Wavefunction delocalization in quantum dot arrays: an asymptotic analysis

Intermediate-band solar cells using quantum dot arrays (QDAs) are theoretically predicted to significantly increase the efficiency with which solar energy can be harvested. In the limit of identical quantum dots, the wavefunction for electrons in a QDA will be fully delocalized. Fully delocalized wavefunctions have been theoretically shown to reduce thermal losses and consequently increase photovoltaic device efficiency. However, even small nonuniformities can cause electrons to localize in a single quantum dot, negating any advantages from delocalized states. In this work a modified Schrödinger equation is used to model a two-dot array with nonuniform quantum dots and solved using perturbation methods. This result is extended to N-dot arrays, and several metrics are constructed to characterize the degree of delocalization. Our results, which compare favorably with numerical simulations, show explicitly how the amount of delocalization depends on key design parameters.     

The design of SAW RAC's using arrays of thin metal dots

Many successful dot array SAW devices including RAC's, resonators and band-pass filters have been reported, but the wide range of dot parameters, including dot dimensions, spacing and pattern, which allows flexibility in design, has not previously been utilized. This paper reports on the design of an RAC (reflecting array compressor) with a TB product of 1000, the emphasis being to exploit some of this flexibility. The design has been implemented using YZ LiNbO₃ with thin metal dots as the reflectors. It makes use of the wide range of reflectivities available from electrical effects in a novel weighting scheme based on dot size. The dots have been designed so that the sensitivity to over-etch is minimal. The array, which is sparse and near-periodic, has been designed to reduce unwanted reflections. This is least successful in the case of SAW to bulk wave reflection and the consequent loss due to this cause is accounted for in the design. The design approach is quite general and should be of use for other devices     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

交流型量子点发光器件：现状、挑战与展望

量子点作为高性能发光材料一直是新型显示技术的研究热点。尽管传统的电致发光型量子点器件一般工作于直流驱动模式,交流驱动的电致发光型量子点器件逐渐受到广泛关注。这是因为交流型器件相比于直流型器件具有独特的优势:可以避免由单向电流注入形成的电荷积聚对发光材料造成的破坏,可有效提高器件的寿命和发光效率;驱动系统中不需要转换器和整流器,可以显著降低功率损耗和产品成本,具有较高的商用价值。基于这些优点,国内外研究人员对交流型量子点发光器件开展了大量研究工作。本综述按照外部载流子注入量子点发光器件的类型,将器件分为双端载流子注入型发光器件,单端载流子注入型发光器件和无载流子注入型发光器件三大类,并对三类器件的器件结构、工作原理、研究进展进行综述。最后分析了交流型量子点发光器件的相关技术挑战,并展望其发展前景。     

Three-dimensional Si/Ge quantum dot crystals

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.     

Multi-section quantum dot superluminescent diodes for spectral shape engineering

The design and operating parameters of a novel multi-contact quantum dot superluminescent diode incorporating a number of features which inhibit lasing are described and compared with that of a single-contact device. Such devices allow the independent tuning of emission power and spectral shape; hence the penetration depth and resolution in optical coherence tomography are decoupled. The emission spectrum of a device utilising chirped quantum dots is shown to be tuned to produce a broadband single Gaussian emission, centred at the required wavelength of 1050 nm, at higher output powers than a singlecontact device.     

Designing spatial correlation of quantum dots: towards self-assembled three-dimensional structures

Buried two-dimensional arrays of InP dots were used as a template for the lateral ordering of self-assembled quantum dots. The template strain field can laterally organize compressive (InAs) as well as tensile (GaP) self-assembled nanostructures in a highly ordered square lattice. High-resolution transmission electron microscopy measurements show that the InAs dots are vertically correlated to the InP template, while the GaP dots are vertically anti-correlated, nucleating in the position between two buried InP dots. Finite InP dot size effects are observed to originate InAs clustering but do not affect GaP dot nucleation. The possibility of bilayer formation with different vertical correlations suggests a new path for obtaining three-dimensional pseudocrystals.     

