Lithium Incorporation in (111)NiO Epitaxial Layers Grown by Pulsed Laser Deposition Technique

Wide bandgap oxides such as ZnO, Ga₂O₃ and In₂O₃ have tremendous potential in UV-optoelectronics, yet the unintentional generation of certain donor type of defects in the film during growth make it challenging to achieve stable p-type doping in these materials. Nickel oxide (NiO) is a wide bandgap semiconductor of bandgap ranging from 3.6 - 4 eV. It is one of the few oxide semiconductors that exhibits stable p-type conductivity and antiferromagnetic properties with Neel temperature of 525K. The material also has high chemical and thermal stability. All these characteristics have made NiO a potential candidate for wide range of device applications such as exchange bias systems, field effect transistors, transparent hole conducting films, spintronics, UV-photodetectors and UV-LEDs. It has to be noted that stoichiometric NiO is an insulator, and p-type conduction in the film can be ascribed to the presence of nickel vacancies. However, controllable p-type doping with native defects is difficult to accomplish due to the formation of other unintentional compensating defects in the film. Intentional doping with monovalent atoms such as lithium will be interesting to explore. Keeping in mind that epitaxial films will give better device performance, there are efforts to grow epitaxial layers of NiO on different substrates such as c-sapphire, MgO and yttria-stabilised zirconia. In fact, the growth of NiO films with high epitaxial qualities have been reported by various techniques such as pulsed laser deposition (PLD), mist chemical vapour deposition, RF magnetron sputtering and molecular beam epitaxy.<br/>Here, we study the incorporation of Li in (111) NiO epitaxial layers grown by PLD technique on c-sapphire substrates as a function of growth conditions. The structural, morphological, electrical and optical properties of the films have been systematically investigated. It has been found that the crystalline quality of these films deteriorates as the growth temperature is lowered. Surface morphology of the films, studied by atomic force microscopy (AFM) and field emission gun secondary electron microscopy (FEGSEM) shows smooth and continuous nature of the films with surface roughness lying between 0.4 -1.4 nm. It has been found that the conductivity of the layers increases as the growth temperature is decreased. The enhancement of conductivity has been ascribed to the increase in density of nickel vacancy with the reduction of temperature. The investigation further suggests that there is a miscibility limit of Li in NiO. Li-clusters are detected in the films beyond a critical concentration of lithium. Further, it has been found that lithium inclusion results in hydrostatic tensile strain in the NiO lattice leading to the reduction of the bandgap. The study also shows that Li incorporation, which has also been verified by secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS), improves the electrical conductivity of the layers.