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Study of some physical properties of ultrasonically spray deposited silver doped lead sulphide thin films
Materials Science in Semiconductor Processing 68 (2017) 288–294
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Study of some physical properties of ultrasonically spray deposited silver doped lead sulphide thin films
MARK
⁎
Emrah Sarica , Vildan Bilgin Canakkale Onsekiz Mart University, Department of Physics, 17020 Canakkale, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrasonic Spray Pyrolysis PbS Thin Films Ag Doping Structural Properties Optical and Electrical Properties Morphological Properties
In this study, undoped and Ag doped PbS thin films at different concentrations were deposited onto glass substrates at 225 °C by using ultrasonic spray pyrolysis technique, in order to investigate the effect of Ag doping on the physical properties of PbS thin films. Structural investigations revealed that all doped PbS:Ag thin films have cubic structure and Ag doping enhances crystalline level of PbS thin films. It was determined that average crystallite size of PbS:Ag thin films increased from 24 nm to 49 nm by increasing Ag doping concentration. Morphological studies showed that surfaces of the films become denser after Ag doping. Optical transmittance and absorption spectra revealed that all deposited thin films have low transmission and high absorbance within the visible region and band gap energy of the PbS:Ag thin films were determined to be in the range of 1.37 eV and 1.28 eV by means of optical method. Electrical conductivity type of PbS:Ag films was determined to be ptype and calculated electrical resistivity was found to be lowest for Ag-doped PbS thin films at 2%.
1. Introduction In recent years, PbS which is the chalcogenide semiconductor material has gained great interest due to its unique properties. PbS has a narrow band gap (∼0.41 eV at room temperature) in bulk form with direct transition [1]. However, its larger exciton Bohr radius (18 nm) results in strong quantum size confinement effect and this phenomenon allows the tuning of the optical band gap by forming nano-sized PbS materials [2]. In addition to having enable to adjust of band gap, the fact that PbS thin films have p-type conductivity as well as high optical absorption coefficient (∼105 cm−1) make them attractive for solar cell applications [2,3]. On the other hand, PbS has been used as a material various applications such as photo-detectors [4], gas sensors [5], FET for heavy metal concentration monitoring [6] etc. As seen from the literature, various deposition techniques including aerosol assisted chemical vapor deposition (AACVD) [7], RF sputtering [8], vacuum evaporation [9], successive ionic layer adsorption and reaction (SILAR) [10], spin coating [11], chemical bath deposition (CBD) [12], spray pyrolysis (SP) [13] have been adopted for deposition of PbS films. It has been also noted that deposition parameters have a significant effect on the physical properties of the PbS. Such that, Beddek et al. investigated the effect of Sulphide concentration and lead source on PbS thin films by using CBD and they stated that crystalline level of PbS thin films prepared by using lead acetate as Pb2+ source enhanced with decreasing thiourea concentration yet the inverse
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behavior was noted for the thin films prepared by using lead nitrate [1]. Another investigation carried out by Ji et al. has shown that increasing source of S2- in the precursor led to increase in crystallite size indicating that enhances structural properties while adversely affects the morphological properties of PbS thin films [14]. In addition, Seghaier et al. expressed that good quality PbS thin films (in terms of structural and morphological properties) can be obtained by optimizing the bath time [15]. Deposition temperature is another important parameter, especially for the spray pyrolysis technique, it may cause to change in not only crystal structure but also in electrical conduction mechanism of PbS thin films as pointed out by Veena et al. [16]. All these variations in crystalline structure are also reflected in optical properties of PbS, because of its strong quantum confinement effect. Intentional doping is another way to tailor the physical properties of semiconductor thin films. In recently, the effect of several doping elements, such as Fe [17], Sr [18], Cu [19], Mg [20], Cd [21], Ni [22,23], Zn [24] on physical properties of PbS thin films have been investigated. Ravishankar et al. showed that doping of Fe+2 ions led to crystalline deterioration and the absorption edge shifted to blue. They also unveiled that Fe doped PbS thin films have ferromagnetic behavior while pure PbS thin films have diamagnetic nature [17]. Touati and et al. and Yucel and Yucel examined the effect of copper and strontium doping on the physical properties of CBD deposited PbS thin films, respectively. They noted that both doping of Cu and Sr resulted in decrease in crystalline size and also, as a result of the optical analysis and also
Correspondence to: Çanakkale Onsekiz Mart Üniversitesi, Fen-Edebiyat Fakültesi, Fizik Bölümü, Terzioğlu Yerleşkesi, 17020 Çanakkale, Turkey. E-mail address: [email protected] (E. Sarica).
http://dx.doi.org/10.1016/j.mssp.2017.06.034 Received 30 April 2017; Received in revised form 13 June 2017; Accepted 19 June 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 68 (2017) 288–294
E. Sarica, V. Bilgin
expressed that optical transmittance increased in the longer wavelengths than 900 nm and blue shift in the band gap occurred as a consequence of Cu and Sr doping [17,18]. In the study carried out by Kumar et al., Ni doped Pb thin films deposited by SILAR method exhibited very low transmittance in the UV region and band gap energy of deposited films were found to be 2.45 eV and 2.67 eV. Moreover, it was stated that both undoped and Ni doped PbS thin films room temperature ferromagnetism [22]. As a common point of these studies, it is stated that crystal structure PbS is deteriorated as a result of doping even if some other properties such as optical and electrical properties are improved Exceptionally, it has seen that the doping of Sb enhances the crystallization of PbS films [25]. However, during the literature review, we have realized that there is a limited work on the effect of Ag doping on the physical properties of PbS thin films and a comprehensive investigation has not been performed yet, in spite of the fact it has been reported to incorporation of Ag has an positive impact on the crystallization of thin film semiconductors sulphides such as SnS thin films [26], In2S3 thin films [27] etc. Ag, which is IB group element, known as good electrical conductor and is an attractive doping element due to its dual behavior in the context of electrical transport mechanism. Such that, it acts as an acceptor when it is substituted with cation ions in the host lattice while acts as donor while existing in the interstitial sites [28]. In addition to that, it has ability to be easily diffuse at room temperature [29]. It has been frequently used as acceptor type dopant for several featured semiconductors such as ZnO [30], CdS [31], ZnTe [32], CdTe [33], etc. Therefore, this novel study aims to investigate the effect of Ag incorporation on the physical properties of PbS thin films, systematically. In accordance with this aim, undoped and Ag doped PbS thin films were deposited by using ultrasonic spray pyrolysis. Also, unlike previous studies, ultrasonic nozzles were chosen instead of conventional nozzles (i.e. perfume atomizer) for better atomization of precursor solution.
