ZnSe-ZnS应变超晶格的Raman散射

本文报导了Znse—ZnS应变超晶格的Raman光学声子谱.我们观测到,随着应变大小的改变,ZnSe和ZnS的纵向光学声子发生频移.ZnSe层中纵向光学声子可发生较大的蓝移,也可发生较小的红移;ZnS层中的纵向光学声子发生较大的红移.这些现象为“应变场下的光学模理论”所解释.文中还报导了在波数为110cm-1处观测到一很强的散射峰,并把它归结为超晶格表面层单斜Se所引起的散射;在其它地方还观测到非晶态Se、三角Se引起的散射峰.     

静压下(CdTe)_2(ZnTe)_4-ZnTe多量子阱结构的多声子共振拉曼散射

用加静高压的方法改变光学能隙来实现共振条件。在以(CdTe)_2(ZnTe)_4短周期超晶格为阱层,(ZnTe)_(4)为垒层的多量子阱结构中观察到高达四阶的类 ZnTe 纵光学声子模的多声子共振拉曼散射。通过对拉曼位移随压力变化的分析,发现在与(CdTe)_2(ZnTe)_4短周期超晶格共振时测得的类ZnTe 纵光学声子模的频率比与 ZnTe 势垒层共振时测得的 ZnTe 纵光学声子模的频率低4cm~(-1)。并将它归结为在短周期超品格中纵光学声子模的限制效应。在与短周期超品格严格的2LO 声子出射共振条件下观察到了类 CdTe 的2LO 声子的共振拉曼峰。     

Fano-like electron–phonon interference in δ-doping GaAs superlattices

The interaction between electron excitations and LO phonons is studied by Raman scattering inδ-doping GaAs superlattices. The Raman spectra measured close to the E₀ +  Δ₀resonance of GaAs present Fano-like coupling of the LO phonons with the quasicontinuum single-particle electron excitations. Due to the self-consistent origin of the electron-energy spectrum in δ-doping superlattices the resonance of the Fano interference was found to be strongly dependent on the electron density as well as the excitation energy.     

The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually matches one of the phonon modes, typically theq = 0 optical mode. These oscillations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent phonon signal can be used as a probe of the surface depletion field and (2) multiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced and acoustic modes can now be excited.     

Control of coherent acoustic phonons in semiconductor quantum wells.

Using subpicosecond optical pump-probe techniques, coherent zone-folded longitudinal acoustic phonons (ZFLAPs) were investigated in an InGaN multiple quantum well structure. A two-pump differential transmission technique was used to generate and control coherent ZFLAP oscillations through the relative timing and amplitude of the two pump pulses. Enhancement and suppression of ZFLAP oscillations were demonstrated, including complete cancellation of generated acoustic phonons for the first time in any material system. Coherent control was used to demonstrate that ZFLAPs are generated differently in InGaN multiple quantum wells than in GaAs/AlAs superlattices.     

Anomalous LO phonon lifetime in AlAs

The temperature dependence of the frequencies and linewidths of the Raman-active longitudinal optical (LO) phonons in GaAs and AlAs have been measured. The low-temperature lifetime of the LO phonon in AlAs is found to be 9.7 ps, very close to the corresponding GaAs value of 9.5 ps. This contradicts early theoretical predictions. The agreement between theory and experiment can be restored when the accidental degeneracy between the AlAs LO phonon frequency and a feature in the two-phonon density of states is taken into account.     

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：     

Intraband coherence of Bloch oscillations after momentum scattering

Resonantly excited secondary emission of light (RSE) in GaAs/AlGaAs superlattices gives a direct proof for the stability of coherent Bloch dynamics beyond the optical coherent regime. The polarization analysis of the RSE signal confirms its nonlinear origin resulting from coherent intraband density dynamics. It is shown that the intraband coherence of Bloch oscillations (BO) persists after momentum scattering of the photoexcited states. A variation of emission angle and excitation density allows to distinguish two momentum scattering processes contributing to the RSE. A static disorder mediated contribution is restricted to directions near the reflection direction. A further contribution of carrier induced momentum scattering is resolved in directions well separated from diffraction directions. The dependence of the RSE as a function of an external electric field demonstrates an enhancement of momentum scattering with increasing field. PACS 78.47.+p; 63.20Kr; 71.35.-y.     

FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

Optical phonon frequencies and damping in AlAs, GaP, GaAs, InP, InAs and InSb studied by oblique incidence infrared spectroscopy

The transverse (TO) and longitudinal (LO) optical phonons in AlAs, GaP, GaAs, InP, InAs and InSb have been measured at room temperature by infrared spectroscopy using an oblique incidence reflectance method. The spectra obtained were then fitted using a novel approach to determine the TO and LO phonon frequencies and damping. The results obtained are found to be more precise than in earlier reflectivity measurements using near-normal angles of incidence and provide information on the damping of both phonons. Apart from the GaAs LO mode, the observed damping parameters are found to be quite different from those predicted by theory. From these results the Lowndes condition governing the relative magnitudes of the TO and LO phonon line widths is found to be violated for all these zincblende semiconductors.     

