Nonlinear Spectroscopy of Si Nanocrystals & Step-Edges Mike Downer University of Texas at Austin

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1 EPI-OPTICS 11 Erice, Sicily July 20, 2010 Nonlinear Spectroscopy of Si Nanocrystals & Step-Edges Mike Downer University of Texas at Austin Si nanostructures have properties & applications different from those of bulk Si Si lasers start to take shape Field-effect LED V gate > V e- injection In vivo bio-sensing Live pancreatic cancer cells V gate > V h+ injection Observation of optical gain in Si nanocrystals embedded in SiO 2 Pavesi et al., Nature 408, 440 (2000) Walters et al, Nature Mat. 4,143 (2005). amine-terminated micelle-encapsulated Si NC, a non-toxic alternative to CdSe NCs Erogbogbo et al, ACS Nano.2, 873 (2008) 17 µm These interesting properties originate at Si NC/SiO 2 interfaces. SHG has a reputation for being interface-specific

2 EPI-OPTICS 11 Erice, Sicily July 20, 2010 Nonlinear Spectroscopy of Si Nanocrystals & Step-Edges Mike Downer University of Texas at Austin Si nanostructures have properties & applications different from those of bulk Si Si lasers start to take shape nanowire Observation of optical gain in Si nanocrystals embedded in SiO 2 Pavesi et al., Nature 408, 440 (2000) Field-effect LED V gate > V e- injection silicon stepped surfaces provide templates for 1D quantum wires, 1 molecular V gate > V h+ injection electronics, 2 atomic-scale memory, 3 quantum computers 4 & other nano-electronic structures 1 McChesney, Nanotech. 13, 545 (02) 2 Kasemo, Surf. Sci. 500, 656 (02) 3 Bennewitz, Nanotech. 14, 499 (02) Walters 4 Ladd, Phys. et al, Rev. Nature Lett. Mat. 89, 4, (2005). (02) In vivo bio-sensing Live pancreatic cancer cells amine-terminated micelle-encapsulated courtesy D. R. T. Zahn, TU-Chemnitz Si NC, a non-toxic alternative to CdSe NCs DNA bases adsorbed at vicinal Si Mauricio et al., Nanoletters 3, 479 (2003) Erogbogbo et al, ACS Nano.2, 873 (2008) 17 µm These interesting properties originate at Si NC/SiO 2 interfaces. SHG has a reputation for being interface-specific step-edges

3 Co-workers Si NCs Si step-edges Theory Junwei Wei Robert Ehlert Bernardo Mendoza CIO, León, México Y. Jiang PhD 2002 Liangfeng Sun PhD 2006 Adrian Wirth (MS 2007) Pete Figliozzi PhD 2007 Jinhee Kwon PhD 2006 Financial Support: Robert Welch Foundation U.S. National Science Foundation Yongqiang An PhD UC-Boulder 2004 W. Luis Mochan U. Nacional Autónoma Cuernavaca, México

4 Their elusive nano-interfaces make Si NCs interesting & challenging (PL) diameter 2 nm 5 nm Radiative double bonds: Wolkin et al., Phys. Rev. Lett. 82, 197 (1999) Luppi & Ossicini, Phys. Rev. B 71 (2005) Bridge bonds: Sa ar et al., Nano Lett. 5, 2443 (2005). Dangling bonds # atoms O O a-sio 2 Si Si Si Si # surface atoms strained SiO x? Si H surface atom fraction c-si a-si? (XPS, Raman) Transition layer(s): Daldosso et al., Phys. Rev. B 68, (2003) (SHG) (SE) Buried nano-interfaces inaccessible to many surface science probes and challenging to described theoretically (e.g. by DFT, Monte Carlo) Here we use multiple complementary spectroscopies

5 Surface & bulk contributions to SHG from planar surfaces are never separated with full rigor... P() P(2) P(0) SHG intensity I pp (2) (2) surf : E E Fourier decomposition Si(001)/SiO azimuthal angle (degrees) p J. E. Sipe et al., Phys. Rev. B 35, 1129 (1987) asymmetric interface electron response t Erley & Daum, PRB 58, R1734 (1998) E 1 E 2 p-polarized interface dipole SHG bulk quadrupolar SHG SiO 2... but empirical separation is usually possible based on: 1. azimuthal anisotropy 2. spectrum 3. sensitivity to interface modification surface + bulk bulk I(2) = a 0 + b 4 sin4 2 SiO x /Si interface thermal oxide Second-harmonic signal (arb. units) Si native oxide thermal oxide + anneal in Ar

