G. Karczewski

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...

Temperature dependent magnetic anisotropy in (Ga,Mn)As layers

It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently low. In such films, the easy axis assumes a standard in-plane orientation when the temperature is raised towards the Curie temperature or the hole concentration is increased by low temperature annealing. These findings are shown to corroborate quantitatively the predictions of the mean-field Zener model for ferromagnetic semiconductors. The in-plane anisotropy is also examined, and possible mechanisms accounting for its character and magnitude are discussed.

Molecular beam epitaxy of high structural quality Bi2Se3 on lattice matched InP(111) substrates

Epitaxial layers of the topological insulator Bi2Se3 have been grown by molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve is Δω=13 arc sec, and the (ω−2θ) scans exhibit clear layer thickness fringes. Atomic force microscope images show triangular twin domains with sizes increasing with layer thickness. The structural quality of the domains is confirmed on the microscopic level by transmission electron microscopy.

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.

Structural properties of cubic MnTe layers grown by MBE

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.

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

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.

Tuning the properties of magnetic CdMnTe quantum dots

We show that CdMnTe self-assembled quantum dots (QDs) can be formed by depositing a submonolayer of Mn ions over a ZnTe surface prior to deposition of the CdTe dot layer. Single-dot emission lines and strongly polarized QD photoluminescence (PL) in an applied magnetic field confirm the presence of Mn in individual QDs. The width of PL lines of the single CdMnTe dots is 3 meV due to magnetic moment fluctuations (MMFs) of the Mn ions. After rapid thermal annealing, the emission lines of individual magnetic QDs narrow significantly to 0.25 meV, showing that effect of MMFs is strongly reduced, most probably due to an increase in the average QD size. These results suggest a way to tune the spin properties of magnetic QDs.

Exciton spin relaxation time in quantum dots measured by continuous-wave photoluminescence spectroscopy

We demonstrate a method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots (QDs) the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state (σ−-polarized) than in the case when the excitons are excited into the higher energy spin state (σ+-polarized). A simple rate equation model gives the exciton spin relaxation time in CdTe QDs equal to τS=4.8±0.3 ns, significantly longer than the QD exciton recombination time τR=300 ps.

High Quality Homoepitaxial GaN Grown by Molecular Beam Epitaxy with NH3 on Surface Cracking.

Epitaxial GaN films have been grown on GaN single-crystal substrates, using on surface cracked ammonia as nitrogen precursor for molecular beam epitaxy. With this approach excellent optical and structural properties are achieved. Low-temperature photoluminescence shows well-resolved excitonic lines with record low linewidths as narrow as 0.5 meV. The transitions are attributed to excitons bound to neutral donors ((D°, X)1 at 3.4709 eV and (D°, X)2 at 3.4718 eV) and to a neutral acceptor ((A°, X) at 3.4663 eV). In addition, free exciton lines are observed at 3.4785 eV, 3.4832 eV, and 3.499 eV for excitons A, B, and C, respectively.

Deep hole traps in p‐type nitrogen‐doped ZnSe grown by molecular beam epitaxy

P‐type nitrogen‐doped ZnSe grown on n+‐GaAs by molecular beam epitaxy has been studied by deep‐level transient spectroscopy (DLTS) and double correlation DLTS. To achieve p‐type doping of ZnSe, we employed an active nitrogen beam produced by a free‐radical plasma source. Four hole traps—with activation energies of 0.22, 0.51, 0.63, and 0.70 eV—were detected by DLTS. Two of these—those at 0.51 and 0.63 eV—have never been observed before in ZnSe. They are probably introduced to the material by nitrogen doping. The properties of the other two traps—at 0.22 and 0.70 eV—support the hypothesis that both of them are associated with native defects, in agreement with earlier reports. To our knowledge this is the first report about direct experimental investigation of deep states in p‐type ZnSe.