Control of electrical properties of BaSi2 thin films by alkali-metal doping using alkali-metal fluorides

Exploration of impurity elements for carrier density control is important for device application of the BaSi2 semiconductor, which is a promising candidate for an earth-abundant solar cell absorber. In this study, we have investigated the doping of BaSi2 films with alkali metals (Li, Na, and K) by deposition of alkali-metal fluorides followed by rapid thermal annealing. Electrical characterization by Hall measurement shows that LiF treatment increases electron density in BaSi2 up to 1020 cm− 3 after annealing at 500 °C while NaF and KF treatments have limited effects with electron densities lower than 1018 cm− 3. The mechanisms of electrical property modification are discussed from structural viewpoints. Secondary ion mass spectroscopy shows that Li atoms slightly diffuse into the film at 400 °C by prolonged annealing, which nevertheless does not accordingly increase the carrier density. Formation of barium fluoride, oxide, and silicon at 500 °C, which is revealed by energy-dispersive X-ray and Raman spectroscopies, brings about the conclusion that a high temperature is needed for the dissociation of LiF through the reaction with BaSi2 and the incorporation of Li atoms probably into the interstitial sites of BaSi2, which would generate electrons. On the other hand, Na and K atoms are found to evaporate at the same time as the dissociation of NaF and KF, respectively, evidenced by energy-dispersive X-ray spectroscopy. BaF2 formation accompanied by the NaF dissociation is detected by X-ray diffraction.