准周期Fibonacci超晶格的低频纵声学模喇曼光谱

本文报导了准周期 Fibonacci 金属 Nb/cu 和非晶半导体 a-Si:H/a-SiN_x:H(x≈1)超晶格纵向低频声学声子模的喇曼散射研究。在弹性连续模型的基础上,理论模拟表明了这些低频声子光谱是对应于周期超晶格中声学模在约化布里渊区的折叠效应。     

Ge_xSi_(1-x)/Si超晶格中周期不均匀对折叠声学声子拉曼线宽的影响

本文报导了在 Ge_Si_(1-x)/Si 超晶格中观察到了超晶格周期不均匀导致的折叠声学声子拉曼谱线展宽,这种谱线展宽与折叠指数(m)有关,折叠声学声子的谱线越宽,m 相同的折叠声学声子具有相同的线宽。     

非晶态半导体超晶格中的纵声学声子折叠效应

报道用喇曼散射技术对a-Si:H/a-SiN:H和a-Si:H/a-SiC:H两种周期性超晶格中纵声学声子折叠模的系列研究结果。获得了a-Si:H/a-SiC:H超晶格的纵声学声子折叠谱。并用弹性连续介质模型对折叠纵声学声子的色散关系进行了理论计算,其结果与实验值符合得很好。同时,对布里渊区中心频隙的存在做了简单的讨论。 关键词：     

ZnSe—ZnTe应变层超晶格的纵光学声子模

本文首次利用室温非共振喇曼后向散射测得具有7.3％晶格失配的ZnSe-ZnTe应变层超晶格限制在ZnSe层中的纵光学声子模。计算了限制效应引起的声子模频率的红移,以及弹性应变引起的声子模的移动,它比前者大得多。ZnSe层所受拉伸应变引起声子频率红移,ZnTe层所受压缩应变引起声子频率蓝移。同时在喇曼光谱中观察到由于这种效应导致出现的纵光学声子折叠模。     

Ge_x Si_(1-x)/Si超晶格中纵声学声子的喇曼散射强度研究

本文用光弹理论,在全面考虑了超晶格中两种材料的声速,质量密度和光弹常数存在差别的基础上,计算了Ge_xSi_(1-x)/Si超晶格中折迭纵声学声子的喇曼散射强度,在高达50cm~(-1)的频率范围内,理论值和实验符合得很好。     

SiGe／Si应变层超晶格的结构和光散射特性

ＳｉＧｅ／Ｓｉ应变层超晶格是一种亚稳态结构。在高温下，原子互扩散导致异质界面展宽；同时，由晶格失配引起的应变会发生弛豫。这两种现象都将导致超晶格结构的变化。本文证明，利用拉曼散射光谱可以定量地反映这一变化。（１）我们首先把界面陡峭超晶格中折叠纵声学（ＦＬＡ）声子的色散关系和散射强度的理论计算推广到界面展宽的情形。然后，我们分析了测量得到的在不同温度下退火的超晶格样品的ＦＬＡ声子散射谱。通过测量谱和计算结果的比较，得到了不同退火温度下异质界面的展宽或扩散长度。我们也从理论上证实并且在实验上观察到：ＬＦＡ声子在超晶格布里渊区边界的能带分裂随异质界面的展宽而减小。（２）我们定量地分析了ＳｉＧｅ／Ｓｉ超晶格中光学声子的散射谱随退火温度的变化。考虑到原子互扩散引起的合金组份变化以及应变弛豫两大因素，我们发现，对上述合金型ＳｉＧｅ／Ｓｉ超晶格，在８００℃下退火１０分钟，超晶格中由于原子互扩散引起的界面展宽是非常严重的。相比之下，晶格弛豫的大小与材料的生长条件有关。对较低温度（４００℃）下生长的超晶格，晶格弛豫量并不大，仅为１６％。这一结果也得到了Ｘ－射线衍射谱的支持。（３）我们讨论了与ＳｉＧｅ／Ｓｉ应变层超晶格的结     

ZnS-ZnSe应变层超晶格的纵声学声子折叠模

本文报道室温下ZnS-ZnSe应变层超晶格的纵声学声子折叠模的喇曼光谱测量,在10～90cm~(-1)范围内得到三级双峰结构。     

高温超导体的喇曼光谱研究

<正> 本文评述高温超导体的喇曼光谱研究工作,较详细地介绍了超导体中声子、电子和磁子的喇曼散射及其与高温超导性的关联.一、引言二、晶体结构与晶格振动谱晶体结构与对称性晶格动力学     

自发有序Ga_（0．5）In_（0．5）P合金的喇曼光谱研究

在室温和非共振条件下测量了自发有序Ｇａ０．５Ｉｎ０．５Ｐ合金的喇曼散射谱。测得的３８０，３６０和３３０ｃｍ－１附近的三个散射峰分别归结于合金中的类ＧａＰ的ＬＯ模、类ＩｎＰ的ＬＯ模和ＴＯＭ。发现有序合金的类ＧａＰ的ＬＯ模的声子频率随着合金的带隙能量的降低而增大。认为这与在有序合金中形成沿［１１１］方向的（ＧＳＰ）１／（ＩｎＰ）１单层超晶格有一定联系。在几种偏振配置下测得的有序合金的喇曼谱的偏振特性与通常的闪锌矿结构的半导体材料的偏振特性类似。     

