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Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method
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Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method M. Madoun a, R. Baghdad a,n, K. Chebbah a, M.A. Bezzerrouk a, L. Michez b, N. Benramdane c a
Laboratoire de Génie Physique, Université Ibn-Khaldoun, 14000 Tiaret, Algeria CINaM, UPR3118 CNRS, Campus de Luminy case 913, 13288 Marseille cedex 9, France c Laboratoire d'Elaboration et de Caractérisation des Matériaux, Département d'Electronique, Université Djillali Liabes, BP89, Sidi Bel Abbés 22000, Algeria b
a r t i c l e i n f o
Keywords: Bismuth sulfide Spay pyrolysis Nanocrystalline thin films Slater model
abstract Bismuth sulfide (Bi2S3) nanocrystalline thin films exhibit a low band gap, a high absorbance coefficient and good dispersity. In this study, the structural, optical and electrical properties of Bi2S3 nanocrystalline thin films prepared from bismuth chloride (BiCl3) and thiourea (CS(NH2)2) solutions and deposited by a spray pyrolysis method, are investigated as a function of the substrate temperature (TS). TS has been increased from 140 to 280 1C by step of 40 1C. Characterizations of the films have been carried out using X-ray diffraction (XRD), scanning electron microscopy, ultra-violet–visible–near infrared (UV–vis–NIR) spectroscopy and electrical resistivity measurements. These studies reveal that Bi2S3 films consist of nanocrystalline grains. Average grain size was calculated using Debye–Scherrer formula. As TS increases, the grain size of Bi2S3 crystallites increases from 40 to 60 nm. In addition, a blue shift of 0.20 eV in the optical band gap energy Eg, which is in agreement with Slater's model, and a decrease in electrical resistivity from 2.61 to 1.05 Ω cm was observed. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction Semiconductor nanostructured thin films are always important in materials science due to their outstanding electronic and optical properties and useful applications in various optoelectronic devices [1,2]. Bismuth sulfide (Bi2S3) is a group V–VI semiconductor that draws much attention for its superior properties such as a high absorption coefficient, a direct band gap energy of 1.2–1.7 eV and a good chemical stability [3,4].
n
Corresponding author. Tel.: +213 7 99 29 94 55; fax: +213 46 42 47 10. E-mail addresses: [email protected], [email protected] (R. Baghdad).
A variety of deposition techniques has been used to grow Bi2S3 nanocrystalline thin films with desirable structural, optical, and electrical properties [5–10]. Every technique has its own advantages and disadvantages. One of the greatest disadvantages is that some of them require very sophisticated instruments along with vacuum facilities, which increases the production cost of the material. On the other hand, the chemical spray pyrolysis technique (SPT) is particularly attractive because of its simplicity and has been, during the last decade, one of the major techniques used for the deposition of a wide variety of materials in nanocrystalline thin films. The prime requisite for obtaining good quality nanocrystalline thin films is the optimization of the preparative conditions viz. substrate temperature, spray rate, concentration of solution etc.
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Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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with Δm¼ m2−m1, where m1 is the mass of the substrate without the deposited film and m2 is the mass of the sample after deposition (film+substrate). ρm is the Bi2S3 density (ρm≅6.8 g cm−3) [13] and A is the area of the deposited film. The measurements of transmittance and reflectance spectra from 200 to 2500 nm wavelength were recorded using a double beam ultra-violet–visible–near infrared (UV–vis–NIR) JASCO type V-570 (resolution: 0.1 nm). Morphology was carried out by a Jeol JSM 5800 scanning electron microscope. The electrical resistivity was measured by the four probe method using a Keithly
In this relation, (h, k, l) are Miller indices of refractor planes appearing on the diffraction spectrum and dhkl their inter-reticular distances. The calculated values for the
*
2000
280 °C
1500
*
1000
*
500
* Bi (012) (002) (501) (251)
2500
(130)
3000 (221) (301) (311) (420) (231) (041) (430) (520)
Δm ρm Α
Fig. 1 shows the XRD pattern of nanocrystalline bismuth sulfide thin films. This spectrum carried out in the θ–2θ configuration, corresponds to as-deposited thin films. The observed broad hump in XRD patterns is due to the amorphous glass substrate. Except for the XRD pattern of the film grown at 140 1C, the diffraction peaks in the other XRD patterns could be indexed by comparing the peak position and relative intensities to the orthorhombic crystal structure of Bi2S3, which is in good agreement with the standard JCPDS Data (card no.: 17-0320) [14]. The spatial group of the crystalline structure corresponds to Pbnm symmetry group. Well defined (220), (130), (211), and (311) peaks are observed in the XRD pattern. These results are in good agreement with those obtained by other groups [10,11,15]. The films preferably grow along the (130) direction. No diffraction peaks corresponding to impurities like BiOCl and BiO3 have been observed, which confirms that the as-synthesized product is a single phase Bi2S3. However, an additional peak appears at TS ¼220 1C due to the formation of Bi metallic phase around 2θ¼25.501, corresponding to (012) plane, indicating desorption of sulfur atoms at high TS. This peak slightly increases with substrate temperature. We have observed a satisfying coherence between our results and the card data. For the orthorhombic lattice parameters evaluation, we have used the quadratic relation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 dhkl ¼ ð1Þ 2 2 2 2 h =a2 þ k =b þ l =c2
