Journal of Physics and Chemistry of Solids 73 (2012) 257–261

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Optical bandgap and electrical conductivity studies on near stoichiometricLiNbO3 crystals prepared by VTE process

Rajeev Bhatt n, S. Ganesamoorthy, Indranil Bhaumik, A.K. Karnal, P.K. Gupta

Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore, MP 452013, India

a r t i c l e i n f o

Article history:

Received 3 March 2010

Received in revised form

18 September 2011

Accepted 24 October 2011Available online 4 November 2011

Keywords:

A. Optical materials

B. Crystal growth

D. Defects

D. Electrical properties

D. Optical properties

97/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jpcs.2011.10.033

esponding author. Tel.: þ91 731 2488657; fa

ail address: rbhatt@rrcat.gov.in (R. Bhatt).

a b s t r a c t

Vapour transport equilibrium (VTE) technique was used to prepare near stoichiometric LiNbO3 (NSLN)

crystals. Simultaneous occurrence of reduction has been observed during the Li-enrichment that results

in the weak absorption bands centred at 1.7, 2.6 and 3.7 eV in the absorption spectrum. Annealing in

oxygen atmosphere resulted in decrease in the intensity of these bands. The indirect and direct band-

gap energies for NSLN crystals evaluated from absorption studies are reported. The energy of the

phonon involved in the indirect transition is �85 meV (685 cm�1). Near room temperature

ac-conductivity measurements reveal lower conductivity for oxygen annealed NSLN crystal in

comparison to as prepared NSLN and CLN specimens. The activation energies for ac-conductivity along

the z-direction for NSLN and CLN crystals in the temperature range 500–1100 K are 1.03 eV and 0.96 eV,

respectively.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

LiNbO3 (LN) is an important functional material and findsdiverse applications in opto-electronics industry [1–2]. It isnormally grown from melt with congruent composition(48.6 mol% Li2O) and exhibits non-stoichiometry. The congruentLiNbO3 (CLN) crystals contain lot of intrinsic defects and disorder,in particular Nb-antisites (NbLi

5þ) and corresponding charge com-pensating Li vacancies (VLi

�1) [2]. Stoichiometric LiNbO3 crystal(SLN), on the other hand, has lesser intrinsic defects, and showsenhanced non-linear optical properties [3] and lower coercivefields �200 V/mm [4]. SLN crystals therefore offer favourableproperties for realisation of periodically poled devices (PPLN) [5],domain engineering and photonic crystals [6], photorefractiveand holographic data storage applications [7].

Near stoichiometric LiNbO3 (NSLN) crystals have been grownby double crucible Czochralaki (DCCz) technique [2], top seededsolution growth (TSSG) technique from Li2O and K2O fluxes [6,8]and post-growth vapour transport equilibrium (VTE) technique[9–12]. However, VTE is an easier and effective method to prepareNSLN crystals with Li/Nb�1 (i.e. close to stoichiometry limit).Recently, Zhang et al. [13] have reported Li-poor vapour transportequilibration to prepare off-congruent Li-poor Mg:LN withdesired Li2O content. Also, Hua et al. [14] have reported Li-poor

ll rights reserved.

x: þ91 731 2488650.

vapour transport equilibration to prepare off-congruent LiNbO3

crystals for integrated optics application.In the present investigation NSLN crystal has been prepared

using VTE process and a signature of mild reduction phenomenonhas been observed along with Li enrichment. The reductionphenomenon has resulted in appearance of weak absorptionbands in the absorption spectra and influenced the electricalconductivity near room temperature. The effect of Li-enrichmenton birefringence, bandgap and electrical conductivity is alsopresented.

2. Experimental Details

Congruent LN single crystal was grown by the Czochralskitechnique. High purity chemicals of Li2CO3 (99.999%) and Nb2O5

(99.99%) were weighed for 48.45 and 51.55 mol%, respectively.These were mixed thoroughly and fired at 1273 K for 24 h. Thesynthesised charge was transferred to a platinum crucible of50 mm diameter and 50 mm height. Growth was carried outalong [00.1] with pull and rotation rates as 3 mm/h and15–5 rpm, respectively.

For VTE process, z-oriented CLN wafer of thickness �1 mmwas cut from the grown boule and polished. The oriented waferwas placed in a platinum crucible containing nearly 300 g of two-phase lithium-rich powder (Li3NbO4þLiNbO3), which wasprepared by solid state reaction of 65 mol % Li2CO3 and 35 mol% Nb2O5 at 1273 K for 24 h. The crucible was covered tightly by aplatinum foil and heated to 1373 K in a resistive heated furnace

R. Bhatt et al. / Journal of Physics and Chemistry of Solids 73 (2012) 257–261258

for 100 h. The VTE-treated sample was then re-polished forsubsequent measurements.

