Silicon DOI 10.1007/s12633-015-9399-z
ORIGINAL PAPER
Structural and Optical Characterization of Chemically Deposited PbS Thin Films A. N. Fouda1,2 · M. Marzook3 · H. M. Abd El-Khalek1 · S. Ahmed3 · E. A. Eid4 · A. B. El Basaty5
Received: 19 May 2015 / Accepted: 28 December 2015 © Springer Science+Business Media Dordrecht 2016
Abstract PbS thin films were deposited on glass substrates by a chemical bath deposition method. The effect of varying the film thickness on the structural and optical properties has been investigated. XRD analysis reveals the crystallinity of the deposited PbS films with (200) preferred crystal orientation. Increasing the film thickness enhances the crystallinity of the films as well as decreases the strain and dislocation density. The surface morphology features were dramatically changed from small spherical grains to bead-like shape. The absence of impurities in the deposited films was confirmed by energy dispersive x-ray spectrometry (EDX) measurements. The optical constants of the deposited films were calculated and a small decrease in the band gap energy was observed with increasing the film thickness. Keywords PbS films · Chemical bath deposition · Surface morphology · XRD
A. N. Fouda
[email protected] 1
Physics Department, Faculty of Science, Suez Canal University, 41522 Ismailia, Egypt
2
Recruitment Department, University of Hail, Hail 2440, KSA Hail Saudi Arabia
3
Basic Science Department, Faculty of Petroleum and Mining Engineering, Suez University, 43721 Suez, Egypt
4
Department of Basic Science, Higher Technological Institute, 10th of Ramadan City, Ramadan Egypt
5
Basic Science Department, Faculty of Industrial Education, Helwan University, 11813 Cairo, Egypt
1 Introduction PbS is one of the most important IV-VI semiconductors with a narrow band gap of 0.41 eV at 300 K [1], and a sufficiently large Bohr radius of 18 nm [2]. This provides strong quantum confinement of holes and electrons, regulating the band gap value by controlling the crystallite size according to the effective mass model [3]. PbS thin films have been a subject of extensive research due to their wide applications in gas sensors [4], infrared radiation detectors [5, 6], optoelectronics, solar cells, diode laser, etc [7, 8]. PbS thin films is p-type semiconductors [9], and their direct band gap is around 2.2 eV [9]. For these reasons, many research groups have shown a great interest in the study and enhancement of this material by various depositional processes. PbS thin films can be deposited by different chemical and physical methods, such as vacuum evaporation [10], successive ionic layer adsorption and reaction (SILAR) [11], electrodeposition [12], chemical bath deposition (CBD) [13, 14], etc. Among these, CBD is the most simple, with low temperature requirement, low-cost, able to deposit thin films on different types of substrates, and convenient for large area deposition. The CBD method is based on successive absorption and reaction of species on the substrate surface from aqueous solution containing Pb2+ and S2− . Film growth takes place by a cluster mechanism or ion-by-ion. In this method, we can deposit good adhesive films with different optical and structural properties by optimizing the bath temperature [15], reactant pH [16], reagent concentration [17], and deposition time [18]. A comparative study has been performed on properties of PbS thin films grown by CBD, TEA was used as the complexing agent in one of the baths, and PbS films were prepared without triethanolamine (TEA) [19]. Ubale et al. presented the
Silicon
synthesis of nanocrystalline PbS thin films using the CBD method where concentrations of Pb2+ and S2− ions, temperature and pH were optimized to obtain good quality PbS thin films on glass [9]. In this work, PbS thin films were deposited by CBD with different film thicknesses at room temperature. The structural and optical properties of the prepared films were studied by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-VIS-NIR spectrophotometry.
