【作者】 杨勇；

【导师】 顾星；

【作者基本信息】 东南大学 ， 材料科学与工程， 2025， 硕士

【摘要】 N极性GaN HEMT器件凭借其天然的背势垒、低欧姆接触电阻、强大的器件缩放能力,以及可采用深凹槽栅结构抑制电流崩塌效应等优势,被视为未来高频通信功率放大器的关键技术。然而,由于MOCVD外延生长的N极性GaN存在高表面粗糙度和高氧背景杂质浓度等问题,导致其距离商业化应用仍有不小差距。基于此,本文围绕N极性GaN缓冲层的表面平整度和高阻特性优化展开研究,并在此基础上构建N极性GaN HEMT外延结构及器件。主要的研究内容和成果如下:（1）采用偏向<11（?）0>晶向4°的斜切C面SiC作为生长衬底,通过两步温度生长法在N极性GaN缓冲层结构中引入LT-GaN,并针对其生长温度、厚度、V/Ⅲ比及生长压力等参数进行优化,成功将N极性GaN缓冲层的表面RMS粗糙度从一步温度生长法的8.4 nm降至5.3 nm;此外,在优化的LT-GaN中继续引入2.85%的Al组分形成LT-AlGaN,将表面RMS粗糙度进一步降低到了2.9 nm,为实现高迁移率的2DEG扫除了主要障碍。对不同的生长底层进行研究后发现,LT-GaN和LT-AlGaN在不同程度上缓解了N极性GaN生长初期承受的台阶聚束效应,揭示了两步温度生长法对N极性GaN缓冲层表面形貌优化的作用机理。（2）通过Cp₂Fe掺杂流量和非故意掺杂u-GaN层厚度的优化,实现了对Fe掺杂记忆效应的有效控制;并基于此进行了N极性GaN/AlGaN异质结的生长和优化工作,最终以30 nm GaN/0.6 nm AlN/25 nm Al_0.28Ga_0.72N的异质结结构,有效缓解了异质结界面粗糙度散射和合金无序散射等机制对2DEG迁移率的限制,成功获得了电子迁移率、2DEG浓度和方块电阻分别为1110.44 cm²/V·s、1.43×10¹³ cm^-2和393.59Ω/sq的N极性GaN HEMT外延结构。（3）通过对器件工艺进行相应的优化,制备了栅长为1μm的N极性GaN HEMT器件。RTA处理前后,Ti/Al/Ni/Au电极与N极性GaN沟道层之间的欧姆接触电阻分别为1.76Ω·mm和0.65Ω·mm,展示出了N极性GaN HEMT结构在易实现低欧姆接触方面的天然优势。在直流测试条件下,器件的最高跨导、阈值电压、导通电阻和最高饱和输出电流密度分别为75.52 mS/mm、-4.20 V、9.29Ω·mm和0.36 A/mm,成功制备了具有直流输出性能的N极性GaN HEMT器件。此外,通过C-V测试进一步验证了N极性GaN HEMT外延结构具备优秀的载流子限阈能力。更多还原

【Abstract】 With inherent advantages such as a natural back-barrier,low ohmic contact resistance,superior scaling potential,and the feasibility of employing deep recess structures to mitigate current collapse,N-polar GaN HEMTs have emerged as a key candidate for next-generation high-frequency power amplifier applications.However,challenges including rough surface morphology and high oxygen background impurity concentrations of MOCVD-grown N-polar GaN still hinder its commercialization.Therefore,this research focuses on optimizing the surface morphology and high-resistivity characteristics of N-polar GaN buffer layers,followed by the development of corresponding HEMT epitaxial structures and devices.The main research contents and results of this thesis are as follows:（1）N-polar GaN films were grown on vicinal carbon-face SiC substrates with a 4°misorientation angle towards the<11（?）0>direction.A two-step temperature growth process was employed to introduce LT-GaN into the N-polar GaN buffer structure,and growth parameters such as temperature,thickness,V/Ⅲratio,and pressure were systematically optimized.This approach reduced the surface RMS roughness from 8.4 nm（using a one-step temperature growth process）to 5.3 nm.Furthermore,by incorporating 2.85%Alcomponent into the optimized LT-GaN to form LT-AlGaN,the RMS roughness was further reduced to 2.9 nm,eliminating a key obstacle to achieving high-mobility 2DEG.Through comparative analysis of different buffer configurations,it was revealed that both LT-GaN and LT-AlGaN effectively mitigated the step-bunching effect during the initial stages of N-polar GaN growth to varying degrees,thereby clarifying the mechanism by which the two-step temperature growth process enhances surface morphology.（2）By optimizing the Cp₂Fe precursor flow rate and the thickness of the unintentionally doped GaN（u-GaN）layer,the memory effect associated with Fe doping was effectively suppressed.Based on the optimized buffer,N-polar GaN/AlGaN heterostructures were subsequently grown and further refined.A heterostructure comprising of 30 nm GaN/0.6 nm AlN/25 nm Al_0.28Ga_0.72N was finally established,which effectively alleviated limitations on 2DEG mobility caused by interface roughness and alloy disorder.As a result,the optimized N-polar GaN HEMT structure exhibited an electron mobility of 1110.44 cm²/V·s,a 2DEG concentration of 1.43×10¹³ cm^-2,and a sheet resistance of393.59Ω/sq.（3）An N-polar GaN HEMT device with a gate-length of 1μm was fabricated through optimized device processing.The specific contact resistances between the Ti/Al/Ni/Au stack and the N-polar GaN channel layer were measured as 1.76Ω·mm before and 0.65Ω·mm after RTA,demonstrating the intrinsic advantage of N-polar GaN HEMTs in achieving low ohmic contact resistance.Under direct current measurement,the device exhibited a peak transconductance of 75.52 m S/mm,a threshold voltage of-4.20 V,an on-resistance of 9.29Ω·mm,and a maximum saturation output current density of 0.36 A/mm,indicating the successful realization of a DC-operable N-polar GaN HEMT.Additionally,C-V characterization confirmed the strong carrier confinement enabled by the optimized epitaxial design.更多还原