With increasing interest in fs-laser based photonic device fabrication and mesoscale composite materials, the goal of the project was to produce hybrid micro- nano- structure via ultrafast laser modification of semiconductor doped glass (SDG) combined with a possible second processing step of heat treatment at elevated temperatures. The aim was to induce precipitation of the dopant semiconductor in micron-scale regions defined by fs-laser processing. Photoinduced chemical changes were monitored with electron microscopy and wave dispersive x-ray spectroscopy (WDS), meanwhile the development of optically active features was monitored with confocal fluorescence spectroscopy in combination with optical microscopy.
3 laser systems were tested, each for a range of parameters: 800 nm, 1 kHz repetition rate, 200 fs pulse width, Ti:Sapphire Spectra-Physics laser; 1030 nm, 1 MHz repetition rate, 750 fs pulse width, Uranus series PolarOnyx fiber laser; 1030 nm, 473 kHz rep-rate, 750 fs pulse width, unknown model PolarOnyx fiber laser.
The SDG substrate was Schott's OG570, re-melted to dissolve the semiconductor into a homogeneous solution. The fs-laser processed samples were heat treated at temperatures in the 500 °C–600 °C range.
1 kHz laser has proven to be unsuitable for space-selective semiconductor precipitation in the chosen SDG. The work discusses some of the reasons that contribute to this negative result, suggesting that the trend would extend to other SDGs.
Lines written with the two high repetition rate lasers, 1 MHz and 473 kHz, have shown elemental segregation that exhibits consistent behavior over a wide processing parameter window. The elements diffuse away from the laser focal volume to form a pattern, 10-150 μm in size, of three chemically distinct zones: Zn-rich, Si-rich, and Na- and K-rich, arranged in that order at an increasing distance from the modifying beam.
Heat treating the fs-laser processed samples resulted in evolution of fluorescence at the Na- and K-rich region. Two peaks were observed in the spectra: a red peak and a blue peak. The red peak was a broad peak centered at 710 nm and did not change position with heat treatment. It appeared in the early stages of the heat treatment and then disappeared as the blue peak emerged. The blue peak initially appeared as a partial peak near the 473 nm pump line then becoming a full peak, as it shifted its position towards longer wavelengths with continued heat treatment. The moving blue peak is assigned to near band edge electron-hole recombination in the CdSxSe 1-x nanocrystals, and is held as evidence of semiconductor precipitation in the photomodified glass. The location of the peak changes as the particles grow at an elevated temperature and degree of quantum confinement diminishes. The stationary 710 nm peak is attributed to Se2- formation, and its disappearance is tied to consumption of selenium as semiconductor nanocrystals nucleate and grow.
A model for this spatially selective semiconductor precipitation behavior is suggested on the basis of glass transition dependence on network modifier concentration. It is posited that increase in a local concentration of sodium and potassium effectively forms a "micro-crucible" with lower glass transition temperature and increased local network mobility sufficient to result in faster semiconductor precipitation dynamics as compared to the surrounding bulk glass. The contributions from local solubility and semiconductor concentration changes are also discussed, but at present there is limited information to fully account for the extent of these contributions.
Overall, the method of introducing chemical segregation with high-repetition rate laser and then exploiting the localized increases in network mobility is shown to be a very robust method for controlled, space-selective semiconductor precipitation. With some further development steps suggested, this processing method can prove a powerful tool in photonic device fabrication.
|Advisor:||Risbud, Subhash H., Krol, Denise M.|
|Commitee:||Risbud, Subhash H., Sen, Sabyasachi, Shackelford, James F.|
|School:||University of California, Davis|
|Department:||Materials Science and Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 74/07(E), Dissertation Abstracts International|
|Subjects:||Nanotechnology, Materials science|
|Keywords:||Cadmium selenide, Fs-laser processing, Glass, Photonics, Quantum dot, Ultrafast laser|
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