二硫化钼纳米点中的缺陷辅助载流子俘获超快动力学（英文）

二硫化钼纳米点正在成为有潜质的半导体材料用于光电设备的应用.然而,关于对其中激子动力学的研究却很少.本文利用飞秒瞬态吸收光谱学来研究二硫化钼纳米点的载流子动力学.结果显示,缺陷辅助的载流子再复合过程与观测到的动力学相符,通过俄歇散射对光激载流子进行俘获至少存在两种不同俘获速率的缺陷.四个过程参与了载流子驰豫,在受到光激发后,立即在～0.5 ps内载流子冷却,然后大部分载流子被缺陷快速俘获,随着泵浦能量的增加,该过程对应的时间从～4.9ps增加到～9.2ps,这可以用缺陷态的饱和来解释.接下来,拥有相对慢的载流子俘获速率的其它类型缺陷对小部分载流子进行俘获,该过程约65 ps.最后,剩余的少量载流子通过直接带间跃迁发生电子-空穴再复合,时间约为1 ns.研究结果可以深入了解二硫化钼纳米点中的载流子动力学基本原理,引导其更多的应用.     

低温生长铝镓砷光折变效应的研究

三元化合物铝镓砷(AlGaAs)是一种可用于全光固体超快诊断技术的重要材料.基于低温外延技术的AlGaAs材料不仅具有低温生长砷化镓(low-temperature grown GaAs, LT-GaAs)超短载流子寿命的特点,并且可以调整材料的禁带宽度,为超快诊断系统的设计增加了极大的灵活性.泵浦-探测实验结果表明,低温外延生长可以有效加速AlGaAs材料的非平衡载流子复合,非平衡载流子弛豫时间小于300 fs,而非平衡载流子的复合时间低至2.08 ps.由于经过特殊的钝化工艺处理,极大地降低了表面复合对载流子衰退过程的影响,而低温外延生长引入的As原子团簇,形成了深能级缺陷,是加速载流子复合的主要因素.基于单复合中心的间接复合理论,建立LT-AlGaAs载流子演化模型,获得与复合速率相关的关键物理参量:载流子俘获面积δ_e=6.6×10~(-14) cm~2, δ_h=4.7×10~(-15) cm~2,计算结果与实验相符.该方法可用于半导体材料载流子演化特性定量分析,有助于推进超快响应半导体材料的优化改进.     

量子阱光学过程的瞬态特性

当量子阱材料注入(例如光注入)大量非平衡载流子时,非平衡载流子的动力学过程可分为两个过程:一是高激发过剩非平衡载流子的快速弛豫过程,即热载流子通过发射纵向光学声子(LO声子)与晶格交换能量、释放能量.描述这个过程的主要物理量是LO声子的散射时间常数,其时间域大约为几个ps到几十个ps,取决于激发强度、量子阱宽度等.另一个过程是弛豫到导带底(价带顶)的载流子通过辐射复合放出光子,即辐射复合发光.描述这个过程的主要物理量是发光寿命。 一、量子阱中辐射复合发光 动力学过程研究 在量子阱结构中,载流子复合发光动力学过程和体材料…     

低温生长砷化镓的超快光抽运-太赫兹探测光谱

本文采用光抽运-太赫兹探测技术系统研究了低温生长砷化镓(LT-GaAs)中光生载流子的超快动力学过程.光激发LT-GaAs薄层电导率峰值随抽运光强增加而增加,最后达到饱和,其饱和功率为54μJ/cm~2.当载流子浓度增大时,电子间的库仑相互作用将部分屏蔽缺陷对电子的俘获概率,从而导致LT-GaAs的快速载流子俘获时间随抽运光强增加而变长.光激发薄层电导率的色散关系可以用Cole-Cole Drude模型很好地拟合,结果表明LT-GaAs内部载流子的散射时间随抽运光强增加和延迟时间(产生光和抽运光)变长而增加,主要来源于电子-电子散射以及电子-杂质缺陷散射共同贡献,其中电子-杂质缺陷散射的强度与光激发薄层载流子浓度密切相关,并可由散射时间分布函数α来描述.通过对光激发载流子动力学、光激发薄层电导率以及迁移率变化的研究,我们提出适当增加缺陷浓度,可以进一步降低载流子迁移率和寿命,为研制和设计优良的THz发射器提供了实验依据.     

