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Carbon ions irradiation induced modifications in structural and electrical resistivity characteristics of ZrN thin films
Materials Science in Semiconductor Processing 39 (2015) 530–535
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Carbon ions irradiation induced modifications in structural and electrical resistivity characteristics of ZrN thin films Shakil Khan a,n, Ishaq Ahmed b,d,e, Noaman Khalid b, Mazhar Mehmood a, Abdul Waheed c, Maaza Malik d,e a
Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan National Centre for Physics (NCP), Islamabad, Pakistan Department of Physics, Islamia College University, Peshawar, Pakistan d UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, P.O. Box 392, Pretoria, South Africa e Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, P.O. Box 722, Western Cape Province, South Africa b c
a r t i c l e in f o
Keywords: Carbon ions irradiation ZrN Cathodic arc ion XRD Electrical resistivity
abstract Zirconium nitride (ZrN) thin films are irradiated with 800 keV energetic carbon (C) ions in a 5UDH-Pelletron accelerator and the ions irradiation induced effects are investigated. The films are irradiated at various C ions fluences, ranging from 1013 to 1015 ions/cm2. The scanning electron microscopy study of the films indicates the development of zirconium (Zr) nanoparticles at ions irradiated region. X-ray diffraction (XRD) patterns of C ions irradiated films also show the formation of (100) and (002) oriented nanocrystalline metallic Zr phases. The irradiated films spectra depict a shift in ZrN peaks towards higher 2θ values, exhibiting that C ions bombardment induces compressive stress in the irradiated films. The appearance of C related peaks in Fourier transform infrared (FTIR) spectra confirms the incorporation of C atoms into ZrN film. Compressive stress has been calculated from the IR peak shift which indicates that higher ion dose ( Z 5 1014 ions/ cm2) produce lower compressive stress relative to the lower ions fluences. Effect of ion dose on the film resistivity is also reported. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction The nitride coatings of transition metals on cutting tools are known for their wear resistance and as a corrosion resistant layer on mechanical components. Among transition metal nitrides, zirconium nitride (ZrN) is an attractive thin film material due to its good thermal stability, higher hardness and lower electrical resistivity [1]. Comparing with other nitrides such as TiN, it has higher negative free energy that consequently leads to n
Corresponding author. E-mail address: [email protected] (S. Khan).
http://dx.doi.org/10.1016/j.mssp.2015.05.062 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
better tribological properties i.e., corrosion and wear resistance [2]. Investigations of the non-equilibrium treatments such as the ion irradiation are important for ZrN, since it is one of the materials that are considered as an inert matrix. Ion irradiation induces a wide variety of structural changes, such as the formation of defects, lattice parameters modification, and alteration in stoichiometry or amorphization [3]. Radiation resistance of ZrN film has been investigated for different kinds of irradiation ions such as Ar, Xe, Kr and Bi ions [4–5]. The irradiation of these ions has proven as reported in the literature that ions irradiation alter its microstructure. The addition of impurity atoms, also affect other characteristics such as electrical
S. Khan et al. / Materials Science in Semiconductor Processing 39 (2015) 530–535
531
and optical properties [4,5]. However Ar, Xe or Kr ions have very high mass numbers; it seems desirable to see the irradiation effects with the bombardment of lighter elements. In the literature, alteration in microstructure of boron nitride by carbon ions (C) irradiation was reported [6], therefore, changes in the microstructure of ZrN due to carbon ions irradiation is also expected. In this study, C ions irradiation induced modifications in the microstructure and electrical resistivity of ZrN thin film have been investigated. The films are irradiated in a 5UDH-Pelletron accelerator by 800 keV energetic C ions at different ions fluences. Investigations of the ions fluence effect on ZrN thin film microstructure are conducted by means of X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Effect of ions dose, on the film resistivity, has been also studied.
initially accelerated to smaller energy 60–70 keV, in a short horizontal section followed by gaining 800 keV energy in the accelerating tubes. The samples were mounted on a high precision (0.011) five-axis goniometer in the chamber, for controlling their orientation relative to the C ion beam. Rutherford backscattering spectroscopy (RBS) analysis was employed to acquire information of the as grown film about film thickness and composition. The surface morphology, crystalline phases and bond information of the films, before and after ions irradiation was studied by scanning electron microscopy, XRD analysis at grazing angle of incidence and FTIR spectroscopy, respectively. Resistivity measurement was performed by means of Four-point probe method.
2. Experimental
3.1. Rutherford backscattering spectroscopy
A commercial cathodic vacuum arc ion (CVA) system equipped with dual cathodes was used for ZrN thin film growth. The details of the synthesis chamber and fabrication method are reported elsewhere [7]. The as-grown ZrN films were irradiated with C ions beam in a 5UDHPelletron accelerator. Graphite cathode in SINCS (source of negative ions by cesium sputtering) as C ion source was employed. The energy of the ions beam was kept at 800 keV and the ions fluence was varied as listed in Table 1. There was 10 4 Pa pressure in the irradiation chamber during the irradiation process. The irradiation was performed at room temperature and irradiation current was maintained at 150 nA. Schematic diagram of the ion irradiation scheme is shown in Fig. 1. In the accelerated chamber, the ion source system is followed by high voltage accelerating terminal inside the tank and the terminal is connected to the tank through ceramic titanium tubes called accelerating tubes. A potential gradient is maintained through these tubes from high voltage to ground, from right of the tank to the terminal as well as from the terminal to left side of the tank. The C ions were generated from ion source and were injected into the accelerating tube by injector. They were
Thickness and stoichiometric measurement of the asgrown film was performed via RBS. The theoretical simulation of the experimental data was performed using SIMNRA software. Fig. 2 shows the RBS experimental and simulated spectra for as deposited specimen. The simulation agrees well with the experimental data. Signals of other elements like sodium (Na), calcium (Ca), magnesium (Mg), oxygen (O) and silicon (Si) appeared in the spectrum originate from the glass substrate, while the signal for nitrogen arises due to the incorporated nitrogen atoms in the deposited film. The bands of zirconium (Zr) in the ZrN film and underlying zirconium layer are clearly distinct, which confirms the formation of well-defined and smooth layer of zirconium at the interface of substrate and zirconium nitride film. The estimated thicknesses of Zr and ZrN layers are 120 nm and 282 nm, respectively. The nitride layer demonstrates the stoichiometric ratio (N/Zr) of 0.8518.
Table 1 Carbon ions irradiation conditions. Sample description
S1 2
Fluence rate (ions/cm )
1 10
S2 13
5 10
S3 13
5 10
S4 14
1 1015
3. Results and discussion
3.2. SEM analysis Fig. 3(a) and (b) shows the irradiated and pristine portions of the films bombarded at a fluence of 1 1013 and 5 1014 ions/cm2, respectively. It demonstrates a clear change in the appearance with irradiation as well as with different ions dose. Apparently, the film roughness increases after ion bombardment giving rise to relatively darker appearance of the irradiated circular portion. The irradiated region contains nanoparticles, which are
Ions production
sample Collimator
Fig. 1. Schematic diagram of the experimental setup.
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S. Khan et al. / Materials Science in Semiconductor Processing 39 (2015) 530–535
generated due to the ions irradiation. The EDS analysis of the film irradiated at 5 1014 ions/cm2 was performed and is shown in Fig. 3(c). It exhibits the peak of Zr, N, C, O, Au, Ca and Si elements. The Ca, O and Si peaks arise from glass substrate, while Au peak originates from gold coating on the film (to avoid charging effect). Any clear difference in the composition was not observed for the pristine and irradiated portion, though it is known that the concentration of lighter elements cannot be determine by EDS with acceptable accuracy.
3.3. XRD study XRD patterns of the as-grown and irradiated ZrN films are shown in Fig. 4(a). X'pert high score analysis software was used for XRD result analysis (peak shifting, FWHM and grain size). The pristine spectrum as shown in X'pert Energy [keV] 400
600
800
1000
1200
1400
1600
1800
900
1,000
1,200
Counts
900 600 300 0
200
300
400
500
600
700
800
Channel Fig. 2. Simulated and experimental RBS spectra of the pristine ZrN film.
