czochralski growth , crystalline perfection and optical c...

$: Czochralski growth , crystalline perfection and optical c ...shodhganga.inflibnet.ac.in/bitstream/10603/6479/8/08_chapter 4.pdf · photorefraction at 325 nm in highly Zn -doped LN$
Chapter – 4 LiNbO 3 & Zn:LiNbO 3

~ 87 ~

Chapter - 4

Czochralski growth, crystalline

perfection and optical characterization

of LiNbO3 and Zn:LiNbO3 NLO single

crystals

Abstract

The pure and Zn-doped (1 mol%) LiNbO3 single crystals have been

grown by Czochralski method. The structure of the crystals has been

confirmed and the strains in both the crystals have been evaluated by

powder X-ray diffraction. The high resolution multicrystal X -ray

diffractometer has been employed to record the rocking for (006)

planes to assess the crystalline perfection of the grown crystals . The

Fourier transform infrared and Raman spectra have been recorded for

investigation of vibrational modes, defects, Li-composition and

incorporation of protons (H+

) in crystals’ lattice . The UV-VIS-NIR

studies have been carried out to study the transparency, band gap and

defectiveness. The birefringence and refractive indices for 1064 and

532 nm wavelengths have been studied using prism coupler

spectrometer. The wavelengths dispersion , optical dielectric constant,

dispersion energy ‘Ed’ and average single oscillator energy ‘E o’ for

the crystals have been studied by the Ellipsometry.

Chapter – 4 LiNbO3 & Zn:LiNbO3

~ 88 ~

4.1 INTRODUCTION

The single crystals are backbone for almost all the advanced scientific and

technological developments. The modern science and technology demand fast data

processing and high density storage devices (Carina et al., 2002). The optical

holographic systems are proposed to be the best to overcome the high density data

handling difficulties (Heerden et al., 1963; Mark Haw, 2003). The oxide single

crystals play an unmatched role for their applications in lasers, nonlinear optical

devices employing for storing, retrieving and processing the information in the form

of photon at the speed of light. The Lithium Niobate (LiNbO3; LN), a nonlinear

optical (NLO) material, has been well proved as a potential and unique material for its

use in advanced photonic device applications like second-harmonic generation (SHG),

high energy lasers (for inertial confinement fusion reactions), electro-optical

switching, optical modulators, holographic data storage, acousto-optic, optical

communication and ferroelectric. Lithium Niobate, due to its role in widespread

photonics era is also termed as ‘silicon of photonics’. Zn doping removes the antisitic

Nb (NbLi4+

) and improves the optical damage resistance significantly and sufficed the

recording of holograms at high-speed with long life-time by resulting in high

photoconductivity and optical band gap of LN (Zhen et al., 2003; Volk et al., 1996).

The SHG efficiency of LN in crystals was found to be increased manifold with Zn-

doping (Nevado, R. & Lifante, 2001). Fabrication of LN waveguides with low

propagation loss and high photorefractive damage resistance has been reported by

diffusion of Zn (Li et al., 2001). The Enhanced SHG has also been observed in

periodically poled Zn-doped LN planar waveguide (Vincent et al., 2007). The Nd3+

co-doped LN:ZnO crystal exhibited continuous-wave laser action at 929 nm suitable

for generation of blue lasers (Jaque et al., 2004). Analogous to In and Na doping, Zn

doping makes LN suitable for ultraviolet recording grating application (Haijun Qiao

et al., 2004) and recently, Feifei, et al. (2010) have demonstrated ultraviolet

photorefraction at 325 nm in highly Zn-doped LN for its holographic applications in

UV region. The polarization independent signal (wavelength) conversion with

ultrafast modulation could be achieved in LN with Zn doping (Masaki et al., 2003).

The cited literature proves the importance of Zn doped LiNbO3 single crystals in


~ 89 ~

photonics. However, the need of systematic study on the defect structure in a carefully

grown LN crystal due to doping particularly with Zn which influences the physical

properties motivated the present investigation which is elaborated in the following.

The properties of the doped single crystals depend on the fact that weather the dopants

are homogeneously distributed in the entire crystal or agglomerated into the clusters.

The homogeneous distribution of dopants in the lattice matrix of host crystal is being

decided by the concentration, size, ionic state of dopants and the lattice sites of

crystals occupied by the dopants. In LN, the dopants distribution is statistical up to

their certain concentration known as ‘threshold concentration’. Above this

concentration of dopants, high geometric strains develop in lattice and the lattice tries

to relax by overcoming the strains, during this process the dynamics of point defects

take place which lead to the agglomeration of point defects and dislocations, and this

ultimately results in the formation of structural grain boundaries (Bhagavannarayana

et al., 2009; Bhagavannarayana et al., 2005). The properties of single crystals are

anisotropic in nature, therefore in the presence of structural defects these get masked

or partially/completely deteriorate (Haixuan et al., 2009; Tsai et al., 2008; Dongfeng

& Xiangke, 2006; Bhagavannarayana et al., 2006; Bhagavannarayana & Kushwaha,

2010) and thereby may reduce the efficiency of device property. Therefore, for the

realization of full efficiency of the device, the crystal must be free from such defects

(Bhagavannarayana, Budakoti et al. 2005).

In the modern era of miniaturized technology of photonics it is very important

to grow the pure and doped bulk single crystals of LN and assess their crystalline

perfection, especially in the case of doped crystals. The single crystals of LN have

been grown by different techniques depending on their application. However, the

Czochralski (CZ) technique has been proved to be the most suitable for the growth of

bulk crystals of LN, over all the available techniques. The structural defects in LN

crystals have been characterized by various techniques (Wenbo et al., 2004; Wallace

et al., 1970; Sidorov et al., 2007; Sugh et al., 1973; Ivanova et al., 1980; Wicks et al.,

1968). High resolution X-ray diffractometry (HRXRD) being non-destructive in

nature is the most suitable technique to characterize bulk single crystals particularly,

possessing the low and very low angle structural grain boundaries which in general


~ 90 ~

arise in the crystals during growth or post growth cooling process

(Bhagavannarayana, Budakoti et al., 2005). As described in earlier chapter (Chapter-

2) LN undergoes the structural changes at its curie temperature i.e. ~1416 K, hence

during the cooling cycle the defects may generate in grown crystals.

