Chapter – 5 Fe:LiNbO 3

~ 117 ~

Chapter – 5

Crystalline perfection and optical

properties of Czochralski grown

Fe:LiNbO3: A photorefractive NLO

single crystal

Abstract

The large size bulk single crystal of Fe (0.05 mol%)doped LiNbO3 has

been grown by using a Czochralski method. The crystal structure has

been confirmed by powder X-ray diffractometry and the strains in the

crystal lattice have been evaluated . The crystalline perfection of the

grown crystal was assessed by a high resolution multicrystal X-ray

diffractometry. For the invest igation of vibrational modes, Li

composition, and concentration of protons (H+

) in the lattice of grown

crystal, Raman and Fourier transform infrared spectroscopy were

employed. The UV-VIS-NIR transmission spectrum was recorded and

the band gap energy was determined. The ordinary, extraordinary

refractive indices and birefringence of crystal has been studied by

using a prism coupler spectrometer . The wavelength dispersion , optical

dielectric constants , dispersion Ed and average single oscillator Eo

energy have been studied by the Ell ipsometry.

Chapter – 5 Fe:LiNbO 3

~ 118 ~

5.1 INTRODUCTION

Lithium Niobate (LiNbO3: LN) single crystals have been extensively

investigated for their application in volume holographic data storage, just after the

invention of optical damage in LN crystals (Ashkin, et al., 1966). The optical

damage/distortion in LN crystals occurs due to an induced change in the refractive

indices, which is more prominent for the extraordinary polarized light over ordinary

polarized. This photorefraction effect is considerably understood as; the light beam

produces the optical ionization of some uncontrollable donor impurities, the released

charge moves by some transport process (diffusion, drift or photovoltaic) to the dark

part of the sample where it is trapped by an acceptor center. The spatial distance

between the ionized donor and charged acceptor gives rise to an internal electric field

and the electric field by means of the electrooptic effect produces the refractive index

change. This photorefraction effect accounts with presence of certain impurities or

color centers which could be optically ionisable. For LN the most active

photorefractive impurities are Fe, Cu and Mn, these appear in simultaneous two

valance states: Fe2+

/Fe3+

, Cu+/Cu

2+, and Mn

2+/Mn

3+. The reduced valance ions act as

charge donors whereas oxidized valance ions act as acceptors. The inhomogeneous

light of proper wavelength produce ionization of donors and the released charges

move along the crystals by means of one of the charge transport processes: inter-band

diffusion, photovoltaic current, or drift in an external electric field. In the end the

charges get trapped in acceptors and this trapping occurs in a different place than

ionization which results a distribution of internal electric field. This electric field

distribution induces a refractive index change distribution via the electrooptic effect.

This process can be used positively to write with light and store the information for

some time in the crystals. In the presence of optical beam, the refractive indices of

crystal change by the photoexcited carriers, which transport along the polar axes of

crystals and subsequently being trapped. In iron doped LiNbO3 (Fe:LN) crystals, the

photoexcited carriers come from Fe2+

impurities and the photorefraction sensitivity is

proportional to its Fe2+

concentration and the photorefraction sensitivity can be

increased by doping the crystals with iron or copper impurities. The light induced

refractive index modulations in LiNbO3 can be used to advantage to obtain optical

Chapter – 5 Fe:LiNbO 3

~ 119 ~

phase conjugation through degenerate four wave mixing. Lithium Niobate has the

advantage of growing large size single crystals which allow for increased capacity and

high angular sensitivity, long storage lifetime among photorefractive crystals and high

diffraction efficiencies. The transition and rare earth metal ions such as Fe, Mn, Cu

and Ce, efficiently modify the photorefraction efficiency, hence improve the data

storage and processing capabilities. The Fe:LN crystals were also found to be suitable

for the pyroelectric infrared sensors and X-ray generation applications (Bayssie et al.,

2005). It is a good candidate for holographic data storage in system like optical

correlators where large numbers of analog images must be accessed at high speed.

The angle multiplexing holograms in LiNbO3 allows parallel access to up to a

thousand images which can be correlated against an input scene in a fraction of

second. The iron doped LiNbO3 crystal has peak sensitivity for writing holograms in

the blue-green wavelength region of electromagnetic radiation spectrum. A series of

metals have been used to influence the photorefraction behavior of LiNbO3, but out of

them iron found to most suitable for the diffraction applications hence for the

holographic recording (Deanna et al., 1998). The transition metals lead to the change

in refractive index and optical transparency of LN, at the same time these lead to the

numerous structural changes in the crystal lattice depending on the dopant

concentration and size of dopant. In spite of available indirect techniques to evaluate

the defects in single crystals, there is scope for more precise and in-depth studies

required by the strategic material of title material. In parallel to the improvement of

photorefraction behavior of crystals it is very important to assess the crystalline

perfection of grown bulk single crystals in the presence of photorefractive impurities.

