Aharanov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, the Aharanov-Bohm (AB) effect is shown to be remarkably robust and persists until 180K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge this is the highest temperature at which the AB effect has been observed in semiconductor structures

Shifting donor-acceptor photoluminescence in N-doped ZnO

We have grown nitrogen-doped ZnO films grown by two kinds of epitaxial methods on lattice-matched ScAlMgO$_4$ substrates. We measured the photoluminescence (PL) of the two kinds of ZnO:N layers in the donor-acceptor-pair transition region. The analysis of excitation-intensity dependence of the PL peak shift with a fluctuation model has proven that our observed growth-technique dependence was explained in terms of the inhomogeneity of charged impurity distribution. It was found that the inhomogeneity in the sample prepared with the process showing better electrical property was significantly smaller in spite of the similar nitrogen concentration. The activation energy of acceptor has been evaluated to be $\approx 170$ meV, which is independent of the nitrogen concentration.Comment: 4 pages, 3 figures, 1 table, RevTeX4, to appear in the July issue of J. Phys. Soc. Jp

Optical and electrical properties of Zn1-xBexSe grown by molecular beam epitaxy

Journal of Physical Studies, 8(4): pp. 384–388. Retrieved September 19, 2006 from http://www.ece.drexel.edu/mml/pubs/jps_paper.pdfThe dopant behaviour in the n-type ZnBeSe:Cl and p-type ZnBeSe:N grown by Molecular Beam Epitaxy (MBE) is investigated. All the samples have been grown on (001) GaAs substrates (conducting or semi-insulating). For the experimental investigation the photoluminescence (PL), Raman Scattering, photoconductivity and I–V measurements are used. From the temperature (9 K–300 K) and exitation intensity dependent on PL measurements it will be shown that donor and acceptor impurities exhibit a somewhat different behaviour and/or properties from those in ZnSe. We argue that (i) the activation energy of both the N acceptor and Cl donor increases in ZnBeSe as concentration of Be increases; (ii) all the indoped and n-type samples exhibit high (>180 K) temperature PL associated with shallow impurity of acceptor type, which also show an increase in the binding energy with Be composition; Raman scattering experiments have shown that at low Be concentrations (x < 0.04) one can use LO photon energy in ZnSe in the analyses of PL spectra from ZnBeSe. From photoconductivity and I–V measurements we show that (i) there are a number of carrier traps in ZnBeSe:Cl; (ii) some of the ZnBeSe samples with Be concentrations over 1% exhibit photoconductivity behaviour associated with the presence of DX centres or other deep trap, i. e., we observe persistent photoconductivity; (iii) volt-ampere dependencies (with and without irradiation) are different for the samples grown on conducting and semi-insulating substrates

Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification

We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution

Quantum Rings in Electromagnetic Fields

This is the author accepted manuscript. The final version is available from Springer via the DOI in this recordThis chapter is devoted to optical properties of so-called Aharonov-Bohm quantum rings (quantum rings pierced by a magnetic flux resulting in AharonovBohm oscillations of their electronic spectra) in external electromagnetic fields. It studies two problems. The first problem deals with a single-electron AharonovBohm quantum ring pierced by a magnetic flux and subjected to an in-plane (lateral) electric field. We predict magneto-oscillations of the ring electric dipole moment. These oscillations are accompanied by periodic changes in the selection rules for inter-level optical transitions in the ring allowing control of polarization properties of the associated terahertz radiation. The second problem treats a single-mode microcavity with an embedded Aharonov-Bohm quantum ring which is pierced by a magnetic flux and subjected to a lateral electric field. We show that external electric and magnetic fields provide additional means of control of the emission spectrum of the system. In particular, when the magnetic flux through the quantum ring is equal to a half-integer number of the magnetic flux quanta, a small change in the lateral electric field allows for tuning of the energy levels of the quantum ring into resonance with the microcavity mode, thus providing an efficient way to control the quantum ring-microcavity coupling strength. Emission spectra of the system are discussed for several combinations of the applied magnetic and electric fields