INVERSE PHOTOEMISSION: CB DOS

Suggested Reading: F. J. Himpsel, “Inverse Photoemission from Semiconductors”, Surf. Sci. Rep. 12 (1990) 1-48

1.Process and Methods

2.Applications: Graphene

3.Practical Drawbacks and Advantages

Required Reading….

Photoemission+LEED+IPES/spin resolved

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

EF

EB

e-

hv

Photoemission allows us to interrogate Filled states of the system

What about the empty states

EF

EB

e-

hv (in) E

E(loss)

Hv(out) = E-ELoss

Photoabsorption:--not surface sensitive--need high energy/flux source (synchrotron--NEXAFS (core π*)

EB

e-

E(loss)

Electron energy loss (EELS)

e- in, E

e- out E = E- Eloss

Direct and inverse photoemission

hv=9.7 eV, Geiger-Müller detector

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

PES and Inverse PES

6Substrate-mediated assembly of doped graphene

Simplified Experimental Setup (Himpsel, Surf. Sci. Rep.)

Rotator

Laser

GaAs B

G-MDetector

G-M Detectors

Spin Gun

MagnetGaAsCrystal

Cs Source

O2

Lin.Transport

Sample MottDetector

h

Dowben Group Facility for spin-polarized inverse photoemission

8

Dowben group uses photoelectrons from GaAs

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.

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

Mapping out the conduction band (k|| = 0)(adopted from Himpsel paper): note slight matrix element effects on intensities as Ei is varied

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.

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

Note formation of π* band

IPES at varying polar angles maps dispersion of CB states. Note lack of dispersion of π* band

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

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

ALD of BN monolayer on Ru(0001)

CVD with C2H4 yields graphene overlayer

STM dI/dV Data: DOS is graphene-characteristic

Expt: HOPG

Graphene/BN/Ru

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

Our data

161770.001

GRAPHENE CHARACTERIZATION

Shallow valley near Fermi level, 0 eV bandgap semiconductor

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

Graphene is a zero band gap semiconductor

VB CB

Raman 2D shows humongous red shiftStrong charge transfer?

Graphene/SiC(0001)

Graphene/BN(111) *

Ef: graphene/BN

Ef: graphene/SiC

*

KRIPES: Graphene/BN/Ru vs. Graphene/SiCData indicates BN Graphene π* Charge Transfer

(0.12 e/Carbon atom!)

π* band is filled!

EF

EF

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

~2.5 eV

σ*

18

IPES+UPS VB and CB DOS, compare to STM

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

n-type 0.03 e-/C atom

No charge transfer (Forbeaux et al.)

p-type 0.03 e-/C atom

substrate

graphene

n-type e-

e-

p-type

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

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).

Conclusions:

1.IPES conduction band DOS—k vector resolved only

2.Minor Cross sectional effects

3.Time consuming, low count rates

4.~Monolayer sensitive

5.PES+IPES can give accurate picture of VB, DOS, and Band gap formation PES: angle resolved or integrated

6.Spin-resolved versions of both IPES and PES possible.

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

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 lookA. Types of ImpuritiesB. Dopant Chemistry and ionization potentialC. Dopant Effects on Fermi Level

II. P-N Junctions and Transistors

III. Doping-induced Insulator-Metal Transitions

CB

VB

Egap

n-type dopant

e-

e-

p-type dopant

h+

hole trap

e-

electron trap

Creates a hole in VB

hv

luminescence

Semiconductor Impurities

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

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

Si

Si

Si

Si

P+

e-

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

Si

Si

Si

Si

P+

e-

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)

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*/(8 єL2 єo

2 h2 n2) n= 1, 2, 3, 4…

•n = 1 binding energy of donor electron = .031 eV (calc) vs. .045 (exp)

1/T

n

EVBM

ECBM

Ed

EF

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

Saturation regime

Intrinsic regime

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

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

Case Study:

B-doped Si(111)(3x3), Kaxiras, et al., PRB 41 (1990) 1262

Theory suggests B sits underneath Si sites

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

IPES and polarized Electrons

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

--Conduction band polarization also important

Santoni, et al. PRB 43(1991) 1305

Spin polarized inverse photoemission and photoemission

e- Spin is conserved during photoemisson

e-Spin is conserved during inverse photoemisson

EF

e-

electrons will not fall into states, and vice versa

e-

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

P = [N - N]/[N+N]

Typical figure ~ 30% (see Dowben paper).

EF

e-

e-

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

Santoni and Himpsel, PRB 43 (1991) 1305

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

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?

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

Removal of excess surface Sb enhances spin polarization near Fermi level

Clean, excess Sb stoichiometricMore sputtering and annealing

P

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.

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.