High Reliability and Fast-Speed Phase-Change Memory Based on [Sb.sub.70][Se.sub.30]/Si[O.sub.2] Multilayer Thin Films.
With the increase of portable electronic devices, people's demand for volatile memory has increased dramatically . 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 [2-4]. Phase-change memory due to read and write with fast speed, high-density 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. The 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.sub.2][Sb.sub.2][Te.sub.5] (GST) has been attracted much attention in PCM research due to its relatively good performance. However, Ge2Sb2Te5 has some problems such as low crystallization temperature (~160[degrees]C) and poor data retention (~85[degrees]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 nitrogen-doped GST, nitrogen is located in the grain vacancies or grain boundaries as a result of thermodynamic stability . Nitrogen doping in [Sb.sub.70][Se.sub.30] film can raise its thermal stability, reducing the RESET current . Additionally, Superlattice-like (SLL) Si/[Sb.sub.80][Te.sub.20] films have been confirmed to have a rapid crystallization speed . Besides, previous studies [14-16] suggest that alternative multilayer phase-change materials can increase reversible phase-change 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.sub.70][Se.sub.30]/Si[O.sub.2] (SbSe/SiO) multilayer thin films were fabricated by the radio frequency (RF) sputtering method. The thermal stability, crystallization characteristics, and optical transition of SbSe/SiO phase-change material were investigated in detail. The investigations of resistance versus temperature (R-T), X-ray diffraction (XRD), and surface topography measurements were carried out.
2. Experimental Details
In this work, SbSe/SiO multilayer thin films with different thickness ratios and [Sb.sub.70][Se.sub.30] and Si[O.sub.2] targets were deposited on Si[O.sub.2]/Si (100) with a thickness of 0.5 cm substrates by the radio frequency (RF) magnetron sputtering system at room temperature. The purity of [Sb.sub.70][Se.sub.30] and Si[O.sub.2] targets was 99.9999%. Prior to the growth of thin films, the deposition rates of SbSe and SiO single layers were predetermined. The total thickness of SbSe/SiO thin films was 50 nm, and the periodicity was 5. The thickness of each individual layer can be designed by controlling the deposition time. The SbSe and SiO layers were deposited alternately to obtain the required number of layers of SbSe/SiO. The base pressure in the deposition chamber was 2 x [10.sup.-4]Pa. All deposition processes were carried out in Ar gas pressure of 0.4 Pa, the flow of 30 SCCM, and the RF power of 30 W. The substrate holder was rotated at an autorotation speed of 20 rpm to ensure the uniformity of deposition.
The amorphous-to-crystalline transition was investigated by in situ temperature-dependent resistance (R-T) measurement using a TP 95 temperature controller (Linkam Scientific Instruments Ltd., Surrey, UK) at a heating rate of 10[degrees]C/min. The size of each measured thin film is 1 cm x 1 cm. The electrode made up of [Si.sub.3][N.sub.4] is set up on the surface of the sample with a diameter of 0.7 [micro]m, and the separation gap between two electrodes is 5 mm. The activation energy ([E.sub.a]) and data retention temperature of ten years can be further gained by measuring the isothermal crystallization curve. The bandgap was obtained by measuring the reflectivity of thin films in the range of 400-2500 nm by the NIR spectrophotometer (7100CRT, XINMAO, China). The crystalline structures of the films were analyzed by X-ray diffraction (XRD, PANalytical, X'PERT Powder). The incidence angle [theta] ranges from 10[degrees] to 30[degrees], and the diffraction patterns were taken in the 2[theta] range from 20[degrees] to 60[degrees] using Cu K[alpha] radiation with a scanning step of 0.01[degrees]C/min. The surface morphology of the films 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 reflectivity of the material. The 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.
3. Results and Discussion
The resistance as a function of temperature (R-T) for SbSe/SiO multilayer thin films with different thickness ratios at a heating rate of 10[degrees]C/min is shown in Figure 1. As can be seen, all thin films first display high resistance values, which are considered to be associated with semiconductor behavior. The resistance decreases sharply as the temperature reaches to a certain value which is referred to as the crystallization temperature [T.sub.c] . Figure 1 illustrates that with the increasing thickness of the SiO ratio, the [T.sub.c] values of the SbSe/SiO thin films increase from 210[degrees]C to 228[degrees]C. This suggests that SiO deposition inhibits the crystallization and increases the crystallization temperature. As is known, the higher Tc represents better thermal stability . Therefore, we can infer that SbSe/SiO multilayer thin films improve the thermal stability. Good thermal stability of the phase-change materials is beneficial to the data retention and the reliability of the PCM devices, which is of great significance in practical application. Besides, the resistances of amorphous and crystallization states were observed to increase with the thickness of the Si[O.sub.2] layer, which is helpful for reducing RESET current according to the joule heating equation . Therefore, PCM devices based on SbSe/SiO multilayer thin films will have lower power consumption.
