Indian Journal of Pure & Applied Physics

Vol. 39, October 200 I, pp. 647-653

iT t;I "NI 'I

Optical absorption ~dge and Urbach tails for ITlo.999Q.afro.OolSe2, ITlo.99sGaPro.oosSe2 and TIGaSe2

~) Gi.irbulak

AtatUrk University, Faculty of Sciences, Department of Physics, 25240 Erzurum, TUrkiye )

----Received 27 November 2000; revised 19 June 200 I ; accepted 6 July 200 I

C TIGaSe2, Tlo.'l'J,)GaPrO.(XlISe2 and Tlo. 'l')~GaPrO(X)~Se2 single crystals were grown by the Stockbarger method. ll1e absorp­tion measurements were carried out in these samples in temperature range 10-320 K with a step of 10K. The phonon ener­gies calculated in TIGaSe2. Tlo.'l'J,)GaPrO.lX))SeZ and Tlo"),),1GaPro.CMl,lSez are 60.0±5, 55.0±5 and 130.0±5 meV respectively. The tirst defect levels (n= I) have been found as 2.259. 2.235. 2.200 and 2.149 eV for Tlu.,),)<)GaPrO(XlISe2 and 2.254 , 2.225. 2. 189 and 2.149 e V for Tlo.'l'J,GaPro.(Xl.ISe2 at 10. 100, 200 and 300 K. At 300 K direct band gap of TIGaSez is 2. 156 eV, and indi­rect band gap is 2.075 eV. There are abrupt changes in the Urbach energy peaks for Tlo,),),)GaPrO(XlISe2 at 100 and 200 K. and Tlo.'l'J,IGaPrO.lXl.lSe2 at 200 and 260 K. There is an abrupt change in the 00 values for Tlo,),),)GaPrO(XlISeZ and Tlo.'J'J5GaPro.(X),SeZ in the temperature range 140-180 K and 220-260 K. ll1ese temperatures obtained from the changing of Urbach energy and 0 0

values may be phase transition temperatures. )

~ B i. /1 ~ s., ~ 3 v-~ f1 et-ed I,' c- VJ s .: _ c. __ 1 Introduction

The ternary semiconducting chalcogenides with

the formula ABX2 (A, B represent metal atoms and X represents chalcogen atoms) have been studied inten­

sively for years l.3. The ternary compounds TIGaS2,

and TIGaSe2, crystallize in two layers and contain 64

atoms4. Layer crystals consist of separate layers with

strong (covalent) bonding between atoms within the

layers . The bonding between them is weak, predomi­

nantly of Van der Waals type. Such crystal structure

causes the specific shape of phonon branches in layer

crystals5. The p-type TIGaSe2 crystals were prepared

by the Bridgman-Stockbarger method6.7

. Large crys­

tals of TIGaSe2 were grown by using the Bridgman method8

Optical absorption spectra of CuInSe2 single

crystals were measured for the samples with - 0.150 :-; x :-; 0.053 , where x represents a degree of non­

stoichiometry in formula Cu l.x1nl+xSe2' The Urbach

tail was observed for all samples between 90 K and

room temperature. The Urbach's energy which repre­

sents an arbitrary intensity of exciton-phonon interac-

tion, was almost constant for Cu-rich samples (x < 0).

while it increasing In composition for the In-rich9

ones (x > 0) . The Steepness constant in Urbach rule

for the absorption spectra 10 of EuSe has been deter­

mined for the first time to be 0 .79.

The energy gap structure and the lattice dynam-. III III VI .

ICS of ternary layer A B C2

semiconductor,

particularly that of TIGaSe2 known to exhibit a strong

anisotropy of the electronic and vibrational spectra

have been attracting considerable attention 6. The

present report gives results obtained while studying

the photoelectrical properties of TIGaSe2 crystals

non-doped and doped with rare-earth impurities .

The long-wavelength tail of the optical absorp­

tion in TIGaSe2 at a = 30-150 cm- I is shown to obey

the Urbach rule ll .12 in the temperature range 4.2-293

K. The anomalous behaviour of the parameters of this

rule suggests the presence of two phase transitions in

TIGaSe2 at 246 K and 101 K besides the known

phase transitions at 120 and 107 K. The presence of

phase transitions in TIGaSe2 at 246 K and 101 K is

648 INDIAN J PURE & APPL PHYS VOL 39, OCTOBER 2001

also comformed by means of the heat capacity meas­

urement I4.15. It has been shown from transition meas­

urement that TIInS2 has incommensurate phase at 220

K. It has been suggested that TIGaSe2 transforms to a

commensurate phase at 120 K.