(211)
d¼
3.1. XRD results
(220)
It is important to take precautions before depositing the films. One of these is the careful cleaning of the substrate as it helps the films to adhere on the substrates most effectively. The substrates were cleaned by diluted hydrochloric acid (HCl), washed by double distilled water and dried. The nanocrystalline thin films Bi2S3 were deposited on amorphous glass substrates with dimensions of 36 25 1 mm3, using the spray technique described in a previous paper [10]. The solution is sprayed intermittently for 15 s, with 3–4 min interval between spraying periods in order to avoid excessive cooling of the glass substrates due to a continuous spraying. We fixed the deposition time equal 60 s. Typically, bismuth chloride (BiCl3) and thiourea [CS(NH2)2] were used as starting materials and were dissolved in double-distilled water at 0.2 M concentration. The prepared solutions of bismuth chloride and thiourea were appropriately mixed to obtain a Bi:S proportion of 2:3. Compressed air at a pressure of 6 104 Pa has been used as a carrier gas. In this preparation process, we have worked under atmospheric pressure and we have changed the substrate temperature from 140 to 280 1C. In order to get uniform Bi2S3 nanostructured thin films, the optimum values for the rate of spray process and the spray nozzle to heating plaque distance were adjusted to 5 cm3/min and 28 cm respectively. The structure of as-prepared films were characterized by using a Philips 1830 powder X-ray diffractometer working at CuKα peak with wavelength λ¼0.15406 nm. The film thickness d varied from 300 to 500 nm and has been estimated using the double weighing method, carried out before and after film deposition.
3. Results and discussion
(101)
2. Experimental details
electrometer model 617. The substrate temperature was measured using a calibrated copper-constantan thermocouple. Liquid nitrogen was used to cool down the sample temperature to about 206 K.
(020) (120)
The (SPT) is particularly attractive because of its simplicity. It is widely used for large scale production of films owing to low production cost. This method is also fast, inexpensive, vacuum-less and therefore, suitable for mass production. For all these reasons, this technique has been chosen for the preparation of our films of Bi2S3 [10–12]. The present investigation aims at studying the effect of the substrate temperature (TS ¼140, 180, 220, 240, 260 and 280 1C) on the structural, optical and electrical properties of Bi2S3 nanocrystalline thin films prepared from bismuth chloride (BiCl3) and thiourea (CS(NH2)2) solutions and deposited using the spray pyrolysis method.
Intensity (arb.units.)
2
260 °C 220 °C 180 °C 140 °C
14.8 22.2 29.6 37.0 44.4 51.8 59.2 66.6 74.0 2θ (degree) Fig. 1. X-ray diffraction patterns of Bi2S3 nanocrystalline thin films at different substrate temperatures (140, 180, 220, 260 and 280 1C).
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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3
Table 1 The films thickness d, lattice constants, average grain size were calculated from XRD patterns. TS (1C)
d (nm)
a (Å)
b (Å)
c (Å)
Grain size (D) (nm)
140 180 220 260 280
300 320 380 460 500
– 11.04 10.783 11.128 11.111
– 11.290 11.318 11.304 11.302
– 3.987 4.109 3.987 4.025
– 40.6 50.4 55.0 59.5
!!! structure's translation vectors modules ð a ; b ; c Þ, forming between their straight angle (α¼β¼γ ¼901) are summarized in Table 1. The mean crystallite size (D) of Bi2S3 nanocrystalline film was calculated of (130) plane using Debye–Scherrer formula [16]: ð2Þ
where B is the full width half maximum (FWHM) (in radian), λ is the X-ray wavelength (CuKα ¼0.154 nm), θ is the Bragg diffraction angle, and C is a correction factor taken as 0.9. The grain size is defined as the dimension of crystallites along the direction perpendicular to the plane. The value obtained from Scherrer formula corresponds only to small crystallites [10,17]. Generally, the peak broadening in XRD analysis originates from the instrumental broadening and physical factors such as crystallite size and lattice strain [18,19]. In the case of physical factors, FWHM of each diffraction peak is expressed as a linear combination of the contributions from lattice strain and crystallite size. Hence, there exist some uncertainties in the crystallite size determined using Scherrer formula and the actual crystallite size will be a little bit higher than the measured value if instrumental and strain broadenings are taken into account. Table 1 summarizes the grain size that has been estimated from the main peak (130). However, grains of varying size could be estimated by scanning electron microscopy (SEM). The density of the synthesized films was not studied in this work, but it could be also inspected by SEM. Generally the films obtained using a spray pyrolysis technique have a low density, which is attributed to the deposition method itself.
2 μm Fig. 2. SEM image of bismuth sulfide nanocrystalline thin films at (a) TS ¼180 1C and (b) TS ¼ 220 1C.
