Dot Ground State Research Articles

Characterization of structure and defects in dot-in-well laser structures

Recent progress in the development of 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, a consequence of the finite dot density, and to carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of using thin InAlAs capping layers and high temperature buffer layers. Both effects are observed to strongly modify the structure of the QDs producing significant improvements in the InAs QDs optical properties at room temperature. Initial attempts at multilayer QD structures showed substantial degradation in optical and electrical properties compared to single layer structures. Analysis by Transmission Electron Microscopy (TEM) has identified the presence of defects arising from the complex interaction of QDs, which propagate through the QD layers into the upper regions of the structure as being the primary cause of the poor electronic device characteristics. The use of high growth temperature spacing layers (HGTSLs) have recently allowed us the fabrication of a defect free five layer-stacked structure with record low threshold current density.

Optical properties of 1.3 μm room temperature emitting InAs quantum dots covered by In0.4Ga0.6As/GaAs hetero-capping layer

Room temperature 1.3 μm emitting InAs quantum dots (QDs) covered by an In0.4Ga0.6As/GaAs strain reducing layer (SRL) have been fabricated by solid source molecular beam epitaxy (SSMBE) using the Stranski–Krastanov growth mode. The sample used has been investigated by temperature and excitation power dependent photoluminescence (PL), photoluminescence excitation (PLE), and time resolved photoluminescence (TRPL) experiments. Three emission peaks are apparent in the low temperature PL spectrum. We have found, through PLE measurement, a single quantum dot ground state and the corresponding first excited state with relatively large energy spacing. This attribute has been confirmed by TRPL measurements which allow comparison of the dynamics of the ground state with that of the excited states. Optical transitions related to the InGaAs quantum well have been also identified. Over the whole temperature range, the PL intensity is found to exhibit an anomalous increase with increasing temperatures up to 100 K and then followed by a drop by three orders of magnitude. Carrier’s activation energy out of the quantum dots is found to be close to the energy difference between each two subsequent transition energies.

Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field

We report magnetic field spectroscopy measurements in carbon nanotube quantum dots exhibiting fourfold shell structure in the energy level spectrum. The magnetic field induces a large splitting between the two orbital states of each shell, demonstrating their opposite magnetic moment and determining transitions in the spin and orbital configuration of the quantum dot ground state. We use inelastic cotunneling spectroscopy to accurately resolve the spin and orbital contributions to the magnetic moment. A small coupling is found between orbitals with opposite magnetic moment leading to anticrossing behavior at zero field.

Exciton dephasing via phonon interactions in InAs quantum dots: Dependence on quantum confinement

We report systematic measurements of the dephasing of the excitonic ground-state transition in a series of InGaAs?GaAs quantum dots having different quantum confinement potentials. Using a highly sensitive four-wave mixing technique, we measure the polarization decay in the temperature range from 5 to 120 K on nine samples having the energy distance from the dot ground-state transition to the wetting layer continuum (confinement energy) tuned from 332 to 69 meV by thermal annealing. The width and the weight of the zero-phonon line in the homogeneous line shape are inferred from the measured polarization decay and are discussed within the framework of recent theoretical models of the exciton-acoustic phonon interaction in quantum dots. The weight of the zero-phonon line is found to decrease with increasing lattice temperature and confinement energy, consistently with theoretical predictions by the independent Boson model. The temperature-dependent width of the zero-phonon line is well reproduced by a thermally activated behavior having two constant activation energies of 6 and 28 meV, independent of confinement energy. Only the coefficient to the 6-meV activation energy shows a systematic increase with increasing confinement energy. These findings rule out that the process of one-phonon absorption from the excitonic ground state into higher energy states is the underlying dephasing mechanism.

Quantum dot superluminescent diodes emitting at 1.3 /spl mu/m

We report ridge-waveguide superluminescent diodes based on five stacks of self-assembled InAs-GaAs quantum dots. Devices with output powers up to 10 mW emitting around 1.3 /spl mu/m are demonstrated. Spectral analysis shows a broad emission peak (26-nm full-width at half-maximum) from the dot ground state at low injection, and an additional peak from the excited state at higher bias. Temperature characteristics in the range 10/spl deg/C-80/spl deg/C are also reported. The experimental curves are in good agreement with simulations performed using a traveling-wave rate equation model.

