Optical and electrical characterization of chemical

Thin Solid Films 526 (2012) 97–102

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Optical and electrical characterization of chemical bath deposited Cd–Pb–S thin films M.A. Barote a,⁎, S.S. Kamble b, A.A. Yadav c, E.U. Masumdar c a b c

Department of Physics, Azad college, Ausa, (M.S.), 413520, India Bharat Ratna Indira Gandhi College of Engineering, Kegaon, Solapur, (M.S.), 413255, India Thin Film Research Laboratory, Department of Physics, Rajarshi Shahu Mahavidyalaya, Latur, (M.S.), 413512, India

a r t i c l e

i n f o

Article history: Received 18 May 2011 Received in revised form 10 November 2012 Accepted 13 November 2012 Available online 28 November 2012 Keywords: Thin films Chemical bath deposition Optical properties Electrical conductivity Cadmium lead sulfide

a b s t r a c t CdzPbyS thin films have been deposited on glass substrates using inexpensive chemical bath deposition technique. The aqueous solution containing precursors of Cd 2+ and Pb 2+ has been used to obtain good quality deposits at optimized preparative parameters. The thin film samples have been characterized through optical absorption, electrical conductivity and thermoelectric power measurement techniques. From optical studies, the absorption coefficient ‘α,’ is found to be of the order of 10 4 cm −1. The optical absorption studies revealed direct band to band transition. The band gap energy is found to vary nonlinearly from 2.47 eV (CdS) to 0.49 eV (PbS) as the composition parameter ‘x’ was increased from 0 to 1. The electrical characterization revealed increased conductivity (σ) with the increased composition parameter up to x = 0.175. The electrical conductivity measurements indicate two types of conduction mechanism, namely grain boundary scattering limited and variable range hopping conductions. The activation energies of the films of different compositions were determined at low and high temperature regions. The activation energies were observed to be in the range of 0.168–0.240 eV and 0.514–0.711 eV respectively. Thermoelectric power measurements highlighted n-type behavior of the as-grown thin film samples. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The ternary derivative materials have boosted a lot of research interest in the field of optoelectronic devices due to the potential of tailoring both the lattice parameters and the band gap by controlling deposition conditions [1,2]. One such prominent ternary candidate is cadmium lead sulfide having wide band gap for photoconducting and photovoltaic devices, IR detectors, photodiodes and laser applications [3]. Thin films of cadmium and lead sulfide are promising photovoltaic materials as their variable band gap can be engineered to match the ideal band gap (≈1.5 eV) required for most efficient solar cell [4]. These materials are used in optoelectronic devices, solar control coatings, gas and humidity sensors and photoelectrochemical solar cells [4–6]. Skyllas-Kazacus et al. [7] are the one who gave detailed data on high cadmium mole fraction in solution. Roger and Crocker carried out an extensive study of the electrical properties of Pb1 − xCdxTe using bulk material [8]. Harman and co-workers reported that, the bulk crystals of Pb1 − xCdxS could be prepared beyond the stable phase boundary limits by quenching [9]. Thin metal chalcogenide films can be obtained by chemical bath deposition (CBD) method [10,11], spray pyrolysis [12], and physical and electrochemical techniques [13,14]. Many researchers have deposited ternary derivative materials in thin film form Cd1−xZnxS

⁎ Corresponding author. Tel.: +91 9422658959. E-mail address: [email protected] (M.A. Barote). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.018

[15], PbS–CuxS [6], Bi2S3–CuxS [16], Cd1−xCuxS [17] and Bi2Se3–Sb2Se3 [18], using simple and inexpensive chemical bath deposition. The overriding advantage of CBD over the other methods is that films can be deposited on different kinds of substrates with variable size and shape [19]. The present study deals with the optical, electrical and thermoelectric analysis of cadmium lead sulfide ternary compounds which is essential imply their active properties in various optoelectronic devices. 2. Experimental details CdzPbyS thin films with the nominal composition Cd1 − xPbxS (0 ≤ x ≤ 1) were deposited onto glass substrates by chemical bath deposition method reported earlier [20]. For the deposition, solutions of cadmium sulfate, lead sulfate and thiourea (all A. R. grade) were mixed in stoichiometric proportion to obtain ‘x’ value from 0 to 1. Triethanolamine was used as complexing agent and pH of the reaction mixture was adjusted to 10.5 ± 0.1. The ultrasonically cleaned glass substrates were mounted on a specially designed substrate holder and were rotated with a constant speed in the reaction mixture. To obtain good quality thin films deposition time, temperature and speed of substrate rotation were optimized. These optimized parameters were 60 min, 80 °C and 65 rpm respectively. The thickness of as-deposited thin films was determined using gravimetric weight difference method with sensitive microbalance. The X-ray diffractograms were obtained for these samples with Philips-PW 1710 X-ray diffractogram using CuKα line (λ = 1.5406 Å) within the 2θ range from 20° to 80°. The microscopic features were

