Ground State Photoluminescence Research Articles

Strain relaxation in InAs∕InGaAs quantum dots investigated by photoluminescence and capacitance-voltage profiling

We present detailed studies of the onset of strain relaxation in InAs∕InGaAs quantum dots. We show that the ground-state photoluminescence (PL) emission redshifts with increasing the InAs coverage before relaxation and blueshifts when relaxation occurs. PL spectra of the relaxed samples show two predominant families of dots with very different temperature-dependent efficiency. By comparison we show that the dots emitting at long wavelength are degraded by relaxation while the dots emitting at short wavelength remain coherently strained. Consequently, the PL spectra are dominated by the dots emitting at short wavelength, leading to the observed blueshift. This result suggests that the relaxation does not occur uniformly. In addition, we show that the relaxation occurs in the dot bottom interface.

Optical spin polarization and exchange interaction in doubly charged InAs self-assembled quantum dots

The work is an experimental study of optical spin polarization in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) with two resident electrons or holes. A capture of a photogenerated electron-hole pair into such a QD creates a negative or positive tetron (doubly charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to be radically different in the dots with the opposite sign of the charge. Particularly, under excitation in a GaAs barrier, the polarization of the ground-state PL is negative (relative to the polarization of exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry; however, it is suppressed in strong fields. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results.

Nanosecond scale carrier dynamics of self-assembled InAs/AlAs quantum dots studied by time-resolved photoluminescence

Photo-excited carrier dynamics in the self-assembled InAs/AlAs quantum dot system are studied by time-resolved photoluminescence within the first two nanoseconds. Several unexpected energy shifts are observed, i.e., an initial redshift of ground state photoluminescence which is followed by a blueshift after ∼1 ns, and a continuous blueshift of the first excited states emission. Moreover, steady-state photoluminescence shows a continuous redshift of the ground state peak with increasing excitation intensity. Our data support the idea that for the carrier dynamics in the InAs/AlAs quantum dot system the existence of AlAs X-states, tunneling and scattering play an important role in the lateral carrier transfer in such a dense quantum dot system.

Optical spin polarization in double charged InAs self‐assembled quantum dots

AbstractThe work is an experimental study of optical spin polarization in InAs/GaAs quantum dots (QDs) with 2 resident electrons or holes. A capture of a photo‐generated electron‐hole pair into such a QD creates a negative or positive tetron (double‐charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to differ radically in the dots with opposite in sign charge. Particularly, under excitation in a GaAs barrier, the ground state PL polarization is negative (relative to the polarization of an exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry, but strong fields suppress it. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the photoluminescence emission from InAs quantum dots in a strained Ga0.85In0.15As quantum well

Photoluminescence from InAs quantum dots in a strained Ga0.85In0.15As quantum well is investigated over a temperature range from 10 to 300 K using low intensity optical excitation. A rate equation model for the carrier dynamics is fitted to the experimental data obtained for the integrated intensity of the photoluminescence at different temperatures. It is found necessary to assume a potential barrier, possibly arising from the strain, at the interface between the dots and the quantum well that makes the carrier capture in dots less effective at low temperatures. In addition, two thermal escape mechanisms for carriers in the dots are identified that have the onsets at 110 K and 220 K, respectively. At low temperatures, the ground state photoluminescence has a large full width at half maximum, while at temperatures above 220 K, the full width decreases as emission from larger dots dominate. The activation energies for different carrier thermal escape channels are estimated using the solutions of the steady-state rate equation system.

Formation trends in quantum dot growth using metalorganic chemical vapor deposition

We discuss the results of a growth matrix designed to produce high quantum dot (QD) density, defect-free QD ensembles, which emit at 1.3 μm using metalorganic chemical vapor deposition (MOCVD). In our study, we balance the nucleation rate and adatom surface migration to achieve high surface densities (1×1011 dots/cm2) and avoid QD coalescence or defects that commonly characterize MOCVD-grown QD ensembles designed for longer wavelength emission. Room-temperature photoluminescence (PL) spectra from corresponding surface QDs depend on QD size and density and show an emission wavelength up to 1600 nm. Ground-state PL from capped QDs is measured at 1.38 μm with a 40 meV linewidth. We demonstrate the ground-state 1.3 μm electroluminescence from a QD light-emitting diode structure grown on n-type GaAs.

