Structural and optical characterization of sprayed ZnS

3 downloads 0 Views 819KB Size Report
The cost-effective spray pyrolysis technique was applied to prepare ZnS thin films ..... by the successive ionic layer adsorption and reaction (Silar) method using ...

Structural and optical characterization of sprayed ZnS thin films Tatjana Dedova, Arvo Mere, Malle Krunks*, Olga Kijatkina, Ilona Oja, and Olga Volobujeva Department of Materials Science, Tallinn University of Technology, 5 Ehitajate Rd., 19086 Tallinn, Estonia ABSTRACT The cost-effective spray pyrolysis technique was applied to prepare ZnS thin films for large-scale applications. Aqueous solutions containing ZnCl2 and SC(NH2)2 were deposited onto glass and n-type Si(100) substrates at 430–600 °C. The structure and phase composition were studied by XRD and FTIR, the surface morphology by SEM. The optical properties were determined from the transmittance and reflectance measurements in the UV–visible and near-IR regions. The phase composition, crystallinity, and surface morphology of the films could be controlled by the Zn : S molar ratio in the stock solution and the deposition temperature. Highly (001) oriented ZnS films with wurtzite structure were grown at temperatures above 500 °C using the Zn : S molar ratio 1 : 2 in the solution. The sprayed films had high optical transmittance in a wide spectral range and the refractive index of up to 1.9 at 632.8 nm. It was also found that the sprayed ZnS films are capable of being used as antireflection coatings in Si-based devices. Keywords: ZnS, thin films, spray pyrolysis, structural properties, optical properties, band gap, refractive index, antireflection coatings

1. INTRODUCTION Zinc sulfide has a direct band gap of 3.7 eV, high refractive index (2.35) and n-type conductivity. It exhibits a high optical transparency in a wide spectral range of 0.4–12 μm. The optical transparency combined with the chemical and thermal stability makes ZnS one of the most suitable materials for optical windows.1 Zinc sulfide is used in electroluminescent displays, light emitting diodes, dielectric filters, detectors, modulators, optoelectronic devices, etc.2 It is also a promising material for photovoltaic devices as an antireflection coating on the silicon cells 3 and an alternative to the CdS buffer layer in the solar cells based on the copper ternaries absorber layer.4,5 ZnS thin films have been deposited by a number of methods, including reactive sputtering,6 chemical vapor deposition,1 and atomic layer deposition.7,8 All these techniques require expensive deposition equipment, particularly if large areas have to be covered. Chemical bath deposition and spray pyrolysis are considered to be simple and inexpensive tools for the production of the ZnS thin films.5,9 However, chemical bath deposition is a low temperature process, and in order to get well-crystallized films the post-deposition annealing is needed. This paper describes the preparation and properties of the zinc sulfide films fabricated by a simple, highly productive, and nonvacuum chemical spray pyrolysis process. The effect of the deposition parameters, such as the growth temperature and molar ratio of zinc and sulfur precursors in the stock solution, on the structural, morphological, and optical properties of the ZnS films is studied.

*E-mail: [email protected] Optical Materials and Applications, edited by Arnold Rosental, Proceedings of SPIE Vol. 5946 (SPIE, Bellingham, WA, 2005).0277-786X/05/$15. doi: 10.1117/12.639065 Proc. of SPIE Vol. 5946 594605-1