70 140 °C 180 °C 220 °C 260 °C 280 °C
60 50 40
50
30
Reflectance(%)
Cλ Bcos θ
Transmitance (%)
D¼
2 μm
20 10
40 30 20 10 0 0
500
1000 1500 2000 Wavelength (nm)
2500
0
3.2. SEM results Information about the structural changes of the films can be also gained from the SEM images. We present in Fig. 2 the SEM images of Bi2S3 nanocrystalline thin films deposited at a substrate temperature of (a) 180 1C and (b) 220 1C. Fig.2 shows that the films are uniform, homogenous and are of nanocrystalline nature and it can be clearly seen that crystallite size increases with increasing substrate temperature as well as with film thickness. Grains in both layers have a particular shape, characterized by a tubular growth in different direction from a knot or a center. The increase in the surface diffusion and mobility of adatoms with TS enables the adatoms to migrate along a long distance and to coalesce with each other in order to
0
500
1000 1500 Wavelength (nm)
2000
2500
Fig. 3. Optical transmittance spectra of bismuth sulfide nanocrystalline thin films deposited at different TS. The inset shows the corresponding reflectance spectra.
form larger crystallites. These results are in very good agreement with XRD which have been discussed earlier. 3.3. UV–vis–NIR results Fig. 3 shows the optical transmittance and reflectance (Fig. 3 inset) of Bi2S3 thin films deposited at different substrate temperatures. All the films exhibited transmittance (reflectance) in range 50–70% (0–50%) above the
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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4
4.0x1010 140 °C 180 °C 220 °C 260 °C 280 °C
3.5x1010
(αhν)2 ((cm-2)(eV2))
3.0x1010 2.5x1010
T = 140 °C Eg = 1.8 eV
2.0x1010 1.5x1010 1.0x1010 5.0x109 0.0 1.0
1.2
1.4
1.6
1.8 hν (eV)
2.0
2.2
2.4
2.6
Fig. 4. Plots of (αhν)2 vs. hν for Bi2S3 nanocrystalline thin films deposited at different TS. (The inset shows an example of the extrapolation of the linear part of the curve (αhν)2 crosses the energy axis at Eg).
where α is the absorption coefficient, d is the thickness of the film, R and T are the reflectance and the transmittance respectively. For a direct gap semiconductor such as Bi2S3 the absorption coefficient has the following spectral dependence [21]:
by extrapolating the linear part of the spectrum (αhν) ¼f (hν) as shown in Fig. 4. In our study, as tabulated in Table 2, the grain size of Bi2S3 thin films increases with TS. This will change the lattice parameters, and hence the band gap of the material [22]. The results of the optical studies revealed that the films formed with lower TS are nearly stoichiometric, while those formed at elevated TS are sub-stoichiometric film may result from the formation of sulfide ion vacancies in the films acting as structural defect. Another reason to explain the decrease of the optical band gap Eg with film TS can come from a quantum size effect observed in thin films of semiconductors. The red shift in the band gap is due to the increase in the crystallite size [23]. On other hand and in our case, blue shifts in Eg values for thin films with small thickness and/or grain size have been reported for many chemically deposited chalcogenide films [12,24,25]. Thickness dependence of band gap can be explained by the presence of at least one of the following effects: (i) presence of a large density of dislocations, (ii) quantum size effect and (iii) change in barrier height due to change in grain size in nanocrystalline films. In the present case, the first effect looks reasonable because in the present case, with small contributions from dislocation density as well as the thickness of the films in the present study is quite large, the quantum size effect can completely be ruled out. The decreasing band gap with grain size as shown in Fig. 5 is exactly similar to its thickness dependence and indicates that the barrier height decreases with increasing grain size. The variation of grain boundary barrier height with grain size is given by Slater [26]:
αðhνÞ ¼ Aðhν−Eg Þ1=2
Eg S ¼ Eb0 þ CðX−f DÞ2
Table 2 The average grain size, experimental optical band gap and theoretical band gap proposed by Slater [26]. TS (1C)
d (nm)
Grain size (D) (nm)
Eg (eV)
Egs Slater [26](eV)
140 180 220 260 280
300 320 380 460 500
– 40.6 50.4 55.0 59.5
1.80 1.75 1.65 1.63 1.61
1.791 1.753 1.667 1.630 1.596
absorption edge. The optical band gap of Bi2S3 thin films showed strong light absorption in the wavelength range between 400 and 800 nm, indicating the feasibility of utilizing nanocrystalline Bi2S3 as an absorber layer in solar cell devices. The optical band gap Eg can be determined from the absorption coefficient α calculated as a function of incident photon energy E(hν). Near the absorption edge region, the absorption spectra α(hν) of the films have been obtained from the optical transmission and reflection measurements at 300 K. The absorption coefficient is obtained from the following equation [20]: ! 1 ð1−RÞ2 α ¼ ln ð3Þ d T
ð4Þ
where A is a constant and Eg is the band gap. From this relation the optical band gap of the films was determined
ð5Þ
where Eb0 is the original barrier height, C is a constant depending on the density of charge carriers and dielectric
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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5
550
1.85 Eg: Experimental EgS: Slater model Thikness d
500
1.75
450
1.70
400
1.65
350
1.60
300
1.55 30
35
40
45 50 Grain size D(nm)
55
60
Thikness d (nm)
Eg, EgS (eV)
1.80
250 65
Fig. 5. Grain size dependence vs. experimental optical band gap, Slater model band gap and the films thickness.
