Light emission from Si-based nanostructures including Si- and Ge-QDs has stimulated considerable interest in the field of silicon-based photonics because of its potential to combine photonic processing with electronic processing on a single chip. Much effort to improve light emission efficiency and its stability has been devoted with deliberate approaches which include not only the confinement of injected carriers but also the use of strained structures and impurity doping. So far, we have reported the formation of Si-QDs with Ge core on ultrathin SiO₂ by controlling the thermal decomposition of pure SiH₄ and 5% GeH₄ diluted with He alternately, and demonstrated fairly broad photoluminescence (PL) spectra in the region of 0.6–0.8 eV originating from radiative recombination of photo-generated carriers through quantized states of Ge core. In this paper, our recent achievements on the formation of double-stacked high density Si-QDs with Ge core on thermally-grown SiO₂ by highly-selective CVD, and characterization results of their electroluminescence (EL) under pulsed bias applications are reviewed.

Si-QDs with Ge core with an areal density as high as ~1×10¹¹cm^-2, of which the average height was ~7nm including a Ge core with a height of ~2.5nm, were formed on thermally-grown SiO₂/c-Si (100) by controlling the early stages of LPCVD using pure SiH₄ for the pre-growth of Si-QDs, and the subsequent thermal decomposition of 5% GeH₄ diluted with H₂ and of pure SiH₄ with keeping a gas pressure as low as 2.7Pa for highly selective depositions of Ge core and Si cap on the pre-grown Si-QDs, respectively. For the surface passivation of the QDs so prepared, ~2nm-thick SiO₂ was formed by exposing the dot surface to remote O₂ plasma at 500˚C. In stacking a layer of QDs, the process steps above mentioned was followed by remote H₂ plasma exposure of SiO₂ to terminate the oxide surface with OH bonds and repeated sequentially.

With excitation by 979 nm laser light, humped–shape PL signals in the energy region from 0.64 to 0.8eV were observed even at room temperature. Notice that, in the energy region over ~1.0eV, no PL signals were detectable even with excitation by 690 nm laser light although PL signals from coreless Si QDs were observed in the energy region of 1.0-1.6eV. In addition, as a result of encapsulation of QDs with SiO₂ having an expansion coefficient smaller than bulk Si and Ge, temperature dependence of PL is markedly weak and very different from phonon-assisted PL from bulk Si and Ge. We also confirm that, with double stacking of a QDs layer, PL intensity just about doubles with keeping the spectral shape and emission energy. From the spectral analysis using a Gaussian curve fitting method, it is revealed that the observed PL spectra can be deconvoluted into mainly four components peaked at 0.68, 0.71, 0.74 and 0.78eV in the double stacked QDs as in the case of single layer of Si-QDs with Ge core. Taking into account the fact that, with Si capping after Ge selective deposition on pre-grown Si QDs, PL signals above ~1.0eV are efficiently eliminated and the signals below ~0.8eV are enhanced significantly, the observed PL is interpreted in terms of recombination through quantized states of the Si-QDs with Ge core reflecting the type II energy band discontinuity between Si clad and Ge core. In fact, with decreasing Ge core size, a distinct blue shift of PL was observed. Based on these results, EL was characterized from the backside through the c-Si substrate after the formation of Al top and bottom electrodes. Current-voltage characteristics show clear rectification properties reflecting work function difference between the Al top electrode and p-Si(100) substrate. With application of pulsed biases in the forward direction, EL signals peaked at ~0.83eV was observed even at room temperature. The EL peak energy was higher than the PL peak energies. No EL was detected under reverse bias conditions. Notice that, with increasing the applied bias, the EL intensity was increased with no significant change in the spectral shape. The result indicates that the observed EL can be explained by radiative recombination between higher order quantized states in Ge core caused by hole injection from the p-Si(100) and electron injection from the top Al-electrode, in which direct bandgap transition might contribute to EL because the emission width of EL was narrower than that of PL. These results imply that Si-QDs with Ge core is greatly promising for their optoelectronic device applications.

Acknowledgement: This work was supported in part by Grant-in Aid for Scientific Research (S) No. 15H05762 of MEXT, Japan.