STRUCTURAL STUDIES IN SINGLE AND MLTLTILAYER THIN FILMS OF CuS, PbS, CdS and CuPc

Transcription

1 STRUCTURAL STUDIES IN SINGLE AND MLTLTILAYER THIN FILMS OF CuS, PbS, CdS and CuPc 6.1 Introduction X-ray diffraction (XRD) technique allows accurate determination of the crystal structure of a material. Various metal substituted phthalocyanines are of interest as potential materials for thin film gas sensors in view of their thermal stability, high conductivity and sensitivity to a variety of gases.'-4 The electrical properties of these materials are dependent on the crystal structure and the requirement for gas sensing is that the structure should be well defined and phase should be stable over wide temperature range. Sulphides can be classified according to their crystal structure into three groups; (1). Wurtzite and halite types. These binary sulphides are covalently bonded and usually have stoichiometrically accurate compositions. The wurtzite type frequently can also crystallize with the sphalerite structure. (2) Other binary sulphides. The bonding in these sulphides is usually a mixture of covalent and ionic bonds with the result that it sometimes has a strong metallic character. The sulphides in this category are usually isostructural with simple AB- or ABr- type compounds. (3) Complex sulphides. The sulphides in this category include those with more complicated structures as well as the ternary sulphides. The bonding in these sulphides can range from nearly pure covalent bonds to more complex

2 127 mixed types. There are two types of CdS and ZnS films, hexagonal and cubic. In hexagonal and cubic CdS, the interatomic separation is For hexagonal ZnS the interatomic distance is 2.33 and that for cubic ZnS it is Lucadomo et. a15 studied the texture in Ti /A1 and Nb/Al multilayer thin films and the role of Cu in it using the X-ray pole studies and transmission electron microscopy. Fujirnoto et.a16 have done the characterization of rnultilayer thin films of Si by grazing incidence X-ray reflectivity. X-ray diffraction determination of the residual stresses and elasticity constants of thin films are also done by Goudeau et. a17, Cheng Jiangong et. a18 and Checchetto et. a1.' In this chapter the structural studies in CuPc, CuS, multilayer CuS-CuPc, PbS, multilayer PbS-CuPc, CdS, multilayer CdS-CuPc and multilayer PbS-CuS thin films are dealt with. 6.2 Theory Every atom in a crystal scatters an X-ray beam incident upon it in all directions. Because, even the smallest crystal contains a very large number of atoms, the chance that these scattered waves would constructively interfere is zero except for the fact that the atoms in crystals are arranged in a regular and repetitive manncr. The possible directions in which a crystal can diffract a beam of monochromatic X-rays is determined by the Bragg condition, where d is the distance between atomic planes parallel to the axis of the incident beam, 8, the angle of incidence relative to the planes in question, n, the order of

3 rellection and A, the wavelength of the incident X-rays. The factor d is related to the (hkl) indices of the plane by the relation, and, It is therefore apparent that the diffraction directions are solely determined by the structure and size of the unit cell. Intensities of the diffracted beams are determined by the positions of the atoms within the unit cell. The consequence of this is that although there are theoretically hundreds of possible diffraction directions, there are particular atomic arrangements, which reduce the intensity of so many diffracted beams to zero. The grain size of the film is calculated from the Full Width-at I-Ialf Maximum (FWHM) of the X- ray peaks using the Scherrer formula" here k is the Scherrer constant (k=0.9), h. = A, 111, 2 the FWHM in radians and 0, the diffraction angle of the most intense peak.