2. EXPERIMENTAL The ZnS films were prepared using the aqueous solutions containing ZnCl2 of the 98% purity and (NH2)2CS of the ≥98% purity. Deionized water was used as a solvent. Molar ratios of the zinc and sulfur sources were 1 : 1 and 1 : 2, with the concentration of ZnCl2 kept constant at 0.05 mol/l. The precursor solution was pulverized onto the substrates placed on the soldered tin bath. Commercial glass slides (30 × 30 × 1 mm3) and n-Si(001) wafers were used as substrates. The deposition temperature was varied in the range of 430–600 ºC using electronic temperature controller and hold with the accuracy of ±10 ºC. The compressed air carrier gas had a flow rate of 8 l/min. The solution spray rate was maintained at 2.5 ml/min. The structure of the films was characterized by x-ray diffraction (XRD) using a Bruker AXS D5005 diffractometer (monochromatic Cu Kα radiation, λ = 1.54056 Ǻ). The XRD patterns were recorded in the 2θ interval of 20–60° with a step of 0.04° and counting time 2 s/step. The surface morphology was examined by scanning electron microscopy (SEM). The measurements were performed on a Zeiss/LEO Supra 35 apparatus. The SEM cross-sectional micrographs served for the estimation of the film thickness. The optical absorption and transmission spectra were measured with a Beckman double-beam UV–visible spectrophotometer in the 300–800-nm region and with a Perkin-Elmer Fourier transform IR (FTIR) spectrophotometer GX1 in the 4000–11000cm-1 region . Refractive indices were calculated from the IR reflectance spectra registered at two angles of incidence (25 and 50°). For comparison, ellipsometric measurements were performed on a high precision DRE ELX-02C ellipsometer equipped with a He-Ne laser source (λ = 632.8 nm).

3. RESULTS AND DISCUSSION 3.1. Structural properties of sprayed ZnS films Figures 1 and 2 present the x-ray diffractograms of the ZnS thin films deposited on glass and Si substrates at different temperatures using two different molar ratios of the precursors in solution. Diffraction peaks were indexed by means of the JCPDS files. The films prepared at temperatures Ts below 460 ºC exhibit broad diffraction peaks belonging to ZnS sphalerite structure. The width of the peaks refers to a poor crystallinity of the films (the XRD patterns are not presented). ZnS W(002)

ZnS S(111)

Intensity, a.u.

Si ZnO 0

Ts= 600 C, Si

Intensity, a.u.

ZnS W (102)

ZnS W(103) 0

Ts = 530 C, Glass

0

Ts= 500 C, Glass

0

Ts= 570 C, Si

Si

0

Ts= 530 C, Glass

0

Ts= 600 C, Si

0

Ts= 500 C, Glass

20

30

40

50

60

0

Ts= 500 C, Si

20

30

40

50

60

2 θ

2 θ

Figure 1. XRD patterns of ZnS films deposited from 1 : 1 solution.

Figure 2. XRD patterns of ZnS films deposited from 1 : 2 solution.

ZnS films grown on the glass substrate at 500 °C using the 1 : 1 solution show only one well-pronounced diffraction peak at 2θ = 28.6° (Fig. 1). The deposition at 530 °C significantly increases the peak intensity. The deposition at temperatures higher than 530 °C was made onto the silicon substrates. As can be seen, at higher deposition temperatures no other reflections of

Proc. of SPIE Vol. 5946 594605-2

the ZnS appear than at 2θ = 28.6° (Fig. 1), which shows that the ZnS films deposited from 1 : 1 solution have the sphalerite structure. This result agrees with the literature data on the ZnS films sprayed from the 1 : 1 solution.2 According to the XRD, an additional reflection belonging to the ZnO phase appears in the sample deposited at 600 °C. The spraying of the Zn : S = 1 : 2 solution at 500 °C onto the glass substrate results in the ZnS film with wurtzite structure, as the (102) and (103) peaks could be cleraly distinguished in addition to the (002) peak of the wurtzite phase (Fig. 2). The films are textured in the (002) direction. The increase of the deposition temperature increases the intensity and decreases the width of the diffraction peaks. The average crystallite size is about 60 nm for the films deposited onto the glass substrate at 530 °C and the silicon substrate at 600 °C, as calculated by the Scherrer formula. No oxide phase was observed in the film deposited at 600 °C onto silicon substrate. This observation suggests that the other process than the oxidation of ZnS is responsible for the formation of the ZnO phase in the films deposited from the 1 : 1 solution.

