Laser-ablated plasma for deposition of aluminum oxide films

Applied Surface Science 143 Ž1999. 56–66

Laser-ablated plasma for deposition of aluminum oxide films A. Misra, R.K. Thareja

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Department of Physics and Centre for Laser Technology, Indian Institute of Technology Kanpur, Kanpur-208 016 (UP), India Received 30 September 1998; accepted 17 December 1998

Abstract Aluminum films are deposited in oxygen background at an ambient pressure of 100 mTorr by pulsed laser deposition technique at various target–substrate distances. Two-dimensional images of the laser-ablated aluminum plumes were recorded using Integrated Charged Couple Device ŽICCD. camera system. The importance of these images in optimising the target–substrate distance for the deposition of various films is reported. An attempt is made to correlate the characteristics of the film with that of laser-ablated plume parameters. We observed that at a critical target–substrate distance, the films obtained are oxygen-rich. q 1999 Elsevier Science B.V. All rights reserved. PACS: 52.50.Jm; 81.15.Fg; 81.15.y z Keywords: Aluminum; Oxide films; Fast Photography; Plasma parameters

1. Introduction In recent years, laser ablation has emerged as a versatile technique for the deposition of various materials including semiconductors, high Tc superconductors, ceramics, ferroelectrics, metal and metal compounds, polymer, biological material and refractive material in the form of a film w1–3x. Laser ablation deposition ŽLAD. technique is fairly simple, inexpensive, free of contamination and fast as compared to other conventional techniques. LAD can be used to deposit films of any material irrespective of

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Corresponding author. Tel.: q91-512-597989; Fax: q91512-590914; E-mail: [email protected]

their optical properties and stoichiometry w3x. The production of multicomponent films w4,5x and low processing temperatures are among the other advantages. Among the drawbacks are the appearances of particulates w6x, non-uniformity of thickness and small area of deposition. Even though LAD has edge over other techniques, parametric studies are still on to optimise the process parameters such as target–substrate distance, laser wavelength, irradiance, pulse width and substrate temperature for deposition of good quality films. Since the source of the films is a laser generated plasma composed of neutrals and ionised atoms, molecules and other species w7x, it is necessary to understand the mechanism of the plasma formation and expansion. The exact nature of the plasma process requires detailed quantitative data on the composition and dynamics of the plume evolu-

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 9 3 0 - 1

A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

tion as it propagates towards the substrate w8x. Saenger w9x has reviewed a number of experimental findings. The effect of various parameters such as target surface topography, target–substrate distance, dimensions of laser spot size and laser parameters, viz., wavelength, fluence, pulse width on the plume angular distribution in vacuum has been discussed by her. Several authors w9–14x have experimentally investigated the motion of plume in different ambient gases at different pressures. Using Monte-Carlo simulation, Kools w15x studied the effect of elastic collisions between the target atom and low pressure gas atoms, on the kinetic energy and spatial distributions of the particles arriving at the substrate. Lindley et al. w16x have used holographic interferometry to investigate the expansion of the laser-ablated aluminum plumes. The expansion was studied in vacuum and in argon gas at various pressures. Aden et al. w17x have reported the formation of shock waves on irradiation of aluminum targets with Nd:YAG. Kim and Kwok w18x have reported a pressure distance relationship for optimum quality films for YBCO and PLZT for laser ablation in oxygen atmosphere using blast wave theory. Several other researchers w11–13,19x have used drag and shock models to explain the plume expansion. To define limits for the formation of shock, Dyer and Sidhu w20x studied experimentally the effect of pressure on the formation of shock and concluded that at high pressures the shock model is a better fit to experimental observations. Geohegan w11x and Wanniarachchi et al. w21x have shown that the drag model is more suited at low pressures. Study of pulsed laser-ablated plume in low ambient pressure of the order of few mTorr is very important because these background pressures are generally used in LAD w22x. Since the high velocity of the shock can deteriorate the quality of the films, it is important to have knowledge of the pressure and distance at which the shock formation takes place. Thareja and Dwivedi w23x have deposited carbon films using LAD in presence of helium and argon at various pressures and attempted to correlate their formation with the characteristics of laser produced carbon plasmas. Bjormander et al. w24x have ¨ grown ferroelectricrsuperconducting heterostructure on single crystal LaAlO 3 . Perovskite La 1yx Ca xMnO 3y d thin films were deposited by Gu et al. w25x on Mg Ž001. substrate and the films were analysed

