Growth and Characterization of Cd 1-x Zn x Te Films

Transcription

1 Universities Research Journal 2011, Vol. 4, No. 4 Growth and Characterization of Cd 1-x Zn x Te Films Nway Han Myat Thin 1 and Pho Kaung 2 Abstract The II-VI polycrystalline semiconducting materials have come under increased scrutiny because of their wide use in the cost reduction of devices for photovoltaic applications. Cd 1-x Zn x Te is one of the II-VI ternary semiconductor materials for high-efficiency solar cells, switching and other optoelectronic devices. Cd 1-x Zn x Te compound films can be fabricated by using the cadmium zinc telluride (Cd 1-x Zn x Te) compounds with the ways of coating the pastes on the glass substrates and sintering the films in a suitable atmosphere. In this matter, Cd 1-x Zn x Te compounds were formulated by the three compositions of (x=0.04, 0.2 and 0.6) and fabricated in the self-assembled vacuum chamber. The surface morphology of the films was characterized with scanning electron microscope. The structural properties and the compositions of Cd 1-x Zn x Te compound powders were analyzed by X-ray diffraction (XRD) and energy dispersive X-ray (EDXRF) analysis. The electrical properties of the films were investigated by photoconductivity measurement. Key words: screen printing, Cd 1-x Zn x Te film Introduction Semiconductors are important because they have been found applicable in almost all branches of industry and area of daily life. In recent years, there has been a rapid development in the field of group II-VI compound semiconductors for their use in photovoltaic devices. The uses of thin film polycrystalline semiconductors have attracted much interest in an expanding variety of application in various electronics and optoelectronics devices and solar cells [1]. The technological interest in polycrystalline based devices is mainly caused by their low production costs. Thin films now occupy a prominent place in basic research and solid state technology. The electrical and physical properties of Group II-VI compound films depend on their chemical composition, defect chemistry and structure, which in turn are related to the film growth techniques and parameters. The Group II-VI compound semiconductors of Cd 1-x Zn x Te compounds can be used for manufacturing of solar cell [2]. There are a variety of deposition techniques that have been used for the growth of Group II-VI compound semiconductor films such as chemical bath deposition, sol-gel techniques, 1.Demonstrator, Department of Physics, Dagon University 2. Pro-Rector, Universities Research Centre, University of Yangon

2 258 Universities Research Journal 2011, Vol. 4, No. 4 MOCVD, sputtering, electrochemical deposition and screen printing technique. Among them a method of screen printing with sintering is one of the cheapest and convenient techniques for fabrication of these films [3]. The study deals with qualitative observations on film characterization, composition and crystallite size variation with different growth conditions from x-ray diffraction, scanning electron microscopy and energy dispersive x-ray fluorescence spectrometry. The electrical properties of the films are investigated by photoconductivity measurement [4]. Experimental Procedure Screen Printing Technique The samples in this study were prepared on borosilicate glass substrates by using screen printing method. Cadmium zinc telluride was fabricated by using the temperature controlled vacuum chamber. Screenprinting, or serigraphy, previously known as silk-screening is a printmaking technique that creates a sharp-edged image using a stencil. The tools are 8" 11" wooden frame, 2" rubber squeegee, masking tape, glass and printing board and mesh counter as shown in figure (1). The important screen printing parameters are the viscosity of the paste, the mesh number of the screen and the snap-off distance between the screen and the substrate. In the present work, the mesh number of the silk screen is 150 meshes per inch and the snap-off distance is 0.7 cm. The semiconducting inks were prepared by using zinc chloride for Cd 1-x Zn x Te with propylene glycol (PG) to form the paste. A paste of the material to be screen printed is pressed though the screen by means of a squeegee. (a) (b)

3 Universities Research Journal 2011, Vol. 4, No (c) (d) Figure 1 Tools for screen printing: (a) Wooden frame (b) Squeegee (c) Glass and printing board (d) Mesh counter Structure of Vacuum System The instrument (TOKUDA, JAPAN) is composed of four sections as shown in figure (2). These four sections are the vacuum chamber, the rotary pump, the electrical system and temperature controller system. The vacuum chamber is mounted on the top of the cabinet and the two systems stay beside the cabinet. Figure 2 Photograph of Vacuum System

