Tokyo Tech News
Published: July 31, 2009
Katsuro Hayashi and colleagues at the Materials and Structures Laboratory demonstrate the selective generation of atomic oxygen from solid oxide sources for the first time. These findings will find a wide range of applications in the electronics industry.
Oxygen is an unsung hero of the electronics industry, being extensively used for producing ultrathin silicon dioxide layers in the microelectronics industry. Notably, "atomic oxygen"—a highly reactive species formed when the bonds of molecular oxygen are broken—is important for the development of automobile catalytic converters, solid-state fuel cells, and related applications.
Currently, atomic oxygen is extracted from gaseous oxygen plasma produced by exposing molecular oxygen to radio-frequency electric fields. This process for generating atomic oxygen is complicated, expensive, and energy intensive and leads to the generation of spurious impurity ions, which damage samples under investigation. So for a long time, researchers have been interested in developing alternative methods for generating atomic oxygen.
Now, Katsuro Hayashi and colleagues at the Materials and Structures Laboratory, Frontier Collaborative Research Center, and ERATO-SORST, Japan Science and Technology Agency, demonstrate for the first time that large quantities of atomic oxygen can be selectively produced by heating yttrium oxide doped zirconia (3% Y2O3-doped ZrO2)—a solid oxide electrolyte permeated with oxygen.
The addition of yttria to pure zirconia leads to the generation of large numbers of oxygen vacancies in the zirconia lattice, resulting in an electrically conducting material with applications ranging from solid oxide fuel cells and thermal coating in gas turbines to nonmetallic ceramic knives and restoration of teeth.
Here, Hayashi and coworkers used quadrupole mass spectrometry (QMS) to analyze the gaseous species emitted from an incandescently heated yttrium oxide doped zirconia tube in a vacuum. More specifically, molecular oxygen with a pressure of between 10 to 3,000 Pa was passed through a zirconia tube with an outer diameter of 2 mm, and the central 10 mm region of the tube was heated to 1,400–1,800°C (figs. 1 and 2). The intensity of atomic oxygen emitted from the hot region of the zirconia surface tube decreased as the distance (d) between the QMS and the zirconia was increased, and, notably, the atomic oxygen intensity was proportional to 1/d2 for separations greater than 40 mm.
But what led Hayashi and colleagues to conduct these experiments? "Originally, we were seeking to induce a singly charged oxygen anionic (O-) emission from solid oxide electrolytes by applying electric fields," says Hayashi. "Our idea was very simple. That is, since O- ions are stable under vacuum conditions, extraction of oxide ions—or other anionic oxygen species—from metal oxides should lead to direct emission of O- ions. Our goal was to replace conventional negative ion sources, which use plasmas and highly reactive electronegative metals."
These results imply that the hot region of the zirconia tube behaves as a "point source" of atomic oxygen. The QMS measurements showed the flux of atomic oxygen emission to be approximately 1017 atoms/cm2/s (higher than plasma-based techniques) and that the atomic oxygen intensity increased when the pressure of the molecular oxygen flowing through the inner tube was increased. These latter results suggest that the atomic oxygen originates from oxygen species diffusing through the zirconia tube due to the oxygen chemical potential between the inner and outer surfaces of the zirconia.
The main advantages of this approach over conventional plasma-based generation include higher energy efficiency; suppression of high energy ionic species that could damage samples; operation over wider vacuum pressures; and elimination of high-frequency power sources, cooling systems, and inert gases. Thus, the Tokyo Tech approach offers a safe and versatile means of producing atomic oxygen using a solid, stable source.
But Hayashi confides the results were unexpected. "We were very much surprised by our findings," says Hayashi. "The O- emission obtained from our apparatus was much less than we had expected. In fact, a theoretician confirmed that extraction of O- ions is difficult, but he said that neutral atoms should be easier. And he was right."
With a view to practical applications, the Tokyo Tech group recently successfully deposited ultrathin layers of insulating oxide layers on silicon surfaces and oxidized the silver regions on crystal oscillator sensors (fig. 3).
"We are manufacturing and testing a compact prototype system for high vacuum applications, such as pulsed laser deposition chambers," says Hayashi. "We are also planning experiments for drastically enhancing the atomic oxygen emission. Collaboration with theoreticians for a deeper understanding of atomic oxygen emission is also important."
This discovery will find a wide range of applications in vacuum technology in areas including low-temperature deposition of nanolayers of insulating oxides on semiconductor surfaces, fabrication of thin films of new oxide materials, and modification of the surface properties of materials.
K. Hayashi, T. Chiba, J. Li, M. Hirano, and H. Hosono. Intense Atomic Oxygen Emission from Incandescent Zirconia. J. Phys. Chem. C. 113, p. 9436 (2009).
Fig. 1: Experimental setup showing the zirconia tube and molecular oxygen feed. The inner zirconia tube is hermetically sealed from the outside by graphite sealants and stainless steel connectors for electrical connections to the internal electrodes.
Fig. 2: Photograph of the apparatus showing the intense light resulting from heating the zirconia tube inside the vacuum chamber.
Fig. 3: Atomic oxygen oxidized the silver layer on a crystal oscillator sensor. The image on the left shows the silver before oxidation, and the one on the right is after oxidation using the zirconia tube.
Materials and Structures Laboratory