Low-Energy Amorphization of Ti1Sb2Te5 Phase Change Alloy Induced by TiTe2 Nano-Lamellae

Increasing SET operation speed and reducing RESET operation energy have always been the innovation direction of phase change memory (PCM) technology. Here, we demonstrate that ∼87% and ∼42% reductions of RESET operation energy can be achieved on PCM cell based on stoichiometric Ti1Sb2Te5 alloy, compared with Ge2Sb2Te5 and non-stoichiometric Ti0.4Sb2Te3 based PCM cells at the same size, respectively. The Ti1Sb2Te5 based PCM cell also shows one order of magnitude faster SET operation speed compared to that of the Ge2Sb2Te5 based one. The enhancements may be caused by substantially increased concentration of TiTe2 nano-lamellae in crystalline Ti1Sb2Te5 phase. The highly electrical conduction and lowly thermal dissipation of the TiTe2 nano-lamellae play a major role in enhancing the thermal efficiency of the amorphization, prompting the low-energy RESET operation. Our work may inspire the interests to more thorough understanding and tailoring of the nature of the (TiTe2)n(Sb2Te3)m pseudobinary system which will be advantageous to realize high-speed and low-energy PCM applications.

dispersedly distribute inside the quintuple-layered building blocks so as to preserve the ordering configuration 8 . To inhibit the Ti precipitation after repeated RESET-SET operations, it would be worthwhile to try to concomitantly raise the Te content of the non-stoichiometric Ti x Sb 2 Te 3 materials. Thus, in this paper, we show the better electrical phase change properties based on a stoichiometric Ti 1 Sb 2 Te 5 material. It is like a pseudobinary compound with 1 : 1 ratio of TiTe 2 and Sb 2 Te 3 components. Even containing higher Ti content (12.5 at.%), the Ti 1 Sb 2 Te 5 based PCM cell has superior endurance characteristic over the Ti 0.56 Sb 2 Te 3 based one. In addition, without significantly sacrificing the SET speed, the RESET energy of the Ti 1 Sb 2 Te 5 based PCM cell is further lowered by 42∼ 47% compared to the Ti 0.4 Sb 2 Te 3 based one. We argue that the richer concentration of TiTe 2 lamellae in c-Ti 1 Sb 2 Te 5 plays a major role in decreasing the RESET energy, meanwhile the lack of Ti-centered octahedrons resided in the quintuple-layered Sb 2 Te 3 lattice may slow down the nucleation rate, however the increasing TiTe 2 lamellae can act as structure-ordering template to enhance the crystal growth rate 11 . Accordingly, we believe that Ti 1 Sb 2 Te 5 material is promising for realizing DRAM-like PCM application once advanced fabrication techniques being applied to further shrink the device dimension. Figure 1 shows the temperature-dependent sheet resistance (R s ) curves of as deposited TiTe 2 , Sb 2 Te 3 , Ti 0.4 Sb 2 Te 3 , and Ti 1 Sb 2 Te 5 films upon in situ annealing with a heating rate of 10 °C/min. As the annealing temperature increases, a continuous decrease in R s is observed for each film. Due to the partial crystallization during the sputtering process, compared to Ti 0.4 Sb 2 Te 3 and Ti 1 Sb 2 Te 5 films, both TiTe 2 and Sb 2 Te 3 films have smaller initial R s and present a smooth decrement in R s . By comparison, one can observe the sudden drop in R s occurs for Ti 0.4 Sb 2 Te 3 and Ti 1 Sb 2 Te 5 films when the temperature reaches T c (both around 186 °C) 9 . The decrease in R s with increasing temperature just before the onset of the crystallization indicates a semiconductor-like behavior. The temperature dependence for the R s in a semiconductor can be expressed by R s = R s0 exp(−E σ /kT) 12 , where R s0 is a pre-exponential factor and E σ is the activation energy for electrical conduction. The fitting results of