Effect of annealing on physical properties of CBD synthesized nanocrystalline FeSe thin films

Materials Science in Semiconductor Processing 27 (2014) 280–287

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Effect of annealing on physical properties of CBD synthesized nanocrystalline FeSe thin films Ashok U. Ubale n, Naina R. Welekar, Amruta V. Mitkari Nanostructured Thin Film Materials Laboratory, Department of Physics, Govt. Vidarbha Institute of Science and Humanities, VMV Road, Amravati 444604, Maharashtra, India

a r t i c l e in f o

Keywords: Thin film Chalcogenides Annealing Chemical synthesis

abstract Nanocrystalline FeSe thin films were successfully prepared by solution growth method using ferric chloride and sodium selenosulphate as cationic and anionic precursors along with complexing agent oxalic acid. The thickness dependent physical properties of FeSe thin films prepared by varying deposition time are discussed. The FeSe films of thickness 161 nm were further annealed to investigate its impact on physical properties. The X-ray diffraction studies showed that, as deposited FeSe films are nano crystalline in nature and their crystallinity increases with thickness as well as with annealing temperature. The morphological studies showed that FeSe exhibits granular surface with channel like features at higher thickness. The electrical resistivity and thermo-emf measurements confirmed that, FeSe films are semiconducting in nature with P-type conductivity. The activation and band gap energies of FeSe films are found dependent on film thickness as well as on annealing temperature. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recently, binary semiconducting Fe–Se ferromagnetic compounds such as FeSe, FeSe2 and Fe3Se4 etc. have attracted scientific community due to their novel physical and chemical properties [1]. Along with semiconductor devices FeSe has find suitable for various types of biomedical and biosensing applications [2]. The iron-selenium alloy system has been studied widely with the help of X-ray diffraction, thermal analysis and susceptibility measurements. On the basis of these studies it has been reported in the literature that, for low concentration of Se i.e. up to 48 at% the system is paramagnetic, for the range 48.8–56.0 at% of Se, the system is ferromagnetic; and for higher concentrations i.e. above 56 at% the alloy is again paramagnetic [3]. Very few reports are available on preparation and characterization of FeSe thin films for structural, electrical and optical properties. Takemura

n

Corresponding author. Tel.: þ91 721 2531706; fax: þ91 721 2531705. E-mail address: [email protected] (A.U. Ubale).

http://dx.doi.org/10.1016/j.mssp.2014.06.035 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

et al. [4] have prepared polycrystalline FeSe thin films on GaAs (100) substrates by molecular beam epitaxy method. From M– H curve analysis it was found that the deposited FeSe films have two phases with different coercive forces. Takemura et al. [5] have reported isotropic magnetization in FeSe thin film on GaAs substrate. Feng et al. [6] have prepared FeSe thin films at 593 K temperature using low pressure metal organic chemical vapor deposition. Hamdadou et al. [7] have prepared iron selenide thin films by selenization of evaporated iron thin films. The deposited films are tetragonal in nature with some orthorhombic crystallites of FeSe2. However, films annealed for 2 h at 773 K temperature showed tetragonal structure of FeSe with no detection of FeSe2. Wu et al. [8] have prepared FeSe films by MOCVD method and reported that, films are ferromagnetic at room temperature and showed P-type conduction with carrier concentration 1021 cm
3. Mahalingam et al. [9] have prepared FeSe2 thin films by electrodeposition method on tin oxide coated conducting glass substrates by varying bath temperature. Ubale et al. [10,11] have reported structural, electrical and optical properties of FeSe thin films grown by Chemical Bath Deposition (CBD) and spray

