19 May 2000 Ž . Chemical Physics Letters 322 2000 166–174 www.elsevier.nlrlocatercplett Possible metallicity in SiO and GeO solids Stefan Mankefors a , Torvald G. Andersson a , Itai Panas b, ) a Department of Physics, Chalmers UniÕersity of Technology, S-412 96 Goteborg, Sweden ¨ b Department of Inorganic EnÕironmental Chemistry, Chalmers UniÕersity of Technology, S-412 96 Goteborg, Sweden ¨ Received 10 February 2000; in final form 28 March 2000 Abstract First principles calculations are performed to determine possible structures and electronic properties of silicon monoxide Ž. and germanium monoxide solids. For both compounds, an orthorhombically distorted SnO s structure is found to display Ž. Ž. the highest stability. The bondings in SiO s and GeO s display considerable degrees of both covalency and ionicity in Ž. Ž. conjunction with significant metallic character in the bc planes. SiO s is predicted to be a better conductor than GeO s . Ž. Possible importance to CMOS technology is emphasized as supposedly insulating thin SiO s layers may display local 2 Ž. phase separation, resulting in metallic perforation due to microscopic SiO s ‘spikes’. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Ž. The possibility of forming SiO s is fundamental 2 for the operation and manufacturing of modern solid Ž. state devices. While SiO s and to some extent 2 Ž. GeO s have been subject to a number of experi- 2 Ž w x. mental and theoretical studies see, e.g., Ref. 1,2 , there is very little information to be found on the corresponding monoxide systems. One reason for the lack of information regarding structure and proper- ties of SiO and GeO is that these solids are metastable Ž. Ž. and easily decomposed into Si s q SiO s and 2 Ž. Ž. Ge s q GeO s , respectively. The possible techno- 2 Ž. logical importance of GeO s is considerably more Ž. limited when compared to SiO s , but it is still rewarding to treat the two systems together since they are both largely unknown and expected to dis- ) Corresponding author. Fax: q 46-31-7722833; e-mail: itai@inoc.chalmers.se play characteristic similarities. Still, controlled oxi- Ž. dation of Ge s is known to be much more difficult Ž. to achieve than is that for Si s . This poses an interesting problem which to some extent affects the detailed results presented below. Ž. The unknown properties of the monoxides of Si s Ž. and Ge s could be of urgent importance for the state-of-the-art complementary metal oxide semicon- Ž . ductor CMOS technology. Indeed, with shrinking dimensions, material properties at Si–SiO –Si inter- 2 faces are expected to eventually become decisive. Ž. Ž. The interface between the Si s and SiO s phases 2 is not well characterized as to how the microscopic composition changes in the cross-over region. The ˚ complex region is believed to extend at least 5–10 A into the oxide. Due to the large surface to volume ratio, however, simple statistical considerations im- Ž. Ž . ply that, if thin enough, a SiO s layer 3–4 nm , 2 will exhibit defect ‘spikes’ consisting of unsaturated Ž. SiO s partly or completely penetrating the SiO x 2 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00435-8