4 I Experiment CuS, PbS, CdS and rnultilayer PbS-CuS films are prepared by chemical deposition technique as described in chapter 3. Thc copper phthalocyanine (CuPc) powder used in this study is obtained from Aldrich chemical company Inc: USA. Thin films of CuPc are deposited at room temperature on to pre-cleaned glass substrates with pre-evaporated high purity silver electrodes, at a base pressure of 1 om5 Torr using a Hind Hivac Vacuum coating unit. The evaporation is carried out by resistive heating of the CuPc powder fiom a molybdenum boat and the rate of sublimation is kept constant. The optimum rate of evaporation is adjusted to be nrn per minute. Powdered samples of CuPc are used as source materials for multilayer CuS- CuPc, PbS-CuPc and CdS-CuPc films. The sulphide films are used as substrates for evaporation. CuPc is evaporated onto these sulphide films with preevaporated high purity silver electrodes, at a base pressure of Ton by resistive heating fiom a molybdenum boat as per the procedure described in section 3.6 of chapter 3. The optimum rate of evaporation is adjusted to be nm per minute. Thickness is measured by Tolansky's multiple beam interference method1 as described in section 2.8- of chapter 2. These films are annealed in air at different temperatures. Annealing is canied out in a hace whose temperature is controlled by a controller cum recorder. The annealing time is two hours. Philips Analytical XD P W3 710 BASED diffractometer using CuK, radiation of wavelength A was used for X-ray diffraction measurements. X-ray diffractograms are recorded for as deposited and annealed CuPc, CuS, mu1 ti layer CuS-CuPc, PbS, multilayer PbS-CuPc, CdS, multilayer CdS-CuPc and

5 130 multilayer PbS-CuS films. The diffractograms obtained arc analysed to determine the structure of the films and the lattice constants. 6.4 Results and Discussion CuPc films The phthalocyanine molecule is a square planar molecule consisting of a divalent metal ion surrounded by four benzopyrrole units as shown in Figure in Chapter 1. The phthalocyanine compounds exist in different phases, the most common are a, p and y-phases, the P form being thermodynamically more stablei2. The first two are most commonly studied forms, having two distinct crystalline struct~res.'~ Figure shows the X- ray dihctogram of as deposited CuPc film of thickness 2180 A. The X-ray used is monochromatic Cu K, radiation from a 25 rna beam of 30 kev electrons. A continuous scanning is employed to observe the preferred orientation of the deposited CuPc crystallites. The scanning speed is fixed at 0.2 degreedminute. Indexing is carried out by comparing the International Centre fbr Diffraction Data-Powder Diffraction File (ICDD-PDF) and the lattice constants are calculated. The film deposited at room temperature is identified to be a-form by Ambily et. al.i4 Miller indices are calculated and given in Table The differences in the d values are attributed to higher X-ray absorption. sample purity, particle size, preferred orientation and crystal texture." The structure of CuPc thin films at room temperature is adjudged to be tetragonal with six molecules per unit cell which is in agreement with Robinson and ~lein'~. Sharp and ~bkowitz'~ have reported that P-CuPc is monoclinic in structure. Well defined diffraction peak in the (001) direction gives the direction of the preferential orientation. The lattice

6 131 parameters are obtained as a = b = A, c = A and P = 90'. The crystal structure assumes a similar configuration reported by Debye and d am'^ and Komiyarna et. a1. l9 Robinson and ~lein'' have reported a = b = A and c = A. As the film thickness increases, no change in intensity is observed in the direction of preferential ~rientation.~' Figure XRD pattern of CuPc film of thickness A Table Calculated and observed values of interplanar distance (d) and (hm) values for a CuPc film of thickness 2180 A Peak A B C D E Calculated 2ep) d ( ~ ) (hkl) (001) (41 1) (500) (510) (520) Wo) Observed d (A)

7 6.4.2 CuS films The X-ray diffraction patterns of the CuS films are recorded using a Philips Analytical X-ray P W 3710 BASED machine. Figures and show the X-ray diffractograms of as deposited and annealed CuS films of thickness 3120 A respectively. For as deposited samples, two peaks are observed and the most intense peak is in the (1 02) direction. These peaks are due to the reflection from different planes. But for annealed samples no peaks are observed and the material is mostly amorphous. The figures indicate that as deposited CuS film have crystalline structure with sharp reflection peaks and annealed samples are mostly amorphous. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of CUS films. The scanning speed is fixed at 0.4 degreestminute. Indexing is carried out using ICDD-PDF file (12-176) and the lattice constants are calculated. Miller indices and interplanar distances are obtained. Tabie a and b give the calculated and observed interplanar distances of as deposited and annealed CUS films respectively. The differences in the d values are attributed to higher X-ray absorption, sample purity, particle size, preferred orientation and crystal texture." The lattice parameters obtained for CuS lilms are a = A, b =4.005A, c = 6.806A and the structure is identified as hexagonal.

