J. Phys. Chem. C 2010, 114, 17899–17904
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Growth and Phase Separation Behavior in Ge-Doped Sb-Te Thin Films Deposited by Combined Plasma-Enhanced Chemical Vapor and Atomic Layer Depositions Seol Choi, Byung Joon Choi, Taeyong Eom, Jae Hyuck Jang, Woongkyu Lee, and Cheol Seong Hwang* WCU Hybrid Materials Program, Department of Materials Science and Engineering and Inter-uniVersity Semiconductor Research Center, Seoul National UniVersity, Seoul 151-744, Korea ReceiVed: August 3, 2010; ReVised Manuscript ReceiVed: September 9, 2010
The growth, phase separation, crystallization behavior, and electrical properties of various Ge-doped Sb-Te thin films were examined. The films were deposited by combined plasma-enhanced chemical vapor and atomic layer depositions at a wafer temperature of 150 °C using Ge(i-C4H9)4, Sb(i-C3H7)3, and Te(i-C3H7)2 as the precursors for Ge, Sb, and Te, respectively. The different compositions were obtained by adjusting the Ge and Sb precursor injection times. Segregated crystalline Sb islands were observed in amorphous Sb-rich Sb-Te thin films containing over 10 atom % of Ge. The crystallization kinetics of the Ge-doped SbTe alloys, which depends on the concentration ratio of Sb/Te and the bond energy between Ge, Sb, and Te, provide clues to help understand the phase separation behavior of the films at growth temperature. Ge17Sb58Te25 and Ge31Sb38Te31 showed the highest resistivity of 67 Ω cm at the as-deposited state and exhibited a crystallization temperature of ∼260 °C. I. Introduction Recently, phase change random access memory (PRAM) has drawn a great deal of attention as a next generation nonvolatile memory.1-3 PRAM has superior memory properties such as nonvolatility, larger endurance, and promising scalability of memory cell size compared to conventional memories.4 The operation principle of PRAM is based on the reversible phase change of several chalcogenide compounds between the crystalline and the amorphous phases by electric stimulus. The crystalline and amorphous phases of these materials have a hugely different electrical resistivity,5 which represents the binary digital data. Among many phase change materials, the pseudobinary (GeTe-Sb2Te3) compound Ge2Sb2Te5 (GST) is the most widely known material. Sb2Te3-based materials like GST show nucleation-dominant crystallization kinetics, which results in a slow crystallization behavior, whereas Sb7Te3-based materials show growth-dominant crystallization kinetics, and thereby a faster phase transition is possible.6 Taking notice of this promising property of Sb7Te3-based materials, several researchers have examined this material to improve the slow switching speed (from amorphous to crystalline) of conventional GST-based PRAM.6-9 The conventional PRAM shows a switching time of ∼100 ns, which is much slower compared to charge-based memory devices such as dynamic random access memory (∼20 ns) or static random access memory ( ∼150 °C, suggesting that the thin films contained small amounts of amorphous material in the as-deposited state. Figure 5b and 5c shows the contours of resistivity and crystallization temperature of as-deposited films, respectively. Considering the intimate relationship between resistivity and crystallinity in the Ge-Sb-Te system, the tendency of resistivity decrease in Figure 5b is in good agreement with the crystallographic phase. Increasing the Ge concentration will result in an increase of resistivity due to the crystallization temperature increase of the film. However, in actual cases the resistivity was not proportionate to Ge concentration. This can be explained by the phase separation of Sb. As the size of the
Figure 5. (a) Variation of the resistivity of various films as a function of annealing temperature, (b) resistivity of as-deposited films, and (c) crystallization temperature of amorphous as-deposited films.
