Electrical, structural, and compositional properties evolution of Al-doped zinc oxide
upon excimer laser annealing, rapid thermal annealing, and damp heat exposure
S. Elhamali
1
, N. Pliatsikas
2
, W. Cranton
3
, R. Ranson
1
, P. Patsalas
2
, D. Koutsogeorgis
1
1
School of Science & Technology, Nottingham Trent University, Clifton Lane, Nottingham, NG11 8NS, UK.
2
Aristotle University of Thessaloniki, Department of Physics, GR-54124 Thessaloniki, Greece
3
Materials and Engineering Research Institute, Sheffield Hallam University, Howard Street, Sheffield, S1 1WB
Email
Zinc oxide based transparent conductive oxides (TCOs), such as aluminium doped
zinc oxide (AZO), have received an increased interest as potential substitutes to ITO. ZnO
based TCOs advantages include relatively low cost and low growth temperature, comparable
electrical conductivity to that of ITO, and easy etchability. The electrical properties of TCOs
are strongly influenced by the structural and compositional characteristics, which are in turn
controlled by the fabrication conditions.
Aiming at high-quality AZO transparent electrodes, the reported work explores the
electrical, structural, and compositional characteristics of AZO samples upon post-deposition
processing via excimer laser annealing (ELA) and rapid thermal annealing (RTA). 180 nm
thick AZO samples were deposited by RF magnetron and a range of parameters was
investigated including the oxygen concentration, RF power, and sputtering pressure. ELA
was performed in air at ambient temperature using a Krypton fluoride laser (λ=248 nm –
pulse duration 25 ns). The ELA parameters investigated were the laser fluence (25–150
mJ/cm
2
) and the number of pulses (1–100), applied to 10 mm x 10 mm treatment areas. RTA
was conducted in nitrogen using an Annealsys rapid thermal annealer (model AS-One), at
different target temperatures (250–400) and for different dwell times (20–120 s). Thereafter,
the long-term stability of AZO samples against damp heat exposure was examined, via
performing the IEC 61646 test (85°C and 85% relative humidity for 1000 h).
ELA with five pulses at 125 mJ/cm
2
, or RTA at 300°C for 20 s resulted in resistivity
reduction from 1x10
-3
Ω.cm for the optimised as-deposited samples to 5x10
-4
Ω.cm. While,
the average visible transmission was improved to > 85 %. Detailed analysis of data collected
via four point probe, Hall Effect, XRD and XPS have revealed the mechanisms of
improvement via annealing. ELA and RTA, with different annealing mechanisms for each,
resulted in average grain size enlargement, microstructural defects reduction, and effective Al
dopant incorporation into the ZnO lattice. Consequently, the free electron density and
mobility are increased and thus the resistivity is decreased. The damp heat stability was
strongly dependant on the samples structural properties.
In conclusions we report two large-area compatible annealing techniques to further
enhance the electro-optical properties of room temperature deposited AZO thin films,
resulting in conductivity values close to that of ITO. The origin of this enhancement is
systematically investigated.
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