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Formation of pyrite (FeS2) thin nano-films by thermal-sulfurating electrodeposition films at different temperature
Materials Letters 59 (2005) 2398 – 2402 www.elsevier.com/locate/matlet
Formation of pyrite (FeS2) thin nano-films by thermal-sulfurating electrodeposition films at different temperature Y.Z. Dong, Y.F. Zheng*, H. Duan, Y.F. Sun, Y.H. Chen Physics Department of Xinjiang University, URMQ, Xinjiang 830046, PR China Received 20 January 2005; accepted 13 March 2005 Available online 18 April 2005
Abstract The electrodeposition of iron disulfide pyrite (FeS2) thin films on indium – tin oxide (ITO) substrate in aqueous solution containing Na2S2O3I5H2O, FeCl2I4H2O reagents was investigated. The deposited material is successively annealed in sulfur atmosphere. In this way, we successfully obtained pyrite thin films with good quality. The structure, optical and electrical property analysis for the resulted films was carried out and some special results have been concluded. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrodeposition; Characterization; Iron disulfide thin films
In recent years, iron disulfide pyrite (FeS2) has been investigated as an alternative solar cell material due to its very high optical absorption coefficient, a 5 105 cm 1 for k 700 nm [1,2] and its suitable bandgap, E g = 0.95 eV. In addition, the abundant, cheap and non-toxic composite elements are also the reasons for enormous interests in pyrite as an absorber material for thin-film solar cells [3]. Several methods have been applied for the preparation of pyrite thin films: thermal evaporation [4], flash evaporation [5], chemical vapor transport (CVT) [6], metal-organic chemical vapor deposition [7], sulfurization of electrodeposited iron films [8], the spray method [9], et al. Despite of this, the conversion efficiency of pyrite (FeS2) solar cells has not exceeded 3% [10]. The reason for the low value can be attributed to phase impurity [11]. That is, FeS2 can crystallize not only into a cubic pyrite structure, but also into an orthorhombic metastable marcasite structure that is
detrimental to photovoltaic applications because of its low bandgap (E g = 0.34 eV). Therefore, new and improved techniques are needed to resolve these problems. Moreover, a simple method should also be developed for the preparation of polycrystalline pyrite thin films with large area using low-cost equipment. In this present work, we conveniently prepared single phase pyrite (FeS2) thin films
**
1200
*
1000
Intensity/cps
1. Introduction
*
ITO FeS
800 600 400
*
*
*
*
***
200
*
*
*
*
0 20
40
60
80
2θ/(°C) * Corresponding author. Tel.: +86 991 8583183. E-mail address: [email protected] (Y.F. Zheng). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.03.025
Fig. 1. XRD patterns of the sample without anneal.
100
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
T
P
P
P P
P P
P
(111)
0
20
P PP M
P P
P P
P
500°C
P PP PP
P
40
P 600°C
P P
P
P P P
P P PP
P
P (333)
1000
M
300°C
(422)
P
T TT
400°C
P P P P
2000
T P
(331) (420) (124) (332)
3000 2000 1000 0
(210)
0
M
P M P
P M
M
(200)
Intensity/cps
400
P
P
(311)
P
T
(222) (023) (321)
M
0 800
(211)
400
T PT P P M
(220)
800
2399
60
80
100
2θ/(°C) Fig. 2. XRD patterns of the sample sulfidated under different temperatures (I: ITO, T: FeS, P: Pyrite, M: Marcasite).
via annealing the electrodeposited thin films in a sulfur atmosphere.
2. Experiments The electrodeposition bath consists of aqueous solution containing FeCl2I4H2O and Na2S2O3I5H2O (analytical reagents). The electrodeposition was carried out in the stirred solution potentiostatically and indium – tin oxide (ITO) substrate was chosen as cathode, the platinum sheet as anode and Ag/AgCl electrode as reference electrode. Prior to the electrodeposition, the substrate was dipping in concentrated HNO3, and then washing with doubly distilled water and rinsing with acetone. During the electrochemical deposition process, the solution was deaerated with argon. The temperature of the bath was kept at 25 -C. The current density was fixed at 0.8 mA/cm2. The pH value of the solution was adjusted by adding dilute hydrochloric acid. The annealing treatment was performed in a closed quartz tube in a sulfur atmosphere (sulfur pressure 8.0 104 Pa) for 10 h.
