On the structural, morphological and electrical properties of tantalum

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Chinese Journal of Physics 55 (2017) 1412–1422

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On the structural, morphological and electrical properties of tantalum oxy nitride thin films by varying oxygen percentage in reactive gases plasma Jamil Siddiqui a,b,∗, Tousif Hussain a, Riaz Ahmad a,b, Zeeshan A. Umar c a

Centre for Advanced Studies in Physics, GC University, 1 Church Road, Lahore, Pakistan Department of Physics, GC University, Katchery Road, Lahore 54000, Pakistan c Department of Physics, Karakoram International University, Gilgit, Pakistan b

a r t i c l e

i n f o

Article history: Received 1 August 2016 Revised 21 October 2016 Accepted 31 January 2017 Available online 19 May 2017 Keywords: DPF device Tantalum oxy nitride thin film XRD SEM EDX Electrical resistivity

a b s t r a c t Tantalum oxy nitride (Ta-O–N) thin films possess unique properties due to combined beneficial aspects of tantalum nitride and tantalum oxide. In the present work, Ta-O–N thin films are synthesized using energetic ions and electrons beams emanated from the hot and decaying plasma in dense plasma focus (DPF) device. Oxygen percentage in the reactive gas admixture [O/(O+N)]% is varied from 10 to 60 for thin film synthesis. Influence of oxygen percentage on structural, morphological, compositional and electrical properties of thin films is also analyzed. Increase of oxygen percentage in the gas admixture (upto 40%) resulted in transformation of thin film from tantalum nitride to tantalum oxy nitride; however; higher oxygen percentages (≥ 50%) caused amorphization in synthesized thin film. Morphological analysis revealed that pure nitrogen environment yields the granular structure of film in nano-meter range whereas escalation of grains is observed with the increase in oxygen percentage. Cross sectional SEM showed better film growth rate at lower oxygen percentages. Compositional profiles exhibited improvement of oxygen content in thin film by increasing oxygen percentage in the admixture. The electrical resistivity of films shifted from conducting (for 0% oxygen) to semiconducting-insulating (for 60% oxygen) material range for maximum oxygen percentage. © 2017 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.

1. Introduction Transitional metal oxynitrides (TM-O–N) are currently gaining investigational interest, not only because of their suitability for diverse applications, but also because their properties may gradually be changed by controllably varying the concentration of the constituents along with the other deposition parameters during the growth process. Tailoring the material composition helps in tuning the mechanical, electrical and optical properties of the film [1]. TM-O–N films exhibit to have capabilities in a much larger domain, than demonstrated by corresponding metallic nitrides and oxides [2]. Main emphasis in oxy nitride films fabrication is based on addition of third element into oxide or nitride compound to enhance any of its property. In TM-O–N group, tantalum oxynitride [3,4] is examined a little less than its companions like titanium [5,6] and silicon oxynitrides [7,8]. Yet the main advantage of tantalum oxynitride is the probability of tuning both metallic



Corresponding author at: Centre for Advanced Studies in Physics, GC University, 1 Church Road, Lahore, Pakistan. E-mail address: [email protected] (J. Siddiqui).

http://dx.doi.org/10.1016/j.cjph.2017.01.009 0577-9073/© 2017 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.