2 Experimental Lead sulfide thin films have been synthesized on glass substrates using the CBD method (Fig. 1). The glass substrate was cleaned using ultrasonic baths of acetone, dichloromethanol and methanol. Then we rinsed it with deionized water. The CBD solution was prepared by sequential addition of 2.5 ml (0.5 M) of lead acetate Pb(CH3 COO)2 , 2.5 ml (2 M) of sodium hydroxide (NaOH), 1 ml (1 M) of triethanolamine (TEA) C6 H15 NO3 , and 3 ml (1 M) of thiourea CH4 N2 S. At room temperature (25 ◦ C), deionized water was added until the total volume of the solution reached 70 ml, and the pH was 11. The cleaned substrates were vertically immersed in a 100 ml beaker, containing the CBD solution. The reactions of the PbS formation on the glass substrate are given as: [P b(CH3 COO)2 .3H2 O + 2NaOH ] → P b(OH )2 +2Na(CH3 COO) + 3H2 O
(1)
:OH −
SC(NH2 )2 + 2H2 O −→ H2 S(g) + CO2(g) ↑ :OH −
+2NH3(g) ↑ −→ S 2− + H2 O
Fig. 1 Schematic diagram describing the procedure for PbS films preparation
(2)
2.5 ml (0.5 M) of lead acetate
2P b(OH)2+ 2[C6 H15 N O 3 ]n →2[P b(T EA)n ]+ 2H2 O+O2 (3)
[P b(T EA)n ] → P b2+ + n (T EA)
(4)
P b2+ + S 2− → P bS
(5)
The samples are labeled S1, S2, S3 and S4 for deposition times of 1h, 2h, 3h, and 4h respectively. The deposited films were cleaned in an ultrasonic bath with deionized water. The obtained films were homogeneous without color degradation, and with good adhesion to the substrate. The color of the films was light brown for S1 and became darker with increasing the deposition time. The structure and the average particle size of the deposited lead sulfide films were determined by X-ray diffraction (XRD) using a P-analytical X’PERT PRO Materials Research Diffractometer. Data were collected in the 2θ/θ geometry and were recorded in the range of 20◦ – ˚ at 45 kV and 70◦ using Cu Kα radiation (λ = 1.54 A) 40 mA. The surface morphology of the PbS films was studied using a FEI QUANTA FEG 250 scanning electron microscope (SEM). Film thickness was measured using interference microscopy and further confirmation was obtained from side-view SEM images as shown in Fig. 4. The thicknesses of S1, S2, S3, and S4 were 47, 83, 105, and 133 nm respectively. The elemental composition of the films was determined by energy-dispersive X-ray analysis (EDX) using a FEI QUANTA FEG 250. The measurements were extended to optical characterizations in the wavelength range from 380
2.5 ml (2 M) of sodium hydroxide
1 ml (1 M) of triethanolamine
3 ml (1 M) of thiourea
Water added
After 1 hour After 2 hours
4 glass plates immersed
After 4 hours After 3 hours
S4
S1 S2
S3
(311)
(222)
(220)
(200)
INTENSITY (arb. units)
(111)
Silicon
room temperature are shown in Fig. 2. All the films are polycrystalline with (200) preferred crystal orientation. The characteristic reflection peaks of PbS are observed at 25.8◦ , 29.9◦ , 42.9◦ , and 50.9◦ for (111), (200), (220), and (311) planes, respectively. An increment in the peak intensity can be observed with increasing the film thickness for S1, S2, and S3. In the case of S4, an irregular trend is observed and this can be attributed to the surface topography of the sample as shown in Fig. 3. Similar behavior was reported by other groups [17]. Generally with increasing the film thickness, an enhancement in the crystallinity was established. The crystallite size was calculated using the well-known Scherrer’s formula:
S4
S3
S2
S1
20
30
40
50
60
70
2θ (degree)
D=
Fig. 2 X-ray diffraction patterns for the S1, S2, S3, and S4
to 1200 nm using a UV/VIS/NIR double beam spectrophotometer (JASCO V-570).