constant of the material, X the barrier width (20–30 nm), D the grain size and f is a fraction of the order of 1/15 to 1/50 depending on the charge accumulation and carrier concentration. After having fitted the experimental curves with the theoretical relation (5), we find the following values for the constants: Eb0 ¼65 meV, C¼0.00132, X¼26 nm as the average barrier width, f¼0.0285 ¼1/35. Thus, the calculated Egs as a function of grain size is compared qualitatively to the experimentally observed band gap variation with grain size in Fig. 5. These results are in striking agreement with those of Slater model [26]. The decrease in band gap will help to increase the carrier concentration across the forbidden gap with the temperature; as a consequence, the resistivity of the films decreases [27]. 3.4. Electrical results In order to estimate the effect of these structural changes on the electrical properties of the films, the XRD and SEM results are correlated to the electrical conductivity measurements. We present in Fig. 6 the variation of land(T) as a function of the inverse of temperature in the range of 150–300 K, for different TS, as indicated. This figure shows that s increases when the TS is raised from 140 1C to 280 1C, in good agreement with crystallization of the films as evidenced by XRD and SEM results discussed above. The plot indicates the semiconducting nature of bismuth sulfide thin films with the increase of film substrate temperature. For the films of TS ¼140, 180 and 220 1C, the conductivity increases from 0.31 to 0.47 (Ω cm)−1, and increases rapidly to 0.91 and 0.93 (Ω cm)−1 for the films grown at 260 an 280 1C respectively (Fig. 7). All the results obtained for conductivity are summarized in Table 3. The low values of conductivity for low TS films may be attributed to the nanocrystalline nature of the films, presence of grain boundary discontinuities, and presence of surface states. Nevertheless, the increase in
conductivity with substrate temperature may be attributed to the improvement in the crystallinity and decrease in the microstructural defects [15,28]. Also, the grain growth with the increase in substrate temperature leads to the reduction in the grain boundary scattering of the charge carriers, thus increasing the mobility and eventually increasing the films conductivity [29]. That is, the Bi2S3 films grown at low TS have very small grain size and large grain boundary regions. The grain boundary regions are highly disordered and have large number of defects, and these defect states acting as an effective carrier traps impede the flow of charge carriers between the grains. However, at higher TS the films have larger grains, which would result in decreasing the defect states and hence and in an increase of the conductivity of the films. Generally in semiconductors the conduction mechanism is highly influenced by intercrystalline grain boundaries and strain fields associated to the dislocation network [8]. The activation energies were calculated using Eq. (6): s ¼ s0 eð−Ea=kb TÞ
ð6Þ
where, Ea is activation energy, s0 is a constant, kB is Boltzmann's constant and T is the absolute temperature. Activation energies for Bi2S3 films are tabulated in Table 3. The activation energy represents the location of trap levels below the conduction band [30]. The observed increase in activation energy with TS clearly suggests the presence of internal structural-related defects and the trap levels due to the evaporation of sulfide with increasing substrate temperature. 4. Conclusion Nanocrystalline bismuth sulfide thin films were grown from BiCl3 and CS(NH2)2, using the spray pyrolysis technique. It was observed that substrate temperature (TS) has a strong influence on the structural, optical and electrical properties of Bi2S3 films. The prepared bismuth sulfide
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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6
1.00 140 °C 180 °C) 220 °C 260 °C 280 °C
1.00
0.14 Conductivity σ (Ω.cm)
ln σ (Ω.cm)-1
0.37
0.05
0.80 0.60 0.40 0.20 0.00 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 1000/T (°K )
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
1000/T (°K-1) Fig. 6. Variation of lns(T) as a function of the inverse of T obtained for Bi2S3 nanocrystalline thin films grown at different TS values as indicated. (The inset shows s(T) vs. 1000/T).
good agreement with Slater's model. The electrical study showed the semiconducting behavior of Bi2S3 thin films. Both electrical conductivity and activation energy increased with the increase of the film thickness and substrate temperature. Hence, the results depict that good quality and defect free films can be improved by the application of TS and a good optimization of the other deposition conditions, which are the prerequisites for nano-device applications.
α (Ω.,cm)-1
1.0 0.9
σRT
0.8
σFP
0.7 0.6 0.5 0.4
References 0.3 0.2 120
140
160
180
200 220 TS (°C)
240
260
280
300
Fig. 7. Variation of electrical conductivity as a function of substrate temperature. s: room temperature conductivity. sFP: conductivity using four probe technique.
