The Optical Properties Of Silicon Nanoporous Pillar Array And Its Cds Nanocomposite System

It is important in basic scientific research and applications that fabrication and physical properties, especially photoelectronic properties of silicon nanomaterials and silicon-based nanocomposite systems are studied. In this paper, silicon nanoporous pillar array (Si-NPA), a typical micron/nanometer composite system with clear hierarchical structures, was prepared by a novel hydrothermal technique. Its physical properties-such as reflectivity, absorption, photoluminescence, dielectric property, electrical structure property, and so on.-was studied systematically. Then ultra thin films of CdS were deposited on Si-NPA by using spin-coating technique and a simple chemical polyphase reaction, and four kinds of uniformed CdS/Si-NPA heterojunctions with different structures were obtained. Similarly to the above Si-NPA, varied physical properties for the heterojunctions materials were also systematically studied and discussed. On this basis, the inherent mechanisms were explored on the Si-NPA and CdS/Si-NPA, which will establish a solid foundation for the applications in many fields such as photoelectric materials, heterojunction diode arrays and silicon-based solar cells. The main points in this paper are as follows:1. Study on the properties of optics and electrics for Si-NPASilicon nanoporous pillar array (Si-NPA) was prepared by using a novel hydrothermal technique. Porosity-an important structural parameter with closed correlation to the properties of physics, chemistry and dynam of Si-NPA-was calculated by the use of reflectivity spectrum and a theory model of effective-medium approximation, which was a novel method presented by us on calculating porosity ofporous materials. At the same time, some optical constants about Si-NPA, such as complex refractivity, complex dielectric function and absorption coefficient, were obtained by reflectivity spectrum and Kramers-Kroning transformation theory. The electrical structural properties of Si-NPA were also studied by the help of the obtained optical constants. The study on the dispersive relation of optics showed that peaks of theimagination of dielectric constant (at 4.05 eV and 5.0 eV respectively in ￡2(co)) were originated from the two peaks located at 3.4 eV and 4.6 eV in ￡2{co) spectrum ofsc-Si, only by the blue-shifts of 0.65 eV and 0.4 eV compared to the latter. The obtained results were well agreed with the theoretical calculation according to the quantum confinement effect. Furthermore, the analysis to the electrical structure by the help of Tauc's theory at absorption edge showed that the structure of Si-NPA has transformed from single-conduction band structure into direct band-width structure with two-conduction bands. The structure of two-band gaps at 2.01ev and 3.06ev was also agreed with photolutninescence peaks, which could further prove the decisive role of quantum confinement effect on the physical properties of Si-NPA.2. The preparation and optical properties of CdS/Si-NPA nanometercomposite system(1) wc-CdS was synthesized by microemulsion method, and then a composite thin film was fabricated by spin-coating nanocrystal CdS onto Si-NPA. The experimental results indicated that the photoluminescence intensity of CdS/Si-NPA composite enhanced by one magnitude compared to single Si-NPA, and only a narrow bule PL peak appeared for CdS/NPA system. (2) Hiring Si-NPA as templates, three kinds of CdS/Si-NPA nanocomposites were prepared by in situ synthesis of CdS via a simple polyphased reaction on the Si-NPA. The first type of CdS/Si-NPA composite film has a composite structure of ring-cluster, which means that CdS nanorings appeared around every silicon pillar's bottom and CdS clusters appeared on the top. The second type was obtained at the same experimental conditions, only by altering CdCl2 aqueous solutionto alcohol solution. CdS nanofilm was uniformly coated on the surface of the substrate, forming a uniform cap-like structure on every silicon's pillar. The third type CdS/Si-NPA film was prepared by using ultraviolet lamp to irradiate at certain energy. CdS nanopoles grew along silicon pillars. Due to the different conductive types between CdS and Si-NPA materials, the prepared CdS/Si-NPA of the latter two types could be belonged to a good kind of nanometer composite system of silicon-based heterojunction semiconductor. The results of PL showed that the PL peak at ~ 495 nm appeared, which comes from photoluminescence of CdS nanocrystalline, meanwhile double red peaks of substrates of the first type film still existed. Besides, the PL peak positions and intensities of the second and third types were same, but their main PL peak has a 20 nm blue shift compared to the PL peak of Si-NPA substrate, which may be caused by the change of the effective dielectric constant during the formation of CdS/Si-NPA structure.