多层In_(0.55)Al_(0.45)As/A_(0.5)Ga_(0.5)As量子点的变温光致发光研究

测量了自组织多层In0.55Al0.45As/Al0.5Ga0.5As量子点的变温光致发光谱,同时观察到来自浸润层和量子点的发光,首次直接观察到了浸润层和量子点之间的载流子热转移.分析发光强度随温度的变化发现浸润层发光的热淬灭包括两个过程:低温时浸润层的激子从局域态热激发到扩展态,然后被量子点俘获;而温度较高时则通过势垒层的X能谷淬灭.利用速率方程模拟了激子在浸润层和量子点间的转移过程,计算结果与实验符合得很好     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

Observation and Modeling of a Room-Temperature Negative Characteristic Temperature 1.3-μm p-Type Modulation-Doped Quantum-Dot Laser

A room-temperature negative characteristic temperature (T₀ ) and ultralow threshold current density (J_th) of 48 Amiddotcm^-2 are demonstrated for a 1.3-mum InAs quantum dot laser. These characteristics are obtained by combining a high-growth-temperature GaAs spacer layer with p-type modulation doping of the quantum dots in multiple layer dot-in-a-well structures. Through a comparison of p-doped and undoped devices, a photon coupling mechanism is proposed to account for the different temperature dependences of Jth for the two devices. Numerical simulations based on a rate equation model, which includes photon coupling between ground and excited quantum dot states, are performed. The simulations are able to account for the very different temperature-dependent Jth behavior of the doped and undoped device     

Dynamics of the wetting-Layer-quantum-dot interaction in InGaAs self-assembled systems

An experiment is described to study the carrier dynamics in an InGaAs quantum-dot-wetting-layer system emitting in the 1-/spl mu/m band at carrier densities typical of laser-diode operating conditions. The temporal evolution of the differential luminescence spectrum generated by the dots when an ultrafast optical pulse is used to perturb the steady-state carrier population of the surrounding wetting layer is measured. This provides information about the dynamic interaction of wetting-layer and quantum-dot populations. Study of the differential luminescence signal shows a marked bottleneck in the transfer of charge carriers into the quantum dots from the two-dimensional layers that surround them when the equilibrium occupancy of the dot states is high. Recovery of the system back to steady state then takes place over a time interval in excess of 600 ps. This situation arises because the dc electrical injection produces a high carrier density in the dot ensemble, and thus a high proportion of the additional photogenerated carriers remain in the wetting layer as the system thermalises. This reservoir of carriers then feeds into the dots as states are emptied by recombination events.     

InAlAs/InGaAs复合限制层对InAs量子点发光性质的影响

系统研究了InAlAs/InGaAs复合限制层对InAs量子点光学性质的影响;发现InAs量子点的基态发光峰位、半高宽以及基态与第一激发态的能级间距都强烈地依赖于InAlAs薄层的厚度和In的组分;得到了室温发光波长在1.35μm,基态与第一激发态的能级间距高达103 meV的InAs量子点的发光特性。这一结果对实现高T_0的长波长InAs量子点激光器的室温激射具有重要意义。     

Optical gain and saturation characteristics of quantum-dot semiconductor optical amplifiers

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs.     

Photoelectric spectroscopy of InAs/GaAs quantum dot heterostructures in a semiconductor/electrolyte system

The photovoltaic effect in the semiconductor/electrolyte junction is an effective method for investigation of the energy spectrum of InAs/GaAs heterostructures with self-assembled quantum dots. An important advantage of this method is its high sensitivity. This makes it possible to obtain photoelectric spectra from quantum dots with high barriers for the electron and hole emission from quantum dots into the matrix even if the surface density of the dots is low (～10⁹ cm^?2). In a strong transverse electric field, broadening of the lines of optical transitions and emission of electrons and holes from quantum dots into the matrix directly from the excited states are observed. The effect of the photovoltage sign reversal was detected for a sufficiently high positive bias across the barrier within the semiconductor. This effect is related to the formation of a positive charge at the interface between the cap layer and electrolyte and of the negative charge on impurities and defects in the quantum dot layer.     

