Preparation and Characterization of MOCVD Copper Zinc Sulphide Thin Films Using Single Solid Source Precursors of CuC10H16N2O2S4 and ZnC10H16N2O2S4.
J.O. Emegha1, J.Damisa1, F.O. Ese2, M.C Okafor3, B. Olofinjana2, M.A. Eleruja2 and S.O Azi1.
1Department of physics, University of Benin, Benin City, Nigeria.2Department of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife, Nigeria.3Federal College of Education (Tech), Asaba, Delta State, Nigeria.Abstract
Metal organic chemical vapour deposition (MOCVD) was used with a single solid source precursors of copper dithiocarbamate (CuC10H16N2O2S4) and zinc dithiocarbamate (ZnC10H16N2O2S4) to prepare pyrolysed copper zinc sulphide thin films. The precursors were characterized by Fourier Transform Infrared Spectroscopy (FTIR), to determine the structures present. The copper zinc sulphide thin films were deposited on soda lime substrates at a temperature of 4200C. Experimental results showed that the compositional studies carried out using EDX and RBS were found to be complementary. The structural analyses revealed that the film is crystalline with an average grain size of approximately 2?m. An optical energy of 2.50 eV was obtained with an absorbance that decreases with wavelength. The electrical characterization gave values of sheet resistance, resistivity and conductivity of the film as 5.13 x10-6?/Sq, 2.27 x 10-1?.cm and 3.61 x 10-2(?.cm)-1 respectively. The findings confirmed that the synthesized precursor is a promising material for depositing high quality copper zinc sulphide thin films.
KEY WORDS: Precursor; Substrate; MOCVD; Copper zinc sulphide; Dithiocarbamate.
Copper Zinc sulphide or CZS has gained much interest due to its versatility and potential applications in electronics and Optoelectronics. The CZS thin films have properties in between its binary chalcogenide constituents of CuS and ZnS with a lattice structure that may be very unstoichiometric 1. The vacancies and interstitials control the conductivity type as an excess of copper or zinc causes either p-type or n-type conductivity. It presents some unusual physical properties such as anomalous band gap energy values, high dielectric constants and high carrier mobility. This makes the material attractive in the fabrication of solar cells, LEDs, sensors and so on 2,3.
Diverse methods of deposition have been used to prepare CZS thin films such as chemical spray pyrolysis 1,2,4. Chemical bath deposition 5,6,7, SILAR 8,9, solution growth technique 10, and Election beam evaporation 11. In the case of CZS preparation using these chemical methods, it is difficult to avoid the undesired side reactions and incorporation of impurities within the precursors, which may cause diffusion of layers and affect the quality of the films. This is probably due to the use of two or more precursors of different properties 12. Other limitations are non uniformity of deposited films and the use of unusually high temperature. For physical method of electron beam evaporation, the plasma-induced defect associated with the films due to the bombardment of energetic ions taking place within the surface of the films is a case of disturbance 13. To overcome these limitations, the development of alternative single solid source precursor has been pursued. A single solid source precursor is a metal-organic molecule which contains all the desired elements for the growth of a compound material, likely with its stoichiometry 14,15. The use of a single solid source precursor has potential advantages over the conventional precursors. First, it offers the unique advantage of mildness, safety and simplified preparation procedure and delivers exact control over stoichiometry of the film 14. Another important attraction of this route is seen in the unusual crystal growth or metastable phase formation of the final products which are sometimes not possible using the conventional technique. For these reasons, the single solid source precursor technique has proven to be precise route to prepare high quality thin films 15,16.
This study reports on the preparation of copper dithiocabamate and zinc dithiocarbamate (50:50) as a single solid source precursors used in the deposition of CZS thin films with MOCVD. Thus, the synthesis using MOCVD is simple and reduced the preparation cost, therefore making it more viable economically when compared with other deposition methods 17.Therefore, the aim of this study is to demonstrate the feasibility of using dithiocarbamate as a single solid source material for the production of CZS films and to study their physical properties.
