Silicon Thin Film Microstructure And The Dangling Bond Defect Research

Si-based thin film solar cells, as the most promising solar cells now, have been intensively studied and applied in many fields,because of their advantages, such as abundant storage of Si in the earth, environmental friendly characteristic, and low manufacturing cost.The thin film has been found to consist of structural inhomogeneities ralated to grain boundaries, micro-voids and dangling bonds, introduced during the prepared process of the Si films.Microstructure defects significantly affect the properties of solar cells.Small-angle X-ray scattering(SAXS)measurements have revealed the presence of micro-voids in the thin films;while grain boundary defects are investigated by TEM as well as HR-TEM. However, the most important defects in the Si films are dangling bonds.The dangling bonds, which are additional combination centers for electrons and holes, increase the capture cross section and reduce life of electrons, and consequently affect the stability of cell properties.There are spins in the un-paired electrons in dangling bonds.Electron spin resonance (ESR) investigations have revealed a very high spin density, up to 1020 cm-3,in the evaporation and sputter deposited a-Si films. The un-doped a-Si films with high density of dangling bonds(1018cm-3 or even higher) possess poor electronic quality and can not fulfill the requirement in component applications.Therefore, it is essential to investigate the dangling bonds and associated defects in the Si films.In this work, Si films were prepared or treated with three different techniques, including ECR-PECVD,ArF excimer laser crystallization and intermediate frequency magnetic sputter deposition. The phase structure, preferential orientation, crystallinity, H concentration, and dangling bond density of the as-prepared Si films were investigated with transimission electron microscope (TEM), X-ray diffreaction (XRD), Raman spectrum, Fourier transform infrared spectrum (FTIR) and electron spin resonance (ESR).The influences of crystallinity, H content, and dangling bond density on properties of Si films were analyzed, with the purpose of achieving a stabilized preparation technique.The main results are:(1)The dangling bond density in the Si films, prepared on the substrate of(100)Si plate by means of ECR-PECVD under 70 sccm Ar flow rate,is minimum of 4.42×016 cm-3,while the H concentration is-10 at.% and the crystallinity of-50%.The Ar flow rate of 70 sccm is optimum in comparison with other flow rates.(2) ArF laser surface treatment causes a reduction of dangling bond density by-1 order of magnitude, compared with un-treated Si films.(3)In the Si films prepared by intermediate frequency magnetic sputter deposition on the substrates of (100) Si and normal glass sheets, the dangling bond density is lower on the substrate of (100) Si sheet.