Synthesis of Zn(1-x)Cd(x)S:Mn/ZnS quantum dots and their application to light-emitting diodes

3.6?nm sized Mn-doped Zn(1-x)Cd(x)S quantum dots (QDs) with the composition (x) of 1, 0.5, 0.2 and 0 were synthesized by a reverse micelle approach. The bandgap energy of Zn(1-x)Cd(x)S:Mn QDs was tuned to a higher energy by increasing the Zn content, and the actual composition of alloyed Zn(1-x)Cd(x)S:Mn QDs was found to be different from the solution composition. Consecutive overcoating of the Zn(1-x)Cd(x)S:Mn QD surface by a ZnS shell was done, and the core/shell structured QDs exhibited quantum yields of 14-30%, depending on the composition of the core QDs. Using CdS:Mn/ZnS QDs, orange and white light-emitting diodes (LEDs) pumped by a near-UV and blue LED chips, respectively, were fabricated and their optical properties are described.     

Carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots on GaAs and Si substrates

We have investigated the carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots (QDs) on GaAs and Si substrates. The carrier dynamics of QDs on GaAs and Si substrates is studied using time-resolved photoluminescence (PL) measurements, revealing shorter exciton lifetimes of QDs on Si substrate. In particular, the activation energy of electrons confined in QDs on the GaAs substrate, as obtained from temperature-dependent PL spectra, is higher than that of electrons confined in QDs on the Si substrate. Both results confirm that defects and dislocations in QDs on the Si substrate provide nonradiative channels.     

Luminescent optical fibers with CdS(Se) quantum dots for spark detectors

We present results of studying the luminescent characteristics of optical fibers with CdS and CdSSe quantum dots (QDs) for spark detectors with up-conversion of the spark flashing spectrum to the long-wavelength range. It is shown that, by selecting a proper QD composition and size, it is possible to shift the luminescence band position toward the region of maximum sensitivity of a Si photodetector.     

Investigation of bonding characteristics between Si quantum dots and a SiO2 matrix

In order to understand and control the properties of Si quantum dot (QD) superlattice structures (SLS), it is necessary to investigate the bonding between the dots and their matrix and also the structures' crystallinities. In this study, a SiOx matrix system was investigated and analyzed for potential use as an all-silicon multi-junction solar cell. Si QD SLS were prepared by alternating deposition of Si rich SiOx (x = 0.8) and SiO2 layers using RF magnetron co-sputtering and subsequent annealing at temperatures between 800 and 1,100 degrees C under nitrogen ambient. Annealing temperatures and times affected the formation of Si QDs in the SRO film. Raman and FTIR spectra revealed that nanocrystalline Si QDs started to precipitate after annealing at 1,100 degrees C for 1 hour. TEM images clearly showed SRO/SiO2 SLS and Si QDs formation in SRO layers after annealing at 1,100 degrees C for 2 hours. XPS analysis showed that Si-Si and Si-O bonding changes occurred above 1,100 degrees C. XPS analysis also revealed that Si QD SLSs started stabilizing after 2 hours' annealing and approached completion after 3 hours'. The systematic investigation of Si QDs in SiO2 matrices and their properties for solar cell application are presented.     

Experiments and modeling of alloying in self-assembled quantum dots

We review the recent advances in the experimental and theoretical investigation of alloy distribution in semiconductor quantum dots (QDs). X-ray diffraction analysis, as well as wet chemical etching, represent two powerful techniques that are able to measure the alloy distribution inside the dots. From a theoretical point of view, determination of the alloy distribution follows from consideration of the thermodynamic quantities involved in the formation and stability of the QD: strain energy, surface energy, internal energy and entropy. Starting from the alloy distribution, the investigation of its role in influencing the electronic and optical properties of QDs is possible. Tight binding and ab initio calculation show the band structure of non-uniform alloyed Ge/Si and InAs/GaAs quantum dots. While for Ge/Si the indirect bandgap does not offer a strong photoluminescence spectra, direct-bandgap materials offer intense light emission, including the range for telecom applications (1.77–1.37 μm). Control of alloying inside the QDs allows for the tailoring of their band structure and photoluminescence spectra, where high alloy gradients induce a blue-shift of the spectra, compared to a more uniform composition.     