Sb particle is very small and because they are percolated, they cannot contribute directly to the electrical conduction in the 4-point probe measurement. However, due to the field concentration effect near the metallic particles, they may enhance the lateral electrical conduction. Therefore, the true electrical resistivity of the amorphous matrix could be larger than the experimentally estimated values. Results show that the Ge17Sb58Te25 film has the highest resistivity of 66.8 Ω cm in the as-deposited state. In Figure 5c, the crystallization temperatures of the amorphous films are shown. The crystallization temperature was determined by the temperature where a sudden decrease of resistivity occurs. From the results it can be seen that the Ge31Sb38Te31 film shows the highest crystallization temperature of ∼260 °C, which suggests a high degree of amorphous phase stability. Generally, films showing a higher resistivity have higher crystallization temperatures. However, there are inconsistent regions in resistivity and crystallization temperature as can be seen in Figure 5b and 5c. As discussed previously, this inconsistency is caused by the phase-separated Sb in the film. Therefore, it can be understood that the crystallization temperature mainly depends on the Ge concentration of the film; the higher the Ge concentration, the higher the crystallization temperature. From these results, a qualitative explanation can be given regarding the composition-dependent crystallization and resistiv-
Ge-Doped Sb-Te Thin Films
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TABLE 2: Thermodynamic Phases in the Ge-Sb-Te System20 mol % designation
phase
Ge Sb Te R γ δ
Ge Sb Te Sb2Te3 Sb1-xTex Sb1-xTex
Gemin Gemax Sbmin Sbmax 99.65 0 0 0 0 0
100 2.5 0.01 3.6 8.0 11.0
0 97.5 0 40.0 40.4 63.3
0.035 100 0.014 40.4 51.0 83.6
Temin
Temax
0 0 99.986 59.6 41.1 16.4
0.015 1.3 1 61.0 49.0 36.7
ity of Ge-ST films. Referring to the Ge solubility of the R, γ, and δ phases of a Sb-Te system in Table 2, it can be assumed that the Ge solubility of each phase is a critical factor that determines the role of Ge in the crystallization of alloy materials. This means that the films with Ge concentrations lower than the maximum solubility of Ge show the properties of a Sb-Te system as if there were no Ge content. Sb2Te3, which has a crystallization temperature of 85 °C,21 is a single R-phase material even though it could include up to 3.6 atom % of Ge. Therefore, as-deposited Sb2Te3 films show a crystalline phase in this deposition system. Also, Sb2Te3 shows a nucleationdominant crystallization behavior with slow growth properties. Only at concentrations larger than 40 atom % can Ge give an amorphous phase stabilizing effect to Sb2Te3-based materials and result in an amorphous phase. This suggests that a very high concentration of Ge is necessary to suppress the nucleation of crystals. Sb7Te3-based materials are more complex. Sb7Te3 films including less than approximately 10 atom % of Ge are single γ- or δ-phased materials with crystallization temperatures of 10322 and 124 °C,9 respectively. Therefore, these films show a crystalline phase. On the other hand, when Ge concentrations are larger than 10 atom %, Sb7Te3-based materials give an amorphous phase with phase separation. The phase-separated element is expected to be Ge from the phase diagram.23 It can be seen from the phase diagram that Ge-doped Sb7Te3 shows a phase separation into Sb7Te3 and Ge at temperatures under ∼530 °C. However, in the results presented in this work, the precipitated particles were identified as Sb. The explanation for this Sb phase separation can be attributed to the following two aspects. First, the bond energies between Ge, Sb, and Te are 264 (Ge-Ge), 302 (Sb-Sb), 397 (Ge-Te), 277 (Sb-Te),24 and 143 kJ/mol (Ge-Sb).25 Due to the relatively low Ge concentration, there will be only a small number of of Ge-Ge bonds in the film, so they can be neglected. Also, the Ge-Sb bond is the weakest, and the Ge-Te bond is the strongest. Therefore, most of the Ge will tend to bond with Te, and some of the Sb will lose bonds with Te. Then, the remaining Sb will prefer bonding with each other because the Sb-Sb bond is the second strongest. Second, the films in this paper were deposited under nonequilibrium conditions. Generally, the crystallization kinetics of nuclei can disturb the equilibrium state in thin film deposition processes containing the vapor phase. The growth-dominant crystallization kinetics of the Sb-rich phase working simultaneously with the bond energy effect mentioned above can disturb the equilibrium state. As a result, Sb precipitates instead of Ge. Now, the micro- and nanoscale phase separations are considered. The case of the microscale phase separation, which is due to the tetravalent Ge atom, may decrease the number of crystallite nuclei and suppress the crystallization process. However, a small amount of Ge is not sufficient to suppress crystallization over the whole film. On the other hand, Sb may
