The Environmental Semiconductor Material Of Ca <sub> 2 </ Sub> Of Si Preparation And Characteristics,

In this dissertation, ecologically friendly semiconducting Ca₂Si thin films are prepared by radio frequency magnetron sputtering system for the first time. The structural, optical and electrical properties of Ca₂Si thin films prepared under different anneal ing conditions are characterized by the X-ray diffractometer, scanning electron microscope, atomic force microscope, spectroscopicellipsometer and four-probe meter, respectively. Then the theoretical calculations of the energy-band structures and optical properties of the Ca₂Si are studied using the first principle methods, and the results of theoretical calculations are compared with those of the experiments.Firstly, the progress on the ecologically friendly semiconducting material Ca₂Si are summarized briefly. Magnetron sputtering technique of preparing Ca₂Si thin films is introduced in detail, and the operating principles and characteristics of the X-ray diffractometer, scanning electron microscope, atomic force microscope, spectroscopic ellipsometer and four-probe meter are introduced briefly.Secondly, Effects of annealing temperature on the growth of the Ca₂Si thin films on p-type Si (100) substrates are studied. Results show that 800℃is optimal thermal processing temperature to grow Ca₂Si thin films when Ca thin films are deposited on Si substrates and annealed for 60 min, and large grains are observed. The optical properties of Ca₂Si are studied by spectroscopic ellipsometry measurements in the photon energy of 1. 5eV-4. 5eV, and the refractive index(n) and extinction coefficient (k) for the Ca₂Si thin film at different annealing temperatures for 60 min are obtained. The refractive indexes of four Samples at different annealing temperatures reach their mini mum at～4. eV. For each sample, the refractive index(n) first decreases and then increases with increasing photo energy, whereas the extinction coefficient (k) changes with photon energy in a contrary way to that of the refractive index (n). The extinetion coefficients of four Samples at different annealing temperature reach their maximum at～3.3eV. The four-probe measurements show the resistivity increases with increasing annealing temperature.Effects of anneal ing time on the growth of the Ca₂Si thin films are also studied. XRD results show single phase Ca₅Si₃ thin films are grown on n-type Si (111) substrates , and single phase Ca₂Si thin films on p-type Si (100) substrates when the Ca thin films are deposited on different Si substrates at an annealing temperature of 800℃. The dielectric constants, refractive index(n) and extinetion coefficient (k) for the Ca thin films on p-type Si (100) substrates with different annealing time at 800℃are obtained. The refractive indexes of four Samples with different annealing time reach their minimum at～4.3eV. For each sample,the refractive index(n) first decreases and then increases with increasing photo energy, at the same time , the extinetion coefficient (k) changes with photon energy in a contrary way to that of n. The resistivities of the samples with different annealing time on p-type Si (100) substrate are studied by four-probe measurements. The results show the resistivity increases with increasing annealing time. The extinction coefficients of four Samples with different annealing time reach their maximum at～3. 3eV though the extinction coefficients are a little complex.Finally, the energy-band structure and optical properties of semiconductor Ca₂Si are studied using the first-principle psendopotential plane-wave method within the density functional theory. The results indicate that both the cubic and the orthorhombic Ca₂Si are direct band gap semiconductor, and the band gap of the orthorhombic Ca₂Si with GGA and LDA is 0. 5663eV and 0. 4117eV , respectively, whereas the band gap of the cubic Ca₂Si with GGA and LDA is 0.3193eV and 0. 1365eV ,respectively. The analysis of the states indicates that the valence bands of Ca₂Si are composed of Ca 4s, 4p, 3d as well as of Si 3p. The conduction bands are mainly composed of the Ca 4s, 4p, 3d states and the contributions of Si 3s, 3p to the conduct i on bands are relatively small. The results of theoretical calculations of optical properties of the Ca₂Si basically agree with those measured by spectroscopic ellipsometry in the photon energy of 1. 5eV-4. 5eV for all samples.