Tianwei Zhou

Ge-Si quantum dots thin film solar cells

Thin film p-i-n solar cells (SCs) with 30 bilayers undoped or p-type self-assembled Ge/Si quantum dots (QDs) were fabricated on n+-Si(001) substrates by ultrahigh vacuum chemical vapor deposition. Compared with the SCs without Ge QDs, the external quantum efficiency in infrared region and the short-circuit current densities of the SCs with Ge QDs increased. However, their open-circuit voltages and efficiencies decreased. The open circuit voltages of p-type Ge/Si QDs SCs recovered significantly at low temperature, which was due to the suppression of recombination centers and longer carrier lifetime.

Single-crystalline Ge1-x-ySixSny alloys on Si (100) grown by magnetron sputtering

Magnetron sputtering was successfully used to grow single-crystalline Ge1-x-ySixSny ternary alloys on Si (100) substrates. The lattice constants of the alloys were calculated by X-ray diffraction and corrected Vegard’s law, respectively, showing that the corrected Vegard’s law is suitable for the Ge1-x-ySixSny lattice constants. Thermal stability investigations showed that the Ge0.85Si0.051Sn0.099 alloy was stable at 500 °C. The Ge1-x-ySixSny can maintain good crystalline quality under moderate annealing temperature, with no indication of phase segregation or Sn precipitation. Optical absorption measurements were carried out at room temperature to determine the band gap energies of the Ge1-x-ySixSny alloys. These results suggest that magnetron sputtering is an effective alternative method to grow Ge1-x-ySixSny alloys for fabrication of novel devices.

Transport mechanism of novel silicon-riched nitride (SRN)/ silicon-riched oxide (SRO) superlattice quantum dot structure

Novel structure of silicon-riched nitride (SRN)/silicon-riched oxide (SRO) is demonstrated using RF reactive magnetron sputtering and post-annealed, to increase the density of Si nanocrystals especially in the separated layer made of isolation medium, thus improving the transport ability of Si nanocrystal material. I-V characteristics shows the current of hybrid SRN/SRO system increases up to 2 orders of magnitude at 1V compared to that of SiNx/Si3N4 structure, proving the importance of the existence of sparser Si NCs in the SRO layer to greatly improve the transport ability. Temperature dependent I-V test reveals that direct tunneling dominates at low electrical field (V<;1V) and the temperature below 180K; while at high electrical field (V>;1V) Poole-Frenkel emission is the dominated one.