Table 3 Summary of electrical conductivity and Ea activation energy values. sRT: room temperature conductivity. sFP: four-probe method conductivity. TS (1C)
d (nm)
rRT (Ω cm)
rFP (Ω cm)
Ea (meV)
140 180 220 260 280
300 320 380 460 500
0.310 0.452 0.474 0.910 0.936
0.382 0.445 0.502 0.933 0.952
0.027 0.042 0.052 0.112 0.081
thin films exhibited good crystallinity with orthorhombic structure. The optical studies showed direct electronic transition, and the band gap energy decreased with the increase in TS, film thickness and grain size, which is in
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Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method M. Madoun a, R. Baghdad a,n, K. Chebbah a, M.A. Bezzerrouk a, L. Michez b, N. Benramdane c a
Laboratoire de Génie Physique, Université Ibn-Khaldoun, 14000 Tiaret, Algeria CINaM, UPR3118 CNRS, Campus de Luminy case 913, 13288 Marseille cedex 9, France c Laboratoire d'Elaboration et de Caractérisation des Matériaux, Département d'Electronique, Université Djillali Liabes, BP89, Sidi Bel Abbés 22000, Algeria b
a r t i c l e i n f o
Keywords: Bismuth sulfide Spay pyrolysis Nanocrystalline thin films Slater model
abstract Bismuth sulfide (Bi2S3) nanocrystalline thin films exhibit a low band gap, a high absorbance coefficient and good dispersity. In this study, the structural, optical and electrical properties of Bi2S3 nanocrystalline thin films prepared from bismuth chloride (BiCl3) and thiourea (CS(NH2)2) solutions and deposited by a spray pyrolysis method, are investigated as a function of the substrate temperature (TS). TS has been increased from 140 to 280 1C by step of 40 1C. Characterizations of the films have been carried out using X-ray diffraction (XRD), scanning electron microscopy, ultra-violet–visible–near infrared (UV–vis–NIR) spectroscopy and electrical resistivity measurements. These studies reveal that Bi2S3 films consist of nanocrystalline grains. Average grain size was calculated using Debye–Scherrer formula. As TS increases, the grain size of Bi2S3 crystallites increases from 40 to 60 nm. In addition, a blue shift of 0.20 eV in the optical band gap energy Eg, which is in agreement with Slater's model, and a decrease in electrical resistivity from 2.61 to 1.05 Ω cm was observed. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction Semiconductor nanostructured thin films are always important in materials science due to their outstanding electronic and optical properties and useful applications in various optoelectronic devices [1,2]. Bismuth sulfide (Bi2S3) is a group V–VI semiconductor that draws much attention for its superior properties such as a high absorption coefficient, a direct band gap energy of 1.2–1.7 eV and a good chemical stability [3,4].
n
Corresponding author. Tel.: +213 7 99 29 94 55; fax: +213 46 42 47 10. E-mail addresses: [email protected], [email protected] (R. Baghdad).
A variety of deposition techniques has been used to grow Bi2S3 nanocrystalline thin films with desirable structural, optical, and electrical properties [5–10]. Every technique has its own advantages and disadvantages. One of the greatest disadvantages is that some of them require very sophisticated instruments along with vacuum facilities, which increases the production cost of the material. On the other hand, the chemical spray pyrolysis technique (SPT) is particularly attractive because of its simplicity and has been, during the last decade, one of the major techniques used for the deposition of a wide variety of materials in nanocrystalline thin films. The prime requisite for obtaining good quality nanocrystalline thin films is the optimization of the preparative conditions viz. substrate temperature, spray rate, concentration of solution etc.
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Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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with Δm¼ m2−m1, where m1 is the mass of the substrate without the deposited film and m2 is the mass of the sample after deposition (film+substrate). ρm is the Bi2S3 density (ρm≅6.8 g cm−3) [13] and A is the area of the deposited film. The measurements of transmittance and reflectance spectra from 200 to 2500 nm wavelength were recorded using a double beam ultra-violet–visible–near infrared (UV–vis–NIR) JASCO type V-570 (resolution: 0.1 nm). Morphology was carried out by a Jeol JSM 5800 scanning electron microscope. The electrical resistivity was measured by the four probe method using a Keithly
In this relation, (h, k, l) are Miller indices of refractor planes appearing on the diffraction spectrum and dhkl their inter-reticular distances. The calculated values for the
*
2000
280 °C
1500
*
1000
*
500
* Bi (012) (002) (501) (251)
2500
(130)
3000 (221) (301) (311) (420) (231) (041) (430) (520)
Δm ρm Α
Fig. 1 shows the XRD pattern of nanocrystalline bismuth sulfide thin films. This spectrum carried out in the θ–2θ configuration, corresponds to as-deposited thin films. The observed broad hump in XRD patterns is due to the amorphous glass substrate. Except for the XRD pattern of the film grown at 140 1C, the diffraction peaks in the other XRD patterns could be indexed by comparing the peak position and relative intensities to the orthorhombic crystal structure of Bi2S3, which is in good agreement with the standard JCPDS Data (card no.: 17-0320) [14]. The spatial group of the crystalline structure corresponds to Pbnm symmetry group. Well defined (220), (130), (211), and (311) peaks are observed in the XRD pattern. These results are in good agreement with those obtained by other groups [10,11,15]. The films preferably grow along the (130) direction. No diffraction peaks corresponding to impurities like BiOCl and BiO3 have been observed, which confirms that the as-synthesized product is a single phase Bi2S3. However, an additional peak appears at TS ¼220 1C due to the formation of Bi metallic phase around 2θ¼25.501, corresponding to (012) plane, indicating desorption of sulfur atoms at high TS. This peak slightly increases with substrate temperature. We have observed a satisfying coherence between our results and the card data. For the orthorhombic lattice parameters evaluation, we have used the quadratic relation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 dhkl ¼ ð1Þ 2 2 2 2 h =a2 þ k =b þ l =c2
(211)
d¼
3.1. XRD results
(220)
It is important to take precautions before depositing the films. One of these is the careful cleaning of the substrate as it helps the films to adhere on the substrates most effectively. The substrates were cleaned by diluted hydrochloric acid (HCl), washed by double distilled water and dried. The nanocrystalline thin films Bi2S3 were deposited on amorphous glass substrates with dimensions of 36 25 1 mm3, using the spray technique described in a previous paper [10]. The solution is sprayed intermittently for 15 s, with 3–4 min interval between spraying periods in order to avoid excessive cooling of the glass substrates due to a continuous spraying. We fixed the deposition time equal 60 s. Typically, bismuth chloride (BiCl3) and thiourea [CS(NH2)2] were used as starting materials and were dissolved in double-distilled water at 0.2 M concentration. The prepared solutions of bismuth chloride and thiourea were appropriately mixed to obtain a Bi:S proportion of 2:3. Compressed air at a pressure of 6 104 Pa has been used as a carrier gas. In this preparation process, we have worked under atmospheric pressure and we have changed the substrate temperature from 140 to 280 1C. In order to get uniform Bi2S3 nanostructured thin films, the optimum values for the rate of spray process and the spray nozzle to heating plaque distance were adjusted to 5 cm3/min and 28 cm respectively. The structure of as-prepared films were characterized by using a Philips 1830 powder X-ray diffractometer working at CuKα peak with wavelength λ¼0.15406 nm. The film thickness d varied from 300 to 500 nm and has been estimated using the double weighing method, carried out before and after film deposition.
