Photoluminescence Study of Si Quantum Dots with Ge Core

Physical properties of nanometer size semiconductor quantum dots (QDs) have been extensively studied to reveal quantum size effect and to open up new device application [1, 2].  In our previous work, we have reported that Si-QDs and dots with Ge core were formed on SiO₂ by controlling the thermal decomposition of pure SiH₄ and 5% GeH₄ diluted with He alternately, and demonstrated unique charged storage characteristics of an individual Si-QD with a Ge core, that is, electrons in a Si clad and holes in a Ge core by means of AFM/Kelvin probe force microscopy (KFM) [3-5].  In this work, we extended our research work to the evaluation of photoluminescence (PL) properties of high density Si-QDs with Ge core.

Hemispherical Si-QDs were first grown on ~4nm-thick SiO₂/p-Si(100) at 560°C from the thermal decomposition of pure SiH₄ with a pressure of 67 Pa.  Subsequently, Ge was deposited selectively on the pre-grown Si-QDs at 400°C using 5 % GeH₄ diluted with He.  After the Ge core deposition, the formation of Si clad was carried out in the same chamber at 560 ºC by LPCVD using SiH₄ under 2.7 Pa.  Finally, the surface of the Si-QDs with Ge core was oxidized at the same temperature by a remote VHF plasma of 1 % O₂ diluted with He, which resulted in conformal coverage with a ~2nm-thick SiO₂ layer.

AFM images confirm that the areal dot density (~1.0×10¹¹cm^-2) remains unchanged after the Ge deposition and subsequent Si-clad formation, while the dot height increases by ~3 nm after the Ge deposition and by ~5 nm after the Si-clad formation.  Room temperature PL spectra peaked at ~0.70 eV was observable from high density Si-QDs with Ge core so-prepared, where 979 nm line of semiconductor laser was used as an excitation light source and laser power was kept at 156 mW.  Considering that the PL spectrum from c-Ge(100) substrates is measured at ~0.67 eV under the same experimental set up.  The observed PL peak is likely to be caused by radiative recombination through quantized states of the Si-QDs with Ge core.  It should be noted that no PL signals originating from the radiative recombination in Si-QDs and/or Si-clad were detectable.  To get a further insight into the origin of PL signals peaked at ~0.7eV, room temperature PL was measured after the sample was annealed at 700 °C under N₂ ambient.  Evidently, the PL peaked at ~0.70 eV disappeared and a new PL signals appeared at ~1.2 eV after annealing.  This indicates that intermixing of Si-QDs and Ge core proceeds resulting in SiGe-QDs.  Based on these results, the room temperature luminescence from the Si-QDs with Ge core is promoted by hole confinement in Ge core reflecting the type II energy band discontinuity between Si clad and Ge core.  

Acknowledgments

This work was supported in part by Grant-in Aids for Scientific Research (A) No. 24246054 and Young Scientists (A) No. 25709023 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration.  In addition, the author deeply appreciates that the samples were fabricated successfully by utilizing the clean room facilities of Hiroshima Univ.  The authors wish to thank Prof. H. Kishida and Dr. T. Koyama for their support in the experiments.

References

[1] S.A. Ding et al., Appl. Phys. Lett., 73 (1998) 3881. 

[2] L.P. Rokhinson et al., Appl. Phys. Lett., 76 (2000) 1591.

[3] S. Miyazaki et al., Thin Solid Films 369 (2000) 55.

[4] Y. Darma et al., Nanotechnology 14 (2003) 413.

[5]  S. Miyazaki et al., Electrochemical Society Trans., 2 (2006) 157.