Effect of supercooling on the microstructural development and optimization of physical properties of melt grown SnSe crystals

The microstructural development of stoichiometric tin monoselenide (SnSe) crystals grown by vertical Bridgman–Stockbarger method using an indigenously fabricated furnace has been investigated under high vacuum (~ 10⁻⁶ mbar). The ampoule translation rate (t_r) and supercooling, ΔT (= T_m − T, where T_m is the melting point and T is the crystallization temperature) were varied in the range, 12–2 mm/h and 20–100 °C respectively. Enhancement of ΔT and t_r led to constitutional supercooling, inducing compositional changes and non-stoichiometry. Low ΔT (20–40 °C) and high t_r (12–10 mm/h) resulted in globules, flakes and cavities. When ΔT = 60 °C and t_r = 9 to 7 mm/h, mounds were formed with closed contours and ripples, due to atomically rough liquid–solid (l-β) interface. Fine tuning of ΔT (60 °C) and t_r (2 mm/h) enabled smooth planar interface, so as to yield good quality crystalline structures with periodic atomic deposition promoting crystal growth, layer-by-layer. Energy dispersive analysis by X-rays and powder X-ray diffraction studies revealed appreciable crystallinity, chemical homogeneity and phase purity. The density of crystals estimated from crystallographic data (6.183 g/cm³) corroborates with that obtained utilizing Archimedes principle. Thermogravimetric and microindentation analyses established thermal and mechanical stability. The low etch pit density (~ 10² cm⁻²) manifests nearly perfect growth of crystals than their melt counterparts. UV–Vis–NIR and PL spectra reflected direct transition with an energy gap of 1.32 eV, validating immense potential of the grown crystals for photovoltaic applications.