Journal of Crystal Growth 310 (2008) 2915–2922 Solidification behavior of falling germanium droplets produced by pulsated orifice ejection method Satoshi Masuda à , Kenta Takagi, Wei Dong, Kenta Yamanaka, Akira Kawasaki Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aoba Aramaki Aobaku, Sendai 980-8579, Japan Received 21 December 2007; received in revised form 12 January 2008; accepted 29 January 2008 Communicated by M. Rettenmayr Available online 5 February 2008 Abstract In order to clarify containerless solidification mechanism of falling semiconductor microdroplets, the monosized droplets of germanium in the size range from 200 to 500 mm were ejected by pulsated orifice ejection apparatus method. The density of grains in the resultant particles was dramatically reduced by decreasing the ejection temperature, i.e. initial temperature. A surface observation and orientation imaging microscopy analysis proposed that such microstructural transition was derived from the dependency of preferred growth direction on the undercooling level: the initial temperature determined the solidification undercooling. Numerical calculation with the classical nucleation theory was performed to discuss this phenomenon, but could not sufficiently elucidate the effect of initial temperature obtained experimentally. r 2008 Elsevier B.V. All rights reserved. PACS: 61.72.uf; 81.10.Mx; 81.20.Ev Keywords: A1. Nucleation; A1. Solidification; B2. Semiconducting germanium 1. Introduction Recently, a lot of attention has been given to semi- conductor microspheres used in the electronic, optical and energy fields. The spherical wafers are expected to reduce the production facility cost and increase the yield rate compared to the conventional flat wafers [1]. Furthermore, the concept of spherical semiconductor circuit has a great potential to create novel devices, because of its high flexibility in device design. In fact, the Ball ICs, 3D-accelerometers and spherical solar cells have been prototyped from the semiconductor spheres [2,3]. Semiconductor particles as a raw material of the spherical wafers require narrow size distribution, low- density grains and high purity as well as mass productivity. Several groups have recently tried to prepare silicon and germanium particles using their original methods involving a drop-tube technique [4–7]. The drop-tube technique is surely suitable for mass production of particles comparing to others such as the levitation methods. However, the drop-tube technique has not satisfied all the requirements, particularly the microstructural control for reduction in grain density. Omae et al. [4] demonstrated that in the particles applicable for the spherical solar cells should consist of a few grains. At an early stage, Li and Herlach [6] obtained the germanium particles by the drop-tube technique as well as the levitation method, and roughly estimated the solidification undercooling levels correspond- ing to several kinds of microstructures in the droplet particles. They additionally found the particles consisting of a single grain and a few grains among the obtained particles, but could not achieve the control of the grains density due to the poly-dispersed particles. Recently, Nagashio et al. [7] tried to prepare the coarse-grained silicon particles with the semi-solid ejection process com- bined with a drop-tube technique. Their study demonstrated ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.01.050 à Corresponding author. Tel.: +81 22 795 7312; fax: +81 22 795 7356. E-mail addresses: masuda@stu.material.tohoku.ac.jp (S. Masuda), ktakagi@material.tohoku.ac.jp (K. Takagi), w-dong@material.tohoku.ac.jp (W. Dong), k_yamanaka@imr.tohoku.ac.jp (K. Yamanaka), kawasaki@material.tohoku.ac.jp (A. Kawasaki).