Electrical Transport Properties of Hexagonal TaSi2 Crystals Based on Structural Stability under High Pressure

LI Xiaoyang LU Yang YAN Hao

LI Xiaoyang, LU Yang, YAN Hao. Electrical Transport Properties of Hexagonal TaSi2 Crystals Based on Structural Stability under High Pressure[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021102. doi: 10.11858/gywlxb.20170571
Citation: LI Xiaoyang, LU Yang, YAN Hao. Electrical Transport Properties of Hexagonal TaSi2 Crystals Based on Structural Stability under High Pressure[J]. Chinese Journal of High Pressure Physics, 2018, 32(2): 021102. doi: 10.11858/gywlxb.20170571
李晓阳, 陆阳, 晏浩. 高压下六方TaSi2晶体基于结构稳定性的电学输运性质[J]. 高压物理学报, 2018, 32(2): 021102. doi: 10.11858/gywlxb.20170571
引用本文: 李晓阳, 陆阳, 晏浩. 高压下六方TaSi2晶体基于结构稳定性的电学输运性质[J]. 高压物理学报, 2018, 32(2): 021102. doi: 10.11858/gywlxb.20170571

Electrical Transport Properties of Hexagonal TaSi2 Crystals Based on Structural Stability under High Pressure

doi: 10.11858/gywlxb.20170571
Funds: 

National Natural Science Foundation of China U1530402

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    Author Bio:

    LI Xiaoyang(1991—), male, master, major in condensed matter physics.E-mail:xiaoyang.li@hpstar.ac.cn

    Corresponding author: YAN Hao(1975—), male, doctor, major in condensed matter physics.E-mail:yanhao@hpstar.ac.cn
  • 摘要: 作为一类稳定的低电阻及高温材料,二硅化钽(TaSi2)被广泛应用于集成电路中。因此,其电学稳定性和结构稳定性同样重要。报导了高压下六方TaSi2晶体基于结构稳定性的电学输运性质。通过同步辐射X射线衍射和拉曼光谱实验研究了TaSi2晶体在压力高达20 GPa时稳定的结晶学结构,并通过原位高压电阻测量发现,当压力增加到16.3 GPa时,TaSi2的电阻率趋于稳定在2 μΩ·cm左右;进一步理论计算了压力下TaSi2的电子结构,以进一步理解其金属性行为。

     

  • As a class of silicide compounds, metal silicides, such as TaSi2, TiSi2, WSi2, Mg2Si, CrSi2 and Ta5Si3, have been extensively investigated due to their low resistivity, superior thermal stability and excellent oxidation resistance[1-4].These silicides have been employed as coating materials for energy and aerospace[5-6], low resistance contacts and interconnects in large scale integrated circuits[7], gate materials for CMOS microelectronic devices[8], strain-sensitive materials for strain gauges[5]and thermoelectric materials for stable transducers[9].Generally, the structural stability and electrical transport properties of materials are more attractive for the semiconductor industry, materials science and physics.In addition to temperature, pressure is an alternative approach to studying structural stability and physicochemical properties of materials, as well as to synthesizing new compounds and exploring novel physical mechanisms[10].Recently, Li et al.[11] found that the hexagonal C40 structure of TaSi2 is stable in a wide pressure range (0-50 GPa).Electronic devices made of metal silicides, like TaSi2, may become available under stress or pressure if electrical transport properties permit.

    The intrinsic electrical properties of metal silicides have long been studied at different temperatures.For TaSi2, superconductivity has been found below the critical temperatures of 0.35 and 4.4 K for bulk single crystals and a thin-film on sapphire as substrate, respectively[12-13].The electrical resistivity of the TaSi2 single crystal along the 〈0001〉 crystallographic direction was less than along the 〈1010〉 direction in the range of 4.2-1100 K and the anisotropy of electrical resistivity at room temperature was almost 100%[14].The current-voltage characteristics of the TaSi2/n-Si junction were also investigated at different temperatures and the junction exhibited Schottky behavior[15].However, the electrical transport properties of metal silicides under high pressure have rarely been reported.

