Intersublevel Spacing Research Articles

In-situheight engineering of InGaAs / GaAs quantum dots by chemical beam epitaxy

This work reports on a chemical beam epitaxy growth study of InGaAs/GaAs quantum dots (QDs) engineered using an in-situ indium-flush technique. The emission energy of these structures has been selectively tuned over 225 meV by varying the dot height from 7 to 2 nm. A blueshift of the photoluminescence (PL) emission peak and a decrease of the intersublevel spacing energy are observed when the dot height is reduced. Numerical investigations of the influence of dot structural parameters on their electronic structure have been carried out by solving the single-particle one-band effective mass Schrodinger equation in cylindrical coordinates, for lens-shaped QDs. The correlation between numerical calculations and PL results is used to better describe the influence of the In-flush technique on both the dot height and the dot composition.

Persistence of In/Ga intermixing beyond the emission energy blueshift saturation of proton-implanted InAs/GaAs quantum dots

Low temperature photoluminescence (PL) measurements are carried out to investigate the influence of the high extent of intermixing induced by proton implantation and subsequent annealing on the optical and electronic properties of the InAs/GaAs quantum dots (QDs). Several QDs structures were proton implanted at various doses (5×1011–1×1015 ions cm−2) with an acceleration energy of 18 keV and then annealed at 700 °C for 30 s. A saturation of the emission energy blueshift is found to occur for implantation doses higher than 5×1013 ions cm−2 accompanied with a continual decrease in the intersublevel spacing energy suggesting that the intermixing process persists beyond the emission energy blueshift saturation. An additional emission peak was found to appear in PL spectra for proton doses higher than 1×1014 ions cm−2 and attributed to the coalescence of closely spaced QDs. Strain assisted predominant lateral intermixing is proposed as the main factor responsible for the observed behavior.

Intermixing of InAs/GaAs quantum dots by proton implantation and rapid thermal annealing

Low-temperature photoluminescence (PL) experiments have been carried out to investigate In/Ga intermixing in InAs/GaAs self-assembled quantum dots (QDs) by studying the changes in the optical properties of the system by rapid thermal annealing (RTA) and by room temperature proton implantation at various doses followed by RTA. The study of the RTA effect on the investigated structure shows that thermal stability can be ensured for an annealing temperature below 675 °C. For higher annealing temperatures, the thermal intermixing is found to change both the optical transition energy and the inter-sublevel spacing of the QD energy levels. By using proton implantation at various doses and subsequent annealing at 675 °C for 30 s, a tunable energy shift up to 130 meV has been obtained. The band gap tuning limit for this system has been achieved for an implantation dose of 5×10 13 cm −2. Regardless of the intermixing technique employed, a pronounced PL peak broadening is found to occur at low annealing temperatures and/or proton implantation doses.

Tunable interband and intersubband transitions in modulation C-doped InGaAs∕GaAs quantum dot lasers by postgrowth annealing process

A bandgap and intersublevel spacing tuned laser has been realized by using a modulation C-doped InGaAs∕GaAs quantum dot structure, which utilizes a postgrowth annealing process. The intermixed laser exhibits comparable light-current characteristics, indicating little detrimental change to the quantum dot laser material, and show a ground state bandgap blueshift of ∼13nm and intersublevel energy spacing reduction of ∼30nm compared to the unannealed device. The differences in the samples during annealing are attributed to the suppression of Ga vacancy propagation for samples with modulation C doping.

Direct Observation of Polarons in Electron Populated Quantum Dots by Resonant Raman Scattering

The general problem of the pairing of strongly interacting elementary excitations producing new quasiparticles such as polarons arises in many areas of solid state physics. Recent interest in polaron formation in semiconductor quantum dots has been motivated by the need to understand the physical nature of the carrier relaxation processes and their role in quantum-dot based devices. We report on the direct observation of polarons in InAs/GaAs self-assembled quantum dots populated by few electrons where the polarons are strongly coupled modes of quantum dot phonons and electron intersublevel transitions. The degree of coupling is varied in a systematic way in a set of samples having electron intersublevel spacing changing from larger to smaller than the longitudinal optical phonon energy. The signature of polarons is evidenced clearly by the observation of a large (12-20 meV) anticrossing for both InAs and GaAs-like quantum dot phonons using resonant Raman spectroscopy.

Thermal-induced intermixing effects on the optical properties of long wavelength low density InAs/GaAs quantum dots

The effect of post-growth rapid thermal annealing on the photoluminescence properties of long wavelength low density InAs/GaAs (001) quantum dots (QDs) with well defined electronic shells has been investigated. For an annealing temperature of 650 °C for 30 s, the emission wavelength and the intersublevel spacing energies remain unchanged while the integrated PL intensity increases. For higher annealing temperature, blue shift of the emission energy together with a decrease in the intersublevel spacing energies are shown to occur due to the thermal activated In–Ga interdiffusion. While, this behaviour is commonly explained as a consequence of the enrichment in Ga of the QDs, the appearance of an additional exited state for annealing temperatures higher than 650 °C suggests a variation of the intermixed QDs's volume/diameter ratio toward QDs's enlargement.

Direct monitoring of the excited state population in biased SiGe valence band quantum wells by femtosecond resolved photocurrent experiments

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9μm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160meV, a value of 550fs was obtained.

