Article pubs.acs.org/cm
Conformal Formation of (GeTe2)(1−x)(Sb2Te3)x Layers by Atomic Layer Deposition for Nanoscale Phase Change Memories Taeyong Eom,† Seol Choi,† Byung Joon Choi,† Min Hwan Lee,‡ Taehong Gwon,† Sang Ho Rha,† Woongkyu Lee,† Moo-Sung Kim,§ Manchao Xiao,⊥ Iain Buchanan,⊥ Deok-Yong Cho,# and Cheol Seong Hwang*,† †
WCU Hybrid Material Program, Department of Materials Science and Engineering, and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-744, Republic of Korea ‡ Mechanical Engineering, School of Engineering, University of California, Merced, California 95343, United States § Air Products Korea, 15 Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do, 446-920, Republic of Korea ⊥ Air Products and Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011, United States # IWE2 & JARA-FIT, RWTH Aachen University, 52056 Aachen, Germany S Supporting Information *
ABSTRACT: Phase change random access memory appears to be the strongest candidate for next-generation high density nonvolatile memory. The fabrication of ultrahigh density phase change memory (≫1 Gb) depends heavily on the thin film growth technique for the phase changing chalcogenide material, most typically containing Ge, Sb and Te (Ge−Sb−Te). Atomic layer deposition (ALD) at low temperatures is the most preferred growth method for depositing such complex materials over surfaces possessing extreme topology. In this study, [(CH3)3Si]2Te and stable alkoxy-Ge (Ge(OCH3)4) and alkoxy-Sb (Sb(OC2H5)3) metal− organic precursors were used to deposit various layers with compositions lying on the GeTe2− Sb2Te3 tie lines at a substrate temperature as low as 70 °C using a thermal ALD process. The adsorption of Ge precursor was proven to be a physisorption type while other precursors showed a chemisorption behavior. However, the adsorption of Ge precursor was still selfregulated, and the facile ALD of the pseudobinary solid solutions with composition (GeTe2)(1‑x)(Sb2Te3)x were achieved. This chemistry-specific ALD process was quite robust against process variations, allowing highly conformal, smooth, and reproducible film growth over a contact hole structure with an extreme geometry. The detailed ALD behavior of binary compounds and incorporation behaviors of the binary compounds in pseudobinary solid solutions were studied in detail. This new composition material showed reliable phase change and accompanying resistance switching behavior, which were slightly better than the standard Ge2Sb2Te5 material in the nanoscale. The local chemical environment was similar to that of conventional Ge2Sb2Te5 materials. KEYWORDS: GeSbTe pseudobinary solid-solution, atomic layer deposition, phase change random access memory, reaction mechanism efficiency (only ∼2%) of this structure.5 Plugging the phase Change (PC) material into a small contact hole (called the confined cell structure) was suggested as the most efficient method to improve the cell efficiency and increase the integration density.6,7 However, the confined cell structure requires the PC material to be deposited with highly conformal, reproducible, and rapid growth properties, which can usually be accomplished by chemical vapor deposition (CVD) and atomic layer deposition (ALD). Of these two techniques, ALD is the more preferred growth method owing to its unique self-limiting growth behavior.8 However, the ALD of a PC material (most typically a Ge−Sb−Te pseudobinary solid solution) is difficult because of the strong bond energy between the metal ions (Ge,
I. INTRODUCTION Innovations in modern information technology require solidstate memories with higher density, faster operating speed, and lower power consumption. Under these circumstances, nonvolatile memory (NVM) is becoming increasingly important in mobile electronic devices.1 Among such NVMs, phase change random access memory (PCRAM) appears to fulfill most of the aspects required for future applications. In addition, as scaling proceeds PCRAM may win out over other technologies, including charge-based memories such as dynamic random access memory and flash memory,2,3 because scaling leads to a reduction in the energy needed for its stable operation. Therefore, the development of an applicable PCRAM is essential to extend Moore’s law over the next few decades. The current PCRAM technology generally adopts a planar structured (so-called T-shaped) cell.4 However, further scaling is difficult with a T-shaped cell due to the extremely low heating © 2012 American Chemical Society
