Alessandro Antonaia

Structural and optical characterization of amorphous and crystalline evaporated WO3 layers

Abstract Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. At low substrate temperature thermal evaporation technique produces an amorphous material mixed to a nanocrystalline phase. When this material is annealed at temperatures higher than 400°C a fully crystalline structure is obtained. Different absorption mechanisms are responsible for colored amorphous and crystalline WO3 layers, small polaron transitions and Drude-like free electron scattering, respectively. Furthermore, both O/W ratio and crystalline structure affect electrochromic efficiency of a WO3 film. It has been confirmed that a hydrogen tungsten bronze structure appears when the material is colored. Finally, a self-bleaching process occurs when a colored WO3 film is exposed to an oxidative environment.

Transport mechanism and IR structural characterisation of evaporated amorphous WO3 films

Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. Absorption by small polaron transitions has been hypothesised to explain the coloration mechanism in amorphous WO3 films and the most reliable model was found by Zhang, who proposed that a WO3−x bleached virgin film (x<0.4) consists mainly of W6+ and W4+, whereas in the coloured state W5+ is also present. In this work, amorphous WO3 films have been deposited by a thermal evaporation technique on glass and c-Si substrates. Electrical, optical and IR properties of these samples have been studied. A hopping mechanism with mobility activation has been hypothesised to justify the discrepancies between electrical and optical properties. Furthermore, our careful analysis of the typical IR absorption peaks in the range 500–1000 cm−1 confirmed the Zhang theory on the coloration mechanism.

Improvement in electrochromic response for an amorphous/crystalline WO3 double layer

Abstract Amorphous WO3 films have been deposited by E-beam evaporation technique at a substrate temperature of 80°C using different base pressure values. The best results have been obtained starting from a mild base pressure (1×10−5 Torr) probably due to the presence of water and oxidative atmosphere in the evaporation chamber. TEM investigations have shown that thermally evaporated WO3 films have an amorphous metastable structure, which subjected to energetic solicitations changes towards a nanocrystalline phase. The amorphous material subsequently annealed at a temperature of 500°C exhibits a fully crystalline structure. Also, in this case good electrochromic properties have been observed. Afterwards a WO3 double layer structure has been realised depositing an amorphous film upon a crystalline layer. The amorphous/crystalline structure properties have been compared with the amorphous and crystalline single layer ones. The colouration response and the transmittance asymptotic value during the bleaching phase have been better for the double layer.

Effect of substrate surface treatment on the nucleation and crystal growth of electrodeposited copper and copper–indium alloys

Abstract Thin films of copper–indium alloy electroplated on molybdenum substrates show different morphologies related to the substrate treatment. An ammonia treatment of the molybdenum electrode can remove a thin oxide layer from the electrode surface, changing the morphological structure of the deposit. The nucleation kinetics of copper were investigated by potentiostatic current–time transients. The deposition process involves instantaneous nucleation for untreated substrates and progressive nucleation for ammonia treated substrates, with diffusion-controlled growth of the three-dimensional nuclei. A higher nucleation density is observed for ammonia-treated molybdenum substrates. The film morphology of the copper–indium alloys can be closely related to the copper nucleation mechanism. On the untreated substrates a low nucleation density together with high adatom mobility lead to the formation of a globular structure. However, when the nucleation density is high and the nuclei are formed continuously during crystal growth, the film evolves towards a porous structure. After the selenization step the film morphology appears related to the precursor structure.

Electrodeposition of copper-indium alloy under diffusion-limiting current control

Abstract The aim of this work was to study a method of electrodeposition of stoichiometric copper-indium alloy to be used as a precursor for obtaining CuInSe2 by selenization. This compound represents one of the most promising materials in terms of low cost and high efficiency for the realization of large-area thin film solar cells for terrestrial applications. However, to avoid the growth of secondary phases such as CuSe and In2Se3, the stoichiometry and morphology of the alloy have to be carefully controlled. We investigated the relationship between the ratio of Cu2+ to In3+ in the solution and the metal ratio in the deposited film. In order to avoid the use of complexing agents we chose to control the stoichiometry of the copper-indium alloys, electrodepositing them under diffusion-limiting current. Since the diffusion currents are, as a first approximation, proportional to the concentrations, there is a simple relationship between the individual diffusion currents of the ions and the stoichiometry of the deposited alloy. Thus we can change the CuIn content in the deposited film by controlling the ratio between the ions in the deposition bath.

Influence of process parameters on properties of reactively sputtered tungsten nitride thin films

Tungsten nitride (WNx) thin films were produced by reactive dc magnetron sputtering of tungsten in an Ar–N2 gas mixture. The influence of the deposition power on the properties of tungsten nitride has been analyzed and compared with that induced by nitrogen content variation in the sputtering gas. A combined analysis of structural, electrical and optical properties on thin WNx films obtained at different deposition conditions has been performed. It was found that at an N2 content of 14% a single phase structure of W2N films was formed with the highest crystalline content. This sputtering gas composition was subsequently used for fabricating films at different deposition powers. Optical analysis showed that increasing the deposition power created tungsten nitride films with a more metallic character, which is confirmed with resistivity measurements. At low sputtering powers the resulting films were crystalline whereas, with an increase of power, an amorphous phase was also present. The incorporation of an ...

Transport properties of polycrystalline tin oxide films

Abstract Device quality, tin oxide polycrystalline layers deposited by spray pyrolysis technique have been characterized in terms of morphological, electrical and transport properties. The set of the investigated samples exhibit high values of the mobility (ranging between 30 and 70 cm2 V−1 s−1). The behaviour of the mobility, reported as a function of the carrier concentration and of the size of the crystallines, reveals a transport governed by the processes at the grain boundaries. Furthermore, the behaviour of the mobility with the temperature strongly suggests a transport mechanism determined by tunneling of the potential barrier existing between the grains, and rules out the importance of the thermionic emission process.

Refractive index measurement in TCO layers for micro optoelectronic devices

The operation of micro-optoelectronic devices relies on optical properties of materials, first of all the wavelength dependent refractive index. Standard measurement techniques are often prone to significant ambiguities caused by the periodic nature of the optical response. In this paper we propose a procedure which exploits transmittance measurements, performed at variable light incidence angle, to reconstruct the complex refractive index as a function of the wavelength. Experiments performed on a thin TCO layer prove the reliability of the method.

Approximate analysis of optical properties for ZnO rough surfaces

TCO (Transparent Conductive Oxides) layers often exhibit very rough surfaces and their optical properties, can not be described by one dimensional analytical models and numerical approaches must be adopted. However, computational time becomes, often, too large. In this paper we propose an automated procedure, based on the interaction between MATLAB and the Sentaurus TCAD environment which finds, in a generic rough surface, the smallest area characterized by the same statistical features of the whole surface so that full 3D analysis can be performed.