oxide. This provides a dense array of reactive hydroxyl groups which are the natural grafting sites for trichlorosilane molecules (CH3 -(CH 2)n-SiCl 3, with n=7 to 17). We have also used degenerate substrates (n+,~10l 9 cm-3 ) in order to realize MIM-like (Metal-Insulator-Metal) structures. The silane molecules are first dissolved at low concentrations (10-4 moll-1) in an inert organic solvent such as hexadecane. Special care is taken to avoid any contact with water moisture during that preparation step in order to avoid undesirable bulk polymerisation and formation of a large polysiloxane network visible to the naked eye in the solution. Extreme cares are taken to use an ultra-clean surface without residual contamination. Wet cleaning with organic and aqueous chemicals and dry cleaning by combining ultraviolet irradiation and ozone atmosphere were used before strating the silanization (i.e. the chemical grafting of trichlorosilane molecules). These carefully cleaned silicon substrates are dipped into the solution immediately after preparation and the chemical reaction is allowed to proceed to completion. Typical time frames are 10-60 minutes. Full details are given in Ref [3]. There are two important controlling parameters to insure good deposition, leading to homogeneous, dense monolayer. First, the temperature of the reactive bath has to be maintained below a critical temperature, specific of the length of the molecule to be grafted but independent of the nature of the solvent [3]. Second, the substrate to be covered has to be pre-hydrated with molecularly thin layer of water to allow (figure 1) : i) the hydrolysis of the chlorosilane head groups to silanols , ii) the covalent bonding of these silanol groups with the surface hydroxyl groups, iii) the in-plane condensation of the unreacted silanol groups between neighbouring molecules, and finally, iv) the lateral mobility necessary for an eventual redistribution of the molecules within the monolayer [6,7]. If all conditions are correct,
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