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Graphene quantum dots (GQDs) are promising luminescent materials for various light-emitting, biological, and energy applications due to their low toxicity compared to cadmium-based semiconductor quantum dots. The practical application and use of GQDs is driven by their luminescent characteristics. As such, a critical need exists to control and tailor the emission properties of these materials to suit the targeted optoelectronic applications. In this review, the fundamental properties, synthesis methods, and strategies for incorporating GQDs into optoelectronic devices are discussed. We present the fundamentals on luminescence mechanism of GQDs, including reflections on the defect-related properties (i.e., oxygen functional groups and odd-numbered carbon rings in the basal plane). We then offer new guidelines for the fabrication of GQDs aimed at increasing the luminescence efficiency. Finally, we introduce strategies for integrating GQDs into optoelectronic devices based on architectural design of the fundamental properties.

Semiconductor quantum dots (QDs) of various material systems are being heavily researched for the development of solid state single photon emitters, which are required for optical quantum computing and related technologies such as quantum key distribution and quantum metrology. In this review article, we give a broad spectrum overview of the QD-based single photon emitters developed to date, from the telecommunication bands in the IR to the deep UV.

We report on stacked multiple quantum dots (QDs) formed inside inverted pyramidal recesses, which allow for the precise positioning of the QDs themselves. Specifically, we fabricated double QDs with varying inter-dot distances and ensembles with more than two nominally highly symmetric QDs. For each, the effect of the interaction between QDs is studied by characterizing a large number of QDs through photoluminescence spectroscopy. A clear red-shift of the emission energy is observed together with a change in the orientation of its polarization, suggesting an increasing interaction between the QDs. Finally, we show how stacked QDs can help influencing the charging of the excitonic complexes.

As with any quantum computing platform, semiconductor quantum dot devices require sophisticated hardware and controls for operation. The increasing complexity of quantum dot devices necessitates the advancement of automated control software and image recognition techniques for rapidly evaluating charge stability diagrams. We use an image analysis toolbox developed in Python to automate the calibration of virtual gates, a process that previously involved a large amount of user intervention. Moreover, we show that straightforward feedback protocols can be used to simultaneously tune multiple tunnel couplings in a triple quantum dot in a computer automated fashion. Finally, we adopt the use of a “tunnel coupling lever arm” to model the interdot barrier gate response and discuss how it can be used to more rapidly tune interdot tunnel couplings to the gigahertz values that are compatible with exchange gates.

InAs quantum dots (QDs) have been investigated using optical spectroscopy, in order to understand the experimental observation of strong enhancement of their photoluminescence efficiency. When a tu ...

We have observed and investigated random telegraph noise in the photoluminescence from InAs quantum dots in GaAs. The luminescence from a single quantum dot, exhibiting switching between two levels, has been spectrally resolved. The random telegraph noise is only observed in the presence of band filling. We find no spectral shift of the emission in the different states. It is only the intensity, mainly for higher energy peaks, that changes. The InAs quantum dots behave similarly to InP/GaInP and InGaAs/GaAs quantum dots, with respect to random telegraph noise.

Excitonic effects are observed in the optical absorption and photoluminescence of strongly confined CdS quantum dots embedded in the polymer Nafion. The three bands identified in the optical absorption spectra could be attributed to 1s e-1s h, 1p e-1p h, and 2s e-2s h transitions of the noninteracting particle model. Photoluminescence spectra show a strong emission band corresponding to electron-hole recombination and a weak band due to emission from defect states. The strength of electron-phonon coupling is small in the regime of strong confinement and decreases with decreasing particle size. ? 2002 American Institute of Physics.

We report on a silicon single-electron tunneling device fabricated in an ultrathin (∼3 nm) silicon-on-insulator (SOI) film whose surface is undulated by an alkaline-based solution. The nanometer-scaled undulation in the ultrathin film results in great SOI thickness variations and brings about large electron-potential fluctuations, due to the difference of the quantum confinement effects from one part to another. Consequently, a number of quantum dots are effectively formed in the undulated ultrathin SOI film. This device shows clear Coulomb blockade oscillations at 80 K, as well as nonvolatile single-electron memory functions even at room temperature. The measurements of the undulation with atomic force microscopy reveal that the undulation has two correlation lengths. Based on the analysis of electrical characteristics, it is concluded that the Coulomb blockade oscillations are dominated by a quantum dot formed by the longer-correlation-length undulation and that single-electron memory effects are due to...

