Surface Science 609 (2013) Contents lists available at SciVerse ScienceDirect. Surface Science

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1 Surface Science 69 () 47 Contents lists available at SciVerse ScienceDirect Surface Science journal homepage: Scanning tunneling spectroscopy an ensity functional calculation of silicon angling bons on the Si()- :H surface Wei Ye a,b,, Kyoungmin Min a,, Pamela Peña Martin b, Angus A. Rockett b, N.R. Aluru a,, Joseph W. Lying a,b,c a Beckman Institute for Avance Science an Technology, University of Illinois at Urbana Champaign, Urbana, IL 68, Unite States b Department of Materials Science an Engineering, University of Illinois at Urbana Champaign, Urbana, IL 68, Unite States c Department of Electrical an computer Engineering, University of Illinois at Urbana Champaign, Urbana, IL 68, Unite States Department of Mechanical Science an Engineering, University of Illinois at Urbana Champaign, Urbana, IL 68, Unite States article info abstract Article history: Receive October Accepte 9 November Available online 7 December Keywors: UHV-STM Silicon angling bon Density functional calculations Scanning tunneling spectroscopy Hyrogen passivate silicon Si() surface Dangling bon wire We stuie electronic properties of atomic-scale angling bon (DB) an DB wires on Si()- :H surfaces using a ultrahigh vacuum scanning tunneling microscope (UHV-STM). The ecay of the near-migap DB-states inuce by an unpaire DB epens on the crystalline orientation of the Si() surface. The ecay length of the DB-states of an unpaire DB wire can be ~. nm along the imer row irection. The perturbation from an unpaire DB to an ajacent paire DB is also emonstrate. The results are in goo agreement with ensity functional calculations. Elsevier B.V. All rights reserve.. Introuction Silicon angling bons (DBs) are common efects observe on the Si() surface. The DB states inuce within the bangap of Si are responsible for the Fermi level pinning at the surface []. It has been shown that by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM), one can selectively pattern silicon (Si) angling bons (DBs) with atomic level precision on the hyrogenpassivate Si() surface [ 4]. For example, an atomic wire can be forme on the H-Si() surface by removing hyrogen atom by atom using the STM tip along a Si imer row. Due to the coupling of the DB states in the wire, the unpaire DB wire shows oneimensional metallic behavior, which coul be use as an atomic size interconnect []. In aition, charge DBs can serve as atomic scale quantum ots (QDs) [6 8] leaing to the construction of a room temperature quantum-ot cellular automata (QCA) cell [6]. Furthermore, Si DB patterns can also be use as templates for subsequent selective chemistry ue to the ifference in reactivity between bare an hyrogen passivate silicon [9]. The potential applications to atomic-scale electronic evices arouse an increase interest in the stuy of Si DBs. Unerstaning the electronic properties of silicon DBs is essential for their integration into future atomic scale electronics. Piva et al. [6] emonstrate by STM that a negatively charge unpaire DB can electrostatically perturb the I-V characteristics of a nearby styrene molecular wire. Recently, Raza et al. [] stuie the same case theoretically an propose that the unpaire DB can introuce a near-migap state in the local ensity of states (LDOS) of neighboring Si atoms. The DB-inuce gap states (DBIGS) can have an aitional electronic contribution to the perturbation within a nm region. Thus the ecay of the near-migap DB state of an unpaire DB has the potential to engineer the electronic properties of nearby atomic scale evices. However, further experimental evience of this behavior is lacking. In this stuy, we use UHV-STM an Spanish Initiative for Electronic Simulations with Thousans of Atom (SIESTA) calculations to investigate theecayofdbigsfromunpairedbs an unpaire DB wires. We also emonstrate the perturbations from an unpaire DB upon a paire DB within nm.. Experimental methos Corresponing author at: Department of Materials Science an Engineering, University of Illinois at Urbana Champaign, Urbana, IL 68, Unite States. Tel.: aress: weiyeuiuc@gmail.com (W. Ye). The silicon use in this stuy is boron ope Si() which has a resistivity of.. Ω cm. The hyrogen passivate Si()- surface was prepare by first egassing in UHV in the preparation 9-68/$ see front matter Elsevier B.V. All rights reserve.

