AMORPHOUS FILM MICROSTRUCTURE AND ITS DEVICE APPLICATIONS: STRAIN SENSOR, MICROBOLOMETER, THIN FILM TRANSISTOR AND SOLAR CELL
Open Access
- Author:
- Shin, Hang-beum
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 26, 2012
- Committee Members:
- Thomas Nelson Jackson, Dissertation Advisor/Co-Advisor
Thomas Nelson Jackson, Committee Chair/Co-Chair
Joan Marie Redwing, Committee Member
Srinivas A Tadigadapa, Committee Member
Mark William Horn, Committee Member - Keywords:
- hydrogenated amorphous silicon
PECVD
autonomous powered circuit
uncooled IR microbolometer
strain sensor array
thin film transistor
flexible substrate
TCAD simulation
high TCR
1/f noise
normalized Hooge parameter
sputtered germanium
ZnO TFT
a-Si:H solar cellcircuit
uncooled IR microbolometer - Abstract:
- This thesis reports materials and device results for several types of thin-film devices: (1) n+ µc-Si:H strain sensors and a-Si:H thin film transistor (TFT) arrays on a flexible polyimide substrates, (2) high temperature coefficient of resistance (TCR) a-Si(C):H and sputtered a-Ge films as sensor materials for uncooled microbolometers, and (3) a-Si:H solar cell integration with plasma enhanced atomic layer deposited (PEALD) ZnO TFTs and circuits. The strain sensor array was fabricated on a flexible polyimide substrate. Six photolithography layers were processed on 4-by-4 inch flexible samples, forming a-Si:H TFTs and n+ µc-Si:H strain bridges. Discrete test structures and arrays with up to 32-by-32 sensors were fabricated. By using Wheatstone bridge sensors with varying orientation for different sensors in an array it was possible to determine both strain magnitude and direction. Strain sensor array operation was demonstrated with a custom interface circuit board and a LabView program from real-time data display. In resistive microbolometer devices, the temperature coefficient of resistance (TCR) is a key property for high sensitivity. Plasma enhanced chemical vapor deposition (PECVD) silicon and sputtered germanium deposited with various deposition parameters were investigated to achieve high TCR. Real-time spectroscopic ellipsometry (RTSE) was installed onto a load-locked PECVD system, and the growth evolution of films was monitored successfully. Depending on the deposition conditions, film phases were correlated with variations in film electrical properties. One major factor that limits microbolometer performance is 1/f noise, and the normalized Hooge parameters of PECVD silicon and sputtered germanium films were evaluated. In a more theoretical approach to understanding amorphous silicon, an a-Si:H bandstructure was modeled using a commercial software, Synopsis Sentaurus, and the temperature dependence of resistivity was simulated using a variety of band structure models. An a-Si:H model was constructed to examine the TCR dependence on several model parameters: band-tail slope / concentration, gap-state slope / concentration, doping concentration, and mobility. Comparison of experimental and modeled conductivity over a wide temperature range can help determine the slope of the band-tail traps. Autonomously powered circuits are of interest for a range of applications. Thin film solar cells currently occupy a smaller portion of the market than solar cells made from bulk materials, but still have large potential for use in mobile and portable electronics. Here, the heterogeneous integration of a-Si:H solar cells and ZnO TFT circuits was demonstrated. The process requires only two additional photolithography steps compared to a standard ZnO TFT process. As a simple integration example, 15 series connected n-i-p solar cells were fabricated to provide the supply voltage for 7-stage ZnO ring oscillator. The resulting a-Si:H/ZnO integration requires only illumination (no other external power) and oscillates at about 28 kHz. This integration of PV cells into functional TFT circuits to create an autonomously powered devices may be groundbreaking for the development of cost-effective low-power sensors.