Step-induced electronic resonance at vicinal Si(001) observed by spectroscopic SHG and RAS

Transcription

1 Step-induced electronic resonance at vicinal Si(001) observed by spectroscopic SHG and RAS Robert Ehlert, Jinhee Kwon and Michael C. Downer Department of Physics, The University of Texas at Austin, Austin TX 78712, USA. Motivation investigation of step-enhanced chemical reactions atomic wires suitable for transport lithography by self assembly of nanostructures Himpsel, F. J et al., Solid State Communications 117(3): (2001). C. Tegenkamp, Journal of Physics-Condensed Matter 21 (2009). J. Viernow et al., Appl. Phys. Lett. 72, 948 (1998); on-invasive in-situ sensors are needed SHG/RAS need to be applied together OSI VIII /11

2 RAS of vicinal Si(001) surface show step induced features RAS spectra of clean and H 2 adsorbed Si(001):6 2 Calculated optical anisotropy and step induced features S r/r [10-3 ] 0 S photon energy [ev] Schmidt, W. G., F. Bechstedt, et al. PRB 63(4) (2001) Jaloviar, S. G., J. L. Lin, et al. (1999). PRL 82(4): can we use SHG to see this step induced resonance and would it provide additional information? OSI VIII /11

3 Combined SHG/RAS probe of stepped Si(001) surfaces in UHV environment + H 2 tunable fs-source monochromator + PMT analyzer PEM PMT Xenon lamp (1.5 to 5 ev) H 2 sticking coefficient quartz Coherent MIRA Ti:Sapphire oscillator nm NOPA 520nm-780nm B.S. gas inlet valve polarizer strain free window analyzer BG39 PMT SHG ports QMS sample M. Dürr et al., Phys. Rev B. 63, (2001) OSI VIII /11

4 RAS sensitive to adsorbates but does not distinguish terrace from step adsorption sites large number of dangling bonds surface very susceptible to contamination RAS is sensitive to adsorbates on Si(001) (2x1) origin of features on modified surface not well understood r/r [10-3 ] 2 clean after 3h in UHV after 6h in UHV 1 after peforming AES E 1 E 0 ' E 2 see also N. Witkowski et al. / Surface Science 600 (2006) step d.b. highly reactive e.g. oxidation proceeds from step edges Chung, C. H., H. W. Yeom, et al. (2006). "Oxidation of Step Edges on Si(001)-c(4 x 2)." Physical Review Letters 97(3): contamination in UHV photon energy 24min 250min 1200min Nishizawa, M., T. Yasuda, et al. (2002). "Origin of type-c defects on the Si(100)-(2x1) surface." Physical Review B 65(16): OSI VIII /11

5 SH [arb. units] Fourier analysis of RA-SHG clearly distinguishes step edge and terrace adsorption sites contamination clean after 3h after 6h after AES H 2 adsorption at step edges SH intensity [arb. units] 2 6 clean 1200L 1800L 2400L azimuthal angle [degree] azimuthal angle [degree] Fourier coefficients [arb. units] a 2 a time [hours] a OSI VIII /11 Fourier coefficients a2 a3 hydrogen exposure [L] a

6 Fourier analysis of SHG data yields spectra of Fourier coefficients a 0 to a 4 within the available wavelength range SH dependence on azimuthal sample rotation Fourier analysis terrace step terrace 16 λ ( nm) strong variations with azimuthal angle can be broken down in Fourier series clean Si(001):6 (2x1) after exposure to H 2 OSI VIII /11

7 RA spectrum resembles line shape and H 2 dependence of step induced 1 st and 3 rd order Fourier coefficients a 1 r/r [10-3 ] 0 SH [arb. units] a photon energy [ev] ab-initio calculations for RAS have shown that feature at 3eV is step induced * resembles line shape and H 2 dependence of a 1 and a 3 * W. G. Schmidt, F. Bechstedt and J. Bernholc, Phys. Rev. B 63, (2001) SH photon energy [ev] may point to common microscopic origin of RAS/SHG OSI VIII /11

8 Simplified Bond Hyperpolarizability Model (SBHM) using abinitio structure calculation fits SHG data with high fidelity SBHM * enables qualitative bond level interpretation of SHG/RAS ab initio DFT calculation provides input structure for SBHM analysis step β rebond step β d.b step β back terrace β dimer terrace β d.b. Results SBHM fits to RA-SHG data at selected wavelength 730nm nm terrace β back. allows empirical fits with varying bond hyperpolarizabilities β of select classes of surface bonds *G. D. Powell and D. E. Aspnes et al., J. Vac. Sci. Technol. B20(4), 1699 (2002) ** R. C. Miller. Phys. Rev. Lett. 5, 17 (1964) 4 840nm 840nm 2 H SBHM fits data with high fidelity & reproduces measured RAS clean OSI VIII /11

9 Hyperpolarizability spectra of individual surface bonds reveal charge transfer at identifiable molecular sites Re[β Re[β step step ] ] -5-3 d.b. d.b. step edge bonds D B step terrace bonds (2x1) terraces Re[β terrace ] d.b Re[β step ] Re[β rebond step ] 1 0 rebond - Re[β terrace dimer ] 0.5Im[β step ] rebond 0.6 Im[β terrace ] dimer Re[β step ] 0.6 back 0.6 Im[β step ] back OSI VIII /11

10 Conclusion showed that SHG is sensitive to adsorbates on Si(001) (2x1):6 and allows to distinguish specific adsorption sites acquired spectroscopic SHG and RAS data together on same clean reconstructed vicinal Si(001):6 surfaces before and after dissociative adsorption of H 2 at D B step edges Fourier analysis revealed step-induced 3 rd order Fourier component shows step d.b. induced resonance, resembling RAS line shape SBHM: reconstructed RAS line shape charge-rich step edge dangling bond dominant contribution to SH signal able to see charge redistribute from step edge dangling bonds into back bonds Results reveal specific underlying connections between RAS and SHG and allow qualitative bond level interpretation OSI VIII /11

11 Future directions Extend RAS/SHG study to other adsorbates, i.e. atomic hydrogen pump/probe Obtain ab-initio calculations of the nonlinear response Research Group Junwei Wei, Jimmy Price, Ming Lei Funding NSF DMR Robert Welch Foundation F-1038 OSI VIII /11