Nanocrystalline CdS: In thin films prepared by the spray-pyrolysis technique

Journal of Luminescence 141 (2013) 27–32

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Nanocrystalline CdS: In thin films prepared by the spray-pyrolysis technique Shadia J. Ikhmayies a,n, Hassan K. Juwhari b, Riyad N. Ahmad-Bitar b a b

Al Isra University, Faculty of Information Technology, Department of Basic Sciences-Physics, Amman 16197, Jordan University of Jordan, Faculty of Science, Physics Department, Amman 1192, Jordan

art ic l e i nf o

a b s t r a c t

Article history: Received 13 February 2012 Received in revised form 22 February 2013 Accepted 28 February 2013 Available online 21 March 2013

Nanocrystalline CdS:In thin films were produced by the spray pyrolysis technique (SP) on glass substrates. The films were characterized by investigating their X-ray diffractograms (XRD), scanning electron microscope images (SEM), energy dispersive analysis by using X-rays (EDAX), transmittance curves and photoluminescence (PL) spectra. The absorbance was deduced from the transmittance measurements and then it was used to estimate the optical bandgap energies. This was done by plotting the first derivative of the absorbance against wavelength of the radiation, where the positions of the minima in this curve refer to the values of the optical bandgap energy. The size of the nanocrystallites was estimated from XRD diffractograms then from the hyperbolic band model using the estimated bandgap energies. Fine-structured PL spectra confirmed the nanocrystalline nature of the films. & 2013 Elsevier B.V. All rights reserved.

Keywords: Semiconductors Thin films II–VI Compounds Photoluminescence Cadmium sulfide X-ray diffraction

1. Introduction Nanocrystalline semiconductors possess a number of novel physicochemical properties such as quantum size effects, sizedependent chemical reactivity, optical non-linearity, efficient photoelectron emission and melting point reduction providing for unique applications of these materials [1]. They are also characterized by large surface-to-volume ratios (surface effects) [2] and are of great interest for applications in optoelectronics [3,4], nonlinear optics, photoelectrochemistry [3], catalysis [4], photovoltaics, and biological sensing [3]. Quantum confinement effects as well as surface effects control optical properties of semiconductor nanocrystals [2,5]. It was indeed shown several years ago that the growth of a highly lattice mismatched semiconductor layer onto a substrate could lead to the spontaneous formation of semiconductor clusters with sizes in the quantum range [6]. Misfit stresses occur in crystalline films due to the geometric mismatch at interface boundaries between crystalline lattices of films and substrate. Therefore a stress is also developed in the film due to the lattice misfit [7]. However, the stress has two components: thermal stress arising from the difference of expansion coefficient of the film and substrate and internal stress due to the accumulation effect of the crystallographic flaws that are built into the film during deposition n

Corresponding author. Tel.: þ962 4711710; fax: þ 962 4711505. E-mail address: [email protected] (S.J. Ikhmayies).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.045

[5]. It has been argued that in these semiconductor nanoparticles the hyperbolic band model gives a better fit to the observed quantum size effect as compared to the effective mass approximation [5,8]. Most studied nanocrystalline semiconductors belong to the II–VI group as they are relatively easy to synthesize and generally prepared as particulates or in thin film form. Kityk et al. [9] studied the thin interface nanolayers separating the crystallite ZnS films prepared by the spray pyrolysis technique and the glass substrates using photoinduced linear and non-linear optical methods. Among II–VI compounds, CdS is one of the most studied materials [10]. Han et al. [11] prepared strongly surface-modified CdS (R-CdS, R: C6H13S–) nanoparticles by using cosurfactant hexanethiol (C6H13SH) as capping reagent in an inverse microemulsion. Kumpf et al. [12] presented a Debye-formula-based method for the structure determination of nanoparticles with diameters of 2–5 nm and applied it to CdS and ZnS nanoparticles. Němec et al. [13] reported on the control of nanocrystal sizes in CdS nanocrystalline films prepared by ammonia-free chemical bath deposition technique. Also Datta et al. [14] achieved size tunability of thiophenol capped CdS nanoparticles (NPs) by controlling the temperatures at the time of synthesis. In this work CdS: In nanocrystalline thin films were produced by the spray pyrolysis (SP) technique on glass substrates at a substrate temperature of 490 1C. Doping of CdS thin films with indium is well known and it is used to increase the conductivity of the films. X-ray diffraction (XRD), scanning electron microscope

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S.J. Ikhmayies et al. / Journal of Luminescence 141 (2013) 27–32

imaging (SEM), energy dispersive analysis by using X-rays (EDAX), transmittance and photoluminescence (PL) measurements have been used to study the properties of the films.

