Self-Limited Growth of Nanocrystals in Structural Heterogeneous

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Self-Limited Growth of Nanocrystals in Structural Heterogeneous Phase-Change Materials during the Heating Process Guoxiang Wang,*,†,‡ Yawen Zhang,†,‡ Chao Li,†,‡ Andriy Lotnyk,§ Yegang Lu,†,‡ and Xiang Shen*,†,‡ †

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Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China ‡ Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China § Leibniz Institute of Surface Engineering (IOM), Permoserstrasse 15, D-04318 Leipzig, Germany ABSTRACT: Typical nanocomposite heterogeneous ZnSb-Al2O3 and ZnSb-ZnO phasechange materials were prepared. A direct comparison of the distinct structures in the amorphous, metastable, and stable states between two different materials was investigated systematically. Upon heating, ZnSb-Al2O3 films show a two-step crystallization process with the formation of the metastable orthorhombic ZnSb phase ahead of the stable trigonal ZnSb phase, while ZnSb-ZnO films could exhibit a one-step crystallization process with the formation of the stable trigonal ZnSb phase when the ZnO-doping concentration is more than 12.3 atom %. In the case of ZnSb-Al2O3, the structural transition to the metastable phase is accompanied by a pronounced increase in the grain size up to 100 nm. Such an increase in the crystal grain size is not found in ZnSb-ZnO films; i.e., the nanocrystals do not grow significantly when the crystallized film is precipitated with the metastable ZnSb phase. By the method of advanced scanning transmission electron microscopy, we clearly find that the grain growth is limited by the separated ZnO domain formation, which improves the amorphous thermal stability significantly with the optimized 10-year data retention ability up to 229.2 °C for (ZnSb)81.8(ZnO)18.2 film.

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INTRODUCTION Recently, chalcogenide phase-change alloys have attracted much attention due to their large differences in optical and/or electronic properties between amorphous and crystalline states.1 They exhibit unique properties and are used as active layers for phase-change memory (PCM), which are based on the ultrafast and reversible transition between amorphous and crystalline states and is the most promising alternative to replace flash technology.2,3 However, for the downscaling of PCM to nanometer size, power consumption has become a critical issue since the phase-change alloys need to be heated up to the melting point during the amorphization step, resulting in the consumption of high programming currents.4 Nanoscale engineering of phase-change alloys is confirmed to be a promising approach for further development of the materials with lower power consumption. The approach is based on the synthesis of nanocomposite phase-change materials, which are designed as the introduction of dielectric materials into phase-change materials. For instance, the conventional Ge2Sb2Te5 layers doped by TiOx,5 HfO2,6 and SiOx7 have been employed to reduce the reset power by decreasing smaller programming volume. Previous studies have frequently been concerned with Ge− Sb−Te alloys,8−10 but Te-free Zn−Sb alloys have also attracted much attention due to their good thermal and electrical properties.11,12 Especially, ZnSb film has higher crystalline resistance, higher crystallization temperature (∼250 °C), better data retention (∼200 °C), and lower melting temperature (∼500 °C); thus, it is expected that ZnSb-based PCM © XXXX American Chemical Society

devices could possess better thermal stability and lower power consumption.12 Besides that, its two-step crystallization behavior, such as the transition from amorphous to metastable ZnSb phase first and then from the metastable to the stable ZnSb phase, will give an opportunity to apply it in multilevel data storage. However, the structural transformation during the crystallization process fails to be controlled accurately owing to its Sb diffusion behavior and voids formation in the metastable ZnSb phase.13 Moreover, the resistance ratio between metastable and stable state for ZnSb films is rather narrow during in situ heating experiments, which is hardly distinguished due to their similar structure.12 To solve the current problems, in this work, the ZnSb− ZnO/Al2O3 films with various amounts of oxides were prepared, and the effect of different oxide dopants on the crystallization behaviors and structural properties of ZnSb film was investigated. The structural transformation during the thermally induced crystallization process has been systematic studied by analyzing the X-ray diffraction pattern, Raman spectra, in situ sheet resistance measurements, and advanced scanning transmission electron microscopy. It was found that the enhanced crystallization behaviors of ZnO-ZnSb films were ascribed to the self-limiting method to form a small phase change volume in which the phase change materials were separated from each other by dielectric ZnO materials. The Received: November 22, 2018 Revised: December 25, 2018 Published: January 4, 2019 A