2 48 W. Ye et al. / Surface Science 69 () 47 chamber at 6 C overnight. The Si sample was subsequently flashe to ~ C several times to form the clean Si()- surface. Finally, the reconstructe silicon sample was expose to atomic hyrogen at 77 C to ensure a hyrogen passivate surface with monohyrie coverage []. The STM experimental ata were collecte within a homebuilt UHV-STM [] with a base pressure of approximately 7 Torr. Electrochemically etche polycrystalline tungsten tips were use. In our experimental setup, the tip is groune through a current preamplifier an the bias voltage is applie to the sample. STM images were acquire in constant current moe, with a typical setpoint tunneling current of pa. Iniviual DB patterns were create by the feeback control lithography (FCL) metho [4]. The feeback loop immeiately terminates patterning once a hyrogen esorbing event is etecte. DB wires were fabricate by repeately using FCL to make a row of DBs. Scanning tunneling spectroscopy (STS) is conucte by collecting tunneling current voltage (I V) spectra at user efine locations within the scan winow. The tip is hel at those locations with the feeback loop temporarily isable. Tunneling current is recore while sweeping sample bias from. V to +. V. All the experiments were carrie out at room temperature.. Theoretical methos To explore the electronic properties an compare them with experimental results of the effect of the Si DBs on the silicon surface, we performe SIESTA calculations base on fully self-consistent ensity functional theory (DFT) methos []. A generalize graient approximation (GGA) [] with revise Perew Burke Ernzerhof (RPBE) is implemente for the parameterization of the exchangecorrelation functional [4]. The core electrons are replace by the norm-conserving pseuopotentials [] an a ouble zeta basis plus polarization (DZP) numerical atomic orbital is employe. A mesh cutoff of 4 Ry is use. The number of k-points is generate with 7 7 Monkhorst Pack for the structural relaxation an Monkhorst Pack for the ban structure calculation [6]. We constructe a layer Si()-( ) unit cell for the unpaire DB an unpaire DB wire case, an use layers for the paire DB an paire plus unpaire DB case. The bulk structure with a 6 unit cell for each ifferent layer is optimize until the maximum resiual force is less than. ev/å. Finally, we set up several DB configurations using the unit cell for each case an compute the ban structure an LDOS. The ban structure is compute at the highest symmetrical irection on the surface brillouin zone of silicon. Aitional structural optimizations after creating DBs are not consiere ue to their negligible effect on the ban structure, except for the cases of paire DBs, an paire an unpaire DBs [7]. For both cases, structures are relaxe until the maximum force is less than. ev/å. After structural optimization, silicon imer length is obtaine as.8 Å with a buckling angle of 7.9, which shows the reasonable agreement with the previous work [8]. Detaile information for each ifferent DB state is shown in Table. DBs are create as the same way in the STM experiments. Table Details of unit cell setup. Unit cell construction Unit cell Number of atoms Unpaire DB 9 4 Unpaire wire DBs a 6 66 Unpaire wire DBs b 6 8 Paire DBs Paire an Unpaire DBs a Along an b across the Si imer row irection. Number of atoms is silicon atoms only. Perioic bounary conitions for both of x an y-irection are applie for all casesexcept for unpaire wire DBs a along the imer irection an unpaire wire DBs b across imer irection. Eges are terminate with hyrogens for those cases. 