4 260 Universities Research Journal 2011, Vol. 4, No. 4 Preparation of Cd 1-x Zn x Te Compound Powder (Composition of x=0.04, 0.2 and 0.6) For preparation of Cd 1-x Zn x Te compounds, cadmium metals, zinc powders and tellurium powders of high purity were used. Firstly high purity cadmium metals were ground by a file and then cadmium powders were obtained. The obtained cadmium powder, zinc powder and tellurium powder were weighed in line with the stoichiometric formula Cd 1-x Zn x Te (where x = 0.04, 0.2, 0.6), and they were crushed with propylene glycol for one hour. The three obtained powders were dried at 120 C for one hour and thermally heated at 600 C for one hour in the temperature controlled vacuum chamber. After cooling down the temperature to room temperature, the three Cd 1-x Zn x Te compounds were obtained. The photographs of the manufacturing process of Cd 1-x Zn x Te are shown in figure (3). (a) File (b) Cadmium Metal (c) Cadmium Powder (d) Zinc Powder (e) Tellurium Powder (f) Cd 0.4 Zn 0.6 Te Compound (g) Cd 0.8 Zn 0.2 Te Compound (h) Cd 0.96 Zn 0.04 Te Compound Figure 3 Photograph of manufacturing process of Cd 1-x Zn x Te Compounds

5 Universities Research Journal 2011, Vol. 4, No Cd 1-x Zn x Te Films Preparation To prepare Cd 1-x Zn x Te pastes Cd 1-x Zn x Te compound powder and zinc chloride (ZnCl 2 ) powder were thoroughly mixed and then a few drops of PG were added. The weight of ZnCl 2 was only 10% of the weight of Cd 1-x Zn x Te powders. ZnCl 2 was used as a flux and PG as a viscous agent. The pastes were kneading well to achieve the viscous agent. Then the pastes were applied to the borosilicate glass substrates under the screen printing method to form coating layers. The prepared films were dried at 120 C for one hour in the temperature controlled vacuum chamber. The reason of drying the sample at lower temperature was to avoid the cracks in the samples. Then the films were sintered at 550 C for 15 minutes in the vacuum chamber. The obtained Cd 1-x Zn x Te films were shown in figure (4). As shown in the figures, the colour of Cd 0.96 Zn 0.04 Te and Cd 0.8 Zn 0.2 Te is dark gray and that of Cd 0.4 Zn 0.6 Te is white. Figure 4 Photographs of (a) Cd 0.96 Zn 0.04 Te film (b) Cd 0.8 Zn 0.2 Te film (c) Cd 0.4 Zn 0.6 Te film Results and Discussion Surface Morphology of Cd 1-x Zn x Te films SEM images of sintered Cd 1-x Zn x Te films (where x = 0.04, 0.2 and 0.6) were shown in figure (5), (6) and (7). From these figures, all the films are adherent crack free form and grained microstructure with small crystallite sizes. These should be assumed that it is spread uniformly on the surface areas. But Cd 0.4 Zn 0.6 Te film has less uniformity than those of Cd 0.96 Zn 0.04 Te film and Cd 0.8 Zn 0.2 Te film. In figure (5), the grain size of this film is nearly 1.5 μm and it has large and clear morphological properties. In figure (6), the grain size of the film is nearly 1 μm and it is composed of

6 262 Universities Research Journal 2011, Vol. 4, No. 4 closely packet particles. In figure (7), the grain size of this film is round about 0.5 μm and it is composed of non-uniformity particles. Based on these three figures, it appears that grain sizes decreases with increasing zinc content in the films. Figure 5 SEM image of Cd 0.96 Zn 0.04 Te film on glass substrate at 550 C Figure 6 SEM image of Cd 0.8 Zn 0.2 Te film on glass substrate at 550 C

7 Universities Research Journal 2011, Vol. 4, No Figure 7 SEM image of Cd 0.4 Zn 0.6 Te film on glass substrate at 550 C EDXRF Analysis of Cd 1-x Zn x Te Cd 0.96 Zn 0.04 Te, Cd 0.8 Zn 0.2 Te and Cd 0.4 Zn 0.6 Te compounds were analyzed by using EDXRF spectrometer. In figure (8), Cd 0.96 Zn 0.04 Te sample showed that the concentration of tellurium is (51.607%), cadmium (37.96%) and zinc (10.228%). A few percent of Se was found in this compound. In figure (9), Cd 0.8 Zn 0.2 Te sample revealed that the concentration of tellurium is (41.742%), cadmium (45.611%) and zinc (12.647%). No impurities peaks were occurred in this sample. In figure (10), Cd 0.4 Zn 0.6 Te sample was indicated by the concentration of tellurium (44.453%), cadmium (30.313%) and zinc (25.010%). There also found a few impurities percent of Se. From these figures, it can be concluded that all the peak heights and percent ratios are in good agreement with the concentration of zinc in their ratios.

