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INVERSE PHOTOEMISSION: CB DOS Suggested Reading: F. J. Himpsel , “Inverse Photoemission from Semiconductors”, Surf. Sci. Rep. 12 (1990) 1-48 Process and Methods Applications: Graphene Practical Drawbacks and Advantages. Required Reading…. Photoemission+LEED+IPES/spin resolved.

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slide1

INVERSE PHOTOEMISSION: CB DOS

  • Suggested Reading: F. J. Himpsel, “Inverse Photoemission from Semiconductors”, Surf. Sci. Rep. 12 (1990) 1-48
  • Process and Methods
  • Applications: Graphene
  • Practical Drawbacks and Advantages
slide2

Required Reading….

Photoemission+LEED+IPES/spin resolved

Band mapping, spin detection using synch. Rad+ PES/IPES

slide3

What about the empty states

e-

EF

EB

hv

Photoemission allows us to interrogate

Filled states of the system

slide4

Photoabsorption:

--not surface sensitive

--need high energy/flux source (synchrotron

--NEXAFS (core π*)

e-

Hv(out) = E-ELoss

E(loss)

EF

EB

hv (in) E

slide5

e-

e- out E = E- Eloss

Electron energy loss (EELS)

E(loss)

EB

e- in, E

slide6

PES and Inverse PES

hv=9.7 eV, Geiger-Müller detector

Direct and inverse photoemission

www.tasc.infm.it/research/ipes/external.php

Substrate-mediated assembly of doped graphene

slide8

Dowben Group Facility for spin-polarized inverse photoemission

Dowben group uses photoelectrons from GaAs

slide9

Important Consideration of IPES: Low Count Rates (Himpsel)

fine structure constant

R = Rydberg Const.

σ(IPES) ~ 10-8 photons/electron: cannot use intense ebeams (sample damage)

σ(PES) ~ 10-3 electrons/photon: can use intense hv sources (synchrotrons)

Bottom line: IPES is not for the impatient, or for unstable samples.

slide10

Himpsel: Fermi edge for Ta: resolution ~ 300 meV (~400 meV for Dowben group): Note Thermal Broadening

slide11

Mapping out the conduction band (k|| = 0)

(adopted from Himpsel paper): note slight matrix element effects on intensities as Ei is varied

slide12

Growth of Graphite (Multilayer graphene) on SiC(0001)—Forbeaux, et al., PRB 58 (1998) 16396

LEED shows that Si evaporation leads to graphitization at ~ 1400 C.

slide13

Same transition followed with IPES (normal emission) Forbeaux, et al.

Note formation of π* band

slide15

Growth of Graphene/BN(0001)/Ru(0001)

(Bjelkevig, et al. J. Phys. Cond. Matt. 22 (2010) 302002)

CVD with C2H4 yields graphene overlayer

ALD of BN monolayer on Ru(0001)

slide16

GRAPHENE CHARACTERIZATION

STM dI/dV Data: DOS is graphene-characteristic

Expt: HOPG

VB

CB

Our data

Shallow valley near Fermi level, 0 eV bandgap semiconductor

Graphene/BN/Ru

D. Pandey et al. / Surface Science 602 (2008) 1607–1613

Graphene is a zero band gap semiconductor

C. Bjelkevig, et al. J.Phys. Cond. Matt. 22 (2010) 302002

1770.001

slide17

Raman 2D shows humongous red shift

Strong charge transfer?

slide18

KRIPES: Graphene/BN/Ru vs. Graphene/SiC

Data indicates BN Graphene π* Charge Transfer

(0.12 e/Carbon atom!)

σ*

π* band is filled!

EF

EF

~2.5 eV

I. Forbeaux, J.-M. Themlin, J.-M. Debever, Phys. Rev. B 58, 16396 (1998)

slide20

By looking at distance of a CB feature from the Fermi level, we can look at charge transfer between graphene and substrate

n-type 0.07 e-/C atom

n-type 0.06 e-/C atom

graphene

n-type

e-

p-type

substrate

e-

n-type 0.03 e-/C atom

No charge transfer (Forbeaux et al.)

p-type 0.03 e-/C atom

Kong, et al. J.Phys. Chem. C. 114 (2010) 2161

slide21

Graphene/MgO(111) : Angle integrated photoemission, and angle-resolved IPES cobmine to show a band gap ! (Kong, et al.)

Why is the π below the σ feature in the VB, and the reverse in the CB ?

Answer: at Many k-values,

π is below σ angle integrated PES gives this result. At k=0, the π and π*

are closer to EF than σ, σ*, so IPES yields this result.

We could do ARPES (need synchrotron, really).

slide22

Conclusions:

  • IPES conduction band DOS—k vector resolved only
  • Minor Cross sectional effects
  • Time consuming, low count rates
  • ~Monolayer sensitive
  • PES+IPES can give accurate picture of VB, DOS, and Band gap formation PES: angle resolved or integrated
  • Spin-resolved versions of both IPES and PES possible.
slide23

PES, IPES and surface states of semiconductors

Surface states of semiconductors can be used in reconstruction, or dangling bonds can have signficant effects—good or bad—in interfacial device properties.

