Top-down fabrication and characterization of Zinc Oxide nanowire field effect transistors

Abstract

Top-down fabrication is used to produce ZnO nanowires by remote plasma enhanced atomic layer deposition (PEALD) over a SiO₂ pillar and anisotropic dry etching. Nanowire field effect transistors (FETs), with channel lengths in the range 18.6 to 1.3 µm, are produced in well-defined locations on a 150 mm diameter silicon wafer. The control of nanowire FET dimensions and locations is seen as the key to wafer-scale nanowire integrated circuit production. Measured electrical results show n-type enhancement behaviour and a breakdown voltage ≥75 V at all channel lengths. This is the first report of high voltage operation for ZnO nanowire FETs. Reproducible, well-behaved electrical characteristics are obtained and the drain current scales with 1/L, as expected for long-channel FETs.
This thesis reports for the first time that semiconducting quality of ZnO thin film can be achieved using remote PEALD at a minimum temperature of 100ºC. Remote PEALD technique offers flexible approach in controlling defects and impurities on the film even at low temperatures which remains a challenge in thermal ALD. Dry etch and remote PEALD processes have been optimised to produce high performance nanowire FET and semiconducting ZnO film. It is demonstrated that using the same CHF3 chemistry, ICP etched nanowires have field-effect mobility six times higher than RIE etched device. The surface roughness from RIE is shown to degrade nanowire FET electrical performance. Experimental results from remote PEALD optimisation show a stoichiometric balanced ZnO film when deposited at substrate temperature of 190oC, zinc precursor dose time of 1s and oxygen plasma time of 4s. Optimized ICP etched nanowire FET with 20 nm width and 10 µm long channel show a high field effect mobility of f 10 cm²/Vs. The electrical results from the pristine state of the nanowires without any post deposition treatments such as passivation demonstrates the feasibility for high performance top-down fabricated NWFETs in line with other unpassivated bottom-up fabricated devices.
The effect of atmospheric oxygen adsorption on nanowire surface has been investigated by measuring FET characteristics particularly the threshold voltage shift and hysteresis under different environments and at different gate bias sweep rates. These top-down unpassivated NWFETs are shown to be electrically reproducible when measured in ambient air even after 3 months of fabrication. The device is shown to be electrically air stable with a shift of threshold voltage of less than 11% for unpassivated and only 2% for passivated after 30-days of fabrication. In addition, passivation improves the field effect mobility by a maximum of 4-fold. Unpassivated device measured in vacuum showed a mobility improvement by 1.8 fold. These results show the electronic transport properties of the top-down fabricated nanowires can be influenced by the surface environments. In addition, hysteresis characteristics on top-down fabricated ZnO nanowire devices have been reported for the first time. Hysteresis measurement is sensitive to gate bias sweep rate. The maximum hysteresis obtained for this top-down ZnO NWFET device is 2.2 V, 0.8 V and 1.6 V when measured in ambient air, vacuum and after passivation, respectively. The hysteresis obtained for the unpassivated top-down fabricated ZnO NWFET in this work is smaller compared to other bottom up devices due to better interface quality of remote PEALD ZnO with SiO₂ gate dielectric.
From this top-down technology, 100 parallel nanowires with channel length of 20 µm are successfully fabricated for biosensing experiments. These devices consistently show n-type enhancement mode characteristics in different solutions. The BSA molecules with negative charges in buffer solution are successfully detected by the channel conductance modulation where the drain current reduced by 12 times. Meanwhile, Lysozyme molecules with positive charges in buffer solution are also successfully detected with an increase of drain current by 21 times. This top-down fabrication approach with low temperature film deposition is promising technology for future low-cost mass manufacturable sensors for health care and biomedical research.

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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