亚单层InGaAs量子点-量子阱异质结构的时间分辨光致发光谱

测定了亚单层InGaAs/GaAs量子点-量子阱异质结构在5K下的时间分辨光致发光谱.亚单层量 子点的辐射寿命在500 ps 至 800 ps之间，随量子点尺寸的增大而增大，与量子点中激子的 较小的横向限制能以及激子从小量子点向大量子点的隧穿转移有关.光致发光上升时间强烈 依赖于激发强度密度.在弱激发强度密度下，上升时间为 35 ps，纵光学声子发射为主要的 载流子俘获机理.在强激发强度密度下，上升时间随激发强度密度的增加而减小，俄歇过程 为主要的载流子俘获机理.该结果对理解亚单层量子点器件的工作特性非常有用. 关键词： 亚单层 量子点-量子阱 时间分辨光致发光谱     

Ge掺杂GaN晶体双光子诱导超快载流子动力学的飞秒瞬态吸收光谱研究

本文利用飞秒瞬态吸收光谱技术,在近红外波段对Ge掺杂GaN(GaN:Ge)晶体进行了超快载流子动力学研究.在双光子激发下,瞬态吸收动力学呈现出双指数衰减,其中慢过程寿命随着泵浦光强增加而增加.瞬态吸收响应随着探测波长而单调增强,并在约1050nm处由空穴吸收占据主导.利用简化模型模拟载流子动力学发现,GaN:Ge中碳杂质形成的深受主能级对空穴有很强的俘获能力,并且引起了缺陷发光.在较适中的载流子注入下,n型GaN中的载流子寿命可以通过控制缺陷浓度和载流子浓度来共同调控,使其可应用于发光二极管和光通信等不同的领域.     

利用可见-近红外-中红外超快光谱揭示离子交换法制备的少层MoS2中缺陷介导的载流子动力学

本文结合可见-近红外-中红外瞬态吸收光谱技术对离子交换法制备的少层MoS₂中缺陷介导的载流子动力学进行了详细的解析. 在近红外瞬态吸收光谱中观察到的宽带漂白信号表明少层MoS₂纳米片带隙中分布着大量的缺陷态. 实验结果明确揭示了载流子被缺陷态的快速捕获以及进一步的复合过程，证明带隙中的缺陷态对MoS₂光生载流子动力学过程起着至关重要的作用. 在中红外瞬态吸收光谱中观察到的正信号到负信号的转变进一步证实了在导带下小于0.24 eV处存在被载流子占据的缺陷态. 这些在少层MoS₂纳米片中存在的缺陷态可以作为有效的载流子捕获中心来辅助光生载流子在皮秒时间尺度内完成非辐射复合过程.     

Ge2Sb2Te5非晶薄膜中超快载流子动力学的飞秒分辨反射光谱研究

利用飞秒时间分辨抽运-探测反射光谱技术研究了室温下Ge₂Sb₂Te₅非晶薄膜中载流子超快动力学及其激发能量密度依赖性.发现光激发后05 ps时间内，反射变化率降到最小值，然后开始迅速增加，在几个皮秒时间内达到大于初始反射率的新的最大值.反射率的减小量、增加量和增加速率均随激发能量密度的增大而增加.利用高密度等离子体的Auger复合及其感应的晶格加热模型较好地定量解释了反射率由最小到最大的快速变化过程，表明高密度等离子体的Auger复合加热 关键词： 抽运-探测光谱 2Sb₂Te₅非晶薄膜')" href="#">Ge₂Sb₂Te₅非晶薄膜 Auger复合 载流子动力学     

