V. K. Kalevich

Time-resolved photoluminescence in self-assembled InAs/GaAs quantum dots under strictly resonant excitation

We investigate the carrier dynamics in self-assembled InAs/GaAs quantum dots under strictly resonant excitation of the ground state. The spectral selectivity of the resonant excitation allows us to study the physical properties of a class of dots characterized by an energy distribution comparable to the excitation laser spectrum. We detect no Stokes shift of the photoluminescence (PL) line. The PL decay time yields a straightforward determination of the radiative recombination time.

Spin-dependent recombination in GaAsN solid solutions

Room-temperature spin-dependent recombination in a series of GaAs1−xNx solid solutions (x = 2.1, 2.7, 3.4%) has been observed as manifested by a more than threefold decrease in intensity of the edge photoluminescence upon switching from circular to linear polarization of the exciting light or upon the application of a transverse magnetic field (∼300 G). The interband absorption of the circularly polarized light is accompanied by the spin polarization of conduction electrons, which reaches 35% with an increase in the pumping level. The observed effects are explained in terms of the dynamic polarization of deep paramagnetic centers and the spin-dependent trapping of conduction electrons on these centers. The electron spin relaxation time, as estimated from the dependence of the edge photoluminescence depolarization in the transverse magnetic field (the Hanle effect) on the pumping intensity, is on the order of 1 ns. According to the adopted theory, the electron spin relaxation time in the presence of spin-dependent recombination is determined by a slow spin relaxation of localized electrons. The sign (positive) of the g factor of localized electrons has been experimentally determined from the direction of the magnetic-field-induced rotation of their average spin observed in the three GaAsN crystals studied.

Spin dynamics in dilute nitride semiconductors at room temperature

We report optical studies in undoped GaAsN epilayers and InGaAsN quantum wells, which show that a strong electron spin polarization can persist at room temperature. This is a direct consequence of the long spin relaxation time of electrons in dilute nitride materials. Introducing less than 1% of nitrogen in the binary (GaAs) or ternary (InGaAs) alloy increases the electron spin relaxation time at T=300K by a factor greater than 20 in as-grown material before annealing. A drastic drop in the electron spin relaxation time is observed for annealed samples.

Optical orientation and spin-dependent recombination in GaAsN alloys under continuous-wave pumping

We present a systematic theoretical study of spin-dependent recombination and its effect on optical orientation of photoelectron spins in semiconductors with deep paramagnetic centers. For this aim we generalize the Shockley-Read theory of recombination of electrons and holes through the deep centers with allowance for optically-induced spin polarization of free and bound electrons. Starting from consideration of defects with three charge states we turn to the two-charge-state model possessing nine parameters and show that it is compatible with available experimental data on undoped GaAsN alloys. In the weak- and strong-pumping limits, we derive simple analytic equations which are useful in prediction and interpretation of experimental results. Experimental and theoretical dependences of the spin-dependent recombination ratio and degree of photoluminescence circular polarization on the pumping intensity and the transverse magnetic field are compared and discussed.

Determination of strain-induced valence-band splitting in GaAsN thin films from circularly polarized photoluminescence

The paper studies the circularly polarized photoluminescence (PL) from dilute GaAsN alloys with nitrogen content of 1%–3.4%, grown on GaAs substrates. The room-temperature PL is found to consist of two bands whose splitting grows with increasing nitrogen content. The analysis of the PL circular polarization has shown that the PL bands originate from the splitting of light- and heavy-hole subbands, induced by an elastic strain in GaAsN layer. The dependence of the energy gap of unstrained GaAsN on the nitrogen content has been calculated using the measured light- and heavy-hole splittings.

Spin-dependent recombination and hyperfine interaction at deep defects

We present a theoretical study of optical electron-spin orientation and spin-dependent Shockley-Read-Hall recombination taking into account the hyperfine coupling between the bound-electron spin and the nuclear spin of a deep paramagnetic center. We show that the number of master rate equations for the components of the electron-nuclear spin-density matrix is considerably reduced due to the restrictions imposed by the axial symmetry of the system under consideration. The rate equations describe the Zeeman splitting of the electron spin sublevels in the longitudinal magnetic field, the spin relaxation of free and bound electrons, and the nuclear spin relaxation in the two defect states, with one and two (singlet) bound electrons. The general theory is developed for an arbitrary value of the nuclear spin I, the magnetic-field and excitation-power dependencies of the electron and nuclear spin polarizations are calculated for the particular value of I = 1/2. The role of the nuclear spin relaxation in each of the both defect states is analyzed. The circular polarization and intensity of the edge photoluminescence as well as the dynamic nuclear spin polarization as functions of the excitation power are shown to have bell-shaped forms