The behaviour of several dopants such as Pr in

TIGaSe2 and other ill-ill··VI compounds is interesting

because of their Pr effect on crystal anisotropy.

2 Experimental Details

TIGaSe2, Tlo.999GaPro.ooISe2 and Tlo.995GaPro.oo5Se2 single crystals were grown by the Stocbarger method. TIGaSe2 compound has a melting point l6 of 820±5 DC. Sealed quartz ampoule was annealed at 1050 DC for 10 hr in the outgassing furnace. The temperature of quartz ampoule was decreased to room temperature in 9 hr. The mixture of stoichiometric Se-TI-Ga-Pr-Se was put into quartz ampoule which subsequently was sealed under a vacuum of 10-6 mbar. A quartz crucible (13 mm in diameter and about 250 mm in length) with carbon coating was used. The crucible was suspended in the middle of a vertical two zone furnace. The tem­perature of furnace was increased to 200 DC and waited for 4 hr and then heated to 320 DC again and waited for 6 hr. The temperature of furnace was increased heated to 920 DC and waited for 34 hr. The temperature of low zone of furnace was lowered to 560°C at a rate of 5 ° C/hr. Both of the furnace zones cooled to 350°C in 68 hr. The solidified ingot was cooled to room tempera­ture in 52 hr. The prepared TIGaSe2, Tlo999GaPro.oo lSe2 and Tlo.995GaPro.oosSe2 single crystal ingots were 12 mm in diameter and about 60 mm in length. Ingots did not have cracks and voids on the surface. Absorption spec­tra were measured on freshly cleaved (00 I) surfaces. The TIGaSe2, Tlo 999GaPrO001 Se2 and Tlo.995GaProoo5Se2 samples used in this study were prepared in 2.0x 1.50, 1.8x2. 1 and 1.75x 1.95 mm2 in size respectively.

The absorption coefficients were obtained from

the transmission data using the relationshipl1

T = (I-R)2exp( -A)=(I-R)2exp( -ad) . .. (I)

where R is the reflectivity , A the absorbance, a. the optical absorption coefficient (cm-I) and d is the

sample thickness . The optical absorption coefficients

determined for all temperatures using the values of R at room temperature by assuming that the temperature

changes from 10 to 320 K produces a small change in

R. The multiple reflection and the interference fringes

the samples were put with a small angle with respect

to the incident beam.

The fundamental absorption edge in most semi­

conductors follows the exponential law. Above the

exponential tail , the absorption coefficient of semi­

conductor has been observed to obey the equation:

ali w=B(liw-Eg )" .. . (2)

where aliw is the absorption coefficient of an

angular frequency of w = 2nv , B is a constant and n

is an index which can be assumed to have values of

112, 3/2, 2 and 3, depending on the nature of elec­

tronic transition responsible for the absorption. n=1/2

for the direct allowed transition (high energy part of

the spectra), n=3/2 for forbidden direc t transition ,

n=2 for the indirect allowed transition (low energy

part of the spectra) and n=3 for forb idden indirect transition 14.

An exponentially increasing absorption edge in

a number of insiulators including ionic crystals ,

semiconductors, and organic crystals follows the empirical expression 15:

[CJ(hV - E )]

a =a exp (I

" kT ... (3)

where a.., and Eo are the characteristic parame­

ters of the materia l, cr is the steepness parameter, k

the Boltzmann constant and T is the temperature.

The steepness parameter CT, characterizes the

steepness of the straight line near the absorption edge

is expressed empirically as a function of tempera­

ture l6:

(j = CJ" -- tanh --' (

2kT J (l1W

) )

llw" 2kT ... (4)

where <Y" is temperature-independent but mate­

rial dependent parameter. Some researcher have

stated that nw" corresponded to the energy of phonons

GORBULAK: OPTICAL ABSORPTION 649

associated with Urbach tail. The absorption coeffi­

cients obtained for a I s-exciton considering only the

quadratic term of the exciton-phonon interaction

operator are very similiar to those expressed by Eq. (3), and the parameter (J/kT for the interaction be­

tween exciton and longitidinal-optical (LO) phonons

coincides with Eq. (4) with a constant factor l6.