Structural and electrooptical characteristics of quantum dotsemitting at 1.3 μm on gallium arsenide

We present a comprehensive study of the structural and emission properties of self-assembled InAs quantum dots emitting at 1.3 μm. The dots are grown by molecular beam epitaxy on gallium arsenide substrates. Room-temperature emission at 1.3 μm is obtained by embedding the dots in an InGaAs layer. Depending on the growth structure, dot densities of 1-6×10¹⁰ cm^-2 are obtained. High dot densities are associated with large inhomogeneous broadenings, while narrow photoluminescence (PL) linewidths are obtained in low-density samples. From time-resolved PL experiments, a long carrier lifetime of ≈1.8 ns is measured at room temperature, which confirms the excellent structural quality. A fast PL rise (τ_rise=10±2 ps) is observed at all temperatures, indicating the potential for high-speed modulation. High-efficiency light-emitting diodes (LEDs) based on these dots are demonstrated, with external quantum efficiency of 1% at room temperature. This corresponds to an estimated 13% radiative efficiency. Electroluminescence spectra under high injection allow us to determine the transition energies of excited states in the dots and bidimensional states in the adjacent InGaAs quantum well     

Self-assembling Ge(Si)/Si(100) quantum dots

The morphological evolution of self-assembled epitaxial quantum dots on Si(100) is reviewed. This intensely investigated material system continues to provide fundamental insight guiding the growth of nanostructured electronic materials. Self-assembled quantum dots are faceted, three-dimensional islands which grow atop a planar wetting layer. Pure Ge growth at higher substrate temperatures results in narrower island size distributions but activates additional strain-relief mechanisms which will alter the optical and electronic properties of the dots. Optical and electrical characterization has shown that electrons and holes are confined to different regions of the dot. This results in a spatially indirect, type II recombination mechanism. Emerging device applications which exploit properties of these nanoscale Ge islands are discussed.     

Spatially resolved spectroscopy on single self-assembled quantum dots

We report about spatially resolved experiments on self-assembled InGaAs quantum dots. Single quantum dots can be investigated by using STM-induced luminescence spectroscopy. The quantum dot occupancy can be increased via the STM tip current, which results in state filling and therefore in the onset of discrete excited state luminescence. In the limit of low injection currents, a single emission line from the ground state of the dot is observed. Using near-field spectroscopy through shadow masks, we have investigated the optical properties of single self-assembled InGaAs quantum dots as a function of occupancy and magnetic field. This allows us to fully resolve diamagnetic/orbital effects, Zeeman splitting, and to determine manybody-corrections. Photoluminescence excitation spectra further reveal a strong contribution of phonon assisted processes in quantum dot absorption.     

Photoinduced and equilibrium optical absorption in Ge/Si quantum dots

The results of studies of the optical absorption spectra in Ge/Si quantum dot structures in the mid-infrared region are reported. Two types of structures different in terms of the method used for quantum dot formation and in terms of barrier layer thickness are explored. The photoinduced absorption associated with the nonequilibrium population of hole states and optical absorption in structures doped to different levels are investigated. Specific features that are associated with occupation of the ground and excited states of quantum dots and exhibit a polarization dependence are observed. From the experimental data, the energy spectrum of holes is determined for structures of both types.     

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm² at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots. Fiz. Tekh. Poluprovodn. 31, 483–487 (April 1997)     

In situ study of the formation kinetics of InSb quantum dots grown in an InAs(Sb) matrix

Formation of InSb quantum dots grown in an InAs matrix by molecular-beam epitaxy that does not involve forced deposition of InSb is studied. Detection of intensity oscillations in the reflection of high-energy electron diffraction patterns was used to study in situ the kinetics of the formation of InSb quantum dots and an InAsSb wetting layer. The effects of the substrate temperature, the shutter operation sequence, and the introduction of growth interruptions on the properties of the array of InSb quantum dots are examined. Introduction of a growth interruption immediately after completing the exposure of the InAs surface to the antimony flux leads to a reduction in the nominal thickness of InSb and to an enhancement in the uniformity of the quantum-dot array. It is shown that, in the case of deposition of submonolayer-thickness InSb/InAs quantum dots, the segregation layer of InAsSb plays the role of the wetting layer. The Sb segregation length and segregation ratio, as well as their temperature dependences, are determined.     