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or non-metallic atomic ratio (Ta/(O + N)) and the non-metallic elemental atomic ratio (N/O) through altering the deposition parameters which leads to a large spectrum of compositions and microstructures thus opening the range of potentially attractive properties. Tantalum nitride is a hard and chemically inert material which exhibits metallic behavior. Owing to low electrical resistivity along with good diffusion barrier property, tantalum nitride (Ta-N) has been applied as diffusion barriers for copper interconnections [9,10]. On the other side, tantalum oxide (Ta-O) films have been comprehensively applied in optical coatings, as well as, dielectric coatings in microelectronic industry due to their high dielectric constant, large band gap and high refractive index [11,12]. Thus the tantalum oxy nitride film is expected to possess unique and attractive features resulting from the combination of beneficial properties of both tantalum nitride and tantalum oxide. The control of reactive gas ratio allows tuning the film structure and hence the electrical, mechanical and optical properties and the photocatalytic behavior of such coatings [13]. The growth of tantalum oxynitride films is based on the idea that the compound can potentially benefit from properties exhibited by the oxide, the nitride, the tantalum phases, or their combination [14]. Banakh [15,16], Venkataraj [17,18], and Chung [19] have reported microstructure, mechanical, optical and electrical properties of tantalum oxy nitride films as a function of the O/(O+N) ratio. The main aim of present work is to synthesize tantalum oxy nitride nano-composite thin films at room temperature by employing high energy density pulsed plasma generated in a dense plasma focus (DPF) device. DPF is a hydro magnetic coaxial plasma accelerator where the high-current electrical discharge efficiently heats and compresses the plasmas into a pinched plasma column where a short lived hot (1 keV–2 keV) and dense (1025 m−3 –1026 m−3 ) plasma is produced. Due to the broad energy spectrum of radiation emanated in the plasma focus, its applications in material processing have shown prominent features of better adhesion and good deposition rates [20,21]. Numerous properties and features of DPF which have made it an exclusive and attractive source for thin film depositions and material processing, have been reported by Rawat in a recent review paper [22]. Various thin films (titanium nitride, hafnium oxide, diamond like carbon (DLC), tantalum nitride, tungsten nitride, niobium nitride, gallium nitride and zirconium silicon nitride) with desired properties have been recently successfully synthesized in DPF device [23–30]. In the presented research, we have synthesized tantalum oxy nitride thin films in DPF device and devoted to a systematic investigation at different oxygen percentage in the reactive gas admixture [(O/O+N) %]. Thin films are characterized in terms of crystallographic structure, composition, surface morphology and electrical resistivity by employing multiple techniques. The relationship between reactive gas ratios, composition, microstructure and resistivity of Ta–O–N films is also discussed and established. 2. Experimental setup Tantalum oxy nitride thin films are synthesized in Mather-type DPF device (schematic diagram presented in Fig. 1). The technical parameters of the device are given as follows; The device is powered by a 30 μF, 15 kV Maxwell capacitor. Operational charging voltage is kept at 12 kV throughout the experiment for all coatings, which leads to an energy of 2.3 kJ and a maximum discharge current of 175 kA. Aggregate inductance of system is about 80 nH. The device employs a copper-made solid cylindrical anodes surrounded by six cylindrical copper rods constituting cathode. The anode was concaved from the tip in order to minimize the impurities in the plasma [31]. A slightly tapered anode from open end is used as it has been reported that tapering enhances the emission of ions, electrons, and x-rays from the pinched plasma [32]. For synthesis of tantalum oxy nitride thin films, tantalum target is placed at the concaved end of the anode. The stainless steel made main chamber is evacuated up to 10−2 mbar (1 bar = 105 Pa) by rotary vane pump. Afterward, reactive gas mixture (O+N) is filled in the chamber. Different (O/(O+N)) percentages with various admixture gas filling pressures are initially used to find out the optimum admixture gas pressure. Strong focusing of the device (ensuring efficient transfer of the energy) is confirmed by sharp spike in the high voltage (HV) probe signal corresponding to the radial collapse phase [33]. In fact the quality of film synthesized by plasma focus device depends upon the pinching status of the device which depends on the fact that how efficiently the energy stored in the capacitor bank, is transferred to the electrode assembly. These basic plasma diagnostic techniques (Rogowski coil and High voltage probe) were continuously utilized during the experiment to make sure that all films are synthesized in same (maximum focusing) condition. The chamber was evacuated after each 4 to 5 shots and then fresh gas (or gases) was filled into the chamber. The optimized admixture filling gas pressure is found to be about 2.0 mbar which is not significantly influenced for various [O/(O+N)] percentages and hence all films are synthesized at a fixed admixture gas pressure of 2 mbar. Silicon wafers are cleaned in ultrasonic bath by rinsing initially in acetone and then in methanol. Silicon substrate samples of 1 × 1 cm2 dimensions are attached on the sample holder at a fixed axial distance of 9 cm from the anode tip and along anode axis. Film synthesis is achieved for various oxygen percentages in the reactive gas admixture. All thin films are synthesized with fifteen plasma focus shots. 2.1. Plasma formation mechanism A qualitative description of plasma formation and thin film synthesis process in the DPF device operated with (O+N) reactive gas admixture is as follows.

