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Final ExaminationApril 18th, 2006
Dominic A. RicciDepartment of Physics
University of Illinois at Urbana-Champaign
Photoemission Studies of Interface Effects on Thin Film Properties
Final examination, April 18, 2006
Threshold of Technology
1947
2006
2020
10-1 m
10-7 m
10-9 m
3.5 milliontransistors
Year Length Scale
Final examination, April 18, 2006
On the Atomic Scale
When physical structures < e- coherence lengthquantum effects manifest
Thin films 1D e- confinement quantum well states
Pure Science Applied Technology
Understand quantum physics
of thin films
Thin films are building
blocks
Quantum wells dominate
properties of thin films
Final examination, April 18, 2006
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
Preview
Final examination, April 18, 2006
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
Overview
Final examination, April 18, 2006
Background
• Photoemission• Surfaces reconstructions and films• Quantum well states
Results
• Schottky barrier tuning• Thermal stability temperature control
Final examination, April 18, 2006
Photoemission Spectroscopy
e-
• Probes electronic states in system• Input: High intensity, monochromatic photons (VUV)• Output: e- emitted – energy, momentum recorded (angle-resolved)
BEhKE KEBE
h
= photoelectron kinetic energy
= electronic state binding energy
= work function
Final examination, April 18, 2006
Photoemission Spectroscopy
e-
• Probes electronic states in system• Input: High intensity, monochromatic photons (VUV)• Output: e- emitted – energy, momentum recorded (angle-resolved)
Photoemission is surface sensitive – ideal for studying thin films
Normal emission hν = 22 eV
BEhKE
h
Final examination, April 18, 2006
Photoemission Spectrum
Typical spectrum – energy relative to Fermi level EF
EF
Final examination, April 18, 2006
Photoemission Requirements
• High intensity monochromatic light
Synchrotron Radiation Center (Stoughton, WI)
• Sample cleanliness
Ultrahigh vacuum chamber (base pressure: 8 x 10-11 torr)
• Electron detection
Hemispherical electron energy analyzer
Overview
Final examination, April 18, 2006
Background
• Photoemission• Surfaces reconstructions and films• Quantum well states
Results
• Schottky barrier tuning• Thermal stability temperature control
Final examination, April 18, 2006
SubstrateSemiconductor 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 @ 1250°C for 7-10 s
• Si has band gap Eg = 1.15 eV
Final examination, April 18, 2006
Deposition
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
Sample
HV
FilamentSupply
Final examination, April 18, 2006
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
α-33-Au
33-In 33-Pb
66-Au -33-Au
25-Au
Used to modify film-substrate boundary
Final examination, April 18, 2006
Pb Film Growth
Metal-reconstructed Si(111) substrates cooled to 60-100 K prior to Pb deposition, then film annealed to 100 K
• Pb is a free-electron-like metal
• Pb/Si interface abrupt w/o intermixing
Pb
Si
Interfactant
Overview
Final examination, April 18, 2006
Background
• Photoemission• Surfaces reconstructions and films• Quantum well states
Results
• Schottky barrier tuning• Thermal stability temperature control
Final examination, April 18, 2006
Quantum Well States
Pb
Si
•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 Ndifferent energies
• Different boundary conditionsdifferent energies
hv
Vacuum
e-
Band Gap
e-
Final examination, April 18, 2006
Quantum Well States
Metaln-type
Semiconductor
EF
VBM
CBM
E0
Eg
Well depth = confinement range E0 between Pb EF and Si valence band maximum
k(E)
En
erg
y (
eV
)
Pb
Si
Si VBM
Fermi Level
ΓL L
Final examination, April 18, 2006
Quantum Well States
EFEE0Energy (eV)
Confined electrons sharp, intense peaks in spectra
Partially confined electrons E < E0
Quantum well resonances broad, less intense peaks
Final examination, April 18, 2006
Atomic Layer Resolution
• Quantum well peak reaches max intensity at integer monolayer film thickness
• Absolute film thickness determination
is , boundary dependenceFinal examination, April 18, 2006
Bohr-Sommerfeld Phase Model
nkNt is 22
Total electronic phase quantized in 2π
Quantum well state energy levels for (N, n)
N = number ML
t = ML thickness (Å)
n = quantum number
)(Ek = e- momentum)(Es = surface phase shift
)(Ei = interface phase shift
Overview
Final examination, April 18, 2006
Background
• Photoemission• Surfaces reconstructions and films• Quantum well states
Results
• Schottky barrier tuning• Thermal stability temperature control
Final examination, April 18, 2006
Schottky Barrier
• Rectifying energy barrier at metal-semiconductor junction
• Barrier height S = Eg – E0 for n-type substrate
Examine Schottky barrier height by varying film-substrate
boundary conditionMetaln-type
Semiconductor
EF
VBM
CBM
E0
Eg
SchottkyBarrier
Final examination, April 18, 2006
Measuring the Barrier Height
Measure E0 Measure S
Two methods using quantum well spectroscopy:
1. Energy level analysis• Interface phase shift depends on E0
• Fit energy levels to obtain barrier height
2. Peak width analysis• E0 < E < EF: small width; E < E0: larger width• Identify threshold to obtain barrier height
