Bloch定理在单层石墨烯多势阱结构体系中的应用

石墨烯材料中观测到Klein隧穿效应,也因其特殊的光学、电学等特性而被广泛研究.本文设计了等高的多势垒体系来分析载流子的隧穿效应.由Bloch定理得到周期性势垒结构中的能带结构,分析载流子的状态和共振隧穿区.数值研究结果表明,通过调制势垒、势阱宽度、势垒高度、以及载流子入射能量等参数来获得所需的隧穿概率,并可进一步分析电导特性.     

CdSe/CdS/ZnS 量子点体系中的超快载流子动力学

利用飞秒泵浦探测技术对CdSe/CdS/ZnS量子点体系中的超快载流子动力学过程进行了研究. 通过选择不同波长的泵浦光分别激发样品壳层和核层,研究了载流子在壳层和核层中的超快动力学过程. 实验结果表明,载流子在CdS壳层导带中弛豫过程非常迅速(约130 fs),时间明显短于载流子在CdSe核层导带中的弛豫时间(约400 fs). 实验中也发现在CdS壳层和CdSe核层的分界面存在一定量的缺陷态.     

Au/SrTiO3/Au界面电阻翻转效应的低频噪声分析

本文研究了Au/SrTiO₃/Au三明治结构中的双极电阻翻转效应, 观察到高、低阻态的电阻弛豫现象. 低频噪声测量表明高、低阻态的电阻涨落表现出1/f行为. 对比试验表明, 高阻态的低频噪声来源于反向偏置肖特基势垒和氧空位的迁移, 强度较大, 低阻态的噪声则源于类欧姆接触底电极区域的氧空位迁移导致的载流子涨落, 强度较低. 同时, 界面上氧空位浓度的弛豫导致了高、低阻态的弛豫过程. 关键词： 电阻翻转效应 低频噪声 氧空位     

InP纳米颗粒的超快动力学和光学非线性

通过飞秒泵浦-探测方法测量了波长为800 nm时InP半导体纳米颗粒激发态的瞬态动力学过程。观察到一个快速的光致漂白建立和一个漂白的恢复过程,分析饱和吸收的来源可能是带填充效应引起跃迁的饱和吸收。对于漂白恢复中的快过程是由于自由载流子的弛豫,而慢成分是由于光激发载流子在很短的时间内受陷于表面态形成的限域载流子的弛豫。通过飞秒光克尔效应(OKE)方法测量材料的超快非线性响应曲线,计算了材料的光学三阶非线性极化率,分析了非线性的来源。     

AlGaN/GaN异质结载流子面密度测量的比较与分析

对MOCVD技术在蓝宝石衬底上生长的不同Al组分AlGaN/GaN异质结进行了范德堡法Hall测量和电容-电压(C-V)测量，发现Hall测量载流子面密度值大于C-V测量值，并且随着AlGaN层Al组分的增加，两种测量值都在增加，同时它们的差值也在增加.认为产生这一结果的原因有两方面.一方面，Ni/Au肖特基金属淀积在AlGaN/GaN异质结上，改变了AlGaN势垒层的表面状态，使得一部分二维电子气(2DEG)电子被抽取到空的施主表面态中，从而减小了AlGaN/GaN异质结界面势阱中的2DEG浓度.随着势垒层Al组分的增加，AlGaN层产生了更多的表面态，从而使得更多的电子被抽取到了空的表面态中.另一方面，由于C-V测量本身精确度受到串联电阻的影响，使得测量电容小于实际电容，从而低估了载流子浓度.     

抽运-探测反射技术研究本征CdTe的载流子动力学

采用抽运-探测反射技术,研究了室温下本征CdTe晶体的光致非平衡载流子布局与光子能量和抽运光强的关系.根据实验结果,发现随着抽运光光子能量的提高,快过程在载流子弛豫过程中所占的比例增大;随着抽运光功率的提高,反射率随之增大,快过程时间常数也随之增大.通过建立简单的本征半导体受激载流子弛豫过程模型,讨论了载流子散射、载流子-声子相互作用和载流子复合等的贡献.在抽运光光子能量为1.49 eV(比CdTe的禁带宽度约高20 meV)时,通过双指数函数拟合,得到了本征CdTe中载流子弛豫过程的快、慢时间常数,分别为2.8 ps和158.3 ps.     