The following empirical expression is often

used to describe the temperature dependence of the energy gapl?:

£ x(T) = £ x(O)-o T:2

f3 ... (5)

where Eu(T) is the energy gap at the sample <:>

temperature, Eg(O) is the energy gap at 0 K and 8 and

f3 are constants depending on the material. The con­

stant (f3) is approximately equal to the Debye tem­

perature e D'

The optical measurements as a function of tem­

perature were made in a c losed-cycle He cryostat. For

optical measurements the Perkin Elmer UV/vS

Lambda 2S Spectrometer was used which works in

the wavelength range of 190-1100 nm. Wavelength

resolution of the spectrometer is approximately better

than ± 0.3 nm. Therefore Eg can be calculated with

an accuracy of approximately better than ± 0.003 eV

considering the wavelength accuracy of our spec­

trometer.

3 Results and Discussion

This paper presents the results of the absorption

experiments and Urbach tai ls In TIGaSez,

Tlo999GaPro.oo lSe2 and Tlo995GaProoo5Sez crystals over the temperature range of 10K to room temperature.

The absorption spectra were measured on freshly cleaved surfaces with thickness 140, 142

and 140 J..lm. The freshly cleaved crystals had a mir­ror-like surface and there was no need of mechanical

treatment. TIGaSez, Tlo999GaProoo lSe2 and Tlo.995 GaPrO.005SeZ samples were found to be of p-type that

by using hot probe techniques. The absorption coeffi­cient spectra have been obtained from the experimen­

tal absorbance values at different sample tempera­

tures using Eq. I.

The typical absorption spectra of TIGaSe2,

Tlo 999GaPrO oO ISez and Tlo 995GaProoo5Se2 samples versus temperatures at 10K and 300 K are shown in

Fig. I. The direct absorption edges of the investigated

compounds shifted considerably when the tempera­

ture was changed from 10 to 320 K. As seen in

Fig. I, the absorption spectra of Tlo.999GaPro.ooISe2

and Tlo995GaProoo5Se2 single crystals, defect and

exciton level s appear at low temperatures (10-120 K).

When added praseodymium to TIGaSe2, these levels

appear. These levels for Tlo.999GaPro oo l Se2 and

Tlo 995GaProoo5Se2 were found to be 2.166 ± 0.003 and 2.181 ± 0.003 eV at 10K respectively. This result

agrees with investigation by Abdullaeva et ai. 19 and Ibragimov and Mamedov20 who have shown the exci­

ton levels at 2.160, 2.181 and 2.154 eV (10 K). On

the other hand , the phonon energies calculated in

TIGaSe2, Tlo 999GaPrO OOlSe2 and Tlo.995GaPro oo<;Se2 are 60.0 ± 5,55.0 ± 5 and 130 ± 5 meV , respectively. Pr

doping in the TIGaSe2 changes the absorption coeffi­

cient peak in the lower energy region. This effect

causes a change in the phonon energy.

The energy gaps of the indirect and direct opti­

cal absorptions for TIGaSe2 are obtained from the

upper and lower parts of Fig. I using the dependen­cies (a l12 versus Jiw) and (a2 versus Jiw) by extrapo-

275

:::--- 245 1

S ~215 ...... ......

185 (J)

0 U

~ 155 0 ...... +' 0..125 H 0 rJJ

..a 95 <t:

TIGaSez 'A 0 co::x:o 10 K • 6 .... 300 K -:,.A J TIo.vevGaPro.OOISez o:::r::ITI 10 K :,. ..... 300K ~ TIo.lIIIsGaPrO.OO5Sez •• 0 6 ~ 10K ofo ~300 K •• .Ae .Ae ..... 0 e

0

0

0

2.15 2.21 2.27 Photon Energy (eV)

Fig. I - Typical absorption spectra obtained at 10 and 300 K for TIGaSc2. Tlo'J'J'JGaPro.!KIISez and Tlo.'J'J,;GaProIKI,;Se2 sampl es

650 INDIAN J PURE & APPL PHYS VOL :19, OCTOBER 2001

lation of the straight lines down to a l/2 = 0 and a 2 = 0 (Eq. 2), respectively. The direct band gaps of

TIGaSe2 are 2.269, 2.244, 2.204 and 2.156 e V at 10,

100, 200 and 300 K, respectively. The indirect band

gaps of TIGaSe2 are 2.204, 2.175, 2 .124 and 2.075

eV at 10, 100, 200 and 300 K respectively. These

result are in agreement with our previous investiga­

tion of Dy doped and undoped21 p-type TIGaSe2.