In view of the above, in continuation of such investigations, in this chapter (i)

growth of good quality bulk Fe:LN single crystals by the Czochralski method, (ii)

evaluation of the structural strains in the crystal lattice, (iii) evaluation of crystalline

perfection by powder X-ray diffractometry and high-resolution X-ray diffractometry

(HRXRD) to reveal more information about the effect of Fe-doping and the associated

antisitic defects, (iv) Raman spectroscopy, (v) Fourier transform infra-red (FTIR)

spectroscopy to study the incorporation of proton into the crystal lattice, (vi) electron

paramagnetic resonance (EPR) spectrometry analysis to study the incorporated iron

Chapter – 5 Fe:LiNbO 3

~ 120 ~

and identify their paramagnetic sites, (vi) the prism coupler experiments to estimate

the difference in the refractive index ∆n accurately in ordinary and extraordinary rays

(vii) optical transmittance and absorbance study in the UV-VIS-NIR wavelength

range and (viii) ellipsometry investigations have been reported. The photorefractive

properties of LiNbO3 single crystals enhance with doping of iron and as reported

(Peithmann et al., 1999), at ~ 0.06 wt.% of Fe2O3 doping, LN exhibited the best

performance for the multiplexing capability related to the holographic applications.

Due to such reported doping level of proven advantage with the required most

efficient physical property, the bulk crystal of LiNbO3 with 0.05 mol% Fe2O3

concentration, has been grown by Czhochralski technique. The suitable correlation

between the crystalline perfection and physical properties has been proposed.

5.2 CRYSTAL GROWTH

The iron doped LiNbO3 single crystal with 0.05 mol% Fe2O3 has been grown

successfully on the Czochralski crystal puller developed at NPL [§§2.1.2.2]. A good

quality 20 mm long with square cross section (5×5 mm2) crystal cut along [001]

shown in Fig. 2.6(b), was used as a seed to grow bulk crystal. The crystal was grown

under the well-controlled temperature and pulling conditions and such growth

conditions facilitated to obtain a 50 mm long and ~ 20 mm diameter bulk single

crystal of Fe:LN without any visible voids or cracks. The photograph of harvested

crystal is shown in Fig 5.1, the used seed crystal intact with the bulk crystal is also

visible. The crystal is visibly quite transparent, but due to Fe doping, it adopted

slightly brown color. For the characterization purpose a Z-cut wafer was obtained

from the grown crystal boule by using a diamond cutter, which was lapped and

optically polished to final thickness of ~500 μm, shown in Fig. 5.2. The yellow arrow

over the crystal in photograph indicates the position of crystal boule from where the

wafer has been obtained. The clear visibility of the letters (NPL INDIA) in the back

ground of polished wafer is the signature of high optical transparency of the grown

bulk crystal.

Chapter – 5 Fe:LiNbO 3

~ 121 ~

Figure 5.1: The photograph of Czochralski grown Fe:LiNbO3 (0.05 mol%) bulk single crystal,

brown color of crystal is due to the present of iron in the crystal lattice.

Figure 5.2: The Z-cut lapped and polished ~500 μm thick wafer

Fe:LN

Fe:LN

Chapter – 5 Fe:LiNbO 3

~ 122 ~

5.3 CHARACTERIZATION STUDIES

To confirm the crystal structure/system of grown crystal (Fe:LN) and evaluate

the effect of dopant on the lattice structure, the powder X-ray diffraction (PXRD)

measurements. The homogeneous powdered specimens were prepared by crushing the

small portions from the crystal boules. The PXRD measurements were performed at

ambient temperature on a Bruker AXS D8 Advanced powder X-ray diffractometer as

described in §3.2 and the diffraction spectra were recorded in the 2θ angular range of

20–80 with step size 0.01º and time/step of 0.1 sec.

The lapped and polished crystal wafer, shown in Fig. 5.2, was subjected to the

high resolution multicrystal X-ray diffractometer (MCD) [§3.3]. As recently

described in the literature (Senthilkumar et al., 2011), the theoretical FWHM is nearly

proportional to the wavelength of the exploring X-ray beam and hence to get more

reliable information from a rocking curve, MoKα1 (λ=0.70926 Å) is better than that of

CuKα1 (λ=1.54056 Å). The omega scan in HRXRD was adopted to record the rocking

curve.

