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High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties
Author’s Accepted Manuscript High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties Chun-Yang Cheng, Jien-Wei Yeh www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31510-5 http://dx.doi.org/10.1016/j.matlet.2016.09.050 MLBLUE21488
To appear in: Materials Letters Received date: 8 August 2016 Revised date: 6 September 2016 Accepted date: 12 September 2016 Cite this article as: Chun-Yang Cheng and Jien-Wei Yeh, High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties Chun-Yang Cheng, Jien-Wei Yeh* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC * Corresponding author: Tel.: +886-3-5719558, E-mail: [email protected].
Abstract Equi-atomic BNbTaTiZr composition was designed to achieve a thin film with exceptional thermal stability of amorphous structure. Amorphous structure of BNbTaTiZr thin film can sustain after 1 hour vacuum annealing at 800 °C. To elucidate the exceptional thermal stability, thermodynamic, topological and kinetic factors are discussed through high entropy effect, atomic size difference and sluggish diffusion phenomena. The amorphous film also displays high electrical resistivity 246 -cm comparing with most amorphous metals. Negative temperature coefficient of resistivity is observed. The reason is related with the increased interatomic spacings with increasing temperature. Keywords: Amorphous materials; Physical vapor High-entropy alloys; Thermal stability; Thin films
deposition;
Sputtering;
1. Introduction The amorphous structure is featured with short-range atomic-scale order in contrast with the long-range order structure of crystalline materials. Suitable designed amorphous materials such as bulk metallic glass (BMG) and/or amorphous metal thin films (AMTFs) could exhibit outstanding mechanical, magnetic properties and corrosion resistance [1-3]. Combining the excellent properties with amorphous nature, they have the potential in advanced applications such as precision gears [1] and diffusion barriers in microeletronics [4, 5]. However, amorphous material associated with poor thermal stability is prone to crystallize at elevated temperatures and lose their merits of amorphous structure. This would limit their use in many applications. For instance, amorphous material with thermal stability to 400 °C is generally commanded for back-end-of-line processing in microeletronics [6]. Therefore, 1
deterring amorphous material from crystallization at elevated temperatures, i.e., enhancing thermal stability of amorphous material, is always the important issue. In 2004, Yeh defined high-entropy alloys (HEAs): a HEA comprises at least five major metallic elements, each of which has an atomic percentage between 5 and 35% [7-9]. In HEAs, high mixing entropy effect could improve the stability of solid solutions at high temperatures [9]. Moreover, severe lattice distortion and sluggish diffusion effects could slow down phase transformation rate [9]. In this study we propose crystallization tendency of amorphous solid solutions at high temperatures could be reduced by taking advantage of high entropy, lattice distortion and sluggish diffusion effects. The thinking route stems from the following considerations. First, equi-mole content of five or more principal elements could maximize mixing entropy to improve the mixing of composing elements. Second, significant atomic size difference could validate the topological instability to form amorphous structure. Third, utilization of higher melting point elements and sluggish diffusion effect could suppress the crystallization process of amorphous structure. Along this line of thinking, high-entropy BNbTaTiZr thin films with amorphous structure are herein designed. Note that Nb, Ta, Ti and Zr have small mixing enthalpy between each other, but B has similar large negative enthalpy with the four metal elements (mixing enthalpies between B and Nb, Ta, Ti, Zr are -54, -54, -58, -71 kJ/mole, respectively [10]). These comparable mixing enthalpies would let B mix well with the four elements during deposition from vapor phase. Thus, sputtering deposition method and rapid thermal annealing (RTA) were chosen to fabricate the homogeneous amorphous films and evaluate the thermal stability of amorphous structures at different temperatures, respectively. In addition, high electrical resistivity is often required in special applications such as thin film resistors in 3C electrical devises [11]. Otherwise, the thickness of thin resisters needs to be reduced to increase the overall resistance. As thinning would bring difficulty in getting uniform film coverage on the surface of substrate, precision control and reliability of electrical resistance would become difficult. This is why small B is chosen for the present composition to increase overall atomic size difference and resistivity. Therefore, both thermal stability and resistivity were investigated in order to verify the proposed design strategy for high thermal stability and high resistivity.