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Fig. 1. Schematic diagram of DPF system.

When electrical energy stored in capacitor bank is delivered to electrode assembly through spark gap switch, the breakdown of gas present in the chamber takes place in the vicinity of insulator sleeve. This breakdown results in the formation of axis symmetric current sheath which is influenced and accelerated by the Lorentz force. The current sheath spreads uniformly outwards towards cathode rods in inverse pinch phase and then accelerates upwards towards the open end of electrodes in axial rundown phase. The sheath collapses radially inwards on reaching the open end of electrode assembly, resulting in the creation of short lived (∼10 ns–50 ns), dense (∼1026 cm−3 ) and hot (∼1 keV) pinched plasma just above the anode tip in final focus phase. Formation of pinched plasma column is subsequently followed by sausage (m= 0) instability which enhances the induced electric field locally, which couples with magnetic field, resulting in the disruption of plasma column [34]. The disruption of plasma column leads to the acceleration of highly energetic ions towards the substrate (top of the chamber), and relativistic electrons (∼100 keV above) in downward direction towards target placed at the anode. During exposure to each plasma focus shot, a sample is processed by an amalgam of high energy ion flux, shock wave and decaying hot plasma [22]. 2.2. Thin film growth and characterization Sample processing is comprised of the following mechanisms; (i) Accelerated ions bombard the sample surface and results in high thermal gradients by sufficient energy transfer. Abrupt heating followed by cooling effects happens in the sample surface. The ion bunches may etch and clean sample surface prior to the deposition of thin films. Besides etching, ion flux may also increase the surface temperature to such high values at which sample evaporates. Evaporated sample material may react with the succeeding bunches of ions in the same plasma focus shot and deposit on the sample surface. (ii) Downward accelerated relativistic electrons alongwith hot and decaying plasma, ablate the target material (tantalum in our experiment) inserted at the anode top. The ablated target material may react with the reactive gas (nitrogen and oxygen) species during their flight to the substrate; forming tantalum oxy nitride and depositing on the substrate sample surface. Hence the energetic ion flux and relativistic electrons emanated from decaying dense plasma are employed for tantalum oxy nitride thin films synthesis. The x-ray diffraction (XRD) technique using Philips X’ Pert PRO MPD theta-theta x-ray diffractometer operated at a voltage of 40 kV and current of 40 mA with Cu Kα (λ = 1.54 A°) radiation is used to characterize the synthesized thin films and reveal the structural analysis of the film. The machine is used in detector scan mode to perform a 2θ scan over the 20° – 80° range. Scanning electron microscopy (SEM) using JEOL JSM-6480 LV with energy dispersive x-ray (EDX) attachment

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is used to investigate the surface morphologies and the elemental compositions of the deposited films. Film thickness is calculated by cross sectional SEM. Electrical resistivity of the films is calculated by employing four probe techniques. The self-designed and fabricated four probe set up consists of four equally spaced tungsten metal tips. These four metal tips are part of an auto mechanical stage which move up and down during measurement. For the accurate measurement, the samples are placed such that the contacts are in the center of the sample. A high impedance current source supplied current through the two outer probes, while a voltmeter measured the corresponding voltage across the two inner probes. The film electrical resistivity is calculated by using the following relation

Resistivity ρ = 4.523(V/I )t

(1)

