Effect of Ge Core Size on Photoluminescence from Si Quantum Dots with Ge Core
Authors: Yamada Kentaro , Kondo Keigo , Makihara Katsunori , Ikeda Mitsuhisa , Ohta Akio , Miyazaki Seiichi 
Published: February 2020

The achievement of intense light emission at room temperature from group-IV materials is a crucial step toward the accomplishment of fully-Si-based optoelectronics. Recently, Ge-nanostructures on Si or embedded in Si have been also studied to realize well-defined three-dimensional hole confinement in Ge-QDs with keeping electron transport efficiency, which can enhance carrier recombination efficiency [4-7]. In our previous paper, we have reported on the formation of Si quantum dots (Si-QDs) with Ge core with an areal density as high as ~1011cm-2and their photoluminescence (PL) properties [8, 9]. We also demonstrated that, in this system, room temperature PL was promoted by hole confinement in Ge core reflecting the type II energy band discontinuity between Si clad and Ge core. In this work, we extended our research work to evaluate impact of Ge-core size on their PL characteristics.
After conventional wet-chemical cleaning steps, ~2.0 nm-thick SiO2 was grown on p-Si(100) by dry O2 oxidation at 850°C. The SiO2 surface was shortly dipped into a 0.1% HF solution to etch back the SiO2 and to obtain uniform surface termination with OH bonds. Subsequently, hemispherical Si-QDs with an areal dot density of ~1011cm-2 were formed from the thermal decomposition of pure SiH4 under 67 Pa at 550˚C. After that, Ge was deposited selectively on the pre-grown Si-QDs at 370°C by using GeH4 diluted with 5% H2 where the GeH4-LPCVD time was varied in the range from 2.5 to 3 min to control core size. After the Ge core deposition, Si clad was formed in the same chamber at 530ºC by LPCVD using SiH4 under 2.7 Pa. Finally, the surface of the Si-QDs with Ge core was oxidized at room temperature by a remote VHF plasma of O2, which resulted in conformal coverage with a ~2.0 nm-thick SiO2 layer.

AFM images confirm that the areal dot density (~1.0×1011cm-2) remains unchanged after the Ge deposition regardless of deposition time. This result implies Ge was deposited selectively on the pre-grown Si-QDs. From the size distribution of the dot evaluated from AFM images taken at each process steps, we confirmed that average dot heights were increased by ~2 nm and ~6 nm with GeH4-LPCVD for 2.5 min and 3 min, respectively, in comparison to the pre-grown Si-QDs.

The Si-QDs with an average Ge dot size of 6nm shows PL signals peaked at ~0.69 eV at room temperature (Fig. 1). The observed PL peak is likely to be caused by radiative recombination of photogenerated electron-hole pairs through quantized states of the Ge core. It should be noted that, with a decrease in the core size down to ~2nm, PL signal was slightly shifted towered the high energy side by ~80 meV, although no significant change in the PL intensity was observed. This result can be interpreted as a result of increased quantized energy reflecting the size of the Ge core rather than the intermixing of the Si clad and Ge core.
In conclusion, we demonstrated room temperature PL from the Si-QDs with different sized Ge core. The PL from the Si-QDs with Ge core is dominated by the recombination of electron-hole pairs between quantized states of Ge core, and emission wavelength can be changed by controlling the Ge core size.

Acknowledgments

This work was supported in part by Grant-in Aid for Scientific Research (S) 15H05762 of MEXT, Japan.

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Figure 1