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Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method
Ceramics International (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method ⁎
Charles Moditswea, , Cosmas M. Muivaa, Pearson Luhangab, Albert Jumaa a b
Department of Physics and Astronomy, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana Department of Physics, University of Botswana, Private Bag UB0704, Gaborone, Botswana
A R T I C L E I N F O
A BS T RAC T
Keywords: CuI thin films Thermal evaporation Annealing temperature Structural properties Optoelectronic properties
High quality transparent conducting CuI thin films were deposited at room temperature via thermal evaporation technique followed by post deposition annealing at different temperatures. The samples were characterised by X-ray diffraction (XRD), UV–Vis spectrophotometry, Scanning electron microscopy and I-V measurements. The structural, morphological and optical properties were studied as a function of the annealing temperature from room temperature (RT) to 200 °C. XRD results revealed that the films were polycrystalline with zinc blende structure of cubic phase. Increasing the annealing temperature increased the crystallite size from 33 to 49 nm whilst the dislocation density and lattice strain shifted to lower values. High transmittance of about 70–80% was exhibited by all films in the entire visible spectral range. The as deposited film possesed the lowest resistivity of 3.0×10−3 Ω cm.
1. Introduction Transparent conductors (TCs) are very important for applications in optoelectronic devices. For a conductor to be transparent in the visible range it must have a band gap of around 3 eV [1]. A lot of research progress has been achieved for n-type TCs, particularly ITO, FTO, CdS, ZnO have received a lot of attention. Despite this progress, reports on their p-type counterparts remain scanty and only a few (CuAlO2, CuO, CuSCN) are known [2,3]. The realisation of transparent junction devices requires both p-type and n-type TCs, as such several studies are being done to attain more and better p-type TCs. CuI is one of the promising non-oxide p-type wide band gap materials with the potential for application in solar cells as hole collectors, light emitting diodes, field emission displays and organic catalyst [4–6]. CuI is a halogenide semiconductor which exists in three crystalline phases that are temperature dependent; zinc blende structure (γ-CuI) for temperatures below 370 °C; wurtzite structure (β-CuI) between 370 and 400 °C, and rock salt structure (α-CuI) for temperatures higher than 400 °C [7]. The zinc blende structure of CuI attainable at low temperatures possesses p-type conductivity. CuI has a band gap of 2.3– 3 eV [8]. The valence band edge of CuI is made from the Cu 3d states (ᴦ15,2) and I 5p states (ᴦ12,1) [9]. Several methods have been used to deposit CuI thin films which include SILAR method [7], thermal evaporation [10], RF-DC coupled magnetron sputtering technique [11], simple complex compound [12],
⁎
ethanol thermal method [13], pulse laser deposition [9] and spray pyrolysis [14]. In these studies thermal evaporation method was used because of its advantages over the other methods which includes catalyst–free growth, simplicity, easy control of deposition parameters (thickness, growth rate etc.), and cost effectiveness [15]. Few studies on CuI thin films can be found in literature. In this paper CuI thin films were fabricated using thermal evaporation and thereafter annealed at different temperatures in open atmosphere. The detailed structural, optical and electrical analyses are therefore furnished in this paper. This paper will be a great addition to the pool of knowledge regarding the p-type TCs. 2. Experimental details 2.1. Sample preparation Marienfeld microscope glass slides measuring 20×15×1 mm were used as substrates for hosting the thin films. Prior to deposition the glass slides were cleaned as described elsewhere [16]. First CuI powders were prepared from potassium iodide (KI) and copper nitrate trihydrate salts (Cu(NO3)2·3H2O). All chemicals were of analytical grade and were used as received from Sigma Aldrich. Aqueous solutions of 0.5 M KI and Cu(NO3)2·3H2O) were made separately using double distilled water as the solvent. The KI solution was added to Cu(NO3)2·3H2O solution and a reddish brown precipitate was formed.
Corresponding author. E-mail address: [email protected] (C. Moditswe).
http://dx.doi.org/10.1016/j.ceramint.2017.01.026 Received 23 November 2016; Received in revised form 23 December 2016; Accepted 6 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Moditswe, C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.026
Ceramics International (xxxx) xxxx–xxxx
C. Moditswe et al.
The precipitate was filtered and washed several times with double distilled water to improve the purity of CuI powder. The obtained powders were dried in an oven at 100 °C for a period of 3 h. The CuI formation can be explained by the following chemical reaction: 2Cu(NO3)2·3H2O(aq) + 4KI(aq) →2CuI(s) + I2
(g)
Table 1 Lattice parameters and mechanical properties of CuI thin films annealed at different temperatures.
+ 4KNO3(aq) + 3H2O(l)
The powders were deposited using Polaron E6700 Turbo Molecular Bench Top Vacuum Evaporator at ambient temperature. Glass substrates were mounted on the tilt/rotating sample stage directly above the evaporation boat at a distance of 10 cm. The evaporation was done at conventional low voltage (LT) and high alternating current (AC) supply of 20 V and 50 A, respectively. A measured amount of CuI powder was put in the evaporation boat. The deposition chamber was evacuated to a base pressure of 10−5 mbar. The source current was increased until a vapour was formed which condensed on the glass substrates to form the desired film. Deposition was done for 2 s from a custom made boat with large surface area. The deposition time was maintained by the use of a switch. In order to get thicker films of around 550 nm the process was repeated three times. After deposition the chamber was allowed to vent to atmospheric pressure before removing the samples, it took roughly 20 min to vent. The obtained films were annealed at different temperatures for a duration of 1 h using OF-02 G forced convection oven.