Surface states usually lie in the band gap—can be affected by dopants

slide24

IMPURITIES IN SEMICONDUCTORS –LECTURE II

  • Cox, Chapt. 7.1,7.2
  • Feynman Lectures on Physics Vol. III, Ch. 14
  • Britney Spear’s Guide to Semiconductor Physics (http://britneyspears.ac/lasers.htm)

I. Impurities in insulators and Semiconductors, a closer look

A.Types of Impurities

B. Dopant Chemistry and ionization potential

C. Dopant Effects on Fermi Level

II. P-N Junctions and Transistors

III. Doping-induced Insulator-Metal Transitions

slide25

Semiconductor Impurities

CB

e-

e-

luminescence

n-type dopant

electron trap

Egap

hv

p-type dopant

hole trap

e-

h+

VB

Creates a hole in VB

slide26

Chung, et al., Surface Science 64 (1977) 588: Oxygen vacancies donate electrons into bottom of conduction band

slide27

Impurity Chemistry: How does that extra electron(hole) get into the conduction (valence) band?

e-

Si

Si

Si

Si

P+

Hydrogenic Model—An N-doner like phosphorous, in tetrahedral coordination, can be thought of as P+with a loosely coordinated valence electron in a Bohr-type orbit

slide28

Orbital diameter can include several lattice spacings

Electron screened from P+ by dielectric response of the lattice (єL)

“Ionization” corresponds to electron promotion to bottom of CB, not to vacuum

Note: EVac – ECBM ~ Electron Affinity (EA)

e-

Si

Si

Si

Si

P+

  • In hydrogenic model, therefore, V(r) = -e2/(4π єL єor)
  • єL = 12 for Si big effect!
  • Kinetic energy = p2/2m* m* = 0.2 me for bottom of Si CB
  • En (Bohr model) = -e4m*/(8єL2 єo2 h2 n2) n= 1, 2, 3, 4…
  • n = 1 binding energy of donor electron = .031 eV (calc) vs. .045 (exp)
slide29

ECBM

Ed

Intrinsic regime

EF

EVBM

Saturation regime

Temperature dependence of # of carriers (n) and Fermi level for an n-type semiconductor (see Cox, Fig. 7.3)

n

1/T 

slide30

Evidence indicates As in bulk-terminated surface sites. As sits on tip

Uhrberg ,et al., PRB 35 (1987) 3945

slide31

Case Study:

B-doped Si(111)(3x3)

, Kaxiras, et al., PRB 41 (1990) 1262

Theory suggests B sits underneath Si sites

slide32

B (hole doped) broken bond surface states now empty, show up in IPES

Undoped, singly-occupied surface states in both PES and IPES

As (n doped) filled surface states only apparent in PES spectra

See also, Kaxiras, et al., PRB 41 (1990) 1262

slide33

IPES and polarized Electrons

--polarization of the valence of fundamental and technological interest.

--Conduction band polarization also important

slide35

Spin polarized inverse photoemission and photoemission

Spin is conserved during inverse photoemisson

e-

Spin is conserved during photoemisson

e-

slide36

e-

e-

 electrons will not fall into  states, and vice versa

EF

Real world intrusion: Typical electron sources have only partial polarization (P):

P = [N - N]/[N+N]

Typical figure ~ 30% (see Dowben paper).

slide37

e-

e-

EF

By using spin up (down) electrons, we can map out the spin down (up) portions of the conduction band

slide38

Inverse photoemission maps out the spin states in the Fe(110) and Fe(111) conduction bands

Santoni and Himpsel, PRB 43 (1991) 1305

slide39

How does surface structure affect the magnetic behavior of magnetic alloys? e.g., Ristoiu, et al. Europhys. Lett. 49 (2000) 624

Unit cell of NiMnSb

--supposed to have very high polarization near Fermi level, but measurements inconsistent

Could surface composition affect this?

Combine spin integrated photoemission with spin-resolved inverse photoemission to find out.

Why this combo: spin-integrated photoemission, spin resolved IPES?

slide40

Ristoiu, et al. MOKE and LEED data for NiMnSb(100) film

Easy axis of magnetization <110>

Spin integrated PES used to determine surface composition

Surface is Sb-rich

slide41

Removal of excess surface Sb enhances spin polarization near Fermi level

More sputtering and annealing

stoichiometric

Clean, excess Sb

P

slide42

NiMnSb conclusions

--surface magnetization very sensitive to surface structure

--surface prep therefore critically impacts MTJ performance

--Need to correlate surface compostion with electronic and magnetic structure.

slide43

Summary

Inverse Photoemission Conduction Band DOS

Can be used in spin-polarized manner spin polarized electrons

Typically need other surface methods (AES, PES, LEED….) to monitor surface composition.

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