8 $ Figure XRD pattern of as deposited CuS film of thickness 3 120A Figure XRD pattern of annealed CuS fil rn of thickness 3 120A

9 Table a. Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited CUS film of thickness 3120 A Peak A B C D E Calculated d (A ) (hkl) ( 102) (110) (103) (1 12) (201) Observed 2W0) d (A) Table b. Calculated and observed values of interplanar distance (d) and (h kl) values for annealed (523 K) CuS film of thickness 3120 A Peak Calculated 0 bserved d (A) (hkl) 2W0) d (A) A ( 1 02) No peaks B (1 10) C (103) D (1 12) (20 1 )

10 6.4.3 Multilayer CuS-CuPc films The X-ray diffractograms using Philips Analytical X-ray PW 3710 BASED machine for as deposited and annealed multilayer CuS- CuPc films of thickness 5340 A are shown in figures and respectively. For as deposited samples and annealed samples, six peaks each are observed and the most intense peak is in the (-1 01) direction. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of the CuS films. The scanning speed is fixed at 0.4 degreedminute. Indexing is carried out by ICDD-PDF file ( & ) and the lattice constants are calculated. Miller indices and interplanar distances are calculated. Table a gives the calculated and observed interplanar distances of as deposited CuS, CuPc and multilayer CuS- CuPc films. Table b gives the calculated and observed interplanar distances of annealed CuS, CuPc and multilayer CuS-CuPc films. The present study confirms that the multilayer CuS-CuPc film has crystalline structure with sharp peaks.

11 Figure XRD pattern of as deposited multilayer CuS-CuPc film thickness 5340A Figure XRD pattern of annealed rnultilayer CuS-CuPc film of thickness 5340A

12 Table a. Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited CuS-CuPc film of thickness 5340 A Peak Calculated (CuS) d (A ) (hkl) Calculated (CuPc) d(a) (hkl) Observed (CuS-CuPc) 20 ( *) d (A) A (- 10 1) B (002) C D E Table b. Calculated and observed values of interplanar distance (d) and (hkl) values for annealed CuS-CuPc film of thickness 5340 A. Peak Calculated (CuS) d (A) (hkl) Calculated (CuPc) d (A) (Nl) Observed (CuS-CuPc) 20 (U) d (A) A (102) (-101) B (1 10) (002) C (103) (1 12) D (112) (113) E (201) (213) (114) (-523) (615) (-618) (130)

13 PbS films The X-ray diffraction patterns of the PbS films are recorded using a Philips Analytical X-ray PW BASED machine. Figures and show the X- ray diffractograrns of as deposited and annealed PbS films of thickness 4150 A respectively. For as deposited samples, two peaks are observed and the most intense peak is in the (111) direction. These peaks are due to the reflection from dirferent planes. But for annealed samples only one peak is observed. The figures indicate that as deposited PbS film have crystalline structure with sharp reflection peaks and annealed samples are less crystalline. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of PbS films. The scanning speed is fixed at 0.4 degreedminute. lndexing is carried out using ICDD-PDF file (5-592) and the lattice constants are calculated. Miller indices and interplanar distances are obtained. Table a and b give the calculated and observed interplanar distances of as deposited and annealed PbS films respectively. The differcnccs in the d values are attributed to higher X-ray absorption, sample purity, particle size, preferred orientation and crystal texture. ' The lattice parameters obtained for PbS films are a = A, b = A, c = A and the structure is identified as cubic.

14 Figure XRD pattern of as deposited PbS film of thickness A Figure XRD pattem of annealed PbS fi l~n of thickness 4 150A

15 Table a. Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited PbS film oft hickness 4150 A Peak A B C D E Calculated d (A ) (hkl) (111) (200) " (220) (311) (222) Observed 2W0) d (A) Table b. Calculated and observed values of interplanar distance (d) and (hkl) values for annealed (523 K) PbS film of thickness 4150 A Peak A B C D Calculated d (A) (hkl) (111) (200) (220) (31 1) (222) Observed 20 ( O) d (A)

16 6.4.5 Multilayer PbS-CuPc films The X-ray diffractograms using Philips Analytical X-ray PW 3710 BASED machine for as deposited and annealed rnultilayer PbS-CuPc films of thickness 6450A are shown in figures and respectively. For as deposited samples three peaks are observed and the most intense peak is in the (-11 1) direction. For annealed samples, there are four peaks and the most intense peak is in the (-101) direction. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to obselve the preferred orientation of the films. The scanning speed is fixed at 0.4 degreeshinute. Indexing is carried out by ICDD-PDF file (5-592 & ) and the lattice constants are calculated. Miller indices and interplanar distances are calculated. Table a gives the calculated and observed interplanar distances of as deposited PbS, CuPc and multilayer PbS-CuPc films. Table b gives the calculated and observed interplanar distances of annealed PbS, CuPc and multilayer PbS-CuPc films. The present study confirms that the multilayer PbS-CuPc film has crystalline structure with sharp peaks.

