As an active material, ZnO nanowires have many advantages in electronic devices due to their low defect density, bottom-up growth, and ability to form single crystals. However, these advantages can only be realized if high quality contacts can be made to the nanowires, which require expensive techniques such as electron beam lithography. To fabricate nano devices on a large scale, this work focuses on combining bottom-up nanowire growth with nanogap fabrication using adhesion lithography (A-Lith); a technique for producing coplanar nanogaps with a spacing of less than 10 nm to tens of nanometers. A-Lith uses self-assembled monolayers to alter the adhesion forces between metals or oxides, which can be overlapped and peeled away from each other to form asymmetric or symmetric nanogaps.

To realize bottom-up nanowire devices using A-Lith, the growth of ZnO nanowires was first optimized by controlling the seed layer geometry to obtain horizontal nanowires, which were used to bridge the nanogap (40 nm) created by A-Lith between ZnO/Ti and Au. The nanowires were grown using the hydrothermal method in which 60 nm thick ZnO, coated with 40 nm Ti (ohmic contact), acted as the seed layer. To make the Schottky contact, 40 nm thick Au was thermally evaporated, resulting in nanowire Schottky diodes. The diodes exhibited rectification ratio of > 10$4$ at low operating voltage of ±1 V with minimal OFF state currents of < 30 pA. Compared to thin film ZnO Schottky diodes of similar dimensions, the fabricated devices showed an improvement in the rectification ratio at ±1 V of at least 10$2$, which is attributed to low device leakage currents.

To test the Schottky device as a multi-purpose sensor, the device was characterized as a function of temperature, which demonstrated the device’s potential as an ambient temperature sensor. Varying the temperature from 20 °C to 120 °C changed the forward and the reverse currents by a factor of 10$4$ and 10$6$, respectively. The upper limit of the temperature detection was 150 °C, after which the device failed. The near UV (NUV) response of the device also showed the device’s capability as a continuous NUV sensor (395 nm) with a transient time of 20 s. Additionally, the device response scaled linearly to the optical power density and physical width of the device.

Finally, the Schottky devices were gated to form gate-tunable Schottky diodes. Both top and bottom gated device structures were fabricated. The top-gate devices exhibited depletion mode of operation, while the bottom-gate devices exhibited operation in both depletion and accumulation modes. The bottom gate was more effective in changing the rectification ratio of the devices, wherein a change of ∼10$\mathrm{<mrow\; data-mjx-texclass="ORD">0.5</mrow>}$ was observed when the gate voltage was varied from -6 V to 6 V. Further, it was noted that the gate was more effective in controlling the reverse bias currents of both the device structures, such that the devices acted like a transistor in the reverse bias region with constant transconductance. The operation of the devices was similar to that of source-gated transistors, in which the current between the drain and source was controlled by modulating the depletion of the Schottky contact by the applied gate potential.