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The thermoluminescence properties of CdS films under nitrogen atmosphere
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1531–1538
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The thermoluminescence properties of CdS films under nitrogen atmosphere H. Metin a,n, S. Erat b,c, F.M. Emen d, V. Kafadar e, A.N. Yazici e, M. Ari f, N. Kulcu g a
Mersin University, Department of Physics, 33343 Mersin, Turkey Laboratory for High Performance Ceramics, EMPA-Swiss Federal Laboratories for Materials Testing & Research, CH-8600 D¨ ubendorf, Switzerland Department of Nonmetallic Inorganic Materials, ETH Z¨ urich-Swiss Federal Institute of Technology, CH-8053 Z¨ urich, Switzerland d Kırklareli University, Department of Chemistry, Kırklareli, Turkey e Gaziantep University, Department of Engineering Physics, Gaziantep, Turkey f Erciyes University, Department of Physics, Kayseri, Turkey g Mersin University, Department of Chemistry, 33343 Mersin, Turkey b c
a r t i c l e in fo
abstract
Article history: Received 11 January 2010 Received in revised form 19 March 2010 Accepted 22 March 2010 Available online 8 April 2010
Chemically deposited cadmium sulphide (CdS) films have been grown on glass at 60 1C and annealed at nitrogen atmosphere at different temperatures. The as-deposited film shows a mix phase of cubic and hexagonal. Once the film subjected to annealing the hexagonal phase becomes dominant and the crystal size increases due to these changes optical band gap energy decreases from 2.44 to 2.28 eV. The electrical conductivity increases depending on temperature and the film annealed at 423 K shows the highest conductivity. Thermoluminescence (TL) intensity of the films was measured after irradiating the films with 90Sr/90Y b-source and the trap depths were calculated after the TL curves deconvoluted by using the computer glow curve deconvolution (CGCD) method. It is observed that the as-deposited film has three different trap depths, at around 0.257, 0.372, and 0.752 eV corresponding to 383, 473, and 608 K, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Thin film CBD Crystal structure Thermoluminescence
1. Introduction Chemical bath deposition (CBD) is known to be a simple, low temperature, and inexpensive large-area deposition technique for group II–VI semiconductors such as CdS [1]. Over the years the CdS thin films have been intensively studied and have been used in electronic and optoelectronic devices. Thin polycrystalline films of CdS can be prepared by different methods such as sputtering [2,3], screen printing [4–6], spray pyrolysis [7,8], spin coating [9], electrodeposition [10,11], chemical bath deposition (CBD), etc [12–24]. Among these various techniques, the CBD is the most successful method used in the production of uniform, adherent, and reproducible large-area thin films for solar-related application [25,26]. The chemically deposited thin films show high resistivity (106 7 O cm) at room temperature [12–14,17,27–32], most probably because of the lattice defects and dislocations in films [33–37]. The physical and chemical properties of the most of the binary metal chalgonide of II–VI semiconductors such as CdS, CdSe, etc. obtained by using the CBD strongly depend on various preparative parameters such as concentration of metal and chalgonide ions, pH of the deposition solution, deposition time and temperature, and gas phase process parameters like temperature and gas partial pressure.
n
Corresponding author. Tel.: + 90 324 361 00 01/4622; fax: +90 324 361 00 46. E-mail address: [email protected] (H. Metin).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.03.025
The photo- and thermal-conductivities of CdS films are related to impurities or vacancies present in the crystal. Accordingly, there have been many reports on the measurements of photoconductivity and thermally stimulated conductivity of CdS thin films but there has not been sufficient work on the thermally stimulated luminescence (TL) properties of this material up to now [38,39]. It is known that the TL intensity from semiconductive materials is generally difficult to measure above the room temperature. Therefore, there has not been sufficient work on the TL properties of CdS, either in bulk or thin film forms, above the room temperature up to now. In a previous investigation, it has been shown that the TL properties of CdS were affected by the annealing in various conditions [38]. In addition, the TL was observed above the room temperature in CdS-doped glasses after exposing the intense light at 300 K [40,41]. In this work CdS films with 1.8 mm thickness were prepared by CBD technique at 60 1C. The films were annealed in nitrogen atmosphere in order to prevent reaction of the film with the oxygen in the air. The structural characterization of the films was carried out by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) techniques. The optical, electrical conductivity, and thermoluminescence properties of these films were investigated systematically. The trap parameters were calculated by the computer glow curve deconvolution (CGCD) method from thermoluminescence (TL) glow curves. Also, the effect of annealing temperature on optical, electrical, morphological, and thermoluminescent properties is the scope of this work and discussed in detail.
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2. Experimental details The CdS films have been deposited on glass substrates (with a size of 75 mm 25 mm 2 mm) using the CBD technique. These substrates were cleaned by following the procedure adopted by Metin and Esen [42] in order to reduce the impurities. Aqueous solutions of 1 M cadmium sulphate, 2.25 M hydrazine, 1.4 M thiourea, and 25% NH3 were prepared without any precipitation to deposit CdS film. The film was deposited on the glass substrate by keeping it vertically in the beaker without stirring at 60 1C for 2 h. This process was repeated nine times to get thicker films and the deposition solution was refreshed for each time. After deposition, the films were rinsed in distilled water and methanol to remove the loosely adhered CdS particles on the surface and finally dried in air. The six as-deposited yellow CdS films with 1.8 mm thickness were uniform and well adherent to the substrates. Five of the films were annealed at 423 KrTr823 K (in steps of 100 K) with 5 K/min heating/cooling rate and 1 h dwelling time using Protherm (PTF 15/ 75/610) type of horizontal tube furnace under 99.999% pure N2 flow with flow rate 100 ml/min. The CdS films were structurally characterized by XRD, in the range of diffraction angle 201 r2y r601 in steps of 0.021 with ˚ using Bruker AXS 40 kV at 30 mA, CuKa1 radiation (l ¼1.5406 A) Advance D8 diffractometer. Elemental composition of the films was measured using PANanalitical Axios Advanced X-ray fluorescence (XRF). The microstructures of these films were characterized using a LEO 440 SEM. The absorption spectra of the CdS films were measured by using UV–visible spectrophotometer (SHIMADZU UV-1700) at room temperature in the wavelength range 190–1100 nm. The electrical measurements of the films were carried out by means of four-point probe technique. Four platinum wires with 0.5 mm diameter were used two of which for detecting potential drop and the others for applying a constant current across the thin films. Keithley 2400 sourcemeter and Keithley 2700 multimeter have been used for applying current and measuring potential difference, respectively. Temperature of the films was measured by using a K type thermocouple, while the film was in the programmable furnace. The data have been recorded through an interface card controlled by a PC. The glow curves of CdS films were measured using a Harshaw QS 3500 manual type TL reader that is interfaced to a PC where all glow curves were analyzed. Glow curve readout was carried out on a platinum planchet at a linear heating rate of 1 1C/s. A continuous flux of nitrogen was used during the TL readouts to reduce chemiluminescence in the heated samples. All the films were irradiated using the same experimental setup of a 90Sr/90Y b-source (2.27 MeV) at a dose rate of approximately 0.04 Gy/s at room temperature.