LT-GaAs飞秒光电导之电场响应特性

利用飞秒光电导自相关技术研究了LT GaAs飞秒光电导开关时间随外加偏置电场的变化规律 .实验结果显示当外加偏置电场从0.5×10⁴ V/cm 上升到9.5×10⁴ V/cm 时，光电导 开关时间开始在200fs附近缓慢变化再迅速增加到750fs.这是由于随着外加电场增加，Fren kel-Poole效应导致的EL2缺陷中心库仑吸引势垒降低和电场增强的碰撞电离效应显著增强 ，导致载流子俘获截面减小，载流子寿命增加之故. 关键词： 光电导自相关技术 载流子俘获时间 Frenkel-Poole效应 电子碰撞电离效应     

CdTe量子点的超快过程研究

使用pumpprobe技术研究了CdTe量子点的瞬态吸收光谱，通过双指数拟合，得到两个弛豫过程，将得到快过程归结为表面俘获载流子跟价带底空穴的直接复合，即表面态辐射过程；慢过程被归结为电子被深表面态俘获，然后以发射荧光或无辐射跃迁的形式与空穴进行复合，即深表面俘获态辐射过程. 并细致的研究了量子点的尺寸对单纯核的弛豫过程的影响.     

Robust Fabrication of Quantum Dots on Few‐Layer MoS2 by Soft Hydrogen Plasma and Post‐Annealing

Apart from unique properties of layered transition‐metal dichalcogenide nanosheets like MoS₂, quantum dots (QDs) from these layered materials promise novel science and applications due to their quantum confinement effect. However, the reported fabrication techniques for such QDs all involve the use of liquid organic solvents and the final material extraction from such liquid dispersions. Here a novel and convenient dry method for the synthesis of MoS₂ quantum dots interspersed on few‐layer MoS₂ using soft hydrogen plasma treatment followed by post‐annealing is demonstrated. The size of MoS₂ nanodots can be well controlled by adjusting the working pressure of hydrogen plasma and post‐thermal annealing. This method relies on the cumulative hydrogen ion bombardment effect which can destroy the hexagonal structure of the top MoS₂ layer and disintegrate the top layer into MoS₂ nanodots and even QDs. Post‐thermal annealing can further reduce the size. Such MoS₂ quantum dots interspersed on few‐layer MoS₂ exhibit two new photoluminescence peaks at around 575 nm because of the quantum confinement effect. This dry method is versatile, scalable, and compatible with the semiconductor manufacturing processes, and can be extended to other layered materials for applications in hydrogen evolution reaction, catalysis, and energy devices.     

Multicolor Light‐Emitting Diodes with MoS2 Quantum Dots

Molybdenum disulfide (MoS₂) quantum dots (QDs) are known for their excitation‐wavelength‐dependent photoluminescent (PL) properties. However, the mechanism of this phenomenon is still unclear. Here, small size MoS₂ QDs with a narrow size distribution are synthesized. Based on the decay study and PL dynamics, a reasonable radiation model is presented to understand the special PL properties, i.e., the carrier recombination in the localized surface defect states generated the PL. Accordingly, this optical property is used to fabricate multicolor light‐emitting devices with the same MoS₂ QDs. The emission color covers the full visible spectrum from blue to red, only by adjusting the thickness of the down‐conversion QD layers.     

Optical detection of picosecond processes of formation of free carriers and primary photolysis products in AgBr(J) nanocrystals

The fastest picosecond processes occurring in the electronic stage of photolysis in photosensitive AgBr(J) nanocrystals—the generation of free carriers and their initial capture by lattice defects and impurity centers with subsequent transition of captured carriers to deeper levels—were identified and quantitatively studied by methods of optical detection for the first time. The experimental characteristic times of these processes for AgBr(J) nanocrystals were shorter than 5 ps in the first stage and about 30 and 300 ps (for the generation and the capture of free carriers, respectively) in subsequent stages. The minimum durations of pulses of destructive nonactinic radiation at which dephotolysis processes can be reliably detected were estimated to be 20 ps for simultaneous illumination by pulses of destructive and exciting radiation and about 5 ps in the case of successive illumination. The decisive role of two-photon absorption in nonlinear processes of formation of hidden images under the action of picosecond pulses of nonactinic radiation was shown experimentally.     

