CHAPTER 6

OPTICAL PROPERTIES OF (l-~-y)(B~0,)-x(Li,O)-y(MC1~) GLASSES

6 . Optical properties of (1-x-y) (B203)-x(Li20)-y(MC12) (M=Cd,Zn) glasses

6.1. Introduction

Most organic coiupounds and many inorganic ions and complexes absorb

radiation in the 1JV-vis. region. A plot of the absorbance by a compound against

wavelength is called its absorption spectrum. This has a shape that is

characteristic of the particular compound or class of compounds. Absorption

spectrometry is a non-destructive' and extremely sensitive technique, and is

therefore ideal li)r the characterisation of small amounts of precious compounds.

Measurement of mt)lecular interactions with the absorbed radiation forms the basic

of UV-vis spectroscopy and could provide a wealth of information about the

molecules.

Optical transmission property of glasses is important for their application as

glass windows and in glass bulbs. The uses of lenses for better vision and glass

liber communication are based upon the refractive, optical dispersion and

transmission properties of glasses. There are many technological areas where the

optical properties of glass play a pivotal role. Selective absorption of various

wavelengths in the visible region gives rise to the appearance of colours in glass.

The source of absorption is of three types: (i) electron transition within the

~~nfilled orbits of transition elements; (ii) plasma resonance; and (iii) electron

transitions across the band gap. There are four possible transition element series in

the periodic table. Of these, the first series from Sc to Ni produces strong colours

because of absorption of selected wavelengths when they are present in a glass

even in amounts less than 1 percentage. The amber colour, commonly called the

carbon-sulfur amber, is obtained by adding 0.2-0.5 percentage of Fe203 plus

Na2S04 (to yield - 0.04 percentage of SO3 in the glass). Colour can be produced in

glasses by precipitating in them colloidal particles of metals such as Au, Ag or Cu.

l'he absorption in these glasses occurs due to the plasma resonance of free

electrons in the metal particles. In semiconductors, electron transitions across the

band gap can also cause absorption in the visible region of the spectrum.

t;xamples of this type are the yellow to red colours in glasses containing small

anlounts of CdS-%nS and CdS-CdSe. Strong absorption bands arising from

electron excitations produce essentially a UV cut off, causing most glasses to

appear opacluc in the UV. These electron excitations are generally of two types:

( i ) intrinsic excitation where electrons are excited from the valence band to

unoccupied states in the conduction band levels (ii) transfer of an electron between

the shells of one ion arid the shells of a neighboring ion (interionic transition).

l'hese ions may bc major constituents or minor constituents. Electron transfer 3+ . between neighboring ~nultivalent transition elements, for instance ~ e ~ ' to Fe , 1s

an example of the latter. Intrinsic absorption occurs when photons have energies

comparable to that of the band gap, Eg. This means roughly 8.9 eV for Si02 and

only about 1 to 3 eV for amorphous semiconductors such as the chalcogenides2.

The effect of a strong disorder on the electronic states of alnorphous

semiconductors is apparently more profound close to the band edges than deep in

the bands. 'l'he two important changes in the electronic states near the band edge

are localization, and snlearing of the band edges. Experimental evidence for the

existence of electronic states in the energy gap of chalcogenide glasses was

provided by 'I'auc and ~ c n t h ' . He has estimated the density of distribution of these

states from the optical and photo-emission measurements. Kunio wakamura4

suggested a strong contribution of the electronic energy state near the band gap on

superionic condudion.

Szukwei et ali presented a systematic investigation of the optical and

elcctric properties of (ie-Se-Te glass system in order to find out the relationship

between the glass composition and band gaps. The experimental results show that

width of gap of (ie3rSe24+,Te40.,As4 (x=0,8,16,24,32) glasses varied with the

\ ariation of Sc- I'e ratio. It is indicated that, the increasing Se content causes an

increase in the value of E,(opt) and E,(ele). The difference between Eg(e1e) and

E,g(opt) are used to measure band tails in the gap. For higher Se concentration the

difference betbveen l,(ele) and E,(opt) are increases to a higher value and causes a

deeper extension in band tail in the gap. This is also an indication of an increase in

the degree of disorder with increasing Se concentration in the glass. E Kh shokr6

also reported the optical properties of glassy Ge20Teso.xSe, thin films. The effects

of composition and thickness of films on optical parameters are described in this

report. A computational method was applied by Al - ~ n i ' for the analysis of optical

absorption spectra of amorphous materials to obtain both the power index r and

,,Dl.