Fig. 1. XRD patterns of PbS:Ag thin films.
using X-ray diffractometer (PANalytical Empyrean), having CuKα radiation (λ= 1.5405 Å). A continuous scan mode was used to collect 2θ data from 20° to 80° with a step of Δ(2θ) = 0.013°. JEOL SEM-7100EDX Scanning Electron Microscope equipped with an EDS setup was used to take surface images and confirm chemical compositions of all PbS:Ag thin films. Optical transmittance and absorbance spectra were recorded by using Shimadzu UV-2600 spectrophotometer to examine optical properties of deposited films. For electrical analysis, I-V measurements were carried out at room temperature and dark conditions by using Keithley 2400 source meter. Hot point probe technique was also adopted to determine electrical majority carrier types of all deposited films.
3. Results and discussions
2. Experimental
3.1. Structural studies
2.1. Growth of thin films
Fig. 1 shows X-ray diffraction patterns of undoped and Ag doped PbS thin films at various doping concentration. The comparison of these patterns with standard JCPDS cards (65-0346) confirmed that the PbS:Ag thin films were formed with fcc cubic structure. As it can be seen from Fig. 1, the increase in peak intensities was observed up to Ag doped at 3%, but the decrease trend in the peaks intensities was also noted for higher doping concentration. Additionally, it should be noted that, highest intensity was recorded for (200) plane for undoped and Ag doped at 1 and 2 at% PbS thin films, however, intensity of (111) oriented peak was found to be highest for further doping concentration. This variation can be clearly seen from variation of calculated texture coefficient (TC) [35] given in Table 1.
Ultrasonic spray pyrolysis technique described in [34] was adopted in order to deposit of undoped and Ag doped (1, 2, 3 and 4 at%) PbS thin films. For this purpose, firstly, glass substrates were cleaned by sonicating in detergent and distilled water mixture and then rinsed into distilled water by ultrasonic cleaner, in which each steps took 30 min. And then, glass substrates were dried in air. Secondly, precursor solution were prepared by mixing appropriate ratio of 0.05 M lead acetate (Pb(CH3CO2)2·3H2O, purchased from Sigma Aldrich), 0.125 M thiourea (NH2CSNH2, purchased from Sigma Aldrich) and 0.05 M silver nitrate (AgNO3, purchased from Sigma Aldrich) aqueous solutions as Pb2+, S2and Ag+ source, respectively. Distilled water used as solvent during the preparation of all precursor solution. In order to obtain Ag doped PbS thin films, AgNO3 aqueous solution was added to the precursor solution in the amount of 1–2–3 and 4 at%. Thin films were deposited by ultrasonically spraying of precursor solution onto pre-heated glass substrates at substrate temperature of 225 °C. For the sake of enhancement atomization of precursor solution, ultrasonic nozzle, which works at 100 kHz frequency, was used. Precursor solution was sprayed for 30 min and flow rate was kept as 5 ml/min. Distance between nozzle and substrate was maintained at 35 cm and compressed air was used as carrier gas with a pressure of 1 atm. 2.2. Thin film characterization The thickness of all deposited films was determined by using PHE 102 Spectroscopic Ellipsometry and found to be 394 nm, 312 nm, 320 nm, 325 nm and 290 nm for undoped and Ag doped PbS thin films at 1, 2, 3 and 4 at%, respectively. In order to carry out structural analyses of deposited films, X-ray diffraction patterns were recorded by
Fig. 2. Variation of crystallite size and micro-strain depending on Ag doping concentration.
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Fig. 3. AFM images of PbS:Ag thin films a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
TC(hkl) =
I(hkl)/ I0(hkl) 1/ N [∑N I(hkl)/ I0(hkl) ]
Table 2 Interplanar spacing (d), full width half-maximum (FWHM), miller indices, crystallite size (D) and micro-strain values (ε).
(1)
Where I(hkl) is the measured intensity of the corresponding orientation, I0(hkl) is the standard intensity JCPDS and N is the number of diffraction peaks. TC(hkl)∼1 represents randomly oriented crystallites, while TC > 1 indicates the preferential orientation for given hkl direction [36]. Calculated TC coefficient for (111) and (200) orientation were found to be slightly higher than 1 and it was seen that TC for (200) is higher than (111) orientation for undoped and Ag doped PbS thin films at 1 and 2 at %, when TC of these two peaks are compared. However, this competition was reversed for the higher Ag doping concentration. Similar results have been reported by others [1,22,23]. Mean crystallite size was calculated by using Scherrer equation [37] and listed in Table 2.
