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David A. Ginley

Bio: David A. Ginley is an academic researcher from National Renewable Energy Laboratory. The author has contributed to research in topics: Physics & Contact resistance. The author has an hindex of 1, co-authored 1 publications receiving 23 citations.

Papers
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TL;DR: In this article, it was shown that nucleation in thermally annealed hydrogenated amorphous silicon occurs in the more well ordered spatial regions in the network, which are defined by the initial inhomogeneous H distributions in the as-grown films.
Abstract: Utilizing the concepts of a critical crystallite size and local film inhomogeneity, it is shown that nucleation in thermally annealed hydrogenated amorphous silicon occurs in the more well ordered spatial regions in the network, which are defined by the initial inhomogeneous H distributions in the as-grown films. Although the film H evolves very early during annealing, the local film order is largely retained in the still amorphous films even after the vast majority of the H is evolved, and the more well ordered regions which are the nucleation center sites for crystallization are those spatial regions which do not initially contain clustered H, as probed by H NMR spectroscopy. The sizes of these better ordered regions relative to a critical crystallite size determine the film incubation times (the time before the onset of crystallization). Changes in film short range order upon H evolution, and the presence of microvoid type structures in the as grown films play no role in the crystallization process. While the creation of dangling bonds upon H evolution may play a role in the actual phase transformation itself, the film defect densities measured just prior to the onset of crystallization exhibit no trends which can be correlated with the film incubation times.

23 citations

Journal ArticleDOI
TL;DR: In this article , the electronic structure of BCM was investigated using the combined effort of first-principles calculations and experimental O K-edge x-ray absorption spectroscopy (XAS), and the computed projected density-of-states (PDOS) and orbital-plots were used to propose a simplified model for orbital mixing between the oxygen and the ligand atoms.
Abstract: BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ (BCM), a non-stoichiometric oxide closely resembling a perovskite crystal structure, has recently emerged as a prospective contender for application in renewable energy harvesting by solar thermochemical hydrogen generation. Using solar energy, oxygen-vacancies can be created in BCM and the reduced crystal so obtained can, in turn, produce H2 by stripping oxygen from H2O. Therefore, a first step toward understanding the working mechanism and optimizing the performance of BCM, is a thorough and comparative analysis of the electronic structure of the pristine and the reduced material. In this paper, we probe the electronic structure of BCM using the combined effort of first-principles calculations and experimental O K-edge x-ray absorption spectroscopy (XAS). The computed projected density-of-states (PDOS) and orbital-plots are used to propose a simplified model for orbital-mixing between the oxygen and the ligand atoms. With the help of state-of-the-art simulations, we are able to find the origins of the XAS peaks and to categorize them on the basis of contribution from Ce and Mn. For the reduced crystal, the calculations show that, as a consequence of dielectric screening, the change in electron-density resulting from the reduction is strongly localized around the oxygen vacancy. Our experimental studies reveal a marked lowering of the first O K-edge peak in the reduced crystal which is shown to result from a diminished O-2p contribution to the frontier unoccupied orbitals, in accordance with the tight-binding scheme. Our study paves the way for investigation of the working-mechanism of BCM and for computational and experimental efforts aimed at design and discovery of efficient water-splitting oxides.
Journal ArticleDOI
TL;DR: In this paper , the ultrathin Ti layer in the 5'nm Ti/100'nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability.
Abstract: Beta gallium oxide (β-Ga2O3) shows significant promise in high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperatures present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300–400 °C) due to a combination of nonstoichiometric Ti/Ga2O3 interfacial reactions and kinetically favored Ti diffusion processes. Here, we demonstrate stable Ohmic contacts for Ga2O3 devices operating up to 500–600 °C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5 nm Ti/100 nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (>500 h) at 600 °C in vacuum (≤10−4 Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550 °C in flowing N2. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift toward rectifying interfaces and other interfacial layers.
04 Apr 2023
TL;DR: In this article , the ultrathin Ti layer in the 5nm Ti / 100nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability.
Abstract: Beta gallium oxide ($\beta$-Ga$_2$O$_3$) shows significant promise in the high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperature present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300-400$^{\circ}$C) due to a combination of non-stoichiometric Ti/Ga$_2$O$_3$ interfacial reactions and kinetically favored Ti diffusion processes. Here we demonstrate stable Ohmic contacts for Ga$_2$O$_3$ devices operating up to 500-600$^{\circ}$C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5nm Ti / 100nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (>500 hours) at 600$^{\circ}$C in vacuum ($\leq$ 10$^{-4}$ Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550$^{\circ}$C in flowing N$_2$. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift towards rectifying contacts and other interfacial layers.