4. Results an iscussion Fig. a shows a fille-state STM image of an unpaire silicon DB fabricate by STM on the Si()- :H surface. The unpaire DB appears as a bright protrusion in the fille-state image ue to its enhance LDOS. The bright feature appears off-center on the imer row, inicating that only one hyrogen was esorbe from a Si imer an an unpaire DB was forme. To investigate the electronic properties of the unpaire DB, we took current voltage (I V) spectra across the DB at each pixel along a line. Fig. c shows the Log I V spectra map taken along the Si imer row irection. The position along the imer row irection is represente on the horizontal axis, the applie sample bias plotte on the vertical axis, an the log I value is color-mappe onto the plane. The unpaire DB has finite local ensity of states (LDOS) at the Fermi level while the Si substrate ~ nm away from the DB shows a semiconucting ban gap of. ev. However, the electronic property of the Si within ~ nm from the center of the unpaire DB is change by the ecay of DBIGS. The lengths of the ecay regions are.9 nm to the left an.6 nm to the right, as shown in Fig. c. The DB switche position to the right Si atom in the same Si imer after recoring several sets of spectra ata. The topographic image is shown in Fig. b. The hyrogen atom intraimer iffusion uner the fiel of the STM tip at room temperature has been observe before [9]. Fig. shows a log I V spectra map in the irection perpenicular to the Si imer rows. The ecay regions are.7 nm an.6 nm to the left an right of the DB respectively, an are aroun % smaller than those along the imer row irections. This is because the Si imers along the imer row irection have a close proximity of.84 Å, while the separation between imers across the imer row irection is 7.68 Å. Thus the DB state of the unpaire DB can ecay further into the Si lattice along the imer row irection. Fig. e shows the LDOS plot of an unpaire DB an the Si substrate. The unpaire DB shows finite (non-zero) LDOS throughout the ban gap of the Si substrate an has a peak at.74 ev. It has been shown that the near migap state for the case of an unpaire DB is very flat [7], an this has also been observe in the calculations. Therefore, we implement DFT-molecular ynamics (MD) simulations to stuy the effect of finite temperature to compare with the experimental result. Coorinates of each step at K from DFT-MD are obtaine to compute the ban structure, an the locations of near migap states are measure to be aroun.6 ev, showing the thermal broaening effect at finite temperature. Simulation results of LDOS of DB state along an across Si imer row irection are shown in Fig. f an g respectively an they show a rapi ecay of the DB contribution to the near migap state in LDOS. The DB state finally isappears at.4 nm from the DB center along the imer row irection. As for the across imer row irection, the transition regions are. nm an. nm to the left an right of the DB, respectively, which are in reasonable agreement with experiment. The transition region at K for along imer row irection is.9 nm an it shows smoother ecay rate of the LDOS peak ue to the DB state than for the groun state case. By tuning the patterning parameters, we can esorb both hyrogen atoms from the same Si imer to form a paire DB. Unlike an unpaire DB, a DB pair is image at the center of the imer row in a fille-state image; an example is shown in Fig. a. Fig. c shows the Log I V spectra map across the paire DBs. A ban gap of.9 ev is observe on the paire DBs, while the ajacent Si substrate continues to exhibit a ban gap of. ev. In contrast to the unpaire DB, there is no apparent ecay region between the paire DBs an the ajacent Si. The LDOS of the paire DBs, shown in Fig. e, consists of two peaks, at. ev an. ev. These are from boning (π) an antiboning (π*) states of the paire DBs. Compare to the Si substrate, the π* state is.4 ev below the conuction ban an the π state is close to the valence ban (.4 ev below). The experimental result is consistent with the calculate ban structure of a paire DB,

