N型4H-SiC低温拉曼光谱特性

Low-Temperature-Dependent Characteristics of Raman Scattering in N-Type 4H-SiC

利用拉曼散射技术对N型4H-SiC单晶材料进行了30～300K温度范围的光谱测量。实验结果表明,随着温度的升高,N型4H-SiC单晶材料的拉曼峰峰位向低波数方向移动,峰宽逐渐增宽。分析认为,晶格振动随着温度的升高而随之加剧,其振动恢复力会逐渐减小,使振动频率降低;原子相对运动会随温度的升高而加剧,使得原子之间及晶胞之间的相互作用减弱,致使声学模和光学模皆出现红移现象。随着温度的升高,峰宽逐渐增宽。这是由于随着温度的升高声子数逐渐增加,增加的声子进一步增加了散射概率,从而降低了声子的平均寿命,而声子的平均寿命与峰宽成反比,因此随着温度的升高峰宽逐渐增宽。声子模强度随温度升高呈现不同规律,E2(LA),E2(TA),E1(TA)和A1(LA)声子模随着温度升高强度单调增加,而E2(TO),E1(TO)和A1(LO)声子模强度出现了先增后减的明显变化,在138K强度出现极大值。分析认为造成原因是由于当温度高于138K时,高能量的声子分裂成多个具有更低能量的声子所致。

In the present paper, Raman scattering techniques for N-type 4H-SiC single crystal material were performed at the temperatures ranging from 30 to 300 K. These measurements revealed that the Raman phonon modes have a redshift and the lin- ewidth gradually broadens with temperature increasing. Based on the experimental results, the reason for the redshift and broad- ening is discussed. With the temperature increasing, the lattice thermal vibration restoring force decreases with the lattice ther- mal vibration increasing, and the interaction between the atom and unit cell is weakened, resulting in the redshift of the acoustic phonon modes and optical phonon modes. The mean number of phonons increase with the temperature increasing, which leads to an increase in scattering probability, in result of decreasing the phonon lifetime. The linewidth and phonon lifetime is inversely proportional, so the linewidth gradually broadens with temperature increasing. Phonon mode intensity with increasing tempera- ture showed different laws that the intensity of E2 （LA）, E2（TA）, E1 （TA） and A1 （LA） phonon mode monotonously increases as the temperature increases, while the intensity of E2 （TO）, E1 （TO） and A1 （LO） phonon mode decreases after 138 K. The reason is that the high-energy phonons split into multiple lower energy phonons.