Comparison Research On Complex CdS Double Layers Prepared By Evaporation And Chemical Bath Deposition

Cadmium sulphide (CdS), as the direct band-gap compound semiconductor, shows good luminence effect and nonlinear optical effect, and shows the phenomena of optical absorption and amplification. Cadmium sulphide as an electronic and optical functional material has become one representative of the second generation of compound semiconductor after the first generation of silicon semiconductor, to which more and more scientists have begun to attach much importance and deep study.The polycrystalline thin film material of CdS is a good semiconductor optoelectronic material, and is often used as the window material of the thin film solar battery. However, a little study is made on the characteristics of light conducting electricity of the polycrystalline thin film material of CdS. Cadmium sulphide, whose band width is 2.45eV, exerts a good response to the visible light region. Therefore, it still needs further investigation that cadmium sulphide fuctions as the invisible material, light-triggering power switch, and sensitized material, etc.In this thesis, study is made on the structure and characteristics of light conducting electricity of the thin film of CdS which is prepared by evaporation and chemical bath deposition. Much discussion and explanation are given to the influences on the thin film from the conditions of film forming temperature, PH value, etc. Such method as XRD,UV/Vis,SEM are adopted to represent the crystal structure, band width of the forbidden band and morphology. Tests are carried out on the light and dark resistance of the thin film, and then the band width of the forbidden band is worked out.There are lots of methods for preparing the thin film of CdS, such as screen printing, thermal spray, magnetron sputtering, vacuum evaporation, chemical bath deposition, close-spaced sublimation, electrodeposit method, etc. Vacuum evaporation and chemical bath deposition are mainly adopted to prepare the thin film of CdS. As a mature technology, vacuum evaporation is easy to operate, and the quality of the thin film is rather stable; while the chemical bath deposition is simple and low cost, which make it easier to prepare the thin film of CdS on a large scale and endow this method with a very good commercial value. But the focus is put on the technological process of preparing homogeneous thin film with high performance. The thin film of CdS prepared by vacuum evaporation and chemical bath deposition in this thesis is of homogeneous quality, and under under the normal temperature the band width of the forbidden band is 2.45eV. In the sunlight, the electric conductivity can reach 10-4σ/?-1cm-1.1. Preparing the polycrystalline thin film of CdS through expriment:It mainly means the preparation process by the chemical bath deposition, in which what is separately prepared are solution I and II. Solution I: cadmium chloride, ammonium chloride, and ammonia hydroxide; solution II: ammonium chloride, ammonia hydroxide, and thiocarbamide. Solution I and II are separately heated by water bath, and PH value is adjusted. Then solution I and II, which are already heated, are mixed together respectively to keep rotor of dependence magnetic force mixing, and the ultrasonic of the power 300W also can be added into.We have studied the influnences from temperature on the rate of initial growth, and studied the relation between the time needed for starting reaction and the solution temperature. See the following diagram:All the thin films formed are compared based on different concentration, temperature and PH value. The concentration of solution I : ammonia hydroxide 0.771mol/L; ammonium chloride 0.038mol/L, cadmium chloride 0.013mol/L. The concentration of solution II: ammonia hydroxide 0.771mol/L, thiocarbamide 0.18mol/L. The quality of the thim film is prepared best when the temperature is 85℃, and PH value is from 9 to 11.Meanwhile the study is made on the relation between the thickness of thin film and deposition time and temperature, which is shown in the following transformation diagram: In the part before the turning point, the thickness of thin film is gaining the asymptotically linear growth with the development of deposition time, that is linear region. Past the turning point, the growth of thin film is decreasing until finally almost stopping, this part is known as saturate region. When making the thin film of CdS, we should make the thin film form in the linear region, which can make sure the optimization and control over the thin film. 2. Morphology and quality comparison of the thin films prepared by the methods of evaporation and CBD2.1 Morphology comparison by SEMThe grain of the thin film of CdS prepared by evaporation is relatively small, and it's difficult to determine the size of the grain by SEM. The grain of the thin film of CdS prepared by CBD is about 30-50nm. The thin films prepared by the two methods have different crystal structure. 2.2 Test result of XRDIn Diagram 5, A curve stands for the XRD image of the thin film prepared by evaporation without annealing treatment. Conducting electricity shows certain cubic crystal system. B refers to the XRD image of the thin film prepared by evaporation with annealing treatment of 350OC for one hour. Not only the peak of the cubic cystal system is increasing, but also the crystallization of the six-ray is slightly strengthened. Diagram Two is XRD image of the thin film CdS prepared by CBD, and the thin film CdS has a good cubic crystal direction.2.3 Test result of optical qualityThe relation of (αhν)2-hνmade by transmission spectrum is shown in the following transformation diagram: Make a tangent line, and the horizontal intercept is the energy gap of the CdS film, Eg=2.45eV, which theoretically proves that it has a good response to the visible light. 2.4 Quality test of the light conducting electricityThe thin film of CdS which has been prepared is coated by the aluminium electrode. The multimeter is used to test the changes of resistance under different sunlight, which proves that the thin film of CdS has a good response in the visible light region. The film's thickness is 100nm and area is 5cm2, the resistance between two sides is around 5k?, and the sheet-resistance measured by four-probe method is 5.6 104?. All these have proven that cadmium sulphide can be used as photoconductive material, and meet the needs of such devices as light-triggering power switch, lightpath, etc.In the test of the composite membrane of CdS/Cu2S, under the same conditions mentioned above, the conductive quality is better. Therefore when making the light-triggering power switch and the relevant devices, we can consider preparing the composite membrane or multilayer film of CdS.