3 W. Ye et al. / Surface Science 69 () a nm c nm.6 nm 4 Position (nm) e Density of States (a.u.) Unpaire DB Si()-x: H b (A). - nm nm.6 nm 4 Position (nm) f g x A,.84 Å B, 7.68 Å C,. Å D,.4 Å DB D C B A H E,.7 Å F,. Å G, 7.68 Å H,.7 Å & I,.4 Å G F E G I DB Fig.. (a) Fille-state STM image of an unpaire angling bon on Si()- :H surface. (V s =. V, I t =pa) (b) STM image of the DB in the same Si imer after switching position from left to right. (c,) Log I V spectra plotte as a function of position in (c) for the unpaire DB in (a) an in () for the DB in (b). Re otte arrows in (a) an (b) enote where the I-V spectra maps were taken. The green otte lines in (c) an () are use to highlight the positions of the center of the unpaire DB an where the Si bangap is fully recovere. (e) DOS-V spectra of the unpaire DB in (a) an silicon substrate. (f) Simulate LDOS of Si atoms along the Si imer irection with the istance from DB. (Inset shows the magnifie view for atoms B,C, an D.) (g) Simulate LDOS of Si atoms across the Si imer irection with the istance from DB. (Inset shows the magnifie view for atoms F,G,H, an I.). which is shown in Fig. f, which shows that the π* state is. ev below the conuction ban an the π state is close to the valence ban (.9 ev below). To stuy the interaction between a paire an an unpaire DB, we introuce a DB cluster.8 nm away from the paire DB in Fig. a. The topographic image is shown in Fig. b. The DB cluster consists of a paire DB an an unpaire DB next to it. The unpaire DB is.9 nm to the right of the paire DB. So the influence from the unpaire DB on the left paire DB is negligible. However, the istance between the right paire DB an the unpaire DB is only.66 nm. We expect a strong interaction between these two DBs. Fig. shows an STS spectra map across both paire DBs in the imer row irection. The left paire DB shows a ban gap of.9 ev experimentally, as expecte, an.9 ev from the simulation. However, the ban gap of the right paire DB is reuce to.78 ev, or to.76 ev by simulation. The LDOS plot in Fig. e inicates a shift of the π state peak to.88 ev. The result is also consistent with the calculate ban structure shown in Fig. g. Fig. a shows an unpaire DB wire consisting of seven unpaire DBs along the imer row irection. An aitional unpaire DB is forme on the next imer row ue to spurious epassivation. STS spectra maps were taken both along the imer row irection an across the imer row irection to reveal the electronic properties of the DB wire. Fig. b shows the log I V spectra map along the imer

4 W. Ye et al. / Surface Science 69 () 47 a c e b f g.. E-Ef, (ev). -. *state state E-Ef, (ev). -. *state Near migap state state Γ J Κ J Γ - Γ J Κ J Γ Fig.. (a) STM topographic image of a paire DB fabricate by removing two hyrogen atoms from the same Si imer. (b) STM topographic image of a DB cluster forme to the right of the paire DB in (a). (c) Log I V spectra map taken on the paire DB, as inicate by the re otte arrow in (a). () Log I V spectra map taken across both paire DBs in (b) along the re otte arrow. (e) DOS-V spectrum for both paire DBs an Si substrate in (b). Simulation results are plotte in (f) for the paire DB an (g) for the paire DB an unpaire DB case. row irection. With strong interaction between the DBs just.84 Å away from each other, the whole DB wire exhibits metallic behavior. The ecay length of the DB state is much longer than that in the single DB case. The transition region, where the ban gap of the Si is reuce ue to DB-state inuce gap states, is in a range of. nm from the ens of the wire, along the imer row. However, in