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M.A. Barote et al. / Thin Solid Films 526 (2012) 97–102

observed through a scanning electron microscope JEOL-JSM-5600 (Japan) with operating voltage 25 kV. The optical absorption spectra were recorded at room temperature using UV–VIS-IR spectrophotometer (Carry-5000 Japan). The absorption coefficient, energy band gap and mode of transition were determined. A two probe press contact method was employed to measure electrical conductivity and thermo-emf of the as-obtained samples in the temperature range of 300–500 K.

3. Results and discussion

(311) (222)

(220)

(200)

Intensity (arb. units)

S(200)

(111)

The X-ray diffraction (XRD) pattern for few representative samples is shown in Fig. 1. These diffractograms reveal the polycrystalline nature of as-deposited thin films, irrespective of the composition parameter ‘x’ over entire range (0 ≤ x ≤ 1). It is observed that, the CdS (x = 0) exhibit cubic and hexagonal crystal structure. It is reported that chemically deposited CdS films depending upon preparative conditions, show cubic, hexagonal or mixed (cubic + hexagonal) crystal structures [21]. For Cd1.35S prepared without Pb, the prominent peaks corresponding to (111), (200), (220) and (311) planes of the material with cubic phases, while peaks corresponding to (002), (111), and (110) planes are of CdS hexagonal structure. For Pb1.56S prepared without Cd, the preferential orientation is along (111), (200), (220), (311), (222), (420) and (422) planes. The XRD pattern fairly matches with the peak positions (2θ) of standard X-ray powder diffraction data (JCPDS) file [22]. No shift in peak position with increased Pb was observed, which indicates that, the Cd1 − xPbxS (0 ≤ x ≤ 1) thin films are of composite type. The average lattice parameters ‘a’ and ‘c’ for hexagonal phase were found to have a non-linear variation with the composition (a = 4.1035 Å to 4.1720 Å and c = 6.6061 Å to 6.7288 Å). The similar trend was observed in case of cubic phase (a = 5.8015 Å to 5.9171 Å). The determined average grain size was found to lie in between 7 and 17 nm. The compositional analysis of as-grown samples was determined by energy dispersive X-ray spectroscopy (EDAX) using JEOL JSM 5600 at the operating voltage of 25 kV for three different locations. EDAX study confirmed that as-obtained thin films are sulfur deficient nature [23]. The elemental composition of Cd1−xPbxS thin films deposited by chemical bath method is given in Table 1. Fig. 2 shows a typical EDAX pattern of chemical bath deposited Cd1−xPbxS (x= 0.175) thin film sample. The possible composition and structure of chemically deposited thin films can vary widely due to the variation in pH value and due to the presence of reducing agent in the reaction mixture [24].

Table 1 Elemental composition of Cd–Pb–S thin films deposited by chemical bath method. Nominal Concentration, x as “Cd1−xPbxS”

Cd1 −xPbxS (0 ≤ x ≤ 1) concentration

As observed atomic % in film by EDAX analysis Cd

Pb

S

0 0.1 0.175 0.3 0.5 0.7 1

Cd1.35S Cd1.77Pb0.15S Cd1.77Pb0.15S Cd1.13Pb0.45S Cd0.68Pb0.61S Cd0.48Pb0.89S Pb1.56S

57.45 60.51 52.03 43.68 29.68 20.15 00.00

00.00 05.25 05.37 17.56 26.56 37.56 60.95

42.55 34.24 42.60 38.76 43.76 42.29 39.05

3.1. Optical properties The optical absorption study of the material provides a simple method for explaining some features concerning the band structure and energy gap of non-metallic materials. These studies constitute most important means of determining the band structures of semiconductors. The optical absorption spectra of as-deposited CdzPbyS thin films have been used to determine the absorption coefficient (α), energy band gap (Eg) and the nature of transition involved. The action spectra were taken in the range of 350–3300 nm. The absorption coefficient is higher for all the film compositions (10 4 cm −1). The wavelength dependence of absorption coefficient for five representative compositions is shown in Fig. 3. It is observed that, the absorption edge of the films varies with the composition parameter ‘x’. The relation between the absorption coefficient ‘α’ and the incident photon energy ‘ ν’ for allowed direct type of transitions can be written as [25] 1=2 αhυ ¼ A hυ−Eg :

