INVERSE PHOTOEMISSION: CB DOS

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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

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Required Reading.
PhotoemissionLEEDIPES/spin resolved
Band mapping, spin detection using synch. Rad
PES/IPES
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What about the empty states
e-
EF
EB
hv
Photoemission allows us to interrogate Filled
states of the system
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Photoabsorption --not surface sensitive --need
high energy/flux source (synchrotron --NEXAFS
(core? p)
e-
Hv(out) E-ELoss
E(loss)
EF
EB
hv (in) E
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e-
e- out E E- Eloss
Electron energy loss (EELS)
E(loss)
EB
e- in, E
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PES and Inverse PES
hv9.7 eV, Geiger-Müller detector
Direct and inverse photoemission
www.tasc.infm.it/research/ipes/external.php
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Simplified Experimental Setup (Himpsel, Surf.
Sci. Rep.)
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Dowben Group Facility for spin-polarized inverse
photoemission
Dowben group uses photoelectrons from GaAs
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Important Consideration of IPES Low Count Rates
(Himpsel)
fine structure constant
R Rydberg Const. s(IPES) 10-8
photons/electron cannot use intense ebeams
(sample damage) s(PES) 10-3 electrons/photon
can use intense hv sources (synchrotrons) Bottom
line IPES is not for the impatient, or for
unstable samples.
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Himpsel Fermi edge for Ta resolution 300
meV (400 meV for Dowben group) Note Thermal
Broadening
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Mapping out the conduction band (k
0) (adopted from Himpsel paper) note slight
matrix element effects on intensities as Ei is
varied
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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.
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Same transition followed with IPES (normal
emission) Forbeaux, et al.
Note formation of p band
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IPES at varying polar angles maps dispersion of
CB states. Note lack of dispersion of p band
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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)
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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)
16071613
Graphene is a zero band gap semiconductor
C. Bjelkevig, et al. J.Phys. Cond. Matt. 22
(2010) 302002
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Raman 2D shows humongous red shift Strong charge
transfer?
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KRIPES Graphene/BN/Ru vs. Graphene/SiC Data
indicates BN? Graphene p Charge Transfer (0.12
e/Carbon atom!)
s
p band is filled!
EF
EF
2.5 eV
I. Forbeaux, J.-M. Themlin, J.-M. Debever, Phys.
Rev. B 58, 16396 (1998)
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IPESUPS? VB and CB DOS, compare to STM
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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
e-
p-type
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
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Graphene/MgO(111) Angle integrated
photoemission, and angle-resolved IPES cobmine to
show a band gap ! (Kong, et al.)
Why is the p below the s feature in the VB, and
the reverse in the CB ? Answer at Many
k-values, p is below s? angle integrated PES
gives this result. At k0, the p and p are
closer to EF than s, s, so IPES yields this
result. We could do ARPES (need synchrotron,
really).
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• Conclusions
• IPES? conduction band DOSk vector resolved only
• Minor Cross sectional effects
• Time consuming, low count rates
• Monolayer sensitive
• PESIPES 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.

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PES, IPES and surface states of
semiconductors Surface states of semiconductors
can be used in reconstruction, or dangling bonds
can have signficant effectsgood or badin
interfacial device properties. Surface states
usually lie in the band gapcan be affected by
dopants
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• IMPURITIES IN SEMICONDUCTORS LECTURE II
• Cox, Chapt. 7.1,7.2
• Feynman Lectures on Physics Vol. III, Ch. 14
• Britney Spears 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
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Semiconductor Impurities
CB
e-
e-
luminescence
n-type dopant
electron trap
Egap
p-type dopant
hv
hole trap
e-
h
VB
Creates a hole in VB
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Chung, et al., Surface Science 64 (1977) 588
Oxygen vacancies donate electrons into bottom of
conduction band
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Impurity Chemistry How does that extra
electron(hole) get into the conduction (valence)
band?
Hydrogenic ModelAn N-doner like phosphorous, in
tetrahedral coordination, can be thought of as P
with a loosely coordinated valence electron in a
Bohr-type orbit
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Orbital diameter can include several lattice
spacings Electron screened from P by dielectric
response of the lattice (?L) 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/(4p ?L
  ?or)
• ?L 12 for Si? big effect!
• Kinetic energy p2/2m m 0.2 me for bottom
  of Si CB
• En (Bohr model) -e4m/(8 ?L2 ?o2 h2 n2) n 1,
  2, 3, 4
• n 1? binding energy of donor electron .031 eV
  (calc) vs. .045 (exp)

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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 ?
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Evidence indicates As in bulk-terminated surface
sites. As sits on tip
Uhrberg ,et al., PRB 35 (1987) 3945
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Case Study B-doped Si(111)(?3x?3)
, Kaxiras, et al., PRB 41 (1990) 1262
Theory suggests B sits underneath Si sites
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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
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IPES and polarized Electrons --polarization of
the valence of fundamental and technological
interest. --Conduction band polarization also
important
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Santoni, et al. PRB 43(1991) 1305
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Spin polarized inverse photoemission and
photoemission
Spin is conserved during inverse photoemisson
e-
Spin is conserved during photoemisson
e-
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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).
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e-
e-
EF
By using spin up (down) electrons, we can map out
the spin down (up) portions of the conduction band
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Inverse photoemission maps out the spin states in
the Fe(110) and Fe(111) conduction bands
Santoni and Himpsel, PRB 43 (1991) 1305
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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?
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Ristoiu, et al. MOKE and LEED data for
NiMnSb(100) film Easy axis of magnetization lt110gt
Spin integrated PES used to determine surface
composition
Surface is Sb-rich
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Removal of excess surface Sb enhances spin
polarization near Fermi level
More sputtering and annealing
stoichiometric
Clean, excess Sb
P
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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.
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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.