Hiroshi Takaoka

Low temperature epitaxy by ionized‐cluster beam

Ionized cluster beam deposition offers the capability of introducing useful kinetic energy and chemical activity during film formation processes. This allows high quality epitaxy of materials at low temperature onto a wide variety of substrate surfaces and even permits the formation of thin film materials not previously possible. Capabilities are demonstrated by epitaxial Al films on various kinds of substrates, very large area deposition of GaAs, composition controlled Cd1−xMnxTe films, and CdTe/PbTe multilayer epitaxial films.

Epitaxial growth of Cd1−xMnxTe films by ionized‐cluster beams and their magneto‐optical properties

Epitaxial growth of Cd1−xMnxTe films with x ranging from 0 to 0.76 has been investigated on sapphire (0001) substrates using the ionized‐cluster beam method. Magneto‐optical properties of the films are discussed using Kramers–Kronig relation. Epitaxial films in the form of a single crystal can be obtained at low substrate temperatures up to 300 °C by the reactive composing of an accelerated CdTe cluster beam and a neutral MnTe cluster beam. When CdTe clusters are only ionized and accelerated, the composition of grown films varies largely, depending on the acceleration voltage. Faraday rotation for Cd1−xMnxTe films represents a band‐edge dispersion, and the intensity increases as Mn concentration x increases; for the film with x=0.76, the maximum value of the Verdet constant was 0.72 °/cm G, which was two orders of magnitude larger than that of a CdTe (x=0) film. The magneto‐optical figure of merit θF/α (H=5 kOe) for a Cd1−xMnxTe (x=0.76) epitaxial film was estimated to be about 400 ° at 660 nm.

Ionized-cluster beam epitaxy

Abstract Film formation by ionized-cluster beam epitaxy is characterized by various processes including sputtering, ion implantation, substrate heating, and migration effects. These processes can be utilized to form good crystalline epitaxial films. The film formation mechanism has been studied from the view point of atomistic model and it is found to be different from that observed with the other methods. Some properties of Si and ZnO are shown as typical examples of the standard type and the reactive type of ionized cluster beam epitaxy.

Optical and thermal properties of BeO thin films prepared by reactive ionized‐cluster beam technique

Preparation of BeO thin films is successfully achieved by the reactive ionized‐cluster beam technique developed by the authors. Transparent BeO films with the c‐axis preferential orientation which is peculiar to a hexagonal structure are obtained on glass substrates, and single‐crystal epitaxial films are formed on sapphire substrates at low substrate temperatures up to 400 °C. Optical and thermal transport properties of films are investigated in detail. A strong reststrahlen reflection peak is observed in the vicinity of wavelength 13.5 μm, from which the frequencies of the transverse and longitudinal vibration modes are determined. Anisotropic lattice thermal conductivities parallel and perpendicular to the c axis of the films are measured, and the values of κ∥ph ?2.6 W/cm deg and κ⊥ph ?0.6 W/cm deg at 300 K are obtained. It is also shown that the thermal conductivities of BeO films are proportional to T−2 below Debye temperature ϑD?1053 K of this material.

Vaporized-metal cluster formation and effect of kinetic energy of ionized clusters on film formation

Abstract Film formation by the ionized cluster beam (ICB) method is studied from the standpoint of the effects of kinetic energy. First, the energy and the size of the vaporized-metal cluster are described. Then the effects of the kinetic energy of the ionized cluster on the initial stage of film formation and the resulting properties of the deposited films are discussed with microscopic and macroscopic observations. Specific differences between ICB deposition and mass-analysed ion beam deposition are also described.

Crystalline and Electrical Characteristics of Silicon Films Deposited by Ionized-Cluster-Beams

Si epitaxial films on Si (111) substrates could be obtained by ionized-cluster-beam deposition at the low substrate temperatures of 300??730?C in a vacuum range of 10-7?10-6 Torr. The crystalline and electrical characteristics of Si epitaxial films were measured through use of the medium-energy ion channeling and backscattering, the Hall effect, and C-V measurements. The results show that epitaxial films of about the same quality as the substrate could be grown. The p-n junction diodes formed by the deposition of n-type Si on p-type Si substrates at low substrate temperatures had sharp junctions.

