Doctor of Philosophy
Simpson, Peter J.
Silicon (Si) nanocrystals (nc) precipitated from silicon-implanted silicon oxide (SiO2) are of interest as a novel light source for illumination, biomedical applications, optical computing, etc. They have some advantages over conventional III-V compound semiconductor nanocrystals produced by colloidal synthesis. They are compatible with Si/SiO2 based semiconductor processing, are stable, non-toxic at point of synthesis and consumption, and their luminescence falls with the infrared transmission window of biological materials. Unfortunately, synthesis of Si-nc embedded SiO2 is uneconomical and is not as amenable to precise control of the size distribution of nanocrystals as is the case for III-V compound colloidal nanocrystals.
The distribution of nanocrystals precipitated out of a sample is affected by the purity of the SiO2 implantation target, the implantation temperature, the implanted Si+ dose and energy, the anneal temperature, the anneal ramp rate, total anneal time, and the use of secondary annealing in a passivating gas atmosphere.
Two studies were conducted within the scope of this thesis. The first concerned the enhancement of Si-nc precipitation by use of a double implant procedure to increase the vacancy concentration in the implanted region. The second involved the development of a combinatorial implantation and characterization procedure to allow many implantation doses to be synthesized at once, thereby more rapidly optimizing synthesis procedure. An exponential dose profile was implanted in thermal oxide and fused silica. Optical absorption profiles were measured for the implanted fused silica and photoluminescence profiles were made for both samples. It was determined that the implanted thermal oxide yielded Si-nc with a quantum confinement mechanism. Both samples also yielded luminescence from another, unspecified mechanism. By comparing the quantum confined luminescence from the thermal oxide in both studies, it was confirmed that the double implant procedure does yield enhanced Si-nc precipitation, even with a much lower concentration of excess Si.
Based on the literature review and experimental results, various recommendations are made for future work.
Summary for Lay Audience
A large machine was used to put very tiny pieces of silicon (the thing computer chips are made of) into silicon oxide or silica (basically, glass but another thing in computer chips). After this was done, the silicon oxide with silicon in it goes into a furnace to be heated. When it gets very hot, the little pieces of silicon move around a little bit inside the silicon oxide and stick together. This turns them into tiny lights that can be seen if a purple laser is shone onto them. The idea behind this is that it might someday be useful to build a light directly into the silicon chips of a computer, or for medical imaging.
For this research, some silicon oxide with more silicon in it than usual was made and it was looked at with the laser and by a machine that can find tiny, atom-size holes in material. In this case the holes are surrounding the silicon that was put into the silicon oxide and is all clumped together. Also in this research, a piece of silicon oxide was made with an amount of silicon put into it that increased from one side to the other. This made it possible to make many samples in one. By comparing the two types of sample we found the kind with the more silicon than usual was better.
After doing this research and reading about lots of other research some ideas were thought up to do a better job in the future.
Gaudet, James M., "Si nanocrystal synthesis via double implantation and variable implantation" (2020). Electronic Thesis and Dissertation Repository. 6963.