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The phase-change kinetics of amorphous Ge2Sb2Te5 and device characteristics investigated by thin-film mechanics
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The phase-change kinetics of amorphous Ge2Sb2Te5 and device characteristics investigated by thin-film mechanics Ju-Young Cho,a Dohyung Kim,b Yong-Jin Park,a Tae-Youl Yang,a Yoo-Yong Leea and ⇑ Young-Chang Jooa,c, a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Process Development Team, Semiconductor R&D Center, Samsung Electronics Co., Ltd., San #16, Banwol-Dong, Hwasung-City, Gyeonggi-Do 445-701, Republic of Korea c Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-742, Republic of Korea
b
Received 29 January 2015; revised 28 April 2015; accepted 30 April 2015
Abstract—For high switching speed and high reliability of phase-change random access memory (PcRAM), we need to identify materials that enable fast crystallization at elevated temperatures but are stable at and above room temperature. Achieving this goal requires a breakthrough in our understanding of the unique crystallization kinetics of amorphous phase change materials as a fragile glass, described as the non-Arrhenius behavior of atomic mobility. It is a highly rewarding task to unravel the unconventional crystallization kinetics and related properties, because these properties can be utilized to predict the device characteristics. This manuscript utilizes the thin-film mechanics to investigate the crystallization kinetics of amorphous Ge2Sb2Te5 phase-change materials doped with Al, Bi, C and N, which is an effective method to analyze the structural changes in amorphous materials. Crystallization temperature, super-cooled liquid region, glass transition temperature and fragility are measured to describe the crystallization kinetics tuned by doping; characteristic fragile-to-strong transition is observed for C and N dopings due to their structural feature as an interstitial dopant. Consequently, doping effects on the phase stability and atomic mobility manifested by the crystallization temperature and the super-cooled liquid region (or 1/fragility) successfully correspond with PcRAM characteristics, i.e., reliability and switching speed, respectively. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ge2Sb2Te5; Crystallization; Thin-film mechanics; Fragility; Doping
1. Introduction The timescale for functional devices is considerably wide from device operation time (few nanoseconds) to device lifetime (few decades), which leads to a technical limitation in their characterization. Especially when unique classes of materials, such as amorphous solids, are employed, this challenge can only be overcome by a theoretical breakthrough. Mechanical film stress analysis in conjunction with the theory of fragile or strong glass reveals the unconventional crystallization kinetics of amorphous solids and further predicts device characteristics by simply employing a simple thin-film characterization (few hours). Amorphous solids have been actively applied in numerous functional devices – flexible, transparent or large-area electronics, bio applications and data storage media – due to the unique properties of amorphous solids originating
⇑ Corresponding
author at: Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea. Tel.: +82 2 880 8986; fax: +82 2 883 8197; e-mail: ycjoo@snu. ac.kr
from the disordered atomic structure, which is distinct from that of crystalline solids. However, amorphous solids suffer from inherent phase instability; consequently, the devices have encountered reliability issues. Moreover, the other functionalities of the devices are controlled by the effect of the disordered structure on the phase stability. Therefore, characterization and tuning of the properties relevant to the phase stability, which governs the device characteristics, are required. Structural control combined with theoretical background yields engineering breakthroughs and appropriate design of amorphous solids for improved performance and reliability of the functional devices. Among the many amorphous material systems, chalcogenides, and particularly the subgroup of phase change materials, exhibit extremely short crystallization times (a few nanoseconds) accompanied by significant changes in optical and electrical properties [1]. Because of this unique combination of properties, phase change materials are employed in rewriteable optical data storage media, more recently, in phase-change random access memory (PcRAM). The extremely fast crystallization of phase change materials at elevated temperatures enables a high SET speed (switching speed from the high-resistance to
http://dx.doi.org/10.1016/j.actamat.2015.04.058 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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low-resistance state), which is critical for the performance of the PcRAM. However, at the same time, the loss of the information stored in the high-resistance state, the so-called data retention loss, inhibits the reliability of PcRAM due to the phase instability of amorphous phase change materials. Crystallization can occur over a very long time (few decades) at a low temperature, which is a major cause of the data retention loss. The unique crystallization kinetics of amorphous phase change materials governing the PcRAM characteristics are attributed to the so-called fragile behavior as reported in previous works of Orava [2] and Wuttig [3] for amorphous Ge2Sb2Te5 (GST), which is the most popular phase change material. The atomic mobility, experimentally observed to be the reciprocal of the viscosity (g), is represented by its characteristic temperature dependence in the amorphous phase, as depicted in the well-known Angell plot [4] in Fig. 1(a). If the viscosity as a function of temperature follows an Arrhenius relationship (e.g., SiO2), it is represented by the strong behavior, and we would only need to understand the relevant activation barrier to describe the crystallization kinetics. However, the viscosity of GST is attributed to a pronounced non-Arrhenius relationship (Vogel–Fulcher–Tammann) with respect to temperature represented by the fragile behavior. The rapid decrease of viscosity with increasing temperature in fragile behavior results in high atomic mobility at elevated temperatures, which is responsible for the extremely fast crystallization kinetics of GST. It is a thrilling and highly rewarding task to unravel the unconventional crystallization kinetics of GST by measuring the relevant properties, because these properties can be utilized to predict the device characteristics. The temperature dependence of viscosity determines the properties relevant to the crystallization kinetics as described in Fig. 1(b). As the viscosity decreases upon heating above the glass transition temperature (Tg), the atomic mobility becomes sufficient for the crystallization to occur at a certain temperature, crystallization temperature (Tx). Consequently, the Tx and the super-cooled liquid region (Tx Tg) are determined by strong and fragile behaviors. These properties determine the reliability and performance of PcRAM devices; Tx can be regarded as a quantitative measure for thermal stability against crystallization at low temperature. In addition, the value Tx Tg can be regarded as a quantitative measure of the atomic mobility during crystallization at elevated temperatures. These properties can be a theoretical breakthrough overcoming the technical limitation of the crystallization analysis, as Tx Tg and Tg reflect the fragile and strong crystallization kinetics and can be measured on the timescale of few hours, not employing the measurement in a few nanoseconds. It is not new adopting the temperature dependence of viscosity for the description of the crystallization kinetics of GST, however, the transition of this trend (fragile or strong) associated with the structural control by doping has never been explored. Doping in amorphous GST has been applied to attain desirable PcRAM characteristics; several substances have been utilized as dopants, including N [5], C [6], Bi [7] and other metals (Al [8], Ag [9–11], Cu [12] and etc.). Each dopant exhibits a distinct effect on the structural features and properties of amorphous GST; however, theoretical and universal understanding is not available. Therefore, revealing the correlation between the
