Influence of thermal annealing on optical constants of Ag doped Ga–Se chalcogenide thinfilms

Optics Communications 295 (2013) 21–25

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Discussion

Influence of thermal annealing on optical constants of Ag doped Ga–Se chalcogenide thin films M.A. Alvi n Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

a r t i c l e i n f o

abstract

Article history: Received 14 October 2012 Received in revised form 19 December 2012 Accepted 20 December 2012 Available online 30 January 2013

The effect of thermal annealing on the structural and optical properties of Ag doped Ga–Se chalcogenide thin films has been studied. It is found that the films partially transform from amorphous to crystalline phase. The X-ray diffraction technique and FESEM have been used to study the transformed phases. The effect of thermal annealing on the optical spectrum of these thin films has been studied in the wavelength spanning from 450 to 1100 nm. It is found that the calculated optical band gap (Eg) decreases while the absorption coefficient and extinction coefficient increases with increasing the annealing temperature. The film transparency decreases with increasing annealing temperature. The decrease in the optical band gap has been explained on the basis of change in nature of the films, from amorphous to polycrystalline state, with increasing the annealing temperature. & 2013 Elsevier B.V. All rights reserved.

Keywords: Chalcogenide thin film Laser-irradiation Dc conductivity Optical band gap

1. Introduction During the past two decades, the amorphous chalcogenides have been the subject of fundamental importance not only to our understanding in the field of low phonon-energy materials but also to their applications in many fields such as medicine, chemistry, optics and optoelectronics, material engineering and science etc. Chalcogenide-based phase change memory (PCM) materials are used in re-writable optical memories. They use the difference in the optical band gap and reflectivity between their crystalline and amorphous phases. They could also be used in electronic non-volatile memory devices because of their possible superior scaling possibilities compared to flash memory. The intensive luminescence of doped chalcogenides in near and mid infrared regions, photo-induced changes of index of refraction as well as high linear and non-linear index of refraction make these materials very useful for optical recording/imaging media, optical circuits and optical computing [1–5]. Phase-change materials are successfully employed in optical data storage and are becoming a promising candidate for future electronic storage applications. Today’s information and knowledge-based society creates a large demand for data storage capacity. This demand is met by a wide range of commercially available data storage technologies. These technologies differ in specific properties, such as the storage capacity and density, and the throughput, i.e. the speed at which

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data can be written, read, and erased which is based on a class of materials that shows a significant change in optical and electronic properties upon undergoing the crystalline-to-amorphous phase transition. The two states of the amorphous and crystalline phases are considered the two bit binary storage data and could be detected optically. Amorphous Se has been found to have tremendous potential in xerography applications and in many industrial fields, such as photo elements, solar technology, metal coatings as well as lubricants and pharmaceuticals [3,4]. It also exhibits a unique property of reversible transformation. This property makes its use in optical memory devices. But in pure state it has disadvantages because of its short lifetime and low sensitivity. To overcome these difficulties some other material has frequently been used as an additive to Se. It is well known that Ga has one of the longest liquid ranges of any metal and has a low vapor pressure even at high temperature. There is a strong tendency for Ga to super cool below its freezing point, so, seeding may be necessary to initiate solidification [5]. The Ga doped materials show very interesting properties; gallium compounds have many technological applications. For example, gallium arsenide (GaAs) and gallium nitride (GaN) are used in electronic components and are important to many areas of research in semiconductor physics and the quantum Hall effect [6–9]. They are also used in optoelectronics in a variety of infrared applications. Likewise gallium selenide (GaSe) has great potential as a non-linear optical material and photoconductor [10,11]. Gallium selenide crystals show great promise as a non-linear optical material and photoconductor. The non-linear optics gives

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M.A. Alvi / Optics Communications 295 (2013) 21–25