high score software image of Fig. 4(b) exhibits (111), (200) and (220) peaks of ZrN with cubic F lattice (Reference code: 01-074-1217). An additional peak at 2θ ¼36.41 may be assigned to hcp (101) Zr from the underlying Zr layer. Irradiated ZrN films spectra depict a shift in the ZrN phase peaks toward higher 2-theta (2θ) values as shown in Fig. 4 (c). Consider the (111) peak in the as grown film spectrum, which appears at 2θ ¼33.5761 (depicted in Fig. 4(b)) and is near to the bulk value 2θ ¼33.4961 (Reference code: 01-074-1217). After ions irradiation, it shifts to 33.7011 and 33.7171 (Fig. 4(c)) at fluence rates of 1 1013 and 5 1013, respectively. However, with further increase in fluence to 5 1014 and 1 1015 ions/cm2, the peaks appear at 33.6681 and 33.6841 (Fig.4(d)), respectively exhibiting that the shift is relatively smaller with respect to the lower ions dose. Additional peaks at 2θ ¼31.921 and 34.741, also appear after ion irradiation and correspond to the (100) and (002) peaks of Zr phase, respectively. Appearance of additional peaks after irradiation, suggest an improvement in crystallinity of the underlying zirconium, its recrystallization or the formation of new crystallites of zirconium in the ZrN layer due to irradiation effects. Ion irradiation has also affected the peak intensities and FWHM as listed in Table 2. For example, the (111) peak intensity and FWHM of pristine ZrN is 52 cts and 0.8411, respectively. After irradiation, the intensity increases (an average value of 68.5 cts). The FWHM decreases to 0.7871, 0.6301, 0.6181 and 0.5891 at a fluence rate of 1 1013, 5 1013, 5 1014 and 1 1015 ions/cm2, respectively. Using Scherrer's formula, the corresponding crystallite sizes are 98, 105, 132, 134 and 141 nm, in sequence. The shift of the (111) peak towards higher angle may be associated with the compressive residual stresses [8,9] generated during ions irradiation. The compressive stress
Fig. 3. SEM images of ZrN film demonstrating the pristine and irradiated portion at fluence (a) 1 1013 ions/cm2 (b) 5 1014 ions/cm2 and (c) typical EDS spectrum.
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Table 2 XRD results of (111) orientation. Sample 2 theta Peak description position (deg)
Peak intensity (cts)
FWHM (deg)
Grain size (nm)
Pristine S1 S2 S3 S4
52 73 66 72 63
0.841 0.787 0.630 0.618 0.551
98 105 132 134 141
33.576 33.701 33.717 33.684 33.596
15
1X10
2852 cm
14
Intensity (arb units)
1041 cm
1740 cm
-1
-1 2923 cm
5X10
-1
-1
13
5X10
13
1X10
Pristine
500
1000
1500
2000
2500
3000
Wavenumber (cm-1) Fig. 5. Effect of the ions fluence on FTIR spectra.
Fig. 4. (a) Effect of different carbon ions fluence on the XRD patterns of ZrN thin film, X'pert high score analysis software images exhibiting the (111) orientation peak position of (b) pristine film (c) irradiated at 5 1013 and (d) irradiated at 1 1015 (ions/cm2).
develops due to lattice contraction i.e, decrease of lattice parameters. The compressive stress seems higher at lower dose. Ison et al. reported similar trend of stress development in the irradiated CdS thin films case [10]. Apart from delivering energy to the lattice, energetic collisions in the cascade region are expected to have very prominent effect, as a result of which the position of (111) peak shifts to higher values. This may partly be associated with desorption of nitrogen (re-sputtering), incorporation of carbon atoms, change in stress state and or increase in grain size. The increase in peak intensity and grain size can be related to the local temperature rise as proposed in thermal spike model. The local rise in temperature (annealing) for a very short span of time increases the total lattice energy. This rise in energy induces crystallization in the material that consequently leads to rise of peaks intensities, larger grain sizes and reduction in defects [11,12]. The appearance of Zr (100) and (002) peaks demonstrate that the crystallization of Zr phases in the irradiated films occur as well. The smaller peaks shift to higher 2θ position above 5 1013 ions/cm2 fluence, could be related to the overlapping of effected zones. The local rise in temperature is
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5 1013, 5 1014 and 1 1015 ions/cm2, respectively, the estimated residual stress in the irradiated films relative to the as grown film is therefore, 2.75, 2.37, 1.75, and 1.37 GPa in sequence. The lower stress value at higher ions fluence indicated that during the film bombardment, the defect produces by the irradiation recover as well. The higher ions irradiation dose relieved some residual stress as already described in XRD analysis. 3.5. Electrical resistivity
Fig. 6. Influence of the ions fluences on film resistivity.
