Samarium doped Sn15Sb85: a promising material for phase change memory applications

The thermal stability of phase change films is a key parameter in phase change memory (PCM) applications. Here, we find that the Sm doped Sn15Sb85 films have a higher crystallization temperature and a better data retention ability in comparison with those of pure amorphous SnSb films, revealing a potential candidate for PCM applications. In addition, it demonstrated that the Sm doping was also propitious to reduce the power consumption and decrease the change of roughness in the crystallization process. Finally, the data storage capability of the Sm0.015(Sn15Sb85)0.985-based PCM cell was obtained with a reversible phase change process between high and low resistance states.


Introduction
In the last few years, chalcogenide-based phase change memory (PCM) has attracted intensive attention as a next-generation substitute for non-volatile random access memory. 1 Compared with other novel solid-state memories, PCM possesses its conspicuous advantages, including fast access time, high density, low power consumption, excellent scalability and high cycle numbers. 2 So far, the most known and used chalcogenide material is Ge 2 Sb 2 Te 5 ternary alloys (GST) due to their exceptional characteristics such as good scalability and high phase change speed. 3 However, it is highly challenging to prepare chalcogenide phase change lms with improved properties. 4 Recently, Sb-based materials, such as Sn-Sb, 5 Ge-Sb, 6 Sb-Te 7 and Ga-Sb, 8 have been considered as promising candidates for PCM applications because of their ultra-high phase speed. Feng Rao et al. reported that a Sn 12 Sb 88 -based cell is able to show quicker set operation speed, better data retention ability, and lower reset power consumption than the cell based on the GST. 5 Yegang Lu et al. have proved that the SET/RESET process of the devices based on Ga 14 Sb 86 alloys could be operated by a 20 ns pulse. 8 Feng Rao et al. demonstrated that the Sc 0.2 Sb 2 Te 5 compound allows an ultrafast writing speed of only 700 picoseconds compared with a conventional PCM device. 9 However, a material with fast crystallization speed is generally not stable. In order to solve this contradiction, doping is equally signicant for improving the performance of Sb-based alloys. 10 Recently, N, 11 O, 12 Al, 13 W, 14 Si 15 and SiC 16 were demonstrated as effective elements for doping.
Rare earth element (RE) is one of a set of seventeen chemical elements in the periodic table, which has been worldwide used in industrial application, even used in fertilizer, feed additives. 17 In phase change memory, we have reported that the Er can signicantly improve the performance of Sb-based alloys. [18][19][20][21] Unfortunately, very little work has been done to elucidate the effect of rare element on the phase change properties. It is urgent to explore the effect and their mechanisms in phase change materials by doping with other rare earth, such as Sm, Pr and etc. In this work, the objective is to systematically investigate the effect of Sm doping on phase change properties in SnSb alloys. The thermal stability, crystal structure, and device were studies in details.

Experiments
The Sm doped Sn 15 Sb 85 thin lms of Sm x (Sn 15 Sb 85 ) 1Àx (0 # x # 0.020) used for our study were made by Magnetron Cosputtering Sm and Sn 15 Sb 85 targets onto square SiO 2 /Si (100) wafers at room temperature. The purity of Sm and Sn 15 Sb 85 targets was 99.999%. The base pressure in the deposition chamber was 2 Â 10 À4 Pa. Sputtering was performed under the Ar gas pressure of 0.3 Pa, the ow of 30 sccm. The thin lm thickness was set to 50 nm through controlling deposition time of Sn 15 Sb 85 . The doped Sm content was altered by adjust the Sm sputtering power, while the sputtering power of Sn 15 Sb 85 was maintained at 30 W. The deposition rate was 2.6 nm s À1 . To ensure the uniformity of deposition, the substrate holder was rotated at an autorotation speed of 20 rpm.
The 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) under Ar atmosphere. The crystalline structures of the lms were analyzed by X-ray diffraction (XRD, PANalytical, X'PERT Powder). In order to assess the chemical bonding states of Sm doped Sn 15 Sb 85 lm, the X-ray photoelectron spectroscopy (XPS, Thermo Scientic, K-Alpha) was carried out. The surface morphology of the lms was examined by atomic force microcopy (AFM, FM-Nanoview 1000). The PCM devices based on the Sm 0.015 (Sn 15 Sb 85 ) 0.985 thin lm with a tungsten heating electrode of 260 nm diameter were fabricated by 0.18 mm CMOS technology. Between the Sm 0.015 (Sn 15 Sb 85 ) 0.985 lm and the top electrode, a 20 nm thick TiN lm was deposited by direct current magnetron sputtering. Current-voltage (I-V) and resistance-voltage (R-V) were conducted using a Keithley 2400 semiconductor parameter analyzer and an Agilent 81104A programmable pulse generator.