17 Figure XRD pattern of as deposited rnultilayer PbS-CuPc film of thickness 6450A Figure XRD pattern of annealed multilayer PbS-CuPc film of thickness 6450A

18 Table a. Peak Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited PbS-CuPc film of thickness 6450 A Calculated (PbS) d (A ) Calculated (CuPc) d(a) (hkl) Observed (PbS-CuPc) 20 ( O ) d (A) Table b. Calculated and observed values of interplanar distance (d) and (hkl) values for annealed PbS-CuPc film of thickness 6450 A. Calculated (PbS Calculated (CuPc) I Observed (PbS-CuPc) Peak d (A) (hkl) 2W0) d(a)

19 144 6,4,6 CdS films The X-ray dihction patterns of the CdS films were recorded using a Philips Analytical X-ray PW 3710 BASED machine. Figures and show the X- ray diffractograms of as deposited and annealed CdS films of thickness A respectively. The peaks in as deposited and annealed films are almost the same. The most intense peak is in the (1 11) direction. Five additional peaks in the range 3 0 ~ ~ are 0 obtained ~ 0 ~ for as deposited films. The figures indicate that CdS film has more or less crystalline structure with sharp peaks. The X-ray used is monochromatic Cu K, radiation from a 25 rna beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of the CdS films. The scanning speed is fixed at 0.4 degreedminute. Indexing is carried out by International Centre for Diffraction Dah-Powder Difiaction File (ICDD-PDF, ) and the lattice constants are calculated. Miller indices and interplanar distances are calculated. Table 6.4.6a and 6.4.6b gives the calculated and observed interplanar distances of CdS thin films. The differences in the d values are attributed to higher X-ray absorption, sample purity, particle size, preferred orientation and crystal texture'5. The lattice parameters obtained for CdS films are a = b = c = 5.45 A and thc structure is identified as cubic. The present study confirms that the structure is cubic tbr CdS films. At very small film thickness ( OOA), the CdS films appear to be amorphous or possess very small cry~tdiites.~'

20 Figure XRD pattem of as deposited CdS film of thickness 6100A Figure XRD pattern of annealed CdS film of thickness 61 00A

21 Table a. Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited CdS film of thickness 6100 A Peak A B C D E Calculated d (A (hkl) (111) (200) (220) (31 1) (222) (400) Observed 20 (O) d (A) Table b. Calculated and observed values of interplanar distance (d) and (hkl) values for annealed (523 K) CdS film of thickness A Peak A B C D E Calculated d (A (111) (200) (220) 1'.6400 (311) (222) (400) 0 bserved 2W0) d (A)

22 6.4.7 Multilayer CdS-CuPc films The X-ray diffractograms using Philips Analytical X-ray PW 3710 BASED machine for as deposited and annealed multilayer CdS- CuPc films of thickness 8250A are as shown in figures and respectively. The peaks identified in as deposited and annealed films are similar. There are nine prominent peaks for as deposited films and seventeen peaks for annealed samples. The most intense peak is in the (213) direction in both cases. The figures indicate that the peaks obtained for CdS-CuPc film is a combination of CdS and CuPc films. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of the CdS films. The scanning speed is fixed at 0.4 degreeslminute. Indexing is carried out using International Centre for Diffraction Data-Powder Diffraction File (ICDD-PDF, and ) and the lattice constants are calculated. Miller indices and interplanar distances are calculated. Table a gives the calculated and observed interplanar distances of as deposited CdS, CuPc and multilayer CdS-CuPc films. Table b. gives the calcula~ed and observed interplanar distances of annealed CdS, CuPc and multilayer CdS-CuPc films. The present study confirms that the multilayer CdS- CuPc film has crystalline structure with sharp peaks.

