Final Examination April 18 th, 2006 Dominic A. Ricci Department 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 m m m 3.5 million transistors YearLength Scale

Final examination, April 18, 2006 On the Atomic Scale When physical structures < e - coherence length quantum effects manifest Thin films 1D e - confinement quantum well states Pure ScienceApplied 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-e- Probes electronic states in system Input: High intensity, monochromatic photons (VUV) Output: e - emitted – energy, momentum recorded (angle-resolved) = photoelectron kinetic energy = electronic state binding energy = work function

Final examination, April 18, 2006 Photoemission Spectroscopy e-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

Final examination, April 18, 2006 Photoemission Spectrum Typical spectrum – energy relative to Fermi level E F EFEF

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 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 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 1250°C for 7-10 s Si has band gap E g = 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 Filament Supply

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 ReconstructionCoverage (ML) Used to modify film-substrate boundary

Final examination, April 18, 2006 Pb Film Growth Metal-reconstructed Si(111) substrates cooled to 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 N different energies Different boundary conditions different energies hv Vacuum e - Band Gap e -

Final examination, April 18, 2006 Quantum Well States Metal n-type Semiconductor EFEF VBM CBM E0E0 EgEg Well depth = confinement range E 0 between Pb E F and Si valence band maximum k(E) Energy (eV) Pb Si Si VBM Fermi Level ΓLL

Final examination, April 18, 2006 Quantum Well States EFEF EE0E0 Energy (eV) Confined electrons sharp, intense peaks in spectra Partially confined electrons E < E 0 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

boundary dependence Final examination, April 18, 2006 Bohr-Sommerfeld Phase Model Total electronic phase quantized in 2 π Quantum well state energy levels for (N, n) = number ML = ML thickness (Å) = quantum number = e - momentum = surface phase shift = 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 = E g – E 0 for n-type substrate Examine Schottky barrier height by varying film-substrate boundary condition Metal n-type Semiconductor EFEF VBM CBM E0E0 EgEg Schottky Barrier

Final examination, April 18, 2006 Measuring the Barrier Height Measure E 0 Measure S Two methods using quantum well spectroscopy: 1.Energy level analysis Interface phase shift depends on E 0 Fit energy levels to obtain barrier height 2.Peak width analysis E 0 < E < E F : small width; E < E 0 : larger width Identify threshold to obtain barrier height

Energy levels differ by ~1 eV among systems known from first- principles calculations (singularity at VBM) Simultaneous fit E(N,n) obtain E 0 for all systems Final examination, April 18, 2006 Energy Level Analysis Normal emission spectra 100 K

Final examination, April 18, 2006 Peak Width Analysis Peak Width (eV) Energy (eV) Widths increase rapidly below E 0 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 electronegativity determine 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 electronegativity determine 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 electronegativity determine charge transfer around metal-semiconductor dipoles = Schottky barrier height from model

Final examination, April 18, 2006 Schottky Barrier Results Comparison of S exp (circles) to S calc (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 < T stability T > T stability Compare Pb films w/ 3 interfactants:

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 Δ N = 2.2 integer sampling Beating pattern Characteristic oscillation in work function, charge density distribution, interlayer lattice spacing, T C

Final examination, April 18, 2006 Quantum Well Spectroscopy Redux Interface phase shift InAuPb A = In and Pb diff. by ~ π Δ N = 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 T stability 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 Friedel-like functional form: Φ = phase shift (interfactant dependent) InAuPb Φ = 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 T C

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 torr) Final examination, April 18, 2006 Ultrahigh Vacuum UHV < torr Stainless steel chamber Series of pumps

Electron detection Hemispherical electron energy analyzer Final examination, April 18, 2006 Energy Analyzer Focusing Lenses Detector/ CCD Camera Sample e-e- SlitsR1R1 R2R2 R0R0

Final examination, April 18, 2006 Deposition XTM Sample Crucible HV Feedback Control Filament Supply 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 atoms/cm 2 for Si(111) surface For film, defined by bulk: 1 ML = 9.43 x atoms/cm 2 for Pb(111) films

Final examination, April 18, 2006 RHEED Surface quality monitored with Reflection High Energy Electron Diffraction (RHEED) 10 keV electron gun Sample on Rotatable Manipulator RHEED Pattern on Phosphor Screen

Final examination, April 18, 2006 Phase Comparison Direct relationship lags by ~ π/2 Thermal stability control via interfacial engineering

Final examination, April 18, 2006 Thermal Stability Analysis Friedel-like functional form: α = 1.77 from free electron model Φ = phase shift (interfactant dependent) InAuPb Φ = In and Pb diff. by ~ π

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