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Structure of the metallic Si(001) surface at high ... Structure of the metallic Si(001) surface at high ... Document Transcript

  • PHYSICAL REVIEW B 78, 033303 2008 Structure of the metallic Si(001) surface at high temperatures: Synchrotron x-ray scattering measurements Yongsam Kim,1 Cheolho Jeon,2 S. H. Uhm,3 C. C. Hwang,3,† N. D. Kim,3 Osami Sakata,4 C. Y. Park,2 J. W. Chung,3 and D. Y. Noh1,* 1 Department of Materials Science and Engineering and School of Photon Science and Technology, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea 2BK21 Physics Research Division and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, Korea 3Physics Department and Pohang Accelerator Laboratory, Pohang University of Science and Technology, San 31 Hyoja Dong, Pohang 790-784, Korea 4 Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan Received 9 May 2008; published 11 July 2008 The structure of the metallic Si 001 surface above 900 K was investigated by synchrotron x-ray scattering. Above 900 K, the 2, 1 integer-order surface peak decreases anomalously while the 3/2, 0 reconstruction order peak increases. These results together with the behavior of the crystal truncation rod profile exclude the structural models of the metallic Si 001 surface based on the enhanced Debye-Waller factor and the transition to the symmetrical dimer. The experimental results are explained by the enhanced dynamic step-edge fluctua- tions induced by the adatom attachments and detachments above 900 K. DOI: 10.1103/PhysRevB.78.033303 PACS number s : 68.35.Md, 61.05.cm, 61.05.cp, 68.47.Fg The dimer structure of the Si 001 surface has attracted The x-ray scattering measurements were carried out at thesubstantial attention since the observation of the metallic BL13XU beamline at SPring-8 in Japan using an ultrahigh-property of the Si 001 surface at high temperatures. Based vacuum UHV chamber equipped with x-ray scattering ca-on their high-resolution electron-energy-loss spectroscopy pability. The energy of the incident x-ray beam was fixed atand ultraviolet photoemission spectroscopy results, Gavioli 12.4 keV using a double bounce Si 111 monochromator.et al.1 reported the surface metallicity of Si 001 surface The sample was cut into a size of 5 20 0.3 mm3 from anabove 900 K. Prior to this discovery of the semiconductor- n-type Si 001 wafer. To obtain a clean surface, the samplemetal transition on the Si 001 surface, Ha and Greene2 had was degassed and flashed at 1473 K by flowing direct currentreported an anomalous decrease of the He atom scattering through it in the UHV chamber with the base pressure of 3intensity around 930 K. They attributed this to a structural 10−10 torr. Detailed information for the surface x-ray scat-phase transition above which the Debye-Waller DW factor tering chamber is described in the Ref. 5.increases abnormally. Later Barbier and Lapujoulade3 pro- We adopt the surface reciprocal-lattice coordinates to de-posed the anharmonicity in surface atomic displacement scribe the scattering data which is related to the reciprocalrather than the DW factor as the reason for the anomalous lattice of the conventional cubic lattice unit cell by b1decrease. = 110 c, b2 = ¯ 10 c, and b3 = 001 c. The momentum- 1 On the other hand, Fukaya and Shigeta4 proposed that the transfer component in the surface plane qin is indexed bytransition to the symmetric 2 1 dimer structure is the originof the semiconductor-metal transition. They claimed that the h , k through qin = hb1 + kb2. The normal component q isasymmetric dimer structure changes to a symmetric one at represented by q = lb3. The temperature dependence of thearound 900 K based on their reflection high-energy electron- integer-order 2, 1 , the half-integer-order 3/2, 0 and 1/2,diffraction RHEED results. The RHEED intensity of the 0 peaks, and the CTR at 0, 0 were measured across the 004 reflection decreased less than the amount expected reported transition temperature.from the simple DW factor. This result was completely op- In order to examine whether the dimer structure changesposite to the Ha and Greene2 result where the He atom scat- to the symmetric one as claimed by Fukaya and Shigeta, wetering intensity decreased more than the DW factor.3 The measured the