local atomic structure and the properties relevant to the crystallization kinetics induced by various dopings, in short, the structure–properties relationship, would provide a thorough explanation of the crystallization kinetics of this unique class of phase change materials as well as further prediction and tuning of the PcRAM characteristics. Energetic and structural changes in amorphous materials result in the significant changes of free volume and mechanical properties compared to crystalline materials. This is how the phase-change stability of amorphous materials can be investigated; mechanical analysis of amorphous materials. Therefore, mechanical analysis is highly promising way to investigate the phase-change characteristics of amorphous materials. For mechanical analysis of volumetric changes of amorphous materials associated with phase-changes, thin-film mechanics can be introduced as an effect method. Thin-film mechanics is effective to detect volumetric changes associated with phase transition as it measures changes in the horizontal direction. Furthermore, overall change of properties can be measured rather than local change. In addition, sample processing is relatively easy compared to other complex systems; To measure the properties relevant to the crystallization kinetics, Tx, Tg and Tx Tg, we propose a mechanical stress analysis measuring volumetric changes in the horizontal direction. This method facilitates the idea that the biaxial stress in the film associated with the volumetric changes would produce a curvature change of the film-substrate constrained system [13]. The curvature could be precisely measured using a multi-beam laser optic sensor. The typical approaches measure the changes in the optical/electrical properties or film thickness; however, precise measurement could be restricted by the small amount of signal due to slight changes in the band structure or small interaction volume. The volumetric changes associated with the phase transitions during heating are described in Fig. 1(c). As the film is constrained to the substrate, the volume expansion of the film corresponds to the evolution of stress in the compressive direction, and vice versa. Upon heating to the glass transition temperature, (Tg), a gradual volume expansion due to the difference between the coefficient of thermal expansion (CTE) between the glass and the super-cooled liquid occurs. By further heating to Tx, crystallization occurs with an abrupt volume shrinkage originating from the density difference between the crystalline and amorphous states [14]. The precise measurement enables the investigation of the temperature dependence of viscosity obtained from the heating rate dependence of Tg. In this study, the crystallization kinetics of the amorphous phase change materials were explored, including the crystallization temperature (Tx), glass transition temperature (Tg), and the super-cooled liquid region (Tx Tg) of amorphous GST films doped with Al, Bi, N and C. In addition, the temperature dependence of viscosity was also determined. The deviation of the viscosity-temperature relationship from the Arrhenius relationship was quantitatively defined by the fragility. The fragile-to-strong transition by doping (depending on the doping types) was examined and related to the device characteristics. Doping effects on Tx and Tx Tg (reciprocal of fragility) exhibit successful matching with PcRAM characteristics, i.e., data retention and SET speed, respectively. A thorough analysis of the experimental data in conjunction
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Fig. 1. (a) Schematic of the logarithm of the viscosity (reciprocal of atomic mobility) as a function of the reciprocal temperature normalized to Tg describing the strong (Arrhenius) and fragile (Vogel–Fulcher–Tammann) behavior [4]. (b) Schematic of the logarithm of the viscosity and (c) the specific volume (or energy in a broad sense) as a function of temperature associated with the glass transition and crystallization in amorphous solids.
with theoretical models helped to build a framework to describe and explain the fragile crystallization kinetics of GST encountered in PcRAM.
2. Experimental 2.1. Sample preparation via sputtering Amorphous Ge2Sb2Te5 thin films with thickness of 300 nm were deposited onto Si substrates with a thickness of 100 lm (thinned by chemical–mechanical polishing) using direct current (DC) or radio frequency (RF) magnetron sputtering at room temperature. Various dopants were inserted into the GST films during deposition. For N doping of GST, DC magnetron sputtering was used, and the working pressure was 3 mTorr. The ambient conditions in the vacuum chamber were controlled by the flow rate of N2 gas (2–12 sccm) with Ar gas (40 sccm) used to control the N concentration. For the Bi-doped GST, DC magnetron sputtering was used, and the working pressure was 3 mTorr. Co-sputtering of the GST target and the Ge2Bi2Te5 target was used. Several DC powers of the Ge2Bi2Te5 target (5–40 W) were used with a fixed DC power of the Ge2Sb2Te5 target (80 W) to control the Bi concentration. For C doping of GST, RF magnetron sputtering was used with a working pressure of 0.5 mTorr. Co-sputtering of the Ge2Sb2Te5 target and the C target was used. Several RF powers of C (11–89 W) with a fixed RF power of the Ge2Sb2Te5 (30 W) were used to control the C concentration. For the Al-doped GST, RF magnetron sputtering was used with a working pressure of 0.5 mTorr. Co-sputtering of the Ge2Sb2Te5 target and the Al target was used. Several RF powers of Al (5–30 W) and a fixed RF power of Ge2Sb2Te5 (30 W) were used to control the Al concentration. 2.2. Mechanical stress analysis via wafer curvature measurement The wafer curvature measurement with multi-beam laser technique was used (kSA Multibeam Optic System) to determine the mechanical stress of the film during thermal cycling in a N2 atmosphere at 10 Torr. The biaxial stress changes (r) were determined by the changes of curvature according to the Stoney formula given by [13] r = Ys t2s j/6tf, where tf is the thickness of the film
and Ys and ts are the biaxial modulus and thickness of the substrate, respectively. As determined from the resolution of curvature, the minimum detectable curvature during heating (from room temperature to 300 °C) is 0.0017 m1. The minimum detectable stress was determined to be 1.0 MPa for ts of 100 lm and tf of 300 nm. A change in the volume of the film of 0.015 % produces a change in thickness of 0.005 % and a stress change of 5 MPa (assuming elastic behavior) in a detectable range (>1.0 MPa).
3. Results and discussion 3.1. Crystallization temperature and super-cooled liquid region The stress change curves as a function of temperature upon heating at heating rates of 1.0, 2.5, 6.5, 13 and 20 K/min for amorphous GST films doped with various concentrations of Al, Bi, C and N are displayed in Fig. 2(a)–(d). Positive and negative values indicate tensile and compressive stresses in the film, respectively. First, we discuss the curve of pure GST film (denoted as pure) in Fig. 2(a). When the temperature increases to 70 °C at a heating rate of 1.0 K/min, the stress is compressive because the film has a larger CTE than the Si wafer (denoted as 1). From 70–120 °C, the stress became tensile, which indicates that volume shrinkage occurred in the film associated with structural relaxation (denoted as 2); in amorphous solids, the density or stress changes with structural relaxation due to their metastable nature. After heating above 150 °C, the stress rapidly becomes more tensile, which indicates that the volume decreased by crystallization (denoted as 4); this temperature is thus Tx, which was confirmed by the increase in the reflected intensity measured at the same time as the stress data. Stress relaxation occurs above Tx to relieve the stress that built up during the crystallization process. For the Al-doped GST (Al-GST) films shown in Fig. 2(a), Tx increased with increasing Al concentration, which was detected by the delayed temperature of the abrupt tensile stress evolution. Conversely, Tx of Bi-doped GST (Bi-GST) decreases with increasing Bi concentration, as illustrated in Fig. 2(b). As indicated in Fig. 2(c), the stress–temperature curves of C-doped GST (C-GST) also reveal increasing values of Tx with increasing
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Fig. 2. Stress–temperature curve of (a) Al-doped, (b) Bi-doped, (c) C-doped and (d) N-doped GST films during heating (the arrows indicate Tx and Tg) for various heating rates (from 1.0 to 20 K/min). The stress changes associated with thermal expansion, structural relaxation, glass transition and crystallization are denoted as 1, 2, 3 and 4, respectively.
