Arne Behrends

Zinc oxide nanorod based photonic devices: recent progress in growth, light?emitting diodes and lasers

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal-organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour-liquid-solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro- and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I-V characteristics of ZnO:P nanowire/ZnO:Ga p-n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

Determination of the absolute internal quantum efficiency of photoluminescence in GaN co-doped with Si and Zn

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the ZnGa acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE above 90% was calculated for several samples having different concentrations of Zn. Accurate and reliable values of the IQE were obtained by using several approaches based on rate equations. The concentrations of the ZnGa acceptors and free electrons were also estimated from the photoluminescence measurements.

ZnO-GaN Hybrid Heterostructures as Potential Cost-Efficient LED Technology

Reliable and reproducible p-type doping is the main challenge for fabricating highly efficient ZnO-based light-emitting diodes. During the last few years, the lack of reliable p-type conductivity in ZnO has initiated research concerning the combination of ZnO and other semiconductors, which can then be doped p-type. One of these concepts is the combination of ZnO with GaN heterostructures aiming at the fabrication of hybrid LEDs. We discuss the problems as well as potential benefits from a combination of ZnO and GaN hybrid heterostructures in a single device. We also present our recent results on ZnO-GaN hybrid LEDs using an inverted LED concept. The hybrid LEDs have an external quantum efficiency of more than 35%.

Photoluminescence from ZnO nanowires

Self-organized ZnO nanowires were grown by metal-organic chemical vapor deposition on sapphire substrate. Steady-state photoluminescence (PL) from the samples with different densities of the nanowires was studied in wide range of temperatures and excitation intensities. At 13K the PL spectrum consisted of sharp exciton lines at 3.354, 3.357, and 3.363eV and a weak red band with a maximum at ∼1.75eV. The peak intensity of the red band was four orders of magnitude lower than that of the strongest exciton line. The authors investigated also the effect of ambient on PL at room temperature. In vacuum the PL intensity increased linearly with the excitation power density in the range from 3×10−4to0.3W∕cm2, whereas in air the PL intensity increased superlinearly. Such behavior is attributed to photoinduced adsorption of species from air which increases the near-surface barrier and depletion region.

Extremely high absolute internal quantum efficiency of photoluminescence in co-doped GaN:Zn,Si

We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phase epitaxy and a detailed study of photoluminescence in this material. We observe an exceptionally high absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of 0.93±0.04 has been obtained from several approaches based on rate equations.