Phase-change memory (PCM)-based neuromorphic devices can be considered to provide a promising solution to break the separation between computing and storage units, which is imposed by the von Neumann architecture based computing systems^{[1, 2]}. The working principle of PCM is to realize the reversible transformation between amorphous and crystalline phases by the resistivity of materials under the action of external electrical pulse, thus completing the information storage technology^[3−5].

Exhibiting stable resistivity in the amorphous state is a necessary property for multi-level PCM^[6]. The reason is that the amorphous phase change materials will experience spontaneous structural relaxation and lead to the resistance drift that significantly deteriorates the device accuracy^{[7, 8]}. For example, the representative Ge₂Sb₂Te₅ (GST) phase-change material has the large resistance drift coefficient of 0.11 and directly limits its practical applications^[9]. Recent results have been shown that the reduction of the resistance drift in PCM can be achieved by enhancing the thermal stability of phase-change materials by N-doping^[10] and BiSb regulation^[11]. Although, the thermal stability can be improved by material optimization, the trends in the thermal stability in dependence of size and composition of phase-change materials as well as changing of optical band gap have not been explored yet in detail.

As reported^{[12, 13]}, the introduction of Sn benefits to reduce the resistance drift coefficient. Moreover, for the crystallization kinetics of chalcogen element, S is the most frequently studied as a candidate for glass-ceramic matrix due to its high crystallization temperature and glass transition temperature^[14]. In this paper, we introduced SnS material into GST to reduce the resistance drift coefficient of GST. We found the origin for the decreased in resistance drift, which can be ascribed to the lower optical band gap and conductivity activation energy.

Pure GST, SnS, and SnS-doped GST thin films with about 140 nm thickness were directly deposited on SiO₂/Si (100) and quartz substrates by magnetron sputtering method (Kurt J. Lesker PVD 75). First, the substrate temperature was kept at room temperature. In each run of the experiment, the base and working pressures in the chamber were set to be 5 × 10⁻⁴ and 0.4 Pa, respectively. The Ar gas flow was set to 47.6 mL/min. Then, we set direct current power for SnS target from 0 to 20 W to adjust SnS-doping concentration, and fix radio frequency power for GST target at 50 W. The as-deposited films were annealed in a rapid-vacuum oven with N₂ atmosphere at 350 °C for 10 min.

The composition of as-deposited samples can be determined by energy dispersive spectroscopy (EDS). The structure was characterized by X-ray diffraction (XRD). The diffraction patterns were taken in the 2θ range of 10°−60° using Cu Kα radiation with a wavelength of 0.154 nm. The transmission spectra of the thin films in the 800−2500 nm spectral range were obtained using a Perkin−Elmer Lambda 950 ultraviolet/visible/near-infrared spectrometer spectrophotometer. A piece of SiO₂ glass with the same thickness as the substrate was used as the reference material for calculating absorption in the thin films. The sheet resistances of as-deposited films as a function of elevated temperature (non-isothermal) and as a function of time at specific temperatures (isothermal) were in situ measured using a four-point probe in a homemade vacuum chamber. The confocal Raman scattering spectra were recorded at room temperature by using a Renishaw inVia type microscopic Raman spectrometer with a laser at a wavelength of 785 nm.

Fig. 1(a) shows the relationship between sheet resistance and annealing temperatures (R−T) in pure GST, SnS and SnS-doped GST thin films. It is found that pure GST film undergoes two-step crystallization upon annealing across three states: high amorphous resistance, then intermediate resistance with face-centered cubic (FCC) structure and low resistance with hexagonal structure. This phase-change behavior enables multilevel data storage properties. In comparison, pure SnS films do not have a phase-change behavior, and the continuous decrease in sheet resistance could be observed with temperature increasing, exhibiting good amorphous stability. When the SnS was doped into GST, we can find that the crystallization temperature (T_c) value for SnS-GST thin film is about 190 °C, a little higher than GST (168 °C). As reported, the T_c of Sn-doped GST was decreased with an increase of Sn doping content^{[15, 16]}, while the T_c of SnS-GST increases on the contrary. Meanwhile, the temperature for 10-year data retention (T_10-y) can be calculated by the extrapolated fitting curve according to the Arrhenius equation^[17]. As shown in Fig. 1(b), T_10-y of SnS-GST films ranges from 85.7 to 103.6 °C, which is higher than that of GST (about 82 °C^[17]). It indicates that memory cells using SnS-GST materials have better thermal stability than GST-based devices. Thus, we believe that the improvement in thermal stability can be ascribed to the presence of S element.

On the other hand, a power-law equation with time can be used to characterize the local resistance drift of amorphous phase-change materials by^[1]:

1R=R0(tt0)ν.

