Surface analysis techniques part I Yaniv Rosen

Surface Analysis Techniques Chemical Analysis SIMS (Secondary ion mass spectroscopy) AES (Auger electron spectroscopy) Structural Analysis LEED (Low-energy electron diffraction) RHEED (Reflection high energy electron diffraction)

Secondary ion mass spectroscopy Sputter sample with high energy ions for example 4KeV Ar . Surface material is released. Use conventional ion mass spectrometers to determine composition. +

Advantages of SIMS Very sensitive – can reach parts per billion range. Ability to do depth profiling.

Disadvantages of SIMS Only ionized particles are measured.

Disadvantages of SIMS Only ionized particles are measured. Sputtering not necessarily even. Different levels of ionization.

Disadvantages of SIMS Only ionized particles are measured. Sputtering not necessarily even. Different levels of ionization. Intrinsically destructive: Dynamic SIMS 1nA/cm² 1µm/hr Static SIMS 1mA/cm² 1Å/hr

Auger Electron Spectroscopy Fire ~100eV-5keV electrons at sample Electron knocked out of atomic core Higher level electron falls into hole. Outer shell electron emitted with excess energy. Measure energy of emitted electron: KE=Ea-Eb-Ec* Small penetration depth. Depth profiling. Ea Eb Ec

Why use AES? Easy to detect 1% impurity in monolayer. Beam of electrons can be focused and moved easily – provides high resolution. Image can be compared simultaneously with SEM (Scanning electron microscope) image. Good transition rates for smaller elements – can get signal for Li. Transition rates for electrons better then those for photons when at smaller atomic number ~<15

AES disadvantages High resolution and fast rates can cause sample damage. Theoretical predictions are complicated. Backscattering can add random factors of two. Inner shells can add peaks.

AES disadvantages High resolution and fast rates can cause sample damage. Theoretical predictions are complicated. Absolute quantification not attempted Backscattering can add random factors of two. Inner shells can add peaks.

Low-energy electron diffraction Fire 20-300eV electrons at sample in Ultra-High Vacuum (UHV~10^-9 torr) Diffraction and elastic scattering occurs Accelerate electrons towards florescent mesh. Pattern should match reciprocal lattice.

LEED Advantages Small mean-free path through the material Same instrumentation as AES – can be placed in same apparatus. Averages over small defects in the periodicity. Only retrieve surface ions

LEED Disadvantages Adsorbates change the configuration. Possible to have multiple configurations from one spectra. Difficult theory when more then one atom in base cell. Possible to use other techniques to determine multiple configurations. Most difficulties arise when have more then one atom per unit cell. More then two big scattering atoms becomes very difficult.

Reflection high energy electron diffraction 30-100KeV. Fire high energy electrons at a shallow angle. Use phosphorus screen to detect diffraction pattern.

RHEED Advantages Electrons have high energy so they do not need help reacting with phosphorus. Sensitive to local defects – used in MBEs (Molecular-beam epitaxy) systems to grow semi-conductors.

RHEED Disadvantages Mostly concerned with qualitative descriptions of surface as opposed to quantitative diffracted beam intensity. Needs high vacuum so electrons are not deflected.

Conclusion Chemical Analysis Structural Analysis SIMS – Destructive but very sensitive to impurities. AES – high resolution for chemical analysis. Uses same instrumentation as LEED. Structural Analysis LEED – Bulk structure analysis. RHEED – Detects local defects.

Photoemission Spectroscopy for Surface Studies

Principle X-ray (or UV) photons excite electrons to continuum states. Electron kinetic energy (eKE) related to binding energy of the initial state by: eKE = hv – BE – δE eKE E=0 hv BE Core Level

Method Energy Analyzer Electrostatic Lens Detector X-rays/ UV Photoelectrons Sample Huefner et. al. 1996

Typical Properties Resolution: XPS ~.25eV (100 - >1000eV) UPS ~.10eV (10 - 40eV) Detection limits of 1 part in 10k to 100k for long measurements Can sample first ~10nm

Limitations Surface properties interfere with attempts to measure bulk properties Sample degradation Charge loss Radiation damage Lower and upper bounds on analysis spot size (micron-mm) UHV requirement, no magnetic field, low electric field

Chemical Shift PES lines affected by surroundings Neudachina et. al. 2005 Uhrberg et. al. 1998

Ad/Desorption Properties Characterize potential energy surface of final ionized state adsorbed onto substrate AB+ + S (A+B+) + S AB + S hv (A+B) + S Fohlisch et. al. 1998

ARPES Conservation laws eKE = hv – BE pef = phv + pei θ θ Damascelli 2004

Valence Band Characterization Damascelli 2004

Inverse XPS Study unoccupied bands Provides complimentary information to photoemission Directly measure density of states above EF Can use analogous techniques to PES Smith 1988