Optical Spectroscopies of Thin Films and Interfaces. Dietrich R. T. Zahn Institut für Physik, Technische Universität Chemnitz, Germany

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1 Optical Spectroscopies of Thin Films and Interfaces Dietrich R. T. Zahn Institut für Physik, Technische Universität Chemnitz, Germany

2 1. Introduction 2. Vibrational Spectroscopies (Raman) 3. Spectroscopic Ellipsometry 4. Reflectance Anisotropy Spectroscopy

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4 Polarisation Dependent Linear Optical Techniques: SE and RAS Spectroscopic Ellipsometry (SE) variable angle of incidence the effective dielectric function: with: Φ 0 - angle of incidence, r p, r s Fresnel coefficients rp and ρ = = tanψ exp( i ) r s 1 ρ 1 + ρ ε~ 2 2 = sin 2 Φ0 + sin Φ0 tan Φ - 0 SE Φ0 RAS Reflectance Anisotropy Spectroscopy (RAS) near normal incidence the RAS signal: r r = r 2 r α α + where α and β correspond to [-110] and [110] directions in the surface plane of a (001) oriented substrate r r β β Substrate

5 Reflectance Anisotropy Spectroscopy RAS also: Reflectance Difference Spectroscopy RDS

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7 RAS Setup

8 RAS and GaAs Surface Reconstructions J.T.Zettler et al. J.Cryst.Growth 195,151 (1998)

9 RAS: Comparison UHV vs. Gas Phase Surface reconstructions also exist under gas phase conditions. I.Kamiya et al. PRB 68, 627 (1992)

10 After A R Turner et al., Phys. Dynamic RA from Si Rev. Lett, , (001)- detection of proportions of (1x2) and (2x1) domains RHEED (top), Dynamic Reflectance Anisotropy (632 nm HeNe laser) (middle) and Reflectance (bottom) recorded from a (001) Si surface during chemical beam epitaxy growth of Si 1-x Ge x under high vacuum conditions Note that the frequency of the RA response is ca. half of that for the RHEED response. This is because the RHEED simply detects smoothness, while the RA detects net surface orientation of Si-Si dimer domains

11 Compare Electron Diffraction and RAS RHEED e- in e- out hν in RAS hν out RHEED loses coherence, RAS unchanged

12 InGaAs(5ML)/GaAs(10ML) M.Zorn et al.j.cryst.growth 145, 53 (1994)

13 GaAs/InGaP HBT J.T.Zettler et al. J.Cryst.Growth 195,151 (1998)

14 Perylene derivatives PTCDA: 3,4,9,10- Perylenetetracarboxylic dianhydride DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diimide C 24 H 8 O 6 C 26 H 14 O 4 N 2 z x y

15 PTCDA and DiMe-PTCDI: Absorption Spectra and Optical Gap Absorbance E optical PTCDA DiMe-PTCDI Energy / ev Energy positions of the absorption peaks* PTCDA DiMe-PTCDI *determined from the fit of absorption spectra

16 RA Spectrum of S-GaAs(001):(2x1) Re( r/r) E 1, E S S [1] Ga S [2] S-GaAs(001):(2x1) Energy / ev E ' 0, E 2 Bulk-like features induced by the reconstructed surface [1] G. Hughes et al., J. Appl. Phys. 78 (3), 1948 (1995) [2] V. L. Berkovits, D. Paget, Appl. Phys. Lett. 61 (15), 1835 (1992)

17 Evolution of RA Spectra upon PTCDA Deposition PTCDA features occur at ultra-low coverage! GaAs PTCDA 10 nm 5 nm 3 nm 1 nm GaAs extremely strong features around ev are interference and/or absorption related!

