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 and Infrared) 3. Spectroscopic Ellipsometry 4. Reflectance Anisotropy Spectroscopy

3 Application of complementary techniques Growth: Organic Molecular Beam Deposition (OMBD) in Ultra-High Vacuum hω Surface Science: e e Valence Band and Core Level Photoemission NEXAFS, LEED, AES IPES organic inorganic hω Optical Spectroscopy: Raman Spectroscopy IR, PL, RAS, SE Electrical Measurements: Current-Voltage (IV) Capacitance-Voltage (CV) DLTS, Admittance

4 Semiconductor technology has changed our world dramatically, e.g. 4 basic building blocks metal semiconductor e.g. MESFET metal oxide semiconductor p-type semiconductor n-type semiconductor semiconductor A semiconductor B e.g. MOSFET e.g. p-n junction, bipolar transistor e.g. heterostructures, optoelectronics

5 Transistors Bell Labs 1947 TI nm Technology generation: L L/ 2 Moore s Law Transistorized PBS, Nov. 8,

6 21 st Century Electronics: Transistors at the nano/molecular scale electron flow Gate Source Drain ~100 nm Texas Instruments ~2000 ~10 nm ~2015?

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8 The Scale of Things 1 meter (1m) 1 mm (10-3 m) 1 µm (10-6 m) 1 nm (10-9 m) human hair (100 µm) wavelength of light (< 1µm) 248 nm -DUV lithography transistor (100 nm -2000) biomolecules (10 s nm) 1 pm (10-12 m) Silicon atom (0.118 nm)

9 Nanotechnology Defined 1 nanometer = meter Nanotechnology has given us the tools to play with the ultimate toy box of nature atoms and molecules. Everything is made from it The possibilities to create new things appear limitless Horst Störmer, Physics Nobel Prize Winner

10 Crystal structure Si a=5.43 A GaAs a=5.63 A Semiconductor Devices, 2/E by S. M. Sze Copyright 2002 John Wiley & Sons. Inc. All rights reserved. Diamond & Zincblende lattices two interpenetrating fcc sublattices one displaced from the other by ¼ of the distance along the diagonal of the cell (a 3/4)

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13 Three Growth Modes If γ s > γ f + γ sf Film γ s < γ f + γ sf γ s > γ f + γ sf With misfit Substrate Layer-by-layer (Frank-Van der Merwe) 3D islanding (Volmer-Weber) γ s : surface energy of substrate γ f : surface energy of film γ sf : interface energy of substrate-film Layer-by-layer followed by 3D islanding (Stranski-Krastanov)

14 Molecular Beam Epitaxy Growth Mechanism Schematic diagram of MBE process

15 Molecular Beam Epitaxy Monitoring equipment (such as mass spectrometer) Vacuum chamber RHEED screen Source flanges

16 Reflection High-Energy Electron Diffraction (RHEED) Screen Image

17 Finding Growth Rates with RHEED 2-d d growth occurs one atomic monolayer at a time Smooth surface gives peaks in RHEED intensity Period of RHEED intensity oscillations corresponds to the time of growth for one layer

18 Molecular Beam Epitaxy (MBE) Thin film growth under ultra high vacuum. Reactants introduced by molecular beams. Create beams by heating source of material in an effusion (or Knudsen) cell. Several sources, several beams of different materials aimed at substrate Can deposit 1 atomic layer or less! Very precisely defined mixture of atoms to give exactly the desired material composition!

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20 Epitaxy: Self-Organized Growth Self-organized QDs through epitaxial growth strains Stranski-Krastanov growth mode (use MBE, MOCVD) Islands formed on wetting layer due to lattice mismatch (size ~10s nm) Disadvantage: size and shape fluctuations, ordering Control island initiation Induce local strain, grow on dislocation, vary growth conditions, combine with patterning (a) (b) (c) AFM images of islands epitaxiall grown on GaAs substrate. InAs islands randomly nucleate. Random distribution of InxGa1-xAs ring-shaped islands. A 2D lattice of InAs islands on a GaAs substrate. P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.

21 Stranski-Krastanov growth of Ge on Si(001) [100] pyramids Wetting layer ~ 2.5 ML Ge, 475 C, (44nm) 2 huts 3D islands formation ~ 3.5 ML Ge, 475 C, (110nm) 2

22 InAs/InGaAs/GaAs Heterostructures Typical sizes: dot height 10 nm, dot width nm Quantum Dots Surface Diffusion and Elasticity

23 MOCVD Growth System Gas handle system Reactor Computer Control Vacuum and Exhaust system

24 Metal-Organic Vapour Phase Epitaxy

25 Epitaxial Growth Techniques -- Metal-Organic Chemical Vapor Deposition (MOCVD) metal-organic compounds as reacting gases material growth temperature about 750~1050 C growth rate controlled by group V carrier H 2 gas flow rate

26 MOCVD growth system

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28 Confinement of Electrons and Holes 1,2,3,... ; * 2 * = = = = n L n k L m m k E zn z π π h h

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30 Organic field-effect transistors Displays (Kodak) Electrically driven organic lasers Organic semiconductors Organic/Inorganic Microwave Diodes I V Metal Organic Interlayer GaAs(100) Plastic solar cells Organic-modified Schottky Diodes

31 First OVPD-OLED OLED U silver magnesium Alq 3 α-npd ITO glass substrate Structure of the large area OVPD- OLED device First large area OVPD-OLED OLED displaying the Logo of TU Braunschweig processed on a substrate size of 35 x 50 mm².

32 despite the progress achieved over the past two decades, molecular electronics remains a research field full of unknowns and even conflicting results. A particular difficulty is that charge transport properties of a molecular device are typically dominated by the property of molecule-electrode contact -- rather than by the molecule itself, therefore the contact geometry, quality, and chemistry become very important. - Kuan, Larade, and Guo, PRB 67, (2003).

33 Optical Spectroscopy

34 Energy E / ev 3,0 2,5 2,2 2,05 1,7 UV IR Wavelength λ / nm 1 ev = 1, J 1 nm = 10-9 m = 10 Å

35 Light Matter Interaction reflected transmitted or absorbed Dielectric Function ε ( ω) = ε ( ω) + iε ( ω) describes light matter interaction r i incident Refractive index: α = 2ωκ c I( x) = I0 exp n ~ + ( ω) = ε ( ω) = n iκ with n real part of refractive index (refraction!) and κ the so-called extinction coefficient (absorption). Absorption coefficient: Light intensity as function of distance x travelled in a medium: ( αx)

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