higher at greater dose that minimizes the defects produces by the ions tracks. In totality, the energy imparted by the incident ions due to irradiation causes an improvement in the film crystallinity as is evident from the sharpening of XRD peaks. 3.4. FTIR spectroscopic analysis FTIR spectroscopy was employed to yield information about the incorporation or existence of incident C atoms chemical bonds with species of the pristine ZrN film. The FTIR spectra of pristine and irradiated films are shown in Fig. 5. The as-grown films spectrum indicates a broad band centered at 689 cm 1 corresponding to stretching vibrations of Zr–N simple bonds [7,13]. The spectrum also depicts a band for ZrO2 at 1040 cm 1, which appears due to thin oxide layer formed on ZrN surface. Oxide layer is formed due to larger heat of formation of ZrO2 ( 261.5 kcal mol 1) as compared to ZrN ( 87.3 kcal mol 1) [14]. After ions irradiation, the main peak is shifted to 711, 708, 703 and 700 cm 1 at fluences 1 1013, 5 1013, 5 1014 and 1 1015 ions/cm2, respectively. In the IR patterns of irradiated films, the 1740 cm 1 and 1585 cm 1 peaks originate from stretching vibration of carbonyl group (C ¼O) and carbon, respectively [15]. The peaks present at 2000–2300 cm 1 could be assigned to the stretching vibration of alkynes (C triple C) or cyanic group (C triple bond N). The observation of the peaks at 2800–2900 cm 1 reflects the stretching vibration of saturated hydrocarbon (C–H) [16]. After ions irradiation, main IR peak sharpened which exhibit that film crystallinity improves and is in agreement with XRD result. The shifting of main peak from 689 cm 1 to about 711 cm 1 at 1 1013 ions/cm2 fluence could be related to compressive stress [17]. A shift of the XRD peaks towards higher angles has already been observed in the XRD spectra. The amount of stress relative to the as grown film can be calculated from the IR peak shift assuming an 8 cm 1 shift corresponds to 1 GPa stress [17]. The observed peak positions in the spectra are 711, 708, 703 and 700 cm 1 at fluences of 1 1013,
The electrical resistivity (measured by four-probe point method) of the pristine and irradiated films is shown in Fig. 6. The as-deposited film exhibits 92.2 mΩ-cm, which decreases to 84.4 mΩ-cm for a film irradiated at a fluence of 1 1013 ions/cm2. It drops more to 77.4 mΩ-cm with further increase of the ions fluence to 5 1013 ions/cm2. As C irradiation increases further to Z5 1014 ions/cm2, the resistivity rises again. The resistivity variation is related with creation of nitrogen vacancies, generation of point defects, self-interstitial, and C impurity atoms (particularly in the cascade regions) [18]. After ions irradiation, the formation of metallic Zr nanocrystalline particles has been observed that may greatly affect the carrier concentration. Therefore, the initial decrease in resistivity may be attributed to the rise of carrier concentration that overcomes the influence of defects. However, at higher ions fluences, the rise of C ions (impurity atoms) incorporation into ZrN matrix may reduce the carrier concentration causing a rise in film resistivity. 4. Conclusions The 800 keV energetic carbon (C) ion irradiation has proven to affect the microstructure as well as the film resistivity. XRD analysis shows that ZrN (111) peak shifts to higher angles after C ion irradiation indicating the development of compressive stress. It was also noticed that the incident ions preferentially sputter the lighter N atoms from ZrN phase thereby forming (100) and (002) oriented metallic Zr nanoparticles. The SEM analysis as well demonstrated the formation of Zr nanoparticles after ions irradiation. Compressive stress has been calculated from the FTIR peak shift and it was observed that higher ion doses (Z5 1014 ions/cm2) produces lower compressive stress. Ion irradiation at r5 1013 ions/cm2 fluence, decreases the film resistivity from 92.2 mΩ-cm to 77.4 mΩ-cm. However, with further increase in C ions fluence (Z5 1014 ions/cm2), it rises again to higher values.
References [1] D. Wu, Z. Zhang., W. Fu, X. Fan, H. Guo, Appl. Phys. A 64 (1997) 593–595. [2] O. Knacke, O. Kubaschewski. K. Hesselmann, Thermochemical Properties of Inorganic Substance, 2nd edition, Springer-Verlag, Berlin, 1991, pp. 2103–2409. [3] M. Popović, M. Novaković, M. Mitrić, K. Zhang, N. Bibić, Int. J. Refract. Met. Hard Mater. 48 (2015) 318–323.
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[4] A. Janse van Vuuren, V.A. Skuratov, V.V. Uglov, J.H. Neethling, S.V. Zlotski, J. Nucl. Mater. 442 (2013) 507–511. [5] S. Camelio, T. Girardeau, L. Pichon, A. Straboni, C. Fayoux, P.h. Guérin, J. Opt. A: Pure Appl. Opt. 2 (2000) 442–448. [6] Ishaq Ahmad, Usman M., S. Rabab Naqvi, Javed Iqbal, L.U. Bo, Long Yan, C.F. Dee, Aslam Baig, J. Nanopart. Res. 16 (2014) 2170–2178. [7] Shakil Khan, Mazhar Mehmood, Ishaq Ahmad, Farhat Ali, A. Shah, Mater. Sci. Semicond. Process. 30 (2015) 486–493. [8] B. Bhattacharya, M.J. Carter, Thin Solid Films 288 (1996) 176. [9] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Palo Alto, 1978. [10] V.V. Ison, A. Ranga Rao, V. Dutta, D.K. Avasthi, Nucl. Instrum. Methods Phys. Res. B 262 (2007) 209–214.
