近周期超晶格中的声学声子及其光散射特性

利用转移矩阵方法计算了近周期超晶格体系（包括有限周期数的超晶格、耦合超晶格、层厚起伏和层厚渐变的超晶格）中声学声子的喇曼散射谱．结果表明上述近周期超晶格的光散射特性与理想严格周期的体系和完全无序的体系都不相同，呈现出许多独特的性质．对有限周期数的超晶格，由于边界的存在，喇曼谱中除了理想超晶格中存在的折叠声学声子峰（主峰）外，还会出现一系列等间距分布的卫星峰．对耦合超晶格体系，理想超晶格中存在的主峰将出现分裂．对层厚起伏变化的超晶格，主峰呈现非对称展宽，展宽主要出现在高波数端．计算结果和实验测得的谱线作了比 关键词：     

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：     

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     

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。     

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。     

半导体超晶格声子激光器的研究进展

太赫兹频率的相干声子在纳米尺度器件的探测和操控领域具有重要的应用价值。半导体超晶格声子激光器是实现太赫兹频率相干声子源稳定输出的重要途径。本文首先回顾了GHz到THz频率范围声学放大的多种方法,然后详细阐述了超晶格声子放大、超晶格声学布拉格镜的工作原理与设计方法以及声子激光器的阈值条件,同时总结了电抽运和光抽运结构器件的研究现状,最后简要讨论了亚太赫兹声子激光器在声-电子领域的应用。分析表明,这种能够产生强相干太赫兹声子的半导体超晶格声子激光器在纳米尺度器件的探测与成像等方面具有广阔的发展前景。     

高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格的荧光谱研究

报道用分子束外延(MBE)技术生长的x=0.4,0.8的高组分稀磁半导体Cd1-xMnxTe/CdTe超晶格低温和室温荧光谱研究结果.基态激子跃迁能级荧光谱实验结果显示高组分超晶格中具有高量子效率和高质量光发射.对激子能级随温度的变化进行了详细研究,给出激子跃迁能量的温度系数.激子能级线型的展宽随温度变化关系可用激子-纵向光学声子耦合模型解释.与光调制反射谱实验结果进行了比较.     

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。     

部分有序(AlxGa1-x)0.51In0.49P(x=0.29)合金的发光衰退过程

利用时间分辨光谱研究了(Al_xGa_1-x)_0.51In_0.49P合金的时间衰退过程,观察到载流子的转移过程和PL谱峰蓝移现象。这和变温发光谱中谱峰的Z-型依赖关系相吻合。这种现象明显表明了载流子的转移过程和子带的存在,证实了我们对超晶格带折叠效应的猜测。子带是由于有序结构的超晶格效应使导带的L带折叠到Γ带,载流子在时间衰退过程中从Γ带转移到L带。时间分辨光谱的蓝移现象同时也揭示了PL变温谱中谱峰反常蓝移现象的来源。     

高压下γ'-Fe4N晶态合金的声子稳定性与磁性

利用基于密度泛函理论的第一性原理研究了高压下有序晶态γ’-Fe₄N合金的晶格动力学稳定性与磁性. 对比没有考虑磁性的γ’-Fe₄N的声子谱, 得出压力小于1 GPa时, 自发磁化诱导了铁磁相γ’-Fe₄N基态晶格动力学稳定. 压力在1.03-31.5 GPa时, Σ线上的点(0.37, 0.37, 0)、对称点X和M 上相继出现了声子谱软化现象. 压力在31.5-60.8 GPa时, 压致效应与自发磁化对诸原子的作用达到了稳定平衡, 表现出了声子谱稳定. 压力大于61.3 GPa时, 随着压力的增大压力诱导体系动力学不稳定性越强. 通过软模相变理论对于γ’-Fe₄N, 在10 GPa下的声学支声子的M点处软化现象的处理, 发现了动力学稳定的高压新相P2/m-Fe₄N. 压力小于1 GPa时高压新相P2/m-Fe₄N 是热力学稳定的相, 且磁矩与γ’-Fe₄N的磁矩几乎相同. 2.9-19 GPa时, P2/m相的焓比γ’相的焓小, 基态结构更稳定. 大于20 GPa时, 两相磁矩几乎相同.     