8 264 Universities Research Journal 2011, Vol. 4, No. 4 Figure 8 EDXRF analysis spectrum of Cd 0.96 Zn 0.04 Te Figure 9 EDXRF analysis spectrum of Cd 0.8 Zn 0.2 Te

9 Universities Research Journal 2011, Vol. 4, No Figure 10 EDXRF analysis spectrum of Cd 0.4 Zn 0.6 Te XRD Analysis of Cd 1-x Zn x Te Compound Figures (11), (12) and (13) show the XRD patterns of Cd 1-x Zn x Te compounds at various compositions of (where x = 0.04, 0.2 and 0.6). XRD analysis showed that all the compounds have highly oriented crystalline with cubic structure. All the peaks heights and peak positions are in good agreement with library file of XRD machine. All three samples were found that they exhibit preferential orientation along the (111) plane. These traces confirmed the formation of compound of Cd 1-x Zn x Te. The presence of sharp structural peaks in these three XRD patterns confirmed that they have the polycrystalline in nature. So, it can be concluded that all these compounds were found to be polycrystalline in nature and have cubic (zinc- blende) structure over entire composition range. It can be also found that a decrease in crystallite size was observed with the increase of zinc content in these compounds.

10 266 Universities Research Journal 2011, Vol. 4, No. 4 Figure 11 XRD Pattern of Cd 0.96 Zn 0.04 Te Compound Figure 12 XRD Pattern of Cd 0.8 Zn 0.2 Te Compound

11 Universities Research Journal 2011, Vol. 4, No Figure (13) XRD Pattern of Cd 0.4 Zn 0.6 Te Compound Photoconductivity of Cd 1-x Zn x Te films Photoconductivity characteristics of CdS film and Cd 1-x Zn x Te films have been measured by using Fluke resistance meter/fluke View Forms model 189 TRUE RMS MULTIMETER. This measurement has been carried out using ohmic contacts with silver paste on the surface of the films. The distance between contacts is 0.5 cm. The illumination level is 200 W for this measurement. The decay of photoconductivity with time at room temperature for Cd 0.96 Zn 0.04 Te, Cd 0.8 Zn 0.2 Te and Cd 0.4 Zn 0.6 Te films are shown in figure (14), (15) and (16). In figure (14), when light falls on the film, the decay of photoconductivity starts at 18.7 ns and it decreases slowly and steadily after a few second until the light is turned off. And then it decreases after a few seconds and steadily with respect to time. In figure (15), when light is turned on the film, the decay of photoconductivity starts at 16.5 ns and it decreases abruptly and then steadily after a few second. When light is turned off, it decreases abruptly and after some time it is almost constant. In figure (16), when light is turned on the film, the decay of photoconductivity starts at 5.9 ns and it decreases slightly until the light is turned off. And then the conductivity decreases after a few seconds and it decreases steadily with respect to time. By analyzing the three figures, the decay of photoconductivity of Cd 0.8 Zn 0.2 Te film is faster and its photosensitivity is better than that of other films.

12 268 Universities Research Journal 2011, Vol. 4, No. 4 light-on light-off Figure 14 Variation of photoconductivity of Cd 0.96 Zn 0.04 Te film with time light-on light-off Figure 15 Variation of photoconductivity of Cd 0.8 Zn 0.2 Te film with time light-on light-off Figure 16 Variation of photoconductivity of Cd 0.4 Zn 0.6 Te film with time

13 Universities Research Journal 2011, Vol. 4, No Conclusion Cd 1-x Zn x Te compounds were successfully fabricated. The structural and the elemental composition of these compounds have been analyzed by using XRD and EDXRF analysis. From XRD analysis of Cd 1-x Zn x Te compounds, the highest peak of (111) plane confirmed that they have cubic zincblende structure and their peak positions are in good agreement with XRD library file. The crystallite sizes of Cd 0.96 Zn 0.04 Te and Cd 0.8 Zn 0.2 Te are nm. The crystallite size of Cd 0.4 Zn 0.6 Te is nm. From these results, it can be concluded that the crystallite size decreases with increase in zinc concentration. From EDXRF analysis, the element compositions of Cd 1-x Zn x Te compounds are in good agreement with their ratios. From SEM images of Cd 1-x Zn x Te films, Cd 0.96 Zn 0.04 Te film is the finest. It can be also concluded that the grain size of Cd 0.4 Zn 0.6 Te is the smallest. Photoconductivity of Cd 1-x Zn x Te films was found fast and high photosensitivity. Thus it can be concluded that the films were suitable for photovoltaic devices fabrication. Acknowledgements The authors would like to thank Professor Dr Khin Mar Kyu (Head of Department of Physics), Dagon University, for her kind permission to carry out this research work. References Arita, T et al, (1991). Solar Energy Materials, 23, 371 Bube, R H, (1960). Photoconductivity of Solid. New York: Wiley Burgelman, M, (2001). Thin Film by Screen Printing Technology. Brussels: University of Gent Salazar, Y A et al, (2006). Brazilian Journal of Physics, 36, 1058