    In this work, we used TaSi2 as an example metal silicide to study electrical stability at high pressure and room temperature.The structural stability of TaSi2 powder was examined by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy under pressures up to 20 GPa.The electrical stability is reflected by the electrical resistances of the sample under different pressures, which were measured using an in situ high-pressure four-probe method.Finally, the electronic energy band structure of TaSi2 at 0 GPa and high pressure was calculated to explain the experimental results.

    A Mao-Bell type symmetric diamond anvil cell (DAC) with 400 μm-diameter anvil culets was used to generate pressure.T301 stainless steel with a thickness of 250 μm was pre-indented to a thickness of ~50 μm to serve as gaskets.A 150 μm diameter hole was laser drilled (λ=1 064 nm) at the center of the pre-indentation to serve as the sample chamber.Then, TaSi2 powder (Alfa Aesar, 99.9%) was loaded into the chamber, and silicone oil was used as a pressure-transmitting medium.A ruby ball preplaced at the center of the culet was employed to determine the pressure by the ruby fluorescence method[16].

    The in situ high-pressure angle-dispersive X-ray diffraction (AD-XRD) experiments were performed at the BL15U1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) using a wavelength of 0.061 99 nm.The sample to detector distance was 170.5 mm.The obtained two-dimensional image data were converted to one-dimensional XRD patterns by Fit2D-WAXD software[17].Rietveld refinements were conducted using the General Structure Analysis System (GSAS) with the user interface EXPGUI package[18-19].The in situ high-pressure Raman scattering spectra were collected by a Raman spectrometer with an excitation laser (λ=532 nm, Renishaw 1000).

    The pressure-dependent electrical resistance was measured using a four-probe method in a symmetric DAC with 500 μm-culet sized anvils.To create an insulated environment, a ~500 μm diameter hole was drilled in the pre-indented gasket, then the wall of the hole and indentation were fully coated with c-BN and the other gasket surface was covered with epoxy (AB gel).After that, four platinum electrodes (25 μm in thickness) were arranged on one side of the gasket to connect the copper leads and the sample in the chamber.There was no pressure transmitting medium inside the chamber.The resistance measurements were carried out on the as-fabricated microcircuit using electronic equipments (Keithley 6221 current source, 2182A nanovoltmeter, and 7001 switch system), and the pressure was determined by the ruby fluorescence.The resistivity was calculated according to the Van der Pauw method[20].

    The first-principle calculations of the electronic structure for hexagonal TaSi2 under pressures of 0 and 15 GPa were performed using the CASTEP code[21] in the Materials Studio package with the geometry optimization.The convergence tolerance in the geometry optimization was 2.0×10-5 eV/atom.The optimization was completed when the forces were less than 0.5 eV/nm and all stress components were less than 0.1 GPa.At each pressure, a generalized gradient approximation of the Perdew-Burke-Ernzerhof (GGA-PBE) version functional of the exchange-correlation was adopted to optimize the lattice parameters for hexagonal C40 which takes the actual situation such as electron correlations into consideration.

    Here, we studied a hexagonal structure of TaSi2 sample.The X-ray diffraction patterns of the sample under compression and decompression are shown in Fig. 1(b).Upon compression to 20.1 GPa all the Bragg peaks shifted towards high angles, revealing the shrinkage of the TaSi2 unit cell.There was no significant variation in the XRD patterns in the number and shape of the diffraction peaks.The Bragg peaks reverted to their previous positions after decompression to ambient pressure.Fig. 1(c) shows a typical GSAS refinement of the XRD pattern at 1.0 GPa.The detailed crystallographic information for low and high-pressure phases of TaSi2 was shown in Table 1.