D=
Kλ βcosθ
Material
2θ (°)
d (Å)
FWHM (°)
hkl
a (Å)
D (nm)
ε×10−3
Undoped PbS
26.50 30.62 43.61 26.52 30.61 43.63 26.56 30.63 43.66 26.54 30.61 43.62 26.56 30.63 43.67
3.3611 2.9173 2.0739 3.3579 2.9185 2.0727 3.3530 2.9161 2.0715 3.3561 2.9185 2.0733 3.3530 2.9161 2.0710
0.4730 0.3542 0.6227 0.2544 0.2180 0.3345 0.2340 0.2146 0.3164 0.2024 0.1795 0.2811 0.1902 0.1770 0.2616
(111) (200) (220) (111) (200) (220) (111) (200) (220) (111) (200) (220) (111) (200) (220)
5.8216 5.8346 5.8659 5.8160 5.8370 5.8626 5.8075 5.8322 5.8592 5.8132 5.8370 5.8643 5.8075 5.8322 5.8575
18 24 14 34 39 27 36 40 28 42 48 32 45 49 34
8.77 5.65 6.79 4.71 3.48 3.65 4.33 3.42 3.45 3.74 2.86 3.06 3.52 2.82 2.85
PbS:Ag (1%)
PbS:Ag (2%) PbS:Ag (3%)
PbS:Ag (4%)
(2)
Where K is a constant (0.94), λ is the wavelength of x-ray (λCuKα=0.154 nm), β is the full width at half maximum (FWHM) of the corresponding diffraction peak, θ is the Bragg angle. As it can be seen from Table 2, mean crystallite size calculated for three highest peaks tended to increase with the Ag doping concentration. Similarly, there are a few studies related to enlargement in crystallite size as a consequence of Ag doping in the several thin films in the literature. Lin et al. have stated that the increase in the crystallite size as a result of Ag doping in the In2S3 thin films due to the difference between ionic radii
of Ag+ (1.26 Å) and In3+ (0.8 Å) [27]. However, we thought that this evaluation was not adequate to explain the increase in crystallization levels expressed in our work, because the Ag+ ion radius is fairly close to the Pb2+ ion radius (1.20 Å) [38]. Kumar et al. have expressed that new nucleating centers may have been formed after the incorporation of Ag and it may have led to improvement in crystalline level of SnS thin film and crystallite size with the Ag doping [39]. We also think that
Table 1 Relative intensity of the peaks according to the (200) oriented peak (I/I(200)) and calculated texture coefficients (TC) of PbS:Ag thin films. ∼2θ
JCPDS
Ag doping concentration Undoped PbS I/I
26.5 30.6 43.5
94.3 100 64.1
(200)
76.03 100 41.83
PbS:Ag (1%) TC
I/I
1.09 1.35 0.88
74.10 100 37.23
(200)
PbS:Ag (2%) TC
I/I
1.34 1.71 0.99
79.46 100 29.05
290
(200)
PbS:Ag (3%) TC
I/I
1.58 1.88 0.85
124.9 100 28.16
(200)
PbS:Ag (4%) TC
I/I
2.21 1.67 0.73
99.31 100 37.71
(200)
TC 1.62 1.54 0.90
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Fig. 4. SEM images of PbS:Ag thin films with insertion of low magnification images a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
Fig. 4 showed that some voids existed for undoped and Ag doped PbS thin films at 1 at% but for further Ag doping concentration, these voids vanished. As a result of morphological analyses with an overall approach, we can say that Ag incorporation significantly influenced surface morphology of the films and surface homogeneity increased. Indeed, this positive effect of the Ag incorporation is compatible with the structural analyses and we think that Ag incorporation promotes crystalline growth and prevents voids formation by increasing the possible nucleation centers. From the EDX spectra given in Fig. 5, it was confirmed that the Pb, S and Ag elements are exist in the compositions of the deposited thin films and the percentages are listed in Table 3. As expected, it was determined that the Ag ratio increased in the deposited films due to the increased Ag concentration in the spray solution.
similar process might have taken place with the increase of crystallization level and enlargement in crystallite size of Ag doped PbS thin films. The formation of the possible new nucleation centers may also be related to the improvement of the surface properties of Ag doped PbS films as it can be clearly seen from the SEM images given in Fig. 4. Micro-strain built in thin films was estimated using following equation
ε = βcotθ/4
(3)
It is well known that there is a correlation between crystallite size and micro strain [39,40]. To be clear, reduced micro strain enhances crystalline level of films while increased strain obstructs crystallization and this correlation can also be clearly seen from Table 2 and Fig. 2. Additionally, a lattice parameter was calculated for all films using Eq. (4) and listed in Table 2.
d 2 = a2 / h2 + k 2 + l 2
3.3. Optical analysis
(4)
Transmittance and absorbance spectra of all films taken in the wavelength range of 350–1400 nm were given in Fig. 6 and Fig. 7, respectively. It was observed that all deposited films have fairly low transmittance. Along with that, it should be noted that several factors that may affect the variation in transmittance of thin films such as film thickness, crystallization level, morphological properties etc. and so these factors should be taken into consideration simultaneously. In more detail, enhancements in crystal structure such as increased crystalline level, and so, reduced defects lead to increase the transmittance, while formation of densely packed grains result in decreased in transmittance [40,43]. In our case, despite of being thinner, the lower transmittance of Ag doped at 2% with respect to undoped PbS thin films may be evidence of disappearance of voids along the surface. However, at higher doping concentrations, transmittance began to increase again, and this increase may be due to an improvement in crystallization structure. Inversely, the highest absorption was recorded for Ag doped PbS thin films at 2%, as it can be seen from Fig. 7. Additionally, average linear absorption coefficient in the visible region was calculated for all deposited films by using Beer-Lambert formula [44] and listed in Table 4. Absorption coefficient was found to be around 105 cm−1 for all films and it was noted that highest for Ag doped PbS thin films at 2%. Tauc's relationship (given in Eq. (5)) was adopted in order to determine optical band gap by extrapolating the linear portion of (αhν)2 vs. (hν) at (αhν)2=0 as given in Fig. 8.
