@@ -7,22 +7,26 @@ These phonons were categorized into two groups and discussed separately, accordi
The negligible-polarity phonons were initially analyzed at the #sym .Gamma point,
The negligible-polarity phonons were initially analyzed at the #sym .Gamma point,
disregarding the incidence configurations.
disregarding the incidence configurations.
This simplification i s justified because the participating negligible-polarity phonon modes
This simplification wa s justified because
ar e nearly identical across all Raman experiments,
th e negligible-polarity phonon modes participated in Raman scattering
with their frequencies differing by only #sym .tilde 0.1 cm#super [ -1] difference in frequency
were nearly identical across all incidence configurations,
(see gray lines in @figure-discont ).
with their frequencies differ ing by only #sym .tilde 0.1 cm#super [ -1]
(see the intersection of gray solid lines and orange dashed lines in @figure-discont b and c).
There a re eight Raman-active negligible-polar modes in 4H-SiC,
There we re eight Raman-active negligible-polarity modes in 4H-SiC at the #sym .Gamma point ,
corresponding to three irreducible representations of the C#sub [ 6v] group (A#sub [ 1] , E#sub [ 1] , and E#sub [ 2] ).
corresponding to three irreducible representations of the C#sub [ 6v] group (A#sub [ 1] , E#sub [ 1] , and E#sub [ 2] ).
We named these modes as
We named these modes as
E#sub [ 2] -1, E#sub [ 2] -2, E#sub [ 1] -1, A#sub [ 1] -1, E#sub [ 1] -2, E#sub [ 2] -3, E#sub [ 2] -4, and A#sub [ 1] -2,
E#sub [ 2] -1, E#sub [ 2] -2, E#sub [ 1] -1, A#sub [ 1] -1, E#sub [ 1] -2, E#sub [ 2] -3, E#sub [ 2] -4, and A#sub [ 1] -2,
in order of increasing frequency.
in order of increasing frequency (see @figure-discont and @figure-raman ) .
The Raman tensor forms of each mode were derived by further considering their representations in the C#sub [ 2v] group,
The Raman tensor forms of each mode were derived by further considering their representations in the C#sub [ 2v] group,
and were summarized in @table-rep .
and were summarized in @table-rep .
#include "figure-discont.typ"
#include "figure-discont.typ"
#include "figure-raman.typ"
#include "table-rep.typ"
#include "table-rep.typ"
// negligible-polarity modes (only modes visible in our experiments are labeled)
Peaks corresponding to seven Raman-active negligible-polarity phonons were observed in our experiments
Peaks corresponding to seven Raman-active negligible-polarity phonons were observed in our experiments
(only the E#sub [ 2] -4 mode was not observed),
(only the E#sub [ 2] -4 mode was not observed),
which is more than all previous experiments (where only five or six peaks were typically reported).
which is more than all previous experiments (where only five or six peaks were typically reported).
@@ -43,7 +47,7 @@ The E#sub[2]-4 modes was calculated
making it not visible in our and all previous experiments.
making it not visible in our and all previous experiments.
The E#sub [ 1] -2 mode was also located close to the E#sub [ 2] -3 mode and has weak Raman intensity,
The E#sub [ 1] -2 mode was also located close to the E#sub [ 2] -3 mode and has weak Raman intensity,
making it also unobservable in previous experiments.
making it also unobservable in previous experiments.
However, the E#sub [ 1] -2 mode was observable in our experiments of y(zx)#overline [ y] ,
However, the E#sub [ 1] -2 mode was observable in our experiments of y(zx)#overline [ y] with extended acquisition time ,
where the scattering of the E#sub [ 2] -3 mode was suppressed while that of the E#sub [ 1] -2 mode was enhanced,
where the scattering of the E#sub [ 2] -3 mode was suppressed while that of the E#sub [ 1] -2 mode was enhanced,
thanks to the different representations of these two modes.
thanks to the different representations of these two modes.
Our experiments reported the observation of the E#sub [ 1] -2 peak for the first time,
Our experiments reported the observation of the E#sub [ 1] -2 peak for the first time,
@@ -58,51 +62,98 @@ For example, the Raman tensor of the E#sub[2]-3 mode was substantially larger
This highlighted a significant gap in established theory
This highlighted a significant gap in established theory
that rigorous symmetry analysis could only predict the non-zero components of the Raman tensors,
that rigorous symmetry analysis could only predict the non-zero components of the Raman tensors,
but not their magnitudes.
but not their magnitudes.
