Effects of post-deposition annealing on crystalline state of GeSn thin films sputtered on Si substrate and its application to MSM photodetector

Ge_1−xSn_x alloy thin films were prepared by co-sputtering from Ge and Sn targets on a Si (100) substrate at room temperature, and were then heated at temperature ranging from 200 ${}^{\circ }C$ to 500 ${}^{\circ }C$ in N₂ ambient to reduce the disorder and defects and increase the crystalline quality of the films. Images obtained by field emission scanning electron microscopy revealed that the as-grown and all annealed samples displayed a densely packed morphology. The atomic percent composition of Sn in the as-grown Ge_1−xSn_x film is 5.7 at$\%$. Energy-dispersive x-ray spectroscopy results showed Sn surface segregation after heat treatment, as the Sn composition is reduced to 3.3 at$\%$ for the film annealed at 500 ${}^{\circ }$C. The Raman analysis showed that the only observed phonon mode is attributed to Ge–Ge vibrations. The Raman spectra of as-sputtered and annealed films revealed their nanocrystalline-amorphous nature. The samples annealed at lower temperature exhibited higher phonon intensity, indicating the improvement of crystallinity of the film. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on the annealed sample at 200 ${}^{\circ }C$ and the as-sputtered sample were studied in the dark and under illumination. Compared with the as-sputtered one, the annealed sample showed lower dark current and higher current gain of 209. The results showed the potentiality of using the sputtering technique to produce GeSn layer for optoelectronics application. (paper)