| Boron nitride (BN) is a widely used force, thermal, optoelectronic functional materials. There are several allotrope phases for BN. Cubic boron nitride (cBN) films have attracted most attention for wide technical applications due to its excellent physical and chemical properties. Low-cost synthesis of high-purity, stress free and large-sized cBN films is still challenging. In addition, due to the lack of large-sized high-quality samples synthesized, the intrinsic properties of orthorhombic boron nitride (oBN) are rarely examined, and consequently, it strongly limits the characterization and applications for oBN. Both physical and chemical vapour deposition methods used for the preparation of the BN films are covered. To realize cBN films, various substrates (such as Si, diamond, etc.) were used as the substrate for BN films growth. The structure and properties of substrate materials has important influence on the growth of BN films, therefore, choice of substrate materials is the key to preparation of high-quality BN films.In this thesis, cBN and oBN films deposited on mainly c-axis oriented graphite substrate by radio frequency (r.f.) magnetron sputtering (MS) were studied. The growth mechanism, electrical and optical properties have been investigated. Some important results are obtained.1. BN films (~1.2 ?m) with a high concentration of cubic phase (~90%) have been successfully deposited on graphite performed at room temperature and zero bias by r.f. MS technique, the corresponding growth rate is ~10 nm/min.2. UV-vis transmission measurement was performed, the optical band gap Eg=5.8 eV estimated by the formula. Field emission results show that the graphite substrate prepared cBN film has excellent field emission properties. It is found that a turn-on field is about 11.5 V/?m, and the emission current density is about 2.8×10-5 A/cm2 as the applied field performed at 30V/?m. Comparably, the current density nearly 5 times greater than that of the cBN film deposited on Si substrate. It is accepted that the improved field emission characteristics of cBN are related to its NEA and sp3 bonding constitutions. In addition, the nanostructural feature also leads to a significant enhancement of field emission. Besides these factors, the nanostructural feature and the metallic graphite provide more electrons injected into cBN layer also leads to a significant enhancement of field emission.3. By modified the experimental conditions such as the r.f power and substrate temperature, we have been synthesized pure and translucent freestanding oBN films with large area at first time by multiple periodic deposition process. The film is ~40 ?m thickness and corresponding growth rate is ~0.6 ?m/h.4. The analysis results of MDI Jade 5.0 showed that the oBN film is orthorhombic structure, and the cell parameters are a = 12.076 ?, b = 7.501 ?, c = 11.055 ? for the product deposited on graphite substrate. UV-vis measurement showed that the optical band gap of oBN is ~3.43 eV. Photoluminescence measurements show that besides the ultraviolet near-band-edge radiative recombination, there are visible emissions arisen (blue, green and red) from defect-related deep-level centers of oBN. It emits multicolor lights whose mixing produces white light. The hardness of the oBN film is estimated to be 11.5 GPa, similar to the value of silicon.
|
PDF Full Text Request