Nano-structured PECVD Silicon Films and Their Device Applications
Open Access
- Author:
- Li, Handong
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 05, 2004
- Committee Members:
- Stephen Joseph Fonash, Committee Chair/Co-Chair
Russell Messier, Committee Member
Jian Xu, Committee Member
Mark William Horn, Committee Member
Jerzy Ruzyllo, Committee Member
Judith Todd Copley, Committee Member - Keywords:
- poly-crystalline silicon
thin film transistor
nano-crystalline silicon
humidity sensor - Abstract:
- In this thesis study, the interaction of Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR PECVD) deposition conditions and morphology of column-void nano-Si films are further established. The morphology and void fraction are characterized systematically in various ways. It is found that these ECR-PECVD deposited Si films have a column/void structure consisting of three levels of nano-structure: a tapering first level agglomerate (cluster) column structure with widest diameters of 70-80nm in the studied thickness range and a second level of columns that make up the clusters, each with a constant diameter of 10-20nm depending on deposition conditions. The third level arises from the fact that these second level columns are built by the third level 10-20nm diameter elongated nano-crystalline Si blocks of various degree of crystal fraction. The clusters are surrounded by interconnected nano-voids with a radius up to 50nm. The films have a large surface to volume ratio and the nano-crystal size, void size, void content, and specific surface area are tunable through adjusting deposition conditions. The void content and specific surface area are as high as 55% and 180m^2/g, respectively. A high deposition pressure and low microwave power deposition conditions gives the largest void content and specific surface area. A growth model is also proposed based on the experimental evidence. This model is used to explain the formation of the tapering first level clusters. The depositions are done at low temperatures and a wide range of substrates can be used for different applications. Since vapors can easily condense in a small void (pore) due to capillary condensation, a humidity sensor was fabricated based on these nano-structured column/void Si films. This humidity sensor has high-sensitivity (6 order of magnitude of conductivity change in the studied relative humidity range), high-speed (response time less than 1 second), and low hysteresis. Its behavior is reproducible. It can be miniaturized and integrated with signal processing circuits. This humidity sensor's sensitivity and speed of response are the highest in open literature. Using the nano-column/void Si as a sacrificial material, a two-step separation technology for transferring high temperature fabricated high performance poly-Si thin film transistors onto flexible plastic substrates was developed. High temperature poly-Si thin film transistor fabrication and transfer tests with this technology have been done. Transferred high performance poly-Si TFTs on plastic substrate were demonstrated and the effects of temperature, light illumination, and mechanical strain on the released TFTs were explored.