Giovanna Scarel

Surface Polarity Shielding and Hierarchical ZnO Nano-Architectures Produced Using Sequential Hydrothermal Crystal Synthesis and Thin Film Atomic Layer Deposition

Three-dimensional nanoscale constructs are finding applications in many emerging fields, including energy generation and storage, advanced water and air purification, and filtration strategies, as well as photocatalytic and biochemical separation systems. Progress in these important technologies will benefit from improved understanding of fundamental principles underlying nanostructure integration and bottom-up growth processes. While previous work has identified hydrothermal synthesis conditions to produce nanoscale ZnO rods, sheets, and plates, strategies to systematically integrate these elements into more complex nano-architectures are not previously described. This article illustrates that amorphous nanoscale coatings formed by atomic layer deposition (ALD) are a viable means to modulate and screen the surface polarity of ZnO crystal faces and thereby regulate the growth morphology during successive hydrothermal nanocrystal synthesis. Using this new strategy, this work demonstrates direct integration and sequential assembly of nanocrystalline rods and sheets to produce complex three-dimensional geometric forms, where structure evolution is achieved by modifying the surface growth condition, keeping the hydrothermal growth chemistry unchanged. Therefore, rational planning of seed layer and feature spacing geometries may allow researchers to engineer, at the nanoscale, complex three-dimensional crystalline and semicrystalline constructs for a wide range of future applications.

Effects of growth temperature on the properties of atomic layer deposition grown ZrO2 films

Zirconium dioxide films are grown in 200 atomic layer deposition cycles. Zirconium tetrachloride (ZrCl4) and water (H2O) are used as precursors. A relatively high dielectric constant (κ=22), wide band gap, and conduction band offset (5.8 and 1.4 eV, respectively) indicate that zirconium dioxide is a most promising substitute for silicon dioxide as a dielectric gate in complementary metal–oxide–semiconductor devices. However, crystallization and chlorine ions in the films might affect their electrical properties. These ions are produced during atomic layer deposition in which the ZrCl4 precursor reacts with the growth surface. It is desirable to tune the composition, morphology, and structural properties in order to improve their benefit on the electrical ones. To address this issue it is necessary to properly choose the growth parameters. This work focuses on the effects of the growth temperature Tg. ZrO2 films are grown at different substrate temperatures: 160, 200, 250, and 350 °C. Relevant modification...

Investigation of interfacial layer development between thin Al2O3 films grown using atomic layer deposition and Si(100), Ge(100), or GaAs(100)

The uncontrolled formation of an interfacial layer between high-κ oxides and semiconductors is a major concern in advanced microelectronics not only for Si-based devices but also for those exploiting the higher mobility of Ge and GaAs. Using transmission electron microscopy, the authors investigate the interfacial layer formed between as-grown thin Al2O3 films, deposited using atomic layer deposition, and oxide free Si(100), Ge(100), and GaAs(100). In particular, they compare the effects of two different oxygen sources (H2O and O3) on interfacial layer formation during the growth process. They show that no interfacial layer can be distinguished unambiguously between the Al2O3 films, grown using TMA and H2O or O3, and all the semiconductor substrates.

Heat recovery mechanism in the excitation of radiative polaritons by broadband infrared radiation in thin oxide films

This work probes radiative polaritons in thin oxide layers as a mean to capture and absorb broadband infrared radiation and transform it into heat. A heat recovery mechanism, based on the Seebeck effect, is used as the tool of the investigation. Heat production challenges the current understanding which views the excitation of radiative polaritons as only accompanied by the emission of electromagnetic radiation. The heat recovery mechanism presented here can inspire the design of infrared energy harvesting devices, similar to photovoltaic cells, and other devices to convert energy from a wide range of the electromagnetic radiation spectrum using thermoelectric power generators.

Chemical and structural properties of atomic layer deposited La2O3 films capped with a thin Al2O3 layer

X-ray diffraction and infrared spectroscopy measurements are conducted in order to assess the crystallographic structure and chemical purity of lanthanum oxide (La2O3) films grown by atomic layer deposition (ALD) on Si substrates. In situ capping with thin aluminum oxide (Al2O3) layer is proved to be beneficial in preventing the formation of lanthanum hydroxide phases. The effect of two process parameters, namely, La2O3 film growth temperature (260–500°C range) and postdeposition annealing temperature (600–1100°C range), on the chemical and structural evolutions of Al2O3∕La2O3∕Si stacks is discussed. This study enables the identification of the optimum ALD growth recipe yielding the highest hexagonal La2O3 phase content, which might be suitable for integration into innovative metal oxide semiconductor devices.X-ray diffraction and infrared spectroscopy measurements are conducted in order to assess the crystallographic structure and chemical purity of lanthanum oxide (La2O3) films grown by atomic layer deposition (ALD) on Si substrates. In situ capping with thin aluminum oxide (Al2O3) layer is proved to be beneficial in preventing the formation of lanthanum hydroxide phases. The effect of two process parameters, namely, La2O3 film growth temperature (260–500°C range) and postdeposition annealing temperature (600–1100°C range), on the chemical and structural evolutions of Al2O3∕La2O3∕Si stacks is discussed. This study enables the identification of the optimum ALD growth recipe yielding the highest hexagonal La2O3 phase content, which might be suitable for integration into innovative metal oxide semiconductor devices.