Annealing temperature (°C)
Lattice parameter a (Å )
Strain in the a-axis, ɛzz (%) ×10−1
FWHM (°)
Crystallite size (nm)
Dislocation density, ρ (line/m2) ×1014
RT 50 100 150 200 Bulk CuI
6.082 6.082 6.081 6.080 6.079 6.063
3.13 3.13 2.97 2.80 2.64 0
0.276 0.220 0.214 0.189 0.185 –
32.81 41.05 42.37 47.79 49.20 –
9.29 5.93 5.57 4.38 4.13 –
structure of cubic phase. The (111) peak is the most dominant peak in the entire temperature range showing that all films are highly oriented along the (111) plane. The dominancy of the (111) face is associated with its low surface energy [18]. The intensity of the dominant peak is observed to increase with annealing temperature. The increased intensity of the (111) peak signifies the improvement of crystallinity with annealing temperature which may be attributed to the increase in the mobility of the adatoms with annealing temperature [19]. The crystallite size was determined from the XRD data using the Debye-Scherrer equation [20].
2.2. Characterisation
D=
The crystal structure of the films was characterised using an X-ray diffractometer (Model D8 Advance, Bruker, Germany) with CuKα radiation (λ=0.154056 nm) at 40 mA and 40 kV. The diffraction angle 2θ range was 20–70° with an increment of 0.02869° and the time per step of 0.2 s. The surface morphology of the thin films was investigated using a Scanning electron microscope (JSM-7100F, JEOL, Japan). The electrical measurements (I-V curves) were collected using a Signatone four point probe equipped with two Keithley electrical meters at room temperature. Optical transmittance data was obtained using UV/Vis/ NIR spectrophotometer (Varian Cary 500, USA) in the wavelength range of 300–800 nm. The thicknesses of the thin films were determined by a surface profilometer (KLA Tencor, D-100, USA).
0. 94λ βcosθ
(1)
Where D is the crystallite size in nm, β is the full width at half maximum (FWHM) in degrees, λ is wavelength of the incident radiation (λ=1.5406 Å ) and θ is the Bragg's diffraction angle. The FWHM was found to decrease from 0.276 to 0.184° as given in Table 1 which corresponds to the increasing crystallite size from 32.81 to 49.20 nm. The increase in crystallite size is believed to be a product of coalescence of small grains due to an increased kinetic energy of the grains owing to increased annealing temperature [19]. Dislocation density (δ) which is defined as the number of dislocation lines per unit volume of the crystal was estimated from the following formula [21]:
3. Results and discussion
δ=
3.1. Structural studies of the films
1 D2
(2)
where D is the crystallite size in nm. Dislocation density was also observed to decrease with annealing temperature as shown in Table 1. Lattice parameter (a=b=c for cubic structure) was determined from the following equation [22]:
Fig. 1 presents XRD patterns for the CuI thin films annealed at different temperatures (RT to 200 °C). Several reflection peaks are observed at 2θ values of 25.42°, 29.44°, 42.12°, 49.84° and 52.22° which corresponds to (111), (200), (220), (311) and (222) planes, respectively and are all indexed to JCPDS data file 06-0246 [17]. The observed pattern shows that the films crystallized in zinc blende
1 h2 + k 2 + l 2 = 2 a2 dhkl
(3)
Where (h, k, l) are Miller indices and dhkl is the interplanar spacing. The lattice parameter a was found to be almost equal to that of bulk CuI obtained from JCPDS data file 06-0246 [17] as shown in Table 1. The slight difference between observed and bulk CuI lattice parameter is attributed to strains in the film. The lattice strain along the a-axis, ɛzz was calculated from the following equation [23,24]:
ɛzz =
(a − a 0 ) a0
×100
%
(4)
Where a and a0 are the observed (strained) and bulk (strain free) lattice parameters, respectively. The observed lattice parameters in Table 1 are elongated meaning that the films are under tensile strain. The tensile strain is observed to decrease with increase in annealing temperatures. It can now be concluded that increasing annealing temperatures improved the crystallinity and lattice relaxation of the
Fig. 1. The XRD patterns of the CuI thin films at different annealing temperatures.
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Fig. 2. SEM images of (a) not annealed and annealed CuI thin films at various annealing temperatures of (b) 50 °C, (c) 100 °C, (d) 150 °C, and (e) 200 °C.
CuI thin films. The pointers of improved crystallinity in this case were the decrease in both dislocation density and lattice strain and an increase in the crystallite size. Improvement in crystallinity with increased annealing temperature is attributed to the recrystallization process at higher annealing temperatures [19].
grain size with increased annealing temperature is observed. The increase in grain size is associated with the increase in surface mobility of the adatoms [25]. The increased grain size is consistent with the results obtained in XRD studies. 3.3. Optical studies
3.2. Morphological studies Fig. 3 shows optical transmittance spectra of CuI thin films annealed at different temperatures in the wavelength range 300– 800 nm. All films are highly transparent in the visible range with an average transmittance of about 70–80%. A sharp absorption edge is observed around 411 nm. Interference patterns were observed in all films indicating that the surfaces of the films were homogenous and
Surface morphologies of the thin films are shown in 2D SEM micrographs at a magnification of × 40, 000 as shown in Fig. 2. All images show compact and dense films with spherical shaped grains. It is evident from the images that the surface morphology is greatly influenced by the annealing temperature. A gradual increase in the 3
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Fig. 3. Transmission spectrum of CuI thin films at different annealing temperatures. The inset shows the absorption spectra of CuI thin films.
Fig. 5. Plot of dT/dλ versus wavelength for CuI thin films at different annealing temperatures. The inset shows a zoom in the peak.
specular in nature [16]. A hump is observed in the absorption spectrum at around 410 nm for all the films (insert of Fig. 3). The hump is attributed to the excitation of electrons from sub bands in the valence band to the conduction band [9]. The absorption coefficient (α) was calculated from the BeerLambert relation [26]:
Table 2 Optical band gap and Urbach energies as a function of annealing temperature.
