In n-type GaN, an upward band bending of about 1 eV is caused by negative charge at the surface. UV light reduces the band bending by creating a surface photovoltage (SPV), which can be measured by a Kelvin probe. Previously, we reported a fast SPV signal of about 0.6 eV in undoped and moderately doped GaN. In this work, we have studied degenerate GaN co-doped with Zn and Si, with a Si concentration of about 10(19) cm(-3) and a Zn concentration of 6x10(17) cm(-3). At room temperature, a fast component of about 0.6 eV was observed. However, after preheating the sample at 600 K for one hour and subsequently cooling the sample to 300 K (all steps performed in vacuum), the fast component disappeared. Instead, a very slow (minutes) and logarithmic in time rise of the SPV was observed with UV illumination. The total change in SPV was about 0.4 eV. This slow SPV transient can be reversibly converted into the "normal" fast (subsecond) rise by letting air or dry oxygen in at room temperature. Possible explanations of the observed unusual SPV transients are discussed.

Gallium nitride samples codoped with zinc and silicon were fabricated by metal organic chemical vapor deposition. Optical properties of these samples with low-level doped acceptor are investigated by temperature dependent photoluminescence (PL) measurements. PL spectrum shows three peaks labeled as near band edge (NBE), blue luminescence (BL), and yellow luminescence (YL). BL originates from Zn-Ga acceptors in GaN:Zn,Si. It is quenched above 250 K for all samples measured at excitation density of P-exc = 0.2 W cm(-2). The concentration of acceptors responsible for the BL band is estimated for relatively low Zn concentrations by comparing integrated PL intensity of NBE and Zn-related BL bands at 200 K. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Owing to its excellent luminescent properties, zinc oxide is a promising material for the development of optoelectronic and photovoltaic devices. ZnO epitaxial layer quality and control of native and dopant point defects is still an issue for direct device production. One of the challenges for high-quality epitaxial layer growth is the lack of high-quality, large and affordable ZnO wafers. The implementation of nanostructures is a promising solution. For instance, high-quality nanopillars with a small footprint on the wafer can be deposited on different types of wafers, including flexible ones. This chapter gives a short overview of the methods of the fabrication of ZnO nanostructures. The focus is not on the properties of the ZnO nanostructures but on the growth approaches and their comparison promoting the choice of the proper growth method for certain applications. The methods presented can be classified in three groups: wet chemical approaches, gas-phase approaches, and implementation of ZnO nanopowders. The recent advances in this area are presented. The most important aspects of manufacturing both quantum wells (QWs) and p-n junctions embedded into nanostructures are also discussed.

Photoluminescence (PL) from high-quality GaN co-doped with silicon and zinc was investigated in detail. The internal quantum efficiency (IQE) of PL was determined from the analysis of the dependencies of the PL intensity on the excitation intensity and temperature, and the simulation of these dependencies with a phenomenological model based on rate equations. The model reproduces an important phenomenon: the quenching of a recombination channel with a high IQE causes a rise in efficiency of all the other PL bands. Quantitative analysis of this phenomenon allows one to determine reliably the absolute IQE of PL. The absolute IQE of the PL in GaN co-doped with Si and Zn exceeds 90%, with the largest contribution coming from the blue luminescence band associated with the Zn-Ga acceptor. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.

Transparent conductive oxides (TCOs) are used for a variety of different applications, e.g., in solar cells and light emitting diodes (LEDs). Mostly, sputtering is used, which often results in a degradation of the underlying semiconductor material. In this work we report on a "soft" method for the fabrication of ZnO films as TCO layers by using metal organic chemical vapor deposition (MOCVD) at particularly low temperatures. The MOCVD approach has been studied focusing on the TCO key issues: fabrication temperature, morphology, optical, and electrical properties. Very smooth ZnO films with rms values down to 0.8 nm were fabricated at a substrate temperature of only 300 degrees C. Ga-doping is well controllable even for high carrier concentrations up to 2 x 10(20) cm(-3), which is above the Mott-density leading to metallic-like behavior of the films. Furthermore all films show excellent optical transparency in the visible spectral range. As a consequence, our MOCVD approach is well suited for the soft fabrication of ZnO-based TCO layers. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present an approach for the growth of ZnO nanowire arrays with a length of up to 30μm on fluorine doped tin oxide substrates with a growth rate of 20μm per hour. The growth was carried out in a vapor phase transport setup at a temperature of 600°C. To achieve an aligned growth on fluorine doped tin oxide substrates we used wet-chemically grown nanowire arrays with typical length of 2 to 3μm as a seeding layer. Additionally the nanowire arrays were used as electrodes for the manufacturing of dye sensitized solar cells and the best achieved efficiencies were 1.83% at 10mW/cm2 and 1.47% at 100mW/cm2.

In this work we report on the optical investigation of Zn doped GaN films fabricated by metal organic chemical vapor deposition. The samples show bright emission in the blue spectral range around 2.9 eV when Si codoping is provided. This emission is suggested to be used for single-photon emission, thus the density of the Zn-Si pairs was drastically reduced leading to a decrease of the blue luminescence. For electrically excited single-photon sources these Zn-Si pairs have to be incorporated into LEDs, therefore we fabricated GaN-based nano-LEDs which show electroluminescence at 430 nm. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.