The UV–vis absorption spectra were recorded using a commer-cial spectrophotometer Cary 5E. The measurements were carried outat room temperature with unpolarised light at normal incidence.Refractive indices were measured at 532 nm wavelength usingMetricon 2010/M prism coupling system. Z-cut polished plate ofthickness 1 mm was used for this study. The density of theVTE-treated sample was determined by the Archimedes principleusing distilled water as the floating medium.

The ac conductivity measurements were performed on z-cutplates of CLN (4.2�6.4�1.0 mm3) and VTE-treated sample(8�7.6�1.0 mm3) using HP 4294 impedance analyser (fre-quency range: 102–106 Hz; temperature range: 500–1100 K).Platinum paste was used for electroding the samples.

Fig. 2. Difference of absorption coefficient spectra for NSLN and CLN crystals.

3. Results and discussion

Fig. 1 shows the absorption coefficient spectra of CLN and VTEprepared near stoichiometric LiNbO3 (NSLN) samples. The absorp-tion coefficient (a in cm�1) was calculated from the roomtemperature transmittance spectra using the relation T�(1�R)2

e�ad, where T is transmittance, R is reflectivity (calculated fromthe Sellmeier’s equation) and d is the sample thickness. Theobserved shift in UV absorption edge (AE) towards the shorterwavelength confirmed the improvement in Li-content of the VTEtreated LN crystal (NSLN). The Li content is estimated from thecomposition dependent AE study [15]. The measured AEs are 305and 318 nm (at a�20 cm�1) for NSLN and CLN samples, whichcorrespond to Li2O content (mol%) �49.64 for NSLN with Li/Nb�0.985 and 48.60 for CLN with Li/Nb �0.945. As CLN containsLi-vacancies and Nb-antisite defects [16,17], annealing in Li-richambient (at �1373 K for 100 h) compensates Li-vacancies (VLi

�)and accordingly reduces NbLi

þ5, hence composition equilibratesclose to the stoichiometric limit (Li/Nb �1) [9]. The density andrefractive index measurements also confirmed decrease in intrin-sic defects (NbLi

þ5 and VLi�). The density decreases from

4.64870.001 g/cm3 for CLN to 4.60270.001 g/cm3 for NSLNcrystal. This is in agreement with the Li-site vacancy model[16]. As refractive index is directly related to density, the extra-ordinary refractive index (ne) was found to decrease from 2.234

5

10

15

20

25

0.8

1.0

1.2

1.4

1.6

α (c

m-1

)

clnnsln

Photon energy (eV)

Abs

. coe

ffi. (

cm-1

)

clnnsln

Photon energy (eV)1.8 4.23.63.02.4

3.02.0 2.51.5

Fig. 1. Absorption coefficient spectra of NSLN and CLN crystals. Inset shows

absorption peaks at 2.6 eV and 1.7 eV in NSLN crystals.

(CLN) to 2.225 (NSLN) with Dne�0.009 measured at 532 nm withan accuracy of 70.001. The ordinary refractive index (no) wasfound nearly the same �2.324 and 2.323 for CLN and NSLNcrystals. However, the birefringence (Dn) has increased from0.090 for CLN to 0.098 for NSLN, making it more suitable forphotonic applications. The decrease in ne confirms the increase inLi-content of the NSLN crystals. Further, refractive index (n) isrelated to the band gap energy (Eg) by the empirical relation:n2E1þc/Eg, where c is a material dependent constant [18]. Theobserved lower ne is in agreement with the shift of AE towardshigher photon energies (higher band gap energy) for NSLN crystal.

Referring to the absorption coefficient spectra (Fig. 1 and inset)and difference of absorption spectra of NSLN and CLN crystals(Fig. 2), three weak absorption bands around 3.7 eV (340 nm),2.6 eV (480 nm) and 1.7 eV (730 nm) are observed in the NSLNspecimen. The origin of these bands is attributed to the reductionphenomenon in the sample that occurs during the high tempera-ture annealing (VTE) process. The reduction process (loss ofoxygen) can be described as OO-1=2O2þV ::

Oþ2e�, where VO��

is an oxygen vacancy with two positive charges and e� issubsequent gain of electrons [19]. These thermally activatedelectrons are then trapped at the different defect sites in thecrystal lattice and subsequently form F centres (3.3 eV) [20],Q-polarons (interaction of two bipolarons) (3.6 eV) [21], bi-polarons [NbLi