1 ⎡1⎤ α= ln ⎢ ⎥ t ⎣T ⎦
(5)
where α is the absorption coefficient, T is the transmittance and t is the film thickness in nm. The optical band gaps were calculated from the following equation [27]:
(αhν )2 = A (hν − Eg )
Annealing temperature (°C)
Band gap from Tauc plot (eV)
Band gap from dT/dλ (eV)
Urbach energy (meV)
RT 50 100 150 200
3.01 3.01 3.01 3.01 3.01
3.02 3.02 3.02 3.02 3.02
42 41 38 36 34
disorder in the film were determined using the following formula and the obtained values are given in Table 2 [28]:
⎛ hν ⎞ α = α0exp ⎜ ⎟ for hν ⎝ EUrb ⎠
(6)
Where hν is the photon energy, A is a constant and Eg is the optical band gap. To determine the band gaps a Tauc plot, (αhν)2 versus hν, was plotted and tangent of the linear part was extrapolated to the energy axes to give the band gap value (Fig. 4). All the band gap values were found to be 3.01 eV for different annealing temperatures. Zi et.al also reported a constant band gap of 3.04 eV with increasing temperature for CuI thin films by vacuum thermal evaporation [17]. Band gap values were also determined from the position of the maximum of the first derivative of transmittance with respect to wavelength (dT/dλ) as shown in Fig. 5 [16]. The derivative method also gives a constant band gap value of 3.02 eV with different annealing temperatures which conforms well with the constant value (3.01 eV) obtained from the Tauc plot method as shown in Table 2. The Urbach tail energies EUrb which denotes the degree of the
<
Eg
(7)
i.e
⎡ dlnα ⎤−1 EUrb=⎢ ⎥ ⎣ dhν ⎦
(8)
where αo is the pre-exponential factor. The values of EUrb were obtained from the slope of the ln (α) against hν (Fig. 6) near the absorption edge. On increasing the annealing temperature from RT to 200 °C the Urbach energy is found to decrease from 42 to 34 meV as shown in Table 2. The decrease in the Urbach energy substantiates the fact that the crystallinity is improving with the increase in annealing temperature and correlates well with results observed in XRD analysis. It can be seen that increased crystallinity decreased the disorder in the
Fig. 6. Plot of ln α versus hν for CuI thin films deposited at different annealing temperatures.
Fig. 4. Tauc plot for the CuI thin films annealed at different temperatures.
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resistivity (highest conductivity) and hence the highest FOM, which makes them suitable for transparent electronics and solar cell applications. Acknowledgements The authors would like to extend their appreciation to Botswana International University of Science and Technology (BIUST) for financing this work. The authors are also grateful to the University of Botswana, Physics Department for access to equipment used for thin film fabrication, electrical and optical characterisation under grant R1068. References [1] J. Robertson, P.W. Peacock, M.D. Towler, R. Needs, Electronic structure of p-type conducting transparent oxides, Thin Solid Films 411 (2002) 96–100. [2] K. Tennakone, G.R.R.A. Kumara, I.R.M. Kottegoda, V.P.S. Perera, G.M.L.P. Aponsu, K.G.U. Wijayantha, Deposition of thin conducting films of CuI on glass, Sol. Energy Mater. Sol. Cells 55 (1998) 283–289. [3] S.L. Dhere, S.S. Latthe, C. Kapenstein, S.K. Mukherjee, A. Venkateswara Rao, Comparative studies on p-type CuI grown on glass substrate by SILAR method, Appl. Surf. Sci. 256 (2010) 3967–3971. [4] J.-H. Lee, D.-S. Leem, J.-J. Kim, High performance top-emitting organic lightemitting diodes with copper iodide-doped hole injection layer, Org. Electron. 9 (2008) 805–808. [5] P.M. Sirimanne, T. Soga, T. Jimbo, Identification of various luminescence centers in CuI films by cathodoluminescence technique, J. Lumin. 105 (2003) 105–109. [6] V.P.S. Perera, K. Tennakone, Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector, Sol. Energy Mater. Sol. Cells 79 (2003) 249–255. [7] R.N. Bulakhe, N.M. Shinde, R.D. Thorat, S.S. Nikam, C.D. Lokhande, Deposition of copper iodide thin films by chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) methods, Curr. Appl. Phys. 13 (2013) 1661–1667. [8] S. Badyopadhyaya, S. Chaudhuri, A.K. Pal, Synthesis of CuInS2 films by sulfurization of Cu/In stacked elemental layers, Sol. Energy Mater. Sol. Cells 60 (2000) 323–339. [9] P.M. Sirimanne, M. Rusop, T. Shirata, T. Soga, T. Jimbo, Characterization of transparent conducting CuI thin films prepared by pulse laser deposition technique, Chem. Phys. Lett. 366 (2002) 485–489. [10] S. Shi, J. Sun, J. Zhang, Y. Cao, A novel application of the CuI thin film for preparing thin copper nanowires, Physica B 362 (2005) 231–235. [11] T. Tanaka, K. Kawabata, M. Hirose, Transparent conductive CuI films prepared by rf-dc coupled magnetron sputtering, Thin Solid Films 281,282 (1996) 179–181. [12] Y.Y. Zhou, M.K. lü, G.J. Zhou, S.M. Wang, S.F. Wang, Preparation and photoluminescence of γ-CuI nanoparticles, Mater. Lett. 60 (2006) 2184–2186. [13] Y. Liu, J. Zhan, J. Zeng, Y. Qian, K. Tang, W. Yu, Ethanolthermal synthesis to γ-CuI nanocrystals at low temperature, J. Mater. Sci. Lett. 20 (2001) 1865–1867. [14] M.N. Amalina, Y. Azilawati, N.A. Rasheid, M. Rusop, The properties of copper (I) iodide (CuI) thin films prepared by