Adsorption Behavior of Ruthenium 2

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface Ce ´ sar Ortega Lo ´ pez a,c , William Lo ´ pez Pe ´ rez b, *, Jairo Arbey Rodrı ´guez M. c a Departamento de Fı ´ sica, Universidad de Co ´ rdoba, Monteria, Colombia b GFMC, Departamento de Fı ´ sica, Universidad del Norte, A.A. 1569 Barranquilla, Colombia c GEMA, Departamento de Fı ´ sica, Universidad Nacional de Colombia, A.A. 5997 Bogota ´ , Colombia 1. Introduction GaN has become an intensively studied material because of its wide range of technological potential. GaN is the basic compound for blue/ultraviolet light emitting devices, solar-blind detectors, metal semiconductor field effect tran sistors and high mobility electron transistors, in which Schottky contacts are presented [1– 5]. The Schottky contacts could be a useful scheme for the design and prod uct ion of high -tempera ture devices, as recent exper i- ments have showed; such experiments have studied the effects on the electrical and structural properties of Ru and Ru/Au Schottky contacts on n-type GaN [6,7]. Growing of GaN by molecular beam epit axy hav e showed that the (0 0 0 1) surf ace is rele vant [8]. There for e, we bel iev e necessary to car ry out ab ini tio stu die s bas ed on the den sit y functi onal theor y (DF T), for elu cid ati ng the s tr uc tu ra l a nd el ec tr on ic pr op e rt ie s of Ru a da to m on GaN(0 0 0 1) surface. For the GaN(0 0 0 1) and GaN(000 ¯ 1) surfaces several reco nstr ucti ons hav e been obser ved [9], who se atomic str uctures have been exp erimen tal ly obtain ed by sca nning tunneling microsco py and reflection high-energ y electron diffraction and simulated with theoretical models [10,11] . The gallium adsorption on GaN(0 0 0 1) surfaces was experimentally analyzed by specular reflection high-ene rgy electron diffraction [12]. Ab initio stu die s hav e det ermine d the energetic all y fav ora ble reconstructions of such surface with (2 Â2) geometry, starting from several possibil ities related to adsor ptio n of galli um and nitrogen atoms [13]. They foundtha t Ga- T 4 modelis moresta blein Ga-rich con ditio ns and N- H 3 model is mor e stable in N-rich conditions. A similar first principle study on the GaN(0 0 0 1)surface rep orts a Ga- adl aye r rec ons tru ction sta ble in Ga- ric h conditions [14]. Northrup et al. reported the stability of laterally con trac ted bilay er stru ctu res unde r Ga-r ich con ditio ns in the GaN(0 0 0 1) surface with 1 Â 1 geometry [15]. Ultimately, the adsorption of a different atomic species to that of the surface is an int ere sti ng exp loration. For exa mpl e, the ads orp tio n of H, In, Si, Mg and eth ane thi ol onsome sur fac es of theGaN hasbeen stu die d [16– 20]. Recently, the phase diagram of the GaN(0 0 0 1) surface was determined by means of ab initio calculations. In such study, the Applie d Surfa ce Scienc e 255 (2009) 3837–3842 A R T I C L E I N F O Article history: Receiv ed 5 September 2008 Received in revised form 15 October 2008 Accept ed 15 October 2008 Available online 31 October 2008 PACS: 68.35.Bs 68.35.Md 68.43.Bc 68.43.Fg 71.15.Mb 73.20.At Keywords: Surface Adsorption;Diffusion DFT GGA Pseudopotential A B S T R A C T We report firstprinci ple s cal cula tions to anal yze therutheni um ads orption and dif fus ion on GaN(0 0 0 1) s urface in a 2 Â2ge ometry. The calc ulat ions were per formed usi ng the general ized gradie nt approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The sur faceis mod eledusing the rep eatedslabs approach. To study the mos t favorable ruth eni um adso rpt ion model we considered T 1 , T 4 and H 3 special sites. We find that the most energetically favorable structure corresponds to the Ru- T 4 model or the ruthenium adatom located at the T 4 site, while the ruthenium adsorption on top of a gallium atom (T 1 position) is totally unfavorable. The ruthenium diffusion on sur faceshows an ener gy bar rierof 0.612 eV. The resu ltan t rec ons truc tion of the ruth eniu m ads orp tion on GaN(0 0 0 1)- 2 Â2 surface presents a lateral relaxati on of some hundredt h of A ˚ in the most stable site. Thecompar ison of thedensityof states andband str ucture of the GaN(0 0 0 1)surface with outrutheni um adatom and with ruthenium adatom is analyzed in detail. ß 2008 Elsevier B.V. All rights reserved. * Corr espon ding autho r. Tel.: +57 5 350950 9; fax: +57 5 3598852. E-mail address: [email protected] (W.L. Pe ´ rez). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.072

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Adsorption Behavior of Ruthenium 2