We have studied the effects of the N2 gas flow rate on the surface morphology of ZnO films deposited by the sputtering of a ZnO target using Ar/N2. Height-height correlation function (HHCF) analysis indicates that introducing a small amount of N2 (<5 sccm) to the sputtering atmosphere enhances adatom migration, leading to a larger grain size in the ZnO films associated with an increase in the lateral correlation length. The HHCF analysis also reveals that films deposited with and without N2 exhibit a self-affine fractal surface structure. We demonstrate that utilizing such ZnO films deposited using Ar/N2 as buffer layers, the crystallinity of ZnO:Al (AZO) films on the buffer layers can be greatly improved. The electrical resistivity of 100-nm-thick AZO films decreases from 1.8×10-3 to 4.0×10-4 Ω·cm by utilizing a ZnO buffer layers prepared at N2 flow rate of 5 sccm.

Hydrogenated ZnO thin films have been successfully deposited on glass substrates via a nitrogen mediated crystallization (NMC) method utilizing RF sputtering. Here we aim to study the crystallinity and electrical properties of hydrogenated NMC-ZnO films in correlation with substrate temperature and H2 flow rate. XRD measurements reveal that all the deposited films exhibit strongly preferred (001) orientation. The integral breadth of the (002) peak from the hydrogenated NMC-ZnO films is smaller than that of the conventional hydrogenated ZnO films fabricated without nitrogen. Furthermore, the crystallinity and surface morphology of the hydrogenated NMC-ZnO films are improved by increasing substrate temperature to 400 °C, where the smallest integral breadth of (002) 2θ–ω scans of 0.83° has been obtained. By utilizing the hydrogenated NMC-ZnO films as buffer layers, the crystallinity of ZnO:Al (AZO) films is also improved. The resistivity of AZO films on NMC-ZnO buffer layers decreases with increasing H2 flow rate during the sputter deposition of buffer layers from 0 to 5 sccm. At a H2 flow rate of 5 sccm, 20-nm-thick AZO films with low resistivity of 1.5×10-3 Ω cm have been obtained.

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To achieve excellent semiconductor device performance, especially for low-temperature processing of semiconductors, the need to devise strategies to engineer the surface and interface and to develop characterization techniques to understand the cause−effect relationship of surface and interface of semiconductor devices remains to be a key issue. Here, we present a nucleation control method, termed nitrogen-mediated crystallization (NMC), to engineer the surface morphology of a ZnO buffer layer and analyze firstand seconddegree statistical surfaces to reveal the morphological relationship between the buffer layer and the buffered AZO film. The surface parameter is generally understood as the surface roughness (roughness average or RMS roughness) or the surface height profile, and our experimental results suggest that the physical properties of the buffered AZO films are strongly influenced by the fractal geometry of the buffer layers and are insensitive to their surface roughness. We demonstrate that the NMC method promotes enhanced surface migration and effectively prevents the development of nonuniform fractal geometry in the ZnO buffer layer, enabling the stress relaxation in the buffered AZO films and mitigating the threedimensional columnar growth. At a low thermally induced kinetic energy, a 90 nm thick AZO film with an ultralow resistivity of 4.4 × 10 −4 Ω•cm can be achieved, indicating its potential for the realization of high-efficiency flexible optoelectronic devices.

Materials selection for aluminum alloys with desired fatigue and other mechanical properties is very difficult. Usually, when fatigue properties are maximized, other mechanical properties should be compromised. In this paper, an artificial neural network was utilized to build two prediction models that has the purpose of predicting fatigue life from composition and inverse design to predict composition from fatigue properties as a tool for materials selection. A first model was built to predict fatigue life using information on alloy composition, heat treatment, and other mechanical properties. The second model is an inversion of the first model, which predicts the material compositions to get the desired fatigue performance and other mechanical properties. Both models produce good performances based on the R2 scoring metric, where the values were found to be 0.92 and 0.96 for the first and second models, respectively. This study proved that the inversion model for predicting composition based on fatigue properties can reach acceptable accuracy and can be used as a materials selection tool. In addition, to investigate how atomic properties can affect fatigue life, the third model was built. It was found that atomic properties, such as electronegativity and the radius of alloying elements, are closely related to fatigue life and can be used to predict fatigue life as well. The significance of our work is that users can design new alloys for specific applications as well as select available alloys based on fatigue property criteria.

Abstract.We study the surface morphology of ZnO thin films deposited by nitrogen mediated crystallization method utilizing atomic force microscopy as a function of nitrogen flow rates. Initially, the surface morphology of ZnO thin film deposited without nitrogen exhibits a bumpy surface with spiky grains where the skewness and kurtosis values were found to be 0.48 and 4.80, respectively. By addition of small amount of nitrogen, the skewness and kurtosis values of the films significantly decrease associated with a flatter topography. Further increase in nitrogen flow rate to 16 sccm has roughened the surface shown mainly by the increase in kurtosis value to be 3.30. These results indicate that the addition of small amount of nitrogen during deposition process has enhanced the adatoms migration on the surface resulting in a superior film with a larger grain size. Two-dimensional power spectral density analysis reveals that all the films have self-affine fractal geometry with total fractal values in the range of 2.14 to above 3.00.

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