Comparisons of buried InAs/GaAs and InAs/InP quantum dots (QDs) utilizing transmission electron microscopy display interesting parallels and differences between the two systems. The higher 7.2% misfit in the InAs/GaAs system produces small (∼20 nm diameter) QDs with a majority displaying a predominately round shape. The lower 3.2% misfit in the InAs/InP system produces larger QDs (∼35 nm diameter) with the majority also displaying a predominately round shape. In both systems, the size of the QDs can be varied by changes in growth procedures and the largest QDs in any population show evidence of faceting

The effect of the As flux on the kinetics of Ga and In cations, and the role of the elastic strain were studied in the capping process of isolated InAs quantum dots with GaAs by molecular beam epitaxy. Using a fixed evaporation geometry and a suitable choice of growth parameters which enhance the anisotropic diffusion of In and Ga cations, we obtained, at variance with current results, the formation of asymmetric GaAs caps. The growth of a second InAs layer led to the formation of vertically aligned couples of dots (one buried, the other uncapped) placed on the right side of the GaAs caps on the surface, with no other dots in different locations.

The quantum dot solar cell concept is proposed as a scheme for increased solar cell efficiency. A theoretical model is presented for a practical p–i–n quantum dot solar cell, based on the self-organized InAs/GaAs system. The advantages of using the quantum dot in the active region for photon absorption in the long-wavelength part of the spectrum, leading to cell efficiency, is discussed.

We present a quantitative technique for the direct compositional analysis of quantum dots (QDs), in which scanning transmission electron microscopy is applied to a capped InAs/GaAs QD layer in a structure also containing InxGa1−xAs/GaAs quantum well (QW) layers to provide an internal calibration of the In content. By obtaining energy dispersive x-ray analysis line scans through both QWs and QDs, the composition of the QDs can be determined by reference to the known composition of the QWs. In this article the method is described and demonstrated using two InAs/GaAs structures in which the QDs are nominally identical, but with different In composition in the calibration QW layers. We find that the QDs in both structures have an In composition of 65%–67% and the associated wetting layers contain approximately 12% In.

We start with an analysis of the Raman spectra of Ge nanocrystals obtained in previous studies and demonstrate that in many cases the observed experimental peak attributed to Ge in fact originates from the Si substrate. We further compare various experimental ways to separate the Ge signal from that of the substrate and suggest optimum conditions for such measurements. Finally, we demonstrate that upon the annealing of an amorphous Ge–Si–O film, Ge nanocrystals are formed. The nanocrystals are randomly oriented and Ge–Si mixing takes place only at the interface with the Si substrate.

Single and multiple layers of self-assembled InAs quantum dots (QDs) produced by the indium-flush technique have been studied by transmission electron microscopy (TEM) in an effort to develop techn ...

The influence of both geometric and offset charge disorder of two-dimensional quantum dot arrays (also known as network tunnel junctions) on their Coulomb blockade voltage Vb is studied using extensive Monte–Carlo simulations. A general increase of Vb with increasing disorder is confirmed, but an exception to the rule is found for intermediate degrees of offset charge disorder. Detailed studies of the Vb distribution reveal a stability of its minimal value against geometric disorder, whereas this figure is considerably increased for high offset charge disorder. Implications of our results for single electron device design are discussed.

