Research On Growth Of High-quality Hexagonal Boron Nitride Films On Diamond Substrates

Hexagonal boron nitride(hBN)is an artificial compound with a structure similar to graphite.Their lattice mismatch rate is only 1.6%,so hBN is considered to be an optimal substrate for graphene.hBN is also an ultra-wide band gap semiconductor(Eg?6 eV)with distinguished chemical and thermal stability,and has considerable application potential in high-power electronic devices and deep-ultraviolet photoelectric devices.Diamond is a carbon-based material with great development potential in the future and has many excellent properties similar to hBN.Therefore,diamond is chosen as substrates to epitaxially grow hBN films to form a heterojunction in this paper,which makes it possible for the corresponding high-frequency,high-power,high-temperature and low-power loss heterojunction devices.Considering that the lattice mismatch between diamond and hBN is very large,and the surface of diamond can be graphitized under high temperature.Since hBN can be used as an excellent substrate for graphene,conversely,the graphite on the diamond surface can also be used as a buffer layer to improve the crystal quality of hBN.Therefore,the growth of high-quality hBN films on graphitized diamond substrates was investigated in this paper.The main research contents and research results are as follows:(1)The graphitization processes of diamond were investigated.The research results show that the heating and cooling rate and the substrates heating temperature will affect the graphitization process of diamond.The faster the heating or cooling rate is,the more favorable it is for the transform from metastable diamond to more stable graphite.When a relatively slow heating and cooling rate is maintained,the diamonds are not graphitized if the temperature is below 1200?.As the temperature is further raised,the phenomenon of graphitization becomes more and more obvious.When the temperature is raised to 1600?and above,the graphitization process will be significantly accelerated,and the square resistance of the diamond surface will be less than 700?/?.Raman spectra show that the intensity of the G band of graphite is also significantly enhanced,and the graphite can attach to the diamond surface uniformly.After heated at 1600?for 10 min,the surfaces of the diamond substrates can be fully graphitized.The thickness of the surface graphite is estimated to be about16.8 nm,making it an ideal substrate for subsequent growth of hBN films.(2)The growth processes of hBN films were investigated.First of all,the diamond substrates were heated at 1600?for 10 min to obtain the deliberately graphitized substrate(DGS),and then the hBN films were grown on DGS.The influences of growth temperature,fluxes of N₂and precursors,carrier gas types,annealing processes on the crystalline quality of hBN films were investigated.The optimum process parameters for hBN films growth are:BCl₃=10 sccm,NH₃=30sccm,N₂=100 sccm,the growth temperature is 1400?,the working pressure is 200Pa,and the growth time is 3 h.By using this process parameters,we have obtained a high-quality hBN film with a thickness of about 10?m.According to calculated results,the distance d between the adjacent hBN(002)basal planes of the film is3.335(?),the average grain size L_calong the c-axis direction is 25.2 nm,and the average transverse grain dimension L_ais 65.0 nm.After annealing,d,L_c,and L_aare optimized to 3.332(?),29.9 nm and 70.1 nm,respectively.In addition,we used the above process parameters to compare the crystal quality of hBN films grown on DGS and directly on diamond substrates,it shows that hBN films with higher crystalline quality can indeed be grown on DGS.DGS can improve the crystalline quality of the films.(3)Based on the high-quality hBN films samples obtained by using the optimum process parameters,an hBN deep ultraviolet photodetector with metal-semiconductor-metal(MSM)structure was prepared.When the detector is irradiated with an ultraviolet-enhanced xenon lamp,the photocurrent increases linearly with the bias voltage.When the bias voltage is 350 V,the photocurrent exceeds 27 nA,and the light-to-dark ratio current is more than 2700.The photodetector also has obvious photoswitching behavior and short rise and decline times.It is further verified that hBN films grown on DGS have high crystalline quality.