Strong carrier confinement and evidence for excited states in self-assembled InAs quantum islands grown on InP(001)

Photoluminescence (PL) and photoluminescence excitation (PLE) experiments are carried out in order to study the electronics states in self-assembled InAs quantum islands (QI's) fabricated on the InP(001) substrate by solid source molecular-beam epitaxy using the Stranski-Krastanov growth mode. The growth conditions of the InAs QI's have been optimized (high growth temperature, low arsenic pressure and growth rate) in order to minimize the island size dispersion. The low-temperature (8 K) PL spectrum of the InAs/InP(001) QI's is characterized by multicomponent transitions. From PLE spectroscopy, we have evidenced excited states associated with a single ground state in agreement with a unimodal size distribution. The full width at half maximum of the ground state PL peak is only 22 meV at 8 K, which reveals a narrow island size dispersion. The integrated PL intensities measured as a function of the temperature have shown a weak intensity decrease between 8 and 300 K. Intensity remains very strong at room temperature, as much as 49% of that at 8 K under nonresonant excitation and up to 90% under quasiresonant excitation. These results are indicating of a strong confinement and/or spatial localization of the carriers in such InAs/InP(001) QI's.

Interdot energy transfer in a system of coupled InAs/GaAs quantum dots

We present new results concerning the carrier transfer between quantum dots (QDs) in dense InAs/GaAs QD arrays with a bimodal size distribution explored by means of steady-state and time-resolved photoluminescence (PL). Interdot carrier transfer involves transitions from the ground state of small QDs into lower lying states of larger QDs, a relaxation channel that saturates at high excitation densities. A staircase-like spectral dependence of the PL decay time is observed and analyzed by taking into account the bimodal QD size distribution. We show that monitoring the spectral behavior of the ground-state PL time constants offers a new and independent tool for distinguishing between PL contributions from different dot size families.

Abnormal temperature behavior of photoluminescence from self-assembled InAs/AlAs quantum dots

We report on the temperature dependence of photoluminescence (PL) from self-assembled InAs/AlAs quantum dots (QDs). In the temperature range of 6–90 K, an abnormal blueshift of the first excited-state emission and an enhancement of the ground-state PL are observed. This is explained by carrier transfer within spatially coupled QDs with a reduced barrier between, which give rise to a small activation energy of about 2 meV. Based on the analysis of the PL intensities, the rapid redshift of the ground- and excited-state emissions with respect to the InAs band gap in the temperature range of 90–283 K is explained by stepwise carrier escape from the QDs via the excited states.

Field dependent carrier dynamics and charged excitons in InAs self-assembled quantum dots

We have used photocapacitance, photocurrent and photoluminescence spectroscopy to study the field-dependent dynamics and radiative recombination of optically excited electron–hole pairs in InAs self-assembled quantum dots (QDs) under various charging conditions. In the investigated field region the excitons relax into the ground state before tunneling or radiative recombination takes place. At high internal fields electrons and holes tunnel out of the QDs while at low fields the excitons recombine in the dot. In the low-field regime a red shift of the QD ground state photoluminescence is observed when the QDs are loaded with electrons. A decrease in the overall intensity indicates a lower oscillator strength of charged excitons in InAs self-assembled QDs. Field effects being responsible for the observed energy shift can be excluded since the energetic position of the ground-state transition remains constant at high fields.

Self-organized quantum dots and quantum dot lasers (invited)

The growth and characterization of two types of self-organized quantum dots, Stranski-Krastanow (S-K) mode grown quantum dots and atomic layer epitaxy (ALE) grown quantum dots, are described. In the S-K mode growth, molecular beam epitaxy has been used. Multilayer stacked structures have been fabricated. S-K mode grown quantum dots showed the ground state photoluminescence (PL) emission wavelength of 1.14 μm and the full width at half maximum of the emission linewidth of 80 meV. ALE mode grown quantum dots are fabricated using metalorganic chemical vapor deposition. A smaller PL linewidth of 40 meV, indicating a higher uniformity in the size and composition of quantum dots, and a room temperature emission at 1.3 μm were obtained. Continuous wave lasing at the ground state at room temperature was achieved with a S-K mode grown quantum dot laser owing to the high dot density enabled by the stacked dot layer structure. Because of the low density of dots, lasing was at 80 K at the higher order sublevel in ALE grown quantum dot lasers. Based on these experimental results, we discuss the key issues involved in these fabrication technologies and the carrier dynamics to achieve the ideal performance for quantum dot lasers.