We present an attempt to control the properties of CdTe/ZnTe self-assembled quantum dots during their formation in the process of molecular beam epitaxy. Namely, the structures were in situ annealed at various temperatures and annealing times after the formation of quantum dots, before the deposition of a capping layer. Depending on the annealing parameters, the dots exhibit different optical properties which were studied by means of spatially resolved photoluminescence. From the analysis of these results, the information about relative changes of the average size and sheet density of quantum dots was extracted.
A theoretical analysis was carried out of an optical transition observed in high-quality GaAs/AlGaAs heterostructures δ-doped with shallow acceptors. The transition involves a 2D electron and a 3D acceptor-localized hole. The wave functions of a bulk Be acceptor were calculated within the spherical model with both the s-like and d-like parts of the envelope taken into account. The electron envelope wave functions resulted from self-consistent calculations of the electrostatic potential and were dependent on the 2D electron concentration, n_s. We show that: (i) including the d-like part of the acceptor envelope relaxes the selection rules of free-to-bound transitions at k=0;(ii) in the magnetic field, the selection rules depend on the number of the electron Landau level;(iii) the ratio of the intensity of the strongest transitions in both circular polarizations is essentially different from 3:1, and strongly depends on n_s. These results show that a description that neglects the d-like part of the acceptor envelope is both qualitatively and quantitatively unjustified.
Photoluminescence measurements were carried out on Be δ-doped GaAs/Al_{0.33}Ga_{0.67}As heterostructure at 1.6 K in magnetic fields (B) up to 4 T. Luminescence originating from recombination of a two-dimensional electron gas and photoexcited holes localized on Be acceptors was analyzed. The degree of circular polarization (γ_C) of the luminescence from fully occupied Landau levels was determined as a function of B and the two-dimensional electron gas concentration, n_s. At B constant,γ_C decreased with the increase in n_s. The intensity of the optical transition considered was calculated with taking into account s- and d-like parts of the acceptor envelope function. It is shown that the presence of the d-like part explains the observed γ_C(n_s) dependence quantitatively. This shows that polarization spectroscopy on acceptorδ-doped heterostructures enables one to test experimentally the contribution of the L>0 component of the envelope in a shallow acceptor description.
We consider, via numerical calculations, a hybrid structure made of a semimagnetic Cd_{1-x}Mn_xTe quantum well deposited in a close proximity to superconducting niobium film. We simulate photoluminescence and the Faraday rotation spectra, modified by the presence of vortices in this type II superconductor. The magnitude of the evaluated effects is small - the vortex induced spectral line shape variation is of the order of 1% at 1 K and 0.1% at 3 K and is expected to occur mainly in the field range between 0.03 T and 0.05 T.
Dynamics of nonequilibrium carriers in high-Al-content AlGaN/AlGaN multiple quantum wells was studied. A set of multiple quantum wells with well widths varying from 1.65 to 5.0 nm was grown by metal-organic chemical vapor deposition. The structures were investigated by photoluminescence spectroscopy under quasi-steady-state conditions. The observed blueshift of the photoluminescence band peak was attributed to the screening of the built-in electric field. The integrated photoluminescence intensity dependence on excitation and temperature showed a strong influence of carrier localization.
Time resolved photoluminescence of double quantum well structure was investigated versus electric and magnetic fields applied across the sample. The emission due to direct excitons (electron and hole are localized within the same quantum well) decays fast at the nanosecond timescale, whereas the recombination kinetics of indirect excitons is much slower and spreads over microseconds. The time evolution of indirect exciton emission is shown to be altered by application of either electric or magnetic field. This reflects the non-trivial effects of exciton localization which leads to the non-exponential decays of the indirect exciton emission.
In the photoluminescence excitation spectra of two-dimensional valence holes with large spin gap and strong disorder we find evidence for quantum Hall ferromagnetism and small skyrmions around the Landau level filling factorν=1. This interpretation is supported by numerical calculations.