rise to a host of optical phenomena. Non-linear optical materials are used in the frequency conversion of laser light. Frequency conversion involves the shifting of the wavelength of a monochromatic source of light, usually laser light, to a higher or lower wavelength of light that cannot be produced from a conventional laser source. One original problem with using gallium selenide in optics is that it is easily broken along cleavage lines and thus it can be hard to cut for practical application [12,13]. It has been found, however, that doping the crystals with Al, In, Pb etc. greatly enhances their structural strength and makes their application much more practical [12]. In the present research work, we incorporated silver doping in GaSe. Ag doped chalcogenide glasses have become attractive materials of fundamental research for their structure, properties and preparation. They have many current and potential applications in optics, optoelectronics, chemistry and biology (optical elements, gratings, memories, micro-lenses, waveguides, bio and chemical-sensors, solid electrolytes, batteries, etc.). The Ag doped chalcogenide glasses can be used as optical memory, materials for holography [14] and can also be used as sensitive electrochemical electrodes or their membranes for sensors in potentiometric measurement [15,16]. The thermal phenomenon is proved to be essential in inducing crystallization in chalcogenide glasses [17]. The crystallization of chalcogenide films is certainly accompanied by a change in the optical band gap. The separation of different crystalline phases with thermal annealing has been observed in ternary glasses. Extensive efforts have been performed to understand the effect of thermal annealing on optical properties of chalcogenide thin films. Khan et al. [18,19] have studied the effect of annealing on optical band gap of amorphous GaSeSb and GeTeTe thin films. Soltan et al. [20] have studied thermal annealing dependence of the structural, optical and electrical properties of selenium tellerium films. Tae and Dae [21] have studied the effect of thermal annealing on the morphology and electrical properties of ZnO/In films. Ghosh et al. [22] have studied the thermal annealing treatment effect on structural, electrical and optical properties of transparent sol–gel ZnO thin films, Dongol et al. [23] have studied the effects of composition and heat treatment on the structural and optical properties of GeTeCu thin films. Pal et al. [24] have studied the thermal annealing effect on the optical properties of high-energy Cu-implanted silica glass while Bhatia et al. [25] systematically studied the effect of thermal annealing on the optical and electrical properties of amorphous semiconducting thin films in the system Ge20Te80  xBix prepared by the flash evaporation technique. Their analysis shows that the dependences of the optical and electrical parameters on bismuth concentration and annealing temperature are different in the two different regions of material compositions indicating structural differences in the two sets of compositions. Shaheen et al. [26] have studied the optical properties of Se–In chalcogenide thin films before and after gamma irradiation. Kotb et al. [27] studied the annealing temperature dependence of Se rich CdSe thin films. Al-Agel [28] has studied the effect of annealing temperature on optical and electrical properties of GaSeIn thin film; Chauhan et al. [29] studied the photo-induced optical changes in GeAsSe thin films. Imran et al. [30] have studied the effect of thermal annealing on optical band gap of SeGaIn thin films. The admirable work on phase change and optical band gap behavior of SeS chalcogenide thin film by Rafea and Farid [31], temperature effect on optoelectronic properties of Ag–In–S thin films by Qasrawi [32], photo and thermal induced effects on AsSSb amorphous thin films by Naik et al. [33] are also deserved to be mentioned. The effect of thermal annealing and by high energy sources on optical properties of chalcogenide thin films have also been investigated by numerous authors [34–39].

The aim of the present work is to further study the effect of thermal annealing on the optical band gap of the Ag doped GaSe thin films and to oversee the gradual change in optical constants during crystallization. The accurate knowledge of the optical constants is necessary not only for understanding the basic mechanisms of these effects, but also for exploiting their interesting technological potentials. Transmission electron microscopy (TEM) and X-ray diffraction were used to determine the structural changes of Ag doped GaSe films under different conditions. The effect of thermal annealing is interpreted on the basis of amorphous to crystalline transformation.

2. Experimental Gallium, selenium and silver elements of purities ‘‘5 N’’ were used to prepare Ga15Se77Ag8 glass by the well established meltquench technique. The glass transition temperature and crystallization temperature of the prepared sample was measured by the non-isothermal Differential Scanning Calorimeter ((Model DSC Plus, Reheometric Scientific Company, UK) measurements. Thin films of thickness 400 nm were prepared by thermal evaporation techniques onto ultrasonically cleaned glass/Si wafer substrates, using an Edwards E 306 coating system operated at 10  6 Torr. The substrate temperature was kept at room temperature during the evaporation process. The evaporation rate as well as the thickness of the prepared films was controlled using a quartz crystal monitor (Edward FTM5) with a constant evaporation rate of 3 nm/s. To study the phase-change of Ga15Se77Ag8 thin films the thermal treatment was performed for two hours in a vacuum furnace under a vacuum of 10  3 Torr at three different temperatures 348, 358 and 368 K. There temperatures were selected from DSC thermograms. The X-ray diffraction techniques and Field Emission Scanning Electron Microscope (FESEM) have been used for structural characterization of as-prepared and annealed thin films. The optical absorbance and transmittance of as-deposited and thermally annealed thin films were measured at normal incidence at room temperature using a double-beam UV–vis– NIR spectrophotometer (A JASCO, V-500) combined with a PC in the wavelength range 400–1100 nm.

