Article pubs.acs.org/JPCC
Surface-Based Control of Oxygen Interstitial Injection into ZnO via Submonolayer Sulfur Adsorption Ming Li and Edmund G. Seebauer* Department of Chemical and Biomolecular Engineering University of Illinois, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Semiconductor surfaces offer efficient pathways for injecting native point defects into the underlying bulk. Adsorption of a suitably chosen foreign element serves to modulate the injection rate, even at small percentages of a monolayer. Through self-diffusion experiments using isotopic exchange with labeled oxygen, the present work demonstrates such behavior in the case of sulfur adsorption on c-axis Znterminated ZnO(0001), wherein the clean surface injects with exceptional efficiency. The experiments provide strong evidence that the injection sites comprise only a small fraction of the total surface atom density and that sulfur adsorption merely blocks those sites. Comparison with related systems shows this simple mechanism is surprisingly uncommon.
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INTRODUCTION Native atomic defects such as vacancies and interstitial atoms affect the function and performance of semiconductors in catalysis,1−3 electronics,4,5 gas sensing,6 and photonics.7 For ZnO specifically,8,9 oxygen vacancies (VO) contribute to parasitic green emission in optoelectronic devices and carrier trapping in photocatalysts and photovoltaic cells.10 Much research effort has sought to manipulate point defect concentrations to enhance material performance11 for such applications. Along those lines, this laboratory has shown that atomically clean semiconductor surfaces can be used to efficiently inject point defects.12−15 The comparatively low atomic coordination at surfaces requires less bond breakage for defect formation and thus a lower activation barrier compared with bulk pathways. For oxygen in rutile TiO2 and wurtzite ZnO, clean surfaces provide especially pathways for interstitial (Oi) injection, leading to near-complete suppression of VO.13−15 “Injection” here implies thermal driven self-diffusion with no other high-energy ion or plasma-based source involved. Related effects occur for silicon interstitial atoms in Si12 and generalize to efficient annihilation of excess interstitials when the bulk is supersaturated with them.12,16 In the cases of Si and TiO2, adsorption of a suitably chosen foreign element serves to modulate the rate of defect exchange with the surface, even at small percentages of a monolayer. However, the underlying mechanism varies among surfaces. For instance, nitrogen adsorption on Si(111) influences a precursor state connected to interstitial injection and annihilation. By contrast, sulfur adsorption on TiO2(110) does not greatly affect Oi injection rates but rather inhibits the annihilation rate of Ti interstitials diffusing up from the bulk. Curiously, no example has yet been observed of simple blocking of injection sites by a foreign adsorbate. The present work provides such an example in the case of sulfur adsorption on c-axis Zn-terminated ZnO(0001), wherein the clean surface injects (Oi) with exceptional efficiency.15 Self-diffusion experiments using isotopic exchange with labeled oxygen (18O2) are © XXXX American Chemical Society
performed in conjunction with variable sulfur coverages. When interpreted in light of recent first-principles calculations of O bonding to ZnO(0001),17 the experiments provide strong evidence that the injection sites comprise only a small fraction of the total surface atom density and that sulfur adsorption merely blocks those sites. Comparisons and contrasts among the materials wherein adsorption affects defect exchange suggest that the reaction between semiconductor surfaces and atomic defects can exhibit chemistry with richness comparable to reactions of surfaces with gases or liquids.
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EXPERIMENTAL SECTION The experiments employed a well-known isotopic gas−solid exchange approach previously described15 to measure O selfdiffusion, which monitors point-defect behavior indirectly. In brief, specimens were mounted in a turbomolecularly pumped ultrahigh vacuum chamber that was equipped with a cylindrical mirror analyzer for Auger electron spectroscopy (AES) and a solid-state electrochemical cell for sulfur deposition.13 Znterminated ZnO(0001) single crystals (CrysTec) of dimensions 1 cm × 0.5 cm × 0.05 cm were degreased by successive 5 min ultrasonic baths in electronic-grade acetone, isopropanol, and methanol before mounting. To minimize surface contamination, the total ambient exposure time was held to
