Preparation And Properties Of Boron Nitride Material With Large Thickness For Neutron Detection

In recent years,the research on the wide band gap semiconductors of the Ⅲ-Ⅴ group has been in full swing.Among them,hBN,a new type of III-V semiconductor material which has a wide band gap,has attracted lots of attention in recent years due to its outstanding physical and chemical properties.hBN itself has low dielectric coefficient,great resistivity,and excellent thermal stability,radiation resistance and chemical stability.The band gap of hBN is about 5.97e V,its intrinsic absorption limit is at 210 nm.hBN has very high absorption coefficient which have great application prospects in deep ultraviolet photoelectric detection field.Because the thermal neutron absorption cross section of ¹⁰B is as high as 3840 barn and the natural abundance is 19.8%,hBN can be used in the domain of neutron detection,and can realize the direct detection of neutrons without scintillator or neutron conversion layer,which is a characteristic that most semiconductor materials do not have.At present,the relevant research on the hBN neutron detector at home and abroad is just getting started,and the research on the growth of large thickness,large area and high-quality hBN bulk materials is still very scarce.In this paper,the growth process of large thickness,large size,and high-quality hBN bulk materials has been deeply studied,and its feasibility in the domain of neutron detectors has been explored.In this paper,the main achievement are as follows:1.High quality hBN thin films were prepared on sapphire substrates by high temperature and low pressure chemical vapor deposition（LPCVD）.The experimental results showed that the optimum growth parameters of hBN thin films were:growth temperature is 1350℃,BCl₃=10 sccm,NH₃=30 sccm,N₂=100 sccm/100 sccm（as carrier gas of BCl₃ and NH₃ respectively）,working pressure is 200 Pa.Under this condition,XRD shows the FWHM of hBN（002）2θdiffraction peak is 0.44°,and the FWHM of E_2g mode vibration peak in Raman spectrum is 29.10 cm^-1.After calculation,the crystal plane spacing d of hBN is 3.33（?）,and hBN average grain size L_c parallel to the c-axis direction is 20.1 nm,the average grain size L_a parallel to the a-axis direction is 69.5 nm.2.With other gorwth technology parameters unchanged,the growth of large thickness and high quality hBN bulk materials was realized by increasing the concentration of reaction source（BCl₃=20 sccm,NH₃=60 sccm）,prolonging the growth time（2-10 h）,and adopting the variable temperature growth mode.The grown hBN bulk material sample has good flatness and transparency,and the maximum thickness is 243μm.The fastest growth rate is 30μm/h.By adopting the variable temperature growth mode,the upper surface temperature of hBN is close to constant,and the quality of hBN is guaranteed during the long time growth.The crystallization quality of hBN is further improved by the post-annealing process.The FWHM of the2θdiffraction peak of（002）crystal plane in the XRD pattern is 0.65°.According to the calculation,the crystal plane spacing d of the large thickness hBN material is 3.33（?）,and the average grain size L_c in the c-axis direction is 10.3 nm.3.Track,energy loss,range,concentration distribution and vacancy damage caused by secondary charged particles(αand ⁷Li particles produced by thermal neutron and ¹⁰B nuclear reaction)in hBN are studied by SRIM.According to the simulation results,due to the high energy of the secondary particles in the material at the beginning,the track is close to a straight line,and after the energy is reduced to a certain extent,the probability of collision with atoms is greatly increased,and the direction of motion of the particles has changed significantly.The range of four particles with different energy in hBN is 4.76μm/3.88μm/2.40μm/2.14μm.The energy loss rate of particles with higher energy first increases and then decreases,and the resulting Bragg peak is clearly observed.At this time,the maximum ionization energy loss rate is 27.5 e V/（?）;⁷Li particle has lower energy,so its ionization energy loss rate decreases in a nearly linear manner.At the same time,the secondary charged particles will also have elastic collision with B and N nuclei to produce recoil nuclei and vacancies,and the recoil nuclei will continue to have cascade collision to produce more vacancies,causing certain damage to the material.4.Based on the optimized growth of hBN bulk material,hBN deep ultraviolet photodetector was prepared.The I-V characteristics of hBN deep ultraviolet photodetectors were tested by using a pen shaped mercury lamp,and the effects of different metal electrodes/electrode fabrication processes on the detector performance were compared.The product of carrier lifetime and mobility（μτ’s figure of merit）and surface composite field s/μwas obtained by using Many’s equation fitting.The results show that the detector using Au as electrode material,mask evaporation as electrode preparation process and rapid annealing furnace for electrode annealing has the best performance.The value ofμτand s/μare 1.44×10^-4cm²/V and 5.03×10³V/cm.