Dietrich R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Germany

Optical Spectroscopies of Thin Films and Interfaces

1. Introduction

2. Vibrational Spectroscopies

(Raman and Infrared)

3. Spectroscopic Ellipsometry

4. Reflectance Anisotropy Spectroscopy

hω

ee

organicinorganic

hω

Electrical Measurements:Current-Voltage (IV)Capacitance-Voltage (CV)DLTS, Admittance

Surface Science:Valence Band and Core Level PhotoemissionNEXAFS, LEED, AESIPES

Growth:Organic Molecular Beam Deposition (OMBD) in Ultra-High Vacuum

Optical Spectroscopy:Raman SpectroscopyIR, PL, RAS, SE

Application of complementary techniques

semiconductor A semiconductor B e.g. heterostructures, optoelectronics

Semiconductor technology has changed our world dramatically, e.g.

4 basic building blocks

p-typesemiconductor

n-typesemiconductor

e.g. p-n junction, bipolar transistor

metal oxide semiconductor e.g. MOSFET

metal semiconductor e.g. MESFET

Transistors

60 nm

Technology generation: L → L/√2

“Transistorized” PBS, Nov. 8, 1999 www.pbs.org/transistor/

Bell Labs 1947

TI 2001

“Moore’s Law”

21st Century Electronics:Transistors at the nano/molecular scale

Gate

DrainSource

~100 nm

Texas Instruments~2000

~10 nm ?~2015

electron flow

The Scale of Things

• 1 meter (1m)

• 1 mm (10-3 m)

• 1 µm (10-6 m)

• 1 nm (10-9 m)

• 1 pm (10-12 m) Silicon atom (0.118 nm)

human hair (100 µm)

biomolecules (10’s nm)

transistor (100 nm -2000)248 nm -DUV lithography

wavelength of light (< 1µm)

Nanotechnology Defined

• “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

1 nanometer = 0.000000001 meter

Si GaAs

Crystal structure

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)

a=5.43 A a=5.63 ASemiconductor Devices, 2/E by S. M. Sze Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.

Three Growth Modes

Substrate

Film

γs: surface energy of substrate γf: surface energy of filmγsf: interface energy of substrate-film

If γs > γf + γsf

Layer-by-layer(Frank-Van der Merwe)

γs < γf + γsf

3D islanding(Volmer-Weber)

Layer-by-layer followed by 3D islanding

(Stranski-Krastanov)

γs > γf + γsfWith misfit

Molecular Beam Epitaxy

Growth Mechanism Schematic diagram of MBE process

Molecular Beam Epitaxy

RHEED screen

Monitoring equipment (such as mass spectrometer)

Vacuum chamber

Source flanges

Reflection High-Energy Electron Diffraction(RHEED)

Screen ImageScreen Image

Finding Growth Rates with RHEEDFinding Growth Rates with RHEED

•• 22--d growth occurs one d growth occurs one atomic monolayer at a timeatomic monolayer at a time

•• Smooth surface gives peaks Smooth surface gives peaks in RHEED intensityin RHEED intensity

•• Period of RHEED intensity Period of RHEED intensity oscillations corresponds to oscillations corresponds to the time of growth for one the time of growth for one layerlayer

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!

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

AFM images of islands epitaxiall grown on GaAssubstrate.

(a) InAs islands randomly nucleate.

(b) Random distribution of InxGa1-xAs ring-shaped islands.

(c) 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.

Stranski-Krastanov growth of Ge on Si(001)

3D islands formation~ 3.5 ML Ge, 475°C, (110nm)2

[100]

huts

pyramids

Wetting layer~ 2.5 ML Ge, 475 °C, (44nm)2

InAs/InGaAs/GaAs HeterostructuresQ

uant

um D

ots

Sur

face

Diff

usio

n an

d E

last

icity

Typical sizes: dot height 10 nm, dot width 30-40 nm

Vacuum andExhaust system

Gas handle system

ComputerControl

Reactor

MOCVD Growth System

Metal-Organic Vapour Phase Epitaxy

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 H2 gas flow rate

MOCVD growth system

Confinement of Electrons and Holes

,...3,2,1;*2*2 2

2221

2

==

==∆

nLnkLmm

kE

zn

z

π

πhh

Organic semiconductors

Displays (Kodak)Organicfield-effect transistors

GaAs(100)

Organic Interlayer

V

IMetalOrganic/Inorganic

Microwave Diodes

Electrically drivenorganic lasers

Organic-modified Schottky DiodesPlastic solar cells

First large area OVPDOVPD--OLEDOLED displaying the Logo of TU Braunschweig processed on a substrate size of 35 x 50 mm².

silver

U

magnesiumAlq3

α-NPDITO

glass substrate

silver

U

magnesiumAlq3

α-NPDITO

glass substrate

Structure of the large area OVPD-OLED device

First OVPDFirst OVPD--OLEDOLED

“…despite the progress achieved over the past two decades…,molecular electronics remains a research field full of unknownsand even conflicting results. A particular difficulty is that chargetransport properties of a molecular device are typically dominatedby the property of molecule-electrode contact -- rather than bythe molecule itself, therefore the contact geometry, quality, andchemistry become very important.”

- Kuan, Larade, and Guo, PRB 67, 121411 (2003).

Optical Spectroscopy

Energy E / eV

1 eV = 1,602×10-19 J

1 nm = 10-9 m = 10 Å

410 495 620 700

Wavelength λ / nm

560

3,0 2,5 2,2 2,05 1,7

UV IR

Dielectric Function

describes light – matter interaction

Light – Matter Interaction

incident

reflected

transmitted or absorbed

( ) ( ) κωεω inn +==~

( )xIxI α−= exp)( 0

cωκα 2

=

( ) ( ) ( )ωεωεωε ir i+=

Refractive index: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: