Journal Information
Vol. 8. Issue 6.
Pages 5909-5915 (November - December 2019)
Share
Share
Download PDF
More article options
Visits
...
Vol. 8. Issue 6.
Pages 5909-5915 (November - December 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.09.065
Open Access
Electrical and morphological characterization of zinc-doped α- Fe2O3 thin films at different annealing temperature
Visits
...
Ibn Shahjahann Arman
Corresponding author
swaran.apeeru@gmail.com

Corresponding author.
, Md. Ataur Rahman, Abu Bakar Md. Ismail
Department of Applied Physics and Electronic Engineering, University of Rajshahi, Rajshahi 6205, Bangladesh
Article information
Abstract
Full Text
Bibliography
Download PDF
Statistics
Figures (9)
Show moreShow less
Tables (6)
Table 1. The resistivity variation of different layer at 450°C temperature with different molar concentration.
Table 2. The resistivity variation of different layer at 550°C temperature with different molar concentration.
Table 3. Stability of Semiconductor Type.
Show moreShow less
Abstract

α Fe2O3 is naturally n-type material. Zn was used to dope α-Fe2O3 (Zn: Fe2O3) to turn it into p-type. The thin film of Zn: Fe2O3 was obtained by spin coating a blend of Zinc acetate (Zn (CH3COO)2 2H2O) and FeCl2 after annealing the film in air. Zn-doped-Fe2O3 was found as a p-type semiconductor by Hall Measurement. Effect of annealing at 450°C and 550°C temperature on electrical properties were also investigated. The resistivity of those films was found to be between 1 and 10Ω-m. It was observed that the resistivity of the films increases with increasing temperature for most of the sample. It was also observed that the resistivity of the film is inversely proportional to the increasing number of film layer and thickness within most of the sample. Atomic Force Microscopy (AFM) was used to observe the surface morphology of the annealed film on glass substrate. The variation of the average roughness of annealed film on the glass substrate was from 60nm to 98nm and the thickness was 160nm to 540nm.

Keywords:
Thin film
Zn-doped-Fe2O3
Resistivity
Full Text
1Introduction

Non-conventional energy sources have a great significance for the world’s future, given the environmental issues related to energy generation and energy’s importance in our society. To overcome a predictable energy crisis, there are intense researches on alternative energy harvesting techniques [1–3]. Solar energy is a good solution for these energy crises [4]. Hematite (α-Fe2O3) is an attractive material for PEC (Photoelectrochemical) studies due to its ban-gap (Eg=2.2eV), lying nearly in the optimum range for the solar splitting of water. Based on this band gap, solar energy has absorbed were sun emits maximum energy. Besides, α-Fe2O3 is naturally abundant in the earth’s crust and is, therefore a low-cost material. It is also corrosion-resistant in acidic and alkaline medium [5–10]. One major drawback of Fe2O3 is its unfavourable photo-response that arises mainly due to its low conductivity and consequent recombination of photo-generated carriers. Moreover to make a homo-junction of Fe2O3, which is naturally, n-type, p-type conduction in Fe2O3 is needed that is long-term stability.

After the first report of Fujishima on photo-assisted electrochemical water oxidation on TiO2 in 1972 [11] researchers have been investigating the wide bandgap metal-oxide semiconductors such as titanium dioxide (TiO2), zinc oxide (ZnO), iron oxide like hematite (α-Fe2O3) and tin dioxide (SnO2) for their application in solar cells, solar fuel, photocatalysis and energy storage devices [12] . In our recent past, Photoresponse of p-Type Zn:Fe2O3 Oxide Thin Films has been investigated by William B. Jr, John P and U. M. Khan [13] .

One of the fundamental aim of this research to make p-type Zn doped Iron(III) Oxide thin films, analyzing their electrical properties and to learn the fabrication which can be used as a solar cell or homojunction material for any kinds of semiconductor device in future. The present communication describes the PEC study on Zn-doped α-Fe2O3 thin films prepared at different doping concentrations (0.1M, 0.005M and 0.0025M). Resistivity was found to depend upon the doping concentration of Zn and thickness. For thin film deposition, there are various kinds of deposition methods like Chemical vapour deposition, Spin coating, Thermal evaporation, Dip coating. Spray pyrolysis etc [14–16]. Here we have applied spin coating methods in this research project, which is one of the easiest techniques for preparing thin films [17]. In this research Zn:Fe2O3 thin films with different doping concentrations were prepared by the spin coating method and doping effects of these films after annealing were also studied. After synthesis Zn:Fe2O3 solution is taken by micropipette and drop cast on glass surface then spin coated.

2Experimental methodology2.1Solution preparation for doping

The p and n-type semiconductor were separately prepared by a simple one-pot spin coating method on ultrasonically cleaned 1×1inch sizes glass substrates. The substrates were also chemically cleaned with the piranha solution (5:1 H2SO4 and H2O2 mixing) for the increase of hydrophilic properties in which enhance water-compatibility. After that, the dried substrates with atmospheric air were available for the use.

0.11 M FeCl2 Solution Preparation:

Ethanol  100ml 
FeCl2  1.3943

0.01M (CH3COO)2Zn.2H2O(Zinc acetate) Solution Preparation:

Ethanol  100ml 
(CH3COO)2 Zn.2H2219.527mg 

When the two solutions were mixed and stirred for 10min. After that Zn doped α-Fe2O3 solution had prepared. In this way, 0.005M and 0.0025M Zn doped α-Fe2O3 solution had prepared. After synthesizing Fe2O3, 5μL of the solution was taken by micropipette and dropped on loaded substrate surface then spin-coated with the following rotational speed and time:

SP1  T1  SP2  T2 
500rpm  37000rpm  30

2.2Annealing

Two sets of the sample were annealed after deposition at two different temperature (4500C & 5500C) for 4h in a thermal annealing furnace (Carbolite CWF-1200). Then they were left to be cooled naturally to the room temperature.

2.3Studies with atomic force microscope (AFM)

Atomic force microscope (AFM) [18] of model (XE 70) park system was used to study the roughness (nm) [19] of the all film surface.

3Results and discussion3.1X-ray diffraction (XRD) analysis

Fig. 1 shows the XRD patterns of Zn-doped Fe2O3. The diffraction peaks at various 2-θ values can be matched to α- Fe2O3 (a=0.5036nm, c=1.3749nm, rhombohedral corundum-type phase, JCPDS no. 86-2368) with slight shift to the left of (104) and (110) face reflection peaks. Similar crystallization has been also reported [20,21]. It indicates that Zn is highly dispersed in the matrix of crystalline α- Fe2O3.

Fig. 1.

XRD 2-θ scan of Zn-doped Fe2O3 film.

(0.13MB).
3.2Resistivity measurement

The electrical resistivity of Zn:Fe2O3 semiconductor measured by using Van Der Pauw’s’ technique [22,23]. The resistivity variation of different layer Zn doped α-Fe2O3 thin film at different temperature and different molar concentration are shown in the following table: Tables 1 and 2

Table 1.

The resistivity variation of different layer at 450°C temperature with different molar concentration.

No. of layer  0.01M Resistivity (Ω-m)  0.005M Resistivity (Ω-m)  0.0025M Resistivity (Ω-m) 
Layer 1  10.8  2.2  1.7 
Layer 3  8.0  1.8  1.1 
Layer 5  4.0  1.1  0.9 
Table 2.

The resistivity variation of different layer at 550°C temperature with different molar concentration.

No. of layer  0.01M Resistivity (Ω-m)  0.005M Resistivity (Ω-m)  0.0025M Resistivity (Ω-m) 
Layer 1  8.2  6.0  0.7 
Layer 3  7.0  2.9  0.6 
Layer 5  7.8  3.1  1.9 

It was observed that the resistivity of annealed films increases with temperature. The increase in resistivity was not linear. It was seen that the resistivity of the films decreased with decreasing the molar concentration.

Resistivity variation of different layer Zn doped α-Fe2O3 thin film at a different temperature and different molar concentration are shown in Figs. 2–4.

Fig. 2.

Resistivity of different molar concentration 0.01M, 0.005M and 0.0025M and different layer at temperature 450°C.

(0.13MB).
Fig. 3.

Resistivity of different molar concentration 0.01M, 0.005M and 0.0025M and different layer at temperature 550°C.

(0.1MB).
Fig. 4.

Resistivity of different layer and different temperature at 0.01M molar concentration.

(0.14MB).

Resistivity at different temperature and different molar concentration in layer 5 as shown in Fig. 5.

Fig. 5.

Resistivity at different temperature and different molar concentration in layer 5.

(0.11MB).

The stability of semiconductor type and carrier concentration were also investigated which is depicted in Table 3. It is seen from the table that in six month time semiconductor type changed to n-type except for Zn:Fe2O3 doped through 0.005M of Zn-acetate.

