(Invited) Development of Germanium-Tin-Related Semiconductor Heterostructures for Energy Band Design in Electronic and Optoelectronic Applications
Authors: Nakatsuka Osamu , Fukuda Masahiro , Sakashita Mitsuo , Kurosawa Masashi , Shibayama Shigehisa , Zaima Shigeaki 
Published: February 2020

Group-IV element semiconductor materials, germanium tin (GeSn) and germanium silicon tin (SiGeSn) have attracted much attention for electronic and optoelectronic applications because of those unique properties; direct transition with a high Sn content, higher carrier mobility than Si and Ge, low growth temperature, and low thermal conductivity [1-3]. GeSiSn realizes the control of energy band structure independently on the lattice constant of alloy. Also, GeSiSn/Ge(Sn) heterostructure provides a type-I energy band alignment with sufficiently large energy band offsets at both conduction and valence band edges simultaneously with the lattice constant matching. That promises an effective carrier confinement structure for quantum well laser and high electron mobility transistor.

On the other hand, the development of crystal growth technology for GeSn-related materials is still a challenge, since the thermal equilibrium solid solubilities of Sn in Ge and Si are very low compared to a required substitutional Sn content higher than 10%. It is necessary to understand the behavior of Sn atoms in the epitaxial growth and post deposition process and to control the stability of Sn at substitutional site in GeSn-related epitaxial layers. In addition, the development of interface engineering technology is required for not only GeSn/GeSiSn heterostructure but also metal/GeSn and insulator/GeSn structures for nano-scale electronic devices.

Recently, we examined the formation of double heterostructure with GeSiSn/GeSn/GeSiSn layers on Ge substrate using a molecular beam epitaxy (MBE) system [4]. The hard x-ray photoelectron spectroscopy measurement reveals that the sufficiently large energy band offset can be formed at the valence band edge of its GeSiSn/GeSn interface. We also demonstrated that the carrier confinement with this GeSiSn/GeSn/GeSiSn double heterostructure effectively improved the efficiency of photoluminescence (PL) [5]. However, we also found that the crystalline quality of the heteroepitaxial layers significantly influence on the PL intensity and it is necessary to understand the key factor of the crystallinity of heterostructures in order to reduce the dislocation and point defect in the epitaxial layers.

For the low temperature growth of GeSn and GeSiSn hetero-epitaxial layers on Ge using MBE, a biaxial compressive strain is often induced into the epitaxial layers, as a GeSn or GeSiSn layer is often grown pseudomorphically on Ge substrate. The compressive strain of GeSn is a disadvantage for enhancing the luminescence efficiency because a higher Sn content is generally required to the indirect-to-direct crossover for a compressively strained GeSn compared to the unstrained case. In order to improve the efficiency of luminescence, it is important not only to enhance the Sn content but also to control the strain structure. We developed the strain relaxation technology of GeSn epitaxial layer on a substrate or buffer layer. We found that the ion implantation into Ge substrate effectively enhance the strain relaxation of GeSn layer. The degree of strain relaxation achieved over 90% for the GeSn layer grown on the Ge substrate after the boron ion implantation with 3x1014 cm-2, while the compressively strained GeSn layer was pseudomorphically grown on the Ge substrate without the ion implantation.

This work was partly supported with a Grant-in-Aid for Scientific Research (S) (No. 26220605) and the Core-to-Core Program ICRC-ACP4ULSI from the JSPS.

References

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[5] M. Fukuda, K. Watanabe, M. Sakashita, M. Kurosawa, O. Nakatsuka, and S. Zaima, Semicond. Sci. Technol. 32, 104008 (2017).