The technologically useful properties of a ceramic material often depend upon the atomic defects contained in its crystalline structure. The present work seeks to further develop recently-discovered methods to control the concentration and movement of atomic defects in semiconducting ceramics such as titanium dioxide and zinc oxide. The methods are based upon manipulation of chemical bonds at the material surface or illumination by ultraviolet light. Controlling the defects will help improve the performance in diverse applications such as solar hydrogen production by water splitting, environmental water remediation by photocatalysis, and gas sensing. This research is taking place in parallel with development of an industry-supported laboratory course for upper-division undergraduates and graduate students called 'Chemistry and Transport in Semiconductor Materials Synthesis.' In addition, several activities to promote the importance of ethics in science and engineering are being pursued.
TECHNICAL DETAILS: Specially synthesized isotopically-labeled structures are used in conjunction with self-diffusion measurements to determine generation and annihilation rates of interstitial atoms or vacancies at surfaces. The experiments involve observation of the isotopic profile evolution by secondary ion mass spectroscopy, with supplementary experiments by photoreflectance to measure the magnitude of near-surface electric fields. Detailed modeling of the defect diffusion-reaction networks underpins experimental interpretation. The experiments examine the effects of controllable chemical adsorption and surface crystallographic orientation upon the insertion and generation of bulk defects mediated by chemically unsaturated surface bonds ? both neutral and electrically charged. In addition, optical stimulation effects are quantified by analogous experiments performed under super-band illumination, and interpreted in terms of changes in the charge state of either the surface or the defects themselves.
Normal 0 false false false EN-US X-NONE X-NONE The technologically useful properties of a solid often depend upon the atomic-scale defects it contains. Examples of defects include atoms that are missing from the normal atomic structure ("vacancies") and extra atoms that squeeze in between others in the structure ("interstitials"). Despite the harmful sound of "defects," their types and concentrations can sometimes be controlled to improve the properties of a material for practical applications. For example, fabrication procedures for silicon-based integrated circuits rely upon defects to aid the diffusion of dopant atoms that are critical to making the devices work. Since defects affect many aspects of semiconductor behavior, the ability to control defect type, concentration, spatial distribution, and mobility is important for practical applications. The practice of such control is termed "defect engineering." Considerable progress has been made in developing this ability for silicon, but the methods still need considerable development for other semiconductors such as ceramic metal oxides. The present work has focused on developing new methods for defect engineering in semiconducting oxides. These materials include solids such as titanium dioxide and zinc oxide, which are both commonly used in sunscreens. However, these materials are also important in energy, environmental, and microelectronics applications, and can be employed to make useful devices for these applications. We have discovered several new ways to accomplish defect engineering that work well at small length scales below about one micrometer (about the size of a bacterium). The mechanisms include illumination with light and controlled reactions of the defects at surfaces. Our work employs both experiments and mathematical computations to better understand fundamental defect behavior, while simultaneously trying to develop the findings to practical applications. Normal 0 false false false EN-US X-NONE X-NONE One discovery we made concerns the way electrically charged defects interact with surfaces that are also charged. It’s well known that like charges repel and that opposites attract. But within semiconductors, this behavior manifests in curious ways. In silicon, for example, we found that the effect of surface charge on nearly defects can be so strong that the amount of charge on each defect actually changes. This change leads to a pile-up of the defects very close to the surface in a way that we can now mathematically model with precision. By contrast, in titanium dioxide and zinc oxide, the effect of the surface on the defects is weaker. Typically the defect charges do not change, and the interaction remains as conventional attraction or repulsion. An attraction also leads to a pile-up of defects near the surface, but not as pronounced as in the silicon case. Since the amount of charge on a surface is something that’s controllable (within limits), by implication we can control these pile-up effects in both silicon and the metal oxides to tailor the concentration of defects near the surface in ways that are best for a particular application. Another discovery we made is that preparing surfaces in special ways can make them very efficient for creating defects and injecting them into the underlying solid. For example, we found that specially prepared surfaces of titanium dioxide can be made so efficient for injecting oxygen interstitial atoms that their concentration exceeds that of oxygen vacancies that normally dominate. We also discovered an indirect way in which surfaces can control oxygen interstitial motion in the solid: by influencing the concentration of titanium defects, which seem to promote the exchange of the oxygen interstitials into the regular atomic structure of the solid. Thus, we are learning how to control both the majority type and mobility of the oxygen defects by principles that should generalize to other kinds of defects in other kinds of ceramic semiconductors. A third discovery we made is that illuminating the surface of a semiconductor can influence its ability to interact either electrically or chemically with defects in the underlying solid. We had already shown in previous grants from NSF that illumination can change the amount of electrical charge a particular defect type supports. But we now know that illumination can also change the surface properties. Although we don’t fully understand all the details of how this process works, we are confident that the effects generalize to a wide variety of semiconductors. In addition to making new technical discoveries, we have sought to make the research impactful outside our immediate technical community. For example, certain selected results of this work have been incorporated into college courses taught at Illinois that deal with semiconductor device fabrication. In addition, the principal investigator has been involved in developing materials connected with research ethics, including an ethics website and related book chapters, review articles and various invited presentations.