These results are in agreement with literature. The

temperature dependence of the experimental and

theoretical (obtained from Eq . 5) indirect and direct

band gap have been shown in Fig . 2. This curve

represents the values of EgC7) found by Eg. (5). In addition, the temperature dependence of the indirect

and direct band gap for TIGaSe2 is presented in

Fig. 2.

The Urbach tail is observed for all samples be­

tween 10-320 K. Typical Urbach tails for various

temperature are shown in Figs. 3(a), (b) and (c). It is

found that all extrapolations of the Urbach tails con­

verge at (Eo, ao) = 2.343 eV, 4014.63 cm'l; 2.234 eV.

1327.01 cm'l and 2.178 eV, 164.64 cm' l as shown in

Figs . 3(a), (b) and (c) respectively. These results

show that absorption spectra of all samples obey

Urbach rule. All of the points intersect the same (Eo,

ao) point at the tempe rature range 10-320 K (see

Figs. 3(a,b,c».

2 .27

.--....2.24 ? Q)

'-" 2 .21

>. tUl .... ~ 2 .18

r,il

p 2.15 o

-+-l

.2 2 .12 0..

2 .09

2.06

TIGaSe2 00000 E,d 00000 E,I -- Tneory

o 50 100 150 200 250 300 Temperature (K)

Fig. 2 - The temperature dependence of the direct and indirect band gap for TIGaSc2 sample

9.00.-----------­(Eo=2.3437 eV; ao=4014 .6:; em-I )

8.20

7.40

ts E 6 .60

5.80

5.00

TIGaSe2 r:x:xx:xJ 10K 00000 80 K ~160K 00000240 K ***** 320 K

-..._ (a)

-fII -.-~"'TTT 4.20 2 .11 2 .17 2.23 2 .29 2.35

5.95

5.70

5.45

~ 5.20 .s

4.95

4.70

4.45

5.10

4.95

~ 4.80 ~

~ 4.65

4.50

4.35

4.20

Pholon Energy (eV)

(Eo=2.234 eV; ag=327.01 em")

Tlo .... GaPro.oo,Se2 = 10K DOOOO BO K ~160K 00000240 K ***** 320 K

(Eo=2.178 eV; ao=164.64 em")

Tlo .... GaPro.oo.Se2 = 10K 00000 BO K ~IBOK 00000240 K ***** 320 K

(e)

6

1.98 2.02 2.06 2.10 2.14 2.18 Photon energy (eV)

Fig. 3 - Typical absorption spectra obtained at 10. RO. 160. 240 and 320 K for (a) TIGaSe2 (b) Tlo'I')<)GaPro'(X)I Se2 (e)

Tln.9');GaProeX)5Se2 samples

GURBULAK: OPTICAL ABSORPTION 651

Fig. 4 shows the Urbach energy kT/a which

corresponds to the inverse of a slope of the Urbach

tail of the Pr-rich sample. (x = 0.001 and x = 0.005)

as a function of temperature. Urbach energy de­

creases with increasing x ratio. Figs . 5 and 6 show

steepness parameters (a ) and ao values of the Pr-rich

sample. (x = 0.001 and x = 0 .005) as a funct ion of

temperature, respectively. Steepness parameter (a )

----~ 84.0 S

'--'72.0 b

"'" ~ 60 .0

>. ~48 . 0 Q)

~ 36.0 ~

~ 24.0 .0 s... ~ 12.0

0.0

TIGaSez ••••• TIo .• GaPro.OOlSea

••••• Tlo._GaPrO.OO6SeZ ••••• • •

•

• • •

••• ....... • • •••

• • • • • • ••• •••• •• • •• •• •••• • ••• • ••• • • • • •• • • • •• • •• • • •• •

• • •• •• •• • o 50 100 150 200 250 300

Tell1perature (K)

Fig. 4 - The Urbach energy as a function of the temperature for TIGaSez. Tlu.'J'I')GaPru.(MI ISeZ and Tlu.'N5GaPru.lM'5Se2

0 .28

:§: 0 .24

'"' .3 0.20 w E f 0.16 l1l

0.. rn 0.12 rn w 1:1 0. 0 .08 OJ Q)

UJ 0 .04

D.OO

TIGaSez • ••••• • Tlu.GaPr .... ,Se. • ••••• • • TIo...GaPro.OOOSez _ -~ •• ..... • ........... :;--4 .. ...

