Carrier transport and trapping in a-Si:H films under plasma processing

Carrier transport is an important factor that determines the performances of solar cells and transistors [1]. It is often limited by carrier trapping, associated with various defects. The defects are created during fabrication processes using various plasmas; however the defect creation kinetics is not known. Here, we demonstrate the detection of the trapped carriers in a-Si:H films under plasma enhanced CVD, and discuss the carrier trapping and defect kinetics. Using an optically pump-probe technique, we detected the trapped carriers in an a-Si:H films during growth by plasma enhanced CVD [2]. An a-Si:H film growing on a glass substrate was illuminated with pump and probe light. The photocurrent induced by the pump was measured throughout the growth and postgrowth annealing [3]. An increment in the photocurrent induced by the pulsed probe was also measured. The trapped carrier density was determined from the increment since it originates from de-trapping of carriers. We found that the trapped carrier density was typically 10$^{18}$cm$^{-3}$. It was dependent on the growth temperature, and minimized at 473K. Interestingly, the detected trapped carriers were distributed uniformly in the direction of growth, and they were reduced during postgrowth annealing. \\[4pt] [1] S. Nunomura et. al., \textbf{Adv. Mater. 26}, 7555 (2014).\\[0pt] [2] S. Nunomura et. al. \textbf{AIP Advances 4}, 097110 (2014).\\[0pt] [3] S. Nunomura et. al. \textbf{Appl. Phys. Express. 6}, 126201 (2013).