Picosecond spectroscopy of semiconductors

The time-resolved reflectivity of picosecond pulses from optically excited carrier distributions can provide important information about the energy relaxation rates of hot electrons and holes in semiconductors. The basic optical properties of non-equilibrium carrier distributions are discussed, and in the specific case of GaAs, a semi-empirical analysis of the reflectivity spectrum is described which estimates the contributions from the principal critical points of the band structure. Using Boltzmann factors to approximate the hot carrier distributions, it is found that the non-equilibrium reflectivity spectrum is a sensitive function of carrier temperature and that it can reverse its sign as the distribution relaxes. These results are in good qualitative agreement with recent experiments employing a mode-locked cw dye laser.     

Hot phonon effects in ZnSe

Femtosecond pump-probe differential reflectivity spectroscopy is used to investigate the ultrafast cooling dynamics of hot photoexcited carriers in a high-quality ZnSe epilayer grown on GaAs. Comparison with a theoretical model based on a balance equation approach indicates that the observed reduction in the electron cooling rate with increasing carrier density (for carrier densities greater than ～3×10¹⁷cm^?3) is due to both screening of the Fröhlich interaction and a non-equilibrium hot longitudinal optic-phonon population generated by the cooling electron distribution.     

Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time-Resolved THz Spectroscopy

The control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump—terahertz (THz) probe spectroscopy—is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H₂ intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer-sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi-Fermi level of laser-excited carriers through their temperature. The photocarriers undergo a disorder-enabled super-collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub-nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two-temperature Drude-Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.     

激光脉冲照射硅时若干物理量的计算

本文计算了由毫微秒和微微秒激光脉冲照射而在硅晶体中产生的晶格温度、载流子浓度和载流子温度的时间演变和空间分布,本文用PDECOL软件包计算一组关于晶格温度、载流子浓度、载流子温度和激光强度的偏微分方程组的数值解。计算表明,热载流子的能量弛豫时间应为1ps或小于1ps,计算的熔化阈值与发表的实验结果是一致的。     

Non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN

Non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN have been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that for electron densities n≥5×10¹⁷cm⁻³, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the effective electron temperature much higher than the lattice temperature. In addition, we find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron-longitudinal optical phonon scattering rate to the much larger ionicity in GaN.     

GaAs和GaAs/AlGaAs多层量子阱中载流子的超快动力学(英文)

The ultrafast photoexcited carrier dynamics in bulk GaAs and GaAs/ AlGaAs multiple quantum well(MQW)structure has been studied using femtosecond laser pulse pump-probe techniques on the samples grown by MBE. A hot carrier cooling time of 1.5ps in MQW is measured at room temperature. Also, optical phonon emission at 33meV is observed in this sample. These results are found to be similar to that observed in bulk GaAs. A comparison of the hot carrier cooling rates for the two cases suggests that the infra-sub-band optical phonon scattering in MQW may play a dominant role in the cooling of highly excited hot carriers for the narrow wells. The experimental results agree well with that predicted by a simple infinite depth square-well model.     

Distribution of electrons between valleys and band-gap narrowing at picosecond superluminescence in GaAs

Band-gap narrowing due to photogeneration of dense hot electron-hole plasma in GaAs was studied. Plasma was generated by picosecond light pulses, and picosecond superluminescence was observed. In this case, the total concentration of photogenerated electron-hole pairs was experimentally proved to be the sole parameter controlling the electron distribution between Γ₆ and L ₆ valleys and the corresponding band-gap narrowing. This was explained by the fact that the carrier temperature and concentration are correlated in the presence of superluminescence.     

Carrier energy relaxation time in quantum-well lasers

Carrier energy relaxation via carrier-polar optical phonon interactions with hot phonon effects in multisubband quantum-well structures is theoretically studied by using both bulk longitudinal optical phonons and confined longitudinal optical phonons. We find that the width and the depth of quantum wells only have moderate effects on carrier energy relaxation rates. Our results also indicate that the difference of energy relaxation rates between the quantum well and the bulk material is not significant. We investigate the effects of longitudinal optical phonon lifetimes on the carrier energy relaxation rate. Neglect of the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time; this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficient due to carrier heating. The implications of our theoretical results in designing high-speed quantum-well lasers are discussed     

Hot phonons and Auger related carrier heating in semiconductoroptical amplifiers

We have directly measured the carrier temperature in semiconductor optical amplifiers (SOAs) via spontaneous emission and we demonstrate an unexpectedly high carrier temperature. The direct correlation of the temperature increase with the carrier density suggests Auger recombination as the main heating mechanism. We have developed a model based on rate equations for the total energy density of electrons, holes, and longitudinal-optical phonons. This model allows us to explain the thermal behavior of carrier and phonon populations. The strong heating observed is shown to be due to the combined effects of hot phonon and Auger recombination in the valence band. We also observe an evolution of the Auger process, as the density is increased, from cubic to square dependence with coefficients C₃ = 0.9 10^-28 cm⁶ s^-1 and C₂ = 2.4 10^-10 cm³ s^-1. This change is explained by the hole quasi-Fermi level entering the valence band     

