High Sensitivity Low-Temperature Hydrogen Sensors Based on SnO₂/κ(ε)-Ga₂O₃:Sn Heterostructure

Abstract

The structural and gas-sensitive properties of n-N SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures were investigated in detail for the first time. The κ(ε)-Ga₂O₃:Sn and SnO₂ films were grown by the halide vapor phase epitaxy and the high-frequency magnetron sputtering, respectively. The gas sensor response and speed of operation of the structures under H₂ exposure exceeded the corresponding values of single κ(ε)-Ga₂O₃:Sn and SnO₂ films within the temperature range of 25–175 °C. Meanwhile, the investigated heterostructures demonstrated a low response to CO, NH₃, and CH₄ gases and a high response to NO₂, even at low concentrations of 100 ppm. The current responses of the SnO₂/κ(ε)-Ga₂O₃:Sn structure to 10⁴ ppm of H₂ and 100 ppm of NO₂ were 30–47 arb. un. and 3.7 arb. un., correspondingly, at a temperature of 125 °C. The increase in the sensitivity of heterostructures at low temperatures is explained by a rise of the electron concentration and a change of a microrelief of the SnO₂ film surface when depositing on κ(ε)-Ga₂O₃:Sn. The SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures, having high gas sensitivity over a wide operating temperature range, can find application in various fields.

1. Introduction

Sustainable development in terms of preserving the environment requires employment of a great number of sensors: biosensors, image sensors, motion sensors, and chemical sensors for indoor and outdoor as well as for industry-relevant gas surveillance and control. Wide bandgap metal oxide semiconductors tin dioxide (SnO₂) and gallium oxide (Ga₂O₃) are of high interest for the development of gas sensors and transparent contacts, finding applications in a number of devices [1,2,3,4,5,6]. Heterostructures based on metal oxide semiconductors allow the advantages of each component to be combined in a single structure [7]. Thus, superior gas-sensitive characteristics can be achieved for heterostructures compared to single semiconductors. It is reasonable to combine semiconductors with high catalytic activity and concentration of electrons, involved in the physico–chemical processes at chemisorption of gas molecules on the semiconductor surface.

SnO₂ is one of the most studied metal oxide semiconductors for gas sensor applications [1] primarily due to its high catalytic activity, which leads to a high gas sensitivity compared to other metal oxides. Chemisorption of gas molecules on the SnO₂ surface occurs with the involvement of free electrons. However, pure SnO₂ does not have a high electron concentration. Localization of electrons in this semiconductor can be achieved by forming heterostructures. Ga₂O₃ with an electron affinity χ = 4.0 eV can be paired with SnO₂, which is characterized by χ = 5.32 eV [8], to form such a heterostructure. In turn, Ga₂O₃ needs to be doped to achieve the required concentration of free electrons. In this case, one can expect an increase in the sensitivity of such heterostructure to gases as compared to pure SnO₂ and Ga₂O₃ films.

Gallium oxide has several polymorphs [9,10,11,12] namely α, β, γ, δ, and κ(ε). Metastable κ(ε)-Ga₂O₃ polymorph is of particular interest for the development of electronic devices due to its fundamental properties [13] such as the thermal stability up to 700 °C; the high bandgap E_g of 4.5–5.0 eV; availability of the ferroelectric properties; the high symmetry of a crystal lattice. κ(ε)-Ga₂O₃ is a novel material in terms of sensors, since its gas sensitivity was researched for the first time in 2022 [14]. We have demonstrated that κ(ε)-Ga₂O₃:Sn films grown by the halide vapor phase epitaxy (HVPE) have a low resistance (i.e., high electron concentration), stable characteristics in the temperature range from 20 °C to 500 °C, and exhibit sensitivity to H₂ at room temperature (RT) [14]. In addition, the κ(ε)-Ga₂O₃ polymorph meets the conditions of heteroepitaxy on a commercially available (0001) Al₂O₃ substrate better than monoclinic β-Ga₂O₃ [15]. Thus, doped κ(ε)-Ga₂O₃:Sn can be chosen to pair with SnO₂ to form a heterostructure.

