拉曼光谱研究n型4H-和6H-SiC晶体的载流子浓度

半导体材料的纵光学声子与等离子体激元耦合模(LOPC模)能够提供材料电学方面的相关信息。本文在室温下测得了n型4H-和6H-SiC的拉曼光谱,分析了掺入的杂质对于SiC晶体拉曼光谱的影响,通过拟合n型4H-和6H-SiC晶体的LOPC模的线型得到等离子体频率,并由此从理论上计算了载流子浓度。载流子浓度的理论计算值与霍尔测量的结果符合得很好。研究结果进一步证实了对于n型4H-和6H-SiC晶体,可以通过分析LOPC模的线形来较准确地给出相关材料的载流子浓度。     

Effect of 6H-SiC （1120） substrate on epitaxial graphene revealed by Raman scattering

A nonpolar SiC(1120) substrate has been used to fabricate epitaxial graphene (EG). Two EGs with layer numbers of 8-10 (referred to as MLG) and 2-3 (referred to as FLG) were used as representative to study the substrate effect on EG through temperature dependent Raman scattering. It is found that Raman lineshifts of G and 2D peaks of the MLG with temperature are consistent with that of a free graphene predicted by theory calculation, indicating that the substrate influence on the MLG is undetectable. While Raman G peak lineshifts of the FLG to that of the free graphene are obvious, however, its lineshift rate (-0.016 cm^-1/K) is almost one third of that (-0.043 cm^-1/K) of a EG on 6H-SiC (0001) in the temperature range from 300 K to 400 K, indicating a weak substrate effect from SiC (1120) on the FLG. This renders the FLG a high mobility around 1812 cm²- ·V^-1-·s^-1 at room temperature even with a very high carrier concentration about 2.95× 10¹³ cm^-2 (p-type). These suggest SiC (1120) is more suitable for fabricating EG with high performance.     

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon--plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×10¹⁸ cm^-3 and 8×10¹⁸ cm^-3 with a carrier mobility of 30--55 cm²/(V·s) for n-type 4H-SiC substrates and 1×10¹⁶--3×10¹⁶ cm^-3 with mobility of 290--490 cm²/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×10¹⁶ cm^-3 with mobility of 380 cm²/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.     

GaN薄膜拉曼散射光谱的研究

利用拉曼散射光谱对在SiC衬底上采用MOCVD异质外延的未故意掺杂GaN薄膜特性进行研究发现E2模式向频率低的方向漂移表明在GaN薄膜中存在张力,由于SiC衬底不平整度增加引起更多位错的出现,从而引起拉曼谱中E2模式的加宽,因此通过选择平整度较好的衬底可以减小缺陷密度,提高薄膜的质量,此外A1(LO)模式的出现与强度可以用来表征未掺杂GaN的薄膜质量。     

Stimulated Raman scattering and spontaneous Raman solitons in ammonia

In this study we report the first observation of spontaneous Raman solitons in stimulated Raman scattering (SRS) by the gas NH₃. The scattered radiation is called Stokes radiation. Raman solitons are of considerable interest, because their existence can be explained by quantum-mechanical fluctuations of the electromagnetic field in vacuum. We have observed spontaneous Raman solitons in a forward SRS configuration for two different molecular transitions of NH₃, the laser emissions at 58 μm and 72.6 μm wavelength. These are optically pumped by 10 μm CO₂-laser pulses with a duration of 100 ns and an energy of 150 mJ. Spontaneous Raman solitons are short spikes in the pump pulse which occur during its depletion. Their origin is the rapid π phase change of the Stokes seed. In contrast to other laboratories we have used single-pass cells. Thus, we have succeeded in observing multiple spontaneous Raman solitons during one pump pulse. Previous experiments with multi-pass cells never showed multiple solitons. Since multiple spontaneous Raman solitons have already been reported in an earlier experiment with a single-pass cell filled with hydrogen at high pressure, we conclude that such multiple Raman solitons can be observed mainly in this type of gas cell. Subsequently, we have performed statistical measurements on the delay time and the height of the spontaneous Raman solitons in the depleted pump pulse for the 58 μm-NH₃ emission. We have compared these statistics with theory and equivalent experimental results of other laboratories. They are in good agreement with the assumption that quantum-mechanical fluctuations are the origin of spontaneous Raman solitons. The most recent theories postulate that the origin of the formation of spontaneous Raman solitons can be explained by the rapid π phase change of the Stokes seed as well as that of the laser or polarization wave. Therefore, we have determined the phase of the spontaneous Raman solitons relative to the depleted pump pulse. Although, such changes of sign of the relative phase have already been observed in an earlier SRS experiment with hydrogen at high pressure, we did not detect any in our experiment. Therefore, we conclude that in this experiment the π phase change occurs in the Stokes or polarization wave.     

