Field In Quantum Dots Research Articles

Explorations on Growth of Blue-Green-Yellow-Red InGaN Quantum Dots by Plasma-Assisted Molecular Beam Epitaxy.

Self-assembled growth of blue-green-yellow-red InGaN quantum dots (QDs) on GaN templates using plasma-assisted molecular beam epitaxy were investigated. We concluded that growth conditions, including small N2 flow and high growth temperature are beneficial to the formation of InGaN QDs and improve the crystal quality. The lower In/Ga flux ratio and lower growth temperature are favorable for the formation of QDs of long emission wavelength. Moreover, the nitrogen modulation epitaxy method can extend the wavelength of QDs from green to red. As a result, visible light emissions from 460 nm to 622 nm have been achieved. Furthermore, a 505 nm green light-emitting diode (LED) based on InGaN/GaN MQDs was prepared. The LED has a low external quantum efficiency of 0.14% and shows an efficiency droop with increasing injection current. However, electroluminescence spectra exhibited a strong wavelength stability, with a negligible shift of less than 1.0 nm as injection current density increased from 8 A/cm2 to 160 A/cm2, owing to the screening of polarization-related electric field in QDs.

Ultrafast Quenching of Excitons in the ZnxCd1-xS/ZnS Quantum Dots Doped with Mn2+ through Charge Transfer Intermediates Results in Manganese Luminescence.

For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of ZnxCd1−xS/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn2+, which was interpreted as the electrochromic Stark shift of the band-edge exciton. The time-delayed rise and decay kinetics of the Stark peak in the manganese-doped QDs significantly distinguish it from the kinetics of the Stark peak caused by exciton–exciton interaction in the undoped QDs. The Stark shift in the Mn2+-doped QDs developed at a 1 ps time delay in contrast to the instantaneous appearance of the Stark shift in the undoped QDs. Simultaneously with the development of the Stark peak in the Mn2+-doped QDs, stimulated emission corresponding to 4T1-6A1 Mn2+ transition was detected in the subpicosecond time domain. The time-delayed Stark peak in the Mn2+-doped QDs, associated with the development of an electric field in QDs, indicates the appearance of charge transfer intermediates in the process of exciton quenching by manganese ions, leading to the ultrafast Mn2+ excitation. The usually considered mechanism of the nonradiative energy transfer from an exciton to Mn2+ does not imply the development of an electric field in a QD. Femtosecond TA data were analyzed using a combination of empirical and computational methods. A kinetic scheme of charge transfer processes is proposed to explain the excitation of Mn2+. The kinetic scheme includes the reduction of Mn2+ by a 1Se electron and the subsequent oxidation of Mn1+ with a hole, leading to the formation of an excited state of manganese.

Surface Modification of a Stable CdSeZnS/ZnS Alloy Quantum Dot for Immunoassay

Quantum dots (QDs) are powerful materials in various bioapplications based on their excellent optical and electronic properties. For the application of various fields of QDs, surface modification of QDs is necessary. However, surface modification in QDs may result in a reduction in quantum yield (QY). This reduction of QY causes many weaknesses in the biological application of QDs. In this study, CdSeZnS/ZnS alloy QDs were used to prepare antibody-conjugated QDs for a sandwich immunoassay. The alloy QDs displayed a QY of 84.5% that was maintained at 83.0% (98.2% of QY was maintained) after surface modification with the anti-rabbit IgG as a model study. Surface-modified QDs successfully detected their corresponding target through antibody-antigen binding. The limit of detection was 1.1 × 10 2 ng mL-1 for rabbit IgG.

Effect of Interfacial Alloying versus "Volume Scaling" on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots.

Auger recombination is a nonradiative three-particle process wherein the electron-hole recombination energy dissipates as a kinetic energy of a third carrier. Auger decay is enhanced in quantum-dot (QD) forms of semiconductor materials compared to their bulk counterparts. Because this process is detrimental to many prospective applications of the QDs, the development of effective approaches for suppressing Auger recombination has been an important goal in the QD field. One such approach involves "smoothing" of the confinement potential, which suppresses the intraband transition involved in the dissipation of the electron-hole recombination energy. The present study evaluates the effect of increasing "smoothness" of the confinement potential on Auger decay employing a series of CdSe/CdS-based QDs wherein the core and the shell are separated by an intermediate layer of a CdSexS1-x alloy comprised of 1-5 sublayers with a radially tuned composition. As inferred from single-dot measurements, use of the five-step grading scheme allows for strong suppression of Auger decay for both biexcitons and charged excitons. Further, due to nearly identical emissivities of neutral and charged excitons, these QDs exhibit an interesting phenomenon of lifetime blinking for which random fluctuations of a photoluminescence lifetime occur for a nearly constant emission intensity.

