Growth rate of equilibrium-like-shaped single crystals of superionic conductor cuprous selenide

Possible burst-like facet growth mode at high temperatures

“…Since the details of our experimental setup for growth of nonstoichiometric spherical Cu 2Àd Se single crystals have been described in detail elsewhere [14], we will review it here only briefly and will pay more attention to the implementation of the digital interferometry. Our crystals have grown (cf.…”

“…1 in Ref. [14]) in free space, on the small capillary tip, at a fixed temperature of around 800 K. The copper atoms influx has been streamed from a piece of pure copper, diffusing to the site of crystal growth by bulk diffusion through the polycrystalline Cu 2Àd Se specimen, driven by the constant chemical potential established between the pure copper and the surface of growing crystal embedded in Se vapour at constant pressure. All temperatures crucial for growth have been kept stable within 1 K.…”

“…Their structural and transport properties have been widely investigated. Particularly, these properties have been investigated as functions of stoichiometry in quite some detail [13,14]. Their bulk diffusivity depends on temperature and stoichiometry, and at 800 K the diffusivity of copper selenide acquires the value D ¼ 6.2 Â 10 À7 m 2 s À1 [15].…”

“…Therefore, the growth of copper selenide proceeds via fast bulk diffusion of metal atoms through the crystal to its surface, which is embedded in the atmosphere containing the chalcogenic component, whose vapour pressure is the equilibrium vapour pressure, and thus defines the stoichiometry of the growing crystal [18]. The centimeter size is reached within several days in the metal atoms flux (solid state) controlled mode of growth, J Cu 5J Se [14]. We note that for one of our crystals, which has grown at higher constant volume growth rate (0.191 mm 3 /h), approximately 4 Â 10 15 copper atoms reach crystal surface per cm 2 per second from inside (for crystal sizes of rE2 mm), which should be compared to approximately 5 Â 10 20 selenium atoms per cm 2 per second reaching the surface from the outside [14].…”

“…The centimeter size is reached within several days in the metal atoms flux (solid state) controlled mode of growth, J Cu 5J Se [14]. We note that for one of our crystals, which has grown at higher constant volume growth rate (0.191 mm 3 /h), approximately 4 Â 10 15 copper atoms reach crystal surface per cm 2 per second from inside (for crystal sizes of rE2 mm), which should be compared to approximately 5 Â 10 20 selenium atoms per cm 2 per second reaching the surface from the outside [14]. This means that the influx of copper atoms is slow enough to enable sufficient time for each copper selenide molecule to relax at the surface.…”