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TL;DR: In this paper, the effect of the microstructure of remote plasma-deposited amorphous silicon films on the grain size development in polycrystalline silicon upon solid-phase crystallization is reported.
Abstract: In this paper the effect of the microstructure of remote plasma-deposited amorphous silicon films on the grain size development in polycrystalline silicon upon solid-phase crystallization is reported. The hydrogenated amorphous silicon films are deposited at different microstructure parameter values R* (which represents the distribution of SiHx bonds in amorphous silicon), at constant hydrogen content. Amorphous silicon films undergo a phase transformation during solid-phase crystallization and the process results in fully (poly-)crystallized films. An increase in amorphous film structural disorder (i.e., an increase in R*), leads to the development of larger grain sizes (in the range of 700-1100 nm). When the microstructure parameter is reduced, the grain size ranges between 100 and 450 nm. These results point to the microstructure parameter having a key role in controlling the grain size of the polycrystalline silicon films and thus the performance of polycrystalline silicon solar cells.

23 citations

Journal ArticleDOI
TL;DR: In this paper, the kinetics of crystal nucleation in high-rate electron beam evaporated amorphous Si for polycrystalline thin film solar cells were systematically studied on SiN and selected ZnO:Al-coated glass substrates with dissimilar surface topographies by employing Raman spectroscopy, transmission electron microscopy, and optical microscopy.
Abstract: The kinetics of crystal nucleation in high-rate electron beam evaporated amorphous Si for polycrystalline thin film solar cells was systematically studied on SiN and selected ZnO:Al-coated glass substrates with dissimilar surface topographies by employing Raman spectroscopy, transmission electron microscopy, and optical microscopy. The influence of the surface topography of the substrate and the disorder of the deposited amorphous Si could be correlated to the respective characteristics of the transient and steady state regime of the nucleation rate. The steady state nucleation rate Iss, its corresponding activation energy EIss, and consequently the size of the grains in the crystallized Si were found to be governed by the interplay between the surface roughness and the deposition temperature. The steady state nucleation rate Iss increased gradually upon increasing the substrate roughness, while lowering the deposition temperature of the amorphous Si on rough textures resulted in a decline of Iss. The tim...

22 citations

Journal ArticleDOI
TL;DR: In this article, an analytical solution for the crystallization kinetics in the special case of plate-shaped samples with a finite thickness is presented. But the analytical solution does not reveal the thickness range which influences the isothermal crystallization mode significantly.
Abstract: The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model is widely used to quantify the isothermal crystallization kinetics. The present work reports an analytical solution for the crystallization kinetics in the special case of plate-shaped samples with a finite thickness. As a result, we obtained an adapted JMAK model revealing the thickness range which influences the crystallization kinetics mode significantly. The analytical solution also provides theoretical bounds for the film thickness, where the assumption of 2D or 3D kinetics is accurate. Finally, the conclusions related to amorphous silicon and amorphous nickel-titanium thin films are reported.

17 citations

Journal ArticleDOI
TL;DR: In this article, the authors report on the deposition of amorphous silicon (a-Si:H) films at ultra-high growth rate (11-60nm/s) by means of the expanding thermal plasma technique, followed by solid phase crystallization (SPC).
Abstract: In this paper, we report on the deposition of amorphous silicon (a-Si:H) films at ultra-high growth rate (11–60 nm/s) by means of the expanding thermal plasma technique, followed by solid-phase crystallization (SPC). Large-grain (∼1.5 μm) polycrystalline silicon was obtained after SPC of high growth rate (∼25 nm/s) deposited a-Si:H films. The obtained results are discussed by taking into account the impact of the a-Si:H microstructure parameter R* as well as of its morphology, on the final grain size development.

11 citations