DOI: 10.1021/acs.cgd.8b01745 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Chemical Composition, Thermal Parameters, and Thermal-Induced Crystalline Phases of the ZnSb-, Al2O3-, and ZnO-Doped ZnSb Films Prepared by Using Different Sputtering Powers sputtering power (W) ZnSb

Al2O3

30 30 30 30 30 30 35 35 35 35 35

0 1 3 5 7 10

appearance phases confirmed by XRD

ZnO

composition

Tx (°C)

Ea (eV)

T10‑yr (°C)

250 (°C)

300 (°C)

350 (°C)

265 270 271 271 271 272 279 289 296 304 309

5.64

201.7

amorphous

metastable

stable

5.47

196.1

5.53

197.3

3 8 12 16 21

ZnSb (ZnSb)98.2(Al2O3)1.8 (ZnSb)95.4(Al2O3)4.6 (ZnSb)94(Al2O3)6 (ZnSb)91.9(Al2O3)8.1 (ZnSb)89.4(Al2O3)10.6 (ZnSb)94.7(ZnO)5.3 (ZnSb)90.5(ZnO)9.5 (ZnSb)87.7(ZnO)12.3 (ZnSb)84.9(ZnO)15.1 (ZnSb)81.8(ZnO)18.2

5.77

213.4

amorphous

metastable

stable

5.90

220.5

6.09

229.2

amorphous amorphous

stable stable

Figure 1. XRD patterns of undoped and Al2O3-doped ZnSb films annealed at different temperatures for 10 min in N2 atmosphere: (a) as-deposited, (b) 250 °C, (c) 300 °C, and (d) 350 °C. target of 50 mm diameter and the Al2O3 or ZnO target of 50 mm diameter, respectively. The film thickness was effectively controlled by a thickness monitor equipped in the chamber and further measured by a surface profiler (Veeco Dektak 150). The concentration of oxide dopants in the as-deposited films was measured by using energy dispersive spectroscopy. The relationship between sputtering powers and the composition of the ZnSb-based nanocomposite films is shown in Table 1. The as-deposited films were annealed in a vacuum oven filled with N2 atmosphere at various temperatures between 250 and 350 °C with a holding time of 10 min at a heating rate of 150 K/s. The structure of as-deposited and annealed films was studied by X-ray diffraction (XRD, D2 Phaser, Bruker, Germany) and Raman spectra (Renishaw inVia, Gloucestershire, UK). The diffraction patterns were taken in the 2θ range of 10−60° using Cu Kα radiation with a wavelength of 0.154 nm. Raman scattering spectra were excited by 785 nm laser and recorded at room temperature using a backscattering configuration. During Raman spectra measurements, the power density on the sample was kept at a low level of ∼0.2 mWμm−2 in order to avoid any structural deformation induced by laser radiation. The microstructure was measured by the advanced

proper ZnO-doping concentration could suppress the intermediate state and tends to directly crystallize into the stable state, while for Al2O3-ZnSb samples, there was no separate domain formation, and it failed to control the grain growth effectively as well as exhibit a two-step crystallization characteristic well with the presence of metastable phase.

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EXPERIMENTAL PROCEDURES

Al2O3- and ZnO-doped ZnSb thin films with a thickness of ∼80 nm were deposited on quartz and SiO2/Si (100) substrates by a magnetron cosputtering method using separated oxide target (Al2O3 or ZnO) and ZnSb target. The size of the quartz and SiO2/Si substrates were both 2 cm × 2 cm, with RMS roughness of less than 0.8 and 0.5 nm, respectively. The substrate temperature was kept at room temperature. The Ar gas flow was set to 47.6 mL/min. In each run of the experiment, the base pressure in the deposition chamber was evacuated to 5.6 × 10−4 Pa, and then Ar gas pressure was introduced to 0.3 Pa for the film deposition. The direct current power (Pdc) and the radio frequency power (Prf) were applied to the ZnSb B



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Figure 2. XRD patterns of undoped and ZnO-doped ZnSb films annealed at different temperatures for 10 min in N2 atmosphere: (a) as-deposited, (b) 250 °C, (c) 300 °C, and (d) 350 °C.