The inllucnce of co~nposition and film thickness on the optical band gap

and the width of the band tail of thin solid films of the chalcogenide glass system

As-Ge-Se was investigated by Abd-El-Rahman et a18. It was found that the

~nagnitude of optical band gap decreases with increasing Se concentration. The

results were interpreted in terms of the change of cohesive energy as a function of

Se concentration. 'I'he cohesive energy was estimated by employing the chemical

bond approach. I t is reported that the increase of Se content leads to an increase in

the width of the band tails.

~edeek ' inlest~gated the electrical and optical properties of amorphous

CieSe2 and Geuo3SezCd 0034 films. Cd incorporation affects greatly the thermal

activation energy but has little effect on the optical gap. The addition of Cd

renders thc absorption edge more broadened and shifts it to lower energies.

Some detailed reports on the study of optical spectra of oxide glasses

including burate glasses are available in the literature. Optical absorption

coefficient of a series of glass specimens prepared from a mixture of ZnO and

were measured as function of photon energy in the range 3.18 - 6.53 eV by

liashed et al"'. Va l~~es of optical energy gap were found to decrease linearly with

tlie ZnO concentration in the glass. This decreasing tendency of energy gap was

explained in terms of more ordering in the glass system. The mechanism of optical

absorption was discussed based on the theory of Davis and ~ o t t " .

Al-Ani and 1 1igazyI2 reported the optical absorption studies in a series of

MgO-P1O5 glasses in the ultraviolet-visible ranges. They reported that the

fundamental absorption edge is a function of glass cornposition and at lower

values of the absorption coefficients cc(o), it follows the so-called Urbach edge.

The value of the width of tail of localised states in the band gap was found not to

vary significantly nith glass compositions. It is reported that the absorption edge

inoves to long wavelength as the percentage of MgO in the glass is increased. In

the higher energy region, the behaviour of a(w) suggests that there are two

different transition energies for electron, namely direct allowed transition and

riondirect transition in K space.

Sharma et ill'.' r-eported the optical spectra and energy band gap in

praseodymium boro phosphate glasses. They reported that the values of optical

band gap and position of the absorption edge are not particularly sensitive to the

~ncorporation of rare earth ions. The effect of addition of PbO on optical properties

of binaq senliconducting glasses in the system V205-TeO have been reported by I4 . - Memon et al . 1 he effects of concentration of lead oxide on refractive index, and

optical phonon frequency have been discussed. The linear and nonlinear refractive

index for Si02-Ti02-Pb0-K20 glass systems has been investigated by Zhu et all5.

The optical properties of superionic glasses of (AgI), (Ag20 n B203)1.s were

studied by Dalba et al '%by optical absorption spectroscopy for various values of x

and n. Thc optical absorption for n = 4 glasses was reported to shift linearly with

increasing concentration (x) of the AgI.

Optical absorption spectra of tellurite glasses containing boric oxide were

studied by Sabry and EI-~arnanoudyl' in order to study the structure of glasses.

.\'he results of these studies show that the fundamental absorption edge is a

function of B203 concentration. The optical band gap increases as the B203

concentration in thc binary system increases from 5 to 30 mole percentages. It is

also reported that there is no sharp absorption edge, but there is a shift of the

absorption edgc to lower wavelengths as B203 was increased. This shift to lower

energies or a change in the absorption band characteristics could be related to a

transition of bridging oxygens to non-bridging oxygens which bind excited

electron less tightly than bridging oxygen.

