In Situ Monitoring of Cu2ZnSnS4 Absorber Formation With Raman Spectroscopy During Mo/Cu2SnS3/ZnS Thin-Film Stack Annealing

In recent years, Cu
₂
ZnSn(S,Se)
₄
(kesterite) has become increasingly popular as a sustainable alternative absorber material. Many processes for kesterite synthesis involve a high temperature annealing step (>450 °C). This study investigates the possibility of Raman spectroscopy as an in situ monitoring technique during high temperature annealing up to 550 °C. Temperature-dependent behavior of Cu
₂
SnS
₃
(CTS) and Cu
₂
ZnSnS
₄
(CZTS) was studied for reference purposes. The synthesis of CZTS was performed by annealing a stacked Mo/CTS/ZnS precursor on a glass substrate. Annealing of the precursor stack resulted in formation of kesterite and could be monitored in situ by its main A-mode at 338 cm
^-1
. At higher temperatures, this mode shifts to lower wavenumbers, is broadened and reduced in intensity. This can be attributed to combined effects of thermal expansion and anharmonic phonon coupling. The shift of the peak position is linearly proportional to the temperature. Thus, given proper calibration, fitting the peak position of the 338 cm
^-1
mode during the process yields the sample temperature. Implementation of in situ monitoring with Raman spectroscopy would be a step forward toward desired process control and monitoring during this crucial high temperature annealing step in kesterite synthesis.