Study On The Epitaxial Growth Of Hexagonal Boron Nitride Films And Its Related Deep Ultraviolet Photoelectric Response

Along with the growing demands in defense,aerospace,information and communication technology,automotive,and consumer electronics,etc,the wide-band semiconductor market has shown a better development prospect.Researchers are increasingly interested in the research of wide-band semiconductor materials.As a promising wide-band semiconductor material for ultraviolet(UV)and deep-UV optoelectronic devices,hexagonal boron nitride(hBN)has attracted much attention due to its unique crystal structure,superior physical and chemical properties.In the early years,hBN powders and ceramic materials were widely used due to their excellent thermal conductivity,chemical stability,refractory properties,insulating properties,and lubricating properties.While the hBN grains not only are small in size but also do not exhibit specific selective orientations.Therefore,it is compulsory to improve the hBN crystal quality so as to be used as optoelectronic functional materials.At present,most of the research hotspots of hBN are focused on the synthesis of two-dimensional(2D)materials and the fabrications of hBN-based heterojunction devices.However,two-dimensional hBN has limitations in deposition rate,transfer,device fabrication,and mechanical strength,etc.When the 2D-hBN are used for dielectric materials or dielectric gate layers,high tunneling currents will be induced,and the 2D-hBN based devices will not meet usage requirements.When hBN are used for neutron detectors or deep-UV photodetectors,the thickness of the material needs to be on the order of microns.If it is used as a substrate material for other devices,the thickness needs to be further increased to provide good support.Also,in order to promote the scale-up of hBN based devices,reducing the cost of epitaxial growth of hBN is the basis for industrialization.Therefore,it is essential to do some research on preparation of hBN films with large sizes,good quality,high growth rate,micron-scale thickness,and low-cost,as well as on the hBN based optoelectronic devices.In this thesis,a series of studies on the hBN growth process,the improvement of hBN crystal quality,the buffer layer technology for hBN film preparation,the annealing process,and hBN-based solar-blind deep-UV photodetectors were carried out using a high-temperature low-pressure chemical vapor deposition(LPCVD)system for direct epitaxy of hBN films on non-catalytic substrates(silicon,sapphire,etc.),with the following details and main results obtained.1.Direct epitaxial growth of hBN on non-catalytic substrates was investigated.Since silicon(Si)is the most commonly used substrate material for semiconductor devices,the epitaxial growth of hBN films on Si substrates is of practical significance.Using(100)Si as the substrate,boron trichloride(BCl₃)and ammonia(NH₃)as the reaction source,high-purity nitrogen(N₂)as the carrier gas and dilution gas,and a working pressure of 100 Pa,hBN films with a thickness of 2.3?m and the preferred c-axis orientation were prepared at 1200°C with the growth temperature optimization.In order to explore the effect of higher growth temperature on the quality of hBN films,the growth process of hBN films on sapphire substrates was performed.The effects of process conditions such as growth temperature,flow rate ratio of group III/V gas source,growth pressure,and carrier gas type on the crystalline quality,surface morphology,and material properties of hBN thin films were systematically investigated by the controlled variable method.The optimized process parameters for direct deposition of hBN on sapphire substrates were finally obtained,that is,using N₂ as the transport gas and dilution gas,a growth temperature of 1350°C,a gas flow ratio of 1:3 for BCl₃ and NH₃,and a gas pressure of 100 Pa in the reaction chamber.2.The growth of hBN thin films on buffer layers was studied.The hBN epitaxial films were grown on the buffer layers of chromium(Cr),tungsten(W),and aluminum nitride(Al N),respectively.The results show that the buffer layer can improve the crystalline quality and reduce the internal stress of hBN films,and the Cr buffer layer has the best effect on the hBN quality.The sputtered Cr buffer layer has(110)crystalline plane selective orientation.In the X-ray diffraction(XRD)?-2?scan characterization results,the position of the(0002)crystalline plane diffraction peak of the hBN film grown on the Cr buffer layer is closer to the standard peak position than that of the sample grown without the buffer layer,and the full width at half maximum(FWHM)decreases by about 55.6%;the Raman spectral characterization results show that the center of E_2g vibration peak of the hBN film is only 3.5 cm^-1 blue-shifted from the standard peak.The blue-shifted wave number is reduced by about 65%.W and Al N buffer layers also facilitate improving the hBN crystal quality and reducing the internal stress of the film.3.The effect of the high-temperature annealing process on hBN films was studied.In order to further improve the crystalline quality of hBN films and release the residual stresses inside the films,the effects of process conditions such as annealing temperature,annealing time,and annealing atmosphere on the crystalline quality and material properties of hBN films were systematically investigated.The results show that the high-temperature thermal annealing process can significantly eliminate the internal defects of the films,promote the fusion of grain boundaries,and effectively improve the optical properties of hBN films.The optimized annealing process conditions are annealing at 1700°C for 10 min under N₂ atmosphere.After the optimized annealing process,the FWHM of the Raman E_2g vibration peak of the hBN film is about 21.8 cm^-1,the transverse crystal domain size is about 108 nm,and the optical band gap is about5.79 e V.Using the high-temperature in situ thermal annealing technique,high-quality hBN films grown along the c-axis were prepared on sapphire substrates.The FWHM of the XRD diffraction peak of the hBN(0002)crystal plane is only 0.18°,the 2?/(?)scan of the hBN(101(?)1)crystal plane shows perfect sixfold symmetry,the FWHM of the E_2gRaman vibration peak is only 12.8 cm^-1,the dielectric strength of the film is up to 7.6MV/cm,and the absorption edge and optical band gap are 213 nm and 5.81 e V,respectively.Furthermore,a method of mechanically peeling the hBN film with a thickness of several microns from the 2-inch sapphire substrate using thermal stress was invented.The peeled hBN film is self-supporting and has good flexibility.4.The hBN-based solar-blind deep-UV photodetectors were fabricated,and their photoelectric characteristics were examined.A metal-semiconductor-metal(MSM)structured deep-UV photodetector was developed based on the prepared silicon-based hBN film.Under a bias of 20V,the dark current of the device is less than 10 p A,the ratio of light to dark current is about 312.The deep-UV photodetectors were fabricated based on hBN films treated with a high-temperature thermal annealing process.After improving the crystal quality of hBN films,the ratio of photocurrent to dark current at20 V bias was more than 2890,which was 9 times higher than that of unannealed hBN film based deep-UV photodetectors.Vertical structured deep-UV photodetectors were manufactured based on hBN films grown on Cr buffer layers.Cr was used as both a buffer layer for depositing hBN and a bottom electrode for the devices.The test results showed that the devices had an excellent photoelectric response and switching characteristics.The flexible hBN deep-ultraviolet photodetector was developed based on the self-supporting hBN film.The observed light and dark currents of the detector hardly changed under the bending states compared to the spreading ones,showing good stability and reliability of the photodetector.