Self-assembled InAs/InP quantum dots and quantum dashes: Material structures and devices

The advances in lasers, electronic and photonic integrated circuits (EPIC), optical interconnects as well as the modulation techniques allow the present day society to embrace the convenience of broadband, high speed internet and mobile network connectivity. However, the steep increase in energy demand and bandwidth requirement calls for further innovation in ultra-compact EPIC technologies. In the optical domain, advancement in the laser technologies beyond the current quantum well (Qwell) based laser technologies are already taking place and presenting very promising results. Homogeneously grown quantum dot (Qdot) lasers and optical amplifiers, can serve in the future energy saving information and communication technologies (ICT) as the work-horse for transmitting and amplifying information through optical fiber. The encouraging results in the zero-dimensional (0D) structures emitting at 980 nm, in the form of vertical cavity surface emitting laser (VCSEL), are already operational at low threshold current density and capable of 40 Gbps error-free transmission at 108 fJ/bit. Subsequent achievements for lasers and amplifiers operating in the O-, C-, L-, U-bands, and beyond will eventually lay the foundation for green ICT. On the hand, the inhomogeneously grown quasi 0D quantum dash (Qdash) lasers are brilliant solutions for potential broadband connectivity in server farms or access network. A single broadband Qdash laser operating in the stimulated emission mode can replace tens of discrete narrow-band lasers in dense wavelength division multiplexing (DWDM) transmission thereby further saving energy, cost and footprint. We herein reviewed the1 progress of both Qdots and Qdash devices, based on the InAs/InGaAlAs/InP and InAs/InGaAsP/InP material systems, from the angles of growth and device performance. In particular, we discussed the progress in lasers, semiconductor optical amplifiers (SOA), mode locked lasers, and superluminescent diodes, which are the building blocks of EPIC and ICT. Alternatively, these optical sources are potential candidates for other multi-disciplinary field applications.     

Semiconductor quantum dots

Since the invention of semiconductor lasers, huge improvements in device performance have been achieved, and a large variety of specialized designs for different applications were conceived. Two major steps have played a key role in the improvement of device properties. The first step was the application of semiconductor heterostructures that allowed the separate optimization of optical and carrier confinement. The second step was the introduction of quantum films, also called quantum wells, in the carrier recombination zone (started in the 1980s). This permitted a strong reduction of threshold current density due to an increased density of states at the laser energy. This effect of increased density of states is related to the partial discretization of the allowed energy states of carriers, i.e., electrons and holes, and is based on quantum mechanical principles. One major advantage of quantum-dot structures results from the full three-dimensional carrier confinement on a nanometer scale. Therefore, a semiconductor quantum dots, InAs dots embedded in GaAs, behave like non- or weakly interacting single atoms. In addition, the realization of device-quality quantum dot structures became possible by the introduction of self-organized growth. Both, molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used.     

Self-assembled Ge/Si dots for faster field-effect transistors

Self-assembled and coherently strained germanium nanostructured dots are grown on prepatterned Si substrates along ordered lines. These precisely aligned nanocrystals are proposed to make up the central unit of a dot-based field-effect transistor (DotFET). The strain-induced band edge splitting and the inherently smaller effective masses of charge carriers in Ge/Si dots promise faster transistors than are possible for conventional pure Si devices. Thick relaxed buffer layers-mandatory for any existing high-speed SiGe field-effect devices-are no longer required. The DotFET is straight-forward, defect-free, and fully compatible with current complementary metal-oxide-semiconductor (CMOS) technology     

Ultra-low density InAs quantum dots

We show that InAs quantum dots (QDs) can be grown by molecular beam epitaxy (MBE) with an ultralow density of sin 10⁷ cm^?2 without any preliminary or post-growth surface treatment. The strain-induced QD formation proceeds via the standard Stranski-Krastanow mechanism, where the InAs coverage is decreased to 1.3–1.5 monolayers (MLs). By using off-cut GaAs (100) substrates, we facilitate the island nucleation in this subcritical coverage range without any growth interruption. The QD density dependences on the InAs coverage are studied by photoluminescence, atomic force microscopy, transmission electron microscopy, and are well reproduced by the universal double exponential shapes. This method enables the fabrication of InAs QDs with controllable density in the range 10⁷–10⁸ cm^?2, exhibiting bright photoluminescence.     

Applications of colloidal quantum dots

This paper addresses a number of major trends underlying the continuing effort to realize practical optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots assembled in a variety of matrix materials. The great diversity of such structures makes it possible to fabricate numerous ensemble-based devices for applications underlying photoluminescent devices, light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar cells. In addition, the application of colloidal quantum dots to allied technologies such as nanobiotechnology is considered for the case of monitoring conformational changes in biomolecules using luminescent quantum dots.     

Ge quantum dots light-emitting devices

Si photonics becomes one of the research focuses in the field of photonics.Si-based light-emitting devices are one of the most important devices in this field.In this paper,we review the Si-based light...     

When sound meets quantum dots

THz acoustic phonons have wavelengths in the nm range and can be used as internal probes to investigate self-assemble quantum dots (QDs) structures. The interaction between delocalised acoustic phonons and an ensemble of localised electronic states yields interferences in the Raman scattering efficiency. Raman scattering interferences provide an image in reciprocal space of the electronic density and therefore allow one to probe the spatial ordering of QDs and the localisation of the electronic states. Spatial correlations functions are obtained by performing inverse Fourier transforms. Characteristic distances can be identified, provided that optical and acoustic wave reflexion effects are taken into account.     

