G. F. Neumark

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.

Acceptor doping in ZnSe versus ZnTe

It is a long‐standing puzzle that ZnSe is difficult to dope p type, while ZnTe—which is very similar to ZnSe—is very easily doped p type. We report ab initio calculations which show that the solubilities of Li and Na acceptors are much greater in ZnTe than the solubilities of the same acceptors in ZnSe. We trace the origin of this difference to the bonding properties of the acceptors with the neighboring chalcogens. Our results also explain the experimentally observed dependence on dopant concentration of the dislocation density in p‐type ZnSe epilayers grown on GaAs.

Modified donor–acceptor pair luminescence in heavily nitrogen‐doped zinc selenide

The luminescence from heavily doped ZnSe:N shows a deep broadband whose position was found to depend strongly on the excitation intensity and sample temperature. The peak was found to shift towards higher energies with increasing intensity, but in contrast to the standard donor–acceptor pair (DAP) model, shifted towards lower energies with increasing temperatures. This behavior is explained using a modified DAP model that takes into account the perturbations of the band and impurity states caused by fluctuations in the concentrations of the ionized impurities. This model led to an estimate of 2.52 eV for the PL peak, with a width of 140 meV, for the case of low temperature and low excitation intensity, in rough agreement with our observations. The effect of these fluctuations on film conductivity is also discussed.

Achievement of low‐resistivity p‐type ZnSe and the role of twinning

Past literature is replete with reports of well‐conducting p‐type ZnSe; however, such reports have either been poorly reproducible and/or have shown puzzling features which are incompatible with accepted semiconductor theory (e.g., p‐type behavior via n‐type dopants, better light emitting diodes from high‐resistivity material, etc.). We have recently noted that ‘‘good’’ p‐type material available to us has been heavily twinned, and moreover has shown ‘‘nonstandard’’ electrical behavior such as a negative differential resistivity. In the present paper the earlier literature is reanalyzed based on the concept that to date such p‐type material has probably always been heavily twinned. A far improved understanding of the earlier work is obtained via the insight gained from these recent (twinning) results.

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)

Diameter Control and Photoluminescence of ZnO Nanorods from Trialkylamines

A novel solution method to control the diameter of ZnO nanorods is reported. Small diameter (2-3 nm) nanorods were synthesized from trihexylamine, and large diameter (50–80 nm) nanorods were synthesized by increasing the alkyl chain length to tridodecylamine. The defect (green) emission of the photoluminescence (PL) spectra of the nanorods varies with diameter, and can thus be controlled by the diameter control. The small ZnO nanorods have strong green emission, while the large diameter nanorods exhibit a remarkably suppressed green band. We show that this observation supports surface oxygen vacancies as the defect that gives rise to the green emission.

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.

Effect of indium on the properties of DX centers in Si‐doped Iny(Ga0.3Al0.7)1−yAs

The addition of In to Si‐doped Al0.3Ga0.7As has been investigated to determine its effect on DX centers. As expected, the persistent photoconductivity of the material is reduced as the band gap decreases with increasing In concentration. In addition, a new deep level transient spectroscopy peak is observed for the first time, which we attribute to DX centers having near In neighbors. This is clear evidence that the DX levels are highly localized states associated with donor impurities, whose properties are very sensitive to the local atomic configuration near the donor atom. This work supports previously published work on the effects of alloy disorder on DX centers, which is the strongest evidence to date for the microscopic configuration of the DX level.