The role of arsenic in the operation of sulfur-based electrical threshold switches

Arsenic is an essential dopant in conventional silicon-based semiconductors and emerging phase-change memory (PCM), yet the detailed functional mechanism is still lacking in the latter. Here, we fabricate chalcogenide-based ovonic threshold switching (OTS) selectors, which are key units for suppressing sneak currents in 3D PCM arrays, with various As concentrations. We discovered that incorporation of As into GeS brings >100 °C increase in crystallization temperature, remarkably improving the switching repeatability and prolonging the device lifetime. These benefits arise from strengthened As-S bonds and sluggish atomic migration after As incorporation, which reduces the leakage current by more than an order of magnitude and significantly suppresses the operational voltage drift, ultimately enabling a back-end-of-line-compatible OTS selector with >12 MA/cm2 on-current, ~10 ns speed, and a lifetime approaching 1010 cycles after 450 °C annealing. These findings allow the precise performance control of GeSAs-based OTS materials for high-density 3D PCM applications.

Vfire of GeS, GeSAs25 and GeSAs43 devices are obtained by the device responses to the 6 V triangular pulses, and Vfire of GeSAs20 is measured by a 6.5 V one.Vth, Vh and Ion of devices with different As concentrations are determined by 3 V, 4.5 V, 4 V and 4 V pulses, respectively.The rising and falling edges of all pulses are 1 μs.The regulation of voltage seems to be similar with that of the Ion.Vfire goes up from 4.3 V to 5 V, goes down to 4.7 V next, and ends up at 4.4 V, while Vth jumps from 1.79 V to 2.79 V and decreases to 2.64 V and then to 2.23 V. Vh basically remained unchanged at 1.5 V. b.The variation of Ion and Ioff with As content.Ion was captured at the point of threshold switching.Moreover, Ioff is measured through DC test and the step is 0.1 V. Ioff refers to the current at 1/2 Vth in the subthreshold region.In concert with the growing trend of As, the mean value of Ioff from 30 devices for each As content starts at 0.16 μA at 1 V, then goes down to 0.11 nA at 1.5 V, which is the turning point, then slowly climbed to 0.13 nA at 1.5 V, and it kept increasing to 0.2 nA at 1.2 V finally.It can be seen from the figure that the addition of As immensely improved the uniformity of Ioff.The ON state current goes up from 0.59 mA to 1.36 mA and then down to 1.11 mA.

