Transcript Lecture 7-Inverse photoemission

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
What about the empty states
e-
EF
EB
hv
Photoemission allows us to interrogate
Filled states of the system
e-
Photoabsorption:
--not surface sensitive
--need high energy/flux source (synchrotron
--NEXAFS (core π*)
Hv(out) = E-ELoss
E(loss)
EF
EB
hv (in) E
e-
e- out E = E- Eloss
Electron energy loss
(EELS)
E(loss)
EB
e- in, E
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
6
Simplified Experimental Setup (Himpsel, Surf. Sci. Rep.)
Dowben Group Facility for spin-polarized inverse
photoemission
Dowben group uses photoelectrons from GaAs
GaAs
B
Rotator
h
G-M
Detector
Sample
Spin Gun
Mott
Detector
Cs Source
Laser
Lin.
Transport
O2
GaAs
Crystal
G-M
Detectors
Magnet
8
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)
CVD with C2H4
yields graphene
overlayer
ALD of BN
monolayer on
Ru(0001)
GRAPHENE CHARACTERIZATION
STM dI/dV Data: DOS is graphene-characteristic
Expt: HOPG
VB
CB
Shallow valley near Fermi
level, 0 eV bandgap
semiconductor
Our data
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
16
Raman 2D shows humongous red shift
Strong charge transfer?
KRIPES: Graphene/BN/Ru vs. Graphene/SiC
Data indicates BN Graphene π* Charge Transfer
(0.12 e/Carbon atom!)
Ef: graphene/BN
σ*
π* band is filled!
EF
 * Graphene/BN(111)
Ef: graphene/SiC
EF
~2.5 eV
*
Graphene/SiC(0001)
I. Forbeaux, J.-M. Themlin,
J.-M. Debever, Phys. Rev. B
58, 16396 (1998)
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
p-type
e-
n-type
graphene
e-
substrate
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
Graphene/MgO(111) : Angle integrated photoemission, and angleresolved 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 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
Semiconductor Impurities
CB
eluminescence
Egap
hv
VB
n-type
dopant
p-type
dopant
e-
eelectron trap
hole trap
h+
Creates a
hole in VB
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
e-
P+
Si
Si
Si
Hydrogenic Model—An N-doner like phosphorous, in tetrahedral coordination,
can be thought of as P+ with a loosely coordinated valence electron in a Bohrtype orbit
Orbital diameter can include several
lattice spacings
Electron screened from P+ by dielectric
response of the lattice (єL)
Si
eP+
Si
Si
“Ionization” corresponds to electron
promotion to bottom of CB, not to vacuum
Note: EVac – ECBM ~ Electron Affinity (EA)
Si
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 єo2 h2 n2) n= 1, 2, 3, 4…
•n = 1 binding energy of donor electron = .031 eV (calc) vs. .045 (exp)
ECBM
Ed
Intrinsic
regime
EF
EVBM
Saturation
regime
n
1/T 
Temperature dependence of # of
carriers (n) and Fermi level for an
n-type semiconductor (see Cox,
Fig. 7.3)
Evidence indicates As in bulk-terminated surface
sites. As sits on tip
Uhrberg ,et al., PRB 35
(1987) 3945
Case Study:
B-doped Si(111)(3x3)
, 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
Spin is conserved during
einverse photoemisson
e-
Spin is conserved during
photoemisson
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).
e-
e-
EF
By using spin up (down) electrons, we can map out the spin
down (up) portions of the conduction band
Inverse photoemission maps out the
spin states in the Fe(110) and Fe(111)
conduction bands
Santoni and Himpsel, PRB 43
(1991) 1305
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
P
stoichiometric
More sputtering
and annealing
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.