15th Int

In the 21st century, we are facing global resource depletion in industry and environment problems. In this lecture, we would discuss problems of sustainability in optoelectronics and report results obtained in basic and application researches of iron silicides as a new technology based on elements of hope that may be one of solutions for problems of scarcity and environment hidden in the present optoelectronics. 1. Sustainable materials Present electronics and optoelectronics have been still supported by rare resources and toxic compound semiconductors such as III-V (GaAs, InGaAs etc.) or IIVI (CdTe etc.) group compound semiconductors. We are surrounded with such electronic or optoelectronic devices in our daily life. Most of semiconductors employed in those devices give huge ecological impact to environment. We have to reconsider these problems that have been hidden inside glamorous looks of optoelectronics and cannot expect its further progress without technological resolution of them. Since 1997, my colleague and myself have stated necessity of new semiconductor technology and named it “Kankyo-Semiconductor” (ecologically friendly semiconductors, in English) [1]. The word “Kankyo” in Japanese means environment. Researchers and scientists mainly in Japan have investigated Kankyosemiconductors that consist of most of abundant resources existing on the earth crust and being less toxic elements such as Silicon, Iron, Aluminum, Calcium, Magnesium, Oxygen etc. in order to establish sustainable optoelectronics. These elements are called “Elements of Hope” as respected concept [2]. Electronics based on sophisticated silicon technology can be regarded as the most sustainable one, therefore, future optoelectronics also adapt to the Si-based electronics, that is Si photonics or Si-optoelectronic integrate circuits (Si-OEICs)[3-6]. The most essential condition of new semiconductors required for Si photonics is compatibility of production processes with Si LSI processes. Moreover, the new semiconductor should be deposited or synthesized directly on Si or related materials, more preferably can epitaxially grow on Si substrates. From these conditions, iron silicides with semiconducting or ferromagnetic properties may be candidates for Si optoelectronics and photonics, furthermore for spintronics as introduced them in the following sections. 2. Binary system of Iron-Silicon An iron-silicon binary system shown in Fig. 1 reminds ourselves to grub up some silicides valuable for applications. There are three disilicides: and FeSi2 with different structures and properties [7]. In a Si rich region, -FeSi2 is a semiconductor with the bandgap of ~0.87eV @300K. This optical property of -FeSi2 may be applicable for some telecom devices such as light emitters and photodetectors near 1.5m-wavelengths. The origin of orthorhombic -FeSi2 from -FeSi2 with a cubic (fluorite) lattice and a metastable metallic phase can be understood by a significant lattice deformation due to a Jahn-Teller effect between the d like electron bands and the lattice system. The equilibrium phase above 900C is -Fe2Si5 (that exists as -FeSi2) with a tetragonal and metallic phase. The -FeSi2 forms by a rapid  to  phase transition above 850C, however, the formation of -FeSi2 by the reverse transition requires very long hours. The monosilicide -FeSi with a B20 structure (distorted NaCl structure) is a semiconductor with a narrow-gap of ~50 meV [8]. The origin of its semiconducting nature has been explained by characteristics of a Kondo insulator due to strong correlation between d-electrons. The narrow bandgap corresponds to the Kondo temperature TK=600 K. The quasi-binary system FeSi-FeGe: Fe (SixGe1-x) shows a metal-insulator transition at x=0.25at%. A Heusler alloy of Fe2(FeSi)=Fe3Si has a DO3 type ordered ferromagnetic phase. The L21 type ordered alloys Fe2MnSi with B site sensitivity and Fe2CoSi with a half metallicity (completely spin polarized state at the Fermi level) can be formed by using lattice site preference characteristics of Mn and Co atoms in the Fe3Si lattice. These silicides written above may be applied to optoelectronics, photonics or spintronics. Moreover, there are great advantages that these silicides can be synthesized directly on a silicon substrate by a silicide reaction where iron atoms are supplied onto or into Si wafers. Fig.1. Iron-silicon binary system and their silicides with some functional properties for electronic, optoelectronics, photonics or spintronics.