Analyse On Grain-boundary States In Poly-Si TFTs Using OEMS Measurement

Polycrystalline silicon thin film transistor (Poly-Si TFT) has broad application prospect in AMLCD for its high mobility, faster swith vilosity and higher current driving ability. However, owing to grain boundary (GB) existing in poly-Si thin film which generates a large number of dangling bonds and defectes in the band gap, poly-Si TFT appears very complex electrical properties as continuous and discrete band gap states formed to be traps. Therefore, research and development on effective acquisition of GB microscopic trap states information in poly-Si TFT and construction of quantitative analysis theory has been the foundation of research and application in both poly-Si thin film material and device. It also becomes a key problem need to be solved as soon as possible.In this paper, Opto-electronic modulation spectroscopy (OEMS) technique is applied in poly-Si TFTs fabricated by SPC method to study the distribution of GB traps and interface states in poly-Si thin film material. Firstly, electrons in GB traps and interface states are excited circularly by infrared light with modulated wavelength to obtain high resolution OEMS response spectrum formed by photon transition and OEMS phase spectrum between optical modulated signals. Secondly, analytical theory of OEMS in poly-Si is built in the foundation of Swanson's theory. Device model of poly-Si TFTs is analyzed, channel current under bias and optical modulation is calculated, OEMS current modulation effect is discussed and expressions of OEMS current first order response to different band gap states are given. Once again, judgement of traps properties using OEMS response and phase spectrums is given through quantitative analysis in optical emission function. Finally, applying the established theory to analyse OEMS spectrum, four discrete GB trap states and one continuous interface state are obtained from 0 eV to 2.4 eV. Meanwhile, model of hydrogenated amorphous silicon between GB is proposed to explain phenomenon of band gap broadening in OEMS from 1.2 eV to 2.4 eV, and four discrete GB trap states and one continuous interface state are obtained. Extracted trap states are in good agreement with documents, and the reliability of OEMS technique is further verified.Therefore, OEMS technique provides a comprehensive and direct analytical method for deep study GB states distribution and characteristics in poly-Si TFT device and further reveal physical mechanism of poly-Si TFTs.