'The refractive index and the optical band gap of the glass system M20-

B203 (M= Li. Na, K, Rb, and Cs) were studied by Terashi~na et all8. It is found

that the optical band gap to become smaller with increasing M 2 0 content for each

glass systems. .l'llcrc has been interest in recent years on halide glasses because of

the possibility of tleveloping low loss IR ~ a v e - ~ u i d e s ' ~ . But the information

available on the optical properties of glasses that contain a mixture of an oxide and

a halide is \cry limited (Dalba et a1)16. Optical properties of ternary and binary

borate glasses have not been extensively studied. The results of a systematic study

of the optical properties of (1-x-y)(B203)-x(Li20)-y(MC12) (M=Cd,Zn) glasses and

the variation of these properties with composition are reported in this chapter.

6.2. Work undertaken in the present study

The optical absorption and transmission spectra of (1-X-y)(B203)-x(Li20)-

y(MCI2), (M= Cd, Ln) glasses of varying compositions were recorded in the UV-

vis region. From the spectra, various optical parameters such as optical energy gap

(E,,,), refractive index (n), optical dielectric constants (&') width of the tail of

localized states in the normally forbidden gap (AE), ratio of carrier concentration

tc, the effective mass ( ~ l m ' ) , and constant B were evaluated. The effects of

cc~mposition of glasses on these parameters are discussed.

6.3. Theory

The behaviour of the electrons in glassy materials has been a subject of

great interest2". ~ o t t " indicated that there a r e some similarities between the

energy band structure of crystalline and glassy non-metallic materials. But, where

as the crystalline materials show well-defined energy bands with sharp conduction

and valence band edges, the glassy materials show band tailing into the forbidden

gap. The most generally accepted model for conduction in amorphous materials

involved a narrow band of localised states near the centre of the band gap. Two

concentrations 01' trapping centers also exist, one is the upper part of the band gap

and other in the lower part, effectively pinning the Fertni levels near the middle of

thc gap. This model is sh~>wn in figure 6.1.

1 igure 6.1. Ilensit) ot states N(E) and mobility p as a function of energy in an ~morphous semiconductor. Localised states are shown shaded.

1.or absorption coeilicient u >lo4 cm-I, the following relation is obeyed 22.1 1 ,3

a(w) = B Po-E,Jr /-ha,

or u ( o ) b o = B (iio-E,,,)', (1)

whereho is the photon energy, B is a constant, E,,, is the optical gap and r is a

number which characterizes the transition process. The exponent r can take the

values 2,3,112. and .3/2 l'or indirect allowed, indirect forbidden, direct allowed, and

direct forbidden transitions, respectively. The constant B is related to the

parameter AI:. which is a measure of extent of band tailing, by the following 2 0 equation .

B = 4nomi,I ncAE, (2)

where o,,,i,, is the extrapolated dc- conductivity at temperature T = oo, n is the

refractive index and c ihc velocity of light. Ignoring the variation of n w i thdo

and taking n,, as thc average value of n, it follows that (n ,~) . ' - AE is a measure of

extent of band tailing.

The reflectance of glass samples can be calculated using the equation

t = (I - R ) ~ exp(-A), (3)

where R is the reflectance, t the transmittance and A the absorbance. Absorption

coefficient u is obtained fro111 the absorbance (A) with accuracy k0.02 through the . relation

A = ad , (4)

where d is the thickness of the sample (measured with an accuracy of k0.005mm).

The optical diclectr~c constant (6) and the square of wavelength (h2) are relatedz3

through the tbllowirrg equation

- n - = ' [ ( I + ~ R ) I ( I - ~ R ) ] ~

- - E', - e2 N h2 / nc2m*, (5)

where E', is the infinitely high frequency dielectric constant, e the electronic

charge, and (NILTI') is the ratio of carrier concentration to the effective mass.

6.1. Experimental details

Fourteen glass samples of different compositions with code numbers BZLl

to BZL7 and UCL I to BCL7 were prepared from appropriate amounts of analar

grade H3BO3, Li2C03, and CdC12 or ZnC12 (Ref: Table 3.1, chapter3). Glass discs

of -0.90 lnin thickness, and diameter -2 cm were made using melt quenching

technique, which is described in section 3 .3 of chapter 3. The glass simples were

annealed at a temperature of about 450°C for 4h. The samples were cooled to

room temperature and were then polished. The grinding and polishing of the glass

samples is only possible with those glasses annealed at 450". I f the annealing

temperature is beLou 450°C it will break during the polishing. The transmission

and absorpt~on spectra of glasses were recorded using Shimadzu uv-160A

spectrophotorneter 111 the wavelength range 200-1000nln (figures 6.1 and 6.2). All

the spectra were recorded at room temperature. The glassy nature of the samples

was confirmed from thc X-ray diffraction pattern (Ref. Figure3.a).

