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(a) Basic monomers of synthetic melanin. (b) Sketch of the experimental set up for capacitance-voltage measurements (left) on melanin based MIS structure and corresponding equivalent circuit (right). The displayed bias configuration refers to MIS structure biased under depletion condition.
(a) Writing step via hole injection for melanin-based MIS structure on pSi when biased with consecutive voltage pulses Vp = −2.5 V of different duration time tp. The inset shows the behavior of the absolute value of the flat band voltage shifts when increasing the pulse duration. (b) Hypothesized band diagram at the equilibrium and after the negative pulse application producing hole injection from silicon and trapping in the melanin layer. The presence of dipolar space charge developing after the application of the negative bias is also represented. (c) Signal retention vs. time after applying a writing pulse of tp = 60 s. The pulse duration has been chosen in consideration of the time needed for the achievement of memory window width saturation (see inset Fig. 2(a)).
(a) Writing step via polarization effect for melanin-based MIS structure on pSi when biased with consecutive voltage pulses Vp = +5 V with different duration time tp. The inset shows the behavior of the absolute value of the flat band voltage shifts when increasing the voltage pulse duration. (b) Band energy diagram at the equilibrium after the application of the voltage pulse and dipole charge distribution inside the melanin layer. The dipolar space charge distribution after the application of the positive bias is represented too. (c) C vs. V curves collected before and after applying a single writing pulse Vp = +5 V with duration tp = 60 s. The inset shows the signal retention vs. time. The arrows indicate the starting condition and those soon after the application of the pulse and up to the achievement of the maximum window width.
C vs. V readouts under vacuum on melanin based MIS devices after the application of positive (Vp = +5.0 V, tp = 10 s, blue) and negative (Vp = −5.0 V, tp = 10 s, red) bias pulses. The C vs. V curve of the un-programmed device (black line) is represented too. A memory window width (around one volt) was detectable only when applying a positive voltage pulse, meaning that this occurs via a persistent polarization effect after the application of the external bias.
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