The EPR is very sensitive for the ionic states of the transition metal ions

present in the same host lattice. Therefore, to infer about the position and ionic state

of dopant ions of iron in LiNbO3 crystal the EPR spectrum of the grown crystal

(Fe:LN) has been recorded employing a Bruker Biospin A300 X-band EPR

spectrometer at the room temperature [§3.7]. The measurements were carried out at a

radio frequency of 9.462 GHz with the modulation frequency of 100 kHz at T=297 K.

A Z-cut rectangular crystal (1×3×5 mm3) was placed in the cavity keeping c-axis

parallel to the applied magnetic field.

The Raman spectral measurements on the Z-cut wafer of the grown crystals

have been carried out using a Renisha inVia Raman Microscope in back-scattering

mode at the room temperature. The details about the spectrometer are given in §3.5.

To study the stretching vibrational mode O–H corresponding to the incorporation of

protons (H+) and to evaluate their concentration, the FTIR spectrometer has been

employed [§3.4]. The spectra were recorded in the wavenumber range of 400 – 4000

Chapter – 5 Fe:LiNbO 3

~ 123 ~

cm-1

for the Z-cut wafer. The optical transparency and absorption spectra for the Z-cut

crystal wafer were recorded over the entire UV-VIS-NIR wavelength spectrum (200-

1100 nm).

LiNbO3 is a photorefractive medium and iron is capable in manipulating the

photorefraction properties of it by inducing the significant change in refractive

indices. The refractive indices of Z-cut wafer were measured on performed using the

Metricon prism coupler spectrometer (Model 2010/M). The standard rutile prism with

refractive index of 2.8659 was used to couple the 632.8 nm He-Ne laser beam to the

crystal wafer, and a pixel array detector was set to collect the emergent beam. The

Z-cut Fe:LN crystal wafer was subjected to an Ellipsometer for the measurement of

the average refractive index and extinction coefficients for the wavelength range of

200 – 1000 nm [§3.9].

5.4 RESULTS AND DISCUSSION

5.4.1 Powder X-ray diffraction analysis

As described in the previous chapter, LiNbO3 crystals have the rhombhohedral

structure at room temperature and belong to the R3c space group 3m point group

(Abrahams et al., 1966; Chemaya et al., 2001). The recorded PXRD spectra for the

grown Fe:LN crystal is shown in Fig 5.3. The patterns have confirmed crystal

structure of grown crystals and both the spectra have similar features indicating no

change in the crystal structure or origination of any other crystalline phase due to Fe

doping. The peaks in the diffraction spectra of Fe:LN crystal are very sharp similar to

that of pure and indicate the good crystallinity of Fe:LN crystal as of LN. However,

there is a slight variation in the peak intensities of the doped crystals owing to the

manipulation of the strains in the crystal lattice.

The strains in the crystal lattice of Fe:LN have been evaluated by using Hall-

Williamson relation ( sin/cos ). The βcosθ vs. sinθ plot is shown in

Fig. 5.4 the η value found to be –3.38×10-4

, which is of the order for pure crystal

(§4.4.1). The –ve value of η indicates the vacancy type of defects in the grown bulk

crystal. These vacancy defects are expected due to the deficiency of Li and creation of

Chapter – 5 Fe:LiNbO 3

~ 124 ~

0

700

1400

2100

2800

20 30 40 50 60 70 800

700

1400

2100

2800

LN (012)

(31

2)

(30

6)

(22

0)

(20

8)

(20

2)

(11

0)

(214)

(30

0)

(01

8)

(11

6)

(11

3)

(02

4)

(122)

(10

4)

(00

6)

Inte

nsi

ty (

a.u.)

2 (degree)

Fe:LN

Fig. 5.3: The indexed PXRD spectra of grown pure and Fe-doped LiNbO3 crystals

0.2 0.3 0.4 0.5 0.60.0000

0.0005

0.0010

0.0015

0.0020

= -3.38x10-4 Fe:LN

Linear Fit

c

os

sin

Fig. 5.4: The β cosθ vs. sinθ graph for evaluation of lattice strains ‘η’ in grown Fe:LN crystal

more Li vacancies owing to the charge compensation of lattice with Fe doping. These

vacancy defects (VLi) become the cause of expansion of crystal lattice around them

and lead to the tensile strains (Bhagavannarayana, 1989). The in-depth investigation

about the crystalline perfection is discussed in the next section (§5.4.2) for HRXRD.

