High Reliability and Fast-Speed Phase-Change Memory Based on Sb 70 Se 30 / SiO 2 Multilayer Thin Films

Sb70Se30/SiO2 multilayer thin films were applied to improve the thermal stability by RF magnetron sputtering on SiO2/Si (100) substrates. ,e characteristics of Sb70Se30/SiO2 multilayer thin films were investigated in terms of crystallization temperature, ten years of data retention, and energy bandgap. It is observed that the crystallization temperature, 10-year data retention, and resistance of Sb70Se30/SiO2 multilayer composite thin films exhibited a higher value, suggesting that Sb70Se30/SiO2 multilayer composite thin films have superior thermal stability. ,e AFMmeasurement suggests that the SbSe (1 nm)/SiO (9 nm) multilayer thin films possess a smaller surface roughness (RMS� 0.23 nm). Besides, it was found that the phase-change time of SbSe (1 nm)/SiO (9 nm) multilayer thin films was shorter than that of GST in the process of crystallization and amorphization.


Introduction
With the increase of portable electronic devices, people's demand for volatile memory has increased dramatically [1].Flash is now the mainstream of the nonvolatile memory market, but flash has several drawbacks such as its long operation time, the high voltage required for writing operations, and the fact used to store charges cannot meet the law of proportional reduction when it is very small [2][3][4].Phasechange memory due to read and write with fast speed, highdensity storage capacity, and compatible with complementary metal-oxide-semiconductor (CMOS), which regarded as the most promising alternative flash memory, becomes the mainstream of the next generation of nonvolatile storage technology [5,6].In order to solve the problems of large operation current and thermal interference, the whole development trend of PCM is in the three-dimensional direction to the nanometer scale.e sulfur compound semiconductor is a key portion of the phase-change memory currently, and its performance directly determines the performance of the phase-change memory [7,8].
Until recently, Ge 2 Sb 2 Te 5 (GST) has been attracted much attention in PCM research due to its relatively good performance.However, Ge 2 Sb 2 Te 5 has some problems such as low crystallization temperature (∼160 °C) and poor data retention (∼85 °C for 10 years), which cannot meet the requirements of high-density storage in the future data age [9,10].To this end, various strategies such as doping and compositing have been performed to improve the performances of phase-change materials.In the case of nitrogendoped GST, nitrogen is located in the grain vacancies or grain boundaries as a result of thermodynamic stability [11].Nitrogen doping in Sb 70 Se 30 film can raise its thermal stability, reducing the RESET current [12].Additionally, Superlattice-like (SLL) Si/Sb 80 Te 20 films have been confirmed to have a rapid crystallization speed [13].Besides, previous studies [14 -16] suggest that alternative multilayer phase-change materials can increase reversible phasechange speed and decrease the whole phase-change process power consumption, which is due to the fact condition that the advantages of different kinds of materials can be complementary.
In this study, Sb 70 Se 30 /SiO 2 (SbSe/SiO) multilayer thin lms were fabricated by the radio frequency (RF) sputtering method.e thermal stability, crystallization characteristics, and optical transition of SbSe/SiO phase-change material were investigated in detail.e investigations of resistance versus temperature (R-T), X-ray di raction (XRD), and surface topography measurements were carried out.

Experimental Details
In this work, SbSe/SiO multilayer thin lms with di erent thickness ratios and Sb 70 Se 30 and SiO 2 targets were deposited on SiO 2 /Si (100) with a thickness of 0.5 cm substrates by the radio frequency (RF) magnetron sputtering system at room temperature.
e purity of Sb 70 Se 30 and SiO 2 targets was 99.9999%.Prior to the growth of thin lms, the deposition rates of SbSe and SiO single layers were predetermined.e total thickness of SbSe/SiO thin lms was 50 nm, and the periodicity was 5. e thickness of each individual layer can be designed by controlling the deposition time.e SbSe and SiO layers were deposited alternately to obtain the required number of layers of SbSe/SiO.e base pressure in the deposition chamber was 2 × 10 −4 Pa.All deposition processes were carried out in Ar gas pressure of 0.4 Pa, the ow of 30 SCCM, and the RF power of 30 W. e substrate holder was rotated at an autorotation speed of 20 rpm to ensure the uniformity of deposition.
e amorphous-to-crystalline transition was investigated by in situ temperature-dependent resistance (R-T) measurement using a TP 95 temperature controller (Linkam Scienti c Instruments Ltd., Surrey, UK) at a heating rate of 10 °C/min.e size of each measured thin lm is 1 cm × 1 cm.e electrode made up of Si 3 N 4 is set up on the surface of the sample with a diameter of 0.7 μm, and the separation gap between two electrodes is 5 mm.e activation energy (E a ) and data retention temperature of ten years can be further gained by measuring the isothermal crystallization curve.e bandgap was obtained by measuring the re ectivity of thin lms in the range of 400-2500 nm by the NIR spectrophotometer (7100CRT, XINMAO, China).
e crystalline structures of the lms were analyzed by X-ray di raction (XRD, PANalytical, X'PERT Powder).e incidence angle θ ranges from 10 °to 30 °, and the di raction patterns were taken in the 2θ range from 20 °to 60 °using Cu Kα radiation with a scanning step of 0.01 °C/min.e surface morphology of the lms was examined by atomic force microcopy (AFM, FMNanoview 1000).A picosecond laser pump-probe system was used to investigate the phase-change time between amorphous and crystalline states, by measuring the re ectivity of the material.e light source used for irradiating the samples was a frequency-doubled model-locked neodymium yttrium aluminum garnet laser operating at 532 nm wavelength at a pulse duration of 30 ps.

