41 lines
3.3 KiB
XML
41 lines
3.3 KiB
XML
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// 在半导体的极性声子模式中，原子间存在长距离的库伦相互作用，导致散射谱在 Gamma 附近不再连续（引用），如图中的彩色线所示。
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// 这导致不同方向的入射/散射光的声子模式不同。
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// 具体来说，当入射光/散射光沿着 z 方向时，起作用的是 A-Gamma 线上的声子模式（图中的左半边的橘线），它们适用于群 C6v。
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// 这时会有一个 E1 模式（TO，振动方向在面内）和一个 A1 模式（LO，沿 z 振动）。
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// 而当沿着 y 方向入射时，起作用的是 Gamma-K 线上的声子模式（图中的右半边的橘线），它们不再适用于群 C6v，而只适用于群 C2v；
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// 它会分裂成沿x、y、z 方向的三个声子模式（图中的右半边的蓝线），它们分别对应于群 C2v 的 A1、B1 和 B2 表示 TODO: 确认这个几个表示的名字。
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// 若考虑到到入射光不是严格沿着 z 方向，而是有一个小的角度（例如 10 度），则此时有一个声子模式沿着 x 方向，另外两个声子模式则为 y-z 两个方向的混合。
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// (没有在图上表示)
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Strong-polar phonon modes caused by different incident light directions are different,
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due to long-range Coulomb interactions between atoms in semiconductors,
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showing discontinuity in the scattering spectra near the #sym.Gamma point (see @figure-discont).
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For incident light propagating along the z direction (phonon modes on the A-#sym.Gamma line),
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symmetry of C#sub[6v] point group applies and leading to two modes (two peeks in Raman spectra),
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including an E#sub[1] mode (pink line in @figure-discont, vibration in-plane)
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and an A#sub[1] mode (green line in @figure-discont, vibration along z-direction).
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When the light is incident along other directions, symmetry in plane was broken and C#sub[6v] symmetry no longer holds,
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and there will be three phonon modes in theory.
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For example, when the light is incident along the y direction (phonon modes on the #{sym.Gamma}-K line),
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symmetry of C#sub[2v] applies and three modes exist in dispersion curves,
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including an A#sub[1] mode (green line in @figure-discont, vibration along z direction),
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a B#sub[2] mode (blue line in @figure-discont, vibration along x direction),
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and a B#sub[1] mode (red line in @figure-discont, vibration in y direction).
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When the light is incident along a direction between z and y,
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three phonon modes will exist, but vibration in the mixed direction.
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Many Raman experiments on 4H-SiC with incident light along the z direction have observed two peaks.
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However, no experiments have reported three peaks with incident light along other directions.
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In our experiments, we found that the third peak only appears when focusing inside the sample.
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Based on its visibility under different polarizations and its distance from the main E2 peak,
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we identify it as the peak we are looking for.
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We also notice that the TO peak shifts slightly depending on the polarization direction.
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This can be explained by birefringence.
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// 我们预测，随着入射方向偏移，LO 峰会向着高频方向移动。此外，我们也注意到 LO 也会与载流子产生影响。
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// 在 n 型半导体中，LOPC 模式将代替 LO 模式；在 p 型半导体中，LO 模式仍然单独存在，但它的半高宽会受到载流子浓度的影响。
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#include "table-pol.typ"
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