J. Phys. IV France 132 (2006) 259–262 C  EDP Sciences, Les Ulis DOI: 10.1051/ jp4:2006132049 Effects of interface roughness on the local valence electronic structure at the SiO 2 /Si interface: Soft X-ray absorption and emission study Y. Yamashita 1 , S. Yamamoto 1 , K. Mukai 1 , J. Yoshinobu 1 , Y. Harada 2 , T. Tokushima 2 , Y. Takata 2 and S. Shin 1,2 1 The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan 2 Riken/SPring-8, Sayo-gun, Hyogo 679-5148, Japan Abstract. We investigated effects of interface roughness on the local valence electronic structures at SiO 2 /Si interface in order to clarify the relation between interface structures and interface electronic properties, by using soft X-ray absorption and emission spectroscopy. For atomically smooth interface, the local valence structures depend on intermediate oxidation states at the interface. For atomically rough interface, on the other hand, the local valence structures at the interface show amorphous-like electronic states irrespective of the intermediated oxidation states. 1. INTRODUCTION Study of SiO 2 /Si interfaces has received much attention not only from an academic view point but also for the technological application of metal-oxide-silicon (MOS) devices, because the interfaces seriously affect the electronic characteristics of MOS devices [1]. Thus, an atomic level understanding of the properties at the SiO 2 /Si interface should be important. It is reported that the roughness at SiO 2 /Si interface influences the performance of MOS devices. Kobayashi’s group and Hahn et al. reported that the interface state density (gap state density) strongly depended on the roughness [2,3]; the rougher interface exhibits larger gap state density.Yasuda et al. successfully produced the SiO 2 /Si(111) interface with low interface state density by depositing SiO 2 layer on the atomically flat Si(111) surface [4]. Therefore, the effects of interface roughness on the interface properties should be clarified on atomic scale, which gives information on the improvement of electronic characteristics of MOS devices. In the present study, we investigated the effects of interface roughness on the local valence electronic structure at SiO 2 /Si interface in order to clarify the relation between the interface structures and the interface properties, by using soft X-ray absorption and emission spectroscopy [5]. 2. EXPERIMENTAL After cleaning wafers with an RCA method, atomically rough and smooth SiO 2 /Si(111) interfaces were prepared by following methods: for the production of atomically rough interfaces, the wafers were simply immersed in a 1% hydrofluoric solution for 30 s and then 1.5 nm-thick oxide layer was prepared by thermal oxidation of the Si(111) substrate. For the preparation of atomically smooth interfaces, the wafers were immersed in a 1% hydrofluoric solution for 30 s followed by immersion in a 40% NH 4 F solution for 15 min., and then a 1.8 nm-thick oxide layer was prepared by thermal oxidation of the Si(111) substrate. We confirmed the atomically rough and smooth SiO 2 /Si(111) interfaces using atomic force microscopy; after the etch of the oxide layer away, mono-atomic steps were clearly observed for the atomically smooth interface but steps were not seen for the atomically rough interface. The synchrotron radiation experiments were performed using BL-27SU at SPring-8 with the approval of JASRI as Nanotechnology Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2006132049