无紫外光照射下n型15R-SiC及6H-SiC晶体的电化学腐蚀

本文在无紫外光照射下通过电化学腐蚀法制备了多孔n型15R-及6H-SiC,并用扫描电子显微镜(SEM),拉曼散射及X射线衍射仪(XRD)对多孔层的结构进行了分析。结果表明:晶体的晶型及氧化条件等因素对多孔结构有较大影响。首次观察到多孔n型15-SiC的半圆管状结构,其孔隙率约是66%。     

Optical spectroscopy study of damage evolution in 6H-SiC by H_2~+ implantation

Lattice defects induced by ion implantation into Si C have been widely investigated in the decades by various techniques. One of the non-destructive techniques suitable to study the lattice defects in Si C is the optical characterization. In this work, confocal Raman scattering spectroscopy and photoluminescence spectrum have been used to study the effects of 134-ke V H_2~+ implantation and thermal treatment in the microstructure of 6 H-Si C single crystal. The radiation-induced changes in the microstructure were assessed by integrating Raman-scattering peaks intensity and considering the asymmetry of Raman-scattering peaks. The integrated intensities of Raman scattering spectroscopy and photoluminescence spectrum decrease with increasing the fluence. The recovery of the optical intensities depends on the combination of the implantation temperature and the annealing temperature with the thermal treatment from 700℃ to 1100℃. The different characterizations of Raman scattering spectroscopy and photoluminescence spectrum are compared and discussed in this study.     

Raman scattering by hot polaritons in NaClO3

Summary By nonlinear mixing of IR and visible radiation,i.e. coherent Raman scattering by polaritons driven by a CO₂ laser, it has been possible to get high-resolution spectra for theF-modes of a NaClO₃ crystal in the region (907÷957) cm⁻¹, where several isotopic modes are present. The obtained independent measurement of the refractive index and absorbance allows an accurate determination of the polariton dispersion curve and its width inq-space. The experimental data confirm the existence of a localized isotopic mode at 931 cm⁻¹ and a strongly damped isotopic mode near 960 cm⁻¹. Finally, an accurate measurement of the dispersion of the second-order nonlinear polarizability in the same energy region has been obtained for the first time. This work was supported by the Italian Consiglio Nazionale delle Ricerche and Ministero della Pubblica Istruzione.     

Raman scattering from microcrystals

This review discusses the size effects on Raman scattering from microcrystals. For ionic microcrystals, the existence of surface phonon modes is predicted from electromagnetic theories. It is shown that Raman spectroscopy is very effective to detect the surface phonon modes. The size effects on nonpolar phonons in covalent microcrystals can also be studied by Raman spectroscopy. However, the relaxation of the wave-vector selection rule or the phonon confinement explains only some of the experimental data. Development of lattice dynamical theories of Raman scattering from microcrystals including surface effects is highly required. Enhancement of Raman intensities arising from the excitation of electromagnetic normal modes of microcrystals is also discussed.     

表面极化声子的喇曼散射研究

本文主要研究表面极化声子的喇曼散射.在探测表面极化声子的衰减全反射方法以及喇曼散射方法的基础上,借鉴了Kretchmann配置的衰减全反射喇曼散射法,提出了Otto配置的衰减全反射喇曼散射法,并以CaF_2单晶体为样品做了一系列实验.同时,本文利用了能量守恒及平行于样品表面的能量守恒关系,利用了格林函数或响应函数所推出的表面极化声子的一般频散关系,理论上给出了CaF_2-空气表面极化声子的一个唯象的频散关系.实验上用特定设计制做的一个样品架对CaF_2样品测定了它与空气界面上的表面极化声子的频散关系,比较结果,理论与实验取得了比较令人满意的一致.     

Soliton experiments in stimulated Raman scattering

Experimental observations of solitons in stimulated Raman scattering are reported. Soliton formation resulted from the introduction of a phase shift in the incident Stokes beam as predicted by theory. Pulse sharpening and retardation on propagation in the Raman medium have been observed along with amplitude diminution. The first two features were predicted and the third was not. Spontaneous soliton formation has been observed in the absense of any amplitude modulation or apparent phase shift in the optical fields, indicating that additional sets of initial conditions may result in soliton formation.Work performed under the auspices of the U.S. Department of Energy.     

