Formation of Large Grain Ge Single Crystal on Insulating Substrate by Liquid-Solid Coexisting Annealing of a-Ge(Sn)

To realize next generation thin-film-transistors (TFTs), seedless rapid-melting growth of Ge on insulator is investigated. By rapid-thermal annealing (RTA) of amorphous GeSn (a-GeSn) layers on insulating substrates at a temperature between the solidification point and the melting point, GeSn layers melt incompletely, which generates some solid nuclei as residue. Once cooling starts, liquid-phase epitaxial growth occurs from these nuclei, which results in growth of large-grain crystals. Since segregation coefficient of Sn in Ge is very small (~0.02), almost all Sn atoms segregate at edges of the grown regions. As a result, almost Sn-free (≤1%) large Ge crystal grains are achieved on insulating substrates by a self-organizing process.  

Introduction

In order to realize next generation TFTs, large-grain crystals of novel functional materials such as SiGe, Ge, and GeSn should be grown on insulating substrates without any crystal seed. Recently, we developed seedless rapid-melting growth of a-SiGe on insulating substrate[1], where a-SiGe layers were deposited on insulating substrates followed by annealing at a temperature in solid-liquid coexisting temperature region of the Si-Ge phase diagram. During the annealing, phase separation into solid and liquid occurred. Since the annealing temperature was below the complete melting point, Si-rich solid nuclei were formed at the maximum temperature of annealing. Subsequently, in the cooling process, the SiGe grew laterally from the nuclei. As a result, large-grain SiGe crystals were obtained. However, lateral Si concentration gradient was introduced due to SiGe segregation (segregation coefficient: ~10) [2].

To suppress the lateral concentration gradient, we came up with an idea of employment of a-GeSn instead of a-SiGe as a starting material. Since the segregation coefficient (~0.02) of Sn in Ge is five-hundred times smaller compared with the SiGe case, we can expect that almost all Sn atoms segregate at the edges of grown regions, which results in pure Ge growth. To examine this idea, we investigate rapid-melting growth of a-GeSn on insulating substrates without any seed.

Experimental Procedure

In the experiment, a-Ge_0.8Sn_0.2 films (thickness: 100 nm) were deposited on Si substrates covered with Si₃N₄ films (thickness: 100 nm) by a molecular beam technique, and patterned into narrow stripe structures (length: 100µm, width: 3µm). After capping with SiO₂ layers (thickness: 800nm) by sputtering, these samples were heat-treated by RTA at 867^oC for 1s, as schematically shown in Fig. 1(a). Here, the annealing temperature (867^oC) was selected from the solid-liquid coexisting temperature region of the Ge-Sn phase diagram.

Results and Discussion

We investigated the crystal structures and Sn concentration distributions in grown layers by electron backscattering diffraction (EBSD) and Auger electron spectroscopy (AES), respectively.

Typical results are shown in Figs. 1(b) and 1(c), respectively. The EBSD image shown in Fig. 1(b) indicates formation of a very large crystal grain (~100 µm). As we expected, AES measurement reveals that almost all Sn atoms segregate at the edges of the stripe.

As a result, a very large Ge crystal grain without gradient Sn concentration profile is realized on an insulating substrate. More detailed analysis of the growth features will be discussed at the presentation.

References

[1] R. Kato, et al., ECS Transactions 50, 431 (2012).

[2] R. Matsumura, et al., Appl. Phys. Lett. 101, 241904 (2012).