where d is the interplanar spacing and hkl is the miller indices [41]. As it can be seen from Table 2, calculated lattice parameter was smaller than reference value (5.938 Å) for all films and so this result revealed that compressive stress exists for both undoped and Ag doped PbS thin films. Resultant stress may have arisen from the difference in thermal expansion between the substrate and the film [42]. 3.2. Surface morphology and elemental analysis Fig. 3 and Fig. 4 illustrate AFM and SEM images taken to perform morphological analysis, respectively. In Fig. 3, dark and bright regions which represent hollow and hill formation were observed through the surface of all deposited films. However, as distinct from undoped PbS thin films, size of black regions seemed to be reduced as a result of Ag doping and it was also observed that the maximum height of the hills on the film surfaces decreased up to amount of 2 at% Ag doping concentration and reached the minimum level, then increases again with increasing Ag doping concentration. From SEM images given in Fig. 4, nano-scaled particles, which are smaller than approximately 100 nm, can be seen easily for undoped films. However, with the Ag incorporation, it can be also seen that the spaces between these particles were filled up and the size of particles which existing along the film surface increased. It can be also said that PbS thin films become denser as the Ag concentration increases. Moreover, inset images given in 291
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Fig. 5. EDS spectra of PbS:Ag thin films. Table 3 Elemental compositions of PbS:Ag thin films. At% (from EDS)
Ag doping concentration Undoped PbS
PbS:Ag (1%)
PbS:Ag (2%)
PbS:Ag (3%)
PbS:Ag (4%)
Pb S Ag
56.3 43.7 0
56.3 42.8 0.8
55.0 43.0 2.0
55.8 41.8 2.4
53.0 43.1 3.9
Fig. 7. Absorption spectra of PbS:Ag thin films. Table 4 Absorption coefficient (α), optical band gap (Eg) and electrical resistivity (ρ) of PbS:Ag thin films. Material
α×105 (cm)−1
Eg (eV)
ρ (Ω cm)
Undoped PbS PbS:Ag (1%) PbS:Ag (2%) PbS:Ag (3%) PbS:Ag (4%)
1.00 0.78 1.73 0.97 0.83
1.37 1.34 1.35 1.37 1.28
30.8 19.1 15.4 21.7 47.2
Fig. 6. Transmittance spectra of PbS:Ag thin films.
(αhν )n = A (hν − Eg)
in semiconductor materials doped with various elements, the widening and shrinkage in the band gap is mainly explained by the Burstein-Moss effect in relation to the carrier concentration [47]. On the other hand, Al-Ghamdi et al. have stated that chalcogenide materials predominantly contain broken bonds and crystal imperfections which lead to formation of localized states within the band gap and it has also been suggested that these localized states may increase as a result of Ag incorporation and thus, band gap shrinkage occurs [48]. In the present study, even though the average crystal size of the Ag-doped PbS thin films at 1% was remarkably increased compared to the un-doped films, this change was not observed in the variation of band gap. This was probably due to the fact that the concentration of the carrier may have increased as seen from the electrical analysis. However, in general, we can say that optical band gap of Ag doped thin films tend to shrinkage compared to undoped PbS thin films, except for Ag doped at 3% and the shrinkage in band gap may be associated with an increase in the average crystal size of the materials with the incorporation of Ag.
(5)
where n is equal to 2 for allowed direct transition, A is a constant, h is Planck constant,ν is frequency and Eg is the band gap energy [44]. As it can be seen from Fig. 8, linear dependence of Tauc's plots showed that all deposited thin films have direct transition and optical band gap of deposited films was estimated between 1.37 eV and 1.28 eV as listed in Table 4. Band gap energy of the films was quite larger than its bulk value (∼0.41 eV). There are many studies in the literature which have indicated that the forbidden energy range is widened due to the quantum size confinement effect of PbS thin films which have relatively large Bohr exciton radius (∼18 nm) [38,45]. It is well known that larger crystals lead to shrinkage in the band gap while smaller ones caused to widening. For this reason, it must be considered that all deposited thin films are polycrystalline which contains various crystals in different orientation and size and so distribution of different sized crystals become important [46]. In addition to the quantum size effect, 292
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Fig. 8. Plots of (αhν)2vs. hν of PbS:Ag thin films a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
ρ = (∆V / ∆I )(d × l/ L)
3.4. Electrical analysis
(6)
where d, l and L symbolize thickness of film, metal contact length and is the distance between metal contacts. As it can be seen resistivity values listed in Table 4, electrical resistivity which is 30.8 Ω cm for undoped PbS thin films decreased up to 2% Ag doping concentration and reach to minimum value of 15.4 Ω cm. It was determined that it began to increase again for higher doping concentrations. As it has been also stated by Zhang et al., Ag+ substitution with Pb2+ may have increased the concentration of acceptor states as a consequence of Ag doping [53]. Therefore, we believe that the hole concentration may have been increased by forming acceptor levels in the forbidden band gap of PbS thin films up to 2% Ag doping ratio and so the electrical resistivity decreased. However, the increase in electrical resistivity at higher doping concentration may have been caused by the fact that the Ag+ ions were not substituted by any more Pb2+ ions and created electrically inactive neutral defects within lattice. A similar result has been reported in the study of Cu-doped PbS thin films by others [54].
In order to conduct electrical investigations of the films, Au was selected as the metal contact because of the work function compatibility between Pb (∼4.8, 4.9 eV) and Au (Φ∼5.1, 5.2 eV) for the ohmic conduction [49–51], and coated in planar form on the film surface with the dc plasma sputtering technique. It was determined that all deposited thin films have p-type conductivity by using hot point probe technique and after that I-V characteristics were taken under dark condition at room temperature by using two point probe method. Electrical resistivity of deposited thin films was calculated by using I-V plots given in Fig. 9 and following equation [52].
4. Conclusion In this work, the effect of Ag doping on the some physical properties of the PbS thin films were investigated systematically. XRD results revealed that incorporation of Ag remarkably improved the structural properties of PbS thin films without changing the crystal structure. Calculated mean crystallite size increased from 24 nm to 49 nm. Similarly, enhancement was also noticed in morphological properties of deposited thin films. Particularly, doping at 2% and higher concentration led to disappear of voids on the surface of the film and it could provide significant advantage in terms of application of PbS thin films in opto- electrical fields. As a result of optical analyses, it was seen that
Fig. 9. I-V plots of PbS:Ag thin films.