Photomodulated reflectance studies of quantum dot in MCLED structures: monitoring cavity-ground state exciton resonance

Self-assembled quantum dots emitting around 1.3μm and embedded in a microcavity structure are analysed by photo-reflectance (PR) measurements performed at different temperatures. The temperature dependence of the PR spectra line-shape and amplitude allows us to determine the tuning condition of the quantum dot ground state transition with the cavity mode. We found that in our structure the perfect tuning is obtained around 250K.

Ultrafast electron capture into p-modulation-doped quantum dots

Electron and hole relaxation kinetics are studied in modulation-doped InAs quantum dots using femtosecond time-resolved photoluminescence experiments. We demonstrate that, as a result of doping, carrier relaxation from the barrier layers to the quantum dot ground states is strongly enhanced due to rapid electron–hole scattering involving the built-in carrier population. Results for p-doped quantum dots reveal a threefold decrease in the room-temperature electron relaxation time relative to corresponding undoped quantum dots. Our findings are promising for the development of high-speed, GaAs-based quantum dot lasers with modulation speeds in excess of 30GHz.

Wavelength-stabilized tilted cavity quantum dot laser

Wavelength-stabilized operation is realized in broad area quantum dot laser diodes with cleaved facets grown in the tilted cavity laser (TCL) design for the range up to 1.3 µm. For two different designs TCL wavelength is chosen to be either in the range of quantum dot excited state transitions (1.16 µm), or in the range of quantum dot ground state transitions (1.27 µm). The shift of the lasing wavelength is 0.165 nm K−1 in a temperature range between −200 °C and 70 °C. The spectral width of the lasing emission in broad area devices is 0.6 nm and the width (FWHM) of the far field is 4° and 42° for lateral and vertical directions, respectively.

Exchange-correlation energy densities for two-dimensional systems from quantum dot ground states

In this paper we present how to extract polarization-dependent exchange-correlation energy densities for two-dimensional systems from reference densities and energies of quantum dots provided by an exact diagonalization. Compared with results from the literature, we find systematic corrections for all polarizations in the regime of high densities.

Intraband relaxation of photoexcited electrons in GaAs/AlGaAs quantum wells and InAs/GaAs self-assembled quantum dots

We present time-resolved measurements of photoinduced intraband absorption in undoped GaAs/AlGaAs quantum well and InAs/GaAs self-assembled quantum dot structures after interband excitation. By using ultrabroadband mid-infrared pulses we determine the temporal evolution of the intersubband absorption spectrum of a double quantum well sample (subband spacing < longitudinal optical phonon energy) after excitation. From these measurements we determine an intersubband relaxation time of 14 ps at T = 5 K. By tuning mid-infrared pulses into resonance with intraband transitions between confined quantum dot states and the wetting layer continuum, the electron population of the quantum dot ground and first excited states is determined as a function of time-delay after the pump. We find that the most efficient relaxation pathway into the quantum dot ground state is the step-wise relaxation through the excited states of the dot. The capture time at T = 5 K is 4.7 ps.

Optically induced intraband electron transfer in self‐assembled InAs quantum dots

In our experiments we superimpose a near-infrared excitation laser field with a mid infrared field on an InAs quantum dot ensemble or on a single quantum dot. In the latter case a cw infrared laser excitation and a glow-bar mid infrared field are superimposed in a confocal micro-photoluminescence configuration on a single quantum dot. By comparing the recombination spectra for the superimposed spectra with the spectra from the laser excitation we detect changes in the count rates for the different excitonic states. The decrease of the luminescence intensity in the lower states together with an increasing intensity in higher excited states suggests a direct electron transfer between these states due to absorption of mid infrared light. For the quantum dot ensembles we use interband pump – intraband probe-time domain spectroscopy to study the electron capture and relaxation dynamics within the ensemble. By tuning the femtosecond infrared pulses into resonance with transitions between confined quantum dot states and the wetting layer continuum we gain knowledge about the electron population in the quantum dots ground state and first excited states as a function of delay time between pump and probe pulse. Our experiments indicate the stepwise relaxation through the excited dot states to be the most efficient relaxation pathway into the dot ground state on a timescale between 1.5 ps and 4.7 ps depending on temperature and excitation density. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier dynamics and high-speed modulation properties of tunnel injection InGaAs-GaAs quantum-dot lasers