E σ s of Ti 0.4 Sb 2 Te 3 and Ti 1 Sb 2 Te 5 are 0.11 eV and 0.13 eV, respectively. The activation energy of electrical transport is simply determined by half of the band gap E σ = E G /2 + Δ E, where E G /2 is the distance from the Fermi level to the conduction band and Δ E is the depth of the trap states 13 . In the case of intrinsic conduction with equal amounts of electrons and holes, the Fermi level is situated at the middle of the band gap. Thus we can roughly estimate the optical band gaps (E OP s) of a-Ti 0.4 Sb 2 Te 3 and a-Ti 1 Sb 2 Te 5 to be ∼ 0.22 eV and ∼ 0.26 eV, respectively, both of which are quite smaller than that of a-Ge 2 Sb 2 Te 5 (∼ 0.70 eV) 14 . Because the carrier density inside the semiconductor is proportional to exp(− E G /2kT), where E G is the electrical band gap which is roughly identical to the E OP , a decrease in the band gap as the temperature approaching to the T c will lead to the generation of a large number of carriers, which makes a major contribution to the quick drop in film resistivity 14 . On this view, one may roughly estimate that a-Ti 0.4 Sb 2 Te 3 and a-Ti 1 Sb 2 Te 5 could have quite faster crystallization (resistivity decrement) speed than that of a-Ge 2 Sb 2 Te 5 , and a-Ti 0.4 Sb 2 Te 3 could be a little quicker than a-Ti 1 Sb 2 Te 5 . Figure 2a compares the SET speed of Ge 2 Sb 2 Te 5 , Ti 0.4 Sb 2 Te 3 , and Ti 1 Sb 2 Te 5 based PCM cells with the same size (BEC D = 190 nm), which has the same trend as aforementioned estimation. As the magnitude of applied voltage pulse reaches 1.3 V and 1.5 V, respectively, both the Ti 0.4 Sb 2 Te 3 and Ti 1 Sb 2 Te 5 cells show the SET speed of ∼ 6 ns. Apparently, under lower bias, the Ti 0.4 Sb 2 Te 3 cell can complete the SET operation more quickly than the Ti 1 Sb 2 Te 5 cell. In contrast, the SET operation of Ge 2 Sb 2 Te 5 cell requires ∼ 75 ns at 1.6 V and ∼ 35 ns even at 2.1 V. In other words, one order of magnitude faster SET speed can still be achieved even by using the Ti 1 Sb 2 Te 5 cell. Supplementary Information Figure S1 shows the cell resistance versus required time curves for SET operation of such three cells.
In terms of the RESET operation, even the Ti 0.   Figure S2. An ∼ 82% reduction of the RESET current is realized for both the Ti 0.4 Sb 2 Te 3 and Ti 1 Sb 2 Te 5 cells (∼ 0.5 mA) compared to that of Ge 2 Sb 2 Te 5 cell (∼ 2.8 mA) with the same D = 80 nm BEC. Moreover, the RESET current of the Ti 1 Sb 2 Te 5 cell (∼ 1.1 mA) is relatively smaller than that of the Ti 0.4 Sb 2 Te 3 cell (∼ 1.3 mA) with the same D = 190 nm BEC. In addition to the improvements in RESET energy and current, the endurance characteristics of the Ti 1 Sb 2 Te 5 cell (∼ 10 7 cycles) is not inferior to that of the Ti 0.4 Sb 2 Te 3 cell 7 , which is obviously far more better than that of the Ti 0.56 Sb 2 Te 3 cell (< 10 6 cycles with severe fluctuation of the RESET state) 9 , as shown in the Supplementary Information Figure S3.
We used Raman spectroscopy (Fig. 3a) and in situ XRD (Fig. 3b) to characterize the c-Ti 1 Sb 2 Te 5 phase. As a CdI 2 -like structure with space group P3m1 space symmetry, c-TiTe 2 has two Raman active modes produced entirely by the Te atoms for both the in-plane, E g peak (∼ 122 cm −1 ), and out of plane, A 1g peak (∼ 143 cm −1 ), with the Ti atoms at rest 15 . Since there is no distinctive shoulder near ∼ 160 cm −1 , the c-TiTe 2 film can be considered as a stoichiometric compound without noticeable defects or impurities 16 . Because c-Sb 2 Te 3 crystal belongs to the space group R3m, it has three Raman active modes, including two out of plane vibrations A 1g (1) peak (∼ 69 cm −1 ) and A 1g (2) peak (∼ 165 