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Homoepitaxial growth on on-axis 4H-SiC substrate using BTMSM source

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Authors

김현우

Advisor
김형준
Major
공과대학 재료공학부
Issue Date
2018-02
Publisher
서울대학교 대학원
Keywords
4H-SiChomoepitaxial growthBTMSMon-axis epitaxypolytype stability
Description
학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 김형준.
Abstract
Silicon Carbide (SiC) is a wide bandgap semiconductor with high melting point, large breakdown field, high thermal conductivity and high saturation electron mobility, and is being studied as a next generation semiconductor material to replace Si. SiC power devices are expected to be very powerful when applied to industrial transportation systems such as aerospace systems, electric vehicles, or next generation power plants due to their superior characteristics. However, this positive effect of applying SiC can only be expected when a SiC device is successfully fabricated. For this, epitaxial growth, which is an essential process in device fabrication, is very important. The performance of the SiC device depends largely on the quality of the epitaxial layer, so that the productivity, reproducibility and quality of the epitaxial layer have a great influence on the improvement of the device. For growth of high quality SiC epitaxial layers, many techniques such as molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), and vapor-phase epitaxy (VPE) have been tried but problems to be solved are still remained. To solve this problem, the author focuses on the homoepitaxial growth of a 4H-SiC epilayer by metal-organic chemical vapor deposition (MOCVD) using bis-trimethylsilylmethane (BTMSM, C7H20Si2).
Currently, for enhancing the polytype stability of SiC homoepitaxy, an off-axis substrate with several degrees off-cut toward the [112 ̅0] direction is commonly used. However, even though the polytype stability is enhanced in this case, other numerous issues still exist. First, the basal plane dislocations (BPDs) are transferred into the epilayer from the substrate. BPDs are known as killer defects, which, when present within the epilayer, largely degrade the forward voltage of the bipolar device. Second, the number of wafers obtained decreases when the substrate is cut along the off-cut direction in the SiC ingot. In order to solve these issues, it is essential to study epitaxy on on-axis substrates. However, because of the low step density on on-axis substrates, there is a possibility of creating unintended polytypes on epitaxial layers. In this study, we investigated the etching characteristics of Si-face and C-face on-axis substrates and report on the effect of these characteristics on the polytype stability of the epilayer grown using bis(trimethylsilyl)methane (BTMSM) source with a high C/Si ratio of 3.5. By understanding the correlation between the etching characteristics and the epilayer qualilty, we controlled the micro-steps of the etched substrates and improved the polytype stability of 4H-SiC up to 99% for Si-face. And we also discussed how to increase polytype stability at C-face by considering factors that affect polytype stability of on-axis epitaxial growth of SiC.
After in-situ H2 etching of the on-axis substrate, micro-steps generated by selective etching of the threading screw dislocations (TSDs) were observed on the surface of the substrate. The micro-steps formed during etching process enhance 4H stability of the grown epilayer by exposing the stacking sequence of 4H-SiC at the side wall and providing sites for step flow growth. However, the step-bunching phenomenon caused by the high-temperature (above 1500 °C) etching process partially eliminates the micro-steps and increases the probability of nucleation of 3C-SiC. In order to solve this problem, we increased the etching duration at the low etching temperature, spreading the micro-steps on the substrate without step-bunching and greatly increased the stability of the 4H-SiC polytype in the epilayer. Improved polytype stability of 4H-SiC up to 99% was finally achieved for Si-face on-axis substrates using BTMSM source.
In case of C-face, it has etching characteristic similar to that of Si-face. Micro-steps were created by selective etching of TSDs and step-bunching phenomenon less occurred at high temperature than Si-face due to low surface energy of C-face. However, the polytype stability of the grown SiC layer on C-face after spreading micro-steps without step-bunching was not improved and was much lower than that of Si-face. This is because high nucleation density of C-face due to the low critical supersaturation ratio. We discussed the factors affecting polytype stability of SiC layer from the viewpoint of competition of step-flow growth and nucleation. In addition, experimental data on silicon carbide epitaxy were considered. By reducing the source flow rate, the stability of the polytype on the C-face on-axis substrate was improved.
Language
English
URI
https://hdl.handle.net/10371/140626
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