Si–based quantum dots (QDs) have attracted much attention as an active element in Si-based optoelectronic applications because their light emission properties due to carrier confinement have potential to combine photonic processing with electronic processing on a single chip. We have focused on CVD formation and characterization of Si-QDs with Ge core and reported their photoluminescence (PL) properties attributable to type II energy-band alignment between the Ge–core and the Si-shell [1-2]. In addition, we have also demonstrated stable electroluminescence in the near–infrared region from diode structures having a 3–fold stacked Si–QDs with Ge core with an areal dot density of ~2.0×10¹¹ cm⁻² under pulsed bias applications [3]. One of the possible ways to enhance the radiative recombination rate and the improve the light emission efficiency is to reduce non–radiative centers and to increase carrier density by impurity doping into the QDs. In this work, in extending our research work to valency-controlled Si–QDs with Ge core, we have studied the effects of boron– and phosphorus–doping to the Ge core on PL properties.

A highly dense layer of Si–QDs with Ge core was formed by controlling thermal decomposition of GeH₄ and SiH₄ alternately on thermally–grown ~2.0 nm–thick SiO_2. During the Ge deposition, delta doping of boron and phosphorus atoms in QDs was carried out by pulse injection using 1% B₂H₆ and 1% PH₃ diluted with He, respectively. No changes in dot size and areal dot density with B or P doping were confirmed by AFM topographic images. Under photoexcitation of undoped QDs with a 979–nm line from a semiconductor laser, a broad PL spectrum, which consists of four Gaussian components originating from radiative recombination through quantized states in QDs, were observed in the energy range from 0.62 to 0.85 eV even at room temperature. For the doped QDs, in addition to the four components seen in undoped QDs, relatively-narrow components peaked at ~0.64 eV and ~0.68eV were observed with B-doping and with P-doping to the Ge core, respectively. Notice that, with an increase in B₂H₆ pulse injection from 1 to 4 times, the integrated PL intensity was enhanced by a factor of 1.4 to 2.4 compared to that of the undoped QDs while no significant change in spectral shape was observable. This can be interpreted in terms of an increase in the number of holes with B-doping to the Ge core since the carrier recombination rates is proportional to the product of the number of electrons and holes confined in each of QDs under weak photoexcitation.

Acknowledgements

The authors wish to thank Dr. Akio Ohta for his contribution to the measurements. This work was supported in part by Grant–in–Aid for Scientific Research (A) 19H00762, 21H04559, and Fund for the Promotion of Joint Intentional Research [Fostering Joint International Research (A)] 18KK0409 of MEXT Japan.

References

[1] S. Miyazaki, K. Makihara, A. Ohta, and M. Ikeda, Technical Digest of Int. Electron Devices Meeting 2016, 826–830 (2016).

[2] S. Miyazaki, K. Yamada, K. Makihara, and M. Ikeda, ECS Transactions, 80 (4) 167–172 (2017).

[3] K. Makihara, M. Ikeda, N. Fujimura, K. Yamada, A. Ohta, and S. Miyazaki, Applied Physics Express 11, 011305 (4pages) (2018).