2.1 Preparation of Precursor
The preparation of ammonium morphilino-dithiocarbomate (intermediate complex) is achieved by modifying the experimental procedure reported by Ajayi et al.18, according to the reaction scheme below.
A 250ml round-bottom beaker with a thermometer was immersed in an ice bath due to the exothermic nature of the reaction and to achieve the desired temperature of between 0 to 50C. Ethanol of 100cm3 was poured into a beaker containing morpholine (9.66 cm3, 9.66g, 0.10mol) which was kept stirring over a period of 30 minute to ensure that the temperature was kept below 50C. Carbon disulphide (6.7 cm3, 8.44g, 0.10mol) was added drop-wise and vigorously stirred. Ammonia solution of 75 cm3 was immediately added to the reaction mixture to obtain a light yellow solution which was then put in a deep freezer where the crude product ammonium morphilino-dithiocarbamate crystallized as a white to pale yellow solid. The product was filtered under gravity and let to dry completely at room temperature. The dried product, ammonium morphilino dithiocabamate was found to be 13.46g (67.3% yield).
For copper dithocarbamate, the prepared intermediate precursor of ammonium morpholino-dithiocarbamate (18.752g, 0.104 mol) was dissolved in a solvent with a 2:1 ratio of acetone and water in a 500cm3 beaker. Copper (II) chloride (8.869g, 0.052 mol) was also completely dissolved in 60cm3 of ethanol separately. The solution of copper (II) chloride in ethanol was gradually added to the solution of ammonium morphilino-dithiocarbamate on a hot plate, vigorously stirred at 600C. There was a spontaneous formation of dark-brown precipitate as the addition proceeds. The precipitate formed was heated for another 30 minutes before cooled to room temperature and filtered under gravity. After 24 hours, the product was oven dried for another 72 hours at 500C to yield copper dithiocarbamate (CuC10H16N2O2S4) with a percentage yield of 78.15% (15.63g). The reaction is represented below :
The same procedure was followed for zinc dithiocarbamate, ammonium morpholino-dithicarbamate (18.499g, 0.103 mol) was dissolved in a solvent with a 5:2 ratio of acetone and water. The mixture was heated on a hot place at 600C to ensure that the compound completely dissolved. Zinc chloride (6.997g, 0.051 mol) was also completely dissolved in 60 cm3 of water separately. The solution of zinc chloride in water was gradually added to the solution of ammonium morpholino-dithiocarbamate on a hot plate and vigorously stirred. There was a spontaneous formation of white precipitate as the addition proceeds. The warm solution was allowed to cooled on it own after which the product was filtered under gravity and allowed to dry in air for about 36hrs. The product, zinc dithiocarbamate (ZnC10H16N2O2S4) had a percentage yield of 58.25%, (11.65g). The reaction scheme is also given below:
2.2 Preparation and characterization of CZS thin films
The Copper zinc suphide thin films were prepared by the pyrolytic decomposition method already reported 19. The stating materials, 50:50 of copper dithiocarbamate and zinc dithiocarbamate, were grounded to fine powder, poured into an unheated receptacle and dried nitrogen gas was bubbled through the set-up at a flow rate of 2.0dm3/minute. The nitrogen borne precursor was transported into the working chamber which was maintained at an appropriate temperature by an electrically heated furnace. The substrates were supported on steel blocks to ensure good and uniform thermal contact. The deposition was maintained for two hours. The whole operation was carried out in a fume closet to reduce some of the handling problems
associated with such compound. Figure (1) shows the experimental set-up of CZS thin film deposition 20.
The infrared spectroscopy of the precursors and films were carry out using a Shimadzu 8400 FTIR Spectrophotometer. Rutherford backscattering technology (RBS) was employed to determine the thickness and the elemental composition of the films. The surface morphology was observed by TECNAI F20 high resolution transmission electron microscope (HRTEM), equipped with an energy dispersive x-ray (EDX) analyser for elements detection. The optical behavior of the film was investigated using Double beam UV-1800 Shimadzu spectrophotometer. Old Jandel four point technique (Model TY242MP) was used for the electrical measurements while the x-ray diffraction pattern was tested by D8 – High resolution x-ray diffractometer with radiation of 1.5406Å.
