Predicted modi®cations in the direct and indirect gaps of Ge A. Zaoui * ,1 , M. Certier, M. Ferhat, W. Sekkal, O. Pag es, H. Aourag Universit e de Metz, LPLI, 8 rue Marconi, Technop^ ole 2000, France Abstract The charge density of Ge was studied at various k-points and for various bands, by the ab initio pseudopotential method, using additionally the interstitial sites. The lowest X c conduction-band points were found to be unique in having a high charge density in the interstitial site. It has been therefore predicted and veri®ed that the X c points move up in energy relative to the C c point when closed-shell atoms (like H) are substituted at the interstitial sites. The cal- culations also indicate the change of the band-gap for HGeH. Ó 2000 Elsevier Science B.V. All rights reserved. Diamond, zinc blende, wurtzite, and carbo- rundom-type binary crystals are the most loosely packed structures consistent with the tetrahedral coordination for both types of atoms [1,2]. Their openness is highlighted by the fact that for the homopolar members the ratio of the volume of touching atomic spheres to that of the unit cell is 0.34, less than half of that for the close-packed ele- ment structures (0.74). These ``homeotect'' struc- tures may be characterized by the existence of four vacant lattice sites (holes) at the tetrahedral in- terstitial sites near the anion A and four near the cation C, both at the normal nearest-neighbor tetrahedral distance. Hence, as we traverse the á111ñ body diagonal in the zinc-blende crystal, with the origin, say at the cation site 0; 0; 0, we encounter the anion at  1 4 ; 1 4 ; 1 4  the sites with co- ordinates  1 2 ; 1 2 ; 1 2  and  3 4 ; 3 4 ; 3 4 , both unoccupied in normal tetrahedral structures. While this minimal packing fraction re¯ects the eectiveness of pure sp 3 tetrahedral bonding in forming stable com- pounds with low coordination number, denser packing is by no means unusual in these structure types [3±12,15,23]. This can be eected by diusion of interstitial impurities (e.g., the 3d impurities or H, He, and Li  in Ge, occupying the vacant sites), by stoichiometric substitution of one type of vacant site We will verify, following Rompa et al., [14] (RSW), if indirect-gap can be made direct or with other nature by ®lling the interstitial sites. We will then demonstrate how insights deduced from simple and (so far) hypothetical ®lled tetrahedral structures (H in Ge) can be applied to hitherto unknown properties of existing ®lled tetrahedral structures, i.e., the prediction of a new class of tetrahedral semiconductors. The goal of this work is to study the electronic and structural properties of HGeH. We selected this host crystal since it has indirect-gap semicon- ductor at zero pressure. Hydrogen was selected as a ®lling atom because of the simplicity of its in- teraction with the host crystal. It is also a com- monly introduced fast diusing [16,17] interstitial species. In semiconductor technology, it is used as the inert atmosphere in crystal growth, for etching, www.elsevier.com/locate/commatsci Computational Materials Science 17 (2000) 400±403 * Corresponding author. Tel.: +39-040-2240-329. E-mail address: zaoui@ts.infn.it (A. Zaoui). 1 Dipartimento di Fisica Teorica, Universita de Trieste, Strada Costiera 11, 34014 Trieste, Italy. 0927-0256/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 0 0 ) 0 0 0 6 0 - 4