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
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*
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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.