C concentration. The stress–temperature curve of N-doped GST (N-GST) in Fig. 2(d) also reveals increases of Tx with increasing N concentration, which is similar to the case of C-GST. Tx increases with Al, C and N dopings but decreases with Bi doping. However, the stress change for C- and N-GST exhibits different behaviors from those of Al- and Bi-GST. Above 150–200 °C, the stress becomes compressive, which was never observed in pure, Al- and Bi-GST. This change is the glass transition, which makes this temperature Tg (denoted as 3). It appears that well-known narrow regions of super-cooled liquid (Tx Tg 0) of pure GST become wider for C and N dopings. Tx and Tx Tg, were obtained from the stress evolution associated with heating of amorphous GST doped with Al, Bi, C and N as illustrated in Fig. 3. Tx increases with Al, C and N dopings, while Bi doping results in a decrease in Tx, as observed in Fig. 3(a). Fig. 3(b) shows the dependence of Tg as a function of Al, Bi, C and N concentration determined from the stress change-temperature curves in Fig. 2. Tg increases with Al, C and N dopings and decreases with Bi doping. Note that values of Tg of pure, Al and Bi-GST in Fig. 3(b) are obtained from the values of Tx because Tx Tg for those samples. Conversely, Fig. 3(c) reveals the completely different effects of doping for Tx Tg. The narrow super-cooled liquid regions (Tx Tg 0) of pure GST are maintained in Al and Bi but widen with C and N dopings. The different effects of doping on Tx and Tx Tg can be explained by different atomic mechanisms.
The temperature at which crystallization occurs increases for various reasons, including structural changes in the amorphous or crystalline state. Doping in GST changes the bonding enthalpy by substitution of bonding [7,15–18]. Al-GST produces additional bonding of Al-Sb and Al-Te [16,17]. In contrast, Bi-GST substitutes bonds from Sb–Te to Bi–Te, which causes weaker bonding [7,15]. The changes of the bonding enthalpies associated with doping explain the changes of Tx by limiting the partial breakage of bonding required for atomic rearrangements. In addition, C doping is known to lead to a higher portion of tetrahedral bonding in amorphous GST, resulting in the stabilization of the amorphous structure and retardation of the crystallization [19,20]. Furthermore, N doping is known to lead to a strain field in the crystalline lattice and destabilization of the crystalline structure, also resulting in the stabilization of the amorphous structure [21,22]. At temperatures above Tg, the super-cooled liquid experiences a rapid increase in atomic mobility as a function of temperature, which is then sufficient for crystallization to occur. Therefore, the temperature dependence of atomic mobility controls crystallization in the super-cooled liquid region. If the increase in atomic mobility above Tg is abrupt, crystallization will be accelerated due to the easy atomic rearrangement, which results in a narrow super-cooled liquid region. Conversely, if the increase in atomic mobility above Tg is gradual, crystallization will be delayed due to difficult atomic rearrangement, which results in a higher Tx Tg. Because the free volume in an
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Fig. 3. (a) Tx, (b) Tg and (c) Tx Tg as a function of dopant concentrations of Al, Bi, C and N in GST films. Schematics of the viscosity as a function of temperature in pure GST in comparison with (d) Al-GST, (e) Bi-GST, (f) C- or N-GST.
amorphous network acts as the center of the atomic arrangement, a larger amount of free volume leads to an abrupt increase in atomic mobility above Tg and a lower Tx Tg. Therefore, the Tx Tg is determined mainly by the free volume in amorphous phase change materials. The super-cooled liquid region can be regarded as a quantitative measure of the atomic mobility during crystallization at high temperatures. Fig. 3(d)–(f) presents schematics of viscosity as a function of temperature and as a function of the doping elements. The viscosities where the glass transition occurs are assumed to be the same value in the figures. Upon increasing the temperature of the glass above Tg, the super-cooled liquid experiences a rapid increase in atomic mobility, as illustrated in Fig. 1(b). The narrow super-cooled liquid region is observed in pure GST because an abundant amount of free volume is contained in pure GST. As the free volume in the amorphous network acts as a center of atomic arrangement [23], a larger amount of free volume leads to an abrupt increase of atomic mobility above Tg and lower Tx Tg. Because Al and Bi are known to be substitutional dopants that occupy sites of constitutive atoms, and the free volume in the system will not be significantly occupied, the narrow super-cooled liquid region is maintained in Al- and Bi-GST. In contrast, C and N are both known to be interstitial dopants and occupy free volume sites [24,25]. These structural features of C and N-GST result in less free volume and the gradual decrease of viscosity as a function of temperature. Consequently, crystallization will be delayed due to hard atomic rearrangement, resulting in a higher Tx Tg. 3.2. Viscosity and fragility To facilitate the discussions regarding the super-cooled liquid region presented in the previous section, it is useful
to represent the atomic mobility of GST as a function of temperature. To identify the effect of doping on the temperature dependence of atomic mobility, the viscosity (g) was examined. Viscosity is the resistance to atomic flow, which is regarded as the reciprocal of atomic mobility. The temperature dependence of g can be utilized to determine the universal criterion for the kinetic stability of an amorphous structure, fragility [4]. Fragility (m) is determined from the deviation of viscosity as a function of temperature from the Arrhenius relationship, as shown in Eq. (1) [4]: d log gðT Þ E m¼ ¼ ð1Þ dðT g =T Þ T ¼T g ln 10 RT g As m approaches 16, the Arrhenius relationship holds, and the liquid is categorized as “strong”. Larger values of m result in non-Arrhenius relationships and are categorized as “fragile”. This non-Arrhenius relaxation causes the activation energy for relaxation to vary with temperature; therefore, the apparent activation energy for relaxation (E) was defined at Tg for the experimental characterization. This apparent activation energy for relaxation (E) can be extracted from the kinetic nature of the glass transition according to the Moynihan plot [26]. A shift of Tg toward higher temperatures as the heating rate increases is used to calculate E in Eq. (2) [26]: ln b ¼
E þC RT g
ð2Þ
where b is the heating rate (K/min) and C is a constant. Fig. 2 shows the dependence of the stress evolution on the heating rate (1.0, 2.5, 6.5, 13 and 20 K/min); with increases in the heating rate, Tg also increases. Linear dependences of ln b on 1/Tg are shown in Fig. 4(a). From the linear slope (=E/R) of Fig. 4(a) at T = Tg, the fragilities were obtained, as depicted in Fig. 4(b). Pure GST has
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m = 47, which indicates that this material is relatively fragile. Compared with pure GST, m significantly decreases with C and N dopings to m = 20 for 10 at.% C and 10 at.% N GST, which indicates that these materials are relatively strong. However, Al and Bi-GST exhibit negligible changes of m. The significant decrease in m with C and N dopings indicates that the highly mobile nature of pure GST can be easily altered. However, Al and Bi doping has a negligible effect on the fragility. The inconsistencies between dopants can be attributed to the different
structural changes associated with the different dopants, especially to the free volume of the amorphous GST, as discussed in Fig. 3(d)–(f). These structural features of C and N-GST result in the gradual temperature dependence of viscosity, which is manifested in higher values of Tx Tg. The calculated values of m can be utilized to extrapolate the viscosity curves as a function of temperature because these curves are important for predicting the crystallization kinetics at high temperatures (similar to the switching temperature of PcRAM). To describe the temperature
Fig. 4. (a) Linear dependences of natural logarithm of the heating rates (b) on 1/Tg for Al-, Bi-, C- and N-GST. (b) Fragility obtained from the linear slopes in (a) at T = Tg for pure and Al-, Bi-, C- and N-GST. Extrapolated curves of the logarithm of the viscosity as a function of Tg/T according to the Vogel–Fulcher–Tammann relation for (c) 8.5 at.% Al GST, (d) 9.6 at.% Bi GST, (e) 10 at.% C GST and (f) 10 at.% N GST in comparison with pure GST and Arrhenius behavior.