Mister Atomizer at different doping concentration, Procedia Eng. 56 (2013) 731–736. [15] F.H. Alsultany, Z. Hassan, N.M. Ahmed, Control growth of catalyst-free ZnO tetrapods on glass substrate by thermal evaporation method, Ceram. Int. 42 (2016) 13144–13150. [16] C. Moditswe, C.M. Muiva, A. Juma, Highly conductive and transparent Ga-doped ZnO thin films deposited by chemical spray pyrolysis, Opt. – Int. J. Light Electron Opt. 127 (2016) 8317–8325. [17] M. Zi, J. Li, Z. Zhang, X. Wang, J. Han, X. Yang, Z. Qiu, H. Gong, Z. Ji, B. Cao, Effect of deposition temperature on transparent conductive properties of γ-CuI film prepared by vacuum thermal evaporation, Physica Status Solidi A 212 (2015) 1466–1470. [18] Z. Zheng, A.R. Liu, S.M. Wang, B.J. Huang, K.W. Wong, X.T. Zhang, S.K. Hark, W.M. Lau, Growth of highly oriented (110) CuI films with sharp exiton band, J. Mater. Chem. 18 (2008) 852–854. [19] A. Bouhdjer, A. Attaf, H. Saidi, Y. Benkhetta, M.S. Aida, I. Bouhaf, A. Rhil, Influence of annealing temperature on In2O3 properties grown by an ultrasonic spray CVD process, Opt. – Int. J. Light Electron Opt. 127 (2016) 6329–6333. [20] S. Tewari, A. Bhattacharjee, Structural, electrical and optical studies on spraydeposited aluminium-doped ZnO thin films, Pramana J. Phys. 76 (2011) 153–163. [21] G.B. Williamson, R.C. Smallman, Dislocation densities in some annealed and coldworked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1 (1956) 34–46. [22] A.S. Vasudeva, Concepts of Modern Engineering Physics, 3rd edition, S. Chand & Company Ltd, New Delhi, 2012. [23] Selected Powder Diffraction Data for Metals and Alloys, JCPDS, USA, 1 (1978) 108. [24] R. Ghosh, D. Basak, S. Fujihara, Effect of substrate-induced strain on the structural, electrical, and optical properties of polycrystalline ZnO thin films, J. Appl. Phys. 96 (2004) 2689–2692. [25] S. Chopra, S. Sharma, T.C. Goel, R.G. Mendiratta, Effect of annealing temperature on microstructure of chemically deposited calcium modified lead titanate thin film,
Fig. 7. Resistivity and FOM of CuI thin films as a function of annealing temperature.
film. 3.4. Electrical studies The electrical resistivity of the CuI thin films was determined from four point probe measurements using the following equation [29]:
V ρ=K t I
(9)
Where ρ is resistivity in Ω cm, V is voltage in volts, I is current in A, t is the thickness of the sample in cm, and K is a constant dependent on the geometry which is equal to π / ln2 for a semi-infinite thin sheet. It is evident from Fig. 7 that the resistivity of the CuI thin films increased with annealing temperature. The as deposited film posseses the lowest resistivity of 3.0×10−3 Ω cm. Cu vacancies (VCu) have been found to play a crucial role as a dominant native acceptors in CuI thin films [30]. The lower resistivity of the as-deposited film is attributed to higher carrier concentration at lower temperatures [17]. As the annealing temperature was increased, the resistivity increased due to the reduction of VCu. Reduction of VCu with increasing annealing temperature is attributed to the evaporation of iodine from CuI thin films at higher annealing temperatures [17,31]. Figure of Merit (FOM) which is a vital index for assessing the performance of TCs was also determined from the following equation [32]:
FOM =
T10 Rsh
(10)
where T is the average optical transmittance in the visible range and Rsh is the sheet resistance of the film. A higher FOM (10−2 Ω−1 range) corresponds to a good quality TC. In response to the increasing resistivity the FOM was observed to decrease with annealing temperature as shown in Fig. 7. The highest figure of merit of was attained for an as-deposited film, which means it is the best sample for use in transparent optoelectronic devices. The as-deposited CuI showed low crystallinity level, considerably high transparency and the highest FOM. This can be a good candidate for transparent electronic devices like thin film transistors (TFTs), which requires such properties. 4. Conclusions In conclusion thermal evaporation method was successfully used to deposit the CuI thin films. All the thin films showed preferential growth along the (111) plane. The grain sizes increased with annealing temperature pointing to increased crystallinity in the films. All films are highly transparent in the visible range with an average transmittance of about 70–80%. The as deposited CuI films showed the lowest 5
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[29] A. Zaier, A. Meftah, A.Y. Jaber, A.A. Abdelaziz, M.S. Aida, Annealing effects on the structural, electrical and optical properties of ZnO thin films prepared by thermal evaporation technique, J. King Saud. Univ. – Sci. 27 (2015) 356–360. [30] Y. Kokubun, H. Watanabe, M. Wada, Electrical Properties of CuI Thin Films, Jpn. J. Appl. Phys. 10 (1971) 864–867. [31] B.L. Zhu, X.Z. Zhao, Transparent conductive CuI thin films prepared by pulsed laser deposition, Physica Status Solidi A 208 (2011) 91–96. [32] T.-K. Gong, S.-B. Heo, D. Kim, Effect of post-deposition annealing on the structural, optical and electrical properties of ZTO/Ag/ZTO tri-layered films, Ceram. Int. 42 (2016) 12341–12344.