Fig. S3
Comparison of the on-current density of GeS, GeSAs20, GeSAs25 and GeSAs43 devices with that of other OTS cells.The on-current density (Jon) quadratically increases with a decrease of the device size.GeSAs shows advantages in terms of current density as well as thermal stability [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] .S8b.However, GeS shows significant differences after annealing with peaks at 210 and 249 cm -1 representing the B2g vibration mode 16 and bond bonding motions of S atoms in GeS4 tetrahedra which appear in crystal phase 17 .Also, GeS still contains amorphous part at this time with peak at 270 cm -1 corresponding to SGe3-S6/3 groups 18 , peak at 288 cm -1 corresponding to SGe3 pyramids 18 , peak at 350 cm -1 corresponding to edge-shared GeS4 18 , and peak at 388 cm -1 corresponding to S3Ge-GeS3 units 18 .Hence, we conclude that GeS crystallizes at 400 ℃, and GeSAs materials enable to withstand thermal shock at 450 ℃, which is consistent with XRD.Intensity (a.u.) Intensity (a.u.) a. b.After annealing at 450 ℃ for 30 min, GeS loses its switching performance, while the compositions containing As still work.By increasing the As content, the tendency of changes of Vfire, Vth and Vh is consistent with that before annealing, and Vfire decreases from 6.5 V to 6.2 V and then to 6 V; Vth decreases from 3.3V to 2.5 V and then to 2 V. c.The device leakage current increases from 3.1×10 -9 A to 1×10 -7 A with increasing As content.The on-current of the devices decreases from 0.71 mA to 0.4 mA, which is also consistent with the trend before annealing.
Fig. S12 Comparisons of operation voltages and currents of GeSAs20 before and after annealing.a. Vfire, Vth and Vh distributions for as-deposited GeSAs20 devices with 5, 10 and 20 nm-thick OTS layers.b.Vfire, Vth and Vh distributions for annealed GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.In 5-nm cells, the Vfire, Vth and Vh values are distributed from 3.95 V to 4.53 V, 2.12 to 2.72 V, and 1.48 V to 2.06 V, respectively.In 10-nm cells, the Vfire, Vth and Vh values are distributed from 6 V to 7.69 V, 2.42 to 4.37 V, and 1.52 V to 2.08 V, respectively.In 20-nm cells, the Vfire, Vth and Vh values are distributed from 7.6 V to 8.4 V, 3.61~5.32V, and 1.64 V to 2.05 V, respectively.c.Ioff of as-deposited GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.Dashed lines correspond to the FF process.Solid lines correspond to Ioff.d.Ioff of annealed GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.Dashed lines correspond to the FF process.Solid lines correspond to Ioff.e. Vfire, Vth and Vh variations, before and after annealing, with thickness.Solid lines correspond to the as-deposited state.Vfire increases from 3.3 V to 5 V and then to 8.8 V with a doubling of the thickness.Vth increases from 2.2 V for 5 nm, to 2.8 V for 10 nm and ends up at 4.8 V for 20 nm.Vh remains almost unchanged.Dashed lines correspond to the annealed state.Vfire increases from 4.26 V to 6.53 V and then to 8.11 V with a doubling of the thickness.Vth increases from 2.35 V for 5 nm, to 3.29 V for 10 nm and ends up at 4.32 V for 20 nm.Vh stays nearly the same.f.Ion and Ioff variations before and after annealing with thickness.Solid lines correspond to the as-deposited state.Ion changes from 1.45 mA to 1.28 mA, and then to 2.3 mA.Ioff decreases from 51.6 nA to 10.8 nA, and then to 0.74 nA.Dashed lines correspond to the annealed state.Ion becomes slightly smaller than those of unannealed cells.Ion increases from 0.42 mA to 0.71 mA, and then to 1.17 mA.While Ioff of the annealed state is lower, it decreases from 8.4 nA to 3.1 nA, and to 1.65 nA.Absorption around 0.29 eV for carbon tetrachloride affects the spectrum.Therefore, it is concluded that the absorption of the deflection medium is superimposed on the PDS spectrum in the region where the transmittance of the deflection medium is 20% or less.From these results, carbon tetrachloride is suitable for evaluating the trap levels of GeSAs.
Fig. S22 The atomic configuration corresponding to the gap state near the

Fig. S2
Fig. S2 Horizontal comparation of voltage and current parameters of GeS, GeSAs20, GeSAs25 and GeSAs43 devices.a.The fluctuation of Vfire, Vth and Vh of 50 individual devices.Vfire of GeS, GeSAs25 and GeSAs43 devices are obtained by the device responses to the 6 V triangular pulses, and Vfire of GeSAs20 is measured by a 6.5 V one.Vth, Vh and Ion of devices with different As concentrations are determined by 3 V, 4.5 V, 4 V and 4 V pulses, respectively.The rising and falling edges of all pulses are 1 μs.The regulation of voltage seems to be similar with that of the Ion.Vfire goes up from 4.3 V to 5 V, goes down to 4.7 V next, and ends up at 4.4 V, while Vth jumps from 1.79 V to 2.79 V and decreases to 2.64 V and then to 2.23 V. Vh basically remained unchanged at 1.5 V. b.The variation of Ion and Ioff with As content.Ion was captured at the point of threshold switching.Moreover, Ioff is measured through DC test and the step is 0.1 V. Ioff refers to the current at 1/2 Vth in the subthreshold region.In concert with the growing trend of As, the mean value of Ioff from 30 devices for each As content starts at 0.16 μA at 1 V, then goes down to 0.11 nA at 1.5 V, which is the turning point, then slowly climbed to 0.13 nA at 1.5 V, and it kept increasing to 0.2 nA at 1.2 V finally.It can be seen from the figure that the addition of As immensely improved the uniformity of Ioff.The ON state current goes up from 0.59 mA to 1.36 mA and then down to 1.11 mA.

