Crystallization Properties of Al-Sb Alloys for Phase Change Memory Applications

Figure 1a shows sheet resistance as a function of temperature for 160 nm thick Al-Sb films with different Al concentration. The as-deposited samples were amorphous with initial resistance ranging from 10⁵ ohm/sq. to 10⁹ ohm/sq. for 10 at% Al to 45 at% Al, respectively. There was a gradual exponential drop in the sheet resistance with increasing temperature followed by a sharp drop at crystallization temperature. The final sheet resistance was dependent on the Al content in the alloy: ∼5 × 10³ ohm sq⁻¹ for Al_0.45Sb_0.55 and ∼10² ohm/sq for Al_0.1Sb_0.9. Samples with 10%–30% Al exhibited distinct resistance plateaus just before the crystallization temperature, which agrees with the previously published results. ⁶ This phenomenon has been attributed to improved crystallization without any additional phase transformation. ⁶ Although the mechanism of these metastable plateaus is not clear, it was shown to be reproducible. It is possible that these plateaus may improve stability of multiple resistance states in memory devices.

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The dependence of two crystallization temperatures, corresponding to low-T (metastable) and high-T resistance drops, on Al content at various heating rates is shown in Fig. 1b. For comparison, similar values for GST films prepared by room temperature sputtering from stoichiometric target at room temperature are shown. It is evident that the crystallization temperature T_C increases with Al% reaching a maximum value of 270 °C at 30% followed by a decrease in Tc at higher Al%. Essentially Al bonding leads to an increase in total bond enthalpy leading to an increase in glass transition temperature that has been shown to be proportional to high crystallization temperature. ⁹ The T_C for all Al% is noticeably higher than that for GST (173 °C–188 °C) which is expected to result in higher thermal stability for Al-Sb based PCM devices. Observed T_C also shifted to higher temperature with the heating ramp rate; this T_C shift is used to derive activation energy for crystallization using the Kissinger method. ¹⁴ The activation energies obtained from Fig. 1c range from 1.8–2.8 eV which are quite similar to that obtained for GST alloys. Figure 1d shows extrapolated failure time for Al-Sb PCM devices. It is clear that high activation energy of Al_0.2Sb_0.8 and Al_0.3Sb_0.7 alloys will allow improved data retention at 138 °C and 169 °C for 10 years. This 10-year projected data retention is similar to what has been shown previously for Al based PCM. ^10,15

Phase structure of the Al-Sb PCM alloys

Figure 2a shows crystalline phases of 300 °C annealed Al-Sb films using grazing-incidence XRD. The absence of any distinct diffraction peaks in the spectra from the as-deposited films (not shown) indicates that this material is amorphous which is consistent with previously published results. ^4,6,13 The annealed films consisted of only two crystalline phases, namely rhombohedral Sb (A7) phase ¹⁶ and cubic zinc-blende AlSb phase as confirmed by simulations (Fig. 2a). The relative fraction of the cubic AlSb phase increases along with increasing Al% as evident from the disappearance of diffraction peaks corresponding to the A7 phase. To quantify the relative volume of the two phases in the films, areas under the (111) AlSb and (012) A7 peaks were obtained from the diffraction data and compared with the simulated volumes under these peaks (using scattering and structure factors) as shown in Fig. 2b. To improve the accuracy of calculations, areas under weaker peaks such as (002) and (101) were also taken into consideration. Comparison of experimental and simulated data indicates that the rhombohedral Sb phase can contain up to 10–15 at.% of Al which is much higher than the equilibrium solubility of Al in Sb at its melting point of 630 ⁰C (∼ 2 at.%). ¹⁷ It can be summarized that non-equilibrium Sb A7 phase forms exist in the low-resistance state of Al-Sb alloys.

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Crystallization of phases after melting of Al-Sb PCM alloys

Al-Sb alloys based PCMs are two phase system consisting of rhombohedral A7 and AlSb cubic phase as shown earlier by XRD results. In-situ XRD analysis was also done on these alloys to further understand the crystallization behavior upon heating and recrystallization after melting. Figures 3a and 3b also confirm the two phases for both Al_0.2Sb_0.8 and Al_0.3Sb_0.7 alloys upon heating to 500 °C. Figures 3a and 3b both show the peaks intensity getting strong for both A7 and cubic phase that suggests grain growth as also shown later in Fig. 5d. The two films were further annealed close to their melting temperatures and then were cooled down to room temperature. Figures 3c and 3d show the color plot of crystallization of the films upon cooling from melting. Both of the alloys show that the A7 phase crystalizes at a different temperature as compared to the cubic phase which stays crystalline even after annealing to 650 °C. Figure 3e further confirms this behavior as the intensity profile corresponding to 631 °C is missing the (012) peak and it starts to appear around 600 °C while the peaks corresponding to cubic phase are present for all the temperatures. These results mean that the AlSb phase crystallizes at a much higher temperature as compared to the A7 phase which can conclude that resistive switching of this two-phase system is mostly due to the crystallization of only the Sb A7 phase.