Appl. Surf. Sci. 230 (2004) 207–214. [26] H. Wei, D. Ding, X. Yan, J. Guo, L. Shao, H. Chen, L. Sun, H.A. Colorado, S. Wei, Z. Guo, Tungsten trioxide/zinc tungstate bilayers: electrochromic behaviors, energy storage and electron transfer, Electrochim. Acta 132 (2014) 58–66. [27] H. Belkhalfa, H. Ayed, A. Hafdallah, M.S. Aida, R. Tala Ighil, Characterization and studying of ZnO thin films deposited by spray pyrolysis: effect of annealing temperature, Opt.-Int. J. Light Electron Opt. 127 (2016) 2336–2340. [28] S. Benramache, A. Rahal, B. Benhaoua, The effects of solvent nature on spraydeposited ZnO thin film prepared from Zn (CH3COO)2, 2H2O, Opt.-Int. J. Light Electron Opt. 125 (2014) 663–666.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method ⁎
Charles Moditswea, , Cosmas M. Muivaa, Pearson Luhangab, Albert Jumaa a b
Department of Physics and Astronomy, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana Department of Physics, University of Botswana, Private Bag UB0704, Gaborone, Botswana
A R T I C L E I N F O
A BS T RAC T
Keywords: CuI thin films Thermal evaporation Annealing temperature Structural properties Optoelectronic properties
High quality transparent conducting CuI thin films were deposited at room temperature via thermal evaporation technique followed by post deposition annealing at different temperatures. The samples were characterised by X-ray diffraction (XRD), UV–Vis spectrophotometry, Scanning electron microscopy and I-V measurements. The structural, morphological and optical properties were studied as a function of the annealing temperature from room temperature (RT) to 200 °C. XRD results revealed that the films were polycrystalline with zinc blende structure of cubic phase. Increasing the annealing temperature increased the crystallite size from 33 to 49 nm whilst the dislocation density and lattice strain shifted to lower values. High transmittance of about 70–80% was exhibited by all films in the entire visible spectral range. The as deposited film possesed the lowest resistivity of 3.0×10−3 Ω cm.
1. Introduction Transparent conductors (TCs) are very important for applications in optoelectronic devices. For a conductor to be transparent in the visible range it must have a band gap of around 3 eV [1]. A lot of research progress has been achieved for n-type TCs, particularly ITO, FTO, CdS, ZnO have received a lot of attention. Despite this progress, reports on their p-type counterparts remain scanty and only a few (CuAlO2, CuO, CuSCN) are known [2,3]. The realisation of transparent junction devices requires both p-type and n-type TCs, as such several studies are being done to attain more and better p-type TCs. CuI is one of the promising non-oxide p-type wide band gap materials with the potential for application in solar cells as hole collectors, light emitting diodes, field emission displays and organic catalyst [4–6]. CuI is a halogenide semiconductor which exists in three crystalline phases that are temperature dependent; zinc blende structure (γ-CuI) for temperatures below 370 °C; wurtzite structure (β-CuI) between 370 and 400 °C, and rock salt structure (α-CuI) for temperatures higher than 400 °C [7]. The zinc blende structure of CuI attainable at low temperatures possesses p-type conductivity. CuI has a band gap of 2.3– 3 eV [8]. The valence band edge of CuI is made from the Cu 3d states (ᴦ15,2) and I 5p states (ᴦ12,1) [9]. Several methods have been used to deposit CuI thin films which include SILAR method [7], thermal evaporation [10], RF-DC coupled magnetron sputtering technique [11], simple complex compound [12],
⁎
ethanol thermal method [13], pulse laser deposition [9] and spray pyrolysis [14]. In these studies thermal evaporation method was used because of its advantages over the other methods which includes catalyst–free growth, simplicity, easy control of deposition parameters (thickness, growth rate etc.), and cost effectiveness [15]. Few studies on CuI thin films can be found in literature. In this paper CuI thin films were fabricated using thermal evaporation and thereafter annealed at different temperatures in open atmosphere. The detailed structural, optical and electrical analyses are therefore furnished in this paper. This paper will be a great addition to the pool of knowledge regarding the p-type TCs. 2. Experimental details 2.1. Sample preparation Marienfeld microscope glass slides measuring 20×15×1 mm were used as substrates for hosting the thin films. Prior to deposition the glass slides were cleaned as described elsewhere [16]. First CuI powders were prepared from potassium iodide (KI) and copper nitrate trihydrate salts (Cu(NO3)2·3H2O). All chemicals were of analytical grade and were used as received from Sigma Aldrich. Aqueous solutions of 0.5 M KI and Cu(NO3)2·3H2O) were made separately using double distilled water as the solvent. The KI solution was added to Cu(NO3)2·3H2O solution and a reddish brown precipitate was formed.
Corresponding author. E-mail address: [email protected] (C. Moditswe).
http://dx.doi.org/10.1016/j.ceramint.2017.01.026 Received 23 November 2016; Received in revised form 23 December 2016; Accepted 6 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Moditswe, C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.026
Ceramics International (xxxx) xxxx–xxxx
C. Moditswe et al.
The precipitate was filtered and washed several times with double distilled water to improve the purity of CuI powder. The obtained powders were dried in an oven at 100 °C for a period of 3 h. The CuI formation can be explained by the following chemical reaction: 2Cu(NO3)2·3H2O(aq) + 4KI(aq) →2CuI(s) + I2
(g)
Table 1 Lattice parameters and mechanical properties of CuI thin films annealed at different temperatures.
+ 4KNO3(aq) + 3H2O(l)
The powders were deposited using Polaron E6700 Turbo Molecular Bench Top Vacuum Evaporator at ambient temperature. Glass substrates were mounted on the tilt/rotating sample stage directly above the evaporation boat at a distance of 10 cm. The evaporation was done at conventional low voltage (LT) and high alternating current (AC) supply of 20 V and 50 A, respectively. A measured amount of CuI powder was put in the evaporation boat. The deposition chamber was evacuated to a base pressure of 10−5 mbar. The source current was increased until a vapour was formed which condensed on the glass substrates to form the desired film. Deposition was done for 2 s from a custom made boat with large surface area. The deposition time was maintained by the use of a switch. In order to get thicker films of around 550 nm the process was repeated three times. After deposition the chamber was allowed to vent to atmospheric pressure before removing the samples, it took roughly 20 min to vent. The obtained films were annealed at different temperatures for a duration of 1 h using OF-02 G forced convection oven.