5 Fig.
Fig. S4 On and off speeds of GeS, GeSAs20, GeSAs25 and GeSAs43 devices.a.To measure the electrical characteristics, a 1.2 kΩ and a 100 Ω resistor were connected in series with GeSAs devices.The 1.2 kΩ resistor is used to limit the current, and the oscilloscope collects the voltage signal across the 100 Ω one.b.Integration of input voltage and output current.The input voltage is a triangular pulse with a rising time of 1 μs.The output voltage signal is captured by the oscilloscope and the result is converted into current, as shown in the figure, from which an obvious switching phenomenon can be observed.c.Enlarged views of the current response of on-and offstates.When the input voltage reaches Vth, the current signal suddenly surges.With a further increase of the input voltage, the upward trend slows down.We interpret the steep increase period as the on-speed.Similarly, the sudden drop of current is taken as the off-speed d.Speed statistics of 30 individual devices of all compositions.The onspeed of devices with different compositions ranges from 7 to 12 ns, and the off speed is in the range of 5~15 ns.
Fig. S6 Endurance of unannealed GeSAs25 and GeSAs43 devices.3.5 V square pulses with 20 ns rising and falling edges, 100 ns pulse width and 50 ns interval were used in the endurance measurements.GeSAs25 and GeSAs43 devices were successfully turned on and off for 10 8 cycles, as shown in Fig. 1f.
Fig. S7 X-ray diffraction (XRD) results of a. GeS, b.GeSAs20, c. GeSAs25 and d.GeSAs43 films annealed at different temperatures.a. XRD results of GeS film samples annealed at 300 ℃ and 350 ℃ for 30 minutes and 400 ℃ for 10 minutes.Clearly, there is a very small crystallization peak at 30° in the GeS sample annealed at 400 ℃, indicating that the crystallization temperature of amorphous GeS is between 350 ℃ and 400 ℃. b-d.XRD results of GeSAs samples processed at 150, 250, 350 and 450 ℃ for 30 minutes and at 500 ℃ for 10 minutes.The XRD results of the annealed GeSAs samples are almost the same as that of the as-deposited samples, which confirms that As incorporation has a positive effect on the thermal stability of a-GeS.

Fig
Fig. S9 I-V curves of GeSAs devices with 60-nm electrodes after annealing at 450 ℃ for 30 minutes.The testing pulses are the same as 200-nm as-deposited GeSAs devices.The devices with As involved can all be turned on normally.The on-current of GeSAs20, GeSAs25 and GeSAs43 are 0.6 mA, 0.45 mA and 0.33 mA, respectively.Voltage (V)