100 90 80 70 60 50 40 30 20 10 0 300

1 2

Zn:S=1:1

1 2

Zn:S=1:2

100 90 80 Transmission, %

Transmission, %

3.2. Optical properties of sprayed ZnS films 3.2.1. Optical transparency and optical band gap Optical transmittance spectra in the wavelength range of 300–800 nm for the ZnS films deposited on glass substrates are presented in Fig. 3.

3

4

3 4

70 60 50 40 30 20 10

400

500

600

700

800

0 300

400

Wavelength, nm a

500

600

700

800

Wavelength, nm b

Figure 3. Transmittance spectra of ZnS films deposited at 430 (1), 470 (2), 500 (3), and 530 ºC (4) with Zn : S molar ratio in solution 1 : 1 (a, left pane) and 1 : 2 (b, right pane).

The transmittance spectra of the sprayed ZnS films reveal that independently of Zn : S molar ratio in the solution optical transmission decreases with increasing the growth temperature. It was found from the SEM measurements that the film thickness decreases as the deposition temperature increases. For example, for the films deposited from the 1 : 1 solution at 470, 500, and 530 ºC the thickness equaled 500, 265, and 215 nm, respectively. The analogous behavior has already been reported for the sprayed films.10,11 The SEM micrographs for the ZnS films deposited at two different temperatures of 470 and 530ºC are presented in Fig.4. It can be seen that the film deposited at higher temperature has higher surface roughness. Thus, the reduced transmission could be explained by the light scattering at the rough surface.

Proc. of SPIE Vol. 5946 594605-3

a b Figure 4. SEM micrographs of the films deposited from 1 : 2 solution at 470 (a) and 530 ºC (b).

In addition to the decrease of the optical transparency in the higher-temperature films, the shift of the absorption edge towards shorter wavelengths occurs (Fig. 3). This observation will be discussed below. The ZnS films deposited at 440–500 ºC exhibit the transparency higher than 70% in visible region and 75–90% in the IR region of 1–2.5 μm (the spectra are not presented). Due to the 70–90-% optical transparency in a wide spectral region, spray deposited ZnS films can be used as an optical window material. The minimum requirement for the transparency of the optical window material has been reported to be 70%.1 The comparison of two sets of samples prepared from 1 : 1 and 1 : 2 solutions, show that in the region of the fundamental absorption edge the decay of the transmittance curve is sharper for the films deposited from the 1 : 2 solution. The observed feature refers to the better crystallinity as it was proved by XRD. The optical band gap energies (Eg) were determined from the absorbance measurements at 300–500 nm. A standard expression for direct transitions between two parabolic bands (αhν)2 = A(hν – Eg) was used for the calculation of Eg. The values of Eg were found by extrapolating the linear part of the (αhν)2 vs hν curve to the abscissa (Fig. 5) and are summarized in Table 1. 2.0x10

15

1.5x10

15

Table 1. Band gap values for the spray deposited ZnS films. Zn:S=1:2

(a*h*v)

2

Ts (ºC)

1.0x10

15

5.0x10

14

Eg=3.67 eV

0.0 2.5

3.0

3.5

430 470 500 530

4.0

Photon energy, eV

Figure 5. (αhν)2 vs hν for the calculation of the ZnS film band gap.

Proc. of SPIE Vol. 5946 594605-4

Eg (eV) Zn : S = 1 : 1 3.56 3.54 3.59 3.59

Zn : S = 1 : 2 3.60 3.60 3.65 3.67

The band gap energies of the films deposited from the 1 : 1 solution are lower than those for the 1 : 2 solution independently of the deposition temperature. The films deposited from the 1 : 1 solution onto the glass substrates at 470–530 ºC show the Eg values 3.54–3.59 eV. The measured values are in a good agreement with those reported for the sprayed films from 1 : 1 solution.2,12 The band gap energies of the films deposited from the 1 : 2 solution at similar temperatures have larger values, 3.60–3.67 eV. These values correspond to the sprayed films with the wurtzite structure and are the highest ever reported for sprayed ZnS. Both sets of samples show the increase in band gap energy with the deposition temperature. This observation corresponds to literature data2 and is obviously caused by the improved crystallinity. 3.2.2. Optical reflectance of sprayed ZnS films The reflectance spectra were recorded in the near-IR region of 11000-4000 cm-1. The reflectance of a bare Si wafer and the wafer with the ZnS films were measured (at the incidence angle of 25º) with respect to the aluminum mirror and are shown in Fig. 6. It appears that the sprayed ZnS film reduce the reflectivity of Si substrate from 40 to about 10%. Of course, the film thickness should be taken into account when preparing the antireflectance coatings. 50