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for the effect of deposition conditions, such as laser fluence, substrate temperature and oxygen pressure on the growth behavior of films. Various techniques such as Raman spectroscopy w26x, Infrared spectroscopy w27x, X-ray analysis w23x, Rutherford Back Scattering ŽRBS. w28x, Scanning Electron Microscopy ŽSEM. w29x, etc., have been routinely used to characterise the films, to get information on the surface stoichiometry, constituent content, thickness, optical properties, etc. Ozegowski et al. w30x used four different pulsed lasers to deposit and study the influence of the laser parameters on plasma parameter and on the surface morphology of the deposited films. Yakolev et al. w31x deposited PZT films by laser ablation deposition and examined the films for their homogeneity and thickness through a comparative use of Raman and Infra-red spectroscopy. Labardi et al. w32x applied scanning force and friction microscopy to ferroelectric films to investigate the morphology and phase structure of ferroelectric films. An important aspect of laser produced plasmas is the characteristic luminous plume. The spatial, temporal and spectroscopic dependence of the visible plume are a consequence of, and provide insight into, the processes of plume expansion in addition to laser–target, laser–plume, plume–ambient and plume–substrate interactions. Thus, photography has emerged as one of the important diagnostic tools to study various features of expanding plasma. Fast photography using gated intensified ICCD has recently become very popular for laser-ablated plasma plume studies both in vacuum and gaseous background w11,12x because of its high sensitivity. We report the study of the expansion dynamics of the laser-ablated aluminum plasma plume in different ambient gas environment and at different pressures using ICCD detector system. Time integrated, twodimensional images of laser-ablated plasma plume are recorded with the ICCD system and are used to obtain position–time plots of the expanding plasma. These plots are further used to compare with the theoretically predicted values of that of shock and drag models. Films are deposited at the predicted target–substrate distances and are found to match well. Deposited films are characterised using Scanning Electron Microscopy ŽSEM., Rutherford Back Scattering ŽRBS. and Raman spectroscopy and the correlation is discussed.

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A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

2. Experimental details The experimental set up used is similar to the one used earlier w33x. A Q-switched Nd:YAG laser ŽSpectra physics DCR 4G. pulse, operating at 10 Hz, Ž l s 1.06 mm, 8 ns pulse width FWHM. delivering energy up to 1 J is focused by a spherical lens Ž f s 60 cm. on to a flat Al target. The focused spot on the target was 260 mm. Target was continuously translated in order to avoid crater formation. Side on views of plume expansion of the overall visible emission from the plasma plume was recorded with gated, ICCD camera system ŽPrinceton Instruments, ICCD-576Gr2.. The detector consists of MultiChannel Plate ŽMCP. with spatial response 200–800 nm and 384 = 576 CCD array. In order to have a better insight of the film deposition process, the images were recorded at various time delays with respect to the ablating laser pulse using a pulse generator ŽPG-200, Princeton Instrument. and fixed gain of MCP. Experiments were carried out in different background gases ŽHe, Air and Ar. and at different ambient pressures. The laser irradiance Žused in this experiment on the target. was 2.35 = 10 10 Wrcm2 . Films of aluminum were deposited on silicon substrate in oxygen atmosphere at 100 mTorr pressure at different distances from the target surface for 10 min each. Raman spectrum of the deposited films was recorded using Spex-1877D Triplemate system with a resolution of 2 cmy1 .