During the growth of LN crystals at very high temperature Li gets evaporated

form the melt and hence affect the composition (stoichiometry) and the crystals

generally grow in congruent phase with prominent Li vacancy defects (VLi). When the

same charge is used many times the grown crystals possess slightly yellow color

which may be due to the expected high concentration of VLi defects. Therefore,

special care has to be taken for the growth of bulk single crystals of pure as well as

doped LN. The dopants influence the photorefraction behavior of crystals, therefore,

in parallel to the defect analysis it is essential to study optical parameters

(transmittance, band-gap, refractive indices, birefringence etc). The pure and Zn-

doped LN (Zn:LN) single crystals have been grown by CZ technique. Crystalline

perfection of the grown crystals has been investigated by HRXRD. The crystals have

been extensively characterized by powder X-ray diffraction, Raman, Fourier

transform infrared (FTIR), UV-VIS-NIR, prism coupler and ellipsometry techniques.

To see the effect of repeated use of the same charge, the band gap and in particular the

concentration of VLi and NbLi4+

antisitic defects has been evaluated for such a crystal

which possesses yellow color.

4.2 CRYSTAL GROWTH

The LN and Zn:LN bulk single crystals have been grown by CZ technique.

The details about the crystal puller and growth process are provided in Chapter 3. The

photographs of the harvested crystals are shown in Fig. 4.1(a)-(d). All the obtained

pure as well doped crystals at the time of harvesting from the system were visibly

quite transparent and free from the cracks. The crystal LN and Zn:LN shown in Fig.

4.1(a) and (d) are grown using the fresh LiNbO3 charge which are quite transparent

and colorless, whereas, the LN crystals shown in Fig. 4.1(b) & (c) are grown from the

multiple time used LiNbO3 charge and possess the slight yellow color, the yellow

color may be due to the presence of high concentration Li vacancies.


~ 91 ~

10 mm

LN

Fig. 4.1(a): Photograph of CZ grown LiNbO3 single crystal using the seed crystal of [001]

direction, from the fresh LiNbO3 charge

Fig. 4.1(b): Photograph of CZ grown LiNbO3 single crystals using multiple times used LiNbO3

charge, the seed of the same set [Fig. 2.6(b)] has been used


~ 92 ~

35 mm

50

mm

31 mm

LN

10 mm

Zn:LN

Fig. 4.1: (c) The Photograph of CZ grown LiNbO3 single crystal from the multiple used charge

and (d) the photograph of Zn:LiNbO3 single crystal grown from the fresh LiNbO3 charge

LN

Zn:LN

Fig. 4.1: (e) the photographs of polished Z-cut wafers (0.5 mm) of pure and Zn-doped (1 mol%)

LiNbO3 single crystals, the letters in background of wafers visualize their high transparency

This is expected because the Li is a light metal and gets evaporated during the crystal

growth process at high melting point. The complete description about the Li vacancies

and antisitic defects in pure and doped crystals has been discussed in the following

sections. For doping of the crystal 1.0 mol% concentration of ZnO was opted which is

well below its threshold concentration (~5.5 mol%). For the characterization purpose

of the structural defects and other physical properties the crystals have been cut to

obtain the Z-cut wafers of the appropriate size from the respective crystal boules. The

wafers were lapped to the thickness of ~500 μm and optically polished. The two such

(c) (d)


~ 93 ~

typical wafers are shown in Fig. 4.1(e) and it is clear from the photograph that crystals

are visibly quite transparent and do not possess any physical cracks.

4.3.1 CHARACTERIZATION STUDIES

To study the crystal structure/ crystal system and effect of dopant on the

structure and unit cell parameters the powder X-ray diffraction (PXRD)

measurements on the powdered specimens of LN and Zn:LN crystals were performed.

The detailed description of the diffractometer is given in §3.2. The homogeneous

powdered specimens of the crystals were prepared by crushing the small portions

from the pure as well as doped crystals. The diffraction spectra for both the crystals

were recorded in 20–80 degree angular range of 2-theta, with step size 0.01º and

time/step of 0.1 s.

For evaluation of the crystalline perfection of grown bulk crystals, a

multicrystal X-ray diffractometer (MCD) has been employed the detail description

about of MCD is provide in §3.3. The rocking curves (RCs) have been recorded for

(006) diffraction planes of Z-cut crystal wafers. The specimen was rotated about the

vertical axis, perpendicular to the plane of diffraction, with minimum angular interval

of 0.4 arc sec. The RCs were recorded in ω-scan, in symmetric Bragg geometry. The

X-ray power, size of the beam and diffraction configuration of the diffractometer was

kept constant for both the specimens during all the experiments. The results have been

described in detail in the forthcoming section.

The Raman spectral measurements of the grown crystals have been carried out

on Renisha inVia Raman Microscope in back-scattering mode at the room

temperature. The details about the spectrometer are given in §3.5. A 785 nm

wavelength diode laser (HP-NIR) was used as source with beam power of 300 mW.

The spot size of beam on the crystal surface was 1 µm and the exposure time was kept

10 s. To study the stretching vibrational mode O–H corresponding to the in-diffused

protons (H+) and to evaluate their concentration the Fourier transform infrared

spectrum of the grown crystal in transmission mode has been recorded on a FTIR

spectrometer (Nicolet-5700) [§3.4] in the wavenumber range of 400 – 4000 cm-1

. The


~ 94 ~

UV-VIS-NIR transmission and absorption spectra for both the crystals were recorded

using a Shimadzu-1601 spectrophotometer in the wavelength range of 200–1100 nm.

The optical transparency, band gap, and the composition of crystals have been

evaluated.

Refractive indices (ordinary and extraordinary) and birefringence of the Z-cut

samples were measured on a Prism coupler spectrometer (PCS) for 1064 and 532 nm

wavelengths. The wavelength dispersion of both the crystals has been studied by

ellipsometry [§3.9]. The average refractive index and extinction coefficients for the

wavelength range of 200 – 1000 nm on have been evaluated.

