Pulsed laser deposition of silicon nanostructures for photovoltaic applications

The research activity was dedicated to synthesis of new nanostrucutred inorganic materials in view of their application in third generation photovoltaics (PV), in particular semiconductor (Si) quantum dots (QD) for QD – based PV (e.g. heterojunctions or QD sensitized solar cell). The aim was to synthesize Si quantum dots (QDs) with controlled structural and electronic/optical properties using Pulsed Laser Deposition (PLD) technique in a background (inert and reactive) atmosphere as in literature, the systematic study of Si nanostructures in different matrices, role of low and high annealing temperatures and their relative photoluminescence (PL) mechanism is still under debate. Synthesis of Si nanoparticles at Room Temperature withstanding its optical properties have always been a problem as they get oxidized easily when exposed to air therefore it is one of the goals to prevent the as-deposited Si nanocrystals from ex-situ oxidation. The PLD was chosen for present study due to its versatility in control of deposited structures. I’ve explored the role of various PLD parameters i.e. different fluence and wavelength of the laser, different target-to-substrate distance and partial pressures of inert and reactive gases and effect of different annealing temperatures on growth of Si nanostructures. With otpimized parameters of PLD, the research activities led to following conclusions. The nanoscale morphology was varied from compact to open porous by varying the pressure from 1-60 Pa as observed by Scanning Electron Microscopy (SEM). The results obtained from photoluminescence characterization show a very interesting blueshift trend in PL energy bands (1.6 - 2.1 eV) while increasing the partial pressures (from 1 to 60 Pa) of both inert and reactive gases. The high energy component shifts with pressure and we attribute it to quantum confinement in Si nanoparticles. This suggests potential for bandgap engineering for PV application. Annealing in vacuum at 400 °C, results in increment of PL intensity, which further increases at 1000 °C, accompanied by a redshift in PL energy bands. This suggests defect reduction after annealing. Raman scattering measurements show amorphous phase in compact films while no crystalline Si signal was detected in porous films and in 1000 °C annealed films. TEM analysis shows the presence of nanocrystals only after annealing the samples at high temperature i.e. 1000 °C. We believe that the Raman signal was missing due to oxidation of the films as porous Si films are oxidation-prone which also makes the PL discussion very complicated. To prevent our films from oxidation was the next challenge. To be sure that the synthesized silicon nanoparticles do not undergo ex-situ oxidation or to be prevented from oxidation, a thin, compact capping Si layer was deposited on the top of the porous (60-150 Pa of Ar and 100-500 Pa of He) films. Raman spectra indicate the contribution of both amorphous and crystalline Si in this case. The Raman shift ranging from 516.3 to 518.5 cm-1 for the crystalline Si peak (whose position in bulk Si is 521cm-1) corresponds to particle size 4 to 6 nm. The presence of Si nanocrystals was confirmed by High Resolution TEM micrographs showing a mixed aSi-SiNPs i.e. Si nanoparticles embedded in amorphous matrix and the nanocrystal size was good in agreement with the size calculated from Raman shift. The Si/O atomic content found by EDS via Scanning TEM was 3/5, suggests that the films were not completely prevented from oxidation. The contribution of different components in PL spectra at 1.64, 1.78 and 1.85 eV was observed. A set of four-layered films with different pressures of Ar and He was also deposited and presence of size distributed Si nanocrystals was realized by Raman and PL results. Therefore the compact capping layer was able to reduce oxidation thus making the presence of nanoparticles at room temperature clearly traceable. These observations also confirm the possibility of tailoring the structural and optical properties of nanocrystalline thin films for multi-layer, solar cell structure. The room temperature deposition of Si QDs on temperature sensitive substrates like plastic will add an advantage in case of hybrid solar cells. In future perspective, the superlattice of Si nanocrystals embedded in oxide, carbide or nitride matrix with different size of Si nanocrystals in each layer would be suggested using optimized PLD parameters.

NA