Effect of the ceria dopant on the structural and dielectric properties of ZnO semico nductors

All the samples exhibit a normal dielectric dispersion behavior, i.e. it decreases with increasing frequency due to the MaxwelleWagner type of interfacial polarization. The ac conductivity data was used to evaluate the maximum barrier height, the minimum hoping distance, and the density of localized states at Fermi level.

Original Article

Effect of the ceria dopant on the structural and dielectric properties of

ZnO semiconductors

B Rajesh Kumara,*, B Hymavathib, T Subba Raoc

a Department of Physics, Gandhi Institute of Technology and Management (GITAM-Deemed to be University), Visakhapatnam, 530045, Andhra Pradesh,

India

b Department of Physics, Anil Neerukonda Institute of Technology and Sciences (Autonomous), Sangivalasa, Visakhapatnam, 531162, Andhra Pradesh, India

c Department of Physics, Materials Research Lab, Sri Krishnadevaraya University, Anantapur, 515003, Andhra Pradesh, India

a r t i c l e i n f o

Article history:

Received 4 June 2018

Received in revised form

31 August 2018

Accepted 4 September 2018

Available online 11 September 2018

Keywords:

Solid state reaction

X-ray diffraction

Structural properties

Dielectric constant

AC conductivity

a b s t r a c t

ZnO doped with different concentrations (2, 4, 6, 8 and 10%) of ceria was synthesized by the conventional solidestate reaction method X-ray diffraction spectra confirm that all the samples have a hexagonal structure The structural properties of the samples were studied from X-ray diffraction data The surface morphology and elemental composition of the ceria doped ZnO samples were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy The variation of the dielectric constant, the dielectric loss and the ac conductivity as functions of frequency in the range of 100 Hze1 MHz for the as-prepared material were studied at room temperature by using impedance spectroscopy All the samples exhibit a normal dielectric dispersion behavior, i.e it decreases with increasing frequency due to the MaxwelleWagner type of interfacial polarization The ac conductivity data was used to evaluate the maximum barrier height, the minimum hoping distance, and the density of localized states at Fermi level

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

ZnO is a wide bandgap semiconductor with 3.37 eV and a large

exciton energy of 60 meV It crystallizes in the hexagonal wurtzite

structure Due to its unique chemical and physical properties, ZnO

had found widespread applicationst in UV light emitters, surface

acoustic wave (SAW) devices, solar cells and gas sensors[1] Doping

of ZnO can improve the structural properties of ZnO for various

applications[2] Rare earth metal-oxide nanoparticles have good

electronic, optical luminescence and magnetic properties due to

their unique electronic structure[3,4] They are widely used in

magnetic, electronic and functional materials owing to their special

characteristics Among the rare earth elements, Cerium oxide

(Ceria, CeO2) has received much attention due to its peculiar optical

and catalytic properties because of the availability of the shielded 4f

levels with only one electron in the 4f state, Ce3þ[5e7] CeO2can be

obtained in crystalline or amorphous forms at low temperatures

with a bandgap close to 3.1 eV (about 400 nm) Furthermore, the

refractive index of CeO2in the visible region is 2.1e2.2, almost the same as that of ZnO (2.0e2.1) This makes it a very attractive ma-terial [8] However, there are very few reports on the crystal structure and dielectric properties of the CeO2 doped nano-composite ZnO semiconductor In the present work, we have syn-thesized the ZnO doped with different concentrations of CeO2by the solidestate reaction method When compared with other conventional methods, the solidestate reaction has an advantage because of its low cost, high yield, and ability to achieve high purity

in making oxide nano powders[9] A systematic investigation on structural and frequency dependent dielectric properties of CeO2 doped ZnO is reported

2 Experimental ZnO doped with 2, 4, 6, 8 and 10% of CeO2were synthesized by the conventional solidestate reaction method The appropriate ratio of the constituent oxides i.e ZnO and CeO2(99.99% Aldrich Chemical, USA) were milled in a planetary ball mill (Retsch PM 200) with tungsten carbide balls (10 mm diameter) at a ball-to-powder weight ratio of 10:1 with a speed of 350 rpm for 24 h These mixed powders were calcined in a programmable SiC furnace at 900C for