4.3 RESULTS AND DISCUSSION

4.4.1 Powder X-ray diffraction analysis

The LiNbO3 crystals have the rhombhohedral structure at room temperature

and belong to the R3c space group 3m point group. In this space group lattice of

LiNbO3 crystal Nb atoms occupy (0,0,0) positions, Li atoms occupy (0,0,1/4)

positions and O atoms occupy (4,1/3,1/2) positions. The lattice parameters of the

hexagonal unit cell of LiNbO3 are a = 5.1483 Å and c = 13.8631 (Abrahams, Reddy et

al., 1966) Å. LiNbO3 undergo the first order phase transition at ~1160°C, due to

which it transforms from the polar ferroelectric 3m symmetry to nonpolar paraelectric

3m symmetry, above the Curie temperature. At this phase transition a substantial

change in the volume of the unit cell take place (Parlinski et al., 2001) due to which

strains are produced in the crystal lattice. Therefore at this phase transition

temperature the growth conditions are to be handled very carefully during their

cooling cycle after the growth, otherwise the crystals undergo the severe problem of

cracking when they are taken out of the furnace. Li is a light element and during the

growth process of crystals it undergoes the evaporation and hence the grown crystals

are deficient in Li. This results in, the missing of Li+ ions from their regular sites and

most of these Li vacant sites in the lattice are occupied by Nb5+

and result in the

NbLi4+

point defects, termed as antisitic defects. For the confirmation of crystal system

and evaluation of strains in the lattice of crystals the PXRD spectra of for the

powdered specimens of LN and Zn:LN crystals were recorded and shown in Fig. 4.2.


~ 95 ~

0

800

1600

2400

3200

20 30 40 50 60 70 800

800

1600

2400

(312)

(306)

(220)

(208)

(202)

(110)

(214

)

(300)

(018)(1

16)

(113)

(024)

(122

)

(104)

(006)

LN

(012

)

Inte

nsi

ty (

a.u

.)

2-theta (degree)

Zn:LN

Fig. 4.2: The indexed PXRD spectra of pure and Zn-doped LiNbO3 crystals

0.0005

0.0010

0.0015

0.0020

0.2 0.3 0.4 0.5 0.60.0005

0.0010

0.0015

0.0020

LN

Linear Fit

c

os

= -7.54x10-4

= 9.38x10-6 Zn:LN

Linear Fit

c

os

sin

Fig. 4.3: β cosθ vs. sinθ plots for for an evaluation of lattice strain (η) in the lattice of crystals

The diffraction peaks are very sharp which indicate good crystallinity of the grown

crystals. All the peaks occurred in LN spectra are also present in Zn:LN, and reveal

that the grown crystals possess the perfect single phase with trigonal (rhombohedral)


~ 96 ~

crystal structure and R3c (C3ν6) space group (Abrahams et al., 1966; Chemaya et al.,

2001).

The strain (η) in the crystal lattice have been evaluated by using Hall-

Williamson relation [§3.2]. The βcosθ vs. sinθ plots are shown in Fig. 4.3. The

evaluated values of η for LN and Zn:LN are respectively -7.54×10-4

and 9.38×10-6

.

The higher –ve value of η for pure LN crystal is owing to the presence of vacancy

type of defects (VLi) in LN which cause expansion of lattice around the defect core

and lead to the tensile strain (Bhagavannarayana, 1989). For Zn:LN the simulated plot

is almost horizontal having very low +ve value of η which infers that the Li vacancies

(VLi) have been removed by doped Zn2+

ions at low concentration (~1 mol%) and due

to their large size over Li+ lead to the slight compressive strain in the vicinity of

defect core. Such low strain values depict that doped crystal is almost free from the

strains and the strains of such order in nearly perfect silicon single crystal was

evaluated by diffuse X-ray scattering technique at authors’ laboratory

(Bhagavannarayana 1989)29. The antisitic NbLi4+

defects in LN have been reported to

be completely wiped out for the threshold concentration (~5.5 mol%) of Zn doping

(Chemaya et al., 2001). The detailed crystalline perfection analysis of both the grown

crystals is given in the flowing §4.4.2 of HRXRD.

4.4.2 High resolution X-ray diffraction analysis

For better understanding about the crystalline perfectness of the pure and

doped single crystals of LiNbO3 in comparison with the ideal (theoretical) LiNbO3

single crystal, the theoretical rocking curves for (006) diffraction planes have been

obtained and shown in Fig 4.4. These diffraction curves have been obtained by

considering the plane wave theory of dynamical X-ray diffraction (Batterman & Cole,

1964) for an ideally perfect crystal. The figure contains two diffraction curves; one

the so called Darwin, where the phenomenon of linear absorption of X-rays is not

taken in to consideration and the other, the well-known Darwin-Prince curve in which

the linear absorption is taken into account. The HRXRD RCs for (006) diffraction

planes of the Z-cut wafers of grown LN and Zn:LN bulk crystals are recorded using

ω-scan in symmetrical Bragg diffraction geometry and are shown in Fig. 4.5.


~ 97 ~

-20 -10 0 10 200.0

0.5

1.0

Perfect

=1.38x10-5 rad

PFH

B

2sin

22

1

LiNbO3

(006) planes

MoK1

2.6"

Darwin

Darwin-Prince

Ref

lect

ivit

y, I/

I 0

Glancing angle , [arc sec]

Fig. 4.4: The Darwin and Darwin-Prince theoretical rocking curves for (006) diffraction planes

of pure LiNbO3, generated using the plane wave theory of dynamical X-ray diffraction

The RC of LN is having a single peak with FWHM value of 62 arc sec, which is quite

higher than that of the theoretical value as shown in Fig. 4.4 (Kushwaha, et al., 2011).

The higher value of FWHM of recorded RC for LN indicates the presence of the point

defects, which are expected due to expected deviation from the stoichiometry of real

life LN crystals. The absence of any additional peak in RC depicts that the grown LN

is almost perfect and free from the structural grain boundaries which are very

common to observe in LN crystals (Bhagavannarayana et al., 2009).

However, asymmetry of the diffraction peak with high scattered intensity

along -ve side in comparison to that along +ve side with respect to the exact Bragg

peak position indicates that the LN crystal predominantly contains vacancy type of

defects (Kushwaha et al., 2011). This is due to the fact that around the vacancy

defects the lattice undergoes tensile stress and the lattice parameter d (interplanar

spacing) increases which results in high scattered intensity (also known as diffuse X-

ray scattering) at slightly lower (Δθ) glancing angles with respect to the exact Bragg

angle (θB). This is because d and sinθB are inversely proportional to each other in the

Bragg equation (2dsinθB = nλ; n and λ being the order of reflection and wavelength of

X-rays respectively, which are fixed). It is important to mention here that the variation

in the lattice parameter (d) is confined only in the vicinity of point defect which gives


~ 98 ~

the scattering intensity only close to the Bragg peak position. Li is a light element and

during the growth process at high temperature (1523 K) it evaporates and lead to

change in the stoichiometry of the grown crystals with lattice defects VLi and NbLi4+

(Nb5+

ions at Li sites) antisitic defects (Chemaya et al., 2001; Nagashio et al., 2004).