3. Result and discussion Side on view of the expanding plume is imaged using ICCD camera system. The position of the plume front is located by comparing the observed plume front with the theoretical front. The evolution of plume near the target surface is simulated following Singh and Narayan’s w35x hydrodynamic model. It is assumed that the initial expansion is unaffected by the presence of ambient gas. The effect of gas is significant after a certain time when the collision between the ambient gas and the plasma constituents is appreciable. We have used the equations of motion and the continuity to transform the expansion for time dependent density, pressure and velocity into force equations that gives the dynamics of the

plume’s maximum density boundary Ž X Ž t ., Y Ž t ., ZŽ t .. during isothermal expansion: XŽ t.

d2 X

1 dX

ž / t

q dt

d2 Y

1 dY

ž / Ž . ž /

sY Ž t.

t

q dt

t

kT0

dt2 d2 Z

1 dZ

sZ t

s

dt2

q dt

dt2

tFt

m

Ž 1.

where t is the laser pulse width and T0 is the isothermal temperature of the plume. The initial dimensions are of the order of 260 mm in transverse direction depending upon focused spot, whereas in the longitudinal direction, it is less than 1 mm. After the termination of the laser pulse, the plasma expansion is assumed to be adiabatic. Thus, equating the plume’s internal energy to the total isothermal energy in plume’s volume at the end of the laser pulse, we arrive at: XŽ t.

d2 X dt2

sY Ž t.

s

d2 Y dt2

sZŽ t .

d2 Z dt2

kT0

X 0 Y0 Z 0

m

X Ž t . Y Ž t . ZŽ t .

gy1

t)t

Ž 2.

where X 0 , Y0 , Z0 are solutions for the plasma boundary at the end of the isothermal region. The equations are solved numerically using Runge–Kutta iteration technique. For the initial expansion Ži.e., in the isothermal region. the spot size of the laser beam on the target Ž260 mm in our case. is taken to be the boundary condition. The simulated velocities and dimensions of isothermal regime are taken to be the initial boundary conditions for adiabatic region. From these simulations, one can evaluate the dimensions of the plume at various time steps. We considered the evaluation of Al-plume boundary at 50, 100, 200 and 500 ns in presence of ambient gases. The results of simulation is shown in Fig. 1, where for times F 1 ns ŽFig. 1 inset. the expansion is more in the X and Y direction, while it is less in Z-direction. As the time progresses, the expansion in Z-direction

A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

59

Fig. 1. Computer-simulated Al-plume front along Z-axis for time delays of 50, 100, 200 and 500 ns. Inset is the computer-simulated plume shape for time delay of 1 ns. T is the position of the target.

increases and after some time Žfew ns after the termination of laser pulse., the expansion in Z-direction is well beyond the expansion in X and Y-directions and is compared with the theoretical simulated plume and is found to be in good correlation as shown in Fig. 2. Fig. 2 shows an ICCD image taken

Fig. 2. Comparison of computer-simulated plume front ŽI. and ICCD image at time delay of 100 ns for laser-ablated Al-plume at a pressure of 100 mTorr of air.

at 100 ns delay with respect to laser pulse and a simulated plume front using Eqs. Ž1. and Ž2.. Fig. 3 shows the intensity contour, the variation of intensity as the plume progresses. Fig. 4 shows the variation of plume’s front edge with time at 10y1 Torr pres-

Fig. 3. Intensity contour at 100 ns time delay at a pressure of 100 mTorr of air.

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A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

Fig. 4. Variation of position of plume front edge with time at 100 mTorr pressure of He, Ar, O 2 and air.