where t is the film thickness. 3. Results and discussion The structural, morphological, compositional and electrical resistivity profiles of synthesized thin films are discussed in the following subsections. 3.1. Structural analysis Crystallographic investigation of the films synthesized with various oxygen ratios in the plasma is made by XRD (Fig. 2). This characterization is consisted of phase identification by comparing the experimental XRD patterns with the patterns from JCPDS-ICDD (Joint Committee on Powder Diffraction Standards - International Centre for Diffraction Data) files using the X‘Pert Highscore program. The bottom most diffractrogram representing film synthesized with 0% oxygen (pure nitrigen), exhibits the emergence of new peaks at ∼28.8°, 34.6°, 47.1° and 76.3° with (110) preferred orientation which reveals the growth of multiphase tanalum nitride film for pure nitrogen plasma environment. As each and every film is grown with 15 plasma focus shots so the formation of multiphase tantalum nitride can be attributed to two probable mechanisms: (i) formation of tantalum nitride during the same plasma focus shot due to reaction between the ablated tantalum ions with energetic nitrogen ions and (ii) formation of TaN and Ta3 N5 due to nitriding of tantalum atoms by energetic nitrogen ions from the following plasma focus shot [35]. Yet its quite difficult to estimate which one of the two is mainly responsible for the film growth. We have previously reported on the parametric statistics of nitrogen ions emanated from pinched plasma column in 2.3 kJ DPF device [26]. The ion beam signal was detected at 9 cm axial distance from anode tip using BPX65 diode detector. The diode signal represented initial X-rays peak followed by (prominent) second and (diminished) third peaks arising due to nitrogen ion pulse corresponding to N+ and N++ charge states respectively. Higher energy observed for ions with lower charge number may be explained by step-by-step charge reduction of those ions during their interaction with plasma. Bhuyan et al. [36] has reported that ion signals in plasma focus device comprises of multiple (N+, N++ and higher) charge state. The estimated ion energy and number density were from 40 keV to 1.2 MeV and 9.7 × 1019 to 1.8 × 1019 m−3 ,

Fig. 2. X-ray diffractograms of films synthesized by different oxygen percentages in the reactive gas admixture.