Results and Discussion
e resistance as a function of temperature (R-T) for SbSe/SiO multilayer thin lms with di erent thickness ratios at a heating rate of 10 °C/min is shown in Figure 1.As can be seen, all thin lms rst display high resistance values, which are considered to be associated with semiconductor behavior.e resistance decreases sharply as the temperature reaches to a certain value which is referred to as the crystallization temperature T c [17]. Figure 1 illustrates that with the increasing thickness of the SiO ratio, the T c values of the SbSe/SiO thin lms increase from 210 °C to 228 °C. is suggests that SiO deposition inhibits the crystallization and increases the crystallization temperature.As is known, the higher T c represents better thermal stability [18].erefore, we can infer that SbSe/SiO multilayer thin lms improve the thermal stability.Good thermal stability of the phase-change materials is bene cial to the data retention and the reliability of the PCM devices, which is of great signi cance in practical application.Besides, the resistances of amorphous and crystallization states were observed to increase with the thickness of the SiO 2 layer, which is helpful for reducing RESET current according to the joule heating equation [19].erefore, PCM devices based on SbSe/SiO multilayer thin lms will have lower power consumption.e plot of logarithm failure time versus 1/k B T, as shown in Figure 2, ts a linear Arrhenius relationship due to its thermal activation nature.In our case, the tted straight line could be described as the following equation [20]: where t, τ 0 , k B , and T are the failure time, a preexponential factor depending on the thin lm's properties, the Boltzmann constant, and the absolute temperature, respectively.e extrapolated tting lines show that the temperature for 10-year data retention of SbSe, SbSe (5 nm)/SiO (5 nm), SbSe (3 nm)/SiO (7 nm), and SbSe (1 nm)/SiO (9 nm) were 141 °C, 160 °C, 165 °C, and 182 °C, respectively.Comparing with SbSe thin lms, SbSe/SiO multilayer thin lms display better reliability of resistance state at higher temperature, which can meet the demands of data-storage applications at higher temperature.e activation energy E a for crystallization provides a good estimation of the archival life stability of an amorphous phase-change material.E a for SbSe, SbSe (5 nm)/SiO (5 nm), SbSe (3 nm)/SiO (7 nm), and SbSe (1 nm)/SiO (9 nm) multilayer thin lms, evaluated by the slope of the tted curves presented in Figure 2, were 5.2, 6.0, 6.4, and 7.5 eV, respectively.Higher E a implies better reliability of the amorphous phase.As shown in Figure 2, the 10-year data retention and E a of SbSe/SiO multilayer lms increase with the SiO thickness ratio, which indicate that SbSe/SiO multilayer lm has the best reliability and is more quali ed for PCM application.e di use re ectivity spectra of SbSe thin lm and SbSe/SiO multilayer thin lms were measured by NIR spectrophotometry in the wavelength ranging from 400 to 2500 nm at room temperature [21].e bandgap energy (E g ) could be determined by extrapolating the absorption edge onto the energy axis, as shown in Figure 3. e conversion of the re ectivity to absorbance data is obtained by the Kubelka-Munk function (K-M) [22]: where R is the re ectivity, K is the absorption coe cient, and S is the scattering coe cient.As shown in Figure 3, the bandgap energy for SbSe, SbSe (5 nm)/SiO (5 nm), SbSe (3 nm)/SiO (7 nm), and SbSe (1 nm)/SiO (9 nm) multilayer thin lms are 1.47, 1.53, 1.58, and 1.66 eV, respectively.With the increase of SiO thickness, E g of amorphous lms spreads more widely.In general, the carrier density inside the semiconductors is proportional to exp(−E a /2KT) [23], and the increase of the bandgap will result in the reduction of carriers, which makes a major contribution to the increase of lm resistivity.us, the activation energy for crystallization is increased, improving the stability of the amorphous phase.
is nding is in accordance with the results from Figure 1. e crystalline structure of SbSe and SbSe (1 nm) SiO (9 nm) thin lms was characterized by XRD. Figure 4 shows the XRD patterns of SbSe and SbSe (1 nm) SiO (9 nm) thin lms annealed for 10 minutes at di erent temperatures.e di erent annealing temperatures correspond to di erent crystallization stages.As can be seen, there is no di raction peak in all the as-deposited thin lms, implying that the lms have not crystallized at all and are still in the amorphous structure [24,25].After annealing above crystallization temperature T c , multiple di raction peaks are observed.e di raction peak (211) belonging to Si appeared in SbSe and SbSe (1 nm) SiO (9 nm) thin lms, which is associated