Received: February 18, 2012 Revised: April 28, 2012 Published: May 9, 2012 2099
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
Chemistry of Materials
Article
GeTe2 deposition, the following sequence was adopted; Ge precursor pulse − Ge precursor purge − Te precursor pulse − Te precursor purge. This comprises one cycle for Ge−Te deposition. A similar sequence was adopted for Sb−Te deposition. No other reaction gas was used. For (GeTe2)(1−x)(Sb2Te3)x pseudobinary film deposition, the Sb−Te cycle was repeated m times and the Ge−Te cycle was repeated n times to comprise one super cycle. P-type (100) Si, 100 nm thick SiO2/Si and 50 nm thick TiN/SiO2/Si, 5 nm thick TiO2/SiO2/Si were used as substrates, where the SiO2, TiN, and TiO2 films were grown by thermal oxidation, sputtering (Applied Materials, Endura 5500), and ALD, respectively. ALD TiO2 was performed using Ti(OC3H7)4 and O2 plasma at 250 °C in another ALD reactor. In particular, the TiN films for the hole pattern structure were deposited by a third ALD system using TiCl4 + N2/H2 plasma at 360 °C. A contact hole substrate with different depths was fabricated in the 400 and 2500 nm thick plasma-enhanced CVD SiO2 layer on a Si substrate by a standard photolithography and dry etching technique. In some cases, ALD TiN (3 nm) or ALD TiO2 (3 nm) was deposited over the contact hole surface to enhance the nucleation of (GeTe2)(1−x)(Sb2Te3)x. The thickness and composition uniformity over 8 in. scale wafer was quite reasonable; (max. − min.)/2average was 0.74 where the decrease in the Sb2Te3 growth rate with increasing Sb−Te subcycle number just corresponds to the recovery of Sb2Te3 growth rate on itself. Here, the growth rate of Sb2Te3 is dependent on the thickness (or composition) of the underlying GeTe2 layer even for the given Sb−Te subcycle number, which is one in this case. Although the exact reason for this is not clearly understood at this moment, the following can be assumed. Choi et al. reported that the chemical adsorption rate of the precursors in a combined PECVD/ALD of a Ge−Sb−Te pseudobinary alloy was enhanced when the growing film was in situ crystallized or grown on an ex situ crystallized layer.11 This was attributed to ease of charge transfer from the electrically conducting (crystallized) layer to the chemically adsorbing molecules. Similar enhancement in the ALD rate of metals on an electrically conducting surface was also reported for other materials.21 As shown in the Supporting Information, Figure S11, and Figure 7, the GeTe2 and GeTe2-rich pseudobinary films were amorphous and have high electrical resistivity while the Sb2Te3 and Sb2Te3-rich pseudobinary films were already crystallized and electrically conducting. Therefore, it can be assumed that the thicker GeTe2 sublayer makes the growing surface more electrically insulating, retarding Sb2Te3 layer growth as in Figure 5c. When only one cycle of GeTe2 deposition was performed, the surface is more like a pseudobinary film with higher electrical conductance compared to the thicker GeTe2 layer, and therefore an enhanced Sb2Te3 layer growth was not caused by the chemical effect. The temperature dependency of growth rate is shown in Figure 5d. The growth rates of GeTe2 and Sb2Te3 were not remarkably changed in the temperature range of 50−80 °C. However, at temperatures higher than 80 °C, desorption of the adsorbate was accelerated and the growth rate was largely decreased, and eventually no film was deposited at temperatures higher than 130 °C. 3.3. Other Properties of Pseudobinary (GeTe2)0.66(Sb2Te3)0.33 Layers. In this section, the structural and phase change properties of the (GeTe2)(1−x)(Sb2Te3)x films are reported. Several (GeTe2)(1−x)(Sb2Te3)x layers were grown
