Gallium selenide, GaSe, is a well-known nonlinear optical crystal widely used for frequency conversion over visible, IR and THz spectral ranges. During last decade, the breakthrough application of GaSe for terahertz (THz) generation over extremely wide spectral range under the pump by near-IR coherent sources was reported. Most recently, efficient THz generation from two collinearly propagating CO₂ laser pulses was demonstrated that resulted by significant improvement of the frequency conversion efficiency of CO2 laser within mid-IR (second, third and forth harmonic generation) GaSe possesses layered crystal structure with high cleavage properties.  Initially, 120-150 g of polycrystalline material was obtained in single-zone horizontal furnace using high purity (99.9999%) gallium (Ga) and selenium (Se). The synthesis was performed in sealed quartz ampoules by direct fusion of the elements and following homogenization for 1 day at 1000˚C. The crystals used in this study were grown by the conventional Bridgman technique in evacuated quartz ampoules 18 mm in diameter. The temperature gradient at the crystallization front was 10 deg/cm and the crystal pulling rate - 6 mm/day. The samples 20-22 µm, 70-80 µm, ~ 0.3-0.5 mm, and 3 mm in thickness were cleaved from the grown ingots parallel to (001) and were studied without any additional treatment and polishing. Micromorphology of cleaved GaSe(001) surface has been observed by AFM with Solver P-47H device in noncontact mode. SEM explorations were implemented using LEO 1430 and JEOL JSM 6700F devices. Structural properties were evaluated by TEM analysis. As it was shown by AFM observation, the cleaved GaSe(001) surface is almost entirely atomically flat with as low rms parameter as ~0.06 nm for an area of 5×5 µm². In several scans, a hillock-type formations were detected with bottom diameter of ~200 nm and height of ~20-35 nm. The hillocks were also detected in high-resolution SEM patterns. The Ga/Se ratio estimated by local chemical composition analysis was as low as 0.55-0.84 that indicates drastic depletion of the hillock material by gallium.  Acknowledgements: This study is supported by SB RAS (Project 46.13) and Ministry of Education and Science of the Russian Federation (Contract 16.518.11.7091).

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