enhance the formation of crystallized nuclei because of its low crystallization temperature. Therefore, a few crystalline Sb nuclei can survive and grow when Ge concentration is low. These crystalline Sb nuclei work as the nucleation sites for Sb precipitation. Among those nuclei, some grow to a larger size while some stay as nuclei because of the competition between each other. This may correspond to both Ge10Sb68Te22 and Ge17Sb68Te15 films. The case of nanoscale phase separation shows that there is phase instability even in the amorphous state when there is a low Ge content. This may correspond to the Ge10Sb68Te22 film. However, when more Ge is added to the film, the nanoscale phase separation suppression becomes more effective. Therefore, in these cases only microscale crystallization occurs, while the rest of the film remains atomically uniform amorphous. This may correspond to the Ge17Sb68Te15 film. Finally, when even more Ge is added to the film, the entire film becomes amorphous without phase separation and the surface will be smooth. Recently, a nanoscale phase separation phenomenon in an amorphous Ge-Sb-Te system was reported.18 In the report, it was suggested that the origin of nanoscale phase separation is due to the tendency of the film to move toward stoichiometric compositions. It was also suggested that the criteria for phase separation is the deviation of the Ge/Sb ratio from unity. According to the report, materials would show phase separation if it contained more Ge than Sb or vice versa, and the degree of phase separation is proportional to the degree of deviation of the Ge/Sb ratio from unity.18 The phase separation of the films in Figure 3 can be understood from this point of view. The Ge/Sb ratio of Ge10Sb68Te22 is ∼0.15, whereas that of Ge17Sb58Te25 is 0.29, and there is no observable nanoscale phase separation in Ge17Sb58Te25 as can be seen in Figure 3c. Therefore, the tendency of phase separation coincides with the assumption made in ref 18. The local Ge/Sb composition ratio of the flat region in the SEM images of Ge10Sb68Te22 and Ge17Sb58Te25 films can be calculated from Table 1, and they were ∼0.17 and ∼0.30, respectively. Therefore, not only the overall but also the local Ge/Sb ratio is smaller in the Ge10Sb68Te22 film. This larger deviation of the Ge/Sb ratio from unity appears to induce the nanoscale segregation of different composition materials (Figure 3e) even in the matrix region for this case, which was not observed in other cases. Thus, it can be concluded that simultaneous phase separations occurred in the microscale (hundreds of nanometers scale) and nanoscale in the amorphous film and the degree of nanoscale phase separation depends on the Ge/Sb ratio. IV. Conclusions It was shown that the crystallization accompanying phase formation and electrical properties of Ge-ST films depend on both the Ge concentration and the Sb/Te ratio. The crystallization behavior of the films was dependent on Ge concentration, but the role of Ge is different in the films with the different Sb/Te ratios. Sb precipitations were observed in Ge-excess films, and this is an abnormal phenomenon according to the phase diagram. The strong bond energy between Ge and Te and the growth-dominated crystallization kinetics gives these results. Also, the Sb precipitations turned out to have a simple cubic structure, not a rhombohedral structure. This simple cubic structure appears under high pressure, and the accumulated stress during deposition acts as the origin of such conditions. It was also found that these scattered Sb precipitations affected the resistivity values of the overall film. The Ge/Sb ratio of the film influences the compositional segregation in amorphous
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matrices. When the ratio deviates largely from 1, even amorphous materials showed nanoscale compositional segregation. Acknowledgment. The authors acknowledge the support of the National Research Program for the Nano Semiconductor Apparatus Development sponsored by the Korea Ministry of Knowledge and Economy, Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0081961), and World Class University program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-2008-00010075-0). References and Notes (1) Hudgens, S.; Johnson, B. MRS Bull. 2004, 29, 829. (2) Lankhorst, M. H. R.; Ketelaars, B. W. S. M. M.; Wolters, R. A. M. Nat. Mater. 2005, 4, 347. (3) Oh, J. H.; Park, J. H.; Lim, Y. S.; Lim, H. S.; Oh, Y. T.; Kim, J. S.; Shin, J. M.; Song, Y. J.; Ryoo, K. C.; Lim, D. W.; Park, S. S.; Kim, J. I.; Kim, J. H.; Yu, J.; Yeung, F.; Jeong, C. W.; Kong, J. H.; Kang, D. H.; Koh, G. H.; Jeong, G. T.; Jeong, H. S.; Kim, K. In Technical DigestInternational Electron DeVices Meeting, 2006; IEEE: Los Alamitos; 2006; Vol. 2.6. (4) Koh, G. H.; Hwang, Y. N.; Lee, S. H.; Lee, S. Y.; Ryoo, K. C.; Park, J. H.; Song, Y. J.; Ahn, S. J.; Jeong, C. W.; Yeung, F.; Kim, Y.-T.; Park, J.-B.; Jeong, G. T.; Jeong, H. S.; Kim, K. International Conference on Integrated Circuit Design and Technology, 2004; IEEE: Los Alamitos; 2004; p 53. (5) Feinleib, J.; Ovshinsky, S. R. J. Non-Cryst. Solids 1970, 4, 564. (6) Khulbe, P. K.; Hurst, T.; Horie, M.; Mansuripur, M. Appl. Opt. 2002, 41, 29–6220. (7) Lankhorst, M. H. R.; Pieterson, L.; Schijndel, M.; Jacobs, B. A. J.; Rijpers, J. C. N. Jpn. J. Appl. Phys. 2003, 42, 863. (8) Raoux, S.; Salinga, M.; Jordan-Sweet, J. L.; Kellock, A. J. Appl. Phys. 2007, 101, 044209. (9) Lee, M. L.; Miao, X. S.; Ting, L. H.; Shi, L. P. J. Appl. Phys. 2008, 103, 043501. (10) Kim, Y. T.; Hwang, Y. N.; Lee, K. H.; Lee, S. H.; Jeong, C. W.; Ahn, S. J.; Yeung, F.; Koh, G. H.; Jeong, H. S.; Chung, W. Y.; Kim, T. K.;
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