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[11] M. Toulemonde, C. Dufour, Phys. Rev. B 42 (1992) 14362. [12] Sanju Rani, N.K. Puri, Somnath C. Roy, M.C. Bhatnagar, D. Kanjilal, Nucl. Instrum. Methods Phys. Res. B 266 (2008) 1987–1992. [13] J.C. Caicedo, A.G. Bejarano, G.M.E. Gomez, P. Prieto, C. Cortez, J. Munoz, Phys. Status Solidi (c) 11 (2007) 4127–4133. [14] G.L.N. Reddy, J.V. Ramana, Sanjiv Kumar, S. Vikram Kumar, V.S. Raju, Appl. Surf. Sci. 253 (2007) 7230–7237. [15] Chiu Hsin-Tien, Cheng-Chung Huang, Mater. Lett. 16 (1993) 194–199. [16] Dan Zhao, Haifeng Hu, Changrui Zhang, Yudi Zhang, Jun Wang, J. Mater. Sci. 45 (2010) 6401–6405. [17] J.H. Edgar, C.A. Carosella, C.R. Eddy Jr., D.T. Smith, J. Mater. Sci.: Mater. Electron. 7 (1996) 247. [18] M. Ohring, The Materials Science of Thin Films, first ed. Academic Press, San Diego, 1992, 461.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Carbon ions irradiation induced modifications in structural and electrical resistivity characteristics of ZrN thin films Shakil Khan a,n, Ishaq Ahmed b,d,e, Noaman Khalid b, Mazhar Mehmood a, Abdul Waheed c, Maaza Malik d,e a
Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan National Centre for Physics (NCP), Islamabad, Pakistan Department of Physics, Islamia College University, Peshawar, Pakistan d UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, P.O. Box 392, Pretoria, South Africa e Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, P.O. Box 722, Western Cape Province, South Africa b c
a r t i c l e in f o
Keywords: Carbon ions irradiation ZrN Cathodic arc ion XRD Electrical resistivity
abstract Zirconium nitride (ZrN) thin films are irradiated with 800 keV energetic carbon (C) ions in a 5UDH-Pelletron accelerator and the ions irradiation induced effects are investigated. The films are irradiated at various C ions fluences, ranging from 1013 to 1015 ions/cm2. The scanning electron microscopy study of the films indicates the development of zirconium (Zr) nanoparticles at ions irradiated region. X-ray diffraction (XRD) patterns of C ions irradiated films also show the formation of (100) and (002) oriented nanocrystalline metallic Zr phases. The irradiated films spectra depict a shift in ZrN peaks towards higher 2θ values, exhibiting that C ions bombardment induces compressive stress in the irradiated films. The appearance of C related peaks in Fourier transform infrared (FTIR) spectra confirms the incorporation of C atoms into ZrN film. Compressive stress has been calculated from the IR peak shift which indicates that higher ion dose ( Z 5 1014 ions/ cm2) produce lower compressive stress relative to the lower ions fluences. Effect of ion dose on the film resistivity is also reported. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction The nitride coatings of transition metals on cutting tools are known for their wear resistance and as a corrosion resistant layer on mechanical components. Among transition metal nitrides, zirconium nitride (ZrN) is an attractive thin film material due to its good thermal stability, higher hardness and lower electrical resistivity [1]. Comparing with other nitrides such as TiN, it has higher negative free energy that consequently leads to n
Corresponding author. E-mail address: [email protected] (S. Khan).
http://dx.doi.org/10.1016/j.mssp.2015.05.062 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
better tribological properties i.e., corrosion and wear resistance [2]. Investigations of the non-equilibrium treatments such as the ion irradiation are important for ZrN, since it is one of the materials that are considered as an inert matrix. Ion irradiation induces a wide variety of structural changes, such as the formation of defects, lattice parameters modification, and alteration in stoichiometry or amorphization [3]. Radiation resistance of ZrN film has been investigated for different kinds of irradiation ions such as Ar, Xe, Kr and Bi ions [4–5]. The irradiation of these ions has proven as reported in the literature that ions irradiation alter its microstructure. The addition of impurity atoms, also affect other characteristics such as electrical
S. Khan et al. / Materials Science in Semiconductor Processing 39 (2015) 530–535
531
and optical properties [4,5]. However Ar, Xe or Kr ions have very high mass numbers; it seems desirable to see the irradiation effects with the bombardment of lighter elements. In the literature, alteration in microstructure of boron nitride by carbon ions (C) irradiation was reported [6], therefore, changes in the microstructure of ZrN due to carbon ions irradiation is also expected. In this study, C ions irradiation induced modifications in the microstructure and electrical resistivity of ZrN thin film have been investigated. The films are irradiated in a 5UDH-Pelletron accelerator by 800 keV energetic C ions at different ions fluences. Investigations of the ions fluence effect on ZrN thin film microstructure are conducted by means of X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Effect of ions dose, on the film resistivity, has been also studied.