* Corresponding author Fax: þ91 8554 255710

E-mail address: rajphyind@gmail.com (B Rajesh Kumar).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.09.001

2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

10 h and then pressed into pellets of 2 mm thickness and 10 mm in

diameter using PVA (polyvinyl alcohol) as a binder at a pressure of

400 kg/cm2 Finally, the CeO2doped ZnO pellets were sintered at

1100 C for 2 h Structural characterization of the samples was

carried out by using a Bruker D8 Advance X-ray Diffractometer with

a CuKa radiation (l¼ 0.154 nm) source operated at 40 kV and

40 mA XRD measurements were performed in the wide range of

Bragg angles 2q(10< 2q< 80) with a scan speed of 2min1 The

surface morphology of the samples has been studied using a

scanning electron microscope (SEM) (Model No Evo 18, Carl Zeiss

Germany) and the elemental composition of the samples was

determined by the energy dispersive X-ray spectroscopy (EDX;

Oxford Instruments, X-max) attached with SEM For electrical

measurements, the powdered samples have been pressed into

pellets of uniform diameter and thickness The pellets were coated

with silver paste on opposite faces in order to established a parallel

plate capacitor geometry and then sintered for 1 h at 100 C

Dielectric measurements of the samples were taken at room

tem-perature using an LCR HI-Tester (HIOKI 3532-50, Japan) in the

frequency range of 100 Hze1 MHz

3 Results and discussion

3.1 Structural analysis

X-ray diffraction (XRD) patterns of ZnO doped with different

concentrations of CeO2are shown in Fig 1(a) The narrow and

sharp XRD peaks confirm that the sample is of high quality with good crystallinity The diffraction peaks at 2q ¼ 31.65, 34.36, 36.21, 47.44, 56.46, 62.80, 66.18, 67.74and 68.87are

iden-tified with the Miller indices (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1), respectively, corresponding to

wurtzite structure In the range of 2q ¼ 28e33, two weaker diffraction peaks are attributed to (1 1 1) and (2 0 0) planes which indicate that the face-centered cubic (FCC) crystalline structure CeO2 exists in the samples according to the standard JCPDS (No.75-0390) card It is noticed fromFig 1(b) that the diffraction peaks are obviously shifted toward lower angles in the range of

2q¼ 31e37, indicating that cerium ions were doped into the ZnO lattice This is due to the fact that the ionic radius of Ce3þ (0.103 nm) is much larger than that of Zn2þ (0.074 nm), which cause an expansion of the lattice parameter in the cerium doped ZnO crystallites [10] The lattice parameters ‘a’ and ‘c’ of the prepared samples are calculated using the equations as reported

in our previous work[11] The lattice parameter‘a’ increases from 0.3254 to 0.3268 nm, whereas ‘c’ increases from 0.5211 to 0.5268 nm with the increase of doping concentrations of CeO2 from 2 to 10% in ZnO

The average crystallite size (D) of CeO2doped ZnO samples was calculated using Scherer formula[12]

where D is the crystallite size, k (¼ 0.94) is the shape factor andl

(¼0.154 nm) is the wavelength of Cu-Karadiation The decrease in crystallite size from 33 to 27 nm with the increase of the Ceria dopant concentration in ZnO is due to the distortion of host ZnO lattice by Ce3þions, which reduces the nucleation and subsequent the growth rate of ZnO This similar behavior was also observed in the previous literature works[13e16] The specific surface area of the crystallites of the samples was determined from XRD data using the Sauter formula[17]

