Preparation of Compositionally GradientThin Films by Ion-Beam Evaporation

Preparation of Compositionally GradientThin Films by Ion-Beam Evaporation

Kiyoshi Yatsui^a, Takehiko Honzawa^a, Makoto Hirai^a, Naoya Honda^a, Tsuneo Suzuki^a, Hisayuki Suematsu^a, Takashi Yunogami^a, b, Weihua Jiang^a

^aExtreme Energy-Density Research Institute, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan ^bInstitute for Technology, Enterprise and Competitiveness, Doshisha University, Imadegawa-Karasuma, Kamigyoku, Kyoto 602-8580, Japan

Abstract. Without mask control by intense, pulsed ion-beam evaporation (IBE) method, the compositionally gradient Si-Ge thin films have been successfully prepared by an ion-beam irradiation on Si and Ge plates. One of the compositionally gradient Si-Ge thin films indicated the compositional gradient (a) as 0.8 at. %/mm. This value was found to become an eighth of that of the previous work. When the synthesis of new phases in a system is attempted by the preparation of the compositionally gradient thin films having lower a, the crystalline of the phases seems to be easily identified. Accordingly, the present results can indicate a new possibility for the preparation of compositionally gradient thin films.

INTRODUCTION

In recent years, functional thin films as semiconductive, fluorescent and magnetic materials have been used in wide area. The functional thin films, which consist of many elements, have been known to indicate excellent properties. To discover novel electronic, optical and magnetic properties, it is necessary to optimize the composition of thin films with many elements. Combinatorial methods, by which the optimization of the composition can be achieved more efficiently and systematically, have become of major interest lately.

In the combinatorial methods, to utilize pulsed laser deposition or sputtering, compositionally gradient thin films have been prepared on substrates by controlling masks that are located between targets and the substrates [1, 2]. However, these methods have several problems: an experimental apparatus is expensive, and the system of the apparatus is very complicated.

Without the mask control by intense, pulsed ion-beam evaporation (IBE) method, we have successfully synthesized compositionally gradient Si-Ge thin films [3, 4], in which the ratio of Si atoms for Ge ones changed from 0 to 100 % on substrates. However, the compositional gradient (a) of the compositionally gradient Si-Ge thin film has been reported to be large value as 6.4 at. %/mm [3]. When the phases in a thin film are identified by X-ray diffraction (XRD), the irradiation width of X-ray is approximately 6 mm under the conditions as divergence angle of 0.5°, diffraction angle of 30° and goniometer radius of 185 mm. In the compositionally gradient Si-Ge thin film with a=6.4 at. %/mm, the composition differs approximately 38 at. % in the irradiation area of X-ray. To clarify each phase in the compositionally gradient Si-Ge thin films, it is necessary to reduce a in such thin films. Accordingly, in this investigation, by controlling the experimental conditions, it was undertaken to decrease a of the compositionally gradient Si-Ge thin films.

EXPERIMENTAL SET-UP

An intense pulsed ion beam was extracted from a magnetically-insulated diode, which was connected to a pulse power generator «ETIGO-II». A polyethylene flashboard was attached to an anode as an ion source. The high voltage of 1 MV (peak) was applied between the anode and cathode with the pulse width of approximately 50 ns. To prevent a current of electrons between the anode and cathode, the transverse magnetic field of approximately 1 T was generated by the cathode as a theta-pinch coil. The ions, which were produced by flashover of polyethylene flashboard, were mainly composed of protons (more than 85 %) and some carbon ions. The ion beam passed through the cathode with vane structure, and was geometrically focused on a target. When the target was set at the distance of 230 mm from the anode, the energy density (E) of the ion beam on the target was approximately 10 J/cm². Ablation plasma induced by the ion-beam irradiation was deposited on the substrates that were kept at RT in a vacuum of 1×10^–2 Pa.

TABLE 1. Experimental conditions.