The temperature of annealing was varied from 200 -C to 600 -C. The shape, crystallinity and composition of the deposited thin films were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Optical absorption spectra of the films were recorded with a U-3400 spectrophotometer at ambient temperature. The glass substrate absorption was subtracted by placing a bare glass in the instrument reference channel. The carrier concentration and the Hall mobility were determined by a HL5500 Hall auto-measuring appliance in magnetic induction of 0.32 T.
3. Results and discussion 3.1. Phase and structure analysis In the X-ray diffraction profile of the sample without annealing treatment (Fig. 1), there are some intensive ITO peaks, indicating the incompactness of the electrodeposited thin films, and other peaks can be characterized as FeS.
Fig. 3. Shape of crystallite at different annealing temperatures: (a) 400 -C, (b) 500 -C, (c) 600 -C.
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
Table 1 The dependence of the grain size of the thin films at different annealing temperatures (hkl)
400 -C (nm)
500 -C (nm)
600 -C (nm)
(111) (200) (210) (211) (220) (311)
15.8 18.9 17.7 15.6 15.8 16.6
71.4 106.2 92.3 68.5 71.4 80.6
66.5 103.7 88.3 63.4 64.5 76.3
Fig. 2 shows the consequence of XRD patterns of the thin films annealed in a sulfur atmosphere at different annealing temperatures for 10 h. For comparison, the XRD pattern derived from the standard ASTM71-2219 card is also given in this figure. The thin film becomes obviously compact when annealed at 300-C, so we cannot observe the peaks of ITO substrates. At the same time, sulfuration reaction occurs and there exists some peaks of pyrite and marcasite in the XRD pattern of the thin films annealed at 300 -C but the intensity of the peaks is weakly. When annealing at a lower temperature (400 -C), insufficient sulfuration process leads to the formation of marcasite transitional phase, which has smaller bandgap (0.34 eV) than pyrite. With the anneal temperature rising from 500 -C to 600 -C, it is found from Fig. 2 that the films annealed at about 500 -C consist of a single-phased pyrite without any contributions from marcasite, and at a higher temperature (600 -C) would bring about the decline of the intensity of the pyrite peaks. Considering above parameters, the optimal anneal temperature 500 -C for formation of a single-phased pyrite can be obtained.
4. Grain size and shape of the thin films
3.0 2.9
absorption coefficient/x105 cm-1
2400
c
2.8 2.7 2.6 2.5
b
2.4 2.3 2.2 2.1
a
2.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
energy/ev Fig. 5. Optical absorption coefficient of the FeS2 thin films annealed at 500 -C.
microstrain important to XRD patterns were considered in the refinement. According to the Popa theory, the shape and the size of crystallite resulted from the refinement are the comprehensive effects of the truly X-ray diffraction of all the crystallites. From this figure, we can observe that the shape of the grain in the thin films annealed at different temperature is of octahedron basically. This is due to the fact that the growth rate along (100) crystal plane is more rapid than that of (111) crystal plane. A quantitative estimation of the grain size of the polycrystalline thin films annealed at different annealing temperature for 10 h is carried based on the refinement result. In this procedure, we have adopted standard silicon reference to extract an instrument broadening for each sample.
Fig. 3 is the grain shape of the films resulted from the Rietveld refinement according to Popa theory [12]. Several parameters, such as cell parameter, texture, crystallite size,
80 70
(αhν)2/A.U
60 50 40 30 20 10 0 1.2 1.4
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
energy/ev 2
Fig. 4. SEM micrograph of the pyrite films annealed at 500 -C.
Fig. 6. The relation between (ahr) and hr of the FeS2 thin films annealed at 500 -C.
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402 Table 2 Band gap of pyrite by preparation techniques Technique
E gd (eV)
OS [13] EPD [14] SVP [13]
1.35 1.28 1.35
Table 1 shows the dependence of the grain size in the thin films at different annealing temperature. It can be seen from this table that the grain sizes significantly increase when the annealing temperature varies from 400 -C to 500 -C, and the maximum value is up to 106.2 nm at 500 -C, different from the value of about 130 nm determined by SEM (Fig. 4). This is in agreement with the fact that the grain size estimated from XRD pattern is generally smaller than that observed by SEM [13]. Furthermore, when the annealing temperature increases to 600 -C, the grain size decreases appreciably, however. Therefore, it can be concluded that the biggest grain sizes of 106.2 nm are when the annealing temperature is at 500 -C.