23 Figure XRD pattem of as deposited multilayer CdS-CuPc film of thickness 8250A Figure XRD pattem of annealed multilayer CdS-CuPc film of thickness 8250A

24 1 Table a. Peak A B C D E Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited CdS-CuPc flm of thickness 8250 A Calculated (CdS) d (A ) (hkl) (111) (200) (220) (31 1) (222) (400) Calculated (CuPc) d (A ) thkl) (-101) (002) (1 12) (1 13) (213) (1 14) (-523) (61 5) (-6 18) (130) Observed (CdS-CuPc) 2U0) d(a) Table b. Calculated and observed values of interplanar distance (d) and (hw) values for annealed CdS-CuPc film of thickness 8250 A Peak A B C D E Calculated (CdS) d (A (hkl) (111) (200) (220) (311) (222) (400) Calculated (CuPc) d (A ) (hkl) (-101) (002) (112) (113) (213) (114) (-523) (615) (-61 8) (130) Observed (CdS-CuPc) 2e(O) d(a)

25 6.4.8 Mul tilayer PbS-CuS films The X-ray di ffractograms using Phil ips Analytical X-ray P W 3710 BASED machine for as deposited and annealed multilayer PbS- CuS films of thickness 9350A are shown in figures and respectively. There are three peaks in the diffractogram for the as deposited and one peak for the annealed PbS-CuS films. Hence the samples are crystalline in nature. The X-ray used is monochromatic Cu K, radiation from a 25 ma beam of 40 kv electrons. A continuous scanning is employed to observe the preferred orientation of the films. The scanning speed is fixed at 0.4 degreedminute. Indexing is canied out by International Centre for Diffraction Data-Powder Diffraction File (ICDD-PDF, & ) and the lattice constants are calculated.table a. gives the calculated and observed interplanar distances of as deposited PbS, CuS, and multilayer PbS-CuS films. Table b. gives the calculated and observed interplanar distances of annealed PbS, CuS, and multilayer PbS-CuS films. The differences in the d values are attributed to higher X-ray absorption, sample purity, particle size, preferred orientation and crystal texturei5. From the X-ray diffractogram, it can be seen that the multilayer PbS-CuS films are crystalline.

26 Figure XRD pattem of as deposited multilayer PbS-CuS film thickness 9350A Figure XRD pattern of annealed multilayer PbS-CuS film of thickness 9350A

27 Table a. Calculated and observed values of interplanar distance (d) and (hkl) values for as deposited PbS-CUS film of thickness9350 A Peak Calculated (PbS) d (A) (hkl) Calculated (CuS) d (A) (hkl) Observed (PbS-CuS) 28(O) d (A) A (1 11) (102) B (200) (110) C (220) (103) D (31 1) (1 12) E (222) (20 1 ) Table b. Calculated and observed values of interplanar distance (d) and (hkl) values for annealed PbS-CUS film of thickness 9350 A Peak Calculated (PbS) d (A) Calculated (CuS) d (A) (hkl) Observed (PbS-CuS) 2e ( *) d (A) A (1 1 1) (102) B (200) (1 10) C (220) (1 03) D (311) (1 12) E (222) (20 1 1

28 6.5 Conclusion X-ray di ffractograrns obtained for as deposited and annealed CuPc, CUS, multilayer CuS-CuPc, PbS, rnultilayer PbS-CuPc, CdS, multilayer CdS-CuPc, and multilayer PbS-CuS are analysed using ICDD-PDF file. The structure of CuPc thin films at room temperature is adjudged to be tetragonal with six molecules per unit cell which is in agreement with the results of Robinson and ~1ein.l~ Well defined diffraction peak in the (001) direction gives the direction of the preferential orientation. The present study confirms that as deposited CuS film have crystalline structure with sharp retlection peaks and annealed samples are mostly amorphous. It is found that as deposited and annealed samples of multilayer CuS-CuPc film has crystalline structure with sharp peaks. The as deposited PbS film have crystalline structure with sharp reflection peaks and annealed samples are less crystalline. The present study confirms that the as deposited and annealed multilayer PbS-CuPc films have crystalline structure with sharp peaks. The present study confirms that the structure of CdS film is cubic and the multi layer CdS-CuPc films are crystal line. From the X-ray di ffractogram, it can be seen that the multilayer PbS-CuS films are crystalline. The corresponding lattice parameters for each sample are al so determined.

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