X-ray photoemission spectroscopy
study on III-Nitride films
Bo Chen
Department of Physics
Boston University
April 29, 2009
Outline
• III-Nitride films (optoelectronic materials, their electronic
structure not fully measured)
• Synchrotron-radiation based spectroscopies
Angle Resolved Photoemission Spectroscopy (ARPES)
• ARPES study on the charge accumulation for wurtzite InN films
Wurtzite InN films: Crystal Structure and Brillouin Zone
A
H
L

K
K
M
M
K
A
H
L
H
• The Wurtzite III-Nitride structure has a hexagonal unit cell
c/a = 1.633, two hexagonal sublattices, coordination number 4
• Bulk and surface Brillouin zones are hexagonal
ΓΣM, ΓTK and ΓΔA directions
Calculated Band Structure for Bulk InN
Energy (eV) relative to VBM
8
A
6
D
4

H
L
T
S
M
2
0
-2
-4
-6

K H
A

M L
A
I. Mahboob, T. D.
Veal, L. F. J. Piper, C.
F. McConville, Hai Lu,
W. J. Schaff, F.
Bechstedt, and J.
Furthmüller, Phys.
Rev. B 69, 201307
(2004)
•The conduction band minimum (CBM) at the Γ-point is much lower than the
conduction band edge at other points in k-space.
•The conduction band, valence bands (heavy-hole, light-hole, and split-off
bands) at Γ-point
K
Charge Accumulation in Wurtzite InN films
• Concept: Electrons accumulate in the near-surface region for certain
semiconductors.
• Physics: Donor-type surface states cause downward band bending. The
Fermi level lies in the conduction band in the accumulation layer (8 nm).
• Model: A 1D quantum well in the direction perpendicular to the surface
and 2D electron gas in the plane parallel to the surface .
X-ray photoemission
• X-ray photoemission spectroscopy (XPS)
is also called Electron Spectroscopy for
Chemical Analysis (ESCA). Kai Siegbahn
developed the instrumentation and theory
of ESCA.
(Nobel Prize in Physics in 1981)
• The energy and the component of the
momentum parallel to the surface are
conserved.
• The Einstein equation:
EB = hν–KE–Фsp
Measuring the kinetic energy of emitted
electrons gives the binding energy of
electrons in the sample.
Angle Resolved Photoemission Spectroscopy (ARPES)
Momentum conservation parallel to the surface gives:
k|| 
2m( KE)
sin  
2

ARPES measures the kinetic energy and the number of emitted
electrons at different emission angles and gives the energy dispersion
and the Fermi surface.
ARPES measurement of Intrinsic Quantum Well States on InN
Photocurrent
intensity map of
states along SM, in
the surface plane.
hn = 69 eV; sample
@ 177 K.
300°C anneal in
UHV for 30 minutes.
Two well-resolved,
nested bands
L. Colakerol et al, Phys. Rev. Lett. 97, 237601 (2006)
Fermi Surface and Constant Energy Contours
from Quantum Well States on InN
-1
EF
(a)
kx (Å )
EF - 0.5 eV
-0.2 -0.1 0.0 0.1 0.2
(c)
0.2
0.4
-1
ky (Å )
0.1
0.0
-0.1
-0.2
0.4
EF - 0.2 eV
EF - 0.7 eV
(b)
(a) Two well defined circular concentric structures
(d)
Conclusion
• In summary, we have discussed a solid state technology of
measuring the electronic structure of III-nitride films using
synchrotron-radiation based spectroscopies.
Reference
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