3. Results and discussion
(101)
2. Experimental details
electrometer model 617. The substrate temperature was measured using a calibrated copper-constantan thermocouple. Liquid nitrogen was used to cool down the sample temperature to about 206 K.
(020) (120)
The (SPT) is particularly attractive because of its simplicity. It is widely used for large scale production of films owing to low production cost. This method is also fast, inexpensive, vacuum-less and therefore, suitable for mass production. For all these reasons, this technique has been chosen for the preparation of our films of Bi2S3 [10–12]. The present investigation aims at studying the effect of the substrate temperature (TS ¼140, 180, 220, 240, 260 and 280 1C) on the structural, optical and electrical properties of Bi2S3 nanocrystalline thin films prepared from bismuth chloride (BiCl3) and thiourea (CS(NH2)2) solutions and deposited using the spray pyrolysis method.
Intensity (arb.units.)
2
260 °C 220 °C 180 °C 140 °C
14.8 22.2 29.6 37.0 44.4 51.8 59.2 66.6 74.0 2θ (degree) Fig. 1. X-ray diffraction patterns of Bi2S3 nanocrystalline thin films at different substrate temperatures (140, 180, 220, 260 and 280 1C).
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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3
Table 1 The films thickness d, lattice constants, average grain size were calculated from XRD patterns. TS (1C)
d (nm)
a (Å)
b (Å)
c (Å)
Grain size (D) (nm)
140 180 220 260 280
300 320 380 460 500
– 11.04 10.783 11.128 11.111
– 11.290 11.318 11.304 11.302
– 3.987 4.109 3.987 4.025
– 40.6 50.4 55.0 59.5
!!! structure's translation vectors modules ð a ; b ; c Þ, forming between their straight angle (α¼β¼γ ¼901) are summarized in Table 1. The mean crystallite size (D) of Bi2S3 nanocrystalline film was calculated of (130) plane using Debye–Scherrer formula [16]: ð2Þ
where B is the full width half maximum (FWHM) (in radian), λ is the X-ray wavelength (CuKα ¼0.154 nm), θ is the Bragg diffraction angle, and C is a correction factor taken as 0.9. The grain size is defined as the dimension of crystallites along the direction perpendicular to the plane. The value obtained from Scherrer formula corresponds only to small crystallites [10,17]. Generally, the peak broadening in XRD analysis originates from the instrumental broadening and physical factors such as crystallite size and lattice strain [18,19]. In the case of physical factors, FWHM of each diffraction peak is expressed as a linear combination of the contributions from lattice strain and crystallite size. Hence, there exist some uncertainties in the crystallite size determined using Scherrer formula and the actual crystallite size will be a little bit higher than the measured value if instrumental and strain broadenings are taken into account. Table 1 summarizes the grain size that has been estimated from the main peak (130). However, grains of varying size could be estimated by scanning electron microscopy (SEM). The density of the synthesized films was not studied in this work, but it could be also inspected by SEM. Generally the films obtained using a spray pyrolysis technique have a low density, which is attributed to the deposition method itself.
2 μm Fig. 2. SEM image of bismuth sulfide nanocrystalline thin films at (a) TS ¼180 1C and (b) TS ¼ 220 1C.