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all deposited films exhibited fairly high absorption. The highest absorption coefficient was noted for Ag doped PbS thin films at 2%. It was also determined that the optical band gap was found to be in the range of 1.37 eV to 1.28 eV, and it is compatible for solar cell applications. Electrical examinations showed that all deposited thin films have p-type conductivity and electrical conductivity increased up to Ag doping concentration at 2%. It can be concluded that lower doping concentration of Ag (2 at%) may lead to improvement of structural, morphological and electrical properties of PbS thin films.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Study of some physical properties of ultrasonically spray deposited silver doped lead sulphide thin films
MARK
⁎
Emrah Sarica , Vildan Bilgin Canakkale Onsekiz Mart University, Department of Physics, 17020 Canakkale, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrasonic Spray Pyrolysis PbS Thin Films Ag Doping Structural Properties Optical and Electrical Properties Morphological Properties
In this study, undoped and Ag doped PbS thin films at different concentrations were deposited onto glass substrates at 225 °C by using ultrasonic spray pyrolysis technique, in order to investigate the effect of Ag doping on the physical properties of PbS thin films. Structural investigations revealed that all doped PbS:Ag thin films have cubic structure and Ag doping enhances crystalline level of PbS thin films. It was determined that average crystallite size of PbS:Ag thin films increased from 24 nm to 49 nm by increasing Ag doping concentration. Morphological studies showed that surfaces of the films become denser after Ag doping. Optical transmittance and absorption spectra revealed that all deposited thin films have low transmission and high absorbance within the visible region and band gap energy of the PbS:Ag thin films were determined to be in the range of 1.37 eV and 1.28 eV by means of optical method. Electrical conductivity type of PbS:Ag films was determined to be ptype and calculated electrical resistivity was found to be lowest for Ag-doped PbS thin films at 2%.
1. Introduction In recent years, PbS which is the chalcogenide semiconductor material has gained great interest due to its unique properties. PbS has a narrow band gap (∼0.41 eV at room temperature) in bulk form with direct transition [1]. However, its larger exciton Bohr radius (18 nm) results in strong quantum size confinement effect and this phenomenon allows the tuning of the optical band gap by forming nano-sized PbS materials [2]. In addition to having enable to adjust of band gap, the fact that PbS thin films have p-type conductivity as well as high optical absorption coefficient (∼105 cm−1) make them attractive for solar cell applications [2,3]. On the other hand, PbS has been used as a material various applications such as photo-detectors [4], gas sensors [5], FET for heavy metal concentration monitoring [6] etc. As seen from the literature, various deposition techniques including aerosol assisted chemical vapor deposition (AACVD) [7], RF sputtering [8], vacuum evaporation [9], successive ionic layer adsorption and reaction (SILAR) [10], spin coating [11], chemical bath deposition (CBD) [12], spray pyrolysis (SP) [13] have been adopted for deposition of PbS films. It has been also noted that deposition parameters have a significant effect on the physical properties of the PbS. Such that, Beddek et al. investigated the effect of Sulphide concentration and lead source on PbS thin films by using CBD and they stated that crystalline level of PbS thin films prepared by using lead acetate as Pb2+ source enhanced with decreasing thiourea concentration yet the inverse
⁎
behavior was noted for the thin films prepared by using lead nitrate [1]. Another investigation carried out by Ji et al. has shown that increasing source of S2- in the precursor led to increase in crystallite size indicating that enhances structural properties while adversely affects the morphological properties of PbS thin films [14]. In addition, Seghaier et al. expressed that good quality PbS thin films (in terms of structural and morphological properties) can be obtained by optimizing the bath time [15]. Deposition temperature is another important parameter, especially for the spray pyrolysis technique, it may cause to change in not only crystal structure but also in electrical conduction mechanism of PbS thin films as pointed out by Veena et al. [16]. All these variations in crystalline structure are also reflected in optical properties of PbS, because of its strong quantum confinement effect. Intentional doping is another way to tailor the physical properties of semiconductor thin films. In recently, the effect of several doping elements, such as Fe [17], Sr [18], Cu [19], Mg [20], Cd [21], Ni [22,23], Zn [24] on physical properties of PbS thin films have been investigated. Ravishankar et al. showed that doping of Fe+2 ions led to crystalline deterioration and the absorption edge shifted to blue. They also unveiled that Fe doped PbS thin films have ferromagnetic behavior while pure PbS thin films have diamagnetic nature [17]. Touati and et al. and Yucel and Yucel examined the effect of copper and strontium doping on the physical properties of CBD deposited PbS thin films, respectively. They noted that both doping of Cu and Sr resulted in decrease in crystalline size and also, as a result of the optical analysis and also
Correspondence to: Çanakkale Onsekiz Mart Üniversitesi, Fen-Edebiyat Fakültesi, Fizik Bölümü, Terzioğlu Yerleşkesi, 17020 Çanakkale, Turkey. E-mail address: [email protected] (E. Sarica).
http://dx.doi.org/10.1016/j.mssp.2017.06.034 Received 30 April 2017; Received in revised form 13 June 2017; Accepted 19 June 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 68 (2017) 288–294
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expressed that optical transmittance increased in the longer wavelengths than 900 nm and blue shift in the band gap occurred as a consequence of Cu and Sr doping [17,18]. In the study carried out by Kumar et al., Ni doped Pb thin films deposited by SILAR method exhibited very low transmittance in the UV region and band gap energy of deposited films were found to be 2.45 eV and 2.67 eV. Moreover, it was stated that both undoped and Ni doped PbS thin films room temperature ferromagnetism [22]. As a common point of these studies, it is stated that crystal structure PbS is deteriorated as a result of doping even if some other properties such as optical and electrical properties are improved Exceptionally, it has seen that the doping of Sb enhances the crystallization of PbS films [25]. However, during the literature review, we have realized that there is a limited work on the effect of Ag doping on the physical properties of PbS thin films and a comprehensive investigation has not been performed yet, in spite of the fact it has been reported to incorporation of Ag has an positive impact on the crystallization of thin film semiconductors sulphides such as SnS thin films [26], In2S3 thin films [27] etc. Ag, which is IB group element, known as good electrical conductor and is an attractive doping element due to its dual behavior in the context of electrical transport mechanism. Such that, it acts as an acceptor when it is substituted with cation ions in the host lattice while acts as donor while existing in the interstitial sites [28]. In addition to that, it has ability to be easily diffuse at room temperature [29]. It has been frequently used as acceptor type dopant for several featured semiconductors such as ZnO [30], CdS [31], ZnTe [32], CdTe [33], etc. Therefore, this novel study aims to investigate the effect of Ag incorporation on the physical properties of PbS thin films, systematically. In accordance with this aim, undoped and Ag doped PbS thin films were deposited by using ultrasonic spray pyrolysis. Also, unlike previous studies, ultrasonic nozzles were chosen instead of conventional nozzles (i.e. perfume atomizer) for better atomization of precursor solution.
