MS414 Materials Characterization (소재분석)

Lecture Note 2: UPS

Byungha ShinDept. of MSE, KAIST

1

2017 Fall Semester

CourseInformationSyllabus1. 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 lectures)3.1. Rutherford Backscattering Spectrometry (RBS)3.2. Secondary Ion Mass Spectrometry (SIMS)

4. Diffraction and imaging techniques (7 lectures)4.1. Basic diffraction theory4.2. 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)

• UV radiation (10 – 45 eV) àemission of photoelectrons from valence orbitals (bands); energy too small for core level photoelectrons

• Also called Molecular Photoelectron Spectroscopy

XPS: core-level as well as valance level

hn: 1486.6 eV (Al Ka) or 1253.6 eV (Mg Ka)hn: 10 – 50 eV

UPS: valance level

XRF AES

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 àcross-section 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 of XPS: FWHM of AlKa ~0.9 eV (~0.25 eV after the crystal monochromator)- Core-hole lifetime broadening: EB is only determined within the natural lifetime width of the core hole (Heisenberg’s uncertainty principle: ∆𝐸 # ∆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

(strongest emission)(strongest emission)

Relative intensity under normal He I operating conditions

(100)(2)(0.5) (<1)

emissionfromneutralHe

emissionfromsinglyionizedHe

• 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

Herbert Kroemer (Recipient of Nobel Prize in Physics in 2000 for developing semiconductor heterostructures)

Photoemission Process (metallic sample)

core levels

valence band

0 eV

(hn – FS) eV(hn – Fa) eV

(FS – Fa) eV (FS – Fa + Eacc) eV

(hn – Fa + Eacc) eV

• Calibrated KE from a metallic sample: Low KE cutoff at FS and high KE cutoff at ~hn

• Normally plotted in BE = hn – KE, Low BE cutoff at ~0 (from EF) and high BE cutoff at hn – FS

• Even uncalibrated KE (or BE): high KE – low KE = hn – FS

Determination of Work Function

Park et al. Appl. Phys. Lett. 68, p. 2699 (1996)

• Work function of ITO determined by UPS: 21.22 eV (He I) – FITO = 21.77 – 5.03 àFITO = 4.48 eV

• Spectrometer not well calibrated

(3.000 V: to distinguish inelastic cutoff from that due to the spectrometer)

Photoemission Process (semiconductor sample)• High KE – Low KE =

hn – FS – (EF – VBM)• Unlike a metallic sample,

work function of a semiconductor sample cannot be determined from UPS spectrum alone àanalyzer needs to be calibrated with a metal.

• Calibrated KE: high KE at hn – (EF – VBM)low KE cutoff at FS

• Calibrated BE (EF at 0 eV): high BE at hn – FSlow BE cutoff at EF – VBM

• Once calibrated, absolute values of work function and surface VBM determined.

0 eV

hn – Eion = hn – [ FS + (EF – VBM) ]hn – (EF – VBM)

Example: UPS from Au (standard) and p-SiAuraw data with He I (21.22eV) Vacc = 15V

photoelectrons from the spectrometer

KE – qVacc (15 eV)

Low KE cutoff at FS

high KE cutoff must be at ~hn

UPS data taken by Dr. Kyoung Soon Choi at KBSI

4.94 eV (0.42 eV smaller than FSof Au, 5.36 eV)

20.8 eV (~0.42 eV smaller than hn, 21.22 eV)

21.5 eV (~0.28 eV larger than hn ??)

low KE cutoff must be at FS

Au

KE’ = KE + 0.42 eV (calibration)BE = hn – KE’

High BE (secondary) cutoff,hn – FS = 21.22 eV – 5.36 eV = 15.86 eV

EF

Example: UPS from Au (standard) and p-Si

Example: UPS from Au (standard) and p-Sip-Si

KE’’ = KE – 15 eV (Eacc) + 0.42 eV (calibration)BE = hn – KE’’

EF

raw data

High BE cutoff hn – FS = 16.52 eV à FS,p-Si = 4.7 eV

Low BE cutoff EF – VBM = 0.23 eV

Vacuum level

CBM

VBMEF

4.7 eV

0.23 eV

Example: Cu(In,Ga)Se2/CdS

12 9 6 3 0 -30

2

4

6

8

10

Inte

nsity

x 1

03 (arb

. uni

ts)

Binding energy (eV)

Valence band offset=1.02eV

0.28eV1.3eV

CIGS_pCIGS/CdS_p

12 9 6 3 0 -30

2

4

6

8

10In

tens

ity x

103 (a

rb. u

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)

Binding energy (eV)

1.3eV1.57eV

Band bending=0.27eV

CIGS/CdSCIGS/CdS_pump

EF

CIGS

CdS

1.02eV

EF

CIGS

CdS

1.3eV1.57eV

Excitationbyfs-laser:CarriergenerationinCIGSflatteningtheband

(EF– EV)

0.28eV

Eg,CIGS1.12eV

Eg,CdS2.4eV

0.27eV

1.02eVunpumped pumped

UPSperformedbyDr.RichardHaightatIBMT.J.WatsonResearchCenter

Example: Organic/organic heterojunction

Mg sub.

Alq3 (100A)CuPc

3.2 eV (bulk Alq3, EF – VBM)

1.5 eV(“bulk” CuPc)

XPS BE shift

Alq3, hn – FS

CuPc,hn – FS

CuPc: a larger FS

Tang et al. Appl. Phys. Lett. 88, p. 232103 (2006)

UPS

With increasing thickness of CuPc,• Alq3: neg. BE shift à upward band

bending (as approaching the interface)• CuPc: neg. BE shift à upward band

bending (as moving away from the interface) or downward band bending (as approaching the interface)

“bulk” CuPc

bulk Alq3

Example: Organic/organic heterojunction

Tang et al. Appl. Phys. Lett. 88, p. 232103 (2006)

from other measurements

UPS low BE cutoff 1.0UPS high BE cutoff (FS)

XPS BE shift

determined automatically

Mg sub.

Alq3 (100A)CuPc