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

2. Experimental For the present work, amorphous glass slides of dimensions 72  25  1 mm were used as substrates for the deposition of FeSe thin films. In CBD method cleaning of substrate is very important as it affects growth rate and adherence of the film. To clean glass slides, they were washed with liquid detergent and then boiled in concentrated chromic acid (0.5 M) for 1 h and then kept in it for next 48 h at room temperature. The substrates were then washed with double distilled water and finally cleaned in ultrasonic cleaner for 15 min. The Na2SeSO3 solution was prepared in distilled water by mixing 100 g of Na2SO3 (sodium sulfate) and 5 g of selenium powder in a beaker of 500 ml capacity. This mixture was then refluxed at 80 1C temperature for 8 h to obtain a clear solution. Finally it was filtered to remove small micro quantity of un-dissolved content of Se powder. For the deposition of FeSe thin films 25 mL of 0.5 M ferric chloride solution was mixed with 25 mL of 0.15 M oxalic acid with constant stirring. To it freshly prepared 25 mL of 0.13 M sodium selenosulphate was added at room temperature. The cleaned glass substrates were immersed in the bath for deposition. Several deposition trials were carried out to optimize pH and concentration of the oxalic acid and ferric chloride to get uniform adhesive FeSe thin films. It was observed that, the deposition process is very critical. It takes place at pHE4 with 0.15 and 0.5 M concentration of oxalic acid and ferric chloride respectively. The films deposited at this pH and concentrations are more uniform and adherent. The well grown adherent dark brown films were retired from the bath by varying the deposition time with interval of 5 h. Fig. 1 shows variation of FeSe film thickness with deposition time. It is observed that, FeSe film thickness linearly increases from 63–161 nm as deposition time increases from 10–35 h. The films of thickness 161 nm were further annealed for 1 h at 473,573 and 673 K temperature, to investigate its impact on physical properties. 3. Results and discussion 3.1. Reaction mechanism In CBD method, the growth process is based on the slow release of metal and selenium ions. The precipitate formation in the solution takes place when ionic product exceeds the solubility product [12,13]. To prepare FeSe thin films selenosulphate (Na2SeSO3) (Se analog of thiosulphate, with one S exchanged by Se) was prepared by dissolving elemental Se in Na2SO3 solution. The ionization

200 160 Thickness (nm)

pyrolysis method. For that they used ethylenediaminetetraacetic acid (EDTA) and acetic acid as complexing agents. As per literature survey only these two reports are available on growth of FeSe thin films at room temperature using EDTA and acetic acid as complex. In the present work, Chemical Bath Deposition method is employed for the preparation of nanostructured FeSe thin films at room temperature using oxalic acid as a complexing agent. The effect of film thickness and annealing temperature on structural, electrical morphological and optical properties of FeSe thin films is discussed.

281

120 80 40 0 5

10

15

20

25

30

35

40

Deposition time (h)

Fig. 1. Variation of FeSe film thickness with deposition time.

of sodium sulfate in water takes place as, Na2 SO3 -2Na þ SO23


ð1Þ

The selenium metal powder was added and refluxed for 8 h at 80 1C to get selenosulphate as, 2 Na þ þSe þ SO23
-Na2 Se SO3 ðSodium selenosulphateÞ ð2Þ The release of metal ions in the solution was controlled by adding complexing agent in the bath. The Fe2 þ ions released by ferric chloride forms [Fe(C2O4)2]2
complex with oxalic acid as, 2O Fe2++ 2

C

O

O OH

C

O

O

C

O

C

+2H+

Fe C

OH

O

C

O

O

O Dioxalato ferrate ion

ð3Þ The sodium selenosulphate dissociates to give selenium hydride ion, which further reacts with dioxalato ferrate ion to give FeSe as, Na2 SeSo3 þ 2H2 O-½H2 Se þ þ þ Na2 So3 þ 2OH


ð4Þ

COOH ½FeðC2 O4 Þ2 2
þ½H2 Se2 þ -½FeSe↓ þ j COOH

ð5Þ

3.2. Structural studies Fig. 2 shows X-ray diffraction patterns of FeSe thin films deposited at room temperature by varying deposition time from 10–35 h. As deposited FeSe thin films are nanocrystalline in nature with tetragonal lattice. The comparison of observed XRD data with standard JCPDS(Card no 85-735) data is shown in Table 1. The FeSe films of thickness 63 and 85 nm are amorphous in nature, however the crystalline nature of the film increases with thickness. In addition with film thickness the XRD patterns showed improvement in the crystal quality of the deposited FeSe. As film thickness increases the density of grain boundary and defects such as pinholes, voids etc also decreases giving

310

102

004

200

002

161 nm

133 nm

121 nm

106 nm 002

Intensity (Arbitrary unit)

from 11–20 nm, as film thickness rises from 106–161 nm. For thickness less than 106 nm, the XRD shows amorphous nature which may be due to random orientations of tiny nano crystallites of FeSe. The sizes of these crystallites were further improved with deposition time showing nanocrystalline nature. Fig. 3 shows XRD patterns of FeSe thin films of thickness 161 nm annealed at 473,573 and 673 K temperatures. The comparison of observed XRD data with standard JCPDS data is listed in Table 2. It is observed that the (002) orientation due to tetragonal lattice becomes prominent with annealing temperature. Similar behavior was reported earlier for electro synthesized iron selenide thin films by Wu et al. [8]. The common orientations observed in CVD and our CBD deposited FeSe films

of

FeSe

thin

films

deposited

by

varying

Standard d (Å)

