Growth And Optoelectronic Properties Of CdS And ZnS Nanostrucutres

Nanosemiconductors has important applications in the field of photoelectric, which has become the the hot research topic in materials science, condensed mateter physics, solide state electronics, and so on. Nano electronics components can bring many benefits, such as high level of integration, lower power consumption, cost savings and so on. However, as a fundamental component of nanodevice, electrical, optical, magnetic and chemical properties of nanomaterials determine the performance of nanodevice in the application. Therefore, under the premise of cost saving, preparation high quality of nanomaterials has become an ambition for a relevant researchers and application developers.Cadmium sulfide (CdS) semiconductor, with band gap 2.42 eV, has important applications in photocatalysis, photoelectric detection, photovoltaic conversion and so on. As the n-type window material, it can be widely used in thin film solar photovoltaic cell, such as cadmium telluride (CdTe) cell, copper indium gallium selenide (CIGS) cell, copper zinc tin sulfur (CZTS)cell and so on. Thus the preparation of high quality CdS window materials has become the premise for high efficiency thin film solar cell.In this thesis, on the base of preparation technology of CdTe film solar photovoltaic cell, CdS window materials are succesfully obtained. The main contents are as follows:(1) A novel in-situ method is developed in growth of CdS, for the first time. The specific process is as follows:close space sublimation method is used to prepare p-type CdTe films, followed by the annealing treatment in H2S/N2 atmosphere, and then n-type CdS films or nanowire structure are grown in-situ on CdTe films to form CdS/CdTe pn heterojunction successfully. Obviously, on the premise of low preparation cost, the new method does give a hope to improve preparation technology of CdTe solar photovoltaic cell. In addition, with the help of in-situ method, we also successfully fabricate other transition metal nanostructures, such as ZnS nanostrucutre and CuS nanostrucutre.(2) Two different methods, high-vacuum evaporation method and close space sublimation method, are used to prepare CdTe films, and then the technological process and photoelectric property of the CdTe films has been investigated in detail. As for high-vacuum evaporation method, the particle size for deposited CdTe thin film is below 15 nm, and the roughness only about 4 nm. On the contrary, for close space sublimation method, its particle size is large, which comes to a few micrometers, even dozens of micrometers, and the roughness is about 0.3 μn. Since the CdTe thin film with large particle size has advantages in crystallinity, battery efficiency and cost, we choose close space sublimation method to prepare CdTe thin films.(3) The effect of Au catalyst on the nanostrucutre formation is also studied perfectly. Au is known as a very important catalyst for the preparation of nanostructure, which can promote or induce the increase in density and great changes in the morphology of the nanostrucutre. We put the CdTe thin film substrate where Au film has been deposited with different thickness under H2S/N2 mixed gas flow to anneal. The results indicate that the thickness of Au film deposited on the CdTe surface influence the morphology of CdS nanostructure. When the thickness is 5 nm, the morphology is nanowires. But when the thickness is 15 nm, the morphology is nanosheets. On the base of results of high resolution transmission electron microscopy, it is clearly seen that CdS nanowires and nanosheets are all single crystal with hexagonal wurtzite structure, and the preferred growth direction of nanowires is toward (110). Meanwhile, combining with the result from photoluminescence, we can conclude that the change of morphology of nanostructure can also modify the luminescence properties.(4) The growth of CdS nanostructure in the absence of template and catalyst is discussed in the paper. Thus CdTe film is directly annealed under H2S/N2 mixed gas flow. With the increase of the annealing temperature, intensity of CdTe diffraction peak gradually reduces until disappears, and that of CdS diffraction peak gradually increases and becomes a main phase. This transition process between CdTe and CdS are also observed in Raman, X-ray photoelectron spectroscopy and photoluminescence patterns. When the annealing temperature is below 400℃, CdS thin films was formed on CdTe thin film surface accompanying with a small amount of nanowire structure. As the annealing temperature further increases, the density of the nanowires significantly increases. When the annealing temperature increases above 500℃, the number of nanowires dramatically reduces, and many nanohole are left on the surface. On the basis of above research, we put forward the growth mechanism, which is the stress deriving from the lattice mismatch between CdTe thin film and CdS thin film prompting the formation of the CdS nanostructure on CdTe thin film surface.CdS/CdTe heterojunction is successfully prepared after the CdTe thin film fabricated on Mo-coated glass substrate undergoes the annealing treatment in H2S atmosphere. The role of each step in the process of preparation is analyzed in detail. The pn heterjunction prepared at low temperature shows normal Ⅰ-Ⅴ curve. However, the pn heterjunction prepared at high temperature shows abnormal Ⅰ-Ⅴ curve, which is attributed to the series of the nancjunction and nanoholes in pn junction.(5) CuS and ZnS nanostructure are obtained succesfully after adopting the in-situ method, and their optical properties are also investigated clearly. The specific scheme is putting Zn foil thermal annealed in H2S/N2 atmosphere. As a contrast, Zn foil with Au coating is performed the same treatment. The results indicate that after high temperature annealing ZnS nanowires with hexagonal structure and sphalerite structure are formed in-situ on Zn foil with no Au coating, and ZnS nanoribbons with hexagonal single structure are formed in-situ on Zn foil with Au coating. The growth mechanism of the nanowire may be attributed to the spontaneous catalysis, and the growth of nanobelts in the length direction is the result of Au and Zn catalysis, while growth in width is associated with the autocatalysis of Zn.