Carrier transport effects and dynamics in multiple quantum well optical amplifiers

The gain recovery dynamics of multiple quantum well semiconductor optical amplifiers, following gain compression caused by ultrashort optical pulse excitation, have been studied for several devices of different structures. Fast, slow, and intermediate time constants are identified. The fast component (0.6 to 0.9 ps) corresponds to cooling of the dense, inverted electron-hole plasma. The slow component (150 to 300 ps) corresponds to replenishment of carriers from the external bias supply, with the dynamics dominated by spontaneous recombination (primarily Auger) of the electron-hole plasma. The intermediate time constant (2 to 14 ps) is caused by carrier capture by the quantum wells and is structure-dependent. For most of the devices, the capture process is dominated by diffusion-limited transport in the cladding/barrier region. The variation of carrier density and temperature also affects the refractive index profile of the devices and, hence, affects the waveguiding properties. Dynamical variation of the mode confinement factor is observed on the fast and slow timescales defined above.     

Investigation of non-equilibrium electron-hole plasma in nanowires by THz spectroscopy

Efficient emission of THz radiation by AlGaAs nanowires via excitation of photocurrent by femtosecond optical pulses in nanowires was observed. Dynamics of photoinduced charge carrier was studied via influence of electron-hole plasma on THz radiation by optical pump THz probe method. It was found that characteristic time of screening of contact field is about 15 ps. Recombination of non-equilibrium occurs in two stages: fast recombination of free electron and holes (with relaxation time about 700 ps), and slow recombination (with relaxation time about 15 ns), which involves a capture of electrons and holes on the defects of crystalline structure of nanowires.     

Ultrafast nonlinear processes in quantum-dot optical amplifiers

Ultrafast gain dynamics in quantum-dot optical amplifiers has been studied by using the pump-probe and four-wave mixing (FWM) techniques. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the quantum dots (QDs). The relevant time constants were evaluated to be ～90 fs, ～260 fs, and ～2 ps, respectively, under a 50 mA bias condition. The compressed gain recovered to 3% of its initial value in 4 ps, and no recovery component slower than 2 ps could be seen in the temporal range tested. This is quite different from the feature in quantum wells, where a very slow component (> 50 ps) exists. This suggests a possibility of enhancing the operation speed of semiconductor optical amplifiers by using QDs as an active layer. The third-order optical susceptibility (χ⁽³⁾) has been evaluated by means of both nonlinear transmission and FWM experiments. The results show that the nonlinearity expressed by χ⁽³⁾/g ₀ is quite similar to that of bulk and quantum wells, which can be explained by the similar relaxation times.     

Large-scale chemical vapor deposition growth of highly crystalline MoS2 thin films on various substrates and their optoelectronic properties

Large-scale growth of mostly monolayer molybdenum disulfide (MoS₂) on quartz, sapphire, SiO₂/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS₂ on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS₂ is monolayer with high quality and uniformity. The MoS₂ field effect transistors based on the as-grown MoS₂ exhibit carrier mobility of 1–2 cm²V^?1s^?1 and On/Off ratio of ~10⁴ while showing large photoresponse. Our results provide a simple approach to realize MoS₂ on various substrates for electronics and optoelectronics applications.     

DYNAMICS OF CARRIER CAPTURE IN AlGaAs/GaAs MULTIPLE QUANTUM WELLS

Dynamics of carrier relaxation and capture in AlGaAs/GaAs multiple quantum wells (MQW) at 80 K is studied using picosecond luminescence and femtosecond absorption saturation measurements. Carriers generated in the wells and in the barriers scatter initially out of the excited states to a quasi-equilibrium state in 35 and 400 femtoseconds, respectively, before they are captured into the bound states of the quantum wells. Carrier capture occurs during carrier cooling and recombination. A carrier capture time of 25 ps has been deduced from time-resolved luminescence.