S¼
6 103

where S is a specific surface area, D is the crystallite size andris the density of ZnO which equals to 5.606 g/cm3 The specific surface area of the samples increases with the increase of CeO2 concen-tration in ZnO The increase in the specific surface area is due to the presence of pores which leads to the decrease in particle size The presence of dislocations strongly influences the material properties generally, a larger dislocation density implies a higher hardness The dislocation density (d) in the sample was determined by using expression[18],

wheredis dislocation density,bis the broadening of diffraction line measured at half of its maximum intensity (in radian),q is the Bragg's diffraction angle (in degree), a is the lattice constant (in nm) and D is the crystallite size (in nm) The dislocation density (d) was found increase from 1.16 1015to 1.51 1015m2with the increase

of the concentration of Ceria from 2 to 10% in ZnO for the (0 0 2) orientation The structural parameters of CeO2doped ZnO calcu-lated from the X-ray diffraction data are given inTable 1

The wurtzite structure of ZnO has two types offirst-neighbour Zn-O bond distances: Zn-Ocalong the c-axis (one bond), Zn-Obin the basal plane (three bonds) and two bond anglesaandb[19]

Fig 1 X-ray diffraction patterns of ZnO doped with (a) 2%, (b) 4%, (c) 6%, (d) 8%, and (e)

B Rajesh Kumar et al / Journal of Science: Advanced Materials and Devices 3 (2018) 433e439 434

Zn Ob1¼hð1=3Þa2þ ðð1=2Þ  uÞ2c2i1=2

(5)

The wurtzite ZnO structure in its basal plane had the lattice

constants‘a’ and ‘c’ in the basal direction, ‘u’ parameter is expressed

as the nearest-neighbor distance or the bond length b divided by c

(0.375 in an ideal crystal), andaandb(109.47in the ideal crystal)

as the bond angles are shown in thefigure as reported in the earlier

work[20]

a¼ ðp=2Þ þ arc cos
n

1þ 3ðc=aÞ2ð1=2  uÞ2o1=21

(6)

b¼ 2arc sin
n

ð4=3Þ þ 4ðc=aÞ2ð1=2  uÞ2o1=21

(7)

where u denotes the cell internal parameter given by

u¼ ½ð1=3Þ
a2=c2

In addition three types of second-neighbour cationeanion

dis-tance connecting the cation M to the anions O b10, b20, b30

Zn Ob1 0¼ ð1  uÞ c ðone neighbour along the c  axisÞ (9)

Zn Ob20¼ ½a2þðucÞ21=2ðsix neighboursÞ (10)

Zn Ob30¼ ½ð4=3Þ a2þð1=2  uÞ2c21=2ðthree neighboursÞ

(11)

The calculated unit cell internal parameters and cationeanion

distances between the nearest and the second-nearest

neigh-bours (given in Å) as well as the bond angles (given in degrees) for

ceria doped ZnO are given inTable 2

3.2 Surface morphology and elemental analysis

Figure 2(a)e (e)shows the SEM images of CeO2doped ZnO The

surface morphologies of the samples appear smooth, but with a lot

of pores It is also observed that the surface of the sample is dense

up to 6% doped CeO2in ZnO, and as the CeO2content is increased to

10%, the surface becomes bumpy and rough.Fig 2(f) shows the EDX

spectra of the 6% CeO2doped ZnO The spectra reveals the presence

of the O, Zn and Ce elements along with their atomic percentage

composition The presence of O Kapeak at 0.56 keV, Zn Lapeak at 1.01, Zn Kapeak 8.68 keV, Ce M peak at 0.88 keV and Ce Lapeak at 4.84 keV was identified No other peaks related to impurities were detected in the spectrum confirming that CeO2 is successfully doped in ZnO