    Figure  1.  (a) Crystal structure of TaSi2 in ambient conditions; (b) Synchrotron XRD patterns of TaSi2 during compression and decompression; (c) Refinement of TaSi2 XRD data at 1.0 GPa
    Table  1.  Rietveld refinement results of TaSi2 under low pressure and high pressure
    Pressure/GPa Atom type Fractional coordinates
    1 Ta (0.5, 0, 0)
    1 Si (0.16 148 66, 0.32 296 3, 0)
    20 Ta (0.5, 0.32 296 30, 0)
    20 Si (0.17 069 90, 0.34 138 9, 0)
     | Show Table
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    The good refinements confirm that the hexagonal structure of TaSi2 is very stable without phase transitions during compression up to 20 GPa and decompression to ambient conditions.These results are consistent with a previous study[11].From the refinements of all the diffraction patterns, the compressive behavior of the TaSi2 lattice cell can be obtained.Fig. 2(a) shows the pressure dependence of the TaSi2 lattice parameters and volume.We found that the a and c lattice parameters and the unit cell volume V decreased with increasing pressure.The normalized lattice parameters as a function of pressure shown in Fig. 2(b) reveal that the a axis was more compressible than the c axis and anisotropic compression increased with increasing pressure.This anisotropy of TaSi2 under compression can be attributed to the relatively compact stacking along the 〈0001〉 direction[11](Fig. 1(a)).Through third-order Birch-Murnaghan equation of state (EOS) fitting we can get the bulk modulus of hexagonal phase of TaSi2 as 203(2) GPa, as shown in Fig. 2(c).In addition, the bulk modulus of TaSi2 is too big to be easily compressed.

    Figure  2.  (a) Pressure-dependent lattice parameters of TaSi2(a0=0.478 4 nm, c0=0.657 0 nm); (b) Evolution of the normalized lattice parameters and volume with pressure for TaSi2; (c) Pressure-dependent unit cell volume of TaSi2

    Highly sensitive Raman spectroscopy was employed to obtain local structure information.Fig. 3(a) shows the Raman spectra of the sample collected during pressurization from ambient to high pressure and release to ambient pressure.From the Raman spectrum at ambient pressure, four Raman peaks can be observed at the centered frequencies of 145.0, 290.0, 355.0 and 503.8 cm-1, denoted as A1, A2, A3 and A4, respectively.The peak A4 could be related to the Si─Si vibration mode.During the compression-decompression cycle, these Raman modes can almost be clearly observed.Upon decompression to ambient pressure, the Raman modes A1, A2 and A3 were found to revert to their original frequencies, but the Si─Si vibration mode A4 was hysteretic.The vibrational frequencies of two obvious Raman peaks (A3 and A4) as a function of pressure are shown in Fig. 3(b) and exhibit blue shift under compression due to the shortening of the Si─Ta─Si and Si─Si bond.These findings indicate the local structural stability of the TaSi2 particles under pressure, which corresponds to the structural stability analysis from XRD results.

    Figure  3.  (a) Pressure-dependent Raman spectra of TaSi2 at room temperature; (b) Pressure-dependent Raman peaks (A3 and A4) of TaSi2 derived from the Raman spectra

    In general, decreasing the distance between the atoms and the interlayers of a crystal material under external pressure or stress can alter its electronic behavior[22].Sometimes, this effect may adversely affect the stability of electronic devices under stress.Here, the electronic transport properties of TaSi2 under different pressures are expressed by the electrical resistance, which was measured by a four-probe method using a fabricated microcircuit in a DAC (Fig. 4 inset).As shown in Fig. 4, the electrical resistance of the sample decreased dramatically with pressure increasing up to 5 GPa due to the gradually closer contact between the TaSi2 particles.In the high-pressure region (5.0-16.3 GPa), the resistance trend was steady with increasing pressure and the resistance reduced by less than half.The resistance during decompression and compression was almost consistent in the high-pressure region.Therefore, the electrical transport properties of TaSi2 are very stable in the high-pressure region during compression and decompression.

    Figure  4.  The resistivity of TaSi2 under pressure at room temperature (The inset (upper right) is a photograph of the four-probe microcircuit in the diamond anvil cell.)