Semiconductor quantum dots

Three-dimensionally confined semiconductor quantum dots have emerged to be a versatile material system with unique physical properties for a wide range of device applications. With the advances in nanotechnology and material growth techniques for both epitaxial and colloidal quantum dots, recently the research has been shifted largely towards quantum dot based devices for practical applications. In this short review, we have tried to assemble a selection of recent advances in the areas of quantum dots for computing and communications, solid state lighting, photovoltaics, and biomedical applications that highlight the state of the art.     

Operation of nanocrystalline-silicon-based few-electron memory devices in the light of electron storage, ejection, and lifetime characteristics

A metal-oxide-semiconductor field-effect transistor memory device using nanocrystalline Si (nc-Si) dots as a floating gate over a short and narrow channel has been fabricated. Its operation at 77 K presents experimental evidence of storing and ejection of electrons associated with the nc-Si dot in the active area of the device. Though the lifetime of a single electron is apparently longer than the case when it is associated with another electron in the same nc-Si dot, a distribution in lifetime has been generally observed for the stored electrons in the nc-Si dots with the present memory devices.     

Site‐Controlled InGaAs Quantum Dots with Tunable Emission Energy

Semiconductor quantum‐dot (QD) systems offering perfect site control and tunable emission energy are essential for numerous nanophotonic device applications involving spatial and spectral matching of dots with optical cavities. Herein, the properties of ordered InGaAs/GaAs QDs grown by organometallic chemical vapor deposition on substrates patterned with pyramidal recesses are reported. The seeded growth of a single QD inside each pyramid results in near‐perfect (<10 nm) control of the QD position. Moreover, efficient and uniform photoluminescence (inhomogeneous broadening <10 meV) is observed from ordered arrays of such dots. The QD emission energy can be finely tuned by varying 1) the pyramid size and 2) its position within specific patterns. This tunability is brought about by the patterning of both the chemical properties and the surface curvature features of the substrate, which allows local control of the adatom fluxes that determine the QD thickness and composition.     

Evaluation of a novel surface acoustic wave gyroscope

A novel type of gyroscope sensor using metal dot arrays on the surface of a surface acoustic wave (SAW) device has recently been independently proposed. In this paper, we report experimental trials of several devices fabricated to evaluate the effects described and also an order-of-magnitude estimate of the sensitivity to be expected. The conclusions are that this device is extremely insensitive as currently proposed     

Transport properties of coupled semiconductor quantum dots

We have measured the electronic transport properties of the coupled quantum dot devices at low temperatures. The interplay between the strong many body spin interaction and the molecular states are probed in linear and non-linear transport regime. We observe the formation of strong coherent molecular states clearly visible in the double dot conductance phase diagram. In our study, the spin configuration in multiply coupled quantum dots could be identified using Kondo phenomenon. In addition, the characteristics of the spin dependent molecular states and phase dependant tunneling have been also observed using non-linear conductance measurement of the double dots. The results suggest the importance of the diverse spin related physical issues in artificial quantum dot devices.     

Two-dimensional array self-assembled quantum dot sub-diffraction waveguides with low loss and low crosstalk

We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantum dot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductor nanoparticles and introducing a signal light, energy coupling of stimulated photons from the QDs enables light transmission along the waveguide. Monte Carlo simulation with randomized inter-dot separation reveals that the optical gain necessary for unity transfer is 3.1 × 10(7)?m(-1) for a 2D (2?μm length by 500?nm width) array compared to 11.6 × 10(7)?m(-1) for a 1D (2?μm length) given 8?nm diameter quantum dots. The theoretical results are borne out in experiments on 2D arrays by measurement of negligible crosstalk component with as little as 200?nm waveguide separation and is indicative of near-field optical coupling behavior. The transmission loss for 500?nm wide structures is determined to be close to 3?dB/4?μm, whereas that for 100?nm width is 3?dB/2.3?μm. Accordingly, higher pump power and gain would be necessary on the narrower device to create similar throughput. Considering existing nanoscale propagation methods, which commonly use negative dielectric materials, our waveguide shows an improved loss characteristic with comparable or smaller dimensions. Thus, the application of QDs to nanophotonic waveguiding represents a promising path towards ultra-high density photonic integrated circuits.     

Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

International comparison GT-RF 78-1: high attenuation (60 dB, 100dB) at 30 MHz in 50 Ω coaxial line

A worldwide comparison of national standard devices in which high attenuation values (up to 100 dB) at the frequency of 30 MHz have been compared is reported. Eleven national laboratories of metrology from the Netherlands, France, Italy, the United Kingdom, Sweden, Poland, Hungary, Australia, the People's Republic of China, the USSR, and the Federal Republic of Germany participated. The transfer standards were two fixed attenuators with an attenuation of about 60 dB and 100 dB and one switchable attenuator with 100 dB. The maximum difference between any two results is 0.064 dB at an attenuation of 60 dB and 0.125 at 100 dB. For a considerable number of laboratories, the deviation between mean values was not larger than 0.001 dB for a 60-dB attenuation step, and not larger than 0.005 dB for a 100-dB attenuation step     

Atomic layer deposition of lead sulfide quantum dots on nanowire surfaces

Quantum dots provide unique advantages in the design of novel optoelectronic devices owing to the ability to tune their properties as a function of size. Here we demonstrate a new technique for fabrication of quantum dots during the nucleation stage of atomic layer deposition (ALD) of PbS. Islands with sub-10 nm diameters were observed during the initial ALD cycles by transmission electron microscopy, and in situ observations of the coalescence and sublimation behavior of these islands show the possibility of further modifying the size and density of dots by annealing. The ALD process can be used to cover high-aspect-ratio nanostructures, as demonstrated by the uniform coating of a Si nanowire array with a single layer of PbS quantum dots. Photoluminescence measurements on the quantum dot/nanowire composites show a blue shift when the number of ALD cycles is decreased, suggesting a route to fabricate unique three-dimensional nanostructured devices such as solar cells.     

Unconventional gap state of trapped exciton in lead sulfide quantum dots

Exciton states in lead selenide (PbSe) and lead sulfide (PbS) quantum dots have been studied extensively. However, relatively less attention has been paid to the states within the quantum dot bandgap. Our experimental results have revealed a single in-gap state which bears confinement dependence yet cannot be explained by dark exciton theory, nor is it a trap state related to quantum dot surface defects as previously observed. A detailed analysis of the temperature dependence of photoluminescence, Stokes shift, absorption and photoinduced absorption indicates the unconventional GS is a new state of a trapped exciton in a QD film. With appropriate design engineering, these trapped excitons might be harvested in solar cells and other optoelectronic devices.     

量子点材料应用于发光二极管的研究进展

量子点材料因具有独特的光学特性而被广泛应用于发光领域,用其作发光层可制成量子点发光二极管。与有机电致发光二极管相比,量子点发光二极管具有发光光谱窄、色域广、稳定性好、寿命长、制作成本低等优势。本文介绍了量子点发光器件在国内外的热点研究方向及取得的成果,并对其发展前景进行展望。     

Approaching the resolution limit of nanometer-scale electron beam-induced deposition

We report the writing of very high resolution tungsten containing dots in regular arrays by electron beam-induced deposition (EBID). The size averaged over 100 dots was 1.0 nm at fwhm. Because of the statistical spread in the dot size, large and small dots are present in the arrays, with the smallest having a diameter of only 0.7 nm at fwhm. To date these are the smallest features fabricated by EBID. We have also fabricated lines with the smallest having a width at fwhm of 1.9 nm and a spacing of 3.2 nm.