Effect of built-in electric field in photovoltaic InAs quantum dot embedded GaAs solar cell

In this paper, three p–i–n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded in the n− base region (B), and the third without QDs (control sample C). QD-embedded solar cells (samples A and B) show broad photoluminescence spectra due to QD multi-level emissions but have lower open-circuit voltages Voc and lower photovoltaic (PV) efficiencies than sample C. On the other hand, the short-circuit current density Jsc in sample A is increased while it is decreased in sample B. Theoretical analysis shows that in sample B where the built-in electric field in QDs is zero, electrons tend to occupy QDs and strong potential variations exist around QDs which deteriorate the electron mobility in the n− base region so that Jsc in sample B is decreased. Hole trapping and electron–hole recombination in QDs are also enhanced in sample B, resulting in a reduced Voc and thus a worse PV effect. In sample A, a strong built-in field exists in QD layers, which facilitates photo-carrier extraction from QDs and thus Jsc is increased. However, QDs in the depletion region in sample A act also as recombination-generation centers so that the dark saturated current density is drastically increased, which reduces Voc and the total PV effect. In conclusion, a nonzero built-in electric field around QDs is vital for using QDs to increase the PV effect in conventional p–i–n GaAs solar cells.

Parametric properties of the electron spin relaxation in InAs quantum dots

The electron spin relaxation rates Γ under different conditions in InAs quantum dots (QDs) with local elastic twists have been calculated by using the perturbation method. It is shown from the calculational results that the rate Γ is dominated by the magnitude of the absolute value of anisotropic electron g factor, |gα|(α=x,y,z) and markedly influenced by the difference between the values of gα, which indicates there is a proper size for InAs QDs to minimize the rate. The relaxation rate is noticeably affected by the orientations of the magnetic field n(θ,φ), and the ratio of the rates can be obtained by changing orientations of the field in QDs, where the maximum in our calculation is Γmax/Γmin≈6.4.

Influence of strain in the reduction of the internal electric field in GaN/AlN quantum dots grown on a‐plane 6H‐SiC

AbstractThe strain state of stacks of GaN/AlN quantum dots (QDs) grown on (0001) and (11$ \bar 2 $0) 6H‐SiC has been investigated by means of Raman spectroscopy. Depending on the orientation of the wurtzite axis with respect to the growth direction it is found that the piezoelectric contribution to the electrostatic potential may either reinforce that arising from the spontaneous polarization or oppose it. The experimental results are compared with a theoretical model for the strain and polarization field in QDs of both orientations that allows the calculation of the electrostatic potential in the QDs. Both the experimental results and the theoretical model indicate that the internal electric field and electrostatic potential are strongly reduced in the QDs grown on (11$ \bar 2 $0) 6H‐SiC as compared to those grown along the wurtzite c‐axis. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of GaN quantum dots in AlN matrix

In the present work steady-state and time-resolved photoluminescence (PL) from self-assembled GaN quantum dots (QDs) embedded in AlN matrix has been studied. The maximum of low-temperature PL is near 2.55 eV that is below the band gap of bulk GaN. The PL peak is not shifted with variation of excitation power. When temperature is increasing the PL peak exhibits a red shift in accordance with a temperature dependence of the band gap of bulk GaN. A huge red shift of the PL maximum from 3.0 down to 2.2 eV is observed as a function of the delay time after the excitation pulse. The PL decay is non-exponential and spectral dependent. The PL decay time varies from about 10 μs to more than 100 μs over the spectrum. The experimental results interpreted in framework of a model taking into account an internal electric field in QDs and transport of the non-equilibrium carriers between QDs.

Quantum optics of a quantum dot: Local-field effects

The role of local fields in quantum electrodynamics of isolated quantum dot (QD) has been analyzed. The system is modeled as a strongly confined in space two-level quantum oscillator illuminated by quantum light. Relation between local and acting fields in QD has been derived in the dipole approximation from the integral Maxwell equations for electromagnetic field operators. A formalism of the electromagnetic field quantization in electrically small scatterers has been developed. As a result, Hamiltonian of the system has been formulated in terms of the acting field with a separate term responsible for the effect of depolarization. Schroedinger equation with that Hamiltonian has been solved in linear approximation. Interaction of QD with different quantum states of light, such as Fock states, coherent states, Fock qubits, entangled states, has been analyzed. It has been shown that the local-fields induce a fine structure of the QD absorption (emission) spectrum: instead of a single line with the frequency corresponding to the exciton transition, a doublet appears with one component shifted to the blue (red). The value of the shift depends only on the geometrical and electronic properties of QD while the intensities of components are completely determined by the quantum light statistics. Itmore » has been demonstrated that in the limiting cases of classical light and single-photon state the doublet is reduced to a singlet shifted in the former case and unshifted in the latter one. A physical interpretation of the predicted effect has been proposed. Possible ways of experimental observation of the effect has been discussed together with the potentiality of its utilization in the quantum information processing.« less