氯基条件下4H-SiC衬底的同质外延生长研究

利用课题组自主研发的热壁低压化学气相沉积(HWLPCVD)系统,在朝[11-20]方向偏转4°的(0001)Si面4H-SiC衬底上进行快速同质外延生长,研究了生长温度及氯硅比(Cl/Si比)对外延生长速率的影响机理.研究发现,外延生长速率随生长温度的提高呈线性增加,而Cl/Si比的改变对生长速率的影响不大.文章进一步探究了Cl/Si比对4H-SiC外延层表面缺陷的影响.较低的Cl/Si比(0.4～2)可以减少或消除三角缺陷,Cl/Si比较高(大于5)时,表面质量反而下降,因而,适当的Cl/Si比对于获得表面形貌良好的4H-SiC外延层至关重要.     

氢气体积流量对n型4H-SiC同质外延生长的影响

在商业化偏4°导电型4H碳化硅(SiC)衬底上采用化学气相沉积(CVD)的方法进行n型4H-SiC同质外延层生长,研究反应过程中氢气(H₂)对外延生长的影响,并使用表面缺陷测试仪、汞探针和红外膜厚仪等设备对外延层进行分析和表征。结果表明,氢气体积流量由基础值80增加到120 L/min,生长速率呈先增加后降低的趋势,生长速率的增加值最大为2μm/h,但缺陷呈先减少后增加的趋势。在高温CVD外延过程中,生长速率阶段性变化的原因：一是生长速率由气相质量转移系数和表面化学反应速率共同决定;二是氢气体积流量过大时,大量的析出氢难以及时离开生长表面,不利于反应物的有效分解和再沉积过程。综上所述,采用100 L/min氢气体积流量的生长工艺可在较高生长速率下制备高质量、厚度均匀性0.91%和载流子浓度均匀性1.81%的SiC外延片。     

生长温度对4HN-SiC同质外延层表面缺陷的影响

采用化学气相沉积(CVD)的方法在直径100 mm4°偏角衬底上生长4HN-SiC同质外延片,研究工艺生长温度对外延层表面缺陷的影响,并使用金相显微镜、表面缺陷测试设备、汞探针和红外膜厚仪进行分析和表征。结果表明,工艺生长温度由1 550℃增加到1 620℃,外延层表面的三角形缺陷密度可降低至0.39 cm~(-2);但随着工艺生长温度的增加,导致外延层边缘的台阶聚集数量和长度也急剧增加。在高生长温度下,外延层表面三角缺陷减少以及边缘台阶聚集增加的原因为:一是衬底表面原子迁移率的增加,减少了衬底表面2D生长;二是硅原子的气相成核受到抑制;三是〈1100〉和〈1120〉方向横向生长速率的差异加剧。综上结果,采用1 550℃生长工艺可在高生长速率下制备厚度均匀性和掺杂浓度均匀性分别为1.44%和1.92%的高质量4HN-SiC同质外延片。     

4H-SiC同质外延中的缺陷

从实验出发,用LPCVD外延系统在偏向<11-20>方向8°的4H-SiC(0001)Si面衬底上,利用CVD技术进行了4H-SiC同质外延生长。外延后在熔融KOH腐蚀液中进行腐蚀,使用SEM和光学显微镜表征方法探讨了CVD法4H-SiC同质外延中的位错、微管和孪晶等缺陷形貌,并分析其形成机理。     

超厚层4H-SiC同质外延材料

缺陷控制是4H-SiC同质外延的关键技术。在所有的4H-SiC外延缺陷中,三角形缺陷对芯片性能的影响最为致命,而且三角形缺陷的尺寸随着4H-SiC外延层厚度的增加迅速增大,从而会导致外延片无缺陷面积急剧下降。因此对于厚层4H-SiC外延,控制三角形缺陷密度至关重要。南京电子器件研究所追踪衬底/外延层界面处的缺陷转化信息,分析了三角形缺陷的主要成因,并通过优化衬底刻蚀时间及温度,有效消除由于衬底表面加工损伤引入的三角形成核几率。     

非刻意掺杂4H-SiC同质外延中的深能级缺陷

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition （HWCVD） have been studied using photoluminescence （PL） technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon （Vc） are revealed by electron spin resonance （ESR） technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.     