Fig. 1. XRD patterns of PbS:Ag thin films.
using X-ray diffractometer (PANalytical Empyrean), having CuKα radiation (λ= 1.5405 Å). A continuous scan mode was used to collect 2θ data from 20° to 80° with a step of Δ(2θ) = 0.013°. JEOL SEM-7100EDX Scanning Electron Microscope equipped with an EDS setup was used to take surface images and confirm chemical compositions of all PbS:Ag thin films. Optical transmittance and absorbance spectra were recorded by using Shimadzu UV-2600 spectrophotometer to examine optical properties of deposited films. For electrical analysis, I-V measurements were carried out at room temperature and dark conditions by using Keithley 2400 source meter. Hot point probe technique was also adopted to determine electrical majority carrier types of all deposited films.
3. Results and discussions
2. Experimental
3.1. Structural studies
2.1. Growth of thin films
Fig. 1 shows X-ray diffraction patterns of undoped and Ag doped PbS thin films at various doping concentration. The comparison of these patterns with standard JCPDS cards (65-0346) confirmed that the PbS:Ag thin films were formed with fcc cubic structure. As it can be seen from Fig. 1, the increase in peak intensities was observed up to Ag doped at 3%, but the decrease trend in the peaks intensities was also noted for higher doping concentration. Additionally, it should be noted that, highest intensity was recorded for (200) plane for undoped and Ag doped at 1 and 2 at% PbS thin films, however, intensity of (111) oriented peak was found to be highest for further doping concentration. This variation can be clearly seen from variation of calculated texture coefficient (TC) [35] given in Table 1.
Ultrasonic spray pyrolysis technique described in [34] was adopted in order to deposit of undoped and Ag doped (1, 2, 3 and 4 at%) PbS thin films. For this purpose, firstly, glass substrates were cleaned by sonicating in detergent and distilled water mixture and then rinsed into distilled water by ultrasonic cleaner, in which each steps took 30 min. And then, glass substrates were dried in air. Secondly, precursor solution were prepared by mixing appropriate ratio of 0.05 M lead acetate (Pb(CH3CO2)2·3H2O, purchased from Sigma Aldrich), 0.125 M thiourea (NH2CSNH2, purchased from Sigma Aldrich) and 0.05 M silver nitrate (AgNO3, purchased from Sigma Aldrich) aqueous solutions as Pb2+, S2and Ag+ source, respectively. Distilled water used as solvent during the preparation of all precursor solution. In order to obtain Ag doped PbS thin films, AgNO3 aqueous solution was added to the precursor solution in the amount of 1–2–3 and 4 at%. Thin films were deposited by ultrasonically spraying of precursor solution onto pre-heated glass substrates at substrate temperature of 225 °C. For the sake of enhancement atomization of precursor solution, ultrasonic nozzle, which works at 100 kHz frequency, was used. Precursor solution was sprayed for 30 min and flow rate was kept as 5 ml/min. Distance between nozzle and substrate was maintained at 35 cm and compressed air was used as carrier gas with a pressure of 1 atm. 2.2. Thin film characterization The thickness of all deposited films was determined by using PHE 102 Spectroscopic Ellipsometry and found to be 394 nm, 312 nm, 320 nm, 325 nm and 290 nm for undoped and Ag doped PbS thin films at 1, 2, 3 and 4 at%, respectively. In order to carry out structural analyses of deposited films, X-ray diffraction patterns were recorded by
Fig. 2. Variation of crystallite size and micro-strain depending on Ag doping concentration.
289
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Fig. 3. AFM images of PbS:Ag thin films a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
TC(hkl) =
I(hkl)/ I0(hkl) 1/ N [∑N I(hkl)/ I0(hkl) ]
Table 2 Interplanar spacing (d), full width half-maximum (FWHM), miller indices, crystallite size (D) and micro-strain values (ε).
(1)
Where I(hkl) is the measured intensity of the corresponding orientation, I0(hkl) is the standard intensity JCPDS and N is the number of diffraction peaks. TC(hkl)∼1 represents randomly oriented crystallites, while TC > 1 indicates the preferential orientation for given hkl direction [36]. Calculated TC coefficient for (111) and (200) orientation were found to be slightly higher than 1 and it was seen that TC for (200) is higher than (111) orientation for undoped and Ag doped PbS thin films at 1 and 2 at %, when TC of these two peaks are compared. However, this competition was reversed for the higher Ag doping concentration. Similar results have been reported by others [1,22,23]. Mean crystallite size was calculated by using Scherrer equation [37] and listed in Table 2.