2. Experimental procedures The 2-inch sputtering target of equ-atomic BNbTaTiZr HEA was prepared with B, Nb, Ta, Ti, and Zr elemental raw materials via vacuum-arc-melting. Melting and solidification were repeated for at least 5 times to ensure the chemical homogeneity. Amorphous BNbTaTiZr thin films were deposited on sapphire substrates by using 2
direct current magnetron sputtering operated at a target power of 150 W and under a base pressure 2×10-6 torr and a working pressure fixed at 5×10-3 torr with an argon gas flow of 40 sccm. Chemical composition of BNbTaTiZr thin film is very close to that of target (Table 1) by the analyses using an electron probe microanalyzer (EPMA, JXA-8500F, JEOL). The film thickness was controlled to be around 700 nm, which was checked with field-emission scanning electron microscope (FESEM, SU8010, Hitachi). The as-deposited samples were placed on a quartz plate which was at the center position of a quartz tube furnace and subjected to RTA at 500, 550, 600, 650, 700, 750, 800, and 850 °C for 1 h below 3 × 10-7 torr, respectively. The heating rate from room temperature to designated annealing one was 10 °C/s. The heating sources were six infrared heating lamps symmetrically distributed around the quartz tube. The error of annealing temperature was controlled to be within ± 0.5 °C by the temperature controller ‘EUROTHERM 3504’. The probe tip of thermal couple was placed at the side of samples to monitor and ensure the accuracy of annealing temperature. The crystallographic structures of as-deposited and annealed samples were characterized utilizing a glancing incident angle X-ray diffractometer (TTRAXIII, Rigaku, Japan) with Cu Kα radiation at 50 kV, 300 mA and at the incident angle of 1°. The structure evolution of BNbTaTiZr thin films were investigated with transmission electron microscope (TEM, JEM-2100F, JEOL). Average surface roughness of BNbTaTiZr thin films was determined via atomic force microscopy in tapping mode (AFM, Model: Dimension ICON, Bruker). The resistivities ranging from 225 to 400 K with an increment of 25 K was measured using four-point probe measurement system (Keithley 4200-SCS). These selected temperatures were controlled by ‘331 Temperature Controller (LakeShore)’ equipped with liquid nitrogen cooling system.
3. Results and Discussion The XRD patterns of BNbTaTiZr thin films at the as-deposited and as-annealed states are shown in Fig. 1. Single diffuse humps are observed for all samples except 850 °C-annealed BNbTaTiZr film, suggesting the amorphous structure of BNbTaTiZr thin films is retained after one-hour annealing at 800 °C. Furthermore, SEM and TEM analyses are performed since it is compulsory to elaborately characterize the amorphous structure of BNbTaTiZr. From SEM plan-view in Fig. 2(b) the BNbTaTiZr film retains uniform and smooth surfaces even after annealing at 800 °C, as compared to its corresponding as-deposited one (Fig. 2(a)). On the other hand, high resolution (HR) images and selected-area-diffraction (SAD) patterns of as-deposited (Fig. 2(c)) and 800 °C-annealed (Fig. 2(d)) BNbTaTiZr thin films reveal uniform salt-and-pepper feature and exhibit diffused rings. These results confirm the maintenance of 3
amorphous structures for BNbTaTiZr thin films after one-hour annealing at 800 °C. In addition, AFM analyses of the two films give very small average surface roughnesses (Ra), 0.44 and 0.46 nm, respectively, suggesting that surface smoothness is minimally influenced by annealing treatment. Such feature demonstrates that the amorphous BNbTaTiZr thin film is promising for applications in which a atomically smooth surface is required, even after annealing to a relatively high temperature [12, 13].
Fig. 1 XRD patterns of BNbTaTiZr thin films in the as-deposited and annealed states.