AlGaN/GaN HEMT器件电流坍塌和膝点电压漂移机理的研究

考虑了势垒层、缓冲层体陷阱及表面电荷的浓度变化对电流坍塌和膝点电压的影响,发现表面电荷和势垒层体陷阱浓度的变化对沟道电子的浓度影响较小,表面电荷浓度变化下的膝点电压的偏移和坍塌强度的大小与势垒层势阱能量的变化有着主要的关系.缓冲层有着比势垒层更强的局域作用,势垒层和缓冲层的体陷阱浓度在一定范围变化时的膝点电压偏移主要是由沟道电子浓度的变化而引起的,但偏移量却比表面电荷浓度变化的情况下小很多.势阱能量的变化是造成膝点电压偏移的重要原因,坍塌强度主要取决于势阱能量和沟道的电子浓度. 关键词： AlGaN/GaN高电子迁移率晶体管 电流坍塌 膝点电压 陷阱俘获     

双势垒InAs/InP纳米线异质结热电子制冷机

研究了具有不同温度和不同化学势的两个热库中电子通过一个双势垒InAs/InP纳米线异质结进行的传输.利用传输矩阵法得到了电子的传输概率,进而计算得到电子传输所产生的热流.通过数值计算给出了热电子制冷机的性能特征曲线.进一步分析了势垒宽度和势阱宽度对制冷机工作性能的影响.研究发现,当势阱宽度一定时,随着势垒宽度变大共振中心能级的位置变大,共振能级宽度变小,同一偏压对应的制冷率变小,相对制冷系数变大.当势垒宽度一定时,随着势阱宽度变大,同一偏压对应的相对制冷系数变小.当势垒和势阱宽度同时变化时,得到的曲线与势垒宽度一定势阱宽度变化时得到的曲线基本相似.这表明制冷率和相对制冷系数主要受势阱宽度变化的影响.     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率,光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S₂态后,经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

HgS/CdS/HgS球状纳米系统电子的能量与寿命以及概率分布

建立了HgS/CdS/ HgS球状纳米系统物理模型和电子状态满足的方程.应用S矩阵理论，探讨了 s态电子的能量和寿命以及概率分布随势垒和势阱宽度的变化规律.结果表明：电子能量和寿 命随垒势宽度增大而增大；电子能量随阱宽增大而减小，而寿命随阱宽增大而增大；层间相 互作用对结果有重要影响. 关键词： 球状纳米系统 电子能量和寿命 电子概率分布 势垒势阱宽度     

Scanning probe microscopy of cleavages of undoped GaInP/AlGaInP and CdS/ZnSSe heterostructures

Cleavages of undoped nanoscale heterostructures with GaInP/AlGaInP and CdS/ZnSSe quantum wells were studied by scanning probe microscopy. A nanorelief formed on the cleavage surface due to elastic stresses in quantum wells was detected by contact methods. The current method yielded more contrast images. It was shown that the current in undoped heterostructures depends on the Schottky barrier on the probe contact with structure layers, the intrinsic carrier concentration in them, the growth substrate doping level, the intensity and spectrum of external illumination.     

垒层厚度对InGaN/GaN多量子阱电注入发光性能的影响及机理

研究了不同垒厚对InGaN/GaN多量子阱电注入发光性能的影响及机理。实验发现,当GaN垒层的厚度从6 nm增大到24 nm时,垒厚的样品发光强度更强,而且当注入电流增加时,适当增加垒厚,可以更显著增加发光强度。进一步结合发光峰位和光谱宽度的研究表明,由于应力和极化效应的存在,当垒层厚度在6~24 nm范围内时,适当增加垒层厚度不仅会使得能带的倾斜加剧,减少电子泄露,而且也会增加InGaN阱层的局域态深度,从而改善量子阱的发光性能。     

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.     

