Indian Journal of Pure & Applied Physics Vol. 50, June 2012, pp. 380-386 Structural and optical properties of ZnS, PbS, Zn 1 x Pb x S, Zn x Pb 1 x S and PbZn x S 1 x thin films Mahdi Hasan Suhail Department of Physics, College Of Science, University of Baghdad, Iraq E-mail: mhsuhail@yahoo.com Received 9 May 2011; revised 6 February 2012; accepted 10 April 2012 The chemical spray pyrolysis technique (CSPT) to prepare ZnS, PbS, Pb x Zn1 x S, (x=0 to 1), PbZn 1 x S x and Pb x Zn 1 x S, thin films with (x = 0.01, 0.02) at glass substrate, has been used. The result of X-ray diffraction obtained that the films are polycrystalline and have cubic with hkl (111) and wurtize with hkl (100) phases for ZnS films, the phase for PbS films is cubic with hkl (200) and the phases for Pb x Zn 1 x S, Pb 1 x Zn x S and PbZn x S 1 x are cubic with hkl (200). The lattice constants (a) have been calculated as 5.939Å for PbS films, 3.36Å and 5.44Å for wurtize and cubic of ZnS films, respectively, 5.91 to 6.2Å as x varied from 0 to 1 for Zn 1 x Pb x S,5.92Å for Zn x Pb 1 x S films with x = 0.02, and 5.921Å for PbZn x S 1 x films with x = 0.01. The absorption spectra used in the wavelength range 200-1100 nm to calculate the optical energy gap which is found to decrease with increase in the film thickness for all the films prepared by this technique. The values of energy gap have been determined and it was between (1.58-1.78 ev) for PbS films, (2.9-3.1eV) for ZnS films. The energy gap of Zn x Pb 1 x S films in the range (2.4-2.9 ev and (2.38-2.75) ev and of PbZn x S 1 x it was (1.57-1.8 ev and (1.4-1.6 ev for x = 0.02 and x = 0.01, respectively as the thickness increases. The optical constants, such as refractive index (n) and extinction coefficient (k) increase and decrease with increase in the film thickness, respectively for all the films except ZnS films which are having inverse properties. The dielectric constants ( r, i ) have the same behaviour of refractive index (n) and extinction coefficient (k), respectively. Keywords: Optical properties, Structural properties, Energy gap, Semiconductor thin film, ZnS thin film, PbS thin films 1 Introduction Semiconductor materials are always the focus in material science due to their outstanding electronic and optical properties and extensively potential application in various devices including light-emitting diodes, single electron transistors and field-effect thin-film transistors. In principle, the electronic and optical properties of semiconductor materials are tunable by varying their shapes and sizes so it is one of the desired goals in material science to realize precise control of the morphology of semiconductor materials 1. ZnS an important semiconductor material with large band gap (3.5 ev), high refractive index (2.35 at 632 nm) high effective dielectric constant (9 at 1 MHz). It s optical properties make it useful as a filter, reflector and planar wave guide 2. A number of techniques have been examined in the search for more reliable and cheapest method of producing thin films. Physical and chemical depositions are the most common methods for transferring material atom by atom from one or more sources to the growth surface of a film being deposited onto a substrate. So we have two general methods which are the physical and chemical depositions 3-5. There are many techniques that have been used to prepare thin films. Among the various thin film deposition techniques, chemical spray pyrolysis technique (CSPT) is one of the principle methods to produce large area and uniform coating 6,7 and deposit a wide variety of materials in thin film form. The prime requisite for obtaining good quality thin film is the optimization of preparative conditions viz. substrate temperature, spray rate, concentration of solution etc. The interest to non-vacuum methods for thin films deposition has increased in the last decade. The solution based process has several advantages: simplicity of process, access to a wide range of metal oxide stoichiometries, precise composition control and applicability to substrate at any size. Two of the most important optical properties are the refractive index n and the extinction coefficient k, which are, generally, called optical constants. Using the fundamental relations of photon transmittance T and absorbance A, I t = I o exp ( t) (1)
