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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Band mapping, spin detection using synch. Rad+ PES/IPES
Photoemission allows us to interrogate
Filled states of the system
--not surface sensitive
--need high energy/flux source (synchrotron
--NEXAFS (core π*)
Hv(out) = E-ELoss
hv (in) E
e- out E = E- Eloss
Electron energy loss (EELS)
e- in, E
hv=9.7 eV, Geiger-Müller detector
Direct and inverse photoemission
Substrate-mediated assembly of doped graphene
Dowben Group Facility for spin-polarized inverse photoemission
Dowben group uses photoelectrons from GaAs
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 for Dowben group): Note Thermal Broadening|| = 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) Note lack of dispersion of
(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 Note lack of dispersion of
STM dI/dV Data: DOS is graphene-characteristic
Shallow valley near Fermi level, 0 eV bandgap semiconductor
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
Raman 2D shows humongous red shift Note lack of dispersion of
Strong charge transfer?
KRIPES: Graphene/BN/Ru vs. Graphene/ Note lack of dispersion of SiC
Data indicates BN Graphene π* Charge Transfer
(0.12 e/Carbon atom!)
π* band is filled!
I. Forbeaux, J.-M. Themlin, J.-M. Debever, Phys. Rev. B 58, 16396 (1998)
IPES+UPS Note lack of dispersion of 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
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).
PES, IPES and surface states of semiconductors angle-resolved IPES cobmine to show a band gap ! (Kong, et al.)
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
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 angle-resolved IPES cobmine to show a band gap ! (Kong, et al.)
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?
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
Orbital diameter can include several lattice spacings into the conduction (valence) band?
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 into the conduction (valence) band?CBM
Temperature dependence of # of carriers (n) and Fermi level for an n-type semiconductor (see Cox, Fig. 7.3)
Uhrberg ,et al., PRB 35 (1987) 3945
Case Study: sits on tip
, Kaxiras, et al., PRB 41 (1990) 1262
Theory suggests B sits underneath Si sites
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 up in IPES
--polarization of the valence of fundamental and technological interest.
--Conduction band polarization also important
Santoni, et al. PRB 43(1991) 1305 up in IPES
Spin is conserved during inverse photoemisson
Spin is conserved during photoemisson
e- up in IPES
electrons will not fall into states, and vice versa
Real world intrusion: Typical electron sources have only partial polarization (P):
P = [N - N]/[N+N]
Typical figure ~ 30% (see Dowben paper).
e- up in IPES
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 magnetic alloys? e.g., Ristoiu, et al. Europhys. Lett. 49 (2000) 624
Easy axis of magnetization <110>
Spin integrated PES used to determine surface composition
Surface is Sb-rich
More sputtering and annealing
Clean, excess Sb
NiMnSb conclusions Fermi level
--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 Fermi level
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.