SHG Spectroscopy Clean surfaces Oxidation SOI wafer
Scan regions Idler: 730-1050 nm 1000-1400 nm Signal: 680-550 nm Ti:Sapphire: 700-1000 nm 600-500 nm SHG set-up
Bergfeld, Daum, PRL 90, 2915 SHG from GaAs
Spectroscopy onsi/sio₂ interfaces 3.4 ev: E₀/E₁ transition at Γ-point 42eV: 4.2 E₂ transition at Γ-point 3.6 ev: Interface transition
Si(100) Si/Ge SiO₂Si/Ge SiO Growth of strained layer
SHG (a arb. units) units) SHG (arb. 10 8 6 4 2 0 4 2 0 Si(111)7x7 Clean SHG Spectroscopy 10 L O 2 p to p 2,0 2,5 3,0 3,5 4,0 4,5 Clean p to s 2,0 2,5 3,0 3,5 4,0 4,5 5,0 SH Photon Energy (ev) ergy (ev) En 4 3 2 1 0-1 -2-3 Interband transitions Surface states below 3.4 ev K Γ M U 2 U 1 /S 1 10 L O 2 2.8 ev (1.4 ev): 1ω S 2 U 1 Surface states 2.0/2.4 ev: 2ω S 3 U 1 34eV: 3.4 2ω E 1 4.4 ev: 2ω E 2 S 2 S 3
Pump-probe: Dynamics of surface states 4 3 Energ gy (ev) 2 1 0 U 2 U 1 /S 1-1 S 2-2 S 3-3 K Γ M Höfer et al. PRB 73 245305 (2006) Saturation and recovery of unoccupied ad-atom state by pump pulse. Intra band 100 fs, interband (surface) 1-2 ps Surface states to bulk 100 ps
χ ( 2) s (2 ω ) SH resonances f exp( iφ ) n n ( ω ω + ½iγ ) n n + f exp( iφ ) m m (2ω ω + ½iγ ) m m ) n, m 10 SHG [a arb. units] 9 8 7 6 5 4 3 p to p polarization Clean Resonances: 2.05 ev SS 1ω resonance 306 3.06 ev SS 2ω resonance 3.34 ev E 1 2ω resonance 4.34 ev E 2 2ω resonance 1ω or 2ω resonances? 2 1 10 L oxygen Constructive / destructive interference 0 1.5 2 2.5 3 3.5 4 4.5 5 SH Photon Energy [ev]
SH resonances its] G [arb. un SH 9 8 7 6 5 4 3 2 1 Resonances: ptoppolarization p polarization 2.15 ev SS 1ω resonance 3.06 ev SS 2ω resonance 3.34 ev E 1 2ω resonance 3.55 ev Si-O 2ω resonance 4.34 ev E 2 2ω resonance Clean 10 L oxygen New resonance from Si-O bonds at the interface Daum et al. PRL 93 097402 0 1.5 2 2.5 3 3.5 4 4.5 5 SH Photon Energy [ev]
Ge(111) linear response Lowest direct transition very weak Spin-orbit splitting of 2.1-eV transition ~0.2 2eV
Ge(111) spectra comparison 2.1-2.4 ev transitions: spin-orbit or interface states? 2.8-3.0 ev transitions: sensitive to surface treatment.
Ge(111) spectra comparison 2.1-2.4 ev transitions: spin-orbit or interface states? 2.8-3.0 ev transitions: sensitive to surface treatment. Additional peak at 3.1 ev? What happens below 2.0 ev?
Second harmonic generation spectroscopy pyon SOI wafers Kjeld Pedersen Thomas Garm Pedersen Aalborg University Denmark
Sn nanocrystals in Si 1,2 1,0 Sn in Si Bulk sample 1,0 0,8 SH HG (arb. units) 0,8 0,6 0,4 0,2 Si epilayer Si wafer Sn nits) SH HG (arb. u 0,6 0,4 0,2 Si epilayer Si device layer Oxide Si wafer 0,0 700 800 900 1000 1100 1200 1300 Pump Wavelength (nm) 0,0 700 800 900 1000 1100 1200 1300 1400 Pump Wavelength (nm) Samples from Arne Nylandsted Conclusion: Don t use SOI wafers for structures for optical characterization (Unless you want to probe the SOI interfaces)
SOI SHG from interfaces 1 380 nm Si SHG (arb b. units) 0,1 400 nm SiO 2 Log scale! Si device layer Oxide Si handling wafer ~380 nm ~400 nm 0,01 700 800 900 1000 1100 1200 1300 1400 Pump Wavelength (nm) SHG oscillations Linear properties SHG sources Can the oscillations be used to isolate SHG from buried interfaces?