SUHAIL: STRUCTURAL AND OPTICAL PROPERTIES OF THIN FILMS 381 where I t is the incident photon energy at thickness (t) inside the material and I o is incident photon energy at surface of material 8. and A=log I o /I t (2) we have = 2.303 A/t (3) The reflectance (R) has been found by using the relationship R+T+A = 1 (4) For normal reflectance, we have 9,10 R= (n 1) 2 / (n+1) 2 (5) By using the above relation, the refractive index can be determined from the relation: n = (1+(R) 1/2 )/(1 (R) 1/2 ) (6) The extinction coefficient is related to the absorption coefficient k by the relation 11. k = /4 (7) where is the incident photon wavelength. The absorption of radiation that leads to electronic transitions between the valence and conduction bands is split into direct and indirect transitions,these transitions are described by the equation 12,13. * op r α( υ ) = ( υ ) (8) h A h E g where h is the incident photon energy, E g op : is optical energy gap, and r is constant which takes the values (1/2, 3/2, 2, 3) depending on the material and the type of the optical transition whether it is direct or indirect. The real and imaginary parts of dielectric constant ( r and i, respectively) can be calculated as follows 12 : N * = n ik (9) Complex dielectric constant * = r i i.. (10) From the relation N * = *, therefore: (n ik) 2 = r i ε i (11) So that r = n 2 k 2. (12) i = 2nk (13) 2 Experimental Details Before starting the deposition, the solutions are mixed according to the film components, (after found their weight by this equation (W=M M wt V/1000, where M is the molarity, M wt the molecular weight, V is the volume), the solutions can be prepared by solving the salts in the distilled water as follows: Thiourea solution CS(NH 2 ) 2 : This solution can be prepared with molarities (0.1 M), from solving (0.761 g) of thiourea in (100 ml) of distilled water. Lead acetate solution Pb(CH 3 COO) 2 3H 2 O: This solution can be prepared with molarities (0.1 M), from solving (2.78 g) of lead acetate in (100 ml) of distilled water. ZnSO 4 7H 2 O solution: This solution can be prepared with molarities (0.1 M), from solving (2.87 g) of ZnSO 4 7H 2 O in (100 ml) of distilled water. After getting the different amount of solutions according to the ratio and volume requirement, we put it on the magnetic stirrer for about 15 min to be sure solutions have been mixed properly. After cleaning the substrates, we place them on the electrical heater surface and leave them for about 15 min so as their temperature reaches the electrical heater temperature. The solution must be put in solution container after that we can start the deposition process. The substrate heater was an electrically controlled block furnace. After the spray process has been completed, we leave the substrate on the surface of heater about 20 min in order to see that chemical interactions and the crystallization process are complete. The structural analysis of the film has been studied by using X-ray diffraction system (XRD-6000- Shemadzu) operating with 0.15418 nm monochromatized Cu-K radiation at 40 kv and 30 ma with Ni filter. The UV/VIS/NIR spectroscopy type Shimadzu was used to measure the absorption of prepared films from wavelength 200 nm up to 1100 nm and so that thickness of thin films,the energy gap and absorption coefficient can be calculated. The extinction coefficient and the refractive index of prepared thin films have been measured by using Eqs (3 and 6), respectively. The real and imaginary parts of the dielectric constant have been measured by using Eqs (12 and 13), respectively, under the conditions of normal incidence, room temperature and uncoated glass slide.