SnO₂/β-Ga₂O₃ and SnO₂/κ(ε)-Ga₂O₃ heterostructures have previously been investigated for the development of power diodes [16,17] and solar-blind avalanche photodetectors with high sensitivity [8]. β-Ga₂O₃ nanostructures covered with ultrathin layers of SnO₂ demonstrated high sensitivity to ethanol at T = 400 °C [18] and to H₂ in the range of T = 25–200 °C [19]. The gas-sensitive properties of SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures have not been studied before.

The purpose of this work is to gain insight into the gas-sensitive properties of SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures.

2. Materials and Methods

The following films were deposited on (0001) single crystal Al₂O₃ substrates: κ(ε)-Ga₂O₃:Sn and SnO₂ thin films as well as SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure. The process of the κ(ε)-Ga₂O₃:Sn films growth was multistage. In the first stage, a 3-µm-thick semi-insulating (SI) GaN layer was deposited on the Al₂O₃ substrate by gas phase deposition employing a homemade reactor. This layer served as a template for the κ(ε)-Ga₂O₃:Sn film growth. In the second stage, a 1-µm-thick κ(ε)-Ga₂O₃ layer in situ doped by Sn was deposited on the SI-GaN layer by HVPE using a hot-wall homemade reactor. Gaseous gallium chloride and oxygen were utilized as precursors. The doping of the κ(ε)-Ga₂O₃ films was carried out during the growth by adding tin. The HVPE growth temperature of the κ(ε)-Ga₂O₃:Sn film was 600 °C. The analysis of current–voltage (I–V) and capacitance–voltage (C–V) characteristics applied at this stage showed that the effective donor concentration N_d of the films was 5.13 × 10²⁰ cm⁻³.

120-nm-thick pure SnO₂ thin films were deposited by means of magnetron sputtering of an Sn (5N) target in an oxygen–argon plasma on Al₂O₃ and κ(ε)-Ga₂O₃:Sn. An Edwards A-500 (Edwards, USA) setup was employed. To prepare SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures, SnO₂ thin films were deposited through a mask with square-shaped slots of 1 mm × 1 mm. The temperature of substrates during the deposition of the film was RT. The working pressure and power were kept at 7 × 10⁻³ mbar and 70 W, respectively. The oxygen concentration in the O₂+Ar mixture was 56.1 ± 0.5 vol. %. The as-deposited SnO₂ films were annealed ex situ at T = 600 °C; for 4 hours in air. The estimates showed that the N_d value of these films was 5.26 × 10¹⁷ cm⁻³.

Pt contacts were deposited on the κ(ε)-Ga₂O₃:Sn and SnO₂ films (see Figure 1) by means of the magnetron sputtering. Pt contacts were chosen on the basis of their high stability at high temperatures and under exposure to various gases, which are of natural surroundings- and industrial relevance.

X-ray diffraction (XRD) analysis of the samples was performed at DRON-6 diffractometer (Bourevestnik, Petersburg, Russia) equipped with a copper anode (CuK_α1, λ = 1.5406 Å). The XRD patterns were registered in θ-2θ scanning mode. The phase composition of the samples was identified by the position of the reflection peaks. XRD θ-2θ curves were processed using the Scherrer method [20] to determine the characteristic size of the block in the direction perpendicular to the plane of epitaxial growth.

The chemical composition of the samples was studied by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out using a hemispherical analyzer included in the ESCALAB 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) laboratory spectrometer. The measurements were carried out using a monochromatized AlK_a radiation (hv = 1486.6 eV). Survey and core (O1s, Sn3d, Ga3d) photoemission (PE) spectra were recorded at the analyzer transmission energy of 100 and 50 eV, respectively. The film’s surface was irradiated with argon ions at an average energy of 3 eV for 60 s before XPS measurements to remove adsorbed atoms and molecules of contaminants. The analysis of the core spectra was processed employing the Avantage Data System software.

Measurement of transmission spectra was carried out using an Ocean Optics (Ocean Insight, Orlando, FL, USA) spectrometric system to determine the E_g of SnO₂ in the wavelength range of λ = 300–600 nm. The transmission spectrum of κ(ε)-Ga₂O₃:Sn films was measured using a UV-VIS two-beam SPECORD (Analytik Jena, Jena, Germany) spectrophotometer in the range of λ = 230–360 nm.