Figure.1: Experimental setup for CZS deposition.
3.0 Results and discussion
3.1 Infrared (IR) spectrophotometryThe IR spectrum in figure 2. shows the different functional groups present in the mixed copper and zinc dithiocarbamate precursors. The measurement was done at room temperature range of 4000 and 500cm-1 in KBr background. A close examination showed a broad peak attributed to O-H stretching vibration at 3448.84cm-121, N-H stretching at 3122.86cm-1, C-H stretching vibration is between 2966.62 and 2852.81cm-1. However, a series of weak bands appear between 2300 and 1700cm-1 reflecting the substitutional pattern of the organic compound in the precursor 22. There are also C=O vibration at 1627.97cm-1, C-C stretching at 1489.10, 1265.35, 1234.48, 1114.89, 1026.16 and 997.23cm-1, out of plane C-H bending 875.71cm-1, Cu-S and Zn-S bands are below 829.42cm-1. This indicates that the ligand was attached to CZS.
Figure 2: Infrared Spectrum of the precursor.
Figure.3: Infrared spectrum of deposited CZS on soda lime substrate.
Figure 3 shows the IR spectra of the deposited films at 4200C. The signals when compared to that of figure 2, showed that the characteristic bands associated with organic ligands were missing, suggesting a complete decomposition of the precursor to produce copper zinc sulphide. An observation that is consistent with the decomposition of metal sulphides 1823.
3.2 Compositional studied of CZS thin film
The EDX spectrum of the deposited films is shown in figure 4. The signals confirmed the presence of copper, zinc and sulphur as the major elements present in the film. The other feature elements evident in the spectrum is due to the nature of the soda lime substrate used 24. The RBS spectrum of the depositeed film is exhibited in figure 6. The elemental analysis measured are copper (31.6%), zinc (31.5%) and sulphur (30.5%). This observation shows that the decomposition of the precursors in nitrogen gas produced CZS thin film. 12. The thickness was found to be 54.00nm using the relation below 25.
Thicknessnm=Atoms per unit areaatomic density cm-2cm-3 (1)
Figure .4: EDX Spectrum of CZS thin film
Figure. 5: RBS Specturm of CZS thin film.
3.3 Optical Characterization
To demonstrate the suitability of the deposited thin film for device applications, the absorbance in the range of 300nm to 1400nm was investigated in a step size of 20. Measurements were taken with a blank substrate before the deposited substrate was introduced. The optical absorbance A, was calculated from the expression.
Absorbance (A)=1og 1T 2 Where T is the transmittance of the film. The variation of the optical absorbance with wavelength for CZS thin film is displayed in figure 6. As seen, it is observed that the optical absorbance decreased with increase in wavelength. Also, we noticed that that value of the optical absorbance is high in ultra violet region and low in the visible and near infrared regions. This, therefore, means an improvement in the transmittance of the film 26. The nature of the film having an enhanced absorbance in the ultra violet region makes the film a good window layer in solar cells fabrication.
The energy gap (Eg) has been observed to follow the form 11:
Where K is a constant, r is frequency, h is plank’s constant while Eg is the energy gap. In equation (3), there is another constant n, known as the power factor of the electronic transition mode 28. It depends on the nature of the material. For CZS film, the value of n is 0.5, being a direct allowed band gap semiconductor 21, 29. Experimentally, the energy gap is determined by a graph of ?hr2 vesus hr. The energy gap value is obtained by extending the linear portion to intercept (hr)-axis at (?hr)2 equal to zero. Figure 7 shows that the energy gap obtained for CZS film is equal to 2.50 eV. The optical energy gap shows that CZS film is a ternary material whose band gap falls between the binary constituents of 1.38 eV for CuS 20, and 3.67 eV for ZnS 23. Thus, conserving the Vegard’s rule of mixture 19. Also, the value of the energy gap obtained is in agreement with the value of 1.8 to 3.52 eV determined by Noriyuki et al. 4 and 2.4 to 2.7 eV obtained by Ezenwa and Okoli 5 using other routes. Similar value was also reported by Sreejith et al. 1.