(6)
˚ Where λ is the wavelength (1.5406 A), β is the full width at half maximum of the peak (in radians), and θ is the Bragg’s diffraction angle. PbS films exhibited a remarkable increase in the crystallite size with increasing the film thickness and it can be attributed to the coalescence between neighboring islands during compacted deposition and atomic mass transport. Moreover, the
3 Results and Discussion Representative XRD of patterns PbS films with different film thickness deposited on glass substrates at Fig. 3 SEM photographs of the S1, S2, S3, and S4 samples
0.9λ β cos θ
(S1)
(S3)
(S2)
(S4)
Silicon Table 1 (hkl), d, a, crystallite size, dislocation density, and lattice strain values of the PbS films
D (nm)
δ × 10−20 (lines/cm2 )
ε × 10−3
5.9746 5.9536 5.9465 5.9559
9.5
1.108
3.78
3.448 2.977 2.104 1.791
5.972 5.954 5.951 5.939
9.9
1.02
3.63
(111) (200) (220) (311)
3.441 2.976 2.105 1.794
5.960 5.952 5.953 5.948
11.6
0.743
3.12
(111) (200) (220) (311)
3.431 2.963 2.098 1.789
5.942 5.927 5.933 5.934
12.7
0.62
2.84
Sample
(h k l)
˚ d(A)
˚ a(A)
(PDF: 05-0592)
(111) (200) (220) (311)
3.427 2.968 2.099 1.790
5.936 5.936 5.936 5.936
S1
(111) (200) (220) (311)
3.449 2.977 2.102 1.796
S2
(111) (200) (220) (311)
S3
S4
dislocation density can be derived from the crystallite size using the formula [20]: δ=
1 D2
(7)
The lattice strain (ε) of S1, S2, S3, and S4 was calculated from the relation [19]: β cos θ ε= 4
Fig. 4 Cross-sectional scanning electron microscopy (SEM) images of S3, and S4
(8)
The calculations were extended to the micro-strain within the samples to demonstrate the effect of dislocation density on the film quality. The decrease in the strain with increasing film thickness reflects the relaxation of thicker films [21]. The lattice parameter (a) of S1, S2, S3, and S4 was calculated for the cubic structure of PbS with inter-planar spacing (d) using h2 + k 2 + l 2 1 = d2 a2
(9)
Silicon EDX (S1)
EDX (S2)
EDX (S3)
EDX (S4)
respectively, which are comparable to the bulk lattice parameter (a = 5.936) [22]. The dependence of a, d, and dislocation density on the film thickness is summarized in Table 1. Figure 4 shows cross-sectional views of samples S3, and S4 which were used to confirm the previously measured thickness by interference microscopy. The surface morphology of the deposited films is shown in Fig. 3. Isolated islands with intermediate spaces can be seen. For S1, regularly well-organized small grains can be observed. With increasing the film thickness, bead-like shape and agglomeration of grains is distinguished. S2, S3 and S4 images show that elongated grains are randomly oriented with coalescence at different regions. With increasing film thickness the surface roughness increases due to the growth of isolated islands together and an agglomeration effect is observed besides elongation of needle shapes. Compositional analysis was determined by EDX (energy dispersive spectrometry) measurements. Figure 5 shows the EDX patterns of S1, S2, S3, and S4. The patterns display several mean peaks at different energy values, which correspond to the Pb and S elements. It is observed that the atomic or weight concentration of each element varies with increasing the film thickness which can be attributed to the rate of chemical reaction. It is clear that the composition the deposited films contains the elements of Pb and S beside the peaks of glass. The composition of the glass consists of 75 % SiO2 , sodium oxide Na2 O, sodium carbonate Na2 CO3 , calcium oxide CaO, and several minor additives [23]. The transmittance and reflectance spectra recorded for S1, S2, S3, and S4 are shown in Fig. 6. The drop in the reflectance around the band edge is related to the increase in transmission, and the observed interference behavior is related to the film thickness and surface morphology. The reflectance decreases with increasing the film thickness in the spectra range from 400 to 1100 nm which can be attributed to the morphological features. The grain size increases with increasing the film thickness [20]. The transmission edge is shifted toward longer wavelength with increasing the film thickness. The band edge shift is related to the decrease in the direct band gap energy. The absorption coefficient was calculated using the following equation [23]: 1 1 α = ln t T
Fig. 5 EDX measurements for S1, S2, S3, and S4
For the (200) plane, the calculated lattice parameters are 5.953, 5.954, 5.952, and 5.927 for S1 , S2, S3, and S4
(10)
Where t is the film thickness and T is the transmittance. The band gap energy of S1, S2, S3, and S4 were calculated using the Tauc equation [24, 25]: αhυ = A(hυ − Eg )1/2
(11)
Silicon Fig. 6 Variation of transmittance and reflectance with wavelength for S1, S2, S3, and S4
100
60
1 2 3 4
80
50 40
R%
T%
60 40
30 20
20 0
1 2 3 4
10 400
600
800
1000
1200
0
400
600
λ [nm]