Optical-phonon transport and localization in periodic and Fibonacci polar-semiconductor superlattices

A transfer-matrix formalism is employed to study optical-phonon transport in a macroscopic continuum model for both periodic and Fibonacci polar-semiconductor superlattices. A phonon bandgap with subband structures is obtained for the periodic superlattices. However, in the Fibonacci superlattices, there is a spectrum trifurcation and self-similarity. The LO phonon localization length is calculated from which we confirm the existence of complete exponential localization of LO phonons.     

Phonon dispersion in quasiperiodic semiconductor superlattices

The phonon spectra of unstrained and strained quasiperiodic semiconductor superlattices (QSSL) have been calculated using one-dimensional linear chain model. We consider two types of quasiperiodic systems, namely cantor triadic bar (CTB) and Fibonacci sequences (FS), constituting of AlAs, GaAs and GaSb of which the latter two have a lattice mismatch of about 7%. The calculations have been made using transfer matrix method and also with and without the inclusion of strain. We present the results on phonon spectra of two component CTB and two as well as three component FS semiconductor superlattices (SSL), thickness and order dependence on LO mode of GaAs, effect of strain on LO frequency of GaAs. The calculated results show that the strain generated due to lattice mismatch reduces significantly the magnitudes of the confined optical phonon frequency of GaAs.     

Electron mobility for a model Al<Subscript>x</Subscript> Ga<Subscript>1-x</Subscript> As/GaAs heterojunction under pressure

A variational method and a memory function approach are adopted to investigate the electron mobility parallel to the interface for a model Al_xGa_1-xAs/GaAs heterojunction and its pressure effect by considering optical phonon modes (including both of the bulk longitudinal optical (LO) in the channel side and interface optical (IO) phonons). The influence of a realistic interface heterojunction potential with a finite barrier and conduction band bending are taken into account. The properties of electron mobility versus Al concentration, electronic density and pressure are given and discussed, respectively. The results show that the electron mobility increases with Al concentration and electronic density, whereas decreases with pressure from 0 to 40 kbar obviously. The Al concentration dependent and the electron density dependent contributions to the electron mobility from the scattering of IO phonons under pressure becomes more obvious. The variation of electron mobility with the Al concentration and electron density are dominated by the properties of IO and LO phonons, respectively. The effect of IO phonon modes can not be neglected especially for higher pressure and electronic density.     

The phonon effects on the impurity states in cylindrical quantum wire with a finite confining potential

The variational method and the effective mass approximation are applied to calculate the binding energies of the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential well. The phonon effects on the impurity states are considered by taking both the couplings of the electron-phonon and the impurity ion-phonon into account. The numerical results for the GaAs cylindrical quantum wire are given and discussed. It is found that the ion-phonon interaction reduces the impurity binding energy and supplies key contribution to the energy shift, but the electron-phonon coupling enhances the binding energy less. Longitudinal optical (LO) phonons play more important role than interface optical (IO) phonons in the impurity potential screening. The polaron effect caused by LO phonons is more important when the wire is thinner, otherwise the LO phonons are dominant for the thicker wires.     

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.     

Dynamical Properties of Coherent Plasmons Coupled with LO Phonons in an InAs/GaAs Strained Superlattice

We have investigated the coherent plasmon photoexcited in the electron miniband of an InAs/GaAs strained superlattice (SL) using a reflection-type pump-probe technique. Under the condition that the center energy of the pump pulses is tuned to the fundamental transition energy of the SL, the coherent plasmon coupled with the longitudinal optical (LO) phonon of GaAs that is the barrier layer is observed. The time-domain signals of the coherent plasmon coupled with the GaAs-like LO phonon drastically change with varying the generation and detection energies. From the energy dependence of the time-domain signals, we discuss the relaxation process of the carriers photoexcited in the miniband of the SL.     

Long polaron lifetime in InAs/GaAs self-assembled quantum dots

We have investigated the polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots by pump-probe midinfrared spectroscopy. A long T1 polaron decay time is measured at both low temperature and room temperature, with values around 70 and 37 ps, respectively. The decay time decreases for energies closer to the optical phonon energy. The relaxation is explained by the strong coupling for the electron-phonon interaction and by the finite lifetime of the optical phonons. We show that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.     

Electron–phonon interaction in CdS/ZnS quantum dot

Electron–phonon effects on the two first electronic states in both CdS and GaAs quantum dots are investigated. Both confined longitudinal optical (LO) and surface optical (SO) phonons are considered. We use the intermediate-type variational approach. We find that, shifts caused by phonon contribution on electronic energies are more significant for CdS quantum dot. We find, also, that, contrary to GaAs based quantum dots, we shouldn’t neglect the SO phonon contribution for CdS based ones, especially for small dots.     

Hot phonon effects in heterolayers

The strength of the hot-phonon effect generated by hot carriers in a two-dimensional heterolayer is estimated, specifically for electrons in GaAs. Both acoustic-mode phonons, at low temperature, and LO phonons are considered. For the former the phonon mean free path is taken to be large compared to the heterolayer thickness. The LO phonons are, however, assumed to decay locally. In both cases the estimates indicate that substantial hot phonon effects are to be expected in realized conditions.     

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C?, Ge?, Zn?, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with $\text{k}\text{.}\text{p}$ band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.