3. Result and discussion 3.1. DSC, X-ray diffraction and FESEM studies A typical DSC trace of Ga15Se77Ag8 glass powder recorded at heating rates of 15 K/min is shown in Fig. 1. The glass transition (Tg) and crystallization temperature (Tc) were determined using the microprocessor of thermal analyzer. Three characteristic phenomena are evident in this DSC thermogram. The first one is the appearance of an endothermic hump, corresponding to the glass transition, which arises due to an abrupt increase in specific heat of the sample. The second and third is the exothermic peak, which arises due to the crystallization of the sample. There are two crystallization peaks in this sample. These well defined peaks confirm the glassy as well as amorphous nature of the prepared sample. The Tg, first crystallization peak (Tc1) and second crystallization peak (Tc1) are found to be 322 K, 375 K, 398 K respectively. The X-ray diffraction pattern of as-prepared and annealed thin films of Ga15Se77Ag8 was performed by using X-ray diffractometer (PhilpsModel-PW1710). Copper target was used as source of X-rays and l ¼1.5406 A˚ (CuKa1). The scanning angle was in the range of 10–1001. A scan speed of 21/min and a chart speed of 1 cm/min were maintained. X-ray diffraction patterns for asprepared and annealed thin films of Ga15Se77Ag8 are illustrated

M.A. Alvi / Optics Communications 295 (2013) 21–25

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1000

As-Prepared thin films

Ga15Se77Ag8 Heating rate 15 K/min

800

Heat Flow (mW)

600 400 200 0 -200 40

60

80

100

120

140

160

Temperature (°C)

Fig. 1. DSC trace for the powdered Ga15Se77Ag8 glass at 15 K/min.

Relative Intensity (Arb. Unit

Ga15Se77Ag8 As-prepaed thin films

Annealed at 368 K for two hours

10

20

30

40

50

60

70

80

90

100

Angle (2θ) (Degrees)

Relative Intensity (Arb Units)

Ga15Se77Ag8 Annealed at 368 K for 2 Hours

10

Fig. 3. Field Emission Scanning Electron Micrograph of Ga15Se77Ag8 thin films: (a) as-prepared and (b) thermally annealed (365 K).

20

30

40

50

60

70

80

90

100

Angle (2θ) (Degrees) Fig. 2. X-ray pattern for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

in Fig. 2(a) and (b). These two figures show correspondingly, the amorphous nature of the as-deposited films and the polycrystalline nature of the annealed thin films. The same behavior was obtained for other annealed films (not shown here). Field Emission Scanning Electron Microscope (QUANT FEG 450, Amsterdam, Netherlands) was used to study the surface morphology of the asprepared and thermally annealed thin films of Ga15Se77Ag8 deposited on Si (1 0 0) wafer substrate and are shown in Fig. 3(a) and (b). The microscope was operated at an accelerating voltage of 20 kV with 10 mm working distance. It had been observed that the surface morphology of the film was changed due to annealing which confirms the crystallization behavior and grains formation during the process. 3.2. Optical studies A JASCO UV–vis–NIR spectrophotometer is used for measuring optical absorption and transmittance of the thin films. In fact the

‘‘absorbance’’ reading (i.e. photometric value) is a measure of the amount of light absorbed by the sample under specified conditions. Optical behavior of material is generally utilized to determine its optical constants, i.e. absorption coefficient, extinction coefficient (k), optical band gap (Eg) etc. The variation of absorbance with wavelength (nm) is shown in Fig. 4. It is observed from this figure that the value of absorbance increases with increasing annealing temperature. The high values of absorbance are obtained in the wavelength region 450–700 nm and then it becomes almost constant. The absorption coefficient has been obtained directly from the absorbance against wavelength using the relation [40–46]

a ¼ Absorbance ðoptical densityÞ=thickness of the film

ð1Þ

The behavior of absorption coefficient as a function of the incident photon energy (hn) for as-prepared and annealed Ga15Se77Ag8 thin films at different temperatures is shown in Fig. 5. It is seen from the figure that the absorption coefficient increases almost linearly with the increase in the photon energy and it also increases by increasing annealing temperatures. The room temperature spectral dependence of the transmittance (T) of the as-deposited and thermally annealed Ga15Se77Ag8 thin films is shown in Fig. 6. It can be noticed from figure that the film transparency decreases with increasing annealing temperature. It has also been observed that the film transparency increases up to 750 nm wavelengths and then it decreases up to

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M.A. Alvi / Optics Communications 295 (2013) 21–25

2.5

0.05

Absorbance

2

Extinction coefficient (k)

As-Prepared Annealed at 348 K Annealed at 358 K Annealed at 368 K

Ga15Se77Ag8

1.5

1

As-Prepared Annealed at 348 K Annealed at 358 K Annealed at 368 K

0.04

Ga15Se77Ag8

0.03

0.02

0.01

0.5

0 1

0 400

500

600

700

800

900

1000

1.4

1.8

Wavelength (nm) Fig. 4. Absorbance against wavelength (nm) for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

