Light emission from silicon nanocrystals: Probing a single quantum dot

Analysis of low-temperature photoluminescence measurements performed on single silicon nanocrystals is presented. The luminescence emission linewidth of Si nanocrystals is found to be less than thermal broadening at low temperature, confirming the atomic-like nature of their energetic states. Beside the main peak the low-temperature spectra reveal a ∼6 meV replica, the origin of which is discussed. For some of the investigated dots, we also observe a ∼60 meV transverse optical (TO) phonon replica. The regular arrangement of individual nanocrystals used in this work enables combined high-resolution transmission electron microscopy (TEM) and low-temperature photoluminescence characterization of the same single quantum dot.     

Gain Measurement and Anomalous Decrease of Peak Gain at Long Wavelength for InAs/GaAs Quantum-Dot Lasers

Mode gain spectrum is measured by the Fourier series expansion method for InAs/GaAs quantum-dot （QD） lasers with seven stacks of QDs at different injection currents. Gain spectra with distinctive peaks are observed at the short and long wavelengths of about 1210nm and 1300nm. For a QD laser with the cavity length of 1060μm, the peak gain of the long wavelength first increases slowly or even decreases with the injection current as the peak gain of the short wavelength increases quickly, and finally increases quickly before approaching the saturated values as the injection current further increases.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays

We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under ∼100 W/cm² laser radiation, the PL intensity (I_PL) increases with irradiation time upto about 500 s and thereafter declines linearly. The wavelength of the PL emission (λ_peak) exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm² 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I_PL and λ_peak are significantly different; I_PL increases to a saturation level, and the magnitude of the blue-shift in λ_peak is reduced. We discuss possible mechanisms underlying these results.     

Surface optical solitons in semiconductor quantum dot layers

A theory of optical solitons under the condition of nonlinear coherent interaction of surface TM-modes with a layer of inhomogeneously broadened semiconductor quantum dots is developed. Explicit analytical expressions for a surface soliton (2π-pulse) in the presence of single and biexciton transitions are obtained in the regime of self-induced transparency with realistic parameters which can be reached in current experiments.     

Surface optical solitons in semiconductor quantum dot layers

A theory of optical solitons under the condition of nonlinear coherent interaction of surface TM-modes with a layer of inhomogeneously broadened semiconductor quantum dots is developed. Explicit analytical expressions for a surface soliton (2π-pulse) in the presence of single and biexciton transitions are obtained in the regime of self-induced transparency with realistic parameters which can be reached in current experiments.     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

Optical spectra in the region of exciton resonances in quantum wells and quantum dots of In0.3Ga0.7As/GaAs heterostructures

The transparency, reflection and luminescence spectra of In_0.3Ga_0.7As structures with 8 nm thickness and quantum wells limited by the barrier layer GaAs of a 9 nm (upper layer) and 100 nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6 eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass m_hh^∗ and m_lh^∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1 ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2 ps.     

Temperature Dependence of Photoluminescence from Single and Ensemble InAs/GaAs Quantum Dots

We investigate the temperature dependence of photoluminescence from single and ensemble InAs/GaAs quantum dots systematically. As temperature increases, the exciton emission peak for single quantum dot shows broadening and redshift. For ensemble quantum dots, however, the exciton emission peak shows narrowing and fast redshift. We use a simple steady-state rate equation model to simulate the experimental data of photoluminescence spectra. It is confirmed that carrier-phonon scattering gives the broadening of the exciton emission peak in single quantum dots while the effects of carrier thermal escape and retrapping play an important role in the narrowing and fast redshift of the exciton emission peak in ensemble quantum dots.     

Single-Photon Emission at Liquid Nitrogen Temperature from a Single InAs/GaAs Quantum Dot

We report on the single photon emission from single InAs/GaAs self-assembled Stranski-Krastanow quantum dots up to 80 K under pulsed and continuous wave excitations. At temperature 8OK, the second-order correlation function at zero time delay, g^（2）（0）, is measured to be 0.422 for pulsed excitation. At the same temperature under continuous wave excitation, the photon antibunching effect is observed. Thus, our experimental results demonstrate a promising potential application of self-assembled InAs/GaAs quantum dots in single photon emission at liquid nitrogen temperature.     

Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.     

Micro-photoluminescence study of electron-spin injection in self-assembled semiconductor quantum dots

We present a micro-photoluminescence (m-PL) study of electron-spin injection under magnetic fields in self-assembled semiconductor quantum dots (QDs) of CdSe. A characteristic band line-up of the CdSe QDs coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of ZnCdMnSe through a ZnSe barrier layer enabled us to inject the electron spins from the DMS-QW into QDs. An experimental evidence of the electron-spin injection was provided by observations of circularly polarized m-PL spectra from individual QDs in this coupled QD structure. We find anti-correlation of PL intensity in between the DMS-QW (spin injector) and the individual QDs (spin receiver).     

Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots

The properties of ZnO quantum dots (QDs) synthesized by the sol-gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The X-ray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye-Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band.     

Growth and characterization of ZnSe/CdSe/ZnSe quantum dots fabricated by using an alternate molecular beam supplying method

ZnSe/CdSe/ZnSe structures inserted CdSe thin layer are fabricated using an alternate molecular beam supply (ALS). Examining the PL peak energy dependence on beam irradiation time in ALS cycle, we studied the initial stage of CdSe growth. When CdSe below the critical thickness is supplied on ZnSe grown on GaAs (1 0 0), two kinds of 2D islands (platelets) appear. We confirmed the alloying of 2D-CdSe islands and 3D-CdSe islands (dots) is prominent under Cd beam irradiation in ALS growth.     

Enhanced radiation hardness of ZnO nanorods versus bulk layers

It is shown that ZnO nanorods grown by MOCVD exhibit enhanced radiation hardness against high energy heavy ion irradiation as compared to bulk layers. The decrease of the luminescence intensity induced by 130 MeV Xe⁺²³ irradiation at a dose of 1.5 × 10¹⁴ cm^–2 in ZnO nanorods is nearly identical to that induced by a dose of 6 × 10¹² cm^–2 in bulk layers. The change in the nature of electronic transitions responsible for luminescence occurs at an irradiation dose around 1 × 10¹⁴ cm^–2 and 5 × 10¹² cm^–2 in nanorods and bulk layers, respectively. High energy heavy ion irradiation followed by thermal annealing is also effective on the quality of ZnO nanorods grown by electrodeposition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of the excitation and of the temperature on the photoluminescence circular polarization of AlInAs/AlGaAs quantum dots

In this paper, we present a study of photoluminescence (PL) from AlInAs/AlGaAs quantum dots (QDs) structures grown by molecular beam epitaxy. Specifically, we describe the effects of the temperature and of the excitation density on the photoluminescence circular polarization. We have found that the circular polarization degree depends on temperature. On the other hand, the study of the excitation density dependent circular polarization PL degree shows that the last increases in the case of the sample of weak dot density. However, in the case of large dot density, it is almost constant in the excitation density range from 0.116 W cm⁻² to 9 W cm⁻².     

Zeeman mapping of exciton localization in self-assembled CdSe quantum dots using diluted magnetic semiconductors

Localization of exciton wavefunctions in self-assembled quantum dots (QDs) has been investigated using CdSe QDs embedded in ZnMnSe. This system was chosen so as to make use of the giant Zeeman splitting in the diluted magnetic semiconductor (DMS) ZnMnSe, which enables one to map how the exciton wavefunction is distributed between the QDs and the surrounding matrix. Two series of CdSe QDs in ZnMnSe were prepared for this investigation by molecular beam epitaxy (MBE), either by varying the CdSe coverage while keeping a constant Mn concentration in ZnMnSe; or by varying the Mn concentration in the matrix while maintaining a constant CdSe coverage. Photoluminescence (PL) experiments show a systematic evolution of the CdSe QDs with increasing CdSe coverage; and also reveal the role of Mn in nucleating (“seeding”) the self-assembly of the QDs. By simultaneously measuring the Zeeman shifts of the PL peaks from both the CdSe QDs and their ZnMnSe matrix, we are able to extract information on exciton localization in the QDs and its dependence on the degree of development of the self-assembled CdSe QDs with increasing CdSe coverage.     

Optical solitons in semiconductor quantum dot waveguides

A theory of self-induced transparency for a TM-mode propagating in a planar semiconductor waveguide sandwiched between two dielectric media is developed. A transition layer between the waveguide and one of the connected media is described using a model of a two-dimensional sheet of quantum dots. Explicit analytical expressions for the optical soliton in the presence of single-excitonic and biexcitonic transitions are obtained with realistic parameters which can be reached in current experiments.     

Giant optical anisotropy in M-plane GaN/AlGaN quantum wells due to crystal-field effect

The optical polarization of GaN/AlGaN wurtzite quantum wells in various orientations is studied using an arbitrarily-oriented [hkil] Hamiltonian potential matrix. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The results reveal the presence of giant in-plane optical anisotropy (polarized normal to [0001]) in the M-plane (i.e., the -oriented layer plane) GaN/Al_0.2Ga_0.8N quantum well, due to the positive crystal-field split energy effect (ΔCR>0). The present theoretical results are consistent with the photoluminescence measurements presented in the literature [B. Rau, et al., Appl. Phys. Lett. 77 (2000) 3343].     

Giant optical anisotropy in R-plane GaN/AlGaN quantum wells caused by valence band mixing effect

This study investigates the optical anisotropy spectrum in the R-plane (i.e., the -oriented layer plane) of GaN/Al_0.2Ga_0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in -oriented GaN/Al_0.2Ga_0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x^′-polarization to y^′-polarization as the well width increases.