2401
The direct energy gaps E gd can be determined by plotting respectively (ahr)2 as a function of energy hr [13] as shown in Fig. 6. a is determined from transmittance spectra for the film samples. The band gap E gd = 1.34 eV has been qualitatively determined by extrapolating the straight line to the energy axis. Compared with the band gap E gd = 0.95 eV [2] of the bulk FeS2, this value from the experiment result moves 0.39 eV high. This inconsistency could be caused by the significant reduction in grain size of the thin films, which leads to the formation of quantum energy. When the grain size reduces to some value, the energy level of the system is quantized and the corresponding band gap is broadened. Takagahara [15] indicated that the electronhole confinement effect most pronounced in a semiconductor quantum dot tends to increase band gap width. A possible mechanism is proposed for the formation of pyrite. It was found that the annealing temperature performed important roles in the growth of pyrite crystals. Table 2 shows E gd obtained by several preparation methods and our conclusion is comparable to these resulted data.
4.1. Optical properties 4.2. Electric properties Fig. 5 shows the optical absorption spectra for the thin films annealed in a sulfur atmosphere at 500 -C. In this figure, the apparent absorption coefficient (a) vs. photon energy (hr) is plotted. The absorption curve generally consists of three regions: weak absorption region in the range of low photon energy, abrupt absorption region in the range of photon energy about 1.6– 2.3 eV and constant absorption region in the range of high photon energy above 2.3 eV whose absorption coefficient (a) is approximately as a constant. The abrupt absorption region indicates that the films are of typical semiconductor characteristic.
Hall measurements have revealed that all thin films of pyrite which are annealed from 300 to 600 -C are of ptype conduction. The group of the Hahn-Meitner-Institut reported that pyrite films with p-type conductivity were obtained by normal-pressure MOCVD [16], while those with n-type conduction by low-pressure MOCVD [17]. Fig. 7 shows carrier concentration and Hall mobility vs. annealing temperature relationship for pyrite thin films prepared in this study. It indicates that the film annealed at 300 -C has a rather high carrier concentration (a) (about 1019 cm 3) due to containing a lot of FeS which is 250
3.00E+019
(a) 200
2.98E+014
150
2.96E+014 100 2.94E+014 50
(b)
2.92E+014
Hall mobility/(cm2/v−s)
Carrier concentration/cm-3
2.50E+019
2.90E+014
0 300
350
400
450
500
550
600
Annealing temperature/°C Fig. 7. The dependence of both Carrier concentration and mobility upon annealing temperature. (a) Carrier concentration; (b) Hall mobility.
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Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
a metal. The carrier concentration decreases gradually from 2.92 1014 to 2.91 1014 cm 3 when the annealing temperature ranges from 400 -C to 600 -C. The carrier concentration of the thin films annealed at 400 -C is higher due to containing a small quantity of marcasite with a band gap E gd = 0.34 eV than the thin films annealed at 500 -C with a single-phase pyrite. The Hall mobility increases markedly with the rising of annealing temperature below 500 -C and a slow decrease above 500 -C. When annealing temperature ranges from 300 -C to 500 -C, crystallization of grains tends to be perfect, defect amount decreases and region of grain boundaries becomes more and more narrow. According to the grain boundary model [18,19], the decreased defect concentration leads to the lowering of potential barrier at the grain boundaries and the decreased density of charge scattering centers from the point defect results in the rise in Hall mobility. On the other hand, the thin films annealed at a higher temperature have enlarged grain size, less grain boundaries and more continuous surface, which could impair the capture of charge carriers and therefore a higher Hall mobility has been obtained in our study. When annealing temperature continue to rise up to 600 -C Hall mobility decreases appreciably owing to surplus sulfur in the thin films with increased defects.