6 Similarly, nano-interface & bulk contributions to SHG from Si NCs are intertwined, and must be distinguished empirically Mochan et al., Phys. Rev. B 68, (03) single nanoparticle: P s uniform nano-composite: l E (2 ) From symmetry alone, P b ( r ) = E 2 + E E P s ( r ) = s ijk (a,b, f )F j F k, P NL = assuming l << r NC << E E n NC [ e (,,a,b, f ) m (,,a,b, f ) q (a,b, f )/6] TEM 00 mode TE 01 mode

7 One group of samples is prepared by Si ion implantation into SiO 2 C. W. White et al., NIM B 141, 228 (1998) - ORNL 1 Multi-energy implant ( kev) yields uniform NC density (simplifies optical analysis) 2 Si ions Samples 1100 C / 1 hr in Ar or Ar + H 2 to precipitate NC formation <d NC > = 3, 5, 8 nm ± 50% TEM Images SHG 1 μm << l coherence glass substrate Int [a. u.] Photoluminescence 486 nm <d nc > = 3nm Ar + H 2 <d NC > = 3 nm NC = 7e18 cm -3 f NC = Ar [nm] PL spectrum is unchanged throughout the excitation range 250 < < 500 nm Lpez et al., Appl. Phys. Lett. 9, 1637 (2002) single 5 nm NC X-ray diffraction confirms crystallinity after annealing

8 We also fabricate Si NC samples on a benchtop by thermolysis of hydrogen silsesquioxane (HSQ) C. M. Hessel et al., Chem. Mater. 18, 6139 (2006); J. Phys. Chem. C 111, 6956 (2007)

9 XPS and Raman scatter document conversion of multiplycoordinated a-si clusters into 4-fold-coordinated c-si NCs Similar measurements by previous investigators XPS Si (IV) Si (III) Si (II) Si (I) Si (0) Kachurin et al., Semiconductors 36, 647 (2002), Fiz. Tekh. Poluprov. 36, 685 (2002) Hessel, J. Chem. Phys. 112, (2008) Raman backscatter 3 nm NC-Si Si 2p Anneal 1150 C After annealing at > 1000 C, negligible sub-oxide is detectable by XPS C 650 C 500 C after Si implant SiO 2 3 nm NC-Si Si-Si bonds Ar nm pump a-si c-si Raman shift (cm -1 )

10 Spectroscopic ellipsometry (SE) shows modified c-si E 1 and E 2 critical points after annealing [1] En Naciri et al. J. Chem. Phys. 129, (2008) [2] Cen et al., Appl. Phys. Lett. 93, (2008) [3] Seino, Bechstedt, Kroll, Nanotech. 20, (2009) previous related SE results E 1 c-si E 2 as-implanted: no E 1,2 critical point features consistent with small a-si clusters 2 a-si after 1100 C anneal in Ar: E 1,2 peaks appear E 1 suppressed, blue-shifted consistent with: - previous SE measurements [1,2] - ab initio calculations of optical properties of Si NCs in SiO 2 [3] after 1100 C anneal in Ar + H 2 : negligible further change SE appears selectively sensitive to c-si core of Si NCs Measured 1,2 determine Fresnel factors used in SHG analysis

11 PL excitation spectrum demonstrates that linear absorption occurs primarily in bulk c-si cores, consistent with SE E 2 suppressed E 1 Photo-excited carriers cross-relax to interface states for PL

12 Conventional single-beam SHG is weak Cross-Polarized 2-beam SHG (XP2-SHG) enhances signal 100 E 2 E1 Single beam SHG 1,2 = 800 nm nc (2) P ( E ) E silica 2 1 w XP2-SHG SH signal (2) P g P (2) nc Three parameters needed to describe XP2-SHG response ( 2 )( E ) E +... n b g e m q ( / 6) ( E ) E +... NC = NC e i single beam Ar Ar + H 2 sample z-scan SHG (counts/s) 100 Ar Ar + H 2 single beam z-scan position (microns) XP2-SHG (counts/s) 10 4 Ar Ar + H z-scan position (microns) Jiang et al., Appl. Phys. Lett., 78, 766 (2001) L Sun et al, Opt. Lett, 30, 2287 (2005) Figliozzi et al, Phy. Rev. Lett. 94, (2005)

