Growth of plate-type β-FeSi2 single crystals by optimization of composition ratio of source materials

doi:10.1016/j.tsf.2007.02.080

Thin Solid Films

Volume 515, Issue 22, 15 August 2007, Pages 8259-8262

https://doi.org/10.1016/j.tsf.2007.02.080 Get rights and content

Abstract

The importance of the β-FeSi₂ bulk single crystals has increased not only to investigate the intrinsic properties of β-FeSi₂ but also to use it as substrate of β-FeSi₂ thin films for optical devices. Though single crystals of β-FeSi₂ are grown by chemical vapor transport (CVT) method, most of those crystals are needle-like and widths of those flat surfaces are 0.5 mm or less. In order to understand the mechanism of the growth process of β-FeSi₂ by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI₂(g) + 2SiI₄(g)→FeSi₂(s) + 5I₂(g), and we found that the crystal growth progresses under the environment where the FeI₂ gas is insufficient compared with a suitable ratio of FeI₂/SiI₄. Then, to raise the partial pressure of FeI₂ gas, the composition ratio of Fe to Si for the source material was increased and we have obtained the plate-type β-FeSi₂ crystals that exceeded a few square millimeters in size.

Introduction

β-FeSi₂ has been aimed at the application to the thermoelectric materials since 1950, and the researches on the bulk crystals have been done. In this decade, it has been aimed at the application to infrared light emitting and/or receiving devices, the researches which pay attention to an optical characteristic of the thin films have been done actively, and various technologies which especially lead to the improvement of the luminescence characteristic have been developed [1], [2], [3]. However, it has been understood that there are some difficulties in developing the efficient optical devices of β-FeSi₂ thin film because of making it on the Si substrate. a) Making a high quality continuous thin film is difficult because of lattice mismatch with the Si substrate [3]. b) When Fe and Si atoms are supplied on the Si substrate, Fe atoms diffuse to the substrate and it causes the non-stoichiometry of the film and a rough interface between β-FeSi₂ thin film and Si substrate. So, it is necessary to install the template layer of β-FeSi₂ [4]. c) In the analysis of the photo luminescence (PL) spectra, influences of PL from defect or dislocation of Si should be considered in order to determine the correct properties of intrinsic PL of β-FeSi₂ [5]. It is thought that these difficulties will be solved if single crystals of β-FeSi₂ could be used as the substrate. Therefore, recently the importance of the β-FeSi₂ bulk single crystals has been raised.

Single crystal growth has been carried out by using chemical vapor transport (CVT) method to evaluate the basic properties of β-FeSi₂ single crystals by many researchers [6], [7], [8]. Especially, the groups of Arushanov et al. and Heinrich et al. have done very detailed researches for electric [7], [8], magnetic [9], thermoelectric properties [10] of β-FeSi₂ single crystals. However, the grown crystals have a needle-like shape with about 10 mm long and within 1 mm diameter, so a wide and flat crystal surface was not obtained easily. On the other hand, Udono and Kikuma succeeded in the single crystal growth of β-FeSi₂ by solution method [11]. The investigation for an optical characteristic has advanced because the crystals have rather round form than those obtained by CVT method and have rather wide flat surfaces of a few square millmeters in size [12]. But, generally, the crystals obtained by solution method contain much impurities than those obtained from vapor growth.

From these background, in this study, our interests are to obtain large size single crystals from CVT method. In order to understand the mechanism of the growth process of β-FeSi₂ by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the results of observation, the composition ratio of Fe to Si in the source materials was increased.

Section snippets

In-situ observations of crystal growth process

The quartz ampoule was 150 mm long with 16 mm inner diameter. It was washed with hydrofluoric acid and rinsed with distilled water. And then the ampoule was heated up to 1000 °C and kept at this temperature for 12 h to evaporate impurities. After cooling down, it was charged with FeSi₂ ingot (3N) 5 g and iodine for transport agent 1 g (31 mg/cm³, 5N) and sealed off under vacuum 4 × 10^− 4 Pa. A schematic drawing of experimental setup is shown in Fig. 1, where an electric furnace for crystal growth

Heating process

As the iodine evaporated, it became a thick reddish brown color in the ampoule when the temperature was raised from room temperature. Following that, the ampoule gradually became transparent above T_H ∼ 240 °C, and it became completely transparent at 390 °C. At this state, in the low temperature edge of the ampoule, the transparent liquid condensation and evaporation were repeated furiously. It is thought that this liquid was SiI₄, that is, the SiI₄ gas saturated in the ampoule above this

Conclusions

The present work is intended to obtain large size single crystals from chemical vapor transport method. In order to understand the mechanism of the growth process of β-FeSi₂ by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI₂(g) + 2SiI₄(g)→FeSi₂(s) + 5I₂(g), and we found that the

Acknowledgments

We thank Dr. I. Oosugi of Salesian Polytechnic for Laue observations and D. Kusano for his assistance in taking the present data. And one of the authors (Y. Hara) would like to acknowledge Dr. H. Negishi of Hiroshima University for his valuable advice and encouragement.

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Growth of plate-type β-FeSi2 single crystals by optimization of composition ratio of source materials

Abstract

Introduction

Section snippets

In-situ observations of crystal growth process

Heating process

Conclusions

Acknowledgments

Thin Solid Films

J. Alloys Compd.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Nature

Growth of plate-type β-FeSi₂ single crystals by optimization of composition ratio of source materials