5. Conclusion The electrodeposition of iron disulfide pyrite (FeS2) thin films on Indium –tin oxide (ITO) substrates in aqueous solution containing Na2S2O3I5H2O, FeCl2I4H2O reagents was performed in this work. It is found that annealing treatment for the deposited thin films in a sulfur atmosphere at optimal temperature is very necessary for formation of a single-phase pyrite thin film. The shape of the grain of the samples after annealing treatment is approximately octahedral and the maximum grain size is 106.2 nm. All pyrite thin films annealed from 300 to 500 -C are all of p-type conduction, and the crystallization of grains tends to be nearly perfect. Thus, the thin films here have ideal carrier concentration about 1014 cm 3 and show the remarkable increment in Hall mobility with the increase of annealing temperature. However, when annealing temperature rises to 600 -C Hall mobility decreases
appreciably owing to increased the mount of defect. Therefore, the most appropriate annealing temperature 500 -C can be concluded, and the direct E gd of the resulting thin film is 1.34 eV.
Acknowledgements This work was supported financially by the National Natural Science Foundation of the Chinese Government, under Grant No. 50062002).
References [1] K. Ellmer, D. Lichtenberger, A. Ennaoui, C. Ho¨Kpfner, S. Fiechter, H. Tributsch, Conf. Proc. 23rd IEEEPVSC, 1993, p. 535. [2] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonso-Vante, K. Bu¨ker, M. Bronold, C. Ho¨opfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289. [3] A. Ennaoui, H. Tributsch, Sol. Cells 13 (1984) 197. [4] A.M. Karguppikar, V.G. Vedeshawar, Phys. Status Solidi A 95 (1986) 717. [5] C. De las Heras, C. Sanchez, Thin Solid Films 199 (1991) 259. [6] S. Fiechter, J. Mai, A. Ennaoui, J. Cryst. Growth 78 (1986) 438. [7] G. Chatzitheodorou, S. Fiechter, R. Ko¨nenkamp, M. Kunst, W. Jaegermann, H. Tribusch, Mater. Res. Bull. 21 (1986) 1481. [8] G. Pimenta, V. Schro¨der, W. Kautek, Ber. Bunsenges. Phys. Chem. 95 (1991) 1470. [9] A. Yamamoto, M. Nakamura, A. Seki, et al., Sol. Energy Mater. Sol. Cells 75 (2003) 451. [10] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonso-Vante, K. Bu¨ker, M. Bronold, C. Ho¨pfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289. [11] B. Thomas, K. Diesner, T. Cibik, K. Ellmer, Solid State Phenom. 51 – 52 (1996) 301. [12] N.C. Popa, J. Appl. Crystallogr. 31 (1998) 176. [13] G. Smestad, A. Ennaoui, S. Fiechter, H. Tributsch, W.K. Hofmann, M. Birkholz, W. Kautek, Sol. Energy Mater. 20 (1990) 149. [14] H. Duan, Y.F. Zheng, Y.Z. Dong, X.G. Zhang, Y.F. Sun, Mater. Res. Bull. 39 (2004) 1861 – 1868. [15] T. Takagahara, Phys. Rev., B 47 (1993) 4569. [16] K. Ellmer, D. Lichtenberger, A. Ennaoui, C.H. Opfner, S. Fiechter, H. Tributsch, Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, 1993, p. 535. [17] B. Thomas, K. Diesner, T. Cibik, K. Ellmer, Solid State Phenom. 51 (1996) 301. [18] J.Y.W. Seto, J. Appl. Phys. 46 (1975) 5247. [19] D. Lichtenberger, K. Ellmer, R. Schieck, S. Fiechter, H. Tributsch, Thin Solid Films 246 (1994) 6.
