SYNTHESIS, CHARACTERIZATION OF NANO LANTHANA … fileTraditional silicon MOS (Metal Oxide...

SYNTHESIS, CHARACTERIZATIONOF NANO LANTHANA ANDSTUDIES OF ENERGY BAND

DIAGRAMS FOR MOS CAPACITORAPPLICATIONS

Keerti Kumar Korlapati1, Bikshalu Kalagadda2,1,2Department of Electronics and Communication Engineering,

1Vaageswari College of Engineering, Karimnagar,2KUCET, Kakatiya University, Warangal,

Telangana, [email protected],

[email protected]

October 11, 2018

Abstract

Most digital applications require MOS devices with highlinearity and capacitors with high specific capacitance perunit area. In this paper Lanthana (La2O3) nano particlesare synthesized by Pechini method and characterized by X-Ray Diffractometer (XRD), Particle Size Analyzer (PSA),Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray spectrometry (EDX), Fourier Transform Infrared Spec-troscopy (FTIR), Thermo Gravimetric and Differential Ther-mal Analysis (TGDTA), Transmission Electron Microscopy(TEM), LCR meter analysis for the purpose of materialanalysis and further application in the device fabrication.Also the La2O3 has been studied for various parameters likeCharge density, Electric field, Device potential and Energy

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International Journal of Pure and Applied MathematicsVolume 120 No. 6 2018, 277-298ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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and are compared against SiO2 for MOS capacitor applica-tions. From the results, it is observed that the MOS devicecontaining La2O3 oxide layer drives high drain current com-pared to that of SiO2 contained MOS device.

Key Words: MOS Capacitor, Lanthana, Charge Den-sity, Pechini Method, Si substrate, Aluminium

1 INTRODUCTION

Traditional silicon MOS (Metal Oxide Semiconductor) device scal-ing has driven the semiconductor industry for the past four decadesand in recent years, new materials and processes has been intro-duced to maintain pace with Moore’s law which states the develop-ment of high density integrated circuits. Aiming the higher deviceperformances like low power consumption and high frequency op-erations, the MOS devices dimensions are going down to deep sub-micron scales [1]. However, the drastic reduction in the dimensionswill not always support the enhancement in device performanceand this is the reason why the present paper shows the comparisonbetween the conventional materials used nano MOS capacitor andnew materials (La2O3 as oxide layer and Al as metal gate contact)used nano MOS capacitor [2-4].The present article addresses synthesis of La2O3 nano particles byPechini method. The material developed has been used in MOScapacitors and the band diagram analysis of two nano MOS capac-itors one using conventional SiO2 oxide as gate oxide and the otherhaving La2O3 as its gate oxide are studied for various parameterslike Charge density, Electric field, Device potential and Energy andare compared against SiO2 for MOS capacitor applications [5-6].This paper is organised as follows. Section 2 describes the Pechinimethod to prepare La2O3. Section 3 presents the characterizationof La2O3 through X-Ray Diffractometer (XRD), Particle Size Ana-lyzer (PSA), Scanning Electron Microscopy (SEM), Energy Disper-sive X-Ray spectrometry (EDX), Fourier Transform Infrared Spec-troscopy (FTIR), Transmission Electron Microscopy (TEM) andLCR meter [7-11]. Section 4 gives Energy Band Diagram studiesof SiO2 and La2O3 contained MOS capacitors [12-16]. Section 5discusses the results and Section 6 concludes the paper.

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2 Pechini Method

Pechini is a synthesis method which was proposed in the year 1967and is the best method to synthesize highly homogeneous and finelydispersed oxide materials. Now-a-days, Pechini method is widelyused in the synthesis of dielectric, fluorescent and magnetic materi-als, high-temperature superconductors and catalysts for the deposi-tion of oxide films and coatings. The method’s advantages includeits relative simplicity and a relatively low temperature of precursortreatment, due to which the process may occur without sintering,resulting in the production of oxide nanocrystal powders [17]. SiO2

has been synthesized by modified Pechini method at low tempera-ture. The method involves mixing of the Lanthanum Nitrate witha Chelating agent, Urea. The initial precursor materials taken wereLanthanum nitrate (LaN3O9), Urea (CH4N2O), and Ethylene gly-col (C2H6O2). All reagents used were of Analytical grade.The materials were Calcinated (also referred to as calcining) to solidmaterials in order to bring about a thermal decomposition, phasetransition, or removal of a volatile fraction. This calcination pro-cess normally takes place at temperatures below the melting pointof the product materials. The La2O3 powders were calcined at750◦C for 2hrs in a high temperature furnace for the phase La2O3

nano particles.

