S. V. Zaitsev

Negative characteristic temperature of InGaAs quantum dot injection laser

The range of negative characteristic temperatures in temperature dependences of threshold current density of low-threshold (In, Ga)As/(Al, Ga)As quantum dot injection lasers has been observed. A model describing the decrease in threshold current density with temperature at low temperatures is proposed.

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

Optical spectroscopy on individual CdSe/ZnMnSe quantum dots

Optical single dot studies in wide-band gap diluted magnetic CdSe/ZnMnSe quantum dots have been performed. Due to the sample design, the photoluminescence energy of the quantum dot signal is energetically below the internal Mn2+ transition, resulting in high quantum efficiencies comparable to nonmagnetic CdSe/ZnSe quantum dots. Magnetic-field- and temperature-dependent measurements on individual dots clearly demonstrate the exchange interaction between single excitons and individual Mn2+ ions, resulting in a giant Zeeman effect and a formation of quasi-zero-dimensional magnetic polarons.

Spectral and dynamic properties of InAs-GaAs self-organized quantum-dot lasers

The spectral and dynamic properties of InAs-GaAs MOCVD-grown vertically stacked self-organized quantum-dot lasers are studied experimentally. A strong mode grouping effect (quasi-periodic modulation of the lasing spectrum) is observed and interpreted as a result of wavelength-dependent losses in the laser waveguide associated with substrate leakage and reflection. Some samples also display a broader spectral modulation which may be attributed to lasing from different groups of dots, or energy levels. Experimental observations are in agreement with a theoretical explanation involving increased optical nonlinearities due to the localized nature of carriers. In relaxation oscillation pulse trains, a substructure is observed which we believe to be a dynamic manifestation of the same carrier localization effects; a preliminary rate-equation simulation supports this interpretation.

Emission properties of InGaAs/GaAs heterostructures with δ⟨Mn⟩-doped barrier

Light-emitting device heterostructures with a ?Mn-doped layer inserted between the Schottky contact and near-surface InGaAs/GaAs quantum well (QW) have been fabricated. The ?Mn-doped layer facilitates hole tunnelling from the Schottky contact to the QW and impedes that of QW electrons in the opposite direction. It leads to a highly enhanced electroluminescence signal from the InGaAs QW. An effective p?d exchange interaction of holes with magnetic moments of Mn ions is found to strongly enhance the effective hole g-factor and the circular polarization of the low (~2?K) temperature emission up to 20% in magnetic fields of 1?2?T.

Dynamic spin polarization by orientation-dependent separation in a ferromagnet–semiconductor hybrid

Integration of magnetism into semiconductor electronics would facilitate an all-in-one-chip computer. Ferromagnet/bulk semiconductor hybrids have been, so far, mainly considered as key devices to read out the ferromagnetism by means of spin injection. Here we demonstrate that a Mn-based ferromagnetic layer acts as an orientation-dependent separator for carrier spins confined in a semiconductor quantum well that is set apart from the ferromagnet by a barrier only a few nanometers thick. By this spin-separation effect, a non-equilibrium electron-spin polarization is accumulated in the quantum well due to spin-dependent electron transfer to the ferromagnet. The significant advance of this hybrid design is that the excellent optical properties of the quantum well are maintained. This opens up the possibility of optical readout of the ferromagnets magnetization and control of the non-equilibrium spin polarization in non-magnetic quantum wells.

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm2 at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots.

Kinetics of radiative recombination in strongly excited ZnSe/BeTe superlattices with a type-II band alignment

We report results of a detailed investigation of type-II superlattices under high density photoexcitation. A strong spectral shift (≈0.5 eV) of the recombination band corresponding to the indirect transition from the ZnSe conduction band to the BeTe valence band in ZnSe/BeTe superlattices with increasing carrier density has been found at T=300 K. The dynamical characteristics of this transition are studied by time-resolved spectroscopy. A model which accounts for the dependence of band bending and lifetimes of spatially separated electrons and holes on the concentration of the photoexcited carriers is developed. Numerical simulations of the photoluminescence kinetics are in very good agreement with experimental results. It turns out that despite the huge band offsets involved, the radiative recombination under high excitation conditions can be nearly as fast as in spatially direct quantum wells.

Quantum dot injection heterolaser with ultrahigh thermal stability of the threshold current up to 50 °C

Gaseous phase epitaxy from metal organic compounds is used to obtain a low-temperature injection laser with an active region based on In0.5Ga0.5As/GaAs quantum dots. Optimizing the growth conditions and geometric parameters of the structure has made it possible to increase the range of ultrahigh thermal stability in the threshold current (the characteristic temperature is T0=385 K) up to 50 °C.

Giant blue shift of photoluminescence in strongly excited type-II ZnSe/BeTe superlattices

A giant blue shift (≈0.5 eV) and a large decrease in the emission time of a spectral band corresponding to radiative recombination of spatially separated electrons and holes are observed in ZnSe/BeTe superlattices at high laser excitation levels. On the basis of numerical calculations, the observed defects are attributed to band bending arising in type-II structures at high carrier density.