Fabrication and Characterization of Microcrystalline Silicon Near Infrared Photodiode Detector Pixel Circuit on Glass Substrate For LargeArea Electronics

This thesis is focused on development of near infrared (NIR) photodetectors on glass substrate at low temperatures for large area electronics applications. In the first part of this thesis we study the optical properties of plasma enhanced chemical vapor deposition (PECVD) prepared hydrogenated microcrystalline silicon (μc-Si:H) material. We demonstrate that μc-Si:H film has absorption coefficient of 10 cm−1 at wavelength of 850 nm, which is more than three orders of magnitude higher than the absorption coefficient in conventionally used hydrogenated amorphous silicon (a-Si:H) material in large area electronics. However, in spite of its high absorption coefficient in NIR region, we demonstrate that metal-semiconductor-metal (MSM) detectors based on μc-Si:H have a weak dynamic range (DR) of operation of about 50 at wavelength of 850 nm per 1 mW/cm of incident optical power density. Furthermore, we demonstrate that NIR DR for μc-Si:H MSM detectors is very close to the one for a-Si:H MSM device and hence μc-Si:H detector is not efficient for NIR light detection. As a result, we focused on photodiodes instead of MSM detectors as an alternative device structure with high DR capability. For this purpose we developed and characterized doped μc-Si:H layers and fabricated an n type-intrinsicp type (n-i-p) μc-Si:H photodiode with intrinsic layer thickness of 343 nm. This device shows an external quantum efficiency (EQE) of 2 % at 850 nm with a DR of almost 500 for 1 mW/cm of incident optical intensity which is an order of magnitude higher than the one for μc-Si:H MSM detector. By increasing the intrinsic layer thickness to about 2 μm we observed that the EQE was increased to 8 % without a notable change in DR due to proportional increase in dark current level as compared to the photocurrent level. By incorporation of 1 μm thick textured AZO back reflector to the photodiode structure, however, we were able to reach EQE of 19.2 % with DR of more than 1000 at 850 nm per mW/cm of incident optical density, which is 20 times higher than the one for μcSi:H MSM device. Furthermore, we developed HSpice circuit model parameter extraction method for our photodiode demonstrating a non ideality factor of 1.54, reverse saturation current of 4.94 × 10−11 A, shunt resistance of 1.35 GΩ, series resistance of 191.3 kΩ, and parallel capacitance of 40 pF for area of 500× 500 μm. In the second part of this thesis, we focused on development of a-Si:H thin film transistor (TFT) in order to fabricate pixel circuits based on our developed photodiode to test its feasibility for implementation of 2D imaging arrays. During the design of our TFT fabrication process, integration of our μc-Si:H photodiode had been taken into consideration and hence a bottom gate structure was adopted compared to the top gate one. In order to design a hybrid TFT/photodiode pixel circuit we needed to obtain accurate HSpice model representation for our TFT. As a result we adopted the HSpice Level 61 transistor model and presented an step by step parameter extraction procedure for our TFT obtaining the 29 TFT parameters in this