In this article, we give the electronic structure and optical transition matrix elements of coupled quantum dots (QDs) arranged as different cubic lattices: simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) superlattices. The results indicate that electron and hole energies of bcc, sc, and fcc superlattices are the lowest, the highest, and the middle, respectively, for the same subband under the same QD density or under the same superlattice constant. For a fixed QD density, the confinement effects in sc, fcc, and bcc superlattices are the strongest, the middle, and the weakest, respectively. There are only one, two, and four confined energy bands, with energies lower than the potential barrier for sc, bcc, and fcc QD superlattices, respectively. The results have great significance for researching and making semiconductor quantum dot devices. (C) 1998 American Institute of Physics. [S0021-8979(98)02119-7]

Photoreflectance measurements have been performed in a number of InAs/GaAs single-quantum wells with nominal thickness L ranging from 0.6 to 2.0 ML. The InAs growth mode was investigated by analyzing the evolution, with increasing coverage, of the optical response associated with the InAs layer. For L⩽1.6 ML, the experimentally derived energies for the optical transition originating in the InAs are consistent with those evaluated in a simple square-well envelope-function scheme. The dependence of the photoreflectance line shape broadening on L is well described up to L=1.4 ML in terms of a disordered InAs/GaAs interface made by interconnected InAs and GaAs islands with a typical size of order 2 nm. For L=1.6 ML, the quantum well spectral features broaden abruptly and vanish for L=2 ML, suggesting the disappearance of the InAs 2D layer in favor of a predominant nucleation of large quantum dots.

We have investigated the third-order optical nonlinearities of high-quality colloidal InP/ZnS core-shell quantum dots (QDs) using Z-scan technique with femtosecond pulses. The two-photon absorption cross-sections as high as 6.2 × 103 GM are observed at 800 nm (non-resonant regime) in InP/ZnS QDs with diameter of 2.8 nm, which is even larger than those of CdSe, CdS, and CdTe QDs at similar sizes. Furthermore, both of the 2.2 nm and 2.8 nm-sized InP/ZnS QDs exhibit strong saturable absorption in near resonant regime, which is attributed to large exciton Bohr radius in this material. These results strongly suggest the promising potential of InP/ZnS QDs for widespread applications, especially in two-photon excited bio-imaging and saturable absorbing.

Memory devices have been fabricated in high-electron-mobility transistors with embedded InAs quantum dots (QDs). We show that memory operations can be fully controlled by gate biases at room temperature, without the need for light excitations to erase memory states. Real-time measurements indicate a charge retention time of a few minutes. Neither such retention time nor the self-consistent simulations can justify the picture that the memory effect is due to charging/discharging of intrinsic QD states. Experiments at a series of gate biases point to the presence of deep levels coexisting in the QD layer(s), which are responsible for the memory effect.

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids, and domes show common morphological characteristics, independent of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots. © 2004 American Institute of Physics. [DOI: 10.1063/1.1829164]

A system composed of a double layer of stacked quantum dots (QDs) of (In,Ga)As and Ga(Sb,As) was investigated by quantitative high-resolution transmission electron microscopy. The layers were grown by metalorganic chemical vapor deposition on a GaAs substrate. The strain field of the lower quantum dots determines the nucleation in the subsequent layer. Investigating this effect, the strain field as well as the composition could be measured at high resolution. Local changes of lattice distances could be quantified with a precision of 0.003nm. The error in determining the local composition inside the InxGa1−xAs resp., GaSbyAs1−y QDs amounts to ΔxIn=0.02 and ΔySb=0.03.

We calculate the near-field optical spectra of excitons and biexcitons in semiconductor quantum dots naturally occurring at interface fluctuations in GaAs-based quantum wells, using a non-local description of the response function to a spatially modulated electro-magnetic field. The relative intensity of the lowest, far-field forbidden excitonic states is predicted; the spatial extension of the ground biexciton state is found in agreement with recently published experiments.

We present optical characterization measurements from three-dimensional pillar microcavities comprising an InP spacer layer sandwiched between two SiO2/Ta2O5 Bragg reflectors. InAs quantum dots placed at the center of the InP spacer are used as broadband emitters. Emission into the lowest-order mode is observed at an energy of 820 meV in the wide-pillar limit, with shifts of approximately 30 meV in the ground state and more than 80 meV in higher-order modes for pillars of 2 μm diameter. Excellent agreement is found between the experimentally observed cavity mode energies as a function of pillar diameter and the predictions of guided-mode theory. Cavity quality factors in excess of 2200 are obtained for large diameter pillars, with a decrease to approximately 500 for pillars of 2 μm diameter.

