Effect of the Surface Energy of Electron-Hole Drops on Their Luminescence Spectrum in Ge

The size of small electron-hole drops near threshold is investigated in Ge using a small shift of their main luminescence line due to their surface energy $\sigma$. The smallest radii measured at different temperatures are compared with the minimum stable radii given by a model taking $\sigma$ and the drop binding energy $\varphi$ into account. We find an excellent agreement if we use the spectroscopic value of $\varphi$ (23 K).