RAMAN SCATTERING INTENSITIES OF FOLDED LONGITUDINAL ACOUSTIC PHONONS IN GexSi1-x/Si SUPERLATTICES

In terms of photoelastic mechanism we have investigated the Raman scattering intensities of the folded longitudinal acoustic (FLA) phonons in Ge_xSi_1-x/ Si superlattices (SLs), taking into account the differences between the acoustic and photoelastic parameters of the two constituents in the SLs. The relative intensities calculated for the FLA phonons are in excellent agreement with the experimental results at the frequencies up to about 50 cm^-1. The broadening of the linewidth arising from the so called strong acoustic attenuation, which was reported previously located around the frequency 15 cm^-1 in Ge_xSi_1-x/Si SLs(x≈0.5), has not been observed in this work.     

Surface phonons in superlattices

We report the existence of surface localized phonons for a superlattice consisting of alternating slabs (parallel to the surface) of two different crystals. The superlattice has a larger periodicity in the direction perpendicular to the slabs and therefore many phonon branches in the folded Brillouin zone. In the gaps existing between these phonon branches appear the surface localized modes.     

Probing unfolded acoustic phonons with X rays

Ultrafast laser excitation of an InGaAs/InAlAs superlattice (SL) creates coherent folded acoustic phonons that subsequently leak into the bulk (InP) substrate. Upon transmission, the phonons become "unfolded" into bulk modes and acquire a wave vector much larger than that of the light. We show that time-resolved x-ray diffraction is sensitive to this large-wave vector excitation in the substrate. Comparison with dynamical diffraction simulations of propagating strain supports our interpretation.     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Phonon-induced optical superlattice

We demonstrate the formation of a dynamic optical superlattice through the modulation of a semiconductor microcavity by stimulated acoustic phonons. The high coherent phonon population produces a folded optical dispersion relation with well-defined energy gaps and renormalized energy levels, which are accessed using reflection and diffraction experiments.     

Effective-mass eigenfunctions in superlattices and their role in well-capture

The rate of capture into the wells of a superlattice is calculated using the effective-mass eigenfunctions and the superlattice folded spectrum of polar optical phonons. The dependence on well-width is followed for the specific case of 100Å barriers. A general trend of diminishing rate with diminishing well-width is obtained, but superimposed on this is a resonance structure (periodic in well-width) associated with ‘nearly’ bound states. The results are significantly different from other published work, principally because the latter has been hitherto on the bulk-phonon spectrum, and on single wells.     

Intensities of folded acoustical raman lines in GaAs/AlAs superlattices: Theory and experiment

We quantitatively analyse forward and backwards Raman scattering results on folded acoustical phonons in GaAs/AlAs superlattices, focusing on both zone center acoustical gaps and backscattering lines intensities. We emphazise the great sensitivity of these quantities to the supercell structure and successfully describe the intensities variation as a function of the superlattice parameters in terms of the softening of the zone center Brillouin selection rules.     

Confinement of acoustical vibrations in a semiconductor planar phonon cavity

Extending the idea of optical microcavities to sound waves, we propose a phonon cavity consisting of two semiconductor superlattices enclosing a spacer layer. We show that acoustical phonons can be confined in such layered structures when the spacer thickness is an integer multiple of the acoustic half-wavelength at the center of one of the superlattice folded minigaps. We report Raman scattering experiments that, taking profit of an optical microcavity geometry, demonstrate unambiguously the observation of a phonon-cavity confined acoustical vibration in a GaAs/AlAs based structure. The experimental results compare precisely with photoelastic model calculations of the Raman spectra.     

On linewidth of phonons associated to a disorder mechanism

An expression for the linewidth of phonons associated to a disorder mechanism in crystals is deduced. The linewidth of these ‘noise’ phonons is a function of the correlation time describing the statistical behavior of the disorder mechanism. A new view point to the application of the fluctuation-dissipation theorem for order-disorder crystals is presented. The relationship between the behavior of the linewidth of these phonons and phase-transitions mechanisms is discussed.     

Anisotropy of long-wavelength optical phonons in GaAs/AlAs superlattices

The angular anisotropy of optical phonons in GaAs/AlAs (001) superlattices is investigated by Raman scattering spectroscopy. Scattering configurations allowed for phonons with wave vectors oriented along the superlattice layers and normally to them are used. For phonons localized in GaAs layers, the theoretically predicted mixing of the LO1 longitudinal modes with TO1 transverse modes in which atomic displacements occur along the normal to the superlattice is observed experimentally. These modes possess noticeable angular anisotropy. For transverse modes in which atoms move in the plane of the superlattice, the angular anisotropy is small.