Ł. Wachnicki

ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.

Titanium oxide thin films obtained with physical and chemical vapour deposition methods for optical biosensing purposes.

This work discusses an application of titanium oxide (TiOx) thin films deposited using physical (reactive magnetron sputtering, RMS) and chemical (atomic layer deposition, ALD) vapour deposition methods as a functional coating for label-free optical biosensors. The films were applied as a coating for two types of sensors based on the localised surface plasmon resonance (LSPR) of gold nanoparticles deposited on a glass plate and on a long-period grating (LPG) induced in an optical fibre. Optical and structural properties of the TiOx thin films were investigated and discussed. It has been found that deposition method has a significant influence on optical properties and composition of the films, but negligible impact on TiOx surface silanization effectiveness. A higher content of oxygen with lower Ti content in the ALD films leads to the formation of layers with higher refractive index and slightly higher extinction coefficient than for the RMS TiOx. Moreover, application of the TiOx film independently on deposition method enables not only for tuning of the spectral response of the investigated biosensors, but also in case of LSPR for enhancing the ability for biofunctionalization, i.e., TiOx film mechanically protects the nanoparticles and induces change in the biofunctionalization procedure to the one typical for oxides. TiOx coated LSPR and LPG sensors with refractive index sensitivity of close to 30 and 3400nm/RIU, respectively, were investigated. The ability for molecular recognition was evaluated with the well-known complex formation between avidin and biotin as a model system. The shift in resonance wavelength reached 3 and 13.2nm in case of LSPR and LPG sensors, respectively. Any modification in TiOx properties resulting from the biofunctionalization process can be also clearly detected.

ZnO, ZnMnO and ZnCoO films grown by atomic layer deposition

Despite many efforts, the origin of a ferromagnetic (FM) response in ZnMnO and ZnCoO is still not clear. Magnetic investigations of our samples, not discussed here, show that the room temperature FM response is observed only in alloys with a non-uniform Mn or Co distribution. Thus, the control of their distribution is crucial for the explanation of contradicted magnetic properties of ZnCoO and ZnMnO reported till now. In this paper, we discuss advantages of the atomic layer deposition (ALD) growth method, which enables us to control the uniformity of ZnMnO and ZnCoO alloys. Properties of ZnO, ZnMnO and ZnCoO films grown by the ALD are discussed.

Contactless electroreflectance of ZnO layers grown by atomic layer deposition at low temperature

Room-temperature contactless electroreflectance (CER) has been applied to study optical transitions in ZnO layers grown by atomic layer deposition at low temperatures on glass, silicon and GaN substrates. A broad CER resonance was clearly observed at the energy of ~3.4 eV for layers deposited at low temperatures (100–240 °C) on glass or silicon. This resonance has been attributed to excitonic/band-to-band absorption in polycrystalline ZnO. A sharp excitonic resonance at ~3.32 eV was observed for monocrystalline ZnO layers deposited on GaN templates at higher temperatures (>250 °C). In addition, the broad CER resonance at ~3.4 eV was also observed for these layers but its intensity decreases when the growth temperature is increased, i.e. c-plane-oriented monocrystalline areas appear in the ZnO layer instead of polycrystalline areas with various surface orientations.

Reduction of Tb4+ ions in luminescent Y2O3:Tb nanorods prepared by microwave hydrothermal method

Abstract Terbium doped yttrium oxide was prepared with the microwave hydrothermal method. The Y 2 O 3 :Tb nanomaterial crystallized as needle-like grains. Bright luminescence in the green region was observed. Significant luminescence intensity increase was obtained after thermal treatment. Reduction of terbium ions was observed after heating in the air atmosphere. Tb 4+ ions were found to be stabilized by crystal impurities. Hydroxyl species were found to have impact on vacancies elimination. The terbium ions were used as optical and magnetic indicator of the material properties.

Modification of Emission Properties of ZnO Layers due to Plasmonic Near-Field Coupling to Ag Nanoislands.