A high-resolution field emission scanning electron microscope (FESEM) Apreo 2S (Thermo Fisher Scientific, USA) operating at an accelerating voltage of 5 kV was employed to study the microrelief of the film surfaces with a high resolution.

Gas sensing measurements of the samples were performed in a dedicated sealed chamber with a volume of 100 cm³, equipped with a micro-probe Nextron MPS-CHH station (Nextron, Busan, Republic of Korea). A ceramic-type heater, installed in the sealed chamber, was used to heat the samples. The accuracy of temperature T control was ±0.1 °C. The experiments were carried out under dark conditions. Streams of pure dry air or gas mixture of pure dry air + H₂ were pumped through the chamber to measure the gas sensing characteristics of the samples. The H₂ concentration in the mixture was controlled by a gas mixing and delivery system Microgas F-06 (Intera, Moscow, Russia). A special generator (Khimelektonika SPE, Moscow, Russia) was used to produce pure dry air. The total flow rate of the gas mixtures through the chamber was 1000 sccm. The relative error of the gas mixture flow rate did not exceed 1.5%. A Keithley 2636 A (Keithley, Solon, OH, USA) source meter was utilized to measure the time dependences of the current I and the I–V characteristics of the samples. An E4980A RLC-meter (Agilent, Santa Clara, CA, USA) was applied to measure the C–V dependences. Additionally, gas sensing measurements of the samples were carried out under exposure to NH₃, CH₄, CO, NO₂, and O₂. A mixture of N₂ + O₂ was used to study the sensitivity of samples to O₂. To study the effect of relative humidity (RH) on the response of the samples to H₂, the pure dry air in one of the channels was passed through a bubbler with distilled water. Then it entered the homogenizer, where it was mixed with the pure dry air and/or pure dry air + H₂ mixture streams from the other channels. Varying the ratio of flows through the channels, we set the desired level of RH in the measuring chamber. An HIH 4000 Honeywell capacitive sensor with an absolute error of ±3.5% was used to measure the RH. Just prior to these measurements, all the samples were subjected to heat treatment at T = 500 °C for 90 s in pure dry air to stabilize the contact properties and regenerate the surface.

3. Results and Discussion

3.1. Structural Properties

Figure 2a illustrates the θ-2θ XRD pattern of the SnO₂/Ga₂O₃ heterostructure deposited on an Al₂O₃ substrate via a GaN template. The peaks at 2θ = 41.8° and 90.9° are associated with the (0006) and (0 0 0 12) reflections of the Al₂O₃ substrate (ICDD # 00-042-1468). A series of peaks at 2θ = 19.2°, 39.0°, 60.0°, 83.6°, and 112.7° correspond to the 002, 004, 006, 008, and 0 0 10 planes of the κ(ε)-Ga₂O₃ phase. (The calculation was made on the basis of the Bragg equation for the case of CuK_α1 anode (λ = 1.5406 Å). The peaks at 2θ = 34.7°, 73.0°, and 126.1° are due to the (0002), (0004), and (0006) reflections of the GaN template (AMCSD # 99-101-0461). Peaks corresponding to SnO₂ could not be distinguished due to possible overlapping by neighboring reflections of other phases. Thus, the (101) reflection of SnO₂ (AMCSD no. 99-100-8661) is close to the (0002) one of GaN, and the (111) reflection of SnO₂ is close to the (004) one of κ(ε)-Ga₂O₃. The auxiliary vertical red lines of equal intensity depicted in Figure 2a are the tabular values of the SnO₂ reflection positions. In addition, difficulties in SnO₂ peaks identification may be caused by the low film thickness and the developed microrelief of the surface. Finally, the possible low crystallinity of the SnO₂ phase may be the reason for the absence of sharp peaks on the XRD pattern. In this case, broad humps of a low intensity may be present.

κ(ε)-Ga₂O₃:Sn and SnO₂ films are characterized by direct optical transitions according to the analysis of transmission spectra (see Figure 2b), where α is the absorption coefficient. E_g values were graphically calculated and proved to be equal to 4.61 ± 0.01 eV and 3.76 ± 0.01 eV for the κ(ε)-Ga₂O₃:Sn and SnO₂ films, respectively.