Polarons in electron-populated quantum dots revealed by resonant Raman scattering

We report on the direct observation of polarons in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ self-assembled quantum dots (QD) populated by few electrons. The polarons are strongly coupled modes of QD phonons and electron intersublevel transitions. The degree of coupling is varied in a systematic way with a set of samples having electron intersublevel spacing changing from larger to smaller than the longitudinal-optical-phonon energy. The signature of polarons is evidenced clearly by the observation of a large $(12--20\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ anticrossing for both InAs and GaAs-like QD phonons using resonant Raman spectroscopy.

Electronic and Structural Properties of Interdiffused Self-Assembled Quantum Dots from Magneto-Photoluminescence

A set of self-assembled InAs/GaAs quantum dot (QD) samples annealed at various temperatures for 30 s was studied using magneto-photoluminescence up to 28 T. Blueshifts increasing with annealing temperature, due to Ga-As interchange at the QD-barrier interface, are correlated with a reduction in inhomogeneous broadening and a reduction in inter-sublevel spacing. These new sample properties allow us to obtain clear crossing patterns closely matched with Fock-Darwin diagrams where the field applied perpendicular to the QD plane lifts the state degeneracies. In the limit that in-plane electron and hole wavefunction extension is the same, the splitting of the p-shell with magnetic field is inversely proportional to the in-plane exciton reduced mass. We use this to obtain the evolution of the latter with intermixing, and compare with predictions of single-particle k*p calculations.

Laser-induced InAs/GaAs quantum dot intermixing

Laser annealing of InAs/GaAs quantum-dot (QD) microstructures has been investigated for selective area tuning of their electronic shell structure. Extensive blueshifts of the QD excited states were observed following 20–40 s laser irradiation. In the most extreme case, we were able to shift the position of the ground state transition by 298 meV, i.e., to the spectral region where the photoluminescence signal originates from the as-grown InAs wetting layer. A reduction from ∼50 to 8 meV of the full width at half maximum of the PL peak corresponding to this transition indicates a drastic change in the structural characteristics of the investigated QD ensemble. The attractive feature of the laser-QD-intermixing technique is that it offers the possibility of obtaining targeted blueshifts and inter-sublevel energy spacing on the lateral scale required in the fabrication of QD-based integrated optoelectronic devices and, possibly, photonic band gap crystals.

Origin of the inhomogenous broadening and alloy intermixing in InAs/GaAs self-assembled quantum dots

The photoluminescence (PL) spectra of a single-layer and a multilayer sample of self-assembled InAs/GaAs quantum dots (QD's) intermixed by thermal annealing at various temperatures, have been investigated. Intermixing is found to change both the optical transition energy and the intersublevel spacing of the QD energy levels. The linewidth (which is due to inhomogeneous broadening) of the PL emission peaks of both samples, are very similar, hence showing that the intermixing process is very homogeneous over 25 layers of QD's, whatever the annealing technique used. Moreover it is observed that the width of the PL peaks decreases for increasing interdiffusion down to about 5 meV for the ground-state transition of the multilayer sample. A reduction of the peak width is also observed for higher-energy states within the same ensemble of dots. The present paper shows that this effect can only be explained by some variation of the effective confining potential. Theoretical calculations have shown that the QD height, rather than the diameter, the volume, the composition or the strain, appears to be the key parameter that controls the sharpness of the PL linewidths in the investigated samples. Our model allows the identification of the main mechanisms involved in the inhomogeneous broadening of the optical transitions for the InAs/GaAs QD system.

Tuning the energy levels of self-assembled InAs quantum dots by rapid thermal annealing

We studied the photoluminescence spectra of rapid-thermal-annealed self-assembled InAs quantum dots at 10 K. For annealing temperatures ranging from 700 to 950 °C, we observed a blueshift in the interband transition energies, a decrease in the intersublevel spacing energies, and a narrowing of photoluminescence linewidths. In this letter, we demonstrate that the tuning of the InAs quantum dots interband transition and intersublevel spacing energies can be achieved by 30 s of rapid thermal annealing. The relation between interband transition energy changes and the intersublevel spacing energies is found to be linear, with a slope close to the ratio of the dots’ height to their diameter.

State-filling and magneto-photoluminescence of excited states in InGaAs/GaAs self-assembled quantum dots

We present the first radiative lifetime measurements and magneto-photoluminescence results of excited states in InGaAs/GaAs semiconductor self-assembled quantum dots. By increasing the photo-excitation intensity, excited state interband transitions up ton= 5 can be observed in the emission spectrum. The dynamics of the interband transitions and the inter-sublevel relaxation in these zero-dimensional energy levels lead to state-filling of the lower-energy states, allowing the quasi-Fermi level to be raised by more than 200 meV due to the combined large inter-sublevel spacing and the low density of states. The decay time of each energy level obtained under various excitation conditions is used to evaluate the inter-sublevel thermalization time. Finally, the emission spectrum of the dots filled with an average of about eight excitons is measured in magnetic fields up to 13 Tesla. The dependences of the spectrum as a function of carrier density and magnetic field are compared to calculations and interpreted in terms of coherent many-exciton states and their destruction by the magnetic field.

State filling and time-resolved photoluminescence of excited states in InxGa1-xAs/GaAs self-assembled quantum dots.

We present radiative lifetime measurements of excited states in semiconductor self-assembled quantum dots. By increasing the photoexcitation intensity, excited-state interband transitions up to n=5 can be observed in photoluminescence. The dynamics of the interband transitions and the intersublevel relaxation in these zero-dimensional energy levels lead to state filling of the lower-energy states, allowing the Fermi level to be raised by more than 200 meV due to the combined large intersublevel spacing and the low density of states. The decay time of each energy level obtained under various excitation conditions is used to evaluate the intersublevel thermalization time. \textcopyright{} 1996 The American Physical Society.