Figure 3. Raman spectra of undoped and Al2O3-doped ZnSb films annealed at different temperatures for 10 min in N2 atmosphere: (a) asdeposited, (b) 250 °C, (c) 300 °C, and (d) 350 °C. scanning transmission electron microscopy (STEM) measurements. The 40 nm thick samples for STEM analysis were deposited on copper mesh with carbon film. They were performed in a probe Cscorrected Titan3 G2 60−300 microscopes equipped with a high-angle annular dark-field (HAADF), bright-field (BF), dark-field (DF), selected area electron diffraction (SAED), and super-X EDX system.

The TEM was operated at 300 kV accelerating voltage with a probe forming aperture of 25 mrad. Sheet resistance of the films as a function of temperature (non-isothermal) or time at specific temperatures (isothermal) was in situ measured at a fixed heating rate of 40 K/min using a four-point probe in a homemade Ar ambient chamber. C



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Figure 4. Raman spectra of undoped and ZnO-doped ZnSb films annealed at different temperatures for 10 min in N2 atmosphere: (a) as-deposited, (b) 250 °C, (c) 300 °C, and (d) 350 °C.

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RESULTS AND DISCUSSION The effect of oxide-doping on the crystallization behavior of ZnSb films was investigated by XRD patterns as shown in Figures 1 and 2. We observe that the as-deposited and 250 °Cannealed pure ZnSb films can maintain a good amorphous nature. When the annealing temperature increases, they crystallize into a metastable orthorhombic ZnSb phase (JCPDS No. 40-809) at 300 °C and then transform into a stable trigonal ZnSb phase (JCPDS No. 5-714) at 350 °C. After oxide-doping, the as-deposited and 250 °C-annealed Al2O3- and ZnO-doped ZnSb films are also in the amorphous state. However, some significant differences can be detected when the samples were annealed at 300 °C. First, all the Al2O3doped ZnSb films exhibit similar crystallization peaks as the pure ZnSb film and are crystallized into a single metastable ZnSb phase. But the crystallization peaks become sharper and stronger, implying that the grain size cannot be controlled well with the increase of Al2O3 doping concentration. For ZnOdoped ZnSb films, only one single characteristic peak in the metastable ZnSb phase can be detected when the ZnO-doping concentration is less than 12.3 atom %; however, they display an amorphous state when the ZnO-doping concentration is beyond 12.3 atom %. This illustrates that doping ZnO suppresses the formation of the metastable phase more effectively and ZnO-doped ZnSb films exhibit a higher thermal stability than Al2O3-doped ZnSb films. When the annealing temperature increases to 350 °C, the Al2O3-ZnSb films with doping concentration are crystallized into a stable ZnSb phase, which implies that the phase transformation from the metastable ZnSb phase to the stable ZnSb phase occurs with a higher annealing temperature. While for ZnO-doped ZnSb films, we can find that the films with