specular CTR, which is sensitive to the atomicdifference between the RHEED and the He scattering inten- positions in the surface-normal direction, at 725 K and at 970sity remains to be explained, and the structural nature of the K. Figure 1 shows the structure factors of the CTRs plottedsemiconductor-metal transition is still controversial. as a function of l. There is no significant difference between In this study, we investigated the structure of the Si 001 the CTR measured at 725 K and that measured at 970 K.surface around 900 K using surface x-ray scattering tech- The CTR data were fit to two models each assuming thenique. The behavior of the specular crystal truncation rod asymmetrical dimer and the symmetrical dimer respectively CTR , an integer-order surface peak, and a half-integer re- using a reported fitting program.6 In each model the positionsconstruction peak was studied across the reported transition of the near surface atoms given in Fukaya and Shigeta’s4temperature. We found that the dimer structure remains to be paper and references therein are used.7 The surface rough-asymmetrical even above the transition. The dynamic fluc- ness caused by the steps separating dimer domains was thetuations of atomic step edges explain the observed behaviors only meaningful free parameter in the fitting. As shown inconsistently. Fig. 1, the asymmetric dimer model fits the CTR much better1098-0121/2008/78 3 /033303 4 033303-1 ©2008 The American Physical Society
  • BRIEF REPORTS PHYSICAL REVIEW B 78, 033303 2008 Intensity (arb.units) 100 (a) 725 K (b) 970 K 1.5 (a) 1150 KStructure Factor |F| asymmetric asymmetric symmetric symmetric 1.0 (arb. units) 10 970 K 0.5 725 K 1 0.0 -0.02 -0.01 0.00 0.01 0.02 o -1 0.1 K( ) 1 2 3 4 51 2 3 4 5 l (r.l.u.) l (r.l.u.) 3.6 Integrated Intensity (b) 2.7 (arb. units) FIG. 1. Specular CTR profiles measured at 725 K a and 970 K b . The solid lines are fits to the asymmetric dimer model and the 1.8dashed lines are fits to the symmetric dimer model. 0.9than the symmetric model both at 725 K and at 970 K. At725 K, the resultant 2 was 1.7, and the root-mean-square 0.0 200 400 600 800 1000 1200 rms roughness is 1.6 0.3 Å. The symmetric model re- Annealing Temperature (K)sults 2 of 5.9 and of 4.0 0.2 Å. For the CTR measuredat 970 K shown in Fig. 1 b , the asymmetric model solid FIG. 2. a 2, 1 peak profiles at 1150, 970, and 725 K. Theline yields 2 of 3.1, and is 5.5 0.2 Å. On the other solid lines are the results of the fits to a Lorentzian curve. bhand, the 2 resulted by the symmetric model dashed line is Temperature dependence of the integrated intensity of the 2, 1as large as 15.0, with = 3.6 0.2 Å. peak. The dashed lines are guide to eye for DW factor. The fitting results illustrate that the dimer structure abovethe metallic transition remains to be asymmetric since the 2 edges. Considering the diamond structure of silicon, theof the symmetric model is about five times larger than that of phase of the x rays reflected from every other 001 atomicthe asymmetric model. This is in contrast to Fukaya and planes changes by , and the scattered x rays from atomicShigeta’s explanation of the surface structure in the high- planes separated by a step edge can interfere destructively.temperature metallic phase based on the symmetric dimer Therefore, the decrease of the peak indicates that the surfacestructure. We also note that the surface became rougher at becomes rougher as the temperature increases above 900 K.970 K than at 725 K. This indicates that either there are more This roughening can result from the step proliferation or thesteps on the surface or the step height is increased at high step-edge fluctuations. Since the average terrace size doestemperatures. not change much as indicated from the behavior of the peak To understand the behavior of the surface in-plane struc- width, it is likely that the meandering fluctuations of the stepture, we investigated the behavior of the 2, 1 surface edges be the major contribution to the roughening. The en-integer-order reflection. The 2, 1 peak, which corresponds hanced dynamic step-edge meandering at elevated tempera-to the He atom scattering intensity, is sensitive to the surface tures have been reported by scanning tunneling microscopyin-plane structure. Figure 2 a shows the transverse