Film-thickness-dependent conduction in ordered Si quantum dot arrays

In recent years, silicon nanostructures have been investigated extensively for their potential use in photonic and photovoltaic applications. So far, for silicon quantum dots embedded in SiO(2), control over inter-dot distance and size has only been observed in multiple bilayer stacks of silicon-rich oxides and silicon dioxide. In this work, for the first time the fabrication of spatially well-ordered Si quantum dots (QDs) in SiO(2) is demonstrated, without using the multilayer approach. This ordered formation, confirmed with TEM micrographs, depends on the thickness of the initially deposited sub-stoichiometric silicon oxide film. Grazing incidence x-ray diffraction confirms the crystallinity of the 5?nm QDs while photoluminescence shows augmented bandgap values. Low-temperature current-voltage measurements demonstrate film thickness and order-dependent conduction mechanisms, showing the transition from temperature-dependent conduction in randomly placed dots to temperature-independent tunnelling for geometrically ordered nanocrystals. Contrary to expectations from dielectric materials, significant conduction and photocarrier generation have been observed in our Si QDs embedded in SiO(2) demonstrating the possibility of forming initial film-thickness-controlled conductive films. This conduction via the silicon quantum dots in thick single layers is a promising result for integration into photovoltaic devices.     

氧化硅衬底上高密度锗量子点的微结构研究

以化学氧化生成的SiO2缓冲层作为衬底,利用分子束外延(MBE)系统通过直接生长以及后期退火的方式获得了高密度(1011cm-2)的锗量子点结构。借助于扫描电镜和电子衍射等进一步研究了其生长机理,与传统的S-K生长模式进行比较并给出了清晰的微观结构示意图。拉曼光谱证实此类微结构中有压应力的存在,而退火后的量子点则应力得到释放。     

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.     

InAs/GaAs nanostructures grown on patterned Si(001) by molecular beam epitaxy

The potential benefit from the combination of the optoelectronic and electronic functionality of III-V semiconductors with silicon technology is one of the most desired outcomes to date. Here we have systematically investigated the optical properties of InAs quantum structure embedded in GaAs grown on patterned sub-micron and nanosize holes on Si(001). III-V material tends to accumulate in the patterned sub-micron holes and a material depletion region is observed around holes when GaAs/InAs/GaAs is deposited directly on patterned Si(001). By use of a 60?nm SiO(2) layer and patterning sub-micron and nanosize holes through the oxide layer to the substrate, we demonstrate that high optical quality InAs nanostructures, both quantum dots and quantum wells, formed by a two-monolayer InAs layer embedded in GaAs can be epitaxially grown on Si(001). We also report the power-dependent and temperature-dependent photoluminescence spectra of these structures. The results show that hole diameter (sub-micron versus nanosize) has a strong effect on the structural and optical properties of GaAs/InAs/GaAs nanostructures.     

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.     

Deep Ultraviolet Emission from Water-Soluble SnO_2 Quantum Dots Grown via a Facile "Top-Down" Strategy

Tin oxide(SnO_2) is a promising wide bandgap semiconductor for next generation ultraviolet(UV) nonpolar optoelectronic devices applications.The development of SnO_2-based optoelectronic devices is obsessed by its low exciton emission efficiency.In this study,quantum confined SnO_2 nanocrystals have been fabricated via pulsed laser ablation in water.The SnO_2 quantum dots(QDs) possess high performance exciton emission at 297-300 nm light in water.The exciton emission intensity and wavelength can be slightly tuned by laser pulse energy and irradiation time.Optical gain has been observed in SnO_2QDs.Therefore,SnO_2 QDs can be a promising luminescence material for the realization of deep UV nanoemitter and lasing 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.     

Optical spectroscopy of single Cd0.6Zn0.4Te/ZnTe quantum dots on Si substrate

Microphotoluminescence (μ-PL) measurements were carried out to investigate the optical properties of single Cd_0.6Zn_0.4Te/ZnTe quantum dots (QDs) grown on Si (001) substrate by using molecular beam epitaxy. The high quality of single Cd_0.6Zn_0.4Te/ZnTe QDs is witnessed by resolution-limited emission, negligible background and absence of measurable spectral jitter or blinking. Polarization-dependent and power-dependent μ-PL spectroscopy measurements were performed to identify the exciton, the biexciton, and the charged exciton in the emission spectra of single QDs. Furthermore a weak linearly polarized line is observed on the low energy side of the neutral exciton and is ascribed to dark exciton recombination.     

Shape control of InGaAs nanostructures on nominal GaAs(001): dashes and dots

A method for fabricating self-assembled InGaAs quantum dashes on a nominal GaAs(001) substrate is presented. InGaAs layers were grown on nominal GaAs(001) substrates at a low temperature to suppress the Stranski-Krastanov transition as well as indium segregation and indium desorption, then annealed at high temperatures to induce self-assembly. While typical direct growth at the annealing temperature has yielded only quantum dot shapes, our approach has enabled us to control the shape of self-assembled nanostructures from quantum dashes to quantum dots and eventually quantum dot-chains. The major factor controlling the shape of InGaAs nanostructures was found to be the thickness of the pseudomorphic In(0.4)Ga(0.6)As layer.     

