Quantum Confinement by an Order-Disorder Boundary in Nanocrystalline Silicon

We predict theoretically and show experimentally the occurrence of quantum confinement in hydrogenated nanocrystalline silicon. We prove that only valence states (positively charged carriers) are confined effectively within the nanograins. The emission associated to confined states is verified by photoluminescence experiments on nanocrystalline samples with controlled grain size. According to the present study, we propose nanocrystalline silicon as a promising material for oxygen-free optoelectronics, silicon-based memories and photovoltaics.

Figure 1

LUMO (top), HOMO (middle), and electronic gap $G$ (bottom) (calculated according to Tauc et al. [38, 39]) of the $nc\mathrm{\text{−}}\mathrm{Si}/a\mathrm{\text{−}}\mathrm{Si}:\mathrm{H}$ system as a function the grain diameter $D$ at 8% (full square symbols) and 12% (empty circle symbols) hydrogen content.

Figure 2

Spatial localization of the HOMO state for an embedded nanocrystal of diameter $D=0.8\mathrm{nm}$ in $a\mathrm{\text{−}}\mathrm{Si}:\mathrm{H}$ (with 8% H). Red large dots represent Si atoms within the crystalline nanograin. Blue triangles refer to H atoms; small red dots represent Si atoms in the amorphous region. Color scale on the right corresponds to electronic probability density.

Figure 3

PL spectra of several $nc\mathrm{\text{−}}\mathrm{Si}/a\mathrm{\text{−}}\mathrm{Si}:\mathrm{H}$ samples (thin lines) with different grains sizes and crystal-to-amorphous ratios. Superimposed thick bands and vertical lines represent, respectively, the theoretically predicted $A$ bands and energy emission from the most abundant grains. Top panel: as-grown samples with different crystal-to-amorphous ratio. Bottom panel: PL spectra evolution during laser annealing of a sample with initial low crystallinity.

Figure 4

Top panel: Calculated nanocrystal size distributions for samples with different grains sizes and crystal-to-amorphous ratio. The vertical arrows indicate the corresponding average diameters measured by XRD. Bottom panel: Calculated nanocrystal size distributions for as-grown and annealed samples.