Table 3.

Stability of Semiconductor Type.

Zn-acetate concentration  As deposited carrier concentration (×1014 1/cm3)  Conduction type  Carrier concentration (×1014 1/cm3) after 6 month  Conduction type 
0.00253.2  P-type  0.71  N-type 
0.0054.75  P-type  2.03  P-type 
0.012.23  P-type  0.56  N-type 
3.3Surface morphology study using AFM

The thickness of the Zn doped α-Fe2O3 thin film varied with annealing temperature and a number of the film layer. Following Fig. 6 has shown the thickness (nm) variation of Zn doped α-Fe2O3 thin film with temperature:

Fig. 6.

Thickness variation of the thin film layer at different annealing.

(0.09MB).

The thickness of the film has been increased with increasing the number of the film layer and decreased with the increase of annealing temperature. This was due to the fact that increasing of annealing temperature reduces various types of defects like voids, crystal defects and dislocation density.

The relation between annealing temperature & the thin film surface roughness was inversely proportional. Variation of the average value of Roughness of Zn doped α-Fe2O3 at 450°C, 550°C and AFM images are shown in Figs. 7–9.

Fig. 7.

Average value of Roughness (nm) of the film surface of different layer at different temperature of the thin film.

(0.07MB).
Fig. 8.

Surface morphology of Zn doped α-Fe2O3 Layer 5 at 450°C near to the Edge (a) and middle of the surface (b).

(0.41MB).
Fig. 9.

Surface morphology of Zn doped α-Fe2O3 Layer -5 at 550°C near to the Edge (a) and middle 0f the surface (b).

(0.38MB).
4Conclusions

The effect of different parameters (annealing temperature, thickness and molar concentration) on the electrical property of the spin-coated thin film of Zn:Fe2O3 on the glass substrate has been investigated. It was observed that the resistivity of annealed films increases with temperature. The increase in resistivity was not linear. It was seen that the Resistivity of the films decreased with decreasing the molar concentration. At higher Zn-acetate concentration this may happen because of lower rate of conversion of Zn-acetate into Zn by dissociation into Zn+2 and 2(CH3COO)- followed by hydrolysis. Similar effect has been reported for the conversion of CuSO4 to Cu+2 by potassium borohydride [24]. Another reason may be the solid solubility limit of Zn into of Fe2O3.

The semiconductor type was not stable for very long time, their properties were converted from p-type to n-type as well as the carrier concentration were degraded with the time to leave due to their solid format of zinc. But the Zn:Fe2O3 doped at 0.005M of Zn-acetate retained its p-type conduction for comparatively long time. It was also observed that the resistivity of the thin film decreases with increasing of film thickness (increasing number of film layer). Films with lower thickness may have defects like lattice defects, voids etc. These defects significantly contribute to resistivity. When the film thickness was increased more atoms are deposited on film surface which reduces the defects (voids, crystal defects [25] and dislocation density [26]) on film. As a result, resistivity decreases.

The surface of Zn:Fe2O3 was investigated with atomic force microscope (AFM) to see the effect of annealing on surface roughness. It was observed that the surface roughness and thickness was reduced after increasing the annealing temperature. This was due to the fact that increasing of annealing temperature reduces various types of defects like voids, crystal defects and dislocation density. So the film becomes more smooth which was expected for thin film.

Acknowledgements

The authors gratefully acknowledge the support from the M.R. Sarkar Thin Film & Solar Energy laboratory, Department of Applied Physics and Electronic Engineering, University of Rajshahi, Rajshahi, Bangladesh.

