Tunable Phase-Change Performance of Sb-Te-Se Film for Phase Change Memory

aFaculty of Electrical Engineering and Computer Science, Ningbo University, Zhejiang, 315211, People’s Republic of China bKey Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo, 315211, People’s Republic of China cState Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China dLaboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Zhejiang 315211, People’s Republic of China

Demand for mass storage has been increasing owing to explosive growth of information in recent days. To meet this demand, phase change memory (PCM) is proposed and considered as one of the most promising next-generation non-volatile memories because of its superior scaling, fast speed, and low power consumption. [1][2][3] Its performance depends on phase change materials with the high resistance contrast between amorphous and crystalline states. Normally, electrical pulse with high intensity and short width is employed to heat up the phase change materials over its melting point. And the fast quenching leads the material to an amorphous state (high resistance RESET state). Medium pulse intensity for longer time is used to induce the amorphous materials into crystalline states (low resistance SET state) if Joule heating is above its crystallization temperature and below the melting point. Low current with essentially no Joule heating can distinguish the resistance states.
Recently, great efforts have been made to explore novel chalcogenide materials and optimize the phase change characteristics in order to improve the device properties of PCM. Among many phase change materials, Ge 2 Sb 2 Te 5 (GST) has attracted a great deal of attention due to its good overall performance. However, the low crystallization temperature (∼160 • C) and relatively slow crystallization should be addressed for further practical applications. 4,5 Sb 2 Te material possesses high crystallization speed and relatively low-melting point, while the drawback of the low crystallization temperature impedes its application for phase change memory. [6][7][8][9] In fact, much research has been performed by doping various elements such as Zn, 10 N, 11 W, 12 Al, 13 Cr 14 into Sb-Te to improve its material properties for PCM. On the other hand, it is reported that Sb-Se material has the advantages of large on/off resistance ratio, low melting point and high crystallization temperature. 15,16 It is expected that the combination of Sb 2 Te with Sb-Se has advantages of the high speed, low power consumption and long archival life for PCM. Indeed, Sb 65 Se 6 Te 29 exhibits the 10years data retention at 87 • C, and the device can be operated under the pulse as short as 10 ns. 17 Very recently, Sb 44 Te 11 Se 45 was proposed for PCM due to its brilliant thermal stability. 18 In fact, the phase change properties such as thermal stability, crystallization speed, and amorphous/crystalline resistivities can be substantially tailored by the change in composition of Sb-Te-Se ternary system. Nevertheless, the z E-mail: lvyegang@nbu.edu.cn effect of Sb-Te-Se composition on its thermal stability, electrical properties, crystalline structure and bond environments is still unclear. It is necessary for us to investigate the phase change behavior of Sb-Te-Se with different compositions before its practical application for PCM. In this paper, four samples with different Se contents were proposed to insightfully elucidate the effect of dopants on the phase structural and chemical bonds from amorphous to crystalline states. The results can serve as a guide to optimize the phase change properties of Sb-Te-Se materials.
Sb-Te-Se films were deposited on SiO 2 /Si substrates by cosputtering Sb 2 Te and Sb 2 Se 3 targets at room temperature. The background and sputtering pressures were 2.7 × 10 −4 and 0.3 Pa, respectively, The Radio Frequency (RF) power on the Sb 2 Te target was fixed to be 40 W, and the direct current power of Sb 2 Se 3 target to be 5, 10, 15 and 20 W respectively. The corresponding compositions of the deposited films measured by energy dispersive spectroscopy (EDS) were Sb 60 Te 31 Se 9 , Sb 57 Te 26 Se 17 , Sb 56 Te 24 Se 20 and Sb 55 Te 22 Se 23. The sheet resistance of as-deposited films as a function of temperature(R-T) was measured by four-point probe method. The crystal structure of the films annealed at different temperature was measured by X-ray diffraction (XRD) with Cu Kα radiation measurement in the 2θ range from 20 • to 60 • . Raman scattering spectroscopy and X-ray photoelectron spectra (XPS) were employed to study the chemical bonding feature of films. The microstructure of the films was measured by transmission electron microscopy (TEM). Fig. 1a shows the sheet resistance as a function of temperature (R-T) with a heating rate of 10 • C/min. Upon heating, a continuous decrease in the resistance is observed for all the films due to the heat active carrier for hopping conduction. 19 And then an abrupt drop in resistance occurs when the temperature reaches their respective crystallization temperature (T c ), which is determined by the minimum of the first derivative of R-T. With increasing Se content, T c raises from 150 • C for Sb 60 Te 31 Se 9 to 180 • C for Sb 55 Te 22 Se 23 . Both amorphous and crystalline resistivities increase with Se content, which is favorable for low power consumption. It is also shown that the resistance ratios between the two states maintain at least 4 orders of magnitude, which helps to enhancing the operation reliability of the device. The thermal stability was measured by fitting the data with Arrhenius equa- 20 where t and τ is the time to failure and a proportional time constant, respectively. The time to failure is defined as the time which the film resistance reaches half of its initial value at the specific temperature. The Arrhenius plot of thermal stability for the as-deposited amorphous Sb-Te-Se films is shown in Fig. 1b 21,22 It indicates that the increasement of Se content can significantly improve the archival life of the amorphous Sb-Te-Se films. Fig. 2 shows XRD patterns of the Sb-Te-Se films annealed at different temperatures. There are no crystallization peaks for the asdeposited films, suggesting its nature of the amorphous state. With increasing annealing temperature, the characteristic peak is observed only for Sb 60 Te 31 Se 9 film at 150 • C. And the crystallization peaks doesn't appear until the annealing temperature increases to 200 • C for other compositions, as shown in Fig. 2. It indicates that the crystallization temperature of the studied films ranges from 150 to 200 • C, which is in good agreement with R-T results. For Sb 56 Te 24 Se 20 and Sb 55 Te 22 Se 23 films, with increasing annealing temperature to 300 • C, rhombohedra Sb 2 SeTe 2 and hexagonal Sb phase can be found and the positions of diffraction peaks do not change which indicates that no further phase transition happens. 17,23 It is reasonable that the incorporation of Sb 2 Se 3 into Sb 2 Te leads to the change in bond configuration as a result of atomic rearrangement, which will be confirmed by Raman and XPS measurements later. The complex cross bond system contributes to the increasement of the crystallization temperature.