ð1Þ

To understand the onset of high photon energy corresponding to the direct band gap energy we plotted (α ν) 2 versus ν as shown in Fig. 4. The straight line nature of the plots over a wide range of photon energy suggested allowed direct type of transition. It is well known that direct transition across the band gap is feasible between the valence and conduction band edges in k space [26]. The optical band gaps have been then determined by extrapolation of the linear regions on ν axis. The variation of band gap with composition parameter ‘x’ is as shown in Fig. 5. The non-linearity of the band gap energy variation with composition has already reported for Cd1 − xZnxS [27,28] and Cd–S–Se [29,26] thin films. Such a kind of broad and fine tunable band gap properties of ternary compounds have potential applications in gas sensors, solar cells, detectors and optoelectronic devices.

Pb1.56S Cd0.48Pb0.89S Cd0.68Pb0.61S Cd1.13Pb0.45S Cd1.77Pb0.15S Cd1.77Pb0.15S Cd1.35S

20

30

40

50

60

70

80

2θ (deg.) Fig. 1. XRD pattern of chemical bath deposited Cd1 −xPbxS (0 ≤ x ≤ 1) thin films.

Fig. 2. Typical EDAX spectrum of chemical bath deposited Cd1.77Pb0.15S thin film.

Coefficient of absorptionα x 104(cm-1)


99

Cd1.35S

6

Cd1.77Pb0.15S Cd1.77Pb0.15S

5

Cd1.13Pb0.45S Cd0.68Pb0.61S

4

Cd0.48Pb0.89S Pb1.56S

3 2 1 0 500

1000

1500

2000

2500

3000

3500

Wavelength (nm) Fig. 3. The action spectra of five typical compositions of CdzPbyS thin films.

The nature of the optical transition in these films has also been determined as lnðαhυÞ ¼ lnðAÞ þ m ln hυ−Eg :

ð2Þ

Straight line plot (Fig. 6) of ln (αhν) vs. ln (hν − Eg) with slope ≈0.5 has also confirmed the direct type of transition. The values of slope for various film compositions are listed in Table 2. 3.2. Electrical transport studies The electrical transport properties of the materials are of great importance in determining whether the material is suitable for our necessities or not. The electrical properties are mainly dependent on the preparative parameters such as film composition, film thickness, deposition temperature, deposition time and basic ingredients in the reaction solution. The electrical conductivity of the material is prime characterization for various device applications. The dc electrical conductivity of a semiconductor at temperature T is given by [30] −Ea σ ¼ σ 0 exp kT

(αhν)2 × 1010 (eV/cm)2

5 4

where, σ0 is the pre-exponential factor, Ea is the activation energy for the generation process and k is Boltzmann constant. We may write, E lnσ ¼ lnσ 0 − a or kT Ea 1000 þ lnσ 0 : lnσ ¼ − T 1000k

ð4Þ

The dc electrical conductivities of the as-deposited thin films in dark were carried out in the range of temperature from 300 to 500 K. The temperature dependence of the dark conductivity is as shown in Fig. 7. The conductivity of all these samples increases with increase in temperature. The electrical conductivity is found to be composition dependent. It is observed that electrical conductivity is increased as the actual Cd content of as-deposited film layers decreases up to composition parameter ‘x’ = 0.175 (7.94 × 10 −6 Ω −1 cm −1) and thereafter it is decreased. The room temperature electrical conductivities for CdS and PbS are 6.13 × 10 −7 (Ω cm) −1 and 1.70 × 10 −6 (Ω cm) −1 respectively. The lower magnitudes of electrical conductivity of the films are attributed to the deposition method itself i.e. generally chemical methods result into lower magnitude of electrical conductivity [21]. The presence of defects viz. structural disorders, dislocations and surface imperfections are also play a vital role in decreasing the conductivity [31]. From Fig. 7 the variation shows an Arrhenius behavior

Cd0.6 8Pb0 .61S Cd1 .77P b0.1 5S Cd 1.1 3P b0 .45 S Cd1.35 S

6

Cd1.77Pb0.15S

Pb1.56S

Cd0.48 Pb0.89 S

ð3Þ

Fig. 5. Variation of energy band gap (Eg) versus actual concentration of Pb in as-grown thin films.