We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In/sub 0.4/Ga/sub 0.6/As-GaAs-AlGaAs heterostructures, which show that at room temperature, injected electrons preferentially occupy the excited states in the dots and states in the barriers layers. The relaxation time of these carriers to the dot ground state is >100 ps. This leads to large gain compression in quantum-dot (QD) lasers and limits the attainable small-signal modulation bandwidth to /spl sim/ 5-7 GHz. The problem can be alleviated by tunneling "cold" electrons into the lasing states of the dots from an adjoining injector layer. The design, growth, and steady-state and small-signal modulation characteristics of tunnel injection In/sub 0.4/Ga/sub 0.6/As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by three-pulse DTS measurements, are /spl sim/ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/I/sub th/ /spl sim/ 7 is f/sub -3 dB/ = 23 GHz and the gain compression factor for this frequency response is /spl epsiv/ = 8.2 /spl times/ 10/sup -16/ cm/sup 3/. The differential gain obtained from the modulation data is dg/dn /spl cong/ 2.7 /spl times/ 10/sup -14/ cm/sup 2/ at room temperature. The value of the K-factor is 0.205 ns and the maximum intrinsic modulation bandwidth is 43.3 GHz. Analysis of the transient characteristics with appropriate carrier and photon rate equations yield modulation response characteristics identical to the measured ones. The Auger coefficients are in the range /spl sim/ 3.3 /spl times/ 10/sup -29/ cm/sup 6//s to 3.8 /spl times/ 10/sup -29/ cm/sup 6//s in the temperature range 15/spl deg/C<T<85/spl deg/C, determined from large-signal modulation measurements, and these values are smaller than those measured in separate confinement heterostructure QD lasers. The measured high-speed data are comparable to, or better than, equivalent quantum-well lasers for the first time.

Linewidth enhancement factor and chirp in quantum dot lasers

We have made a comparative study of the linewidth enhancement factor (LEF) and chirp in quantum dot (QDL’s) and quantum well lasers (QWL’s). The simulations are based on the quasiequilibrium approximation and on semiempirical transition energies and amplitudes of InGaAs quantum pyramid structures. We have accounted for the carriers confined in the active material as well as for the carriers in all the other material layers. It is found that in the quasiequilibrium approximation inhomogeneous broadening leads to asymmetric population of the quantum dot ground state. If the QDL is operated at the gain maximum, the asymmetry leads to nonzero chirp even for a single bound resonance state located at a large distance from other resonances. Our calculations show that, by detuning the laser emission to ∼15 nm shorter wavelengths with a frequency selective cavity and by tailoring the resonance energies and inhomogeneous broadening, the LEF and chirp of a QDL can be made very small. This detuning does not add a substantial penalty to the efficiency of the laser. For QWL’s, a similar reduction of chirp is generally not feasible due to the fundamentally different density of states. Therefore QDL’s have an important advantage over QWL’s as directly modulated light sources in applications where the stability of the emission wavelength is critical.

Shot noise in tunneling through a quantum dot array

The shot noise suppression in a sample containing a layer of self-assembled InAs quantum dots has been investigated experimentally and theoretically. The observation of a non-monotonic dependence of the Fano factor on the bias voltage in a regime where only few quantum dot ground states contribute to the tunneling current is analyzed by a master equation model. Under the assumption of tunneling through states without Coulomb interaction this behaviour can be qualitatively reproduced by an analytical expression.

Improved performance of MBE grown quantum-dot lasers with asymmetric dots in a well design emitting near 1.3 μm

To improve the performance of InAs/GaInAs quantum-dot lasers emitting near 1.3 μm, the influence of growth parameters and of the dot layer design were investigated. As starting point a symmetrically designed “dots in a well” (DWELL) structure was used. The thickness of the Ga 0.85In 0.15As layer grown before the InAs dot layer formation turned out to be one of the key parameters to control the dot morphology. The best optical properties were achieved by an asymmetric quantum well design with a 1 nm thick GaInAs layer below and 5 nm above the InAs dot layer. With this design the linewidth of the dot ground state transition could be reduced to 30 meV and the level separation could be increased to 75 meV. In comparison to lasers with symmetric DWELLs, the threshold current could be reduced by 50%, and an increased T 0 value of 130 K up to 40°C could be obtained.

Spin Dynamics in CdSe/ZnSe Quantum Dots: Resonant Versus Nonresonant Excitation

We have studied the exciton spin dynamics in CdSe/ZnSe quantum dots, comparing strictly resonant and nonresonant excitation. In case of strictly resonant excitation, excitons are generated in the quantum dot ground state, and because of the zero-dimensionality of the system no transient shift of the photoluminescence signal can be seen. No loss of the spin information is observed within the time window under investigation, if one excites the quantum dot eigenstates. Interestingly, even in case of nonresonant excitation, a high, time-independent polarization degree is obtained. We found maxima in the absolute value of the polarization degree if the laser excess energy amounts to a multiple of LO-phonon energies.