Since phase separation (Sb 2 SeTe 2 and Sb) occurred in Sb 56 Te 24 Se 20 and Sb 55 Te 22 Se 23 films, it is necessary to clarify the bond vibrations of both samples using Raman measurement. Fig. 3 shows Raman spectra of Sb 56 Te 24 Se 20 and Sb 55 Te 22 Se 23 films annealed at different temperatures. For as-deposited and 150 • C annealed films, it is shown that a broad peak in the range from 100 cm −1 to 180 cm −1 which is related to the vibration of amorphous Sb-Te bonds. 24 And the broad band located at around 190 cm −1 corresponds to the heteropolar Sb-Se bond vibrations in the SbSe 3/2 -pyramides. 25,26 The spectra with   the feature of broad band suggest that the as-deposited films are in amorphous states, which is consistent with the R-T and XRD results. The broad peak at 150 cm −1 is divided into three different sharp peaks located at around 114,141 and 172 cm −1 , when both samples were annealed at 200 • C. Meanwhile, the broad band at 190 cm −1 disappears, suggesting part of Sb-Se bond broken in SbSe 3/2 -pyramides. It is interesting that the peak at 167 cm −1 shifts to high wave number with further increasing annealing temperature for both compositions, as shown in Fig. 3. It is reasonable that Se atoms in SbSe 3/2 enter into Sb-Te and form chemical bonds with Te. 27 Thus, rhombohedra Sb 2 SeTe 2 and hexagonal Sb phase can be found in XRD patterns. The bond recombination accounts for the enhancing crystallization temperature for Sb-Te-Se films with higher Se concentration. 28 It is shown that Sb 55 Te 22 Se 23 can serve as the representative of Sb-Te-Se material, and we will further discuss its bond environment and microstructures in next section.
X-ray photoelectron spectra (XPS) of Se 3d, Sb 3d and Te 3d for the Sb 55 Te 22 Se 23 film were investigated to analyze the chemical bonding features. The binding energy of 53.8 eV in Fig. 4a, which is lower than that of Se-Se homopolar binding energy of pure Se, 29 are related to Se 3d of Sb-Se for Sb 55 Te 22 Se 23 . After incorporation of Sb 2 Se 3 into Sb 2 Te, the binding energy of both Sb 3d and Te 3d shifts to high value for Sb 55 Te 22 Se 23 compared with Sb 2 Te, as shown in Figs. 4b and 4c. It has been reported that the binding energy of an element increases with increasing electronegativity of the neighboring bonding atom. 30 The electronegativities of Se, Sb and Te are 2.55, 2.05 and 2.1 respectively. 31 It indicates that the addition of Sb 2 Se 3 into Sb 2 Te suffers from atomic reconfiguration and eventually changes the bonding state, which is in agreement with Raman results.
The microstructures of crystalline Sb 55 Te 22 Se 23 film deposited on carbon membrane substrate are studied by TEM. Fig. 5a shows the TEM image of the film annealed at 250 • C for 3 min. The incoherent elastic scattering (Rutherford scattering) of electrons leads the dark and bright contrast in the TEM image. 32 The black particles which are crystallized after annealing with a size of several tens of nanometers are uniformly distributed in the film. The selected area electron diffraction (SAED) pattern in Fig. 5b indicates the rhombohedra Sb 2 SeTe 2 and Sb phase, which is in agreement with XRD results. The embedded structure of Sb 2 SeTe 2 and Sb phases is also confirmed by the high-resolution TEM (HRTEM) images, as shown in Figs. 5c and 5d.
Phase separation may lead to the deterioration of the endurance and reliability of the device due to the non-uniform composition in programming region as a result of facility in element migrating or void formation, especially in 10 nm node. 33 However, the phase separation of Sb from the composition may help to improve phase change speed. Sb is characterized by an explosive crystallization which is a thermal process with the heat that is released at the crystallization front serving to drive the crystallization of the surrounding amorphous region further. 34 Sb crystallization may serve as a template for subsequent crystallization to reduce the crystallization time.
In summary, phase change characteristics of Sb-Te-Se with different compositions are systematically investigated. The crystallization temperature, the 10-year data retention, and resistivities of both amorphous and crystalline states increase with increasing Se content. The selected composition Sb 55 Te 22 Se 23 exhibits higher crystallization temperature (about 180 • C), better data retention (102.6 • C), and relative large crystalline resistivity, which can effectively reduce the write current. The bonding recombination of incorporation of Sb 2 Te into Sb 2 Se 3 accounts for the improved thermal stability of Sb-Te. The embedded phases of Sb 2 SeTe 2 and Sb show the uniform distribution in Sb-Te-Se film. With large resistance ratio and good thermal stability performance, Sb-Te-Se material is considered to be a promising phase change material for PCM.