18 Biomolecules - DNA Bases Guanine (G) C 5 H 5 N 5 O Cytosine (C) Cytosine (C) C 4 H 5 N 3 O Adenine (A) C 5 H 5 N 5 Thymine (T) C 5 H 6 N 2 O 2

19 Substrate preparation flat Si(111) vicinal Si(111)-3 ( 3 (-6 ) Degreasing Isopropanol,, DI-Water RCA 1 H 2 O 2 (30%):NH 4 OH(28%):H 2 O (1:1:5) - 10 min at 80 C H-termination 2 min in HF(40%) solution RCA 2 H 2 O 2 (30%):HCl(35%):H 2 O (1:1:5) - 10 min at 80 C H:Si(111) LEED (1x1) H:Si(111)-6 H-termination HF(49%):H 2 O (1:30) - 30sec I=5mA, E=29eV I=5mA, E=36eV

20 Organic Molecular Beam Deposition (OMBD) Guanine (G) T Knudsen cell ~ 510 K rate ~ 2 nm/min Cytosine (C) T Knudsen cell ~ 410 K rate ~ 0.2 nm/min Adenine (A) T Knudsen cell ~ 400 K rate ~ 1.5 nm/min Thymine (T) T Knudsen cell ~ 365 K rate ~ 0.8 nm/min silicon substrates were kept at room temperature during the OMBD process. the organic layer thicknesses are in the range nm.

21 in situ RAS monitoring of Guanine grown onto vicinal H:Si(111) Re( r/r) nm 8 nm 6 nm 4 nm H:Si(111)-6 E ' 0,E 1 E 2 With increasing layer thickness: increase in the magnitude of the RAS signal, a slight shift of E 2 towards lower energy Energy / ev

22 10 3 Re( r/r) nm 8 nm 6 nm 4 nm 2 nm H:Si(111)-6 Simulation of r/r Effect of Thickness E ' 0,E 1 E 2 3-phase model a,b (1) substrate: H:Si(111)-6 measured r r (d=0) (2) overlayer: Cauchy layer ε,n (λ),k = 0,d o (3) ambient,ε a =1 o B n(λ) o = A + ; A =1.45, B = λ Energy / ev simulation r r (d) ~ r measured (d=0)*f(ε,n,d) a T. Yasuda, D. E. Aspnes, D. R. Lee, C. H. Bjiorkman, G. Lucovsky, J. Vac. Sci. Technol. A 12 (1994) 1152 b T. U. Kampen, U. Rossow, M. Schumann, S. Park, D. R. T. Zahn, J. Vac. Sci. Technol. B 18 (2000) 2077 r o o

23 10 3 Re( r/r) in situ RAS monitoring of Cytosine grown onto vicinal H:Si(111)-6 10 nm 8 nm 4 nm 2 nm H:Si(111)-6 E ' 0,E 1 E 2 absorption band Energy / ev With increasing layer thickness: increase in the magnitude of the RAS signal, a slight shift of E 2 towards lower energy, larger anisotropy signal compared to guanine due to absorption contribution.

24 in situ RAS monitoring of Thymine grown onto vicinal H:Si(111) Re( r/r) H:Si(111) nm 1 nm 2 nm E ' 0,E 1 E Energy / ev Re( r/r) H:Si(111)-6 2 nm 4 nm 7 nm 10 nm With increasing layer thickness: extremely large anisotropies in the absorption range of Thymine. surface roughness contribution.

25 10 3 Re( r/r) in situ RAS monitoring of Adenine grown onto vicinal H:Si(111)-6 H:Si(111) nm 1 nm 1.5 nm 2 nm E ' 0,E 1 E Energy / ev Re( r/r) H:Si(111)-6 2 nm 4 nm 7 nm 8 nm 10 nm With increasing layer thickness: large anisotropies in the absorption range of Adenine, weaker RAS signal compared to Thymine.

26 in situ RAS monitoring of Adenine grown onto vicinal H:Si(111) Re( r/r) H:Si(111)-3 2 nm 4 nm 6 nm 8 nm 9.5 nm E ' 0,E 1 E 2 Lower offcut angle induces smaller anisotropy by a factor of almost 2 compared to H:Si(111) Energy / ev

27 10 3 Re( r/r) in situ RAS monitoring of Adenine grown onto flat H:Si(111)-0.35 H:Si(111) 4 nm 6 nm 8 nm 10 nm Lower offcut angle induces smaller anisotropy by a factor of almost 40 compared to H:Si(111) Energy / ev

28 10 3 Re( r/r) RAS spectra of 4 nm Adenine: Effect of Vicinality H:Si(111)-0.35, (x10) H:Si(111)-3 H:Si(111) Energy / ev Re( r/r) H:Si(111)-0.35 H:Si(111)-3 H:Si(111) Offcut Angle / the magnitude of the RAS signal scales linearly with the offcut angle.