382 INDIAN J PURE & APPL PHYS, VOL 50, JUNE 2012 3 Results and Discussion 3.1 Structural and optical properties of ZnS films 3.1.1 X-ray diffraction results It is observed that for ZnS film of thickness (1300) nm was found to exhibit diffraction peaks associated with (100), (002) (101) and (103) reflections for the hexagonal structure and (014) reflection for the cubic structure of which the intensity of the (014) orientation is predominant (Fig. 1 ). The lattice constant (a) for ZnS thin films was (3.36Å) for wurtize hexagonal structure and (5.44Å) for cubic phase which are found to be in agreement with the result obtained by Ubale et al. 9. for ZnS films by chemical bath deposition using thiourea and zinc acetate as S 2 and Zn +2 source. 3.1.2 Optical characteristics It is observed that the optical energy gap decreases with increase of film thickness, as listed in Table 1. This may be due to the possibility of structural defects in the films arisen during the time of their preparation, which could give rise to the allowed states near the conduction band in the band gap. In the case of much thicker films, these allowed states could well merge with the conduction band resulting in the reduction of the energy band gap 14. This is similar to the results of many other researchers like Nadeem and Ahmed 4 for ZnS thin films which were prepared by Resistive Heating Technique. The refractive index was determined from the reflectance data and listed in Table 2. The increase of the film thickness causes an overall decrease in the refractive index. The decrease is due to the overall decrease in the reflectance with the increase of film thickness. This behaviour is similar to the work of Ubale et al. 9 for ZnS thin films by thermal evaporation technique. The variation of extinction coefficient with the photon wavelength decreases as the film thickness increases which are due to the different effective thickness of the thin films 16. Table 1 presents the values of the real and imaginary parts of the dielectric constant at 600 nm wavelength and have the same behaviour of refractive index and extinction coefficient. 3.2 Structural and optical properties of PbS films 3.2.1 X-ray diffraction results For PbS film of thickness (1038) nm, it was observed from Fig. 2 that five diffraction peaks associated with (111), (200), (202), (311), and (222) also there are another two weak peaks associated with (400) and (331) are shown. All the diffraction peaks can be indexed to a face-centered-cubic rock-saltstructured PbS which coincide with the work of Ken- Tye Yong et al. whose study is of PbS Nanowires 17. The different peaks for PbS film are indexed in Table 1 Values of refractive index (n), extinction coefficient (k), real ( r ) and imaginary ( i ) dielectric constant for ZnS films (at 650 nm wavelength) Thickness (nm) n K*10 4 r i E g (ev) 797 1.304 19 1.708 0.004 3.1 1036 1.282 12 1.640 0.003 3.0 1306 1.274 09 1.620 0.002 2.9 Table 2 Comparison of crystallographic data for PbS thin films with the JCPDS card 5-592 ( Ref.18) Standard data Observed (d) and (a) value for PbS film d (Å) hkl d (Å) hkl a (Å) 3.429 111 3.429 111 5.939 2.969 200 2.974 200 5.948 2.099 202 2.100 202 5.939 1.790 311 1.792 311 5.943 1.712 222 1.718 222 1.483 400 1.486 400 1.361 331 1.362 331 Fig. 1 X-ray diffraction pattern for ZnS thin film Fig. 2 X-ray diffraction pattern for PbS thin film
SUHAIL: STRUCTURAL AND OPTICAL PROPERTIES OF THIN FILMS 383 Table 2 as well as the corresponding values of the interplanar distance d hkl and compared with the standard values of ASTM data. The lattice constant (a) for PbS thin films was 5.939Å. This value is found to be in good agreement with the standard value as shown in Table 2 which is 5.94Å. No obvious characteristic diffraction peaks from other impurities can be detected. The strong and sharp diffraction peaks suggest that the as-obtained products are well crystalline. 