According to XPS analysis, the composition of the SnO₂ film includes Sn and O elements only. However, carbon (C) as a common contaminant was also observed in the subsurface layer a few nanometers thick. C atoms completely disappear after argon-etching for 60 s. Ga, Sn, O, and C lines were observed in the survey PE spectra of κ(ε)-Ga₂O₃:Sn film. The Sn concentration in this film appeared to be about 3 at. %, which indicates a high level of doping. Thus, the chemical analysis has shown that there are no third-party impurities in the composition of κ(ε)-Ga₂O₃:Sn and SnO₂ films, which confirms the high purity of the deposited films.

The analysis of the chemical state of Sn based on the Sn3d_5/2 PE line revealed the energy position of the main maximum of Sn at 486.5 and 486.3 eV (Figure 2c). The obtained values are in good agreement with the literature data [21,22] and correspond to the higher oxidation state of Sn–SnO₂ oxide. A lower value of the SnO₂ energy position for the κ(ε)-Ga₂O₃:Sn film indicates the effect of Ga₂O₃ on the charge state of SnO₂. Previously, we have observed a similar effect of the Sn3d_5/2 PE line shift to low binding energies of the SnO₂ film doped with rare-earth elements and platinum group metals [21,22]. Analysis of the Ga chemical state in the κ(ε)-Ga₂O₃:Sn film based on the Ga3d PE line showed that Ga corresponds to the higher Ga₂O₃ oxide [23] (Figure 2d).

FESEM images of the SnO₂ films surface deposited on Al₂O₃ substrates and κ(ε)-Ga₂O₃:Sn film are displayed in Figure 2e,f, respectively. The microrelief of the SnO₂ film on Al₂O₃ (Figure 2e) contains small spherical grains with a diameter of ~35 nm and large agglomerates with a characteristic size of ~300 nm. Whereas the microrelief of the SnO₂ film deposited on a κ(ε)-Ga₂O₃:Sn one (see Figure 2f) is represented by small grains with a diameter of ~35 nm only. The formation of large agglomerates for these structures was not observed.

3.2. Gas-Sensitive Properties of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure

The I–V characteristics of the SnO₂ thin films equipped with Pt contacts are linear in the range of applied voltages U = −40–40 V at RT as well as at higher T. Contrary to this, the I–V characteristics of the κ(ε)-Ga₂O₃:Sn films equipped with Pt contacts are nonlinear. The dependence of ln(I) on U^1/4 is linear, indicating the presence of a Schottky barrier at the Pt/κ(ε)-Ga₂O₃:Sn interface [24]. The I value through the Pt/κ(ε)-Ga₂O₃:Sn/Pt structures exceeds 0.1 A at U > 12 V which leads to the samples self-heating.

The SnO₂/κ(ε)-Ga₂O₃:Sn structure equipped with Pt contacts is a n-N isotype heterojunction and the Schottky barriers are connected in series. The I–V characteristics of such heterostructures are nonlinear and asymmetric as can be seen in Figure 3a. The I(U = 4 V)/I(U = −4 V) ratio reaches the value of ~2 × 10³ at T = 25 °C, then drops by half as T increases to 150 °C. The increase in reverse current with T rising is significantly higher than the increase in forward current. The forward-bias region of the I–V characteristics is approximated by the following function: I_f = A₁ × exp(B₁U), where I_f is a forward current; and A₁ and B₁ are the constants: A₁ = (3.0 ± 0.4) × 10⁻⁶ A and B₁ = 0.88 ± 0.02 V⁻¹ at T = 25 °C. The reverse-bias region of the I–V characteristics can be approximated by a similar function of I_r = A₂ × exp(B₂|U|), where I_r is a reverse current; A₂ and B₂ are the constants: A₂ = (2.0 ± 0.4) × 10⁻⁹ A and B₁ = 0.89 ± 0.04 V⁻¹ at T = 25 °C. The forward-bias mode of the structure corresponds to the application of a positive potential to the SnO₂/Pt interface.

Exposure to H₂ leads to a reversible increase in the I through heterostructures at T = 25–200 °C. Figure 3b shows the change in the I–V characteristics of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure when exposed to 10⁴ ppm of H₂ at T = 150 °C. The type of the functions, approximating the forward and reverse branches of the I–V characteristics, does not change with increasing T to 200 °C and under exposure to 10⁴ ppm of H₂. The A₁ and A₂ increase, but B₁ and B₂ values decrease with T. The A₁ and A₂ values increase, whereas B₁ and B₂ practically do not change when exposed to H₂.