Piezoelectric models for semiconductor quantum dots

The importance of fully coupled and semi-coupled piezoelectric models for quantum dots are compared. Differences in the strain of around 30% and in the electron energies of up to 30 meV were found possible for GaN/AlN dots.     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

Electronic properties of interacting quantum dots

The electronic properties of two interacting dots connected to leads are studied in the Kondo regime. The configuration is such that one dot is inserted into the lead while the other is side-connected to it. The situation, where both dots are in the Kondo regime is investigated. We find that the development of a Kondo state related to the connected dot is mediated by the inserted dot. In this case, an anti-resonance appears in the density of states of the inserted dot, at the Fermi level. The equation of motion method is used to calculate the Green's functions of the system.     

Intersubband optics in parabolic quantum dots

The infrared intersubband optical properties of parabolic cylinder and parabolic rotational quantum dots are studied using a single-band effective-mass theory. This allows us to study the impact of shape and size on the transition energies and polarization selection rules. Contrary to earlier work on spheroidal dots, we predict that the change in the polarization of intersubband optical transitions as a function of the parabolic quantum dot height and cross section is here due to level crossing.     

Optical anisotropy of InGaAs quantum dots

Polarization studies of InGaAs/GaAs quantum dots (QDs) synthesized in the submonolayer deposition mode (SMLQDs) on a singular GaAs (100) surface are carried out using photoluminescence spectroscopy. The influence of the effective In content in InGaAs SMLQDs and the effect of a wide-gap AlGaAs matrix on the optical anisotropy of the QDs are investigated. The highest degree (>15%) of optical anisotropy between the [011] and [0 $ \bar 1 $ 1] directions in the emission corresponding to the ground state of InGaAs/GaAs SMLQDs is observed for an effective In content of ～40%. The use of a wide-gap AlGaAs matrix resulted in an increase in the optical anisotropy of InGaAs SMLQDs by a factor of 1.5. It is found that vertical stacking of In(Ga)As/AlGaAs SMLQDs in the vertical-coupling mode (with spacer-layer thicknesses of 5–10 nm) leads to a further increase in the degree of optical anisotropy, which becomes as high as 25% on average. According to the data of transmission electron microscopy, the optical anisotropy of the ground-state photo-luminescence is predominantly caused by the anisotropy of the lateral dimensions of QDs in the [011] and [0 $ \bar 1 $ 1] directions.     

Optical study of InP quantum dots

Results of photoluminescence (PL) studies of self-organized nanoscale InP islands (quantum dots, QDs) in the In_0.49Ga_0.51P matrix, grown on a GaAs substrate by metalorganic vapor phase epitaxy (MOVPE), are presented. Dependences of the PL efficiency on temperature in the range 77–300 K and on excitation level at pumping power densities of 0.01–5 kW/cm² have been obtained. The PL spectra are a superposition of emission peaks from QDs and the wetting layer. Their intensity ratio depends on the pumping power and temperature, and the emission wavelength varies in the range 0.65–0.73 μm. At 77 K and low excitation level, InP QDs exhibit high temperature stability of the emission wavelength and high quantum efficiency. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 2, 2001, pp. 242–244. Original Russian Text Copyright ? 2001 by Vinokurov, Kapitonov, Nikolaev, Sokolova, Tarasov.     

On way to ideal quantum dots

Temperature and concentration self-assembling conditions of tetrahedral 1Sb4Ga clusters in multi-valley zinc blende AlN:(Ga, Sb) in the ultra dilute Sb limit are represented. Tetrahedral clusters should occur as a result of the second-order transition. The final stage of self-assembling when all Sb atoms are in 1Sb4Ga clusters also has to be reached as a result of the second-order transition at finite temperatures.     

Electrical properties of semiconductor quantum dots

A method, which makes it possible to obtain semiconductor particles V ≈ 10^?20 cm³ in volume (quantum dots) with a concentration of up to 10¹¹ cm^?2 and electrical contacts to each of them, is suggested. High variability in the electrical properties of such particles from a metal oxide (CuO or NiO) after the chemisorption of gas molecules is found.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.     

Four-electron quantum dots in magnetic fields

Making use of hyperspherical coordinates, the energy spectrum of a four-electron configuration in a harmonic quantum dot (QD) as a function of the dot size and the strength of a magnetic field are investigated. Discontinuous ground-state transitions induced by an external magnetic field and the quantum dot size effect are shown. An important aspect of the size effect is the crossover of energy levels. The present results are useful to understand the optical and magnetic properties of QD materials.     

MBE growth of tensile-strained Ge quantum wells and quantum dots

Germanium (Ge) has gained much interest due to the potential of becoming a direct band gap material and an efficient light source for the future complementary metal-oxide-semiconductor (CMOS) compatibl...     

PbS量子点的化学制备及其太阳能光伏特性

通过化学溶液体系中反应温度与原料配比的控制合成了第一吸收峰在833～1700 nm范围内可调的PbS量子点.利用X射线衍射(XRD)、透射电子显微镜(TEM)和高分辨透射电子显微镜(HRTEM) 、吸收光谱等手段研究了化学溶液法制备的PbS量子点形貌、尺寸分布以及近红外吸收等特性.所获得的量子点尺寸分布均匀, 直径在2.6 ～7.0 nm范围内可调.基于PbS量子点的红外吸收特性, 通过表面修饰方法在原子层沉积技术(ALD)生长的TiO2薄膜上构筑了FTO/TiO2/PbS/Au光伏器件结构, 并初步研究了光电流与量子点特征吸收的关系等光电转换特性.