13 Three independent z-scan measurements determine g, NC and SHG signal growth in the glass is affected by phase mismatch The peaks result from relaxation of phase mismatch when boundaries of the sample fall within the 2-beam overlap region An analogous enhancement underlies 3rd harmonic microscopy with focused beams Barad, Appl. Phys. Lett. 70, 922 (1997) Peak heights are asymmetric because of linear absorption of SH light by NCs and interference of SHG signals generated by NCs and silica E 2 E1 Measurements (scans): 1 substrate 2 exit 3 entrance glass 2 SHG Intentisty (a.u.) -h/2 0 h/2 L. Sun et al., Optics Lett. 30, 2287 (2005) z -h -h/2 0 h/2 h Position of beam overlap

14 Examples at 2 wavelengths illustrate extraction of fitting parameters nc / g and SH Intensity [a.u.] = 630 nm glass nc@ent nc@exit fit nc@ent fit nc@exit fit glass SH Intensity [a.u.] = 540 nm glass nc@ent nc@exit fit nc@ent fit nc@exit fit glass -h -h/2 0 h/2 h Deviation of beam overlap from sample center -h -h/2 0 h/2 h Deviation of beam overlap from sample center nc = ± g = ( ± ) nc g = ± = ( ± ) SE determined Fresnel factors used in this analysis

15 SHG spectra lack E 1,2 critical point resonances 3 nm Si NCs in SiO 2 E 1 E 2 Kramers-Kronig consistency between NC / g and as-implanted: like a-si SHG spectrum of a-si: Erley & Daum, Phys. Rev. B 58, R1734 (1998) H H

16 SHG spectra lack E 1,2 critical point resonances 3 nm Si NCs in SiO 2 E 1 E 2 annealed in Ar: enhanced NC near known SiO x resonances SH reflection coefficient SHG spectrum of Si(111)-(1x1)-H during oxidation Bergfeld, PRL 93, (2004) p-in/ p-out 3.5 ev H O H SHG spectrum of thermally oxidized planar Si(001) after anneal in Ar Erley & Daum, PRB 58, 1743 (1998) annealed in Ar/H 2 : minimal H-effect consistent with previous SHG s-in/p-out Second-harmonic photon energy (ev)

17 E 1 (3.4 ev) and 3.8 ev resonances appear in SHG spectra of 5 nm diameter Si NCs E 1 E 2! (5nm) NC = 3e18 cm "3 E ev! (3nm) NC = 7e18 cm "3

18 SHG per NC is >10 stronger from 5 nm NCS than from 3 nm NCs Ideal single particle SHG scaling: (5/3) 6 = 21 [Dadap, PRL (1999)] 7/3

19 Spectroscopic XP2-SHG is sensitive to nc-si/sio 2 interfacial features* not observed by other spectroscopies that appear in recent MD simulations Djurabekova & Nordlund, Atomistic simulation of the interface structure of Si NCs embedded in amorphous silica, Phys. Rev. B 77, (2008) ~ 10% undercoordinated bonds, Si=O bonds also present ~ 10% suboxide* PE/atom (ev) d[nm] = a-si c-si 2 nm d = 1.3 nm NC a-si interior r/r 0 *elevated PE/atom a-si shell S-SHG validates & guides simulations of the nc-si/sio 2 interface

20 Conclusions about Si embedded nanocrystals < 2 nm 3 nm 5 nm SiO 2 SiO x c-si a-si XP2-SHG SE/PLE Raman XPS featureless spectrum emergent 3.8 ev resonance featureless background strong 3.8 ev resonance residual bkgd broad featureless spectrum E 1 and E 2 resonances E 1 and E 2 resonances broad 480 cm -1 peak sharp 520 cm -1 peak 480 cm -1 tail sharp 520 cm -1 peak E 1 resonance 480 cm -1 tail > 8 nm interface region stabilizes and thins with increasing d NC