We report a high detectivity of 3×1011 cm Hz1/2/W at 78 K for normal-incidence quantum dot infrared photodetectors with ten layers of undoped InAs/InGaAs/GaAs quantum dot active regions. The high detectivity seen at 1.4 V corresponds to photoresponse peaks at 9.3 and 8.7 μm for positive and negative bias, respectively.

We report results of measurements of Hall mobility in a set of doped and undoped GexSi1−x/Si quantum dot superlattices (x=0.50 and 0.73). The dome-shaped Ge quantum dots have the characteristic base size of 40 nm and height of about 4 nm. The molecular beam epitaxy grown structures consist of 5–20 layers of Ge quantum dots separated by 20-nm-thick Si layers. The position of δ doping varies for different samples. The average measured in-plane Hall mobility for p-type structures is 140 cm2 V−1 s−1 at 300 K and 2.4×103 cm2 V−1 s−1 at 77 K. Relatively large values and temperature dependence suggest that in given quantum dot structures the carrier transport is likely of the band conduction type rather than hopping type. These results are important for proposed optoelectronic and thermoelectric application of quantum dot superlattices.

InAs quantum dots (QDs), grown by molecular-beam epitaxy on GaAs(114)A surfaces, were studied in situ by atomically resolved scanning tunneling microscopy. At their mature stage, the QDs present a complicated but regular shape being bound by flat {110}, (111)A, and {2 5 11}A facets, and a steep part composed of rather variable combinations of {110}, (111)A, {111}B, and {2 5 11} surfaces. The QD shape can be derived from mature InAs QDs on GaAs(001).

We demonstrate an approach to manipulate the quantum states of single self-assembled quantum dots via strain. We fabricate a micromachined air-bridge with microelectromechanical systems (MEMS), in which quantum dots are embedded. The air-bridge is deformed by electrostatic force, which produces additional strain on the dots to modify the confining potential. Our method with MEMS technique will allow functional manipulation of the electronic states through the direct modification of the confining potential.

The properties of InAs quantum dots (QDs) have been studied for application in intermediate band solar cells. It is found that suppression of plastic relaxation in the QDs has a significant effect on the optoelectronic properties. Partial capping plus annealing is shown to be effective in controlling the height of the QDs and in suppressing plastic relaxation. A force balancing model is used to explain the relationship between plastic relaxation and QD height. A strong luminescence has been observed from strained QDs, indicating the presence of localized states in the desired energy range. No luminescence has been observed from plastically relaxed QDs.

This letter reports on theoretical calculations of the oscillator strength associated with electron intraband transitions in (In,Ga)As/GaAs quantum dots. We study the effect of dot size and lateral separation between adjacent dots on the oscillator strength. The calculations indicate that transitions induced by p-polarized light from the electronic ground state to the first excited state are stronger than those induced by s-polarized light for large size dots with wide lateral interdot spacing. This situation changes, however, for small size dots in close proximity with one another. We discuss the relevance and implication of these results for applications in quantum-dot structures designed for mid-infrared detection.

A complementary metal-oxide-semiconductor-compatible method is proposed to form atomic-scale germanium (Ge) quantum dots (

We report on the polarization dependence of intraband photoresponse of (In,Ga)As/GaAs quantum-dot device structures for light polarized parallel and perpendicular to the layers. Strong photoresponse due to intersublevel transitions induced by both s- and p-polarized infrared light was observed. Within the plane of the layers, it is found that the photoresponse for s-polarized light aligned along the [110] crystallographic direction is virtually identical to that in the [110] direction, suggesting that, at least in the x-y plane, the dots are symmetric. The devices studied were found to operate up to a temperature of around 100–105 K.

The effect of the growth temperature on the formation of uniform self-assembled Ge dots on Si (001) substrates was studied. The ratio of pyramid dots to dome dots varies with the growth temperature from 500 to 700 °C. Temperature of 600 °C was optimum to form uniform self-assembled Ge dots, and is attributed to the enhanced diffusion kinetics. Highly uniform Ge dots with height deviation of ±3% were obtained at this growth temperature. Discontinuity in characteristic length was found in an Arrhenius plot between 600 and 625 °C, and it is due to intermixing of Si with Ge which occurred at high temperature.