D=
Kλ βcosθ
Material
2θ (°)
d (Å)
FWHM (°)
hkl
a (Å)
D (nm)
ε×10−3
Undoped PbS
26.50 30.62 43.61 26.52 30.61 43.63 26.56 30.63 43.66 26.54 30.61 43.62 26.56 30.63 43.67
3.3611 2.9173 2.0739 3.3579 2.9185 2.0727 3.3530 2.9161 2.0715 3.3561 2.9185 2.0733 3.3530 2.9161 2.0710
0.4730 0.3542 0.6227 0.2544 0.2180 0.3345 0.2340 0.2146 0.3164 0.2024 0.1795 0.2811 0.1902 0.1770 0.2616
(111) (200) (220) (111) (200) (220) (111) (200) (220) (111) (200) (220) (111) (200) (220)
5.8216 5.8346 5.8659 5.8160 5.8370 5.8626 5.8075 5.8322 5.8592 5.8132 5.8370 5.8643 5.8075 5.8322 5.8575
18 24 14 34 39 27 36 40 28 42 48 32 45 49 34
8.77 5.65 6.79 4.71 3.48 3.65 4.33 3.42 3.45 3.74 2.86 3.06 3.52 2.82 2.85
PbS:Ag (1%)
PbS:Ag (2%) PbS:Ag (3%)
PbS:Ag (4%)
(2)
Where K is a constant (0.94), λ is the wavelength of x-ray (λCuKα=0.154 nm), β is the full width at half maximum (FWHM) of the corresponding diffraction peak, θ is the Bragg angle. As it can be seen from Table 2, mean crystallite size calculated for three highest peaks tended to increase with the Ag doping concentration. Similarly, there are a few studies related to enlargement in crystallite size as a consequence of Ag doping in the several thin films in the literature. Lin et al. have stated that the increase in the crystallite size as a result of Ag doping in the In2S3 thin films due to the difference between ionic radii
of Ag+ (1.26 Å) and In3+ (0.8 Å) [27]. However, we thought that this evaluation was not adequate to explain the increase in crystallization levels expressed in our work, because the Ag+ ion radius is fairly close to the Pb2+ ion radius (1.20 Å) [38]. Kumar et al. have expressed that new nucleating centers may have been formed after the incorporation of Ag and it may have led to improvement in crystalline level of SnS thin film and crystallite size with the Ag doping [39]. We also think that
Table 1 Relative intensity of the peaks according to the (200) oriented peak (I/I(200)) and calculated texture coefficients (TC) of PbS:Ag thin films. ∼2θ
JCPDS
Ag doping concentration Undoped PbS I/I
26.5 30.6 43.5
94.3 100 64.1
(200)
76.03 100 41.83
PbS:Ag (1%) TC
I/I
1.09 1.35 0.88
74.10 100 37.23
(200)
PbS:Ag (2%) TC
I/I
1.34 1.71 0.99
79.46 100 29.05
290
(200)
PbS:Ag (3%) TC
I/I
1.58 1.88 0.85
124.9 100 28.16
(200)
PbS:Ag (4%) TC
I/I
2.21 1.67 0.73
99.31 100 37.71
(200)
TC 1.62 1.54 0.90
Materials Science in Semiconductor Processing 68 (2017) 288–294
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Fig. 4. SEM images of PbS:Ag thin films with insertion of low magnification images a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
Fig. 4 showed that some voids existed for undoped and Ag doped PbS thin films at 1 at% but for further Ag doping concentration, these voids vanished. As a result of morphological analyses with an overall approach, we can say that Ag incorporation significantly influenced surface morphology of the films and surface homogeneity increased. Indeed, this positive effect of the Ag incorporation is compatible with the structural analyses and we think that Ag incorporation promotes crystalline growth and prevents voids formation by increasing the possible nucleation centers. From the EDX spectra given in Fig. 5, it was confirmed that the Pb, S and Ag elements are exist in the compositions of the deposited thin films and the percentages are listed in Table 3. As expected, it was determined that the Ag ratio increased in the deposited films due to the increased Ag concentration in the spray solution.
similar process might have taken place with the increase of crystallization level and enlargement in crystallite size of Ag doped PbS thin films. The formation of the possible new nucleation centers may also be related to the improvement of the surface properties of Ag doped PbS films as it can be clearly seen from the SEM images given in Fig. 4. Micro-strain built in thin films was estimated using following equation
ε = βcotθ/4
(3)
It is well known that there is a correlation between crystallite size and micro strain [39,40]. To be clear, reduced micro strain enhances crystalline level of films while increased strain obstructs crystallization and this correlation can also be clearly seen from Table 2 and Fig. 2. Additionally, a lattice parameter was calculated for all films using Eq. (4) and listed in Table 2.
d 2 = a2 / h2 + k 2 + l 2
3.3. Optical analysis
(4)
Transmittance and absorbance spectra of all films taken in the wavelength range of 350–1400 nm were given in Fig. 6 and Fig. 7, respectively. It was observed that all deposited films have fairly low transmittance. Along with that, it should be noted that several factors that may affect the variation in transmittance of thin films such as film thickness, crystallization level, morphological properties etc. and so these factors should be taken into consideration simultaneously. In more detail, enhancements in crystal structure such as increased crystalline level, and so, reduced defects lead to increase the transmittance, while formation of densely packed grains result in decreased in transmittance [40,43]. In our case, despite of being thinner, the lower transmittance of Ag doped at 2% with respect to undoped PbS thin films may be evidence of disappearance of voids along the surface. However, at higher doping concentrations, transmittance began to increase again, and this increase may be due to an improvement in crystallization structure. Inversely, the highest absorption was recorded for Ag doped PbS thin films at 2%, as it can be seen from Fig. 7. Additionally, average linear absorption coefficient in the visible region was calculated for all deposited films by using Beer-Lambert formula [44] and listed in Table 4. Absorption coefficient was found to be around 105 cm−1 for all films and it was noted that highest for Ag doped PbS thin films at 2%. Tauc's relationship (given in Eq. (5)) was adopted in order to determine optical band gap by extrapolating the linear portion of (αhν)2 vs. (hν) at (αhν)2=0 as given in Fig. 8.
