Electronic and optical properties of $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_2$ nanostructures. II. Electron-hole recombination at the $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_2$ quantum-well–quantum-dot transition

A photoluminescence (PL) study of crystalline $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_2$ quantum wells has been carried out for thicknesses in the $3.9–0.6\mathrm{nm}$ range. We show that quantum confinement plays a great role on emission properties of these narrow quantum wells in term of PL line energy and quantum efficiency. In particular, for the very-low-thickness domain, a set of discrete and high-energy lines is observed between 1.20 and $1.60\mathrm{eV}$ and viewed as resulting from two phenomena: the thickness fluctuations of the silicon layer and the appearance of structural barriers in the plane of the thinnest wells due to the formation of a two-dimensional distribution of Si nanocrystals embedded in $\mathrm{Si}{\mathrm{O}}_2$. A strong increase in the photoluminescence efficiency is measured for wells pertaining to the very-low-thickness domain.

Figure 1

Cross section of a SOI QW obtained after several cycles of oxidation and sacrificial deoxidation of Si. Each point corresponds to two unresolved atomic columns.

Figure 2

Well PL as a function of $l_z$ in a variable thickness QW (ellipsometric measurements of $l_z$ label each spectra). Laser density power is $\mathcal{P}=4\mathrm{kW}{\mathrm{cm}}^{-2}$ and $T\approx 6\mathrm{K}$. All spectra have been corrected from the setup response to the 3400 blackbody.

Figure 3

Solid curve: PL spectra of the $2.1\text{−}\mathrm{nm}$ QW (see Fig. 2). Circles: fit of the spectrum using a five-Gaussian curve set.

Figure 4

Electronic processes at stake in narrow QW’s in the scheme of band-to-band transitions (large thickness) and of activated carrier trapping on interfacial centers, assumed to be the Si and O atoms of the $\mathrm{Si}\mathrm{O}$ bonds, as deduced from Ref. 17 (small thickness). Inset: formation of the 2D nanoplates assembly for the thinnest QW’s and corresponding band diagrams. The scale of length along the QW is contracted.

Figure 5

AFM images of a QW surface $(\text{field}=2\mu \mathrm{m})$ obtained after removal of the top oxide. Images (a), (b), and (c) correspond to scanned zones with decreasing mean thickness. Bright surfaces are flat and are made of crystalline silicon, whereas the dark underlying surfaces correspond to $\mathrm{Si}{\mathrm{O}}_2$. The step between the Si and $\mathrm{Si}{\mathrm{O}}_2$ surfaces arising from the oxide etching is about $6\mathrm{nm}$.

Figure 6

Gain in the QW quantum efficiency with respect to substrate as a function of thickness. Inset: overlap of electron and hole wave functions. Right: probability ${\Pi }_1^{e,h}$ of finding an electron (respectively, a hole) in a $p_z=\hslash k_z$ state for various localization degrees. Left: edge positions of the ${\Pi }_1^{e,h}$ functions as a function of thickness in the first Brillouin zone, with $p_0=\hslash 2\pi ∕a_{\mathit{Si}}$, impulsion at the edge of the first Brillouin zone.