Wetting layers effect on InAs/GaAs quantum dots

FEM combining with the K·P theory is adopted to systematically investigate the effect of wetting layers on the strain-stress profiles and electronic structures of self-organized InAs quantum dot. Four different kinds of quantum dots are introduced at the same height and aspect ratio. We found that 0.5 nm wetting layer is an appropriate thickness for InAs/GaAs quantum dots. Strain shift down about 3%∼4.5% for the cases with WL (0.5 nm) and without WL in four shapes of quantum dots. For band edge energy, wetting layers expand the potential energy gap width. When WL thickness is more than 0.8 nm, the band edge energy profiles cannot vary regularly. The electron energy is affected while for heavy hole this impact on the energy is limited. Wetting layers for the influence of the electronic structure is obviously than the heavy hole. Consequently, the electron probability density function spread from buffer to wetting layer while the center of hole's function moves from QDs internal to wetting layer when introduce WLs. When WLs thickness is larger than 0.8 nm, the electronic structures of quantum dots have changed obviously. This will affect the instrument's performance which relies on the quantum dots' optical properties.     

Impact of GaNAs strain compensation layer on the electronic structure of InAs/GaAs quantum dots

The strain and electron energy levels of InAs/GaAs(001) quantum dots (QDs) with a GaNAs strain compensation layer (SCL) are investigated. The results show that both the hydrostatic and biaxial strain inside the QDs with a GaNAs SCL are reduced compared with those with GaAs capping layers. Moreover, most of the compressive strain in the growth surface is compensated by the tensile strain of the GaNAs SCL, which implies that the influence of the strain environment of underlying QDs upon the next-layer QDs’ growth surface is weak and suggests that the homogeneity and density of QDs can be improved. Our results are consistent with the published experimental literature. A GaNAs SCL is shown to influence the strain and band edge. As is known, the strain and the band offset affect the electronic structure, which shows that the SCL is proved to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the strain compensation technology can be applied to the growth of stacked QDs, which are useful in solar cells and laser devices.     

Encapsulation of highly confined CdSe quantum dots for defect free luminescence and improved stability

This is the first report on the generation of trap states and their effective elimination in highly confined CdSe quantum dots in order to obtain enhanced and stable optical properties prepared by aqueous route. Surface plays an important role in optical properties of quantum dots (QDs) and surface modification of quantum dots can improve optical properties. In present work luminescent CdSe QDs were prepared using 2-Mercaptoethanol (2-ME) as stabilizing agent and encapsulated by polymer. Different concentrations of 2-ME were used to tune the emission spectra with respect to their reduced size. Addition of 2-ME to CdSe QDs enhances the trap emission and quenching band edge emission due to (i) increased surface to volume ratio and; (ii) presence of high concentration of sulfide ions as confirmed from EDX analysis as sulfide ions possesses the hole scavenging characteristics. Polymer encapsulation of QDs was carried out to make them stable and to improve their optical properties. Even though there are previous reports addressing the improved optical properties by polymer encapsulation and silica encapsulation but experimentally it has not been reported yet experimentally. In this work we have synthesized and characterized water soluble polymer encapsulated QDs and proved the facts experimentally. Photoluminescence spectroscopy clearly reveals the role of polymer encapsulation in boosting the optical properties of CdSe QDs. FTIR spectra validate the presence of biocompatible functional groups on CdSe4/PEG (Polymer encapsulated QDs).     

The influence of strain-reducing layer on strain distribution and ground state energy levels of GaN/AlN quantum dot

This article deals with the strain distributions around GaN/AlN quantum dots by using the finite element method. Special attention is paid to the influence of Al_0.2Ga_0.8N strain-reducing layer on strain distribution and electronic structure. The numerical results show that the horizontal and the vertical strain components are reinforced in the GaN quantum dot due to the presence of the strain-reducing layer, but the hydrostatic strain in the quantum dot is not influenced. According to the deformation potential theory, we study the band edge modifications and the piezoelectric effects. The result demonstrates that with the increase of the strain reducing layer, the transition energy between the ground state electron and the heavy hole increases. This result is consistent with the emission wavelength blue shift phenomenon observed in the experiment and confirms that the wavelength shifts toward the short wavelength range is realizable by adjusting the structure-dependent parameters of GaN/AlN quantum dot.     

Electronic structure and optical properties of zinc-blende GaN quantum dots

The energy levels of zinc-blende GaN quantum dots (QDs) are studied within the framework of the effective-mass envelope-function approximation. The dependence of the energy of electron and hole states on the quantum dot (QD) size is presented. The selection rules for optical transitions are given and the oscillator strengths of the dipole-allowed transitions for various QD radii are calculated with the wavefunctions of quantized energy levels. The theoretical absorption spectrum of GaN QDs is in good agreement with the existing experimental result.     

Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.     

1.5 μm emission from InAs quantum dots with InGaAsSb strain-reducing layer grown on GaAs substrates

InGaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs from the GaAs is reduced due to the tensile strain induced by the InGaAsSb SRL, because the lattice constant of InGaAsSb is closer to InAs lattice constant than that of GaAs, resulting in a significant red shift of photoluminescence peaks of the InAs QDs. The emission wavelength from InAs QDs can be controlled by changing the Sb composition of the InGaAsSb SRL. The 1.5 μm band emissions were achieved in the sample with an InGaAsSb SRL whose Sb compositions were above 0.3. The calculation of the electron and the hole wave functions using the transfer matrix method indicates that the electron and the hole were localized around InAs QDs and InGaAsSb SRL.     

Influence of Mg content on optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the structural parameters (the dot height, the barrier thickness between the coupled wurtzite ZnO QDs and Mg content x in the barrier layers) is calculated in detail. The results elucidate that Mg content have a significant influence on the exciton state and optical properties of ZnO coupled QDs. When Mg content x increases, the strong built-in electric field increases and leads to the redshift of the effective band gap of the Mg_xZn_1−xO layer. These theoretical results are useful for design and application of some important photoelectronic devices constructed by using ZnO strained QDs.     

White light-emitting quantum dot diodes and tuning of luminescence processes

A novel white light-emitting diode based on a large Stokes shift (~200 nm) and using pure green light-emitting CdSeS quantum dots (QDs) with an Ag/ZnSnO/QDs/spiro-TPD/ITO structure has been fabricated in which ZnSnO and spiro-TPD are served as the electron and hole transport layer, respectively. The large Stokes shift of the CdSeS QDs excludes potentially Förster resonance energy transfer process, which allows spiro-TPD to act as both an emitter and hole transport layer. The devices exhibit a wide EL spectrum consisting of three components: blue emission from spiro-TPD, green emission from QD band–band recombination, and red emission from QD surface-state recombination. We further found that as the intensity ratios among these three components vary with bias the color of the QD light-emitting diodes is tunable. The device displays a good white light-emitting characteristic with CIE coordinates of (0.281, 0.384) at an appropriate bias.     

Electronic structures of alloy quantum dots with nonuniform composition

In this study, the significant effect of the nonuniform composition in alloy quantum dots (QDs) on electronic structure is analyzed in depth. The equilibrium composition profiles in experimentally observed dome and barn shaped GeSi/Si QDs are determined by combining the finite element method and the method of moving asymptotes. Due to the composition variation, the total band edge of heavy hole is dominated by the band offset and spin-orbit coupling rather than the strain effect. The numerical results reveal that the wave function of heavy hole trends to be localized in the Ge-rich region at the top of the large QD. Moreover, the size effect gradually compensates the composition effect as the size of QD decreases.     

Electronic structure and optical gain of InAsPN/GaP(N) quantum dots

The electronic structure and optical gain of InAsPN/GaP(N) quantum dots (QDs) are investigated in the framework of the effective-mass envelope function theory. The strain distribution is calculated using the valence force field (VFF) method. With GaP barrier, for smaller InAsPN QDs, the minimum transition energy may occur at a lower phosphorous (P) composition, but for larger QDs, the transition energy increases as P composition increases due to the increased bandgap of alloy QDs. When the nitrogen (N) composition increases, the transition energy decreases due to the stronger repulsion between the conduction band (CB) and the N resonant band, and the transition matrix element (TME) is more affected by the transition energy rather than N–CB mixing. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into the GaP barrier. With this GaP_0.98N_0.02 barrier, the conduction band offset is reduced, so the quantum confinement is lower, resulting in a smaller transition energy and longer wavelength. At the same time, the TME is reduced and the optical gain is less than those without N in the barrier at a low carrier density, but the peak gain increases faster when the carrier density increases. Finally it can surpass and reach a greater saturation optical gain than those without N in the barrier. This shows that incorporating N into GaP barriers is an effective way to achieve desirable wavelength and optical gain.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_x Zn_(1-x) O Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg x Zn 1-x O cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg x Zn 1 x O QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L 3.5 nm, but the interband emission wavelength has a red-shift when L 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