GaAs/AlAs超晶格的近共振喇曼散射研究

本文介绍GaAs/AlAs超晶格的室温近共振喇曼散射测量结果。由于超晶格中Fr?hlich相互作用的共振增强效应,GaAs LO声子偶模的散射得到了很大的增强。和前人的结果一样,在偏振谱我们观察到了偶模。但和前人的结果不同,在退偏振谱中我们观察到的是奇模,而不是偶模。从而证明了在近共振条件下LO声子限制模仍遵从与非共振时一样的选择定则。二级喇曼散射实验结果表明,在偏振谱中二级谱是由两个偶模组合而成,而在退偏振谱中的二级谱与前人的结果不同,由一个奇模与一个偶模组合而成。上述结果与最近提出的黄朱模型的预言是一 关键词：     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

Folding of acoustic-phonon modes in GaAs/AlAs (311)A superlattices in the direction perpendicular to nanofacets

GaAs/AlAs superlattices grown on a nanofaceted (311)A surface are studied by atomic-force microscopy and Raman scattering spectroscopy. The spectra of backscattered light in the direction perpendicular to the nano-facets exhibit peaks corresponding to folded acoustic phonons. The period estimated from the positions of these peaks is 1.6 nm, which corresponds to half of the spacing between the nanofacets.     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

Angular dependence of Raman scattering selection rules for long-wavelength optical phonons in short-period GaAs/AlAs superlattices

The angular dependence of Raman scattering selection rules for optical phonons in short-period (001) GaAs/AlAs superlattices is calculated and experimentally studied. Experiments are performed using a micro-Raman setup, in the scattering geometry with the wavevectors of the incident and scattered light lying in the plane of superlattices (so-called in-plane geometry). Phonon frequencies are calculated using the Born model taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra are calculated in the framework of the deformation potential and electro-optical mechanisms. Calculations show an angular dependence of the selection rules for optical phonons with different directions of the wavevectors. Drastic differences in the selection rules are found for experimental and calculated spectra. Presumably, these differences are due to the Fröhlich mechanism in Raman scattering for short-period superlattices.     

Low frequency Raman scattering from acoustic phonons confined in ZnO nanoparticles

We report here the first observation of the low frequency Raman scattering from acoustic phonons in semiconducting zinc oxide (ZnO) nanoparticles without embedding in any solid matrix. ZnO nanoparticles (size 5-10 nm) with nearly spherical shape have been synthesized using a chemical route. A shift in the phonon peaks toward higher frequencies along with broadening was observed with a decrease in particle size. The size dependence of the acoustic phonons in ZnO nanoparticles is explained using Lamb's theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low frequency Raman scattering originates from the spherical (l = 0) and quadrupolar vibrations (l = 2) of the spheroidal mode due to the confinement of acoustic vibrations in ZnO nanoparticles.     

Anomalous behavior of phonons in NbCu metallic superlattices

Brillouin and Raman scattering investigations have been performed on NbCu metallic superlattices. Anomalies have been observed in the surface acoustic phonons and bulk acoustic phonons versus superlattice wavelength. These anomalies occur simultaneously at the same superlattice wavelength ≈ 19 Å.     

Acoustic,optical, and interface phonons in BeTe/ZnSe superlattices

Raman scattering in a number of BeTe/ZnSe type-II superlattices which share no common cations or anions in the interfaces was studied. Folded acoustic phonons; LO phonons of the first, second, and third order in the ZnSe layers; and Kliewer-Fuchs-type electrostatic interface phonons were observed when excited in resonance with the direct exciton transition in the ZnSe layers. Nonresonant excitation produced LO phonons in the ZnSe and BeTe layers and a high-frequency mechanical interface mode, assigned tentatively to a local vibration of the interface Be-Se bond.     

Raman scattering from crystalline and amorphous (GaSb)1-xGex semiconducting films

The first reported Raman scattering experiments have been performed on single crystal and amorphous films of the metastable alloy (GaSb)_1-xGe_x with compositions across the pseudobinary phase diagram. In crystalline films, the optical phonons exhibit a “one-two” type mode behavior. Broadening and softening of the Raman peaks with increasing alloy concentrations are attributed to a relaxation of q-vector selection rules due to substitutional disorder on both the cation and anion sublattices. Additionally, disorder induced scattering from the zone-edge acoustic phonons was observed. In amorphous alloy films, the reduced Raman spectra were compared to the one-phonon densities of states of the end-member crystals. The resulting apparent lack of polarization dependence indicated that the amorphous films were of the random network type.     

First-principles studies of the vibrational properties of amorphous carbon nitrides

Raman spectra of amorphous carbon nitride films (a-C:N) resemble those of typical amorphous carbon (a-C), and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C:N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car-Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.