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dependence of viscosity (g) in a fragile liquid with non-Arrhenius behavior, the Vogel–Fulcher–Tammann equation generally best fits the experimental data of a super-cooled liquid with non-Arrhenius behavior [27]: DT 0 ð3Þ g ¼ g0 exp T T0 where D and T0 are fitting parameters. For the extrapolation, the obtained fragilities were used to define the linear slope of the logarithm of the viscosity as a function of Tg/T (known as an Angell plot) at Tg. For further extrapolation, g = 1012 Pa s at T = Tg and g = 104 (s) at T 1 are assumed. The extrapolated curves of the logarithm of the viscosity as a function of Tg/T according to the Vogel–Fulcher–Ta mmann relation for 8.5 at.% Al, 9.6 at.% Bi, 10 at.% C and 10 at.% N GST in comparison with pure GST and Arrhenius behavior are presented in Fig. 4(c)–(f). Fig. 4(c)–(f) reveal large deviations of pure, Al-GST and Bi-GST from Arrhenius behavior, which results in relatively rapid decreases in g with temperature. Fig. 4(e) and (f) reveal smaller deviations for C-GST and N-GST from Arrhenius behavior, which results in gradual decreases in g with temperature. The viscosity at high temperature determined can be utilized to estimate the SET time during the operation procedure (details in Supporting Materials). Assuming isotropic crystal growth in a sphere with radius 15 nm (Vcell 2.5 104 nm3) for SET switching and the Stokes– Einstein relation (D = kT/3pag, a is the jump frequency determined to be 0.268 nm [28], only valid at T P Tm), the SET time can be estimated. The SET times were determined from the viscosities of the GST films in Fig. 4(c)–(f) with various dopant concentrations at T = 900 K indicated as open circles. The calculated SET times at 900 K varied from 4.4 ns for pure GST to 33 ls for 10 at.% N GST, indicating a difference of approximately four orders of magnitude (less than two orders of magnitude in real devices). This large deviation between the calculated and measured SET time likely originates from the temperature where the SET occurs being shifted with doping because of the changes in resistance of the high-resistance state. 3.3. Structure–property relationships The effects of doping on the crystallization behavior of GST described above enabled the construction of “structu re–property relationship” maps of amorphous Ge2Sb2Te5 doped with various doping elements, which are presented in Fig. 5(a) and (b). Fig. 5(a) and (b) present maps of amorphous Ge2Sb2Te5 tuned using various dopants as a function of the crystallization temperature (Tx) and super-cooled liquid region (Tx Tg) and reciprocal of fragility (m), respectively. The vertical axis describes the thermal stability against crystallization at low temperature (long lasting), and the horizontal axis describes the atomic mobility rate at high temperature (fast transforming). In Fig. 5(a), pure Ge2Sb2Te5 is located in the left region but moves to the upper-right with C and N dopings, to the upper-left with Al doping, and to the lower-left with Bi doping. The various effects of doping are related to the structural changes of amorphous Ge2Sb2Te5, which depend upon the type of dopant. Both interstitial (C and N) and Al (substitutional) dopants increase Tx, which contribute to
Fig. 5. (a) Map of amorphous Ge2Sb2Te5 tuned by various dopants as a function of crystallization temperature (Tx) and super-cooled liquid region (Tx Tg). (a) Map of amorphous Ge2Sb2Te5 tuned by various dopants as a function of crystallization temperature (Tx) and reciprocal of the fragility. (c) Map of PcRAM characteristics as a function of 10-yr data retention and SET time for various phase change materials and dopants (details in Supporting Materials), demonstrating the successful match with (a) and (b). The SET time* is determined for Roff/Ron 102 and includes the data obtained from optical experiments.
the thermal stability against crystallization, while Bi (substitutional) doping decreases Tx, which means lower thermal stability. However, only interstitial dopants significantly change the portion of the free volume, which contributes to the atomic mobility of the system manifested by the changes in Tx Tg and fragility.
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The characteristics of PcRAM are predicted; a higher Tx is generally regarded as higher thermal stability against crystallization, which can achieve higher 10-yr data retention. In addition, the high atomic mobility at high temperature manifested by Tx Tg and fragility can be regarded as the crystallization time, which is related to the SET speed. Fig. 5(c) presents a map of the PcRAM characteristics tuned by various dopings as a function of 10-yr data retention and SET time for various phase change materials and dopants. The lines were obtained from the reported results of other researchers; if both the 10-yr data retention and the SET time were reported in the same reference, these data are marked as triangles for Sb–Te alloys, circles for Ge–Sb–Te alloys, and squares for Ge–Te alloys (details and references in Supporting Materials). The pure phase change materials located in the lower-left region move to the upper-right with C and N dopings, to the upper-left with Al doping or to the lower-left with Bi doping. C and N (interstitial) dopings in phase change materials generally result in an increase in the 10-yr data retention but is disadvantageous for a low SET time. However, substitutional dopants results in different effects according to their structural features; Bi doping slightly decreases the 10-yr data retention while maintaining or even lowering the SET time. In contrast, Al doping slightly increases the 10-yr data retention while maintaining or even lowering the SET time, which is advantageous for reliability and performance. The comparison of PcRAM characteristics in Fig. 5(c) demonstrates that the effects of doping on Tx and Tx Tg (or 1/m) successfully match the data retention and SET speed, respectively. Consequently, the “structure–property relationship” maps of amorphous Ge2Sb2Te5 showing the effects of doping on Tx and Tx Tg (or 1/m) in Fig. 5(a) and (b) successfully match the PcRAM characteristics of the data retention and SET speed in Fig. 5(c), respectively.
4. Conclusion In conclusion, the crystallization kinetics of amorphous phase change materials were explored, including the crystallization temperature, glass transition temperature, super-cooled liquid region and fragility of amorphous Ge2Sb2Te5 (GST) films doped with Al, Bi, C and N using mechanical stress analysis. Fragile-to-strong transition is observed for C and N dopings due to their structural feature as an interstitial dopant, while substitutional Al and Bi dopings maintained the fragile behavior of GST. In addition, the viscosity and fragility were also determined. Doping effects on the thermal stability and atomic mobility manifested by Tx and Tx Tg (or 1/m) successfully correspond with PcRAM characteristics, i.e., the data retention and SET speed, respectively. The mechanical stress analysis in this study can provide guidelines to predict the device characteristics, using thin film characterization techniques, which is a simple task. Acknowledgments This work was supported by IT R&D program of MOTIE/KEIT [10039200, Development of high performance phase change materials].
