Towards Growth of High Quality Cubic Boron Nitride using Fluorine Chemistry

Boron Nitride (BN) is an ultra-wide bandgap material having a bandgap of 6.4 eV and 5.9 eV for the cubic (c-BN) and hexagonal (h-BN) phases, respectively. The ultra-wide bandgap of both BN phases indicates a high breakdown voltage, possibly even greater than diamond [1]. Moreover, epitaxial c-BN on diamond is expected to have high thermal conductivity. Diamond and c-BN have a small lattice mismatch of 1.4 percent [2]. Therefore, diamond is a candidate for use as a substrate for growth of c-BN. However, the choice of an appropriate substrate among different types of diamond as well as crystallographic orientation can affect the quality of the grown c-BN films. Also, depending on the growth technique, it might be essential to use either p-type or n-type diamond to achieve high-quality c-BN films. The last but not the least in growing high-quality c-BN films is the method utilized to clean the substrate which is of great significance, too.<br/>Physical vapor deposition (PVD) is one of the techniques that has shown success in growing c-BN layers. Magnetron sputtering, ion-assisted pulsed laser deposition (IAPLD), ion beam-assisted deposition (IBAD) and mass selected ion beam deposition (MSIBD) are some methods under PVD category that have been exploited to grow pure c-BN [3]. However, utilizing high energy ion bombardment as well as high bias voltage is a must in such methods. Therefore, we should expect high compressive stress on the films which leads to defect formation and delamination of very thick films. Also, slow growth rate of PVD methods, few nm/hrs, prevent us from getting thick, pure c-BN films. To overcome the above obstacles Chemical vapor deposition (CVD) is also suggested for growing c-BN. These methods include DC jet plasma CVD, RF plasma CVD, inductively coupled PECVD and electron cyclotron resonance microwave plasma CVD (ECR MPCVD). The advantage of CVD over PVD is that ion bombardment and high bias voltage are not present to impose comressive stress and the resulting defects. Besides that, faster growth rate of CVD techniques opens up a lot of opportunities to get thick films of c-BN in short processing times [3]. Among the CVD methods, ECR MPCVD has been the most successful technique in getting thich and pure c-BN films due to the fact that processing is done in very low pressure environment down to even Torr. This reduces the contaminants that can interfere with the reaction and improves the precursors reaction on the substrate surface.<br/><br/>In this study we have explored achieving both thick and high quality c-BN films on polycrystalline boron-doped diamond at fast, 100 nm/hrs, and slower, 50 nm/hrs, growth rate. ECR MPCVD method in combination with Fluorine chemistry using H₂, BF₃, N₂, He and Ar gases was utilized. Substrate is negatively biased, i.e. -60 V, with respect to the grounded chamber to facilitate growth of c-BN layers. Fluorine atoms that manage to reach the substrate etch the sp² B-N bondings formed during the growth facilitating sp³ B-N bond formation. That is achieved by use of limited H₂ flow rate. On the other hand, Cleaning the substrate is critical in getting a high quality interface. High flow rate of H₂ plasma cleaning in ECR MPCVD chamber resulted in damage to diamond surface and that prevented us from getting smooth interface. However, when H₂ flow rate was reduced we observed fewer damage to the substrate and that improved the quality of c-BN/diamond interface.<br/>Then, we increased flow rates of the gas precursors while keeping gas ratios and applied voltage bias fixed. This resulted in growth of c-BN at a faster rate. The H₂:BF₃ ratios of 2:2 and 4:4 sccm resulted in 50 nm/hrs and 100 nm/hrs of BN, respectively. We then utilized in-situ X-ray photoemission spectroscopy (XPS) to confirm the presence of only sp³ bonding states at the surface of the grown film (Fig.1). Also, data acquired from ellipsometry confirms the growth rates of the two scenarios mentioned above.