cm −1 ), and one in-plane vibration E g (1) peak (∼ 112 cm −1 ) 17 . Compared to the Raman curves of the c-TiTe 2 and c-Sb 2 Te 3 films, the c-Ti 1 Sb 2 Te 5 film has five Raman active modes identically peaked corresponding to E g and A 1g of TiTe 2 and A 1g (1), A 1g (2), and E g (1) of Sb 2 Te 3 , respectively, as shown in Fig. 3a. The coexistence of c-TiTe 2 and c-Sb 2 Te 3 Raman active modes in c-Ti 1 Sb 2 Te 5 no doubt shall originate from the two separated phases which is already observed in c-Ti 0.4 Sb 2 Te 3 8 . In fact, our in situ XRD result of Ti 1 Sb 2 Te 5 film clearly proves such phase separation phenomenon as shown in Fig. 3b, where both HEX-Sb 2 Te 3 and HEX-TiTe 2 diffraction peaks can be identified. Nevertheless we did not observe such distinct phase separation in Ti 0.4 Sb 2 Te 3 through the same in situ XRD measurement 7 . The phase separation in Ti 0.4 Sb 2 Te 3 occurs in nano-scale dimension 8 , where the TiTe 2 lamellae segregate into no more than 10 nm-width belts and most of the TiTe 2 lamellae (< 2 nm in width) inlay with the Sb 2 Te 3 quintuple-layered blocks. Due to the similar HEX lattice structures of TiTe 2 and Sb 2 Te 3 , and also considering the lower doping content of Ti (∼ 7.4 at.%) in Ti 0.4 Sb 2 Te 3 , less concentration of the TiTe 2 lamellae may result in undetected XRD signal. Note that the pure TiTe 2 alloy has quite high T m (> 1200 °C) 18 and the HEX-phase of its thin film can be maintained even at 700 °C (higher than the T m ∼ 618 °C of Sb 2 Te 3 ) 8 , thus there is no segregated Te or Ti phase being observed in c-Ti 1 Sb 2 Te 5 as shown in Fig. 3b. Extending this analysis to the stoichiometric Ti 1 Sb 2 Te 5 = (TiTe 2 ) y Ti 1-y Sb 2 Te 5-2y (0 < y < 1), where (TiTe 2 ) y part also stands for the segregated TiTe 2 lamellae, one can easily find that, in Ti 1−y Sb 2 Te 5−2y part (= Ti 1−y Te 2−2y Sb 2 Te 3 ), if Ti 1−y Te 2−2y is assigned as the Ti-centered octahedrons accommodated in Sb-Te quintuple-layered blocks, it will be contradictory to keep the rest Sb-Te part in a ratio of 2 : 3 without Te deficiency. In other words, since there is no Ti or Te phase separation, it is more appropriate to describe the c-Ti 1 Sb 2 Te 5 as (TiTe 2 ) 1 (Sb 2 Te 3 ) 1 in which all the Ti atoms are supposed to be contained in the separated TiTe 2 lamellae. Of course, this is a rough estimate, however, we may still get a reasonable inference that compared to the c-Ti 0.4 Sb 2 Te 3 , c-Ti 1 Sb 2 Te 5 should have a higher concentration of the c-TiTe 2 lamellae but a lower concentration of the "solid-solute" Ti-centered octahedrons.
Note that the quasi-two-dimensionalc-TiTe 2 is semimetallic 19 (see Fig. 1 also) with a quite low thermal conductivity (∼ 0.12 W/mK) 20 . The TiTe 2 lamellae could act like the embedded nano-electrodes in c-Ti 1 Sb 2 Te 5 to conduct the electrical current to the adjacent Sb 2 Te 3 grains to generate Joule heat. The heat dissipation could be concomitantly refrained by those low thermal-conductive TiTe 2 lamellae so as to effectively enhance the thermal efficiency of the RESET operation. Not surprisingly the c-Ti 1 Sb 2 Te 5 (∼ 1/2 = 50.0% concentration of TiTe 2 lamellae) based PCM cell can accomplish substantial reduction of the RESET energy compared to the c-Ti 0.4 Sb 2 Te 3 (< ∼ 0.4/1.4 ≈ 28.6% concentration of TiTe 2 lamellae) based PCM cell. On the contrary, the lack of survived Ti-centered octahedrons in Sb-Te rich amorphous matrix after RESET operation may slow down the nucleation rate for recrystallization process 8,9 , leading to a relatively slower SET speed for the Ti 1 Sb 2 Te 5 based PCM cell as compared to that of the Ti 0.4 Sb 2 Te 3 based one.
