In recent years, encryption technology has become more and more important, and the method to use the circular polarized light in optical communications has been proposed. Spin LEDs are expected as on-chip circular polarized light generating devices, and we have been focusing on a Ge as an active material because it is capable of being monolithically integrated on a Si substrate and emitting light in the telecommunications wavelength band. Particularly, the (111) surface orientation of Ge allows for the epitaxial growth of high-quality ferromagnetic materials on Ge, making it possible to inject spin-polarized electrons from ferromagnetic materials into Ge (111) [1]. Furthermore, it has been proven that the use of strained-SiGe can increase the spin lifetime because of its ability to suppress intervalley scattering [2]. In this study, we fabricate strained-SiGe/Ge-on-Si (111) LEDs and obtain very strong EL emissions at room temperature.

Experimental

The sample was prepared using a solid source molecular beam epitaxy (MBE). First, a high-quality tensile-strained Ge layer was grown on a Si (111) substrate using two-step growth method, where a Ge buffer layer of 40 nm was grown on a p-type Si (111) substrate at a temperature of 350℃ and a Ge layer of 500 nm was grown at 700℃. Secondly, a Si_0.15Ge_0.85 layer of 40 nm and a P-doped Ge layer of 500 nm were grown at lower temperature of 350℃ to prevent relaxation of the SiGe. After the growth of the p-i-n structure, a P delta-doping layer of 2×10¹⁴ cm^-2 was inserted to obtain a low resistivity Ohmic contact. Before P delta-doping, an ultra-thin Si (UT-Si) layer of 2 monolayers was inserted in order to avoid surface segregation of P atoms [3]. Finally, 7 nm thick Ge was grown for the capping. The fabricated multilayers were defined into a mesa diode by photolithography and reactive ion etching. AuSb and AuGa were deposited as top and back contacts, respectively.

Results and Discussions

Current-voltage output characteristics were evaluated and good diode characteristics were obtained. Figure 1 shows the room temperature EL spectra for the Si_0.15Ge_0.85/Ge LEDs with various injected currents. It was found that strong EL emissions covering the communication wavelength band of 1500-1600 nm was obtained. The EL peak intensity increases with the injected current and it is drastically enhanced above 350 mA as shown in inset of Fig. 1. We consider that recombination efficiency can be improved by the strained Si_0.15Ge_0.85 layer. While exact light emission wavelengths cannot be known with the current experiments due to the detector upper limit, characterizations in the longer wavelength range are now under investigation. Moreover, we expect further enhancements of EL intensity by introducing Si_0.15Ge_0.85/Ge multiple quantum wells in the active layer. It is also expected that by changing compositions of Si and Ge in SiGe, we can control the emission wavelength range, indicating that the SiGe/Ge LEDs are very attractive for application to on-chip circular polarized LEDs.

This work was supported in part by Grant-in-Aid for Scientific Research (19H02175, 19H05616, 20K21009) from MEXT, Japan.

[1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001 (2018).

[2] T. Naito et al., Phys. Rev. Appl. 13, 054025 (2020).

[3] M. Yamada et al., Appl. Phys. Lett. 107. 132101 (2015).