The plot of logarithm failure time versus 1/[k.sub.B]T, as shown in Figure 2, fits a linear Arrhenius relationship due to its thermal activation nature. In our case, the fitted straight line could be described as the following equation :
t = [[tau].sub.0] exp ([E.sub.a]/[k.sub.B]T), (1)
where t, [[tau].sub.0], [k.sub.B], and T are the failure time, a pre-exponential factor depending on the thin film's properties, the Boltzmann constant, and the absolute temperature, respectively. The extrapolated fitting 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[degrees]C, 160[degrees]C, 165[degrees]C, and 182[degrees]C, respectively. Comparing with SbSe thin films, SbSe/SiO multilayer thin films display better reliability of resistance state at higher temperature, which can meet the demands of data-storage applications at higher temperature. The activation energy [E.sub.a] for crystallization provides a good estimation of the archival life stability of an amorphous phase-change material. [E.sub.a] for SbSe, SbSe (5 nm)/SiO (5nm), SbSe (3nm)/SiO (7nm), and SbSe (1nm)/SiO (9nm) multilayer thin films, evaluated by the slope of the fitted curves presented in Figure 2, were 5.2, 6.0, 6.4, and 7.5 eV, respectively. Higher [E.sub.a] implies better reliability of the amorphous phase. As shown in Figure 2, the 10-year data retention and [E.sub.a] of SbSe/SiO multilayer films increase with the SiO thickness ratio, which indicate that SbSe/SiO multilayer film has the best reliability and is more qualified for PCM application.
The diffuse reflectivity spectra of SbSe thin film and SbSe/SiO multilayer thin films were measured by NIR spectrophotometry in the wavelength ranging from 400 to 2500 nm at room temperature . The bandgap energy ([E.sub.g]) could be determined by extrapolating the absorption edge onto the energy axis, as shown in Figure 3. The conversion of the reflectivity to absorbance data is obtained by the Kubelka-Munk function (K-M) :
K/S = [(1 - R).sup.2]/2R, (2)
where R is the reflectivity, K is the absorption coefficient, and S is the scattering coefficient. As shown in Figure 3, the bandgap energy for SbSe, SbSe (5nm)/SiO (5nm), SbSe (3nm)/SiO (7 nm), and SbSe (1nm)/SiO (9nm) multilayer thin films are 1.47,1.53,1.58, and 1.66 eV, respectively. With the increase of SiO thickness, [E.sub.g] of amorphous films spreads more widely. In general, the carrier density inside the semiconductors is proportional to exp(-[E.sub.a]/2KT) , and the increase of the bandgap will result in the reduction of carriers, which makes a major contribution to the increase of film resistivity. Thus, the activation energy for crystallization is increased, improving the stability of the amorphous phase. This finding is in accordance with the results from Figure 1.
The crystalline structure of SbSe and SbSe (1 nm) SiO (9 nm) thin films was characterized by XRD. Figure 4 shows the XRD patterns of SbSe and SbSe (1 nm) SiO (9 nm) thin films annealed for 10 minutes at different temperatures. The different annealing temperatures correspond to different crystallization stages. As can be seen, there is no diffraction peak in all the as-deposited thin films, implying that the films have not crystallized at all and are still in the amorphous structure [24, 25]. After annealing above crystallization temperature [T.sub.c], multiple diffraction peaks are observed. The diffraction peak (211) belonging to Si appeared in SbSe and SbSe (1 nm) SiO (9 nm) thin films, which is associated with the Si[O.sub.2]/Si substrate. As presented in Figure 4, the diffraction peak (012) belonging to Sb appears in SbSe and SbSe/SiO thin films, which suggests that Sb is excessive [4, 26]. From the XRD patterns of SbSe (1 nm) SiO (9nm) multilayer thin films, it can be inferred that the SiO exists as the amorphous phase in all SbSe (1 nm) SiO (9 nm) multilayer thin films since no SiO diffraction peaks are observed , implying that the phase transition does not occur in the SiO layers. Due to the interface holding effect in the SbSe/SiO multilayer thin films, the SiO layers will impede the propagation of carriers and increase the resistance. This may suggest that SiO layers play an important role to improve the crystallization temperature and thermal stability. In general, the SbSe/SiO multilayer thin films have better thermal stability than SbSe thin film, which will be beneficial to the reliability of the PCRAM.