In this work we report on the fabrication and characterization of a n-ZnO/p-GaN heterojunction LED. The p-GaN layer was fabricated using MOCVD on Al2O3 with Mg as the acceptor whereas the ZnO nanostructures were grown in a very simple vapor transport system without any additionally doping. Room temperature electroluminescence (EL) measurements show green deep band emission centered at 2.3 eV which is clearly visible with the naked eye when the structure is forward biased. Cathodoluminescence mapping was performed to explain the absence of the band edge emission in the EL spectrum. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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%.

GaN and ZnO are both wide band gap semiconductors with interesting properties concerning optoelectronic and sensor device applications. Due to the lack or the high costs of native substrates, alternatives like sapphire, silicon, or silicon carbide are taken, but the resulting lattice and thermal mismatches lead to increased defect densities which reduce the material quality. In contrast, nanostructures with high aspect ratio have lower defect densities as compared to layers. In this work, we give an overview on our results achieved on both ZnO as well as GaN based nanorods. ZnO nanostructures were grown by a wet chemical approach as well as by VPT on different substrates – even on flexible polymers. To compare the growth results we analyzed the structures by XRD and PL and show possible device applications. The GaN nano‐ and microstructures were grown by metal organic vapor phase epitaxy either in a self‐organized process or by selective area growth for a better control of shape and material composition. Finally we take a look onto possible device applications, presenting our attempts, e.g., to build LEDs based on GaN nanostructures.

In this work we studied PL in ZnO layers and nanostructures, including ZnO homoepitaxial layers on ZnO substrate and ZnO–Zn1−xMgxO single quantum well (SQW) structures grown on sapphire substrates by MBE, and ZnO nanowires grown on sapphire by MOCVD. The external quantum efficiency (QE) of PL in O-face ZnO layers exceeded that in Zn-face ZnO layers by two orders of magnitude at low temperatures. In a sample with SQW the combined external QE from the 4.6-nm-wide SQW and 50-nm-thick Zn1−xMgxO barriers achieved 28% at 15K. The highest external QE was observed in one of the samples with ZnO nanowires—52% at 15K and 2% at 300K. Contribution of defect-related PL bands in ZnO nanowires samples was extremely low.

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.

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.

Single ZnO nanowires grown by vapor phase transport were contacted by Ti/Au electrodes patterned by using e-beam lithography. The current-voltage characteristics display a rectifying behavior, likely due to an interfacial insulating layer between the metal and the semiconductor. The contact resistance was determined to 85 M Omega, the deduced carrier concentration yields 10(15) cm(-3) which is low and hints towards Fermi-level pinning at surface states. Spectrally-resolved photoconduction measurements reveal an increase in current only for illumination with light at an energy above the band gap energy. Scanning electron micrographs of the metal-nanowire interface show a smearing of the electrodes; however, energy dispersive X-ray element maps could not expose the elemental composition of the blurry regions.

Self-organized ZnO nanopillars were grown on a-plane Al"2O"3 in a vertical MOCVD reactor using diethylzinc and N"2O as precursors. This is the very first Thomas Swan reactor that is specially designed for the growth of ZnO and GaN. The influence of different growth parameters including growth time, VI/II-ratio, substrate temperature and reactor pressure on the shape of the nanostructures is investigated by field emission scanning electron microscopy. An explanation for the formation of nanopillars is given resulting in high substrate temperatures and low VI/II-ratios for the best results. It is shown that the density, length and diameter can be controlled choosing appropriate growth parameters.

An overview of our nanostructures and thin layers fabrication technologies including MOCVD, vapour phase transport, MBE, chemical growth, electrodeposition is presented. The developed technologies provide control on the nanopillar characteristics, and still leaving the self-organisation mechanism untouched. Also fabrication of complicated heterostructures like quantum wells both as epitaxial layers or embedded into nanopillars was demonstrated. First results of the device applications of ZnO nanostructures as well as implementation of alternative ZnO nanostructures like nanopowders are discussed.

Zinc oxide (ZnO) nanorods grown by vapor transport were contacted individually with Ti/Au electrodes structured by e-beam lithography. The rectifying behaviour observed in the current–voltage characteristics is likely caused by an interfacial insulating layer between the ZnO nanowires and the contact material. Energy-dispersive X-ray spectroscopy (EDX) measurements performed on electrically measured and non measured reference nanowires reveal that no electromigration of Au or Ti into the nanowires occurs.

For thulium-doped fiber amplifier (TDFA) design, knowledge of the gain distribution within the active fiber is of great value. For the first time, the distributed gain along highly thulium-doped fibers was measured with high-resolution reflectometry. The technique of coherent optical frequency-domain reflectometry is a nondestructive and noninvasive method well matched to this task due to its dynamic range, spatial resolution, and measurement range. Using thulium-doped fibers with different Tm 3+ -dopant concentrations, we show precise measurements of Rayleigh backscattering levels for obtaining the optimum gain-length ratio for S-band TDFA pumped by 1050-nm laser diodes