4
Fig. 2 FE-SEM images, HR-TEM images and selective area diffraction patterns of as-deposited and 800 °C-annealed BNbTaTiZr thin films. The electrical resistivity (300) of BNbTaTiZr thin films is 246 -cm at 300 K. It’s much greater than those of pure constituent elements [14-17] and falls in the range anticipated for amorphous metals [1] The normalized resistivity versus temperature plot of BNbTaTiZr thin films between 225 K and 400 K is shown in Fig. 3(a). Apparently, the resistivity of BNbTaTiZr decreases with increasing temperature from 248 to 244 -cm, suggesting the presence of negative thermal coefficients of resistivity (TCR), -9.87×10-5 K-1, in contrast to positive TCRs of most crystalline metals. Mooij predicted from 133 data points that an amorphous metal with a resistivity greater than 150 -cm displays a negative TCR (known as Mooij correlation) [18]. However, Tsuei used more than 500 data points to make the statistical plot and concluded that the critical resistivity point is not universal and might be in the range between 30 and 400 -cm or more. He utilized two competing effects, inelastic scattering and thermal excitation, to explain his conclusion for the possible range of critical resistivity [19]. Although it still lacks of good theoretical explanation for the mechanism of negative TCRs, thermal expansion to let the structure have larger interatomic spacings for free electrons to pass through is believed to be the main mechanism. Thermal activation of more electrons from localization bonds (strong metal-B bonds) at higher temperatures is considered as the 5
second positive factor to decrease the resistivity. As for the higher thermal vibration to increase the resistivity at higher temperature, it is considered as a negative but negligible effect since electron scattering by the topological disorder of amorphous structure would overwhelm the scattering due to thermal vibration effect. The thermal stability of amorphous BNbTaTiZr thin films is exceptional. By convenience, we define Th as the highest experimental temperature at which amorphous structure remains. Hence, Th of BNbTaTiZr is 800 °C. If we compare Th of BNbTaTiZr films with the reported Th of amorphous metal thin films (AMTFs) despite the different annealing time [20-24] (Fig. 3(b)), a large improvement of 200 °C to the best Ta42.6Ni43.5Si13.9 AMTF is seen. Core effects of high-entropy alloys are responsible for the exceptional thermal stability of amorphous BNbTaTiZr thin films as mentioned in Introduction [9]. The strong high entropy effect of BNbTaTiZr promotes mutual solubility among constituent elements, further stabilizing solid solution phase especially at high temperatures. In addition, atomic size difference among the five elements (B = 0.98 Å, Nb =1.46 Å, Ta = 1.47 Å, Ti = 1.46 Å and Zr = 1.60 Å) is significant [25]. The atomic size difference is +14.8 % between the largest size and the average one, and -29.7 % between the average size and the smallest one. On the basis of the hard ball model proposed by Kao et al. [26, 27], this size fluctuation is enough to induce the merging of the 2nd atomic shell with 3rd one and the 4th shell with 5th one, respectively. This demonstrates the significant size difference is sufficient to cause topological instability from crystalline to amorphous in BNbTaTiZr thin films. Sluggish diffusion effect also plays a complimentary role in aiding the formation and stability of an amorphous structure, which consists of two aspects. Firstly, efficient atomic packing retards the process of atomic rearrangement stemming from following factors: (1) aforementioned significant atomic size difference, and (2) negative overall mixing enthalpy (Hmix), -36.32 kJ/mole, is large [10, 28]. Secondly, refractory elements such as Nb and Ta with much higher melting points inherently have slower diffusion rate. Therefore, the design strategy mentioned in Introduction for thermal stable and high-resistivity amorphous structure is verified by the present alloy and the related principles are helpful for similar composition design.
6
Fig. 3 (a) The normalized resistivity of BNbTaTiZr thin films versus temperature, and (b) Th comparison of thin films between the present study and reported data [20-24]. Annealing time is shown in parentheses.
4. Conclusion High-entropy BNbTaTiZr thin films preserve amorphous structures after annealing at 800 °C for one hour, as confirmed by XRD, SEM, TEM and AFM analyses. The electrical resistivity and negative TCR of BNbTaTiZr thin films at room temperature is 246 -cm and -9.87×10-5 K-1, respectively. The exceptional thermal stability of amorphous structure for BNbTaTiZr thin films is due to the combination effect of high entropy, significant atomic size differences and enhanced sluggish diffusion. The high resistivity and negative TCR are related with the large atomic size difference and increased interatomic spacings with increasing temperature, respectively. All these support our design strategy for compositions with stable amorphous structure and higher resistivity. The strategy is thus promising for designing various functional films in high temperature applications.
Acknowledgement The authors thank the financial support from the Ministry of Science and Technology, Taiwan, under the Project no. NSC-102-2221-E-007-047-MY3.
References [1] Suryanarayana C, Inoue A. Bulk Metallic Glasses. Boca Raton: CRC Press Taylor and Francis Group; 2011.