Chapter – 5 Fe:LiNbO 3

~ 125 ~

-400 -200 0 200 4000

400

800

1200

Dif

frac

ted X

-ray

inte

nsi

ty [

c/s]

Glancing angle [arc sec]

Fe:LiNbO3

(006) planes

MoK1

(+,-,-,+)

80"

Vacancy

defect

(a)

VLi

a

b

M

b

c O2-

MLi+ Nb5+

(b)

Fig. 5.5: (a) The recorded rocking curve for (006) diffraction planes of Z-cut specimen and (b) the

schematic of hexagonal unit cell of doped crystal and bonding nature around Li vacancy,

arrangement of constituent atoms projected on (0001) plane (Dongfeng & Xiangke, 2006). M

denotes iron at Li+ site and VLi represents a Li vacancy

5.4.2 High resolution X-ray diffraction analysis

Figure 5.5(a) shows the HRXRD rocking curve for (006) diffraction planes of

Fe:LN crystal recorded in a symmetrical Bragg geometry. The single peak shows that

the crystal does not contain any structural grain boundaries and cracks though LN is

prone to get boundaries and cracks as it undergoes structural phase transition

associated with volume changes during the cooling cycle. The full width at half

maximum of the diffraction curve is 80 arcsec which is quite high in comparison with

Chapter – 5 Fe:LiNbO 3

~ 126 ~

that of the theoretical RC (Batterman & Cole, 1964) (2.6 arcsec) [Fig. 4.4] or with

that of undoped specimen (Bhagavannarayana, Ananthamurthy et al., 2005) and

indicates that the crystal is not free from microscopic defects, i.e. point defects and

their aggregates (Lal & Bhagavannarayana, 1989; Lal & Ramanan, 2000;

Bhagavannarayana, Choubey et al., 2005). To understand these defects, first it is

worth to observe the asymmetry of the curve. For a particular angular deviation ()

of glancing angle with respect to the peak position, the scattered intensity is much

more in the negative direction in comparison to that of the positive direction. This

feature clearly indicates that the crystal contains predominantly vacancy type of

defects than that of interstitial defects.

This can be well understood by the fact that due to vacancy defects, as shown

schematically in the inset, the lattice around these defects undergo tensile stress and

the lattice parameter d (interplanar spacing) increases and leads to give more scattered

(also known as diffuse X-ray scattering) intensity at slightly lower glancing angles

with respect to the exact Bragg angle (θB) as d and sinθB are inversely proportional to

each other in the Bragg equation (2d sinθB = nλ; n and λ being the order of reflection

and wavelength respectively which are fixed) (Bhagavannarayana, Parthiban et al.,

2008). It may be mentioned here that the variation in lattice parameter is confined

very close to the defect core which gives only the scattered intensity close to the

Bragg peak. Long range order could not be expected and hence change in the lattice

parameter is also not expected (Bhagavannarayana, Kushwaha et al., 2010). It is

worth to mention here that the Fe-dopants are more or less statistically distributed in

the crystal. If the dopants or impurities are not statistically distributed but distributed

here and there as macroscopic clusters, then the strain generated by such clusters is

larger leading to cracks and structural grain boundaries which can be seen very clearly

in HRXRD curves with additional peak(s) as observed in our recent study on urea-

doped crystals in ZTS at various levels of doping (Bhagavannarayana & Kushwaha,

2010). However, in the present experiments the RC of Fe:LN crystal does not contain

any additional peak and indicates the absence of clustering of Fe at macroscopic level.

The above analysis clearly indicates that the Fe:LN crystal contains vacancy

defects. In the Fe:LN crystal, Fe could exist in two states Fe2+

and Fe3+

. The Fe3+

and

Chapter – 5 Fe:LiNbO 3

~ 127 ~

Fe2+

ions (64.5 and 78 pm, ionic radii) respectively replace three and two Li+ ions (76

pm; ionic radius) leaving two and one vacancies respectively in the crystal lattice due

to charge compensation (Haixuan et al., 2009). The crystal lattice with incorporated

metal ion (M) and Li+ vacancy (VLi) is shown in Fig. 5.5(b) through the schematic of

hexagonal unit cell and projected view of (0001) plane of crystal lattice (Dongfeng &

Xiangke, 2006). The asymmetry of rocking curve also confirms the congruent nature

of the grown crystal, otherwise for stoichiometric phase, the iron ions would have

replaced the Nb5+

ions of smaller ionic radius (76 pm) and give higher scattered

intensity in +ve direction with respect to the peak position.

This is well understood from the converse explanation given to the vacancies

in the above paragraph. The larger iron ions occupied in place of smaller Nb5+

ions

could compress lattice around them leading to decrease in d parameter which gives

more scattering at larger angles. It may be mentioned here that in our earlier

investigations (Bhagavannarayana, Ananthamurthy et al., 2005) we had grown Fe-

doped LN single crystals which were having the structural grain boundaries in the as-

grown state. However, in the present investigation due to incorporation of resistive

heater [RPGH], which results a low thermal gradient and helped in to grow bulk

crystals free from structural defects like voids, cracks and structural grain boundaries.