Hole capture rate of GaInAs/InP strained quantum-well lasers

The property of hole capture of quantum wells is important in the static properties of lasers above threshold, such as the differential efficiency and light output power. We investigate experimentally the hole capture rate and its influence on the carrier overflow in the optical confinement layers for compressive-strained, tensile-strained and unstrained GaInAs/GaInAsP/InP quantum-well lasers emitting at 1.5 m by measuring the spontaneous emission from the optical confinement layers above threshold. The carrier density in the optical confinement layers increases with current owing to finite hole capture rates. This increase is dependent on well thickness and barrier height determined by the strain. This increase is comparable in the tensile-strained and unstrained lasers with relatively low threshold, while in the compressive-strained laser it is about double that in the other two types. The dependence of this increase on threshold carrier density is also observed, that is the carrier density in the optical confinement layers increases rapidly in high-threshold samples, in particular, in the tensile-strained laser with large hole barrier height. From these results, laser operation with high output power and high efficiency is expected by reducing threshold carrier density in the tensile-strained laser and by increasing well numbers in the compressive-strained laser as long as the inhomogeneous injection between wells is not severe. By fitting measurements with theory, the hole capture time is estimated as 0.1 to 0.25 ps in these strained and unstrained lasers.     

Model calculations of diffusion limited trapping dynamics in quantum well laser structures

We present a phenomenological theoretical model to treat the trapping of carriers into quantum wells of semiconductor laser structures. We consider explicitely the transport within the barrier layers by solving the continuity equation with the appropriate boundary conditions taking into account surface recombination, radiative and nonradiative recombination in the barrier layers and trapping of carriers into the quantum wells. The experimental findings for the trapping dynamics in GaAs/AlGaAs quantum well structures can be consistently interpreted by the model calculations.     

多量子阱中注入载流子的非均匀分布

根据多量子阱中注入载流子的输运机制,计算了多量子阱中注入载流子的非均匀分布.引入不均匀度参量Asy来衡量载流子分布的不均匀程度,分析了各种敏感因素对载流子非均匀分布的影响.指出注入载流子分布的非均匀性,随量子阱数、注入电流、量子垒高度的增加而显著增加,随工作温度的升高而减小.     

Direct observation of variations of optical properties in single quantum dots by using time-resolved near-field scanning optical microscope

We study the variations of optical properties of self-assembled In_0.5Ga_0.5As single quantum dots (QDs) in the spatial and time domains by combining a near-field scanning optical microscope with an ultrafast pulsed laser. Through the examinations of several tens of QDs, we find that the variations of photoluminescence (PL) intensity strongly depend on the condition of the initial carrier creation. The differences in quantum efficiency and those in the carrier flow rate into QDs cause the large distribution of PL intensity when the carriers are excited in the barrier layers. From the results of time-resolved PL decay measurements, we find that there are two types of QDs exhibiting quite different PL decay profiles.     

Injection, intersubband relaxation and recombination in GaAs multiple quantum wells

Results of continuous excitation, time-delayed and time-resolved cathodoluminescence experiments on undoped and Be-doped GaAs multiple quantum wells of thickness 5–11 nm are reported for temperatures 5–300 K and excitation intensities 1–10³ W/cm². Comparison is made with similar investigations of bulk epitaxial GaAs layers. Increasing structural localisation of the carriers is identified to lead to a qualitative change of the type of recombination. Increasingly faster radiative excitonic recombination leads to a bypass of impurity and trap capture processes in undoped as well as in doped material, at low temperatures as well as at room temperature. Despite the exponential character of excitonic decay, the luminescence transients are found to be very complex due to an interplay of intersubband scattering and recombination processes and time-dependent carrier temperature. Transients are analysed in detail, excitonic lifetimes and intersubband scattering times are derived. It is argued that both the lifetime reduction in quantum wells and the novel process of recombination heating lead to a strongly increased quasiequilibrium temperature of excited carriers in the wells as compared to bulk material. Injection of carriers from 18 nm GaAlAs barriers to GaAs wells is found to occur in less than 10^-12 s without loss.