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Pulse measurements

Al-Sb alloy was used as a phase change layer in PCM mushroom cells to evaluate its electrical properties. Figure 4a presents the fixed pulse width programming R-V characteristics from reset state of 20 nm thick Al_0.3Sb_0.7 film deposited in amorphous state on a 120 nm TiN heater and capped with in situ deposited TaN electrode. With the increase of pulse amplitude the low resistance set state appears for pulse voltage as small as ∼1.2 V for different pulse widths. With further increase in pulse amplitude the resistance returns to the high resistance reset state. Al_0.3Sb_0.7 shows larger resistance contrast with similar pulse parameters compared to other Al doped Sb-rich PCM materials. ^10,13,15 The set and reset operation of Al-Sb based device requires pulses with lower amplitude than what has been shown for conventional GST ^18,19 which can lead to lower power consumption in Al-Sb cells. Higher reset resistance of Al-Sb alloy plays an important role in reducing the energy for set and read operation and also leading to low power consumption. It should be noted, that the over half of the volume of the Al_0.3Sb_0.7 alloy consists of cubic AlSb phase which is semiconductor with a relatively wide bulk bandgap of 1.6 eV that can be larger due to the effect of crystalline confinement. Due to the large AlSb bandgap the Al-Sb alloy may be expected to constitute a highly resistive matrix. Figure 4b presents reversible switching properties of Al-Sb based mushroom cell. A very high reset/set resistance ratio of >1000 was observed on Al_0.3Sb_0.7 cells as compared to ∼100–500 for GST. ^15,18,20 Te-poor PCM has also been shown to have low resistance contrast, ²¹ and this three orders of magnitude resistance change for Te-free Al-Sb alloy based PCM shows a great promise toward providing an adequate margin for programming multiple resistance states for multileveled storage devices. The randomness in the set and the reset state can be reduced by better pulse programming and by introducing a metal nitride liner as a drift stabilizer. ²² This large resistance contrast and potential for lower power consumption makes Al-Sb alloys a promising candidate for analog memory application devices.

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Dark-field cross section TEM of a confined cell (Fig. 5a) shows fine crystalline grains in the device region where the switching occurred after ∼100 reversible switching cycles when the device was left in the low resistance set state. It should be noted, that the cell with Al top electrode typically failed after a few hundred cycles likely due to low chemical stability of Al/Al-Sb interfaces and/or Al electromigration. Replacement of the top electrode with TaN improved endurance to at least 10⁶ cycles which will be discussed elsewhere. However, even with the relatively unstable Al electrode the microstructure and chemistry are not affected significantly by Al mass transport as shown below. The polycrystalline granular structure is also visible in Al_0.3Sb_0.7 layer over the Si₃N₄ film which extends up to ∼0.5 μm from the active device region, presumably due to excess heating during the switching. A histogram of grain sizes is shown in Fig. 5c with an average grain size of ∼ 8 nm.

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The evolution of the crystalline grain size in blanket films of Al20%, 30%, and 40% during annealing from 200 °C–500 °C was also obtained by in situ synchrotron XRD measurements (Fig. 5d). Figure 5d reveals two distinct onsets: Firstly corresponding to nucleation with relatively wide temperature range of unchanged grain size and secondly implying grain growth. The grain growth of both Sb A7 and cubic AlSb phases were obtained for Al_0.4Sb_0.6. There is very little change in the grain size of the Sb A7 phase crystallites up to 450 °C (from 5–10 nm) but the grain size increases sharply by a factor of 4–5 in the temperature range of 450 °C–500 °C. Crystallites composed of cubic zinc blende (ZB) AlSb phase, however, do not show any appreciable grain growth even up to 500 °C. This is attributed to the high 1065 °C melting point of cubic AlSb. Since the AlSb grain stays stable upon annealing, this suggests that grain growth is due to agglomeration of A7 Sb grains. The resistance change during switching is therefore dominated by the Sb A7 phase crystallites rather than cubic AlSb crystallites.

Figure 6 illustrates redistribution of Al and Sb in the Al_0.3Sb_0.7 confined cell after ∼100 cycles switching. Al Kα and Sb Lα line intensity profiles were obtained by energy dispersive X-ray spectroscopy (EDX) from a cross-section TEM sample of a PCM device (Figs. 6a and 6b). The profiles across the Al-Sb layer on TiN (inside the cell) and Si₃N₄ (1 μm away from the cell) show a change of Al profile across the PCM layer thickness (Fig. 6c). The 2x change of Al fraction slope across the layer thickness is quiet prominent and could likely be an influence of thick Al contact. The change in Al-Sb composition is significantly less than in typical Te-containing GST alloys², indicating higher chemical stability of Al-Sb PCM materials.

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