零偏4H-SiC衬底的同质外延方法

基于水平热壁化学气相沉积(CVD)技术,采用原位刻蚀方法,在3英寸(1英寸=2.54 cm)(0001)Si面零偏4H-SiC衬底上生长了高质量的同质外延层,并对其主要工艺参数和生长机制进行了探讨。利用微分干涉相差显微镜、喇曼散射及湿法腐蚀等表征手法对样品进行了测试分析。测量结果表明,4H-SiC占整个外延表面积的99%以上,此外,该工艺消除了4H-SiC同质外延层中的基面位错,提高了外延层的质量。同时对零偏4H-SiC衬底的同质外延的工艺过程和理论进行了研究和讨论,实验发现,生长前的原位刻蚀、初始生长参数、碳硅原子比以及生长温度对于维持外延层晶型、避免3C-SiC多型的产生具有重要影响。     

Al掺杂4H-SiC同质外延的缓冲层

利用扫描电子显微镜(SEM)、X射线双晶衍射谱(XRD)和光致发光谱(PL)对在4H-SiC单晶衬底上采用CVD同质外延的4H-SiC单晶薄膜的特性进行研究,发现外延层有很好的晶格结构和完整性。由于Al原位掺杂引起的晶格失配和衬底和外延的晶向偏离,在样品的衬底和外延的界面出现了约1μm的缓冲层,使得XRD摇摆曲线距主峰左侧约41arcs出现了强衍射峰。缓冲层中存在大量的堆垛缺陷和位错,引入缺陷能级,使室温PL测试为"绿带"发光。通过PL全片扫描发现缓冲层在整个样品中普遍存在且分布均匀。     

化学气相沉积4H-SiC同质外延膜的生长及其特征

采用常压化学气相沉积(APCVD)方法在偏向<11-20>晶向8°的p型4H-SiC(0001)Si-面衬底上进行同质外延生长.霍尔测试的结果表明,非有意掺杂的外延膜层导电性为n型.XRD测试显示各个样品只在位于2θ=35.5°附近出现一个谱峰,表明外延膜是SiC单晶.在低温PL谱中,对于在较低温度下外延生长的4H-SiC样品,在1.8～2.4eV范围内出现很宽的谱峰.而在该样品的Raman谱中,也观察到了典型的3C-SiC的特征峰,表明该样品含有立方相SiC的混晶,这与PL谱获得的结果相吻合.     

水平冷壁CVD生长4H-SiC同质外延膜

采用自行设计的水平冷壁低压化学气相沉积（LPCVD）方法在偏向〈11-20〉晶向8°的n型4H-SiC（0001）衬底上进行了同质外延生长．在5.3e3Pa的低压下，外延膜生长速率超过3μm/h．电容电压法测试表明在非有意掺杂外延膜中净施主浓度为8.4e16cm－3．Nomarski显微镜观察表明厚外延膜的表面光滑，生长缺陷密度很低．AFM测试显示表面均方根粗糙度为0.3nm，没有观察到宏观台阶结构．Raman谱线清晰锐利，表现出典型的4H-SiC特征．在低温PL谱中，近带边区域出现很强的自由激子峰，表明样品是高质量的．     

Generation of Defects in Heavily Al-Doped 4H-SiC Epitaxial Layers Grown by the Low-Temperature Halo-Carbon Method

Application of the low-temperature halo-carbon epitaxial growth technique to produce heavily doped p-type epitaxial layers was investigated. While growth at 1300°C facilitated Al incorporation, possible degradation of crystalline quality had to be evaluated. A quantitative measure of the defects generated during epitaxial growth was obtained using molten potassium hydroxide (KOH) etching. The trend with increased doping was for an increase in threading dislocations and Al precipitates. The degradation initially involved an increase in generation of threading dislocations, which was followed at higher doping by generation of defects giving rise to smaller KOH etch pits. The properties of the small-etch-pit defects were reminiscent of Al precipitates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to establish that the small-etch-pit defects were localized at various depths inside the epilayers. Energy-dispersive spectroscopy (EDS) indicated that these small-etch-pit defects contained a higher concentration of aluminum.     

Electroluminescence Spectral Imaging of Extended Defects in 4H-SiC

To understand the nature of various extended defects and their impact on the electronic or optoelectronic characteristics of semiconductor devices, the investigation of spectral properties is required. However, electroluminescence spectroscopy does not provide spatial or structural information. The lack of such information can lead to incorrect assignment of a luminescence band and therefore misinterpretation of the nature of the emitting defect. Here we report on the collection and analysis of real-color and spectrally selective monochromatic electroluminescence (EL) images from 4H-SiC PiN diodes. The former provides the approximate spectral properties from the color of the various defects with high spatial resolution, while the latter enables simultaneous collection of both structural and spectral properties from extended defects, thereby assisting in providing the correct assignment of the various spectral features observed. This effort enabled the observation of the formation of a green EL emission located at C-core partial dislocations (PDs) that occurred during the stacking fault (SF) expansion process.     