Reflectance, %

40 1

30 3

20

2 5

10

4 0 11000 10000 9000 8000 7000 6000 5000 4000 Wavenumber, cm

-1

Figure 6. Reflectance spectra of a bare Si substrate (1) and the ZnS films on the Si deposited at 460 (2), 500 (3), 540 (4), and 600 ºC (5).

3.2.3. Determination of the refractive indices The refractive indices of the ZnS films were determined by the interference fringe method.13 In order to measure the fringe shift, the reflectance spectra were recorded in the near-IR region at two angles of incidence, 25 and 50º. The reflectance measurements and calculations were performed for the samples that were prepared on the Si substrates at 500–600 ºC and had, according to XRD data, a single-phase composition. The refractive indices for the ZnS films were calculated by 13

[(

n = sin 2α 1ν 12 − sin 2α 2ν

2 2

) /(ν

1

−ν

2

) ]1 / 2 ,

where α1 and α2 are two angles of incidence, and ν1 and ν2 are the fringe maximum or minimum values at the angles α1 and α2, respectively. Figure 7 illustrates the reflectance spectra at two angles of incidence for the ZnS film deposited at 570 ºC. The fringe minima values ν1 and ν2 were found using Perkin-Elmer spectrometer software. The results of the calculations are summarized in Table 2.

Proc. of SPIE Vol. 5946 594605-5

Table 2. Refractive indices of a ZnS film. 50

ν1

Reflectance, %

40

Zn : S ratio in the solution

ν2

50 deg

30

Refractive index Ts (ºC)

Fringe method (near-IR region)

Ellipsometry (632.8 nm)

500 540 570 500 540 570 600

1.29 1.48 1.49 1.24 1.82 1.73 1.56

NA* NA NA 1.25 1.93 1.77 1.65

1:1 20

0 11000

1:2

25 deg

10

10000

9000

8000

7000

Wavenumber, cm

6000

5000

4000

-1

*

NA – not analysed

Figure 7. Reflectance spectra of a ZnS films on the Si substrate measured at the incidence angles of 25 and 50°.

The films prepared from 1 : 1 solution at temperatures 540–570 °C had the refractive index close to 1.5. The films prepared at lower temperatures (up to 500 °C) had a significantly lower refractive index and, accordingly, lower density. The use of the 1 : 2 spray solution and the deposition temperatures of 540 and 570 °C results in the films with the refractive index of 1.82 and 1.73, respectively. The attained results indicate that the 1 : 2 solutions allow one to grow more dense films. For comparison, the ellipsometric data were obtained for the ZnS films prepared from 1 : 2 solution (Table 2). The refractive indexes at 632.8 nm correlate well with those found by the fringe method. There are no literature data on the refractive indices of the ZnS thin films prepared by spray pyrolysis. The values we obtained are close to those reported for the ZnS films fabricated by the successive ionic layer adsorption and reaction (SILAR) technique,14 being, however, lower than those reported for the ZnS films prepared by ALD.15

4. CONCLUSIONS Highly (002) oriented ZnS films with the wurtzite structure were prepared by spray pyrolysis at 500–600 °C the aqueous solution of ZnCl2 and SC(NH2)2 with the Zn : S molar ratio 1 : 2. It was the first time when the ZnS films with Eg = 3.67 eV and n = 1.93 (at 632.8 nm) were obtained by the spray technique. Sprayed films exhibit the ~80-% optical transparency at the wavelengths from 0.5 to 2.5 µm making low-cost spray technique attractive to prepare optical windows. In addition, sprayed ZnS films are applicable as antireflection coatings and components of solar cell.

