1. X-ray photoemission spectroscopy study on III-Nitride films
Bo Chen
Department of Physics
Boston University
April 29, 2009

2. 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

3. Wurtzite InN films: Crystal Structure and Brillouin Zone
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

4. Calculated Band Structure for Bulk InNG H K S M L T
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, (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

5. 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 .

6. (Nobel Prize in Physics in 1981)
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.

7. Angle Resolved Photoemission Spectroscopy (ARPES)
Momentum conservation parallel to the surface gives:
ARPES measures the kinetic energy and the number of emitted electrons at different emission angles and gives the energy dispersion and the Fermi surface.

8. ARPES measurement of Intrinsic Quantum Well States on InN
Photocurrent intensity map of states along GSM, in the surface plane. hn = 69 eV; 177 K.
300°C anneal in UHV for 30 minutes.
Two well-resolved, nested bands
L. Colakerol et al, Phys. Rev. Lett. 97, (2006)

9. Fermi Surface and Constant Energy Contours from Quantum Well States on InN
(a) Two well defined circular concentric structures

10. Conclusion
In summary, we have discussed a solid state technology of measuring the electronic structure of III-nitride films using synchrotron-radiation based spectroscopies.

11. Reference
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4. 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, (2004)
5. “Surface Analysis – The Principal Techniques”, J. C. Vickerman, Chapter 2-4, John Wiley & Sons Ltd. (1997)
6. K. E. Smith and S. D. Kevan, Prog. Solid State Chem. 21, 49 (1991); Angle Resolved Photoemission, edited by S. D. Kevan (Elsevier, Amsterdam, 1991).
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11. K. Siegbahn et al. , ESCA: atomic, molecular and solid state structure studied by means of electron spectroscopy, Nova Acta Regiae Societatis Scientiarum Upsaliensis, Ser. IV, 20, (1967).