Results and discussion
To analyze crystallization temperature (T c ) of Sm doped Sn 15 Sb 85 lms, the sheet resistance as a function of temperature is shown at a heating rate of 20 C in Fig. 1(a). With increasing the Sm doping, the results show the T c increase signicantly. In general, the T c is dened by the derivative of the resistance with respect to temperature (dR/dT). 22 In Fig. 1(b), it can be seen that as Sm content increases, the T c increase from 201 C of pure Sn 15 Sb 85 to 214, 226, 242 C for Sm 0.005 (Sn 15 Sb 85 ) 0.995 , Sm 0.010 (Sn 15 Sb 85 ) 0.990 and Sm 0.015 (Sn 15 Sb 85 ) 0.985 . It obviously indicates a better thermal stability of amorphous phase by Sm doping, which will be propitious to achieve more thermal stable PCM cells. In a previous paper, 19 we conrmed that the Er dopants (the content of $2.4%) can increase the T c of 43 C compared with pure Sn 15 Sb 85 . In this work, it shows that the Sm dopants ($1.5%) can increase the T c of 41 C, which indicates that Sm dopants would have much remarkable inuence. Fig. 1(a) also shows that the amorphous and crystalline resistance increases with increasing the Sm content, which reveals much less pulse current to realize the phase transition by Joule heating.
In order to further estimate the retention of amorphous Sm doped Sn 15 Sb 85 lms, the crystallization of isothermal annealing at different temperature was studied as shown in Fig. 2. It observes that the initial period remains unchanged due to the nucleation process of crystalline grains. Aer the process, the grains of the lm grow rapidly meaning the sudden decrease of the resistance. For directly expressing the retention, it usually can be measured by the Arrhenius equation: where t, s, k B and T are the failure time, pre-exponential factor depending on the materials' properties, Boltzmann constant and absolute temperature. 23 The failure time is dened by the time when the resistance decreases to its half value. As shown in the inset of Fig. 2, the E a and temperature for 10 years archival life (T ten ) can be determined by the linear t of amorphous resistance as a function of 1/k B T. It can be shown that the E a is 3.36, 3.23, 4.84, 4.88 eV and the T ten is 119, 132, 161, 174 C, respectively. Thus, the increase of E a and T ten indicates the PCM cells based on Sm doped Sn 15 Sb 85 lms can store information much longer than the pure Sn 15 Sb 85 alloys. Compared with Er 0.018 (Sn 15 Sb 85 ) 0.082 of T ten ($149 C), the Sm 0.015 (Sn 15 Sb 85 ) 0.085 possess much higher T ten ($175 C). The superior enhancement of thermal stability may ascribe to the much bigger Sm 3+ ion compared with Er 3+ ion. 24 In addition, to compete with NOR-Flash memory, the T ten for PCM cells should be higher than 125 C. 25 In this perspective, the Sm doped Sn 15 Sb 85 alloys with high T ten are promising candidate for PCM application. The crystalline structure of Sm doped Sn 15 Sb 85 lms can be veried by XRD. But it is difficult to investigate the structure of 50 nm-thick lm because of its low mass and small scale. Thus we choose the typical XRD pattern of 300 nm-thick Sn 15 Sb 85 and Sm doped Sn 15 Sb 85 to investigate the phase structure. 26 Fig. 3(a) shows the rhombohedra phase of Sb only exists aer annealing at 280 C, revealing only the amorphous to rhombohedra phase process. More importantly, the intensity for (110) planes decrease signicantly by increasing the Sm dopants, meaning the reversible phase change can be realized more difficult by Sm doping, which may be helpful in enhancing the thermal stability. Fig. 3(b) shows the evolution of the XRD pattern as the function of the heating temperature. For the as deposited lm, there is no peak in the XRD pattern, meaning the amorphous structure. Then, it is apparent that the lms crystallize into the rhombohedra phase when the annealing temperature up to 210 C. In addition, the intensity of (110) plane increases with increasing the annealing temperature, indicating the increase of grain size. These results are consistent with those reports for Er doped SnSb lms. In a previous work, we reported that the improvement of performance of SnSb alloys by Er doping was due to the existence of Er-Sb bonding. 