We report both decrease and increase in the 2D carrier gas density in a simple (Cd,Mn)Te/(Cd,Mg)Te heterostructure with (Cd,Mn)Te quantum well. The two effects were achieved by light with different photon energies. The quantum wells were 10 nm wide with 2D hole gas supplied by surface states. For the sample with 25 nm cap layer thickness, it was possible to tune the hole gas concentration from almost empty well (hole density below 1×10^{10} cm^{-2}) to 45×10^{10} cm^{-2}. The illumination with 425 nm wavelength almost doubled the hole gas density from the initial 24×10^{10} cm^{-2}. The depletion mechanism was most effective for illumination with the orange (575 nm) light.
We report studies on electric field built in GaN/Al_{0.09}Ga_{0.91}N structure of nominally 6 nm wide quantum well. The sample was grown in horizontal metal-organic chemical vapor deposition reactor using innovative technology that decreases the density of screw dislocations. Firstly, using visible and mid infra-red interference pattern along the sample, the layer thickness and consequently the quantum well width was determined to vary linearly with the position. Secondly, photoluminescence spectra was taken at different positions. Correlation of those two measurements allows us to determine the built-in electric field to be 0.66 MV/cm, which is considerably larger than previously reported for similar structures.
GaN/AlGaN single quantum disks on GaN nanorods were grown on Si (001) substrate with native SiO_2 layer by a plasma-assisted molecular-beam epitaxy under nitrogen-rich conditions. The transmission electron microscopy observations show single GaN nanorods images with an average thickness of 4 nm for the GaN single quantum disk and nanorod diameter of 15 nm. The observed photoluminescence spectra at 8 K show a peak at 3.475 eV, attributed to an exciton recombination in GaN. A strong peak was observed at 3.542 eV. This peak is attributed to the quantum confinement of excitons in the GaN quantum disks.
We present theoretical studies of effects of the nonlinear elasticity and the electromechanical coupling on the optical properties of InGaN/GaN and AlGaN/AlN quantum wells. In these structures, due to the lattice misfit between constituents, the quantum wells are compressively strained and the intrinsic hydrostatic pressure is present. Therefore, the nonlinear elasticity is investigated by taking into account the pressure dependence of elastic stiffness tensor for the strained quantum wells. We show that this effect leads to (i) decrease in the volumetric strain and (ii) increase in the polarization-induced built-in electric field in the quantum wells. Consequently, the interband transition energies in the quantum wells decrease when the nonlinear elasticity of nitrides is considered. On the other hand, we show that the effect of electromechanical coupling, i.e., co-existence of ordinary and converse piezoelectric effects results in increase in the interband transition energies in the considered quantum wells. It turns out that the influence of the nonlinear elasticity on the optical properties is stronger than the influence of electromechanical coupling for InGaN/GaN quantum wells, while for AlGaN/GaN the opposite situation is observed.
We report on optical studies of exciton localization and recombination kinetics in two single 2.2 nm thick Al_{x}Ga_{1-x}N/Al_{x+0.1}Ga_{0.9-x}N quantum well structures (x=0.55 and 0.6) grown by plasma assisted molecular beam epitaxy on a c-sapphire substrate. Strong localization potential inherent for both the quantum well and barrier regions results in merging of the quantum well and barrier emission spectra into a single broad line centered at 285 nm (x=0.55) and 275 nm (x=0.6). Time-resolved photoluminescence measurements revealed surprising temperature stability of the photoluminescence decay time constant ( ≈ 400 ps) relevant to the recombination of the quantum well localized excitons. This observation implies nearly constant quantum efficiency of the quantum well emission in the whole range from 4.6 to 300 K.
Linear, third-order nonlinear and total optical absorption coefficients associated with intra-conduction band in wurtzite unstrained (In,Ga)N-GaN coupled quantum wells are calculated. Based on the effective-mass and the one-band parabolic approximations, the well and the barrier-widths effects are investigated variationally under finite confinement potential barrier. The results indicate that the structure size have a great influence on the optical properties. The results reveal also that larger optical absorption coefficients are obtained compared to single quantum well and a significant red-shift is obtained as the structure size increases. It is found that the modulation of the absorption coefficients can be easily obtained by adjusting the barrier and/or the well widths.