Observed d (Å)

hkl

63 85 106 121

– – 1.3795 1.8825 1.3795 2.2254 1.3795 1.1906 2.759 1.8825 1.3795

– – 1.3692 1.8745 1.3783 2.2181 1.3645 1.1891 2.761 1.8864 1.3769

– – 004 200 004 102 004 310 002 200 004

133

161

310

573K

Table 1 Comparison of standard and observed XRD data of as deposited FeSe thin films. Film thickness (nm)

004

673K

200

Fig. 2. XRD patters deposition time.

102

63 nm

Intensity (Arbitrary unit)

85 nm

473K 303K

20

40

60

80

2 Fig. 3. XRD patterns of FeSe thin film of thickness 161 nm annealed at various temperatures.

Table 2 Comparison of standard and observed XRD data of annealed FeSe thin films. Annealing temperature (K) Standard d (Å) Observed d (Å) hkl

more perfect crystal. Also it is observed that, intensity of peaks is increased with thickness, which attributed to the improvement in polycrystalline nature of film. The (004) orientation of FeSe is observed at 106 nm film thickness however (002), (102), (200), and (310) orientations are appeared above 121 nm thickness. The (102), (310) orientations are observed only at thickness 133 nm. These results are in good agreement with tetragonal lattice structure of CVD and CBD deposited FeSe thin films by Wua et al. [8] and Ubale et al. [10,11] respectively. In present work, the crystallite size of the deposited film was determined using the Debye–Sheerer formula. It was observed that, the crystallite size of the film increases

673

573

473

303

2.759 2.2254 1.3795 1.1906 2.759 2.2254 1.8825 1.3795 2.759 2.2254 1.8825 1.3795 2.759 1.8825 1.3795

2.748 2.219 1.368 1.189 2.755 2.221 1.8830 1.368 2.749 2.220 1.8795 1.369 2.659 1.8698 1.388

002 102 004 310 002 102 200 004 002 102 200 004 002 200 004

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

283

Fig. 4. SEM images of FeSe thin films deposited by varying deposition time.

are (102) and (002). It is observed that, the poor crystalline quality of the as deposited FeSe films was improved after annealing. The crystallite size of the film of thickness 161 nm is increased from 20–27 nm as annealing temperature rises from 303–673 K. Fig. 4 shows SEM images of FeSe thin films deposited by varying deposition time. The surface of the film is quite

rough and covers complete glass substrate. It is observed that, the granular structure of thin film increases with thickness. At higher thickness, the agglomeration of grains at some places is observed on homogeneous background of closely packed circular grains. The remarkable overgrowth is observed above 85 nm film thickness and becomes maximum at 133 nm. However, above 133 nm

284

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

Fig. 5. SEM images of FeSe thin film of thickness161 nm annealed at various temperatures. 4200

7

SeKa

2400

63 nm 85 nm 106 nm 121 nm 133 nm 161 nm

6.5

6 Fela

log

600

SeKa

FeKeSc

1200

SeKb

1800

FeKb

Counts

3000

FeKa

3600

5.5

5

0 0

1

2

3

4

5

6

7

8

9

10 11 12

13 14

KeV

4.5

Fig. 6. EDAX spectra of FeSe thin film of thickness 161 nm. 4

thickness, the development of channel like features is observed. These channels on the film surface may be due to overgrowth or may be due to strain of the deposited material with substrate surface. Ouertania [1] have reported similar morphology for FeSe2 thin films deposited by selenization. Fig. 5 shows that roughness of the film increases with annealing temperature. The smaller tiny crystallites are merged together to form bigger grains with development

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3

3.1

1/T × 1000 (K )

Fig. 7. Variation of log ρ versus 1/T  1000(K
1) of FeSe films deposited by varying deposition time.

of channel features. It is observed that the annealing temperature enhances the porous nature of the film. Similar morphology was reported by Hamdadou et al. [7] for iron selenide thin films prepared by selenization

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

technique. It was reported that, the tetragonal phase of FeSe becomes hexagonal after annealing for 2 h at 773 K in vacuum. However, in our investigation it is observed that on air annealing at 673 K temperature, FeSe film shows agglomeration of grains with channel like features without changing its tetragonal phase. Fig. 6 shows typical EDAX spectra of FeSe thin film of thickness 161 nm. The elemental analysis was carried out only for Fe and Se elements. The average atomic percentage of Fe: Se was found to be 53:47, which showed that CBD deposited FeSe films are deficient in Se.