In order to address the limitations of existing theories,
In order to address the limitations of existing theories,
a method for estimating the magnitudes of Raman tensors was proposed.
a method for estimating the magnitudes of Raman tensors was proposed.
By analyzing the local environment of individual atoms,
By analyzing the local environment of individual atoms,
this approach decomposed their contributions to the Raman tensor into two parts:
this approach decomposed their contributions to the Raman tensor into two parts:
a dominant component (invariant across similar environments, denoted as $ a _ i $ , where $ i in { 1 , 2 , 3 , 4 } $ )
a dominant component (invariant across similar environments, denoted as $ a _ i $ , where $ i in { 1 , 2 , 5 , 6 } $ )
and several secondary components (reflecting environmental variations,
and several secondary components (reflecting environmental variations,
denoted as $ epsilon _ i $ , $ eta _ i $ , and $ zeta _ i $ , where $ i in { 1 , 2 , 3 , 4 } $ ,
denoted as $ epsilon _ i $ , $ eta _ i $ , and $ zeta _ i $ , where $ i in { 1 , 2 , 5 , 6 } $ ,
and $ | epsilon _ i | + | eta _ i | + | zeta _ i | << | a _ i | $ was assumed).
and $ | epsilon _ i | + | eta _ i | + | zeta _ i | << | a _ i | $ was assumed).
Detailed derivations were provided in @appd-predict , with results summarized in @table-nopol .
Detailed derivations were provided in @appd-predict , with results summarized in @table-nopol .
Notably, the E#sub [ 2] -3 mode was the only mode that retains the $ a _ i $ term,
Notably, the E#sub [ 2] -3 mode was the only mode that retains the $ a _ i $ term,
indicating a constructive interference of contributions from the local environment of individual atoms.
which indicating a constructive interference of contributions from the local environment of individual atoms.
This stood in contrast to other negligible-polarity modes where such contributions tend to cancel out,
This stood in contrast to other negligible-polarity modes where such contributions tend to cancel out,
thereby explaining the exceptionally high Raman tensor magnitude observed for the E#sub [ 2] -3 mode.
explaining the exceptionally high Raman tensor magnitude observed for the E#sub [ 2] -3 mode.
To further investigate the Raman spectra,
To achieve a more precise investigation of the Raman spectra
an analysis of negligible-polarity phonons off the #sym .Gamma point was conducted
and prepare for analyzing impurity and charge carrier effects,
the analysis of negligible-polarity phonons off the #sym .Gamma point was conducted
by comparing experimental and calculated results under various lazer incidence directions.
by comparing experimental and calculated results under various lazer incidence directions.
The E#sub [ 2] -3 peak searved as a calibration reference under various experiments,
The E#sub [ 2] -3 peak searved as a calibration reference under various experiments,
since its position remained virtually invariant between normal and edge incidence
since its position was calculated to be virtually invariant between normal and edge incidence
(with a shift of only #sym .tilde 0.004 cm#super [ -1] ).
(with a shift of only #sym .tilde 0.004 cm#super [ -1] ).
The E2 -1, E2 -2, and A1 -1 modes exhibited observable frequency shifts. These experimental results are in good agreement with our calculations, as shown in Figure X and Table Y."
#include "figure-e12.typ"
The E#sub [ 2] -1, E#sub [ 2] -2, and A#sub [ 1] -1 modes exhibited observable shifts,
The E#sub [ 2] -1, E#sub [ 2] -2, and A#sub [ 1] -1 modes exhibited observable shifts,
and the experimental results were in good agreement with our calculations, as shown in figure and table .
and the experimental results were in good agreement with our calculations, as shown in @ fig-nopo-diff .
Our results further confirmed the accuracy of both our experiments and calculations.
Our results further confirmed the accuracy of both our experiments and calculations.
