Narrow Luminescence Linewidth of a Silicon Quantum Dot

Single-dot luminescence spectroscopy was used to study the emission linewidth of individual silicon nanocrystals from low temperatures up to room temperature. The results show a continuous line narrowing towards lower temperatures with a linewidth as sharp as 2 meV at 35 K. This value, clearly below the thermal broadening at this temperature, proves the atomiclike emission from silicon quantum dots subject to quantum confinement. The low temperature measurements further reveal a $\sim 6\mathrm{m}\mathrm{e}\mathrm{V}$ replica, whose origin is discussed. In addition, an $\sim 60\mathrm{m}\mathrm{e}\mathrm{V}$ TO-phonon replica was detected, which is only present in a fraction of the dots.

Figure 1

(a) SEM images (tilt view) of Si nanopillars after first oxidation step. ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ was removed by HF etching in order to see remaining Si core. (b) Enlarged image of a pillar. Second oxidation is supposed to consume the “neck” between the nanocrystal at the top of the pillar and its basement, thus creating an isolated Si quantum dot embedded in a ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ matrix. (c) Optical microscope image of the white light reflection on pillar arrays (top view). Distance between neighboring pillars is $1\mu \mathrm{m}$. (d) Corresponding PL image under UV-light excitation. Each bright spot can be traced back to a certain pillar.

Figure 2

Statistical breakdown of the number of dots with different emission photon energies (step 0.1 eV) measured at 80 K. Solid line shows a typical ensemble PL spectrum from nanocrystals embedded in silicon dioxide for comparison (from 4).

Figure 3

PL spectra of two different single Si nanocrystals at (a),(b) 35 K and at (c),(d) 80 K. FWHM of Lorentzian fits are shown. Extracted linewidth values are deconvoluted with the system resolution.

Figure 4

(a) Lorentzian FWHM values of the main emission line for different dots at temperatures below 50 K. Dashed line shows the energy equivalent of temperature in this range. Only dots with a resolvable acoustic phonon peak are shown. (b) Temperature dependence of the exciton linewidth for a dot without a TO-phonon replica (acoustic phonon line is not resolved at 50 K for this dot). The solid line is a fit based solely on Bose statistics for a low frequency acoustic mode ($\hslash \omega =6\mathrm{m}\mathrm{e}\mathrm{V}$).