The Feature Article [1] describes the current state of the Atomic Layer Deposition (ALD) technique for producing high-quality thin layers with the focus on oxide materials such as ZrO2 and other rare earth oxides, SnO2 and ZnO for high-k dielectrics, gas sensors and various optoelectronic applications.



The cover picture is a photograph of a thin-film-on-silicon sample. The inset schematically illustrates the ALD growth of an oxide film on a surface where L refers to the precursor ligand.



The first author, Lauri Niinisto, is Professor of Inorganic Chemistry at Helsinki University of Technology. His current research is focused on the preparation and characterization of thin films for optoelectronic and electronic devices.

Advanced electronic and optoelectronic materials by Atomic Layer Deposition: An overview with special emphasis on recent progress in processing of high-k dielectrics and other oxide materials

A comparative study on lanthanide oxide thin films grown by atomic layer deposition

Optical properties of TiO2, Y2O3 and CeO2 thin films deposited by electron beam evaporation

This Review highlights basic and transition metal conducting and semiconducting oxides. We discuss their material and electronic properties with an emphasis on the crystal, electronic, and band structures. The goal of this Review is to present a current compilation of material properties and to summarize possible uses and advantages in device applications. We discuss Ga2O3, Al2O3, In2O3, SnO2, ZnO, CdO, NiO, CuO, and Sc2O3. We outline the crystal structure of the oxides, and we present lattice parameters of the stable phases and a discussion of the metastable polymorphs. We highlight electrical properties such as bandgap energy, carrier mobility, effective carrier masses, dielectric constants, and electrical breakdown field. Based on literature availability, we review the temperature dependence of properties such as bandgap energy and carrier mobility among the oxides. Infrared and Raman modes are presented and discussed for each oxide providing insight into the phonon properties. The phonon properties also provide an explanation as to why some of the oxide parameters experience limitations due to phonon scattering such as carrier mobility. Thermal properties of interest include the coefficient of thermal expansion, Debye temperature, thermal diffusivity, specific heat, and thermal conductivity. Anisotropy is evident in the non-cubic oxides, and its impact on bandgap energy, carrier mobility, thermal conductivity, coefficient of thermal expansion, phonon modes, and carrier effective mass is discussed. Alloys, such as AlGaO, InGaO, (Al xIn yGa1− x− y)2O3, ZnGa2O4, ITO, and ScGaO, were included where relevant as they have the potential to allow for the improvement and alteration of certain properties. This Review provides a fundamental material perspective on the application space of semiconducting oxide-based devices in a variety of electronic and optoelectronic applications.

A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3

The increasing use of reflecting optics in ultraviolet (UV), visible, and infrared (IR) devices has stimulated a great amount of research on coatings for front-surface mirrors. Methods for measuring the reflectance of front-surface mirrors at various wavelengths and angles of incidence are discussed, and techniques for preparing reflecting films with maximum reflectance and durability are described. Data on the UV, visible, and IR reflectance of the most frequently used mirror coatings, Al, Ag, Au, and Rh, are presented. The application of single-layer and multilayer dielectric overcoatings for increasing their durability and normal-incidence reflectance are described, and the effect of these coatings on reflectance at higher angles of incidence is treated. It is shown that Al and Ag overcoated with rather thin layers of silicon oxides or Al2O3 (t = 1000–2000 A) have, in the IR from 8 to 12 μm, practically the same high reflectance as the unprotected metal at close to normal incidence but greatly decreased reflectance at angles larger than 40°. Since only the parallel component is responsible for the IR-reflectance decrease, such film combinations are suitable for producing highly efficient reflection polarizers for the IR, such as for CO2 10.6-μm laser radiation. The effect of water absorption in dielectric overcoatings on the mirror reflectance at 3 μm is discussed.

Reflectance and preparation of front-surface mirrors for use at various angles of incidence from the ultraviolet to the far infrared

Sc(2)O(3) and Y(2)O(3) films were prepared by evaporation from resistance heated boats in a residual gas atmosphere of oxygen. Films of much lower absorptance were obtained than from coatings produced in high vacuum. The short wavelength limit of high transparency was between 300 nm and 400 nm, while the long wavelength absorption edge was reached at about 11 microm. Refractive indices of 1.8-2.0 were measured for both oxides. The influence of annealing on the optical- properties of the films was investigated. The films were amorphous and proved to be mechanically very stable and water resistant. Electron micrographs showed smooth and uniform surfaces. By antireflection coatings of yttria the transmittance of silicon was increased from 52% to 99.4% at the maximum. Possible advantages by the application of Sc(2)O(3) and Y(2)O(3) instead of SiO for antireflection coatings on GaAs laser surfaces are discussed.

W. Heitmann

Papers

Reactively evaporated films of scandia and yttria.