Figure 6: Absorbance against wavelength of CZS ting film
Figure 7: Square of absorption coefficient against photon energy for CZS thin film.
3.4 Electrical Characterization
The electrical characterization of the film was carried out using the four probe method. Errors were minimized in the current-voltage measurements by taking the reading several times and average values of the voltage and current were recorded. The average resistivity (?), of the film was calculated by multiplying the thickness (?), and the sheet resistance (Rs)as follows:
Resistivity =Rs×? (4)Where ? is the thickness from RBS study. The reciprocal of the resistivity was taken as the conductivity value.
The average voltage and current generated in the study were determined as 1.74 ×10-2V and 15.38 × 10-9A respectively. The sheet resistance was calculated to be 5.13 × 106 ?/square, the resistivity was found to be 27.70 ?.cm and the conductivity was also calculated to be 3.61 ×10-2 (?.cm)-1. The electrical conductivity falls within the range of 10-13 to 102 reported for semiconductor thin films 30, suggesting that the deposited film is conductive. The rather high resistive properties of the deposited films indicate that CZS film could find application as semiconductor sensors either as gas or touch sensor 31. The observed high resistivity may be due to the high impurity content within the film.
3.5 Surface Morphology
The surface morphology of the deposited film was observed by high resolution transmission electron microscopy (HRTEM). The HRTEM image of the deposited film is shown in figure 8. From the image, it can be observed that the film consists of grains that are ellipsoidal in nature and are randomly distributed throughout the substrate. The size of the grains ranges from 2.5µm to about 6.0µm with an average of approximately 4.0µm. It is evident from the corresponding selected area electron diffraction (SAED) pattern in figure 9 that the film is polycrystalline in nature. The presence of Debye – Scherrer ring shows that the deposited CZS film is composed of cluster of grains 4, 32.
Figure 8: HRTEM image of the deposited Film
Figure 9: SAED Pattern of the deposited Film
3.6 X-ray Diffraction Studies
The x-ray diffraction pattern of the film is shown in figure 10. Sharp peaks occur at angle 2?= 28.500, 33.010, 47.500, 56.500, 69.50, 77.000 and 78.500. Corresponding to diffraction line of (111), (200), (220), (311), (222), (400), (332) and (420) plane from the shalerite ZnS diffraction data file of 005-0566.
As seen, it can be inferred that there was a phase separation from the deposited copper zinc sulphide to binary zinc sulphide, which is in agreement with reported literature on ternary thin film materials 33 – 35. Also, it is clear that the deposited film is polycrystalline 36. This measurement collaborated with the SAED studies. Furthermore, the broad hump observed in the spectrum within the range 190?20?380 is due to the amorphous nature of the substrate (soda line) used in the deposition 33, 34.
Figure 10: XRD Spectrum of CZS thin film.
Copper zinc sulphide(CZS) thin films have been prepared and deposited at 4200C on soda lime substrate by the pyrolysis of a mixture of copper dithiocarbamate and Zinc dithocarbonate (50:50) using MOCVD technique. Thus, providing a new and cheap method of depositing CZS films. Characterization confirmed that the film is crystalline with a direct optical band gap of 2.50 eV and an average thickness of 54.00 µm. In this case, the elemental analyses showed a film that consist of copper, zinc and sulphur in various proportion that has good adhesion with the substrate. The electrical conductivity determined with four point method was 3.61×10-2?.cm-1 with a high resistivity of 27.70 (?.cm).
The authors are grateful to the Department of Physics and Engineering Physics (Material Science Group), Obafemi Awolowo University, Ile-Ife for allowing the use of their MOCVD facilities and equipments for carrying out this work.
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