(152h) Effects of Alkane Dielectrics in Chemically-Sensitive Field-Effect Transistors Functionalized with Metal-Organic Frameworks

There is a growing demand for distributed gas-sensing networks with sensors that have ultra-low-power consumption, high sensitivity, and selectivity. One such device, the chemically sensitive field-effect transistor (CS-FET), meets these criteria(1). The CS-FET is structurally similar to a standard field-effect transistor, except the gate metal is replaced by a sensing layer whose work function shifts when gas molecules adsorb. However, the CS-FET is susceptible to baseline drift from humidity adsorption at the hydrophilic SiO₂ gate dielectric, which can cause a 50% increase in source-drain current at just 40% relative humidity. A separate issue is that standard sensing materials are non-selective metals. Development of sensing materials that are selective and do not suffer humidity drift is needed to improve the performance of the CS-FET devices.

Towards addressing these problems, we have investigated the potential of metal-organic framework (MOF) sensing layer in CS-FET devices. MOFs are an exciting class of materials for chemical sensing because the adsorbate-driven work-function-shift is highly analyte-MOF specific. Furthermore, we have functionalized the SiO₂ dielectric of the CS-FET with hydrophobic self-assembled monolayers octadecyl-trichloro-silane (OTS) and dimethyl-dichloro-silane (DDMS). These monolayers can be subsequently coated with MOF sensing layers; in our case, HKUST-1 is used as the chemically sensitive layer for humidity because HKUST-1 is known to have a strong affinity for water(2).

The CS-FET devices are functionalized with DDMS or OTS. Sensors with just OTS have nearly no response to humidity, with about less than a 5% increase in source-drain current at 40% humidity. Sensors with just DDMS have a 10% increase in current starting at 40% humidity. Once these SAM-modified devices are coated with an HKUST-1 sensing film, we find distinct sensing dynamics and, surprisingly, different sensitivities. With OTS and HKUST-1, the device responds within seconds and is quite sensitive, achieving nearly 800% initial current at 40% relative humidity compared to dry air. However, with DDMS, the response is much slower and much less sensitive, with only 2x initial current at 40% relative humidity. We attribute the better dynamics to the better coverage of OTS compared to DDMS on the SiO₂. The difference in sensitivity is due to a band bending effect induced by the monolayer in the underlying silicon channel. Band bending is confirmed by differences in the electrical characteristics of the devices.

Inspired by these results we have begun to work on sensors that replace the SiO₂ dielectric completely with an alkane dielectric, 1-octadecene, that grows directly on silicon. Preliminary results show that a device with just this monolayer has even a weaker response to humidity than OTS. We plan to evaluate this monolayer as a dielectric for an HKUST-1-based humidity sensor and evaluate the effect of the monolayer on the electrical properties of the CS-FETs.

References:

1. H. M. Fahad, N. Gupta, R. Han, S. B. Desai, A. Javey, Highly Sensitive Bulk Silicon Chemical Sensors with Sub-5 Nanometer Thin Charge Inversion Layers. ACS Nano. 12, 2948–2954 (2018).
2. D. W. Gardner, X. Gao, H. M. Fahad, A.-T. Yang, S. He, A. Javey, C. Carraro, R. Maboudian, Transistor‐based work function measurement of metal‐organic frameworks for ultra‐low‐power, rationally designed chemical sensors. Chem. – A Eur. J. (2019), doi:10.1002/chem.201902483.