Wavelength (nrn)

Figure 6.1 l'ransmission and absorption spectra of BCL glasses

300 600 800 1 000

Wavelength (nm) Figure 6.2. 7'ransmission and absorption spectra of BZL glasses

6.5. Results and discussion

The various optical parameters of the glass samples were calculated and the

results are given in table 6.1. The plots (Figures 6.3-6.6) obtained by plotting the

quantity (cthv)'" as a function of hv are found to consist of two straight lines a and

b. Thc nature of these plots are found to be connecting the maximum points for the

value of r=2 suggesting the absorption process to be due to an indirect allowed

transition. The optical energy gap (E,,,) of the glass samples and the constant B

were obtained from the extrapolation of the linear region (b) and from the slopes

of the above derived curves, respectively. The values of E,,, (determined with an

accuracy of 0.008 eV), as a function of the glass composition are shown in figures

6.7 and 6.8. There is a tendency for the optical band gap to become larger with

increasing LizO or LnClz content for BZL glasses.

Table 6.1. Optical energy gap (E,,,), infinitely high frequency dielectric constant (c,), refractive index (no), constant (B), ratio of carrier concentration to thc effectiuc mass ( ~ l m * ) , and the extent of band tailing (AE) of glasses of different compositions.

- Sample E,,,, (eV) E', no B (Nlmn)xl~22 AE(c~-'~v-"~) Code .- -.

(cm-lev-'") x 1 o - ~ BZLl 2.97 3.241 1.65 394.5 1.645 1.540 BZL2 3.32 3.063 1.83 481.8 0.944 1.130 BZL3 3.71 4..387 2.01 872.8 , 1.181 0.570 BZL4 3.53 13.02 3.63 2276 0.391 0.121 BZL5 3.18 2.374 1.54 788.8 1.385 0.823 BZL6 3.76 2.815 1.87 1266 1.995 0.422

5.023 2.13 768.8 1.540 0.61 1 I ; ::I:, 3.073 1.66 812.8 0.963 0.742 BCL2 3.17 1.773 1.25 1205 0.559 0.664 BCL3 3.12 3.091 1.75 903.8 0.671 0.632 BCL4 3.24 2.926 1.74 301.6 0.376 1.910 BCL5 3.18 2.310 2.06 1165 0.388 0.417 BCL.6 3.35 2.715 1.42 513.8 2.137 1.370 BCL7 2.83 -~ 2 1.88 66.54 0.126 7.990

But the values obtained for E,,, are found to exhibit a nlaxi~nu~n at 10 mole

percentage of LizO (BCL2) and minimum with 15-mole percentage of Li20

(HCL3). Similarl) for CdC12 variation, Eopt are found to exhibit a nlaximum at 5-

mole percentage of CdC12 (BCL5) and minimum with 15 mole percentage of

CdClz (BCL7). The optical band gap of the glass system M20-B203

(M=Li,Na,l<,Rb,C>,Ag) were studied by Terashi~na et a1". It is reported that there

1s a tendency for the optical band gap to become smaller with increasing M 2 0

content for each glass system. The compositional dependence of refractive index

11, as shown in figures 6.9 and 6.10 reveals that no increases with increasing

concentration of' 1 i20 or ZnC12 in BZL glasses. For BCL glasses, no shows a

~naxirnum at 15 mole percentage of L i20 (BCL3) and a minimum value at 10 mole

percentage of LizO (BCL2), while for CdC12 variation, no shows a maximum at 5

mole percentage of CdC12 (BCLS) and a minimum value at 10 mole percentage of

CdClz (BCL,6).

Figure 6.3. I'lot o f ' ( a h ~ ) ' ' ~ with hv for different mole percentage of L i20 in BCL glasses.