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respectively. The nitrogen ions emission from a low energy plasma focus device having ion energy in the range 5–700 keV with most probable ion energy of 25 keV and the ion densities of the order of 1017 m−3 has been reported [37]. Kelly et al. [38] has made the measurements of the ion energy distribution in a 4.75 kJ PF operating with nitrogen (performed with a Thomson analyser) and identified N+, N++ and N+++ as the main ion species with a characteristic energy in the range 0.17–4.0 MeV, and total (time integrated) average ion flux 8 × 1012 ions/stereorad. Further measurements of the nitrogen ion energy spectrum in the same PF device using a TOF detector also resulted in the similar findings [39]. The peak of silicon (200) also exists at the same angle (∼28. 8°) where Ta3 N5 (201) do exist which indicates a probable phase transformation of the silicon substrate. The transitory increase in silicon surface temperature caused by combination of intense flux of instability accelerated energetic ions of filling gas species along with decaying highly energetic dense pinched plasma column and fast moving shock wave followed by the fast cooling may cause phase transformation in silicon [40]. The instability accelerated ion flux emanated from disrupted plasma consequences in the heating of surface up to several thousand degrees centigrade in a very short time immediately followed by fast melting, rapid cooling and resolidification. The ion irradiation process is, therefore, equivalent to the transient thermal annealing, due to which, atoms on the surface layer are rearranged, hence developing new phases [41]. Thus both tantalum nitride and silicon can contribute to this peak. On addition of small amount (10%) of oxygen in the plasma environment, the intensity of most preferred phase TaN (110) of film shows broadening along with some decrease in its intensity. A minor peak corresponding to TaON (221) is also initiated at ∼55.7° for 10% oxygen in gas mix. A further increase in oxygen percentage in the gas mix to 20% and then 30% shows degradation in most of TaN phases and significant enhancement of TaON (221) phase. Oxygen, being one of the filling gas species in plasma focus, supplies ions in almost all ionization states [42]. Bertalot et al. [43] has studied anisotropic ion emission of two DPF devices. Thomson mass spectrogram revealed the presence of N+ , O+ , N++ , O++ and higher charge states for both nitrogen and oxygen ions in the plasma. The transformation of tantalum nitride film into tantalum oxynitride film with increasing oxygen percentage in the plasma is attributed to the replacement of nitrogen atoms in the lattice by oxygen ones. An increase of the fraction of the insulating oxide phase, considering tantalum oxynitride as a composite material film, may also be a plausible justification for the nitride to oxynitride transformation trend of the film. On increasing the oxygen percentage in plasma, the percentage of the ionic bonds in tantalum oxynitride increases, while the part of the covalent Ta– N bonds is reduced. As oxygen is more electronegative than nitrogen, therefore the electronic charge transfer from Ta towards O is higher than for Ta–N bonds, resulting in an increase of ionicity in Ta–O–N [44]. The refractive indexes of tantalum nitride films widey differ from those of tantalum oxy nitride films. It has been reported that for partially crystallized materials in the TaN phase, the real part of the dielectric function first displays a decrease at low energy and then a large band after 2.5 eV Furthermore, it has also been observed and reported that the real part of the dielectric function, and therefore the refractive index, increases at lower energy when the amount of crystalline TaN is decreased [1,45]. The film synthesized with 40% oxygen reveals a further reduction in TaN (221) phase alongwith the emergnece of several new phases of TaON. Two new reflection peaks corresponding to (102) and (401) phases of TaO are also observed in addition to the tantalum oxynitride phases showing transition of initially grown tanalum nitride to tantalum oxynitride/tantalum oxide film for increasing oxygen percentage. Bousquet et al. [1] have shown that in 1.5–4.75 eV range, refractive index exhibits a maximum which is shifted to higher energies when O incorporation increases. For as-deposited films, this maximum shifts from 2.75 to 3.4 eV for oxynitride materials and to more than 4.75 eV for oxides. The TaN (111) reflection peak shows a slight downshifting from its initial position and now apears as a minor shoulder peak of newly grown phase of TaON (102). This downshifting of TaN (111) designates a higher lattice parameter maintaining the same crystal structure. It is suggested that for 40% oxygen, more oxygen is incorporated into the TaN phase, significant part of oxygen atoms occupy the vacant sites of TaN, resulting in expansion of the lattice. An increase of oxygen ratio to 50% reveals very weakly crystalline tantalum oxynitride film growth. Whereas strong amorphization of the grown film is observed at 60% oxygen ratio. The extra oxygen introduced to the chamber promotes the oxidation of the target-surface. Formation of tantalum oxide on the target can act as an electrostatic shell, which in turn can affect the sputtering yield and the kinetic energy of species which impinge on substrate with a reduction of the sputtering rate. The lesser energy of species reacting on substrate, the lesser crystallinity of films is expected. Also, the oxygen can enter in to the tantalum nitride lattice through a mechanism involving a vacancy creation process by substituting a nitrogen atom in the lattice. During the process, the mechanism of ingress of oxygen into the lattice is by diffusion [46]. On the other hand, the ionic radius of oxygen (rO = 0.140 nm) is almost ten times higher than that of nitrogen (rN = 0.01–0.02 nm) [47]. Thus, the oxygen causes an expansion of the crystal lattice through point defects. As the oxygen content increases, the density of point defects in the film increases and hence crystallinity of the film is disturbed. Gueho et al. [48] have reported the deposition of coatings with large compositional range attained by tuning the reactive flow rate ratio. Though successive formation of TaN, Ta3 N5 , TaON, and Ta2 O5 was achieved while the amount of oxygen was increased in the reactive gas mix, yet, the exceeded addition of oxygen in the plasma caused the films to become amorphous. A decrease in the refractive index alongwith amorphization of oxynitride films has been observed due to progressive substitution of nitrogen by oxygen in the oxynitride films [17,49]. The same findings have also been observed and reported experimentally as well as theoretically for TaO1 − x N1+x films [50,51]. Cubillos et al. [52] have also reported crystalline to amorphous transformation of grown transition metal oxynitride film when oxygen content was increased to 50% and beyond in gas admixture.

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Table 1 Average thickness of film synthesized with various oxygen percentages in the gas admixture. Oxygen percentage in the gas mix

Average film thickness

0% 10% 20% 30% 40% 50% 60%

1.09 ± 0.03 μm 1.25 ± 0.06 μm 1.16 ± 0.03 μm 1.10 ± 0.07 μm 1.03 ± 0.10 μm 0.78 ± 0.13 μm 0.69 ± 0.12 μm

