【International Papers】Gas-sensitive properties of PECVD nanocrystalline Ga₂O₃ thin films doped with Zn

      Researchers from the National Research Tomsk State University have published a dissertation titled "Gas-sensitive properties of PECVD nanocrystalline Ga₂O₃ thin films doped with Zn" in Results in Surfaces and Interfaces.

Background

      Gallium oxide (Ga₂O₃) has attracted considerable attention as a promising material for power electronics and gas sensors due to its outstanding fundamental properties, which even surpass those of well-established SiC and GaN. Gas sensors based on Ga₂O₃ exhibit high sensitivity to various gases over a wide temperature range (from room temperature to 1000 °C) and demonstrate excellent chemical and thermal stability. However, pure β-Ga₂O₃ thin films show limited gas sensitivity, so it is often necessary to enhance their response by introducing dopants or additives of other semiconductors with smaller bandgaps and catalytic activity, such as SnO₂, In₂O₃, or ZnO. ZnO is a widely studied semiconductor for gas sensors, known for its high chemical and thermal stability, and is commonly used to modify other metal oxides to improve gas-sensing performance. Previous studies have focused on Ga-doped ZnO thin films, which exhibit high gas sensitivity, whereas research on Zn-doped Ga₂O₃ (ZGO) remains limited. Existing reports indicate that ZGO can achieve high responses to O₂ at moderate operating temperatures (400–500 °C). Metal oxide thin films used in gas sensors are typically polycrystalline, and the grain size has a significant effect on gas-sensing performance; films with nanocrystalline grains (10–100 nm) often show enhanced sensitivity. Therefore, nanocrystalline Zn-doped Ga₂O₃ (nc-ZGO) thin films represent a promising material for high-performance sensor layers. In this study, nc-ZGO thin films were deposited via PECVD, and their gas-sensitive properties were systematically investigated for the first time, providing a foundation for the development of high-performance gas sensors.

Abstract

      Nanocrystalline Ga₂O₃ thin films doped by Zn (nc-ZGO) were synthesized by plasma-enhanced chemical vapor deposition. Their gas-sensitive properties have been comprehensively investigated. The thin films exhibit high response and operation speed under exposure to industry-relevant gases such as CO, H₂, NH₃ and NO₂. The highest responses are achieved under H₂ exposure. The responses to 500 ppm and 10⁴ ppm of H₂ are 2.99 a. u. and 24.5 a. u., respectively, at the maximum response temperature of 500 °C. The sum of the response and recovery times do not exceed 27.4 s. The nc-ZGO thin films show only a weak drift of their gas-sensitive characteristics during long-term exposure to CO, H₂, and NO₂ as well as and a weak dependence of the gas-sensitive characteristics on the humidity level in the 10–50 % relative humidity range. A mechanism of the sensing effect of nc-ZGO thin films is proposed within the current paradigm.

Highlights

      ● Nanocrystalline Ga₂O₃films heavily doped with Zn (ZGO) were synthesized by PECVD.

      ● ZGO films surface morphology is characterized by grains with a size of 12–25 nm.

      ● Responses to 500 ppm and 10⁴ppm of H₂are 2.99 and 24.5 at 500 °C.

      ● Response and recovery times in sum do not exceed 27.4 s at 500 °C.

      ● ZGO films demonstrate long-term stability of the gas-sensitive characteristics.

Conclusion

      The gas-sensitive properties of nanocrystalline Zn-doped PECVD deposited Ga₂O₃ thin films to a number of habitat and industry-relevant gases have been comprehensively studied. The concentration of Zn in the films is 1.64 at%, the impurity homogeneously distributed over the area of nanocrystalline Ga₂O₃ thin films. The surface morphology of Zn-doped nanocrystalline Ga₂O₃ thin films is presented by grains with the size of 12–25 nm. The samples are characterized by high transmittance, which reaches almost 90 % at λ = 450 nm. The Zn doped nanocrystalline Ga₂O₃ thin films exhibit high response and speed performance under exposure to CO, H₂, NH₃, and NO₂. The highest responses were achieved under exposure to H₂. At the maximum response temperature corresponding to 500 °C, the responses to 500 ppm and 104 ppm of H₂ were 2.99 a. u. and 24.5 a. u., respectively. The sum of the response and recovery times did not exceed 27.4 s. Compared to sensors based on nanocrystalline films of other metal oxides, the feature of the sensors based on Zn-doped nanocrystalline Ga₂O₃ thin films is a weak drift of gas-sensitive characteristics during long-term tests under exposure to CO, H₂, and NO₂, at relatively high operating temperatures and small grain sizes. In addition, the samples are characterized by a weak dependence of gas-sensitive characteristics on humidity in the relative humidity range of 10–50 %. Within the current paradigm, the mechanism of the sensing effect of Zn doped nanocrystalline Ga₂O₃ thin films is proposed. The gas-sensitive characteristics achieved for sensors based on Zn doped nanocrystalline Ga₂O₃ thin films indicate their potential for detecting pre-explosive H₂ concentrations in extreme operating conditions and ensuring safety in nuclear and hydrogen energy applications.