SHG Spectroscopy. Clean surfaces Oxidation SOI wafer

Transcription

1 SHG Spectroscopy Clean surfaces Oxidation SOI wafer

2 Scan regions Idler: nm nm Signal: nm Ti:Sapphire: nm nm SHG set-up

3 Bergfeld, Daum, PRL 90, 2915 SHG from GaAs

4 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

5 Si(100) Si/Ge SiO₂Si/Ge SiO Growth of strained layer

6 SHG (a arb. units) units) SHG (arb 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 Interband transitions Surface states below 3.4 ev K Γ M U 2 U 1 /S 1 10 L O 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 ev: 2ω E 2 S 2 S 3

7 Pump-probe: Dynamics of surface states 4 3 Energ gy (ev) U 2 U 1 /S 1-1 S 2-2 S 3-3 K Γ M Höfer et al. PRB (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

8 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] p to p polarization Clean Resonances: 2.05 ev SS 1ω resonance ev SS 2ω resonance 3.34 ev E 1 2ω resonance 4.34 ev E 2 2ω resonance 1ω or 2ω resonances? L oxygen Constructive / destructive interference SH Photon Energy [ev]

9 SH resonances its] G [arb. un SH 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 SH Photon Energy [ev]

10 Ge(111) linear response Lowest direct transition very weak Spin-orbit splitting of 2.1-eV transition ~0.2 2eV

11 Ge(111) spectra comparison ev transitions: spin-orbit or interface states? ev transitions: sensitive to surface treatment.

12 Ge(111) spectra comparison ev transitions: spin-orbit or interface states? ev transitions: sensitive to surface treatment. Additional peak at 3.1 ev? What happens below 2.0 ev?

13 Second harmonic generation spectroscopy pyon SOI wafers Kjeld Pedersen Thomas Garm Pedersen Aalborg University Denmark

14 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, Pump Wavelength (nm) 0, 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)

15 SOI SHG from interfaces 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, Pump Wavelength (nm) SHG oscillations Linear properties SHG sources Can the oscillations be used to isolate SHG from buried interfaces?

16 SOI wafers Fast IC s transistor insulation CMOS MEMS Si Photonics. Si device layer Oxide ~200 nm ~200 nm Si handling wafer

17 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

18 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, Wavelength (nm)

19 Linear properties Reflection multilayer structure Si SiO₂ Si

20 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, θ=50 o θ=60 o θ=70 o Wavelength (nm) Fit to data: d Si =203 nm d oxide =402 nm θ=30 o θ=70 o Wavelength (nm)

21 SHG set-up Scan regions Idler: nm Signal: nm Excitations: 1-5 ev

22 Refractive index absorption 2ω 7 ω 6 Si ve Inde ex Refracti n i 0 n r 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

23 SHG spectra SOI wafer, 200 nm Si, 400 nm SiO 2 SHG (arb. units) E 2 Interface E 7 E 1 70 o 60 o 50 o 40 o 30 o Pump Wavelength(nm) Resonances at critical points + oscillations

24 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, Pump Wavelength(nm)

25 SHG sources Sum of radiations from dipole sheets ω ω 2ω Interface contributions Bulk contributions

26 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=( *cos(4*pi/180*col(a)))^ Rotational Angle (deg.) Bulk contribution ~10%

27 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 nm Min at 620 nm 1E Wavelength (nm)

28 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

29 Si(111)/oxide interface 10 Si(111)7x7 SHG (a arb. units) Clean 10 L O 2 p to p SHG (arb b. units) p to s Clean 10 L O SH Photon Energy (ev)

30 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

31 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

32 Interface contributions units) SHG (arb χ (2) constant θ=50 o 7 Exp 6 1st interface 3rd interface 5 2nd interface Wavelength (nm) Only 1st interface for λ<850 nm Peak at 900 nm: 2nd and 3rd interface

33 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, Wavelength (nm) 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

34 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, d Si =300 nm d SiO2 =300 nm 2 χ (2) = Wavelength (nm) Wavelength (nm) 1st interface 2nd 3rd 0, 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

35 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