doping concentration less than 12.3 atom % exhibit a two-step phase transition process, but ZnSb films with ZnO-doping concentration (>12.3 atom %) can directly crystallize from an amorphous phase into a stable ZnSb phase at the temperatures higher than 350 °C. The appearance phases in the crystalline films at respective temperatures are summarized in the Table 1. On the basis of the analysis above, it can be concluded that Al2O3 tends to stabilize the metastable ZnSb phase, while ZnO will accelerate the phase transformation from metastable-tostable ZnSb phase. The amorphous, metastable, and stable phases for ZnSb-, Al2O3-, and ZnO-doped ZnSb films can be further distinguished by the structural variation in Raman spectra as shown in Figures 3 and 4. All the as-deposited and 250 °C-annealed films exhibit a wide broadening peak located at ∼155 cm−1, implying the amorphous state. Increasing the annealing temperature to 300 °C, the Raman spectra of pure and Al2O3-doped ZnSb films exhibit two peaks A and B located at ∼113 and ∼150 cm−1, which are ascribed to the vibration of the metastable ZnSb phase.14 The locations of two Raman peaks remain unchanged, but the intensity of Raman peaks becomes obviously stronger, implying that the addition of Al2O3 fails to control the growth of ZnSb grains. For ZnOdoped ZnSb films, they exhibit similar Raman peaks as that observed in pure ZnSb when the ZnO-doping concentration is less than 12.3 atom %, but they show a broadening peak when the doping concentration increases to 15.1 and 18.2 atom %, indicating the amorphous state. As for the samples annealed at 350 °C, the ZnSb-Al2O3 films exhibit three vibration peaks marked as C, D, and E, which are located at ∼112, ∼149, and ∼170 cm−1, respectively. It is in good agreement with the vibration peaks in pure ZnSb D



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annealed at 350 °C, implying that the Raman peaks are ascribed to the vibration of the stable ZnSb phase.14 However, the intensity of Raman peaks was slightly increased, while for ZnO-doped ZnSb films, the three typical Raman peaks’ intensity becomes gradually weaker with higher ZnO-doping concentration, illustrating that the evolution of Raman peaks may be influenced by the compression stress. 15 The compression is originated from the interface between ZnSb domain boundaries and the separated oxides. In order to identify the nanocomposite structure, the advanced STEM experiments were conducted on the 300 °C-annealed Al2O3- and ZnO-doped ZnSb samples to determine the microstructure. Figures 5 and 6 show the bright

Figure 6. (a)BF-TEM, (b) DF-TEM, (c) SAED, and (d) HRTEM image of (ZnSb)87.7(ZnO)12.3 film annealed at 300 °C.

and black grains with a size of 5−10 nm, which are smaller than those in Figure 5a,b. The reduced grain size after ZnO addition in the nanocomposite sample can be readily seen from the continuous diffraction rings in the SAED pattern depicted in Figure 6c. Figure 6d shows the HRTEM image with various small nanocrystalline ZnSb grains (yellow dotted line) and dispersed ZnO particles (red dotted line). Increasing the annealing temperature up to 350 °C, the BF- and DF-TEM images of crystalline (ZnSb)87.7(ZnO)12.3 film can be seen in Figure 7a,b, respectively, showing the larger crystalline grains with a size of 20−30 nm and clearer grain boundaries. The polycrystalline grain structure with clear grain boundaries between A, B, and C grains can be detected in HRTEM image of Figure 7c. By enlarging the A, B, and C grains, a clear lattice arrangement of stable ZnSb (200) structure is found for A grain as shown in Figure 7d, and B and C are determined to be crystalline material ZnO by measuring the EDX composition on them as shown in Figure 7e,f. This result can be more clearly observed in the HAADF image as shown in Figure 8a. The phase separation that ZnSbrich domains were enclosed by oxide-rich domains is found in ZnSb-ZnO mixed film, and the grain boundary can be detected clearly with an obvious clear separation of black domain from each other by white one. We measured the chemical composition based on EDX measurements in the white and dark domains in Figure 8a. The image shows the areas with white (Sb-rich Zn 38.7 Sb 41.2 O 20.1 ) and black (O-rich Zn33.5Sb9.1O57.4 and Zn34.0Sb6.3O59.7), respectively. It can be ascribed that the Sb out-diffusion led to the composition segregation,13,17 resulting in the formation of the O-rich black area and Sb-rich white area, respectively. Figure 8b−d shows, respectively, Zn, Sb, and O elemental mapping from the same area as shown in Figure 8a. Apparently, the green color in Figure 8b indicates that the Sb-element is deficient in the black crystalline grains in Figure 8a, while the uniform red color in Figure 8c indicates that Zn is uniformly distributed in the crystalline film and filled into the black area. Likewise, oxygen-