Tunneling Atomic Force Microscopy Studies on Surface Growth Pits Due to Dislocations in 4H-SiC Epitaxial Layers

The morphological and electrical properties of surface growth pits caused by dislocations in 4H-SiC epitaxial layers were characterized using tunneling atomic force microscopy. The characteristic distribution of the tip current between the metal-coated atomic force microscopy tip and the SiC was observed within a large surface growth pit caused by a threading screw dislocation. The current was highly localized inside the pit and occurred only on the inclined surface in the up-step direction near the pit bottom. This paper discusses the causes and possible mechanisms of the observed tip current distribution inside surface growth pits.     

4H-SiC MESFET结构外延生长技术

利用热壁式CVD技术生长4H-SiC MESFET结构外延材料,TMA和氮气分别用作p型和n型掺杂源.外延层厚度使用SEM进行表征,SIMS及汞探针C-V用作外延层掺杂浓度测试.通过优化生长参数,成功生长出高质量的MESFET结构外延材料并获得陡峭的过渡掺杂曲线,并给出了部分器件测试结果.     

4H-SiC中点缺陷的第一性原理研究

基于密度泛函理论和第一性原理,对4H-SiC晶格中5种点缺陷(VC,VC-C,填隙B,替位B和替位P)的晶格常数、电荷布居、能带等微观电子结构进行了计算,并从缺陷形成能、杂质电离能和载流子浓度等角度分析了这些点缺陷对材料性能的影响.计算结果表明:在这些点缺陷中,C空位的形成能最低,仅为5.929 1 eV,属于比较容易形成的一类点缺陷.同时在能带图中可以看到填隙B的禁带中央附近出现了一条新能带,这条新能带的产生促使填隙B成为5种缺陷中禁带宽度最小的缺陷,有利于SiC半导体器件中载流子的输运.在3种杂质点缺陷中,替位P的电离能最小.掺杂杂质电离能越小,电离程度越深,产生的载流子浓度也越大,这一结论在载流子浓度的计算结果中也得到了验证.     

Diffraction Contrast of Threading Dislocations in GaN and 4H-SiC Epitaxial Layers Using Electron Channeling Contrast Imaging

Forescattered electron channeling contrast imaging (ECCI) offers the potential for imaging and analyzing extended defects in a scanning electron microscope (SEM). Indeed, it is shown that ECCI is able to determine the Burgers vector of threading dislocations with the aid of carefully determined experimental parameters and accompanying image simulations. Simulations are compared with ECC images from samples with features that are relatively easily studied and modeled: those based on specially engineered 4H-SiC mesa substrates. These mesas serve as substrates for both homoepitaxial 4H-SiC layers and heteroepitaxial GaN layers in which images of threading dislocations (TDs) have been recorded using ECCI and found to strongly resemble diffraction contrast simulations of TD intensity profiles.     

4H-SiC MESFET结构外延生长技术

利用热壁式CVD技术生长4H-SiC MESFET结构外延材料,TMA和氮气分别用作p型和n型掺杂源.外延层厚度使用SEM进行表征,SIMS及汞探针C-V用作外延层掺杂浓度测试.通过优化生长参数,成功生长出高质量的MESFET结构外延材料并获得陡峭的过渡掺杂曲线,并给出了部分器件测试结果.     

4°偏角4H-SiC单晶快速外延生长工艺研究

高质量快速SiC外延生长工艺技术是目前高压电力电子器件研制的关键工艺技术。采用HCl气体作为含Cl化合物,研究了不同温度、气相y(C)/y(Si)摩尔比和刻蚀工艺等对于SiC外延层质量的影响。通过优化外延工艺参数,采用原位HCl刻蚀工艺,获得了SiC单晶外延生长速率达32μm/h的快速外延生长工艺,外延层表面平滑,表面粗糙度仅0.218 nm,晶片外延层厚度不均匀性小于0.4%。     

Comparative Study of Conventional and Quasi-Freestanding Epitaxial Graphenes Grown on 4H-SiC Substrate

Semiconductors - The structural and some other characteristics of quasi-freestanding single-layer graphene obtained by annealing of the buffer layer in the flow of hydrogen are studied in...     

Sources of Epitaxial Growth-Induced Stacking Faults in 4H-SiC

Successive reactive-ion etching and microphotoluminescence (PL) intensity mapping have been performed in order to investigate the sources of epitaxial growth-induced stacking faults (SFs) in thick 4H-SiC epilayers. Three kinds of SFs, i.e., 4SSFs, 3SSFs, and 2SSFs, have been identified in the samples. Two of these (3SSFs and 2SSFs) show similar nucleation behaviors, and their formation may be due to stress within the epitaxial layer. In contrast, 4SSFs nucleate at the epilayer–substrate interface and might be related to an unknown dislocation, which shows a rounded-shape etch pit in the substrate.