Ł. Kłopotowski

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

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.

Magnetophotoluminescence study of intershell exchange interaction in CdTe/ZnTe quantum dots

T. Kazimierczuk, ∗ T. Smoleński, M. Goryca, 2 L. K lopotowski, P. Wojnar, K. Fronc, A. Golnik, M. Nawrocki, J.A. Gaj, † and P. Kossacki Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Hoża 69, 00-681 Warsaw, Poland Laboratoire National des Champs Magntiques Intenses, Grenoble High Magnetic Field Laboratory, CNRS, 38042 Grenoble, France Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/64, 02-688 Warsaw, Poland (Dated: June 28, 2011)

Optical injection of spin-polarized carriers across a strongly mismatched heterostructure

Abstract We observed an effective injection of spin-polarized carriers from II–VI Zn 0.97 Mn 0.03 Te diluted magnetic semiconductor spin aligning layer into III–V-based, GaAs-based quantum well structure. By using circular polarized excitation and detection, we demonstrate that the injection of spin-polarized carriers indeed proceeds through the II–VI/III–V interface in spite of a huge lattice mismatch (∼7.8%) which is decorated by a great number of dislocations. This indicates that spins are quite robust and maintain their polarization memory even after passing through a dense array of misfit dislocations.

Light- and environment-sensitive electrospun ZnO nanofibers

One-dimensional (1D) ZnO nanostructures have been widely studied because of their electronic and optoelectronic applications. This report discusses the morphology, optical, electrical and sensory properties of polycrystalline ZnO nanofibers (NFs). We observed that the electrospun ceramic NFs interband emission increases with the nanocrystal size, consistent with decreasing of the surface-to-volume ratio. The observation is novel for the electrospun ceramic NFs. The chemical composition and structural characterization reveal that the NFs consist of ZnO wurzite nanocrystals, whose mean diameters increase from 7 to 22 nm with calcination temperature. Emission properties are studied by cathodo- and photoluminescence. The NFs are applied to construct light, gas and liquid sensors. We find an increase of the NFs conductivity by three orders of magnitude under UV illumination as a result of desorption of molecular oxygen from the nanocrystal surface. We study the influence of oxygen on NF conductivity by purging the NFs with air or nitrogen. We show that the flow of nitrogen removes the oxygen resulting in an important increase of the conductivity. Also, we study the dynamics of this process with and without UV illumination. We show sensitivity of the NFs to liquid environment by studying the conductivity of NFs immersed in water and ethanol and find an increased conductivity with respect to a dry air environment. These light- and environmental-sensitive ZnO NFs have useful optical and electronic properties for building high-performance sensors.

Optical anisotropy and pinning of the linear polarization of light in semiconductor microcavities

We report strong experimental evidence of the optical anisotropy in a CdTe-based microcavity: the polarization of light is pinned to one of the crystallographic axes independently of the polarization of the excitation. The polarization degree depends strongly on the excitation power, reaching almost 100% in the stimulated regime. The relaxation time of the polarization is about 1 ns. We argue that all of this is an effect of a splitting of the polariton doublet at k = 0. We consider different sources for the splitting and conclude that the most likely one is optical birefringence in the mirrors and/or the cavity.

In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g-factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Tunneling of spin polarized excitons in CdTe based asymmetric double quantum well structure

Abstract We have studied by means of cw magnetospectroscopy the tunneling of photoexcited carriers from a Cd1−xMnxTe spin-aligning QW to a CdTe QW through a Cd1−yMgyTe barrier. PLE results suggest that we deal with the tunneling of excitons as whole entities and that the effectiveness of the transfer is highest when exciton energy detuning equals two LO phonon energies. We observed that, during tunneling, photocarriers keep a substantial amount of their spin polarization, amounting to about 10%. The results prove that the injecting spin polarized excitons from spin-aligning QW to another QW via tunneling effect is feasible, and may lead to considerable spin populations.

Spin Splitting Anisotropy in Single Diluted Magnetic Nanowire Heterostructures

We study the impact of the nanowire shape anisotropy on the spin splitting of excitonic photoluminescence. The experiments are performed on individual ZnMnTe/ZnMgTe core/shell nanowires as well as on ZnTe/ZnMgTe core/shell nanowires containing optically active magnetic CdMnTe insertions. When the magnetic field is oriented parallel to the nanowire axis, the spin splitting is several times larger than for the perpendicular field. We interpret this pronounced anisotropy as an effect of mixing of valence band states arising from the strain present in the core/shell geometry. This interpretation is further supported by theoretical calculations which allow to reproduce experimental results.

Stark spectroscopy and radiative lifetimes in single self-assembled CdTe quantum dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

Influence of exciton spin relaxation on the photoluminescence spectra of semimagnetic quantum dots

We present a comprehensive experimental and theoretical studies of photoluminescence of single CdMnTe quantum dots with Mn content x ranging from 0.01 to 0.2. We distinguish three stages of the equilibration of the exciton-Mn ion spin system and show that the intermediate stage, in which the exciton spin is relaxed, while the total equilibrium is not attained, gives rise to a specific asymmetric shape of the photoluminescence spectrum. From an excellent agreement between the measured and calculated spectra we are able to evaluate the exciton localization volume, number of paramagnetic Mn ions, and their temperature for each particular dot. We discuss the values of these parameters and compare them with results of other experiments. Furthermore, we analyze the dependence of average Zeeman shifts and transition linewidths on the Mn content and point out specific processes, which control these values at particular Mn concentrations.