5.4.3 EPR analysis

The dopants in lithium niobate crystals produce the photochromic centers and

are able to change the photorefractive properties and hence the holographic storage

efficiency (Buse et al., 1998; Staebler & Phillips, 1974; Lee et al., 2000). The electron

spin resonance found to be a suitable method to characterize these photochromic

centers (Myeongkyu et al., 2001). The recorded EPR spectrum of the grown Fe:LN

crystal is shown in Fig. 5.6. The originated spectrum with clear peaks is due to the

presence of dopants in Fe3+

form at the Li sites of LN. All the resonance peaks

corresponding to their respective transitions are quite clear, which supports the

homogeneous distribution of dopants in the entire crystal lattice in tune with HRXRD

results. A small sharp resonance line at ~3400 G with magnified view in the inset is

corresponding to Li with g=2.0023. The observed spectrum of Fe3+

depicts that the

doped ions are in axial symmetry at the substitutional sites of Li+ with the

Chapter – 5 Fe:LiNbO 3

~ 128 ~

0 1k 2k 3k 4k 5k-2

-1

0

1

2In

tensi

ty (

arb.

unit

s) X

10

6

Magnteic Field (Gauss)

Fe:LN

3400-0.4

-0.2

0.0

Inte

nsi

ty

Field (Gauss)

Fig. 5.6: EPR spectrum recorded at room temperature: inset shows the close view of the signal at

~ 3380 gauss owing to the Li metal

symmetry axis parallel to the crystallographic c-axis (Herrington et al., 1972).

5.4.4 Raman spectroscopic analysis

The recorded Raman spectra of Fe:LN along with that of LN are shown in Fig.

5.7, and the vibrational frequencies with mode assignment are given in Table 5.1. The

observed modes in the present crystal are in well agreement with various reports

available in the literature (Scott et al., 2004; Sidorov & Palatnikov et al., 2003;

Schlarb et al., 1993). Compared to that of LN crystal the spectra Fe:LN crystal has

slightly different features. The peaks at 186 and 629 cm-1

corresponding to A1(TO)

and E(TO) modes are absent in Fe:LN crystal spectrum, which indicates that the

degeneracy of the transverse phonon modes has been severely affected by Fe doping

(Scott et al., 2004).

The peak parameters (intensity & FWHM) of Raman spectra are known to be

very sensitive for the structural changes of LN crystals, particularly those induced by

the deviations from stoichiometry and structural defects (Sidorov et al., 2007). In the

present crystal (Fe:LN) the peaks corresponding to the (ETO) and (A1LO) are slightly

broader than those of LN and Zn:LN crystals, reason may be the presence of lattice

distortions as investigated by PXRD and HRXRD. The FWHM peaks at 150 and 878

cm-1

have been used to evaluate the Li concentration (CLi) (Schlarb et al., 1993).

Chapter – 5 Fe:LiNbO 3

~ 129 ~

100 200 300 400 500 600 700 800 900 1000

30.0k

60.0k

90.0k

878 cm-1

28.84 cm-1

11.28 cm-1

16.65 cm-1

150

150 cm-1

29.70 cm-1

878

LN

Fe:LN

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1

)

LN

Fe:LN

Fig. 5.7: The Raman spectra for pure and Fe-doped single crystals. The magnified views of 150

and 878 cm-1

peaks are given at the top of main spectra

Table 5.1 Raman scattering modes with their assignments and relative intensities

Vibrational

mode

ν (cm-1

) CLi (mol%)

LN Fe:LN LN Fe:LN

E(TO) 150 151.3 47.7 45.1

A1(TO) 186 - - -

E(TO) 235 235 - -

E(TO) 260 264 - -

A1(LO) 271 270 - -

E(TO) 321 321 - -

E(TO) 367 367 - -

E(TO) 431 431 - -

E(TO) 579 581 - -

E(TO) 629 - - -

A1(LO) 878 874 48.0 47.8

Chapter – 5 Fe:LiNbO 3

~ 130 ~

It has been found to be 45.13 and 47.83 mol%, for the peaks at 150 and 878 cm-1

respectively. These evaluated CLi values for the individual peaks are lower than those

for LN as well Zn:LN crystals. The results clearly indicate that Fe doping lead to the

formation of vacancy defects for charge compensation of the lattice (Haixuan et al.,

2009).

5.4.5 Fourier transform infrared analysis

The recorded FTIR spectra of pure as well as doped crystals are shown in Fig.