Infrared and thermoelectric power generation in thin atomic layer deposited Nb-doped TiO2 films

Infrared radiation is used to radiatively transfer heat to a nanometric power generator (NPG) device with a thermoelectric Nb-doped TiO2 film deposited by atomic layer deposition (ALD) as the active element, onto a borosilicate glass substrate. The linear rise of the produced voltage with respect to the temperature difference between the “hot” and “cold” junctions, typical of the Seebeck effect, is missing. The discovery of the violation of the Seebeck effect in NPG devices combined with the ability of ALD to tune thermoelectric thin film properties could be exploited to increase the efficiency of these devices for energy harvesting purposes.

Wetting properties induced in nano-composite POSS-MA polymer films by atomic layer deposited oxides

Due to their unique properties, nano-composite polyhedral oligomeric silsequioxane (POSS) copolymer films are attractive for various applications. Here we show that their natural hydrophobic character can become hydrophilic when the films are modified by a thin oxide layer, up to 8 nm thick, prepared using atomic layer deposition. A proper choice of the deposition temperature and thickness of the oxide layer are required to achieve this goal. Unlike other polymeric systems, a marked transition to a hydrophilic state is observed with oxide layers deposited at increasing temperatures up to the glass transition temperature (∼110 °C) of the POSS copolymer film. The hydrophilic state is monitored through the water contact angle of the POSS film. Infrared absorbance spectra indicate that, in hydrophilic samples, the integral of peaks corresponding to surface Al–O (hydrophilic) is significantly larger than that of peaks linked to hydrophobic species.

Excitation of Radiative Polaritons by Polarized Broadband Infrared Radiation in Thin Oxide Films Deposited by Atomic Layer Deposition

This work contributes to the understanding of infrared radiation interaction with matter and its absorption for energy harvesting purposes. By exciting radiative polaritons in thin oxide films with polarized infrared radiation, a further evidence is collected that a link exists between radiative polaritons and the heat recovery mechanism hypothesized in previous research. In the voltage transient occurring when the infrared radiation is turned on, the observed time necessary to reach the maximum voltage and the voltage intensity versus angle of incidence exhibit a mismatch when generated by polarized and nonpolarized infrared radiation. The existence of collective charge oscillation modes in the semiconductor-based elements of the thermoelectric power generators supporting the heat recovery mechanism is suggested as the main source of the discrepancy.

Origin of the low frequency radiation emitted by radiative polaritons excited by infrared radiation in planar La2O3 films

Upon excitation in thin oxide films by infrared radiation, radiative polaritons are formed with complex angular frequency ω, according to the theory of Kliewer and Fuchs (1966 Phys. Rev. 150 573). We show that radiative polaritons leak radiation with frequency ω(i) to the space surrounding the oxide film. The frequency ω(i) is the imaginary part of ω. The effects of the presence of the radiation leaked out at frequency ω(i) are observed experimentally and numerically in the infrared spectra of La(2)O(3) films on silicon upon excitation by infrared radiation of the 0TH type radiative polariton. The frequency ω(i) is found in the microwave to far infrared region, and depends on the oxide film chemistry and thickness. The presented results might aid in the interpretation of fine structures in infrared and, possibly, optical spectra, and suggest the study of other similar potential sources of electromagnetic radiation in different physical scenarios.

Fabrication of nanopower generators using thin atomic layer deposited films

Infrared (IR) power generation is emerging as a useful method to harvest IR light and transform it into usable energy available day and night. Here, the authors continue the effort to fabricate thin thermoelectric TiO2/TiN nanolaminate films via atomic layer deposition (ALD) and, specifically, focus on the effects of film sheet resistance Rs on the voltage produced by nanopower generator devices with these films as their active elements. By changing the number and the thickness of the TiO2/TiN nanolaminate, the authors control the sheet resistance Rs over 3 orders of magnitude. The authors observe that the voltage produced by nanopower generator devices increases with Rs and exhibits two roughly linear regimes. In the first regime, when Rs   1 kΩ/◻, where the slope characterizing the increase of voltage with sheet resis...