19 To explore the change of chemical bonding states of Sb and Sn atoms, the XPS spectra were measured as shown in Fig. 4. Aer Sm doping, Fig. 4(a) shows there is the chemical shi of 0.07 eV based on Sb 3d core-level. In addition, the peak of Sn-Sb bonds also shi to higher energies by Sm doping. This result obviously means that the Sm-Sb bond existence in Sm doped SnSb alloy by substitution of Sb-Sb and Sn-Sb bond. Fig. 4(b) indicates that the peak of Sn 3d shi to high energies by Sm doping, which reveals the formation of Sn-Sm bond in lms. This result is also consistent with the Er doped SnSb alloys. Thus, the formation of Sm-Sb and Sm-Sb may be the key reason for improving the performance of the materials.
In general, the electrical properties depend on not only the phase change materials but also the contact quality between the materials and the electrode. Thus, minimizing the surface roughness of phase change materials is essential for PCM device performance, which can maintain good contact quality.   crystallization leads to an increase in grain size and roughness. More importantly, these results reveal that by Sm doping, the change of roughness is smaller than pure Sn 15 Sb 85 . In view of this, the Sm doping will be propitious to make high performance PCM devices. At last, T-shaped PCM cells based on Sm 0.015 (Sn 15 -Sb 85 ) 0.985 alloys fabricated by 0.18 mm CMOS technology was utilized to test and verify their electrically induced phase change abilities. The inset of Fig. 6 shows the schematic diagram of the PCM cells. Fig. 6(a) shows the DC current-voltage (I-V) curve, which has a clear threshold switching behaviour at a threshold voltage, V th . The V th is 1.5 V based on the Sm doped Sn 15 Sb 85 PCM device, which is almost the same as that of Sn 12 Sb 88 based PCM device ($1.6 V). 5,8 Fig. 6(b) shows the R-V curves of the device. When the reset threshold voltage ($2.5 V for 300 ns pulse width), the set state of PCM device (10 3 -10 4 U) can be swily transferred to the reset state (10 5 -10 6 U). This results also indicate the device based on the Sm doped Sn 15 Sb 85 have the same properties in the PCM cell.

Conclusions
In summary, we have systematically demonstrated the effect of Sm doping on phase change performance in Sn 15 Sb 85 alloys. The Sm doped Sn 15 Sb 85 lms have the higher T c , E a and T ten , which ensure much better data retention for PCM application. The amorphous and crystallization resistance are proved to be larger than of pure SnSb alloys, which will reduce the power consumption for PCM cell. Compared with pure SnSb system, the XRD measurement certied that the Sm dopant don't change the phase structure and improve the thermal stability. The XPS tests prove the existence of Sb-Sm and Sn-Sm bond, which give rise to performance. The AFM tests prove that the Sm doped thin lm have much lower change of roughness in crystallization process. The reversible phase change between set and reset state is obtained based on the Sm doped Sn 15 Sb 85 PCM devices. Therefore, the Sm doped SnSb alloys are attractive candidates for PCM application. In addition, to clearly conrm rare earth doping effect on electrical properties, such as Pr, Ce and etc., further works should carefully check the phase change properties and the device characteristics.

Conflicts of interest
There are no conicts to declare.