3.3 Room temperature frequency dependent dielectric properties The dielectric constant, dielectric loss tangent and ac electrical conductivity of CeO2doped ZnO were studied as a function of fre-quency from 100 Hz to 1 MHz using the impedance spectroscopy at room temperature The dielectric constant (εr) was calculated using the following formula[21]

where Cpis the capacitance of the specimen, d is the thickness of the pellet, A is the cross-sectional area of the sample and εo is permittivity of free space 8.85 1012F.m1 Dielectric loss or the imaginary dielectric constant (ε”) was calculated using the relation

where tandis the dielectric loss tangent proportional to the loss of energy from the appliedfield into the sample Fig 3(a) and (b) shows the variation of the real and the imaginary part of dielectric constant (εrandε”) as a function of frequencies for CeO2doped ZnO The variation of the real and the imaginary dielectric constant values with the frequency shows a similar behavior The dielectric constant values are found to be decreased with the increase of frequency At low frequency, the dielectric constant is high due to the accumulation of charges at the grain boundary, and at the interface of the electrode sample and the electrode which is also called space-charge polarization As the frequency increases, the dielectric constant decreases due to the diminishing of the space-charge polarization, indicating the electronic and atomic contri-bution domination According to MaxwelleWagner interfacial model, a dielectric medium consists of double layers having well-conducting grains which are separated by poorly well-conducting grain boundaries[22,23] The charge carriers can easily migrate to the grains by an external electricfield, but they are accumulated at the grain boundaries by resulting a large polarization with a high dielectric constant value The larger value of the dielectric constant

at low frequencies is due to the grain structure and the porosity It is also noticed that the dielectric constant decreases with the increase

Table 1

Structural parameters of CeO 2 doped ZnO nanocomposite semiconductor.

Concentration

of ceria (%)

Lattice parameter

Volume (Å 3 )

method, D (nm)

Specific surface area, S (m 2 /g)

Dislocation density,

d(0 0 2) (  10 15 m2)

Table 2

Calculated cationeanion distances between nearest and second-nearest neighbours (given in Å) as well as bond angles (given in degrees) for CeO 2 doped ZnO.

Concentration of ceria (%) Zn-O b Zn-O b 1 Zn-O b 1

0

Zn-O b 2 0

Zn-O b 3 0

a() b()

of the concentration of ceria upto 6% in ZnO because of the high

density of defects in ZnO But the further increase of ceria

con-centration from 8 to 10% in ZnO, the values of dielectric constant is

found to be increased This increase in the dielectric constant may

be due to the less electronegativeness of Ce (0.89) than that of Zn

(1.78) that makes the ionic bonds of Zn-O-Ce weaker than Zn-O-Zn

bonds[24]

The dielectric loss tangent (tand) represents the energy

dissipa-tion in the dielectric system The variadissipa-tion of tandwith frequency for

CeO2doped ZnO is shown inFig 4 The dielectric loss is high in the

lower frequency region of 100 Hze1 KHz and sharply decreased with

the increasing frequency compared with the dielectric constant The

dielectric loss decreases with the increase of the frequency up to

10 kHz and then increases at higher frequencies Accordingly, the dielectric loss at low and moderate frequencies is characterized by high values of the dielectric loss due to the contribution of ion jump and conduction loss of ion migration However, at higher frequencies the ion vibrations may be the source causing the dielectric loss[25]

As the CeO2dopant concentration increases from 2 to 6% in ZnO the dielectric loss decreases, whereas the dielectric loss increases with the increase of the ceria dopant concentration from 8 to 10% A shift

of the peak towards lower frequencies is observed The decrease of the dielectric loss tangent with the increase in frequency as seen in 2,

4, 6% of CeO doped ZnO is attributed as due to the space charge Fig 2 SEM images of ZnO doped with (a) 2%, (b) 4%, (c) 6%, (d) 8%, and (e) 10% CeO 2 (f) EDS of ZnO doped with 6% CeO 2