which is almost 15 times higher than that in the case where Sb2Te3 was grown on itself (Figure 2). The molar growth rates were also taken from these growth rates. The slopes were divided by the molar mass to calculate molar layer density growth rate. The molar growth rate of the GeTe2 component was 1.59 × 10−10 mole cm−2 cy−1 on SiO2 and 1.43 × 10−10 mole cm−2 cy−1 on TiN, and for the Sb2Te3 component it was 4.79 × 10−10 mole cm−2 cy−1 on SiO2 and 4.31 × 10−10 mole cm−2 cy−1 on TiN. Remarkably, when compared to GeTe2, approximately 3 more times of Sb2Te3 molecules are deposited for every single super cycle performed. Figure 5b shows the variations of the molar layer density increase per subcycle of the (GeTe2)(1‑x)(Sb2Te3)x layers as a function of the x value. Here, the Ge−Te: Sb−Te cycle ratio of 8:1, 4: 1, 2: 1, 1: 1, 1: 2, and 1: 4 resulted in the x values of 0.12, 0.27, 0.5, 0.74, 0.76, and 0.81, respectively. The growth rates were taken from the slopes of the layer density-cycle number graphs and the slopes were divided with the molar mass of each component to calculate the molar layer density growth rate. Figure 5b clearly shows that the molar growth rate of GeTe2 is quite independent of x values meaning that the underlying layer composition hardly influenced the adsorption rate of the Ge precursor. This again corroborates the physical adsorption nature of Ge-precursor. However, the growth of Sb2Te3 is very dependent on the composition of the underlying layer; it appears that the growth rate of (GeTe2)(1−x)(Sb2Te3)x showed a peak value at (GeTe2)0.26(Sb2Te3)0.74 and deviations from that composition decreases the growth rate. Thus, the x value and growth rate of Sb2Te3 could be classified into two region with reference to x = 0.74. First, in the high x value region, the growth rate of the Sb2Te3 component was abruptly decreased when increasing the x value to x = 0.74, 0.76, and 0.81. As shown in the Table 1, it Table 1. Steady-State Molar Growth Rates of GeTe2 and Sb2Te3 Layers on Various Sb−Te Cycle Numbers in High x Region molar growth rate (× 10−10mole cm−2 cy−1)
x
Ge− Te cycle no.
Sb−Te cycle no. (A)
0.74 0.76 0.81 1
1 1 1 0
1 2 4 1
GeTe2 1.43 2.02 1.71
Sb2Te3 (B) 4.24 3.25 1.95 0.32
Sb2Te3, first cycle (C) 4.24 5.99 5.06
Sb2Te3, two or more cycles (A × B−C) × (A-1)−1 0.51 0.92 0.32
was possible to estimate the growth rates of the Sb2Te3 component at first Sb−Te cycle, and after two or more Sb− Te cycles on the given GeTe2 layer Here, the x = 1 data came from the slope of the Sb2Te3 layer deposition results in Figure 2a, and the Ge−Te and Sb−Te cycle number was the number of cycles of each component performed in a single super cycle. The molar growth rate of Sb2Te3 at the first Sb−Te cycle was calculated by multiplying by 2.97 the growth rate of GeTe2. Here, the 2.97 is the molar growth rate ratio of Sb2Te3 and GeTe2 when the Ge−Te:Sb−Te cycle ratio was 1: 1. The molar growth rate of Sb2Te3 at two or more Sb−Te cycle was calculated by the dividing the growth rate of Sb2Te3 after the first cycle with the number of super cycles performed minus one, e. g. when the number of cycles of Sb−Te and Ge−Te were 1 and 4, respectively, then (4 times the growth rate of Sb2Te3 (= super cycle growth rate of Sb−Te) minus 2.97 times 2106
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
Chemistry of Materials
Article
at a growth temperature of 70 °C under the modified film growth conditions as described below. The modification of the process condition is basically due to engineering considerations. Under the standard conditions given in section III-2, the Ge precursor efficiency was as low as 0.24%, primarily because of the weak interaction with the film surface. Here, the efficiency was calculated as the ratio between the deposited amount of Ge atoms and supplied amount of Ge atoms in the precursor. In addition, the growth rate, defined as film thickness increase per process time, of (GeTe2)0.66(Sb2Te3)0.33 was as low as 0.0051 nm s−1. These are extremely unfavorable values for possible application to mass production. Therefore, the Ge precursor injection time was shortened to 2 s from 4 s, and the precursor purge time was also shortened to 1 s. Also, the Sb precursor purge time was shortened to 2 s from 5 s, and the Te precursor purge time was also shortened to 5 s from 10 s in the ALD process window. On the other hand, the Sb and Te precursor injection times were conserved to 2 and 1 s respectively. With these process conditions, several films with x = 0.25, 0.33, and 0.66 are deposited, and the Ge−Te:Sb−Te cycle ratios for those films were 4: 1, 2: 1, and 1: 2, respectively. As will be discussed in the following