4þ–NbNb4þ] (2.6 eV) and small polarons [NbLi

4þ](1.6 eV) [22], depending upon the number of trapped electrons.The observed absorption bands in NSLN are in good agreementwith the bands reported in reduced CLN crystals [20–23]. Thereduced CLN shows strong absorption bands along with a shift infundamental AE towards longer wavelength (redshift). Fig. 3shows the effect of oxygen annealing on the absorption spectraof NSLN and oxygen annealed (at 950 1C for 6 h) NSLN crystals.The oxygen annealed NSLN crystal shows decrease in the inten-sity of 1.7 eV, 2.6 eV and 3.7 eV absorption bands, thereforeconfirms the reduction phenomena in VTE prepared NSLNcrystals.

In order to precisely determine the band gap energies, absorp-tion spectra near the AE is analysed by the empirical relation [24]:ahvpðhv�EgÞ

m, where hn is the photon energy, Eg is the band gapenergy and m is an exponent determined by the nature of theelectron transition during the absorption process i.e. m¼1/2 fordirect transition and m¼2 for indirect transition. The allowedindirect transitions near the fundamental AE are explained by the

0.00

0.05

0.10

0.15

0.20

0.000

0.002

0.004

0.006

0.008clnnslnannealed nsln

Nor

. abs

. coe

ffi.(c

m-1

)

hv (eV)clnnslnannealed nsln

Nor

. abs

. coe

ffi. (

cm-1

)

Photon energy (eV)1.8 4.23.63.02.4

1.5 3.02.52.0

Fig. 3. Absorption coefficient spectra of NSLN and oxygen annealed NSLN crystals

showing effect of oxygen annealing on the reduction induced defects.

Fig. 4. Typical E–k diagram for LiNbO3 crystal depicting interband indirect and

direct transitions. The direct transition (vertical) is represented by DT and the

indirect transition by IDT along with the associated phonon (horizontal line).

0

5

10

15

20

25

E+

E-

Phonon emission

Phonon absorption

cln 3.74 eVnsln 3.91 eV

(αhν

)1/2 (c

m-1

eV

)1/2

clnnsln

Photon energy (eV)2.5 5.04.54.03.53.0

Fig. 5. The dependence of (ahn)1/2 with incident photon energy (hn) showing two

regions corresponding to phonon absorption and emission during photon absorp-

tion process near the band edge.

0.0

20.0k

40.0k

60.0k

80.0k

100.0k

120.0k

140.0k

cln 3.93 eVnsln 4.10 eV

Photon energy (eV)

(αhν

)2 (cm

-1 e

V)2

clnnsln

3.50 4.504.254.003.75

Fig. 6. Dependence of (ahn)2 on incident photon energy (hn).

Table 1Stoichiometry dependent band gap characteristics of LiNbO3 crystals.

Sample AE

(nm)

E1

(eV)

E2

(eV)

Egind

(eV)

Egd

(eV)

EP

(meV)

no ne Li/Nb

CLN 318 3.65 3.82 3.74 3.93 85 2.323 2.234 0.945

nSLN 305 3.83 3.99 3.91 4.10 80 2.324 2.225 0.985

R. Bhatt et al. / Journal of Physics and Chemistry of Solids 73 (2012) 257–261 259

relation:

ahvpðhv�Eind

g 7hOÞ2 for hvZEindg 7hO

0 otherwise

(

where Eindg is the indirect band gap energy and hO is the energy of

the phonons absorbed (þ sign) or emitted (� sign) to conservethe momentum for the transition. Fig. 4 shows a typical E–k

diagram for LiNbO3 crystals depicting direct (DT) and indirect(IDT) transitions involving phonons (horizontal line). The funda-mental interband transition in LN crystal is represented by thevalence-band maximum at the G point and the conduction-bandminima located at 0.4G–K in reciprocal (momentum) space [25],as shown in Fig. 4. However, the direct transition usually takeplace at G point (k¼0).

