photoemission studies of interface effects on thin film properties

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


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


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


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


Background

• Photoemission• Surfaces reconstructions and films• Quantum well states

Results

• Schottky barrier tuning• Thermal stability temperature control


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


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


Photoemission Spectrum

Typical spectrum – energy relative to Fermi level EF

EF


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


Background


Results



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


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


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


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


Background


Results



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-


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


Quantum Well States

EFEE0Energy (eV)

Confined electrons sharp, intense peaks in spectra

Partially confined electrons E < E0

Quantum well resonances broad, less intense peaks


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


Background


Results



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


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


Energy Level AnalysisNormal emission spectra

Pb/Au-6x6/Si(111) @ 100 K

)()( 00 EEEEBAEi

nkNt is 22

)(),( EEk s


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


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



Pb

Si

Pb

Si

Pb

Si

Pb

Si

Pb

Si

Pb

Si

-

+

Au

Si

Au

Si

Pb

Si

Au

Si

Au

Si

Au

Si

+

-


dipoles

2/)(

])1([

FrF

SiPbM CCQ

Q

C

• = avg. charge state of interfacial Si

• = electronegativity• = interfactant concentration



Pb

Si

Pb

Si

Pb

Si

Pb

Si

Pb

Si

Pb

Si

-

+

Au

Si

Au

Si

Pb

Si

Au

Si

Au

Si

Au

Si

+

-


dipoles

2/)(

])1([

FrF

SiPbM CCQ

Q

C

gcal EQ

S2

1

• = Schottky barrier height from model

calS


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


Background


Results



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


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


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


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


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


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



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


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


Title


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


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)


Ultrahigh Vacuum

• UHV < 10-9 torr

•Stainless steel chamber

• Series of pumps

• Electron detection

Hemispherical electron energy analyzer


Energy Analyzer

FocusingLenses

Detector/CCD Camera

Sample

e-

Slits R1 R2

R0


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


RHEEDSurface quality monitored with

Reflection High Energy Electron Diffraction (RHEED)

10 keV electron gun

Sample on Rotatable Manipulator

RHEED Patternon Phosphor Screen


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



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


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

photoemission studies of interface effects on thin film properties

Documents

evfinal examination

filmsfinal examination

films quantum

work functionfinal examination

e emitted energy

substrate quantum

properties schottky

physical film properties