2. Experimental part The precursor solution of CdS thin films was prepared by dissolving 2.06  10−2 mol of extra pure CdCl2  H2O (MERCK Art. 2011) and 2.24  10−2 mol of thiourea (NH2)2CS (497% S) in 350 ml of distilled water. Indium chloride InCl3 (MERCK Art.12471) was used as a doping compound. The ratio of the concentration of indium ions to that of cadmium ions in the solution is 1.0  10−4. This ratio is not necessarily the same as that in the films. The solution was sprayed intermittently by using the spraying system described in [15] on glass substrates at a substrate temperature Ts ¼490 1C, where the substrates were ultrasonically cleaned with methanol for at least 15 min before the deposition of the films. The spraying process took a long period of time because we were spraying for 10 s, waiting 2–3 min and then spraying again by the same way. This method of spraying gave us highly transparent films of narrow distribution of grain size as will be seen in the following sections. The transmittance of the films was measured by using a double beam Shimadzu UV 1601 (PC) spectrophotometer with respect to a piece of glass of the same kind as the substrates in the wavelength range 300–1100 nm. The films' thickness was estimated by using Lambert law for absorption in a semiconductor which is applicable at wavelengths lower than the cut-off wavelength. The linearity of the relation between ln(T/T0) where T/T0 is the relative transmittance against thickness means that Lambert law is valid. We made this plot for CdS films of known thickness prepared by thermal evaporation at three different values of wavelength that are lower than the cut-off value: 420, 435 and 450 nm. The best linear fit was obtained for the wavelength λ ¼435 nm. So at this wavelength we took the absorption coefficient from Ref. [16] which is about 1.1  105 cm−1 for the undoped CdS thin films, since we have light doping (the ratio of In to Cd in the films is about 7  10−2 as obtained from the EDAX compositional analysis). This estimation results with an uncertainty in the film thickness of about 73 nm. Two sets of films were produced; each set consists of 5 films prepared at the same time. One of them of thickness around 140 nm, and the other of thickness around 500 nm. X-ray measurements were made with a Philips PW1840 Compact X-ray diffractometer system with Cu Kα (λ ¼1.5405 Å). The measurements were recorded at a diffraction angle 2θ from 0 1 to 60 1 The SEM images and compositional analysis were made by using FEI scanning electron microscope (Inspect F 50) which is supplied by energy dispersive analysis by X-rays (EDAX). The PL spectra were recorded at T ¼23 K by a system which consists of an Air Product He cryostat DISPLEX DE-202 capable of cooling down to 10 K, where the Ar ion laser of wavelength 488 nm was used as an excitation source. The laser power was 10 mW and the diameter of the laser beam on the sample was about 2 mm. The PL signal was collected by a multi-channel optical spectrometer (an Avantes Fiberoptic Spectrometer AVSS2000) which hosts two gratings. The first grating has a range: 640–1280 nm and the second grating has a range: 190–860 nm. The spectrometer resolution (FWHM) ranges from 0.3–10 nm depending on the recorded region and the grating.

3. Results and discussion Fig. 1 displays the SEM images for two of the as-deposited CdS: In thin films from the two sets, where the nancrystalline nature of

Fig. 1. SEM images of the as-deposited CdS:In thin films of thickness, (a) 500 nm and (b) 140 nm. Note the bar in the bottom of each image represents the scale which is 1 μm.

the films is apparent. The film of thickness about 500 nm appears with larger grains, while that of thickness around 140 nm appears with a large density of smaller grains and a small density of large grains. Fig. 2 displays the EDAX spectra of the two films and Table 1 shows the elemental concentrations of sulfur, cadmium and indium in the films. The ratios of indium to cadmium in the two films are approximately the same (≈7  10−2) and the thicker film is cadmium rich, while the thinner one is sulfur rich. Fig. 3 displays the X-ray diffractogram for two of the asdeposited CdS:In thin films from the two sets. The films show a mixed (cubic and hexagonal) phase with different directions of crystal growth. The grain size was estimated by using Scherrer formula: d¼

λ Dcos θ

ð1Þ

S.J. Ikhmayies et al. / Journal of Luminescence 141 (2013) 27–32

29

Fig. 2. EDAX spectra of CdS:In thin films: (a) film thickness¼500 nm and (b) film thickness¼140 nm.