scan of and low energy electron microscopy.9–12 In the reference, thethe 2, 1 peak measured at l = 0.15. Since the 2, 1 at l = 0 meandering of the SB type of step becomes so large thatwith no momentum-transfer component in the normal direc- adjacent step edges occasionally collide to create steps oftion lies exactly on the surface and x-rays cannot reach in double-atomic layer above 900 K.12 If we make a crude ap-and out of the surface, we adopted the grazing incident x-ray proximation that the integrated intensity shown in Fig. 2 bscattering geometry with a finite l.8 The 2, 1 peak profile corresponds roughly the scattering intensity from the step-was fit to a Lorentzian curve, and the results are represented free region of a terrace, the integrated intensity might beby the solid lines. The peak dwindles as the temperature related to the step meandering width W by 1 − W 2, where L Lincreased from 725 to 1150 K, while there is no significant is the typical terrace width as schematically illustrated in Fig.change in the peak width. From the half width at half maxi- 3.mum HWHM of the peak, the average terrace size is esti- Interestingly, the half-integer order from the 2 1 recon-mated to be about 1100 Å. struction increases as the temperature increases. Figure 4 a As illustrated in Fig. 2 b , the integrated intensity of the shows the profiles of the 3/2, 0 reconstruction peak at 1150,peak, the area under the peak, decreases as the temperature 970, and 725 K, respectively. All the diffraction profile isincreases to around 800 K following the normal DW factor. obtained with l fixed at 0.5 r.l.u. . Compared to the 2, 1Around 900 K, however, the integrated intensity decreases integer-order peak, the 3/2, 0 peak is much sharper. Theabruptly and deviates from the DW factor effect indicated by width obtained by fitting the peak to a Lorentzian is 8.5the dashed line. This behavior is similar to the reported He 10−4 Å−1, which is about 6.5 times smaller than the widthatom scattering results.2,3 The diffraction intensity at the 2, of the integer-order peak. The instrumental resolution in this1 peak, which is in the middle of the bulk 131 and the direction is estimated to be 1.9 10−4 Å−1. The sharp peak ¯ 131 reflection, is quite sensitive to the presence of the step indicates that the 2 1 order is correlated over large length 033303-2
  • BRIEF REPORTS PHYSICAL REVIEW B 78, 033303 2008 (a) L TABLE I. Calculated structure factors F at 1/2, 0 l = 0.5, W 3/2, 0 l = 0.5, and 2, 1 l = 0.15 for asymmetrical and symmetrical dimer models f Si is atomic form factor of Si . 1 2 2 1 F 1/2,0 l=0.5 F 3/2,0 l=0.5 F 2,1 l=0.15 2 1 asymmetrical 1.29 f Si 2.15 f Si 3.62 f Si SB symmetrical 1.47 f Si 2.00 f Si 3.63 f Si SA (b) L SB The model based on the transition from the asymmetrical dimer to the symmetrical dimer is also excluded. Table I shows the structure factors at the three reciprocal space po- 2 1 sitions calculated from the asymmetrical and the symmetrical 2 1 dimer models using the atomic positions reported by Fukaya 1 2 and Shigeta’s result. The structure factor is calculated in a SA unit cell constructed by the 110 , 2 ¯ 10 , and 001 unit 1 1 W SA vectors in real space. At 3/2, 0 , the symmetrical dimer re- sults in a smaller structure factor than the asymmetrical FIG. 3. Schematic diagrams of surface domains with step fluc- dimer contracting the experimental result. At the 1/2, 0 , thetuations a below and b above 900 K L is typical terrace width structure factor of the symmetric dimer is larger than that ofand W is step meandering width . the asymmetric dimer by about 12%, which is not enough toin the direction of the scan, the 110 direction, parallel to the explain the observed data. Both models produce a similartypical step-edge direction. The 2 1 order is well correlated value of structure factor at the integer-order 2, 1 peak,on a single terrace. The peak intensity at 1150 K is signifi- while the experimental value diminishingly small at highcantly larger than the peak measured at 970 K. Similar be- temperature.havior is also observed at the 1/2, 0 fractional-order peak. We attribute the increase of the fractional-order peaks to The temperature dependence of the integrated intensity of the increase of the correlation in the 2 1 order. As shown inthe 3/2, 0 and