Reduction of Tin Oxide by Hydrogen Radicals

Abstract

The effect of a reducing hydrogen ambient on textured tin oxide thin films on glass substrates has been investigated. Hydrogen treatments were done at 230 and 430 °C by hot wire (HW) and rf plasma-decomposed hydrogen with pure H 2 as source gas. By these treatments the possible reduction of the substrate during the deposition of a-Si:H for solar cells is simulated. Ion beam techniques revealed that the exposure to HWdecomposed H-radicals leads to the formation of a tin-rich surface layer of 40 nm in 1 min at both 230 and at 430 °C. The loss of oxygen is higher for the high-temperature treatment. The optical transmission at a wavelength of 800 nm is reduced from 80% to less than 20%, while the sheet resistance increases from 6 to 8 Ω/0. At both temperatures the reduction of fluorine-doped tin oxide (FTO) by a HW-treatment occurs faster than by rf plasma-decomposed H. The H radical concentration, which is higher for the HW-decomposed hydrogen as compared to rf plasma-decomposed hydrogen, is the most important factor in determining the rate of the reduction process. For short exposures to H radicals, the transparency and conductivity of the tin oxide may be completely restored by means of reoxidation in air at 400 °C. In contrast, prolonged exposure to H-radicals induces irreversible loss of transparency and conductivity, concomitant with formation of granulelike particles of metallic tin on the surface. A thin plasma-deposited a-Si:H-layer was found to effectively protect the FTO-layer against reduction due to HW-generated H-radicals.

Influence of helium dilution of silane on microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by HW-CVD

“…The band gap linearly increases from 1.5 to 1.7 eV when the hydrogen content in the film increases from 2 to 11 at.%. Then up to a hydrogen contents of 20 at.%, the band gap remains constant [58]. However, in the present study, the band gap remains as high as 2.0 eV or more despite the hydrogen content in the film is <2.2 at.%.…”

Synthesis of Hydrogenated Nanocrystalline Silicon Films By HW-CVD Without Hydrogen Dilution of Silane

“…Then up to a C H of 20 at. %, E g remains constant [8,9]. However, in the present study, E g remains as high as 2.1 eV although C H in the film is < 6 at.…”

Highly Conducting Phosphorous Doped nc-Si:H Thin Films Deposited at High Deposition Rate by Hot-Wire Chemical Vapor Deposition Method

“…Then up to a hydrogen contents of 20 at.%, the band gap remains constant. 47 Thus, only the number of Si-H bonds or hydrogen content in the film cannot account for band gap of HW-CVD deposited nc-Si:H films. Furthermore, there are several ambiguities about the band gap of nc-Si:H films because the material contains both phases, amorphous and crystalline and their properties vary with the volume fraction of these phases.…”