initially accelerated to smaller energy 60–70 keV, in a short horizontal section followed by gaining 800 keV energy in the accelerating tubes. The samples were mounted on a high precision (0.011) five-axis goniometer in the chamber, for controlling their orientation relative to the C ion beam. Rutherford backscattering spectroscopy (RBS) analysis was employed to acquire information of the as grown film about film thickness and composition. The surface morphology, crystalline phases and bond information of the films, before and after ions irradiation was studied by scanning electron microscopy, XRD analysis at grazing angle of incidence and FTIR spectroscopy, respectively. Resistivity measurement was performed by means of Four-point probe method.
2. Experimental
3.1. Rutherford backscattering spectroscopy
A commercial cathodic vacuum arc ion (CVA) system equipped with dual cathodes was used for ZrN thin film growth. The details of the synthesis chamber and fabrication method are reported elsewhere [7]. The as-grown ZrN films were irradiated with C ions beam in a 5UDHPelletron accelerator. Graphite cathode in SINCS (source of negative ions by cesium sputtering) as C ion source was employed. The energy of the ions beam was kept at 800 keV and the ions fluence was varied as listed in Table 1. There was 10 4 Pa pressure in the irradiation chamber during the irradiation process. The irradiation was performed at room temperature and irradiation current was maintained at 150 nA. Schematic diagram of the ion irradiation scheme is shown in Fig. 1. In the accelerated chamber, the ion source system is followed by high voltage accelerating terminal inside the tank and the terminal is connected to the tank through ceramic titanium tubes called accelerating tubes. A potential gradient is maintained through these tubes from high voltage to ground, from right of the tank to the terminal as well as from the terminal to left side of the tank. The C ions were generated from ion source and were injected into the accelerating tube by injector. They were
Thickness and stoichiometric measurement of the asgrown film was performed via RBS. The theoretical simulation of the experimental data was performed using SIMNRA software. Fig. 2 shows the RBS experimental and simulated spectra for as deposited specimen. The simulation agrees well with the experimental data. Signals of other elements like sodium (Na), calcium (Ca), magnesium (Mg), oxygen (O) and silicon (Si) appeared in the spectrum originate from the glass substrate, while the signal for nitrogen arises due to the incorporated nitrogen atoms in the deposited film. The bands of zirconium (Zr) in the ZrN film and underlying zirconium layer are clearly distinct, which confirms the formation of well-defined and smooth layer of zirconium at the interface of substrate and zirconium nitride film. The estimated thicknesses of Zr and ZrN layers are 120 nm and 282 nm, respectively. The nitride layer demonstrates the stoichiometric ratio (N/Zr) of 0.8518.
Table 1 Carbon ions irradiation conditions. Sample description
S1 2
Fluence rate (ions/cm )
1 10
S2 13
5 10
S3 13
5 10
S4 14
1 1015
3. Results and discussion
3.2. SEM analysis Fig. 3(a) and (b) shows the irradiated and pristine portions of the films bombarded at a fluence of 1 1013 and 5 1014 ions/cm2, respectively. It demonstrates a clear change in the appearance with irradiation as well as with different ions dose. Apparently, the film roughness increases after ion bombardment giving rise to relatively darker appearance of the irradiated circular portion. The irradiated region contains nanoparticles, which are
Ions production
sample Collimator
Fig. 1. Schematic diagram of the experimental setup.
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S. Khan et al. / Materials Science in Semiconductor Processing 39 (2015) 530–535
generated due to the ions irradiation. The EDS analysis of the film irradiated at 5 1014 ions/cm2 was performed and is shown in Fig. 3(c). It exhibits the peak of Zr, N, C, O, Au, Ca and Si elements. The Ca, O and Si peaks arise from glass substrate, while Au peak originates from gold coating on the film (to avoid charging effect). Any clear difference in the composition was not observed for the pristine and irradiated portion, though it is known that the concentration of lighter elements cannot be determine by EDS with acceptable accuracy.