21 Stepped (vicinal) Si surfaces are attractive templates for nanofabrication investigation of step-enhanced chemical reactions atomic wires suitable for transport lithography by self assembly of nanostructures Non-invasive in-situ sensors that provide atomic-scale information over the dimensions of a wafer are needed

22 Optical metrology bridges the nano-scale & wafer-scale Reflectance Anisotropy Spectroscopy r r r r = 2 r r r + r r/r [10-3 ] Aspnes & Studna, PRL 54, 1956 (1985) r E in r E in RAS nano-scale probe STM E in E in step-edges cyclopentene H steps Si(001):6 clean o Photon energy (ev) SHG intensity (arb.u.) optical metrology 780 nm steps cyclopentene [110] H 2 Si(001):6 o clean azimuthal angle (deg) wafer-scale nanomanufacturing nano-patterned surface

23 tunable fs-source We combine SHG & RAS probes of stepped Si(001) surfaces in UHV monochromator + PMT analyzer PEM RAS PMT Xenon lamp (1.5 to 5 ev) quartz Coherent MIRA Ti:Sapphire oscillator nm NOPA 520nm-780nm B.S. gas inlet valve polarizer strain free window analyzer BG39 PMT SHG SHG ports QMS sample RAS & SHG track contamination non-invasively completely reversible

24 We have studied 3 very different surface chemical reactions on Si(001):6 o upper D B lower terrace step terrace 1) H 2 ~ 1000 L H 150 C bonds selectively to step-edge db s Dürr et al., Phys. Rev. B 63, (2001) terrace dangling bonds terrace dimers step dangling bonds step rebonds back bonds 2) H 2 + atomic H ~ 10 6 L H 2 (saturation dose) passivates step AND terrace db s, leaving steps and dimers intact atomic H can react with step rebonds and/or terrace dimers. Which happens first??? 3) H 2 + C 5 H 8 (cyclopentene) C 5 H 8 bonds by 2+2 cycloaddition to terrace dimers, forming crystalline organic monolayer step-adsorbed H may help modify & control C 5 H 8 adsorption C 5 H 8

25 SHG/RAS Case Study #1: Dissociative adsorption of H 2 at D B steps of Si(001):6 o H 2 H 2 H 2 H 2 H 2 H 2 clean Si(001):6 o M. Durr, U. Höfer et al., Phys. Rev B. 63, (2001)

26 SHG intensity In the absence of first principles theory, Simplified Bond Hyperpolarizability Model (SBHM) provides SHG - RAS interpretation at the molecular bond level E in s p ˆb k, ab initio DFT calculation provides input structure for SBHM analysis step dangling bond p-in/p-out i = 45 o step rebond 0 o 180 o 360 o ˆb j, step d.b step back step rebond Powell et al., Phys. Rev. B 65, (2002) E j 2 terrace back. 0 o 180 o 360 o ˆk terrace d.b. terrace dimer ˆb l, 0 o 180 o 360 o A chemical bond is the basic polarizable unit Induced axial SH polarization of bond j: terrace dimer p j (2 ) = j ˆbj ( ˆb j E in ) 2 ˆb j = bond unit vector j = axial hyperpolarizability Far-field SH radiation of bond j : E j 2 = eikr r 2 Total far-field SH radiation: E j 2 = eikr r 2 ( I ˆk ˆk) p (2 ) j ( I ˆk ˆk) Simplifications: - transverse hyperpolarizabilities neglected - local field corrections folded into s - boundary conditions not treated rigorously j p j (2 )

27 SHG dictionary for Si(001): 6o

28 Multi-parameter fitting (like Wall St. investing) requires government regulation based on Bond physics & chemistry Bonds that are parallel to E, charge-rich and non-centrosymmetric contribute most strongly... Kramers-Kronig consistency.. Consistency for Multiple Angles () and Polarizations (MAP) SHG intensity [arb.u.] SHG intensity [arb.u.] 1 = 52.5 o = 45 o = 30 o p-in/p-out clean Si(001):6 o o 180 o 360 o 0 o 180 o 360 o 0.8 azimuthal angle (degrees) after exposure to 1500 L H 2 at RT p-in/p-out 0 0 o 180 o 360 o azimuthal angle (degrees) 0.2 s-in/s-out clean Si(001):6 o azimuthal angle (degrees) db step rebond e step = 0.3 rebond m step = 0 = 0 db H 2 terrace = 0.2 dimer terrace = s-in/p-out clean Si(001):6 o 0 0 o 180 o 360 o azimuthal angle (degrees) db step rebond e step = 0.3 rebond m step = 0.2 = 0.4 db terrace = 0.2 dimer terrace = -0.2

