Biexciton Emission Research Articles

A spin-polarized bi-exciton in a semiconductor quantum dot

We discuss the role of spin-polarized multi-exciton complexes in the emission spectra ofsemiconductor quantum dots. The spin-polarized bi-exciton complex composed of twospin-polarized electrons and two spin-polarized holes confined in a single, parabolic,self-assembled quantum dot is discussed in detail. The configuration-interaction approach isused to compute the energies, wavefunctions, and emission spectra as a function of themagnetic field. It is shown that the spin-polarized bi-exciton emission spectrumdiffers from the unpolarized spectrum due to the Pauli blocking and exchangeinteractions. The emission from spin-polarized bi-exciton allows for the measurement ofground and excited single-exciton states, and its magnetic field dependence allowsfor the differentiation of the spin-polarized bi-exciton and tri-exciton spectrum.

MBE growth of In(Ga)As quantum dots for entangled light emission

Radiative biexciton decay in a single semiconductor quantum dot (QD) is a process by which entangled pairs of photons can be generated for quantum information applications. The observation of entangled light from a QD requires minimal splitting of exciton states and the ability to isolate the neutral biexciton and exciton photoluminescence (PL) emission of the individual dot. As a consequence, the growth of QDs for this purpose is subject to simultaneous constraints on areal dot density, dot emission energy, and wetting-layer (WL) emission energy. In this work we will describe modifications to the molecular beam epitaxial (MBE) growth of In(Ga)As QDs performed to address these requirements, for the realization of samples which generate entangled light of increasing quality.

Natural quantum dots in the InAs∕GaAs wetting layer

We report on microphotoluminescence study of excitons localized by potential fluctuations in a wetting layer (WL), which accompanies InAs∕GaAs quantum dots (QDs). Linear polarization of spectral lines due to localized excitons enable us to identify a neutral excitonic and biexcitonic emission. A charged exciton has also been identified. High resolution transmission electron microscopy measurements of the investigated structure reveal lateral fluctuations of In content in the WL, as well as its broadening. Both effects give rise to potential fluctuations, which can confine excitons observed in our measurements. The potential fluctuations can be regarded as “natural” QDs in the WL.

Biexciton kinetics in GaN quantum wells: Time-resolved and time-integrated photoluminescence measurements

We report on the experimental achievement of narrow ultraviolet excitonic and biexcitonic emission from $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ quantum wells. The very large biexciton binding energy (up to $12\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$) inhibits thermalization between the two species allowing to study the recombination kinetics of the biexciton gas. By performing resonant and nonresonant time resolved measurements, we elucidate the interplay between biexciton formation and exciton acoustic-phonon mediated relaxation within localized states.

Complex behavior of biexcitons in GaN quantum dots due to a giant built-in polarization field

In-depth optical spectroscopic studies of single polar $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ quantum dots (QDs) grown by molecular beam epitaxy are carried out by means of low-temperature microphotoluminescence. Luminescence linewidths as low as $700\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ are obtained allowing thorough characterization of the QD electronic properties. Biexciton emission is observed for a wide range of dot size. It is shown that the binding energy $({E}_{XX}^{b})$ exhibits two regimes. The main one is governed by the dot height through the quantum confined Stark effect leading to a variation of ${E}_{XX}^{b}$ from $+3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the smallest dots to $\ensuremath{-}11\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the largest ones. A secondary variation of opposite sign is demonstrated for dots having the same height but different lateral size.

Single-Photon Emission from a Single InAs Quantum Dot

Excitation power-dependent micro-photoluminescence spectra and photon-correlation measurement are used to study the optical properties and photon statistics of single InAs quantum dots. Exciton and biexciton emissions, whose photoluminescence intensities have linear and quadratic excitation power dependences, respectively, are identified. Under pulsed laser excitation, the zero time delay peak of second order correlation function corresponding to exciton emission is well suppressed, which is a clear evidence of single photon emission.

Photoluminescence from a single InGaAs epitaxial quantum rod

Microphotoluminescence (μ-PL) experiment has been performed on a structure with InGaAs/GaAs epitaxial quantum rods (quantum dots with the aspect ratio as high as 4.1) grown by depositing short-period InAs/GaAs superlattice by molecular beam epitaxy on GaAs substrate. The exciton and biexciton emission from a single quantum rod has been detected via the excitation power dependence of the μ-PL spectra. The origin of the single rod lines has been confirmed by a rate equation model. For a number of quantum rods within the investigated ensemble, the biexciton binding energy has been determined to be in the range of 1.0–2.2meV.

Photoluminescence of exciton and biexciton in GaN nanocolumns

We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology.