Theoretical and Experimental Study of Time- and Temperature-Dependent Photoluminescence in ZnO

In this work, we investigate the dynamics of photo-excited carriers in ZnO. Specifically, we study the luminescence spectrum and the effect of temperature on the luminescence rise time. For comparison, experimental time- resolved photo-luminescence studies on ZnO samples are performed. In the theoretical model, interaction with a laser pulse is treated coherently and a generalized Monte Carlo simulation is used to account for scattering processes. The scattering mechanisms included are carrier interactions with polar optical phonons and acoustic phonons, and carrier–carrier Coulomb interactions. We observed a good agreement between the experimental and simulation results for the photo-luminescence spectrum. Furthermore, as the temperature increases, the luminescence rise time decreases, mostly due to the weaker effect of polar optical scattering at lower temperature.     

Polar optical phonon scattering of charge carriers in alloy semiconductors: Effects of phonon localization

The effects of spatial localization of phonons or their correlation functions to finite distances in alloy semiconductors on polar optical phonon scattering of hot carriers are modeled analytically. Despite the possibility of increased numbers of carrier-scattering-active phonon modes, it is demonstrated that phonon localization in alloys should have little if any effect on the total polar optical scattering rate for charge carriers coupled to equilibrium phonon populations. Further, it is demonstrated that phonon localization may have a beneficial effect on hot carrier transport by reducing the possibility of exciting nonequilibrium phonon populations. These results are obtained without assuming any specific functional form or degree of phonon localization; rather, calculations rely on the inherent orthogonality and mathematical completeness of the classical vibrational modes over the crystal lattice degrees of freedom.     

Nonlinear gain coefficients in semiconductor lasers: effects ofcarrier heating

Nonlinear gain coefficients due to the effects of carrier heating are derived from the rate equations of carrier energy transfer in semiconductor lasers. We find that, in the modulation responses of semiconductor lasers, stimulated recombination heating will affect the resonant frequency and damping rate in a same form as the effects of spectral hole burning, while free carrier absorption heating will only affect the damping rate. The effects of injection heating and nonstimulated recombination heating are also discussed. The carrier energy relaxation time is calculated from first principles by considering the interactions between carriers and polar optical phonons, deformation potential optical phonons, deformation potential acoustic phonons, piezoelectric acoustic phonons. At the same time, the hot phonon effects associated with the optical phonons are evaluated because their negligible group velocity and finite decay time. We show that the carrier-polar longitudinal optical phonon interaction is the major channel of carrier energy relaxation processes for both electron and holes. We also point out the importance of the longitudinal optical phonon lifetime in evaluating the carrier energy relaxation time. Neglecting the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time, this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficients due to carrier heating. The effects of spectral hole burning, stimulated recombination heating, and free carrier absorption heating on limiting the modulation bandwidth in semiconductor lasers are also discussed     

Cooling,Scattering, and Recombination—The Role of the Material Quality for the Physics of Tin Halide Perovskites

Tin‐based perovskites have long remained a side topic in current perovskite optoelectronic research. With the recent efficiency improvement in thin film solar cells and the observation of a long hot carrier cooling time in formamidinium tin iodide (FASnI₃), a thorough understanding of the material's photophysics becomes a pressing matter. Since pronounced background doping can easily obscure the actual material properties, it is of paramount importance to understand how different processing conditions affect the observed behavior. Using photoluminescence spectroscopy, thin films of FASnI₃ fabricated through different protocols are therefore investigated. It is shown that hot carrier relaxation occurs much faster in highly p‐doped films due to carrier–carrier scattering. From high quality thin films, the longitudinal optical phonon energy and the electron–phonon coupling constant are extracted, which are fundamental to understanding carrier cooling. Importantly, high quality films allow for the observation of a previously unreported state of microsecond lifetime at lower energy in FASnI₃, that has important consequences for the discussion of long lived emission in the field of metal halide perovskites.     

Silicon surface emission of hot electrons

A new emission model, based on a probabilistic treatment of electron trajectories, is developed for hot electron emission from a silicon surface. Primary electrons, generated thermally or optically, are heated by a normal electric field and cooled by phonon collisions and by impact ionization. Unlike the case in previous models, the semiconductor field need not be uniform, and multistage phonon processes are included. The model provides hot electron energy distributions both for electrons transported in the non-equilibrium layer at the silicon surface and for electrons emitted into the oxide. It shows that the most probable trajectory of an emitted electron is not one of zero collisions (as assumed in the analysis of Verwey et al. [2]), but one involving the generation of many optical phonons. The model, used together with photoexcited hot electron measurements (as developed by Ning and Yu [6]), also provides an accurate method for determining phonon and ionization mean free paths.     