B Rajesh Kumar et al / Journal of Science: Advanced Materials and Devices 3 (2018) 433e439 436

polarization This behavior, as well as the low loss factor compared to

that of the undoped ZnO makes the prepared samples suitable for

high-frequency device applications With the further increase of

doping concentrations of CeO2in ZnO, the dielectric loss is found to

increase Peaks in the dielectric loss as observed indicate the

char-acteristic feature of the Debye-type relaxation process It is noticed

that the peak shifts toward lower frequencies with the increase of

CeO2concentration in ZnO The curves observed inFig 4are called

the Debye curves and um is the maximum angular frequency,

um¼ 2pfmax, fmaxis the relaxation frequency which is given by

wheretis the relaxation time The relaxation time for the 2, 4, 6, 8

and 10% ceria doped ZnO is found to be 2.44 106, 2.65 106,

3.88 106, 3.98 106and 2.56 106s, respectively

The ac conductivity (sac) of the samples was calculated from the relation[26]

whereεris the relative permittivity (or the real dielectric constant),

εo(¼8.85  1012F/m) is the permittivity of the free space and

u¼ 2pf is the angular frequency The dependence of the ac con-ductivity (sac) on the frequency for the ceria doped ZnO samples is shown inFig 5 The ac conductivity of CeO2doped ZnO samples increases with the increase of the frequency showing the semi-conducting behavior This increase in the electrical conductivity of the samples is related to the increase in the drift mobility of the electrons and holes by the hopping conduction [27] That the ac conductivity decreases with the increase of the concentration of CeO2from 2 to 6% in ZnO may be attributed as due to the fact that the dopants can introduce defects in the ZnO lattice These defects lead to segregation at the grain boundaries due to the diffusion process resulting from cooling and sintering processes With the further increase of the CeO2concentration from 8 to 10%, the ac conductivity is found to increase as a consequence of the electron hopping i.e., the number of the hopping charge carriers increases at the grain boundaries and correspondingly the conductivity in-creases rapidly

According to the Jonscher's power law, the frequency depen-dence of the conductivity is given by the expression[28]

wheresdcindicates the dc conductivity when s¼ 0, the electrical conduction is frequency independent (dc conduction) and the second term is the frequency dependent ac conductivity Here, A is known as the strength of polarizability and s is the temperature dependent parameter The parameters A and s can be obtained from the plot of lnsac versus lnu and the equation is known as a Jonscher's power law The values of the exponent s,sdcand A ob-tained byfitting the equation(5)are given inTable 3 Thefit quality

is usually evaluated by comparing the squared value of the linear correlation coefficient (R2) (see Table 3) Different models have been studied to explain the conduction mechanism on the basis of the parameter s Among these models, the quantum mechanical tunneling (QMT), the correlated barrier hopping (CBH), the small polaron hopping and the overlapping largee polaron (OLP) are the most applicable ones [29e32] If s is temperature independent, quantum mechanical tunneling is expected The CBH is usually associated with a decrease in s with the temperature Small polaron (SP) conduction is predominant if s increases with the temperature

In the OLP conduction mechanism, s decreases with the tempera-ture reaching a minimum value and then increases again In our

Fig 3 Frequency dependent (a) real dielectric constant and (b) imaginary dielectric

constant for ZnO doped with CeO 2

Fig 4 Frequency dependent tandwith different concentrations of CeO 2 doped ZnO.

Fig 5 Variation of ac conductivity with frequency for ZnO doped with CeO

system, the decreasing trend of s with increasing ceria dopant in

ZnO implies the CBH mechanism of conduction In this model, the

conduction of carriers takes place through the barriers separating

the localized sites In case of the present samples, the value of s

decreases from 0.63 to 0.24 with the increase of concentrations of

CeO2from 2 to 10% in ZnO, suggesting that the conduction

phe-nomenon in the material under study is due to the correlated

barrier hopping Therefore, the conduction in the system may be

considered as due to the short-range translational type hopping of

charge carriers This indicates that the conduction process is a

thermally activated one A similar trend in the ac conductivity has

been observed in many nanocrystallized semiconductor materials

[33,34]