High pressure study of cubic BN and SIC (Raman scattering and EDS)

Abstract

First-order Raman spectra and lattice parameters of BN and SiC were measured simultaneously up to a pressure of 34 and 45 GPa. The bulk moduli, their pressure derivatives and the volume dependence of Gruneisen parameters of LO and TO phonons were obtained. The average Gruneisen parameter for BN does not change much; on the contrary, the same parameter for SiC decreases under pressure. The definite connection between the crystal stability, the pressure behaviour of the optical phonons and the electron structure of the ionic cores is pointed out.     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

Raman scattering by hot and thermal polaritons in crystal quartz

Summary Nonlinear mixing of IR and visible radiation,i.e. coherent Raman scattering by polaritons driven by a CO₂ laser, has been used to obtain the dispersion curve and its width inq-space of the polariton associated to theE-phonon at 1065 cm⁻¹ in crystal quartz. It is shown in this paper that a direct method to determine indipendently, with high precision, the refractive index and absorbance of a crystal can be obtained in this way. The results are compared with accurate data obtained from Raman scattering by polaritions in thermal equilibrium and very good agreement is found between the two measurements. It is finally shown that nonlinear-mixing techniques turn out to be completely consistent with the simple picture of scattering of light by hot polaritons. This work was supported by the Italian Consiglio Nazionale delle Ricerche del Ministero della Pubblica Istruzione.     

Raman scattering in defect chalcopyrite crystals

Summary Raman-scattering measurements on the defect chalcopyrite compounds CdGa₂Se₄, CdGa₂S₄, ZnGa₂S₄ are reported. The phonon picture is consistent with the space group. A comparison between corresponding phonons suggests a simple model for the vibrational dynamics. Preliminary Raman results in the mixed compound Cd _x Zn_1−x Ga₂S₄ and in resonance conditions in CdGa₂Se₄ are presented. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

钠分子的共振喇曼散射

本文用80mW6238.2(?)的He-Ne激光激发钠蒸气,用一种新颖的棱镜反射光路,从后向首次观察到了56O0～8200(?)钠分子的共振喇曼谱.     

Raman scattering of neutron irradiated 6H-SiC

Confocal micro-Raman spectroscopy was preformed to investigate the structural damage of SiC neutron irradiated with the fluences of 1.72×10¹⁹ and 1.67×10²⁰ n/cm². In addition to characteristic peaks, several additional signals related to Si–Si, Si–C, and C–C vibration modes were monitored. The vibration mode associated with C_SiV _C complexes was identified to be the unambiguous peak at 575 cm^?1 which appears initially in the sample post-irradiation annealed at 800 °C. The defect-induced phonon confinement effect results in an asymmetric broadening with a low-frequency tail of the optical phonon peaks. The sigmoidal thermal recovery behavior of the optical phonon frequencies indicates that the reduction of FLO_0/6–FTO_2/6 splitting originates mainly from the isolated vacancies and interstitials.     

分子间费米共振增强二元溶液体系的受激拉曼散射研究

研究了液芯光纤内不同体积比的甲苯和间二甲苯二元混合溶液的受激拉曼散射.实验结果表明:在不同的体积比之下二元溶液的环呼吸振动模式1002 cm^-1,甲基的CH伸缩振动模式2920 cm^-1 以及芳香环CH对称伸缩振动模式3058 cm^-1的拉曼带同时产生受激拉曼辐射,并且2920 cm^-1 和 3058 cm^-1 拉曼带的一阶受激拉曼散射阈值要低于1002 cm^-1拉曼带的二阶 关键词： 分子间费米共振 二元溶液 受激拉曼散射 拉曼散射截面     

拉曼光谱方法研究SiC晶体的晶型

本文采用拉曼光谱方法对改进Lely生长法制备的SiC单晶的结构进行了分析,结果表明:晶体的结构为6H,样品的内部缺陷较多,其中存在4H-SiC。另外我们还测量了SiC晶体的高阶拉曼谱,并对其进行了分析归属。从分析结果来看拉曼光谱是研究SiC的晶体结构和生长质量有效快捷的方法。     

Raman analysis of defects in n-type 4H-SiC

This paper employs micro-Raman technique for detailed analysis of the defects (both inside and outside) in bulk 4H-SiC. The main peaks of the first-order Raman spectrum obtained in the centre of defect agree well with those of perfect bulk 4H-SiC, which indicate that there is no parasitic polytype in the round pit and the hexagonal defect. Four electronic Raman scattering peaks from nitrogen defect levels are observed in the round pit (395\,cm$^{-1}$, 526\,cm$^{-1}$, 572\,cm$^{-1}$, and 635\,cm$^{-1})$, but cannot be found in the spectra of hexagonal defect. The theoretical analysis of the longitudinal optical plasmon--phonon coupled mode line shape indicates the nonuniformity of nitrogen distribution between the hexagonal defect and the outer area in 4H-SiC. The second-order Raman features of the defects in bulk 4H-SiC are well-defined using the selection rules for second-order scattering in wurtzite structure and compared with that in the free defect zone.