paragraph, these conditions produced pseudobinary films with highly promising structural and electrical properties. In the GeTe-Sb2Te3 pseudo binary system, it is already wellknown that compositions with identical Ge and Sb atomic concentrations have promising properties as phase change materials. Therefore, in this work, the property of the (GeTe2)0.66(Sb2Te3)0.33 system was intensively investigated. Here, the super cycle number of the deposition process was denoted as ncys (one combined (Ge−Te) step and one combined (Sb−Te) step correspond to one ncys, respectively). Figure 6a shows the variations of layer density and thickness in the (GeTe2)0.66(Sb2Te3)0.33 film as a function of ncys on SiO2
and TiN substrates. The growth rate of the (GeTe2)0.66(Sb2Te3)0.33 layer, estimated from the slope, was 250 ng/cm2ncys (0.42 nm/ncys) on SiO2 and 240 ng/cm2ncys (0.43 nm/ncys) on TiN. The films grew linearly with ncys having almost no incubation, indicating fluent nucleation and growth. The growth rate of the GeTe2 and Sb2Te3 components was also estimated. The growth rate of the GeTe2 component was 61 ng/cm2ncys (0.15 nm/ncys) on SiO2 and 62 ng/cm2ncys (0.15 nm/n cy s ) on TiN, and the growth rate of the Sb 2 Te 3 component was 130 ng/cm2ncys (0.22 nm/ncys) on SiO2 and 117 ng/cm2ncys (0.20 nm/ncys) on TiN. These results are not significantly different from what were shown in Figure 5 even though the process conditions were modified. Figure 6b shows the change in the rms roughness of the (GeTe2)0.66(Sb2Te3)0.33 films as a function of ncys when grown on the SiO2, TiN and TiO2 substrates. The layers had better nucleation behavior on TiO2 and TiN than on SiO2 and showed a generally smoother surface. Up to ncys = 100, the roughness increased because of island type growth but it decreased when further deposition was performed by a coalescence of the islands into a continuous film. Eventually, the film became very smooth at 87 nm for the TiO2 substrate (rms roughness ∼0.8 nm). Figure 6c shows the AES depth profile results of a 45 nm thick layer grown on a TiN substrate. The AES depth profile results on SiO2 and TiO2 substrates are included in the Supporting Information (Figure S9). As can be seen in the figure, the film has a highly uniform composition along the thickness direction. In addition, there was negligible Si-contamination and only ∼1% oxygen suggesting that the film really grew via the suggested ALD mechanism (eqs 3 and 4). Some of the XRR results for the estimation of film density are shown in the Supporting Information (Figure S10). The as-deposited film density on SiO2, TiN and TiO2 substrates was ∼4.40, 4.82, and 4.42 g cm−3, respectively, which are relatively lower than the bulk value at the same composition (= 5.29 g cm−3). The density of a film grown on a SiO2 substrate measured after heat treatment at 200 °C for 10 min under an air atmosphere was ∼4.58 g cm−3, which corresponds to the volumetric shrinkage of only 4%. Electrical measurements were performed to observe the phase change characteristics of the pseudobinary (GeTe2)(1−x)(Sb2Te3)x layers, and to see the competitiveness of this material these electrical properties are compared with those of the conventional Ge2Sb2Te5, which was deposited by plasmaenhanced chemical vapor deposition.10 Figure 7a shows the variation of resistivity as a function of annealing temperature (ρ-T) in (GeTe2)(1−x)(Sb2Te3)x films with various x values. The film with a large x value (0.66) was already crystallized at the asdeposited state, and has a low ρ value even at the lowest temperature. The films with lower x values of 0 (= GeTe2), 0.25, and 0.33 showed an abrupt decrease in ρ at ∼220, 160, and 150 °C, respectively, suggesting useful phase change properties. The resistivity contrast ratio was as high as ∼1 × 106 and the transition was very abrupt. It is important to point out that as the GeTe2 content increases the resistivity tends to show a larger and more rapid drop. In comparison with the Ge 2 Sb 2 Te 5 film, the resistivity drop of the (GeTe2)(1−x)(Sb2Te3)x layer was more abrupt and clear, and the contrast ratio was also higher. The crystallization behavior according to the annealing temperature of the films with four different x values, as determined by XRD, are reported in the Supporting Information (Figure S11).
