AVS 53rd International Symposium
    Advanced Surface Engineering Wednesday Sessions
       Session SE2-WeA

Paper SE2-WeA9
The Role of Oxygen Impurities in Anisotropic Etching of Crystalline Silicon by Atomic Hydrogen, and in the Deposition of Single-Phase Nanocrystalline Silicon

Wednesday, November 15, 2006, 4:40 pm, Room 2007a

Session: Pulsed Plasmas in Surface Engineering
Presenter: S. Veprek, Technical University Munich, Germany
Authors: S. Veprek, Technical University Munich, Germany
Ch.L. Wang, Consultant
M.G.J. Veprek-Heijman, Technical University Munich, Germany
Correspondent: Click to Email
Nanocrystalline silicon, nc-Si (and germanium) was originally prepared by means of chemical transport in hydrogen plasma, which combines the etching of silicon in the charge zone with the formation of monosilane as the main product, Si(s) + 4H → SiH@sub 4@, and the decomposition reaction SiH@sub 4@ → nc-Si + 2H@sub 2@ in the deposition zone.@footnote 1-4@ The etching rate is highest at a temperature of about 60-80 °C and it strongly decreases to essentially zero above 350 °C.@footnote 2@ After a sufficiently long dwell time, partial chemical equilibrium with monosilane concentration of about 0.3 - 0.5 mol.% is established in glow discharge hydrogen plasma at a temperature of 40 - 80 °C, regardless if monosilane, or solid silicon and hydrogen are used as reactants@footnote 4@ In the present paper we shall show, that minor oxygen impurities of about 5 - 10 at. ppm (atomic parts per million) strongly decrease the reaction rate, and above 50 - 60 ppm of oxygen this reaction cannot occur. The etching by atomic hydrogen is isotropic below about 5 ppm of oxygen impurities, whereas controllable addition of a few 10 ppm of oxygen in combination with negative bias to the crystalline silicon results in highly anisotropic etching with thin oxide acting as side-wall passivation. Oxygen impurities also hinder the formation of single-phase nc-Si and cause occurrence of a significant fraction of a-Si in the deposited films. Oxygen impurities of about > 0.5 at. % in the hydrogen plasma can completely hinder nc-Si to form. In a pure hydrogen glow discharge plasma, single-phase nc-Si without any detectable a-Si tissue can be deposited under conditions close to the partial chemical equilibrium. @FootnoteText@@footnote 1@S. Veprek and V. Marecek, Solid-State Electronics 11(1968)683. @footnote 2@S. Veprek and A. P. Webb, Chem. Phys. Lett. 62(1979)173.@footnote 3@Z. Iqbal, A. P. Webb and S. Veprek, Appl. Phys. Lett. 36(1980)163. @footnote 4@J. J. Wagner and S. Veprek, Plasma Chem. Plasma Process. 2(1982)95.