Photoemission Studies of Interface Effects on Thin Film Properties

1
Photoemission Studies of Interface Effects on
Thin Film Properties
Final Examination April 18th, 2006 Dominic A.
Ricci Department of Physics University of
Illinois at Urbana-Champaign
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Threshold of Technology
Year
Length Scale
3.5 million transistors
Final examination, April 18, 2006
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On the Atomic Scale
When physical structures lt e- coherence
length quantum effects manifest
1D e- confinement quantum well states
Thin films
Pure Science
Applied Technology
Quantum wells dominate properties of thin films
Understand quantum physics of thin films
Thin films are building blocks
Final examination, April 18, 2006
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Film Properties

• Schottky barrier height
• Rectifying energy barrier at metal-semiconductor
  junction
• Confines electrons in film
• Determines transport properties in solid-state
  devices
• Thermal stability temperature
• Annealing temperature at which smooth film
  structure roughens
• Relevant to robustness under technological
  operating conditions

Final examination, April 18, 2006
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Preview
Thin Pb films grown on metal (Au, In,
Pb)-terminated Si(111) probed with angle-resolved
UV photoemission

• Terminating metal serves as interfactant layer
  between film and substrate
• Quantum well states depend on boundary
  conditions
• Same film, same substrate, different
  interfactant isolates the interface effect on
  properties
• Schottky barrier and thermal stability measured
  via quantum well spectroscopy

Control electronic and physical film properties
with interfacial engineering
Final examination, April 18, 2006
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Overview

• Background
• Photoemission
• Surfaces reconstructions and films
• Quantum well states
• Results
• Schottky barrier tuning
• Thermal stability temperature control

Final examination, April 18, 2006
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Photoemission Spectroscopy

• Probes electronic states in system
• Input High intensity, monochromatic photons
  (VUV)
• Output e- emitted energy, momentum recorded
• 
  (angle-resolved)

e-
photoelectron kinetic energy
electronic state binding energy
work function
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Photoemission Spectroscopy

• Probes electronic states in system
• Input High intensity, monochromatic photons
  (VUV)
• Output e- emitted energy, momentum recorded
• 
  (angle-resolved)

e-
Normal emission h? 22 eV
Photoemission is surface sensitive ideal for
studying thin films
Final examination, April 18, 2006
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Photoemission Spectrum
Typical spectrum energy relative to Fermi level
EF
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Photoemission Requirements
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Overview

• Background
• Photoemission
• Surfaces reconstructions and films
• Quantum well states
• Results
• Schottky barrier tuning
• Thermal stability temperature control

Final examination, April 18, 2006
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Substrate
Semiconductor substrate n-type Si(111) 7 x
7

• n-type e- charge carrier
• (111) surface plane in Miller indices
• 7 x 7 surface reconstruction periodicity
• (n x m) n bulk units by m bulk units relative
• to surface 1 x 1 unit cell
• Formed by heating in vacuo _at_ 1250C for 7-10 s
• Si has band gap Eg 1.15 eV

Final examination, April 18, 2006
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Deposition
Sample

• Metal deposited on clean Si(111) surface with
  molecular beam epitaxy (MBE)
• Material evaporated from e-beam-heated
  crucible
• Amount deposited measured in monolayers (ML)
• Atomic layer

HV
Filament Supply
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Reconstructions
Sub-monolayer amounts of metal are deposited on
clean Si(111)-7 x 7 at RT, then annealed, to form
reconstructions
Reconstruction Coverage (ML)
0.42
0.76
0.96
0.96
0.33
0.33
Used to modify film-substrate boundary
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Pb Film Growth
Metal-reconstructed Si(111) substrates cooled to
60-100 K prior to Pb deposition, then film
annealed to 100 K
Interfactant

• Pb is a free-electron-like metal
• Pb/Si interface abrupt w/o intermixing

Final examination, April 18, 2006
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Overview

• Background
• Photoemission
• Surfaces reconstructions and films
• Quantum well states
• Results
• Schottky barrier tuning
• Thermal stability temperature control

Final examination, April 18, 2006
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Quantum Well States

• Metal e- confined in film between vacuum and
  semiconductor band gap
• Particle-in-a-box discrete energies at
  integer monolayer film thicknesses
• Different film thicknesses N
• different energies
• Different boundary conditions
• different energies

hv
Vacuum
Band Gap
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Quantum Well States
Well depth confinement range E0 between Pb EF
and Si valence band maximum
CBM
EF
Eg
E0
VBM
n-type Semiconductor
Metal
Final examination, April 18, 2006
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Quantum Well States
Confined electrons sharp, intense
peaks in spectra
Energy (eV)
EF
E
E0
Partially confined electrons E lt E0 Quantum well
resonances
broad, less intense peaks
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Atomic Layer Resolution

• Quantum well peak reaches max intensity at
  integer monolayer film thickness
• Absolute film thickness determination

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Bohr-Sommerfeld Phase Model
Total electronic phase quantized in 2p
Quantum well state energy levels for (N, n)
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Overview

• Background
• Photoemission
• Surfaces reconstructions and films
• Quantum well states
• Results
• Schottky barrier tuning
• Thermal stability temperature control

Final examination, April 18, 2006
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Schottky Barrier
Schottky Barrier

• Rectifying energy barrier at metal-semiconductor
  junction
• Barrier height S Eg E0 for n-type
  substrate

