Trap Level Spectroscopy in Amorphous Semiconductors (Elsevier Insights)
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Add your birthday. Buy it Again. This book focuses on their imaging applications and related properties. It examines the two groups of amorphous semiconductors that are of most commercial interest:. This action might not be possible to undo.
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Are you sure you want to continue? Upload Sign In Join. Home Books Science. Save For Later. Whilst the conduction band minimum CBM in Cu 2 O is formed from spherical overlapping Cu 4 s orbitals, the valence band maximum VBM is due to non-spherical Cu 3 d orbitals which have spatial directivity, and thus they are sensitive to bonding angle disorder 8. As in disordered silicon where the VBM is mainly composed of non-spherical p orbitals 9 , this creates a broad distribution of localised tail states near the VBM of Cu 2 O films.
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The Urbach energy E u is a parameter reflecting the width of the tail states and thin Cu 2 O films show an E u larger than the thermal energy 8 , Thus, multiple carrier trapping and thermal release of holes in tail states i. 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.
In this paper, an analytical study on the conduction mechanism and the density of copper vacancies in Cu 2 O thin films is presented in order to allow an in-depth discussion on the change in electrical characteristics of Cu 2 O by high-temperature annealing. The density of copper vacancies N V C u as a function of T A 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 not only the dominant mobility degradation mechanism in Cu 2 O but also the main cause of the mobility improvement by post-deposition annealing.
Specifically, using Equations 3 and 4 , p trap is given as follows,. Since valence band states of light holes are situated at the top of the valence band 18 , the majority of holes are produced from the light hole band. The hole density trapped at the tail states p trap DOS can be calculated using Equation 8 and the Fermi-Dirac distribution function i. Here, we assumed that all ionised donor-like tail states filled with a hole i.
This yields the solution of Equation 10 as. N V is the effective density of states for free carriers in the valence band and is calculated using This suggests that GLC significantly affects hole transport in as-deposited Cu 2 O but the effect of GLC on hole transport becomes insignificant after high-temperature annealing. Inset shows the schematic van der pauw geometry for the Hall measurement. This suggests that GLC becomes insignificant after high-temperature annealing. The calculated results see Fig.
To explain the reduction in the GLC effect with an increase in T A , changes in the grain size L and the energy barrier height at the grain boundary E B were examined. SEM images see Fig.
In order to examine the change in L quantitatively, L was extracted from the Scherrer equation 23 i. The grain sizes from the SEM images see Fig. Using this equation, the E B is expressed as. E B was calculated using the extracted values i.
In b , the red dot line shows the thermal energy at room temperature. In order to provide a quantitative insight into the reduction in V Cu with an increase in T A , the density of copper vacancies, N V Cu , was extracted using the following method. Since Cu 2 O films have valence band tail states i. In addition, since another possible hole producer the oxygen interstitial, O i has deep acceptor levels, it can be assumed that V Cu with the shallow acceptor level dominates the generation of holes in Cu 2 O 1. Using Equations 12 , 20 and 22 , N V C u is expressed as.
Using these parameters i. In conclusion, this paper shows that grain-boundary-limited conduction becomes insignificant and carrier transport is governed by trap-limited conduction after high-temperature annealing. This is explained by a considerable reduction in the energy barrier height at the grain boundaries and an increase in the grain size by high-temperature annealing, suggesting that the GLC effect on hole transport in Cu 2 O can be reduced significantly by post-deposition annealing.
In addition, an increase in annealing temperature gives rise to a decrease in the total hole concentration, suggesting a reduction in copper vacancies, which is the main origin of holes in Cu 2 O. An extraction method for the density of copper vacancies N V C u is proposed and the consequent calculation of N V C u quantitatively shows a significant decrease in copper vacancy density with annealing. The chamber was pumped to a base pressure of 6.