Where A is a constant, h is Planck’s constant, υ is the frequency of the photon, and Eg is the energy gap. The variation of (αhυ)2 versus hυ (photon energy) is shown in Fig. 7. The extrapolation of the curve is used to calculate the energy gap [26]. As the thickness decreases, the calculated band gap energy for the prepared samples ranges from 2.34 to 2.75 eV as tabulated in Table 2. The energy gap decreases with increasing the film thickness. This is related to the increment in the crystallite size with increasing the film thickness. The reported band gap energy of bulk PbS ranges from 0.4 to 0.6 eV. The obtained higher values can be attributed to the quantum confinement effect. After a certain limiting size of particle size which is associated with the exciton Bohr radius, the space between band levels is changed. The Brus Model of nano-structure band gap
16
2.0x10
16
1.8x10
16
1.6x10
S(1) S(2) S(3) S(4)
1.4x10
16
2
1000
1200
gives the relation between bulk band gap and nano-structure band gap: 1.78e2 h2 π 2 1 1 − Eg (nano) = Eg (bulk) + + ∗ mh ξr 2r 2 m∗e (12) Where m∗e and m∗h are the effective masses of the electron in the conduction band and the hole in the valence band, respectively; r is the radius of the particle; and ξ is the dielectric constant of PbS (value of 17.3). Due to the quantum localization (i.e. the kinetic energy) of the second term, Eg is shifted to higher energies. The third term represents the screened Coulomb interaction between holes and electrons, which shifts Eg to lower energies [27, 28]. The refractive index (n) is related to the optical reflectance (R), by the following relation [29]: 1 2 1+R 4R 2 −k (13) n= + 2 1−R (1 − R) Where (k) is the extinction coefficient which is related to the absorption coefficient (α) and the wavelength (λ) by:
16
( h )
800 λ [nm]
k=
1.2x10
16
1.0x10
αλ 4π
(14)
15
8.0x10
Table 2 Energy gap values of S1, S2, S3, and S4
15
6.0x10
15
4.0x10
Sample
Thickness (nm)
Eg (eV)
S1 S2 S3 S4
47 83 105 133
2.75 2.61 2.44 2.34
15
2.0x10
0.0
1.5
2.0
2.5
3.0
h [eV] Fig. 7 Plot of (αhv)2 vs. hv of S1, S2, S3, and S4
3.5
4.0
Silicon 8
1.4
6 5 4 3 1 2 3 4
2 1 0
400
600
800 λ
1000
1.0 0.8 0.6 0.4 0.2 0.0
1200
400
600
800 λ
1000
ε1 = n2 − k 2
,
[nm]
ε2 = 2nk
(15)
The variation of these two parameters with wavelength is shown in Fig. 9. The values of the real part (ε1 ) increases with wavelength while the imaginary part (ε2 ) of the dielectric constant decreases with wavelength of all samples except S1. Since the values of the refractive index are larger than the k values, the behavior of ε1 is similar to n. The variation of the absorption coefficient has a direct effect on the values of ε2 .
8 1 2 3 4
50
1200
the film thickness, due to the increase of the absorption with increasing the film thickness, which agrees with previous reports [30, 31]. The spectra of the extinction coefficient for different thickness of S1, S2, S3, and S4 show a dominant peak around 380–416 nm as shown in Fig. 8b. The real (ε1 ) and imaginary ε2 ) parts of the dielectric constant of the prepared films can be calculated from [32]:
60
1 2 3 4
7 6
40
5
30
2
1
1 2 3 4
1.2
[nm]
The variation of refractive index (n) with λ is shown in Fig. 8a. The retardation of light in the film is very high in the NIR region and decays to a relatively low rate in the visible region. The n value of S1, S2, S3, and S4 extended from 2.1 to 6.8. The change of refractive index with wavelength opens the door to usage in optoelectronic devices, since the wave propagation is strongly influenced by the distribution of the refractive index. It is clear from the figure that the refractive index increases as the thickness decreases in the visible region and it decreases as the thickness decreases in the IR region except for the smaller film thickness. The scattering of light within the smaller thickness exhibited a different trend because of its smaller and uniform grain distribution. The extinction coefficient as a function of the incident wavelength is shown in Fig. 8b. From the figure, we see that the extinction coefficient is related to the absorption of light within the films. k values decrease with increasing
Fig. 9 Variation of the real (ε1 ) and imaginary (ε2 ) parts of dielectric constant with wavelength for S1, S2, S3, and S4
extinction coefficient ( )
7
refractive index (n)
Fig. 8 Variation of refractive index (n) and extinction coefficient (k) with wavelength for S1, S2, S3, and S4
4 3
20
2 10 0
1 400
600
800 [nm]
1000
1200
0
400
600
800 [nm]
1000
1200
Silicon
4 Conclusions The effect of varying the film thickness on the structural, surface morphology and optical properties of PbS films was investigated. The deposited films by chemical bath deposition were polycrystalline with (200) preferred crystal orientation. The thicker films exhibited bigger crystalline size which influences the optical constants. The purity of the films was confirmed by EDX measurements and the surface morphology is changed from small spherical shaped grains to bigger agglomerated rod-like grains with increasing the film thickness. The change of refractive index with wavelength reflects the possibility of usage in optoelectronic devices.
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