)(Ei
• 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
Final examination, April 18, 2006
Energy Level AnalysisNormal emission spectra
Pb/Au-6x6/Si(111) @ 100 K
)()( 00 EEEEBAEi
nkNt is 22
)(),( EEk s
Final examination, April 18, 2006
Peak Width Analysis
Pe
ak W
idth
(e
V)
Energy (eV)
-33-Au
66-Au
-33-Au
25-Au
33-In
33-Pb
• 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
Final examination, April 18, 2006
Interface Dipole Model
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
-
+
Au
Si
Au
Si
Pb
Si
Au
Si
Au
Si
Au
Si
+
-
Interface species concentration and electronegativitydetermine charge transfer around metal-semiconductor
dipoles
• = avg. charge state of interfacial Si
• = electronegativity• = interfactant concentration
Final examination, April 18, 2006
Interface Dipole Model
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
-
+
Au
Si
Au
Si
Pb
Si
Au
Si
Au
Si
Au
Si
+
-
Interface species concentration and electronegativitydetermine charge transfer around metal-semiconductor
dipoles
2/)(
])1([
FrF
SiPbM CCQ
Q
C
• = avg. charge state of interfacial Si
• = electronegativity• = interfactant concentration
Final examination, April 18, 2006
Interface Dipole Model
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
Pb
Si
-
+
Au
Si
Au
Si
Pb
Si
Au
Si
Au
Si
Au
Si
+
-
Interface species concentration and electronegativitydetermine charge transfer around metal-semiconductor
dipoles
2/)(
])1([
FrF
SiPbM CCQ
Q
C
gcal EQ
S2
1
• = Schottky barrier height from model
calS
Final examination, April 18, 2006
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
Overview
Final examination, April 18, 2006
Background
• Photoemission• Surfaces reconstructions and films• Quantum well states
Results
• Schottky barrier tuning• Thermal stability temperature control
Final examination, April 18, 2006
Thermal Stability Temperature
Annealing temperature at which smooth film structure roughens
Thermal energy allows atomic rearrangement
T < TstabilityT > Tstability
Compare Pb films w/ 3 interfactants:
/Si(111)33-In 6/Si(111)6-Au
/Si(111)33-Pb
Final examination, April 18, 2006
Electronic Stability
Thermal stability
Total film electronic energy
Quantized electronic structure
• Quantum well energy levels change with N• Layer-to-layer variation in total electronic energy• Thickness-dependent thermal stability
Final examination, April 18, 2006
Thickness Oscillations in Pb Films
• e- fill quantum wells w/ increasing N
• “Shell effect” – periodic oscillation in total energy and film properties
• ΔN = 2.2 ML @ integer sampling
• Beating pattern
• Characteristic oscillation in work function, charge density distribution, interlayer lattice spacing, TC
Final examination, April 18, 2006
Quantum Well Spectroscopy Redux
)()( 00 EEEEBAEi Interface phase shift
In Au PbA = -1.70 0.29 2.21
• In and Pb diff. by ~π
• ΔN = 1 equivalent to phase change of π
Final examination, April 18, 2006
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
Final examination, April 18, 2006
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
Final examination, April 18, 2006
Thermal Stability Analysis
FN
DtNkCNT F
)2sin()(
Friedel-like functional form:
Φ = phase shift (interfactant dependent)
In Au Pb
Φ = -1.354 0.942 1.529
•In and Pb diff. by ~π
Thermal stability control via interfacial engineering
Recapitulation
Final examination, April 18, 2006
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
Title
Final examination, April 18, 2006
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
Final examination, April 18, 2006
Synchrotron Radiation
• High intensity monochromatic light
Synchrotron Radiation Center (Stoughton, WI)
• Magnet-confined e- ring
• Monochrometers at beamlines
• Sample cleanliness
Ultrahigh vacuum chamber (base pressure: 8 x 10-11 torr)
Final examination, April 18, 2006
Ultrahigh Vacuum
• UHV < 10-9 torr
•Stainless steel chamber
• Series of pumps
• Electron detection
Hemispherical electron energy analyzer
Final examination, April 18, 2006
Energy Analyzer
FocusingLenses
Detector/CCD Camera
Sample
e-
Slits R1 R2
R0
Final examination, April 18, 2006
Deposition
XTM Sample
CrucibleHV
FeedbackControl
FilamentSupply
Current Monitor
Filament
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 forPb(111) films
Final examination, April 18, 2006
RHEEDSurface quality monitored with
Reflection High Energy Electron Diffraction (RHEED)
10 keV electron gun
Sample on Rotatable Manipulator
RHEED Patternon Phosphor Screen
Final examination, April 18, 2006
Phase Comparison
FN
DtNkCNT F
)2sin()( nNtEk is 2)(2
is Direct relationship
lags by ~π/2 )0()0( is
Thermal stability control via interfacial engineering
Final examination, April 18, 2006
Thermal Stability Analysis
FN
DtNkCNT F
)2sin()(
Friedel-like functional form:
α = 1.77 from free electron model
Φ = phase shift (interfactant dependent)
In Au Pb
Φ = -1.354 0.942 1.529
•In and Pb diff. by ~π
Final examination, April 18, 2006
Phase Comparison
FN
DtNkCNT F
)2sin()( nNtEk is 2)(2
is Direct relationship
Thermal stability control via interfacial
engineering
Φ can be determined from quantum well energy levels