Annealing temperature (°C)
Lattice parameter a (Å )
Strain in the a-axis, ɛzz (%) ×10−1
FWHM (°)
Crystallite size (nm)
Dislocation density, ρ (line/m2) ×1014
RT 50 100 150 200 Bulk CuI
6.082 6.082 6.081 6.080 6.079 6.063
3.13 3.13 2.97 2.80 2.64 0
0.276 0.220 0.214 0.189 0.185 –
32.81 41.05 42.37 47.79 49.20 –
9.29 5.93 5.57 4.38 4.13 –
structure of cubic phase. The (111) peak is the most dominant peak in the entire temperature range showing that all films are highly oriented along the (111) plane. The dominancy of the (111) face is associated with its low surface energy [18]. The intensity of the dominant peak is observed to increase with annealing temperature. The increased intensity of the (111) peak signifies the improvement of crystallinity with annealing temperature which may be attributed to the increase in the mobility of the adatoms with annealing temperature [19]. The crystallite size was determined from the XRD data using the Debye-Scherrer equation [20].
2.2. Characterisation
D=
The crystal structure of the films was characterised using an X-ray diffractometer (Model D8 Advance, Bruker, Germany) with CuKα radiation (λ=0.154056 nm) at 40 mA and 40 kV. The diffraction angle 2θ range was 20–70° with an increment of 0.02869° and the time per step of 0.2 s. The surface morphology of the thin films was investigated using a Scanning electron microscope (JSM-7100F, JEOL, Japan). The electrical measurements (I-V curves) were collected using a Signatone four point probe equipped with two Keithley electrical meters at room temperature. Optical transmittance data was obtained using UV/Vis/ NIR spectrophotometer (Varian Cary 500, USA) in the wavelength range of 300–800 nm. The thicknesses of the thin films were determined by a surface profilometer (KLA Tencor, D-100, USA).
0. 94λ βcosθ
(1)
Where D is the crystallite size in nm, β is the full width at half maximum (FWHM) in degrees, λ is wavelength of the incident radiation (λ=1.5406 Å ) and θ is the Bragg's diffraction angle. The FWHM was found to decrease from 0.276 to 0.184° as given in Table 1 which corresponds to the increasing crystallite size from 32.81 to 49.20 nm. The increase in crystallite size is believed to be a product of coalescence of small grains due to an increased kinetic energy of the grains owing to increased annealing temperature [19]. Dislocation density (δ) which is defined as the number of dislocation lines per unit volume of the crystal was estimated from the following formula [21]:
3. Results and discussion
δ=
3.1. Structural studies of the films
1 D2
(2)
where D is the crystallite size in nm. Dislocation density was also observed to decrease with annealing temperature as shown in Table 1. Lattice parameter (a=b=c for cubic structure) was determined from the following equation [22]:
Fig. 1 presents XRD patterns for the CuI thin films annealed at different temperatures (RT to 200 °C). Several reflection peaks are observed at 2θ values of 25.42°, 29.44°, 42.12°, 49.84° and 52.22° which corresponds to (111), (200), (220), (311) and (222) planes, respectively and are all indexed to JCPDS data file 06-0246 [17]. The observed pattern shows that the films crystallized in zinc blende
1 h2 + k 2 + l 2 = 2 a2 dhkl
(3)
Where (h, k, l) are Miller indices and dhkl is the interplanar spacing. The lattice parameter a was found to be almost equal to that of bulk CuI obtained from JCPDS data file 06-0246 [17] as shown in Table 1. The slight difference between observed and bulk CuI lattice parameter is attributed to strains in the film. The lattice strain along the a-axis, ɛzz was calculated from the following equation [23,24]:
ɛzz =
(a − a 0 ) a0
×100
%
(4)
Where a and a0 are the observed (strained) and bulk (strain free) lattice parameters, respectively. The observed lattice parameters in Table 1 are elongated meaning that the films are under tensile strain. The tensile strain is observed to decrease with increase in annealing temperatures. It can now be concluded that increasing annealing temperatures improved the crystallinity and lattice relaxation of the
Fig. 1. The XRD patterns of the CuI thin films at different annealing temperatures.
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Fig. 2. SEM images of (a) not annealed and annealed CuI thin films at various annealing temperatures of (b) 50 °C, (c) 100 °C, (d) 150 °C, and (e) 200 °C.
CuI thin films. The pointers of improved crystallinity in this case were the decrease in both dislocation density and lattice strain and an increase in the crystallite size. Improvement in crystallinity with increased annealing temperature is attributed to the recrystallization process at higher annealing temperatures [19].
grain size with increased annealing temperature is observed. The increase in grain size is associated with the increase in surface mobility of the adatoms [25]. The increased grain size is consistent with the results obtained in XRD studies. 3.3. Optical studies
3.2. Morphological studies Fig. 3 shows optical transmittance spectra of CuI thin films annealed at different temperatures in the wavelength range 300– 800 nm. All films are highly transparent in the visible range with an average transmittance of about 70–80%. A sharp absorption edge is observed around 411 nm. Interference patterns were observed in all films indicating that the surfaces of the films were homogenous and
Surface morphologies of the thin films are shown in 2D SEM micrographs at a magnification of × 40, 000 as shown in Fig. 2. All images show compact and dense films with spherical shaped grains. It is evident from the images that the surface morphology is greatly influenced by the annealing temperature. A gradual increase in the 3
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Fig. 3. Transmission spectrum of CuI thin films at different annealing temperatures. The inset shows the absorption spectra of CuI thin films.