Figure 6. (a) Variation in the total layer density and thickness for (GeTe2)0.66(Sb2Te3)0.33 film as a function of ncys on the SiO2 and TiN substrates. (b) Change in the rms roughness of the (GeTe2)0.66(Sb2Te3)0.33 films on the SiO2, TiN and TiO2 substrates as a function of ncys. (c) AES depth profile results of the 45 nm-thick (GeTe2)0.66(Sb2Te3)0.33 layer with deposited on the TiN/SiO2 substrate. 2107
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
Chemistry of Materials
Article
Local atomic configurations were examined by the XANES spectra of the crystalline (GeTe2)0.66(Sb2Te3)0.33 layer and Ge2Sb2Te5. They showed that the local bonding structure of the two materials are similar although (GeTe2)0.66(Sb2Te3)0.33 layer had a higher vacancy concentration. This suggests that this new composition layer can have fluent phase change properties as already shown above (see the Supporting Information, Figure S12). 3.4. Complete Filling of Contact Holes with Pseudobinary (GeTe2)(1−x)(Sb2Te3)x Layer. Figure 8a shows the cross-
Figure 7. Electrical switching behaviors of (GeTe2)(1−x)(Sb2Te3)x films. (a) Real-time resistivity change of the (GeTe2)(1−x)(Sb2Te3)x with x = 0 (= GeTe2), 0.25, 0.33, and 0.66, and Ge2Sb2Te5 films. The as-deposited films with x = 0, 0.25, and 0.33 had an amorphous structure while the as-deposited film with x = 0.66 had a crystalline structure. The Ge2Sb2Te5 film was deposited by PECVD. (b-d) The time evolution of the current with the CAFM set up of the 40 nm thick Ge2Sb2Te5 and (GeTe2)0.66(Sb2Te3)0.33 on TiN electrode under a pulsed bias with 10 V amplitude. b) shows the common current response under pulses on both films. Inset figure shows the laser annealing data. It shows the crystallization occurred at ∼500 ns and a laser power of ∼20 mW. x = 0.66 and film thickness was ∼120 nm. c) shows the current image of the (GeTe2)0.66(Sb2Te3)0.33 film after applying 10 V pulses for 1s using a top-to-bottom configuration. d) shows the crystallization time of the two films from the CAFM.
The nanoscale phase change behavior of the (GeTe2)0.66(Sb2Te3)0.33 films was confirmed by pulsed voltage application using the CAFM under vacuum conditions (∼1 × 10−6 Torr). Here, a 40 nm thick as-deposited amorphous film on a TiN layer was used to confirm the electrically induced amorphous−crystalline transformation. Figure 7b shows the commonly observed current response under the application of a pulsed voltage (starting at t = 0 s) with a 10 V amplitude. The initial peak (around t = 0 s) is attributed to the stray capacitance of the measurement setup itself, which was confirmed by performing an open circuit test. The crystallization time from the current responses is generally in the order of a few hundreds of nanosec under 10 V bias. A local current image obtained after pulse application (Figure 7c) clearly shows that conductive paths were actually created through the amorphous (GeTe2)0.66(Sb2Te3)0.33 layer as a result of local biases. Pulses with a 10 V amplitude were applied for 1 μs before CAFM mapping. Repeated pulse responses on different locations showed that the crystallization time observed on the (GeTe2)0.66(Sb2Te3)0.33 layer was slightly shorter than on a Ge2Sb2Te5 layer with the same thickness. (The average of 20 responses for each film was ∼350 ns and ∼470 ns. See Figure 7d.) A more delicately designed heat-insulating cell is expected to have a much shorter crystallization time than the film deposited directly on a flat TiN layer (used in the current experiment). A pulsed laser crystallization experiment on the same films showed similar crystallization times (data not shown).
Figure 8. Cross-section transmission TEM images showing the conformal deposition of (GeTe2)0.66(Sb2Te3)0.33 film on contact hole. (a) Completely filled contact hole with the TiN interfacial layer when ncys = 200. (b) EDS line profile of the three elements along the line shown in a). (c) Cross-section TEM image of the (GeTe2)0.66(Sb2Te3)0.33 film (ncys = 60) on the contact hole with an aspect ratio of 20. (d) EDS profile of the three elements along the (GeTe2)0.66(Sb2Te3)0.33 in the side wall of the hole marked by numbers in c.