sure of He, Ar, Air and O 2 . From the slope of the curve at different time delays, one can determine the velocity of the expanding front. Several theoretical investigations have been carried out in the past to understand the formation and evolution properties of the plasma w9,10,14,16,34– 36x. The steps involved in the process being laser interaction with target and energy absorption, evaporation leading to vaporisation of target material, absorption of laser by vapor giving rise to highly dense plasma, expansion of plasma thus formed and subsequent deposition on the substrate. To understand plume dynamics in vacuum, several models have also been proposed w13,37x. The propagation of the laser-ablated plume in gaseous ambient towards the substrate is a complex hydrodynamic problem. In the case of expansion in vacuum, the plume particles collide with themselves resulting in the observed angular distribution w9x. In the case of expansion in an ambient atmosphere, collision between the particles of the ambient gas and plume also takes place, which attenuates and slows down the plume particles. Expansion in an ambient gas is studied using classical drag and shock model w13,37x. In shock wave model, rapidly expanding plume behaves as a

piston which generates a shock wave in the ambient. The ambient gas is compressed into a thin shell between the shock and the plume front. If we consider our expansion to be conical at earlier times with vertex at the focusing point of the laser beam and spherical at later times, the shock thickness w13,38x is given by:

° ~

Ds

2g

ž /

R

gq1

1r3

y 1 for conical expansion

¢R Ž g y 1. r3Ž g q 1. for spherical expansion Ž 3. where D is the shock thickness, g is the specific heat ratio of the vapor Ž1.2–1.3., and R is the position of the expanding front. The temperature and density w39x of the shocked region are given by: Ts s

2g

Ž g y 1. 2 M q 1 T0 g q 1 Ž g q 1.

rs s r 0

gq1

ž / gy1

Ž 4.

Ž 5.

A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

where M is the mach number Žs Vra., a is the speed of sound, V Žcan be estimated from R–t plots. is the velocity of the expanding front, T0 is the background gas temperature, rs is the density of the shocked region and r 0 is the density of the gas at normal temperature and pressure ŽNTP.. From the values of temperature and density of the shocked gas, one can estimate the extent of diffusion. Diffusion coefficient w13x is defined as: D s D 0 Ž TsrT0 .

5r4

Ž r 0rrs .

Ž 6.

and diffusion range is given by: Dr s Ž 4 Dt .

1r2

,

Ž 7.

where Dr is the diffusion range, t is the time. D 0 for He, Ar, air and O 2 are 1.601, 0.169, 0.165 and 0.2 cm2 sy1 , respectively w40x. The diffusion of the gas into the compressed region increases with time. It is necessary for the formation of shock between the ambient and the ablated species that the compressed region thickness should be significantly larger than the mean free path. Since the mean free path l for the case of oxygen atmosphere at a pressure P is

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l s 5.95 = 10y3 rP ŽTorr. cm, it implies that the shock is generated at later times at low pressures, i.e., away from target surface and forms near to the target for higher pressures. The blast wave theory predicts the position of the shock front R as: R s j 0 Ž E0rr 0 .

1r5 2r5

t

Ž 8.

where, j 0 is a constant and depends upon specific heat Žg ., E0 is the laser energy and r 0 is ambient density w37x. From the propagation distance R, d Rrdt the speed of propagation can be estimated. It follows from Eq. Ž8. that for a particular experimental condition j 0 , E0 and r 0 being fixed, R can be written as R s q 0.2 t n , where q is a constant. The exponent n can be taken as a parameter, which can be varied to fit the theoretical curve to experimental data. Fig. 5 shows the fit of R s q 0.2 t n Žshock model. to the experimentally observed data, one notices that the model is best fitted for 0.22 F n F 0.28 for 100 mTorr pressure of argon, oxygen and air and n s 0.47 for He. For ideal blast wave, one

. and experimentally-observed plume front location for laser ablated Al-plume at a pressure of 1 Fig. 5. Plot of R s q 0.2 t n Ž mTorr in presence of He, Ar, O 2 and air.