3 Characterization of La2O3 nano par-

ticles

La2O3 nano particles obtained were characterized through X-RayDiffractometer (XRD), Particle Size Analyzer (PSA), Scanning Elec-tron Microscopy (SEM), Energy Dispersive X-Ray spectrometry(EDX), Fourier Transform Infrared Spectroscopy (FTIR), ThermoGravimetric and Differential Thermal Analysis (TGDTA), Trans-mission Electron Microscopy (TEM) and LCR meter.

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3.1 X-Ray Diffractometer (XRD)

Fig.1 shows the XRD result of the La2O3 nano particles and thestructure is in pure cubic phase. Peaks are observed at 23◦, 28◦,32◦, 40◦, 48◦ and 54◦ respectively corresponding to the (h k l) val-ues of the peaks (1 0 1), (2 2 2), (3 0 0), (4 0 0), (4 0 4) and(6 2 2) respectively. The lattice parameters were in good agree-ment with JCPDS card number 04-0856, having lattice parametersa = b = c = 11.420A and α = β = γ = 90◦. The crystallite size iscalculated by Debye Scherrers formula given in Eq. 1.

D =Kλ

βcos(θ)(1)

Where D is the average crystallite size of the particle, λ is thewavelength of the radiation, β is the full width at half maximum(FWHM) of the peak, θ is the Bragg’s angle. The average crystallitesize of the sample synthesized by Pechini method is calculated to be18 nm. The strain and crystallite size of the sample are measuredfrom the Williamson Hall equation given in Eq. 2.

βcosθ =Kλ

t+ 2εsinθ (2)

Where β is the full width at half maximum (FWHM) of theXRD corresponding peaks, K is Debye-Scherer’s constant, t is thecrystallite size, λ is the wave length of the X-ray radiation, ε isthe lattice strain and θ is the Bragg angle. With this, the averagecrystallite size was found to be 18 nm and the strain was 11.7x10−2.The lattice parameters of the formed nano particles is measured bythe below formula given in Eq. 3.

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a2=

4(h2 + hk + k2)

3a2+

1

c2(3)

The measured lattice parameters values are a = b= 0.3819 nm andc = 0.61196 nm which are similar to the values observed from theXRD pattern shown in Fig. 1.

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Fig. 1 XRD Patterns of La2O3 particles synthesized by Pechinimethod

3.2 Particle Size Analyzer (PSA)

The synthesized La2O3 nano particles were ultra sonicated and sus-pended in the ethanol solution. The sizes of the agglomerated col-loids in the suspensions were estimated using particle size analyzer(PSA). From the analysis the particle size is found to be 36nm andis in good accordance with the crystallite size of La2O3 i.e. twicethat of the crystallite size. The particle size distribution and aver-age particle Size of La2O3 nano particles results are shown in Fig.2.

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Fig.2 Particle size distribution and average particle Size of La2O3

nano particles

3.3 Scanning Electron Microscopy (SEM)

The grain size, shape and surface properties like morphology areobserved using SEM image. The SEM images of synthesized La2O3

nano particles using Pechini Method are shown in Fig. 3

Fig. 3 SEM images of La2O3 nano particles

It can be inferred from the SEM images shown in Fig. 3 that theparticles are porous and it is the significant property of insulatormaterial.

3.4 Energy Dispersive X-ray spectrometry (EDX)

The elemental composition of nano powders can be obtained fromEDX pattern shown in Fig. 4. The elemental composition per-centages of La2O3 nano particles are given in Table 1. Hence theexistence of La2O3 can be observed from the peaks seen in Fig. 4and also from the compositions in Table 1.

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Fig. 4 EDX Pattern of La2O3 nano particles

Table 1 Elemental composition of La2O3 nano particles

3.5 Fourier Transform Infrared spectroscopy (FTIR)

FTIR analysis has been done in the wave number range from 450/cmto 4000/cm. The sample was admixed with KBr, thoroughly mixedand pelletized by pressing under sufficient pressure before FTIRanalysis. La2O3 nano particles are analysed by FTIR spectrom-eter and are shown in Fig. 5. The very weak absorption bands

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at 3567.66/cm is assigned to O-H stretching vibration of watermolecules, due to presence of moisture in the sample. Very weakbending vibrations of water molecules appeared at 1560.06/cm,medium strong band positions in the range of 1430/cm to 1560/cmare possibly due to stretching vibrations of ions. The narrow ab-sorption peak observed around at 1120/cm can be ascribed to theC=O bonding. The medium to strong absorption bands at 620/cmwere because of La-O stretching. Hence the existence of abovementioned bands identify the presence of La2O3.

