4H-SiC厚外延膜中扩展缺陷产生及转化机理研究

Silicon carbide is a desirable material for high power and high frequency devices due to its wide band gap, high break-down field and high thermal conductivity compared to silicon. Furthermore, the higher junction operating temperatures possible with SiC(~400 ℃), in comparison with silicon(~100℃), reduce the cooling requirements of SiC-based power systems, allowing compact, high performance modules to be realized. In recent years, there has been strong interest in high-power devices with blocking voltages in excess of 10kV.In all these devices, an epitaxial layer of 80-100 um is required to achieve large blocking voltages..An application of particular interest is that of high voltage, bipolar switching devices, where the material properties of 4H-SiC provide a significant advantage over those of Si-based technology. Here, a critical problem area has been the high forward voltage drop that is developed across the thick, low-doped n? drift region, which leads to excessive heat dissipation when the switch is in the on-cycle. The forward voltage drop is minimized by conductivity modulation, where a high concentration of minority and majority carriers are injected into the drift region under high forward bias, thus raising the conductivity of the layer. However, the concentration of carriers that can be sustained is limited by the minority carrier lifetime, which is controlled by material defects, such as half loop arrays, slip bands and the morphology defects. Thus, high defect concentrations in the drift layer lead ultimately to a high forward voltage drop and high heat dissipation..In this project, the generation and transformation mechanism of on defects in 4H-SiC thick homoepitaxial layers will be carried out with theoretical and experimental methods. Firstly, the defects will be simulated with Materials Studio software from the first principle, and the model will be set up with theoretical analysis and experimental results. Secondly, the defects in 4H-SiC thick homoepitaxial layers will be characterized with cathode luminescence spectra, scanning electron microscope and KOH etching.On the base of the model and characterization of the defect mechanism, the process of 4H-SiC thick homoepitaxial layers, such as the substrate etching, temperature, C/Si ratio and gas flow parameters will be optimized. And the high quality 4H-SiC thick homoepitaxial layers will be achieved with the method of defects controlling. .The study of 4H-SiC thick homoepitaxial layers growth has important academic significance and application. The development of 4H-SiC high power electronic devices may be promoted with the method of defects controlling, which could provide a theoretical base for the development of a new generation of SiC-based power devices.

高功率SiC基功率器件的实现普及，会使能源转换领域将会发生巨大变化，极大地促进能源节约型社会的建设进程。基于SiC的高功率器件须在SiC的厚外延上进行制作，厚外延材料质量的优劣（缺陷多少）直接决定了SiC电力电子器件的性能。本项目在现有4H-SiC同质外延生长和表征研究的基础上，采用理论和实验相结合的方法，建立厚外延中缺陷的产生和转化模型，揭示缺陷的演化变化规律，提出4H-SiC厚同质外延中缺陷的控制方法；优化4H-SiC厚膜外延材料生长技术，生长出4H-SiC厚同质外延薄膜，并制备出相同结构的高功率4H-SiC PiN原型，验证厚外延中缺陷的演化机理和缺陷控制方法。本项目在4H-SiC厚同质外延中缺陷的产生、转化机理和缺陷控制方法等方面寻求突破现有研究的瓶颈，制备出高质量的4H-SiC厚外延薄膜，有望促进4H-SiC高功率电力电子器件的发展，为新一代SiC基功率器件的发展做出贡献。

SiC作为半导体材料具有许多优良的性能，如宽禁带、高热导率、高饱和漂移速率等，是制备高温、高频、高功率的电子器件的理想材料之一。基于SiC的高功率器件须在SiC的厚外延上进行制作，厚外延材料质量的优劣（缺陷多少）直接决定了SiC电力电子器件的性能。. 本项目在现有4H-SiC同质外延生长和表征研究基础上，采用理论和实验相结合的方法，建立出厚外延中缺陷的产生和转化模型，揭示缺陷的演化变化规律，提出4H-SiC厚同质外延中缺陷的控制方法；优化4H-SiC厚膜外延材料生长技术，生长出4H-SiC厚同质外延薄膜，并制备高功率4H-SiC PiN 二极管，研究了缺陷对器件特性的影响。主要的研究成果如下：. （1）根据VP508腔体结构特点，建立了流场和温度场分布模型；结合grove模型，建立了4H-SiC的CVD外延速率模型。分析了外延层厚度的分布规律，并指出恰当控制主气流速和托盘转速的关系有助于提高外延层的厚度均匀性。. （2）通过对不同缺陷的伯格斯矢量和弹性应变能的分析，研究了位错缺陷在厚膜外延生长中的延伸和转化机理。研究结果表明：TSD不能转化为TED而BPD可以转化为TED。采用Raman，电子背散射衍射等无损方法对三角形缺陷结构进行分析，完善了材料缺陷的表征体系。上述研究为缺陷控制方法的研究提供了理论指导。.（3）研究了生长工艺参数对外延缺陷的影响。通过优化氢刻蚀工艺和提高生长温度等方法降低了材料表面的划痕、刻蚀坑、三角形缺陷等材料缺陷，降低外延层的表面粗糙度。利用选择性刻蚀方法，使外延材料的BPD密度由1.7×104/cm2降低到5.3×103/cm2。.（4）通过增大硅烷流量和丙烷流量提高生长速率，并加入HCl抑制表面硅滴的形成。提出了新型低压外延生长方法进一步控制材料缺陷。生长的50μm厚外延材料，表面粗糙度小于0.2nm，厚度不均匀性为1.3%-1.8%之间，快速外延生长速率达到25-30μm/h。.本项目在4H-SiC厚同质外延中缺陷的产生、转化机理和缺陷控制方法等方面突破现有研究瓶颈，制备出高质量的4H-SiC厚外延薄膜，促进4H-SiC高功率电力电子器件的发展，为新一代SiC基功率器件的发展奠定了基础。