Ultra-low power ZnO TFT based Ozone Sensors

Open Access
Author:
Rosado, Alexander O
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 31, 2017
Committee Members:
  • Thomas Nelson Jackson, Thesis Advisor
Keywords:
  • Sensor
  • Low-power
  • TMD Etching
  • Thin-film Transistor
Abstract:
Zinc oxide thin film transistor (ZnO) based sensors have been fabricated to detect ozone (O3) for ranges around hundreds part per billion (ppb) with < 1 µW power consumption. The sensor works by ZnO surface depletion due to charge trapping adsorbates from O3 (i.e. O2-). The low power consumption of these sensors is due to a 25 milliseconds recovery via ultraviolet (UV) light pulse per 40 seconds on a light-emitting diode (LED) of 365 nm wavelength which desorbs charge traps, removes the surface depletion and resets the sensor to its baseline current. Reliable properties for optimum sensor performance include good sensitivity for small concentration ranges of target gas, good electrical stability when unexposed to target gas, and good target gas selectivity. In addition to showing very good sensitivity, these sensors show very good electrical stability after post-deposition annealing of ZnO and partial-contact passivation, leaving the ZnO sensing area open to ambient, which also shows negligible hysteresis and improves the electrical charge transport of the as-deposited ZnO. Finally, a path to attain gas selectivity is via the use of an organic film, such as indigo dye, as a filter deposited over the gas sensors is demonstrated. Due to its low power sensor operation, it provides a path for energy harvesting applications. Among other types of materials with potential transistor based sensor applications are transition metal dichalcogenides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2). Due to the bandgap’s number of layer dependence with these materials, much of the interest in is optoelectronics oriented. A reliable etching process is shown via reactive ion etching (RIE) towards layer-by-layer etching, where the etch rate of both chemical vapor deposition (CVD) grown and exfoliated TMDs differ significantly. By taking advantage of the knowledge of device preparation for ZnO thin film transistors (TFT) and the etch rate recipe of TMDs, device fabrication of TMD based transistors can be reliably prepared by patterning the TMD film and acquiring the desired TMD film thickness for device performance. The RIE process consists of using argon (Ar): tetrafluoromethane (CF4) plasma, which is useful for fast patterning, and O2 plasma, which is primarily beneficial for slow and controllable layer-by-layer thinning of the TMD film after patterning. Using the Ar: CF4 plasma, variations were observed for CVD grown with respect to the exfoliated material, where O2 plasma showed no variation between CVD and exfoliated. The patterning and thinning of TMDs provides a pathway to a controllable and reproducible process towards fabrication of TMD based devices such as transistors, which is the first step in moving towards the material’s potential applications for electronics and optoelectronics.