S. Chen, W. Behn, W. Fu, C.P.Y. Wong, K.E.J. Goh, P. Grutter
McGill University,
Canada
Keywords: 2D materials, optoelectronics, semiconductors
Summary:
Persistent photoconductivity is a light-induced phenomenon observed in a wide variety of materials and it is characterized by a slow drop in conductivity after illumination is terminated. Monolayer transition metal dichalcogenides are direct bandgap semiconductors [1] that have garnered widespread scientific attention for novel optoelectronic applications. However, the response time of optoelectronic devices fabricated from transition metal dichalcogenides is often limited by persistent photoconductivity with rise times and fall times ranging from milliseconds to days [2, 3, 8]. Thus, it is essential to elucidate the mechanisms responsible for persistent photoconductivity in two-dimensional transition metal dichalcogenides and understand how to tune its properties. Some proposed mechanisms for persistent photoconductivity include charge trapping in adsorbed molecules [3, 6, 7], trapping in SiO2 [2,5] and trapping from defects in the flake itself [7]. Yet, there is no consensus on the relative importance of these mechanisms in realizing persistent photoconductivity. In this study, we use Kelvin probe force microscopy to visualize the slow reshuffling of charge upon illumination and termination of illumination in mechanically-exfoliated and chemical vapor-grown monolayer WS2 on silicon dioxide substrates. We aim to study how illumination intensity, illumination wavelength, doping level, and sample thickness influence the characteristics of persistent photoconductivity. Source and drain electrodes are patterned onto WS2 flakes for photocurrent measurements. Time-resolved surface potential and source-drain photocurrent measurements are compared to reveal the mechanisms responsible for persistent photoconductivity in two-dimensional transition metal dichalcogenides. Initial measurements show that surface photovoltage and photocurrent saturate at different illumination intensity.