Quantum confinement in germanium nanocrystals

The electronic structure of Ge nanocrystals is studied using a sp(3) tight binding description. Analytical laws for the confinement energies, valid ov er the whole range of sizes, are derived. We validate our results with ab i nitio calculations in the local density approximation for smaller clusters. Comparing to experimental data, we conclude that, similar to the case of s ilicon: (a) the blue-green photoluminescence (PL) of Ge nanocrystals comes from defects in the oxide and (b) the size dependent PL in the near infrare d probably involves a deep trap in the gap of the nanocrystals. We predict that the radiative lifetimes remain long in spite of the small difference ( 0.14 eV) between direct and indirect gaps of bulk Ge. (C) 2000 American Ins titute of Physics. [S0003-6951(00)04134-6].