CBM
EF
E0
Eg
VBM
Examine Schottky barrier height by varying
film-substrate boundary condition
n-type Semiconductor
Metal
Final examination, April 18, 2006
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Measuring the Barrier Height
Measure E0 Measure S

• Two methods using quantum well spectroscopy
• Energy level analysis
• Interface phase shift depends on E0
• Fit energy levels to obtain barrier height
• Peak width analysis
• E0 lt E lt EF small width E lt E0 larger width
• Identify threshold to obtain barrier height

Final examination, April 18, 2006
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Energy Level Analysis
Normal emission spectra Pb/Au-6x6/Si(111) _at_ 100 K

• Energy levels differ by 1 eV among systems
• known from
  first-principles calculations
• (singularity at VBM)
• Simultaneous fit E(N,n)
• obtain E0 for all systems

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Peak Width Analysis

• Widths increase rapidly below E0 threshold
  
• provides measurement of Schottky barrier
• Weighted avg. with heights from energy level
  measurements

Differences observed among systems due to
interface effect
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Interface Dipole Model
Interface species concentration and
electronegativity determine charge transfer
around metal-semiconductor dipoles
Final examination, April 18, 2006
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Interface Dipole Model
Interface species concentration and
electronegativity determine charge transfer
around metal-semiconductor dipoles

• avg. charge state of
  interfacial Si
• electronegativity
• interfactant concentration

Final examination, April 18, 2006
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Interface Dipole Model
Interface species concentration and
electronegativity determine charge transfer
around metal-semiconductor dipoles

• avg. charge state of
  interfacial Si
• electronegativity
• interfactant concentration

• Schottky barrier height from
  model

Final examination, April 18, 2006
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Schottky Barrier Results
Comparison of Sexp (circles) to Scalc (line)
yields agreement
Interface dipole model reproduces measurements
with only chemical parameters (concentration,
electronegativity)
Schottky barrier tuning via proper interfactant
selection
Final examination, April 18, 2006
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Overview

• Background
• Photoemission
• Surfaces reconstructions and films
• Quantum well states
• Results
• Schottky barrier tuning
• Thermal stability temperature control

Final examination, April 18, 2006
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Thermal Stability Temperature
Annealing temperature at which smooth film
structure roughens Thermal energy allows atomic
rearrangement
T gt Tstability
T lt Tstability
Compare Pb films w/ 3 interfactants
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Electronic Stability
Quantized electronic structure
Total film electronic energy
Thermal stability

• Quantum well energy levels change with N
• Layer-to-layer variation in total electronic
  energy
• Thickness-dependent thermal stability

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Thickness Oscillations in Pb Films

• e- fill quantum wells w/ increasing N
• Shell effect periodic oscillation in total
  energy and film properties
• ?N 2.2 ML _at_ integer sampling
• Beating pattern
• Characteristic oscillation in work function,
  charge density distribution, interlayer lattice
  spacing, TC

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Quantum Well Spectroscopy Redux
In Au Pb A -1.70 0.29 2.21

• In and Pb diff. by p
• ?N 1 equivalent to phase change of p

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Measuring Thermal Stability

• Quantum well peak intensity monitored as
  function of T as film annealed
• Sudden drop off at Tstability as film rearranges
  to more stable thicknesses

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Thermal Stability Analysis

• Oscillation phase reversal in Pb/In/Si(111)
  system
• odd N more stable
• Oscillation amplitude larger in Pb/Au/Si(111)
  system
• stable above RT

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Thermal Stability Analysis
Friedel-like functional form
F phase shift (interfactant dependent)
Thermal stability control via interfacial
engineering
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Recapitulation
Thin Pb films grown on metal (Au, In,
Pb)-terminated Si(111) probed with angle-resolved
UV photoemission

• Used interfactant layers to alter film-substrate
  boundary condition and change film quantum
  electronic structure
• Schottky barrier tuning
• Thermal stability temperature manipulation

Control electronic and physical film properties
with interfacial engineering
Final examination, April 18, 2006
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Title
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Future Directions

• Pure science
• Use quantum well spectroscopy to probe other
  film properties to identify non-classical
  behavior
• Applications to technology
• Control film properties, e.g. superconducting TC

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Synchrotron Radiation

• Magnet-confined e- ring
• Monochrometers at beamlines

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Ultrahigh Vacuum

• UHV lt 10-9 torr
• Stainless steel chamber
• Series of pumps

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Energy Analyzer
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Deposition
Metal deposited on clean Si(111) surface with
molecular beam epitaxy

• Amount deposited measured in
• monolayers (ML)
• For reconstruction, defined in substrate units
• 1 ML 7.83 x 1014 atoms/cm2 for Si(111) surface
• For film, defined by bulk
• 1 ML 9.43 x 1014 atoms/cm2 for
• Pb(111) films

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RHEED
Surface quality monitored with Reflection High
Energy Electron Diffraction (RHEED)
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Phase Comparison
Direct relationship
Thermal stability control via interfacial
engineering
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Thermal Stability Analysis
Friedel-like functional form
a 1.77 from free electron model F phase
shift (interfactant dependent)
In Au Pb F -1.354 0.942 1.529

• In and Pb diff. by p

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Phase Comparison
Direct relationship
F can be determined from quantum well energy
levels
Thermal stability control via interfacial
engineering
Final examination, April 18, 2006