3.2 1 Optical characteristics It is observed from Table 3 that the optical energy gap decreases with the increase of film thickness this behavior is similar to result of Ubale et al. 9 and energy gap is found to be in the range 1.88-2.28 ev, while the energy range found by the authors is in the range 1.58-1.78 ev. This difference refers to our thicker films and different technique. Table 3 Values of refractive index (n), extinction coefficient (k), real ( r ) and imaginary ( i ) dielectric constant for PbS films (at 650 nm wavelength) Thickness (nm) n K*10-4 r i E g (ev) 550 1.424 44.78 2.028 0.0127 1.78 710 1.424 34.45 2.028 0.0098 1.68 880 1.484 35.20 2.203 0.0104 1.58 From Table 3, we can observe that the refractive index increases with increase of the thickness but not systematically this is due to reflectance. The extinction coefficient decreases as the film thickness increases which is due to the different effective thickness of the thin films, but not systematically as given in Table 3. This behaviour is similar to the work of Debnath et al. 20 thin films using Electron Beam Evaporation Technique. The real and imaginary parts of the dielectric constant for PbS thin films show in Table 3 and have the same behaviour of refractive index and extinction coefficient. 3.3 Structural and optical properties of Zn 1 x Pb x S films 3.3 1 X-ray diffraction results X-ray diffraction pattern for Zn 1 x Pb x S film (x = 0 to 1) was carried out with thickness 2000 nm and shown in Fig. 3. The 100% intensity of peak (200) and lattice constant of these peak are listed in Table 4. The value of d hkl and lattice constant increase with increasing the amount of zinc in the compound (Table 4). 3.3.2 Optical characteristics From Table 5, it can be observed that the refractive index and the extinction coefficient increase with increase in the thickness which is due to the different effective of the amount of lead in the films. The variations of the real and imaginary parts of the dielectric constant with the incident photon wavelength show increase with increase in the amount of the lead in the film. The absorption coefficient has a high value at low-wavelengths (visible region) and starts decreasing gradually towards the high Table 4 Crystallographic data for Zn 1 x Pb x S thin films Standard data Observed (d) and (a) value for Zn 1 x Pb x S film d (Å) hkl d (Å) Concentration (x) a (Å) 2.96 200 2.955 1 5.91 2.96 200 2.966 0.8 5.932 2. 96 200 2.972 0.6 5.944 2. 96 200 2.977 0.4 5.9543 2. 96 200 3.01 0.2 6.02 2. 96 200 3.012 0 6.20 Table 5 Values of refractive index n, extinction coefficient k, real r and imaginary i dielectric constant for Zn 1 x Pb x S films with x=0.01 at 700 nm wavelength Fig. 3 X-ray diffraction pattern for Zn 1 x Pb x S thin film Concentration (x) n K*10-4 r i E g (ev) 0.2 1.32 62 2.1 0.046 3.0 0.4 1.38 92 2.3 0.042 2.84 0.6 1.45 126 2.7 0.035 2.68 0.8 1.48 162 3.1 0.025 2.40
384 INDIAN J PURE & APPL PHYS, VOL 50, JUNE 2012 wavelength of the ZnS thin films. But when you add the amount of lead to the film, according to the values of x, we observed that we get shift in the edge absorption, where the value at x = 0 the edge at wavelength 380 nm and pushed towards the wavelengths 600 nm when x = 0. 9, and this deviation is causing the change in the energy gap. When we increase the amount of lead in the film we note that the energy gap decreases gradually to reach at x = 0. 9 to 2.43 ev. This is attributed to the fact that the force bond of S-Zn is found to be greater than the force bond of S-Pb. This result is because the zinc has electronegativity larger than for the lead. The difference in electronegativity for bond S-Zn is 0.9 while the difference in electronegativity of the bond S-Pb is 0.3. Therefore, the energy required to break the bond S-Zn is higher than the energy need to break the bond S-Pb, in addition to the lead has atomic number (82) which is greater than zinc (30), and the large number of atomic electrons means that the link be less to the nuclei which led to the transfer of an electron in an atom-level external lead easier than electrons transfer to levels of an atom of zinc. 3.4 Structural and optical properties of Zn x Pb 1 x S films 3.4.1 X-ray diffraction results X-ray diffraction pattern for Zn x Pb 1 x S film (x = 0.02) was carried out with thickness1500 nm and shown in Fig. 4. All the different peaks for Zn x Pb 1 x S film are indexed in Table 6 as well as the corresponding values of the inter planar spacing d(hkl) and compared with the values of PbS film (111), (200), (202), with d value 3.418Å, 2.965Å and 2.095Å, respectively and with obvious characteristic diffraction peak from impurities from which can be detected another unknown peak with its (2 = 33.278) d value is 2.690Å which is due to the ratio of Zn. The lattice constant (a) for Zn x Pb 1 x S thin films is 5.92Å which is less than the lattice constant for PbS film due to existence of Zn. From Table 6, we can see that adding the Zn to the PbS leads to decrease the lattice constant. 