Electron microscopy investigation of sputtered and glow-discharge amorphous silicon-tin alloys

Abstract

The alloying of amorphous silicon with tin for systematic modification of the optical band gap for multijunction thin film solar cell applications has recently attracted interest. This is a comparative study of the microstructural properties of both the RF sputtered and glow discharge a-Si:Sn thin films. Sputtered films were deposited at 20 mTorr and 4 mTorr dynamic pressures of Ar-10 vol.% H2. Glow discharge films were deposited using SiH4, H2 and Sn(CH3)4 gas mixtures. Details of both the sputtering and glow discharge deposition conditions have been recently published elsewhere.In this work, Sn precipitation was observed in all hydrogenated sputtered alloys. Figure 1 shows the increase of the density of precipitates as a function of Sn concentration. These micrographs also reveal a non-coalesced (cauliflower) microstructure in the matrix. Figure 2 shows edge on view SEM micrographs of one of the sputtered films, which shows Sn precipitates originating at the substrate and in the bulk of the growing film. In a device structure, this would lead to electrical shorting and prevent the application of this material to solar cells.