2016 Fall Semester MS482 Materials Characterization ( 재료분석 ) Lecture Note 2: UPS Byungha Shin Dept. of MSE, KAIST 1
Course Information Syllabus 1. Overview of various characterization techniques (1 lecture) 2. Chemical analysis techniques (8 lectures) 2.1. X-ray Photoelectron Spectroscopy (XPS) 2.2. Ultraviolet Photoelectron Spectroscopy (UPS) 2.3. Auger Electron Spectroscopy (AES) 2.4. X-ray Fluorescence (XRF) 3. Ion beam based techniques (4 lecture) 3.1. Rutherford Backscattering Spectrometry (RBS) 3.2. Secondary Ion Mass Spectrometry (SIMS) 4. Diffraction and imaging techniques (7 lectures) 4.1. Basic diffraction theory 4.2. X-ray Diffraction (XRD) & X-ray Reflectometry (XRR) 4.3. Scanning Electron Microscopy (SEM) & Energy Dispersive X-ray Spectroscopy (EDS) 4.4. Transmission Electron Microscopy (TEM) 5. Scanning probe techniques (1 lecture) 5.1. Scanning Tunneling Microscopy (STM) 5.2. Atomic Force Microscopy (AFM) 6. Summary: Examples of real materials characterization (1 lecture) * Characterization techniques in blue are available at KARA (KAIST analysis center located in W8-1)
Ultraviolet Photoelectron Spectroscopy (UPS) UPS: valance level XPS: core-level as well as valance level hv: 10 50 ev XRF hv: 1486.6 ev (Al Ka) or 1253.6 ev (Mg Ka) AES UV radiation (10 45 ev) à emission of photoelectrons from valence orbitals (bands); energy too small for core level photoelectrons Also called Molecular Photoelectron Spectroscopy
UPS vs. XPS UV is more efficient in emitting photoelectrons from valence bands. Emission of photoelectrons from valence bands also occurs in XPS. However, kinetic energy of such photoelectrons too large à crosssection of valence band photoelectrons small compared to UV light. Much better energy resolution with UPS (~10 mev) than with XPS (~0.5 ev). Why? - Broader excitation source: FWHM of Al Ka ~0.9 ev (~0.25 ev after the crystal monochromator) - Core-hole lifetime broadening: E B is only determined within the natural lifetime width of the core hole (Heisenberg s uncertainty principle: E D t ħ ) and it is very short for the empty core hole state. UPS often more surface-sensitive than XPS
UV Light Source Most commonly used UV light source: resonance lines of rare gases produced by discharge or microwave lamp
UV Light Source Most commonly used UV light source: He I at 21.22 ev and He II at 40.81 ev Higher discharge voltages and currents and lower He gas pressures produce a higher intensity of He II radiation emission from neutral He emission from singly ionized He Relative intensity under normal He I operating conditions (0.5) (<1) (2) (100) (strongest emission) (strongest emission)
Herbert Kroemer (Recipient of Nobel Prize in Physics in 2000 for developing semiconductor heterostructures)
Photoemission Process (metallic sample) (hn F S ) ev (hn F a ) ev (hn F a + E acc ) ev 0 ev (F S F a ) ev (F S F a + E acc ) ev valence band core levels Calibrated KE from a metallic sample: Low KE cutoff at F S and high KE cutoff at ~hn Normally plotted in BE = hn KE, Low BE cutoff at ~0 (from E F ) and high BE cutoff at hn F S Even uncalibrated KE (or BE): high KE low KE = hn F S
Determination of Work Function (3.000 V: to distinguish inelastic cutoff from that due to the spectrometer) Work function of ITO determined by UPS: 21.22 ev (He I) F ITO = 21.77 5.03 à F ITO = 4.48 ev Spectrometer not well calibrated Park et al. Appl. Phys. Lett. 68, p. 2699 (1996)
Photoemission Process (semiconductor sample) hn E ion = hn F S (E F VBM) 0 ev hn (E F VBM) High KE Low KE = hn F S (E F VBM) Unlike a metallic sample, work function of a semiconductor sample cannot be determined from UPS spectrum alone à analyzer needs to be calibrated with a metal. Calibrated KE: high KE at hn (E F VBM) low KE cutoff at F S Calibrated BE (E F at 0 ev): high BE at hn F S low BE cutoff at E F VBM Once calibrated, absolute values of work function and surface VBM determined.
Example: UPS from Au (standard) and p-si Au raw data with He I (21.22eV) V acc = 15V KE + E acc (15 ev) UPS data taken by Dr. Kyoung Soon Choi at KBSI photoelectrons from the spectrometer high KE cutoff at ~hn 4.94 ev (0.42 ev smaller than F S of Au, 5.36 ev) Low KE cutoff at F S 20.8 ev (~0.42 ev smaller than hn, 21.22 ev) 21.5 ev (~0.28 ev larger than hn??)
Example: UPS from Au (standard) and p-si Au Low BE (secondary) cutoff, hn F S = 21.22 ev 5.36 ev = 15.86 ev KE = KE + 0.42 ev (calibration) BE = hn KE E F
Example: UPS from Au (standard) and p-si p-si raw data KE = KE 15 ev (E acc ) + 0.42 ev (calibration) BE = hn KE E F Vacuum level 4.7 ev Low BE cutoff hn F S = 16.52 ev à F S,p-Si = 4.7 ev High BE cutoff E F VBM = 0.23 ev E F CBM 0.23 ev VBM
Example: Cu(In,Ga)Se 2 /CdS Intensity x 10 3 (arb. units) 10 8 6 4 2 0 E g, CIGS 1.12 ev unpumped 12 9 6 3 0-3 CIGS 0.27 ev 1.02 ev Band bending= 0.27eV (E F E V ) 1.57eV Binding energy (ev) E F 1.57 ev CdS CIGS/CdS CIGS/CdS_pump 1.3eV Intensity x 10 3 (arb. units) 10 8 6 4 2 0 Valence band offset= 1.02eV 1.3eV 12 9 6 3 0-3 Binding energy (ev) CIGS E g, CdS 2.4 ev Excitation by fs-laser: 0.28 ev Carrier generation in 1.02 ev CIGS flattening the band CIGS_p CIGS/CdS_p CdS 0.28eV E F 1.3 ev pumped UPS performed by Dr. Richard Haight at IBM T. J. Watson Research Center
Example: Organic/organic heterojunction CuPc Alq 3 (100A) XPS BE shift Mg sub. UPS bulk CuPc CuPc, hn F S bulk Alq 3 CuPc: a larger F S With increasing thickness of CuPc, Alq 3 : neg. BE shift à upward band bending CuPc: pos. BE shift à downward band bending 1.5 ev ( bulk CuPc) Alq 3, hn F S 3.2 ev (bulk Alq 3, E F VBM) Tang et al. Appl. Phys. Lett. 88, p. 232103 (2006)
Example: Organic/organic heterojunction CuPc Alq 3 Mg from other measurements UPS low BE cutoff 1.0 UPS high BE cutoff (F S ) XPS BE shift determined automatically Tang et al. Appl. Phys. Lett. 88, p. 232103 (2006)