Lukasz Wachnicki

Label-free sensitivity of long-period gratings enhanced by atomic layer deposited TiO 2 nano-overlays

In this paper, we discuss an impact of thin titanium dioxide (TiO(2)) coatings on refractive index (RI) sensitivity and biofunctionalization of long-period gratings (LPGs). The TiO(2) overlays on the LPG surfaces have been obtained using atomic layer deposition (ALD) method. This method allows for a deposition of conformal, thickness-controlled, with well-defined optical properties, and high-RI thin films which are highly desired for optical fiber sensors. It has been found that for LPGs working at a dispersion turning point of higher order cladding modes only tens of nanometers of TiO(2) overlay thickness allow to obtain cladding mode transition effect, and thus significant improvement of RI sensitivity. When the TiO(2) overlay thickness reaches 70 nm, it is possible to obtain RI sensitivity exceeding 6200 nm/RIU in RI range where label-free sensors operate. Moreover, LPGs with TiO(2)-enhanced RI sensitivity have shown improved sensitivity to bacteria endotoxin (E. coli B lipopolysaccharide) detection, when TiO(2) surface is functionalized with endotoxin binding protein (adhesin) of T4 bacteriophage.

The influence of growth temperature and precursors' doses on electrical parameters of ZnO thin films grown by atomic layer deposition technique

In this paper we report on the low-temperature growth (Ts=30-250^oC) of zinc oxide thin films by atomic layer deposition method using two different organic zinc precursors: diethylzinc and (for comparison) dimethylzinc, and deionized water as an oxygen precursor. An evident influence of growth temperature and precursors" doses on electron concentration and Hall mobility of obtained zinc oxide layers is presented. The lowest achieved room-temperature electron concentration was at the level of 10^1^6cm^-^3 with mobility up to 110cm^2/Vs.

Hafnium dioxide as a passivating layer and diffusive barrier in ZnO/Ag Schottky junctions obtained by atomic layer deposition

This paper reports on ZnO/Ag Schottky junctions obtained by the low temperature atomic layer deposition process. Introducing the thin (from 1.25 to 7.5 nm) layer of hafnium dioxide between the ZnO layer and evaporated Ag Schottky contact improves the rectification ratio to about 105 at 2V. For the ZnO/Ag junctions without the HfO2 interlayer, the rectification ratio is only 102. We assign this effect to the passivation of ZnO surface accumulation layer that is reported for ZnO thin films.

Photovoltaic properties of ZnO nanorods/p-type Si heterojunction structures

Summary Selected properties of photovoltaic (PV) structures based on n-type zinc oxide nanorods grown by a low temperature hydrothermal method on p-type silicon substrates (100) are investigated. PV structures were covered with thin films of Al doped ZnO grown by atomic layer deposition acting as transparent electrodes. The investigated PV structures differ in terms of the shapes and densities of their nanorods. The best response is observed for the structure containing closely-spaced nanorods, which show light conversion efficiency of 3.6%.

Electrical parameters of ZnO films and ZnO-based junctions obtained by atomic layer deposition

This work reports on the zinc oxide layers grown by atomic layer deposition (ALD) from dimethylzinc (Zn(CH3)2, DMZn) or diethylzinc (Zn(C2H5)2, DEZn) and deionized water precursors. These films are suitable for nanoelectronic applications, e.g. selecting elements in the new generation of non-volatile 3D memories constructed in the cross-bar architecture. This architecture imposes strict requirements on the parameters of obtained ZnO layers. Growth temperature must be below 200 °C, electron concentration not higher than 1017 cm−3 and mobility above 10 cm2 V−1 s−1. This is possible when the ALD growth method is used. We demonstrate the correlations between the structural, optical and electrical properties of ALD-ZnO layers. Their control allows us to obtain Schottky junctions with silver, whose parameters are suitable for the applications mentioned above. The ideality factor of about η ≈ 2.65 was calculated for the Schottky diodes based on the pure thermionic emission theory.

Kinetics of anatase phase formation in TiO2 films during atomic layer deposition and post-deposition annealing

Anatase phase formation in TiO2 films obtained by atomic layer deposition (ALD) is investigated. At growth temperature close to 200 °C, the anatase phase of TiO2 originates from crystalline seeds formed in an amorphous layer. These seeds, formed in the initial stages of a film growth, allow an expansion of the anatase phase in the amorphous parts and their transformation. This expansion occurs either during a further growth process or during a post-deposition annealing at relatively low temperatures (160–220 °C). The process of a lateral expansion of the anatase phase within the amorphous one was found to be thermally activated with an activation energy of 1.5 eV.

Trap levels in the atomic layer deposition-ZnO/GaN heterojunction—Thermal admittance spectroscopy studies

In this work, a n-ZnO/p-GaN heterojunction is analyzed using admittance spectroscopy techniques. Capacitance transient measurements performed at 10 kHz reveal four majority-carrier deep levels, the most important one located at approximately 0.57 eV below the ZnO conduction band (CB) edge with a density about two orders of magnitude below the doping level (NT = 4 × 1015 cm−3). The others, located at 0.20 eV, 0.65 eV, and 0.73 eV, are about three orders of magnitude below the doping level (NT = 4–9 × 1014 cm−3).

Low-Temperature Cathodoluminescence Investigations of High-Quality Zinc Oxide Nanorods

We present results of cathodoluminescence (CL) investigations of high-quality zinc oxide (ZnO) nanorods obtained by an extremely fast hydrothermal method on a silicon substrate. A scanning electron microscopy (SEM) system equipped with CL allows direct comparison of SEM images and CL maps, taken from exactly the same areas of samples. Investigations are performed at a temperature of 5 K. An interlink between sample microstructure and emission properties is investigated. CL confirms a very high quality of ZnO nanorods produced by our method. In addition, the presence of super radiation effects in ZnO nanorod arrays is suggested.

Analysis of scattering mechanisms in zinc oxide films grown by the atomic layer deposition technique

In this work, the analysis of the temperature-dependent electrical conductivity of highly crystalline zinc oxide (ZnO) thin films obtained by the Atomic Layer Deposition (ALD) method is performed. It is deduced that the most important scattering mechanisms are: scattering by ionized defects (at low temperatures) as well as by phonons (mainly optical ones) at higher temperatures. Nevertheless, the role of grain boundaries in the carrier mobility limitation ought to be included as well. These conclusions are based on theoretical analysis and temperature-dependent Hall mobility measurements. The presented results prove that existing models can explain the mobility behavior in the ALD-ZnO films, being helpful for understanding their transport properties, which are strongly related both to the crystalline quality of deposited ZnO material and defects in its lattice.

Hybrid disordered blends formed from fullerene porous layers and zinc oxide grown by atomic layer deposition

Thermally evaporated fullerene C60 porous films served as templates for a hybrid (molecular-inorganic) disordered blend formation. C60 films were covered with zinc oxide (ZnO) grown by atomic layer deposition. ZnO filled every pore in the C60 layer which led to the formation of C60–ZnO films with separate and distinguishable phases of C60 and ZnO constituents. Morphological, structural, optical, and electrical properties of the so-obtained films were investigated. Deposition of ZnO polycrystalline films on C60 porous layers resulted in the formation of ZnO with additional structural defects, compared to the films grown on planar substrates, which affected the electrical transport in the ZnO–C60 layers.