Formation of pyrite (FeS2) thin nano-films by thermal-sulfurating electrodeposition films at different temperature Y.Z. Dong, Y.F. Zheng*, H. Duan, Y.F. Sun, Y.H. Chen Physics Department of Xinjiang University, URMQ, Xinjiang 830046, PR China Received 20 January 2005; accepted 13 March 2005 Available online 18 April 2005
Abstract The electrodeposition of iron disulfide pyrite (FeS2) thin films on indium – tin oxide (ITO) substrate in aqueous solution containing Na2S2O3I5H2O, FeCl2I4H2O reagents was investigated. The deposited material is successively annealed in sulfur atmosphere. In this way, we successfully obtained pyrite thin films with good quality. The structure, optical and electrical property analysis for the resulted films was carried out and some special results have been concluded. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrodeposition; Characterization; Iron disulfide thin films
In recent years, iron disulfide pyrite (FeS2) has been investigated as an alternative solar cell material due to its very high optical absorption coefficient, a 5 105 cm 1 for k 700 nm [1,2] and its suitable bandgap, E g = 0.95 eV. In addition, the abundant, cheap and non-toxic composite elements are also the reasons for enormous interests in pyrite as an absorber material for thin-film solar cells [3]. Several methods have been applied for the preparation of pyrite thin films: thermal evaporation [4], flash evaporation [5], chemical vapor transport (CVT) [6], metal-organic chemical vapor deposition [7], sulfurization of electrodeposited iron films [8], the spray method [9], et al. Despite of this, the conversion efficiency of pyrite (FeS2) solar cells has not exceeded 3% [10]. The reason for the low value can be attributed to phase impurity [11]. That is, FeS2 can crystallize not only into a cubic pyrite structure, but also into an orthorhombic metastable marcasite structure that is
detrimental to photovoltaic applications because of its low bandgap (E g = 0.34 eV). Therefore, new and improved techniques are needed to resolve these problems. Moreover, a simple method should also be developed for the preparation of polycrystalline pyrite thin films with large area using low-cost equipment. In this present work, we conveniently prepared single phase pyrite (FeS2) thin films
**
1200
*
1000
Intensity/cps
1. Introduction
*
ITO FeS
800 600 400
*
*
*
*
***
200
*
*
*
*
0 20
40
60
80
2θ/(°C) * Corresponding author. Tel.: +86 991 8583183. E-mail address: [email protected] (Y.F. Zheng). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.03.025
Fig. 1. XRD patterns of the sample without anneal.
100
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
T
P
P
P P
P P
P
(111)
0
20
P PP M
P P
P P
P
500°C
P PP PP
P
40
P 600°C
P P
P
P P P
P P PP
P
P (333)
1000
M
300°C
(422)
P
T TT
400°C
P P P P
2000
T P
(331) (420) (124) (332)
3000 2000 1000 0
(210)
0
M
P M P
P M
M
(200)
Intensity/cps
400
P
P
(311)
P
T
(222) (023) (321)
M
0 800
(211)
400
T PT P P M
(220)
800
2399
60
80
100
2θ/(°C) Fig. 2. XRD patterns of the sample sulfidated under different temperatures (I: ITO, T: FeS, P: Pyrite, M: Marcasite).
via annealing the electrodeposited thin films in a sulfur atmosphere.
2. Experiments The electrodeposition bath consists of aqueous solution containing FeCl2I4H2O and Na2S2O3I5H2O (analytical reagents). The electrodeposition was carried out in the stirred solution potentiostatically and indium – tin oxide (ITO) substrate was chosen as cathode, the platinum sheet as anode and Ag/AgCl electrode as reference electrode. Prior to the electrodeposition, the substrate was dipping in concentrated HNO3, and then washing with doubly distilled water and rinsing with acetone. During the electrochemical deposition process, the solution was deaerated with argon. The temperature of the bath was kept at 25 -C. The current density was fixed at 0.8 mA/cm2. The pH value of the solution was adjusted by adding dilute hydrochloric acid. The annealing treatment was performed in a closed quartz tube in a sulfur atmosphere (sulfur pressure 8.0 104 Pa) for 10 h.
The temperature of annealing was varied from 200 -C to 600 -C. The shape, crystallinity and composition of the deposited thin films were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Optical absorption spectra of the films were recorded with a U-3400 spectrophotometer at ambient temperature. The glass substrate absorption was subtracted by placing a bare glass in the instrument reference channel. The carrier concentration and the Hall mobility were determined by a HL5500 Hall auto-measuring appliance in magnetic induction of 0.32 T.
3. Results and discussion 3.1. Phase and structure analysis In the X-ray diffraction profile of the sample without annealing treatment (Fig. 1), there are some intensive ITO peaks, indicating the incompactness of the electrodeposited thin films, and other peaks can be characterized as FeS.
Fig. 3. Shape of crystallite at different annealing temperatures: (a) 400 -C, (b) 500 -C, (c) 600 -C.