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A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

expects n s 0.4. The discrepancy observed in the exponent n may be due to instability, which grows at moderate laser intensities and high pressures. However, the simple shock model does not explain the experimental observation that the plume propagation will cease at a finite distance, as observed in ICCD images in Fig. 6. To explain the experimental observation of finite propagation of the plume, we use drag model. In drag model, plume is taken as an ensemble and is assumed to experience a viscous force proportional to its velocity in the background gas, where dÕrdt s yb Õ, with Õ the speed of the ensemble and b the damping coefficient. In other words, the plume progressively slows down and eventually comes to rest due to the drag force. The distance at which the propagation ceases at a finite distance from the target

Fig. 6. ICCD images Žnegative. at various time delays Žns. for laser-ablated plasma plume at a pressure of 100 mTorr of helium.

is called stopping distance or plume length. On introducing this force term in equation of motion of plume edge, the position of the plume edge is found to be w37x: X s X max  1 y exp Ž yb t . 4

Ž 9.

where, X max is the stopping distance of the plume edge, and b is called slowingrdamping coefficient. Eq. Ž9. can also be written as: X Ž t . s Xmax w 1 y eyt r t 0 x

Ž 10 .

where X max and b are related by X max s V0rb , V0 being the initial velocity at t s 0 and t 0 s 1rb is the time constant at which the velocity drops to 1re of its initial value. Fig. 7 shows fit of Eq. Ž9. to the experimentally observed plume front which leads to the determination of Xmax , the stopping distance. It is observed that the stopping distance Ž Xmax . of plume for the ambient pressure of 1 mTorr is maximum, while it is least for 100 Torr, in all ambient gas environments. If we compare the stopping distances in terms of background gas at same pressures, one observes that the stopping distance in case of He Ž3.19 cm. is largest and is least for Ar Ž1.82 cm.. While observing the images, one finds that as the pressure increases, the stopping distance decrease and there is an increase in intensity of the plume, this is due to confinement of the plasma in a small region which results in the increase in emission intensity. Hence, it seems that one of the controlling parameters ‘stopping distance’ for defining the film characteristics, can easily be estimated from real time observations and classical drag model. From R–t plots for the luminous front, Fig. 4, the velocity of the expanding front is estimated to be 7.29 = 10 6 cmrs, 6.84 = 10 6 cmrs, 6.75 = 10 6 cmrs and 6.21 = 10 6 cmrs for He, air, O 2 and Ar, respectively, at 10y2 Torr. The measured velocity is used to calculate the vapor density, temperature and pressure using the hydrodynamic relations of adiabatic shock expansion. Assuming that vapor pressure far exceeds the ambient pressure, which is true in our case, the maximum velocity attainable is given w41x by Vmax : Vmax s 2 ar Ž g y 1 .

Ž 11 .

A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

Fig. 7. Plot of Eq. Ž4. Ž mTorr of air.

. and experimentally-observed Ž\. plume front location for laser-produced Al-plume at a pressure of 100

where a is the speed of sound w a s Žg kTsrm.1r2 x. Eq. Ž9. is used to estimate the surface temperature Ts of the target, here g is the specific heat ratio of the vapor, m is the mass of the solid, and k the Boltzmann constant. Knowing the speed of the front at different time from R–t plot and using the following mass and momentum conservation equations, one can determine r v , Pv and Tv : Pv s 2 rg V 2r Ž 1 q gv . Tv s Pvr Ž R r v .

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Ž 12 . Ž 13 .

where, r denotes the density, P the pressure, T the temperature, and R is the gas constant. The subscript v and g denote vapor and gas, respectively w42x. The plasma pressure decreases with increase in time and ambient pressures. Similarly, the vapor temperature also decreases with increase in time and ambient pressure. The vapor temperature Ž; 2.089 = 10 5 K, at 29 ns after the laser pulse in 100 mTorr of oxygen pressure. is less as compared to the temperature Ž; 1.922 = 10 6 K at 29 ns after the laser pulse in 100 mTorr of oxygen pressure. of the shock front for all times and pressures, indicating that the temperature in the shocked region is more and is high

enough for chemical reaction to take place at the edge, in case of metals in oxygen ambient giving rise to metal oxides. To sum up, it follows that plume particles eventually diffuse into the compressed region, Eq. Ž7.. The temperature of the shocked front is higher than the plasma vapor temperature for all times. Further, at and after stopping distance, the gas particles also diffuse into the compressed region. Thus, inside the compressed region, due to high temperature and presence of both the plasma and the gas particles, there is a finite probability for the plasma particles to undergo physical and chemical changes. In order to verify the above, we deposited aluminum films in oxygen atmosphere at various target–substrate distances Ž1.0, 2.0 and 3.0 cm.. The distance of 2.0 cm corresponds to a distance where the plume propagation will cease based on drag model. The other two distances Ž1.0 and 3.0 cm. are chosen to compare the quality of the film with that of 2.0 cm film. Fig. 8 shows typical SEM photographs. The films deposited at a distance of 1.0 cm are denser as compared to the films deposited at 2.0 cm and 3.0 cm from the target. However, if one

