Transverse Quantum Confinement in Semiconductor Nanofilms: Optical Spectra and Multiple Exciton Generation

Abstract

We report absorption and photoluminescence spectra of Si and SnO$_2$ polycrystalline nanofilms in the UV-Vis-NIR range, featuring discrete bands resulting from transverse quantum confinement. The film thickness ranged 3.9 nm to 12.2 nm, depending on the material. The results are interpreted within the particle-in-a-box model, with the box width equal to the layer mass thickness. The energy levels and transitions scale as the inverse square of the film thickness. The calculated values of the effective electron mass are independent on the film thickness and equal to 0.17m$_o$ (Si) and 0.21m$_o$ (SnO$_2$), with m$_o$ the mass of the free electron. The uncertainties in the effective mass values are ca. 2.5%, determined by the film thickness calibration. The second calculated model parameter, the quantum number $n$ of the HOMO, was also thickness-independent: 8.00 (Si) and 7.00 (SnO$_2$). This indicates that the Fermi level should also scale as the inverse square of the film thickness in these nanofilms. The observed transitions all start at the level $n$ and correspond to {\Delta}$n$ = 1, 2, 3, etc. The photoluminescence bands exhibit large Stokes shifts, moving to higher energies with increased excitation energy. The photoluminescence quantum yields exceed unity, showing evidence of multiple exciton generation from each absorbed photon. A prototype Si-SnO$_2$ nanofilm photovoltaic cell demonstrated an increase of the photoelectron yield with the photon energy, showing evidence of multiple exciton generation.