Widely tunable emissions of colloidal Zn(x)Cd(1-x)Se alloy quantum dots using a constant Zn/Cd precursor ratio

The widely tunable emissions of Zn(x)Cd(1-x)Se alloy quantum dots (QDs), which emit green to red wavelengths from 534 to 620 nm, are reported. Green-, yellow-, orange-, and red-emitting QDs were synthesized by varying a point of time for oleylamine (as a co-surfactant) addition and a Se precursor amount, and keeping a constant Zn/Cd precursor ratio. With reaction time the alloying and particle growth of the alloy QDs progressed simultaneously in the opposite direction in the variation of their band gap. However, the band gap energies of all QDs were observed to be gradually blue-shifted due to the slight dominance of alloying over the particle growth effect. The compositions of alloy QDs were estimated based on their sizes and band gap energies. Zn(x)Cd(1-x)Se core QDs were also overcoated with a ZnSe shell with a higher band gap to enhance their quantum yields.     

Formation of Ge quantum dots array in layer-cake technique for advanced photovoltaics

We report a simple and manageable growth method for placing dense three-dimensional Ge quantum dot (QD) arrays in a uniform or a graded size distribution, based on thermally oxidizing stacked poly-SiGe in a layer-cake technique. The QD size and spatial density in each stack can be modulated by conditions of the Ge content in poly-Si(1-x)Ge(x), oxidation, and the underlay buffer layer. Size-dependent internal structure, strain, and photoluminescence properties of Ge QDs are systematically investigated. Optimization of the processing conditions could be carried out for producing dense Ge QD arrays to maximize photovoltaic efficiency.     

Semiconductor self-assembled quantum dots: present status and future trends

Progress in controlling the size, shape, and composition of quantum dots (QDs) as well as their positioning will be crucial to further advances in the fields of quantum information and device applications. The growth of QDs into lattices using controlled positioning of the QD nucleation centers is a possible method. QD positioning is also much needed for further development of QD microcavities and photonic-crystal based devices that are used for quantum information applications. This article discusses the prospects for progress in these fields that may be realized if a better control over the positioning and self-positioning of quantum dots is achieved.     

Silicon quantum dot/crystalline silicon solar cells

Silicon (Si) quantum dot (QD) materials have been proposed for 'all-silicon' tandem solar cells. In this study, solar cells consisting of phosphorus-doped Si QDs in a SiO(2) matrix deposited on p-type crystalline Si substrates (c-Si) were fabricated. The Si QDs were formed by alternate deposition of SiO(2) and silicon-rich SiO(x) with magnetron co-sputtering, followed by high-temperature annealing. Current tunnelling through the QD layer was observed from the solar cells with a dot spacing of 2?nm or less. To get the required current densities through the devices, the dot spacing in the SiO(2) matrix had to be 2?nm or less. The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful fabrication of (n-type) Si QD/(p-type) c-Si photovoltaic devices is an encouraging step towards the realization of all-silicon tandem solar cells based on Si QD materials.     

Detecting quasi-periodic {11<Emphasis Type="Italic">n</Emphasis>} (<Emphasis Type="Italic">n</Emphasis> = 7–11) faces in samples with Ge/Si quantum dots by grazing X-ray reflectometry

High-resolution grazing X-ray reflectometry is used to obtain experimental and theoretical data on the intensity of specular and diffuse reflection from self-organized structures grown by molecular beam epitaxy with single-layer (non-overgrown) and multilayer (overgrown) Ge/Si quantum dots (QDs). Using the positions of diffuse scattering peaks in the direct space, the slopes of quasi-periodic faces have been determined to within ±0.1° by a method employed previously for the investigation of In(Ga)As/GaAs quantum dots. Finding the quasi-periodic {11n} (n = 7−11) faces (typical of the growth of ordered QDs) in the samples with disordered Ge/Si quantum dots is evidence of the generality of mechanisms of QD formation in different systems.     

