= Introduction

4H-SiC 性能很好、器件应用广泛，因此需要开发原位的、非破坏性的表征技术。

4H-SiC 是一种具有优良性质的半导体材料，包括宽禁带、高临界电场强度、高热导率和沿 c 轴的高电子迁移率，因此受到了广泛的研究。
受益于外延技术的发展和新能源产业的需求增长，4H-SiC 在功率电子器件中得到了广泛应用。
然而，4H-SiC 器件的性能仍然受到缺陷的限制，如何在生长和工作条件下避免缺陷的产生仍然是一个挑战。
此外，用于表征 SiC 掺杂的方案（如二次离子质谱（SIMS）和霍尔效应测量）通常是破坏性的且耗时，因此迫切需要开发原位和非破坏性的表征技术。
// 随着尺寸缩小，4H-SiC 中的缺陷对器件性能的影响更加显著。（纳米材料？

The 4H-silicon carbide (SiC) has long attracted a lot of research
  thanks to its wider bandgap, higher critical electric field strength,
  higher thermal conductivity, and higher electron mobility along the c-axis
  than silicon (Si) and gallium arsenide (GaAs) as well as other SiC polytypes
  @casady_status_1996 @okumura_present_2006.
It has been widely used in power electronic devices
  thanks to the development of epitaxy technology and the increasing application in the new energy industry
  @tsuchida_recent_2018 @harada_suppression_2022 @sun_selection_2022.
However, the performance of 4H-SiC devices remains constrained by the presence of defects,
  which may be introduced during the growth process @nishio_triangular_2020 @demenet_tem_2005
  or arise under device operating conditions
    @mahadik_ultraviolet_2012 @okada_dependences_2018 @iwahashi_extension_2017 @caldwell_driving_2010
    @miyanagi_annealing_2006 @iijima_correlation_2017.
Furthermore,
  conventional methods for characterizing the doping properties of SiC
    (such as secondary ion mass spectrometry (SIMS) @kudriavtsev_quantitative_2003 @kim_characteristics_2024
    and Hall effect measurements @noguchi_comparative_2021 @asada_hall_2016),
  are destructive and time-consuming.
Therefore, there is a pressing need to develop an in-situ and non-destructive characterization techniques.

拉曼主要体现声子的信息，并且早已经有应用，主要用来区分 SiC 的多型。

声子（量子化的原子振动）在理解晶体的原子结构以及热电性质方面起着重要作用。
声子可以通过多种实验技术来探测，包括 EELS、IR 吸收谱等。
拉曼光谱是最常用的方法，它提供了一种无损、非接触、快速和局部的声子测量方法。
早有关于拉曼的研究，且拉曼已被广泛用于区分 SiC 的多型。

Phonons (quantized atomic vibrations) play a fundamental role
  in understanding the atomic structure as well as the thermal and electrical properties of semiconductors.
They could be probed by various experimental techniques,
  such as electron energy loss spectroscopy @yan_single-defect_2021 @egoavil_atomic_2014
  and infrared absorption spectroscopy @pluchery_infrared_2012 @tong_temperature-dependent_2018.
Among these techniques,
  Raman spectroscopy is the most commonly used method,
  // TODO: 增加一些引用，可以先不用收集文献给这里，最后把其它部分的全拿过来就行了。
  as it provides non-destructive, non-contact, rapid and spatially localized measurement of phonons
  that near the #sym.Gamma point in reciprocal space.
Studies in Raman scattering of 4H-SiC have been conducted since as early as 1968 @feldman_phonon_1968
  and nowadays have been widely employed to identification of different SiC polytypes
  @guo_characterization_2012 @yan_study_2016 @hundhausen_characterization_2008 @nakashima_raman_2013.

拉曼谱中有更多信息。有一些新的研究，但他们还有不足。

近年来，更多信息被从拉曼光谱中挖掘出来。
LO 声子峰或 LOPC 峰已经被证明与自由载流子的类型和浓度有关，它们已经被用于估计离子注入层的厚度和 n 型 SiC 的掺杂浓度。
3C-SiC 的一类层错的拉曼和 EELS 光谱已经被研究，但更多类型的层错及其在 4H-SiC 中的拉曼光谱还没有被研究。
有人提出了可能的 N 掺杂原子的拉曼峰，但并没有在实验上对比验证，同时 Al 或其它点缺陷的拉曼峰也缺少系统的研究。
此外，拉曼光谱上仍有一些不知来源的峰；同时，也缺少一些理论上预测应该存在的峰。
// TODO: 总结更多文献

In recent years, increasingly rich information has been extracted from Raman spectra of 4H-SiC.
Longitudinal optical phonon–plasmon coupling (LOPC) peek
  has been utilized to rapidly estimate the doping concentration in n-type SiC @harima_raman_1995
    and to identify doping type in different layers @song_depth_2020,
  while the influence of free carriers on the LO phonon peek in p-type SiC
    has not been systematically investigated yet.
Peeks associated with one type of stacking faults in 3C-SiC have been investigated @yan_single-defect_2021,
  while other types of stacking faults and their Raman spectra in 4H-SiC remain unexplored.
Peeks associated with nitrogen dopants have been proposed @_n-sic_2010
  but have not been experimentally verified,
  while systematic studies on the Raman spectra of aluminum or other point defects are still lacking.
Moreover, certain phonon modes predicted by theory remain unobserved,
  while there are still some unidentified peaks in the Raman spectra.

本文通过三种方式，研究 4H-SiC 中带缺陷和不带缺陷的声子。我们第一次做到了什么什么。

In this paper, we explored the phonon in 4H-SiC by three ways:
  symmetry analysis, first-principles calculations, and Raman experiment.
We first investigated the phonon modes in perfect 4H-SiC,
  and then explored the phonon modes associated with defects and doping.
// TODO: 描述自己做了什么，强调自己是第一次做到了什么。
