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The two main polymorphs of boron nitride are hexagonal (h-BN) and cubic (c-BN) phases and both show attractive material properties to be employed under extreme environments. h-BN is also known as “white graphene” due to the matching hexagonal crystal symmetry and shows a layered structure which provides advantages as a lubricant material and as gate dielectrics for two-dimensional (2D) transistor channels. On the other hand, c-BN is analogous to another important elemental carbon material, diamond. While the mechanical strength of c-BN is lower than diamond, the thermal and chemical stability is even superior. Both h-BN and c-BN films are therefore strong candidates for applications under harsh ambient, including high-temperature, high-pressure, and corroding factors. BN films and coatings are conventionally grown using high-temperature CVD methods with typical substrate temperatures over 1000 oC. To reduce the deposition temperatures of this material while preserving the attractive crystal properties, alternative lower temperature physical and chemical vapor deposition strategies have been offered in the literature. Plasma-enhanced atomic layer deposition (PEALD) provides significant advantages over other methods such as precision thickness control, ultimate conformality, while reducing the growth temperature substantially. In our previous work, we had demonstrated the crystalline synthesis of h-BN films at substrate temperatures as low as 350 oC, while no growth was observed at lower substrate temperatures. In this work, we aim to demonstrate the feasibility of growing c-BN at substrate temperatures reaching 200 oC via a customized PEALD reactor featuring a large-diameter capacitively-coupled hollow-cathode plasma source with a relatively short source-to-substrate distance. We used Si(100), sapphire, and glass as the substrates to deposit the films. The growth temperature is kept between 150 and 250 °C, 10 sccm of N2 flow is used as a carrier gas and Ar-purging gas flow was kept at 50 sccm. The triethylboron (TEB) precursor and various nitrogen plasma chemistries including different compositions of N2, H2, and Ar gases are utilized. The optimized growth condition is determined from in-situ monitored saturation studies, where the growth-per-cycle (GPC) parameter is noted for different precursor pulse time (15 and 30 ms), rf-plasma power (100 W to 250 W) and plasma-gas chemistries (N2-only, N2/H2, Ar/N2, Ar/N2/H2 at varying ratios). While the 200 °C as-grown BN films exhibited nanocrystalline c-BN with relatively weak crystal peaks as verified by XRD measurements, vacuum annealing at 5 mTorr N2 flow and 700 °C transformed the film structure into single-phase polycrystalline c-BN. The spectroscopic ellipsometer, x-ray reflectivity (XRR), and x-ray photoelectron spectroscopy (XPS) studies will be presented as well as the influence of post-deposition vacuum annealing to further improve the crystalline structure of c-BN films. Basic electrical properties of the grown BN films will be characterized using metal-insulator-semiconductor (MIS) diode temperature-dependent current-voltage (I-V) measurements. A specific effort will be devoted to enhance the as-grown film crystal properties via implementing novel atomic layer plasma annealing processes, which showed its potential for the ultrawide bandgap Ga2O3 alloy. Another research question to be investigated is whether there is a possibility of tuning the material phase from cubic to hexagonal by carefully adjusting growth and in-situ Ar-plasma annealing conditions. |