3.4.2 Optical characteristics Decrease in extinction coefficient and imaginary dielectric constant with increase the film thickness, this is due to the effect of different thickness. It s energy gap decreases with increase in the film thickness and it is in the range 2.38-2.75 ev. At x = 0.02, there is the same behaviour of optical constants for Zn x Pb 1 x S with x = 0.01 9 (Table 7). It is observed from Table 8 that the optical energy gap decreases with the increase of film thickness. The energy gap of Zn x Pb 1 x S films in the range 2.4-9 ev. Table 6 Comparison of crystallographic data for Zn x Pb 1 x S thin films with PbS film Obser ved (d) and (a) value for PbS film Observed (d) and (a) value for Zn x Pb 1 x S film d (Å) hkl a (Å) d (Å) hkl a (Å) 3.429 111 5.939 3.418 111 5.92 2.974 200 5.948 2.965 200 5.93 2.100 202 5.939 2.095 202 5.925 2.690 unknown Table 7 Values of refractive index n, extinction coefficient k, real r and imaginary i dielectric constant for Zn x Pb 1 x S films with x=0.01 (at 700 nm wavelength) 790 1.411 37 1.992 0.0107 2.75 1030 1.417 29 2.009 0.0084 2.62 1300 1.421 24 2.078 0.0068 2.38 Table 8 Values of refractive index n, extinction coefficient k, real r and imaginary i dielectric constant for Zn x Pb 1 x S films with x=0.02 (at 700 nm wavelength) Fig. 4 X-ray diffraction pattern for Zn x Pb 1 x S thin film (x = 0.02) 790 1.31 23 1.72 0.0061 2.9 1030 1.40 28 1.96 0.0078 2.6 1300 1.53 36 2.34 0.0112 2.4
SUHAIL: STRUCTURAL AND OPTICAL PROPERTIES OF THIN FILMS 385 Table 9 Comparison of crystallographic data for PbZn x S 1 x thin films with PbS film Observed (d) and (a) for PbS films Observed (d) and (a) for PbZn x S 1 x films d (A ) hkl a (Å) d (Å) hkl a (Å) 3.429 111 5.939 3.419 111 5.921 2.974 200 5.948 2.965 200 5.930 2.100 202 5.939 2,094 220 5.922 Table 10 Values of refractive index n, extinction coefficient k, real r and imaginary i dielectric constant for PbZn x S 1 x films with x=0.01 (at 700 nm wavelength) Fig. 5 X-ray diffraction pattern for PbZn x S 1 x thin film (x = 0.01) The optical behaviour of a material is, generally, utilized to determine its optical constants [i.e. refractive index (n), extinction coefficient (k), real ( r ), and imaginary ( i ) dielectric constants]. From Table 8, it can be observed that the refractive index and the extinction coefficient increase with increase in the thickness as listed in Table 8 which is due to the different effective thickness of the thin films. The variation of the real and imaginary parts of the dielectric constant with the incident photon wavelength shows increase with increase in thickness. From Tables 4 and 6, we can see that due to decrease in the ratio of Zn the optical constants increase. While the energy gap decreases with decrease in Zn ratio this behaviour is a good agreement with the result of Al-Ajubory for Zn x Pb 1 x S films which were prepared by spray pyrolysis technique 21. 3.5 Structural and optical properties of PbZn x S 1 x films 3.5.1 X-ray diffraction results X-ray diffraction pattern for PbZn x S 1 x film (x = 0.01) was carried out with X-ray diffraction in the range 2 between 20 to 50 as shown in Fig. 5. It is observed from Fig. 5 that for PbZn x S 1 x film of thickness (1500) nm was found to exhibit three diffraction peaks associated with (111), (200), and (202), with d value 3.419, 2.965 and 2.094Å, respectively. All the diffraction peaks can be indexed to a face-centered-cubic rock. The different peaks for PbS film are indexed in Table 9 as well as the corresponding values of the interplanar spacing d (hkl) and compared with the PbS film. The lattice constant (a) for PbZn x S 1 x film was 5.921Å which is close to the lattice constant of PbS 790 1.324 364 1.754 0.0096 1.4 1030 1.451 341 2.105 0.0144 1.5 1300 1.465 360 2.148 0.0096 1.6 Table 11 Values of refractive index (n), extinction coefficient (k), real ( r ) and imaginary ( i ) dielectric constant for PbZn x S 1 x films with x=0.02 (at 700 nm wavelength) 790 1.355 42 1.8369 0.0115 1.57 1030 1.370 35 1.8777 0.0096 1.64 1300 1.467 42 2.1547 0.0125 1.80 film. Also from Table 9, it could be seen due to existence of Zn, lattice constant would decrease. 