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.actamat.2015.04.058.
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The phase-change kinetics of amorphous Ge2Sb2Te5 and device characteristics investigated by thin-film mechanics Ju-Young Cho,a Dohyung Kim,b Yong-Jin Park,a Tae-Youl Yang,a Yoo-Yong Leea and ⇑ Young-Chang Jooa,c, a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Process Development Team, Semiconductor R&D Center, Samsung Electronics Co., Ltd., San #16, Banwol-Dong, Hwasung-City, Gyeonggi-Do 445-701, Republic of Korea c Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-742, Republic of Korea
b
Received 29 January 2015; revised 28 April 2015; accepted 30 April 2015
Abstract—For high switching speed and high reliability of phase-change random access memory (PcRAM), we need to identify materials that enable fast crystallization at elevated temperatures but are stable at and above room temperature. Achieving this goal requires a breakthrough in our understanding of the unique crystallization kinetics of amorphous phase change materials as a fragile glass, described as the non-Arrhenius behavior of atomic mobility. It is a highly rewarding task to unravel the unconventional crystallization kinetics and related properties, because these properties can be utilized to predict the device characteristics. This manuscript utilizes the thin-film mechanics to investigate the crystallization kinetics of amorphous Ge2Sb2Te5 phase-change materials doped with Al, Bi, C and N, which is an effective method to analyze the structural changes in amorphous materials. Crystallization temperature, super-cooled liquid region, glass transition temperature and fragility are measured to describe the crystallization kinetics tuned by doping; characteristic fragile-to-strong transition is observed for C and N dopings due to their structural feature as an interstitial dopant. Consequently, doping effects on the phase stability and atomic mobility manifested by the crystallization temperature and the super-cooled liquid region (or 1/fragility) successfully correspond with PcRAM characteristics, i.e., reliability and switching speed, respectively. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ge2Sb2Te5; Crystallization; Thin-film mechanics; Fragility; Doping
1. Introduction The timescale for functional devices is considerably wide from device operation time (few nanoseconds) to device lifetime (few decades), which leads to a technical limitation in their characterization. Especially when unique classes of materials, such as amorphous solids, are employed, this challenge can only be overcome by a theoretical breakthrough. Mechanical film stress analysis in conjunction with the theory of fragile or strong glass reveals the unconventional crystallization kinetics of amorphous solids and further predicts device characteristics by simply employing a simple thin-film characterization (few hours). Amorphous solids have been actively applied in numerous functional devices – flexible, transparent or large-area electronics, bio applications and data storage media – due to the unique properties of amorphous solids originating
⇑ Corresponding
author at: Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea. Tel.: +82 2 880 8986; fax: +82 2 883 8197; e-mail: ycjoo@snu. ac.kr
from the disordered atomic structure, which is distinct from that of crystalline solids. However, amorphous solids suffer from inherent phase instability; consequently, the devices have encountered reliability issues. Moreover, the other functionalities of the devices are controlled by the effect of the disordered structure on the phase stability. Therefore, characterization and tuning of the properties relevant to the phase stability, which governs the device characteristics, are required. Structural control combined with theoretical background yields engineering breakthroughs and appropriate design of amorphous solids for improved performance and reliability of the functional devices. Among the many amorphous material systems, chalcogenides, and particularly the subgroup of phase change materials, exhibit extremely short crystallization times (a few nanoseconds) accompanied by significant changes in optical and electrical properties [1]. Because of this unique combination of properties, phase change materials are employed in rewriteable optical data storage media, more recently, in phase-change random access memory (PcRAM). The extremely fast crystallization of phase change materials at elevated temperatures enables a high SET speed (switching speed from the high-resistance to
http://dx.doi.org/10.1016/j.actamat.2015.04.058 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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low-resistance state), which is critical for the performance of the PcRAM. However, at the same time, the loss of the information stored in the high-resistance state, the so-called data retention loss, inhibits the reliability of PcRAM due to the phase instability of amorphous phase change materials. Crystallization can occur over a very long time (few decades) at a low temperature, which is a major cause of the data retention loss. The unique crystallization kinetics of amorphous phase change materials governing the PcRAM characteristics are attributed to the so-called fragile behavior as reported in previous works of Orava [2] and Wuttig [3] for amorphous Ge2Sb2Te5 (GST), which is the most popular phase change material. The atomic mobility, experimentally observed to be the reciprocal of the viscosity (g), is represented by its characteristic temperature dependence in the amorphous phase, as depicted in the well-known Angell plot [4] in Fig. 1(a). If the viscosity as a function of temperature follows an Arrhenius relationship (e.g., SiO2), it is represented by the strong behavior, and we would only need to understand the relevant activation barrier to describe the crystallization kinetics. However, the viscosity of GST is attributed to a pronounced non-Arrhenius relationship (Vogel–Fulcher–Tammann) with respect to temperature represented by the fragile behavior. The rapid decrease of viscosity with increasing temperature in fragile behavior results in high atomic mobility at elevated temperatures, which is responsible for the extremely fast crystallization kinetics of GST. It is a thrilling and highly rewarding task to unravel the unconventional crystallization kinetics of GST by measuring the relevant properties, because these properties can be utilized to predict the device characteristics. The temperature dependence of viscosity determines the properties relevant to the crystallization kinetics as described in Fig. 1(b). As the viscosity decreases upon heating above the glass transition temperature (Tg), the atomic mobility becomes sufficient for the crystallization to occur at a certain temperature, crystallization temperature (Tx). Consequently, the Tx and the super-cooled liquid region (Tx Tg) are determined by strong and fragile behaviors. These properties determine the reliability and performance of PcRAM devices; Tx can be regarded as a quantitative measure for thermal stability against crystallization at low temperature. In addition, the value Tx Tg can be regarded as a quantitative measure of the atomic mobility during crystallization at elevated temperatures. These properties can be a theoretical breakthrough overcoming the technical limitation of the crystallization analysis, as Tx Tg and Tg reflect the fragile and strong crystallization kinetics and can be measured on the timescale of few hours, not employing the measurement in a few nanoseconds. It is not new adopting the temperature dependence of viscosity for the description of the crystallization kinetics of GST, however, the transition of this trend (fragile or strong) associated with the structural control by doping has never been explored. Doping in amorphous GST has been applied to attain desirable PcRAM characteristics; several substances have been utilized as dopants, including N [5], C [6], Bi [7] and other metals (Al [8], Ag [9–11], Cu [12] and etc.). Each dopant exhibits a distinct effect on the structural features and properties of amorphous GST; however, theoretical and universal understanding is not available. Therefore, revealing the correlation between the