Table 1. A₁, A₂, B₁, and B₂ at T = 150 °C and under exposure to 10⁴ ppm H₂.

Conditions	A1 (A)	A2 (A)	B1 (V−1)	B2 (V−1)
Dry pure air	(1.1 ± 0.1) × 10−4	(1.1 ± 0.1) × 10−6	0.65 ± 0.03	0.65 ± 0.02
Dry pure air + 104 ppm H2	(2.6 ± 0.3) × 10−4	(1.3 ± 0.1) × 10−6	0.66 ± 0.03	0.63 ± 0.03

shows the A₁, A₂, B_1, and B₂ values at T = 150 °C and under exposure to 10⁴ ppm of H₂.

To assess the effect of H₂ on the I through the SnO₂/κ(ε)-Ga₂O₃:Sn structures, the current response S_I was calculated based on the experimental I–V characteristics by the following ratio:

S_I = I_H/I_air,

(1)

where I_H is the current of the charge carrier through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the gas mixture of pure dry air + H₂; I_air is the current of the charge carrier through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in pure dry air. The S_I values calculated on the basis of the experimental I–V characteristics and time dependences of currents at a fixed U (see Figure 4a) coincide. The S_I value depends on the magnitude and direction of the applied voltage (see Figure 3c). The highest response in the range of T = 100–125 °C was observed at U = 0.5 V, whereas the highest S_I was observed at U = 0.75 V at T = 150 °C. At room temperature, the maximum S_I was also noticed at U = 0.5 V (see Figure 3c, insertion). The response decreases exponentially with the applied voltage in the range of U = 1–5 V. S_I values are significantly lower at the reverse-bias mode and decrease slightly with an increase in the reverse voltage |U_r| from 0.25 V to 2.5 V. Moreover, the response increases slightly with a further increase in |U_r| to 5 V.

The temperature dependences of the sample’s response to 10⁴ ppm of H₂ are presented in Figure 4b. The κ(ε)-Ga₂O₃:Sn films show the highest response to H₂ at T = 25 °C. The S_I of κ(ε)-Ga₂O₃:Sn films exceeds those of SnO₂ films in the temperature range of T = 25–50 °C; meanwhile, the S_I of κ(ε)-Ga₂O₃:Sn films decreases and S_I of SnO₂ films increases drastically with further increase in T. Sensitivity of SnO₂ and κ(ε)-Ga₂O₃:Sn films is based on reversible chemisorption of H₂ molecules on the semiconductor’s surface according to the mechanisms described in refs. [14,25]. High sensitivity to H₂ at moderate temperatures (T = 300 °C) is characteristic of the SnO₂ thin films. Low S_I for the κ(ε)-Ga₂O₃:Sn films are caused by a significant influence of the bulk conductivity G_b, which does not depend on the charge state of the surface. The dependence of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures response to H₂ on temperature is characterized by a maximum at T = 125 °C. These samples demonstrate the highest S_I in the range of T = 75–125 °C.

The experimental results displayed in Figure 4c prove that the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures are characterized by the high speed of operation compared to the SnO₂ thin films when exposed to H₂. The response t_res and recovery t_rec times were calculated to assess the speed of operation by the method described in ref. [9]. The calculated t_res and t_rec values can only be used to compare the speed of sensors operation at similar experimental conditions. t_res and t_rec decrease exponentially with T. t_rec and t_res + t_rec of the SnO₂/κ(ε)-Ga₂O₃:Sn structures are significantly lower than those of SnO₂ thin films at T = 25–200 °C. SnO₂ films are characterized by low t_res. The speed of operation for the κ(ε)-Ga₂O₃:Sn films was not evaluated due to their low responses at T > 50 °C. These samples are of interest for developing room temperature H₂ sensors. The t_res and t_rec of these films under exposure to 10⁴ ppm of H₂ at T = 25 °C are 349.2 s and 379.6 s, respectively. Obviously, SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures are the most interesting for highly sensitive H₂ sensors with high speed of operation and low operating temperatures. Therefore, our further attention will be focused on these structures.