Fig. 5 shows the plot of (ahn)1/2 versus hn for CLN and NSLNsamples showing photon absorption processes near the bandedge. The presence of two adjoining linear regions (dotted lines)near the AE represents phonon assisted (absorbed or emitted)photon absorption. The straight line fitting and their respectiveintercepts at a¼0 (i.e. E1 and E2) with the energy axis is used toevaluate the indirect band gap Eind

g and associated phonon energy

Ep using the relations: E1¼Eindg �Ep and E2¼Eind

g þEp. The calcu-lated indirect band-gap energy Eind

g for CLN and near NSLN is 3.74and 3.91 eV, respectively (Table 1). The associated phonon energyis 0.085 eV (685 cm�1) for CLN and 0.080 eV (645 cm�1) forNSLN. The phonons with energy �645 cm�1 have been observedin the LN crystals by Raman and IR spectra [26]. The observed Eind

g

and Ep are in good agreement with the reported values(Eind

g �3.78 eV [22] and Ep�0.08 eV [27]) for CLN crystals. The directband gap energy is evaluated from the plot of (ahn)2 versus hn(Fig. 6). The steep rise of the absorption and its linear fit at higherphoton energy depict the direct allowed interband transition.

102 103 104 1051E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

clnnslnannealed nsln

σ' [S

/cm

]

Freq. [Hz]

343K

Fig. 7. Frequency dependent electrical conductivity of CLN, NSLN and oxygen

annealed NSLN crystals at 343 K.

-22

-21

-20

-19

-18

-17

clnnslnannealed nslnlinear fit

ln (σ

') S

/cm

1000/T [K]2.0 3.43.23.02.82.62.42.2

Fig. 8. Arrhenious plot of electrical conductivity of CLN, NSLN and oxygen

annealed NSLN crystals with temperature in the range 300–500 K.

-17

-16

-15

-14

-13

-12

-11

NSLNCLNLinear fit

ln (σ

) (S/

cm)

1000/T (K)0.9 1.41.31.21.11.0

Fig. 9. Arrhenious plot of electrical conductivity of CLN and NSLN in the

temperature range 600–1100 K.

R. Bhatt et al. / Journal of Physics and Chemistry of Solids 73 (2012) 257–261260

The intercept of the straight lines with the energy axis yields directband-gap (Egd) energies of 3.93 eV for CLN and 4.10 eV for NSLN.

Like optical, electrical properties are also sensitive to defectsand their migration. The defect induced scattering has stronginfluence on the electrical conduction processes [28–30]. Fig. 7shows the near room temperature (343 K) ac conductivity (sac)with frequency for CLN, NSLN and oxygen annealed NSLN crystal.The ac conductivity is approximated by Jonscher’s power-lawbehaviour:sac � sdcþAos, where A is the temperature dependentconstant, o is the probing frequency and s (0oso1) is anexponent that depends on both frequency and temperature. Theconductivity is frequency independent in the low-frequencyregion representing the dc contribution. The estimated near roomtemperature dc-conductivities for CLN (�2.0�10�10 S-cm�1)and NSLN (1.6�10�10 S-cm�1) are nearly same and is higherthan the oxygen annealed NSLN crystal (8.6�10�12 S-cm�1).However, with the increase in frequency the conductivity of NSLNincreases in comparison to CLN. The lower sdc for oxygenannealed NSLN specimen is attributed to lowering of defects onannealing. This is in agreement with Ref. [29], wherein, increasedconductivity is reported for reduced material.

The activation energy (Ea) for electrical conduction is esti-mated from the Arrhenius relation:s¼ so expð�Ea=kTÞ, where so

is the pre-exponential factor and k is the Boltzmann constant.Fig. 8 shows the plots of ln(s) against the inverse of temperature(1000/T) for z-cut NSLN, oxygen annealed NSLN and CLN crystalsin the temperature range 300–500 K measured at probingfrequency of 100 Hz. The slope of the fitted straight line yieldsEa, which is lowest for NSLN (�0.11 eV) in comparison to oxygenannealed NSLN (�0.14 eV) and CLN (�0.18 eV) specimen. In spiteof increased Li content and decreased intrinsic defects, theobserved decrease in Ea for NSLN in comparison to CLN crystalindicates the presence of other defects, which are identified asreduction induced F/Fþ-centres and polarons (NbLi

4þ). A slightlyhigher Ea and lower electrical conductivity for oxygen annealedNSLN reveal decrease in the concentration of these defects afterthe oxygen annealing. Also, a slightly lower Ea and comparabledc-conductivity for NSLN (with respect to CLN crystal) indicatethe role of reduction induced defects on the electrical conductionnear room temperature in the NSLN crystals. This is in agreementwith Ref. [31], where electrical conduction in lithium niobate atlow temperature (�400 K) has been attributed to the hoppingmechanism of polarons and electrons. Fig. 9 shows the plots of