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
Table 1 The dependence of the grain size of the thin films at different annealing temperatures (hkl)
400 -C (nm)
500 -C (nm)
600 -C (nm)
(111) (200) (210) (211) (220) (311)
15.8 18.9 17.7 15.6 15.8 16.6
71.4 106.2 92.3 68.5 71.4 80.6
66.5 103.7 88.3 63.4 64.5 76.3
Fig. 2 shows the consequence of XRD patterns of the thin films annealed in a sulfur atmosphere at different annealing temperatures for 10 h. For comparison, the XRD pattern derived from the standard ASTM71-2219 card is also given in this figure. The thin film becomes obviously compact when annealed at 300-C, so we cannot observe the peaks of ITO substrates. At the same time, sulfuration reaction occurs and there exists some peaks of pyrite and marcasite in the XRD pattern of the thin films annealed at 300 -C but the intensity of the peaks is weakly. When annealing at a lower temperature (400 -C), insufficient sulfuration process leads to the formation of marcasite transitional phase, which has smaller bandgap (0.34 eV) than pyrite. With the anneal temperature rising from 500 -C to 600 -C, it is found from Fig. 2 that the films annealed at about 500 -C consist of a single-phased pyrite without any contributions from marcasite, and at a higher temperature (600 -C) would bring about the decline of the intensity of the pyrite peaks. Considering above parameters, the optimal anneal temperature 500 -C for formation of a single-phased pyrite can be obtained.
4. Grain size and shape of the thin films
3.0 2.9
absorption coefficient/x105 cm-1
2400
c
2.8 2.7 2.6 2.5
b
2.4 2.3 2.2 2.1
a
2.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
energy/ev Fig. 5. Optical absorption coefficient of the FeS2 thin films annealed at 500 -C.
microstrain important to XRD patterns were considered in the refinement. According to the Popa theory, the shape and the size of crystallite resulted from the refinement are the comprehensive effects of the truly X-ray diffraction of all the crystallites. From this figure, we can observe that the shape of the grain in the thin films annealed at different temperature is of octahedron basically. This is due to the fact that the growth rate along (100) crystal plane is more rapid than that of (111) crystal plane. A quantitative estimation of the grain size of the polycrystalline thin films annealed at different annealing temperature for 10 h is carried based on the refinement result. In this procedure, we have adopted standard silicon reference to extract an instrument broadening for each sample.
Fig. 3 is the grain shape of the films resulted from the Rietveld refinement according to Popa theory [12]. Several parameters, such as cell parameter, texture, crystallite size,
80 70
(αhν)2/A.U
60 50 40 30 20 10 0 1.2 1.4
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
energy/ev 2
Fig. 4. SEM micrograph of the pyrite films annealed at 500 -C.
Fig. 6. The relation between (ahr) and hr of the FeS2 thin films annealed at 500 -C.
Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402 Table 2 Band gap of pyrite by preparation techniques Technique
E gd (eV)
OS [13] EPD [14] SVP [13]
1.35 1.28 1.35
Table 1 shows the dependence of the grain size in the thin films at different annealing temperature. It can be seen from this table that the grain sizes significantly increase when the annealing temperature varies from 400 -C to 500 -C, and the maximum value is up to 106.2 nm at 500 -C, different from the value of about 130 nm determined by SEM (Fig. 4). This is in agreement with the fact that the grain size estimated from XRD pattern is generally smaller than that observed by SEM [13]. Furthermore, when the annealing temperature increases to 600 -C, the grain size decreases appreciably, however. Therefore, it can be concluded that the biggest grain sizes of 106.2 nm are when the annealing temperature is at 500 -C.
2401
The direct energy gaps E gd can be determined by plotting respectively (ahr)2 as a function of energy hr [13] as shown in Fig. 6. a is determined from transmittance spectra for the film samples. The band gap E gd = 1.34 eV has been qualitatively determined by extrapolating the straight line to the energy axis. Compared with the band gap E gd = 0.95 eV [2] of the bulk FeS2, this value from the experiment result moves 0.39 eV high. This inconsistency could be caused by the significant reduction in grain size of the thin films, which leads to the formation of quantum energy. When the grain size reduces to some value, the energy level of the system is quantized and the corresponding band gap is broadened. Takagahara [15] indicated that the electronhole confinement effect most pronounced in a semiconductor quantum dot tends to increase band gap width. A possible mechanism is proposed for the formation of pyrite. It was found that the annealing temperature performed important roles in the growth of pyrite crystals. Table 2 shows E gd obtained by several preparation methods and our conclusion is comparable to these resulted data.
