Optical Characterization Techniques

1. Optical Characterization Techniques Reflection Microscopy Ellipsometry Reflectance Emission PL UPS Incident Photons Absorption Photoconductivity Inelastic Scattering Raman Brillouin Transmission Transmittance FTIR

2. Optical Characterization Techniques

3. Optical Characterization Techniques • Advantages : • Non-destructive • No contacts required • High sensitivity, • < 1012 cm-3 impurity detection

4. Spectroscopy From Hollas, Fig. 3.1, p. 42

5. Spectroscopy • Need a dispersive element to separate wavelengths • 4 ways of performing spectroscopy Prism Grating Interferometer Michelson Interferometer (FTS) Fabry-Perot Interferometer

6. Prisms • Remember Newton • Wavelengths are separated by the wavelength dependence of refractive index • Dispersion, dn/dl • Resolving power, R = b dn/dl From Hollas, Fig. 3.3, p. 44

7. Grating Monochromator • Wavelengths are separated spatially by a diffraction grating • Requires a lock-in amplifier for good S/N LaPierre, Ph.D. thesis

8. Grating Monochromator • Resolving power: • R = l/Dl=mN • m = diffraction order • N = # of grooves • R ~ 104 •  can resolve • 0.05 nm from l = 500 nm • Usually used in the uv, visible and near-infrared region • Mid- and far-infrared dispersion is more effectively performed by FTS

9. Interferometers • Interference can result in dispersion of wavelengths • Remember oil slicks

10. Fabry-Perot Interferometer • Constructive interference occurs in transmission • for wavelengths satisfying (normal incidence): 2d = mlo/n1 multi-wavelength source, Dl plate spacing d = mlo/2n1 n1 I detector l lo

11. Fabry-Perot Interferometer • The F-P transmission is given by the Airy function • F = 4R/(1-R)2 • = 4pn1d/ lo (normal angle of incidence) R = mirror reflectivity 1 1 + Fsin2(d/2) T = T d

12. Fabry-Perot Interferometer • Want a high coefficient of finesse for a narrow transmission • F = 4R/(1-R)2 •  Want high reflectivity mirrors T d

13. Fabry-Perot Interferometer Resolving power: R = l/Dl½ R = l / (2l/mp√F) R = mp√F / 2 R > 106 is achievable  can resolve 0.0005 nm from l = 500 nm T 1 0.5 Dl½ = 2l/(mp√F) l

14. Fourier-Transform Spectroscopy (FTS) M2 Michelson interferometer BS source M1 P frequency spectrum source interferogram I I laser (long lc) w mirror position I I LED (short lc) w mirror position • large lc (e.g., laser)  wide interferogram • short lc (e.g., LED)  narrow interferogram

15. Fourier-Transform Spectroscopy (FTS) • The frequency spectrum is the Fourier transform • of the interferogram Fourier transform frequency spectrum source interferogram I I laser (long lc) w mirror position I I LED (short lc) w mirror position

16. FTS • Reference spectrum is taken with sample removed from system • Sample is placed in one arm of interferometer • Intensity (interferogram) is measured at detector, P, as a function of mirror position • Fourier transform (FFT) of interferogram gives spectral content of input (includes absorption spectrum from sample) M2 sample input wavelengths (broadband source) M1 BS P

17. FTS • FTS usually used in the near-, mid-, and far-infrared wavelength range • Also called Fourier transform infrared spectroscopy (FTIR spectroscopy) • Strong absorption by H2O; must purge optical path with N2 • Windows in the system must be transparent to the wavelengths of interest

18. FTS from Hollas, Table 3.1, p. 60

19. FTS • Advantages of FTS compared to monochromator or F-P : • Whole spectrum is measured at once (Fellget advantage) • Large energy throughput; large input/output aperture (Jacquinot advantage) • Better S/N ratio • Fast measurement • Resolving power limited by mirror displacement (d) : • R = l / Dl = 2d / l • e.g., mirror displacement of d = 0.5 cm  R = 20000 •  Dl = 0.025 nm at 500 nm