International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 5, May 2013 ISSN 2319 - 4847
4
4
0.01
4
0.03
1
2
3
1,2,3
Department of Physics, College of Science,
Al-Mustansiriyah University, Baghdad, Iraq
BSTRACT
This includes the preparation of glass substrates, followed by the preparation of GeSe4 alloy then GeSe4 doping with Te in different compositions using vacuum evaporation method after that measure the optical properties before and after doping with
Te with annealing isothermal.
To measure the optical properties used visible spectro-photometer device type (uv-880 spectrometry) to measure (absorbance and transmittance spectrum) in the range (300-900)nm for GeSe
4
, GeSe
4
Te
0.01 and GeSe
4
Te
0.03 thin films.
In semiconductors fabrication many electronic devices depended on the energy gap, such as solar cells and detectors and optical diodes.
The term of "thin film" is used to describe a layer or several layers of atoms for certain substance whose thickness ranges between (10nm) and less than or equal (1μm).The physical characteristics of the thin films are different from those of their characteristic materials in their bulk [1].
Among the amorphous chalcogenide alloys, mostly, selenium (se) based materials are preferred because it is unique property of reversible phase transformation between the amorphous and crystalline phases. The choise of (se) based alloys is due to the device application like rectifier, photocells and other application [2]. But pure selenium has disadvantages such as it is low photo-sensitivity and also selenium is not stable in standard operational conditions because its glass transition is close to room temperature. These problems can be overcome by alloying selenium with tellurium, which gives higher photo-sensitivity and smaller aging effects as compared to pure selenium [3]. However, these alloys are found to have some significant problems. The two serious problems are the limited reversibility and low glass transition. These problems can be removed by adding a third element as a chemical modifier in se-Te alloys [4].
1Germanium (Ge) : is a chemical element of forth group in periodic table, with atomic number 32 ,atomic weight
72.59, density 5.32gm/cm
3
, melting point 1220k o
, Boiling point 3103 k o
, and energy gap 0.744ev discovered by
C.Winkler in1886[5].
2Selenium (Se): is a chemical element of sixth group in periodic table, the chalcogenide glasses (sulfur,selenium, tellurium)with atomic number 34 ,atomic weight 78.96, density 4.81gm/cm
3
, melting point 490k o
, Boiling point 958 k o
, and energy gap is (1.89 and 2.2)ev depended on alloy composition and amount of impurities additive to selenium discovered by J.Berzelins in 1817[6].
3 Tellurium (Te): is a chemical element of sixth group in periodic table, the chalcogenide glasses (sulfur, selenium, tellurium)with atomic number 52 ,atomic weight 127.6, density 6.25gm/cm
3
, melting point 723k o
, Boiling point 1663 k o
. crystalline and amorphous tellurium has aband gap between (0.5-0.8)ev while in other studies it is found that crystalline tellurium (C-Te) has aband gap between (0.33-0.37)ev, discovered by MVon Reichenstein in 1782[7].
Tellurium thin films are also used as selective surfaces and selective surfaces are surfaces which have a high absorption in the visible to near infrared region and a low emissivity in the infrared region[8].
Previous studies : many researches studied these materials over years:
In 2003 Salman F.studied the A.C conductivity and dielectric properties of glassy alloys Se x
Te
79-x
Ge
21
with
(x=11,14,and 17) at temperature range (300-450)k o
and over a wide range of frequencies(50-500)Hz. Experiment indicated that AC conductivity and the dielectric constant depend on temperature frequency and Se content[9]. a.
In 2004 kumar D. et al studied the temperature dependence of photoconductivity in amorphous thin films of Se
80x
Te
20
Ge x
(x=5,10,15,and 20) prepared by thermal vacuum evaporation technique and indicated that photosensitivity increases and the density of defect state decreases with increasing of germanium concentration[10].