where d is the grain size (diameter), λ is the X-ray wavelength used, D is the angular line width of the half-maximum intensity and θ is the Bragg angle. The strongest reflection in diffractogram in Fig. 3(a) is from the C(311)/H(112) plane the lines C(311) and H(112) are very close to each other and could not be distinguished. The next strongest reflection in the same diffractogram is from the H(111) plane. The intensity of these lines is much smaller in diffractogram (b) and the C(311)/H(112) line is no longer the preferential orientation. The decrease in the intensity of these lines is due to the decrease in size of the grains related to these directions with the decrease in film thickness. The presence of the weak lines in the X-ray diffractograms confirms that the films contain nanocrystallites of smaller radii than those related to the strongest lines. The estimated values of the grain size for both films (diameter) are inserted in Table 2. As the table shows the grain size has larger values and wider distribution in the case of the thicker film. This means that one has to decrease the thickness of the films to get a narrow range of particles' sizes. It is important to notice that the estimated grain sizes from the strongest two peaks in the

Table 1 The elemental concentrations of sulfur, cadmium and indium in films of different thickness obtained from EDAX analysis. Element

Thickness t ¼500 nm

t¼ 140 nm

S Cd In

[at%]

Error (%)

[at%]

Error (%)

55.5 41.5 3.0

0.7 3.0 0.8

46.21 50.15 3.65

0.8 4.4 1.1

diffractogram of the thicker film are not confidential, because Scherrer formula does not give correct results for very sharp peaks. The transmittance and absorbance curves are shown in Fig. 4 where they are recorded in the wavelength range 300–1100 nm. It is important to mention here that for sulfide films deposited on the glass substrate, nano-confined effects begin to play substantial role in the optical spectra near the band edge [17]. As the figure

S.J. Ikhmayies et al. / Journal of Luminescence 141 (2013) 27–32

H(101)

30

0.05

First Derivative

H(202)

H(103)

H(102)

H(100)

Arbitrary units

C(111)/H(002)

C(311)/H(112)

0.10

0.00 400

600

λ(nm)

a -0.05

b 0.4

0 0

10

20

30

40

50

60

2θ (°)

Table 2 Values of the grain size (diameter) estimated from the XRD diffractograms. t ¼140 nm

First Derivative

0.2

Fig. 3. X-ray diffractograms for CdS:In thin films of doping ratio¼1  10−4 deposited at Ts ¼ 490 1C. (a) t ¼500 nm and (b) t ¼140 nm.

t ¼500 nm d (nm)

Peak

d (nm)

H(202) C(311)/H(112) H(103) C(220) H(102) H(101) C(111)/H(002) H(100)

– 17 4 14 7 12 9 13

H(202) C(311)/H(112) H(103) C(220) H(102) H(101) C(111)/H(002) H(100)

13 71 12 12 16 50 11 6

60 6

500 nm 140 nm

4

Absorbance

8

T%

-0.2

Table 3 The minima found in the first derivative of the absorbance and the calculated energy bandgaps and particles' radii for the film of thickness 140 nm.

80

20 2 0 0 800

600

Fig. 5. The first derivative of the absorbance curves in Fig. 3. (a) The film of thickness 140 nm and (b) the film of thickness 500 nm.

Number

λmin (nm)

Eg (eV)

R (nm)

1 2 3 4 5 6 7 8

503.5 488.5 416.5 365 356.5 339 328.5 314

2.463 2.538 2.977 3.397 3.478 3.658 3.775 3.949

23.2 13.9 6.1 4.5 4.3 3.9 3.7 3.4

10

600

500

λ(nm)

-0.6

100

400

400

-0.4

Peak

40

0.0 300

1000

λ (nm) Fig. 4. Transmittance and absorbance curves of the as-deposited CdS:In thin films prepared by the spray pyrolysis technique.

shows, the absorbance decreases with decreasing thickness along with tailing toward longer wavelength values. This tailing indicates the presence of smaller crystallites in the samples [10]. Also the oscillatory structure in the absorbance curves is a signature of the size quantization effect, and each peak in the absorption spectrum corresponds to transitions to different excited states of

the conduction band [10].To confirm these expectations, the first derivative of the absorbance curves was plotted in Fig. 5. First, it is noticed that the well known splitting of the valence band of the hexagonal phase (A, B, and C) of CdS is apparent in Fig. 5, and the positions of these excitonic peaks can be found by using the second derivative of the absorbance, but this is not the subject of this article. The minima of the first derivative were displayed in Tables 3 and 4 and used to find the optical bandgap energies. So the first two values in each of the two tables represent the bandgap energies including the exciton binding energy. As we see there is a wide range of bandgap energies that is 2.46–3.97 eV. The increase in the bandgap energy could be explained as follows; the nanoparticles have the crystalline structure of their bulk counter parts and hence are characterized by the fully occupied valence band and an empty conduction band separated by the energy gap (Eg). However, the charge carriers in these bands e.g.,

S.J. Ikhmayies et al. / Journal of Luminescence 141 (2013) 27–32

Table 4 The minima of the first derivative of the absorbance and the calculated energy bandgaps and particles' radii for a film of thickness 500 nm. Number

