【International Papers】O₂ sensors for λ-probe based on β-Ga₂O₃ microcrystals fabricated from к-Ga₂O₃ epitaxial film by thermal annealing

      Researchers from the National Research Tomsk State University have published a dissertation titled " O₂ sensors for λ-probe based on β-Ga₂O₃ microcrystals fabricated from к-Ga₂O₃ epitaxial film by thermal annealing " in Sensors and Actuators: B Chemical.

Author's Introduction

      Aleksei V. Almaev is the Head of the Laboratory of Metal Oxide Semiconductors at the Research and Development Center for Advanced Technologies in Microelectronics of the Tomsk State University. He received his Ph.D. in 2018 in Physical and Mathematical Sciences. His research interests are in the area of metal oxides, related devices and functional coatings. He has published more than 40 scientific articles in peer-reviewed international and national journals and conferences within the last 5 years.

Article Introduction

      The λ probe is used to analyse the composition of combustion gases mixture in internal combustion engines. It has been demonstrated that it is sufficient to measure the O₂ concentration in the mixture to control the λ. Many research groups have developed sensors based on semiconducting metal oxides to measure the O₂ concentration in the exhaust gases mixture. The SMO-based sensors for the λ-probe must meet the requirements of λ-probe operation, high operating temperatures of 600–900 °C and effect of composition of exhaust gases mixture.

      Monoclinic β-Ga₂O₃ is a very attractive material for high-temperature O₂ sensors due to its high sensitivity and stability. Metastability of other Ga₂O₃ polymorph films causes the practical lack of interest in the study of their gas-sensitive properties. We previously shown that pure κ-Ga₂O₃ 500 nm-thick films grown by halide vapour phase epitaxy had a developed surface and exhibited sensitivity to various gases, including O₂. However, the low operating temperatures of 400–550 °C and the low response to O₂ do not allow to consider κ-Ga₂O₃ films as sensitive elements of λ-probes. At the same time, our previous results indicate that the formation of heteropolymorphic structures containing at least two polymorphs in their composition are effective ways to enhance the sensitivity of the metastable polymorphs of Ga₂O₃. It can be concluded that one of the methods to create heteropolymorphic structures is the annealing of the Ga₂O₃ metastable polymorphs at conditions when the transition of one polymorph into another is not yet complete.

      In the this work we have investigated in detail the gas-sensitive properties of β-Ga₂O₃ microcrystals obtained by means of annealing at 1000°C of thick epitaxial κ-Ga₂O₃ films on GaN/Al₂O₃ template with deposited Pt contacts. As the results of our researches it was shown, that β-Ga₂O₃ microcrystals on GaN self-formed during annealing (30 min at 1000°C in air) of κ-Ga₂O₃/GaN heterostructure. The β-Ga₂O₃ microcrystals with a latheral size of 4–12 µm demonstrated increased electrical conductance and higher gas response compared to the epitaxial κ-Ga₂O₃ and β-Ga₂O₃ films. The response of β-Ga₂O₃ microcrystals to 5 vol.% of O₂ was 11.33 at the temperature of 650 °C. The gas-sensitive characteristics of the β-Ga₂O₃ microcrystals change weakly under cyclic O₂ exposure, as well as subjected to different relative humidity of the gas mixture in the range of 8–70% and depend significantly on the applied voltage polarity. The studied samples also showed high responses to NO and CO₂. The responses at temperature of 650°C to 1 ppm of NO and 1 vol% of CO₂ were 1.7 and 10.4, correspondingly. At this temperature oxygen chemisorbed on the surface of the β-Ga₂O₃ microcrystals in the form of O^2-, causing the formation of a layer depleted of charge carriers on the surface of the semiconductor, which leads to an increase in its resistance. The obtained effect was considered in terms of a promising use as sensitive elements of λ-probes. The β-Ga₂O₃ microcrystals were able to respond to changes in the O₂ concentration in the mixture simulating exhaust gases at λ = 1. The structural and phase changes induced by the high-temperature annealing were reviewed as the key factors for the high sensitivity of β-Ga₂O₃ microcrystals to O₂.