Fig. 1. X-ray diffractograms of the as-deposited and the annealed CdS films at different temperatures.
3. Results and discussion 3.1. Structural and morphological studies The X-ray diffraction patterns of the as-deposited and annealed films are shown in Fig. 1. The as-deposited and annealed CdS films (To623 K) show a mixture of cubic (C) phase (zinc blende-type) and hexagonal (H) phase (wurtize-type) at low temperature. It has been reported that chemically deposited CdS films onto the glass substrate are either hexagonal or cubic phase [28,29,32,42–46]. It is known that there are two possible ways for the deposition of CdS films during CBD process: (i) ‘‘ion-by-ion’’ process (heterogeneous reaction) and (ii) ‘‘cluster-by-cluster’’ process (homogeneous reaction), which results in pure hexagonal or mixed hexagonal-cubic phase and pure cubic phase, respectively [8,30]. The diffraction
patterns of the films were checked against the JCPDS 06-0314 and JCPDS 10-0454 reference patterns. The peaks due to the hexagonal (0 0 2) plane or cubic (1 1 1) plane diffraction appear at approximately at 26.661 for the as-deposited and annealed film at 423 and 523 K. The intensity of the peaks increases with increase in annealing temperature, which causes a sharpening of the Bragg reflections, revealing an increase in the crystallinity of the film. Also, the hexagonal phase becomes dominant with increase in the temperature and at T4623 K the structure becomes pure stable hexagonal phase. The phase transformation from cubic to hexagonal was also observed for the chemically deposited CdS films before [12,13,22,24,27,31]. The crystallite size of the films was determined from X-ray diffraction data. We use the standard (1 1 1) C reflection at 2y ¼26.661. The evaluation of the crystallite size was quantified
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by using Scherrer’s formula Dhkl ¼
Kl b cos y
ð1Þ
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where K is the Scherrer constant, b is the full-width at halfmaximum (FWHM) of the Bragg reflection under the consideration in radians, l is the wavelength of X-ray used, and y is the Bragg
Table 1 The structural parameters of as-deposited and annealed CdS films. Annealing temperature (K)
FWHM b 10 3 (rad)
Crystallites size (nm)
Dislocation density (1014 lines/m2)
Number of crystallites/unit area (1016 m 2)
Strain (10 3)
As-deposited 423 523 623 723 823
5.651 4.986 4.322 2.659 1.828 1.329
25.21 28.60 33.00 53.60 78.00 107.20
15.73 12.22 9.18 3.48 1.64 0.87
11.23 7.69 5.01 1.17 0.38 0.15
1.26 1.14 0.96 0.59 0.41 0.30
Fig. 2. The SEM micrographs of the CdS films (a) as-deposited and at (b) 423 K, (c) 523 K, (d) 623 K, (e) 723 K, and (f) 823 K.
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angle. Eq. (1) was used for the calculation of the crystallite sizes using K¼0.9. The crystallite size of the as-deposited film was 25.21 nm and increases by almost a factor of 4 with the annealing temperature. Furthermore, using these data the dislocation density (d ¼ 1=D2hkl ), the number of crystallites per unit surface area (N ¼ d=D3hkl , where d is the film thickness), and strain (e ¼ b cos y/4) in the films were determined. The dislocation density is a measure of the quantity of the dislocation present in a material and in the order of 1010 m 2 in a metal. The dislocation density and the strain on the surface of the films are well explained in the literature [9,17,22,47–49]. The crystal size increases with increase in annealing temperature, which results in decrease in the defects like dislocation density and strain in the film (Table 1). XRF was used to determine the elemental composition of the films and it was found that the ratio of Cd/S was almost constant within the detection limit and sensitivity of the instrument. The SEM images of as-deposited and annealed CdS films are shown in Fig. 2. It is seen that the substrate is well covered because of the growth mechanism of ‘‘ion-by-ion’’. Although the distribution of the well-defined grains is not uniform throughout the entire region, the films do not show any voids, pinholes or cracks, except for the one annealed at 823 K where the sintering effect already started. The as-deposited film is made of oblate structured units of 0.7 mm and 100 nm thicknesses. The globules which look like cauliflowers are getting bigger with increasing annealing temperature. The oblate structured units start to be packed to form globules. Especially, for the film annealed at 723 K it is easy to see these new globules with 1 mm diameter. 3.2. Optical properties The optical transmission of the as-deposited and annealed CdS films was measured using UV–visible spectrophotometer at room temperature. The transmittance (T) of the as-deposited and annealed CdS films is shown in Fig. 3 as a function of wavelength. The thickness of the films was calculated from the transmission interference using following equation:
and li + 1, respectively [49,50]. The thickness of the CdS film increases linearly with a growth rate of 100 nm/h (Fig. 4). The wavelength dependent linear absorption coefficient a of the as-deposited and annealed CdS films will now be analyzed in order to determine the optical band gap of the film and the nature of the optical transitions involved by using this equation: It ðd,hnÞ ¼ Io eaðhnÞd
ð3Þ
where d is the thickness of the film, hv is photon energy, and It and Io are the intensity of transmitted and incoming light, respectively. Tauc [51] has identified three distinct regions in the absorption spectrum of amorphous semiconductors: (a) the weak-absorption tail which originates from defects and impurities, (b) the exponential edge region (1 cm 1 o E a o E104 cm 1) which is strongly related to the structural randomness of the glassy material, and (c) the high absorption region which determines the optical energy gap. In the exponential edge region the absorption coefficient a(hn) is well described by the exponential law hv ð4Þ a ¼ a0 exp E0 known as the Urbach law [52]. Here a0 is a constant, hn is the incident photon energy, and E0 is called Urbach energy, which characterizes the slope of the exponential edge region and is width of the band tails of the localized states. The Urbach tail of the absorption edge is usually ascribed to the optical electronic transitions between the extended states and the near edge localized states. The formation of localized states with energies at the boundaries of the energy gap is one of the effects of the structural disorder on the electronic structure of amorphous materials. This is the reason why the Urbach energy (E0) is frequently used as a measure of the degree of structural disorder. E0 is given by the relation given below E0 ¼
1 dðlnðaÞÞ dðhvÞ
ð5Þ
where d is the film thickness, and n(li) and n(li + 1) are the refractive indices at the two adjacent maxima (or minima) at li
The graph of ln (a) versus hv being the photon energy falling the energy region where a0 o104 cm 1 is given in Fig. 5. The value of E0 is calculated from the slope of the linear plot illustrated in Fig. 5. The as-deposited film has E0 ¼0.22 eV and decreases with increase in annealing temperature up to 723 K and then starts to increase (see Table 2).
