CuO is a multifunctional metal oxide excellent for chemiresistive gas sensors. In this work, we report CuO-based NO 2 sensors fabricated via chemical vapor deposition (CVD). CVD allows great control on composition, stoichiometry, impurity, roughness, and grain size of films. This endows sensors with high selectivity, responsivity, sensitivity, and repeatability, low hysteresis, and quick recovery. All these are achieved without the need of expensive and unscalable nanostructures, or heterojunctions, with a technologically mature CVD. Films deposited at very low temperatures (≤350 °C) are sensitive but slow due to traps and small grains. Films deposited at high temperatures (≥550 °C) are not hysteretic but suffer from low sensitivity and slow response due to lack of surface states. Films deposited at optimum temperatures (350−450 °C) combine the best aspects of both regimes to yield NO 2 sensors with a response of 300 % at 5 ppm, sensitivity limit of 300 ppb, hysteresis of <20%, repeatable performance, and recovery time of ∼1 min. The work demonstrates that CVD might be a more effective way to deposit oxide films for gas sensors.

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Unlike most metal oxides, copper oxides (Cu2O and CuO) show p-type conductivity, which is required for many electronic applications. Cu2O has been reported to have relatively high hole mobility (256 cm2 V–1 s–1). Unfortunately, the thin-film deposition of pure Cu2O is not trivial. Pure-phase Cu2O is formed in a narrow pressure–temperature window, only under precise oxygen potential. To obtain pure-phase Cu2O, we have deposited Cu using chemical vapor deposition (CVD) and performed postdeposition oxidation without breaking the vacuum. As Cu2O is very sensitive to oxygen potential, the conditions for oxidation were derived from thermodynamic simulations to obtain pure-phase Cu2O. Hall measurement illustrates a significant Hall mobility of 80.9 cm2 V–1 s–1 for Cu2O. Films are intrinsically p-type doped with a carrier density of 2.3 × 1016 cm–3. To show its device application, thin-film transistors were fabricated on the Cu2O and CuO films, showing typical p-channel accumulation mode transistor characteristics with field-effect mobilities of 4.3 × 10–2 and 2.4 × 10–3 cm2 V–1 s–1, respectively. Overall, material and electrical characterization show that metal-organic CVD along with oxidation is a promising option to achieve pure-phase Cu2O that can be used for electronic applications.

There are very few p-type semiconductors available compared to n-type semiconductors for positive sensing response for oxidizing gases and other important electronic applications. Cupric oxide (CuO) is one of the few oxides that show p-type conductivity, useful for sensing oxidizing gases. Many researchers obtained CuO using the chemical and solid-state routes, but uniformity and large-area deposition have been the main issues. Chemical vapor deposition is one such technique that provides control on several deposition parameters, which allow obtaining thin films having crystallinity and uniformity over a large area for the desired application. However, CuO-chemical vapor deposition (CVD) is still unfathomed due to the lack of suitability of copper precursors based on vapor pressure, contamination, and toxicity. Here, to address these issues, we have taken four Cu complexes (copper­(II) acetylacetonate, copper­(II) bis­(2,2,6,6-tetramethyl-3,5-heptanedionato), copper­(II) ethylacetoacetate, and copper­(II) tert-butylacetoacetate), which are evaluated using thermogravimetry for suitability as a CVD precursor. The decomposition behavior of the complexes was also experimentally confirmed by depositing CuO thin films via CVD. Phase purity, decomposition, volatility, growth rate, and morphological characteristics of the films are investigated in detail. Analysis suggests that copper­(II) tert-butylacetoacetate has the highest vapor pressure and growth rate at a low temperature, making it the most suitable precursor for high-throughput CVD. Further, to investigate the role of these precursors, films deposited using Cu complexes were subjected to gas sensing. The CuO gas sensor fabricated on glass shows pronounced NO2 sensing. The sensing results of CuO films have been explained from the standpoint of roughness, morphology, and unpassivated bonds present on the surface of films and vapor pressure of precursors. The higher density of surface state and the lower resistivity of the Cu­(tbaoac)2 film lead to a sensor with higher responsivity and sensitivity (down to 1 ppm). These precursors can probably be utilized to improve the performance of other metal oxide gas sensors, especially Cu2O and Cu-III-O2.