Igor L. Kuskovsky

Quantum confinement in ZnO nanorods

The colloidal-synthesized ZnO nanorods with radius of 1.1±0.1nm (less than the bulk exciton Bohr radius, aB∼2.34nm) have been studied by optical methods combined with simple model calculations. The quantum confinement has been observed in these nanorods. The exciton binding energy is shown to be significantly enhanced due to one-dimensional confinement. Additionally, it is suggested that the green luminescence in ZnO involves free holes.

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.

Aharonov-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, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. 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 solid-state and molecular nanostructures.

Long Electron−Hole Separation of ZnO-CdS Core−Shell Quantum Dots

The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core-shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron-hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core-shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.

Optical Aharonov-Bohm effect in stacked type-II quantum dots

Excitons in vertically stacked type-II quantum dots experience the topological magnetic phase and demonstrate the Aharonov-Bohm oscillations in the emission intensity. Photoluminescence of vertically stacked ZnTe/ZnSe quantum dots is measured in magnetic fields up to 31 T. The Aharonov-Bohm oscillations are found in the magnetic-field dependence of emission intensity. The positions of the peaks of the emission intensity are in a good agreement with numerical simulations of excitons in stacked quantum dots.

Reversible ultraviolet-induced photoluminescence degradation and enhancement in GaN films

UV-induced modifications in undoped metalorganic chemical vapor deposition grown GaN on sapphire are observed from 9 to 160 K. The photoluminescence intensities of bound excitons (3.476, 3.482 eV), the yellow band (2.2 eV) and the blue band (2.9 eV) change with time when a fresh sample is irradiated by 325 nm (He–Cd laser). The free exciton peak at 3.488 eV is unchanged by laser irradiation. Initially the blue and donor-bound exciton emission degrade rapidly and the yellow luminescence increases, each at the same rate. Later, the yellow luminescence degrades and the donor-bound exciton emission increases very slowly, at the same rate. Mechanisms are proposed that may explain the luminescence pathways and defects involved.

Wide Bandgap Light Emitting Materials and Devices

III-Nitride Light-Emitting Diodes on Novel Substrates (X.A. Cao) III-Nitride Micro-Cavity Light-Emitters (H.X. Jiang, J.Y. Lin) Nitride emitters - recent process (T. Wang) ZnSeTe rediscovered:from isoelectronic centers to quantum dots (Gu, Kuskovsky, Neumark) Optical Properties of ZnO alloys (J. Muth, A. Osinsky)

High crystalline quality ZnBeSe grown by molecular beam epitaxy with Be-Zn co-irradiation

High crystalline quality ZnBeSe epilayers with di!erent compositions were grown on GaAs substrates by molecular beam epitaxy using Be}Zn co-irradiation of the III}V surface and a ZnSe bu!er layer. A (1]2) re#ection high-energy electron di!raction pattern was formed after the Be}Zn co-irradiation indicating the formation of Be and Zn dimers on the GaAs surface. A two-dimensional growth mode was observed throughout the growth of the ZnSe bu!er layer and ZnBeSe epilayer. Narrow X-ray linewidth as low as 23 arcsec with the etch pit density of mid 104 cm~2 were obtained. The linewidth of the dominant excitonic emission is about 2.5 meV at 13 K for the near-lattice-matched ZnBeSe layer. For a nitrogen-doped sample, capacitance}voltage measurements showed a net acceptor concentration of 2.0]1017 cm~3. In addition, the use of a BeTe bu!er layer and of a Zn-irradiation with a ZnSe bu!er layer were also investigated. ( 2000 Elsevier Science B.V. All rights reserved.

Enhancement of p-type doping of ZnSe using a modified (N+Te)δ -doping technique

Delta doping techniques have been investigated to enhance the p-type doping of ZnSe. Tellurium was used as a codopant for improving the nitrogen doping efficiency. The net acceptor concentration (NA−ND) increased to 1.5×1018 cm−3 using single δ doping of N and Te (N+Te), while it was limited to 8×1017 cm−3 by δ doping of N alone. A promising approach was developed in which three consecutive δ-doped layers of N+Te were deposited for each δ-doping cycle. An enhancement in the (NA−ND) level to 6×1018 cm−3 has been achieved in ZnSe using this technique. The resultant layer has an average ZnTe content of only about 3%. This doping method shows potential for obtaining highly p-type doped ohmic contact layers without introducing significant lattice mismatch to ZnSe. Low-temperature photoluminescence spectra reveal some Te-related emissions.

Effects of Be on the II–VI/GaAs interface and on CdSe quantum dot formation

The effects of Be on the II–VI/GaAs interface and on CdSe quantum dot (QD) formation were investigated. A (1×2) surface reconstruction was observed after a Be–Zn coirradiation of the (001) GaAs (2×4) surface. ZnBeSe epilayers grown after the Be–Zn coirradiation show very high crystalline quality with x-ray rocking curve linewidths down to 23 arcsec and a low etch pit density of 4×104 cm−2, and good optical quality with a band-edge photoluminescence (PL) emission peak linewidth of 2.5 meV at 13 K. However, ZnBeSe epilayers grown after Zn irradiation alone have poor crystalline quality and poor optical properties. Atomic force microscopy measurements show that CdSe QDs grown on ZnBeSe have higher density and smaller size than those grown on ZnSe. A narrower PL emission peak with higher emission energy was observed for the CdSe QDs sandwiched by ZnBeSe. These results indicate that the formation of CdSe QDs as well as the II–VI/GaAs interface are modified by the presence of Be.