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A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

Fig. 8. SEM photographs for the Al films deposited at 1.0, 2.0 and 3.0 cm in oxygen ambient at a pressure of 1 mTorr.

observes at higher magnification Ž=10,000., the single particle at 2.0 cm seems to be larger than the particles at other two distances. Further, the films deposited at 2.0 cm show a kind of cluster formation due to reaction of particles with oxygen, which is not observed at other distances. Fig. 9 shows a typical

RBS spectrum of the film deposited at 1.0 cm. The thickness of the film deposited at 1.0, 2.0 and 3.0 cm ˚ 2800 A˚ and 1850 A, ˚ respectively. The are 3800 A, chemical composition of the film deposited at 2.0 cm showed the composition of Al:O to be 2:3, implying that background oxygen has reacted with the plasma

A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

particles resulting change of chemical composition of the film which may be responsible for formation of clusters. While the film deposited at 1.0 and 3.0 cm showed no such compositional changes. The film deposited at all three distances showed the formation of SiO 2 layer at the interface of the Si and Al. This layer is formed most probably due to the presence of Si substrate in oxygen ambient. Fig. 10 shows a typical Raman spectra. Raman spectra taken of a film for target–substrate distance of 2.0 cm showed the formation of a band at 972 cmy1 signifying aluminum oxide in the films w43x. The band at 972 cmy1 is not observed for the films deposited at other two distances. We are not very sure of the peak observed at 1082 cmy1 , maybe this is an overtone of Si-peak Žsubstrate. which is observed at 521 cmy1 . The peak at 1082 cmy1 is observed in all films. To conclude the analysis based on SEM, RBS and Raman spectroscopy suggest that at stopping distance which define the plume length, one observes a change in chemical properties of the film. The film thickness at stopping distance is more than that of the films deposited at distances larger than stopping distance, while it is comparable to the films deposited at distances less than the stopping distance, thus indicating that ceramic films deposited at plume distances will never be oxygen-deficient. The velocity of the expanding particles at stopping distances is minimum and hence, the fear of deterioration of the

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Fig. 10. Raman spectrum for the Al films deposited 2.0 cm in oxygen ambient at a pressure of 1 mTorr.

substrate surface due to fast impinging plasma particles is also the least.

4. Conclusion In this paper we have reported the results of expansion of Al-plasma in different ambient atmosphere and pressures. Fast photography using ICCD is used to record time integrated images of the propagating plasma plume. The evolution of the observed expanding plasma plume compares well with simulated plumes. The expansion of the laserablated plume is explained in the light of the classical drag and shock models. The stopping distance, plume length is determined using the two models. Since the velocity at stopping distances is minimum, the possibility of the film deterioration is the least. The characterization of the films using SEM, RBS and Raman spectroscopy suggest that the films deposited at the stopping distance are oxygen-rich Žin our case. and are of good quality. The finding of deposition characteristics may have potential for the growth of good quality metal-oxide, ceramic and superconducting films.

Acknowledgements Fig. 9. RBS spectrum for the Al films deposited at 2.0 cm in oxygen ambient at a pressure of 1 mTorr.

Authors would like to thank Dr. M.S. Navati and Mr. T. Som for their help in experimentation with

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A. Misra, R.K. Tharejar Applied Surface Science 143 (1999) 56–66

the Micro-Raman and RBS set up, respectively. One of the authors, AM, wish to acknowledge UGC, New Delhi for research fellowship.

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