local atomic structure and the properties relevant to the crystallization kinetics induced by various dopings, in short, the structure–properties relationship, would provide a thorough explanation of the crystallization kinetics of this unique class of phase change materials as well as further prediction and tuning of the PcRAM characteristics. Energetic and structural changes in amorphous materials result in the significant changes of free volume and mechanical properties compared to crystalline materials. This is how the phase-change stability of amorphous materials can be investigated; mechanical analysis of amorphous materials. Therefore, mechanical analysis is highly promising way to investigate the phase-change characteristics of amorphous materials. For mechanical analysis of volumetric changes of amorphous materials associated with phase-changes, thin-film mechanics can be introduced as an effect method. Thin-film mechanics is effective to detect volumetric changes associated with phase transition as it measures changes in the horizontal direction. Furthermore, overall change of properties can be measured rather than local change. In addition, sample processing is relatively easy compared to other complex systems; To measure the properties relevant to the crystallization kinetics, Tx, Tg and Tx Tg, we propose a mechanical stress analysis measuring volumetric changes in the horizontal direction. This method facilitates the idea that the biaxial stress in the film associated with the volumetric changes would produce a curvature change of the film-substrate constrained system [13]. The curvature could be precisely measured using a multi-beam laser optic sensor. The typical approaches measure the changes in the optical/electrical properties or film thickness; however, precise measurement could be restricted by the small amount of signal due to slight changes in the band structure or small interaction volume. The volumetric changes associated with the phase transitions during heating are described in Fig. 1(c). As the film is constrained to the substrate, the volume expansion of the film corresponds to the evolution of stress in the compressive direction, and vice versa. Upon heating to the glass transition temperature, (Tg), a gradual volume expansion due to the difference between the coefficient of thermal expansion (CTE) between the glass and the super-cooled liquid occurs. By further heating to Tx, crystallization occurs with an abrupt volume shrinkage originating from the density difference between the crystalline and amorphous states [14]. The precise measurement enables the investigation of the temperature dependence of viscosity obtained from the heating rate dependence of Tg. In this study, the crystallization kinetics of the amorphous phase change materials were explored, including the crystallization temperature (Tx), glass transition temperature (Tg), and the super-cooled liquid region (Tx Tg) of amorphous GST films doped with Al, Bi, N and C. In addition, the temperature dependence of viscosity was also determined. The deviation of the viscosity-temperature relationship from the Arrhenius relationship was quantitatively defined by the fragility. The fragile-to-strong transition by doping (depending on the doping types) was examined and related to the device characteristics. Doping effects on Tx and Tx Tg (reciprocal of fragility) exhibit successful matching with PcRAM characteristics, i.e., data retention and SET speed, respectively. A thorough analysis of the experimental data in conjunction
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Fig. 1. (a) Schematic of the logarithm of the viscosity (reciprocal of atomic mobility) as a function of the reciprocal temperature normalized to Tg describing the strong (Arrhenius) and fragile (Vogel–Fulcher–Tammann) behavior [4]. (b) Schematic of the logarithm of the viscosity and (c) the specific volume (or energy in a broad sense) as a function of temperature associated with the glass transition and crystallization in amorphous solids.
with theoretical models helped to build a framework to describe and explain the fragile crystallization kinetics of GST encountered in PcRAM.
2. Experimental 2.1. Sample preparation via sputtering Amorphous Ge2Sb2Te5 thin films with thickness of 300 nm were deposited onto Si substrates with a thickness of 100 lm (thinned by chemical–mechanical polishing) using direct current (DC) or radio frequency (RF) magnetron sputtering at room temperature. Various dopants were inserted into the GST films during deposition. For N doping of GST, DC magnetron sputtering was used, and the working pressure was 3 mTorr. The ambient conditions in the vacuum chamber were controlled by the flow rate of N2 gas (2–12 sccm) with Ar gas (40 sccm) used to control the N concentration. For the Bi-doped GST, DC magnetron sputtering was used, and the working pressure was 3 mTorr. Co-sputtering of the GST target and the Ge2Bi2Te5 target was used. Several DC powers of the Ge2Bi2Te5 target (5–40 W) were used with a fixed DC power of the Ge2Sb2Te5 target (80 W) to control the Bi concentration. For C doping of GST, RF magnetron sputtering was used with a working pressure of 0.5 mTorr. Co-sputtering of the Ge2Sb2Te5 target and the C target was used. Several RF powers of C (11–89 W) with a fixed RF power of the Ge2Sb2Te5 (30 W) were used to control the C concentration. For the Al-doped GST, RF magnetron sputtering was used with a working pressure of 0.5 mTorr. Co-sputtering of the Ge2Sb2Te5 target and the Al target was used. Several RF powers of Al (5–30 W) and a fixed RF power of Ge2Sb2Te5 (30 W) were used to control the Al concentration. 2.2. Mechanical stress analysis via wafer curvature measurement The wafer curvature measurement with multi-beam laser technique was used (kSA Multibeam Optic System) to determine the mechanical stress of the film during thermal cycling in a N2 atmosphere at 10 Torr. The biaxial stress changes (r) were determined by the changes of curvature according to the Stoney formula given by [13] r = Ys t2s j/6tf, where tf is the thickness of the film
and Ys and ts are the biaxial modulus and thickness of the substrate, respectively. As determined from the resolution of curvature, the minimum detectable curvature during heating (from room temperature to 300 °C) is 0.0017 m1. The minimum detectable stress was determined to be 1.0 MPa for ts of 100 lm and tf of 300 nm. A change in the volume of the film of 0.015 % produces a change in thickness of 0.005 % and a stress change of 5 MPa (assuming elastic behavior) in a detectable range (>1.0 MPa).
3. Results and discussion 3.1. Crystallization temperature and super-cooled liquid region The stress change curves as a function of temperature upon heating at heating rates of 1.0, 2.5, 6.5, 13 and 20 K/min for amorphous GST films doped with various concentrations of Al, Bi, C and N are displayed in Fig. 2(a)–(d). Positive and negative values indicate tensile and compressive stresses in the film, respectively. First, we discuss the curve of pure GST film (denoted as pure) in Fig. 2(a). When the temperature increases to 70 °C at a heating rate of 1.0 K/min, the stress is compressive because the film has a larger CTE than the Si wafer (denoted as 1). From 70–120 °C, the stress became tensile, which indicates that volume shrinkage occurred in the film associated with structural relaxation (denoted as 2); in amorphous solids, the density or stress changes with structural relaxation due to their metastable nature. After heating above 150 °C, the stress rapidly becomes more tensile, which indicates that the volume decreased by crystallization (denoted as 4); this temperature is thus Tx, which was confirmed by the increase in the reflected intensity measured at the same time as the stress data. Stress relaxation occurs above Tx to relieve the stress that built up during the crystallization process. For the Al-doped GST (Al-GST) films shown in Fig. 2(a), Tx increased with increasing Al concentration, which was detected by the delayed temperature of the abrupt tensile stress evolution. Conversely, Tx of Bi-doped GST (Bi-GST) decreases with increasing Bi concentration, as illustrated in Fig. 2(b). As indicated in Fig. 2(c), the stress–temperature curves of C-doped GST (C-GST) also reveal increasing values of Tx with increasing
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Fig. 2. Stress–temperature curve of (a) Al-doped, (b) Bi-doped, (c) C-doped and (d) N-doped GST films during heating (the arrows indicate Tx and Tg) for various heating rates (from 1.0 to 20 K/min). The stress changes associated with thermal expansion, structural relaxation, glass transition and crystallization are denoted as 1, 2, 3 and 4, respectively.