Fig. 3. The transmittance versus wavelength for the as-deposited and the annealed CdS films.
Fig. 4. Time dependence of the film thickness for CdS films.
d¼
li li þ 1 2½nðli Þli þ 1 nðli þ 1 Þli
ð2Þ
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In the high absorption region (where absorption is associated with interband transitions), the form of the absorption coefficient a(hn) was given in quadratic form by Tauc [51] and discussed in more general terms by Mott and Davis [53], who use the equation of the form
aðhnÞ ¼ AðhnEg Þn
3.3. Electrical properties The temperature dependent electrical conductivity of the films was measured by the four-point probe technique. The conductivity of the films has been determined by using the following expression [54]:
ð6Þ
where A is a constant, a is absorption coefficient, hv is the photon energy, and n is a constant equal to 1/2 for direct band gap semiconductor. The value of the energy where a ¼0 is known as band gap energy Eg. The a2 versus hn, the effect of the as-deposited and the annealed CdS films is given in Fig. 6. It can be seen in Fig. 6 that the absorption edge of the annealed films has obviously shifted towards larger wavelengths with increase in annealing temperature. Also, it is obvious that the band gap energy value of the as-deposited film is 2.44 eV and systematically decreases to 2.28 eV for film at 823 K (Table 2), which is in agreement with the literature [9,22,47]. At high temperature (823 K) the film is seen as degraded in terms of optical absorption at the forbidden region. It is obvious that an increase in the crystallite size gives rise to a decrease in band gap energy. The extinction coefficient k¼
1535
al
ð7Þ
s¼
1 I 2ps V
ð8Þ
where I is the current, V is the potential drop, and s is the distance between the probe tips. The conductivity results of the thin films indicate that the samples have typical semiconductor properties, as can be seen in Fig. 7. The values lie in between 8 10 3–1.2 10 1 S/m. The conductivity of the films starts to increase exponentially from approximately 600 K for the as-deposited film and reaches to highest values at 625 K. The film annealed at 423 K shows the highest conductivity. The activation energy Ea of the films has also been determined using the results obtained from the electrical conductivity measurements. We have used a standard equation to calculate Ea, which can be given as
s ¼ s0 expðEa =kB TÞ
ð9Þ
of the CdS films was obtained taking into account of the a values at l ¼550 nm for the as-deposited and the annealed CdS films and is listed in Table 2. The increase in the extinction coefficient with increase in annealing temperature is paralleled by the change in the absorbance of the films.
where s is the conductivity of the thin film samples, s0 the preexponential factor, Ea the activation energy, kB the Boltzmann constant, and T temperature. The activation energy results of the samples are given in Table 2. The results show that the activation energy decreases with increase in annealing temperature up to 523 K and starts to increase (0.245 eV) at 623 K and then again starts to decrease.
Fig. 5. The ln (a) versus hv for the as-deposited and the annealed CdS films.
Fig. 6. The a2 versus hn for the as-deposited and the annealed CdS films.
4p
Table 2 The optical and electrical parameters of as-deposited and annealed CdS films. Annealing temperature (K)
Eg (eV)
Urbach tail E0 parameter (eV)
Absorption coefficient 104 (cm 1) (l ¼550 nm)
Extinction coefficient (l ¼550 nm)
Activation energy Ea (eV)
As-deposited 423 523 623 723 823
2.44 2.42 2.38 2.35 2.30 2.28
0.22 0.19 0.14 0.11 0.78 1.22
0.4017 0.6676 0.7355 0.5831 1.9297 3.2637
0.0175 0.0292 0.0322 0.0255 0.0844 0.1430
0.265 0.150 0.066 0.245 0.152 0.106
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Fig. 7. The temperature dependence of the electrical conductivity of as-deposited and annealed CdS films.
3.4. Thermoluminescence properties It is well known that the kinetic analysis of the trapping states is correctly obtained from the TL glow curves of the samples if the contributions from the glass substrates and detection system are both eliminated. Therefore, the readout cycle was repeated after each evaluation in order to guarantee that no remaining TL signal was still present on the sample and the second readout without the irradiation of sample with b-rays is considered to be the background of the reader plus sample and glass, which was subtracted from the first one and all of the analysis have been carried out after the subtraction. The recorded glow curves were analyzed using the computer glow curve deconvolution (CGCD) method [55]. The CGCD program uses a linear least-square minimization procedure to determine peak area, activation energy, frequency factor, and kinetic order and the following model was used in the program to analyze the
glow curve: Ea ðb1Þs kB T 2 Ea exp 1þ IðTÞ ¼ n0 s exp kB T b Ea kB T b b1 kB T 0:99201:620 Ea
ð10Þ
where n0 (m 3) is the number of the trapped electrons at t¼0, s (s 1) is the frequency factor, Ea (eV) is the activation energy, T (K) is the absolute temperature, kB (eV K 1) is the Boltzmann’s constant, b (1C s 1) is the heating rate, b is the kinetic order, and I is the TL peak intensity. The experimental and the deconvoluted glow curves of the films are shown in Fig. 8. As seen the TL glow curves of the asdeposited film exhibit two strong TL glow peaks at around 473 and 608 K and a low intensity glow peak at around 383 K when heated at a constant heating rate of 1 1C/s. After a careful investigation of
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Fig. 8. A typical analyzed glow curve of as-deposited and the annealed CdS film measured after 5 min ( E9 Gy) b-irradiation at RT. The glow curve was measured by heating the sample to 673 K at a rate of 1 1C/s. In the figure, open circles represent the experimental points.