Photoluminescence study of strain-induced GaInNAs/GaAs quantum dots

Strain-induced Ga 0.8 In 0.2 N y As 1-y quantum dots on GaAs were fabricated by metal organic vapor phase epitaxy. Nitrogen concentration ywas varied between 0% and 1.3%. The effect of nitrogen concentration on the optical properties of the quantum dots was investigated by continuous-wave and time-resolved photoluminescence measurements. Carrier localization in the states below the band edge of the nitrogen-containing quantum well has been observed. These states are thought to originate from the variation of the quantum well width or from the fluctuation of the composition. Such variations have been identified in GaInNAs quantum wells on GaAs without stressor islands. The measured energy difference of the quantum well and quantum dot ground-state peak energies increase with increasing temperature.

Stability of spin states in quantum dots

We have investigated electronic transport through a Coulomb-blockaded quantum dot in which interactions are strong. Linear changes in conductance peak spacings with in-plane magnetic field are observed and interpreted in terms of Zeeman splitting of single-particle levels. Thereby, the measurements allow tracking changes in the dot's ground-state spin as the dot is gradually opened to the leads and the electron number is changed. Spin states have been identified in the weak- $(kTg\ensuremath{\Gamma}),$ intermediate- $(\ensuremath{\Gamma}\ensuremath{\approx}kT),$ and strong- $(\ensuremath{\Gamma}gkT)$ coupling regime. It is found that ground states with spin $S=0$ or $S=1/2$ are most likely, while larger total spins $Sg~1$ can occasionally occur, despite the large number of 50--100 electrons. A g factor close to the bare bulk GaAs value has been determined experimentally for the majority of the spin states. A perpendicular magnetic field applied to the dot in the same state allows the investigation of spin-pair candidates under conditions where orbital effects dominate the evolution of conductance peaks. Strong correlations in the position and in the amplitude of neighboring peaks allow the final identification of spin pairs. The method of combining parallel and perpendicular magnetic fields for identifying spin states and spin-pairs works well for intermediate and strong coupling of dot states to the leads while the data in the weak-coupling regime is less conclusive. Our results indicate that the spin degree of freedom is remarkably stable and the spin states are well described within a single-particle picture.

Nano-Optics on Individual Quantum Objects - From Single to Coupled Semiconductor Quantum Dots

Some recent highlights of our optical studies on single and coupled semiconductor quantum dots are reviewed. In the first part, we concentrate on the role of spins and spin-spin interaction in nonmagnetic and magnetic single quantum dots. In the case of strictly resonant excitation of the ground state in self-assembled CdSe/ZnSe quantum dots, we find an exciton spin relaxation time, which exceeds the recombination lifetime significantly. Linear polarization has to be used for these experiments, as the electron-hole exchange interaction lifts the spin degeneracy and the eigenstates are linear combinations of spin-up and spin-down excitons. In a magnetic quantum dot, the exchange interaction between carrier spins and the spins of magnetic ions is shown to be responsible for giant magneto-optical effects. We demonstrate the formation of zero-dimensional magnetic polarons and we succeeded in measuring the magnetization on a scale of a few nanometers using the characteristic photoluminescence signal of individual quantum dots as experimental monitor. The second part is devoted to pairs of single quahtum dots. On one hand, single exciton tunneling within an individual quantum dot pair is demonstrated studying single pairs of vertically correlated strain-induced and self-organized quantum dots. On the other hand, we show that in a pair of lithographically defined single dots with strongly different g-factors the energy spacing between the dot ground states can be tuned in an external magnetic field by about 10 meV, giving access to a controlled coupling between two individual quantum dots.

Exciton relaxation and dephasing in quantum-dot amplifiers from room to cryogenic temperature

We present an extensive experimental study of the exciton relaxation and dephasing in InGaAs quantum dots (QDs) in the temperature range from 10 K to 295 K. The QDs are embedded in the active region of an electrically pumped semiconductor optical amplifier. Ultrafast four-wave mixing and differential transmission spectroscopy on the dot ground-state transition are performed with a sensitive heterodyne detection technique. The importance of the population relaxation dynamics to the dephasing is determined as a function of injection current and temperature. Above 150 K dephasing processes much faster than the population relaxation are present, due to both carrier-phonon scattering and Coulomb interaction with the injected carriers. Only at low temperatures (<30 K) does population relaxation of multiexcitons in the gain regime fully determine the dephasing.