2.2

2.6

3

Energy (hν)

1100

Fig. 7. Extinction coefficient (k) against photon energy (hn) for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

200

10000

Ga15 Se 77 Ag8

180

Ga15Se77Ag8

As-Prepared

160

As-Prepared Annealed at 348 K

8000

Annealed at 348 K

140

Annealed at 358 K Annealed at 368 K

Annealed at 358 K

120

6000

(αhν)1/2

Absorption coefficient (α) (cm-1)

12000

4000

Annealed at 368 K

100 80

2000

60

0 1

1.4

1.8

2.2

2.6

3

Energy (hν)

40 20

Fig. 5. Transmittance (T%) against wavelength (nm) for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

0

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

Energy (hν) Fig. 8. (ahn)1/2 against photon energy (hn) for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

100

80

As-prepared Annealed at 348 K

Ga15Se77Ag8

In the high absorption region, from which the optical band gap can be determined, the absorption is characterized by following equation [40–46]

Annealed at 358 K Annealed at 368 K

T%

60

ðahnÞ1=r ¼ BðhnEg Þ

40

20

0 400

500

600

700

800

900

1000

1100

Wavelength (nm) Fig. 6. Absorption coefficient (a) against photon energy (hn) for as-prepared and thermally annealed Ga15Se77Ag8 thin films.

1000 nm wavelength and becomes constant for all annealing temperatures. The extinction coefficient, k can be calculated by using the following relation [40–42] k ¼ al=4p

ð2Þ

where l is the wavelength of the incident light. The values of k at different annealing temperatures in Ga15Se77Ag8 thin films at 500 nm wavelength are given in Table 1 and its variation with photon energy is shown in Fig. 7.

ð3Þ

where n is the frequency of the incident beam (o ¼2pn), B is a constant and r is a parameter that depends on both the type of the transition (direct or indirect) and the profile of the electronic band. The linear plots of (ahn)1/2 versus hn for as-deposited and thermally annealed Ga15Se77Ag8 thin films, as shown in Fig. 8, indicate that the absorption mechanism is a non-direct transition. The optical energy gap of the non-direct transition can be obtained from the intercept of the (ahn)1/2 versus hn plots with the energy axis at (ahn)1/2 ¼0 and is tabulated in Table 1. It can be noticed that the optical band gap decreases from 1.56–0.96 with increasing the annealing temperature up to 368 K. The dangling bonds are responsible for the formation of some defects in the amorphous film. Such defects produced localized states in amorphous thin films. The presence of high concentration of localized states in the band structure is responsible for the decrease of the optical energy gap. The shift in the absorption edge toward lower energies might be explained on the basis of increased tailing of the band edge into the gap. Thermal annealing of amorphous solids at temperatures higher than the glass transition temperature and lower than crystallization temperature causes crystallization in the amorphous films. The major change in the optical

M.A. Alvi / Optics Communications 295 (2013) 21–25

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Table 1 Optical parameters of as-prepared and annealed thin films of Ga15Se77Ag8 chalcogenide glass. Optical constants

As-Prepared

Annealed at 348 K

Annealed at 358 K

Annealed at 368 K

Absorption coefficient (a) (cm  1) at 500 nm Extinction coefficient (k) (10  2) at 500 nm Transmittance (T%) at 500 nm Absorbance at 500 nm Optical band gap (Eg) (eV)

5348 2.13 8.52 1.07 1.56

6961 2.77 4.05 1.39 1.35

7754 3. 09 2.81 1.55 1.17

9254 3.68 1.41 1.85 0.96

energy gap after thermal annealing could be attributed to the thermally induced crystalline phase change.

4. Conclusion The XRD and FESEM studies indicate that the as-deposited Ga15Se77Ag8 thin films are amorphous while the annealed films indicate a phase change and the appearance of homogeneously distributed crystalline phase. The relative reflectance increases after crystallization. The absorption mechanism is found to be indirect transition. The crystallization of Ga15Se77Ag8 thin films by thermal annealing is accompanied by a decrease in the optical band gap. The decrease in the band gap with thermal annealing is due to crystallization of the film. The optical transmittance and absorbance are found to be sensitive with the annealing temperature. The values of absorption coefficient and extinction coefficient increase with increasing the annealing temperature. All these parameters are very important in characterizing a material for its applications in various optoelectronic devices.

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Acknowledgment This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant no. (244/ 130/1432). The author, therefore, acknowledge with thanks DSR technical and financial support.

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