However, the expected change in the stoichiometry is very meager, and LN crystal is

congruent in nature with a small deficiency of Li. This result is supported by the

calculation of Li concentration using Raman and birefringence analysis.

The RC for Zn:LN [Fig. 4.5] has quite different features with scattered

intensity in a broad angular region (~300 arcsec) of glancing angle. Instead of a single

peak as for LN, it consists a shoulder (as an inseparable peak merged with the main

peak) at the +ve side of the peak position. This indicates that the grown Zn:LN crystal

possesses a low angle grain boundary. On de-convolution of the curve we get two

peaks separated by 70 arcsec. The solid curve is the convoluted curve obtained by the

Lorentzian fit of the data points and it is well fitted with the experimental points. The

main peak with FWHM of 86 arcsec corresponds to the main crystal domain and the

smaller peak with FWHM 70 arcsec is due to the low angle internal structural grain

boundary which is mis-oriented from the main crystal domain by 70 arcsec. Such

structural grain boundaries are very common to observe in real crystals particularly in

ferroelectric crystals, which undergo structural phase transitions associated with

volume changes, during post-growth cooling cycle.

The dopants in crystal generate heavy stresses in the lattice and the lattice tries

to relax which results in the formation of dislocations and boundaries. Segregation of

dopants takes place along the grain boundaries through the process of guttering

(Bhagavannarayana & Kushwaha, 2010; Saraev et al., 2003; Grange et al., 1990). The

radius of dopant ions Zn2+

is In the present case of Zn doped crystal Zn:LN, when the

Zn2+

ions (74 pm) comparable to Li+ ion (76 pm) and larger than that of Nb

5+ (64 pm)

occupy the VLi or Nb 4

Li positions in the crystal, the lattice around the defect core

undergoes compressive stress as observed in §4.4.1 of PXRD. Such type of structural

grain boundaries were successfully investigated by HRXRD and X-ray topography in

pure LN which could be removed successfully by the post growth annealing process


~ 99 ~

0

100

200

300

400

-300 -200 -100 0 100 200 3000

100

200

300

400

Vacancy

defect

62"

LN (006) planes

MoK1

(+,-,-,+)D

iffr

acte

d X

-ray I

nte

nsi

ty [

c/s

]

Grain-

Boundary

70"

70"Zn:LN (006) planes

MoK1

(+,-,-,+)

86"

Dif

fracte

d X

-ray i

nte

nsi

ty [

c/s

]

Glancing angle [arc sec]

Fig. 4.5: The RC’s recorded for the (006) diffracting planes of LN, the inset indicates the vacancy

defect (VLi) with expansion of the lattice around the defect core, and of Zn:LN, the inset shows the

schematic of a low angle grain boundary (α << 1’)

with very low rates of heating and cooling (Bhagavannarayana et al., 2009). The

HRXRD analysis indicates that both the grown crystals (LN & Zn:LN) are almost

perfect and decipher their suitability for device applications. Though the specimens

contain a low angle boundary, the relatively low angular spread of around 300 arcsec

of the diffraction curve and the low FWHM values show that the crystalline


~ 100 ~

perfection is reasonably good. It may be mentioned here that such low angle

boundaries (which are unlikely to degrade the properties) could be detected with

resolved peaks in the diffraction curves only because of the high resolution of the

diffractometer, characterized by very low values of wavelength spread (i.e. ∆λ/λ ≈

10-5

) and horizontal divergence (<< 3 arcsec) for the exploring or incident beam.

4.4.3 Raman spectroscopy analysis

The LiNbO3 primitive unit cell with two formula weights (10 atoms) results in

30 degrees of freedom, 27 of which assigned as phonon modes and other three as

acoustic phonons. The optical modes of LN for R3c are given by the relation:

EAAoptical 954 21 . Only A1 and E modes are Raman active therefore only 13

phonon peaks are expected in the spectra, the degeneracy between longitudinal optical

(LO) and transverse optical (TO) phonons has been lifted due to the long-range

electrostatic fields because of ionic nature of LN (Scott et al., 2004). The Raman

spectra recorded for the pure as well as doped Z-cut single crystal wafers are shown in

Fig. 4.6 and the vibrational frequencies with assignments are given in Table 4.1. Both

the spectra consist of similar features with the all fundamental vibrational modes

corresponding to LiNbO3 structure. The peak parameters of the Raman spectra are

known to be very sensitive for the structural changes of LiNbO3 crystals, particularly

those induced by the deviations from stoichiometry and structural defects (Sidorov et

al., 2007). Significant differences in the peak intensities of the recorded spectra for

both the crystals have been observed, with no significance change in the peak

positions. The intensity of the line at ~186 cm-1

is said to be strongly dependent on

structural defects and its low intensity for LN may be attributed to the presence of

stresses in the lattice owing to the VLi defects revealed by PXRD and HRXRD. The

relaxation of the lattice from strains of Zn:LN crystal might have led to the increase in

the intensity of this peak. The increase in the intensity of the other peaks also may be

attributed to the same reason.

The observed modes in the present crystals are in well agreement with various

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

2003; Schlarb et al., 1993). Except the variation in the intensities of the peaks, there is


~ 101 ~

200 400 600 800 10000.0

30.0k

60.0k

90.0k

11.28 cm-1

LN

Zn:LN

150 cm-1

12.03 cm-1

878 cm-1

28.83 cm-1

28.84 cm-1

878

Inte

nsi

ty (

a.u.)