Figure 5. (a) BF-TEM, (b) DF-TEM, (c) SAED, and (d) HRTEM image of (ZnSb)89.4(Al2O3)10.6 films annealed at 300 °C.

field (BF)- and dark filed (DF)-TEM, SAED, and HRTEM images for 300 °C-annealed (ZnSb)89.4(Al2O3)10.6 and (ZnSb)87.7(ZnO)12.3 films, respectively. As shown in Figure 5a, many white contents are believed as oxygen-rich Al2O3 to distribute everywhere around the black grains. From the DFTEM image as shown in Figure 5b, the bright grains with the size of ∼100 nm can be observed clearly and mixed by each other. In Figure 5c, the discontinuity diffraction rings in SAED image correspond to the metastable ZnSb phase, implying that the grain size is large. However, according to the HRTEM image as shown in Figure 5d, it is impossible to measure the interplanar distances because no separated domain can be formed in the annealed Al2O3-ZnSb film, in which the high annealing temperatures will result in bond relaxation and rearrangement in the nonstoichiometric oxide.16 Diffusion of the oxygen is expected, resulting in the inhomogeneous element distribution and imperfect interface between ZnSb and AlxOy. From the viewpoint of phase change behavior, it can be concluded that Al−O bonds inside ZnSb matrix do not result in significant suppression of the crystalline phase, which is good agreement with the XRD and Raman analysis. Figure 6a,b shows the BF- and DF-TEM images of 300 °Cannealed ZnO-doped ZnSb film. It can be found many white E



Article

Figure 8. (a) HAADF image of (ZnSb)87.7(ZnO)12.3 film annealed at 350 °C; the corresponding elemental mapping images: (b) Sb, (c) Zn, and (d) O.

figure of Figure 9a. This can be explained by the uneven distribution of AlxOy in the matrix. The subtle second decrease in electrical resistance at 350 °C depicts the phase transformation from the metastable to the stable phase. Fortunately, we find that the phase-change property of Al2O3-ZnSb films undergo transitions across three states from high amorphous resistance, through intermediate resistance (metastable phase) at 300 °C, to low resistance (stable phase) upon annealing at 350 °C, which results in a two-step crystallization process in Al2O3-doped ZnSb films, just as the two distinct resistance steps revealed in Ge50Te50/Ge8Sb9221 and Ge2Sb2.04Te4.7422 films for multilevel data storage application. In comparison, ZnO-doped ZnSb films exhibit an obvious increase in Tx with the ZnO-doping concentration increasing during the whole crystallization process just as shown in inset figure of Figure 9b. Moreover, the Tx values are obviously higher than that for Al2O3-doped ZnSb films. On the other hand, it can be also found that the intermediate metastable states gradually disappear with the more ZnO-doping concentration when the heating temperature is beyond 300 °C. The phenomenon indicates that ZnO dopants can promote the crystallization of stable trigonal ZnSb phase with the suppression of metastable phase in the ZnSb film. The enhanced crystallization behavior is also helpful to improve the data retention ability, just as shown in Figure 9c,d. The crystalline activation energy (Ea) and maximum temperature for 10-year data retention (T10‑yr) can be determined by the extrapolated fitting the data based on the Arrhenius equation.23

Figure 7. (a) BF-TEM, (b) DF-TEM, and (c) HRTEM image of (ZnSb)87.7(ZnO)12.3 film annealed at 350 °C; (d), (e), and (f) is the enlarged lattice of A, B, and C crystals selected in (c), respectively.

