Research On Epitaxial Growth And Properties Of Hexagonal Boron Nitride By MOCVD

As a group Ⅲ nitride semiconductor with two-dimensional characteristics,hexagonal boron nitride（h-BN）has become a research hotspot because of its wide band gap,good thermal conductivity,chemical stability,thermal stability,and dielectric properties.It has a wide range of research value in the fields of ultraviolet optoelectronics,quantum optics,and new electronic devices.Nowadays,research on h-BN films mainly focuses on metal substrates.However,the growth mechanism and characteristics of h-BN films on insulating substrates such as sapphire need to be further studied.Therefore,in this paper,the growth mechanism and related characteristics of h-BN films were explored in depth using a metalorganic chemical vapor deposition（MOCVD）system on sapphire substrates,providing a theoretical basis for the epitaxy and properties of h-BN.Meanwhile,the applications of h-BN as a released layer and in-situ dielectric layer were innovatively studied to promote the development of h-BN applications.Specific research contents are as follows:（1）The epitaxial growth mechanism of h-BN on the sapphire by MOCVD system was investigated.With the aim of obtaining the key points of epitaxy of h-BN,the effects of growth temperature,pressure,and Ⅴ-Ⅲ ratio on the growth of h-BN were studied in the continuous growth mode and pulse growth mode.The growth conditions of h-BN at conventional temperature were explored.Compared to continuous growth mode,pulsed growth mode can improve the crystal quality and surface morphology of h-BN films by inhibiting gas phase prereaction.Meanwhile,the growth process and surface morphology of h-BN films were analyzed using the pulse method,and the growth stage change model of h-BN films was proposed.This provided a theoretical basis for the growth of h-BN.（2）For the MOCVD-grown h-BN film,the material properties of the h-BN film were studied.The h-BN grown in pulse growth mode contains certain C impurities introduced by triethylboron（TEB）unintentionally.By experiments and the first-principles calculation of the energy band,the position of the C atom occupying the N atom in the film was determined.The luminescence of zero phonon line（ZPL）and phonon sideband（PSB）caused by vibration of the C impurity defect appears in h-BN by photoluminescence spectroscopy（PL）,which also proves the potential of h-BN grown by MOCVD as a single photon emitter substrate.This provides guidance for the device application of h-BN.（3）The band alignment between h-BN andε-Ga₂O₃,GaN,and AlN was investigated,providing a theoretical basis for the application of h-BN in optoelectronic devices.The valence band（△E_V）and the conduction band offsets（△E_C）were calculated.The△E_V and△E_C of ε-Ga₂O₃/h-BN heterojunction were -1.60 eV and 2.77 eV;The△E_V and△E_C of GaN/h-BN heterojunction were -0.39 eV and 2.67 eV;The△E_V and△E_C of AlN/h-BN heterojunction were -1.35 eV and 0.93 eV,respectively.All of them were band type-Ⅱ（staggered-gap）heterojunctions.Provide guidance for the application of h-BN to the device.（4）The nucleation growth mechanism of AlN and GaN on the h-BN film was studied in detail as the release layer and buffer layer.Due to the lack of hanging bonds on the surface of the 2D h-BN,Ga and Al migrated rapidly across h-BN surface,while GaN was difficult to nucleate on the h-BN.Because of the stronger bond energy of AlN,AlN nucleation can be achieved on the h-BN.The fast migration rate of Al atoms made it difficult to anneal the nucleating layer of AlN,and the nucleating islands aggregated into sheets.Because of the large number of dislocations and grain boundaries in the further epitaxial AlN and GaN,it was difficult to merge,resulting in more holes on the surface.The quality of upper GaN can be improved by adjusting the growth conditions of the AlN nucleation layer to optimize the nucleation layer.Finally,the electronic mobility of the epitaxial AlGaN/GaN heterojunction was 1617 cm²·V^-1·s^-1,and the carrier concentration was about 1×10¹³ cm^-2.Both the GaN and AlN films can be easily released from the substrate by the h-BN release layer.（5）The lossless vdW substrate transfer technology based on the h-BN release layer was explored.A large-scale wafer-level flexible vacuum UV MSM photodetector based on a porous AlN film had been successfully fabricated.The device performance did not deteriorate significantly after substrate transfer.Under the light intensity of 275μw/cm²,the wavelength of 185 nm ultraviolet irradiation,the photo-to-dark current ratio（PDCR）was approximately 10³ and the responsivity can reach 46 mA/W.In addition,the device can work well under bending conditions.The maximum curvature it can withstand can reach 1/4 cm^-1.Provide guidance for lossless vd W transfer techniques.（6）Innovatively,the properties of the h-BN dielectric layer grown in situ MOCVD were studied.By controlling growth conditions of AlGaN and h-BN,the decomposition of AlGaN was inhibited and the in-situ h-BN passivation layer was deposited on the AlGaN/GaN heterojunction successfully.The GaN MIS-HEMT devices were fabricated to characterize the dielectric capacity of h-BN films.The h-BN film may affect the threshold of devices due to the appearance of h-BN film delamination.The high temperature during h-BN growth may lead to a slight deterioration of the AlGaN barrier layer,thus affecting the dynamic performance of the device.In addition,the h-BN dielectric layer has the same excellent breakdown field strength characteristics as the LPCVD-SiN_x dielectric layer.Provide experimental reference and theoretical basis for the study of in-situ deposition and properties of h-BN dielectric layer.