Preparation and Field Emission Characteristics of Boron Nitride Films on Various Substrates
Post Date: 27 Dec 2010 Viewed: 541
Boron nitride has a lot of excellent properties and an extensive application foreground as a kind of wide band gap semiconductor material. It primarily has two kinds of crystalline constructions: sp2 hybridized structurecorresponds hexagonal boron nitride (h-BN) and sp3 hybridized structure rr corresponds cubic boron nitride (c-BN). h-BN is similar with graphite carbon, ,it has good thermal stability, thermal conductivity, good lubricity. So it could be an ideal electrical insulating material. And c-BN is appropriate to be used in radio resistance and ultra high temperature high pressure technique fields. There are some reports that h-BN have a high sound wave transmission velocity and excellent light transmittance. That"s means that it"s a good candidate for surface acoustic wave device substrate. Cubic boron nitride has very high hardness (only smaller than diamond), little friction coefficient, high thermal conductivity, excellent chemical stability and anti-oxidation at high temperature. It enables c-BN an ideal hard tool coating. c-BN film also has extraordinary electrical performance, such as good insulativity, wide band gap and negative electron affinity which was found recently. It have an extension application foreground in high-temperature, large power electrical device and field emission flat display or other new type electrical device. i"A lot of researchers successively report various physical vapor deposition and chemical vapor deposition techniques to obtain c-BN film or film that containing c-BN, such as radio frequency sputtering deposition(RF sputtering), ion beam assisted deposition(IBAD), ion plating , pulse laser ablation deposition(PLD), plasma assisted chemical vapor deposition(PACVD). Atpresent, there are some problems in synthesizing c-BN films: 1. Nucleation and growth mechanism is not very clear. 2. Poorly adhesion. 3. Deviation of stoichiometric composition. 4. Interlayer between cubic phase and substrate and lamella upon cubic phase which is sp2-bonded. This limits the application in super hard coating materials and electrical filed. 5. It is commonly little as a few tens nanometers for c-BN crystal grains grown by various techniques and this restricts its application.We deposit a series of boron nitride films on molybdenum and NiMnCo alloy substrates by changing deposition conditions. Fourier transform infrared spectroscopy (FTIR) indicates that all of films have the same structure of hexagonal construction. It have a strong influence for growth of films such as Sputter power, bias voltage, deposition pressure, gas component, deposition temperature and deposition time. The results of serial experiments indicate that it is good for film growth in such conditions: bias voltage set at -50V, deposition power at 150W, deposition time lasted 120min, total pressure is 4Pa, Na percentage is 50%, substrate temperature set at 500 C. BN films growth on molybdenum substrates have a better quality than films growth on NiMnCo alloy substrates.BN films deposited by magnetron arc plasma enhanced chemical vapor deposition. Deposition pressure have a range of 50~100Pa, N/B flux ratio is 25:1 or 50:1, bias voltage various from 0V to -150V, substrates temperature fixed at 900 C, deposition lasted 10~30min. the structure of as-grown films is found to be hexagonal boron nitride uniformly by FTIR analysis. With longer deposition time, higher substrate temperature and higher N/B flux ratio, crystalline is improved. It is also found that bias voltage have a negligibleeffect on film growth.Cubic phase appeared when arc current was increased from 10A to 18A, it can be concluded that arc current is very important for synthesizing c-BN. otherwise, appropriate N/B flux ratio and substrate temperature can accelerate nucleation and growth of c-BN. it should be noted that bias voltage is free when deposition.