A. Bakin

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.

Influence of exciton-phonon coupling on the energy position of the near-band-edge photoluminescence of ZnO nanowires

Room-temperature near-band-edge photoluminescence of ZnO is composed of contributions from free-exciton recombination and its longitudinal-optical phonon replica. By tracking the photoluminescence of ZnO nanowires from 4K up to room temperature, the authors show that the relative contributions of these emission lines show a strong variation for samples grown under different conditions. The varying coupling strengths of the excitons and phonons thus lead to a significant shift of the energy position of the room-temperature photoluminescence. They verify that this is not caused by laser heating or stress/strain but is most probably related to crystalline imperfections in the surface region.

Magnetic property investigations on Mn-doped ZnO Layers on sapphire

The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10−5emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples.

Photoluminescence properties: Catalyst-free ZnO nanorods and layers versus bulk ZnO

In this contribution, we compare the photoluminescence properties of ZnO nanorods and epilayers with those of bulk ZnO. Owing to the high aspect ratio (length of 4–14μm, diameter of 80–500nm), the characterized ZnO nanorods show very good optical properties. Due to the high surface-to-volume ratio in ZnO nanorods, surface excitons dominate at low temperature. The optical properties of nanorod ensembles improve with increasing nanorod length. The photoluminescence emission from free A excitons was intense in the ZnO layer at 13K.

Influence of ZnO seed crystals and annealing on the optical quality of low-temperature grown ZnO nanorods

The influence of ZnO seed crystals and postgrowth annealing on low-temperature aqueous chemically grown ZnO nanorods is analyzed. At the seed crystal/nanorod interface a high density of structural defects leads to emission at 3.332 eV, attributed to excitons bound to structural defects. This peak is absent for seed crystals, very pronounced for rods of shorter lengths grown on seed crystals, and reduced for longer nanorods. After annealing in oxygen and nitrogen atmosphere, the near-band-edge excitonic transitions sharpen and deep-level emission is strongly reduced. Time-resolved photoluminescence measurements show a striking similarity between donor-bound excitons and excitons bound to structural defects.

Determination of the specific resistance of individual freestanding ZnO nanowires with the low energy electron point source microscope

A low energy electron point source microscope is used to determine the electrical conductivity of freestanding ZnO nanowires. The nanowires were contacted with a manipulation tip and I-V curves were taken at different wire lengths. From those, the specific resistance was calculated and separated from the contact resistance. By comparing the specific resistances of ZnO nanowires with diameters between 1100 and 48nm, a large surface contribution for the thin nanowires was found. A geometric model for separation between surface and bulk contributions is given.

Structural characterization of ZnO films grown by molecular beam epitaxy on sapphire with MgO buffer

The structural characteristics of the ZnO film grown on sapphire substrate using a thin MgO buffer layer were studied using transmission electron microscopy and high-resolution x-ray diffraction. The growth was carried out in a modified plasma-molecular beam epitaxy system. The observed misfit dislocations were well confined at the sapphire overgrown interface exhibiting domain matching epitaxy, where the integral multiples of lattice constants match across the interface. The main extended defects in the ZnO film were the threading dislocations having a mean density of 4×109cm−2. The formation of the MgO buffer layer as well as the ZnO growth were monitored in situ by reflection high-energy electron diffraction. The very thin ∼1nm, MgO buffer layer can partially interdiffuse with the ZnO as well as react with the Al2O3 substrate forming an intermediate epitaxial layer having the spinel (MgO∕Al2O3) structure.

Optical investigations and exciton localization in high quality Zn1−xMgxO–ZnO single quantum wells

This work investigates the photoluminescence properties of Zn1−xMgxO–ZnO single quantum wells, which have been fabricated by molecular-beam epitaxy. With increasing temperature from 13to300K the single quantum well-related emission peaks exhibit an irregular S-shaped (redshift-blueshift-redshift) behavior, which is in contrast with that ascribed to band gap shrinkage (redshift). In order to clarify the origin of this behavior, the temperature dependence of the integral photoluminescence intensity of the quantum well emission was studied and the relevant activation energies were calculated and correlated to its full width at half maximum, band offsets, and monolayer fluctuations.

Controlled low-temperature fabrication of ZnO nanopillars with a wet-chemical approach

Aqueous chemical growth (ACG) is an efficient way to generate wafer-scale and densely packed arrays of ZnO nanopillars on various substrate materials. ACG is a low-temperature growth approach that is only weakly influenced by the substrate and even allows growth on flexible polymer substrates or on conducting materials. The advanced fabrication of wafer-scale and highly vertically aligned arrays of ZnO nanopillars on various substrate materials is demonstrated. Moreover, it is possible to control the morphology in diameter and length by changing the growth conditions. Photoluminescence characterization clearly shows a comparatively strong band-edge luminescence, even at room temperature, that is accompanied by a rather weak visible luminescence in the yellow/orange spectral range.

High-speed InP-based resonant tunneling diode on silicon substrate

A technology for high speed and high performance III-V semiconductor devices on silicon substrate has been developed. It consists of an InP-on-Si quasi-substrate exhibiting an XRD FWHM as low as 86 arcsec, followed by a low-temperature (370/spl deg/C) grown InAlAs layer. The surface roughness is reduced to 1.9 nm along with an almost complete elimination of surface defects. The applicability is experimentally verified for InP-based resonant tunneling diodes exhibiting a speed index of 32 ps/V indicating a potentially low-cost technology for high functionality circuits operating above 10 Gb/s.