The ac conductivity and frequency exponent expressions of CBH

model are given by the following equations[35,36]

s¼ 1  ½6kBT=ðWmkBTlnð1=utoÞÞ (17)

where kBis the Boltzmann's constant, T is the temperature,tois the

characteristics relaxation time of the carriers and Wmis the binding

energy or the maximum barrier height, which is defined as the

energy required to remove an electron completely from one site to

another site For a small value of T,Wm>> kBT ln (1/ut0) and,

therefore, the equation(3)can be written as

where Wmis the maximum barrier height or binding energy The

values of Wmwere calculated by putting values of s and T (¼303 K)

in the above equation Using the values of the binding energy,

minimum hopping distance Rminwas calculated with the equation

whereεois the permittivity of free space and ε is the dielectric

constant.Fig 6(a) represents the variation of Rminwith frequency

for ZnO doped with different concentrations of CeO2 It is noticed

that Rminincreases with the increase of ceria dopant in ZnO A low

value of Rmin in the lower frequency region was obtained

(~1010m) A continuous dispersion with the increase in frequency

has been observed This can be attributed to the conduction

phe-nomenon originating from the short-range mobility of charge

car-riers A sigmoidal increase in the value of Rminwith the frequency

approaches to a saturation value These observations are related to

a lack of a restoring force which governs the mobility of charge

carriers with the action of an induced electricfield[37] The Rmin

values at 10 KHz are found to be increased from 1.36 to 7.20 nm

with the increase of doping concentrations of CeO2from 2 to 10% in

ZnO

According to the AustineMott formula based on the CBH model,

the ac conductivitysac(u) is expressed in terms of the hopping of

electrons between a pair of localized states N(EF) at the Fermi level

The ac conductivitysacis related to the number of sites per unit

energy per unit volume N(EF) at the Fermi level and is given by the

equation[38]

sacðuÞ ¼ ðp=3Þ e2ukBTððNðEFÞÞ2a5ðln fo=uÞ4 (20)

where e is the electronic charge, fothe photon frequency andais the localized wave function The density of the localized states N(EF) was calculated according to equation(20)assuming fo¼ 1013Hz,

a¼ 1010m1.Fig 6(b) illustrates the variation of N(EF) with fre-quency for the ceria doped ZnO samples, where the values of N(EF) decreases with the increasing frequency It is also observed that the obtained value of N(EF) increases with the increasing ceria doping amount The high N(EF) values suggest that the hopping between the pairs of sites dominates the mechanism of the charge transport

[39]

4 Conclusion The CeO2 doped ZnO samples were prepared by the conven-tional solidestate reaction method The XRD patterns showed that these samples had a hexagonal wurtzite structure Since rare earth elements have larger ionic radii as compared with zinc, the incor-poration of trivalent cerium ions in the ZnO host lattice can cause a significant distortion therein The dielectric properties of the samples reveal that the dielectric constant and the dielectric loss of the samples decreases with the increase of frequency, whereas the electric conductivity increases with frequency The decrease of the dielectric loss tangent with an increase in frequency seen in the

Table 3

Fitting parameters obtained from experimental data of ac conductivity as a function of frequency using the Jonscher's power law and calculated values of W m and R min at

10 KHz.

Fig 6 Variation of (a) the minimum hopping distance, R min and (b) the density of states at Fermi level N(E F ) with frequency for ZnO doped with CeO 2

B Rajesh Kumar et al / Journal of Science: Advanced Materials and Devices 3 (2018) 433e439 438

ceria doped ZnO samples is attributed to the space charge

polari-zation A Debye-like relaxation in the dielectric loss was observed

for the ceria doped ZnO samples with a peak at a maximum

fre-quencyum, which is shifted to the lower frequency with increasing

the doping concentration of ceria in the Debye curves As CeO2

doping increased up to 6% in ZnO, the loss factor decreased to a

large extent From these results, we concluded that the 6% ceria

doped ZnO is a much better dielectric material than the parent ZnO

and suitable for high frequency device applications

Acknowledgements

The authors gratefully acknowledge the Director, Central

facil-ities for research and development, Osmania University,

Hyder-abad, India for providing the facility to perform SEM analysis

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