3.3. XRD study XRD patterns of the as-grown and irradiated ZrN films are shown in Fig. 4(a). X'pert high score analysis software was used for XRD result analysis (peak shifting, FWHM and grain size). The pristine spectrum as shown in X'pert Energy [keV] 400
600
800
1000
1200
1400
1600
1800
900
1,000
1,200
Counts
900 600 300 0
200
300
400
500
600
700
800
Channel Fig. 2. Simulated and experimental RBS spectra of the pristine ZrN film.
high score software image of Fig. 4(b) exhibits (111), (200) and (220) peaks of ZrN with cubic F lattice (Reference code: 01-074-1217). An additional peak at 2θ ¼36.41 may be assigned to hcp (101) Zr from the underlying Zr layer. Irradiated ZrN films spectra depict a shift in the ZrN phase peaks toward higher 2-theta (2θ) values as shown in Fig. 4 (c). Consider the (111) peak in the as grown film spectrum, which appears at 2θ ¼33.5761 (depicted in Fig. 4(b)) and is near to the bulk value 2θ ¼33.4961 (Reference code: 01-074-1217). After ions irradiation, it shifts to 33.7011 and 33.7171 (Fig. 4(c)) at fluence rates of 1 1013 and 5 1013, respectively. However, with further increase in fluence to 5 1014 and 1 1015 ions/cm2, the peaks appear at 33.6681 and 33.6841 (Fig.4(d)), respectively exhibiting that the shift is relatively smaller with respect to the lower ions dose. Additional peaks at 2θ ¼31.921 and 34.741, also appear after ion irradiation and correspond to the (100) and (002) peaks of Zr phase, respectively. Appearance of additional peaks after irradiation, suggest an improvement in crystallinity of the underlying zirconium, its recrystallization or the formation of new crystallites of zirconium in the ZrN layer due to irradiation effects. Ion irradiation has also affected the peak intensities and FWHM as listed in Table 2. For example, the (111) peak intensity and FWHM of pristine ZrN is 52 cts and 0.8411, respectively. After irradiation, the intensity increases (an average value of 68.5 cts). The FWHM decreases to 0.7871, 0.6301, 0.6181 and 0.5891 at a fluence rate of 1 1013, 5 1013, 5 1014 and 1 1015 ions/cm2, respectively. Using Scherrer's formula, the corresponding crystallite sizes are 98, 105, 132, 134 and 141 nm, in sequence. The shift of the (111) peak towards higher angle may be associated with the compressive residual stresses [8,9] generated during ions irradiation. The compressive stress
Fig. 3. SEM images of ZrN film demonstrating the pristine and irradiated portion at fluence (a) 1 1013 ions/cm2 (b) 5 1014 ions/cm2 and (c) typical EDS spectrum.
S. Khan et al. / Materials Science in Semiconductor Processing 39 (2015) 530–535
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Table 2 XRD results of (111) orientation. Sample 2 theta Peak description position (deg)
Peak intensity (cts)
FWHM (deg)
Grain size (nm)
Pristine S1 S2 S3 S4
52 73 66 72 63
0.841 0.787 0.630 0.618 0.551
98 105 132 134 141
33.576 33.701 33.717 33.684 33.596
15
1X10
2852 cm
14
Intensity (arb units)
1041 cm
1740 cm
-1
-1 2923 cm
5X10
-1
-1
13
5X10
13
1X10
Pristine
500
1000
1500
2000
2500
3000
Wavenumber (cm-1) Fig. 5. Effect of the ions fluence on FTIR spectra.
Fig. 4. (a) Effect of different carbon ions fluence on the XRD patterns of ZrN thin film, X'pert high score analysis software images exhibiting the (111) orientation peak position of (b) pristine film (c) irradiated at 5 1013 and (d) irradiated at 1 1015 (ions/cm2).
develops due to lattice contraction i.e, decrease of lattice parameters. The compressive stress seems higher at lower dose. Ison et al. reported similar trend of stress development in the irradiated CdS thin films case [10]. Apart from delivering energy to the lattice, energetic collisions in the cascade region are expected to have very prominent effect, as a result of which the position of (111) peak shifts to higher values. This may partly be associated with desorption of nitrogen (re-sputtering), incorporation of carbon atoms, change in stress state and or increase in grain size. The increase in peak intensity and grain size can be related to the local temperature rise as proposed in thermal spike model. The local rise in temperature (annealing) for a very short span of time increases the total lattice energy. This rise in energy induces crystallization in the material that consequently leads to rise of peaks intensities, larger grain sizes and reduction in defects [11,12]. The appearance of Zr (100) and (002) peaks demonstrate that the crystallization of Zr phases in the irradiated films occur as well. The smaller peaks shift to higher 2θ position above 5 1013 ions/cm2 fluence, could be related to the overlapping of effected zones. The local rise in temperature is
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5 1013, 5 1014 and 1 1015 ions/cm2, respectively, the estimated residual stress in the irradiated films relative to the as grown film is therefore, 2.75, 2.37, 1.75, and 1.37 GPa in sequence. The lower stress value at higher ions fluence indicated that during the film bombardment, the defect produces by the irradiation recover as well. The higher ions irradiation dose relieved some residual stress as already described in XRD analysis. 3.5. Electrical resistivity
Fig. 6. Influence of the ions fluences on film resistivity.