Active doping of B in silicon nanostructures and development of a Si quantum dot solar cell

Active doping of B was observed in nanometer silicon layers confined in SiO(2) layers by secondary ion mass spectrometry (SIMS) depth profiling analysis and confirmed by Hall effect measurements. The uniformly distributed boron atoms in the B-doped silicon layers of [SiO(2) (8 nm)/B-doped Si(10 nm)](5) films turned out to be segregated into the Si/SiO(2) interfaces and the Si bulk, forming a distinct bimodal distribution by annealing at high temperature. B atoms in the Si layers were found to preferentially substitute inactive three-fold Si atoms in the grain boundaries and then substitute the four-fold Si atoms to achieve electrically active doping. As a result, active doping of B is initiated at high doping concentrations above 1.1 × 10(20) atoms cm( - 3) and high active doping of 3 × 10(20) atoms cm( - 3) could be achieved. The active doping in ultra-thin Si layers was implemented for silicon quantum dots (QDs) to realize a Si QD solar cell. A high energy-conversion efficiency of 13.4% was realized from a p-type Si QD solar cell with B concentration of 4 × 10(20) atoms cm( - 3).     

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.     

Long-wavelength quantum-dot lasers

Quantum dots (QD) of (InGaAs/GaAs) on GaAs substrate with long-wavelength emission (1300 nm) have been fabricated using metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) for use in surface-emitting laser diodes. QDs are obtained by employing two different approaches, seeding and overgrowth with a quantum well, yielding similar recombination spectra. Despite the shift to long wavelengths, a large separation (80 meV) between excited states is maintained. The introduction of such QDs into a vertical cavity leads to a strong narrowing of the emission spectrum. Lasing from 1300-nm QD VCSEL is reported.     

Enhancement of Single-Photon Purity and Coherence of III-Nitride Quantum Dot with Polarization-Controlled Quasi-Resonant Excitation (Small 5/2023)

III-Nitride semiconductor-based quantum dots (QDs) play an essential role in solid-state quantum light sources because of their potential for room-temperature operation. However, undesired background emission from the surroundings deteriorates single-photon purity. Moreover, spectral diffusion causes inhomogeneous broadening and limits the applications of QDs in quantum photonic technologies. To overcome these obstacles, it is demonstrated that directly pumping carriers to the excited state of the QD reduces the number of carriers generated in the vicinities. The polarization-controlled quasi-resonant excitation is applied to InGaN QDs embedded in GaN nanowire. To analyze the different excitation mechanisms, polarization-resolved absorptions are investigated under the above-barrier bandgap, below-barrier bandgap, and quasi-resonant excitation conditions. By employing polarization-controlled quasi-resonant excitation, the linewidth is reduced from 353 to 272 µeV, and the second-order correlation value is improved from 0.470 to 0.231. Therefore, a greater single-photon purity can be obtained at higher temperatures due to decreased linewidth and background emission.     

Narrow emission linewidths of positioned InAs quantum dots grown on pre-patterned GaAs(100) substrates

We report photoluminescence measurements on a single layer of site-controlled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on pre-patterned GaAs(100) substrates with a 15 nm re-growth buffer separating the dots from the re-growth interface. A process for cleaning the re-growth interface allows us to measure single dot emission linewidths of 80 μeV under non-resonant optical excitation, similar to that observed for self-assembled QDs. The dots reveal excitonic transitions confirmed by power dependence and fine structure splitting measurements. The emission wavelengths are stable, which indicates the absence of a fluctuating charge background in the sample and confirms the cleanliness of the re-growth interface.     

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.     

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.     

Visible colloidal nanocrystal silicon light-emitting diode

We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot-organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast.     

Emission wavelength trimming of self-assembled InGaAs/GaAs quantum dots with GaAs/AlGaAs superlattices by rapid thermal annealing

The effects of rapid thermal annealing on InGaAs quantum dots grown by atomic layer molecular beam epitaxy to the structural transformation and optical properties are investigated. No misfit dislocation was observed from either the as-grown or annealed dots. The size and composition of the quantum dots become more uniform upon annealing mainly from the height fluctuation as predicted by the theoretical model. Large bandgap blue shifts, resulted from the In and Ga interdiffusion, were observed with the preservation of three-dimensional carrier confinement. The GaAs/AlGaAs superlattice was found to minimize the defect diffusion and dot interdiffusion during the high-temperature epitaxial overgrowth.     

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.     

Microscopic Origin of the Fast Blue‐Green Luminescence of Chemically Synthesized Non‐oxidized Silicon Quantum Dots

The microscopic origin of the bright nanosecond blue‐green photoluminescence (PL), frequently reported for synthesized organically terminated Si quantum dots (Si‐QDs), has not been fully resolved, hampering potential applications of this interesting material. Here a comprehensive study of the PL from alkyl‐terminated Si‐QDs of 2–3 nm size, prepared by wet chemical synthesis is reported. Results obtained on the ensemble and those from the single nano‐object level are compared, and they provide conclusive evidence that efficient and tunable emission arises due to radiative recombination of electron–hole pairs confined in the Si‐QDs. This understanding paves the way towards applications of chemical synthesis for the development of Si‐QDs with tunable sizes and bandgaps.