•

• • • • •

•• •

• • • .. ...

... ...

• • •

... ...

... ...

50 100 150 200 250 300 Temperature (K)

Fig. 5 - The tClllpe rature ve rsus the S tcep ness paraillet ers (a)

fur TIG aScz. TI".,,,.,Ga Pr,, .. M11 Scz amJ Tlu.'MGaPro.""Se~

and ao values decrease with increasing x ratio . There

is abrupt change in the Urbach energy peak for

TI 0999GaPrO.OO ISe2 at 100 and 200 K. and

Tlo.995GaPro.oo5Se2 at 200 and 260 K. However, there

are an abrupt change IJ1 the ao values for

Tlo.999GaPrO.OOISe2 and Tlo.995GaPro.oo5Se2 in the tem­

perature ranges 140-180 K and 220-260 K. The va l­

ues obtained from the Urbach energy and a o values

change may be phase trans iti on temperatures . The

maximum values obtained from heat-capacit/ 2

measurements are 340, 253 , 117.2, 108.9, 106 and

101 K. The phase tran si tion II found in at 250 K was

blurred, whereas in thi s study a sharp heat-capac ity

peak has been obtained for a TIGaSe2 si ngle crystal.

The values obtained from the energy peak change

may be phase transition temperature. The Urbach

energy of TIGaSe2 is increased by PI' (Fig. 4). The

Urbach energy increases with increasi ng temperature .

Fig. 7(a and b) and Fig. 8(a and b) show some

typical absorption spectra containing the sample

temperatures 10, 100,200 and 300 K. Fig. 7(b) and

Fig. 8(b) obtai ned from the difference between ab­

sorpt ion constants in TIGaSe2-Tlo.999GaPro.oo l Se2 and

TIGaSe2-Tlo.995GaPro.oo5Se2 at the same temperature. Fig. 7(b) and Fig. 8(b) , the first defect levels (11 = I)

have been found as 2.259, 2.235 , 2.200 and 2. 149 eV

for Tlo.999GaPrOOOI Se2 and 2.254, 2.225, 2.189 and

1.15 -. • • 1.00 - •

• •

TlGaSe. ..... Tlo._GaPr •. OO!Se. _ .. Tlo._GaPro.DOlISe • ...............

0 .B5 - • •

0.55 -

0.40 -

• • • • . ... ...... .. ... .. ",'" .................. " ...• ~ ... Ii· ••• ""11

0 .25 .... ...-- ... --~"''''''' ......... . ...........

0 .10 I t I I o 50 100 150 200 250 300

T (K)

ri g. 6 - The au va lllcs ;IS a runcl ion or the ICll1pCra turc I'm

TIGaSc2' TI" "."GaPrll IHIi SeZ and TI"'1J5GaPr".I",Sc: sample,

652 IND IAN J PURE & APPL PHYS VOL 39. OCTO BER 200 1

305J~~~~~~-------------160 n....caPrO'OIISe.

275

I

S 245 o

.2 15 ...... ...... Q.)

8 185

§ 155 :;::; p.. H 125 o UJ

~ 95

~120

! * ,*

,..* 9 0

Ii' :j BO e

/: I~ , , I

,..* /6 ,DO g / * I

::J 40 " * I/)'I i :' *,' I .tl " I b., -< 0 " * I

2.00 2. U 2 .28 :' ;1 Photon energy (eV) "

TIGaSe. / = 10K " =100 K :' ~200 K " ***** 300 K ,-Tlo .... GaPrO.OO1Sea , i' -- 10 K ,". ,,"

- - 100 K .' ,

:-,~ ,~ ~gg ~ ./ /// .,' "," /

/

" "

o 0

o o

o

o 0

o o

o o

".,-:>' / / (a ) 65~'~--~--~-~"~~~~~~~~

2.01 2. 05 2 .09 2. 13 2. 17 2.2 1 2.25 2.29 Phot on En e r gy (eV)

Fig. 7 - T he optical absorption spectra ve rsus photon energy (a) TIGaScTTI II .!I')~GaPrll '( KIISc2 samples (h) Pr spec truill ror

x=O.OO I

2. 149 e V for Tl o.99sGaPro .oosS e~ at the 10, 100, 200 and 300 K. The second defect levels (n = 2) have been foun d as 2. 159 and 2. 157 eV for Tlo.99I)GaPro.oOtS e2 at the 10, 100 K and 2.180, 2. 157 and 2.178 e V for Tlo.995GaProoosSe2 at the 10, 100 and 200 K. The defect levels whi ch ex ist in TIGaSe2:Pr but not ex ist in pure TIGaSe2 can be observed with adding PI' in the range 10-320 K.

