Growth of Cubic Boron Nitride by Plasma Assisted Chemical Vapor Deposition on Silicon, Silicon Carbide and Diamond Substates
Michael Spencer
Morgan State University – Baltimore, Maryland, USA
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Cubic Boron Nitride (cBN) is an ultra-wide bandgap semiconductor that has many excellent material properties, including an indirect bandgap that allows N and P type doping, high-predicted breakdown field high predicted electron saturated high thermal conductivity and material hardness second only to diamond. The indirect bandgap together with a high breakdown field give the material a high Baliga Figure of Merit making it attractive for next generation power conversion devices while the high saturation velocity coupled with the high thermal conductivity give the material a high Johnson Figure of Merit indicating its potential for applications in high power high frequency devices. In this work we report on the growth of cubic boron nitride using a custom designed Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor. The Electron Cyclotron Resonance (ECR) plasma was formed using a permanent magnet. The growth chemistry consisted of Boron Trifluoride (BF3), Nitrogen (N2), Hydrogen (H2) together with inert gases of Helium (He) and Argon (Ar) (to sustain the plasma). The growth was performed at growth pressures of 1-10mTorr and growth temperatures of 700-9000C. During the growth a bias voltage of -50V was applied to the substrate holder resulting in a plasma ion current of 60ma or 100ma depending on the growth conditions. Silicon (Si (111) and Si (100)), Silicon Carbide (4H-SiC and 3C-SiC) and single and polycrystalline diamond substrates were loaded together during an individual run. Using gas flow rates of .25 sscm (BF3), 12.4 sscm (N2) 1 sscm (H2) 34 sscm (He) 2.5 sscm (Ar) growth of Boron Nitride was obtained on all the substrates. Using optical interreference techniques the thickness of the BN films was measured as .1-.3um indicating a growth rate of 50nm to 100nm/hr. depending on the amount of BF3 flow. Energy Dispersive X-Ray analysis (EDX) was performed in an electron microscope on the grown substrates and on small commercial platelets used in cutting applications. The results from EDX study showed that B/N ratio of ~2:1 was present on the commercial sample as well as the films grown on the polycrystalline diamond substrates, a boron nitride signature was detected on all substrates on which growth was attempted. The surface morphology was studied using optical and scanning electron microscopes (SEM) as well as atomic force microscopy (AFM). Films grown on Si, 3C-SiC and diamond showed severe delamination, but stable growth was obtained on polycrystalline diamond and 4H- SiC substrates. Fourier Transform Infrared Reflectance (FTIR) measurements were made on all the samples. FTIR measurements on polycrystalline diamond substrates in the best cases showed a 100% cBN signature. Other samples on polycrystalline diamond showed a mixed hexagonal boron nitride (h-BN), cBN signature.
Email: michael.spencer@morgan.edu