Table 3 The values of the kinetic parameters of TL peaks of as-deposited and the annealed CdS thin films determined by the CGCD method.
As-deposited P1 P2 P3 423 K P1 P2 P3 623 K P1 P2 P3 723 K P1 P2 P3
TM (K)
E (eV)
b
383 473 608
0.26 0.37 0.75
1.42 1 1.05
348 453 608
0.29 0.48 1.18
1.026 1 1.987
373 473 608
0.37 0.35 0.88
2 1 1.124
383 473 608
0.38 0.43 0.77
2 1 1.124
this figure, it is observed that the glow curve structure of the film is described by a linear combination of at least three glow peaks between RT and 673 K and a best fit was always obtained by assuming that the first (P1) and third peaks (P3) are of general-order kinetics and the second peak (P2) is of first-order kinetics. The trapping parameters obtained after fitting are listed in Table 3. It is seen that the trapping parameters are affected by the annealing temperature. For example, the changes in the trap depths of P1, P2,
Fig. 9. Comparison of the glow curves of CdS films annealed at different temperatures (D E9 Gy).
and P3 are in the range of 0.26–0.38, 0.35–0.48, and 0.75–1.18 eV, respectively. The TL glow curves of the annealed films are compared in Fig. 9. As it is shown in the figure, the TL intensity decreases with increase in annealing temperature. The TL of CdS films obtained in the present study can be explained by the following process. As mentioned previously, the
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CdS is a wide band gap material (semiconductive material) and therefore it is very difficult to measure the TL intensity of it above room temperature. However, when the CdS thin film was grown on glass by using the chemical bath deposition technique, they form a glass matrix plus CdS thin film composition. The crystallite size of the as-deposited film in the given study is found in the order of 25 nm, which confirms the nano-crystalline form of the developed samples. Nanomaterials are defined as those materials whose length scales lie within the nanometric range, i.e. in the range between one to a hundred nanometers [56]. It is well known that within this region, the materials exhibit different behavior from that of their bulk materials. In the literature there are extensive studies on the luminescence properties of semiconductor nanocrystals [57,58]. In those studies, it was found that the luminescence process in semiconductor nanocrystals is a very complex phenomenon and it was attributed to that the luminescence of semiconductor nanocrystals is generally arising from the deep traps of surface states whose energy levels lie within the band gap of semiconductor [55]. In nanophosphors the number of the ions on the surface of samples quickly increases as the crystallite size decreases. As mentioned previously, the crystallite size of the deposited film in the given study increases with increase in annealing temperature and some of the typical glow curves obtained after different annealing temperatures are shown in Fig. 9. As seen from this figure, the structures of the TL glow curves of CdS nanocrystals remain constant without any observable change in the structure of the glow curves for different annealing temperatures but the TL intensity decreases with increase in annealing temperatures. Now, during the ionizing irradiation of the CdS films, the electron–hole pairs are generated within the crystal structure of the films by excitation of electron from the valence band to the conduction band. The excited electrons and holes from the surface ions are easily trapped at the surface states. So, the trapped carriers at the surface states are released by heating the sample and they recombine with each other and give out luminescence, which is known as thermoluminescence. The number of surface ions and surface states increases due to the increase in the surface to volume ratio with decreasing crystallite size, so the number of trapped charged particles at surface state increases, thus TL efficiency is increased. Therefore, the TL of CdS nanophosphor may be related with the surface states.
4. Conclusion The as-deposited CdS film shows a mixture phase of 50% hexagonal and 50% cubic where the hexagonal phase becomes dominant upon annealing. This goes along with increasing crystallite size and a reduction of the dislocation and strain of the films. Structural phase transformation from metastable cubic phase to the stable hexagonal phase is observed at TZ623 K. The optical band gap of the as-deposited film (2.44 eV) decreases with increase in annealing temperature due to an increase in crystallite size, which also results in decrease of TL efficiency of the film. The film annealed at 423 K shows highest electrical conductivity and also highest TL efficiency. Therefore, it might be suggested that 423 K is the best annealing temperature for these CdS films for the application.
Acknowledgements Financial support from the the Mersin University Research Found (contract no: BAP-FEF FB (HM) 2004-3), the Erciyes University Research Found (contract no: FBA-07-041), the National Swiss Foundation (NSF No. 200021-116688 for S. Erat), and Turkish Council of Scientific and Industrial Research (TUBITAK, 107T392) is gratefully acknowledged.