We also used the two-dimensional finite element method (FEM) to simulate and compare the RESET operations of the Ge 2 Sb 2 Te 5 , Ti 0.4 Sb 2 Te 3 , and Ti 1 Sb 2 Te 5 based PCM cells, as shown in Fig. 4. The Joule heat is mainly generated in the phase change films. The thermal transfer obeys the standard heat conduction equation: 21 where κ, is the thermal conductivity, c, the specific heat, ρ, the density, t, the time, T, the temperature, and Q, the Joule heat per unit volume and per unit time, which is called the heat density. The key material parameters for FEM simulations include κ of c-Ge 2 Sb 2 Te 5 (∼ 0.46 W/mK) 21 , c-Sb 2 Te 3 (∼ 0.78 W/mK) 22 , and c-TiTe 2 (∼ 0.12 W/mK) 20 , c of c-Ge 2 Sb 2 Te 5 (∼ 1.20 J/cm 3 K) 23 , c-Sb 2 Te 3 (∼ 1.02 J/cm 3 K) 24 , and c-TiTe 2 (assumed to be ∼ 1.80 J/cm 3 K of c-TiSe 2 ) 25 , and ρ of c-Ge 2 Sb 2 Te 5 (∼ 6.2 g/cm 3 ) 21 , c-Sb 2 Te 3 (∼ 6.5 g/cm 3 ) 26 , and c-TiTe 2 (∼ 6.3 g/cm 3 ) 27 . The phase change film layers of the models are divided into grid shape to represent the poly-crystalline morphology. Each grid denotes the small crystal grain. ∼ 29% and ∼ 50% of the grids in the c-Ti 0.4 Sb 2 Te 3 and c-Ti 1 Sb 2 Te 5 layers are randomly chosen to be the c-TiTe 2 grains, respectively, as shown in Fig. 4b,c. Constant voltage pulse is applied to the axis-symmetric mushroom-type (T-shaped) cells. It can be observed the highest peak temperature is achieved in the Ti 1 Sb 2 Te 5 based cell (Fig. 4c) while the Ge 2 Sb 2 Te 5 based cell (Fig. 4a) has the lowest peak temperature. Apparently, more heat can be generated and confined in the phase change film layer as c-TiTe 2 concentration increases, therefore lower energy is needed for the RESET operation. In summary, the pseudobinary Ti 1 Sb 2 Te 5 phase change alloy shows drastically decreased RESET energy, while increasing the SET speed, of the PCM cell compared to the Ge 2 Sb 2 Te 5 based one. Without significantly reducing the SET speed as compared to the Ti 0.4 Sb 2 Te 3 based PCM cell, nearly half of the RESET power can be saved on the Ti 1 Sb 2 Te 5 based one. These improvements are achieved by introducing more nano-scale separated TiTe 2 lamellae. We believe with more thermally stable TiTe 2 lamellae the efficiency of electric conduction and heat inhibition could be greatly enhanced for the low-energy RESET operation. We expect the speed/power to be further increased/decreased significantly on thorough investigations of the (TiTe 2 ) n (Sb 2 Te 3 ) m pseudobinary system and device dimension scaling techniques. In this regard, for example, a superlattice or multilayered structure constructed by alternate TiTe 2 /Sb 2 Te 3 stacking film instead of the co-sputtering one will be a great help. It may also be possible to search topological superconducting properties on a finely-tuned TiTe 2 /Sb 2 Te 3 superlattice sample. Methods Ti 0.4 Sb 2 Te 3 films were deposited by co-sputtering of pure Ti and Sb 2 Te 3 targets. By adding an additional pure Te target, three-target co-sputtering technique was used to fabricate the Ti 1 Sb 2 Te 5 films. Similarly, TiTe 2 films were obtained by co-sputtering of pure Ti and Te targets. For Sb 2 Te 3 and Ge 2 Sb 2 Te 5 films, respective pure alloy target was used for sputtering. The compositions of all films were measured by X-ray fluorescence spectroscopy using a Rigaku RIX 2100 system. The temperature-dependent sheet resistance changing trends of TiTe 2 , Sb 2 Te 3 , Ti 0.4 Sb 2 Te 3 , and Ti 1 Sb 2 Te 5 films with the same 150 nm thickness were studied by Linkam LMP 95 hot stage. For real-time observation of structure transition in Ti 1 Sb 2 Te 5 film, vacuum in situ X-ray diffraction (XRD) measurement with a 20 °C/min heating rate was performed on 300-nm-thick film (deposited on Si substrate at room temperature) using PANalytical X'Pert PRO diffractometer with a Cu Kα (λ = 0.15418 nm) radiation source. The diffraction data were collected in the 2θ range of 10°-60° with a scanning step of 0.02°. Raman spectroscopy (Thermo Fisher DXR) was performed on 300-nm-thick film samples at room temperature using an Ar + laser (wavelength 532 nm) with ∼ 1 μ m 2 beam spot.
T-shaped PCM cells with diameter (D) = 190 (80) nm tungsten plug bottom electrode contact (BEC) were fabricated using 0.13 μ m complementary metal-oxide semiconductor technology. In all the PCM cells, the thickness of the switching material films is around 170 nm. The 15-nm-thick TiN and 300-nm-thick Al films were used as top electrode for all cells. All the electrical measurements were performed by using the Keithley 2600C source meter (measuring cell resistance), the Tektronix AWG5002B pulse generator (generating voltage pulse with a minimum width of ∼ 6 ns), the homemade constant current driver (generating current pulse with a maximum magnitude of ∼ 10 mA), and the Tektronix 7054 digital phosphor oscilloscope (measuring transient voltage drop across the cell when current pulse is applied).