3.2. SEM analysis The surface morphologies of the grown films for different oxygen percentage in the plasma are shown in Fig. 3. A granular surface morphology exhibiting uniformly distributed grains (∼few tens of nm to 100 nm) is revealed for film grown with 0% oxygen i.e. pure nitrogen plasma environment. The film surface appears to be quite smooth owing to the even dissemination of the grains. A slight incomplete coverage of the substrate surface by grains defines lesser dense morphology of the film. The film also possesses a couple of pores which possibly exists due to the irradiation of energetic ions followed by successive plasma focus shots. The sample is exposed to highly energetic ion pulses (∼hundred ns duration) in each and every plasma focus shots which results in fleeting surface heating to several thousand degree centigrade in short duration; followed by rapid melting and re-solidification [53]. The ion irradiation mechanism is similar to transient thermal annealing which leads to atomic diffusion on surface layer and hence formation of nanoparticles in the film surface. For 10% oxygen addition in the gas mix, whitish grey granular morphology is observed. The change of color from dark grey to whitish gray (for all percentages of oxygen) is dedicated to the addition of oxygen in the film. The film surface for 10% oxygen content is not as compact as for film synthesized in pure nitrogen environment. This non-compact film growth can be described by taking into consideration the existence of so-called poisoning of the tantalum target. A competitive phenomenon between removal by sputtering and formation of nitride and oxide layers at the target surface [54]. Almost similar granular morphology with grains escalation is observed for the films synthesized with 20%, 30% and 40% oxygen in the gas mix. This growth is consistent with the XRD findings which showed increase in tantalum oxy nitride phases with a maximum for 40% oxygen. But, for 50% oxygen content, the film possesses only a couple of extremely large structures surrounded by a few nano-scaled grains which could be assigned to a quasi-amorphous structure grown along with few nano-crystallites embedded in an amorphous matrix [19]. This behavior seems to be consistent with the XRD findings of film growth with 50% oxygen content. For further increase in oxygen percentage (to 60%), micro structures are revealed and nano-structures are almost vanished; possibly suggesting amorphization of the deposited film. Apparently, on coarseness basis, the films may be divided into two distinct regions; (i) corresponding to comparatively lower surface roughness of films for low oxygen percentages upto 40% (including that of TaN) and (ii) region where roughness grows significantly, corresponding to the highest oxygen fractions (50% and 60%). This increase in roughness due to enhancement of the grown structures on the surface, may be result of the increase of coating disorder/defects promoted by the possible formation of amorphous oxide phases and the inclusion of oxygen atoms within the grain boundaries or in the TaN or TaON lattice [54]. Furthermore, films growth mode for 50% and 60% oxygen is characterized by an island-type film formation, resulting in increases of the surface roughness. Cross-sectional SEM is carried out for measuring the average film thickness. A typical cross sectional SEM micrograph for film synthesized with 0 percent oxygen is represented in Fig. 4. Growth of dense thin film is depicted by the cross-sectional micrograph. The variation in film thickness as a function of oxygen percentage in the gas mix is depicted in Table 1. Average thickness of about 1.09 ± 0.03 μm is revealed for the film synthesized with 0 percent oxygen by fifteen plasma focus shots which highlights a good deposition rate of about 72 nm per plasma focus shot. A little variation in the film thickness is observed for variation in oxygen percentage in the gas mix up to 40 percent which may be attributed to the shot- to- shot variation in the pinching/focusing competency of the DPF device [55]. However a significant reduction in the film thickness is observed for 50 and 60 percent oxygen. The attenuation in the deposition rate (may be related to the formation of tantalum oxide) is believed to be a consequence of reduced sputter rate. Reduction in thickness with increase in oxygen percentage in the gas mix designates that effective species sputtered from the target and reaching the substrate are decreased with increasing oxygen percentage. Increasing oxygen percentage causes reduction in the energy acquired by particles and their mobility while travelling to the substrate. In fact it becomes rather difficult for the sputtered species to reach substrate leading to a decreasing growth rate and hence reduction in the film thickness [56,57]. 3.3. EDX analysis Synthesized thin films were investigated for their compositional profiles by employing EDX spectroscopy. The results confirmed the presence of tantalum, oxygen and nitrogen in the synthesized films for various oxygen percentages in the gas

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Fig. 3. Scanning electron micrographs for film synthesized with (a) 0% (b) 10% (c) 20% (d) 30% (e) 40% (f) 50% (g) 60% of oxygen.

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Fig. 4. Cross-sectional SEM of the thin film synthesized with zero percent oxygen in the gas admixture.