The temperature coeffi cients (8) were ca lcu­lated for direct and indirect band gaps as 5.1 43x I 0-4

eV/K and 5.40x I 0-4 eV/K fo r TIGaSe2 fro m a sati s­fac tory fittin g of the experimental curve using the above Eq.(5) and these results agree with results of Guse inov ef al . 23. However, for TI GaSe2 sample, ~ were calcul ated at 125 K and 105 K respecti ve ly.

TIGaSez and Tlo.999GaPrOOo l Se2' ,

The direct and indirect energy gaps for TI GaSe2 decrease towa rDS lower values as the increasing tem­perature and the temperature coefficient has a nega­tive sign (Fig. 2). As seen from Fig. I, the indirect band gap of TIGaSe2 and Tlo.999GaPro.00 1Se2 is smaller than that of TIGaSe2. That is, the region that was calcul ated the indi rect band gap of TIGaSe2 single

305.------------------------------,

275

I S 245 o

...... ...... Q.)

215

8 185

§ 155 . ...., ,.J p.. H 125 o rJJ

~ 95

120 -r-----------,

~100

's .2.. 80

* * * " " "

o 0

o o ... * ' 6 860 *:' P.

u *: 0 I • .l 6' ~ 40 • / I ::I 0 . 0 ,+ ~ I ~ 9. .. DO

~ 20 '6. 0 "ott I~ ~ 0

~ 0 0 A .: * " I

1.97 2.07 2.17 2.27 ",1 6. I 0 TlGaSe, Photon energy (eV) . .' 6. /

~~gg ~ 'ff' ; ,,/ ,,/ : 0 ***** 300 K /" ... -;. ~-".. l:l Tlo .... GaPro.aor;Sct ,' ... I * 6. 0 -- 10 K ,', I * " 0

- - 100 K ",,' I : " 0

:-_=,~ ~gg ~ ./ ,'I t f 0 .,' ... '" I

o

• , I

(a) 65 ,

2.01 2. 05 2. 09 2. 13 2. 17 2.2 1 2.25 2 .29 Photon Energy (eV)

Fig. 8 - T he o pt ica l absorpti on spcctra versus pho ton encrgy (a) TIGaSerTIII.'n,GaPro.INI.ISc1 samplcs (b) Pr spec trum I'm

x=().005

crystal is sub-region of absorpti on spectrum. Thi s sub-region may be called the region that defects (ex­ci ton defects) exist commonly as well. In these sam­ples, praseodymium creates the defects in thi s region and ideal absorption spectrum will change in to the non-idea l. The reason of thi s change may be frolll the increase of Pr rati o.

The other important results are th at 1- the defect levels and defect absorpti on peaks which results from PI' has been observed (Fig. 7(b) ,and 8(b). 2- the ab­sorpti on peak of PI' has been came to li ght (Fig. 7b and 8b). The defect levels whic l~ ex ist in TIGaSeo but not ex ist in ' pure TIGaSe2 can be observed with add­ing PI' in the range 10-320 K. The rare earth (RE) elements cause the new appearance and changin g of the absorption peak intensity, Tlo')99GaPrOO(llSeZ and Tlo99<)GaPr000 1Se2. As seen in Fi g. l , the reason of thi s increase in the slopes is due to di sappearing impuri­ti es ex isting in TIGaSe2 crystal by adding PI'. Thi s result shows that there are many impurit ies in TIGaSe2 single crystal and these impuri ties in TIGaSe2 are created by additing PI'. These are ex­pected results because of the behaviour of the PI' atoms in the TIGaSe2.

GURBULAK: OPTICAL ABSORPTION 653

Acknowledgement

This work was supported by the Atatiirk Uni­

versty Research Fund, Nos . 2000/33 and 2000/34.

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