70 140 °C 180 °C 220 °C 260 °C 280 °C
60 50 40
50
30
Reflectance(%)
Cλ Bcos θ
Transmitance (%)
D¼
2 μm
20 10
40 30 20 10 0 0
500
1000 1500 2000 Wavelength (nm)
2500
0
3.2. SEM results Information about the structural changes of the films can be also gained from the SEM images. We present in Fig. 2 the SEM images of Bi2S3 nanocrystalline thin films deposited at a substrate temperature of (a) 180 1C and (b) 220 1C. Fig.2 shows that the films are uniform, homogenous and are of nanocrystalline nature and it can be clearly seen that crystallite size increases with increasing substrate temperature as well as with film thickness. Grains in both layers have a particular shape, characterized by a tubular growth in different direction from a knot or a center. The increase in the surface diffusion and mobility of adatoms with TS enables the adatoms to migrate along a long distance and to coalesce with each other in order to
0
500
1000 1500 Wavelength (nm)
2000
2500
Fig. 3. Optical transmittance spectra of bismuth sulfide nanocrystalline thin films deposited at different TS. The inset shows the corresponding reflectance spectra.
form larger crystallites. These results are in very good agreement with XRD which have been discussed earlier. 3.3. UV–vis–NIR results Fig. 3 shows the optical transmittance and reflectance (Fig. 3 inset) of Bi2S3 thin films deposited at different substrate temperatures. All the films exhibited transmittance (reflectance) in range 50–70% (0–50%) above the
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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4
4.0x1010 140 °C 180 °C 220 °C 260 °C 280 °C
3.5x1010
(αhν)2 ((cm-2)(eV2))
3.0x1010 2.5x1010
T = 140 °C Eg = 1.8 eV
2.0x1010 1.5x1010 1.0x1010 5.0x109 0.0 1.0
1.2
1.4
1.6
1.8 hν (eV)
2.0
2.2
2.4
2.6
Fig. 4. Plots of (αhν)2 vs. hν for Bi2S3 nanocrystalline thin films deposited at different TS. (The inset shows an example of the extrapolation of the linear part of the curve (αhν)2 crosses the energy axis at Eg).
where α is the absorption coefficient, d is the thickness of the film, R and T are the reflectance and the transmittance respectively. For a direct gap semiconductor such as Bi2S3 the absorption coefficient has the following spectral dependence [21]:
by extrapolating the linear part of the spectrum (αhν) ¼f (hν) as shown in Fig. 4. In our study, as tabulated in Table 2, the grain size of Bi2S3 thin films increases with TS. This will change the lattice parameters, and hence the band gap of the material [22]. The results of the optical studies revealed that the films formed with lower TS are nearly stoichiometric, while those formed at elevated TS are sub-stoichiometric film may result from the formation of sulfide ion vacancies in the films acting as structural defect. Another reason to explain the decrease of the optical band gap Eg with film TS can come from a quantum size effect observed in thin films of semiconductors. The red shift in the band gap is due to the increase in the crystallite size [23]. On other hand and in our case, blue shifts in Eg values for thin films with small thickness and/or grain size have been reported for many chemically deposited chalcogenide films [12,24,25]. Thickness dependence of band gap can be explained by the presence of at least one of the following effects: (i) presence of a large density of dislocations, (ii) quantum size effect and (iii) change in barrier height due to change in grain size in nanocrystalline films. In the present case, the first effect looks reasonable because in the present case, with small contributions from dislocation density as well as the thickness of the films in the present study is quite large, the quantum size effect can completely be ruled out. The decreasing band gap with grain size as shown in Fig. 5 is exactly similar to its thickness dependence and indicates that the barrier height decreases with increasing grain size. The variation of grain boundary barrier height with grain size is given by Slater [26]:
αðhνÞ ¼ Aðhν−Eg Þ1=2
Eg S ¼ Eb0 þ CðX−f DÞ2
Table 2 The average grain size, experimental optical band gap and theoretical band gap proposed by Slater [26]. TS (1C)
d (nm)
Grain size (D) (nm)
Eg (eV)
Egs Slater [26](eV)
140 180 220 260 280
300 320 380 460 500
– 40.6 50.4 55.0 59.5
1.80 1.75 1.65 1.63 1.61
1.791 1.753 1.667 1.630 1.596
absorption edge. The optical band gap of Bi2S3 thin films showed strong light absorption in the wavelength range between 400 and 800 nm, indicating the feasibility of utilizing nanocrystalline Bi2S3 as an absorber layer in solar cell devices. The optical band gap Eg can be determined from the absorption coefficient α calculated as a function of incident photon energy E(hν). Near the absorption edge region, the absorption spectra α(hν) of the films have been obtained from the optical transmission and reflection measurements at 300 K. The absorption coefficient is obtained from the following equation [20]: ! 1 ð1−RÞ2 α ¼ ln ð3Þ d T
ð4Þ
where A is a constant and Eg is the band gap. From this relation the optical band gap of the films was determined
ð5Þ
where Eb0 is the original barrier height, C is a constant depending on the density of charge carriers and dielectric
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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5
550
1.85 Eg: Experimental EgS: Slater model Thikness d
500
1.75
450
1.70
400
1.65
350
1.60
300
1.55 30
35
40
45 50 Grain size D(nm)
55
60
Thikness d (nm)
Eg, EgS (eV)
1.80
250 65
Fig. 5. Grain size dependence vs. experimental optical band gap, Slater model band gap and the films thickness.
