| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 99 === Boron-aluminum nitride BxAl1-xNy(0.001≦x≦0.07, 0.85≦y≦1.0 5)films having wurtzite type structure are proposed. The material has higher hardness, higher sound velocity and wider band gap than hexagonal aluminum nitride. This study relates to a co-sputtered c-axis wurtzite BxAl1-xNy coating which is utilized for light emitting devices or surface acoustic wave devices.
First, the AlN-on-Diamond architecture is ideal for growing GaN by MOCVD for the manufacture of LED with very high power (e.g. 30 W). Due to the boron doping can shrink AlN lattice to make it closer to diamond. The BxAl1-xNy layer was used to bridge the gap of lattice mismatch between diamond and AlN. We demonstrated the viability by coating a diamond film with a buffer layer of oriented AlN that incorporated BN to enhance the texturing. The properties and growth mechanism of BxAl1-xNy film on diamond were discussed. In addition, the relationship between the microstructures and the sputtering condictions were discussed.
The results indicated that c-axis oriented BxAl1-xNy film could be prepared by co-sputtered system and the boron content in BxAl1-xNy film increased with increase of RF sputtering power. The lattice constant of BxAl1-xNy films were smaller than pure AlN, suggesting the substitution of smaller boron atoms at Al positions. Because diamond has a significantly tighter lattice than BxAl1-xNy, the initial deposition was under tremendous compressive stress, which prevented the formation of the crystalline phase. The BxAl1-xNy film on diamond substrates shows a continuous variation in structure, from randomly oriented nano grains to c-axis oriented columns. If the sputtered atoms have higher energy from target to a substrate, the growth films will obtain higher quality of c-axis orientation structure. Therefore, the thickness of randomly oriented nano grains decreased with increasing sputtering power, nitrogen concentration, substrate temperature and bias voltage. As the film was deposited under substrate bias voltage or at high nitrogen concentration, the BxAl1-xNy film with higher film quality than AlN was observed.
Second, SAW devices which exhibit a large piezoelectric coupling constant and a high SAW velocity property are needed in the future. In this study, c-axis orientated BxAl1-xNy film on 128° Y-X LiNbO3 and diamond substrate were used to boost the SAW operation frequency. The resulting films exhibit a higher piezoelectric coefficient d33 and higher Young』s modulus than AlN films. Moreover, the greater rigidity of BxAl1-xNy film further boosts the resonance frequency of LiNbO3 and diamond SAW device. The LiNbO3 SAW velocity increases along with the films thickness (at a fixed IDT wavelength), but the K2 value shows a slight decrease. Considering the SAW wavelength (?=2 ?m), the center frequency and surface acoustic velocities of BxAl1-xNy on diamond are 4.43 GHz and 8860 m/s. These results are better than that on AlN/diamond devices. Furthermore, the K2 value (0.5%) of BxAl1-xNy on diamond SAW device showed a good performance.
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