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 5, May 2013 ISSN 2319 - 4847 b.
In 2007 Maan A.S. et al studied the optical absorption of Te
5
(In x
Se
100-x
)
95
glassy alloy (x=10,15,20 and 25) prepared by thermal vacuum evaporation technique. These measurements are made on the as-prepared and annealed samples; it is observed that annealing affects the optical properties and causes a reduction in the energy range of tail states and an increase in the optical energy gap in all the alloys. The optical band gap is found to decrease up to x=20 and there after it increases. This behaviour has been explained to be increasing in content and average bond energy of the system[11]. c.
In 2009 Mainika .et al studied the effect of impurity addition of Ag, Bi and Ge on the optical properties of vacuum evaporation thin films of Se
80
Te
20
,(Se
80
Te
20
)
98
M
2
where (M= Ag, Bi and Ge)have been studied using transmission spectra at normal incidence and they found that optical energy gap decreases while refractive index increases on the incorporation of (Bi). On the other hand optical energy gap increase while refractive index and extinction coefficient decreases on incorporation of Ag and Ge in Se-Te system[2]. d.
In 2008 Mean A.S. et al studied the dielectric properties of In
40
Se x
Te
60-x glassy alloys (x=10,20, and 30)these measurements are carried out as a function of temperature and frequency in the range (120-350) k o
and (0.1-10)KHz respectively. The experiments result indicates that dielectric constant
ε
1 and dielectric loss
ε
2 are almost similar in nature in all the alloys. At a particular temperature and frequency, values of
ε
1 and
ε
2
are maximum at x=20 and decrease as x is changed to 10 and 30[12].
The most interesting optical processes in thin films are the absorption and emission of light. Absorption permits a determination of the optical band gap of the semiconductor and it generates charge carriers [13].
The optical band gap, refractive index and extinction coefficient are the most significant parameters in amorphous semi conducting thin films. The optical behavior of material is utilized to determine its optical constants. Films are ideal specimen for reflectance and transmittance type measurements.
Therefore, an accurate measurement of the optical constant is extremely important [14].
The optical properties: a.
Reflectance(R): the reflectance which can be defined as the ratio of the reflected intensity to the incident intensity[15]:
R=

 refl
 refl

 inc
 inc

I
R
I
O
…………… .(1) where I
R
: is the intensity of reflected light at the interface where the refractive indices are different. and I o
: is the intensity of incident light at the interface where the refractive indices are different[16]. b.
Transmission (T): the transmission coefficient is defined as the ratio of transmitted power to incident power,
I
T
T=
I
O
…………….(2) and given by [17]:
( 1  R )
2
T=
1  R
2 exp(   t exp(  2  t )
)
……………….(3) where α : absorption coefficient, t : sample thickness.
c.
Absorbance: the absorption is defined as (Beer- Lambert low ):
A= -log(
I
T
I o
) ………………..(4) where I
T
: is the intensity of the transmitted light through the material and I o
: is the incident intensity of light[16].
Optical constant: a.
Refractive index (n) and Extinction coefficient (k): the refractive index can be calculated by using equation depending on the reflectance (R) and extinction coefficient (k) as in the following equation [16] :
R = n  1
2 n  1

( n  1 )
( n  1 )
2
2
 k
 k
2
2
…………..(5) and the extinction coefficient (k) can be calculated from the relation :

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 5, May 2013 ISSN 2319 - 4847 k=

4 
………… (6) b.
Absorption coefficient (α): A bsorption coefficient [α(hν)]can be defined as the relative rate of decrease in light intensity [L(hν)] along it is propagation path[18]:
 
1
L ( h  )
 d [ L ( h  )]
…….(7) dx and given by [19]:
 
2 .
303 * A
…….(8) t
But
1
A=log (
T
) ……(9) and t : thickness of thin film. c.
Real and Imaginary part of dielectric constant ε
1 and ε
2
:
The dielectric constant represents the ability of material to polarization, to obtain the dielectric constant[20]:
ε = ε
1
+ iε
2
……………(10)
where
ε
1 and
ε
2
: are the real and imaginary parts of the dielectric function respectively.
ε
1
= n2-k2 ………..(11) ,
ε
2
=2nk ………(12) n : refractive index , k : extinction coefficient.
This includes the preparation of glass substrates, followed by the preparation of GeSe4 alloy then GeSe4 doping with
Te in different compositions using vacuum evaporation method after that measure the optical properties before and after doping with Te with annealing isothermal.
To measure the optical properties used visible spectro-photometer device type (uv-880 spectrometry) to measure
(absorbance and transmittance spectrum) in the range (300-900)nm for GeSe
4
, GeSe
4
Te
0.01 and GeSe
4
Te
0.03 thin films.
In semiconductors fabrication many electronic devices depended on the energy gap, such as solar cells and detectors and optical diodes. From equation (8) the absorption coefficient for each wavelength is obtained and the incident photo energy E(hν) as a function of the wavelength (λ) from the relation:
1240
E(ev)=
 ( nm )
…………(13) and the optical constants calculated from transmittance and absorbance spectrum at normal incidence over the range
(300-900)nm for GeSe4 , GeSe4 Te0.01 and GeSe4 Te0.03 thin films. Refractive index (n) is calculated using equation
: n =
1 
1 
R
………(14)
R and extinction coefficient (k) from equation (6) and the dielectric constant can be calculated in its real and imaginary parts
( ε
1 and
ε
2
) from equation (11,12)respectively.
In studying the optical properties of GeSe
4
, GeSe
4
Te
0.01 and GeSe
4
Te
0.03 thin films. This part includes the results of transmission and absorption measurements and their relation to wavelength and from these measurements it can be calculated many optical constants, like absorption coefficient, refractive index, extinction coefficient, dielectric constant.
1Transmission measurements: the influence of tellurium concentration on GeSe
4
thin films on the optical properties of the prepared films with annealing and without annealing the transmission spectra as a function of wavelength in the range (400-900)nm, is shown in figure (1) and from this figure the transmission increase with increasing wavelength and have maximum transmission at(800nm)because cholcogenide glasses and especially Se-Te alloys are generally transparent from the visible up to infrared[21]. Fig(1) also shows a decrease in transmission with increasing (Te) concentration because (Te) thin films are used as selective surfaces and selective surfaces are surfaces which have a high absorption in the visible to near infrared region and a low emissivity in the infrared region and this means that the transmission decrease and absorption increase with increasing (Te) concentration[7]. And show the transmission