3.5 2 Optical constants of PbZn x S 1 x thin films at (x= 0.01, 0.02) The optical constants for compound PbZn x S 1 x for x = 0.01, 0.02 are shown in Tables (10 and 11).The refractive index increases with increase in film thickness while extinction coefficient decreases with increase in film thickness and both real and imaginary dielectric constants have the same behaviour of refractive index and extinction coefficient, respectively.the energy gap decreases with increase in film thickness. It can be seen that increasing Zn ratio leads to increase in the energy gap (Tables 10 and 11). The energy gaps for PbZn x S 1 x with x = 0.01, 0.02 are closed to the energy gap of PbS films because the ratio of Zn is very small as compared to ratio of Pb and S. 4 Conclusions The following conclusions may be drawn from the results obtained: XRD-studies showed that all films are polycrystalline with cubic phase except ZnS films have cubic and hexagonal phase. Optical studies reveal that these films have a direct band gap and it is observed that the bad gaps decrease with increase in
386 INDIAN J PURE & APPL PHYS, VOL 50, JUNE 2012 the film thickness. The transmittance is high in vis- NIR regions and the high transmittance properties make film a good material for thermal control window coating for cold climates and anti-reflective coating. The refractive index (n) increases with increase in the film thickness, while the extinction coefficient (k) decreases with increase in the thickness for all the films except ZnS films which are inversely. Spray pyrolysis method for production of the solid films is a good method for preparation of thin films suitable for scientific studies and for many applications in technology and industry. References 1 Popescu V, Nascu H I & Darvasi E, J Optoelectronics & Advanced Mater, 8(3) (2006) 1187. 2 Pawan Kumar, Aravind Kumar, Dixit P N & Sharma T P, Indian J Pure & Appl Phys, 44 (2006) 690. 3 Choudhury N & Sarma B K, Indian J Pure & Appl Phys, 46 (2008) 261. 4 Nadeem M Y & Waqas Ahmed, Turk J Phy, 24 (2000) 651. 5 Mattox D M, Handbook of physical vapour deposition (PVD) processing Copyright by Noyes Publications. Printed in USA, 1998. 6 Barote M A, Yadav A A & Masumdar E U, Chalcogenide Lett, 8( 2) (2011) 129. 7 Thangaraju B & Kallannan P, Cryst Res Tech, 35(1) (2000) 71. 8 Ezekoye B A, J Sci &Tech, 23(2) (2003) 41. 9 Ubale A U, Sangawar V S & Kulkarni D K, Bull Mater Sci, 30( 2) (2007) 147. 10 Elidrissi B, Addou M, Regragui M,Bougrine J C, et al., Mater Chem Phys, 68 (2001) 175. 11 Gumus C, Ozkedir O M, Kavak H & Ufuktepe Y, J Optoelectronics & Advanced Mater, 8(1) (2006) 299. 12 Ezema F I, Asogwa P U, Ekwealor A B C, et al., J Chem Tech & Metall, 42(2) (2007) 17. 13 Sapra S, Nanda J & Sarma D D, Solid State and Structural Chemistry unit, IISC-Bangolare 560 012, India 2002. 14 Sahay P P, Tewari S & Nath R K, Cryst Res Tech, 42(7) (2007) 723. 15 Yun S J, Dey S & Nam K, Electronics and Telecommunications Research Institute, Taejon 305-600. 16 Bandic Z & Megill T, Appl Phys Lett, 72(22) (2006) 2662. 17 Ken-Tye Yong, Yudhisthira Sahoo, Kaushik Roy Choudhury, Mark T. Swihart, John R Minter & Paras N Prasad, Chem Mater, 18 (2006)5965. 18 Ubale A U, Junghare A R, Wandibhasme N A, et al., Turk J Phys,31 (2007) 279. 19 Ni Y, Liu H, Wang F, et al., Cryst Res Tech, 39(3) (2004) 200. 20 Debnath S, Islam M R & Khan M S R, Bull Mat Sci, 30(4) (2007) 315. 21 AL-Jubory A A D, Study the structural & optical properties of Zn 1 x Pb x S thin film, MSc Thesis, University of Anbar, Anbar, Iraq, 2005.