Role of surface chemistry in determining the heteroepitaxial growth of Ir films on a-plane α-Al₂O₃ single crystals

Heteroepitaxy has increased in importance as the development of the semiconductor industry has advanced and a number of heteroepitaxial devices have been developed. In this study, Ir films of (001) or (111) preferred orientation were deposited onto a-plane α-Al₂O₃ surfaces pretreated via annealing in air and/or Ar⁺ ion etching. Ir (001) films were epitaxially grown on α-Al₂O₃ substrates with an in-plane orientation relationship of Ir (001)[110]//α-Al₂O₃ (112¯0)[0001]. In contrast, (111) films were non-epitaxial with the out-of-plane orientation relationship of Ir (111)// α-Al₂O₃ (112¯0). The effects of annealing in air and Ar⁺ ion etching on Ir film growth were then analyzed. Annealing in air can lead to a terrace morphology and a lower Al/O atomic ratio of 1/2 than 2/3 for α-Al₂O₃ surfaces, and a better Ir film crystallinity due to the step-flow mechanism; however, this was not the key factor for Ir heteroepitaxy. Ar⁺ ion etching was regarded as an irreplaceable pretreatment procedure. The surface chemistry of α-Al₂O₃ substrates was characterized via X-ray photoelectron spectroscopy. Carbon adsorbate (C = O, C[sbnd]C, C[sbnd]O, etc.) on substrate surfaces was drastically reduced after etching. Interfacial structures were observed by transmission electron spectroscopy. Two different stacking sequences were observed along the out-of-plane direction of α-Al₂O₃ (112¯0) surface, representing the 〈111〉 and 〈001〉 crystal directions, respectively; C element enrichment was observed at the interface without etching. Finally, the interaction between Ir films and α-Al₂O₃ substrates, which is probably influenced by the surface chemistry, is considered to be the growth mechanism of Ir films. This study provides significant insights into the role of the surface chemistry in the epitaxial growth of films.