J. Kossut

PHOTOLUMINESCENCE STUDY OF CDTE/ZNTE SELF-ASSEMBLED QUANTUM DOTS

We report on optical properties of CdTe self-assembled quantum dots (SADs) grown by molecular beam epitaxy on ZnTe. Formation of SADs was achieved by deposition of 1.5–2.5 monolayers of CdTe at a substrate temperature of 420 °C and by applying growth interrupts for few seconds in Cd flux. The resulting dots have a typical diameter of 2 nm and a sheet density of 1012 cm−2. At T=2 K the photoluminescence (PL) spectra consist of two emission lines. The high-energy line originates from excitonic recombination in a wetting layer while the low-energy emission PL band is assigned to recombination in SADs. The increase in temperature up to 70 K does not affect the SADs-related emission intensity. It shifts, however, the PL peak energy towards low energies and causes a significant narrowing of the PL linewidth, from 80 meV at 1.9 K to 50 meV at 130 K. The activation energy of the thermal quenching of SADs-related PL emission was found to be equal to 47 meV. This value is three times greater than the one observed i...

CdSe quantum dots in a Zn1−xMnxSe matrix: new effects due to the presence of Mn

Abstract We report detailed photoluminescence (PL) and magneto-PL data on a new II–VI quantum dot (QD) system involving diluted magnetic semiconductors (DMSs). CdSe dots embedded in ZnMnSe exhibit relatively weak PL emission compared to analogous non-DMS systems, such as CdSe/ZnSe. However, we observe a dramatic increase in the PL intensity emitted by excitons confined in such QDs when a magnetic field is applied. We explain this effect in terms of an efficient transfer of excitation between the excitons and the Mn ions. In addition to this, we report on the role that Mn plays in CdSe dot nucleation, providing a new handle for optimization of the growth of the CdSe QDs.

Structural and optical evidence of island correlation in CdTe/ZnTe superlattices

The properties of superlattices consisting of 2 monolayer wide CdTe insertions into ZnTe spacer barriers with thickness ranging from 3 to 75 monolayers are investigated by means of transmission electron microscopy and photoluminescence spectroscopy. We show that quasi zero-dimensional CdTe islands form in this highly lattice-mismatched system. For spacer thickness smaller than 25 monolayers, the islands are vertically correlated along the axis tilted by 40° with respect to the growth direction, while for thicker ZnTe spacers no correlation is observed. The electronic coupling between the correlated islands manifests itself by the appearing of an additional emission band at energies lower to those corresponding to uncorrelated dots. The optical spectroscopy data reveal zero-dimensional localization of excitons by the electronically coupled islands. The decay time of the excitonic recombination is found to be over an order of magnitude longer in the case of the coupled islands than in the case of isolated ones.

Cathodoluminescence study of diluted magnetic semiconductor quantum well/micromagnet hybrid structures

Cathodoluminescence (CL) was studied in hybrid structures consisting of a diluted magnetic semiconductor (DMS) Cd1−xMnxTe (x=0.06 and 0.09) quantum well buried 300 A below the surface on which Fe islands with micrometric dimensions were deposited. The CL at T=10 K collected from areas far away from the Fe island was consistent with the photoluminescence spectra obtained prior to Fe deposition as were the raster scans and spot excited CL spectra taken in nonmagnetized structures close to the Fe islands. After a magnetization at a magnetic field of 3 T, the CL peak related to DMS quantum well (QW) shifts by up to 4 meV to lower energy only when the exciting beam is focused close to edges of an island. The observed shifts are interpreted as due to a fringe field, affecting the DMS QW, of magnetic domains formed in the Fe islands. The experiments prove a feasibility of the concept of usage of the fringe fields to achieve further confinement of excitons in submicron DMS/ferromagnet hybrid structures.

Spin coherence of a two-dimensional electron gas induced by resonant excitation of trions and excitons in Cd Te ∕ ( Cd , Mg ) Te quantum wells

The mechanisms for generation of long-lived spin coherence in a two-dimensional electron gas (2DEG) have been studied experimentally by means of a picosecond pump-probe Kerr rotation technique.

Structural properties of cubic MnTe layers grown by MBE

\mathrm{Cd}\mathrm{Te}∕(\mathrm{Cd},\mathrm{Mg})\mathrm{Te}

Photoluminescence study and structural characterization of p-type ZnO doped by N and/or As acceptors

quantum wells with a diluted 2DEG were investigated. The strong Coulomb interaction between electrons and holes, which results in large binding energies of neutral excitons and negatively charged excitons (trions), allows one to address selectively the exciton or trion states by resonant optical excitation. Different scenarios of spin coherence generation were analyzed theoretically, among them the direct trion photocreation, the formation of trions from photogenerated excitons, and the electron-exciton exchange scattering. Good agreement between experiment and theory is found.

High mobility 2D electron gas in iodine modulation doped CdTe/CdMgTe heterostructures

Abstract We report on the growth of zinc-blende MnTe on (001) GaAs substrates by MBE. Layers with thickness up to 8 μm were grown. The growth was performed for various ratios of Mn and Te fluxes. For high values of the Te Mn flux ratio, Raman scattering spectra showed a presence of tellurium lines corresponding to crystalline tellurium. For low flux ratios the presence of Te precipitates was less evident. These results were confirmed by X-ray diffraction studies. The lattice parameter of cubic MnTe and its temperature dependence were determined by X-ray diffraction. From the analysis of the rocking curves, for various Bragg reflections, the fluctuations of the lattice parameter and mosaicity were separated out.

Tuning the properties of magnetic CdMnTe quantum dots

ZnO doped with N and/or As layers was fabricated by thermal oxidation of ZnTe films grown by MBE on different substrates. Hall effect measurements demonstrated p-type conductivity with a hole concentration of ~5 × 1019 cm−3 for ZnO:As and ZnO:As:N on GaAs substrates and ~6 × 1017 cm−3 for ZnO:N on ZnTe substrates. The concentration of N and As atoms in ZnO is estimated to be ~1020 cm−3. This suggested that simple substitutional N atoms form acceptor impurities with a smaller efficiency than an As-related complex, probably AsZn–2VZn. In particular, we were able to distinguish between nitrogen and arsenic acceptor-related luminescence. Optical studies showed meaningful differences of the PL features in samples with different acceptors, grown on different substrates.

Introduction to the physics of diluted magnetic semiconductors

Abstract We report on growth and characterization of modulation doped CdTe/Cd 1− y Mg y Te quantum well structures. Well resolved Shubnikov—de Haas oscillations and quantum Hall effect have been observed. In the best CdTe/Cd 1− y Mg y Te structures the modulation doping enabled fabrication of a two-dimensional electron gas with mobility exceeding 10 5 cm 2 /V s. This is the highest mobility reported in wide-gap II–VI materials.