5.8. In both spectra the broad bands at ~3484 cm-1

indicate the presence of OH- ions

related defects (Klauer & Wӧhlecke, 1994) and the FWHM of the peaks for LN and

Fe:LN crystals, respectively are 33 and 34 cm-1

. Similar to those of LN and Zn:LN

crystals the absorption band of Fe:LN does not have the separate peak, however the

FWHM has been slightly increased. The increase in the FWHM of band indicates

increase in the lattice distortion as compared to that of LN and Zn:LN crystals, this is

clear from the HRXRD and PXRD which reveal prominence of VLi defects. Figure

5.9 shows the schematic for the nearest neighbors of O2-

anion in the lattice of LN

crystal, and the absorption constant (κ) corresponding to the 0 – 1 stretching

vibrational band of OH at 3484 cm-1

. The κ value is used to evaluate the OH- ion

concentration in the Fe:LN crystal and found to be 1.53×1019

ions/cm3, which is

slightly higher than that of evaluated for LN (1.17×1019

ions/cm3) as well as Zn:LN

crystals (0.97×1019

ions/cm3) (Bollmann & Stohr, 1977). The slight higher

concentration of OH- is owing to the presence of large Li vacancies as revealed by

HRXR and Raman analyses. The band at 1746 cm-1

is owing to the characteristic

Nb–O overtone band which has been prominently occurred in Fe:LN crystal with high

absorption intensity. The above mentioned results are in good correlation with

crystalline perfection investigations by HRXRD and PRXRD.

5.4.6 UV-VIS-NIR analysis

The recorded transmittance spectrum for the wafer is shown in Fig 5.10(a).

The spectrum indicates that the grown crystal is highly transparent for the wavelength

range from 375 to 1100 nm, whereas below 375 nm, it is highly absorbing. The high

transparency indicates that most of the Fe is statistically distributed in the entire

crystal without forming macroscopic clusters. The magnified view of the spectrum in

Chapter – 5 Fe:LiNbO 3

~ 131 ~

4000 3000 2000 10000

20

40

60

80

Tra

nsm

itta

nce

(%

)

Wavenumber (cm-1)

LN

Fe:LN

3520 3480 3440 3400

34 cm-1

33 cm-1

Fig. 5.8: The FTIR spectra of pure and Zn-doped recorded in transmission mode and the inset in

figure shows magnified view of absorption band corresponding to O-H stretching vibration

Oxygen

Nb

proton

Li

below plane above plane

1

23

6

45

3540 3510 3480 3450 34200.00

0.15

0.30

0.45

0.60

(

cm-1)

(cm-1)

LN

Fe:LN

Fig. 5.9: The schematic view of an oxygen (001) plane in hexagonal unit cell indicating

the distorted different O–O bonds lengths and possible lattice cites feasible for proton

incorporation (Klauer & Wӧhlecke, 1994). The graph set-right shows the nature of the absorption

constant for OH- absorption band for pure and Fe-doped LN single crystals

360-600 nm range in the inset of Fig. 5.10(a) visualizes the sharp absorption for 482

nm. The 482 nm photons excite the electrons of Fe2+

to the conduction band and

trapped by Fe3+

ions. This absorption is attributed to the presence of iron in the form

of Fe2+

ions (antisitic defects) (Myeongkyu et al., 2001) and the spin forbidden d-d

transition of Fe3+

ions. However, from the spectrum it is clear that there is no clear

broad band except a slight lowering of transparency,

Chapter – 5 Fe:LiNbO 3

~ 132 ~

250 375 500 625 750 875 10000

20

40

60

80

100

300 325 350 375 400 4250

40

80

3.00 3.25 3.50 3.75 4.000

5

10

15

T (

%)

(nm)

LN

Fe:LN(a)

360 420 480 540 600

65

70

(b)

(

cm-1)

(nm)

LN

Fe:LN

20

= 369 nm

(c)

(h)2

x1

08 (

eVm

-1)2

h (eV)

LN

Fe:LN

Fig. 5.10: (a) the optical transparency spectrum of pure and Fe doped LN wafers; the inset shows

the sharp absorption valley occurring due to Fe doping, (b) absorption co-efficient spectrum in

UV-region and (c) (αhυ)2 vs. hυ spectrum indicates the energy band gap of crystal

which has been otherwise reported due to the transfer transitions (Schirmer et al.,

1991) of Fe2+

-Nb5+

.