constant of the material, X the barrier width (20–30 nm), D the grain size and f is a fraction of the order of 1/15 to 1/50 depending on the charge accumulation and carrier concentration. After having fitted the experimental curves with the theoretical relation (5), we find the following values for the constants: Eb0 ¼65 meV, C¼0.00132, X¼26 nm as the average barrier width, f¼0.0285 ¼1/35. Thus, the calculated Egs as a function of grain size is compared qualitatively to the experimentally observed band gap variation with grain size in Fig. 5. These results are in striking agreement with those of Slater model [26]. The decrease in band gap will help to increase the carrier concentration across the forbidden gap with the temperature; as a consequence, the resistivity of the films decreases [27]. 3.4. Electrical results In order to estimate the effect of these structural changes on the electrical properties of the films, the XRD and SEM results are correlated to the electrical conductivity measurements. We present in Fig. 6 the variation of land(T) as a function of the inverse of temperature in the range of 150–300 K, for different TS, as indicated. This figure shows that s increases when the TS is raised from 140 1C to 280 1C, in good agreement with crystallization of the films as evidenced by XRD and SEM results discussed above. The plot indicates the semiconducting nature of bismuth sulfide thin films with the increase of film substrate temperature. For the films of TS ¼140, 180 and 220 1C, the conductivity increases from 0.31 to 0.47 (Ω cm)−1, and increases rapidly to 0.91 and 0.93 (Ω cm)−1 for the films grown at 260 an 280 1C respectively (Fig. 7). All the results obtained for conductivity are summarized in Table 3. The low values of conductivity for low TS films may be attributed to the nanocrystalline nature of the films, presence of grain boundary discontinuities, and presence of surface states. Nevertheless, the increase in
conductivity with substrate temperature may be attributed to the improvement in the crystallinity and decrease in the microstructural defects [15,28]. Also, the grain growth with the increase in substrate temperature leads to the reduction in the grain boundary scattering of the charge carriers, thus increasing the mobility and eventually increasing the films conductivity [29]. That is, the Bi2S3 films grown at low TS have very small grain size and large grain boundary regions. The grain boundary regions are highly disordered and have large number of defects, and these defect states acting as an effective carrier traps impede the flow of charge carriers between the grains. However, at higher TS the films have larger grains, which would result in decreasing the defect states and hence and in an increase of the conductivity of the films. Generally in semiconductors the conduction mechanism is highly influenced by intercrystalline grain boundaries and strain fields associated to the dislocation network [8]. The activation energies were calculated using Eq. (6): s ¼ s0 eð−Ea=kb TÞ
ð6Þ
where, Ea is activation energy, s0 is a constant, kB is Boltzmann's constant and T is the absolute temperature. Activation energies for Bi2S3 films are tabulated in Table 3. The activation energy represents the location of trap levels below the conduction band [30]. The observed increase in activation energy with TS clearly suggests the presence of internal structural-related defects and the trap levels due to the evaporation of sulfide with increasing substrate temperature. 4. Conclusion Nanocrystalline bismuth sulfide thin films were grown from BiCl3 and CS(NH2)2, using the spray pyrolysis technique. It was observed that substrate temperature (TS) has a strong influence on the structural, optical and electrical properties of Bi2S3 films. The prepared bismuth sulfide
Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i
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6
1.00 140 °C 180 °C) 220 °C 260 °C 280 °C
1.00
0.14 Conductivity σ (Ω.cm)
ln σ (Ω.cm)-1
0.37
0.05
0.80 0.60 0.40 0.20 0.00 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 1000/T (°K )
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
1000/T (°K-1) Fig. 6. Variation of lns(T) as a function of the inverse of T obtained for Bi2S3 nanocrystalline thin films grown at different TS values as indicated. (The inset shows s(T) vs. 1000/T).
good agreement with Slater's model. The electrical study showed the semiconducting behavior of Bi2S3 thin films. Both electrical conductivity and activation energy increased with the increase of the film thickness and substrate temperature. Hence, the results depict that good quality and defect free films can be improved by the application of TS and a good optimization of the other deposition conditions, which are the prerequisites for nano-device applications.
α (Ω.,cm)-1
1.0 0.9
σRT
0.8
σFP
0.7 0.6 0.5 0.4
References 0.3 0.2 120
140
160
180
200 220 TS (°C)
240
260
280
300
Fig. 7. Variation of electrical conductivity as a function of substrate temperature. s: room temperature conductivity. sFP: conductivity using four probe technique.
Table 3 Summary of electrical conductivity and Ea activation energy values. sRT: room temperature conductivity. sFP: four-probe method conductivity. TS (1C)
d (nm)
rRT (Ω cm)
rFP (Ω cm)
Ea (meV)
140 180 220 260 280
300 320 380 460 500
0.310 0.452 0.474 0.910 0.936
0.382 0.445 0.502 0.933 0.952
0.027 0.042 0.052 0.112 0.081
thin films exhibited good crystallinity with orthorhombic structure. The optical studies showed direct electronic transition, and the band gap energy decreased with the increase in TS, film thickness and grain size, which is in
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Please cite this article as: M. Madoun, et al., Temperature effect on structural and optoelectronic properties of Bi2S3 nanocrystalline thin films deposited by spray pyrolysis method, Materials Science in Semiconductor Processing (2013), http: //dx.doi.org/10.1016/j.mssp.2013.04.004i