(Invited) Ge1-xSnx Optical Devices: Growth and Applications

Ge has been considered as a promising candidate for the active region in optical interconnect applications such as lasers and photodetectors, since its band gap corresponds to a wavelength with less loss in SiO₂ waveguides. However, to increase the efficiency of these applications, a direct band gap material is required.

Incorporation of Sn into Ge has been considered to obtain a direct band gap material [1,2]. According to theoretical calculations [3], the critical Sn content to achieve the direct transition depends on the strain. When the GeSn layer is fully relaxed, around 11% of Sn is required. On the other hand, when the layer experiences compressive strain, the critical Sn content increases. One option to obtain highly strain-relaxed GeSn with high Sn content is to increase the thickness of the GeSn layer. In addition, the active region of the optical devices has to be thick to obtain a high efficiency. However, it is well known that Sn segregation to the surface easily occurs [4], which results in Sn precipitation/agglomeration on the surface when a thick layer is grown.

The Sn agglomeration is the result of the collision among Sn atoms on the surface. Therefore, we focused on the freezing of Sn atoms during GeSn growth. GeSn layers were grown with Ge₂H₆ and SnCl₄ precursors on Ge(001) substrates at 320°C by atmospheric CVD in an Epsilon like equipment from ASM. The target thickness of the GeSn layers is 200 nm. N₂ and H₂ with different flows ranging from 10 to 40 slm were used as carrier gases to study the impact of the partial pressure of the precursors on Sn agglomeration. It is expected that using low carrier gas flows enhances the Sn freezing due to the higher Ge₂H₆ and SnCl₄ partial pressures. In addition, it is expected that switching the carrier gas from N₂ to H₂suppresses the Sn migration on the surface due to the resulting surface H-termination.

We found opposite tendencies for the surface Sn agglomeration as a function of the H₂ and N₂ carrier gas flow. A lower N₂ suppresses the surface Sn agglomeration. As we expected, a high partial pressure of the precursors enhances the freezing of Sn atoms into the subsurface. On the other hand, when H₂ is used as a carrier gas, a higher H₂ flow suppresses the surface Sn agglomeration. The growth temperature (320°C) is close to the temperature at which adsorbed H starts to desorb thermally from the Ge surface (340°C) [5, 6]. However, a high H₂ partial pressure may suppress the desorption of surface H which is provided by the deposition by Ge₂H₆. The Sn segregation should be suppressed by the surfactant effect of surface H [6]. The different behavior of these two carrier gases will be discussed in detail.

[1] D. W. Jenkins and J. D. Dow, Phys. Rev. B, 36, 7994 (1987).

[2] M. R. Bauer, et al., Solid State Commun. 127, 355 (2003).

[3] Y. Shimura, et al., J. Appl. Phys. submitted.

[4] E. Kasper, et al., Thin Solid Films, 520, 3195 (2012).

[5] L. Surnev and M. Tikhov, Surf. Sci. 138, 40 (1984).

[6] A. Sakai and T. Tatsumi, Appl. Phys. Lett. 64, 52 (1994).