7
[2] Chu JP, Jang JSC, Huang JC, Chou HS, Yang Y, Ye JC, et al. Thin Solid Films 2012;520:5097-122. [3] Lee J, Huang KH, Hsu KC, Tung HC, Lee JW, Duh JG. Surf Coat Technol 2015;278:132-7. [4] Nicolet MA. Thin Solid Films 1978;52:415-43. [5] Wittmer M. J Vac Sci Technol, A 1984;2:273-80. [6] Farcy A, Carpentier JF, Thomas M, Torres M, Torres J, Ancey P. Microelectron Eng 2008;85:1940-6. [7] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Adv Eng Mater 2004;6:299-303. [8] Yeh JW. JOM 2013;65:1759-71. [9] Murty BS, Yeh JW, Ranganathan S. High-Entropy Alloys. Boston: Butterworth-Heinemann; 2014. [10] Takeuchi A, Inoue A. Mater Trans 2005;46:2817-29. [11] Matsuda K, Sato K, Doi T, Ogata K, Konishi K. National Technical Report 1980;26:283-8. [12] Cowell EW, Alimardani N, Knutson CC, Conley JF, Keszler DA, Gibbons BJ, et al. Adv Mater 2011;23:74-+. [13] Alimardani N, Cowell EW, Wager JF, Conley JF, Evans DR, Chin M, et al. J Vac Sci Technol, A 2012;30:01A113. [14] Schauer A, Roschy M. Thin Solid Films 1972;12:313-&. [15] Singh B, Surplice NA. Thin Solid Films 1972;10:243-&. [16] Morton N, James BW, Wostenholm GH, Nichols RJ. Journal of Physics F-Metal Physics 1975;5:85-92. [17] Desai PD, James HM, Ho CY. J Phys Chem Ref Data 1984;13:1097-130. [18] Mooij JH. Phys Status Solidi A-Appl Res 1973;17:521-30. [19] Tsuei CC. Phys Rev Lett 1986;57:1943-6. [20] Chu JP, Liu CT, Mahalingam T, Wang SF, O'Keefe MJ, Johnson B, et al. Phys Rev B 2004;69:113410. [21] Chu JP, Lo CT, Fang YK, Han BS. Appl Phys Lett 2006;88:012510. [22] Chen GJ, Jian SR, Jang JSC, Shih YH, Chen YT, Jen SU, et al. Intermetallics 2012;30:127-31. [23] Rajan ST, Kumar AKN, Subramanian B. Crystengcomm 2014;16:2835-44. [24] McGlone JM, Olsen KR, Stickle WF, Abbott JE, Pugliese RA, Long GS, et al. J Alloys Compd 2015;650:102-5. [25] Kittel C. Introduction to solid state physics. 8th ed. New York: John Wiley & Sons; 2004. [26] Kao SW, Chen YL, Chin TS, Yeh JW. Ann Chim Sci, Mat, 2006;31:657-68. 8
[27] Kao SW, Yeh JW, Chin TS. J Phys-Condes Matter 2008;20:145214. [28] Yang X, Zhang Y. Mater Chem Phys 2012;132:233-8.
9
Table 1 Chemical composition of BNbTaTiZr target and thin films
BNbTaTiZr Target Thin films
Concentration (at%) B
Nb
Ta
19.6 ± 0.3 19.8 ± 0.1 20.3 ± 0.1 19.3 ± 0.2 20.2 ± 0.1 20.1 ± 0.1
Ti
Zr
20.1 ± 0.1 20.2 ± 0.1 19.7 ± 0.1 20.7 ± 0.2
Highlights
New strategy is proposed to design and synthesize BNbTaTiZr thin film.
Amorphous structure of BNbTaTiZr film exhibits excellent thermal stability.
Amorphous BNbTaTiZr film displays high resistivity.
The mechanisms and expectation of the design strategy are elucidated.
10
PII: DOI: Reference:
S0167-577X(16)31510-5 http://dx.doi.org/10.1016/j.matlet.2016.09.050 MLBLUE21488
To appear in: Materials Letters Received date: 8 August 2016 Revised date: 6 September 2016 Accepted date: 12 September 2016 Cite this article as: Chun-Yang Cheng and Jien-Wei Yeh, High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High-entropy BNbTaTiZr thin film with excellent thermal stability of amorphous structure and its electrical properties Chun-Yang Cheng, Jien-Wei Yeh* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC * Corresponding author: Tel.: +886-3-5719558, E-mail: [email protected].