ln(s) versus inverse of temperature (1000/T) for NSLN and CLNcrystals in the temperature range 600–1100 K measured at probingfrequency of 100 Hz. Here NSLN crystal shows higher Ea�1.03 eVthan the CLN (�0.96 eV) crystal. Accordingly, the lower electricalconductivity is observed for NSLN (�3.20�10�6 S-cm�1) in com-parison to CLN (�5.02�10�6 S-cm�1) crystal measured at 700 K.The observed Ea and electrical conductivity for CLN are in agreementwith the values presented in Ref. [32]. The conduction in LN crystalin this temperature range is dominated by Li ions (Ea�1.17 eV [33])and protons [Hþ] (Ea�0.87 eV [34]). The observed lower electricalconductivity and higher activation energy for NSLN are because oflower Li vacancies and Hþ ions (proton annihilation as observedfrom FTIR spectra [13]) due to reduction. It appears from theelectrical conductivity measurements that reduction induced defectsare contributing to the electrical conduction near the room tem-perature but not in the high temperature region. (Table 2)

The electrical–optical correlation for polaron conduction is eval-uated from the RH-theory [35], which describes the dependence of

Table 2Electrical conductivity and activation energy for CLN and NSLN crystals.

Sample Ea (eV) @ 100 Hz

300–500 K

Ea (eV) @100 Hz

600–1100 K

sdc (S/cm) 343 K sdc (S/cm)

estimated

sdc (S/cm) 700 K

NSLN 0.11 1.03 1.6�10�10 4.2�10�11 3.2�10�6

NSLN annealed 0.14 – 8.6�10�12 –

CLN 0.18 0.96 2.0�10�10 – 5.0�10�6

R. Bhatt et al. / Journal of Physics and Chemistry of Solids 73 (2012) 257–261 261

optical absorption coefficient (a) with electrical conductivity as

a¼ so

nceo

sinhð_o=kBTÞ

ð_o=kBTÞexp �

_2o2

16EakBT

!

where so is the DC conductivity, eo the free space permittivity, c thevelocity of light, Ea the activation energy for dc electrical conductivity,o the angular frequency of the incident light and T the absolutetemperature. The polaron related absorption band at photon energy(Eopt) in the absorption spectra is approximated to activation energyby Eopt � 4Ea with full width at half maximum (FWHM) of theabsorption band as D�8(EakBT)1/2 [35]. The measured absorptioncoefficient is used to estimate the conductivity from the aboveequation using the values of refractive index and activation energyat that frequency. RH-theory, therefore predicts an activation energy(Ea)�0.65 eV for the electrical conduction (near room temperature)for the absorption band at 2.6 eV, which is higher than the measuredEa�0.11 eV (at 100 Hz) near room temperature. The estimateddc conductivity related to this band (a�1.2 cm�1, n�2.3 andEa�0.65 eV) at room temperature is �1.1�10�11 S/cm, which isan order of magnitude lower than the experimentally observed value�1.6�10�10 S/cm measured at 343 K for NSLN crystal and themeasured FWHM �0.7 eV of the 2.6 eV absorption band is alsolower than the estimated FWHM �1.01 eV. As the experimentallyobserved conductivity is higher than the estimated value for 2.6 eVband, it can be inferred that the reduction induced defects arecontributing reasonably to the electrical conduction processes nearroom temperature. However, for 1.6 eV absorption band (a�1 cm�1,n�2.1 and Ea�0.4 eV) the estimated electrical conductivity at roomtemperature is �4.0�10�8 S/cm, which is higher than the experi-mentally observed value 1.6�10�10 S/cm. This discrepancy may bedue to the inherent approximations used in the RH-theory [35,22].

4. Conclusion

NSLN crystals have been prepared by the VTE technique. Theabsorption spectra show the presence of weak absorptions bandsat 3.7, 2.6 and 1.7 eV in the VTE prepared NSLN crystals. Theseabsorption bands are correlated to reduction phenomenon thatoccurs during annealing in Li rich ambient. The annealing of NSLNcrystals in oxygen atmosphere resulted in decrease in the inten-sity of 2.6 eV and 1.7 eV absorption bands confirming the reduc-tion phenomenon during VTE process. The indirect and directband gap energies for CLN and VTE prepared near SLN areevaluated. The ac conductivity measurements reveal lower sdc

for oxygen annealed NSLN crystal in comparison to NSLN and CLNspecimens at around 343 K. The activation energy for ac conduc-tion is higher for CLN in the low temperature region, whereas in

the high temperature region it is higher for NSLN crystal. Thereduction induced defects are contributing to the electrical con-duction near the room temperature but not in the high tempera-ture region.

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