Figure 6.4. BCL

Flgure 6.5. Plot of ( u h v ) ' with hv for different nlole percentage of LizO in BZL glasse\

hv(eV) Figure 6.6. Plot of ( ~ h v ) " ~ with hv for different mole percentage of ZnClz in BZL

glasses.

+UZO\criaion in E L glass + W I Z wia60n in B2L qlass

I . , . , . , I . , . , ,

4 6 8 10 12 14 16 18 20 22

mole percentage of x or y figure 6.7. Variation of' optical band gap (Eopt) with mole percentage of Li10 (x)

or CdClz (y) in BCL glasses.

Mole percentage of x or y Figure 6.8. Variation of' optical band gap (E,,,) with mole percentage of LizO (x)

or ZnClz (y) in BZL glasses.

Mole percentage of x or y

Figure 6.9. Variation of refractive index (no) with mole percentage of Li,O (x) or CdC12 (y) in UCL glasses.

Mole percentage of x or y

1,igure 6.10 Var~at~on of refractive index (no) with mole percentage of LizO (x) or LnClz (5) I I I BZL glasses.

Equation (5 I is ~erified and the variation of &'with h2 is shown in figures

6.11-6.14. 'l'he values of E', and e2 N / nc2m* determined from the extrapolation

of the plot to A' 0 and their slopes, respectively. Then the quantity of (Nlm*) is

computed and its \ d u e \ are listed in Table 6.1 as functions of glass composition.

I'he derived values ol (n ,~) . ' , which is a measure of the extent of band tailing

(AE). are found to be decreasing with increasing concentration of Li20 or ZnCI2 in

BZL glass. But the values of AE show a maximum and minimum with increasing

concentration of CdCI: or Li20 content in BCL glasses (Table 6.1). The sinall

values of band tailing energy indicate the presence of sharp localised states in the

band gap. The changes in the borate network produced by the addition of Li20

may be the reason for the generation of localised states in the band gap. The

introduction of oxygen Srom a modifier oxide to boric oxide glasses converts

boron from a 3-coordinated state to a 4-coordinated state. In the B 0 4 and BO,

groups, the oxygens are fully bridging for small concentration of modifying

oxygen and it will give inore rigidity to the structure. Further addition of modifier

oxide will causes creation of non-bridging oxygens in the structure and gets more

disorder in the glass structure. From table 1 it is seen that there is almost a

systematic variation in E,,, and AE indicating that the degree of disorder is

markedly influenced by the change in concentration of Li20, ZnClz or CdC12. The

observation that the magnitude of the band tailing is small points to the generation

of states in the band gap which are localised near the mobility edge. Variation of

AE with composition of the glass also shows that the extent of the localised states

is disordered depcndcnt, which is in agreement with the theory of Mott and

r)avis20.

2.0 3.0 4.0 5.0 6.0 7.0 8.0

2 (nrn2)

Figure 6.1 1 . Variation of dielectric constant (E') with hZ in BCL glasses.

' ' ' 1 ' ' 1 ' ' ~ 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

h2 ( n m 2 ) ( ~ 1 ~ - 5 ) IFigure 6 . I? . Variation of dielectric constant (E') with h2 in 13CI. glasscs

F i r I . I . \ ' ; i~~;ition ol'iliclcctric constant (c') wit11 A' ill I IZI slasscs.

Figure 6.14. Variation of dielectric constant (E') with k2 in BZL glasses.

6.6 . Conclusion

The optical gap (E,,,) and band tailing (AE) of the glass systems (I-x-y)

(B203)-x(Li20)-y(MC12) (M=Cd,Zn) were found to vary with the variation of

I,i20, CdC12, and ZnC12 concentration in the glass composition. The refractive

index (no) is found to increase with increasing concentration of LizO or ZnClz

content in the BZL glasses, while for BCL glasses, it shows a maximum and a

minimum value for increasing concentration of Li20 or CdCL2. It is observed that

the transition in (1-x-yj(B203)-x(Li20)-y(MC12) (M=Cd,Zn) glass system is

indirect allowed in naturc. 'The values of band tailing (AE) are found to be very

small leading to large values of optical band gap.

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