Raman shift (cm-1)

LN

Zn:LN

150

Fig. 4.6: The Raman spectra for both the crystals recorded in back scattering mode shows all

thirteen vibrational modes. At top shows the magnified view of peaks at 150 and 878 cm-1

the line-

widths (Γ) of these used to evaluate the composition of the crystals

no inconsistency in the spectra due to Zn doping and reveals no change in the basic

structure of crystal lattice as analyzed by PXRD and HRXRD. The E(TO) and A1(LO)

modes respectively at ~150 & 878 cm-1

are used to calculate the Li concentration

(CLi) in both the grown crystals using the empirical formulae: CLi[mol%] = 53.03 -

0.4739Г [cm-1

] for 150 cm-1

and CLi[mol%] = 53.29 - 0.1837Г [cm-1

] for 878 cm-1

(Schlarb et al., 1993) indicated by arrows in figure. Here Г is the linewidth (full width

at half maximum) of the peak, its values for the mode E(TO) for LN and Zn:LN are

respectively 11.28 and 12.03 cm-1

, whereas for A1(LO), these are 28.84 and 28.83

cm-1

. The slight variation in Г is owing to the change in translational symmetry due to

the presence of point defects (VLi, NbLi and ZnLi), which influence the interionic

potential and hence the selection rules of transitions (Chemaya et al., 2001; Schlarb et

al., 1993). Evaluated CLi values are given in Table. 4.1, and indicate that the grown

crystals have the congruent composition


~ 102 ~

Table 4.1 Raman scattering modes and evaluated CLi for LN and Zn:LN bulk single crystals

(Li/Nb = 0.954 & 0.948 respectively for LN and Zn:LN), slightly off from the

stoichiometric composition (Li/Nb = 1) (Scott & Burns, 1972; Balanevskaya et al.,

1983). The CLi results for LN with deficiency of Li indicating the congruent nature of

crystal is in correlation with PXRD and HRXRD analysis in §4.4.1 & §4.4.2

indicating the presence of tensile stresses and vacancy defects.

4.4.4 Fourier transform infrared analysis

In LiNbO3 crystals, due to thermal diffusion of H+ ions, the fixed holograms

have a non-infinite lifetime at room temperature. However, this life time is too longer

to make room temperature direct measurements and hologram lifetime is found to be

a function of the H+ ion concentration (H0), of the concentration of iron and relative

valance ratio, and of the diffusion coefficient of H+ ions (DH) through the

relation,

1

0

2

12

t

FN

H

, where Ʌ hologram fringe spacing. So, low

hydrogen concentration and large Nt (=[Fe2+

][Fe3+

]/[Fe]) are required for long lasting

holograms (Arizmendi et al., 2004; Miguel et al., 2002).

Modes of vibration ν (cm

-1) CLi (mol%)

LN Zn:LN LN Zn:LN

E(TO) 150 149 47.7 47.4

A1(TO) 186 187 - -

E(TO) 235 235 - -

A1(TO) - 253 - -

E(TO) 260 - - -

A1(LO) 271 270 - -

E(LO) - 296 - -

E(TO) 321 321 - -

E(TO) 367 367 - -

E(TO) 431 431 - -

E(TO) 579 578 - -

E(TO) 629 628 - -

A1(LO) 878 879 48.0 48.0


~ 103 ~

4000 3000 2000 10000

20

40

60

80 LN

Zn:LN

Wavenumber (cm-1

)

Tra

nsm

itta

nce

(%

)

3520 3480 3440 3400

33 cm-1

31 cm-1

Fig. 4.7: 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.

Therefore, in parallel to the defect analysis of the grown crystals, it is very important

to analyze them for the presence of OH- ions. The recorded FTIR spectra of pure as

well as doped crystals are shown in Fig. 4.7. 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 Zn:LN are respectively 33 and 31 cm-1

. The

magnified view of these bands is shown in the inset of figure. Although the absorption

bands do not have any separated peaks but have comprised of a clear absorption

hump, at approximately ~14 cm-1

away from the main peak position of ~3468 cm-1

.

The relative intensities of the observed peaks are reported to be independent of the

temperature (Herrington et al., 1973).

The spectra infer the aspects about the incorporation of H+ into the crystal

lattice that the transitions are of electric dipole nature and the band is the 0–1

stretching vibrational band of OH- and there are two different transitions near 3484

cm-1

(2.87 μm). It is also reported that the energy levels are not strongly coupled to

phonons (Herrington et al., 1973). Due to relatively large polarizabilities of the O2-

ions and the coulomb forces occupying an anion site, the OH- bond is believed

parallel to the shortest O2-

–O2-

bond favored. The OH bond stretching, normally in

inorganic and organic crystals, is with the energies of the order of 3000–3100 cm-1

.


~ 104 ~

Oxygen

Nb

proton

Li

below plane above plane

1

23

6

45

3540 3510 3480 3450 34200.00

0.25

0.50

LN

Zn:LN

(cm-1)

(

cm-1)

Fig. 4.8: The schematic of an oxygen (001) plane in hexagonal unit cell indicating different O–O

bond lengths and possible lattice cites feasible for proton incorporation (Klauer & Wӧhlecke,

1994). Nature of the absorption constant for OH- absorption band occurred in FTIR spectra

But in LiNbO3 the energy of the band is comparatively higher and only slightly less

than that of free hydroxyl ions (3650 cm-1

), which indicate that OH- ions form very

weak hydrogen bonds only. Figure 4.8 shows the schematic for the nearest neighbors

of O2-

anion in LN. As it is clear from the schematic the oxygen atoms are in the

distorted closed-packed hexagonal structure of LN due to which the sites of hydroxyl

ions are nonequivalent and hence result in components of OH band in spectra. This

stretching energy difference is reported to be very less (~13 cm-1

) (Herrington et al.,

1973).

In our crystals as the peaks in the absorption band are not resolved and form

only a hump which indicates that the crystals’ lattice is not distorted to a great extent

and therefore the transitions corresponding to the O-H stretching of the incorporated

protons at different sites in the lattice are not distinguishable, the hump in case of the

Zn:LN is seem to be diminished and the FWHM of the entire band reduced to 31 cm-1

than that of 33 cm-1

for LN crystal. Such feature of the absorption band indicates that

the distortion of the crystal lattice decreased with Zn doping, by diminishing the

vacancies due to Li ion vacancies and Nb 4

Li antisitic defects. This is well understood

by the evaluation of OH- ions concentration incorporated in the lattice by using the

absorption constant ‘κ’ corresponding to the maxima of bands (shown in Fig. 4.8).