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Chopra, in: Thin Film Phenomenon, McGraw-Hill, New York, 1969. [35] G. Harbeke (Ed.), Polycrystalline Semiconductors: Physical Properties and Applications, Springer, Berlin, 1985. [36] G.I. Rusu, M.E. Popa, G.G. Rusu, I. Salaoru, Appl. Surf. Sci. 222 (2003) 218. [37] R.S. Rusu, G.I. Rusu, J. Optoelectron. Adv. Mater. 7 (2) (2005) 823. [38] C.H. Lee, S.K. Kim, H.Y. Du, G.N. Jeon, J. Phys. D: Appl. Phys. 24 (1991) 422. [39] D.E. Arafah, R. Ahmad-Bitar, Semicond. Sci. Technol. 13 (1988) 322. [40] T. Miyoshi, K. Ushigusa, K. Migita, M. Tamechika, N. Tokuda, T.J. Kaneda, Mater. Sci. 43 (2008) 203. [41] T. Miyoshi, Y. Makidera, T. Kawamura, N. Matsuo, T. Kaneda, J. Lumin. 102– 103 (2003) 614. [42] H. Metin, R. Esen, J. Cryst. Growth 258 (1–2) (2003) 141. [43] R. Devi, P.K. Kalita, P. Purakayastha, B.K. Sarma, J. Optoelectron. Adv. Mater. 10 (11) (2008) 3077. [44] B.G. Wagh, D.M. Bhagat, Curr. Appl. Phys. 4 (2004) 259. [45] Y.A. Kalandaragha, M.B. Muradov, R.K. Mamedov, M. Behboudnia, A. Khodayari, J. Optoelectron., Adv. Mater.– Rapid Commun. 2 (1) (2008) 42. [46] D. Patidar, N.S. Saxena, K. Sharma, T.P. Sharma, J. Optoelectron. Adv. Mater. – Rapid Commun. 1 (7) (2007) 329. [47] V. Bilgin, S. Kose, F. Atay, I. Akyuz, Mater. Chem. Phys. 94 (2005) 103. [48] S. Chandramohan, R. Sathyamoorthy, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Asokan, V. Ganesan, T. Shripathi, U.P. Deshpande, Appl. Phys. A 94 (2009) 703. [49] H. Metin, M. Ari, S. Erat, S. Durmus, M. Bozoklu, A. Braun, J. Mater. Res. 25 (1) (2010) 189. [50] J.N. Pankove, in: Optical Processes in Semiconductors, Dover, New York, 1971. [51] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum Press, London and New York, 1974, p. 175. [52] F. Urbach, Phys. Rev. 92 (1953) 1324. [53] N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, 287, Clarendon, Oxford, 1979. [54] H. Metin, F. Sat, S. Erat, M. Ari, J. Optoelectron. Adv. Mater. 10 (10) (2008) 2622. [55] A.N. Yazici, M. 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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
The thermoluminescence properties of CdS films under nitrogen atmosphere H. Metin a,n, S. Erat b,c, F.M. Emen d, V. Kafadar e, A.N. Yazici e, M. Ari f, N. Kulcu g a
Mersin University, Department of Physics, 33343 Mersin, Turkey Laboratory for High Performance Ceramics, EMPA-Swiss Federal Laboratories for Materials Testing & Research, CH-8600 D¨ ubendorf, Switzerland Department of Nonmetallic Inorganic Materials, ETH Z¨ urich-Swiss Federal Institute of Technology, CH-8053 Z¨ urich, Switzerland d Kırklareli University, Department of Chemistry, Kırklareli, Turkey e Gaziantep University, Department of Engineering Physics, Gaziantep, Turkey f Erciyes University, Department of Physics, Kayseri, Turkey g Mersin University, Department of Chemistry, 33343 Mersin, Turkey b c
a r t i c l e in fo
abstract
Article history: Received 11 January 2010 Received in revised form 19 March 2010 Accepted 22 March 2010 Available online 8 April 2010
Chemically deposited cadmium sulphide (CdS) films have been grown on glass at 60 1C and annealed at nitrogen atmosphere at different temperatures. The as-deposited film shows a mix phase of cubic and hexagonal. Once the film subjected to annealing the hexagonal phase becomes dominant and the crystal size increases due to these changes optical band gap energy decreases from 2.44 to 2.28 eV. The electrical conductivity increases depending on temperature and the film annealed at 423 K shows the highest conductivity. Thermoluminescence (TL) intensity of the films was measured after irradiating the films with 90Sr/90Y b-source and the trap depths were calculated after the TL curves deconvoluted by using the computer glow curve deconvolution (CGCD) method. It is observed that the as-deposited film has three different trap depths, at around 0.257, 0.372, and 0.752 eV corresponding to 383, 473, and 608 K, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: Semiconductor Thin film CBD Crystal structure Thermoluminescence
1. Introduction Chemical bath deposition (CBD) is known to be a simple, low temperature, and inexpensive large-area deposition technique for group II–VI semiconductors such as CdS [1]. Over the years the CdS thin films have been intensively studied and have been used in electronic and optoelectronic devices. Thin polycrystalline films of CdS can be prepared by different methods such as sputtering [2,3], screen printing [4–6], spray pyrolysis [7,8], spin coating [9], electrodeposition [10,11], chemical bath deposition (CBD), etc [12–24]. Among these various techniques, the CBD is the most successful method used in the production of uniform, adherent, and reproducible large-area thin films for solar-related application [25,26]. The chemically deposited thin films show high resistivity (106 7 O cm) at room temperature [12–14,17,27–32], most probably because of the lattice defects and dislocations in films [33–37]. The physical and chemical properties of the most of the binary metal chalgonide of II–VI semiconductors such as CdS, CdSe, etc. obtained by using the CBD strongly depend on various preparative parameters such as concentration of metal and chalgonide ions, pH of the deposition solution, deposition time and temperature, and gas phase process parameters like temperature and gas partial pressure.
n
Corresponding author. Tel.: + 90 324 361 00 01/4622; fax: +90 324 361 00 46. E-mail address: [email protected] (H. Metin).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.03.025
The photo- and thermal-conductivities of CdS films are related to impurities or vacancies present in the crystal. Accordingly, there have been many reports on the measurements of photoconductivity and thermally stimulated conductivity of CdS thin films but there has not been sufficient work on the thermally stimulated luminescence (TL) properties of this material up to now [38,39]. It is known that the TL intensity from semiconductive materials is generally difficult to measure above the room temperature. Therefore, there has not been sufficient work on the TL properties of CdS, either in bulk or thin film forms, above the room temperature up to now. In a previous investigation, it has been shown that the TL properties of CdS were affected by the annealing in various conditions [38]. In addition, the TL was observed above the room temperature in CdS-doped glasses after exposing the intense light at 300 K [40,41]. In this work CdS films with 1.8 mm thickness were prepared by CBD technique at 60 1C. The films were annealed in nitrogen atmosphere in order to prevent reaction of the film with the oxygen in the air. The structural characterization of the films was carried out by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) techniques. The optical, electrical conductivity, and thermoluminescence properties of these films were investigated systematically. The trap parameters were calculated by the computer glow curve deconvolution (CGCD) method from thermoluminescence (TL) glow curves. Also, the effect of annealing temperature on optical, electrical, morphological, and thermoluminescent properties is the scope of this work and discussed in detail.