Here R₀ and t₀ are constants which have a dependence on the initial state of materials. R and ν is the test resistance and the resistance drift coefficient, respectively. The time dependent drift of the amorphous resistance of GST and SnS-doped GST thin films at 50 °C is shown in Fig. 1(c). The ν value for the amorphous GST is 0.06, close to the previous work^[9]. The resistance drift coefficient is obviously decreased with the addition of SnS (0.010−0.033), which may be ascribed to the optical band gap and conductivity activation energy.

The origin of the amorphous resistance drift can be ascribed to the variation in optical band gap, which can be characterized by using the well-known Tauc’s equation^{[18, 19]}.

2(αhν)12=B12(hν−Eopt).

Here E_opt, B and hv is the optical band gap, a constant which depends on the transition probability and incident photon energy, respectively. Fig. 2(a) shows the E_opt of pure SnS, GST and SnS-doped GST thin films. We can obtain the E_opt for as-deposited amorphous sample by extrapolating the linear portion to the energy axis at (αhν)12=0. It reveals that the E_opt values of the undoped SnS and GST thin films are 1.03 and 0.65 eV, respectively, while the E_opt value of SnS-GST thin films ranges from 0.43 to 0.65 eV.

Fig. 2(b) shows the electrical conductivity as a function of the reciprocal temperature from 30 to 140 °C for the pure GST and SnS-doped GST films, where the conductivity can be expressed by an Arrhenius-type relation^[20]:

3σ=σ0exp(−EσkBT).

Here σ₀, k_B, E_σ and T is a temperature-independent constant, the Boltzmann constant, the electrical conduction activation energy and the absolute temperature, respectively. From the plot of lnσ versus 10³/T, the E_σ can be obtained based on the slope (E_σ/k_B) of the plot. The obtained E_σ values are plotted in Fig. 2(b). The E_σ of GST, (SnS)_8.8(GST)_91.2, (SnS)_35.1(GST)_64.9, and (SnS)_54.6(GST)_45.4 are ~0.383, ~0.253, ~0.342, and ~0.363 eV, respectively.

The comparison of the E_opt and E_σ for all the studied films strongly suggests that the activation energy of conductivity is linear proportional to the band gap and inversely proportional to the resistance drift. The conduction activation energy increases, the band gap also increases, but the resistance drift becomes larger. Therefore, in order to significantly reduce the resistance drift, it is necessary to further reduce the band gap.

In order to evaluate the fundamental phase-change properties in dependence of crystallization, we investigate the amorphous and crystalline structures of the SnS-doped GST thin films by XRD patterns in Fig. 3. No crystallization peaks were detected in as-deposited SnS-doped GST thin films, displaying the good amorphous nature. For 200 °C-annealed SnS-doped GST films, we can observe diffraction peaks ascribed to a FCC structure. As the annealing temperature is further increased to 350 °C, it is interesting to see that only the hexagonal GST peaks can be observed in the crystalline (SnS)_8.8(GST)_91.2 with low SnS-doping concentration, forming a mixture of GST nanocrystals and SnS amorphous phases, while a new SnS crystalline phase begins to appear in the (SnS)_35.1(GST)_64.9 and (SnS)_54.6(GST)_45.4 films with high SnS-doping concentration, forming a dual-phase coexistence of crystalline GST and SnS.

To confirm further the dual-phase coexistence in SnS-GST thin films, Raman spectra of SnS and SnS-GST films annealed at 200 and 350 °C was measured as shown in Fig. 4. For pure SnS film in Fig. 4(a), we find that two Raman peaks at 95 and 218 cm⁻¹ can be ascribed to the vibration of SnS crystals. For (SnS)_8.8(GST)_91.2 films in Fig. 4(b), the Raman mode at 105 cm⁻¹ can be ascribed to A₁ mode of GeTe₄ corner-sharing tetrahedral and a broad peak located at 156 cm⁻¹ is associated with Sb-Te vibrations in SbTe₃ units^[21] when the annealing temperature increases up to 350 °C. Noteworthy, four peaks located at 95, 218, 105 and 156 cm⁻¹ can be observed in (SnS)_35.1(GST)_64.9 and (SnS)_54.6(GST)_45.4 in Figs. 4(c) and 4(d) and ascribed to the vibration of SnS and GST crystals, respectively. This well confirms the dual-phase coexistence in SnS-doped GST thin films, which is consistent with the XRD analysis. This dual-phase coexistence structure can improve the amorphous stability of the system to counteract the effects of structural relaxation, thus reduce resistance drift.

The electrical, optical and structural properties of pure GST and SnS-doped GST films have been investigated systematically. The results show that the addition of SnS into the GST films leads to a smaller electrical activation energy and narrower band gap, which directly causes the decrease in resistance drift. Indeed, the drift coefficient was reduced from 0.06 for GST to 0.01 for SnS-doped GST thin films. Moreover, X-ray diffraction patterns and Raman spectra reveal that the improvement amorphous thermal stability in SnS-doped GST films originates from the formation of dual-phase coexistence behavior in amorphous SnS-dopd GST thin films. This helps to alleviate resistance drift in amorphous phase-change materials for phase-change memory application.