(111) NiO epitaxial layers embedded with crystallographically oriented Ni-clusters are grown on c-sapphire substrates using pulsed laser deposition technique. It has been found that certain growth and post-growth cooling conditions can be adjusted to vary the size, shape and density of these clusters. (111) Ni epitaxial layer can also be deposited on c-sapphire substrate by this technique when no oxygen is supplied into the chamber during growth. Structural and magnetic properties of the clusters are examined by a variety of techniques including atomic probe tomography and superconducting quantum interference device magnetometry. All cluster embedded films are found to exhibit ferromagnetic behaviour even at room temperature. Magnetic characteristics of these films depend strongly on the density, size and shape of the clusters. It has also been observed that electrical conductivity of these samples enhances by several orders of magnitude when the cluster density crosses the percolation threshold. While, strong ferromagnetic behaviour is maintained even in highly resistive samples. Therefore, the material system has a potential to serve as an efficient spin-injector, the missing link for the realization of semiconductor based spintronics.

Li-doped NiO/[Formula: see text]-Ga2O3 polycrystalline bilayer thin-film pn-heterojunctions with different Li-doping concentrations are grown on Si-substrates using the pulsed laser deposition technique. Resistive switching property of these devices has been investigated in detail. This study shows that the Li-doping concentration in NiO layer significantly influences the performance of these devices. For an optimum Li-doping of 1.5%, a stable memory window of ∼102 with endurance of more than 100 cycles and long retention time can be achieved. The coefficient of variation ([Formula: see text]) of SET and RESET voltages also found to ∼ 20% and ∼ 40%, respectively, satisfying the acceptability benchmark. A transition from complementary resistive switching (CRS) to bipolar resistive switching (BRS) after multiple sweeping operations has been observed in devices with intermediate Li-doping concentrations. Observation of CRS has been explained in terms of the formation of Li-rich metallic layer at the NiO/Ga2O3 interface as a result of out-diffusion of Li. Redistribution of the Li-ions from the Li-rich interfacial zone to whole of the NiO layer after first few sweeping cycles must be the reason for CRS-to-BRS transition. Results further suggest that return to high resistive state via Poole–Frenkel (PF) pathway during the RESET process might be the key to achieve high performance in p–n junction based resistive switching devices. This study, thus, presents Li-doping as a possible route to modulate the resistive switching property of bilayer Li:NiO/Ga2O3 based memory devices.

NiO and Ni cluster embedded NiO epitaxial films grown on c-sapphire substrates by PLD technique are annealed at different temperatures in oxygen environment. Structural, morphological and electrical properties of these samples are investigated as functions of the annealing temperature. It has been observed that in all cases, sample surface becomes smoother with the increase in annealing temperature. Resistivity of the Ni-cluster free NiO epilayers is found to decrease with the increase in annealing temperature beyond ~300℃. Annealing at 450℃, results in about two orders of magnitude decrease of resistivity from the highly insulating state of the as grown. Resistivity shows a gradual increase when annealed at temperatures higher than 450℃. Reduction of resistivity with annealing can be attributed to the enhancement of the density of Ni-vacancy, which are known to act as shallow acceptors in NiO. Formation energy of the Ni-vacancies, which has been estimated from the ln⁡(ρ^(-1) ) versus 1/T_A plot, comes out to be 623 meV. Density of 60°-dislocations in these films increases sharply when annealed above 500℃. Reduction of conductivity of these films for annealing above 500℃ can thus be explained in terms of the enhancement of the rate of scattering of carriers by these defects. Resistivity of the Ni-cluster embedded highly conducting NiO epilayers are found to increase rapidly by more than several orders of magnitude beyond an annealing temperature of ~200℃. This has been ascribed to percolation driven metal-to-insulator transition arising as a result of the reduction of the cluster density with annealing.

Incorporation of Lithium in (111)NiO epitaxial layers grown by pulsed layer deposition (PLD) technique on c-sapphire substrates is studied as functions of growth conditions. The effect of Li-inclusion on the structural, morphological, electrical and optical properties of the films have been systematically investigated. It has been found that the concentration of Li in the film is more at lower growth temperatures. However, the crystalline quality deteriorates as the growth temperature is lowered. The investigation suggests that there is a miscibility limit of Li in NiO. Beyond a critical concentration of Lithium, Li-clusters are detected in the films. Further, it has been found that inclusion of Li gives rise to hydrostatic tensile strain in the NiO lattice that results in the reduction of the bandgap. The study also suggests that Li incorporation improves the electrical conductivity of NiO layers. Ni-vacancy defects also play important role in governing the conductivity of these samples.