285

The thermal activation energy decreases from 0.21– 0.18 eV as annealing temperature increases. Figs. 9 and 10 shows variation of induced thermo-emf with temperature difference applied across the FeSe film. The increase in thermo-emf with film thickness as well as with annealing temperature is attributed to the decrease in electrical resistivity. It was observed that the thermoemf generated at 136 K applied temperature difference for 6.2 303 K

3.3. Electrical studies

573 K 5.7

ρ ¼ ρ0 exp ðEa=KTÞ

ð6Þ

Where, ρ is the resistivity at temperature T, ρ0 is constant, K is Boltzmann constant, and Ea is activation energy. Table 3 shows that thermal activation energy of FeSe film decreases from 0.32–0.21 eV as film thickness rises from 63–161 nm. This decrease in activation energy may be due to improved crystalline quality of the film. Activation energy is a function of crystal perfection and poor crystal quality generally produces increased activation energy. It is very obvious that as film thickness increases the density of grain boundary and defects such as pinholes, voids etc also decreases giving more perfect crystal. In the present case, the XRD studies showed improvement in the crystal quality of the FeSe film with thickness. Fig. 8 shows the variation of log of resistivity of annealed FeSe thin films with reciprocal of temperature. It is observed that room temperature resistivity of FeSe thin film of thickness 161 nm is of the order of 4.8  104 Ω cm and it decreases to 5.7  104 Ω cm as film was annealed at 673 K temperature for 1 h. Table 4 shows the variation of thermal activation energy of FeSe with annealing temperature. Table 3 Variation of grain size, activation energy and band gap energy of FeSe thin film with thickness. Thickness (nm)

Deposition time (hr)

Grain size (nm)

Eg (eV)

Activation Energy (eV)

161 133 121 106 85 63

32 27 20 19 17 12

20 19 15 11 – –

2.14 2.17 2.24 2.34 2.48 2.53

0.21 0.22 0.24 0.25 0.27 0.32

log

673 K

5.2

4.7

4.2 2.2

2.4

2.6

2.8

3

1/T × 1000 (K )

Fig. 8. Variation of log ρ versus 1/T  1000(K
1) of FeSe film of thickness 161 nm annealed for 1 h at different temperatures. Table 4 Variation of grain size, activation energy and band gap energy of FeSe thin film of thickness 161 nm with annealing temperature. Thickness (nm)

Annealing temperature (K)

Grain size Eg (nm) (eV)

Activation energy (eV)

161 – – –

303 473 573 673

20 21 24 27

0.21 0.20 0.18 –

2.14 2.04 1.96 1.94

16

161 nm 133 nm

12

121nm Thermo emf (mV)

Fig. 7 shows the variation of log of electrical resistivity of FeSe with reciprocal of temperature. It was observed that the resistivity of the as deposited FeSe film of thickness 63 nm at 333 K is of the order of 4.8  106 Ω cm and it decreases to 3.5  105 Ω cm as film thickness increases to 161 nm. The higher value of resistivity of nanocrystalline FeSe film is may be due to coarsely grained material [6]. The rise in resistivity at lower thickness was attributed to the fine grains of FeSe. Also it was observed that, resistivity decreases with rise in temperature indicating its semiconducting nature. The thermal activation energies were calculated using the relation,

106 nm 85 nm 8

4

0 45

65

85

105

125

145

Temperature difference (K)

Fig. 9. Variation of thermo-emf versus temperature difference applied across FeSe film.