We have investigated in detail the dependence of the photoluminescence decay times in stacked quantum dot (QD) structures, showing that they strongly depend on the emission energy. We propose that the dispersion of the lifetimes has to be related to the presence of vertical disorder in the quantum dot column. Due to the disorder the energy distribution of the carrier extended states shrinks and localized states are created in the tails. This idea is supported by a theoretical model, which takes into account the presence of a vertical disorder along the QD columns. (C) 2003 American Institute of Physics.

We report the growth of InP self-assembled quantum dots (QDs) on In0.5Al0.5P and In0.5(Al0.6Ga0.4)0.5P matrices, lattice matched on 0°, 2°, 6°, and 25° off-axis (100) GaAs substrates by metalorganic chemical vapor deposition. The influence of the substrate misorientation on the morphology and cathodoluminescence (CL) properties of the InP QDs was investigated. We find that the density of QDs grown on both In0.5Al0.5P and In0.5(Al0.6Ga0.4)0.5P matrices increases with the misorientation angle up to 6° off (100). At the same time, the dispersion of the QD size is getting larger for the growth on an In0.5Al0.5P matrix, but not for the growth on an In0.5(Al0.6Ga0.4)0.5P matrix. The InP QDs grown on In0.5(Al0.6Ga0.4)0.5P on 25° off-axis substrates are two-dimensionally well ordered. Moreover, the ordering improves and the density of QDs increases with an increase in the deposition of InP. The room-temperature CL intensity of InP QDs grown on a 25° off In0.5(Al0.6Ga0.4)0.5P is much stronger than that for InP QDs...

We present a detailed study of the growth of Si quantum dots (Si QDs) by low pressure chemical vapor deposition on alumina dielectric deposited by atomic layer deposition. The Si QDs density is very high, 1012 cm−2, for a mean diameter between 5 and 10 nm. Al2O3/Si QD stacks have been integrated in memory devices as granular floating gate. The devices demonstrate good charge storage and data retention characteristics.

We report the observation of a spatially-ordered bidimensional array of self-assembled InP quantum dots grown on slightly In-rich InGaP layers. The alignment of InP dots is observed along [100] and [010] directions. This effect is enhanced when 2° off vicinal substrates are used; it is also strongly dependent on growth temperature. Our results suggest that the density and size of CuPt-type atomically ordered regions as well as compositional modulation of InGaP layers play an important role on the spatial alignment of InP/InGaP quantum dots.

Self-organized vertical and lateral ordering in PbSe/Pb1−xEuxTe quantum-dot superlattices is investigated as a function of PbSe dot layer thickness. An efficient lateral ordering and face centered cubic-like dot stacking occurs only for PbSe thicknesses between 4–6 monolayers. For smaller thicknesses, no correlations are formed, whereas for larger thicknesses the dots are vertically aligned along the growth direction. These transitions are explained by changes in interlayer dot interactions as a function of the dot size.

The luminescence polarization properties of GaInP islands have been investigated. The islands, which form during overgrowth of InP quantum dots, were studied using scanning tunneling luminescence (STL) and photoluminescence (PL). STL from these islands shows emission at an energy below the main emission peak of the bulk GaInP. The linear PL polarization anisotropy was measured at low temperature, for which the emission from the islands shows high polarization anisotropy. The intensity maximum for the emission occurs for light polarized parallel to the elongation of the islands. The observed linear PL polarization anisotropy indicates the presence of highly ordered domains of GaInP in the islands.