Fig. 5. Plot of dT/dλ versus wavelength for CuI thin films at different annealing temperatures. The inset shows a zoom in the peak.
specular in nature [16]. A hump is observed in the absorption spectrum at around 410 nm for all the films (insert of Fig. 3). The hump is attributed to the excitation of electrons from sub bands in the valence band to the conduction band [9]. The absorption coefficient (α) was calculated from the BeerLambert relation [26]:
Table 2 Optical band gap and Urbach energies as a function of annealing temperature.
1 ⎡1⎤ α= ln ⎢ ⎥ t ⎣T ⎦
(5)
where α is the absorption coefficient, T is the transmittance and t is the film thickness in nm. The optical band gaps were calculated from the following equation [27]:
(αhν )2 = A (hν − Eg )
Annealing temperature (°C)
Band gap from Tauc plot (eV)
Band gap from dT/dλ (eV)
Urbach energy (meV)
RT 50 100 150 200
3.01 3.01 3.01 3.01 3.01
3.02 3.02 3.02 3.02 3.02
42 41 38 36 34
disorder in the film were determined using the following formula and the obtained values are given in Table 2 [28]:
⎛ hν ⎞ α = α0exp ⎜ ⎟ for hν ⎝ EUrb ⎠
(6)
Where hν is the photon energy, A is a constant and Eg is the optical band gap. To determine the band gaps a Tauc plot, (αhν)2 versus hν, was plotted and tangent of the linear part was extrapolated to the energy axes to give the band gap value (Fig. 4). All the band gap values were found to be 3.01 eV for different annealing temperatures. Zi et.al also reported a constant band gap of 3.04 eV with increasing temperature for CuI thin films by vacuum thermal evaporation [17]. Band gap values were also determined from the position of the maximum of the first derivative of transmittance with respect to wavelength (dT/dλ) as shown in Fig. 5 [16]. The derivative method also gives a constant band gap value of 3.02 eV with different annealing temperatures which conforms well with the constant value (3.01 eV) obtained from the Tauc plot method as shown in Table 2. The Urbach tail energies EUrb which denotes the degree of the
<
Eg
(7)
i.e
⎡ dlnα ⎤−1 EUrb=⎢ ⎥ ⎣ dhν ⎦
(8)
where αo is the pre-exponential factor. The values of EUrb were obtained from the slope of the ln (α) against hν (Fig. 6) near the absorption edge. On increasing the annealing temperature from RT to 200 °C the Urbach energy is found to decrease from 42 to 34 meV as shown in Table 2. The decrease in the Urbach energy substantiates the fact that the crystallinity is improving with the increase in annealing temperature and correlates well with results observed in XRD analysis. It can be seen that increased crystallinity decreased the disorder in the
Fig. 6. Plot of ln α versus hν for CuI thin films deposited at different annealing temperatures.
Fig. 4. Tauc plot for the CuI thin films annealed at different temperatures.
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resistivity (highest conductivity) and hence the highest FOM, which makes them suitable for transparent electronics and solar cell applications. Acknowledgements The authors would like to extend their appreciation to Botswana International University of Science and Technology (BIUST) for financing this work. The authors are also grateful to the University of Botswana, Physics Department for access to equipment used for thin film fabrication, electrical and optical characterisation under grant R1068. References [1] J. Robertson, P.W. Peacock, M.D. Towler, R. Needs, Electronic structure of p-type conducting transparent oxides, Thin Solid Films 411 (2002) 96–100. [2] K. Tennakone, G.R.R.A. Kumara, I.R.M. Kottegoda, V.P.S. Perera, G.M.L.P. Aponsu, K.G.U. Wijayantha, Deposition of thin conducting films of CuI on glass, Sol. Energy Mater. Sol. Cells 55 (1998) 283–289. [3] S.L. Dhere, S.S. Latthe, C. Kapenstein, S.K. Mukherjee, A. Venkateswara Rao, Comparative studies on p-type CuI grown on glass substrate by SILAR method, Appl. Surf. Sci. 256 (2010) 3967–3971. [4] J.-H. Lee, D.-S. Leem, J.-J. Kim, High performance top-emitting organic lightemitting diodes with copper iodide-doped hole injection layer, Org. Electron. 9 (2008) 805–808. [5] P.M. Sirimanne, T. Soga, T. Jimbo, Identification of various luminescence centers in CuI films by cathodoluminescence technique, J. Lumin. 105 (2003) 105–109. [6] V.P.S. Perera, K. Tennakone, Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector, Sol. Energy Mater. Sol. Cells 79 (2003) 249–255. [7] R.N. Bulakhe, N.M. Shinde, R.D. Thorat, S.S. Nikam, C.D. Lokhande, Deposition of copper iodide thin films by chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) methods, Curr. Appl. Phys. 13 (2013) 1661–1667. [8] S. Badyopadhyaya, S. Chaudhuri, A.K. Pal, Synthesis of CuInS2 films by sulfurization of Cu/In stacked elemental layers, Sol. Energy Mater. Sol. Cells 60 (2000) 323–339. [9] P.M. Sirimanne, M. Rusop, T. Shirata, T. Soga, T. Jimbo, Characterization of transparent conducting CuI thin films prepared by pulse laser deposition technique, Chem. Phys. Lett. 366 (2002) 485–489. [10] S. Shi, J. Sun, J. Zhang, Y. Cao, A novel application of the CuI thin film for preparing thin copper nanowires, Physica B 362 (2005) 231–235. [11] T. Tanaka, K. Kawabata, M. Hirose, Transparent conductive CuI films prepared by rf-dc coupled magnetron sputtering, Thin Solid Films 281,282 (1996) 179–181. [12] Y.Y. Zhou, M.K. lü, G.J. Zhou, S.M. Wang, S.F. Wang, Preparation and photoluminescence of γ-CuI nanoparticles, Mater. Lett. 60 (2006) 