the irection perpenicular to the imer rows, the ecay length of the DB states is comparable to the single DB case. As shown in Fig. c, the length is about. nm. Simulation results from stuying the LDOS ecay of the unpaire DB wire state along an across Si imer row irection are shown in Fig., an e respectively. As shown for an unpaire DB, the migap DB state contribution ecays rapily. The DB state finally isappears at.9 nm for along imer row an. nm to the left an. nm to the right for across imer row irection from the outermost DB respectively, which shows reasonable agreement with experiment.. Conclusions In summary, we investigate the lateral ecay of DB states aroun an unpaire DB. The ecay length is anisotropic on the Si() surface. The near-migap state of an unpaire DB ecays ~.9 nm into ajacent Si atoms along the Si imer row, while the DB states isappear ~.4 nm across the Si imer rows, an simulations agree

5 W. Ye et al. / Surface Science 69 () 47 a b c A,.84 Å B, 7.68 Å C,. Å D,.4 Å E, 9. Å x e... F,.7 Å G,. Å H, 7.68 Å I, 7.68 Å J,.7 Å K,.4Å x - DB wires.. DB wires Fig.. (a) STM topographic image of an unpaire DB wire along the imer row irection. The scanning parameters are pa an. V. Log I V spectra maps are plotte as a function of position for (b) along the imer row irection an for (c) perpenicular to the imer row irection. The otte re arrows enote where the spectra were taken. () Simulate LDOS of Si atoms along the Si imer irection with the istance from unpaire wire DBs. (Inset shows the magnifie view for atoms B, C, D, an E.) (e) Simulate LDOS of Si atoms across the Si imer irection with the istance from unpaire wire DBs. (Inset shows the magnifie view for atoms F, G, H, I, J, an K.) with these experimental values. With the increasing interactions along the imer row irection, the ecay length of an unpaire DB wire increases to ~. nm along the Si imer row irection, while the ecay length in the irection perpenicular to the imer row is comparable to that of the unpaire DB. We also emonstrate that an unpaire DB can perturb the electronic property of an ajacent paire DB, reucing the bangap of the paire DB from.9 ev to.78 ev. Another paire DB.9 nm away from the unpaire DB remains unperturbe. References [] R.J. Hamers, U.K. Kohler, J. Vac. Sci. Technol., A Vac. Surf. Films 7 (4) (989) 84. [] J.W. Lying, T.C. Shen, J.S. Hubacek, J.R. Tucker, G.C. Abeln, Appl. Phys. Lett. 64 () (994). [] T.C. Shen, C. Wang, G.C. Abeln, J.R. Tucker, J.W. Lying, P. Avouris, R.E. Walkup, Science 68 (7) (99) 9. [4] M.C. Hersam, et al., Nanotechnology () () 7. [] T. Hitosugi, T. Hashizume, S. Heike, H. Kajiyama, Y. Waa, S. Watanabe, T. Hasegawa, K. Kitazawa, Appl. Surf. Sci. (998) 4. [6] P.G. Piva, G.A. DiLabio, J.L. Pitters, J. Zikovsky, M. Rezeq, S. Dogel, W.A. Hofer, R.A. Wolkow, Nature 4 (74) () 68. [7] M.B. Haier, J.L. Pitters, G.A. DiLabio, L. Livaaru, J.Y. Mutus, R.A. Wolkow, Phys. Rev. Lett. (4) (9) [8] H. Kawai, Y.K. Yeo, M. Saeys, C. Joachim, Phys. Rev. B 8 (9) () 96. [9] M.A. Walsh, M.C. Hersam, Annu. Rev. Phys. Chem. 6 () (9) 9. [] H. Raza, K.H. Bevan, D. Kienle, Phys. Rev. B 77 () (8) 4. [] J.W. Lying, S. Skala, J.S. Hubacek, R. Brockenbrough, G. Gammie, Rev. Sci. Instrum. 9 (9) (988) 897. [] M.S. José, et al., J. Phys. Conens. Matter 4 () () 74. [] J.P. Perew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (8) (996) 86. [4] B. Hammer, L.B. Hansen, oslash, J.K. rskov, Phys. Rev. B 9 () (999) 74. [] N. Troullier, J. Martins, Phys. Rev. B 4 () (99). [6] H.J. Monkhorst, J.D. Pack, Phys. Rev. B () (976) 88. [7] H. Raza, Phys. Rev. B 76 (4) (7) 48. [8] D.Q. Ly, L. Paramonov, C.J. Makatsoris, Phys. Conens. Matter (9) 86. [9] U.J. Quaae, K. Stokbro, C. Thirstrup, F. Grey, Surf. Sci. 4 () (998) L7.