ACKNOWLEDGMENTS This work was supported by the Estonian Science Foundation (Grant No. 5612).

REFERENCES

1. Y. Drezner, S. Berger, and M. Hefetz, “A correlation between microstructure, composition and optical transparency of 2.

CVD-ZnS”, Mater. Sci. Eng. B 87, pp. 59–65, 2001. B. Elidrissi, M. Addou, M. Regragui, A. Bougrine, A. Kachouane, and J. C. Bernède, “Structure, composition and optical properties of ZnS thin films prepared by spray pyrolysis”, Mater. Chem. Phys. 68, pp. 175–179, 2001.

Proc. of SPIE Vol. 5946 594605-6

3. U. Gangopadhyay, K. Kim, D. Mangalaraj, and J. Yi, “Low cost CBD ZnS antireflection coating on large area commercial mono-crystalline silicon solar cells”, Appl. Surf. Sci. 230, pp. 364–370, 2004. 4. R. N. Bhattacharya and K. Ramanathan, “Cu(In,Ga)Se2 thin film solar cells with buffer layer alternative to CdS”, Sol. Energy, 77, pp. 679-683, 2004. 5. M. Rusu, W. Eisele, R. Würz, A. Ennaoui, M. Ch. Lux-Steiner, T. P. Niesen, and F. Karg, “Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)2 solar cell”, J. Phys. Chem. Sol. 64, pp. 2037–2040, 2003. 6. L.-X. Shao, K.-H. Chang, and H.-L. Hwang, “Zinc sulfide thin films deposited by RF reactive sputtering for photovoltaic applications”, Appl. Surf. Sci. 212–213, pp. 305–310, 2003. 7. J. A. Lahtinen, A. Lu, and T. Tuomi, “Effect of growth temperature on the electronic energy band and crystal structure of ZnS thin films grown using atomic layer epitaxy”, J. Appl. Phys. 58, pp. 1851–1853, 1985. 8. V. Balek, J. Fusek, O. Kříž, M. Leskelä, L. Niinistö, E. Nykänen, J. Rautanen, and P. Soininen, “Emanation thermal analysis in the characterization of zinc sulfide thin films prepared from different precursors”, J. Mater. Res. 9, pp. 119– 124, 1994. 9. J. Cheng, D.B. Fan, H. Wang, B.W. Liu, Y.C. Zhang, and H. Yan, “Chemical bath deposition of crystalline ZnS thin films”, Semicond. Sci. Technol. 18, pp. 676–679, 2003. 10.M. Krunks and E. Mellikov, “Zinc oxide films by the spray pyrolysis method”, Thin Solid Films 270, pp. 105–109, 1995. 11.M. Krunks, O. Bijakina, T. Varema, V. Mikli, and E. Mellikov, “Structural and optical properties of sprayed CuInS2 films”, Thin Solid Films 338, pp. 125–130, 1999. 12.A. El Hichou, M. Addou, J. L. Budendorff, J. Ebothé, B. El Idrissi, and M. Troyon, “Microstructure and cathodolumenscence study for sprayed Al and Sn doped ZnS thin films”, Semicond. Sci. Technol., 19, pp. 230–235, 2004. 13.H. J. Harrick, “Intenference fringe method“, Appl. Opt. 10, pp. 2344–2356, 1971. 14.S. Lindroos, Y. Charreire, D. Bonnin, and M. Leskelä, “Growth and characterization of zinc sulfide thin films deposited by the successive ionic layer adsorption and reaction (Silar) method using complexed zinc ions as the cation precursor”, Mater. Res. Bull. 33, pp. 453–459, 1998. 15.D. Riihelä, M. Ritala, R. Matero, and M. Leskelä, “Introducing atomic layer epitaxy for the deposition of optical thin films”, Thin Solid Films 289, pp. 250–255, 1996.

Proc. of SPIE Vol. 5946 594605-7

Suggest Documents