Abstract Equi-atomic BNbTaTiZr composition was designed to achieve a thin film with exceptional thermal stability of amorphous structure. Amorphous structure of BNbTaTiZr thin film can sustain after 1 hour vacuum annealing at 800 °C. To elucidate the exceptional thermal stability, thermodynamic, topological and kinetic factors are discussed through high entropy effect, atomic size difference and sluggish diffusion phenomena. The amorphous film also displays high electrical resistivity 246 -cm comparing with most amorphous metals. Negative temperature coefficient of resistivity is observed. The reason is related with the increased interatomic spacings with increasing temperature. Keywords: Amorphous materials; Physical vapor High-entropy alloys; Thermal stability; Thin films
deposition;
Sputtering;
1. Introduction The amorphous structure is featured with short-range atomic-scale order in contrast with the long-range order structure of crystalline materials. Suitable designed amorphous materials such as bulk metallic glass (BMG) and/or amorphous metal thin films (AMTFs) could exhibit outstanding mechanical, magnetic properties and corrosion resistance [1-3]. Combining the excellent properties with amorphous nature, they have the potential in advanced applications such as precision gears [1] and diffusion barriers in microeletronics [4, 5]. However, amorphous material associated with poor thermal stability is prone to crystallize at elevated temperatures and lose their merits of amorphous structure. This would limit their use in many applications. For instance, amorphous material with thermal stability to 400 °C is generally commanded for back-end-of-line processing in microeletronics [6]. Therefore, 1
deterring amorphous material from crystallization at elevated temperatures, i.e., enhancing thermal stability of amorphous material, is always the important issue. In 2004, Yeh defined high-entropy alloys (HEAs): a HEA comprises at least five major metallic elements, each of which has an atomic percentage between 5 and 35% [7-9]. In HEAs, high mixing entropy effect could improve the stability of solid solutions at high temperatures [9]. Moreover, severe lattice distortion and sluggish diffusion effects could slow down phase transformation rate [9]. In this study we propose crystallization tendency of amorphous solid solutions at high temperatures could be reduced by taking advantage of high entropy, lattice distortion and sluggish diffusion effects. The thinking route stems from the following considerations. First, equi-mole content of five or more principal elements could maximize mixing entropy to improve the mixing of composing elements. Second, significant atomic size difference could validate the topological instability to form amorphous structure. Third, utilization of higher melting point elements and sluggish diffusion effect could suppress the crystallization process of amorphous structure. Along this line of thinking, high-entropy BNbTaTiZr thin films with amorphous structure are herein designed. Note that Nb, Ta, Ti and Zr have small mixing enthalpy between each other, but B has similar large negative enthalpy with the four metal elements (mixing enthalpies between B and Nb, Ta, Ti, Zr are -54, -54, -58, -71 kJ/mole, respectively [10]). These comparable mixing enthalpies would let B mix well with the four elements during deposition from vapor phase. Thus, sputtering deposition method and rapid thermal annealing (RTA) were chosen to fabricate the homogeneous amorphous films and evaluate the thermal stability of amorphous structures at different temperatures, respectively. In addition, high electrical resistivity is often required in special applications such as thin film resistors in 3C electrical devises [11]. Otherwise, the thickness of thin resisters needs to be reduced to increase the overall resistance. As thinning would bring difficulty in getting uniform film coverage on the surface of substrate, precision control and reliability of electrical resistance would become difficult. This is why small B is chosen for the present composition to increase overall atomic size difference and resistivity. Therefore, both thermal stability and resistivity were investigated in order to verify the proposed design strategy for high thermal stability and high resistivity.