#include "figure-nopo-diff.typ"
#include "figure-nopo-diff.typ"
为了进一步探索拉曼光谱, 离开Gamma点的弱极性声子的分析也被进行, 通过比对不同方向入射下的实验和计算结果。
=== Strong-polar Phonons
沿着不同方向入射的话,强声子的模式是不同的。
// 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。
// 这导致不同方向的入射/散射光的声子模式不同。
// 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。
// 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)。
// 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;
// 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。
// 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。
// (没有在图上表示)
半导体中的强极性声子模式强烈地依赖于入射光的方向,
这是由于半导体中原子之间的长程库伦相互作用所致,
表现在散射光谱中, Gamma点附近不连续( 见@figure-discont )。
具体来说,当入射光沿着 z 方向时,起作用的是 A-Gamma 线上的声子模式(图中的上半部分的橘线),它们适用于群 C6v。
这时会有一个 TO 模式( C6v 中的 E1) 和一个 LO 模式( C6v 中的 A1) 。
而当沿着 y 方向入射时,起作用的是 Gamma-K 线上的声子模式(图中下半部分的橘线),它们不再适用于群 C6v, 而只适用于群 C2v;
这时将会出现三个模式, 包括一个LO( 沿y方向振动, 对应C2v中的A1) 和两个TO模式( 根据振动方向, 命名为B2-x和B2-y, 对应C2v中的B1和B2) 。
当入射光不是严格沿着坐标轴方向, 而是在xOy面内呈现一定的夹角时( 掠入射, 以及考虑了斜切的正入射) , 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。
Strong-polarity phonon modes caused by different incident light directions are different,
due to long-range Coulomb interactions between atoms in semiconductors,
showing discontinuity in the scattering spectra near the #sym .Gamma point (see @figure-discont ).
For incident light propagating along the z direction (phonon modes on the A-#sym .Gamma line),
symmetry of C#sub [ 6v] point group applies and leading to two modes (two peeks in Raman spectra),
including an E#sub [ 1] mode (pink line in @figure-discont , vibration in-plane)
and an A#sub [ 1] mode (green line in @figure-discont , vibration along z-direction).
When the light is incident along other directions, symmetry in plane was broken and C#sub [ 6v] symmetry no longer holds,
and there will be three phonon modes in theory.
For example, when the light is incident along the y direction (phonon modes on the #{ sym . Gamma } -K line),
symmetry of C#sub [ 2v] applies and three modes exist in dispersion curves,
including an A#sub [ 1] mode (green line in @figure-discont , vibration along z direction),
a B#sub [ 2] mode (blue line in @figure-discont , vibration along x direction),
and a B#sub [ 1] mode (red line in @figure-discont , vibration in y direction).
When the light is incident along a direction between z and y,
three phonon modes will exist, but vibration in the mixed direction.
将理论/计算结果与实验对比。
我们将计算与实验结果进行了对比。衬底中的LO峰与plasmon耦合形成LOPC峰, 因此与计算结果大不相同。
对于TO峰, 在正入射中, 它与E2-3模式的距离为xxx; 在掠入射中, 它与E2-3模式的距离为xxx。
Many Raman experiments on 4H-SiC with incident light along the z direction have observed two peaks.
However, no experiments have reported three peaks with incident light along other directions.
In our experiment, we found the third, and it satisfied properties we expected.
In our experiments, we found that the third peak only appears when focusing inside the sample.
E1 的情况。
注意到在正入射中, 理论上不能被观察到的E#sub [ 1] -1模式也被观察到了。
与弱极性的 E1-1 模式类似,我们也认为这是由于入射光并非完全沿 z 轴入射所致。
但与弱极性 E1-1 模式不同的是,强极性 E1-1 模式在 xy 的偏振下并没有更强反而更弱。
这是因为E1这时不再是严格的E1模式, 而是分裂成了两个相近的模式。
我们的计算表明, 在2度的入射角下, E1分裂的两个模式非常接近。
其中某个模式会怎样怎样,另一个会怎样怎样。
// 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。
// 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。
#include "table-pol.typ"
为了检验我们实验和计算的准确性,我们测量和计算了不同入射方向下弱极性峰位的微小移动。
在我们的计算中, 对比于正入射, 肩入射中, E2-1与A1-1会有可观测的蓝移, 同时E2-2会有可观测的红移。
E2-1 E2-2 的计算结果与实验吻合较好, A1-1与实验结果略有出入。
这可以被解释为频率移动的原因不同。
对于 E2 模式,其极性为零,色散曲线在 Gamma 点附近连续, 因此频率取决于色散曲线在Gamma附近的曲率;
而 A1 模式具有弱但非零的极性,它在 Gamma 点附近不连续,
计算频率偏移需要同时考虑色散曲线曲率和阶跃大小的影响。
在 A1-1 模式中,阶跃大小占主导。
而在重掺杂n型衬底中, 阶跃大小被载流子屏蔽显著减小, 因此导致理想模型的计算结果与实验结果不符。
通过将载流子屏蔽效应纳入计算,计算结果与实验结果吻合较好,如图所示。
在强极性声子中:
在强极性声子中: