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(Color online) (a) Schematic illustration of process steps for fabricating flexible single crystal silicon transistors with doped contacts on plastic substrates. A spin-on dopant (SOD) provides the phosphorus dopant. A spin-on glass (SOG) serves as a mask to control where dopant diffuses into the silicon. After doping, the silicon is removed from the wafer and transfer printed onto a plastic substrate where device fabrication is completed; (b) optical images of an array of devices on a plastic substrate.
(Color online) (a) Width normalized resistance measured between two contacts as a function of L on uniform, highly doped thin film. The intercept of a linear fit to these data gives a contact resistance. Inset shows the test structures for evaluating the contact resistance; (b) phosphorus concentration in a silicon film with patterned doping, as evaluated by TOF-SIMS.
(a) Typical current–voltage characteristics of a single crystal silicon transistor with doped contacts on a PET substrate, with and . From bottom to top, varies from ; (b) transfer curves of devices with channel lengths, from top to bottom, of 97, 72, 47, 22, 7, and . The channel width in each case is ; (c) width-normalized resistance of devices in the ON state as a function of channel length at different gate voltages. The solid lines represent linear fits. The scaling is consistent with contacts that have negligible influence on device performance for this range of channel lengths. Inset shows the sheet conductance , determined from the reciprocal of the slopes of the linear fitting in (c), as a function of gate voltage; (d) effective mobility, evaluated in the linear regime, as a function of channel length for the devices with undoped (triangle) and doped (square) contacts.
Bending and fatigue tests of single crystal silicon transistors with doped contacts on flexible PET substrates, for several different channel lengths: (a) normalized effective mobility as a function of bend induced strain (bend radius). Negative and positive strains correspond to tension and compression, respectively; (b) normalized effective mobility after bending (to a radius of ; 0.98% strain) and unbending the devices several hundred times. No significant change in device properties is observed.
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