Low temperature growth of A1n and Al2O3 films by the simultaneous use of a microwave ion source and an ionized cluster beam system

Abstract We have prepared aluminium nitride (A1N) and oxide (Al2O3) films at a substrate temperature of 100°C by the simultaneous use of a microwave ion source and an ionized cluster beam system. For A1N, transparent and amorphous films with high packing density were obtained. Both A1N and Al2O3 films were chemically and thermally stable. Furthermore, the Al2O3 films could be made polycrystalline by increasing the incident energy of the oxygen ion beam and the acceleration voltage of ionized aluminium clusters. The electrical resistivity of the A1N and Al2O3 films was higher than 5 × 10 13 Ω cm and the breakdown voltage was larger than 3 × 106V cm−1. The chemical reaction between aluminium and reactive gas (i.e. nitrogen and oxygen) could be promoted by ionizing the reactive gas. Also, the ionization of aluminium clusters could improve the quality of the films as a result of combination with the reactive gas ions even at a low substrate temperature.

New developments in ionized-cluster beam and reactive ionized-cluster beam deposition techniques☆

Abstract Ionized-cluster beam (ICB) and reactive ionized-cluster beam (R-ICB) deposition techniques are described from the standpoint of the ion-based technique, as applied to the production of thin film devices. In ICB deposition, clusters (macroparticles consisting of approximately 103 atoms loosely coupled together) instead of atomic or molecular particles are used after ionization, resulting in a remarkable improvement of epitaxial film growth and of the quality of deposited films with strong adhesion. This paper describes in detail the influences of the ion content and the acceleration voltage on nucleation and film properties. MnBi films as magneto-optical memories and ZnO epitaxial films as optical devices are discussed as practical applications of the ICB and R-ICB deposition techniques.

Ionized-cluster beam epitaxy of CdTe and its application to CdTe/PbTe multilayer structure

Abstract CdTe films and CdTe/PbTe multilayered structures have been grown on Si(111) and InSb(111) substrates by ionized-cluster beam (ICB) epitaxy. The crystal properties of CdTe films are studied, and it was found that they are of good crystallinity and effective as a buffer layer for CdTe/PbTe multilayered structures. The formation of the multilayered structures was confirmed by Rutherford backscattering measurements. Also, optical absorption measurements and the theoretical calculations revealed the existence of an n = 1 miniband in the potential well of the structure. The ICB technique has several unique features for film formation, and it was found to have a high potential for preparing functional thin films and devices.

Film formation technique by ionized-cluster beam

The ionized-cluster beam (ICB) technique (deposition and epitaxial growth) does not use ionized atoms or molecules as in the conventional ion plating method. The cluster is formed from a loosely coupled aggregate of about 103 atoms by an adiabatic expansion of pure evaporant material without any inert gas mixing, through a nozzle into high vacuum (10−7−10−5 Torr). We have previously shown that the crystal structure could be changed from an amorphous state to a single crystal by changing the acceleration voltage for the deposition at a particular substrate temperature. These experimental data suggested to us the possibility of forming a hydrogenated amorphous Si film useful for electron devices. By using the ICB technique, hydrogenated amorphous Si can be prepared at hydrogen pressure lower than 10−4 Torr, whereas in the conventional fabrication methods such as glow discharge and reactive sputtering, hydrogenated amorphous Si film is deposited from reactive gas plasma at the pressure higher than 10−2 Torr. The operation condition requiring high gas pressure may cause serious contamination by gases, and complicates the diagnosing of the deposition conditions. On the other hand, the ICB technique is preferable to overcome these difficulties. In this paper, the required deposition conditions of amorphous Si films are described from a standpoint of the behaviour of incident clusters onto a substrate. The thermally stable amorphous Si film properties are also shown.