Abstract
The ability to dope a semiconductor depends on whether the Fermi level can be moved into its valence or conduction bands, on an energy scale referred to the vacuum level. For oxides, there are various suitable n-type oxide semiconductors, but there is a marked absence of similarly suitable p-type oxides. This problem is of interest not only for thin-film transistors for displays, or solar cell electrodes, but also for back-end-of-line devices for the semiconductor industry. This has led to a wide-ranging search for p-type oxides using high-throughput calculations. We note that some proposed p-type metal oxides have cation s-like lone pair states. The defect energies of some of these oxides were calculated in detail. The example SnTa2O6 is of interest, but others have structures more closely based on perovskite structure and are found to have more n-type than p-type character.Graphic abstract
Highlights
The valence band edge of Ga2O3 is just too deep to allow p-type doping (Figure 1), but p-type doping can form blocking undoped layers which are of use in device design.[50]
For use in electronic devices, the most important factors are the possible compensation of doping by compensating native defects
The stability range matters because this sets the amount of Fermi energy shift that is allowed before the formation energy of compensating native defects constrains the doping limits
Summary
There has been considerable effort to develop transparent oxide semiconductors for use in large area electronics such as displays or solar cell electrodes.[1–5] This field has recently been extended to include back-end-of-line (BEOL) semiconductor devices.[6–9] A typical application of these electronic rather than optical devices is that they act as switches with a low off-state leakage current for charge storage devices (down to 10−20 A).[7]. SnO acts as a p-type semiconductor, and its Sn 5s states hybridize with the oxygen 2p states to give a mixed Sn s/O 2p valence band maximum with a low m* value.[23,24]. The key point is that the stability range of SnTa2O6 is large, so that EF can vary over a wide energy range before compensating native defects constrain the doping This is why when compensating defects rather than m* values are the dominant criteria in doping, it is the stability range of the oxide that matters, not the local band extrema
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