半导体红外量子点发展现状与前景

半导体量子点具有独特的光学与电学性质,特别是红外量子点良好的光稳定性和生物相容性等优点使其在光电器件、生物医学等领域受到广泛关注。综述了吸收或发射光谱位于红外波段的量子点在激光、能源、光电探测以及生物医学等方面的应用现状与前景,归纳了适用于红外量子点材料的制备方法,并对比了不同方法在应用中的优势。半导体红外量子点材料选择丰富、应用形式多样:InAs量子点被动锁模激光器在1.3 μm波长处产生7.3 GHz的近衍射极限脉冲输出;InAs/GaAs量子点双波长激光器可泵浦产生0.6 nW的THz波;PbS量子点掺杂光纤放大器可在1.53 μm中心波长处实现10.5 dB光增益,带宽160 nm;CdSeTe量子点敏化太阳能电池、异质结Si基量子点太阳能电池的总转换效率可达8%和14.8%;胶质HgTe量子点制成的量子点红外探测器(QDIP)可实现3 μm~5 μm中波红外探测,Ge/Si量子点可实现3 μm~7 μm红外探测;CdTe/ZnSe核壳量子点可用于检测DNA序列的损伤与突变。半导体红外量子点上述应用形式的发展,将进一步促进红外光电系统向高效、快速、大规模集成的方向演进,也将极大地促进临床医学中活体成像检测的应用推广。     

Photoluminescence and magneto-optical properties of multilayered type-II ZnTe/ZnSe quantum dots

Multilayered Zn–Se–Te structures grown by migration enhanced epitaxy are studied by temperature- and excitation-dependent photoluminescence (PL) as well as magneto-PL. The PL consists of two bands: a blue band, overlaid with band edge sharp lines, dominant at low temperatures and high excitation, and a green band, which appears at elevated temperature and low excitation. Upon varying excitation intensity by four orders of magnitude, the green band peak energy shifts by ∼60 meV, indicating recombination of excitons in type-II quantum dots (QDs); no significant shift is observed for the blue band. Therefore, the green emission is attributed to ZnTe/ZnSe type-II QDs, which co-exist with isoelectronic centers, responsible for the blue and band edge emissions. The existence of type-II ZnTe/ZnSe QDs is further confirmed by magneto-PL, for which the observed oscillations in the PL intensity as a function of magnetic field is explained in terms of the optical Aharonov–Bohm effect.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/MgxZn1-xO Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg_xZn_1-xO cylindrical quantum dots (QDs) for four different Mg compositions: x=0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg_xZn_1-xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L<3.5 nm, but the interband emission wavelength has a red-shift when L>3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L>3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

InAs/GaAs柱形岛的制备及特性研究

利用固源分子束外延（MBE）的方法经SK模式自组装生长由多层InAs／GaAs量子点组成的柱形岛.具体分析了GaAs间隔层厚度，生长停顿时间以及InAs淀积量对发光峰波长的影响.原子力显微镜（AFM）结果显示柱形岛表面的形状和尺寸都比较均匀；室温下不同高度的柱形岛样品的发光波长分别达到1.32和1.4μm，而单层量子点的发光波长仅为1.1μm，充分说明了量子点高度对发光波长的决定性影响，这为调节量子点发光波长提供了一种直观且行之有效的方法. 关键词： 柱形岛 生长停顿 间隔层厚度 PL谱     

A detailed study of physical properties of ZnS quantum dots synthesized by reverse micelle method

In this article, zinc sulfide nanocrystal quantum dots were synthesized by reverse micelle method using polyvinyl pyrrolidone as surfactant. The various crystallite properties of these nanocrystals such as, size, d-spacing, lattice parameter, microstrain, intrinsic stress, X-ray density, specific surface area, dislocation density, porosity, and agglomeration number have been analyzed using X-ray diffraction spectrum. The transmission electron microscopy was used to calculate the size and monitoring morphology of the nanocrystals, while the scanning electron microscopy was utilized to investigate the surface morphology of nanoclusters. The various optical properties of zinc sulfide quantum dots such as absorption coefficient, extinction coefficient, optical band gap energy, Urbach energy, and threshold wavelength have been analyzed using UV-visible data. The photoluminescence was used to study the emission spectra of produced ZnS quantum dots. Moreover, Furrier Transform-Infrared studies revealed that ZnS quantum dots are pure.     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Energy-selective charging of type-II GaSb/GaAs quantum dots

The hole confinement in type-II self-organized GaSb/GaAs quantum dots (QDs) was investigated by combining optical excitation and time-resolved capacitance spectroscopy. The experimental results indicate energy-selective charging even for type-II QDs. With increasing excitation energy the apparent hole activation energy decreases, which is attributed to light absorption in sub-ensembles of QDs with decreasing hole localization. The large localization energy of about 450 meV and the possibility of optical-multiplexing makes type-II GaSb/GaAs QDs a potential material system for QD memory concepts.