where d is the interplanar spacing and hkl is the miller indices [41]. As it can be seen from Table 2, calculated lattice parameter was smaller than reference value (5.938 Å) for all films and so this result revealed that compressive stress exists for both undoped and Ag doped PbS thin films. Resultant stress may have arisen from the difference in thermal expansion between the substrate and the film [42]. 3.2. Surface morphology and elemental analysis Fig. 3 and Fig. 4 illustrate AFM and SEM images taken to perform morphological analysis, respectively. In Fig. 3, dark and bright regions which represent hollow and hill formation were observed through the surface of all deposited films. However, as distinct from undoped PbS thin films, size of black regions seemed to be reduced as a result of Ag doping and it was also observed that the maximum height of the hills on the film surfaces decreased up to amount of 2 at% Ag doping concentration and reached the minimum level, then increases again with increasing Ag doping concentration. From SEM images given in Fig. 4, nano-scaled particles, which are smaller than approximately 100 nm, can be seen easily for undoped films. However, with the Ag incorporation, it can be also seen that the spaces between these particles were filled up and the size of particles which existing along the film surface increased. It can be also said that PbS thin films become denser as the Ag concentration increases. Moreover, inset images given in 291
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Fig. 5. EDS spectra of PbS:Ag thin films. Table 3 Elemental compositions of PbS:Ag thin films. At% (from EDS)
Ag doping concentration Undoped PbS
PbS:Ag (1%)
PbS:Ag (2%)
PbS:Ag (3%)
PbS:Ag (4%)
Pb S Ag
56.3 43.7 0
56.3 42.8 0.8
55.0 43.0 2.0
55.8 41.8 2.4
53.0 43.1 3.9
Fig. 7. Absorption spectra of PbS:Ag thin films. Table 4 Absorption coefficient (α), optical band gap (Eg) and electrical resistivity (ρ) of PbS:Ag thin films. Material
α×105 (cm)−1
Eg (eV)
ρ (Ω cm)
Undoped PbS PbS:Ag (1%) PbS:Ag (2%) PbS:Ag (3%) PbS:Ag (4%)
1.00 0.78 1.73 0.97 0.83
1.37 1.34 1.35 1.37 1.28
30.8 19.1 15.4 21.7 47.2
Fig. 6. Transmittance spectra of PbS:Ag thin films.
(αhν )n = A (hν − Eg)
in semiconductor materials doped with various elements, the widening and shrinkage in the band gap is mainly explained by the Burstein-Moss effect in relation to the carrier concentration [47]. On the other hand, Al-Ghamdi et al. have stated that chalcogenide materials predominantly contain broken bonds and crystal imperfections which lead to formation of localized states within the band gap and it has also been suggested that these localized states may increase as a result of Ag incorporation and thus, band gap shrinkage occurs [48]. In the present study, even though the average crystal size of the Ag-doped PbS thin films at 1% was remarkably increased compared to the un-doped films, this change was not observed in the variation of band gap. This was probably due to the fact that the concentration of the carrier may have increased as seen from the electrical analysis. However, in general, we can say that optical band gap of Ag doped thin films tend to shrinkage compared to undoped PbS thin films, except for Ag doped at 3% and the shrinkage in band gap may be associated with an increase in the average crystal size of the materials with the incorporation of Ag.
(5)
where n is equal to 2 for allowed direct transition, A is a constant, h is Planck constant,ν is frequency and Eg is the band gap energy [44]. As it can be seen from Fig. 8, linear dependence of Tauc's plots showed that all deposited thin films have direct transition and optical band gap of deposited films was estimated between 1.37 eV and 1.28 eV as listed in Table 4. Band gap energy of the films was quite larger than its bulk value (∼0.41 eV). There are many studies in the literature which have indicated that the forbidden energy range is widened due to the quantum size confinement effect of PbS thin films which have relatively large Bohr exciton radius (∼18 nm) [38,45]. It is well known that larger crystals lead to shrinkage in the band gap while smaller ones caused to widening. For this reason, it must be considered that all deposited thin films are polycrystalline which contains various crystals in different orientation and size and so distribution of different sized crystals become important [46]. In addition to the quantum size effect, 292
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Fig. 8. Plots of (αhν)2vs. hν of PbS:Ag thin films a) undoped PbS b) Ag doped at 1% c) Ag doped at 2% d) Ag doped at 3% e) Ag doped at 4%.
ρ = (∆V / ∆I )(d × l/ L)
3.4. Electrical analysis
(6)
where d, l and L symbolize thickness of film, metal contact length and is the distance between metal contacts. As it can be seen resistivity values listed in Table 4, electrical resistivity which is 30.8 Ω cm for undoped PbS thin films decreased up to 2% Ag doping concentration and reach to minimum value of 15.4 Ω cm. It was determined that it began to increase again for higher doping concentrations. As it has been also stated by Zhang et al., Ag+ substitution with Pb2+ may have increased the concentration of acceptor states as a consequence of Ag doping [53]. Therefore, we believe that the hole concentration may have been increased by forming acceptor levels in the forbidden band gap of PbS thin films up to 2% Ag doping ratio and so the electrical resistivity decreased. However, the increase in electrical resistivity at higher doping concentration may have been caused by the fact that the Ag+ ions were not substituted by any more Pb2+ ions and created electrically inactive neutral defects within lattice. A similar result has been reported in the study of Cu-doped PbS thin films by others [54].
In order to conduct electrical investigations of the films, Au was selected as the metal contact because of the work function compatibility between Pb (∼4.8, 4.9 eV) and Au (Φ∼5.1, 5.2 eV) for the ohmic conduction [49–51], and coated in planar form on the film surface with the dc plasma sputtering technique. It was determined that all deposited thin films have p-type conductivity by using hot point probe technique and after that I-V characteristics were taken under dark condition at room temperature by using two point probe method. Electrical resistivity of deposited thin films was calculated by using I-V plots given in Fig. 9 and following equation [52].
4. Conclusion In this work, the effect of Ag doping on the some physical properties of the PbS thin films were investigated systematically. XRD results revealed that incorporation of Ag remarkably improved the structural properties of PbS thin films without changing the crystal structure. Calculated mean crystallite size increased from 24 nm to 49 nm. Similarly, enhancement was also noticed in morphological properties of deposited thin films. Particularly, doping at 2% and higher concentration led to disappear of voids on the surface of the film and it could provide significant advantage in terms of application of PbS thin films in opto- electrical fields. As a result of optical analyses, it was seen that
Fig. 9. I-V plots of PbS:Ag thin films.
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all deposited films exhibited fairly high absorption. The highest absorption coefficient was noted for Ag doped PbS thin films at 2%. It was also determined that the optical band gap was found to be in the range of 1.37 eV to 1.28 eV, and it is compatible for solar cell applications. Electrical examinations showed that all deposited thin films have p-type conductivity and electrical conductivity increased up to Ag doping concentration at 2%. It can be concluded that lower doping concentration of Ag (2 at%) may lead to improvement of structural, morphological and electrical properties of PbS thin films.
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