A simple fabrication method of silver (Ag) nanoislands on ZnO films is presented. Continuous wave and time-resolved photoluminescence and transmission are employed to investigate modifications of visible and UV emissions of ZnO brought about by coupling to localized surface plasmons residing on Ag nanoislands. The size of the nanoislands, determining their absorption and scattering efficiencies, is found to be an important factor governing plasmonic modification of optical response of ZnO films. The presence of the Ag nanoislands of appropriate dimensions causes a strong (threefold) increase in emission intensity and up to 1.5 times faster recombination. The experimental results are successfully described by model calculations within the Mie theory.

High-k oxides by atomic layer deposition—Applications in biology and medicine

Wide band gap oxides grown by atomic layer deposition (ALD) are intensively studied for applications as insulators (gate oxides in field effect transistors) and as resistance switching materials in electronic memories. For these applications, compositions of dielectric films (laminar structures of HfO2, TiO2, ZrO2, and Al2O3) can also be used. Our research shows that the same layers show antibacterial activity comparable to ZnO. For mentioned application, it is important that the ALD allows uniform coating of various materials, including temperature sensitive ones (e.g., perishable fabrics) and equipment and instruments (including implants) used in hospitals and in a food industry. Here, the authors present the antibacterial activity of ALD-deposited metal oxides, promising a range of potential applications in medicine, veterinary, broader health care, and food industry.

Tuning properties of long-period gratings by atomic layer deposited zinc oxide nano-coating

This work presents an application of thin zinc oxide (ZnO) films obtained using atomic layer deposition (ALD) for effective tuning of spectral response and the refractive-index (RI) sensitivity of long-period gratings (LPGs). The technique allows for an efficient and well controlled deposition at monolayer level of excellent quality nano-films as required for optical sensors. The effect of ZnO deposition on spectral properties of the LPGs is discussed. We correlated the increase in ZnO thickness with the shift of the LPG resonance wavelength and proved that similar films are deposited on fibers and silicon reference samples in the same process run. The thin overlay effectively changes the distribution of the cladding modes and thus also tunes the device’s RI sensitivity. The tuning can be simply realized by varying number of cycles, which is proportional to thickness of the high-refractive-index (n<1.9 in infrared spectral range) ZnO film. The advantage of this approach is precision in determining the film thickness resulting in RI sensitivity of the LPGs.

Refractive index sensitivity of optical fiber lossy-mode resonance sensors based on atomic layer deposited TiOx thin overlay

This work presents an optical fiber refractive index sensors based on lossy–mode resonance (LMR) effect supported by titanium oxide (TiOx) thin overlay. The TiOx overlays of different thickness were deposited on core of polymer-clad silica (PCS) fibers using atomic layer deposition (ALD) method. Based on numerical simulations, a number of structures differing in the location of exposed core area and the thickness of TiOx coatings were designed. For fabricated structures the spectral response to external refractive index (next) was measured. The maximum sensitivity reaches 634.2 nm/RIU (next range: 1.357 - 1.402 RIU; TiOx coating thickness: 260.9 nm; investigated spectral range: 500-800 nm) and it highly depends on the thin-film thickness.

Oxide-based materials by atomic layer deposition

Thin films of wide band-gap oxides grown by Atomic Layer Deposition (ALD) are suitable for a range of applications. Some of these applications will be presented. First of all, ALD-grown high-k HfO2 is used as a gate oxide in the electronic devices. Moreover, ALD-grown oxides can be used in memory devices, in transparent transistors, or as elements of solar cells. Regarding photovoltaics (PV), ALD-grown thin films of Al2O3 are already used as anti-reflection layers. In addition, thin films of ZnO are tested as replacement of ITO in PV devices. New applications in organic photovoltaics, electronics and optoelectronics are also demonstrated Considering new applications, the same layers, as used in electronics, can also find applications in biology, medicine and in a food industry. This is because layers of high-k oxides show antibacterial activity, as discussed in this work.