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013 ISSN 2319 - 4847 without (Te) increasing with increased in time annealing and stable after (550nm) and decrease with increased time annealing but with (Te) the transmission decrease with increased in (Te) concentration and time annealing.
GeSe GeSeTe0.03
GeSeTe0.01
40
36
120
60 100
32
28 80 20Sec
50
24
20Sec 30Sec
20Sec
60
20 40
16
30Sec
40Sec
30Sec
40Sec
40
40Sec
30
12
20
8
20
4
10
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000
ג wav e
0
400 500 600 700 wav e
800 900 1000
Fig.(1)
2Absorbance measurements: it is possible to separate three distinct regions in the absorption edge spectrum of amorphous semiconductors. The first is the weak absorption tail, which originates from defects and impurities.
(Existence of the weak absorption tails in the band gaps of the films under study may be attributed to the amorphous nature, and randomly distributed impurities in the films) second is the exponential edge region, which is strongly retated to the structural randomness of the amorphous compound. Third is the high absorption region from which optical energy gap width can be determined from figure (2) illustrates absorbance spectrum for GeSe
4
, GeSe
4
Te
0.01 and
GeSe
4
Te
0.03 thin films as a function of wavelength, the prepared films show high absorption at visible spectrum and low absorption at the near infrared region and absorption increase with increasing (Te) concentration and time annealing for all thin films[22,23].
GeSe GeSeTe0.03
GeSeTe0.01
4.3
3.9
3.6
3.3
3
2.7
2.4
2.1
1.8
1.5
400 500 600 700 800 900 1000
20Sec
30Sec
40Sec
4
3.7
3.4
3.1
2.8
2.5
400 500 600 700 wav e
800 900 1000
20Sec
30Sec
40Sec
4.2
3.9
3.6
3.3
3
2.7
2.4
2.1
1.8
1.5
400 500 600 700 800 900 1000
20Sec
30Sec
40Sec wav e wave
Fig.(2)
3Reflectance measurements: from figure(3) illustrates reflectance spectrum for GeSe4 , GeSe4 Te0.01 and GeSe4
Te0.03 thin films as a function of wavelength. The prepared films show high reflection at visible spectrum and low reflection at the near infrared region and reflection increase with increasing (Te) concentration and time annealing for all thin films[23].
GeSeTe0.03
GeSeTe0.01
GeSe
1 1 1
0.8
0.95
0.95
20Sec 20Sec
0.9
30Sec
40Sec
0.9
30Sec
40Sec
0.6
0.4
20Sec
30Sec
40Sec
0.85
0.85
0.2
0.8
400 500 600 700 800 900 1000
0.8
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000 wave wave wave
Fig.(3)
4Absorption coefficient measurements: Analysis of optical absorption spectra is one of the most productive tools understanding and developing the band structure and energy gap of both crystalline and amorphous non-metallic material, the threshold at the low energy side of optical absorption spectra is called optical absorption edge and corresponds to separation in energy between the bottom of the conduction band and the top of the valance band.
Absorption coefficient is calculated by using equation (8), figure(4) show the variation of(α) with wavelength, from the figure the absorption coefficient(α) increases at visible spectrum and low absorption coefficient at the near infrared region and absorption coefficient increase with increasing (Te) concentration and time annealing for all thin films.

