Bringing quantum dots up to speed [Breaking the phonon bottleneckwith high-speed modulation of quantum-dot lasers]

In this article, we will focus on the carrier relaxation time in quantum dots (QDs), its probable mechanism, and the implications for the performance characteristics of directly modulated QD lasers and other QD devices. The electron and hole bound states and general predictions of carrier capture time into them will be presented, followed by a discussion of intersubband carrier relaxation in QDs. The modulation characteristics of QD lasers as a function of temperature will be described, and these modulation results will be discussed in terms of the temperature, composition, and size dependence of the relaxation time in QDs, including possible methods for designing QDs to overcome this relaxation time barrier. Also, the performance characteristics of other possible QD devices, such as intersubband lasers and detectors, will be examined in terms of our current understanding of the relaxation time in QDs     

Numerical Analysis of the Frequency Chirp in Quantum-Dot Semiconductor Lasers

We present a numerical model for the analysis of the chirp dynamics of quantum-dot (QD) semiconductor laser under large signal current modulation. The model is based on the multipopulation rate equation formalism, and it includes all the peculiar characteristics of the active QD material such as the inhomogeneous broadening of the gain spectrum, the presence of an excited state confined in the QDs and the presence of nonconfined states due to the wetting layer and the barrier. In this paper the model is applied to the analysis of the chirp of two QD single-mode lasers emitting from the ground state and from the excited state, respectively. In order to make comparisons of the chirp in various operating conditions, we define some equivalent parameters for quantifying the adiabatic and transient contributions to the chirp. These parameters are then used to analyze the chirp as function of the bias current, of the modulation depth and of the modulation frequency. All the various simulation results show that the carrier accumulation in the QD states, poorly involved in the stimulated emission process and the carrier dynamics in these states, can cause a nonzero chirp under current modulation even for the ideal condition of zero linewidth enhancement factor (or -parameter) at the laser threshold.     

Specific Features of XGM in QD-SOA

The peculiarities of a cross-gain modulation (XGM) in quantum-dot semiconductor optical amplifier (QD SOAs) related to the inhomogeneous broadening are theoretically studied. We have solved numerically the electron rate equations for the QD SOA model including an excited state, a ground state and a continuum wetting layer (WL), and the propagation equations for the pump and the signal lightwaves. It is shown that XGM in QD SOA is possible for a comparatively large detuning close to the inhomogeneous broadening because in such a case QDs with substantially different resonant frequencies interact directly through WL.     

Analysis of the Linewidth-Enhancement Factor of Long-Wavelength Tunnel-Injection Quantum-Dot Lasers

We have studied the linewidth-enhancement factor of 1.3-mum tunnel-injection quantum-dot (QD) lasers utilizing a rate-equation model that takes into account the injection of electrons directly into the QDs from a coupled quantum well, the presence of wetting layer states, and nonequilibrium carrier relaxation in the QDs. In a conventional separate confinement heterostructure QD laser, plasma effects, which result from a large portion of the injected carriers preferably occupying the barrier and wetting layer states, largely determine the values of the linewidth-enhancement factor and lead to a strong dependence of the linewidth-enhancement factor on injection current. In a tunnel-injection QD laser, however, due to the injection of "cold" electrons directly into the lasing states of the QDs, both the values of linewidth-enhancement factor and the dependence on injection current are substantially reduced. The calculated linewidth-enhancement factors of conventional separate confinement heterostructure and tunnel-injection QD lasers are in excellent agreement with reported experimental values. Our analysis elucidates the role of tunnel injection in achieving near-zero alpha-parameter, which would be important in the design of chirp-free high-speed QD lasers     

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.     

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     

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.     

Small-signal field effect in GaAs/InAs quantum-dot heterostructures

InAs quantum dots (QDs), incorporated into the space-charge region of an epitaxial n-GaAs film at different distances (5?C300 nm) from the surface, decrease the potential barrier for the electrons located in n-GaAs. For tunnel-thin coating layers this decrease is related to tunneling through QD energy levels. For thick layers this decrease is caused by negative charging of the QDlevels and defects located near QDs. The decrease in the barrier increases the efficiency of electron capture by surface states and shifts the frequency dispersion of mobility under the field effect, related to this capture, toward higher frequencies. When QDs are incorporated near the barrier??s base, they manifest themselves in the relaxation of the small-signal field effect. Some parameters of the QD levels are determined. Defect formation is revealed in the layers adjacent to QDs.     