Quantum computers are analog devices; thus they are highly susceptible to accumulative errors arising from classical control electronics. Fast operation--as necessitated by decoherence--makes gating errors very likely. In most current designs for scalable quantum computers it is not possible to satisfy both the requirements of low decoherence errors and low gating errors. Here we introduce a hardware-based technique for pseudo-digital gate operation. We perform self-consistent simulations of semiconductor quantum dots, finding that pseudo-digital techniques reduce operational error rates by more than two orders of magnitude, thus facilitating fast operation., Comment: 4 pages, 3 figures

An electrically injected InAs/GaAs self-organized quantum-dot photonic crystal microcavity light-emitting diode operating at 1.04 μm is demonstrated. Light–current characteristics are obtained for devices with two- and five-defect period cavities with maximum light output of 0.17 μW measured in the surface-normal direction. Near-field images were also obtained for an injection current of 8.35 mA, showing light confinement within a few periods of the photonic crystal defect microcavity.

We study the characteristics of the fluctuations, observed in the low-temperature magnetoconductance of an open quantum-dot molecule formed from a pair of split-gate quantum dots. The evolution of these fluctuations suggests a decrease in the typical area for coherent interference with decreasing dot-coupling strength. We discuss this behavior in terms of a transition from multi- to single-dot interference as a function of the interdot coupling.

The growth of vertically aligned self-assembled PbSe/Pb1−xEuxTe quantum-dot superlattices is found to result in a pronounced hexagonal lateral ordering tendency accompanied by a narrowing of the size distribution. In addition, the lateral dot separations and dot densities become tunable by changes in the spacer thickness with an almost twofold density increase for spacer thicknesses increasing from 100 to 275A. Similar marked changes are also found for PbSe dot sizes and shapes. This could provide additional means for the tuning of the optical and electronic properties of the dots.

A method to achieve ordering and selective positioning of single InAs self-assembled quantum dots (QDs) has been developed. The selective growth was achieved by manipulating the strain distribution on the sample surface. The QDs are formed on predesigned mesas with added strain. Single dots were obtained on small mesas. Using this technique, two-dimensional single QD arrays have been achieved.

In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth.

We present a pathway for the formation of ordered Ge quantum dots on Si(111)–(7×7) substrate. Self-assembled growth of Ge quantum dots on the Si(111)–(7×7) surface has been investigated using scanning tunneling microscopy. The Ge is grown on the substrate by solid phase epitaxy at room temperature. It has been found that the deposited submonolayer Ge can aggregate and form ordered Ge quantum dots on the surface through controlling the annealing temperature. The formation of ordered Ge quantum dots is due to the preferential adsorption sites of Ge on Si(111)–(7×7). The formed Ge quantum dots may have a great potential in the application of nanodevices.

The interdiffusion effects on the band alignment of the GeSi dots embedded in Si matrix were studied by temperature- and excitation-power-dependent photoluminescence measurements. A different power-dependent behavior of the photoluminescence for the as-grown and the annealed samples was observed. It was suggested that the band alignments of the dots changed from type II to type I after annealing due to the Ge/Si interdiffusion. The decrease of the valence band offset, which was also induced by the Ge/Si interdiffusion, was observed from the temperature-dependent photoluminescence measurements.

We report continuous-wave (cw) room-temperature operation of electrically pumped AlGaAs/GaAs microcylinder lasers using self-assembled InAs/GaAs quantum dots (QDs) as the active material. The threshold currents are lower than those of similar devices with an InGaAs quantum well fabricated for comparison. This is discussed in terms of nonradiative surface recombination at the sidewall of the resonator which has less influence in the case of the QD devices since the lateral carrier confinement suppresses lateral carrier diffusion.

We show that the combination of different electric fields in In0.5Ga0.5As/GaAs quantum-dot electroluminescent devices dramatically blueshifts the emission wavelength even though the photoluminescence occurs at the expected value of 1.3 μm at room temperature. Systematic photoluminescence (PL), electroluminescence (EL), and photocurrent measurements demonstrate that the electric field associated with the built-in dipole in the dots, directed from the base of the dots to their apex, and the device junction field lead to the depletion of the ground state. As a consequence, structures grown on n-type GaAs substrates exhibit electroluminescence only from the excited states (whereas the photoluminescence comes from the ground level). Instead, by growing the same device structure on p-type GaAs substrates, i.e., by reversing the direction of the built-in electric field of the device, the effect of the permanent dipole is strongly reduced, thus allowing us to obtain EL emission at the designed wavelength of 1.3 μ...