3 2 1 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

hν (eV) Fig. 4. Plot of (α ν)2 versus ν for Cd1 −xPbxS (0 ≤x ≤ 1) thin films.

Fig. 6. Variation of ln (α ν) versus ln ( ν − Eg) for Cd1−xPbxS (0 ≤ x ≤ 1) thin films.

100


Table 2 Optical and electrical parameters of chemical bath deposited Cd–Pb–S thin films. Nominal concentration, x as “Cd1−xPbxS”

0 0.05 0.1 0.15 0.175 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eg (eV)

2.47 2.38 2.28 2.17 2.13 2.05 1.86 1.64 1.47 1.22 1.03 0.87 0.62 0.49

Power factor

0.51 0.48 0.52 0.54 0.51 0.53 0.52 0.46 0.57 0.48 0.55 0.58 0.47 0.53

Activation energy LT (eV)

HT (eV)

0.240 0.213 0.193 0.176 0.168 0.173 0.181 0.185 0.188 0.193 0.201 0.205 0.208 0.212

0.711 0.645 0.578 0.523 0.514 0.545 0.566 0.578 0.622 0.631 0.638 0.647 0.668 0.676

consisting of high and low temperature regions. The activation energies of electrical conduction have been determined from these plots in high and low temperature regions and listed in Table 2, for each of the composition parameter ‘x’. The linearity of lnσ against 1/T in the high temperature region indicates that, the conductivity in this region exhibit activated behavior while in the low temperature region conductivity exhibits non-activated behavior. In low temperature regions, between 300 K and 380 K the temperature dependence conductivity for all compositions increases slightly with the small activation energies. In this region the increase in conductivity with temperature is due to the intrinsic nature of as-grown films. Also at low temperatures, the charge carriers in conduction band may too few to give rise to an appreciable conduction, which suggests that, the conduction is due to Mott's variable range hoping in localized states near the Fermi level. This variable range hoping mechanism is characterized by Mott's expression [32] as, 1=4 T σ ¼ σ 0 exp − T0

ð5Þ

where, T0 ¼ λα3 =kNðEf Þ here, λ is a dimensionless constant, k is Boltzman's constant, N(Ef) is the density of localized states at Ef, α is the degree of localization and T0 the degree of disorder.

Conductivity × 10−6 (Ω cm)−1

Mobility × 10−3 (cm2/V s)

0.613 1.885 3.732 6.761 7.944 4.262 2.671 2.611 2.450 2.323 2.074 1.843 1.751 1.702

1.414 2.659 4.336 5.761 6.213 5.383 3.138 3.132 3.095 3.063 2.933 2.880 2.865 2.814

The value σ0 is obtained by Touraine [33] is,

2

σ 0 ¼ 3e ν

NðEf Þ 1=2 8παkT

where, ‘e’ is the electron charge and ‘ν’ is the Debye frequency. Fig. 8 represents the plot of ln (σT 1/2) versus 1/T 1/4 for the as-deposited thin films. From these curves it is clear that ln (σT 1/2) versus 1/T 1/4 is a linear relation. This is in good consonance with the Mott's variable range hoping process. At low temperature charge carriers do not have sufficient energy for excitation to the adjacent band and hence they move from one impurity to another with the help of phonon. In high temperature region (380 K and 500 K) the conductivity of the film sample increases sharply. At these temperatures the low mobility of charge carriers is easily compensated by creation of large number of charge carriers and this result in increased conductivity of the film [34]. A polycrystalline film material contains a large number of microcrystallites with grain boundaries between them. At the grain boundary the incomplete atomic bonding can act as trap centers. These trap centers trap the charge carriers at the grain boundaries, and hence a local space charge region can be built up. The grain boundary potential model proposed by Seto [35] is given by, pffiffiffi −Ea σ T ¼ σ 0 exp kT

Fig. 7. Plot of log σ versus (1000/T) of CdzPbyS thin films.

ð6Þ

Fig. 8. Variation of log (σT1/2) vs. (T−1/4) for chemical bath deposited Cd1−xPbxS (0 ≤ x ≤ 1) thin films.