We report transmission measurements of GaN quantum well grown on sapphire substrate in the 220-325 GHz frequency band at low temperatures. A significant enhancement of the transmitted beam intensity with the applied voltage on the devices under test is found.
We report on polarization spectra of spontaneous terahertz electroluminescence from uniaxially deformed Ge(Ga). At compressive pressure of about 3±0.3 kbar in the [111] direction, near the impurity breakdown threshold, the linear polarization degree attains ≈80-90% for the main lines of the terahertz emission.
We present a study of time-dependent transmission spectra of a modulation-doped Cd_{1-x}Mn_xTe/Cd_{1-y-z}Zn_yMg_zTe quantum well with variable hole gas concentration. We study the influence of pump pulses on excitonic absorption in subpicosecond time scale. A spectrally broad probe pulse of duration of 40 femtoseconds was used to record the absorption spectra at controlled delay. Studies of temporal evolution of exciton energies revealed coherence decay of linearly polarized excitons and thermalization of non-equilibrium exciton states. We found that a characteristic timescale for thermalization of non-equilibrium populations of photocreated excitons is between 0.8 and 3.6 ps. The timescale of this process depends on the hole concentration in quantum well: for higher hole concentration the decay is faster. Long-lived photo-induced magnetization accompanied by heating of the magnetic system was also observed.
The static and dynamic properties of excitons and trions in a 80 nm wide Cd_{1-x}Mn_xTe/Cd_{0.7}Mg_{0.3}Te quantum well with extremely small Mn content (x=0.00027) have been studied by means of time-integrated and time-resolved photoluminescence experiment at low and elevated temperatures. The trion binding energy has been estimated to be 2.6 ± 0.8 meV. The exciton and trion lifetimes have been measured to be ≈ 150 ps, and ≈ 200 ps, respectively. The temperature dependence of both lifetimes together with the multicomponent character of the PL decay process suggest a spatial localization of excitons and trions in the investigated quantum well.
We present contactless surface photovoltage spectroscopy and photoreflectance studies of 10 nm wide, p-type doped asymmetric GaAs/InGaAs/AlGaAs quantum well structures. The MBE grown structures differ in spacer thickness between the quantum well and the reservoir of holes. The doping causes that quantum well is placed in electric field. The surface photovoltage spectroscopy measurements gave us detailed information about the optical transitions between confined states and between confined and unconfined states. The comparison of experimental and numerical analysis allows us to identify all features present in the surface photovoltage spectroscopy and photoreflectance spectra. It has been found that spacer layer thickness has significant influence on surface photovoltage spectroscopy spectra.
Using the effective mass and parabolic band approximations, the binding energy of a shallow donor impurity is calculated in a GaAs-(Ga,Al)As quantum well wire of rectangular transversal section, under the combined effects of two independent axially-applied intense laser radiation fields and a static electric field oriented in the cross-section plane. The lateral size of the rectangular cross-section is assumed to be larger than 10 nm, in such a way that the uncorrelated electron motion along the x and y directions can be considered uncoupled. The impurity-related states are calculated by means of a variational procedure using a three-dimensional hydrogen-like trial wave function. The intense laser field effects are introduced via the combination of the Floquet method for the laser-modified confinement potential shape and the inclusion of a two-interaction centers model for the Coulombic coupling. It is shown that, according to the polarization of the incident radiation, the quantum well wire can evolve from a single 1D-heterostructure towards a configuration of two-well defined or four-well defined laser-induced parallel coupled quantum well wires. The obtained results also show that the binding energy is strongly dependent on the impurity position and on the strength of the intense laser field parameter.
We present time-dependent reflectance spectra of wide (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Interactions between excitons, trions, and carriers are studied for exciton densities up to 10^{11} cm^{-2}. The resonant excitation at different excitonic lines is analyzed.
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