| Hexagonal boron nitride(h BN) is both an excellent thermal conductor and a good electric insulator. Since the lattices of h BN and graphene match very well, h BN is usually used as the substrate or the insulated gate of graphene devices. Moreover, h BN is also a promising material for the deep ultraviolet optoelectronic devices and surface acoustic wave devices. Therefore, the growth and preparation of high-quality h BN material becomes an attractive research topic. Due to rigorous growth conditions, it is difficult to synthesize high-quality and large-size h BN crystals. In order to obtain large-area h BN material, many chemical or physical technologies are used for depositing h BN films.However, h BN thin films are unstable sometimes, especially in moist air, which confines applications of h BN films. The phenomenon that h BN thin films were attacked by water vapor was observed in previous literatures. These researches mainly focused on the chemically vapor-deposited boron nitride(BN), and the hydrolysis of BN by moisture leads to the formation of ammonium borate hydrates or boric acid crystals. However, the stabilities of h BN films prepared by physically vapor-deposited were hardly reported. In this dissertation, the stabilities of BN films deposited on silicon by RF magnetron sputtering will be discussed.Four BN thin film samples(labeled as sample I, II, III, and IV) prepared by RF magnetron sputtering were kept in the air with the humidity of 80% for 24 hours, 48 hours, 72 hours and 96 hours respectively. Compared with the fresh prepared BN films, the FTIR of the sample I had obvious characteristic peaks at 720 cm-1, 1203 cm-1,1265 cm-1 and 3230cm-1 besides the IR absorption peaks at 761cm-1 and 1370 cm-1 which corresponded with TO modes of h BN. Here, the peaks at 720 cm-1 and 1265 cm-1 come from the absorption of boron oxide, and the peaks at 1203 cm-1 and 3230cm-1 correspond to the absorption of boric acid and the hydroxyl group respectively. It indicates that BN thin films absorb the water vapor in the moist air and contain the components of boron oxides and boric acids. The absorption peaks of h BN and boron oxides tend to weaken with the keeping time in the moist air, while the peaks at 1203 cm-1 and 3230cm-1 enlarge gradually, moreover, the peak at 1370 cm-1 is blue-shifted to 1420 cm-1 and enhanced. The peak at 1420 cm-1 is also from the absorption of the boron acid. That indicates more and more boron nitrides and boron oxides change into boric acids. According to the areas of peaks at 1265 cm-1, 1370 cm-1 and 1420 cm-1, the changes of the contents of h BN,B2O3 and H3BO3 in films were obtained. Based on these analyses, the primary cause of the instability of BN films is a great deal of oxygen impurities which induces many boron oxides with high water absorption in BN films, finally, boron oxides react with the vapor and form boric acid.In order to improve the stability of BN films, we applied the following methods: i> Sinter the BN target at 2000℃ to get rid of the oxygen impurity in the target; ii> Purify the growth chamber with the nitrogen; iii> Bake the inner walls of the growth chamber to remove the adsorptions. After these technology processes, the novel prepared BN films possess perfect stability and water-resistance. XPS results show that B atoms are bonded with N atoms directly, and a small quantity of C and O impurities mainly come from the surface adsorptions. The property of BN films did not change even after being soaked in deionized water. When the BN films grown on the substrate of silicon are soaked in deionized water for about four hours, the whole BN films will peel off the substrate and float on the surface of water, which is convenient for transferring BN films between different substrates.The h BN films prepared by RF magnetron sputtering are often nonstoichiometric, and there exist N vacancies in films. The BN materials synthesized by other methods have also similar problems. For solving this problem, the mixture of argon and nitrogen was used as the working gases, and the relation between the stoichiometry of BN films and the mixing ratio of the working gases was investigated. In experiments, the total gas flow was constant, and the mixing ratio of argon and nitrogen were set as 2:1, 1:1, 1:2, and 1:3 respectively. The results of XPS show that the content of N in BN films is increasing and O impurities are decreasing with raising the proportion of nitrogen in the working gases, and the stoichiometry of BN is close to 1:1. The crystalline phase of the prepared h BN films is pure, and the c axis orientation is preferential in the growth of h BN films. The prepared h BN films is transparent for the visible light, and its intrinsic absorption edge is less than 210 nm, and the absorption factor is larger than 105cm-1, and the optical bandgap is about 6.0 e V. The h BN films are P-type,and the p-h BN/n-Si heterojunction shows excellent rectifying characteristics, the rectification ratio can reach 105. |
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