摘要
SiC器件和其它半导体器件一样,材料的质量对于SiC器件的制备和性能具有非常重要的意义。高质量的SiC单晶是制备高性能SiC器件的基础,有利于SiC器件研制和进一步推广应用。目前,4H-SiC同质外延单晶薄膜的研究已经取得了可喜的成绩。但是国内才刚起步,且由于同质外延的表征测试存在一定困难:还没有一套系统完整的表征测试方法;对于非刻意掺杂4H-SiC同质外延中存在的影响器件性能的本征深能级,国际上也存在比较大的争议,不同样品的结果也差异很大;对于深能级的起源没有定论,也没有提出一套很好的解决办法。国内对于这一方面的研究还是空白。
在此背景下,本文对制备4H-SiC同质外延薄膜的机理、方法和特性进行了系统的理论分析和实验验证;提出一套系统完整的4H-SiC同质外延薄膜表征测试方法;对影响器件性能的非刻意掺杂n型4H-SiC外延中的本征深能级等相关问题进行了广泛深入的研究。主要的研究成果如下:
1.在理论分析的基础上,对4H-SiC同质外延生长的关键工艺进行了实验研究,得出影响这些材料参数的主要因素。确定了关键参数变化趋势,制定了完整的工艺流程。
2.建立了系统的4H-SiC同质外延薄膜表征测试方法。通过不同掺杂浓度样品Raman光谱中LOCP的不同,提出了一种简易测试外延厚度的方法。利用FTIR反射谱对4H-SiC同质外延晶体薄膜的质量进行评价,完善了用干涉条纹的频率和强弱的方法来计算外延薄膜的厚度。通过van der Paul法霍尔测试技术,对低N掺杂的n型4H-SiC同质外延的迁移率和霍尔系数进行测试表征。用汞(Hg)探针C-V测试获取4H-SiC同质外延纵向杂质浓度分布的信息,并通过多点测试,得到外延片的掺杂浓度均匀性。对样品进行了电阻均匀性测试,测试结果显示电阻率分布较为均匀,最小和最大电阻率分别为0.3768Ω·cm和0.3972Ω·cm,误差仅为1.04%。HRXRD测试显示4H-SiC同质外延薄膜半高宽(FWHM)为36 s,距主峰左侧约41″处出现另外一个衍射峰。经计算分析,该衍射峰偏移主要是由外延和衬底的晶向偏移引起的。借助SEM、AFM和X射线光电子能谱仪对4H-SiC样品表面形貌以及元素进行了定性和定量的测试分析。借助SIMS对Al掺杂和高N掺杂的4H-SiC样品中的主要掺杂杂质浓度及薄膜厚度进行了测试,验证了光学测试的结果。
3.研究了非刻意掺杂4H-SiC同质外延禁带中形成的缺陷深能级。对样品进行10K到室温的变温光致发光测量,发现4H-SiC存在由本征深受主能级引起的“绿带”发光现象。借助PL mapping对样品的峰值波长(Peak Lambda)和发光强度(Peak Int)进行了测量,结果表明在4H-SiC外延大部分区域存在本征深能级缺陷,且分布均匀。通过理论分析表明存在两个深能级,分别位于导带下E1 = 0.942eV,E2 = 1.190eV处。深能级缺陷形成的原因主要是由于样品中存在着大量的碳空位VC。“绿带”发光谱与碳空位VC及其扩展缺陷都相关,是由二者络合形成的。
4.为了减少外延层中的C空位,进行了C离子注入4H-SiC外延的理论研究和工艺设计。通过研究离子注入理论和工艺特性,采用蒙特卡罗(Monte Carlo)方法,借助Trim模拟软件对C离子注入4H-SiC的分布进行了统计计算,计算模拟了不同注入能量下C注入4H-SiC的平均投影射程、标准偏差以及浓度分布。在此基础上,提出采用三次C离子注入消除非刻意掺杂4H-SiC中深能级缺陷的方法和工艺流程,包括离子注入工艺参数(能量、剂量)的确定、注入掩膜层SiO2厚度的选取原则,退火温度和时间以及退火保护。成功地进行了注入实验。
5.研究了C离子注入和高碳硅比生长4H-SiC对深能级的影响。发现1600℃的退火温度对注入引起的晶格损伤起到了很好的修复作用,并可以有效地激活碳离子,使其占据C空位。高温退火使C空位被占据有效地减少了C空位衍生缺陷。通过碳离子注入成功消除了非刻意掺杂n型4H-SiC外延薄膜中的深能级缺陷。
研究了生长4H-SiC外延薄膜合适的碳硅比区间。综合考虑增大C/Si对4H-SiC外延质量的影响和实验成本两个方面,选取C/Si为1.5和3两种样品,找出增大C/Si对于非刻意掺杂4H-SiC深能级缺陷变化的趋势和效果。结果表明,增大生长碳硅比,能在一定程度上抑制碳空位的产生,减少本征深能级缺陷,但受外延晶体质量等条件限制,有一定的局限性。
As other semiconductor device, the quality of silicon carbide (SiC) material plays a very important role in the fabrication and performance of SiC device. The high quality SiC single crystal material is the base for achieving SiC devices with high performance, and making for the study and the further applications of SiC devices. At present; there are obviously progresses about the formation on the growth of 4H- SiC homoepitaxial layers. But many difficulties and problems are also existed on the characterization of the 4H-SiC homoepitaxial layers materials. It is necessary to systematically develop a series of characterization methods for characteristics of 4H-SiC homoepitaxial layers. Until now, there are still different opinions for the intrinsic deep energy level in unintentionally doped 4H-SiC. The results obtained from measurements are different for different samples. Also the origin of intrinsic deep energy level is not quite clear. However the related researches are just in beginning in mainland China.
