Atomic layer deposition of Sn-doped germanium diselenide for an As-free Ovonic threshold switch with low off-current

This work introduces the atomic layer deposition (ALD) of Sn-doped GeSe₂ (SnGS2) films for developing arsenic-free Ovonic threshold switch (OTS) devices with low off-current (I_off) and desirable threshold voltage (V_t). The undoped GeSe₂ film exhibits high V_t and low endurance characteristics despite its low I_off originating from a large mobility gap. Such problems could be overcome by appropriate doping, and thus, Sn was doped into the GeSe₂ film using a supercycle consisting of SnN_x and GeSe₂ subcycles. The co-injection of NH₃ with Se-precursor ([(CH₃)₃Si]₂Se) during the GeSe₂ subcycle generated a reactive H₂Se intermediate, which enhanced the reactivity and transformed the pre-deposited SnN_x to SnSe₂. The overall cation-to-anion ratio of the deposited films remained at approximately 1 : 2, with amorphous structure and smooth surface morphology. The planar OTS device using SnGS2 films with a Sn concentration of 9.6%, for which the crystallization temperature was >400 °C, showed the lowest V_t of ∼3.5 V and I_off of ∼12.5 nA at half V_t and demonstrated switching endurance over 10⁶ cycles. Furthermore, a vertical-type OTS device was fabricated using the same SnGS2 film using the excellent step coverage of ALD, exhibiting similar switching characteristics to its planar counterpart.