Abstract
A quantitative and analytical investigation on the conduction mechanism in p-type cuprous oxide (Cu2O) thin films is performed based on analysis of the relative dominance of trap-limited and grain-boundary-limited conduction. It is found that carrier transport in as-deposited Cu2O is governed by grain-boundary-limited conduction (GLC), while after high-temperature annealing, GLC becomes insignificant and trap-limited conduction (TLC) dominates. This suggests that the very low Hall mobility of as-deposited Cu2O is due to significant GLC, and the Hall mobility enhancement by high-temperature annealing is determined by TLC. Evaluation of the grain size and the energy barrier height at the grain boundary shows an increase in the grain size and a considerable decrease in the energy barrier height after high-temperature annealing, which is considered to be the cause of the significant reduction in the GLC effect. Additionally, the density of copper vacancies was extracted; this quantitatively shows that an increase in annealing temperature leads to a reduction in copper vacancies.
Highlights
Cu2O having a significantly lower hole mobility compared with this theoretical limit
In a recent report[8], we showed that high-temperature annealing in vacuum leads to a significant improvement in the field-effect mobility and a reduction in the off-state current, mainly resulting from a film mobility (i.e. Hall mobility, μHall) enhancement and a decrease in intrinsic carrier density, respectively
The effect of grain-boundary-limited conduction (GLC) on μHall is quantified using a GLC coefficient which is extracted from the difference between ptrap(Hall) and ptrap(DOS)
Summary
Cu2O having a significantly lower hole mobility compared with this theoretical limit. The effect of the potential barriers such as grain boundary scattering (i.e. grain-boundary-limited conduction, GLC) impedes hole transport[5, 6]. Multiple carrier trapping and thermal release of holes in tail states (i.e. trap-limited conduction, TLC) degrades transport in Cu2O11–13. In a recent report[8], we showed that high-temperature annealing in vacuum leads to a significant improvement in the field-effect mobility and a reduction in the off-state current, mainly resulting from a film mobility (i.e. Hall mobility, μHall) enhancement and a decrease in intrinsic carrier density (i.e. free hole concentration, pfree), respectively. The density of copper vacancies NVCu as a function of TA was extracted using an equation derived from the charge neutrality condition, with consideration for ionized valence band tail states, and the formula for the ionized acceptor concentration. This work is important for an understanding of the dominant mobility degradation mechanism in Cu2O and the main cause of the mobility improvement by post-deposition annealing
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