Short wavelength emission of AlGaInP quantum dots grown on GaP substrate

We report on the growth of AlGaInP quantum dots (QDs) with Al contents between 0% and 10% on GaP substrate by gas-source molecular beam epitaxy and the investigation of their morphological and low temperature photoluminescence properties. These high areal density QDs show short wavelength emission between 575 and 612 nm depending on their composition. The authors interpret the QD emission as originating from indirect type-II transitions. This interpretation is supported by a single-band effective-mass model, which allows us to describe the role of differing barrier composition in the QD emission. Time-resolved photoluminescence measurements are performed and discussed with respect to the calculations.     

Optimization of process parameter for synthesis of silicon quantum dots using low pressure chemical vapour deposition

Si quantum dots-based structures are studied recently for performance enhancement in electronic devices. This paper presents an attempt to get high density quantum dots (QDs) by low pressure chemical vapour deposition (LPCVD) on SiO ₂ substrate. Surface treatment, annealing and rapid thermal processing (RTP) are performed to study their effect on size and density of QDs. The samples are also studied using Fourier transformation infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM) and photoluminescence study (PL). The influence of Si–OH bonds formed due to surface treatment on the density of QDs is discussed. Present study also discusses the influence of surface treatment and annealing on QD formation.     

Photoluminescent silicon quantum dots in core/shell configuration: synthesis by low temperature and spontaneous plasma processing

Quantum confinement in zero-dimensional silicon nanocrystals (nC) in the quantum dot (QD) configuration has triggered a tremendous interest in nanostructured device technology. However, the formation of Si-QDs eventually proceeds through multi-step routes and involves high temperature processing that impedes preferred device configuration. The present work demonstrates the formation of nC-Si QDs of controlled size, density and distribution through one-step and spontaneous plasma processing, at a low substrate temperature (300?°C) compatible for device fabrication. Direct growth of nC-Si/SiO(x) core/shell quantum dots embedded in the a-Si matrix, 6.4-3.7 nm in diameter and with number density in the range ～ 6 × 10(9)-1 × 10(11) cm(-2) has been accomplished, following a novel route where He dilution to SiH(4) in RF plasma CVD has been found instrumental. On gradual reduction in the size of QDs, splitting of the energy bands widens the optical band gap and induces visible photoluminescence that appears controllable by tuning the size and density of the dots. This low temperature and spontaneous plasma processing of nC-Si/SiO(x) core/shell QDs that exhibit the quantum size effect in photoluminescence is being reported for the first time.     

Optical property of silicon quantum dots embedded in silicon nitride by thermal annealing

We present the effects on the thermal annealing of silicon quantum dots (Si QDs) embedded in silicon nitride. The improved photoluminescence (PL) intensities and the red-shifted PL spectra were obtained with annealing treatment in the range of 700 to 1000 °C. The shifts of PL spectra were attributed to the increase in the size of Si QDs. The improvement of the PL intensities was also attributed to the reduction of point defects at Si QD/silicon nitride interface and in the silicon nitride due to hydrogen passivation effects.     