higher at greater dose that minimizes the defects produces by the ions tracks. In totality, the energy imparted by the incident ions due to irradiation causes an improvement in the film crystallinity as is evident from the sharpening of XRD peaks. 3.4. FTIR spectroscopic analysis FTIR spectroscopy was employed to yield information about the incorporation or existence of incident C atoms chemical bonds with species of the pristine ZrN film. The FTIR spectra of pristine and irradiated films are shown in Fig. 5. The as-grown films spectrum indicates a broad band centered at 689 cm 1 corresponding to stretching vibrations of Zr–N simple bonds [7,13]. The spectrum also depicts a band for ZrO2 at 1040 cm 1, which appears due to thin oxide layer formed on ZrN surface. Oxide layer is formed due to larger heat of formation of ZrO2 ( 261.5 kcal mol 1) as compared to ZrN ( 87.3 kcal mol 1) [14]. After ions irradiation, the main peak is shifted to 711, 708, 703 and 700 cm 1 at fluences 1 1013, 5 1013, 5 1014 and 1 1015 ions/cm2, respectively. In the IR patterns of irradiated films, the 1740 cm 1 and 1585 cm 1 peaks originate from stretching vibration of carbonyl group (C ¼O) and carbon, respectively [15]. The peaks present at 2000–2300 cm 1 could be assigned to the stretching vibration of alkynes (C triple C) or cyanic group (C triple bond N). The observation of the peaks at 2800–2900 cm 1 reflects the stretching vibration of saturated hydrocarbon (C–H) [16]. After ions irradiation, main IR peak sharpened which exhibit that film crystallinity improves and is in agreement with XRD result. The shifting of main peak from 689 cm 1 to about 711 cm 1 at 1 1013 ions/cm2 fluence could be related to compressive stress [17]. A shift of the XRD peaks towards higher angles has already been observed in the XRD spectra. The amount of stress relative to the as grown film can be calculated from the IR peak shift assuming an 8 cm 1 shift corresponds to 1 GPa stress [17]. The observed peak positions in the spectra are 711, 708, 703 and 700 cm 1 at fluences of 1 1013,
The electrical resistivity (measured by four-probe point method) of the pristine and irradiated films is shown in Fig. 6. The as-deposited film exhibits 92.2 mΩ-cm, which decreases to 84.4 mΩ-cm for a film irradiated at a fluence of 1 1013 ions/cm2. It drops more to 77.4 mΩ-cm with further increase of the ions fluence to 5 1013 ions/cm2. As C irradiation increases further to Z5 1014 ions/cm2, the resistivity rises again. The resistivity variation is related with creation of nitrogen vacancies, generation of point defects, self-interstitial, and C impurity atoms (particularly in the cascade regions) [18]. After ions irradiation, the formation of metallic Zr nanocrystalline particles has been observed that may greatly affect the carrier concentration. Therefore, the initial decrease in resistivity may be attributed to the rise of carrier concentration that overcomes the influence of defects. However, at higher ions fluences, the rise of C ions (impurity atoms) incorporation into ZrN matrix may reduce the carrier concentration causing a rise in film resistivity. 4. Conclusions The 800 keV energetic carbon (C) ion irradiation has proven to affect the microstructure as well as the film resistivity. XRD analysis shows that ZrN (111) peak shifts to higher angles after C ion irradiation indicating the development of compressive stress. It was also noticed that the incident ions preferentially sputter the lighter N atoms from ZrN phase thereby forming (100) and (002) oriented metallic Zr nanoparticles. The SEM analysis as well demonstrated the formation of Zr nanoparticles after ions irradiation. Compressive stress has been calculated from the FTIR peak shift and it was observed that higher ion doses (Z5 1014 ions/cm2) produces lower compressive stress. Ion irradiation at r5 1013 ions/cm2 fluence, decreases the film resistivity from 92.2 mΩ-cm to 77.4 mΩ-cm. However, with further increase in C ions fluence (Z5 1014 ions/cm2), it rises again to higher values.
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