Single-dot absorption spectroscopy and theory of silicon nanocrystals

Photoluminescence excitation measurements have been performed on single, unstrained oxide-embedded Si nanocrystals. Having overcome the challenge of detecting weak emission, we observe four broad peaks in the absorption curve above the optically emitting state. Atomistic calculations of the Si nanocrystal energy levels agree well with the experimental results and allow identification of some of the observed transitions. An analysis of their physical nature reveals that they largely retain the indirect band-gap structure of the bulk material with some intermixing of direct band-gap character at higher energies.

Figure 1

Comparison of the experimentally obtained absorption curve (red circles to the right) with a calculated one (broadening 50 meV) for a $\sim 3\mathrm{nm}$ diameter Si nanocrystal (green curve) exhibiting best agreement. The room-temperature photoluminescence spectrum (PL) of this nanodot is presented as red circles to the left. The PL peak position is close to the calculated band gap (green peak at 1.88 eV). A typical featureless ensemble absorption [24] is also given for comparison (dashed line).

Figure 2

Left: Cross-sectional TEM image of a silicon nanocrystal taken along the [110] direction from an SOI sample. Si (111) plane lattice fringes visible (scale bar 2 nm). Right: Photoluminescence image of $\sim 50\times 50\mu {\mathrm{m}}^2$ sample area. Bright points correspond to luminescence from individual Si quantum dots, formed randomly in a thinned SOI layer.

Figure 3

Typical (top) room-temperature and (bottom) low-temperature photoluminescence (linewidth indicated) and absorption spectra of two different individual silicon quantum dots. Four steps on the absorption curves can be distinguished and the black line is a fit based on four Gaussians (see text).

Figure 4

Left: Calculated absorption curves (broadening 50 meV) for a nanodot of $2.6\times 2.6\times 3$ nm dimensions (purple) and for a 3 nm diameter nanocrystal (green) on a log scale. Small shape variations slightly modify the absorption curve. Right: The exciton spectrum counterpart including many-body effects (broadening 1 meV). Dashed lines represent the experimentally obtained peaks.

Figure 5

Projections of the calculated conduction band states to bulk directlike Bloch functions for a $2.6\times 2.6\times 3$ nm Si nanodot in oxide matrix. The intermixing of $\mathrm{\Gamma }$ and $X$ components is stronger for higher energy.