2. Experimental procedures The 2-inch sputtering target of equ-atomic BNbTaTiZr HEA was prepared with B, Nb, Ta, Ti, and Zr elemental raw materials via vacuum-arc-melting. Melting and solidification were repeated for at least 5 times to ensure the chemical homogeneity. Amorphous BNbTaTiZr thin films were deposited on sapphire substrates by using 2
direct current magnetron sputtering operated at a target power of 150 W and under a base pressure 2×10-6 torr and a working pressure fixed at 5×10-3 torr with an argon gas flow of 40 sccm. Chemical composition of BNbTaTiZr thin film is very close to that of target (Table 1) by the analyses using an electron probe microanalyzer (EPMA, JXA-8500F, JEOL). The film thickness was controlled to be around 700 nm, which was checked with field-emission scanning electron microscope (FESEM, SU8010, Hitachi). The as-deposited samples were placed on a quartz plate which was at the center position of a quartz tube furnace and subjected to RTA at 500, 550, 600, 650, 700, 750, 800, and 850 °C for 1 h below 3 × 10-7 torr, respectively. The heating rate from room temperature to designated annealing one was 10 °C/s. The heating sources were six infrared heating lamps symmetrically distributed around the quartz tube. The error of annealing temperature was controlled to be within ± 0.5 °C by the temperature controller ‘EUROTHERM 3504’. The probe tip of thermal couple was placed at the side of samples to monitor and ensure the accuracy of annealing temperature. The crystallographic structures of as-deposited and annealed samples were characterized utilizing a glancing incident angle X-ray diffractometer (TTRAXIII, Rigaku, Japan) with Cu Kα radiation at 50 kV, 300 mA and at the incident angle of 1°. The structure evolution of BNbTaTiZr thin films were investigated with transmission electron microscope (TEM, JEM-2100F, JEOL). Average surface roughness of BNbTaTiZr thin films was determined via atomic force microscopy in tapping mode (AFM, Model: Dimension ICON, Bruker). The resistivities ranging from 225 to 400 K with an increment of 25 K was measured using four-point probe measurement system (Keithley 4200-SCS). These selected temperatures were controlled by ‘331 Temperature Controller (LakeShore)’ equipped with liquid nitrogen cooling system.
3. Results and Discussion The XRD patterns of BNbTaTiZr thin films at the as-deposited and as-annealed states are shown in Fig. 1. Single diffuse humps are observed for all samples except 850 °C-annealed BNbTaTiZr film, suggesting the amorphous structure of BNbTaTiZr thin films is retained after one-hour annealing at 800 °C. Furthermore, SEM and TEM analyses are performed since it is compulsory to elaborately characterize the amorphous structure of BNbTaTiZr. From SEM plan-view in Fig. 2(b) the BNbTaTiZr film retains uniform and smooth surfaces even after annealing at 800 °C, as compared to its corresponding as-deposited one (Fig. 2(a)). On the other hand, high resolution (HR) images and selected-area-diffraction (SAD) patterns of as-deposited (Fig. 2(c)) and 800 °C-annealed (Fig. 2(d)) BNbTaTiZr thin films reveal uniform salt-and-pepper feature and exhibit diffused rings. These results confirm the maintenance of 3
amorphous structures for BNbTaTiZr thin films after one-hour annealing at 800 °C. In addition, AFM analyses of the two films give very small average surface roughnesses (Ra), 0.44 and 0.46 nm, respectively, suggesting that surface smoothness is minimally influenced by annealing treatment. Such feature demonstrates that the amorphous BNbTaTiZr thin film is promising for applications in which a atomically smooth surface is required, even after annealing to a relatively high temperature [12, 13].
Fig. 1 XRD patterns of BNbTaTiZr thin films in the as-deposited and annealed states.
4
Fig. 2 FE-SEM images, HR-TEM images and selective area diffraction patterns of as-deposited and 800 °C-annealed BNbTaTiZr thin films. The electrical resistivity (300) of BNbTaTiZr thin films is 246 -cm at 300 K. It’s much greater than those of pure constituent elements [14-17] and falls in the range anticipated for amorphous metals [1] The normalized resistivity versus temperature plot of BNbTaTiZr thin films between 225 K and 400 K is shown in Fig. 3(a). Apparently, the resistivity of BNbTaTiZr decreases with increasing temperature from 248 to 244 -cm, suggesting the presence of negative thermal coefficients of resistivity (TCR), -9.87×10-5 K-1, in contrast to positive TCRs of most crystalline metals. Mooij predicted from 133 data points that an amorphous metal with a resistivity greater than 150 -cm displays a negative TCR (known as Mooij correlation) [18]. However, Tsuei used more than 500 data points to make the statistical plot and concluded that the critical resistivity point is not universal and might be in the range between 30 and 400 -cm or more. He utilized two competing effects, inelastic scattering and thermal excitation, to explain his conclusion for the possible range of critical resistivity [19]. Although it still lacks of good theoretical explanation for the mechanism of negative TCRs, thermal expansion to let the structure have larger interatomic spacings for free electrons to pass through is believed to be the main mechanism. Thermal activation of more electrons from localization bonds (strong metal-B bonds) at higher temperatures is considered as the 5