The dependence of the SnO₂/κ(ε)-Ga₂O₃:Sn structure response on the H₂ concentration n_H2 is linear (Figure 4d,e) in the n_H2 range of 100–30000 ppm. The I_air and I_H of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure decrease by 30% and 28%, respectively (see Figure 4f), during a cyclic exposure to H₂ (five cycles). At the same time, the current response decreased by only 17%. The observed decrease in response during cyclic exposure to H₂ is caused by the manifestation of chemisorbed hydrogen atoms with high binding energy. The temperature of 125 °C is not sufficient for the complete desorption of these hydrogen atoms from the semiconductor surface. Short-term heating of the structure at high temperatures can be used to regenerate the surface of semiconductors and for full desorption of H atoms [26]. The results of the long-term tests of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures at T = 125 °C and when exposed to 10⁴ ppm of H₂ demonstrated opposite changes of S_I. The samples after the experiments were stored in sealed packages. The long-term tests lasted 8 weeks with an interval between the experiments of 7–8 days. Just prior to each measurement the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures were subjected to heat at T = 500 °C for 90 s. There were increases in response from ~30 arb. un. to 47 arb. un. during the long-term tests. Response increases mostly due to a decrease in I_air. The most significant changes in response were in the first 4 weeks of testing.

The responses of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure to NO₂, CH₄, NH₃, CO, and O₂ gases at T = 125 °C was measured to evaluate its selectivity (Figure 4g). Noteworthy, is that I through heterostructure increases reversibly when exposed to 10⁴ ppm of CH₄, NH₃h and CO. The response to these gases has been calculated by equation (1). The I_H was replaced by I_g, where I_g is the charge carrier current through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the gas mixture of pure dry air + reducing gas (CH₄, NH₃, or CO). The responses to CH₄, NH₃, and CO are insignificant compared to the S_I to H₂, which equates to 29.92–46.98 arb. un. at T = 125 °C and n_H2 = 10⁴ ppm.

It was found, that the I value of the SnO₂/κ(ε)-Ga₂O₃:Sn reversibly decreases when exposed to NO₂ and O₂. The responses to NO₂ (S_NO2) and O₂ (S_O2) have been calculated by the following equations, correspondently:

S_NO2 = I_air/I_NO2,

(2)

S_O2 = I_N/I_O2,

(3)

where I_NO2 is the charge carrier current through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the gas mixture of pure dry air + NO₂; I_N is the charge carrier current through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the nitrogen atmosphere; I_O2 is the charge carrier current through the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the gas mixture of N₂ + O₂. S_I ratio when exposed to 100 ppm of H₂ at T = 125 °C happened to be 26.7 times lower than those for 100 ppm of NO₂ (Figure 4g). The SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure also demonstrated relatively high response to O₂. The response to O₂ appears to be higher than to CH₄, NH₃, or CO at same concentration values. Hence, we have shown that SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure is also attractive for developing highly sensitive NO₂ and O₂ sensors operating at low temperatures.

An increase in RH leads to a drop in the response of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure to H₂ (Figure 4h). The most significant decrease in S_I occurs when RH increases from 0 to 34 %. In the range of RH = 34–90.0%, the response varies slightly.

Furthermore, the effect of 10⁴ ppm of H₂ on the C-V characteristics of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures at T = 125 °C and signal frequencies f = 1 kHz, 10 kHz, and 1 MHz have been studied. The results are illustrated in Figure 4i,j. Evidently, exposure to H₂ leads to a reversible increase in the electrical capacity of the structures. The capacitive response S_C has been calculated by the following equation:

S_C = C_H/C_air,

(4)

where C_H is the electrical capacitance of SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in the gas mixture of pure dry air + H₂; and C_air is the electrical capacitance of structures in pure dry air. Visibly, the S_C (see Figure 4i) is significantly lower than the S_I (Figure 3c). At f = 10 kHz and 1 MHz the capacitive response varies weakly. The highest S_C value is observed in the range of U = 0.95–5.00 V at f = 1 kHz and has a maximum at U = 2.9 V.