~ 105 ~

The concentration of OH- ions was evaluated by using the relation: n(OH

-) = (3 ± 1) ×

1019

κ(3484 cm-1

) cm-3

, here n is the density of the OH- ions and these have been

found to be 1.17×1019

ion/cm3 and 0.97×10

19 ion/cm

3 respectively for LN and Zn:LN

(Bollmann et al., 1976; Bollmann & Stohr, 1977; Herrington et al., 1973). The lower

OH- ion concentration in Zn:LN indicates that due to the occupancy of VLi and

Nb 4

Li sites by Zn2+

ions, the diffusion of H+ into the lattice of Zn:LN reduced. This

result is in good agreement with the evaluated strains in the crystals’ lattice by PXRD

and Raman investigations. The band at 1746 cm-1

is owing to the characteristic Nb–O

overtone band. Such features of the FTIR spectra assure the perfect molecular

structure of grown crystals and display a good correlation with crystalline perfection

assessed by HRXRD and PXRD techniques and will be further verified by UV-VIS

studies as follows.

4.4.5 UV-VIS-NIR optical analysis

The optical transmission spectra are recorded in the wavelength range of 250-

1100 nm and are shown in Fig. 4.9. The spectra depict that both the crystals are highly

transparent in the entire visible region. In comparison to the pure crystal Zn-doped

crystal possesses slightly higher transparency in the entire visible as well as UV

regions. This enhancement in transparency due to Zn doping is attributed to increase

in the photoconductivity (Simon et al., 1995). The calculated absorption coefficient

(α) exhibits the interesting feature with blue shift of the cut-off wavelength to 324 nm

for Zn:LN compared to that of 321 for LN. This shows increase in optical band gap of

LN due to Zn doping. This behaviour of Zn-doping is just opposite to that of Fe

doping as we have observed in our recent studies on Fe:LN, which exhibits red-shift

(Kushwaha et al., 2011). This shift in the absorption edge may be elucidated

by rZ /2* , here sZZ *; Z*, Z, s , and r are the effective nuclear charge

number, the atomic ordinal number of the ion, the shield factor, and the radius of ion,

respectively. The valance electron transition energy from 2p orbits of O2-

to 4d orbits

of Nb5+

decides the absorption edge in LiNbO3 (Zhen et al., 2003), therefore the

position of absorption edge considerably get affected by the symmetry of the valence

electron shells.


~ 106 ~

250 375 500 625 750 875 10000

25

50

75

100

300 310 320 330 340 3500

25

50

75

100

3.7 3.8 3.9 4.0 4.1 4.20

6

12

18

T (

%)

(nm)

LN

Zn:LN

(

cm-1

)

(nm)

LN

Zn:LN

(h)2

x1

08 (

eVm

-1)2

h (eV)

LN

Zn:LN

Fig. 4.9: From top to bottom; the optical transmittance (T) spectra in the entire UV-VIS-NIR

wavelength range, absorption coefficient (α) vs. wavelength (λ) plots, λ20 (wavelength

corresponding to 20 value of α) used to evaluate the crystals’ composition and defectiveness, and

(αhν)2 vs. hν plots used for optical band gap ‘Eg’ evaluation

The dopant ions with higher ability to polarize O2-

ion, relative to the original lattice-

site ion, decreases the transition energy of valance electron and vice-versa.

In the present case, the Zn2+

ions with lower Z* and hence lesser ability to

polarize the O2-

ions in comparison to that of Nb5+

, the replacement of Nb 4

Li (present

in pure crystal) by Zn2+

results in decrease the polarization ability of O2-

ions


~ 107 ~

300 350 400 4500

5

10

15

20

25

(cm

-1)

(nm)

LN - Yellow

20

= 336 nm

3.0 3.2 3.4 3.6 3.8 4.00

2

4

6

8

10

(

h)2

x10

7 (

eVm

-1)2

LN - Yellow

h (eV)

Fig. 4.10: The absorption coefficient, cut off wavelength and (αhν)2 vs. hν for the yellow crystal,

λ20 has been used to evaluate the defectiveness of crystal

in the consequence of it the energy required for transition of valance electron

increased (Zhen et al., 2003) and hence increases the band gap of crystal. It is

important to mention here that such behaviour of Zn doping is true for its

concentrations below the threshold value. The Fig. 4.10 shows absorption coefficient

spectrum of the grown yellow crystal [Fig. 4.1(c)]. The absorption edge for this

yellow crystal is significantly shifted towards the longer wavelengths. This behavior

of the absorption edge may be due the presence of VLi and NbLi4+

with high

concentration. The direct band gaps of LN and Zn:LN crystals have been calculated

using the Tauc’s relation:

)()( 2 hEAh g (4.1)

and by plotting (αhν)2 vs. hν (Fig. 4.10), where α is the absorption coefficient, ν is the

frequency of incident radiation, h is the Plank’s constant, Eg represents the optical

band gap and A is a constant (Kushwaha, Maurya et al., 2011). The Eg values have

been evaluated by extrapolating the linear part of the plots to the abscissa (hv) as

shown in figure and found to be 3.91 and 3.97 eV respectively for LN and Zn:LN.

This increase in the band gap of LN by Zn-doping at 1 mol% (which is well

below the threshold concentration of ~6 mol%) could be attributed to the occupancy

of Zn2+

ions at Li+ sites in the form of Zn which are otherwise occupied by Nd

5+ in

the form of Nb 4

Li (Zhen, Xu et al., 2002). At high concentrations, Zn facilitates LN

for excellent photorefraction applications in UV region of electromagnetic radiation


~ 108 ~

Table 4.2 Evaluated Li/Nb, C(VLi) and C(NbLi) values for the grown crystals

(Haijun et al., 2004). From the Fig. 4.10 it is clear that the band gap for the yellow

crystal is very less than that of transparent pure LN crystal.

The Li/Nb ratio for the grown crystals has been evaluated by using the

empirical formula,

2

20

29.81

54.3011

Nb

Lisuitable for nominally pure as well

doped crystals with concentration below threshold value, here λ20 is the wavelength

(nm) corresponding to the absorption coefficient of 20 cm-1

. The Li/Nb values are

found to be 0.954 and 0.959 respectively for LN and Zn:LN crystals and used to

estimate the concentration of VLi and NbLi which act as defects in crystals, using the

formulae (4.2) for pure and (4.3) & (4.4) for doped crystals (Salloum et al., 2010).