The sharpness of the valley indicates the existence of iron in the crystal lattice

in Fe3+

form (Pape et al., 2005). The absorption coefficient in Fig 5.10(b) indicates

Chapter – 5 Fe:LiNbO 3

~ 133 ~

the increase in the optical absorption and shifting of the absorption edge towards

higher wavelengths for grown Fe:LN crystal as compared to that of pure LN crystal

similar to that of reported (Andreas et al., 2004). Here the absorption edge shifting

towards larger wavelengths may be elucidated by rZ /2* , as explained in the earlier

chapter. The polarization abilities of the ions Nb5+

, Fe3+

, Fe2+

, and Li+ are reported to

be 52.5, 42.46, 30.22, and 4.2 respectively, as evaluated by sZZ * . In the

present case in Fe:LN crystal the Fe2+

/Fe3+

ions occupy the normal Li+ sites and form

FeLi+/FeLi

2+ accordingly. As the polarizability of Fe

2+/Fe

3+ have the higher ability to

polarize the O2-

ions in comparison to that of Li+ and the replacement of Li

+ by

Fe2+

/Fe3+

results in increase the polarization ability of O2-

ions in the lattice of

crystals.

Hence the energy required for transition of valance electron decreased (Zhen

et al., 2003) and resulted in the shifting of the absorption towards longer wavelengths.

The direct band gaps of Fe:LN crystal in contrast to the LN crystal has been

calculated using the relation: )()( 2 hEAh g and by plotting (αhν)2 vs. hν [Fig.

5.10(c)] (Kushwaha, Maurya, Vijayan et al., 2011). The Eg value has been found to be

3.49 eV which is much less than that of 3.91 and 3.94 eV respectively for LN and

Zn:LN crystals [§4.4.5] (Arizmendi, 2004). The high transparency with sharp

absorption at 482 nm of grown crystal is much higher as compared to the earlier

reported LN crystal (Kar et al., 2008) with much lower iron concentration (~ 0.028

mol%). Therefore, the Fe:LN crystal grown in the present study seems to be very

good for the optical information processing (Shiuan et al., 1999) and holographic data

storage applications, the device level quality of crystal is inferred by HRXRD.

5.4.7 Prism coupler birefringence analysis

Iron doped Lithium niobate is an excellent photorefractive crystal and its

refractive index varies greatly in the presence of transition metals, even in the trace

amount (Kösters et al., 2009). Therefore, it is important to measure the refractive

index of the grown crystals very accurately, the prism coupler found to be an ideal

method to calculate the ordinary and extraordinary refractive indices with high

accuracy (Ulrich & Torge 1973; Mingfu et al., 2009; Hidetoshi et al., 1983).

Chapter – 5 Fe:LiNbO 3

~ 134 ~

Fig. 5.11: The recorded reflection patterns for TE and TM modes for the z-cut wafer [Fig. 5.2]

The optically polished wafer surface was made in good contact with the base of right

angle rutile prism and the laser beam (632.8 nm) was coupled into the crystal wafer.

When the laser beam in the prism matches the propagation constant of modes in the

wafer, the beam is coupled into the wafer. The transverse electric (TE) and transverse

magnetic (TM) guided-mode spectra are shown in Fig. 5.11. The sharp fall in the

intensity in the spectra indicates the good coupling of crystal wafer with the prism and

there is no scattering of beam at the coupling interface. The located knee positions in

the spectra were used to calculate the refractive indices. The calculated ordinary and

extraordinary refractive indices nTE and nTM are found to be 2.2880 and 2.2047

respectively with the difference (Δn = nTE - nTM) of 0.0833. Both of the calculated

Chapter – 5 Fe:LiNbO 3

~ 135 ~

2.25

2.50

2.75

0.30 0.45 0.60 0.75 0.900.0

0.5

1.0

1.5

LN

Fe:LN

n

(m)

LN

Fe:LN

Fig. 5.12: The variation of average refractive index ‘n’ and extinction coefficients ‘κ’ with

wavelength recorded for (006) planes of grown pure and Fe-doped LiNbO3 single crystals

indices for our crystal wafer are slightly higher than that of earlier reported for pure

LN crystal (Wong, 2002; Imbrock et al., 2004; Li et al., 2009). The evaluated indices

and recorded reflection spectra indicate that the grown crystal is of good optical

quality and free from the macroscopic structural defects like voids, dislocations and

grain boundaries as revealed by HRXRD. However the slight increment in nTE and

nTM may be attributed to the doping of iron. The incorporated iron in Fe:LN crystal

exists in Fe2+

and Fe3+

two valance states which cause the redistribution of electronic

charge in the crystal and create the space charge fields. The space charge field

interacts with the incident electromagnetic radiation and change the refractive indices

no and ne through electro-optic effect (Buse et al., 1998). The electrons are excited

from Fe2+

ions and redistributed due to the drift in electric field or photovoltaic effect

and finally trapped in Fe3+

ions, which gives rise to build up space charge field

(Kösters et al., 2009; Imbrock et al., 2004). In earlier studies on Mg doped lithium

niobate, the change in refractive indices (Δn) for 532 nm wavelength found to be

decreased, whereas for 1064 nm its value increased (Sen et al., 2004).

