【作者】 陈曦；

【导师】 宋俊峰；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2021， 博士

【摘要】 随着国防、航空航天、信息与通信技术、汽车和消费电子等众多行业需求的不断增长,宽禁带半导体市场展现出较好的发展前景,科研人员对宽禁带半导体材料的研究兴趣愈发浓厚。作为在紫外及深紫外光电器件领域具有良好应用前景的宽禁带半导体材料,六方氮化硼(hBN)由于其独特的晶体结构、物理和化学性质,引起了人们的广泛关注。起初,由于优异的热导率、化学稳定性、耐火特性、绝缘特性和润滑特性,hBN粉末和陶瓷材料得到广泛应用。此类hBN晶粒尺寸很小,且不需要有特定的择优取向。若想将hBN应用于光电功能材料领域,必须提高其晶体质量。目前,hBN的研究热点多集中在二维hBN的合成及异质结器件的制备。但二维hBN在沉积速度、转移、器件制备和机械强度等方面存在着一定的局限性。当用于介电材料或栅极介电层时,二维hBN可能会使隧穿电流过大,使器件不符合使用要求。当用于中子探测器或深紫外光电探测器时,hBN的厚度需达到微米量级。若作为其他器件的衬底材料,其厚度还需要进一步增加,以起到良好的支撑作用。同时,为了促进hBN器件的规模化发展,降低外延成本也是产业化的基础。从这些应用的角度来看,开展大尺寸、高质量、高生长速率、微米量级厚度、低成本的hBN薄膜及其光电器件的研究具有重要意义。本论文采用高温低压化学气相沉积(LPCVD)系统,在非催化衬底(硅、蓝宝石等)上直接外延hBN薄膜,对生长工艺、晶体质量、缓冲层技术、退火工艺和hBN基日盲型深紫外光电探测器等方面开展了系列研究,具体研究内容和得到的主要结果如下:1.非催化衬底直接外延生长hBN的研究。硅(Si)是半导体器件最常用的衬底材料,在硅衬底上外延生长hBN薄膜具有实际意义。以(100)Si为衬底、三氯化硼(BCl3)和氨气(NH3)为反应源、高纯氮气(N2)作为载气和稀释气体,工作压强为100 Pa,通过优化生长温度,在1200℃条件下,制备出厚度为2.3μm、c轴择优取向的hBN薄膜。为了探索更高生长温度对hBN薄膜质量的影响,进行了蓝宝石衬底上hBN薄膜的生长工艺研究。通过控制变量法系统地研究生长温度、III/Ⅴ族气源流量比、生长压强和载气类型等工艺条件对hBN薄膜结晶质量、表面形貌和材料性质的影响。最终获得了在蓝宝石衬底上直接沉积hBN的优化工艺参数,即:使用N2作为输运气体和稀释气体,生长温度为1350℃,BCl3和NH3的气体流量比为1:3,反应室气体压强为100 Pa。2.缓冲层上生长hBN薄膜的研究。分别采用铬(Cr)、钨(W)和氮化铝(Al N)作为缓冲层生长了hBN外延薄膜。结果表明缓冲层能够提高hBN薄膜的结晶质量、减小薄膜内部应力,Cr缓冲层的效果最佳。溅射的Cr缓冲层具有(110)晶面择优取向,在X射线衍射(XRD)θ-2θ扫描表征结果中,与未使用缓冲层生长的样品相比,Cr缓冲层上生长的hBN薄膜(0002)晶面衍射峰位置更接近标准峰位,半峰宽(FWHM)下降了约55.6%;拉曼(Raman)光谱表征结果表明hBN薄膜E2g振动峰中心峰位仅比标准峰位蓝移3.5 cm-1,蓝移波数下降了约65%。W和Al N缓冲层也有助于提高hBN晶体质量及减小薄膜内部应力。3.hBN薄膜高温后退火工艺的研究。为了进一步提高hBN薄膜的结晶质量、释放薄膜内部的残余应力,系统地研究了退火温度、退火时间和退火气氛等工艺条件对hBN薄膜的晶体质量和材料性质的影响。结果表明高温热退火工艺可以显著消除薄膜的内部缺陷,促进晶界融合,有效改善hBN薄膜的光学性能。优化的退火工艺条件是:在N2氛围下,1700°C退火10分钟。通过优化的退火工艺处理后,hBN薄膜Raman E2g振动峰的FWHM约为21.8 cm-1,横向晶畴尺寸约为108 nm,光学带隙约为5.79 e V。利用高温原位热退火技术,在蓝宝石衬底上制备出沿c轴择优生长的高质量hBN薄膜,hBN(0002)晶面XRD衍射峰的FWHM仅为0.18°,hBN(101(?)1)晶面的2θ/(?)扫描结果呈现完美的六重对称性,Raman E2g振动峰的FWHM仅为12.8 cm-1,薄膜的介电强度高达7.6 MV/cm,吸收边和光学带隙分别为213 nm和5.81 e V。同时发明了一种完整剥离2英寸hBN薄膜的方法,剥离后的hBN薄膜不仅能够自支撑,而且具有良好的柔韧性。4.hBN基日盲型深紫外光电探测器的研究。基于制备的硅基hBN薄膜研制出金属—半导体—金属(MSM)结构深紫外光电探测器,在20V偏压下,器件的暗电流小于10 p A,在紫外增强型氙灯的辐照下,器件显示出良好的深紫外光电响应特性,光暗电流之比约为312。基于高温热退火工艺处理的hBN薄膜制备了深紫外光电探测器,在提高hBN薄膜晶体质量后,探测器在20 V偏压时的光暗电流之比大于2890,是未退火hBN薄膜深紫外光电探测器的9倍。基于Cr缓冲层生长的hBN薄膜制备了垂直结构深紫外光电探测器,Cr不仅作为沉积hBN的缓冲层,还作为器件的底电极,测试结果表明器件具有良好的光电响应和开关特性。基于自支撑hBN薄膜研制出柔性hBN深紫外光电探测器。探测器在弯折和展平的状态下,观测到的光、暗电流没有明显变化,表现出良好的稳定性和可靠性。更多还原

【Abstract】 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 2.3μm 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）θ-2θscan 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 2θ/（?）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.更多还原