Fig. S11
Fig. S11 Distributions and change trends of operation voltages and currents for GeS devices with different thicknesses.a. Vfire, Vth and Vh distributions for asdeposited GeS devices with 5, 10 and 20 nm thick OTS layers.Vfire of 5-nm, 10-nm and 20-nm GeS devices are obtained by the device responses to the 4.5 V, 6 V and 8 V triangular pulses.Vth, Vh and Ion of devices with different As concentrations are determined by 2.5 V, 3 V and 4 V pulses, respectively.The rising and falling edges of all pulses are 1 μs.Moreover, Ioff is measured through DC test and the step is 0.1 V. b.Ioff of deposited GeS devices with 5, 10 and 20 nm thick OTS layer.Dashed lines correspond to the FF process.Solid lines correspond to Ioff.c. Vfire, Vth and Vh variations with thickness.When the As content is zero, Vfire rises from 2.8 V, and Vth rises from 1.75V, which increase nonlinearly with thickness, and Vh is independent of thickness.d.Ion and Ioff variations with thickness.Ion increases and Ioff decreases with increasing thickness.
Fig. S13 Comparisons of operation voltages and currents of GeSAs25 before and after annealing.a. Vfire, Vth and Vh distributions for as-deposited GeSAs25 devices with 5, 10 and 20 nm thick OTS layers.b.Vfire, Vth and Vh distributions of annealed GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.In 5-nm cells, the Vfire, Vth and Vh values are distributed from 3.41 V to 3.61 V, 1.61 to 1.92 V, and 1.32 V to 1.62 V, respectively.In 10-nm cells, The Vfire, Vth and Vh values range from 5.76 V to 6.4 V, 1.92 to 2.97 V and 1.4 V to 1.7 V, respectively.In 20-nm cells, The Vfire, Vth and Vh values range from 6.93 V to 8.47 V, 2.91 to 4.35 V, and 1.42 V to 1.84 V, respectively.c.Ioff of asdeposited GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.Dashed lines correspond to the FF process.Solid lines correspond to Ioff.d.Ioff of annealed GeSAs20 devices with 5, 10 and 20 nm thick OTS layers.Dashed lines correspond to the FF process.Solid lines correspond to Ioff.e. Vfire, Vth and Vh variations before and after annealing with thickness.Solid lines correspond to the as-deposited state.Vfire increases from 3.45 V to 4.73 V and then to 7.82 V with a doubling of the thickness.Vth increases from 2.23 V for 5 nm, to 2.64 V for 10 nm and ends up at 4.16 V for 20 nm.Vh remains almost unchanged.Dashed lines correspond to the annealed state.Vfire increases from 3.84 V to 6.01 V and then to 7.8 V with a doubling of the thickness.Vth increases from 2.16 V for 5 nm, to 2.51 V for 10 nm and ends up at 3.65 V for 20 nm.Vh stays nearly the same.f.Ion and Ioff variations before and after annealing with thickness.Solid lines correspond to the as-deposited state.Ion increases from 1.45 mA to 1.28 mA, and then to 2.3 mA.Ioff decreases from 56 nA to 13 nA, and then to 0.6 nA.Dashed lines correspond to the annealed state.Ion becomes slightly smaller than those of unannealed cells.Ion increases from 0.42 mA to 0.7 mA, and then to 0.93 mA.While Ioff for the annealed state is lower, it changes from 31 nA to 48 nA, and then to 6.9 nA.
Fig.S20The electrical conductivity activation energy of a-GeS, a-GeSAs20, a-GeSAs25 and a-GeSAs43 films.The electrical conductivity activation energy can be drawn from the Arrhenius equation: σ = σ0exp(-Ea/kBT).Ea is the activation energy for electrical conduction.kB is the Boltzmann constant and T is the absolute temperature.The value increases from 0.79 to 0.93 eV, then drops to 0.8 eV, and ends up at 0.76 eV with increasing As content, which is consistent with the variations of Ioff and the bandgap.
Fig. S21 FTIR and PDS results of GeS films measured in different environments.a. Fluorinert FC43 was used for a previous PDS experiment 19 , whereas carbon tetrachloride was used in this work.FTIR results for FC43 and carbon tetrachloride for an optical path length of 10 mm are shown.The PDS in this region is subject to the absorption of the deflection medium.Below 0.33 eV, FC43 does not transmit light.There is a region where the transmittance is low, even at 0.33 eV or more.Carbon tetrachloride, on the other hand, has a transmittance of about 80% in most regions above 0.3 eV.b.As shown by the vertical lines, there is a good agreement between the absorption position of FC43 and the peak position of the PDS spectrum.c.It can be seen that carbon tetrachloride has almost no effect in the high-transmittance region.Absorption around 0.29 eV for carbon tetrachloride affects the spectrum.Therefore, it is concluded that the absorption of the deflection medium is superimposed on the PDS spectrum in the region where the transmittance of the deflection medium is 20% or less.From these results, carbon tetrachloride is suitable for evaluating the trap levels of GeSAs.