Free carrier absorption and lattice vibrational modes in bulk ZnO

The electronic properties of semiconductors are highly dependent on carrier scattering mechanisms determined by crystalline structure, band structure, and defects in the material. Experimental characteristics of lattice vibrational modes and free carrier absorption in single-crystal ZnO samples obtained from different sources are presented in this work to provide a further understanding of carrier scattering processes pertaining to electronic properties. Infrared absorption measurements indicate strong absorption peaks due to a combination of optical and nonpolar phonon modes in the 9–13 μm spectral region. The Raman spectra obtained for these samples similarly reveal the presence of these phonon modes. Infrared absorption measurements also demonstrate free carrier absorption in the 3–9 μm spectral region for higher conductivity samples, where a λ^m dependence is observed with m=2.7–3, indicating both longitudinal optical phonon scattering and ionized impurity scattering. From these results, we show that infrared absorption can be used as a routine nondestructive technique to determine the material characteristics and quality of bulk ZnO.     

Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

Understanding the lattice dynamics and phonon transport from the perspective of chemical bonds is essential for improving and finding high‐efficiency thermoelectric materials and for many applications. Here, the coexistence of global and local weak chemical bonds is elucidated as the origin of the intrinsically low lattice thermal conductivity of non‐caged structure Nowotny–Juza compound, α‐MgAgSb, which is identified as a new type of promising thermoelectric material in the temperature range of 300–550 K. The global weak bonds of the compound lead to a low sound velocity. The unique three‐centered Mg? Ag? Sb bonds in α‐MgAgSb vibrate locally and induce low‐frequency optical phonons, resulting in “rattling‐like” thermal damping to further reduce the lattice thermal conductivity. The hierarchical chemical bonds originate from the low valence electron count of α‐MgAgSb, with the feature shared by Nowotny–Juza compounds. Low lattice thermal conductivities are therefore highly possible in this series of compounds, which is verified by phonon and bulk modulus calculations on some of the compositions.     

单晶硅表面载流子动力学的超快抽运探测

利用800 nm波长的飞秒抽运探测技术测量了单晶硅表面50 ps内的瞬态反射率变化,研究了表面载流子的超快动力学过程.基于自由载流子密度变化过程建立的反射率模型可以很好地描述瞬态反射率变化,说明受激自由载流子超快响应的贡献主导了反射率的变化过程.经拟合获得了样品的表面复合速度(SRV)为1.2×106cm/s.建立了耦合的载流子输运模型,探讨了单晶硅表面热载流子的密度、温度随时间的演化过程.研究表明,表面复合过程是影响本征单晶硅表面载流子动力学的重要因素.     

Emission dynamics and optical gain of 1.3-μm (GaIn)(NAs)/GaAslasers

The ultrafast emission dynamics of a 1.3-μm (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9·10^-4 eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment     

Effects of carrier heating on laser dynamics-a Monte Carlo study

The static and dynamic properties of semiconductor quantum-well (QW) lasers have traditionally been analyzed by using rate equations that couple cold carriers to photons in the lasing cavity. This assumption of cold carriers, however, has often been disputed because it does not account for heating due to carrier relaxation, hot phonon effects, and spectral hole burning. All these processes affect laser performance significantly by modifying the gain because gain depends on carrier temperature as well as spectral broadening. In this paper, we study the carrier dynamics of QW lasers using a Monte Carlo method and conclude that hot carrier effects in semiconductor lasers are important and need to be considered for the analysis and design of semiconductor lasers     

三束飞秒激光辐照下铜膜内电子非平衡热输运

首先采用有限元法数值计算了铜膜内的电子温度和晶格温度分布变化,揭示了铜膜内电子非平衡热输运时间随飞秒激光光束参量的变化情况。仿真结果表明,铜膜内的电子非平衡热输运时间会随着泵浦光束数量及脉冲能量密度的增加而增加,并且使用三束飞秒泵浦激光作用时,电子非平衡热输运时间比单脉冲作用时的电子非平衡热输运时间增加了3倍。其次使用三束飞秒激光泵浦的泵浦-探测实验系统进行验证。实验结果表明:通过用具有一定延时的三束飞秒泵浦激光作用铜膜时,铜膜表面的瞬态反射率出现三次突变,使电子非平衡热输运时间得到极大延长,从而大幅度消除激光加工热障,并提高加工的质量、精度和效率。