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     

GaN/AlN量子点结构中的应变分布和压电效应

从Ⅲ-Ⅴ族氮化物半导体压电极化对应变的依赖关系出发,采用有限元方法计算了GaN/AlN量子点结构中的应变分布,研究了其自发极化、压电极化以及极化电荷密度.结果表明,应变导致的压电极化和Ⅲ-Ⅴ族氮化物半导体所特有的自发极化将导致电荷分布的变化,使电子聚集在量子点顶部,空穴聚集在量子点下面的湿润层中,在量子点结构中产生显著的极化电场,并讨论了电场的存在对能带带边的形状以及能级分布的影响.     

Photomultiplication-Type Organic Photodetectors with Fast Response Enabled by the Controlled Charge Trapping Dynamics of Quantum Dot Interlayer

A novel photomultiplication (PM)-type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM-type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM-inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM-type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD-interlayer PM-type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast-responding PM-type OPDs but also provides comprehensive understanding of the underlying device physics.     

GaN／AIN量子点结构中的应变分布和压电效应

从Ⅲ-Ⅴ族氮化物半导体压电极化对应变的依赖关系出发，采用有限元方法计算了GaN／AIN量子点结构中的应变分布，研究了其自发极化、压电极化以及极化电荷密度．结果表明，应变导致的压电极化和Ⅲ-Ⅴ族氮化物半导体所特有的自发极化将导致电荷分布的变化，使电子聚集在量子点顶部，空穴聚集在量子点下面的湿润层中，在量子点结构中产生显著的极化电场，并讨论了电场的存在对能带带边的形状以及能级分布的影响．     

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．     

Quantum-Dot Lasers—Desynchronized Nonlinear Dynamics of Electrons and Holes

We analyze the complex turn-on behavior of semiconductor quantum-dot (QD) lasers in terms of a nonlinear rate equation model for the electron and hole densities in the QDs and the wetting layer, and the photons. A basic ingredient of the model is the nonlinearity of the microscopic carrier–carrier scattering rates. With the framework of detailed balance, we analytically relate the microscopic in- and out-scattering rates. We gain insight into the anomalous nonlinear dynamics of QD lasers by a detailed analysis of various sections of the 5-D phase space, accounting for density-dependent carrier scattering times. We show that the strongly damped relaxation oscillations are characterized by a desynchronization of electron and hole dynamics in the dots. Analytic approximations for the steady-state characteristics are also derived.      

Self-Consistent Analysis of Carrier Confinement and Output Power in 1.3-$mu{hbox {m}}$ InAs–GaAs Quantum-Dot VCSELs

A self-consistent model comprising rate equations and thermal conduction equation is used to analyze the influence of self-heating on the carrier occupation, quantum efficiency, and output power of 1.3- $mu{hbox {m}}$ InAs–GaAs quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). The simulation results show that the poor hole confinement in QDs is due to the thin wetting layer, and increase in QD density and layer number can significantly improve the self-heating effect and quantum efficiency of the device. The output power of the QD VCSEL is mainly determined by the quantum efficiency. High output power can be achieved by the high number of QD layers and QD density. However, there exists an optimized number of QD layers ($sim$15) to achieve the highest output power.      

量子点阵列光致荧光谱温度依赖性研究

多模量子点阵列的光致荧光(PL)光谱的温度依赖性研究对于实现高效的量子点光电器件有着非常重要的意义.利用速率方程模型模拟不同密度量子点阵列中的载流子动力学过程.研究表明,高密度量子点阵列中不同尺寸量子点族的PL强度表现不同的温度依赖关系;而低密度量子点阵列不同点族PL强度均随温度衰减.高密度量子点阵列中,载流子被热激发到浸润层后,部分地被大量子点再俘获,即在量子点族间转移;低密度量子点阵列中不同量子点族间的载流子转移受到限制.不同量子点族光致荧光强度比的最大值强烈地依赖于量子点的激活能差.     