Fig. 5 FTIR spectrum of La2O3 nano particles

3.6 Thermo Gravimetric and Differential Ther-mal Analysis (TGDTA)

The TG analysis of La2O3 nano particles synthesized at constantusing Mixture of Fuels Method and using Pechini Method is repre-sented in Fig. 6. The temperature range is 50◦C to 800◦C. The ini-tial weight loss observed at 450◦C corresponds to that of loss of car-bonaceous compounds. The peak observed after 450C correspondsto decomposition of covalently bond organic material, mainly car-bon which was converted into CO2 at the time of synthesis. FromDTA curves of La2O3 nano particles the exothermic peak present

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in between 500◦C to 600◦C can be observed due to desorption anddecomposition of carbonaceous materials.In TG analysis, the initial weight loss observed is below the 450◦C,it shows that the loss of water which is evaporated on the surface ofthe sample. Beyond this temperature the peak increment is becauseof adsorption due to the decomposition of covalently bound organicmaterial. The weight loss of the La2O3 nano particles synthesizedis calculated to be 19.5%.

Fig. 6 TG/DTA curves of La2O3 nano particles

3.7.Transmission electron microscopy (TEM) Analysis The TEManalysis show the agglomerated sample in nano range. Fig. 7 showsthe TEM micrograph of La2O3 nano particles existing on the syn-thesized sample.

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Fig. 7 TEM micrograph at different magnifications

3.7 LCR Meter for Dielectric Analysis

The dielectric studies have been carried out on the as prepared sam-ples of La2O3. The analysis was conducted in the frequency rangeof 1 KHz to 2 MHz. The dielectric constant which is dependent oncapacitance was observed to be very high at lower frequencies andgradually decrease as the frequency is increased. Finally as the fre-quency reached to 1.5 MHz, the nano particles start stabilizing andbecame nearly stable at 2 MHz. The temperature was maintainedconstant at room temperature. From the dielectric analysis shownin Fig. 8, the maximum dielectric constant of the synthesized sam-ple is observed to be 30.65 F/m approximately which is very highcompared to that of SiO2.

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Fig. 8 Plot of Dielectric constant Vs. Frequency of the samples ofLa2O3 nano particles

4 Band Energy Diagram studies of SiO2

and La2O3 contained MOS capacitors

The performance of the MOS capacitors containing SiO2 and La2O3

as their gate oxide material has been estimated through energyband diagrams by considering parameters like charge density (),electric field (E), potential (V) and energy (eV). The nano MOScapacitors are built layer by layer using simulation software devel-oped by Richard G. Soutwick et. al. of Boise State Universitywhich has the inbuilt database of substrates, oxide layers and gatemetal contact materials. For the first nano N-MOS capacitor, con-ventional materials i.e., Silicon (Si) of 50nm thickness as substrate,Silicon dioxide (SiO2) of 2nm thickness as oxide layer and Alu-minium of 2nm thickness as gate contact material are selected. Forthe other N-MOS capacitor, conventional materials i.e., Silicon (Si)of 50nm thickness as substrate, Lanthanum Oxide (La2O3) of 2nmthickness as oxide layer and Aluminium of 2nm thickness as gatecontact material are selected. The structures of SiO2 contained

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MOS capacitor and La2O3 contained MOS capacitor are shown inFig. 9 and Fig. 10 respectively.

Fig. 9 Structure of SiO2 contained MOS capacitor

Fig. 10 Structure of La2o3 contained MOS capacitor

The simulation is run over the gate voltage -2.0 to +2.0V rangeat 300K temperature. The properties of the materials used are

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tabulated in Table 2. The parameters of the SiO2 contained MOScapacitor to measure the performance are compared against the pa-rameters of the La2O3containedMOScapacitorandareshowninTable3.

Table 2 Properties of the materials used for MOS Capacitorcontaining SiO2 and La2O3 as gate oxide materials

The charge density of the SiO2 contained MOS capacitor isshown in Fig. 11. In Fig. 11, the blue line represents the chargedensity of Aluminium (Al) gate contact, red line represents thecharge density of SiO2 and the dark green line represents the chargedensity of Si substrate. For the applied gate voltage of +2V, Fig.11 shows the charge density of 0.00000312C/cm2 for Al, 0C/cm2for SiO2 (since dielectric is an insulator) and -0.00000175C/cm2for Si and then reached zero at and after 2.5nm transition distanceapproximately.

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Fig. 11 The charge density of the SiO2 contained MOS capacitor

Fig. 12 The charge density of the La2O3 contained MOS capacitor

The charge density of the La2O3 contained MOS capacitor isshown in Fig. 12. In Fig. 12, the blue line represents the chargedensity of Aluminium (Al) gate contact, pink line represents thecharge density of La2O3 and the dark green line represents thecharge density of Si substrate. For the applied gate voltage of +2V,Fig. 12 shows the charge density of 0.000018C/cm2 for Al, 0C/cm2

for SiO2 (since dielectric is an insulator) and -0.000064C/cm2 for

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Si and then reached zero at and after 1.5nm transition distanceapproximately.

