Heavy Doping of Li⁺-ion into NiO Epitaxial Thin Films via Unequilibrium Room-temperature Processing for New Functionalization

Abstract

NiO is a typical material for new p-type oxide semiconductors. Conductivity of NiO can be raised with Li⁺ doping. In case of Li-heavy doping, we can obtain Li_xNiO₂(0.5< × <1.0). Recently the importance of LiNiO₂ has been increased as an electrode material for rechargeable lithium cells.

In this work, we tried to fabricate a novel NiO material with Li⁺-heavily doped by applying the pulsed laser-induced room temperature (R.T.) film process. Previously, we have succeeded in the epitaxial growth of various oxide thin films at R.T. such as Sn-doped In₂O₃ transparent electrodes [1]. Although the many studies have been made on the deposition of NiO epitaxial thin film at low temperatures [2], there are few reports on fabrication and the conductive characteristic for Li-heavily doped NiO epitaxial films. The film deposition at R.T., which is the unequilibrium vapor phase process, is expected to result in different crystal structure and characteristics from the films grown at high-temperatures.

A composition-adjusted thin film of Li_xNi_1-xO(0.10< × <0.40) was deposited on a sapphire (α-Al₂O₃)(0001) or MgO(100) substrates by pulsed laser deposition (PLD) technique in 10⁻⁶ Torr of oxygen at R.T. and the high temperatures of 350 and 515°C. Crystalline properties of thin films deposited at R.T. or high temperatures were examined using reflection high energy electron diffraction (RHEED) and X-ray diffraction. For the Li-heavily doped NiO films(x>0.30) grown at R.T., a clear streak RHEED pattern showing epitaxial growth was observed. But the Li-heavily doped NiO films grown at high temperatures, exhibited the ring RHEED pattern, which indicates the policrystal growth of films. Electric conductivity of various Li-doped NiO thin films deposited at R.T. or high temperatures on sapphire (0001) substrates were measured by two-probe method. The interesting results were obtained that conductivity of the film was increased remarkably with an increase of Li-doping for R.T. deposition, but was not changed so much regardless of Li-doping for high-temperature depositions.