Table 2 Elemental percentages of tantalum, nitrogen and oxygen present in thin films. Elemental percentage

0%

10%

20%

30%

40%

50%

60%

Nitrogen (at. %) Oxygen (at. %) Tantalum (at. %)

49 7 44

42 18 40

34 25 41

25 38 37

16 46 38

15 68 17

12 75 13

Fig. 5. EDX spectrum of film synthesized for 40% oxygen in the gas admixture.

mix. The variations in tantalum, oxygen and nitrogen contents in the thin films for different oxygen percentages in the gas mix are depicted in Table 2. The little oxygen contribution in film for 0% oxygen may be due to the residual gas pressure due to non-ultra high vacuum conditions during the experiments (Fig. 5). A gradual decrease in nitrogen content in the films for increasing oxygen percentage is also revealed by the spectra which depicts the replacement of nitrogen atoms in the lattice by oxygen ones. It reveals that the oxygen dominates the elemental composition in the Ta–N–O films because the oxygen has high affinity and more negative enthalpy as compared to nitrogen [58]. It is also observed that tantalum content is almost same for 0 to 40% oxygen but reduced for higher oxygen percentages (50 and 60%). This attenuation in tantalum may be attributed to the poisoning of the target for too much oxygen presence; and hence resulting in lower sputtering from it.

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Fig. 6. Electrical resistivity of thin films synthesized with various oxygen percentage in the gas admixture.

3.4. Electrical resistivity The variation in electrical resistivity of thin films synthesized with various oxygen percentages in the gas mix is depicted in Fig. 6. The electrical resistivity of the un-exposed silicon substrate is found to be about 8.6 Ω cm (in semi-conductor range). The resistivity of film synthesized with 0 percent oxygen (pure nitrogen) is only about 8.8 × 103 μΩ cm (in conducting material range) owing to the formation of conductive tantalum nitride for 0 percent oxygen (see XRD spectra). The resistivity of Ta–O–N films is observed to be increased with increasing oxygen percentage in the gas admixture. It may also be noted that electrical resistivity is drastically increased to about 2.09 × 105 μΩ cm and 3.68 × 105 μΩ cm (∼in semi conductinginsulating material range) for the films synthesized with fifty and sixty percent oxygen respectively. The enormous increase in electrical resistivity for 50% and higher oxygen content may be dominated by Ta-O phase formation as found by XRD analysis. Greater oxygen percentages (≥ 50%) results in Ta-O phase formation associated with additional oxygen defects and ionic-covalent bonding which results in decrease in carrier concentration and mobility [19]. 4. Conclusions The relationship between the crystallographic structure, morphology, composition and electrical resistivity of tantalum oxy nitride thin films grown in plasma focus device, is investigated. The films are synthesized with various oxygen percentages [O/(O+N)]% in the reactive gases admixture. XRD spectra reveal growth of multi- phase tantalum nitride film with (110) preferred orientation for pure nitrogen environment. For 10 to 40% oxygen, the tantalum oxy nitride thin films are grown whereas amorphization of film is observed for higher (≥ 50%) oxygen percentages. SEM analysis shows that granular surface morphology of thin film synthesized in pure nitrogen environment. For addition and increase of oxygen in the reactive gas admixture, escalation of grains is observed. Cross sectional SEM examination shows a growth rate of about 72 nm per plasma focus shot for pure nitrogen environment. The film thickness is decreased for 50 and 60% oxygen in the reactive gas admixture. EDX spectra reveal the existence of tantalum, nitrogen and oxygen in the synthesized thin films for different oxygen percentages in the gas admixture. Oxygen content in the film is improved with the increase in oxygen percentage. Electrical resistivity measurements made by four probe method, show variation in electrical resistivity of thin film with changing oxygen percentages. Minimum electrical resistivity of about 8.8 × 103 μΩ cm is observed for film synthesized with 0% oxygen. The electrical resistivity is increased with increase in oxygen percentage whereas the maximum electrical resistivity of about 3.68 × 105 μΩ cm is observed for 60% (maximum) oxygen. The high-resistivity films may be used for the dielectric or photocatalyst application, while the low-resistivity films are suitable for barrier applications. Acknowledgments The authors would like to thank Prof. Dr. Shahid Rafique, Chairman, Department of Physics, University of Engineering and Technology, Lahore for the technical support in the Four-probe analysis. Dr. Uzma Ikhlaq is also acknowledged for technical support in XRD analysis. One of the authors (Jamil Siddiqui) is grateful to the HEC for providing financial support during his PhD research work.

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