Paper EL+TF+AS+EM+SS+PS+EN+NM-MoM6
Optical Modeling of Plasma-Deposited ZnO: Extended Drude and its Physical Interpretation
Monday, October 29, 2012, 10:00 am, Room 19
| Session: |
Spectroscopic Ellipsometry for Photovoltaics and Semiconductor Manufacturing |
| Presenter: |
H.C.M. Knoops, Eindhoven University of Technology, the Netherlands |
| Authors: |
H.C.M. Knoops, Eindhoven University of Technology, the Netherlands
M.V. Ponomarev, Eindhoven University of Technology, the Netherlands
J.W. Weber, Eindhoven University of Technology, the Netherlands
N. Leick, Eindhoven University of Technology, the Netherlands
B.W.H. van de Loo, Eindhoven University of Technology, the Netherlands
Y.G. Melese, Eindhoven University of Technology, the Netherlands
W.M.M. Kessels, Eindhoven University of Technology, the Netherlands
M. Creatore, Eindhoven University of Technology, the Netherlands
|
| Correspondent: |
Click to Email |
High-quality transparent conductive oxides such as ZnO are important due to their electrical and optical properties. To improve these properties the responsible physical processes have to be understood. Traditionally, charge-carrier-scattering processes are investigated by combining morphology data and Hall measurements. This contribution discusses the extensive optical modeling of plasma-deposited ZnO and how its interpretation directly provides insight into the relevant charge-carrier-scattering processes at different length scales. The interpretation is generalized to the concept of frequency-dependent resistivity, which is used to explain the applicability of different Drude models.
Thin films (50-1000 nm) of Al-doped and undoped ZnO were deposited using an expanding thermal plasma MOCVD process.1 Conditions of high pressure and high diethyl zinc flow allowed for dense films with low electrical resistivities (e.g., 4×10-4 Ω cm at 300 nm). The films were analyzed with variable-angle spectroscopic ellipsometry (SE) (0.75 – 5.0 eV), FTIR reflection spectroscopy (0.04 – 0.86 eV), Four-point-probe (FPP), and Hall measurements.
The SE and FTIR data were combined and fitted with classical and extended Drude2 models. The high intensity of the Drude in the FTIR range resulted in a high sensitivity with which the carrier concentration and mobility could even be determined for thin (~40 nm) undoped ZnO films. An extended Drude model was needed to correctly model the SE energy range, which was explained by the dominance of ionized impurity scattering and a reduction of this scattering for higher photon energies. The grain-boundary-scattering mobility could be determined by the difference between optical and Hall mobilities.3 When combined with FPP results, the effective mobility can be determined from these optical techniques without the use of Hall measurements. The optical response above the band gap was modeled by a PSEMI or Tauc-Lorentz oscillator model, where a broadening and shift of the transition was seen for increasing carrier concentration.4
These insights and a generalized view of electron scattering in ZnO at different length scales will be presented.
1. Ponomarev et al., J. Appl. Phys. Submitted (2012)
2. Ehrmann and Reineke-Koch, Thin Solid Films 519, 1475 (2010)
3. Steinhauser et al., Appl. Phys. Lett. 90, 142107 (2007)
4. Fujiwara and Kondo, Phys. Rev. B 71, 075109 (2005)