29 Full SBHM fits SHG data with high fidelity clean Si(001): 6 o = 42 o Si(001): 6 o with H-terminated steps p-in/p-out

30 Strict regulation: derive RAS response from SHG data Fitted bond hyperpolarizability spectra j step terrace *Miller s theorem: j linear bond polarizabilities = j,2 j, j, E 2 linear (Î ˆk ˆk) ~ r ~ r ~ r j 110 j,2 j,2 ˆbj ˆbj E in 2 Ehlert, JOSA B 27, 981 (2009) *R. C. Miller, Appl. Phys. Lett. 5, 17 (1964) spectral range of SHG data

31 Hyperpolarizability spectra show charge transfer from step dangling bond to 3 underlying bonds when H 2 dissociatively adsorbs at step-edges clean 1000 L H 2 With SHG-RAS-SBHM, we watch charge transfer accompanying the formation of specific stepedge chemical bonds.

32 SHG/RAS/SBHM Case Study #2: Reaction of atomic H with H 2 -saturated Si(001):6 o surface 1000 L H ?? monohydride terraces convert to 1 1 dihydride, breaking dimers Borenzstein et al., Phys. Rev. Lett.95, (2005) H symmetrized dimers 2 1-reconstructed terraces re-bonded (r) D B steps intact Pehlke & Kratzer, Phys. Rev. B 59, 2790 (1999) top view side view atomic H catalyzes conversion of r-d B to non-rebonded D B, S B and S A steps Laracuente & Whitman, Surf. Sci.545, 70 (2003)

33 SHG: Rebonds break immediately LEED: Dimers break later spot spli+ng p-in/p-out rebond contribution 2x1 spots 2x1 spots clean (2x1) full H 2 +50L 2x1 spots + 50L 300C No remaining SHG signature of rebond after ~ 100 L atomic H 2x1 spots gone! + 50L dimers broken

34 SHG-MAP shows shows decreasing rebond, increasing backbond, expression during H exposure pp ss L H L atomic H Si(001):4 o 780nm 45 o incidence sp azimuthal angle [radians] step rebond step backbond

35 RAS is a spectator as the rebond breaks

36 RAS-SHG-SBHM Case Study #3: monitor and control of nanofabrication of organic monolayers on Si(001) SHG Intensity [arb. u.] SH intensity [arb.units] pp 780nm Si(001):6 clean H2 H2+Cyclopentene Cyclopentene azimuthal angle STM shows C 5 H 8 bonds to Si=Si terrace dimers to form an ordered monolayer. clean vicinal Si(001):6 o 300 C after C 5 H 8 adsorption SHG, on the other hand, shows C 5 H 8 reacts immediately & strongly with step rebonds of the clean Si(001):6 o surface... H 2 pre-adsorption at step db s partially protects steps from reacting with C 5 H 8 C 5 H 8 Hamers et al. (2000). Acc. Chem. Res. 33(9): Lu et al., Phys. Rev. B 68, (2003)

37 Summary: Noninvasive optical spectroscopy of nano-interfaces I. 0-D: Si NCs embedded in SiO 2 importance: Si LEDs, bio-sensors method: XP2-SHG + SE, Raman, XPS, PL results: new SHG evidence for a-si and SiO x nano-interfacial transition regions Figliozzi et al., Phys. Rev. Lett. 94, (2005); Wei et al., in preparation II. 1-D: step-edges of vicinal Si importance: templates for molecular electronics, quantum wires & computers method: SHG & RAS & SBHM results: visualization of formation of step Si-H bond and of breaking of step rebond; - control & optical monitoring of cyclopentene nano-lithography by self-assembly Kwon et al., Phys. Rev. B 73, (2006). Ehlert et al., J. Opt. Soc. Am. B 27, 981 (2009) & more in the oven. Yu Suzuki

38 END