Film surface roughness is of great significance for the device performance due to the electrode-film interface affected by the induced stress during the phase-change process [28, 29]. The microstructures of the SbSe thin film and SbSe (1 nm)/SiO (9 nm) multilayer thin films before and after crystallization have been detected by AFM. The SbSe thin film and SbSe (1 nm)/SiO (9nm) multilayer thin films were annealed at 240[degrees]C for 10 min. Figure 5 shows the AFM images of as-deposited and annealed SbSe thin film and SbSe (1 nm)/SiO (9 nm) multilayer thin films. The surfaces of amorphous SbSe thin film and SbSe (1 nm)/SiO (9 nm) multilayer thin films are smooth relatively, with the root-mean-square (RMS) surface roughness of 0.33 and 0.23 nm, respectively. After crystallization, the RMS of SbSe thin film increases to 0.44 nm. By contrast, the annealed SbSe (1 nm)/SiO (9 nm) multilayer thin film has a smaller RMS (0.37 nm). Above points imply that the internal stress change of SbSe (1 nm)/SiO (9 nm) multilayer thin film is much smaller, which is helpful to the fatigue performance of phase-change memory. These values of the RMS are lower than the other phase-change films, such as [Sb.sub.2]-[Te.sub.3]  and [Ge.sub.10][Sb.sub.90] , indicating well smooth surface for PCM devices.
In the phase change, the electrical resistivity changes are accompanied by optical reflectivity. 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 . That is to say, the power consumption and operation speed of PCM are mainly determined by the reset and set processes, respectively. Figure 6 shows the normalized reflectivity evolution of the SbSe (1 nm)/SiO (9 nm) multilayer thin film. 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 reflectivity. As can be seen from Figure 6(a), the reflectivity dropped, implying the transition from crystalline-to-amorphous state. The amorphization time was 1.60 ns and 0.96 ns, which corresponds to the irradiation fluences of 15.5mJ/[cm.sup.2] and 23.8mJ/[cm.sup.2], respectively. As shown in Figure 6(b), the crystallization time was observed at 1.7 ns under the irradiation fluence of 6.34mJ/[cm.sup.2]. It has been reported that the crystallization time of GST is 23.1 ns . Therefore, we can infer that the SbSe (1nm)/SiO (9nm) multilayer thin film possesses the faster phase-change speed.
In summary, SbSe/SiO multilayer thin films were prepared by the radio frequency (RF) sputtering method. Phase-change behavior was studied by in situ temperature-dependent resistance measurements. The crystallization temperature, activation energy, and 10-year data retention temperature of the SbSe/SiO multilayer thin films were proved to be larger than those of conventional SbSe thin film, which indicates the SbSe/SiO multilayer thin films have better thermal stability in comparison with SbSe thin film. The AFM measurement shows that the SbSe (1 nm)/SiO (9 nm) multilayer thin films possess better surface roughness (0.23 nm) than that of SbSe thin film. Meanwhile, the picosecond laser measurement suggests that the crystallization time of SbSe (1 nm)/SiO (9nm) multilayer thin films is shorter than that of GST thin film. The results indicate that the SbSe/SiO multilayer thin films are a promising candidate for high-reliability and low-consumption PCM device applications.
The authors would like to share their original data in a native format (.xls and .txt). The data used to support the findings of this study can be directly obtained by requesting it from the corresponding author.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was supported by Doctoral Scientific Research Foundation (Project KYY160015), Natural Science Foundation of Jiangsu Province (no. BK20160288), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (no. 16KJB130002), and Six Talent Peaks Project in Jiangsu Province (no. JXQC-013).
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Dan Zhang, (1) Yifeng Hu, (2) Haipeng You, (2) Xiaoqin Zhu, (2) Yuemei Sun, (2) Hua Zou, (2) and Yan Zheng (3)
(1) School of Materials and Engineering, Jiangsu University of Technology, Changzhou 213001, China
(2) School of Mathematics and Physics, Jiangsu University of Technology, Changzhou 213001, China
(3) School of Automotive and Traffic Engineering, Jiangsu University of Technology, Changzhou, Jiangsu 213001, China
Correspondence should be addressed to Yan Zheng; email@example.com
Received 4 April 2018; Accepted 23 May 2018; Published 21 June 2018
Academic Editor: Kohji Tashiro
Caption: FIGURE 1: R-T curves of SbSe and SbSe/SiO multilayer thin films at a heating rate of 10[degrees]C/min.
Caption: FIGURE 2: Plots of failure times as a function of reciprocal temperature of SbSe and SbSe/SiO multilayer thin films.
Caption: FIGURE 3: The Kubelka-Munk function of amorphous SbSe and SbSe (1 nm) SiO (9 nm) multilayer thin films.
Caption: FIGURE 4: XRD patterns of thin films annealed at different temperatures for 10 min in Ar atmosphere: (a) SbSe and (b) SbSe (1 nm) SiO (9 nm).
Caption: FIGURE 5: AFM topographic images: (a) SbSe; (b) annealed SbSe; (c) SbSe (1 nm)/SiO (9nm); (d) annealed SbSe (1 nm)/SiO (9nm).
Caption: FIGURE 6: Reversible reflectivity evolution of SbSe (1 nm) SiO (9 nm) thin film: (a) amorphization and (b) crystallization.
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|Title Annotation:||Research Article|
|Author:||Zhang, Dan; Hu, Yifeng; You, Haipeng; Zhu, Xiaoqin; Sun, Yuemei; Zou, Hua; Zheng, Yan|
|Publication:||Advances in Materials Science and Engineering|
|Date:||Jan 1, 2018|
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