2184–2186. [13] Y. Liu, J. Zhan, J. Zeng, Y. Qian, K. Tang, W. Yu, Ethanolthermal synthesis to γ-CuI nanocrystals at low temperature, J. Mater. Sci. Lett. 20 (2001) 1865–1867. [14] M.N. Amalina, Y. Azilawati, N.A. Rasheid, M. Rusop, The properties of copper (I) iodide (CuI) thin films prepared by Mister Atomizer at different doping concentration, Procedia Eng. 56 (2013) 731–736. [15] F.H. Alsultany, Z. Hassan, N.M. Ahmed, Control growth of catalyst-free ZnO tetrapods on glass substrate by thermal evaporation method, Ceram. Int. 42 (2016) 13144–13150. [16] C. Moditswe, C.M. Muiva, A. Juma, Highly conductive and transparent Ga-doped ZnO thin films deposited by chemical spray pyrolysis, Opt. – Int. J. Light Electron Opt. 127 (2016) 8317–8325. [17] M. Zi, J. Li, Z. Zhang, X. Wang, J. Han, X. Yang, Z. Qiu, H. Gong, Z. Ji, B. Cao, Effect of deposition temperature on transparent conductive properties of γ-CuI film prepared by vacuum thermal evaporation, Physica Status Solidi A 212 (2015) 1466–1470. [18] Z. Zheng, A.R. Liu, S.M. Wang, B.J. Huang, K.W. Wong, X.T. Zhang, S.K. Hark, W.M. Lau, Growth of highly oriented (110) CuI films with sharp exiton band, J. Mater. Chem. 18 (2008) 852–854. [19] A. Bouhdjer, A. Attaf, H. Saidi, Y. Benkhetta, M.S. Aida, I. Bouhaf, A. Rhil, Influence of annealing temperature on In2O3 properties grown by an ultrasonic spray CVD process, Opt. – Int. J. Light Electron Opt. 127 (2016) 6329–6333. [20] S. Tewari, A. Bhattacharjee, Structural, electrical and optical studies on spraydeposited aluminium-doped ZnO thin films, Pramana J. Phys. 76 (2011) 153–163. [21] G.B. Williamson, R.C. Smallman, Dislocation densities in some annealed and coldworked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1 (1956) 34–46. [22] A.S. Vasudeva, Concepts of Modern Engineering Physics, 3rd edition, S. Chand & Company Ltd, New Delhi, 2012. [23] Selected Powder Diffraction Data for Metals and Alloys, JCPDS, USA, 1 (1978) 108. [24] R. Ghosh, D. Basak, S. Fujihara, Effect of substrate-induced strain on the structural, electrical, and optical properties of polycrystalline ZnO thin films, J. Appl. Phys. 96 (2004) 2689–2692. [25] S. Chopra, S. 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Fig. 7. Resistivity and FOM of CuI thin films as a function of annealing temperature.
film. 3.4. Electrical studies The electrical resistivity of the CuI thin films was determined from four point probe measurements using the following equation [29]:
V ρ=K t I
(9)
Where ρ is resistivity in Ω cm, V is voltage in volts, I is current in A, t is the thickness of the sample in cm, and K is a constant dependent on the geometry which is equal to π / ln2 for a semi-infinite thin sheet. It is evident from Fig. 7 that the resistivity of the CuI thin films increased with annealing temperature. The as deposited film posseses the lowest resistivity of 3.0×10−3 Ω cm. Cu vacancies (VCu) have been found to play a crucial role as a dominant native acceptors in CuI thin films [30]. The lower resistivity of the as-deposited film is attributed to higher carrier concentration at lower temperatures [17]. As the annealing temperature was increased, the resistivity increased due to the reduction of VCu. Reduction of VCu with increasing annealing temperature is attributed to the evaporation of iodine from CuI thin films at higher annealing temperatures [17,31]. Figure of Merit (FOM) which is a vital index for assessing the performance of TCs was also determined from the following equation [32]:
FOM =
T10 Rsh
(10)
where T is the average optical transmittance in the visible range and Rsh is the sheet resistance of the film. A higher FOM (10−2 Ω−1 range) corresponds to a good quality TC. In response to the increasing resistivity the FOM was observed to decrease with annealing temperature as shown in Fig. 7. The highest figure of merit of was attained for an as-deposited film, which means it is the best sample for use in transparent optoelectronic devices. The as-deposited CuI showed low crystallinity level, considerably high transparency and the highest FOM. This can be a good candidate for transparent electronic devices like thin film transistors (TFTs), which requires such properties. 4. Conclusions In conclusion thermal evaporation method was successfully used to deposit the CuI thin films. All the thin films showed preferential growth along the (111) plane. The grain sizes increased with annealing temperature pointing to increased crystallinity in the films. All films are highly transparent in the visible range with an average transmittance of about 70–80%. The as deposited CuI films showed the lowest 5
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Appl. Surf. Sci. 230 (2004) 207–214. [26] H. Wei, D. Ding, X. Yan, J. Guo, L. Shao, H. Chen, L. Sun, H.A. Colorado, S. Wei, Z. Guo, Tungsten trioxide/zinc tungstate bilayers: electrochromic behaviors, energy storage and electron transfer, Electrochim. Acta 132 (2014) 58–66. [27] H. Belkhalfa, H. Ayed, A. Hafdallah, M.S. Aida, R. Tala Ighil, Characterization and studying of ZnO thin films deposited by spray pyrolysis: effect of annealing temperature, Opt.-Int. J. Light Electron Opt. 127 (2016) 2336–2340. [28] S. Benramache, A. Rahal, B. Benhaoua, The effects of solvent nature on spraydeposited ZnO thin film prepared from Zn (CH3COO)2, 2H2O, Opt.-Int. J. Light Electron Opt. 125 (2014) 663–666.
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