Development of a-Si:H films during plasma-enhanced CVD from hydrogen-diluted silane


Development of a-Si:H films during plasma-enhanced CVD from hydrogen-diluted silane

Panchev, B.; Danesh, P.; Grambole, D.; Schmidt, B.

The application of hydrogenated amorphous silicon (a-Si:H) in the photovoltaic devices provided a basis for an extensive process development. Plasma enhanced chemical vapor deposition (PECVD) from pure silane is the most often used method for preparation of a-Si:H films. Recently, the benefits of strong dilution of silane with hydrogen have been demonstrated for the decrease of the light-induced degradation of the a-Si:H solar cells. The growth of a-Si:H films in a hydrogen-rich plasma is accompanied by two processes - in-situ hydrogenation and hydrogen etching. The hydrogenation leads to a relatively high hydrogen concentration in the films. The hydrogen etching promotes the morphological transition of amorphous network to the microcrystalline during the film growth. It has been suggested that regions with a higher structural order develop gradually as a-Si:H grows and these serve as sites for microcrystal growth. Such a gradual change has also been observed for the defect density and a clear relationship between the electronic properties and the medium-range order of the amorphous network has been established.
The aim of the present work is to find more evidence about the structural and compositional development of a-Si:H during the film growth. We have studied the thickness dependences of the interconnected void network and the depth profiles of the hydrogen concentration. The intrinsic mechanical stress in the films has also been measured, since it is related with both the structural order and the hydrogen content and is a parameter especially interesting for the large-area device processing.
a-Si:H films were deposited by PECVD from 10% silane in hydrogen - a dilution, at which a completely amorphous material can still be prepared. The substrate temperature was varied in the range of 150-270 °C. The films were deposited with a thickness in the range of 100-600 nm, the most important one for the device preparation.
The hydrogen and its depth distribution have been studied using nuclear reaction analysis. The dependence of the interconnected void network on the film thickness has been established by means of field-assisted ion exchange. This method is based on the silver-sodium ion exchange in the glass substrate and is sensitive to the interconnected void strings crossing the entire film thickness. The depth profiles of stress have been obtained by measuring the radii of curvature of a series of samples with different film thickness. The effect of the substrate temperature on the hydrogen concentration, void network and on the intrinsic stress has been studied, as well.
The obtained results suggest that there is no apparent correlation between the hydrogen concentration and the structural properties of the material. The temperature dependences are rather unusual and are discussed in view of the specificity of a-Si:H growth process in the hydrogen-rich plasma.

  • Poster
    5th European Conference on Photovoltaic Devices - High Efficiency Solar Cells, Tomar, Portugal, September 08-13, 2001

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Publ.-Id: 4891