4.1. Optical properties 4.2. Electric properties Fig. 5 shows the optical absorption spectra for the thin films annealed in a sulfur atmosphere at 500 -C. In this figure, the apparent absorption coefficient (a) vs. photon energy (hr) is plotted. The absorption curve generally consists of three regions: weak absorption region in the range of low photon energy, abrupt absorption region in the range of photon energy about 1.6– 2.3 eV and constant absorption region in the range of high photon energy above 2.3 eV whose absorption coefficient (a) is approximately as a constant. The abrupt absorption region indicates that the films are of typical semiconductor characteristic.
Hall measurements have revealed that all thin films of pyrite which are annealed from 300 to 600 -C are of ptype conduction. The group of the Hahn-Meitner-Institut reported that pyrite films with p-type conductivity were obtained by normal-pressure MOCVD [16], while those with n-type conduction by low-pressure MOCVD [17]. Fig. 7 shows carrier concentration and Hall mobility vs. annealing temperature relationship for pyrite thin films prepared in this study. It indicates that the film annealed at 300 -C has a rather high carrier concentration (a) (about 1019 cm 3) due to containing a lot of FeS which is 250
3.00E+019
(a) 200
2.98E+014
150
2.96E+014 100 2.94E+014 50
(b)
2.92E+014
Hall mobility/(cm2/v−s)
Carrier concentration/cm-3
2.50E+019
2.90E+014
0 300
350
400
450
500
550
600
Annealing temperature/°C Fig. 7. The dependence of both Carrier concentration and mobility upon annealing temperature. (a) Carrier concentration; (b) Hall mobility.
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Y.Z. Dong et al. / Materials Letters 59 (2005) 2398 – 2402
a metal. The carrier concentration decreases gradually from 2.92 1014 to 2.91 1014 cm 3 when the annealing temperature ranges from 400 -C to 600 -C. The carrier concentration of the thin films annealed at 400 -C is higher due to containing a small quantity of marcasite with a band gap E gd = 0.34 eV than the thin films annealed at 500 -C with a single-phase pyrite. The Hall mobility increases markedly with the rising of annealing temperature below 500 -C and a slow decrease above 500 -C. When annealing temperature ranges from 300 -C to 500 -C, crystallization of grains tends to be perfect, defect amount decreases and region of grain boundaries becomes more and more narrow. According to the grain boundary model [18,19], the decreased defect concentration leads to the lowering of potential barrier at the grain boundaries and the decreased density of charge scattering centers from the point defect results in the rise in Hall mobility. On the other hand, the thin films annealed at a higher temperature have enlarged grain size, less grain boundaries and more continuous surface, which could impair the capture of charge carriers and therefore a higher Hall mobility has been obtained in our study. When annealing temperature continue to rise up to 600 -C Hall mobility decreases appreciably owing to surplus sulfur in the thin films with increased defects.
5. Conclusion The electrodeposition of iron disulfide pyrite (FeS2) thin films on Indium –tin oxide (ITO) substrates in aqueous solution containing Na2S2O3I5H2O, FeCl2I4H2O reagents was performed in this work. It is found that annealing treatment for the deposited thin films in a sulfur atmosphere at optimal temperature is very necessary for formation of a single-phase pyrite thin film. The shape of the grain of the samples after annealing treatment is approximately octahedral and the maximum grain size is 106.2 nm. All pyrite thin films annealed from 300 to 500 -C are all of p-type conduction, and the crystallization of grains tends to be nearly perfect. Thus, the thin films here have ideal carrier concentration about 1014 cm 3 and show the remarkable increment in Hall mobility with the increase of annealing temperature. However, when annealing temperature rises to 600 -C Hall mobility decreases
appreciably owing to increased the mount of defect. Therefore, the most appropriate annealing temperature 500 -C can be concluded, and the direct E gd of the resulting thin film is 1.34 eV.
Acknowledgements This work was supported financially by the National Natural Science Foundation of the Chinese Government, under Grant No. 50062002).
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