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

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 LixNiO2(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 LixNi1-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.

Inelastic Scattering Components in the Si(111)-(7×7) RHEED Pattern by the Energy Filtering Method

Energy loss spectra for several parts of the Si(111)-(7 × 7) RHEED pattern, the (0 0) specular spot, the (3/7 3/7) superspot in the zeroth Laue zone, the (0 1) fundamental spot in the first Laue zone, and the Kikuchi line and background have been measured by the recently developed retarding type energy filter in the condition of the [Formula: see text] azimuth with a 10 kV incident electron beam. It was found that there are some differences in their spectra. Energy loss spectra of the (0 0) and (3/7 3/7) spots show surface plasmon loss peaks of silicon dominantly, and the spectrum of the (0 1) spot shows the same but includes weak bulk plasmon peaks. The spectrum of the Kikuchi line mainly shows bulk plasmon peaks and that of the background has no distinct structure in the profile. Glancing angle dependences of their profiles were also measured and discussed. The experimental data show that there is a relation between the quasielastic component of the diffraction beam and the pass length of the electron beam in a vacuum region near the surface where the electron interacts with the surface plasmon. The quasielastic component of the diffraction beam decreases as the incident glancing angle and/or takeoff angle become grazing.

RHEED Study of the Si(100) Surface Structure with Temperature

The structural properties of clean silicon (100) surface have been investigated by RHEED. The clean surface is reconstructed and the RHEED pattern shows superstructures associated to (2×1) and (1×2) domains. We recorded the intensity of the integer, as well as half order rods of the RHEED pattern, as a function of the crystal temperature from room temperature to 1500 K. The (1×2) domain grows at the expense of the others until it predominates. Between 1170 K and 1350 K the RHEED intensity of half order spots varies quickly and passes through a maximum. Such a behavior could be interpreted as originating from step bunching due to the heating mode (Joule effect by using DC current). At around 1350 K the half order spot decreases abruptly. The analysis of the intensity profile change versus temperature shows that the half width increases as the intensity decreases until the spot disappears. Analysis suggests that at arround 1400 K there is a lack of order at the crystal surface, which could originate in an incomplete melting.

STRUCTURE ANALYSIS OF ${\rm Si}(111)(\sqrt{3}\times\sqrt{3}$)–Al BY ENERGY-FILTERED RHEED

Rocking curves of energy-filtered reflection high-energy electron diffraction (EF-RHEED) intensities from the Si(111)([Formula: see text])–Al surface have been measured. The EF-RHEED apparatus is a recently developed one, equipped with a retarding field analyzer to exclude inelastic scattering component from an entire RHEED pattern. The energy-filtered rocking curves measured at E loss =90 eV and 0 eV are compared. It is found that when the E loss value comes close to 0 eV the relative intensities of reflection beams become remarkably weak at the lower angle side. Three structure models, which have already been presented as the T4 model, and additionally two simple rigid sphere models, are examined by dynamical calculations of RHEED. It is found that Hanada et al.'s structure model is the most favorable one among them. In the analysis of zero-loss rocking curves, especially, an extended imaginary potential distribution into the vacuum is tentatively used to express the absorption due to surface plasmon loss and fairly good agreements between the calculated and experimental rocking curves are obtained.

Evidence of 2D-3D transition during the first stages of GaN growth on AlN

In order to identify the strain relaxation mechanism, Molecular Beam Epitaxy of wurtzite GaN on AlN was monitored in situ using Reflection High Energy Electron Diffraction (RHEED). In the substrate temperature range between 620°C and 720°C, a Stransky-Krastanov (SK) transition was evidenced, resulting in a 2D-3D transition after completion of 2 monolayers, with subsequent coalescence of 3D islands, eventually resulting in a smooth surface. Quantitative analysis of the RHEED pattern allowed us to determine that island formation is associated with elastic relaxation. After island coalescence, a progressive plastic relaxation is observed. The size and density of 3D islands was varied as a function of the growth parameters. AFM experiments revealed that the size of the GaN islands, about 8 nm large and 2 nm high, was small enough to expect quantum effects. It was found that capping of the islands by AlN resulted in a smooth surface after deposition of a few monolayers allowing us to grow a »superlattice» of islands by periodically repeating the process.