Low temperature crystallization of germanium on plastic by externally applied compressive stress

Abstract

The conventional Cu-induced crystallization of a-Ge has been facilitated by different types of external stress mechanically applied to the flexible substrate. It has been observed that in the case of compressive stress, crystallization becomes possible at temperatures as low as 130 °C and evolves as stress becomes more stringent. High electrical conductance and a hole mobility of 110 cm2/V s show the crystallinity of the Ge film, further confirmed by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses. The temperature of the annealing process (between 130 and 180 °C) expedites the process (from 6 to 1 h) as it is increased, but the principal mechanism seems to be independent of temperature. Temperatures higher than 180 °C are detrimental to the plastic substrate, polyethylene terephthalate. Evolution of cracks in Ge layer has been studied as the main consequence of the interfacial stress between Ge layer and substrate. The crack density was minimized by patterning the a-Ge layer before annealing. The main explanation of the physical phenomenon accounting for crystallization is believed to be the externally applied compressive stress, which reinforces the inherently present internal stress between the layer and the substrate.