the 1/2, 0 is shown in Fig. 4 b . The in- Fig. 3 schematically, the reconstruction order changes fromcrease of the fractional-order peak at high temperatures ex- the 2 1 dimer to the 1 2 dimer across a step edge, whichcludes the models based on the enhanced DW factor. If the changes back to 2 1 dimer at the following step edge.DW factor is enhanced anomalously at high temperatures as There are two possible configurations forming the 2 1 re-proposed in Refs. 3 and 13–15, the fractional-order peaks as construction out of the bulk terminated 1 1 surface. On awell as the integer-order peak should decrease. single terrace, two possible 2 1 domains separated by do- main boundary coexist. The 2 1 order on two terraces sepa- rated by a terrace of 1 2 dimer are not correlated typically. Intensity (arb. units) 6 (a) 1150 K At high temperatures, the adatoms are quite mobile as evi- denced by reported increase in the mobility of the step 4 edges.12 The increase in the 3/2, 0 peak indicates that the 970 K 2 1 order propagate across the 1 2 terrace as the step 2 edge collides frequently at high temperatures. Figure 5 725 K shows the behavior of the full width at half maximum 0 FWHM of the 3/2, 0 peak. The width decreases slightly -0.008 0.000 0.008 o -1 indicating that the 2 1 domain size along the step-edge K direction increases, although it is difficult to extract quanti- 1.6 tative value since the width is already close to the instrumen- (b) Integrated Intensity -3 1.2 1.2x10 (arb. units) 0.8 -4 -1 o 9.0x10 x2 0.4 FWHM -4 6.0x10 0.0 200 400 600 800 1000 1200 Annealing Temperature (K) 3.0x10 -4 200 400 600 800 1000 1200 FIG. 4. a 3/2, 0 peak profiles at 1150, 970, and 725 K the Annealing Temperature (K)solid lines are fitted by a Lorentzian curve . b Temperature depen-dence of the integrated intensity 1/2, 0 and 3/2, 0 b at l FIG. 5. FWHMs of 3/2, 0 peak profiles as a function of an-= 0.5 the dashed lines are guide to eye for DW factor . nealing temperature. 033303-3
  • BRIEF REPORTS PHYSICAL REVIEW B 78, 033303 2008tal resolution. The 2 1 domain size inside a terrace in- detachment and attachment. The observed structural infor-creases at high temperatures. In summary, we have observed mation of the Si 001 surface might cast clue in understand-that the integer-order reflection decreases anomalously while ing the metallic properties of Si 001 surface at high tem-the fractional-order reconstruction peak increases on a peratures.Si 001 surface at temperatures above 900 K. Our resultsexclude the models based on the enhanced Debye-Waller This work was supported by the Korean Science and En-factor and the transition of the asymmetrical to symmetrical gineering Foundation KOSEF through the National Re-dimer in describing the high-temperature metallic Si 001 search Laboratory Program No. M10400000045–04J0000–surface. The specular CTR data were fit well to a surface of 04510 . J.W. Chung acknowledges the support from thethe asymmetrical dimer structure with enhanced surface Korea Research Foundation Grant No. KRF-2006–312-roughness. The observed experimental results are explained C00513 .by the dynamic step-edge fluctuations caused by the adatom*dynoh@gist.ac.kr 9 N. Kitamura, B. S. Swartzentruber, M. G. Lagally, and M. B.†cchwang@postech.ac.kr Webb, Phys. Rev. B 48, 5704 1993 . 1 L. Gavioli, M. G. Betti, and C. Mariani, Phys. Rev. Lett. 77, 10 H. J. W. Zandvliet, B. Poelsema, and H. B. Elswijk, Phys. Rev. B 3869 1996 . 51, 5465 1995 . 2 J. S. Ha and E. F. Greene, J. Chem. Phys. 91, 571 1989 . 11 3 C. Pearson, B. Borovsky, M. Krueger, R. Curtis, and E. Ganz, L. Barbier and J. Lapujoulade, J. Vac. Sci. Technol. A 8, 2662 Phys. Rev. Lett. 74, 2710 1995 . 1990 . 12 4 Y. Fukaya and Y. Shigeta, Phys. Rev. Lett. 91, 126103 2003 . N. C. Bartelt and R. M. Tromp, Phys. Rev. B 54, 11731 1996 . 13 P. Zeppenfeld, K. Kern, R. David, and G. Comsa, Phys. Rev. 5 O. Sakata et al., Surf. Rev. Lett. 10, 543 2003 . 6 E. Vlieg, J. Appl. Crystallogr. 33, 401 2000 . Lett. 62, 63 1989 . 14 L. Yang and T. S. Rahman, Phys. Rev. Lett. 67, 2327 1991 . 7 M. T. Yin and M. L. Cohen, Phys. Rev. B 24, 2303 1981 . 15 8 H. Dosch, B. W. Batterman, and D. C. Wack, Phys. Rev. Lett. G. Helgesen, D. Gibbs, A. P. Baddorf, D. M. Zehner, and S. G. 56, 1144 1986 . J. Mochrie, Phys. Rev. B 48, 15320 1993 . 033303-4