Nickel oxide (NiO) films are prepared on n-type GaN by an e-beam evaporation technique and its structural and chemical characteristics analysed by XRD, TEM and XPS measurements first at room temperature. XRD and TEM results reveal that the NiO films are oriented and that the NiO/n-GaN interface has a good quality. XPS analysis demonstrated that the NiO films clearly showed Ni 2p_3/2 and 2p_1/2 peaks at 854 eV and 872 eV along with the O 1s peak at ∼529.1 eV. Then, we fabricated an Au/NiO/n-GaN heterojunction Schottky diode with a NiO insulating layer and compared its electrical properties with the Au/n-GaN Schottky junction. The Au/NiO/n-GaN heterojunction presents excellent rectifying behaviour with a low reverse-leakage current compared to the Au/n-GaN Schottky junction. Calculation revealed that a higher barrier height is achieved for the Au/NiO/n-GaN heterojunction than for the Au/n-GaN Schottky junction, implying the barrier height was modified by the NiO insulating layer. Using Cheung's and Norde functions and an Ψ_S–V plot, the barrier heights are estimated and we found that the values are comparable with one another. The results suggest that the interface state density (N_SS) of the Au/NiO/n-GaN heterojunction decreases compared to the Au/n-GaN Schottky junction, which indicates the NiO insulating layer plays a significant role in the reduced N_SS. The results demonstrate that Poole–Frenkel emission governs the reverse leakage current in both junctions, which could be associated with structural defects and trap levels in the insulating layer.

Recently, significant progress has been made in increasing the figure-of-merit (ZT) of various nanostructured materials, including thin-film and quantum dot superlattice structures. Studies have focused on the size reduction and control of the surface or interface of nanostructured materials since these approaches enhance the thermopower and phonon scattering in quantum and superlattice structures. Currently, bismuth–tellurium-based semiconductor materials are widely employed for thermoelectric (TE) devices such as TE energy generators and coolers, in addition to other sensors, for use at temperatures under 400 K. However, new and promising TE materials with enhanced TE performance, including doped zinc oxide (ZnO) multilayer or superlattice thin films, are also required for designing solid-state TE power generating devices with the maximum output power density and for investigating the physics of in-plane TE generators. Herein, we report the growth of Al₂O₃/ZnO (AO/ZnO) superlattice thin films, which were prepared by atomic layer deposition (ALD), and the evaluation of their electrical and TE properties. All the in-plane TE properties, including the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ), of the AO/ZnO superlattice (with a 0.82 nm-thick AO layer) and AO/ZnO films (with a 0.13 nm-thick AO layer) were evaluated in the temperature range 40–300 K, and the measured S, σ, and κ were −62.4 and −17.5 μV K⁻¹, 113 and 847 (Ω cm)⁻¹, and 0.96 and 1.04 W m⁻¹ K⁻¹, respectively, at 300 K. Consequently, the in-plane TE ZT factor of AO/ZnO superlattice films was found to be ∼0.014, which is approximately two times more than that of AO/ZnO films (ZT of ∼0.007) at 300 K. Furthermore, the electrical power generation efficiency of the TE energy generator consisting of four couples of n-AO/ZnO superlattice films and p-Bi_0.5Sb_1.5Te₃ (p-BST) thin-film legs on the substrate was demonstrated. Surprisingly, the output power of the 100 nm-thick n-AO/ZnO superlattice film/p-BST TE energy generator was determined to be ∼1.0 nW at a temperature difference of 80 K, corresponding to a significant improvement of ∼130% and ∼220% compared to the 100 nm-thick AO/ZnO film/p-BST and n-BT/p-BST film generators, respectively, owing to the enhancement of the TE properties, including the power factor of the superlattice film.

CdS quantum dots (QDs) are grown on mesoporous TiO₂ films by 3 to 13 repeated cycles of in situ chemical bath deposition (CBD). The overall energy conversion efficiency of CdS quantum dot-sensitized solar cells (QD-SSCs) increases as the number of cycles increases to the eight — peaking at 2.13%. This is attributable to efficient light harvesting and charge-collection resulting from enhanced light absorption and faster charge transport. However a further increase of CBD cycles to thirteen reduces QD-SSCs performance, despite better light absorption in the long wavelengths. This is attributed to decreased charge-injection efficiency, which is due to reduced driving forces of carrier injection, increased trap-mediated recombination in the QDs, and hindered ion transport.