Surface structures of silicon nitride thin ®lms on Si(111) Guangjie Zhai 1 , Jianshu Yang 2 , Nelson Cue, Xue-sen Wang * Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 17 August 1999; received in revised form 3 February 2000; accepted 3 February 2000 Abstract After exposing Si(111) to NH 3 at about 1075 K, a 10.2-A Ê surface periodic structure (the `8=3 £ 8=3' reconstruction) is observed in scanning tunneling microscopy images. When the nitridation temperature is above 1125 K, a silicon nitride ®lm with a surface superstructure with a period of 30.7 A Ê and a minimum step height of 2.9 A Ê is obtained. Our systematic analyses suggest that the 30.7-A Ê superstructure is the 4 £ 4 reconstruction on the Si 3 N 4 (0001) surface of the crystalline silicon nitride (b -Si 3 N 4 ) thin ®lm formed on Si(111), while the 10.2-A Ê periodic structure is an incomplete nitridation phase on Si(111) surface. The surface structure has been found to be sensitive to the impurities in NH 3 . q 2000 Elsevier Science S.A. All rights reserved. Keywords: Silicon nitride; Silicon; Ceramic; Epitaxy; Scanning tunneling microscopy; Low energy electron diffraction 1. Introduction In recent years, the nitridation process on silicon surfaces has been studied intensively [1±14] because of its scienti®c and technological values. Silicon nitride and silicon oxyni- tride are expected to be more stable dielectric layers on silicon when compared to silicon oxide, both in a strong electric ®eld and at a high working temperature [15,16]. They should also be more effective as diffusion barriers to impurities. The physical and chemical properties of the sili- con nitride ®lms are closely related to the surface and inter- face structures of the sample during the nitridation process, which consequently affect the performance of the electronic devices. Meanwhile, the surface structure of a silicon nitride ®lm is itself an important research topic. Our current knowl- edge on the surface properties of ceramic materials is rather primitive in general compared to that of metals and semi- conductors [17,18]. Although a sizeable Si 3 N 4 specimen is not available, epitaxial growth of Si 3 N 4 ®lm on Si(111) is theoretically possible because of good match in lattice para- meter between Si(111) and Si 3 N 4 (0001) [19,20]. Thus, the surface of a silicon nitride ®lm provides a model system for ceramic surface studies. In addition, many applications involve deposition of metals and semiconductors on the silicon nitride ®lms. For example, high quality GaN has been grown recently on silicon nitride ®lm on a Si(111) substrate [21], which should lead to the prospect of integrat- ing optoelectronic devices with silicon-based circuits. The growth mode of a metal or semiconductor overlayer should be strongly affected by the surface property of the silicon nitride ®lm. Nitridation occurs when a silicon sample is exposed to gases such as NH 3 [3,5±7,9±12], NO [2,9,13], and N 2 H 4 [14], or to N atoms and ions [1,3,4,8,16]. The nitridation process on Si has been analyzed with X-ray and ultraviolet photoelectron spectroscopy, Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) [1± 10]. After nitridation, the Si 2p binding energy shows an increase of more than 2 eV. The Si LVV Auger peak shifts correspondingly from 91 to 84 eV. Although NH 3 or N 2 H 4 molecules dissociate and react readily with Si surfaces at 300 K or even colder [5±7,14], the reaction occurs at the adsorption sites and the original surface reconstruction on Si surfaces is still observable. More dramatic surface structural change cannot happen until the sample temperature is raised above 850 K for hydrogen desorption and Si out-diffusion to occur [1,3]. On a Si(111) surface, an `8 £ 8' LEED pattern is observed when the nitridation temperature is above 1075 K [1,3,11,22], indicating that a superstructure with periodicity of 30.7 A Ê (3.84  A £ 8) exists on the ®lm surface. Thin Solid Films 366 (2000) 121±128 0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S0040-6090(00)00852-X www.elsevier.com/locate/tsf * Corresponding author. Fax: 1852-2358-1652. E-mail address: phwangxs@ust.hk (X. Wang) 1 Present address: Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 Present address: Surface Physics Laboratory, Fudan University, Shang- hai 200433, China.