The structural, optical, and electrical properties of ZnO are intimately intertwined. In the present work, the structural and transport properties of 100 nm thick polycrystalline ZnO films obtained by atomic layer deposition (ALD) at a growth temperature (Tg) of 100–300 °C were investigated. The electrical properties of the films showed a dependence on the substrate (a-Al2O3 or Si (100)) and a high sensitivity to Tg, related to the deviation of the film stoichiometry as demonstrated by the RT-Hall effect. The average crystallite size increased from 20–30 nm for as grown samples to 80–100 nm after rapid thermal annealing, which affects carrier scattering. The ZnO layers deposited on silicon showed lower strain and dislocation density than on sapphire at the same Tg. The calculated half crystallite size (D/2) was higher than the Debye length (LD) for all as grown and annealed ZnO films, except for annealed ZnO/Si films grown within the ALD window (100–200 °C), indicating different homogeneity of charge carrier distribution for annealed ZnO/Si and ZnO/a-Al2O3 layers. For as grown films the hydrogen impurity concentration detected via secondary ion mass spectrometry (SIMS) was 1021 cm−3 and was decreased by two orders of magnitude after annealing, accompanied by a decrease in Urbach energy in the ZnO/a-Al2O3 layers.
Flash memories are the preferred choice for data storage in portable gadgets. The charge trapping nonvolatile flash memories are the main contender to replace standard floating gate technology. In this work, we investigate metal/blocking oxide/high-k charge trapping layer/tunnel oxide/Si (MOHOS) structures from the viewpoint of their application as memory cells in charge trapping flash memories. Two different stacks, HfO2/Al2O3 nanolaminates and Al-doped HfO2, are used as the charge trapping layer, and SiO2 (of different thickness) or Al2O3 is used as the tunneling oxide. The charge trapping and memory windows, and retention and endurance characteristics are studied to assess the charge storage ability of memory cells. The influence of post-deposition oxygen annealing on the memory characteristics is also studied. The results reveal that these characteristics are most strongly affected by post-deposition oxygen annealing and the type and thickness of tunneling oxide. The stacks before annealing and the 3.5 nm SiO2 tunneling oxide have favorable charge trapping and retention properties, but their endurance is compromised because of the high electric field vulnerability. Rapid thermal annealing (RTA) in O2 significantly increases the electron trapping (hence, the memory window) in the stacks; however, it deteriorates their retention properties, most likely due to the interfacial reaction between the tunneling oxide and the charge trapping layer. The O2 annealing also enhances the high electric field susceptibility of the stacks, which results in better endurance. The results strongly imply that the origin of electron and hole traps is different—the hole traps are most likely related to HfO2, while electron traps are related to Al2O3. These findings could serve as a useful guide for further optimization of MOHOS structures as memory cells in NVM.
A worldwide effort is under way to understand the activation of acceptor states in ZnO with the motivation to achieve persistent p‐type conductivity. In this study, cathodoluminescent (CL) imaging, electron microscopy (SEM), secondary‐ions mass spectrometry (SIMS), and X‐ray diffraction (XRD) are used to compare ZnO:N films subjected to a rapid thermal annealing process (RTP) in N2 and O2 atmosphere at 400–900 °C. The study, performed for ZnO:N films with nitrogen concentration of 2 × 1018 at cm−3 grown under O‐rich conditions is directed to establish the optimal atmosphere and temperature at which acceptor‐related CL is enhanced and correlated with structural properties. XRD shows that crystallite size increases from ≈100 to ≈250 nm with increasing annealing temperature up to 800 °C. The low‐temperature (LT) CL maps reveal that the acceptor‐ and donor‐related CL mostly derives from different crystallites. Both annealing medium and temperature influence acceptor‐related CL intensity, which is higher under oxygen annealing. It is observed that the intensity of acceptor‐related CL increases with annealing temperature up to 800 °C and then decreases. Noticeable donor‐related emission appears only after RTP at 700 °C and becomes prominent for RTP at 900 °C
The origin of unintentional hydrogen incorporation in ZnO films grown by thermal atomic layer deposition is investigated by comparing layers deposited using H2O and D2O as oxygen precursors. Secondary ion mass spectroscopy measurements of as‐grown and 800 °C annealed layers provide evidence that the hydrogen contribution originating from the oxygen precursor is weakly chemically bound and is mostly removed by rapid thermal annealing conducted at 800 °C for 3 min in oxygen atmosphere. On the other hand, the remaining hydrogen introduced by ethyl groups originating from the metalorganic zinc precursor withstands such treatment.
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