Fig. 13 The Electric Field of the SiO2 contained MOS capacitor

The Electric Field of the SiO2 contained MOS capacitor isshown in Fig. 13. In the Fig. 13, the bottom blue line repre-sents the electric field of Al, red line represents the electric fieldin SiO2 and the dark green line represents the electric field in Sisubstrate. For the applied gate voltage of +2V, the graph showsthe rise of electric field till 8.8MV/cm and the same field is main-tained in SiO2up to 5nm transition distance approximately andthen reached zero.

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Fig. 14 The Electric Field of the La2O3 contained MOS capacitor

The Electric Field of the La2O3 contained MOS capacitor isshown in Fig. 14. In the Fig. 14, the bottom blue line repre-sents the electric field of Al, pink line represents the electric fieldin La2O3 and the dark green line represents the electric field in Sisubstrate. For the applied gate voltage of +2V, the graph shows therise of electric field till 8.7MV/cm and the same field is maintainedin La2O3 up to 5nm transition distance approximately. Then asudden rise of electric field in Si substrate is observed at 5nm dis-tance approximately and then reached zero which clearly infers theinsulating property of La2O3 dielectric material.

Fig. 15 Energy of the SiO2 contained MOS capacitor

The Energy of the SiO2 contained MOS capacitor is shown inFig. 15. In the Fig. 15, the blue line represents the energy bandof Al, red polygon region represents the energy band of SiO2 andthe dark green line represents the energy band of the Si substrate.The Al material has the energy of -6.1eV till the distance of 2.5nmapproximately and the energy band bending can be observed inSiO2 due to the applied gate bias voltage. As the positive voltageis applied, the energy band shifts downward and allows the electronsto flow towards the other side (towards the oxide layer) if the emptybands are present below the Fermi level. From the Fig. 15 it can beinferred that there is a possibility for the electrons to tunnel towards

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the SiO2 layer as less energy of SiO2 i.e., -6.1eV has been observedcompared to that of Si substrate which is -4.7eV approximately.The height of tunnelling barrier is 10.8eV which is observed fromthe energy band gap between the Si substrate and SiO2 oxide layer.The Energy of the La2O3 contained MOS capacitor is shown in Fig.16. In the Fig. 16, the blue line represents the energy band of Al,pink polygon region represent energy band of La2O3 and the darkgreen line represents the energy bands of Si substrate. Similar tothe SiO2 contained nano MOS capacitor, Si has the energy of -6.1eVand band bending can be observed in La2O3 material. From Fig.16, it can be inferred that there is possibility of electrons to tunnelfrom Si substrate to Al contact through the oxide layer. The heightof the tunnelling barrier is 7.6eV which is lesser compared to thatof SiO2 contained MOS capacitor. So, the transition probability oftunnelling is lower in La2O3 contained MOS capacitor because ofthe higher barrier height and less barrier width.

Fig. 16 The Energy of the La2O3 contained MOS capacitor

The Potential of the SiO2 and La2O3 contained MOS capacitorsare shown in Fig. 17 and Fig. 18 respectively.From the Fig. 17 and Fig. 18, it can be inferred that there is nomuch difference in the potential of the nano MOS capacitors and

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hence the channel conductance and the ability of the oxide materialto shield the electric field depends on the applied drain voltage.

Fig. 17 The Potential of the SiO2 contained MOS capacitor

Fig. 18 The Potential of the La2O3 contained MOS capacitor

5.Results and Discussion The parameters like flat band voltage,equivalent oxide thickness, total capacitance and threshold voltageextracted for both the capacitors are shown in Table 3.

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Table 3 Parameters of SiO2 and La2O3 contained MOS capacitors

From the results shown in Table 3, it can be inferred that La2O3

contained MOS capacitor shows much more capacitance that SiO2

contained MOS capacitor which in turn infers that La2O3 MOScapacitor is better suitable for nano electronic devices since morecharge can be stored in an area equal to that of the SiO2 containedMOS capacitor. It also can be inferred from the results shown inTable 3 that the leakage current associated with La2O3 containedMOS capacitor is low due to high threshold voltage compared tothat of the SiO2 contained MOS capacitor.

5 CONCLUSION

La2O3 nano particles are synthesized by Pechini method and char-acterized by various techniques and its suitability for nano elec-tronic devices has been tested by developing two n-type nano MOScapacitors. One being SiO2 contained MOS capacitor and the otherbeing La2O3 contained MOS capacitor. Both the MOS devices aresimulated for charge density, electric field, energy and potential.Respective graphs are compared. From the comparison it is knownthat the nano MOS capacitor containing La2O3 oxide layer driveshigh drain current compared to the nano MOS capacitor contain-ing SiO2 oxide layer. It has been observed that the La2O3 nanoparticles have enhanced properties and significantly high dielectricconstant which is needed for nano electronic applications. Hence itcan be concluded that the La2O3 nano particles can be coated to athin film and can be perfectly replace SiO2 as oxide layer in MOSdevices.

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