Effect of Oxygen on the Gas-Sensitive Properties of α-Ga2O3/ε-Ga2O3 Structures
Authors: Almaev Aleksei V. , Nikolaev Vladimir I. , Stepanov Sergey I. , Yakovlev Nikita N. , Pechnikov Alexei I. , Chernikov Evgeny V. , Kushnarev Bogdan O. 
Published: July 2021

Miniature O2 sensors with low energy consumption are of practical interest for the chemical and metallurgical industries, development of systems for analyzing the performance of internal combustion engines and as functional elements of artificial lung ventilation devices. The requirements for miniaturization, high sensitivity, speed and relative cheapness are satisfied by O2 sensors based on β-Ga2O3. The chemical and thermal stability of β-Ga2O3 allows developing gas sensors with extremely high operating temperatures of 400-1100 °C ensuring high reproducibility of their characteristics and high speed of operation. In turn, the high operating temperatures of O2 β-Ga2O3 sensors are their drawback causing high energy consumption.
Previously, we studied the effect of H2 on the gas-sensitive properties of the α-Ga2O3/ε-Ga2O3 structure with Pt contacts grown by the halide vapor phase epitaxy (HVPE) on the patterned sapphire substrates (PSS) [1]. Low operating temperatures, weak dependence of properties on the humidity and control of selectivity by means of voltage changes indicate that the research of α-Ga2O3/ε-Ga2O3 structures as sensitive elements of gas sensors is promising. We presume that in addition to increasing the applied voltage to the structures, the sensitivity of α-Ga2O3/ε-Ga2O3 to certain gases, in particular O2 can be increased by reducing the level of doping with a donor impurity. Thus this work is devoted to research the effect of O2 on gas-sensitive properties of α-Ga2O3/ε-Ga2O3 structures doped with Sn.
Ga2O3 films were grown by the HVPE in Perfect Crystals LLC. PSS of (0001) orientation and 430 microns in thickness were used as substrates. During the synthesis, the films were doped with Sn. To measure the gas-sensitive properties of the α-Ga2O3/ε-Ga2O3 structures Pt contacts were formed on their surface.
The samples consisted of a mixture of α and ε phases with the orientation (0001). Using scanning and transmission electron microscopy, it was found that the α-phase forms columnar structures at the top of the sapphire cone, and the ε-phase fills the gaps between the columns. The characteristic triangular shape of columnar structures indicates that they consist of α-Ga2O3 with trigonal symmetry. ɛ-Ga2O3 has a grain but not polycrystalline structure taking into account the XRD results.
Figure 1 shows the I-U characteristics of α-Ga2O3/ε-Ga2O3 structures at the exposure of O2 in a wide range of concentrations C
O2 from 2 to 100 % at temperature T = 200 °C. The current through the samples decreases with C
O2 increasing. In the range of the T = 180 - 200 °C, at T = 200 °C, the highest response to O2 are observed. It is worth noting that the operating temperatures of the α-Ga2O3/ε-Ga2O3 structures in the reaction to O2 are significantly lower than the operating temperatures of O2 β-Ga2O3 sensors. These low values of T are an advantage in developing gas-analytical systems with low energy consumption. The concentration dependences of the response at the T = 180 - 200 °C are approximated fairly accurately by a power function: response ~ C
O2

b
, where b is the exponent that depends on the temperature.
Figure 1-Effect of O2 on I-U characteristics of α-Ga2O3/ε-Ga2O3.
A decrease in the Sn impurity concentration in α-Ga2O3/ε-Ga2O3 from ~4×1018
cm-3
to ~1.5×1017
cm-3
leads to a significant sensitivity to O2 in the temperature range from 180 to 220 °C at low values of the applied voltage U ≤ 7.5 V. The I-U characteristics of α-Ga2O3/ε-Ga2O3 structures in a dry gas mixture N2 + O2 are described by the model of MSM structures based on the theory of thermoelectronic emission in diode approximation with the resistance of the semiconductor layer exceeding the resistance of the space charge regions at the metal/semiconductor interface. The sensory effect consists in the chemisorption of oxygen molecules on the surface of ε-Ga2O3 that according to SEM has a grain structure. As a result, the energy barrier at the grain boundaries of ε-Ga2O3 increases leading to a decrease in the current. The studied structures showed high sensitivity to relatively low concentrations (0.745 %) of H2 and CO at the T = 180-220 °C and practically did not react to the effects of NO2 and CH4.
This research was financially supported by the Russian Scientific Foundation, grant No 20-79-10043.
[1] A V Almaev, V I Nikolaev, S I Stepanov, A I Pechnikov, A V Chikiryaka, N N Yakovlev, V M Kalygina, V V Kopyev and E V Chernikov. J. Phys. D: Appl. Phys. 53 (2020) 414004 (9pp) https://doi.org/10.1088/1361-6463/ab9c69

Figure 1