In this dissertation, the formation mechanism, technology, and characteristics on the 4H- SiC homoepitaxial layers are studied theoretically and experimentally. A series of test and characterization methods for characteristics of 4H-SiC homoepitaxial layers are presented. The intrinsic deep energy level in unintentionally doped 4H-SiC homoepitaxial layers, which has a great effect on the device performance, is comprehensively studied. The main studies and contributions of this dissertation are as follows.
1. The key processes of growth are studied by experiments based on the theoretical analyse, and the major factors to affect the key processes are obtained. The trend of parameters and process are achieved.
2. Several methods for the characterization of 4H-SiC homoepitaxial layers are investigated. A new simple method is presented according to the difference LOCP peak in Raman spectrum with different dopping concentrations. The quality of 4H-SiC homoepitaxial layers is evaluated and the depth is calculated using the intensity and frequency of interference fringes in FTIR spectrum. The resistivity mapping measurements reveal a good homogeneity of electrical properties in the main area of the wafer, with the maximum resistivity 0.3972Ω·cm and the minimun resistivity 0.3768Ω·cm. The maximum error of resistivity is only 1.04%.
The FWHW of rocking curve is 36 second measured by X-ray diffraction (XRD), and another peak is found at the left, 41 second away from main peak. It is caused by the misorientation between the substrate and epilayer through mathematical analysis. The surface topography and element of 4H-SiC homoepitaxial layers are qualitatively and quantitatively analysed by SEM, AFM and XPS, and the concentration of heavy N-doping 4H-SiC and the doping of Al-doped sample are measured by secondary ion mass spectroscopy (SIMS). The thickness of different doping layers can also be obtained according to the concentration, which is used to verify the results from FTIR method.
3. The intrinsic deep energy level in the bandgap of unintentionally doped 4H-SiC is investigated. An undoped 4H-SiC homoepitaxial layer grown by hot-wall chemical vapor deposition has been studied using photoluminescence (PL) technique with temperature varied from 10K to 240K. The broadband green luminescence has been observed. The broadband green luminescence may be composed of two Gauss-type spectra by using nonlinear optimization technique. It shows that the broadband green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 0.942 eV and 1.190 eV below the conduction band, respectively. The ends of two energy levels are expanding and superimposed to each other. Vacancies of carbon (VC) are found by electron spin resonance (ESR) technique at 110K. The results strongly imply that vacancy of C and the extended defects are responsible for the green band luminescence.The The peak lambda and intensity of PL mapping indicated the intrinsic deep level energy exist in almost the whole wafer, and it is evenly distributed.
4. In order to reduce vacancy of carbon, the theory and process design of 4H-SiC homoepitaxial layer implanted by carbon ion are studied. From the analysis of theory and process ion-implantation, the ion implantation range, location of peak concentration and longitudinal straggling of carbon are calculated with the Monte Carlo simulator TRIM. Then the process flow for removing deep energy level in undoped 4H-SiC homoepitaxial layer by three times carbon ion-implantation is proposed, including the design of implantation energy, dose, and depth of the SiO2 resist mask, annealing temperature, annealing time and annealing protection.
5. The effect of carbon ion implantation and high C/Si ratio on deep energy level in the growth of 4H-SiC is investigated. The deep energy level in 4H-SiC material can be significantly improved by with implantation of carbon atoms into a shallow surface layer. The damage of crystal lattice can be repaired well, and the carbon ions are effectively actived after the 1600°C annealing. The vacancies of carbon and their related defects can be decreased by the process of carbon ion implantation and annealing.
C/Si ratio to be suitable for the growth of 4H-SiC epilayer is discussed according to the quality of 4H-SiC epilyers and the experiment cost. C/Si=1.5 and C/Si=3 are selected to find the tendency of intrinsic deep energy level with the C/Si ratio increasing. The results indicate that the vacancies of canbon are suppressed and the density of deep energy level defects is decreaed in carbon rich growth conditions. But this method is limited by severl factors, such as quality of 4H-SiC epilyers and concentration of N dopping.
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