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

Aðhυ
EgÞn hυ

ð7Þ

Where, hυ is photon energy, A and n are constants, Eg is band gap energy. For allowed direct transition n¼½ and for allowed indirect transition n¼2. Fig. 12 shows the plots of (αhυ)2 verses hυ for FeSe films deposited by varying deposition time. The nature of graph confirmed that CBD deposited FeSe has direct optical band gap. The optical band gap energy estimated by extrapolating the linear 700 161 nm 600

133 nm 121 nm

500

106 nm 85 nm 63nm

400

-9

Fig. 11 shows the variation of optical absorbance (αt) of FeSe thin films deposited by varying deposition time in the wavelength range 350–1090 nm. It was observed that absorption coefficient of FeSe film deposited at 10 h is of the order of 1.5  104 and it increases to 2.2  104 cm
1 as deposition time was increased to 35 h. It may be because for bulk materials the density of states is a continuous function, but when confinement appears quantization arises, and thus for quantum films the density of states

α¼

-1 2

3.4. Optical studies

shows discrete nature. The variation of optical absorption with wavelength obeys the relation,

-

the FeSe film of thickness 161 nm is 12.5 mV and it decreases to 1.9 mV when thickness of FeSe film becomes 85 nm. Also for the 136 K applied temperature difference the thermo-emf of FeSe thin film annealed at 573 K temperature is 15.8 mV and it increases to 17.2 mV when film is annealed at 673 K temperature. The polarity of thermo-emf developed across the junction shows that FeSe film exhibits P-type conductivity. The rise in thermo-emf with thickness and annealing temperature is may be due to improved crystallinity of the film.

( h ) x 10 (eV cm )

286

2

20

303 K

16 Thermo- emf (mV)

300

200

573 K 12 673 K

100 8

0 1

4

1.5

2

2.5

3

3.5

4

h (eV) 0 50

70

90

110

130

150

Fig. 12. Variation of (αhυ)2 versus hυ for FeSe films deposited by varying deposition time.

Temperature difference (K)

Fig. 10. Variation of thermo-emf versus temperature difference applied across annealed FeSe film.

3 303 K

3

2.5

161 nm

473 K 573 K

133 nm

673 K

121 nm 106 nm 2

2

1.5

t

63 nm

t

85 nm

1

1 0.5

0 300

500

700

900

1100

Wavelength , nm) Fig. 11. Variation of αt versus wavelength for FeSe films deposited by varying deposition time.

0 300

500

700

900

1100

Wavelength , nm) Fig. 13. Variation of αt versus wavelength for FeSe film of thickness 161 nm annealed at different temperatures.

A.U. Ubale et al. / Materials Science in Semiconductor Processing 27 (2014) 280–287

287

and acceptor states in the forbidden zone which may reduce the band gap energy [15,16].

350

303 K 300

4. Conclusion

473 K 573 K

In conclusion, nanocrystalline FeSe thin films were deposited at room temperature onto glass substrates by solution growth method using oxalic acid as a complexing agent. The structural investigation showed that, the as deposited FeSe films are nano crystalline in nature and its crystallinity increases with thickness as well as with annealing temperature. The morphological studies showed granular film surface with channel features. The FeSe films are semiconducting in nature with P-type conductivity. A shift of 0.39 eV in the optical band gap energy and a decrease in electric resistivity from 4.8  106–3.5  105 Ω cm and increase in grain size of FeSe crystallites from11–20 nm were observed when the thickness was varied from 63–161 nm.These properties are also found to be dependent on the annealing temperature.

673 K

200

150

h

2

x 10-9 (eV-cm-1)2

250

100

50

References 0 1

1.5

2

2.5

3

3.5

4

h eV) 2

Fig. 14. Variation of (αhυ) versus hυ for FeSe film of thickness 161 nm annealed for 1 h at different temperatures.

portion of the graph at α ¼0 changes from 2.53–2.14 eV depending on film thickness (Table 3). The rise in band gap with decrease in thickness may be due to size quantization that appears as a result of decreased crystallite size. Fig. 13 shows the variation of optical absorbance (αt) of annealed FeSe thin films measured in the wavelength range 350–1090 nm. It is observed that absorption coefficient of FeSe film annealed at 303 K is of the order of 1.5  104 and increases to 5.5  103 cm
1 as the annealing temperature was increases to 673 K. Fig. 14 shows plots of (αhυ)2 verses hυ for annealed FeSe films. It is observed that the optical band gap energy decreases from 2.14– 1.94 eV as the annealing temperature increases from 303– 673 K (Table 4) the value of Eg is slightly higher than the value reported by S.M. Pawar [14]. The annealing of film may generates more number of vacancies that forms donor

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