Sustained electron tunneling at unbiased metal-insulator-semiconductor triboelectric contacts

Generating sufficient current density for powering electronic devices remains as one of the critical challenges of mechanical energy harvesting techniques based on piezo and triboelectricity, mainly due to the high impedance of the insulating material systems. Here we report on producing sustainable tunneling current using an unbiased, triboelectrically charged metal-insulator-semiconductor (MIS) point contact system, consisting of p-type silicon, silicon oxide and a metal tip. The native thin oxide (~ 1.6 nm) on the silicon surface provides a natural pathway for quantum mechanical tunneling of the triboelectrically generated electrons into the silicon substrate. Lateral back and forth sliding motion of the tip, irrespective of the direction of motion, generates a constant direct current (d.c.) with very high current density. The measured current shows an exponential decay with the thickness of oxide layer deposited with atomic layer deposition (ALD), confirming the quantum mechanical tunneling mechanism. It is proposed that the contact potential difference enhanced by triboelectric charging provides potential difference between metal point contact and the substrate. With single metallic micro probe sliding on a moderately doped p-type silicon, an open circuit voltage (V_oc) of 300–400 mV and a short-circuit direct current (I_sc) of 3–5 μA (a corresponding high current density, J, in the order of 1–10 A/m²) have been observed. It is predicted from conductive-atomic force microscopy (C-AFM) experiment that the theoretical J can be as high as 10⁴ A/m². This new concept has the potential as a green energy harvesting technique where a broad range of material candidates and device configurations could be used.