second positive factor to decrease the resistivity. As for the higher thermal vibration to increase the resistivity at higher temperature, it is considered as a negative but negligible effect since electron scattering by the topological disorder of amorphous structure would overwhelm the scattering due to thermal vibration effect. The thermal stability of amorphous BNbTaTiZr thin films is exceptional. By convenience, we define Th as the highest experimental temperature at which amorphous structure remains. Hence, Th of BNbTaTiZr is 800 °C. If we compare Th of BNbTaTiZr films with the reported Th of amorphous metal thin films (AMTFs) despite the different annealing time [20-24] (Fig. 3(b)), a large improvement of 200 °C to the best Ta42.6Ni43.5Si13.9 AMTF is seen. Core effects of high-entropy alloys are responsible for the exceptional thermal stability of amorphous BNbTaTiZr thin films as mentioned in Introduction [9]. The strong high entropy effect of BNbTaTiZr promotes mutual solubility among constituent elements, further stabilizing solid solution phase especially at high temperatures. In addition, atomic size difference among the five elements (B = 0.98 Å, Nb =1.46 Å, Ta = 1.47 Å, Ti = 1.46 Å and Zr = 1.60 Å) is significant [25]. The atomic size difference is +14.8 % between the largest size and the average one, and -29.7 % between the average size and the smallest one. On the basis of the hard ball model proposed by Kao et al. [26, 27], this size fluctuation is enough to induce the merging of the 2nd atomic shell with 3rd one and the 4th shell with 5th one, respectively. This demonstrates the significant size difference is sufficient to cause topological instability from crystalline to amorphous in BNbTaTiZr thin films. Sluggish diffusion effect also plays a complimentary role in aiding the formation and stability of an amorphous structure, which consists of two aspects. Firstly, efficient atomic packing retards the process of atomic rearrangement stemming from following factors: (1) aforementioned significant atomic size difference, and (2) negative overall mixing enthalpy (Hmix), -36.32 kJ/mole, is large [10, 28]. Secondly, refractory elements such as Nb and Ta with much higher melting points inherently have slower diffusion rate. Therefore, the design strategy mentioned in Introduction for thermal stable and high-resistivity amorphous structure is verified by the present alloy and the related principles are helpful for similar composition design.
6
Fig. 3 (a) The normalized resistivity of BNbTaTiZr thin films versus temperature, and (b) Th comparison of thin films between the present study and reported data [20-24]. Annealing time is shown in parentheses.
4. Conclusion High-entropy BNbTaTiZr thin films preserve amorphous structures after annealing at 800 °C for one hour, as confirmed by XRD, SEM, TEM and AFM analyses. The electrical resistivity and negative TCR of BNbTaTiZr thin films at room temperature is 246 -cm and -9.87×10-5 K-1, respectively. The exceptional thermal stability of amorphous structure for BNbTaTiZr thin films is due to the combination effect of high entropy, significant atomic size differences and enhanced sluggish diffusion. The high resistivity and negative TCR are related with the large atomic size difference and increased interatomic spacings with increasing temperature, respectively. All these support our design strategy for compositions with stable amorphous structure and higher resistivity. The strategy is thus promising for designing various functional films in high temperature applications.
Acknowledgement The authors thank the financial support from the Ministry of Science and Technology, Taiwan, under the Project no. NSC-102-2221-E-007-047-MY3.
References [1] Suryanarayana C, Inoue A. Bulk Metallic Glasses. Boca Raton: CRC Press Taylor and Francis Group; 2011.
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Table 1 Chemical composition of BNbTaTiZr target and thin films
BNbTaTiZr Target Thin films
Concentration (at%) B
Nb
Ta
19.6 ± 0.3 19.8 ± 0.1 20.3 ± 0.1 19.3 ± 0.2 20.2 ± 0.1 20.1 ± 0.1
Ti
Zr
20.1 ± 0.1 20.2 ± 0.1 19.7 ± 0.1 20.7 ± 0.2
Highlights
New strategy is proposed to design and synthesize BNbTaTiZr thin film.
Amorphous structure of BNbTaTiZr film exhibits excellent thermal stability.
Amorphous BNbTaTiZr film displays high resistivity.
The mechanisms and expectation of the design strategy are elucidated.
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