section TEM images of the (GeTe2)0.66(Sb2Te3)0.33 layer grown on a contact hole structure (opening diameter ∼65 nm, depth ∼400 nm, an aspect ratio of ∼6) at ncys = 200, where a 3-nm thick ALD TiN interlayer was deposited on the SiO2 hole before (GeTe2)(1−x)(Sb2Te3)x ALD. Figure 8b shows the EDS composition analysis line scan results along the line indicated in Figure 8a. The (GeTe2)0.66(Sb2Te3)0.33 layer has an amorphous microstructure, and very uniform thickness and chemical composition along the depth direction in the hole. The small voids (seam) at the center region of the filled material are due to the bowing shape of the patterned hole. Other examples of the conformal film growth with different ncys on different types of hole substrates are shown in the Supporting Information (Figure S13). 2108
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
Chemistry of Materials
■
The hole filling ability of the present ALD process shown in panels a and b in Figure 8 is already more than adequate for the present or near-term requirements for the PCRAM. However, the ultimate conformal deposition capability of the this ALD process was examined using a much more severe hole pattern (opening diameter ∼120−150 nm and hole depth of 2500 nm, giving an aspect ratio of ∼20). Panels c and d in Figure 8 show cross-section TEM images and EDS composition results of the locations shown in the image (ncys = 60). A further increase in ncys just clogged the entrance of the hole and no further filling was achieved due to the severe bowing of this hole pattern. Even on such an extreme geometry, there was completely uniform deposition (thickness and chemical composition) over the entire surface. This demonstrates the supreme film growth properties of this ALD process.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This study was supported by the National Research Program for the Nano Semiconductor Apparatus Development sponsored by the Korea Ministry of Knowledge and Economy (10034831), and the Converging Research Center Program through the National Research Foundation of Korea (NRF) (2011K000610).
■
4. CONCLUSIONS In summary, low-temperature (∼70 °C) thermal ALD of Ge− Sb−Te phase changing material layers, with composition lying on the GeTe2−Sb2Te3 tie line, has been realized using silyl-Te and alkoxy-Ge and alkoxy-Sb metal−organic precursors ([(CH3)3Si]2Te, Ge(OCH3)4, Sb(OC2H5)3) without the use of any reaction gas. The strong affinity between the silyl group in the Te precursor and oxygen atoms in the Ge and Sb precursors provide an efficient thermal ALD reaction route for the system. In addition, all the precursors are in liquid form and are stable at the vaporization temperature, in contrast with previous reports,14 suggesting superior mass-production compatibility of this ALD process. This chemistry-specific ALD process was quite robust against process variations resulting in highly conformal, smooth, and reproducible film growth over a contact hole structure with an extreme geometry. The binary GeTe2 layer on TiN shows a very long incubation cycles, while binary Sb2Te3 nucleates immediately on the same substrate. However, when they were combined to form the pseudobinary film and Sb2Te3 layer deposited first, almost no incubation was observed and a highly linear growth behavior was achieved. The overall film composition was well represented by (GeTe2)(1−x)(Sb2Te3)x suggesting the chemical reaction route for the binary layers works well in the pseudobinary case, and continuous variation in composition (value of x) was achieved through adjustment in the relative number of Ge−Te and Sb−Te cycles. However, interestingly, the Sb2Te3 layer showed an accelerated growth rate, which is almost 15 times higher than that on itself, on the (GeTe2)0.26(Sb2Te3)0.74, whereas GeTe2 layer growth was quite independent of the underlying layers. The adsorption of Ge precursor molecules proceeds via a physical route, while those of Sb and Te precursors occurred in a chemical way. However, the smaller degree of desorption of the Ge precursors on the Sb−Te surface enabled an ALD-like saturated growth behavior. This new composition material showed a reliable phase change and accompanying resistance switching behavior, which were slightly better than the standard Ge2Sb2Te5 material in the nanoscale.