λmin (nm)

Eg (eV)

R (nm)

1 2 3 4 5 6 7 8 9 10 11

503.5 490 464 449.5 437 426.5 417.5 400.5 390 365.5 312

2.463 2.531 2.672 2.759 2.838 2.907 2.970 3.096 3.179 3.393 3.974

23.2 14.4 9.4 8.0 7.2 6.6 6.2 5.5 5.2 4.5 3.4

3000

500 nm 140 nm

2500

PL(Counts)

2000 1500 1000 500 0 1.4

1.6

1.8

2.0

2.2

2.4

2.6

λ(nm)

1200

500 nm 140 nm

PL(Counts)

1000 800 600 400 200 0 1.0

1.2

1.4

1.6

1.8

λ(nm) Fig. 6. PL spectra of the as-deposited CdS:In thin films recorded by: (a) the 2nd grating and (b) the first grating.

the electrons in the conduction band and holes in the valence band experience an overall confining potential due to the finite size of these particles. As a result there will be size-dependent discrete states in the conduction and valence bands resulting in the effective enhancement of the bandgap; the so-called quantum size effect [8]. As Tables 3 and 4 show, the bandgap energy varies from that of the bulk CdS:In (≈2.46 eV) to approximately that of the glass substrate (≈3.4 eV). This result is consistent with the results obtained by Kityk et al. [9] for ZnS films on glass substrates when thin ZnS nanocrystalline layers of 1–2 nm thick are concerned, where a continuous variation of bandgap was observed between

31

3.6 and 4 eV due to a great deal of trapping levels whose discrete energies are located within the bandgap energy of ZnS. It has been argued that in these semiconductor nanoparticles the hyperbolic band model gives a better fit to the observed quantum size effect as compared to the effective mass approximation [8]. So it was used to estimate the size of the nanocrystallites. The equation derived for the bandgap, Egn of nanocrystallites according to [10] is:  1=2  π 2 Egn ¼ E2gb þ 2ℏ2 Egb =mn R

ð2Þ

where Egb is the bandgap for the bulk semiconductor and equals 2.4 eV [10], R is the particle radius, and mn the effective electron mass. Taking mn =me ¼ 0:2 for CdS [7] where me is the mass of a free electron. The calculated values of particles' radii are inserted in Tables 3 and 4 for the two films of thickness 140 and 500 nm respectively. As the tables show, the values are restricted in the range 3.4–23.2 nm, which are approximately consistent with those estimated from the XRD diffractograms except for the strongest line in the diffractogram of the thicker film. Also it is noticed that more distribution in the particles' size was observed in the case of the thicker film as found from the XRD diffractograms, or the range of particles' sizes becomes narrower as the thickness gets smaller. The accuracy of these values is limited because the bulk bandgap and the absorption coefficient that were used in the calculations are those of undoped CdS, while our films are indium doped, and it is known that doping increases the bandgap energy. Fig. 6 shows the PL spectra for the same two films of thickness 140 and 500 nm recorded by the two gratings. The difference between the two curves in (a) and (b) is obvious; that is the PL signal of the thinner film is weaker and contains a considerable fine structure. Since the ratios of indium to cadmium in both films are approximately the same, we expect that there will be no difference in the PL spectra shown in Fig. 6 due to indium doping. The influence of indium doping on the PL spectra and other properties of CdS:In thin films was investigated in our previous works [18,19], where it was found that indium doping decreases the intensity of the PL signal. To explain this result we have to mention that samples prepared by spray pyrolysis (i.e. a chemical route) are known to have a size distribution. The PL spectrum shown is the summation of the spectra of individual crystallites. In the case of the thicker film a larger size distribution is present (as seen in Tables 2 and 4), so the PL spectrum of the thicker film does not show any sharp features originating from individual crystallites but show wide terraces. On the other hand, the spectrum of the thinner film in (a) and (b) shows an obvious fine structure. It is well known that the size of the crystallites decreases with decreasing film thickness, so it is expected that the average size of the nanoparticles is smaller in the case of the thinner film and hence the fine PL could be observed.

4. Conclusions Nanocrystalline CdS:In thin films were produced on glass substrates by the spray pyrolysis technique at a substrate temperature of 490 1C. The films were characterized by investigating their XRD diffractograms, SEM images, EDAX compositional analysis, transmittance and absorbance curves and PL spectra. The sizes of the nanoparticles obtained from XRD diffractograms are comparable with the values estimated from the hyperbolic band model. Fine-structured PL spectra confirm the presence of nanocrystallites and demonstrate that the size distribution becomes narrower when the film thickness decreases.

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