3.3. The Mechanism of the Sensory Effect

Initially, the resistance of the Pt/κ(ε)-Ga₂O₃:Sn interface is low and the Pt/SnO₂ contact is ohmic. The change in the potential barrier at Pt/κ(ε)-Ga₂O₃:Sn and Pt/SnO₂ interfaces upon exposure to gases can be neglected. The observed high responses of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure at T = 25–175 °C; are due to the formation of the n-N isotypic heterojunction, where SnO₂ is the base.

Diffusion of H atoms up to the SnO₂/κ(ε)-Ga₂O₃:Sn interface at T = 25–175 °C; is unlikely. Changes of I and C upon exposure to H₂ occur mainly due to the chemisorption of gas molecules on semiconductor’s surface. In the air atmosphere within a temperature range of T = 25–175 °C, the oxygen chemisorbs mainly in a molecular form on metal oxide semiconductors surface and captures electrons from their conduction band [25,27]. The reaction of reversible chemisorption of oxygen molecules can be represented as follows:

O₂ + S_a + e⁻ ↔ O₂⁻_(c),

(5)

where S_a is a free adsorption center; e is the electron charge; O₂⁻_(c) is the chemisorbed oxygen ion. As a result of reaction (4), an electron-depleted region is formed in the near-surface part of the semiconductor. A negative charge on the surface causes the upward of energy bands bending at eV_s, where V_s is the surface potential and eV_s~N_i², where N_i is the surface density of chemisorbed oxygen ions. In our case, the Debye length L_D for SnO₂ exceeds the grain size and the effect of grain boundaries on the transport of charge carriers in a semiconductor can be ignored. Oxygen chemisorption weakly changes the electrical conductivity of the κ(ε)-Ga₂O₃:Sn films due to the low contribution of the surface conductivity G_s to the total conductivity of G_t. Thus, changes in the current during the chemisorption of gases are mainly due to changes in the concentration of charge carriers in the SnO₂.

G_t = G_b + G_s and the relationship between G_t and eV_s in our case is described by the following equation [28]:

G_t = G_b × [1 − (L_D/D) × [eV_s/(kT)]],

(6)

where D is the SnO₂ film thickness; k is the Boltzmann constant. An increase in the eV_s due to the oxygen molecule’s chemisorption on the SnO₂ surface leads to a drop of G_t. The increase in G_t when exposed to H₂ is caused by the interaction of H₂ molecules with previously chemisorbed O₂⁻_(c) on the SnO₂ surface. This interaction can be represented as follows:

2H₂ + O₂⁻_(c) → 2H₂O + e⁻.

(7)

As a result of reaction (7), a neutral H₂O molecule is formed and desorbed, an electron returns to the conduction band of SnO₂, the eV_s decreases and the finally G_t increases. When exposed to NO₂ the following reactions take place [29]:

NO₂ + S_a + e⁻ → NO₂⁻,
NO₂⁻ → O⁻_(c) + NO,
O⁻_(c) + O⁻_(c) → O₂⁻_(c) + e⁻ + S_a.

(8)

NO₂ molecules chemisorb onto free adsorption centers and capture electrons from the SnO₂ conduction band. Meanwhile, eV_s is proportional to (N_i + N_NO2)² in the mixtures of air + NO₂, where N_NO2 is the surface density of chemisorbed NO₂⁻ ions [25,27]. An additional negative charge on the surface of the SnO₂ film leads to a greater decrease in G_t. Further, NO₂⁻ ions are dissociated to form chemisorbed O⁻_(c) ions and gaseous NO molecules. In the low temperature region, atomic O⁻_(c) ions associated to O₂⁻_(c) form and a free electron e⁻, which returns to the conduction band of the semiconductor.

The κ(ε)-Ga₂O₃:Sn film is a source of electrons that are involved in reactions (5) and (8) with the gas molecules on the SnO₂ surface. This causes a high response of heterostructure at T = 75–175 °C. The base region (SnO₂) is filled with electrons from the κ(ε)-Ga₂O₃:Sn film with T rising, G_b of SnO₂ increases and the response decreases. SnO₂ films deposited on κ(ε)-Ga₂O₃:Sn are characterized by the absence of large agglomerates (see Figure 2e,f). This leads to an increase in the specific surface area of SnO₂ and the surface density of adsorption centers for gas molecules.