5.4.8 Wavelength dispersion analysis

The refractive index (RI) and extinction coefficient (κ) for Fe:LN crystal along

with that of LN crystal is shown in Fig. 5.12.

Chapter – 5 Fe:LiNbO 3

~ 136 ~

4.0

4.8

5.6

6.4

0.30 0.45 0.60 0.75 0.900.0

1.5

3.0

4.5

6.0

r

LN

Fe:LN

LN

Fe:LN

(m)

i

Fig. 5.13: The variation of real ‘εr’ and imaginary ‘εi’ components of the dielectric

constant with the wavelength, evaluated using n and κ parameters

0 2 4 6 8 100.23

0.24

0.25

0.26

0.27

0.28

(h)2 (e

2V

2)

1/(

n2-1

)

LN

Fe:LN

Linear fit: LN

Linear fit: Fe:LN

Fig. 5.14: The (n2-1)

-1 vs. (hν)

2 plots for pure and doped crystals straight lines are the

linear fit to the data points

From the figure it is clear that above 400 nm the average RI values for Fe:LN crystal

are almost constant over the entire wavelength range. Below 600 nm the RI values for

Fe:LN crystal are lower than that of pure, however for the wavelength above 600 nm

these values are slightly higher. This behaviour of RI indicates that in contrast to Zn-

Chapter – 5 Fe:LiNbO 3

~ 137 ~

doping, Fe-doping lead to the enhancement in the photorefraction behaviour of LN

crystals and makes them suitable for the holographic applications for the visible and

near infrared wavelengths. The extinction coefficient values for Fe:LN crystal are

higher than that of pure crystal over the entire wavelength range. The real (εr) and

imaginary (εi) parts of optical dielectric constant of the grown crystal are shown in

Fig. 5.13. Compared to that of pure crystal the εr value for Fe:LN crystal is almost

constant over the entire wavelength region above 300 nm.

The average single oscillator strength energy for electronic transition (Eo) and

the dispersion energy or oscillator strength (Ed) which is the measure of average

strength of interband optical transitions, have been obtained from the 1/(n2-1) vs. (hv)

2

plots [Fig. 5.14] in the (hv)2 range of 1–10 e

2V

2. The dotted lines are linear to the data

points and give Eo and Ed. In comparison to those of pure and Zn-doped crystal the

Fe:LN crystal shows the anomalous plot of 1/(n2-1) vs. (hv)

2 and therefore we are

unable to get the proper linear fit to these plots in the defined energy range. Therefore

for Fe:LN crystal it could not be possible to get the either exact value of slope or

intercept for the linear fit. Hence, it was not possible to obtain the Ed and Eo

parameters for grown Fe:LN crystal. This behaviour of crystal may be due to the

presence of Fe ions in the crystal lattice which induce the photorefraction behaviour

via the photovoltaic effect. From the crystalline perfection investigations it is clear

that Fe:LN crystal has abundance of VLi defects, which might have also been cause

for the unusual behaviour of the plot.

5.5 CONCLUSION

The iron doped good quality bulk single crystal of congruent LiNbO3 has been

successfully grown by an indigenously developed LTG two zone furnace by

employing indigenously developed crystal puller. The HRXRD revealed that the

grown crystal has good crystalline perfection and does not contain macroscopic

defects like grain boundaries and the dopants have occupied the vacant Li+ sites. The

obtained crystalline perfection also indicates the proper distribution of Fe in the

crystalline lattice. The EPR analysis confirmed the proper axial incorporation of

dopants at lithium sites. The vibrational modes of the crystal have been analyzed by

Chapter – 5 Fe:LiNbO 3

~ 138 ~

Raman spectroscopy and used to evaluate the Li concentration. The incorporation of

H+ has been investigated by FTIR and evaluated concentration of OH

- ions has been

correlated with the crystalline perfection, vacancy defects due to Fe doping. Prism

coupler studies depicted the slight increment in refractive indices of LN with iron

doping for ordinary and extraordinary rays with refractive index change (Δn) of

0.0833. The UV-VIS-NIR studies revealed that the crystals are highly transparent for

the wavelength range from 375 to 1100 nm and shows the lowering of band gap due

to Fe-doping. The present studies reveal that the grown crystal with two zone low

thermal gradient furnace has very good device properties needed for holographic data

storage applications.