Carrier Dynamics of Quantum-Dot, Quantum-Dash, and Quantum-Well Semiconductor Optical Amplifiers Operating at 1.55 μm

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (0-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In-GaAsP-InP quantum well (2-D), all of which operate near 1.55-mum wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer.     

InAs/GaAs自组织量子点激发态的激射

将覆盖层引入生长停顿的量子点结构作为激光器有源区来研究量子点激光器受激发射机制 .由于强烈的能带填充效应 ,光致发光谱和电致发光谱中观察到对应于量子点激发态跃迁的谱峰 ,大激发时其强度超过基态跃迁对应的谱峰 .最后激发态跃迁达到阈值条件 ,激射能量比结构相似但不含量子点的激光器低 ,表明量子点激光器中首先实现受激发射是量子点的激发态     

In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点的形貌和光学性质

提出了利用分子束外延方法生长In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点，并分析量子点的形貌和光学性质随GaAs隔离层厚度变化的特点。实验结果表明，随着耦合量子点中的GaAs隔离层厚度从2 nm增加到10 nm，In0.5Ga0.5As量子点的密度增大、均匀性提高, Al原子扩散和浸润层对量子点PL谱的影响被消除，而且InAlAs材料的宽禁带特征使其成为InGaAs量子点红外探测器中的暗电流阻挡层。由此可见，选择合适的GaAs隔离层厚度形成InGaAs/InAlAs应变耦合量子点将有益于InGaAs量子点红外探测器的研究。     

InAs/GaAs自组织量子点激发态的激射

将覆盖层引入生长停顿的量子点结构作为激光器有源区来研究量子点激光器受激发射机制．由于强烈的能带填充效应, 光致发光谱和电致发光谱中观察到对应于量子点激发态跃迁的谱峰,大激发时其强度超过基态跃迁对应的谱峰．最后激发态跃迁达到阈值条件, 激射能量比结构相似但不含量子点的激光器低,表明量子点激光器中首先实现受激发射是量子点的激发态．     

Below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot solar cells

A new approach to derive the below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot (QD) devices using room temperature external quantum efficiency measurement results is presented. The significance of incorporating an extended Urbach tail absorption in analyzing QD devices is demonstrated. This tail is used to evaluate the improvement in the photo‐generated current. The wetting layer and QD absorption contributions are separated from the tail absorption. Several absorption peaks due to QD excited states and potentially different size QDs are observed. An inhomogeneous broadening of 25 meV arising from the variance in the size of QDs is derived. Copyright © 2014 John Wiley & Sons, Ltd.     

The sandwich InGaAs/GaAs quantum dot structure for IR photoelectric detectors

A new possibility for growing InAs/GaAs quantum dot heterostructures for infrared photoelectric detectors by metal-organic vapor-phase epitaxy is discussed. The specific features of the technological process are the prolonged time of growth of quantum dots and the alternation of the low-and high-temperature modes of overgrowing the quantum dots with GaAs barrier layers. During overgrowth, large-sized quantum dots are partially dissolved, and the secondary InGaAs quantum well is formed of the material of the dissolved large islands. In this case, a sandwich structure is formed. In this structure, quantum dots are arranged between two thin layers with an increased content of indium, namely, between the wetting InAs layer and the secondary InGaAs layer. The height of the quantum dots depends on the thickness of the GaAs layer grown at a comparatively low temperature. The structures exhibit intraband photoconductivity at a wavelength around 4.5 μm at temperatures up to 200 K. At 90 K, the photosensitivity is 0.5 A/W, and the detectivity is 3 × 10⁹ cm Hz^1/2W^?1.