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2. Experimental details The CdS films have been deposited on glass substrates (with a size of 75 mm 25 mm 2 mm) using the CBD technique. These substrates were cleaned by following the procedure adopted by Metin and Esen [42] in order to reduce the impurities. Aqueous solutions of 1 M cadmium sulphate, 2.25 M hydrazine, 1.4 M thiourea, and 25% NH3 were prepared without any precipitation to deposit CdS film. The film was deposited on the glass substrate by keeping it vertically in the beaker without stirring at 60 1C for 2 h. This process was repeated nine times to get thicker films and the deposition solution was refreshed for each time. After deposition, the films were rinsed in distilled water and methanol to remove the loosely adhered CdS particles on the surface and finally dried in air. The six as-deposited yellow CdS films with 1.8 mm thickness were uniform and well adherent to the substrates. Five of the films were annealed at 423 KrTr823 K (in steps of 100 K) with 5 K/min heating/cooling rate and 1 h dwelling time using Protherm (PTF 15/ 75/610) type of horizontal tube furnace under 99.999% pure N2 flow with flow rate 100 ml/min. The CdS films were structurally characterized by XRD, in the range of diffraction angle 201 r2y r601 in steps of 0.021 with ˚ using Bruker AXS 40 kV at 30 mA, CuKa1 radiation (l ¼1.5406 A) Advance D8 diffractometer. Elemental composition of the films was measured using PANanalitical Axios Advanced X-ray fluorescence (XRF). The microstructures of these films were characterized using a LEO 440 SEM. The absorption spectra of the CdS films were measured by using UV–visible spectrophotometer (SHIMADZU UV-1700) at room temperature in the wavelength range 190–1100 nm. The electrical measurements of the films were carried out by means of four-point probe technique. Four platinum wires with 0.5 mm diameter were used two of which for detecting potential drop and the others for applying a constant current across the thin films. Keithley 2400 sourcemeter and Keithley 2700 multimeter have been used for applying current and measuring potential difference, respectively. Temperature of the films was measured by using a K type thermocouple, while the film was in the programmable furnace. The data have been recorded through an interface card controlled by a PC. The glow curves of CdS films were measured using a Harshaw QS 3500 manual type TL reader that is interfaced to a PC where all glow curves were analyzed. Glow curve readout was carried out on a platinum planchet at a linear heating rate of 1 1C/s. A continuous flux of nitrogen was used during the TL readouts to reduce chemiluminescence in the heated samples. All the films were irradiated using the same experimental setup of a 90Sr/90Y b-source (2.27 MeV) at a dose rate of approximately 0.04 Gy/s at room temperature.
Fig. 1. X-ray diffractograms of the as-deposited and the annealed CdS films at different temperatures.
3. Results and discussion 3.1. Structural and morphological studies The X-ray diffraction patterns of the as-deposited and annealed films are shown in Fig. 1. The as-deposited and annealed CdS films (To623 K) show a mixture of cubic (C) phase (zinc blende-type) and hexagonal (H) phase (wurtize-type) at low temperature. It has been reported that chemically deposited CdS films onto the glass substrate are either hexagonal or cubic phase [28,29,32,42–46]. It is known that there are two possible ways for the deposition of CdS films during CBD process: (i) ‘‘ion-by-ion’’ process (heterogeneous reaction) and (ii) ‘‘cluster-by-cluster’’ process (homogeneous reaction), which results in pure hexagonal or mixed hexagonal-cubic phase and pure cubic phase, respectively [8,30]. The diffraction
patterns of the films were checked against the JCPDS 06-0314 and JCPDS 10-0454 reference patterns. The peaks due to the hexagonal (0 0 2) plane or cubic (1 1 1) plane diffraction appear at approximately at 26.661 for the as-deposited and annealed film at 423 and 523 K. The intensity of the peaks increases with increase in annealing temperature, which causes a sharpening of the Bragg reflections, revealing an increase in the crystallinity of the film. Also, the hexagonal phase becomes dominant with increase in the temperature and at T4623 K the structure becomes pure stable hexagonal phase. The phase transformation from cubic to hexagonal was also observed for the chemically deposited CdS films before [12,13,22,24,27,31]. The crystallite size of the films was determined from X-ray diffraction data. We use the standard (1 1 1) C reflection at 2y ¼26.661. The evaluation of the crystallite size was quantified
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by using Scherrer’s formula Dhkl ¼
Kl b cos y
ð1Þ
1533
where K is the Scherrer constant, b is the full-width at halfmaximum (FWHM) of the Bragg reflection under the consideration in radians, l is the wavelength of X-ray used, and y is the Bragg
Table 1 The structural parameters of as-deposited and annealed CdS films. Annealing temperature (K)
FWHM b 10 3 (rad)
Crystallites size (nm)
Dislocation density (1014 lines/m2)
Number of crystallites/unit area (1016 m 2)
Strain (10 3)
As-deposited 423 523 623 723 823
5.651 4.986 4.322 2.659 1.828 1.329
25.21 28.60 33.00 53.60 78.00 107.20
15.73 12.22 9.18 3.48 1.64 0.87
11.23 7.69 5.01 1.17 0.38 0.15
1.26 1.14 0.96 0.59 0.41 0.30
Fig. 2. The SEM micrographs of the CdS films (a) as-deposited and at (b) 423 K, (c) 523 K, (d) 623 K, (e) 723 K, and (f) 823 K.