Nucleation sequence of InAs quantum dots on cross-hatch patterns

InAs quantum dots (QDs) are grown via molecular beam epitaxy on cross-hatch pattern (CHP) templates that result from lattice-mismatched epitaxy of In(x)Ga(1-x)As on (100)-GaAs substrates. Growth of InAs on low-(x = 0.10) and medium-(x = 0.13) mismatch CHPs with InAs thickness grading from sub- to beyond critical thickness show different stages of QD nucleation that is dictated mainly by surface steps. Tangential surface stress fields arising from the buried network of (110) misfit dislocations (MDs) at the InGaAs/GaAs interface are simulated in two dimensions and found to have a direct correlation to QD height at various locations, implying sequential QD nucleation at the surface intersection of the glide plane of dislocation T-section, cross-hatch intersection, threading dislocation, [1-10] MD line, and [110] MD line, followed by nucleation on the flat areas. Deviations from this nominal sequence is possible due to material anisotropy and are accounted for in the stress calculation by dislocation-specific scaling factors.     

Argon-plasma-induced InAs/InGaAs/InP quantum dot intermixing

We report the first study of argon (Ar)-plasma-enhanced intermixing of InAs/InGaAs/InP self-assembled quantum dots (QDs) in an inductively coupled plasma reactive ion etch system. The Ar-plasma exposure creates point defects, which propagate into the QD structure and enhance the intermixing between the QDs and their barrier layers, hence tuning the energy bandgap of the QDs. By optimizing the plasma exposure time and the annealing temperature, we observe (i) a blueshift of 160?nm and an increase in the photoluminescence (PL) intensity of the QD samples immediately after Ar-plasma exposure for 90?s, and (ii) a further increase in the blueshift of 330?nm, accompanied by 2.5-times increase in the PL intensity and 37?nm narrowing in the PL linewidth after subsequent rapid thermal annealing at 720?°C. The ability to generate a large blueshift without degrading the material quality shows that Ar-plasma exposure is an efficient post-growth technique for tuning the energy bandgap of QD structures.     

The polarization response in InAs quantum dots: theoretical correlation between composition and electronic properties

III-V growth and surface conditions strongly influence the physical structure and resulting optical properties of self-assembled quantum dots (QDs). Beyond the design of a desired active optical wavelength, the polarization response of QDs is of particular interest for optical communications and quantum information science. Previous theoretical studies based on a pure InAs QD model failed to reproduce experimentally observed polarization properties. In this work, multi-million atom simulations are performed in an effort to understand the correlation between chemical composition and polarization properties of QDs. A systematic analysis of QD structural parameters leads us to propose a two-layer composition model, mimicking In segregation and In-Ga intermixing effects. This model, consistent with mostly accepted compositional findings, allows us to accurately fit the experimental PL spectra. The detailed study of QD morphology parameters presented here serves as a tool for using growth dynamics to engineer the strain field inside and around the QD structures, allowing tuning of the polarization response.     

Activation energy and carrier dynamics of CdTe/ZnTe quantum dots on GaAs and Si substrates

We investigate the activation energy and carrier dynamics of CdTe/ZnTe quantum dots (QDs) grown on GaAs and Si substrates. The activation energy of the electrons confined in QDs on the Si substrate, as obtained from the temperature-dependent photoluminescence (PL) spectra, is lower than that of electrons confined in QDs on the GaAs substrate. Time-resolved PL measurements used to study the carrier dynamics show shorter exciton lifetimes for QDs on the Si substrate. This behavior is attributed to the fact that defects and dislocations in the QDs on the Si substrate provide nonradiative channels.     

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.     

Tailoring of morphology and emission wavelength of AlGaInAs quantum dots

We present a study of the growth, morphology and optical properties of Al(x)Ga(1-x-y)In(y)As quantum dots (QDs) for a wide range of Al and In concentrations (0≤x≤0.34 and 0.43≤y≤0.60). Short emission wavelengths between 660 and 940?nm and QD surface densities up to 1.1 × 10(11)?cm(-2) have been achieved. Our results show that by varying both the Al concentration and the In concentration an independent adjustment of strain and QD band gap is possible. This additional degree of freedom can be employed for tailoring AlGaInAs QDs with the desired emission wavelength, surface density and average size. AlGaInAs QDs thus offer new possibilities for future QD device design.