%)(100)/(5

)/(44)( mol

NbLi

NbLiVC Li

& 4/)()( LiLi VCNbC (4.2)

%)(100)/(5

)/)(4(34)( mol

NbLi

NbLiyyVC Li

(4.3)

%)(100)/(5

)/(21)( mol

NbLi

NbLiyNbC Li

(4.4)

where, y is the molar percentage of dopant in the crystal, which is 1 mol% ZnO in the

present case. The evaluated defect concentrations are given in Table 4.2. The lower

concentrations of VLi and NbLi in doped crystal indicate that the doped crystal

comprises lesser point defects compared to that of pure crystal, as illustrated by

PXRD and HRXRD, lead to the fall in the lattice strains. The reduced concentration

of NbLi manifests the slight increase in the band gap. The significant blue shift of the

absorption edge in stoichiometric as well congruent crystals and interesting

photorefractive behavior in UV region has been reported with varied Mg doping

(Salloum et al., 2010; Jingjun et al., 2000). The value λ20 for yellow crystal is very

Crystal λ20 (nm) Li/Nb C(VLi) (at %) C(NbLi) (at %) Eg (eV)

LN 319 0.954 0.62 0.15 3.91

Zn:LN 318 0.959 0.48 0.07 3.97

LN-Yellow 336 0.820 2.47 0.62 3.56


~ 109 ~

high compared to that of LN as well as Zn:LN. The ratio of Li/Nb for yellow crystal is

found to be significantly less i.e. 0.820. The tabulated values of C(VLi) and C(NbLi)

[Table 4.2] are very high compared to those of LN and Zn:LN and these results

clearly decipher that the grown yellow LN crystal contains more Li vacancies and

NbLi antisitic defects and hence these results clearly reveal that when we use the same

lithium niobate residual charge many times for the growth of single crystals, the

grown crystals have abundance of intrinsic defects and get yellow color due to color

centers at Li vacancies.

4.4.6 Prism coupler birefringence analysis

Lithium niobate is an excellent nonlinear optical (second harmonic generation:

SHG) crystal due to its birefringence nature. The Nb 4

Li and VLi intrinsic defects in LN

influence the photorefraction behavior by manipulating photo-induced charge

transport and thereby the change in refractive index. The NbLi act as electron shallow

traps and VLi as hole traps. The birefringence of lithium niobate crystals increases

significantly in the presence of transition metals as dopants (Buse et al., 1998;

Peithmann et al., 2002; Peithmann et al., 1999; Haijun et al., 2004; Koster et al.,

2009; Jaque et al., 2004). The ordinary (no) and extraordinary (ne) refractive indices

of grown LN and Zn:LN crystals have been measured for the wavelengths 532 and

1064 nm and are plotted in Fig. 4.11 and the actual values are tabulated in Table 4.3.

The measured refractive indices are in well agreement with earlier reports (Schlarb et

al., 1995; Weis et al., 1985; Volk et al., 2001). The refractive index values are found

to be higher for 532 nm compared to those for 1064 nm. The no for both pure and Zn-

doped LN crystals are almost same at both the wavelengths 532 nm and 1064 nm,

whereas ne changed in doped crystal to lower values. The birefringence

( oe nnn ) of crystals are found to be higher for LN in magnitude at 532 nm in

comparison at 1064 nm. In contrast to LN, ∆n for Zn:LN found to be increased at 532

nm whereas at 1064 nm, it is decreased. This may be due to the fact that the higher

energy photons are able to excite the charge carriers from deeper level rather than the

photon of lower energy and lead to the photorefraction in LiNbO3 in ultraviolet

wavelength range (Haijun et al., 2004). The ∆n values at different wavelengths of the

pure crystal for have been used to evaluate the stoichiometry (CLi) of LN crystals


~ 110 ~

2.226

2.268

2.310

2.352

no

LN

Zn:LN

2.142

2.184

2.226

2.268

ne

532 1064-0.056

-0.070

-0.084

-0.098

n

Wavelength (nm)

Fig. 4.11: The ordinary (no), extraordinary (ne) and birefringence (∆n) parameters of pure LN

and Zn:LN crystals at 532 and 1064 nm wavelengths measured by the prism coupler spectrometer

Table 4.3 The evaluated no, ne, ∆n and CLi values for the grown single crystals

Crystal λ (nm) no ne Δn CLi (mol%)

LN 532 2.324 2.234 -0.090 48.13

1064 2.231 2.156 -0.075 47.92

Zn:LN 532 2.323 2.227 -0.096 -

1064 2.233 2.167 -0.066 -


~ 111 ~

using the relation nbaCLi )()( , where a(λ) and b(λ) are the wavelength

dependent parameters and the values of these parameters are taken from the literature

for the respective wavelength used to measure birefringence and CLi values are listed

in Table 4.3. The CLi for LN are of the same order as obtained by the Raman analysis

and indicate that the grown crystals are only slightly off from their stoichiometry and

is congruent in nature. The PXRD and HRXRD structural analysis along with their

optical behaviour infer the congruent nature of grown crystals which is further

evidenced by the composition evaluation.

4.4.7 Wavelength dispersion analysis

The wavelength dispersion analysis of the grown crystals is analysed by

ellipsometry and the optical constants have been evaluated. The optical constants

define how light interacts with a material. The complex refractive index is a

representation of the optical constants of material and is represented by ñ = n + iκ.

The real part ‘n’ is the index of refraction, defines the phase velocity of light in

material: v = c/n, where v is the speed of light in material and c is the speed of light in

vacuum. The imaginary part ‘κ’ is the extinction coefficient, determines how fast the

amplitude of the wave decreases. The extinction coefficient is directly related to the

absorption of material and is related to the absorption coefficient by: α = 4πκ/λ,

where, α is the absorption coefficient and λ is the wavelength of light. The measured n

and κ are plotted as a function of wavelength for LN and Zn:LN crystals and are

shown in Fig. 4.12. The obtained average RI values for both the crystals are of the

order of reported in literature and in the above section evaluated by prism coupler

measurements. The RI values are higher at lower wavelengths compared to that in

high wavelength region. In the high wavelength region RI values for both the crystals

are similar, whereas at lower wavelengths these values are higher for Zn:LN crystal.