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angle. Eq. (1) was used for the calculation of the crystallite sizes using K¼0.9. The crystallite size of the as-deposited film was 25.21 nm and increases by almost a factor of 4 with the annealing temperature. Furthermore, using these data the dislocation density (d ¼ 1=D2hkl ), the number of crystallites per unit surface area (N ¼ d=D3hkl , where d is the film thickness), and strain (e ¼ b cos y/4) in the films were determined. The dislocation density is a measure of the quantity of the dislocation present in a material and in the order of 1010 m 2 in a metal. The dislocation density and the strain on the surface of the films are well explained in the literature [9,17,22,47–49]. The crystal size increases with increase in annealing temperature, which results in decrease in the defects like dislocation density and strain in the film (Table 1). XRF was used to determine the elemental composition of the films and it was found that the ratio of Cd/S was almost constant within the detection limit and sensitivity of the instrument. The SEM images of as-deposited and annealed CdS films are shown in Fig. 2. It is seen that the substrate is well covered because of the growth mechanism of ‘‘ion-by-ion’’. Although the distribution of the well-defined grains is not uniform throughout the entire region, the films do not show any voids, pinholes or cracks, except for the one annealed at 823 K where the sintering effect already started. The as-deposited film is made of oblate structured units of 0.7 mm and 100 nm thicknesses. The globules which look like cauliflowers are getting bigger with increasing annealing temperature. The oblate structured units start to be packed to form globules. Especially, for the film annealed at 723 K it is easy to see these new globules with 1 mm diameter. 3.2. Optical properties The optical transmission of the as-deposited and annealed CdS films was measured using UV–visible spectrophotometer at room temperature. The transmittance (T) of the as-deposited and annealed CdS films is shown in Fig. 3 as a function of wavelength. The thickness of the films was calculated from the transmission interference using following equation:
and li + 1, respectively [49,50]. The thickness of the CdS film increases linearly with a growth rate of 100 nm/h (Fig. 4). The wavelength dependent linear absorption coefficient a of the as-deposited and annealed CdS films will now be analyzed in order to determine the optical band gap of the film and the nature of the optical transitions involved by using this equation: It ðd,hnÞ ¼ Io eaðhnÞd
ð3Þ
where d is the thickness of the film, hv is photon energy, and It and Io are the intensity of transmitted and incoming light, respectively. Tauc [51] has identified three distinct regions in the absorption spectrum of amorphous semiconductors: (a) the weak-absorption tail which originates from defects and impurities, (b) the exponential edge region (1 cm 1 o E a o E104 cm 1) which is strongly related to the structural randomness of the glassy material, and (c) the high absorption region which determines the optical energy gap. In the exponential edge region the absorption coefficient a(hn) is well described by the exponential law hv ð4Þ a ¼ a0 exp E0 known as the Urbach law [52]. Here a0 is a constant, hn is the incident photon energy, and E0 is called Urbach energy, which characterizes the slope of the exponential edge region and is width of the band tails of the localized states. The Urbach tail of the absorption edge is usually ascribed to the optical electronic transitions between the extended states and the near edge localized states. The formation of localized states with energies at the boundaries of the energy gap is one of the effects of the structural disorder on the electronic structure of amorphous materials. This is the reason why the Urbach energy (E0) is frequently used as a measure of the degree of structural disorder. E0 is given by the relation given below E0 ¼
1 dðlnðaÞÞ dðhvÞ
ð5Þ
where d is the film thickness, and n(li) and n(li + 1) are the refractive indices at the two adjacent maxima (or minima) at li
The graph of ln (a) versus hv being the photon energy falling the energy region where a0 o104 cm 1 is given in Fig. 5. The value of E0 is calculated from the slope of the linear plot illustrated in Fig. 5. The as-deposited film has E0 ¼0.22 eV and decreases with increase in annealing temperature up to 723 K and then starts to increase (see Table 2).
Fig. 3. The transmittance versus wavelength for the as-deposited and the annealed CdS films.
Fig. 4. Time dependence of the film thickness for CdS films.
d¼
li li þ 1 2½nðli Þli þ 1 nðli þ 1 Þli
ð2Þ
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In the high absorption region (where absorption is associated with interband transitions), the form of the absorption coefficient a(hn) was given in quadratic form by Tauc [51] and discussed in more general terms by Mott and Davis [53], who use the equation of the form
aðhnÞ ¼ AðhnEg Þn
3.3. Electrical properties The temperature dependent electrical conductivity of the films was measured by the four-point probe technique. The conductivity of the films has been determined by using the following expression [54]:
ð6Þ
where A is a constant, a is absorption coefficient, hv is the photon energy, and n is a constant equal to 1/2 for direct band gap semiconductor. The value of the energy where a ¼0 is known as band gap energy Eg. The a2 versus hn, the effect of the as-deposited and the annealed CdS films is given in Fig. 6. It can be seen in Fig. 6 that the absorption edge of the annealed films has obviously shifted towards larger wavelengths with increase in annealing temperature. Also, it is obvious that the band gap energy value of the as-deposited film is 2.44 eV and systematically decreases to 2.28 eV for film at 823 K (Table 2), which is in agreement with the literature [9,22,47]. At high temperature (823 K) the film is seen as degraded in terms of optical absorption at the forbidden region. It is obvious that an increase in the crystallite size gives rise to a decrease in band gap energy. The extinction coefficient k¼
1535
al
ð7Þ
s¼
1 I 2ps V
ð8Þ
where I is the current, V is the potential drop, and s is the distance between the probe tips. The conductivity results of the thin films indicate that the samples have typical semiconductor properties, as can be seen in Fig. 7. The values lie in between 8 10 3–1.2 10 1 S/m. The conductivity of the films starts to increase exponentially from approximately 600 K for the as-deposited film and reaches to highest values at 625 K. The film annealed at 423 K shows the highest conductivity. The activation energy Ea of the films has also been determined using the results obtained from the electrical conductivity measurements. We have used a standard equation to calculate Ea, which can be given as
s ¼ s0 expðEa =kB TÞ
ð9Þ
of the CdS films was obtained taking into account of the a values at l ¼550 nm for the as-deposited and the annealed CdS films and is listed in Table 2. The increase in the extinction coefficient with increase in annealing temperature is paralleled by the change in the absorbance of the films.
where s is the conductivity of the thin film samples, s0 the preexponential factor, Ea the activation energy, kB the Boltzmann constant, and T temperature. The activation energy results of the samples are given in Table 2. The results show that the activation energy decreases with increase in annealing temperature up to 523 K and starts to increase (0.245 eV) at 623 K and then again starts to decrease.
Fig. 5. The ln (a) versus hv for the as-deposited and the annealed CdS films.
Fig. 6. The a2 versus hn for the as-deposited and the annealed CdS films.
4p
Table 2 The optical and electrical parameters of as-deposited and annealed CdS films. Annealing temperature (K)
Eg (eV)
Urbach tail E0 parameter (eV)
Absorption coefficient 104 (cm 1) (l ¼550 nm)
Extinction coefficient (l ¼550 nm)
Activation energy Ea (eV)
As-deposited 423 523 623 723 823
2.44 2.42 2.38 2.35 2.30 2.28
0.22 0.19 0.14 0.11 0.78 1.22
0.4017 0.6676 0.7355 0.5831 1.9297 3.2637
0.0175 0.0292 0.0322 0.0255 0.0844 0.1430
0.265 0.150 0.066 0.245 0.152 0.106
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Fig. 7. The temperature dependence of the electrical conductivity of as-deposited and annealed CdS films.