    A previous study reported that the resistivity of a TaSi2 single crystal at ambient pressure and temperature was approximately 20 μΩ·cm along the 〈0001〉 crystallographic direction and 40 μΩ·cm along the 101¯0 direction[23].Hence, TaSi2 exhibits metallic behavior.In our case, the pressure-dependent resistivity of the sample consisted of many particles, as shown in the Fig. 4 inset.Thus, the resistivity under pressures of 5.0 and 16.3 GPa was about 2.8 and 1.7 μΩ·cm, respectively.That is to say, the resistivity of TaSi2 decreased by one order of magnitude at room temperature from ambient pressure to 5.0 GPa.The metallicity of TaSi2 obviously increased with the increase of applied pressure.Therefore, it is conceivable that electronic devices made of TaSi2 may work well under pressure and release less waste heat.

    To further understand TaSi2 electrical stability and illuminate the underlying physical mechanism of its resistivity-pressure relationship, its band structures under ambient pressure and high pressure were calculated by first-principle calculations.Fig. 5 shows the electronic band structure of TaSi2 at ambient pressure and 15 GPa.Their topological geometries are considerably similar, i.e., the electronic energy band structure is very stable under high pressure.The difference is that the electronic energy band broadens under high pressure compared to that under ambient pressure, which is due to shortening of the lattice parameters. Moreover, the Fermi surface of TaSi2 under ambient pressure and 15 GPa locates below the top of the valence band and the valence-band maximum crosses the conduction-band minimum, i.e., the band gap disappears.This demonstrates that TaSi2 shows metallic behavior, which can contribute to the low-resistivity of TaSi2 under ambient and high pressure.

    Figure  5.  Calculated band structure of TaSi2 at (a) 0 GPa and (b) 15 GPa

    In summary, we studied the crystallographic structural stability and electrical transport properties of metallic TaSi2 under high pressure using angle-dispersive synchrotron XRD, Raman spectroscopy, and four-probe resistance measurements as well as first-principle calculations.The in situ high-pressure XRD and Raman characterizations demonstrated that the structure was stable up to 20 GPa, consistent with a previously reported result.The resistivity of TaSi2 was steady at ~2 μΩ·cm under pressures from ambient pressure to 16.3 GPa.First-principle calculations showed that the topological geometries of the TaSi2 electronic structure under ambient and high pressure were similar and their valence-band maximums were located over the Fermi surface, which was responsible for its electrical stability and metallic behavior.

  • Figure  1.  (a) Crystal structure of TaSi2 in ambient conditions; (b) Synchrotron XRD patterns of TaSi2 during compression and decompression; (c) Refinement of TaSi2 XRD data at 1.0 GPa

    Figure  2.  (a) Pressure-dependent lattice parameters of TaSi2(a0=0.478 4 nm, c0=0.657 0 nm); (b) Evolution of the normalized lattice parameters and volume with pressure for TaSi2; (c) Pressure-dependent unit cell volume of TaSi2

    Figure  3.  (a) Pressure-dependent Raman spectra of TaSi2 at room temperature; (b) Pressure-dependent Raman peaks (A3 and A4) of TaSi2 derived from the Raman spectra

    Figure  4.  The resistivity of TaSi2 under pressure at room temperature (The inset (upper right) is a photograph of the four-probe microcircuit in the diamond anvil cell.)

    Figure  5.  Calculated band structure of TaSi2 at (a) 0 GPa and (b) 15 GPa

    Table  1.   Rietveld refinement results of TaSi2 under low pressure and high pressure

    Pressure/GPa Atom type Fractional coordinates
    1 Ta (0.5, 0, 0)
    1 Si (0.16 148 66, 0.32 296 3, 0)
    20 Ta (0.5, 0.32 296 30, 0)
    20 Si (0.17 069 90, 0.34 138 9, 0)
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  • 收稿日期:  2017-04-26
  • 修回日期:  2017-05-03

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