Temperature dependence of photoluminescence of QD arrays

It is essentially important to understand the temperature dependence of the photoluminescence of multimodal quantum dot (QD) arrays for the realization of efficient photonic devices. In this paper, the dynamics processes of different density multimodal QD arrays were fitted by using the rate equation model. It is shown that, in high density QD arrays, the intensity of photoluminescence of different QD families has different temperature dependence, and the intensity of photoluminescence is quenched as the temperature increases in low density QD arrays. In high density QD arrays, as the temperature increases, the carriers will be thermally excited into the wetting layer from QDs, and then some of them will be recaptured by the big scale QDs; carrier coupling takes place between the different QD families, while in low density QD arrays, the carrier transfer between different QD families will be limited. Temperature dependence of the maximum of the ratio of photoluminescence intensity of different QD families strongly depends on the difference of thermal activation energies.     

Theoretical and experimental study of the effect of InAs growth rate on the properties of QD arrays in InAs/GaAs system

The dependence of properties of quantum dot (QD) arrays in an InAs/GaAs system on the InAs growth rate has been investigated theoretically and experimentally. The derived kinetic model of the formation of coherent nanoislands allows the calculation of the average size, surface density of islands, and wetting layer thickness as functions of the growth time and conditions. Optical properties of InAs/GaAs QDs have been studied for the case of two monolayers (ML) of the material deposited at different growth rates. Predictions of the theoretical model are compared with the experimental data. It is shown that with two ML of the deposited material the characteristic lateral size of QDs decreases and the thickness of the residual wetting layer increases with rising growth rate.     

Synthesis and functionalization of epitaxial quantum dot nanostructures for nanoelectronic architectures

We describe methods for assembly of quantum dots (QDs) into arrays of any symmetry, and methods for nanoscale doping of individual QDs. We have previously shown how the Ga⁺ focused ion beam (FIB) can template Si(1 0 0) surfaces for controlled Ge QD nucleation. Local Ga-induced reduction of the wetting layer thickness also suppresses QD nucleation away from the templating sites. This allows synthesis of arrays of any defined geometry and set of spatial frequencies, with positional control of each QD element to ca. 10 nm. We have also applied these methods to “quantum dot molecule” (QDM) structures, that comprise four QDs surrounding a central surface pit and that form spontaneously under conditions of limited adatom mobility in GeSi/Si(1 0 0). Again, the positions and spacings of the QDMs can be controlled by local FIB templating. This creates hierarchical order over multiple lengths scales, from ultra-small dimensions inaccessible to conventional lithography (i.e. nm for QD spacing to tens of nm for individual QD sizes), to much greater length scales (hundreds of μm) over which controlled arrays of QDMs can be fabricated. The ability to bring individual QDs in the QDM into very close proximity (of order nm) has potential applications to nanoelectronic architectures based on electron/hole tunneling or spin interactions. We are developing methods for electronic and magnetic functionalization of these nanostructures using a mass-selected FIB, where ions of different species can be separated from liquid metal alloy sources (e.g. Si from AuSi, B and As from PdAsB, and Mn and Ge from MnGe).     

Quantum Junction Solar Cells: Development and Prospects

Nanocrystals, called semiconductor quantum dots (QDs), contain excitons that are three-dimensionally bound. QDs exhibit a discontinuous electronic energy level structure that is similar to that of atoms and exhibit a distinct quantum confinement effect. As a result, QDs have unique electrical, optical, and physical characteristics that can be used in a variety of optoelectronic device applications, including solar cells. In this review article, the stable and controllable synthesis of QD materials is outlined for upscaling solar cells, including material development and device performance enhancement. It includes a systematic variety of device structures for the fabrication of solar cells, such as QD, hybrid QD/organic, hybrid QD/inorganic, perovskite QD, and hybrid 2D MXene QD/perovskite. The mechanisms for the improvement of stability by QD treatment are examined. For example, the 2D MXene QD and/or Cu_1.8S nanocrystal doping significantly increases the long-term light and ambient stability of perovskite solar cells, resulting from improved perovskite crystallization, reduced hole transport layer (HTL) aggregation and crystallization of films, and reduced UV-induced photocatalytic activity of the electron transport layer (ETL). For the advancement of QD solar cells and their interaction with various materials, the conclusions from this review are crucial. Finally, future prospects for the development of QD solar cells as well as current challenges are discussed.