This behaviour of RI for doped crystal is, because Zn enhances the photorefraction

behaviour of LN in the ultraviolet region by removing the localized energy states

present in the optical band gap caused by the vacancies and antisitic defects in the

lattice. The κ increases with increase in the energy and above 300 nm it remains

almost constant, it is clear from the plots that κ for Zn:LN is slightly lesser over the

entire energy range.


~ 112 ~

2.25

2.50

2.75

0.30 0.45 0.60 0.75 0.900.0

0.5

1.0

n

LN

Zn:LN

(m)

LN

Zn:LN

Fig. 4.12: The variation of average refractive index ‘n’ and extinction coefficients ‘κ’ with

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

4.5

5.4

6.3

7.2

0.30 0.45 0.60 0.75 0.900

2

4

r

LN

Zn:LN

i

(m)

LN

Zn:LN

Fig. 4.13: The variation of real ‘εr’ and imaginary ‘εi’ components of the dielectric constant

with the wavelength. These components have been evaluated from n and κ parameters

The linear response of the matter to electromagnetic radiation is best described

by the complex frequency-dependent dielectric constants (ε = εr + εi ) where, εr and εi

are respectively the real and imaginary parts of optical dielectric constant.


~ 113 ~

0 2 4 6 8 10

0.22

0.24

0.26

0.28

(Eo & E

d)

LN= 8.96 & 32.6 eV

LN

Zn:LN

Linear fit - LN

Linear fit - Zn:LN

1/(

n2-1

)

(h)2 (e

2V

2)

(Eo & E

d)

Zn:LN= 7.71 & 26.1 eV

Fig. 4.14: The (n2-1)

-1 vs. (hν)

2 plots for and the linear fit to these plots for the evaluation

of dispersion energy ‘Ed’ and average single oscillator energy ‘Eo’ of LN and Zn:LN crystals

These are related to the more directly measureable parameters, refractive index and

extinction coefficient through the following relations (Wolton et al., 1963; Wemple et

al., 1977):

22 nr and ni 2 (4.5)

The variations of εr and εi with wavelength are shown Fig. 4.13. Both εr and εi have

higher values at lower wavelengths and decrease very fast with increase in

wavelength but for the wavelengths above 400 nm these remain almost constant, and

follow the similar behaviour respectively of n and κ. The εr values for doped specimen

are higher over the lower wavelength range up to 600 nm. However εi values are

almost same for both the crystals. The dispersion plays an important role in the

research for optical materials due a significant factor in optical communication and in

designing the devices for spectral dispersion. The single-oscillator parameters for the

crystals were calculated and analyzed using Wemple-DiDomenico model

(DiDomenico et al., 1969):

22

0

02

)(1

hvE

EEn d

(4.6)

where, h is Plank’s constant, Eo is the average single oscillator energy for electronic

transitions and Ed is the dispersion energy or oscillator strength which measures the

average strength of interband optical transitions. These parameters for both the

crystals have been obtained by plotting 1/(n2-1) vs. (hv)

2 in Fig. 4.14 for the 1 – 10


~ 114 ~

e2V

2 range of (hv)

2. The light and dark straight lines are the linear fit to the plots

respectively for pure and doped crystals, have been extended to the ordinate at zero of

abscissa. The linear fit plot for Zn:LN exhibits higher slope and intercept on the

ordinate compared to that of pure. The Eo and Ed have been calculated from the slope

(EoEd)-1

of linear fit curves and intercept the (Eo/Ed) on ordinate cut by the extension

of linear fit. The oscillator strength and dispersion energies of crystals strongly

depend on their structures (Wemple & DiDomenico, 1969). The evaluated Eo and Ed

parameters for pure crystal are slightly higher than that of reported values, however

for the Zn-doped crystal these parameters are in good agreement with reported one.

(Wemple & DiDomenico, 1971). The deviation in these parameters for the pure

crystals can be correlated with the presence of abundance of Li+ ion vacancies and

NbLi4+

antisitic defects in pure crystals as described in §4.4.1 and §4.4.2. However, in

Zn-doped crystal these point defects were removed and resulted in widening of optical

band-gap of crystal and dispersion parameters have attained the perfect values. The

normalized dispersion energy ‘β’ evaluated by a general empirical expression; Ed = β

NcZaNe, where Ne is the effective valance electrons per anion, Za anion valancy and Nc

the coordination number of the nearest cation. Its value for Zn:LN crystal is 0.27 and

falls well in the category ionic oxide crystals, whereas for LN it is 0.34 slightly higher

due to the higher dispersion nature of crystal.

4.4 CONCLUSION

The bulk single crystals of pure and Zn-doped (1 mol%) LiNbO3 have been

successfully grown by indigenously developed CZ puller assisted with radio

frequency heating furnace and a post growth resistive heater. Both the crystals are

visibly quite transparent. The PXRD analysis confirmed the crystal structure and

space group which are same for both the crystals. The powder XRD results revealed

stresses in the crystals and found to be tensile strain in LN due to vacancy defects

whereas compressive strain Zn:LN crystal due to Zn-doping. The HRXRD revealed

that that the crystalline perfection of both pure and doped crystals is reasonably good.

LN contains the vacancy defects owing to the Li deficiency and free from the

macrodefects like grain boundaries, however Zn:LN contains low angle grain


~ 115 ~

boundaries. The Raman analysis for the defects analysis found to be consistent with

PXRD and HRXRD results and illustrate the congruent composition of the grown

crystals. The O–H stretching vibrations in crystals originated due to the incorporation

of protons (H+) in the crystal lattice and have been assessed by FTIR. The

concentration of OH- ions in both the crystals has been evaluated, which is slightly

higher for the pure crystal.

Compared to that of LN the optical transparency of Zn:LN increased over the

entire recorded UV-VIS-NIR spectrum. Band gap was found to be slightly increased

due to Zn doping. The enhanced optical transparency and band-gap envisage the

suitability of Zn:LN for SHG applications. The birefringence of the doped crystals

found to be increased for lower wavelengths however, at higher wavelengths it is

found to be decreased, such behaviour of doped crystal makes it suitable for the

photorefractive holographic data storage applications in the ultraviolet region. The

wavelength dispersion behavior of grown crystals has been revealed that the average

single oscillator energy for electronic transitions (Eo) and dispersion energy or

oscillator strength (Ed) of optcal transitions. The Eo and Ed values found to be strongly

dependent on the crystalline perfection/ lattice defects.

~ 116 ~

czochralski growth , crystalline perfection and optical c...

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