C concentration. The stress–temperature curve of N-doped GST (N-GST) in Fig. 2(d) also reveals increases of Tx with increasing N concentration, which is similar to the case of C-GST. Tx increases with Al, C and N dopings but decreases with Bi doping. However, the stress change for C- and N-GST exhibits different behaviors from those of Al- and Bi-GST. Above 150–200 °C, the stress becomes compressive, which was never observed in pure, Al- and Bi-GST. This change is the glass transition, which makes this temperature Tg (denoted as 3). It appears that well-known narrow regions of super-cooled liquid (Tx Tg 0) of pure GST become wider for C and N dopings. Tx and Tx Tg, were obtained from the stress evolution associated with heating of amorphous GST doped with Al, Bi, C and N as illustrated in Fig. 3. Tx increases with Al, C and N dopings, while Bi doping results in a decrease in Tx, as observed in Fig. 3(a). Fig. 3(b) shows the dependence of Tg as a function of Al, Bi, C and N concentration determined from the stress change-temperature curves in Fig. 2. Tg increases with Al, C and N dopings and decreases with Bi doping. Note that values of Tg of pure, Al and Bi-GST in Fig. 3(b) are obtained from the values of Tx because Tx Tg for those samples. Conversely, Fig. 3(c) reveals the completely different effects of doping for Tx Tg. The narrow super-cooled liquid regions (Tx Tg 0) of pure GST are maintained in Al and Bi but widen with C and N dopings. The different effects of doping on Tx and Tx Tg can be explained by different atomic mechanisms.
The temperature at which crystallization occurs increases for various reasons, including structural changes in the amorphous or crystalline state. Doping in GST changes the bonding enthalpy by substitution of bonding [7,15–18]. Al-GST produces additional bonding of Al-Sb and Al-Te [16,17]. In contrast, Bi-GST substitutes bonds from Sb–Te to Bi–Te, which causes weaker bonding [7,15]. The changes of the bonding enthalpies associated with doping explain the changes of Tx by limiting the partial breakage of bonding required for atomic rearrangements. In addition, C doping is known to lead to a higher portion of tetrahedral bonding in amorphous GST, resulting in the stabilization of the amorphous structure and retardation of the crystallization [19,20]. Furthermore, N doping is known to lead to a strain field in the crystalline lattice and destabilization of the crystalline structure, also resulting in the stabilization of the amorphous structure [21,22]. At temperatures above Tg, the super-cooled liquid experiences a rapid increase in atomic mobility as a function of temperature, which is then sufficient for crystallization to occur. Therefore, the temperature dependence of atomic mobility controls crystallization in the super-cooled liquid region. If the increase in atomic mobility above Tg is abrupt, crystallization will be accelerated due to the easy atomic rearrangement, which results in a narrow super-cooled liquid region. Conversely, if the increase in atomic mobility above Tg is gradual, crystallization will be delayed due to difficult atomic rearrangement, which results in a higher Tx Tg. Because the free volume in an
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Fig. 3. (a) Tx, (b) Tg and (c) Tx Tg as a function of dopant concentrations of Al, Bi, C and N in GST films. Schematics of the viscosity as a function of temperature in pure GST in comparison with (d) Al-GST, (e) Bi-GST, (f) C- or N-GST.
amorphous network acts as the center of the atomic arrangement, a larger amount of free volume leads to an abrupt increase in atomic mobility above Tg and a lower Tx Tg. Therefore, the Tx Tg is determined mainly by the free volume in amorphous phase change materials. The super-cooled liquid region can be regarded as a quantitative measure of the atomic mobility during crystallization at high temperatures. Fig. 3(d)–(f) presents schematics of viscosity as a function of temperature and as a function of the doping elements. The viscosities where the glass transition occurs are assumed to be the same value in the figures. Upon increasing the temperature of the glass above Tg, the super-cooled liquid experiences a rapid increase in atomic mobility, as illustrated in Fig. 1(b). The narrow super-cooled liquid region is observed in pure GST because an abundant amount of free volume is contained in pure GST. As the free volume in the amorphous network acts as a center of atomic arrangement [23], a larger amount of free volume leads to an abrupt increase of atomic mobility above Tg and lower Tx Tg. Because Al and Bi are known to be substitutional dopants that occupy sites of constitutive atoms, and the free volume in the system will not be significantly occupied, the narrow super-cooled liquid region is maintained in Al- and Bi-GST. In contrast, C and N are both known to be interstitial dopants and occupy free volume sites [24,25]. These structural features of C and N-GST result in less free volume and the gradual decrease of viscosity as a function of temperature. Consequently, crystallization will be delayed due to hard atomic rearrangement, resulting in a higher Tx Tg. 3.2. Viscosity and fragility To facilitate the discussions regarding the super-cooled liquid region presented in the previous section, it is useful
to represent the atomic mobility of GST as a function of temperature. To identify the effect of doping on the temperature dependence of atomic mobility, the viscosity (g) was examined. Viscosity is the resistance to atomic flow, which is regarded as the reciprocal of atomic mobility. The temperature dependence of g can be utilized to determine the universal criterion for the kinetic stability of an amorphous structure, fragility [4]. Fragility (m) is determined from the deviation of viscosity as a function of temperature from the Arrhenius relationship, as shown in Eq. (1) [4]: d log gðT Þ E m¼ ¼ ð1Þ dðT g =T Þ T ¼T g ln 10 RT g As m approaches 16, the Arrhenius relationship holds, and the liquid is categorized as “strong”. Larger values of m result in non-Arrhenius relationships and are categorized as “fragile”. This non-Arrhenius relaxation causes the activation energy for relaxation to vary with temperature; therefore, the apparent activation energy for relaxation (E) was defined at Tg for the experimental characterization. This apparent activation energy for relaxation (E) can be extracted from the kinetic nature of the glass transition according to the Moynihan plot [26]. A shift of Tg toward higher temperatures as the heating rate increases is used to calculate E in Eq. (2) [26]: ln b ¼
E þC RT g
ð2Þ
where b is the heating rate (K/min) and C is a constant. Fig. 2 shows the dependence of the stress evolution on the heating rate (1.0, 2.5, 6.5, 13 and 20 K/min); with increases in the heating rate, Tg also increases. Linear dependences of ln b on 1/Tg are shown in Fig. 4(a). From the linear slope (=E/R) of Fig. 4(a) at T = Tg, the fragilities were obtained, as depicted in Fig. 4(b). Pure GST has
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m = 47, which indicates that this material is relatively fragile. Compared with pure GST, m significantly decreases with C and N dopings to m = 20 for 10 at.% C and 10 at.% N GST, which indicates that these materials are relatively strong. However, Al and Bi-GST exhibit negligible changes of m. The significant decrease in m with C and N dopings indicates that the highly mobile nature of pure GST can be easily altered. However, Al and Bi doping has a negligible effect on the fragility. The inconsistencies between dopants can be attributed to the different
structural changes associated with the different dopants, especially to the free volume of the amorphous GST, as discussed in Fig. 3(d)–(f). These structural features of C and N-GST result in the gradual temperature dependence of viscosity, which is manifested in higher values of Tx Tg. The calculated values of m can be utilized to extrapolate the viscosity curves as a function of temperature because these curves are important for predicting the crystallization kinetics at high temperatures (similar to the switching temperature of PcRAM). To describe the temperature
Fig. 4. (a) Linear dependences of natural logarithm of the heating rates (b) on 1/Tg for Al-, Bi-, C- and N-GST. (b) Fragility obtained from the linear slopes in (a) at T = Tg for pure and Al-, Bi-, C- and N-GST. Extrapolated curves of the logarithm of the viscosity as a function of Tg/T according to the Vogel–Fulcher–Tammann relation for (c) 8.5 at.% Al GST, (d) 9.6 at.% Bi GST, (e) 10 at.% C GST and (f) 10 at.% N GST in comparison with pure GST and Arrhenius behavior.
