Phosphorene for flexible nanoelectronics

Few-layer black phosphorus (BP) has attracted ever more attention since its debut last year as a new 2D layered semiconductor.^{1, 2} The puckered crystal structure distinguishes its physical properties from plane-structured graphene with a thickness-tuned bandgap ranging from 0.3 to ∼2eV. Its exceptional electrical properties include high hole mobility (∼1000 cm²/Vs) and high field-effect current modulation (10⁵).^{2, 3} These properties enable both high-speed and low-power nanoelectronic applications beyond the demonstrated performance capability of graphene or transitional metal dichalcogenides (TMDs).

We have successfully prepared the first flexible devices and circuits based on few-layer BP on a highly bendable polyimide (PI) substrate (see Figure 1). To fabricate BP FETs, we first mechanically exfoliated few-layer black phosphorus onto the flexible polyimide substrate as channel material, and then used e-beam lithography to define the source and drain electrodes. An embedded bottom gate was used for field-effect control. Few-layer BP obtained by mechanical exfoliation from high-quality bulk BP crystal is extremely sensitive to the ambient environment, with degradation typically commencing within one hour of exposure. We therefore developed an effective encapsulation strategy, including immediate polymer coating after exfoliation and aluminum oxide encapsulation after source/drain metal contact lift-off to help maintain the pristine properties of the BP flakes.⁴

Figure 1. Image (left) and schematic illustration (right) of flexible black phosphorus (BP) devices on a polyimide (PI) substrate.

We measured the electrical performance of our flexible BP devices, including DC current transfer and output characteristics, in ambient environment at room temperature, with low-field mobility extracted from the measured transfer characteristics. The high carrier mobility and high field-effect current modulation obtained from our flexible BP devices enable future high-speed and low-power flexible smart systems. Our devices afford maximum mobility of ∼310 cm²solVs, more than five times higher than the fastest TMD flexible transistors.^{5, 6} In addition, the flexible BP devices were mechanically robust after 5000 bending cycles at 1.5% tensile strain with no significant performance degradation observed: see Figure 2(a).

Figure 2. (a) Transfer characteristics—output drain current (I_d) plotted against input gate voltage (V_g)—of the flexible BP device with the highest achieved hole mobility (μ_p), before and after 5000 cycles of bending at 1.5% strain. (b) Plot of the input (V_in) and output voltage (V_out) of a flexible BP non-inverting voltage amplifier. S: Source. D: Drain. 8.7 X: Magnification between output and input signal amplitudes.

The unique ambipolar transport characteristics (that is, that both positively and negatively charged carriers can conduct current under certain voltage bias conditions) and high on/off ratio of BP devices made it possible to produce an ambipolar digital inverter with an inverter gain of 4.6. For analog circuits, the strong current saturation few-layer BP together with high transconductance enables voltage amplifiers: see Figure 2(b). (Transconductance indicates the capability of the device to convert gate voltage input into drain current output.) The amplification factor of 8.7 is the highest value reported for flexible thin-film amplifiers from 2D materials.^7–9

Essential circuit blocks for modern communication systems include analog frequency doublers and amplitude-modulated (AM) demodulators, both of which are enabled by the ambipolar transport characteristics.¹⁰ We have successfully demonstrated frequency doublers with operation frequency of 124kHz and a BP receiver that demodulated a baseband music signal.

In summary, we have investigated flexible BP devices and fundamental digital and analog circuits that for the first time feature outstanding electrical and mechanical performance. Our findings show phosphorene is a promising 2D semiconductor for advanced flexible nanoelectronics. Our next step will be to improve the operation frequency of the circuits from sub-MHz to GHz by optimizing the device structure, for instance, by employing an isolated embedded gate structure and top gate structure.

This work is supported in part by the Office of Naval Research under contract N00014-1110190, and the National Science Foundation NASCENT Engineering Research Center under Cooperative Agreement EEC-1160494.