Metal oxides gas sensing properties particularly for In2O3 and ZnO nanostructures and nanostructured thin films are reviewed. Fabrication methods for these most commonly used metal oxides are presented, followed by a study on how growth techniques lead to nanostructures and nanostructured polycrystalline films with surface features of nanometer scale for film thickness bellow 1μm. The study continues with a discussion on how, a broad range of morphological parameters, affect the thin film response to various gases. After an overview, the study focus on thin films prepared by reactive dc magnetron sputtering and pulsed laser deposition in different growth conditions. In2O3 and ZnO thin films prepared for ozone sensing exhibit resistivity changes of five to eight orders of magnitude at room temperature after exposure to UV light and subsequent ozone treatment. Structural properties, i.e., crystallinity and microstructure investigated by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) are studied. The nanostructure and nanostructured surfaces are highly controlled by the deposition parameters, which, control the transport properties, and thus the sensing characteristics as measured by conductometric techniques. Analyses on the sensing response of nanostructures and nanostructured In2O3 and ZnO films for different gases are presented. Experiments on Surface Acoustic Wave (SAW) devices based on In2O3 and ZnO thin films fabricated on LiNbO3 substrates indicate the capability of achieving sensing levels in the low ppb range.
Photorefractive materials consistute a fast growing branch of nonlinear optics. The materials most commonly designated as photorefractive involve a charge-transport-induced non-linearity. Recent work on simple oxides such as InOx, prepared by dc sputtering, has demonstrated changes in their conductivity of more than six orders of magnitude after low power UV illumination and subsequent oxidation in and Ozon atmosphere. The structural changes on these films induced by growth parameters were studied by x-ray diffraction and Atomic Force Microscopy (AFM). Between Room Temperature (BRT) and 300°C it was found that there is a preferred growth along the (222) axis while AFM revealed a roughness increase as a function of film thickness and a tendency for nanoscale grains to be overgrown by larger neighbors. Based on these light induced charge-transport changes of InOx, ambient holographic recording process characteristics were obtained using a UV laser radiation at 325nm. It was realized that there exists a direct correlation of the recording efficiency with conductivity changes under ambient conditions. Evidence is provided for the presence of two coexisting processes in the recording regime.
The desirable electrical properties of InOx thin films and their response towards oxidizing gases has promoted InOx to be recognized as a promising material for gas sensors. In this study, InOx films in the thickness range of 10-1000 nm were deposited onto Corning 7059 glass substrates by dc magnetron sputtering. Their structural, electrical, and O3 and NO2 sensing properties were analyzed. Structural investigations carried out by XRD and AFM showed a strong correlation between crystallinity, surface topology and gas sensitivity. Moreover, the electrical conductivity exhibited a change of over six orders of magnitude during the processes of photoreduction and oxidation. The films deposited on alumina transducers were calibrated towards O3 and NO2 at temperatures from 50-300 °C. The sensors show promising characteristics as they exhibited reproducible and stable responses. The 50 nm thin film had a response of over 10 towards 50 ppb of ozone operating at 50°C, while the 20 nm film had a response of over 22 towards 0.1 ppm of NO2 at 100°C.
In high quality otpical coating systems for the DUV-spectral range, MgF2 is one of the preferred deposition materials. MgF2-coatings exhibit relatively low optical losses as well as high stability and laser induced damage thresholds. In the present joint research effort of several European laboratories, the potentiality of MgF2 is evaluated in respect to the production of improved optical coatings for applications in laser technology and semiconductor lithography. For this purpose, single layers of MgF2 were deposited on superpolished fused silica and CaF2-substrates by ion beam sputtering, boat and e-beam evaporation in different laboratories. Besides photometric inspections, the samples were characterized by an optical scatter measurement facility at 193 nm and 633 nm. The structural properties were assessed using AFM, XRD, and adapted TEM-techniques invovling conventional thinning methods for the layers. For the measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed.
ZnSe-based laser diodes have recently encountered strong competition from those grown from GaN related materials. These two material systems behave in a very different way as far as defect generation and propagation are concerned. For ZnSe-based materials the lifetime of a laser-diode is very sensitive to the density of pre-existing extended defects in the epitaxial material. Therefore, fabrication of a long- lived ZnSe-based laser diode requires an elimination of extended defects as well as making low-resistivity components in order to minimize device heating. We discuss the molecular beam epitaxy growth and characterization of ZnSe-based epitaxial structures on various III-V buffer layers lattice matched to GaAs. The status of our ZnSe-based laser diodes and microcavity LEDs will also be discussed.
The measurement of intrinsic laser induced damage thresholds (LIDT) in optical components for continuous wave (CW) CO2 radiation has been investigated. A combination of analytical and numerical models showed that the temperature rise is mainly determined by the surface absorption in transmissive as well as reflective components, and is proportional to the ratio of power to linear dimension (P/d) of the irradiated spot rather than to the conventional power/area (P/d2) parameter. The former ratio therefore represents the correct power scaling law for LIDT measurement in CW laser systems. The precise time domain within which this law holds is a function of spot diameter. This prediction has been confirmed by experimental LIDT tests on well characterized uncoated ZnSe substrates and copper mirrors, and on coated ZnSe windows and copper mirrors. Measured P/d values, though lower than predicted by modelling are considerably higher than those inferred from the technical literature, and show that transmissive components may be used at much higher powers than are at present believed. The results indicate that surface absorption occurs primarily in the sub-surface processing layer. This has been shown by transmission electron microscopy and spectroscopic ellipsometry to be a few hundred nm in depth.
This paper reports progress towards characterizing the laser induced damage threshold (LIDT) for zinc selenide and copper components in CW/C02 laser optical systems. The work has been undertaken by industrial partners and research centres supported by the CRAFT initiative of the European Commission. Specimen surfaces have been characterized by a wide range of experimental methods, including absorption calorimetry. Thermal modelling on the basis of these observations indicates that the temperature rise in irradiated components is dominated by surface absorption, and that the maximum temperature rise is proportional to the ratio of the total power P to focal spot size d rather than to the areal power density. A CW/C02 laser damage facility has been constructed by one of the industrial partners. Preliminary. experiments have determined P/d values at the LIDT an order of magnitude less than those predicted by the measured level of surface absorption.