3.4. Thermoluminescence properties It is well known that the kinetic analysis of the trapping states is correctly obtained from the TL glow curves of the samples if the contributions from the glass substrates and detection system are both eliminated. Therefore, the readout cycle was repeated after each evaluation in order to guarantee that no remaining TL signal was still present on the sample and the second readout without the irradiation of sample with b-rays is considered to be the background of the reader plus sample and glass, which was subtracted from the first one and all of the analysis have been carried out after the subtraction. The recorded glow curves were analyzed using the computer glow curve deconvolution (CGCD) method [55]. The CGCD program uses a linear least-square minimization procedure to determine peak area, activation energy, frequency factor, and kinetic order and the following model was used in the program to analyze the
glow curve: Ea ðb1Þs kB T 2 Ea exp 1þ IðTÞ ¼ n0 s exp kB T b Ea kB T b b1 kB T 0:99201:620 Ea
ð10Þ
where n0 (m 3) is the number of the trapped electrons at t¼0, s (s 1) is the frequency factor, Ea (eV) is the activation energy, T (K) is the absolute temperature, kB (eV K 1) is the Boltzmann’s constant, b (1C s 1) is the heating rate, b is the kinetic order, and I is the TL peak intensity. The experimental and the deconvoluted glow curves of the films are shown in Fig. 8. As seen the TL glow curves of the asdeposited film exhibit two strong TL glow peaks at around 473 and 608 K and a low intensity glow peak at around 383 K when heated at a constant heating rate of 1 1C/s. After a careful investigation of
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Fig. 8. A typical analyzed glow curve of as-deposited and the annealed CdS film measured after 5 min ( E9 Gy) b-irradiation at RT. The glow curve was measured by heating the sample to 673 K at a rate of 1 1C/s. In the figure, open circles represent the experimental points.
Table 3 The values of the kinetic parameters of TL peaks of as-deposited and the annealed CdS thin films determined by the CGCD method.
As-deposited P1 P2 P3 423 K P1 P2 P3 623 K P1 P2 P3 723 K P1 P2 P3
TM (K)
E (eV)
b
383 473 608
0.26 0.37 0.75
1.42 1 1.05
348 453 608
0.29 0.48 1.18
1.026 1 1.987
373 473 608
0.37 0.35 0.88
2 1 1.124
383 473 608
0.38 0.43 0.77
2 1 1.124
this figure, it is observed that the glow curve structure of the film is described by a linear combination of at least three glow peaks between RT and 673 K and a best fit was always obtained by assuming that the first (P1) and third peaks (P3) are of general-order kinetics and the second peak (P2) is of first-order kinetics. The trapping parameters obtained after fitting are listed in Table 3. It is seen that the trapping parameters are affected by the annealing temperature. For example, the changes in the trap depths of P1, P2,
Fig. 9. Comparison of the glow curves of CdS films annealed at different temperatures (D E9 Gy).
and P3 are in the range of 0.26–0.38, 0.35–0.48, and 0.75–1.18 eV, respectively. The TL glow curves of the annealed films are compared in Fig. 9. As it is shown in the figure, the TL intensity decreases with increase in annealing temperature. The TL of CdS films obtained in the present study can be explained by the following process. As mentioned previously, the
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CdS is a wide band gap material (semiconductive material) and therefore it is very difficult to measure the TL intensity of it above room temperature. However, when the CdS thin film was grown on glass by using the chemical bath deposition technique, they form a glass matrix plus CdS thin film composition. The crystallite size of the as-deposited film in the given study is found in the order of 25 nm, which confirms the nano-crystalline form of the developed samples. Nanomaterials are defined as those materials whose length scales lie within the nanometric range, i.e. in the range between one to a hundred nanometers [56]. It is well known that within this region, the materials exhibit different behavior from that of their bulk materials. In the literature there are extensive studies on the luminescence properties of semiconductor nanocrystals [57,58]. In those studies, it was found that the luminescence process in semiconductor nanocrystals is a very complex phenomenon and it was attributed to that the luminescence of semiconductor nanocrystals is generally arising from the deep traps of surface states whose energy levels lie within the band gap of semiconductor [55]. In nanophosphors the number of the ions on the surface of samples quickly increases as the crystallite size decreases. As mentioned previously, the crystallite size of the deposited film in the given study increases with increase in annealing temperature and some of the typical glow curves obtained after different annealing temperatures are shown in Fig. 9. As seen from this figure, the structures of the TL glow curves of CdS nanocrystals remain constant without any observable change in the structure of the glow curves for different annealing temperatures but the TL intensity decreases with increase in annealing temperatures. Now, during the ionizing irradiation of the CdS films, the electron–hole pairs are generated within the crystal structure of the films by excitation of electron from the valence band to the conduction band. The excited electrons and holes from the surface ions are easily trapped at the surface states. So, the trapped carriers at the surface states are released by heating the sample and they recombine with each other and give out luminescence, which is known as thermoluminescence. The number of surface ions and surface states increases due to the increase in the surface to volume ratio with decreasing crystallite size, so the number of trapped charged particles at surface state increases, thus TL efficiency is increased. Therefore, the TL of CdS nanophosphor may be related with the surface states.
4. Conclusion The as-deposited CdS film shows a mixture phase of 50% hexagonal and 50% cubic where the hexagonal phase becomes dominant upon annealing. This goes along with increasing crystallite size and a reduction of the dislocation and strain of the films. Structural phase transformation from metastable cubic phase to the stable hexagonal phase is observed at TZ623 K. The optical band gap of the as-deposited film (2.44 eV) decreases with increase in annealing temperature due to an increase in crystallite size, which also results in decrease of TL efficiency of the film. The film annealed at 423 K shows highest electrical conductivity and also highest TL efficiency. Therefore, it might be suggested that 423 K is the best annealing temperature for these CdS films for the application.
Acknowledgements Financial support from the the Mersin University Research Found (contract no: BAP-FEF FB (HM) 2004-3), the Erciyes University Research Found (contract no: FBA-07-041), the National Swiss Foundation (NSF No. 200021-116688 for S. Erat), and Turkish Council of Scientific and Industrial Research (TUBITAK, 107T392) is gratefully acknowledged.
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