Abstract:
The aim of the study is to synthesize and characterize high quality undoped, Ce doped, Sm doped, Ce
and Sm co-doped, and Ce and Al co-doped ZnO thin films and nanorods for use in optoelectronic
devices, using the low-cost sol-gel and chemical bath deposition techniques.
Undoped, Ce doped (up to 2.0 at.%), Sm doped (up to 2.0 at.%), Ce and Sm co-doped (up to
2.0 at.% each), and Ce and Al co-doped (up to 7.0 at.% Ce and 1.0 at.% Al) ZnO thin films on glass
and n-Si substrates were investigated. X-ray diffraction revealed the incorporation of the dopant
into the ZnO lattice, with the exception of the highly doped Ce and Al co-doped sample, where the
lattice parameters decreased and CeO2 was observed. Doping increased the lattice parameter and, at
low concentration (2.0 at.% or less), improved crystalinity, but not at higher doping
concentration. Room temperature photoluminescence spectroscopy revealed both UV and visible light
emission from the Ce and Al co-doped samples, while for the other films only a peak in the UV (at
400 nm) was observed. The visible light emission was at 450 - 850 nm and was attributed to defect
formation. Schottky diodes fabricated on the films deposited on n-Si showed good rectification
behaviour for all films. The ZnO films co-doped with Ce and Sm manifested excellent rectification
of six orders of
magnitude, a barrier height of 0.82 eV, ideality factor of 1.62 and series resistance of 60 Ω.
ZnO nanorods that were grown as un-doped, Sm doped (up to 5.5 at.%), Ce doped (up to 10 at.%), Ce
and Sm co-doped (up to 0.8 at.%) were grown by means of chemical bath deposition on glass,
ITO-coated glass and n-Si substrates that were seeded with ZnO by sol-gel spin coating. X-ray
diffraction revealed the incorporation of the dopants into the ZnO lattice. In all cases doping
increased the lattice parameter and improved crystalinity. Raman spectroscopy showed the
characteristic E2 (high) mode peak was the prominent and its position shifted towards a lower wave
number after doping. For all samples, the photoluminescence showed peaks in both the UV and
visible (450 - 800 nm) region. Schottky diodes fabricated on the nanorods deposited on both the
ITO-coated glass substrates and the n-Si substrate showed good rectification. The 1.5 at.% Sm doped
ZnO on ITO had an ideality factor of 2.5 and barrier height of 0.72 eV, while the 10.0 at.% Ce
doped ZnO on ITO had
the lowest ideality factor of 1.34, Schottky barrier height of 0.856 eV and series resistance of
130 Ω.
The I-V characteristics of the Ce and Sm co-doped nanorods grown on n-Si showed an increase in the
generation-recombination current with increasing dopant concentration. The current transport
mechanism in the diode was dominated by approximately ohmic leakage conduction mechanism at a lower voltage (0.0 to 0.6 V), while at a voltage higher than 0.6 V, the space-charge limited current and
the trap-filled limit voltage mechanism dominated.