SOI wafers Fast IC s transistor insulation CMOS MEMS Si Photonics. Si device layer Oxide ~200 nm ~200 nm Si handling wafer
SHG measurements on SOI interfaces B. Jun, IEEE Trans Nucl Sci, 51, 3231 B. Jun, Appl. Phys. Lett. 85, 3095 N. Tolk, Microelectronic Engineering 84, 2089
Linear properties Ellipsometry c parame eters (s 1 and s 2 ) 0,8 SOI wafer Fit to eliipsometric data: 209 nm Si 0,4 410 nm SiO 2 0,0-0,4 psometri Elli -0,8-1,2 400 500 600 700 800 Wavelength (nm)
Linear properties Reflection multilayer structure 1 2 3 4 Si SiO₂ Si
Linear reflection data from Palik Refle ection 1,0 0,8 0,6 04 0,4 0,2 θ=30 o θ=40 o Refle ection Refle ection 0,0 0,8 0,6 04 0,4 0,2 0,0 0,8 0,6 0,4 0,2 0,0 600 800 1000 1200 1400 1600 600 800 1000 1200 1400 1600 θ=50 o θ=60 o 600 800 1000 1200 1400 1600 400 600 800 1000 1200 1400 1600 θ=70 o 600 800 1000 1200 1400 1600 Wavelength (nm) Fit to data: d Si =203 nm d oxide =402 nm θ=30 o θ=70 o 600 800 1000 1200 1400 1600 Wavelength (nm)
SHG set-up Scan regions Idler: 720-1400 nm Signal: 680-500 nm Excitations: 1-5 ev
Refractive index absorption 2ω 7 ω 6 Si ve Inde ex Refracti 5 4 3 2 1 n i 0 n r 200 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm) Transparent to pump light in whole region Transparent to pump light in whole region Strong absorption of SHG for pump shorter than 800 nm
SHG spectra 12 11 SOI wafer, 200 nm Si, 400 nm SiO 2 SHG (arb. units) 10 9 8 6 5 4 3 E 2 Interface E 7 E 1 70 o 60 o 50 o 40 o 30 o 2 1 0 500 600 700 800 900 1000 1100 1200 1300 1400 Pump Wavelength(nm) Resonances at critical points + oscillations
SHG spectra 10 SOI wafer, 200 nm Si, 400 nm SiO 2 SHG 1 Log scale! 70 o 60 o 50 o 40 o 30 o 0,1 E 2 E 1 Interface 0,01 500 600 700 800 900 1000 1100 1200 1300 1400 Pump Wavelength(nm)
SHG sources Sum of radiations from dipole sheets ω ω 2ω Interface contributions Bulk contributions
Rotational anisotropy Bulk contribution 1,0 SHG (a arb. unit ts) 0,8 0,6 0,4 02 0,2 0,0 p to p λ p =750 nm θ=60 o y=(0.9+0.1*cos(4*pi/180*col(a)))^2 0 45 90 135 180 225 270 315 360 Rotational Angle (deg.) Bulk contribution ~10%
Effect of linear reflections 1 st interface Reflectio on 1 0,1 0,01 SHG R(2ω) R(ω) θ=50 o + Min at 550 nm + Min at 1000 nm + Peak at 750-800 nm Min at 620 nm 1E-3 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm)
SHG Si/SiO₂ interface response Natural oxide W. Daum, PRB 59, 2915 Si(100) interface resonance 60 nm thermal oxide Depends on interface formation Interface resonance part of response function in near IR (eg. 800 nm) 105 nm dry/wet/dry +annealing
Si(111)/oxide interface 10 Si(111)7x7 SHG (a arb. units) 8 6 4 2 Clean 10 L O 2 p to p SHG (arb b. units) 0 4 2 0 20 2.0 25 2.5 30 3.0 35 3.5 40 4.0 45 4.5 p to s Clean 10 L O 2 2.0 2.5 3.0 3.5 4.0 4.5 5.0 SH Photon Energy (ev)
Model SHG from dipole sheets Sipe s model: J. Opt. Soc. Am. B4, 481 (1987) Phys. Rev. B35, 1129 (1987) Field in medium 1 from source in medium i z 1 2 P z i i n E = E + 1 i, 1 i Other models: Bethune, J. Opt. Soc. Am. B6, 910 Yeganeh, Phys. Rev. B46, 1603 Wierenga, Physica B204, 281
SHG model (continued) P (2ω) = i χ (2) ijk E j ( ω) E k ( ω) f l iφl ( exp( ) 2 ) χ = 2 2 l 2ω ω l iγ l l E, E, : 1 2 Interface χ eff = a1ξ + a2γ + a3 zxx + a4 zzz + a5 xyz Bulk Anisotropic Bulk Isotropic Surface Isotropic Depends on angle of incidence
Interface contributions units) SHG (arb. 10 9 8 χ (2) constant θ=50 o 7 Exp 6 1st interface 3rd interface 5 2nd interface 4 3 2 1 0 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm) Only 1st interface for λ<850 nm Peak at 900 nm: 2nd and 3rd interface
Fit to experiments 10 SHG model: 3 interfaces θ=50 o 10 θ=30 o SHG (arb b. units) 1 2nd and 3rd interface 0,1 b. units) SHG (ar 1 0,1 0,01 0,01 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm) 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm) Si device layer Oxide Si handling wafer 1st 2nd 3rd Same χ² ²for both angles and dfor all ll3i interfaces No bulk contribution
Looking for buried interfaces SHG (arb. units s) 1,0 0,8 0,6 0,4 0,2 d Si =200 nm d SiO =400 nm SiO 2 χ (2) =1 1st interface 2nd 3rd SHG (arb. units) 1,0 d Si =100 nm 0,8 =200 nm )dsio 2 0,6 0,4 0,2 χ (2) =1 SHG (arb. units) 1,0 0,8 0,6 0,4 0,2 0,0 500 600 700 800 900 1000 1100 1200 1300 1400 d Si =300 nm d SiO2 =300 nm 2 χ (2) =1 500 600 700 800 900 1000 1100 1200 1300 1400 Wavelength (nm) Wavelength (nm) 1st interface 2nd 3rd 0,0 500 600 700 800 900 1000 1100 1200 1300 1400 Wavelength (nm) 0,0 We can find a wavelength to test 3rd interface 2nd interface is difficult to reach Si device layer Oxide Si handling wafer 1st 2nd 3rd
Conclusions Oscillations in SHG from SOI wafer Multiple reflections in linear field Multiple reflections in SH field Variations in χ² of Si Only first interface for λ<800 nm Wavelength for2 nd and 3 rd interface can be found λ p,θ