Why Do Crystals Grow Faster in Space？ ― A New Mechanism of SiGe Crystal Growth Revealed by Experiments in “Kibo”

Experiment at Kibo

The first scientific article relating to the results of the Hicari-II ^*1 experiment, conducted using the Gradient Heating Furnace (GHF) ^*2 installed in the Japanese Experiment Module ‘Kibo’ on the International Space Station (ISS), has been published in the journal Materialia. This article is an open access (see the paper for details).

When silicon–germanium (SiGe) semiconductor crystals were grown in microgravity, a high growth‑rate — not observed in ground experiments—was detected 1.5 (open circle) and 5 (triangle) hours after the onset of crystal growth (Fig. 1).
Hicari‑II experiment revealed that the higher growth rate originates from the transient constitutional supercooling ^*3 enhanced under microgravity.

These results quantitatively reveal both the conditions leading to the constitutional supercooling and the associated increase in growth rate under microgravity. The findings enable estimation of growth rates that prevent polycrystallization and solidification instabilities that degrade crystal quality, thereby advancing semiconductor crystal-growth research in space.

Why Does Crystal Growth Rate Increase in Space？

Fig. 1 Time dependence of the SiGe crystal growth rate in Hicari-II Run#2, with time zero defined as the onset of growth.
The dashed line indicates the interface stabilized growth rate under conditions on ground. In microgravity, a growth rate of 0.15 mm/h (triangles) was achieved without interface destabilization, corresponding to an approximately 20% increase when compared to terrestrial experiments.

In Hicari-II experiment, if the SiGe solution melt is in equilibrium (i.e., stable temperature and concentration distributions over the melt), the crystal growth rate is calculated by atomic diffusion constant and the temperature gradient at the solidification front in the melt.
While the melt deviates from equilibrium condition, excess solute atoms transported to the crystal growth interface, resulting in an increased growth rate.

A previous ISS experiment using semiconductor indium-gallium-antimonide (InGaSb) reported higher growth rates in microgravity than on ground. To elucidate this phenomenon, for example, increased atomic diffusion coefficients ([1]) or changes in the temperature gradient in space were proposed.

In the present experiment, however, the growth rate observed after several tens of hours (0.12 mm/h) was nearly similar to the rate on the ground as shown in Fig. 1. This result indicate that similar melt concentration and temperature distributions on ground have occurred even in the ISS.

Precise observations of the solidification interface and the concentration (marked by temperature pulse during growth) provided further insight that the temperature gradient on the solidification interface remained nearly constant throughout the experiment.

A well-known mechanism for solute condensation is Soret effect^*4 in which a temperature gradient induces mass transport. However, phase field simulations conducted by co-authors from National Institute for Materials Science (NIMS) showed that the Soret effect is negligible and does not contribute significantly to concentration inhomogeneity under our SiGe crystal growth experiment in ISS.

Instead, due to the lack of natural convection, the melt near the seed crystal remained in a solute-condensation (constitutional supercooling) for several hours after growth onset. That localized constitutional supercooling near the solidification interface induced an increase in the growth rate.

Utilizing Microgravity Environment for Semiconductor Crystal Growth

This work highlights that crystal growth from the melt in microgravity, careful attention must be paid to solute segregation caused by the absence of convective mixing.
Such conditions can increase growth-rate and cause growth interface instability even without spontaneous nucleation within the melt.

Conversely, microgravity offers a unique opportunity: it allows the controlled creation of solute rich conditions, which can be harnessed to deliberately enhance crystal growth rates (Fig. 2).

This work provides important insights into the efficient manufacturing of semiconductor crystals through a deep understanding of crystal growth phenomena enabled by microgravity.

Fig. 2 Schematic of the SiGe crystal growth experiment (including melt and crystal) on ground and under microgravity conditions.
In the ISS experiment, faceted, non uniform (triangle-wave) growth patterns were observed in the SiGe crystal. Under stable growth conditions, the interface becomes smooth.

*1 Study on SiGe crystal growth under microgravity (Hicari-II)
Principal Investigator: ARAI Yasutomo (Senior Researcher, JAXA)

*2 Gradient Heating Furnace (GHF)

*3 Constitutional supercooling: a condition in which the melt temperature is below the liquidus temperature of the alloy due to solute condensation

*4 Soret effect: a phenomenon in which a temperature gradient induces atomic or mass transport

Reference
[1] R. Ghritli, Y. Okano, Y. Inatomi, A. Sekimoto, S. Dost, Estimation of the diffusion coefficient of GaSb in InSb melt using Bayesian optimization and the ISS experimental results, J. Cryst. Growth 573 (2021) 126280.

Paper Information

The results of the Hicari-II experiment have been published in Materialia as an open-access paper.

Journal Materialia, Volume 46, May 2026, 102709
Title Increasing SiGe Crystal Growth Rate Utilizing Microgravity
Authors ARAI Yasutomo, KINOSHITA Kyoichi, TAISHI Toshinori, MATSUOKA Yusuke, ABE Taichi, TSUKADA Takao, and KUBO Masaki
DOI 10.1016/j.mtla.2026.102709

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