Crystallization of Amorphous Selenium in the Cylindrical Morphology

Crystal Growth in Amorphous Selenium Thin Films─Reviewed and Revisited: Direct Comparison of Microscopic and Calorimetric Measurements

“…Consequently, the activation energy of crystal growth E G can be associated with the activation energy of the self-diffusion processes ED (as reported in refs – , directly in relation to the Se thin films) to verify the validity of the Stokes–Einstein relationship, − which identifies the correlation between the diffusion coefficient D and reciprocal value of shear viscosity η. Considering the potential breach of the Stokes–Einstein formalism, Ediger et al have introduced the decoupling parameter ξ defined as where E η is the activation energy of the viscous flow, f p is the probability of the structural entity, newly attached to the crystal growth interface remaining within the crystalline phase, and Δ G lc is the difference between the Gibbs energies of undercooled liquid and crystalline phases. , In eq , ξ = 1 corresponds to the Stokes–Einstein concept (usually valid at high temperatures, near the thermodynamic equilibrium), and ξ < 1 indicates a higher mobility of the structural units compared to what would be dictated by the sole impact of shear viscosity (from the mechanistic point of view, this behavior can be explained by the existence of the structured domains in the supercooled liquid/glass).…”

“…In order to investigate the above-reported difference further, the available literature data on the crystal growth rate u r in a-Se thin films were added in the plot together with the information about the relevant conditions (film thickness, substrate material, and illumination)see Figure A,B. The data , in Figure A show that the illumination of the a-Se thin film during the crystallization can accelerate the growth rate by up to 1 order of magnitude. The actual thickness of the thin a-Se films seems to be largely irrelevant on its own.…”

“…However, based on Figure A, the substrate material appears to be the dominant factor for the resulting growth rate of the Se crystals. The substrates with a potential nanoporosity that in addition contain sodium (soda-lime white glass, mica = phyllosilicate mineral with the general formula (Na/K/Ca) 2 (Al/Mg/Fe) 4–6 (Si/Al) 8 O 20 (OH/F) 4 ) lead to the very high crystal growth rates in the deposited a-Se films, whereas the polymeric substrates (Kapton, Mylar = biaxially oriented polyethylene terephthalate) result in the growth rates very similar to that in bulk selenium. Note that the a-Se growth rate data from the literature were measured at the free surface, at the Mylar/Se interface, and also perpendicularly to the substrate (hence the seeming deviation of the third data-line attributed to the literature in Figure A).…”

“…The increasing nucleation and crystal growth rates are associated with the higher number of the so-called dangling bonds, that is, unsaturated bonds at the ends of the Se chains and/or broken rings. This concept can be perceived as a common unifying feature for all the above-described aspects influencing the crystal growth rate: the photo-crystallization is caused by light producing a hole–electron pair either via breaking the Se–Se bond (resulting in the formation of two dangling bonds) or by the loss of one electron from the 3p lone pair of the di-coordinated Se atom (resulting in the formation of one dangling bond); , the deposition conditions (mainly the substrate temperature) have a crucial effect on the ring/chain ratio; , certain impurities saturate the Se dangling bonds, suppressing the nucleation and crystal growth; , different impurities (Na + ) can accelerate the crystal growth, for example, by activating new dangling bonds via plasticization and untangling of the chain structure. Based on this concept, the large diversity in the u r values displayed in Figure A,B can be reasonably explained.…”

“…After reaching the free surface of the thin film, the growth velocities are reported in the following order: u s ≥ u i ≫ u t , where the subscripts refer to the surface lateral growth, interface lateral growth, and perpendicular growth along the film thickness, respectively. However, the actual quantitative descriptions of the crystal growth, namely, the growth rate values, wildly differ throughout the available literature − including some very recent results . In accordance with this situation, the aim of the present paper is twofold: First, the temperature dependence of the crystal growth rate in several a-Se thin films will be carefully and precisely determined under a variety of well-defined conditions, and a thorough analysis of the literature data will be made based on the comparison with the present results.…”