Electron-nuclear spin dynamics of Ga2+ paramagnetic centers probed by spin-dependent recombination: A master equation approach

Similar to nitrogen-vacancy centers in diamond and impurity atoms in silicon, interstitial gallium deep paramagnetic centers in GaAsN have been proven to have useful characteristics for the development of spintronic devices. Among other interesting properties, under circularly polarized light, gallium centers in GaAsN act as spin filters that dynamically polarize free and bound electrons reaching record spin polarizations (100\%). Furthermore, the recent observation of the amplification of the spin filtering effect under a Faraday configuration magnetic field has suggested that the hyperfine interaction that couples bound electrons and nuclei permits the optical manipulation of its nuclear spin polarization. Even though the mechanisms behind the nuclear spin polarization in gallium centers are fairly well understood, the origin of nuclear spin relaxation and the formation of an Overhauser-like magnetic field remain elusive. In this work we develop a model based on the master equation approach to describe the evolution of electronic and nuclear spin polarizations of gallium centers interacting with free electrons and holes. Our results are in good agreement with existing experimental observations. In regard to the nuclear spin relaxation, the roles of nuclear dipolar and quadrupolar interactions are discussed. Our findings show that, besides the hyperfine interaction, the spin relaxation mechanisms are key to understand the amplification of the spin filtering effect and the appearance of the Overhauser-like magnetic field. Based on our models results we propose an experimental protocol based on time resolved spectroscopy. It consists of a pump-probe photoluminescence scheme that would allow the detection and the tracing of the electron-nucleus flip-flops through time resolved PL measurements.

Spin-dependent recombination in GaAs1–xNx alloys at oblique magnetic field

We have studied experimentally and theoretically the optical orientation and spin-dependent Shockley–Read–Hall recombination in a semiconductor in a magnetic field at an arbitrary angle α between the field and circularly polarized exciting beam. The experiments are performed at room temperature in GaAs1–xNx alloys where deep paramagnetic centers are responsible for the spin-dependent recombination. The observed magnetic-field dependences of the circular polarization ρ(B) and intensity J(B) of photoluminescence can be approximately presented as a superposition of two Lorentzian contours, normal and inverted, with their half-widths differing by an order of magnitude. The normal, narrow, Lorentzian contour is associated with depolarization of the transverse (to the field) component of spin polarization of the localized electrons, whereas the inverted, broad, Lorentzian is due to suppression of the hyperfine interaction of the localized electron with the own nucleus of the defect. The ratio between the height of one Lorentzian and depth of the other is governed by the field tilt angle α. In contrast to the hyperfine interaction of a shallow-donor-bound electron with a large number of nuclei of the crystal lattice, in the optical orientation of the electron-nuclear system under study no additional narrow peak appears in the oblique field. This result demonstrates that in the GaAsN alloys the hyperfine interaction of the localized electron with the single nucleus of the paramagnetic center remains strong even at room temperature. For a theoretical description of the experiment, we have extended the theory of spin-dependent recombination via deep paramagnetic centers with the nuclear angular momentum I = 1/2 developed previously for the particular case of the longitudinal field. The calculated curves ρ(B), J(B) agree with the approximate description of the experimental dependences as a sum of two Lorentzians, and an additional narrow shifted peak does not appear in the computation as well.

Spin Repolarization Due to Pauli Blocking in Quantum Dots

We have studied the spin dynamics in self-organized InAs/GaAs quantum dots (QD) by time-resolved photoluminescence (PL). After optical orientation of carriers photogenerated in the GaAs barrier by 1.2 ps light pulses, an unexpected increase of the QD excited-state luminescence polarization is observed. This effect is interpreted in terms of Pauli blocking which prevents the relaxation of electrons in the QD ground state already occupied by another electron with the same spin orientation. A theoretical model qualitatively describing the experimental results is developed.

Electron Spin Redistribution Due to Pauli Blocking in Quantum Dots and Quantum Wells

We have investigated the electron spin dynamics on the discrete energy levels of self-organized InAs quantum dots (QDs) covered by the thin InGaAs layer and embedded in the GaAs matrix. The InGaAs layer forms the external quantum well (QW). In particular, we report on the electron mean spin redistribution over the QD and QW energy spectrum resulting in a drastic increase of electron spin polarization of the QD excited levels and the QW ground state. The electron polarization is determined by measuring the degree of circular polarization of the QD and QW emissions after excitation by 1.5 ps circular polarized pulses in the GaAs barrier.