Photoluminescence of Multilayer InAs Quantum Dots

采用MBE设备生长了多层InAs/GaAs量子点结构,测量了其变温光致发光谱和时间分辨光致发光谱。结果表明多层量子点结构有利于减小发光峰的半高宽,并且可以提高发光峰半高宽和发光寿命的温度稳定性。实验发现,加InGaAs盖层后,量子点发光峰的半高宽进一步减小,最小达到23.6 meV,并且发光峰出现红移。原因可能在于InGaAs盖层减小了InAs岛所受的应力,阻止了In组分的偏析,提高了InAs量子点尺寸分布的均匀性和质量,导致载流子在不同量子点中的迁移效应减弱。Multilayer InAs/GaAs quantum dots structures were grown by molecular-beam epitaxy(MBE).The steady-state and time-resolved photoluminescence of the samples were measured at various temperatures.Results showed that multilayer structures could not only narrow the photoluminescence FWHM(full width at the half maximum) but enhance the stability of the photoluminescence lifetime and FWHM.As for the quantum dots with InGaAs cap layer,the photoluminescent spectra became narrower(the narrowest FWHM was only 23.6meV) and the photoluminescent wavelength became longer.The possible reason for the above phenomena was that the InGaAs cap layer could both release the strains in InAs islands and inhibit segregation of In components,resulting in the weaker migration among different quantum dots

Electron microscopy of quantum dots

This brief review describes the different types of semiconductor quantum dost systems, their main applications and which types of microscopy methods are used to characterize them. Emphasis is put on the need for a comprehensive investigation of their size distribution, microstructure, chemical composition, strain state and electronic properties, all of which influence the optical properties and can be measured by different types of imaging, diffraction and spectroscopy methods in an electron microscope

The study of optical properties on InAs self-organized quantum dots

由于半导体量子点具有零维电子特性，它不仅成为基本物理研究的重要对象，也成为研制新一代量子器件的基础。正因如此，量子点材料及器件成为目前国际上最前沿的研究课题之一。GaAs基InAs自组织量子点因其成本低廉、器件工艺成熟，成为替代InP基材料、制备光纤通信用1.3－1.55µm发光激光器的热门材料之一。本文采用分子束外延技术制备了高质量的GaAs基InAs自组织量子点材料。利用原子力显微镜（AFM）、扫描电子显微镜（SEM）、变温及时间分辨的光致发光谱（PL）等手段，分别研究了InGaAs应变层（指在InGaAs层上生长量子点，下同）、InGaAs盖层、InGaAs/InAlAs联合...Quantum dots (QDs), with zero-dimensional electronic properties, have stimulated great interest due to their important roles in fundamental physical research and for developing novel devices. In recent years, it has been one of frontier topics of materials science to study the characterization of self-organized quantum dots and device applications. GaAs-based In(Ga)As self-organized QDs have becom...学位：理学博士院系专业：物理与机电工程学院物理学系_凝聚态物理学号：B20022400

Low dimensional structures for optical and electrical applications

Ankara : The Department of Physics and the Institute of Engineering and Sciences of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 64-70.Low dimensional structures such as quantum dots have been particularly attractive because of their fundamental physical properties and their potential applications in various devices in integrated optics and microelectronics. This thesis presents optical and electrical applications of low dimensional structures. For this purpose we have studied silicon and germanium nanocrystals for flash memory applications and InAs quantum dots for optical modulators. As a quantum dot, nanocrystals can be used as storage media for carriers in flash memories. Performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. Charge and discharge dynamics of PECVD grown nanocrystals were studied. Electron and hole charge and discharge currents were observed to differ significantly and strongly depend on annealing conditions chosen for the formation of nanocrystals. Our experimental results revealed that, discharge currents were dominated by the interface layer acting as a quantum well for holes and route for direct tunneling for electrons. On the other hand, possibility of obtaining quantum dots with enhanced electro-optic and/or electro-absorption coefficients makes them attractive for use in light modulation. Therefore, waveguides of multilayer InAs quantum dots were studied. Electro-optic measurements were conducted at 1.5 µm and clear Fabry-Perot resonances were obtained. The voltage dependent Fabry-Perot measurements revealed that 6 V was sufficient for full on/off modulation. Electroabsorption measurements were conducted at both 1.3 and 1.5 µm. Since the structure lases at 1285 nm, high absorption values at 1309 nm were obtained. The absorption spectrum of the samples was also studied under applied electric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.Akça, İmranM.S

Photovoltaic Performance of a Nanowire/Quantum Dot Hybrid Nanostructure Array Solar Cell

Abstract An innovative solar cell based on a nanowire/quantum dot hybrid nanostructure array is designed and analyzed. By growing multilayer InAs quantum dots on the sidewalls of GaAs nanowires, not only the absorption spectrum of GaAs nanowires is extended by quantum dots but also the light absorption of quantum dots is dramatically enhanced due to the light-trapping effect of the nanowire array. By incorporating five layers of InAs quantum dots into a 500-nm high-GaAs nanowire array, the power conversion efficiency enhancement induced by the quantum dots is six times higher than the power conversion efficiency enhancement in thin-film solar cells which contain the same amount of quantum dots, indicating that the nanowire array structure can benefit the photovoltaic performance of quantum dot solar cells

Stacking of multilayer InAs quantum dots with combination capping of InAlGaAs and high temperature grown GaAs

We are reporting the growth of multilayer stacks of quantum dots (10 periods) with a combination capping of In0.21Al0.21Ga0.58As (30 angstrom) and GaAs (70-180 angstrom) grown by solid source molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) has been used for the insitu monitoring of quantum dot (QD) formation in heterostructure samples. The samples were also characterized by other exsitu techniques like cross sectional transmission electron microscopy (XTEM) and photoluminescence measurements (PL). For a heterostructure sample with thin barrier thickness (<100 angstrom), an XTEM image showed the stacking of QDs only up to the 5th layer and in the upper layers there was hardly any formation of dots. We presume the stoppage of dot formation is due to the uneven surface of the InAlGaAs alloy overgrown on the InAs QDs, as a result of the local compositional deviations of the Group-III atoms. Samples grown with thicker barriers (> 100 angstrom of GaAs) showed good stacking of islands until the tenth layer. The thick GaAs layer overgrown on the InAlGaAs at 590 degrees C is believed to remove the surface modifications of the quaternary layer thereby creating a smoother surface front for the growth of subsequent QD layers