with the SiO 2 /Si substrate.As presented in Figure 4, the diffraction peak (012) belonging to Sb appears in SbSe and SbSe/SiO thin lms, which suggests that Sb is excessive [4,26].From the XRD patterns of SbSe (1 nm) SiO (9 nm) multilayer thin lms, it can be inferred that the SiO exists as the amorphous phase in all SbSe (1 nm) SiO (9 nm) multilayer thin lms since no SiO di raction peaks are observed [27], implying that the phase transition does not occur in the SiO layers.Due to the interface holding e ect in the SbSe/SiO multilayer thin lms, the SiO layers will impede the propagation of carriers and increase the resistance. is may suggest that SiO layers play an important role to improve the crystallization temperature and thermal stability.In general, the SbSe/SiO multilayer thin lms have better thermal stability than SbSe thin lm, which will be bene cial to the reliability of the PCRAM.
Film surface roughness is of great signi cance for the device performance due to the electrode-lm interface affected by the induced stress during the phase-change process [28,29].e microstructures of the SbSe thin lm and SbSe (1 nm)/SiO (9 nm) multilayer thin lms before and after crystallization have been detected by AFM. e SbSe thin lm and SbSe (1 nm)/SiO (9 nm) multilayer thin lms were annealed at 240 °C for 10 min.Figure 5 shows the AFM images of as-deposited and annealed SbSe thin lm and SbSe (1 nm)/SiO (9 nm) multilayer thin lms.e surfaces of amorphous SbSe thin lm and SbSe (1 nm)/SiO (9 nm)  Advances in Materials Science and Engineering multilayer thin lms are smooth relatively, with the rootmean-square (RMS) surface roughness of 0.33 and 0.23 nm, respectively.After crystallization, the RMS of SbSe thin lm increases to 0.44 nm.By contrast, the annealed SbSe (1 nm)/SiO (9 nm) multilayer thin lm has a smaller RMS (0.37 nm).Above points imply that the internal stress change of SbSe (1 nm)/SiO (9 nm) multilayer thin lm is much smaller, which is helpful to the fatigue performance of phasechange memory.ese values of the RMS are lower than the other phase-change lms, such as Sb 2 -Te 3 [30] and Ge 10 Sb 90 [31], indicating well smooth surface for PCM devices.
In the phase change, the electrical resistivity changes are accompanied by optical re ectivity.In this study, the switching speed of the phase-change materials was investigated by picosecond laser technology.Since the reset operation needs more power and shorter time than the set one in the resistance switching process of PCM devices, the reset power and the set speed have been attracted more attention [32].at is to say, the power consumption and operation speed of PCM are mainly determined by the reset and set processes, respectively.Figure 6      Advances in Materials Science and Engineering multilayer thin lm.In the picosecond laser test, the time of 0 is a reference for data recording, and it does not correspond to the time of phase transition.What is concerned in present study is the time interval from crystalline state to amorphous state which appears as abrupt change in re ectivity.As can be seen from Figure 6(a), the re ectivity dropped, implying the transition from crystalline-toamorphous state.e amorphization time was 1.60 ns and 0.96 ns, which corresponds to the irradiation uences of 15.5 mJ/cm 2 and 23.8 mJ/cm 2 , respectively.As shown in Figure 6(b), the crystallization time was observed at 1.7 ns under the irradiation uence of 6.34 mJ/cm 2 .It has been reported that the crystallization time of GST is 23.1 ns [33].erefore, we can infer that the SbSe (1 nm)/SiO (9 nm) multilayer thin lm possesses the faster phase-change speed.

Conclusion
In summary, SbSe/SiO multilayer thin lms were prepared by the radio frequency (RF) sputtering method.Phasechange behavior was studied by in situ temperaturedependent resistance measurements.e crystallization temperature, activation energy, and 10-year data retention temperature of the SbSe/SiO multilayer thin lms were proved to be larger than those of conventional SbSe thin lm, which indicates the SbSe/SiO multilayer thin lms have better thermal stability in comparison with SbSe thin lm.
e AFM measurement shows that the SbSe (1 nm)/SiO (9 nm) multilayer thin lms possess better surface roughness (0.23 nm) than that of SbSe thin lm.Meanwhile, the picosecond laser measurement suggests that the crystallization time of SbSe (1 nm)/SiO (9 nm) multilayer thin lms is shorter than that of GST thin lm. e results indicate that the SbSe/SiO multilayer thin lms are a promising candidate for high-reliability and low-consumption PCM device applications.

Figure 1 :
Figure 1: R-T curves of SbSe and SbSe/SiO multilayer thin lms at a heating rate of 10 °C/min.