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 5, May 2013 ISSN 2319 - 4847
GeSeTe0.01
GeSe
GeSeTe0.03
7E+11
7E+11
6E+11
20Sec
30Sec
40Sec
7E+11
6E+11
5E+11
6E+11
5E+11
5E+11
4E+11
3E+11
2E+11
1E+11
4E+11
3E+11
2E+11
1E+11
0
400 500 600 700 800 900 1000
20Sec
30Sec
40Sec
4E+11
3E+11
2E+11
1E+11
20Sec
30Sec
40Sec
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000 wave
WAVE w ave
Fig.(4)
5Optical constants :
 Extinction coefficient (k):
Extinction coefficient is measured by using equation (6), figure(5) illustrates variation of(k) as a function of wavelength, extinction coefficient increase with increasing wavelength in visible and near infrared region which is increased with increasing the tail states width that resulted the increasing tellurium concentration and time annealing[23].
Ge SeTe0.01
GeSe GeSeTe0.03
3E+14
2.5E+14
2.5E+14
3.5E+14
3E+14
2.5E+14
2E+14
3.5E+14
2E+14
1.5E+14
1E+14
5E+13
20Sec
30Sec
40Sec
2E+14
1.5E+14
1E+14
5E+13
20Sec
30Sec
40Sec
1.5E+14
1E+14
5E+13
20Sec
30Sec
40Sec
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000 w ave w ave wave
Fig.(5)
 Refractive index (n):
Refractive index can be determined using equation (14), from figure(6) it is clear that refractive index increase with increasing wavelength (600nm) and decrease in higher wavelengths, and (n)increases with increasing tellurium concentration and time annealing , this may be due to the change in crystallite size, stoichiometry and internal strain and because the decreasing of optical band gap with increasing Te concentration[22,23] .
GeSe
GeSe4Te
GeSeTe0.03
40
30
20
10
0
400
90
80
70
60
50
500 600 700 800 900 1000
20Sec
30Sec
40Sec
80
70
60
50
40
30
20
10
0
400 500 600 700 800 900 1000
20Sec
30Sec
40Sec
80
70
60
50
40
30
20
10
0
400 500 600 700 800 900 1000
20Sec
30Sec
40Sec w ave w ave wave
Fig.(6)
 Dielectric constant:
The complex dielectric constant is fundamental intrinsic material property. The real part of it is associated with the term that how much it will slow down the speed of light in the material and imaginary part gives that how a dielectric absorb energy from electric field due to dipole motion. The real and imaginary parts of the dielectric constant were determined using the relation (11,12) respectively, so that values of (ε
1
,ε
2
) depend on refractive index (n) and extinction coefficient(k)[22], from figure (7,8) show the variation of real part of dielectric constant ε
1
and imaginary part of dielectric constant ε
2 with wavelength respectively, from these figures it is clear that (ε
1
,ε
2
) increase with increasing wavelength (400-700)nm with increasing in Te concentration and increa sing time annealing for (ε
1
) but in (ε
2
) increasing reach to (400-900)nm wavelength, that is similar behavior of the previous experiments[21,22].
GeSeTe0.03
GeSe GeSeTe0.01
6000
6000 7000
6000
5000
4000
3000
2000
20Sec
30Sec
40Sec
5000
4000
3000
2000
20Sec
30Sec
40Sec
5000
4000
3000
2000
1000
20Sec
30Sec
40Sec
1000
1000
0
400 500 600 800 900 1000
0
400 500 600 800 900 1000
0
400 500 600 800 900 1000
700 w ave
700 wave
700 w ave
Fig.(7)

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 5, May 2013 ISSN 2319 - 4847
Ge Se Te 0.03
GeSeTe0.01
GeSe
5E+16
4E+16
5E+16
3E+16
4E+16
4E+16
3E+16
3E+16
4E+16
20Sec
30Sec
40Sec
20Sec
30Sec
40Sec
2E+16
3E+16
3E+16
2E+16
3E+16
2E+16
2E+16
2E+16
20Sec
30Sec
40Sec
1E+16
2E+16
1E+16
1E+16
5E+15
5E+15
5E+15
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000
0
400 500 600 700 800 900 1000 w ave wave w ave
Fig.(8)
In studying the optical properties of GeSe
4
, GeSe
4
Te
0.01 and GeSe
4
Te
0.03 thin films. This part includes the results of transmission and absorption measurements and their relation to wavelength and from these measurements it can be calculated many optical constants, like absorption coefficient, refractive index, extinction coefficient, dielectric constant.
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