Wavelength tunable InAs/InP(1 0 0) quantum dots in 1.55-μm telecom devices

This paper reviews the growth, characterization and device applications of self-assembled InAs/InP(1 0 0) quantum dots (QDs) formed by MOVPE. The problematic As/P exchange reaction during QD growth is suppressed by the insertion of a GaAs interlayer together with optimum growth conditions. This produces QDs with continuously tunable emission over the 1.55-μm wavelength region for fiber-based telecom applications. Device quality of these QDs is proven by continuous wave lasing at room temperature from the as-cleaved facets of Fabry–Pérot narrow ridge-waveguide lasers implementing widely stacked QDs as gain medium. The low transparency current density of 6 A/cm 2 per QD layer and low loss of 4.2 cm −1 are accompanied by a 80-nm wide gain spectrum. The deeply etched QD lasers possess similar threshold current densities as the shallowly etched ones and do not deteriorate with time, revealing that device performance does not suffer from sidewall recombination. This allows the fabrication of mono-mode and more compact devices with small bending radii, as demonstrated by the operation of a QD ring laser with 40-GHz free spectral range. Unpolarized emission from the cleaved side, important for the realization of polarization insensitive semiconductor optical amplifiers, is obtained by close stacking of QDs due to vertical electronic coupling. Sharp exciton–biexciton emission from a single QD around 1.55 μm is observed with clearly resolvable peaks above 70 K, which is required for single photon sources working at liquid nitrogen temperature for fiber-based quantum cryptography systems.

Bipolariton laser emission from a GaAs microcavity

Biexciton emission properties were studied in a single GaAs quantum well semiconductor planar microcavity by photoluminescence measurements at low temperatures. At high pump intensity a bipolariton emission appears close to the lower polariton mode. This new mode appears when we detune the cavity resonance out of the lower polariton branch, showing a laser-like behavior. Very small linewidths were measured, lying below 110 μeV and 150 μeV for polariton and bipolariton emission respectively. The input/output power (I/O) measurements show that the bipolariton emission has a weaker coupling efficiency compared to previous results for polariton emission. Varying the pump laser polarization, we were able to show the selection rules for the biexciton particle creation in the quantum well. Simultaneous photoluminescence and near-field measurements show that the polariton and bipolariton emission are spectrally and spatially separated.

Photoluminescence of individual doped GaAs∕AlGaAs nanofabricated quantum dots

Dilute arrays of GaAs∕AlGaAs modulation-doped quantum dots with same sizes fabricated by electron beam lithography and low impact reactive ion etching exhibit highly uniform luminescence lines. Single quantum dots display spectral emission with peak energies and linewidths linked largely to the geometrical diameter of the dot and to the built-in electron population. Multicharged excitonic and biexcitonic emission intensities have activation energy of about 2meV. These results highlight the potential of high quality nanofabricated quantum dots for applications in areas that require fine control of optical emission.

Exciton and Biexciton Emissions from Single GaAs Quantum Dots in (Al,Ga)As Nanowires

We report the optical properties of GaAs quantum dots embedded in (Al,Ga)As nanowires grown by the vapor–liquid–solid method. We used the micro-photoluminescence (PL) technique to observe PL peaks, which are assigned as exciton and biexciton emissions from single quantum dots. In addition, unusual features appear in the excitation power dependence of the energies and linewidths of the two PL peaks. The PL also depends on the optical polarization axis, indicating that the nanostructures have a highly asymmetrical shape. The results show that our method is a promising way of engineering the positions and optical properties of GaAs/(Al,Ga)As nanostructures.

Multiexcitons in type-II colloidal semiconductor quantum dots

The spectroscopy and dynamics of multiple excitations on colloidal type-II $\mathrm{Cd}\mathrm{Te}∕\mathrm{Cd}\mathrm{Se}$ core-shell quantum dots (QDs) are explored via quasi-cw multiexciton spectroscopy. The charge separation induced by the band offset redshifts the exciton emission and increases the radiative lifetime. In addition, we observe a significant modification of multiexciton properties compared with core-only or type-I QDs. In particular, the Auger recombination lifetimes are significantly increased, up to a nanosecond time scale. While in type-I QDs the Auger lifetime scales with the volume, we find for type-II QDs a scaling law that introduces a linear dependence also on the radiative lifetime. We observe a blueshift of the biexciton emission and extract biexciton repulsion of up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in type-II QDs. This is assigned to the dominance of the Coulomb repulsion as the positive and negative charges become spatially separated, which overwhelms the correlation binding term. Higher electronic excited states can remain type I even when the lowest transition is already type II, resulting in a different size dependence of the triexciton emission. Finally, we discuss the possibilities of ``multiexciton band gap engineering'' using colloidal type-II QDs.

Enhanced Zn–Cd interdiffusion and biexciton formation in self-assembled CdZnSe quantum dots in thermally annealed small mesas

By controlled annealing of small ZnSe mesas with embedded CdZnSe quantum dots (QDs) at considerably low temperatures, significant changes of the QD luminescence have been observed. To investigate the spectral evolution of single exciton lines several thermal annealing steps were performed successively, and large energy shifts of single exciton (X) lines due to annealing were traced. In annealed QDs the biexciton (XX) emission is drastically enhanced. Biexciton binding energies less than 10meV were recorded for the thermally annealed CdZnSe QDs, indicating a considerable change in the QD confinement. The pronounced energy shifts of the QD luminescence is attributed to the Cd–Zn interdiffusion between the CdZnSe QDs and the surrounding ZnSe matrix. In small mesas interdiffusion activation energy (EA) of less than 1eV was determined. This value of EA is half of that recorded for the Cd–Zn interdiffusion in large QD ensembles, indicating that the sidewalls of the etched mesas play an important role in the observed diffusion process.

