Structural properties of microcrystalline silicon in the transition from highly crystalline to amorphous growth

The growth of microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition depends on the deposition conditions and yields films with variable content of crystalline grains, amorphous network, grain boundaries and voids. The changes in the structural properties of a series of films grown under a variation of the dilution of the process gas silane in hydrogen, which induces a transition from highly crystalline to amorphous growth, were investigated. The evolution of the crystalline volume fraction was quantitatively analysed by Raman spectroscopy and X-ray diffraction. The results confirm the need for proper correction of the Raman data for optical absorption and Raman cross-section. Transmission electron microscopy was used to investigate the characteristics and the variation in the microstructure. Upon increasing the silane concentration the strong columnar growth with narrow grain boundaries degrades towards the growth of small irregularly shaped grains enclosed in an amorphous matrix. Simultaneously, the initial film growth becomes predominantly amorphous. Spherical voids with a diameter up to 14 nm and 'crack-like' cavities (which accumulate at the film-substrate interface in the highly crystalline growth regime) were observed. Relaxation reactions of strained bonds within a growth zone of the order of a few tens of nanometres and preferential etching are proposed to govern the growth process.