101

-0.5 Cd1.35S Cd1.77Pb0.15S Cd1.77Pb0.15S Cd1.13Pb0.45S Cd0.68Pb0.61S Cd0.48Pb0.89S Pb1.56S

log(σT1/2)(Ω-1cm-1K1/2)

-1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 2.0

2.1

2.2

2.3

2.4

2.5

1000/T (K-1) Fig. 9. Variation of log (σT1/2) vs. (1000/T) for chemical bath deposited Cd1−xPbxS (0 ≤x ≤ 1) thin films.

where, σ0 is the pre-exponential factor, Ea is the activation energy, k is Boltzman's constant and T is absolute temperature. The plot of ln (σT 1/2) versus 1000//T have been plotted in Fig. 9. Obviously in the high-temperature region, grain boundary limited conduction is dominant conduction mechanism. The thermoelectric power is the ratio of thermally generated voltage to the temperature difference in the semiconductor, which gives the information about charge carriers in the deposited material. In thermoelectric power measurements, the open circuit thermo-voltage generated by the film samples when a temperature gradient is applied across a length of a sample is measured. The thermoelectric power increases with increase in temperature. The polarity of thermally generated voltage for CdzPbyS thin films at hot end is positive indicating that, the as-deposited thin films are of n-type [36]. The variation of thermo-emf as a function of temperature difference is shown in Fig. 10. TEP was also used to evaluate the carrier mobility (μ) and carrier concentration (n) using the relation [37], " ( !)# −K 2πmc kT3=2 A þ ln 2 TEP ¼ e nh3

ð7Þ

Fig. 11. Carrier density variations with actual concentration of Pb in as-obtained thin films.

electron and T is the absolute temperature. After substitution of various constants Eq. (4) simplifies to logn ¼

400

Cd1.35S Cd1.77Pb0.15S Cd1.77Pb0.15S Cd1.13Pb0.45S Cd0.68Pb0.61S Cd0.48Pb0.89S Pb1.56S

Thermo-emf (μV)

350 300 250

ð8Þ

The carrier density (n) at room temperature was determined for all samples. The variation of carrier density with composition for chemically deposited CdzPbyS thin films is as shown in Fig. 11. The carrier density is found to be of the order of 1015 cm−3. The carrier density is increased with decrement in actual Cd content of as-deposited thin film layers up to the composition parameter ‘x’ = 0.175. The smaller carrier density is characteristic of the compensation type semiconductor involving deep donor or deep acceptors [38]. Moreover, structural defects and grain boundaries, whose number are generally larger in deposited materials may also be responsible for reducing carrier density, since they are capable of trapping carriers [35]. The electron mobility has been calculated using the standard relation, μ¼

where, k is thermal conductivity, A is a thermoelectric factor, n is electron density, h is Plank's constant, mc* is the effective mass of the

3 logT−0:005TEP þ 15:719: 2

σ ne

ð9Þ

where μ is the electron mobility, σ is the electrical conductivity and n is the carrier density. The electron mobility is found to be a function of composition. The electron mobility for chemical bath deposited Cd–Pb–S thin films is found to be in the range of 1.414–6.213 × 10 −3 cm 2/V s. The smaller values of electron mobility are due to presence of the grain boundaries and structural defects which cause scattering of charge carriers. The calculated values of electron mobility at room temperature are listed in Table 2. 4. Conclusion

200 150 100 50 0 20

40

60

80

100 120 140 160 180 200

Temperature difference (oC) Fig. 10. Thermo-emf variation as a function of temperature difference for CdzPbyS thin films.

CdzPbyS thin films with nominal composition as Cd1 − xPbxS (0 ≤ x ≤ 1) have been deposited by simple and inexpensive chemical deposition route. The optical studies indicated direct energy band gap which strongly depends on the composition parameter ‘x’. The band gap energy was tailored in the range 2.47 eV (CdS) to 0.49 eV (PbS) which is a prime need for various device application viz. solar cell, photoelectrochemical cell, IR detectors and lasers. The electrical conductivity of as-deposited thin films was found to be increased with the composition parameter ‘x’ up to 0.175. The room temperature electrical conductivity for optimized sample was measured to

102


be 7.94 × 10 −6 Ω −1 cm −1. The conductivity measurements revealed two types of conduction mechanism, namely grain boundary scattering limited and variable range hoping conductions. n-Type conduction of as-grown Cd–Pb–S thin film layers was highlighted by the thermoelectric power measurements. The wide and fine tunability of the energy band gap as well as the uneven changes in the conductivity of ternary Cd–Pb–S thin film layers have potential applications in a variety of optoelectronic devices.

[17] [18] [19] [20] [21] [22] [23] [24] [25]

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