Size‐dependence of anisotropic exchange interaction in InAs/GaAs quantum dots

AbstractA comprehensive study of the exchange interaction between charge carriers in self‐organized InAs/GaAs quantum dots is presented. Single quantum‐dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to the energetic structure of the charged excited exciton (hot trion). A varying degree of intermixing within the hot trion states leads to varying degrees of polarization of the corresponding emission lines. The emission characteristics change from circularly polarized for small quantum dots to elliptically polarized for large quantum dots. The findings are explained by a change of magnitude of the anisotropic exchange interaction and compared to the related effect of fine‐structure splitting in the neutral exciton and biexciton emission. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton fine structure and biexciton binding energy in single self-assembled InAs∕AlAs quantum dots

The exciton and biexciton emissions of a series of single quantum dots of InAs in an AlAs matrix have been studied. These emissions consist of linear cross polarized doublets showing large values of both the biexciton binding energy and the fine-structure splitting. At increasing exciton emission energy, corresponding to decreasing dot size, the biexciton binding energy of 9meV decreases down to zero, reflecting a possible crossover to an antibinding regime. Simultaneously the fine-structure splitting diminishes from a value of 0.3meV down to zero, at the same energy, suggesting a common origin for the two effects.

Microphotoluminescence of exciton and biexciton around 1.5μm from a single InAs∕InP(001) quantum dot

We report on the fabrication by low-pressure metalorganic vapor phase epitaxy of InAs∕InP(001) quantum dots (QDs) emitting around 1.5μm, and on the observation of microphotoluminescence (μPL) from a single QD in this wavelength range. The QDs are diamond shaped, with a density of 6.3∙109cm−2. μPL experiments were carried out on a QD sample covered with a gold mask containing apertures (diameter of 200nm). Well defined peaks corresponding to the emission of single QDs were recorded between 1.24 and 1.6μm. The analysis of the pump power dependence of their intensity allows us to discriminate between exciton and biexciton emission lines.

Temporal behavior of biexciton luminescence under two-photon excitation in CuCl quantum dots

Temporal behavior of luminescence of biexcitons under two-photon resonant excitation has been measured in CuCl quantum dots. The non-exponential decay profile was observed, and its fast decay component becomes dominant with shortening the decay time for higher pump intensity. This characteristic feature of the biexciton emission is observable only for the two-photon resonant excitation with realization of population inversion in biexciton–exciton system at the initial stage and indicates the possibility of Dicke's superradiance, which is originated from the coherent interaction of diploes among many quantum dots.

Fine structure and magneto-optics of exciton, trion, and charged biexciton states in single InAs quantum dots emitting at1.3μm

We present a detailed investigation into the optical characteristics of individual InAs quantum dots (QDs) grown by metalorganic chemical vapor deposition, with low temperature emission in the telecoms window around 1300 nm. Using micro-photoluminescence (PL) spectroscopy we have identified neutral, positively charged, and negatively charged exciton and biexciton states. Temperature-dependent measurements reveal dot-charging effects due to differences in carrier diffusivity. We observe a pronounced linearly polarized splitting of the neutral exciton and biexciton lines (~250 ueV) resulting from asymmetry in the QD structure. This asymmetry also causes a mixing of the excited trion states which is manifested in the fine structure and polarization of the charged biexciton emission; from this data we obtain values for the ratio between the anisotropic and isotropic electron-hole exchange energies of (Delta1)/(Delta0)= 0.2--0.5. Magneto-PL spectroscopy has been used to investigate the diamagnetic response and Zeeman splitting of the various exciton complexes. We find a significant variation in g-factor between the exciton, the positive biexciton, and the negative biexciton; this is also attributed to anisotropy effects and the difference in lateral extent of the electron and hole wavefunctions.

Intrinsic exciton transitions in high-quality ZnO thin films grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates

High-quality ZnO thin films have been grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates. Free-exciton absorption and exciton-LO phonon absorption peaks are observed in the films at room temperature, indicating that the exciton states are stable even at room temperature. Three excitonic transitions associated with valence bands A, B, and C are clearly revealed in the reflectance spectrum measured at low temperatures. This result indicates that the ZnO thin films have a perfect wurtzite crystal structure. Biexciton emission is observed in the photoluminescence spectra at low temperatures, from which the biexciton binding energy is estimated to be 14.5meV, in good agreement with previous results. Exciton-LO (Ex-LO) and exciton-2LO (Ex-2LO) photon emission peaks are observed at low temperature. The energy difference between the Ex-LO and Ex-2LO bands is about 72.5meV, which coincides with previously reported values of the LO phonon energy for ZnO thin films.