Research On Growth Of Hexagonal Boron Nitride Films With Transition Metals As Buffer Layers

Hexagonal boron nitride?h-BN?is a synthetic semiconductor material with a graphite-like structure.The lattice mismatch ratio of h-BN to graphene is only 1.7%,so h-BN is considered to be used as an excellent substrate material for the growth of graphene.As a wide bandgap semiconductor,h-BN has excellent properties such as high resistivity,superior thermal endurance and radiation hardness.Besides,its dielectric strength is up to 8MV/cm and the intrinsic absorption edge is located in the deep ultraviolet region.In virtue of those distinguished characteristics,h-BN is a prospective optoelectronics and electronics material.However,the research of the growth process of h-BN films is still in the exploration stage,resulting in h-BN films with poor crystal quality and small sizes.Therefore,the study of improving the growth process to obtain high-quality and large-area h-BN films has attracted extensive interests from researchers.In this paper,we develop a novel growth method of h-BN films,which is using transition metals as buffer layers to grow h-BN films.Besides,the effects of growth temperature,annealing process and the type of buffer layers on the quality of h--BN films were studied.The main research contents and results are as follows:1.The effect of different transition metal as a buffer layer on the crystallinity of h-BN films was investigated.The h-BN films were respectively prepared on Cr,Mo and W buffer layers and sapphire substrates by low pressure chemical vapor deposition.The results of X-ray diffraction?XRD?show that the h-BN film has more excellent crystal quality grown on the buffer layer than that directly on the sapphire substrate,and we obtained the best crystalline quality h-BN films grown on Cr buffer layer,the corresponding?002?peak is located at 26.67°with a full width half maximum?FWHM?of 0.617°.2.The effect of growth temperature on the crystallinity of h-BN films was explored.h-BN films were individually grown on Cr,Mo and W buffer layers and directly on sapphire substrates at temperatures of 1200??1350?and 1450?.The results of XRD show that the improvement in the crystallinity of h-BN films can be achieved by increasing growth temperature,and the trend is independent of substrate selection.Therefore,we get the best crystalline quality h-BN films at the growth temperature of 1450?.3.The effect of quality of Cr as a buffer layer on the crystallinity of grown h-BN film was probed.Cr buffer layers were prepared on the sapphire substrates at different sputter temperature and subsequently thermal annealed,then the h-BN films were synthesized on Cr at growth temperature of 1450?.The results of XRD patterns and Raman spectra show that a significant improvement in the crystallinity of h-BN films can be achieved by grown on the high-quality Cr buffer layer.Under the optimized growth conditions,the peak of XRD is centered at 26.67°with a FWHM of0.617°,and the peak of Raman spectrum is closed to 1370.5 cm^-11 with a FWHM of45.5cm^-1.4.The effect of annealing process on the crystallinity of h-BN films was studied.hBN films were thermal annealed at a high temperature of 1650?.The results of XRD patterns and Raman spectra show that the FWHM of XRD peak falls to 0.422°and that of Raman peak shifts to 25.2 cm^-1.Therefore,thermal annealing at high temperature can succeed in improving the quality of h-BN films.