References
[1]
S. Chen, L. Shen, S.S. Guo, Mao.
Semiconductor-based photocatalytic hydrogen generation.
Chem Rev, 110 (2010), pp. 6503
[2]
P. Zhao, C.X. Kronawitter, X. Yang, J. Fu, B.E. Koel.
WO3–α-Fe2O3 composite photoelectrodes with low onset potential for solar water oxidation.
Phys Chem Chem Phys, 16 (2014), pp. 1327
[3]
Y.K. Hsu, Y.C. Chen, Y.G. Lin.
Novel ZnO/Fe2O3 core–shell nanowires for photoelectrochemical water splitting.
ACS Appl Mater Interfaces, 7 (2015), pp. 14157
[4]
S. Chinnammai.
A study on energy crisis and social benefit of solar energy.
Int J of Envi Sci and Dev, 104 (2014), pp. 406-411
[5]
E. Murad, J. Cashion.
Iron Oxides.
Mössbauer Spectroscopy of Environmental Materials and Their Industrial Utilization, (2004), pp. 159-188
[6]
Y. Izumi.
A critical review of CO2 photoconversion: catalysts and reactors.
Coord Chem Rev, 257 (2013), pp. 171
[7]
L. Mai, X. Tian, X. Xu, L. Chang, L. Xu.
Nanowire electrodes for electrochemical energy storage devices.
Chem Rev, 114 (2014), pp. 11828
[8]
I.A. Raid, Y. Najim, M. Ouda.
Spray pyrolysis deposition of α Fe2O3 thin film.
e-J Surf Sci Nanotech, 6 (2008), pp. 96-98
[9]
L. Jing, W. Zhou, G. Tian, H. Fu.
Surface tuning for oxide-based nanomaterials as efficient photocatalysts.
Chem Soc Rev, 42 (2013), pp. 9509
[10]
S. Kumari, C. Tripathi, A.P. Singh, D. Chauhan, R. Shrivastav, S. Dass, et al.
Characterization of Zn-doped hematite thin films for photoelectrochemical splitting of water.
Cur Sci, 91 (2006), pp. 1062
[11]
A. Fujishima, K. Honda.
Electrochemical photolysis of water at a semiconductor electrode.
Nature, 238 (1972), pp. 37
[12]
J.W. Edward, Crossland, N. Noel, V. Sivaram, T. Leijtens, J.A. Alexander Webber, et al.
Mesoporous TiO2 single crystals delivering enhanced mobility and optoelectronic device performance.
Nature, 495 (2013), pp. 215
[13]
WBI Jr, J.P. Baltrus, S.U.M. Khan.
Photoresponse of p-Type zinc-doped iron(III) oxide thin films.
J Am Chem Soc, 126 (2004), pp. 10238
[14]
S.O. Kasap.
Electrical Engineering Materials and Devices.
2nd Ed, (2001),
[15]
B.G. Streetman.
Solid State Electronic Devices.
Prentice Hall, (2019),
[16]
S. Tolansky.
Multiple Beam Interferometry of surfaces and films.
Oxford University Press, (2019),
[17]
J.G. Gotlling, W.S. Nicol.
Dependence of image quality on horizontal range in a turbulent.
J Opt Soc Am, 56 (1966), pp. 1227
[18]
J.D. Miller, S. Veeramasuneni, J. Drelich, M.R. Yalamanchili, G. Yamauchi.
Effect of roughness as determined by atomic force microscopy on the wetting properties of PTFE thin films.
Poly Engg & Sci, 36 (1996), pp. 1849
[19]
J.L. Hutter, J. Bechhoefer.
Calibration of atomic‐force microscope tips.
Rev Sci Instrum, 64 (1993), pp. 1868
[20]
A. Kelly, K.M. Knowles.
Crystallography and Crystal Defects.
John Wiley & Sons, (2019),
[21]
P.P. Sarangi, B. Naik, N.N. Ghosh.
Low temperature synthesis of single-phase α-Fe2O3 nano-powders by using simple but novel chemical methods.
Powder Tech, 192 (2009), pp. 245
[22]
L.J. van der Pauw.
A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape.
Philips Res Rept, 13 (1958), pp. 1
[23]
A.A. Ramadan, R.D. Gould, A. Ashour.
On the Van der Pauw method of resistivity measurements.
Thin Solid Film, 239 (1994), pp. 272
[24]
Q.L. Zhang, Z.M. Yang, B.J. Ding.
Preparation of copper nanoparticles by chemical reduction method using potassium borohydride.
Trans Nonferrous Met Soc, 20 (2010), pp. 240
[25]
W. Bollmann.
Crystal Defects and Crystalline Interfaces.
(1961), pp. 37-142
[26]
A. Kelly, K.M. Knowles.
Crystallography and Crystal Defects.
2nd Ed, (2012), pp. 241-332
Copyright © 2019. The Authors
Journal of Materials Research and Technology

Subscribe to our newsletter

Article options
Tools
Cookies policy
To improve our services and products, we use cookies (own or third parties authorized) to show advertising related to client preferences through the analyses of navigation customer behavior. Continuing navigation will be considered as acceptance of this use. You can change the settings or obtain more information by clicking here.