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dependence of viscosity (g) in a fragile liquid with non-Arrhenius behavior, the Vogel–Fulcher–Tammann equation generally best fits the experimental data of a super-cooled liquid with non-Arrhenius behavior [27]: DT 0 ð3Þ g ¼ g0 exp T T0 where D and T0 are fitting parameters. For the extrapolation, the obtained fragilities were used to define the linear slope of the logarithm of the viscosity as a function of Tg/T (known as an Angell plot) at Tg. For further extrapolation, g = 1012 Pa s at T = Tg and g = 104 (s) at T 1 are assumed. The extrapolated curves of the logarithm of the viscosity as a function of Tg/T according to the Vogel–Fulcher–Ta mmann relation for 8.5 at.% Al, 9.6 at.% Bi, 10 at.% C and 10 at.% N GST in comparison with pure GST and Arrhenius behavior are presented in Fig. 4(c)–(f). Fig. 4(c)–(f) reveal large deviations of pure, Al-GST and Bi-GST from Arrhenius behavior, which results in relatively rapid decreases in g with temperature. Fig. 4(e) and (f) reveal smaller deviations for C-GST and N-GST from Arrhenius behavior, which results in gradual decreases in g with temperature. The viscosity at high temperature determined can be utilized to estimate the SET time during the operation procedure (details in Supporting Materials). Assuming isotropic crystal growth in a sphere with radius 15 nm (Vcell 2.5 104 nm3) for SET switching and the Stokes– Einstein relation (D = kT/3pag, a is the jump frequency determined to be 0.268 nm [28], only valid at T P Tm), the SET time can be estimated. The SET times were determined from the viscosities of the GST films in Fig. 4(c)–(f) with various dopant concentrations at T = 900 K indicated as open circles. The calculated SET times at 900 K varied from 4.4 ns for pure GST to 33 ls for 10 at.% N GST, indicating a difference of approximately four orders of magnitude (less than two orders of magnitude in real devices). This large deviation between the calculated and measured SET time likely originates from the temperature where the SET occurs being shifted with doping because of the changes in resistance of the high-resistance state. 3.3. Structure–property relationships The effects of doping on the crystallization behavior of GST described above enabled the construction of “structu re–property relationship” maps of amorphous Ge2Sb2Te5 doped with various doping elements, which are presented in Fig. 5(a) and (b). Fig. 5(a) and (b) present maps of amorphous Ge2Sb2Te5 tuned using various dopants as a function of the crystallization temperature (Tx) and super-cooled liquid region (Tx Tg) and reciprocal of fragility (m), respectively. The vertical axis describes the thermal stability against crystallization at low temperature (long lasting), and the horizontal axis describes the atomic mobility rate at high temperature (fast transforming). In Fig. 5(a), pure Ge2Sb2Te5 is located in the left region but moves to the upper-right with C and N dopings, to the upper-left with Al doping, and to the lower-left with Bi doping. The various effects of doping are related to the structural changes of amorphous Ge2Sb2Te5, which depend upon the type of dopant. Both interstitial (C and N) and Al (substitutional) dopants increase Tx, which contribute to
Fig. 5. (a) Map of amorphous Ge2Sb2Te5 tuned by various dopants as a function of crystallization temperature (Tx) and super-cooled liquid region (Tx Tg). (a) Map of amorphous Ge2Sb2Te5 tuned by various dopants as a function of crystallization temperature (Tx) and reciprocal of the fragility. (c) Map of PcRAM characteristics as a function of 10-yr data retention and SET time for various phase change materials and dopants (details in Supporting Materials), demonstrating the successful match with (a) and (b). The SET time* is determined for Roff/Ron 102 and includes the data obtained from optical experiments.
the thermal stability against crystallization, while Bi (substitutional) doping decreases Tx, which means lower thermal stability. However, only interstitial dopants significantly change the portion of the free volume, which contributes to the atomic mobility of the system manifested by the changes in Tx Tg and fragility.
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The characteristics of PcRAM are predicted; a higher Tx is generally regarded as higher thermal stability against crystallization, which can achieve higher 10-yr data retention. In addition, the high atomic mobility at high temperature manifested by Tx Tg and fragility can be regarded as the crystallization time, which is related to the SET speed. Fig. 5(c) presents a map of the PcRAM characteristics tuned by various dopings as a function of 10-yr data retention and SET time for various phase change materials and dopants. The lines were obtained from the reported results of other researchers; if both the 10-yr data retention and the SET time were reported in the same reference, these data are marked as triangles for Sb–Te alloys, circles for Ge–Sb–Te alloys, and squares for Ge–Te alloys (details and references in Supporting Materials). The pure phase change materials located in the lower-left region move to the upper-right with C and N dopings, to the upper-left with Al doping or to the lower-left with Bi doping. C and N (interstitial) dopings in phase change materials generally result in an increase in the 10-yr data retention but is disadvantageous for a low SET time. However, substitutional dopants results in different effects according to their structural features; Bi doping slightly decreases the 10-yr data retention while maintaining or even lowering the SET time. In contrast, Al doping slightly increases the 10-yr data retention while maintaining or even lowering the SET time, which is advantageous for reliability and performance. The comparison of PcRAM characteristics in Fig. 5(c) demonstrates that the effects of doping on Tx and Tx Tg (or 1/m) successfully match the data retention and SET speed, respectively. Consequently, the “structure–property relationship” maps of amorphous Ge2Sb2Te5 showing the effects of doping on Tx and Tx Tg (or 1/m) in Fig. 5(a) and (b) successfully match the PcRAM characteristics of the data retention and SET speed in Fig. 5(c), respectively.
4. Conclusion In conclusion, the crystallization kinetics of amorphous phase change materials were explored, including the crystallization temperature, glass transition temperature, super-cooled liquid region and fragility of amorphous Ge2Sb2Te5 (GST) films doped with Al, Bi, C and N using mechanical stress analysis. Fragile-to-strong transition is observed for C and N dopings due to their structural feature as an interstitial dopant, while substitutional Al and Bi dopings maintained the fragile behavior of GST. In addition, the viscosity and fragility were also determined. Doping effects on the thermal stability and atomic mobility manifested by Tx and Tx Tg (or 1/m) successfully correspond with PcRAM characteristics, i.e., the data retention and SET speed, respectively. The mechanical stress analysis in this study can provide guidelines to predict the device characteristics, using thin film characterization techniques, which is a simple task. Acknowledgments This work was supported by IT R&D program of MOTIE/KEIT [10039200, Development of high performance phase change materials].
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.actamat.2015.04.058.
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