■
Article
REFERENCES
(1) Burr, G. W.; Kurdi, B. N.; Scott, J. C.; Lam, C. H.; Gopalakrishnan, K.; Shenoy, R. S. IBM J. Res. Dev. 2008, 52 (4), 449−464. (2) Ovshinsky, S. R. Phys. Rev. Lett. 1968, 21 (20), 1450. (3) Lankhorst, M. H. R.; Ketelaars, B. W. S. M. M.; Wolters, R. A. M. Nat. Mater. 2005, 4 (4), 347−352. (4) Pirovano, A.; Lacaita, A. L.; Benvenuti, A.; Pellizzer, F.; Hudgens, S.; Bez, R. In International Electron Devices Meeting, 2003. IEDM ’03 Technical Digest; Dec 8−10, 2003 ; IEEE: New York, 2003; pp 29.6.1−29.6.4. (5) Sadeghipour, S. M.; Pileggi, L.; Asheghi, M. In The Tenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronics Systems, 2006. ITHERM ’06; San Diego, CA, May 30−June 2, 2006 ; IEEE: New York, 2006; pp 660−665. (6) 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.; Park, Y.-K.; Kim, K.-N.; Kong, J.-T. Jpn. J. Appl. Phys. 2005, 44 (4B), 2701−2705. (7) Li, Y.; Hwang, C.-H.; Li, T.-Y.; Cheng, H.-W. Nanotechnology 2009, 20 (28), 285701. (8) Suntola, T. Sci. Rep. 1989, 4, 261−312. (9) Choi, B. J.; Choi, S.; Shin, Y. C.; Hwang, C. S.; Lee, J. W.; Jeong, J.; Kim, Y. J.; Hwang, S.-Y.; Hong, S. K. J. Electrochem. Soc. 2007, 154 (4), H318−H324. (10) Choi, B. J.; Choi, S.; Shin, Y. C.; Kim, K. M.; Hwang, C. S.; Kim, Y. J.; Son, Y. J.; Hong, S. K. Chem. Mater. 2007, 19, 4387−4389. (11) Choi, B. J.; Choi, S.; Eom, T.; Ryu, S. W.; Cho, D.-Y.; Heo, J.; Kim, H. J.; Hwang, C. S.; Kim, Y. J.; Hong, S. K. Chem. Mater. 2009, 21, 2386−2396. (12) Kim, R.-Y.; Kim, H.-G.; Yoon, S.-G. Appl. Phys. Lett. 2006, 89 (10), 102107. (13) Longo, M.; Salicio, O.; Wiemer, C.; Fallica, R.; Molle, A.; Fanciulli, M.; Giesen, C.; Seitzinger, B.; Baumann, P.; Heuken, M. J. Cryst. Growth 2008, 310 (23), 5053−5057. (14) Pore, V.; Hatanpäa,̈ T.; Ritala, M.; Leskelä, M. J. Am. Chem. Soc. 2009, 131 (10), 3478−3480. (15) Ritala, M.; Pore, V.; Hatanpäa,̈ T.; Heikkilä, M.; Leskelä, M.; Mizohata, K.; Schrott, A.; Raoux, S.; Rossnagel, S. M. Microelectron. Eng. 2009, 86 (7−9), 1946−1949. (16) Leskela, M.; Pore, V.; Hatanpaa, T.; Heikkila, M.; Ritala, M.; Schrott, A.; Raoux, S.; Rossnagel, S. ECS Trans. 2009, 25 (4), 399− 407. (17) Knapas, K.; Hatanpäa,̈ T.; Ritala, M.; Leskelä, M. Chem. Mater. 2010, 22 (4), 1386−1391. (18) Chieh-Fang, C.; Schrott, A.; Lee, M. H.; Raoux, S.; Shih, Y. H.; Breitwisch, M.; Baumann, F. H.; Lai, E. K.; Shaw, T. M.; Flaitz, P.; Cheek, R.; Joseph, E. A.; Chen, S. H.; Rajendran, B.; Lung, H. L.; Lam, C. In IEEE 2009 International Memory Workshop; Monterey, CA, May 10−14, 2009 IEEE: New York, 2009; pp 1−2. (19) Lencer, D.; Salinga, M.; Grabowski, B.; Hickel, T.; Neugebauer, J.; Wuttig, M. Nat. Mater. 2008, 7 (12), 972−977.
ASSOCIATED CONTENT
S Supporting Information *
Additional figures, equations, and information (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. 2109
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
Chemistry of Materials
Article
(20) Fukumoto, H.; Tsunetomo, K.; Imura, T. J. Phys. Soc. Jpn. 1987, 56 (1), 158−162. (21) Kim, S. K.; Han, J. H.; Kim, G. H.; Hwang, C. S. Chem. Mater. 2010, 22 (9), 2850−2856.
2110
dx.doi.org/10.1021/cm300539a | Chem. Mater. 2012, 24, 2099−2110