elemental mapping across the whole area exhibits relatively uniform yellow in Figure 8d, indicating that O is also homogeneously dispersed over the entire region and filled into the black area. This illustrates that the black domains are mainly dominated by the dispersed ZnO particles; however, the white domains are rich in ZnSb grains. The phase separation that ZnSb-rich nanocrystals were surrounded by the ZnO-rich phase has been confirmed in annealed ZnSb-ZnO composite films, and the segregated domains exhibited a relatively uniform size. The formed ZnSb/ZnO interfaces can increase the crystallization temperature owing to the restriction of surface atomic motion on ZnSb nanocrystals caused by coherent bonding with the surrounding atoms at the interface.18−20 The variation of sheet resistance as a function of the increasing temperature (R−T) was measured in situ at a heating rate of 40 K/min for oxide-doped ZnSb films. As shown in Figure 9a,b, R−T curves give direct information about the phase evolution during the crystallization process. The electrical resistance of the Al2O3/ZnO-doped ZnSb films first decreases gradually with the heating temperature increasing. The first abrupt drop near the crystallization temperature (Tx) illustrates that the film is crystallized from the amorphous to the metastable phase. But we can see that no obvious increase in the Tx can be detected in the ZnSb films with more Al2O3-doping concentration just as shown in inset

t = τ exp(Ea /kBT )

where t is the time to failure, τ is a proportional time constant, Ea is the crystalline activation energy, kB is the Boltzmann constant, and T is the thermodynamic temperature. The failure time t was defined as the time that the sheet resistance decreases to half of its initial value at the specific temperature F



Article

Figure 9. Sheet resistance as a function of temperature for (a) Al2O3- and (b) ZnO-doped ZnSb films; the corresponding insets are the relation between crystallization temperature (Tx) and oxide contents, respectively. The Arrhenius extrapolation at 10 years of data retention for (c) Al2O3and (d) ZnO-doped ZnSb films.

T. As shown in Figure 9c,d, the Ea and T10‑yr values of the amorphous Al2O3- and ZnO-doped ZnSb films are determined and listed in Table 1. It is obvious that the ZnO-doped ZnSb films exhibit a better thermal stability than Al2O3-doped ZnSb fil m s . E s p e c i a l l y , th e d a t a r e t e n t i o n a b i l i t y o f (ZnSb)81.8(ZnO)18.2 film reaches 229.2 °C for 10 years, which is already sufficient to meet the long-term data storage requirements for automotive electronics, i.e., at least 10 years at 120 °C.24

volume even after crystallization into the stable phase. The enhanced crystallization behaviors can improve the thermal stability with a higher crystallization temperature up to 309 °C and 10-year data retention of 229.2 °C for (ZnSb)81.8(ZnO)18.2 film.

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AUTHOR INFORMATION

Corresponding Authors

*(G.X.W.) E-mail: [email protected]. *(X.S.) E-mail: [email protected].

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CONCLUSIONS The phase change process and microstructure characterization of ZnSb-Al2O3 and ZnSb-ZnO thin films with different oxidedoping concentration have been investigated. The different oxide-doping has a distinct effect on the structure and properties of ZnSb thin films. First, the amorphous state of all the ZnSb-Al2O3 samples is kept until 250 °C, while the ZnSb-ZnO samples can keep the amorphous state up to 300 °C with a high ZnO-doping concentration of >12.3 atom %. Second, after crystallization, the metastable phase can be stabilized in the 300 °C-annealed ZnSb-Al2O3 films, but it disappeared in the ZnSb film with ZnO-doping concentration of >12.3 atom %. Third, the phase transformation from the metastable to the stable phase occurs at 350 °C for ZnSbAl2O3 and ZnSb-ZnO, but the ZnSb films with ZnO-doping concentration of >12.3 atom % can directly transform from amorphous into the stable ZnSb phase. Phase separate domain formation (ZnSb-rich domains are enclosed by ZnO-rich domains) is found clearly in ZnSb-ZnO mixed film, which gives a self-limiting method to form smaller phase change

ORCID

Guoxiang Wang: 0000-0003-0042-4025 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61604083, 61775111, 6177030933, and 61775109), the 3315 Innovation Team in Ningbo City, Zhejiang Province, China, and was sponsored by the K. C. Wong Magna Fund in Ningbo University. We thank Yimin Chen, Ph.D., from Faculty of Science, Ningbo University, for editing the English text of a draft of this manuscript.

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Self-Limited Growth of Nanocrystals in Structural Heterogeneous

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