Table 2. Gas-sensitive characteristics of structures based on Ga₂O₃ polymorphs.

Structure	ng (ppm)	T (℃)	Response (arb. un.)	Ref.
H2
α-Ga2O3:Sn	104	350	80	[9]
α-Ga2O3:Si	3 × 104	400	69.3	[30]
β-Ga2O3	500	RT	7.9 × 105	[31]
β-Ga2O3	2000	500	ΔI = 1.4 (mA)	[32]
β-Ga2O3	3 × 104	600	4	[33]
β-Ga2O3	200	300	6.3	[34]
β-Ga2O3:Cr2O3	2500	500	60	[35]
β-Ga2O3/Pd nanoclusters	104	625	~103	[36]
β-Ga2O3/SiO2 (filter)	5000	700	~103	[37]
α-Ga2O3/κ(ɛ)-Ga2O3:Sn	2500	125	1.25	[38]
κ(ɛ)-Ga2O3	104	500	9.44	[14]
κ(ɛ)-Ga2O3:Sn	104	RT	1.2
Pt/β-Ga2O3/GaN	1000	RT	229.8	[39]
β-Ga2O3/SnO2	1000	400	8	[18]
β-Ga2O3/SnO2	1000	200	7075.5	[19]
β-Ga2O3/WO3	1000	200	4.1	[40]
SnO2/κ(ε)-Ga2O3:Sn	1000	125	5.7	This work
	104		47
NO2
β-Ga2O3/ZnO	10	300	73.5	[41]
β-Ga2O3	200	800	5.1	[42]
β-Ga2O3/La0.8Sr0.2CoO3			25.7
SnO2/κ(ε)-Ga2O3:Sn	100	125	3.7	This work

shows a comparison of the responses and optimal operating temperatures of structures based on the Ga₂O₃ polymorphs when exposed to H₂ and NO₂ [9,14,18,19,30,31,32,33,34,35,36,37,38,39,40,41,42]. n_g is a gas concentration. SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure in comparison with other items listed in this table is characterized by relatively high sensitivity to H₂ and NO₂ at a relatively low operating temperature. Ga2O3-based structures with higher responses to gases are characterized by high operating temperatures [9,30,34,35,36,37,41,42], low speed of operation [18,19], or are diode-type sensors based on high-cost materials [31,32,39]. SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures demonstrate relatively high responses to H₂ and NO₂ at lower temperatures in comparison with the heterostructures based on other metal oxides (see

Table 3. Gas-sensitive characteristics of heterostructures based on different metal oxide semiconductors.

Structure	ng (ppm)	T (℃)	Response (arb. un.)	Ref.
H2
CeO2/In2O3	50	160	20.7	[43]
SnO2/ZnO	100	350	18.4	[44]
Pd/BN/ZnO	50	200	13	[45]
SnO2/NiO	500	500	114	[46]
rGO/ZnO-SnO2	100	380	9.4	[47]
RGO/ZnO	200	150	3.5	[48]
Al2O3/CuO	100	300	2.37	[49]
SnO2/κ(ε)-Ga2O3:Sn	1000	125	5.7	This work
	104		47
NO2
m-WO3/ZnO	1	150	167.8	[50]
WO3/SnO2	200	200	186	[51]
WO3/MWCNT composite	5	150	18	[52]
ZnO/SWCNT composite	50	150	5	[53]
Sb2O3/In2O3 nanotubes	1	80	47	[54]
MoS2/In2O3 nanotubes	50	RT	209	[55]
PdO/SnO2 nanotubes	100	RT	20.3	[56]
TiO2/ZnO nanotubes	5	RT	2.05	[57]
In2O3/ZnO	50	200	78	[58]
NiO/In2O3	10	145	532	[59]
SnO2/κ(ε)-Ga2O3:Sn	100	125	3.7	This work

). Nano-structured heterostructures are characterized by higher responses to NO₂ but generally do not differ in high speed of operation.

4. Conclusions

The structural and gas-sensitive properties of the n-N SnO₂/κ(ε)-Ga₂O₃:Sn heterostructures were investigated for the first time. The κ(ε)-Ga₂O₃:Sn and SnO₂ films were obtained by the halide vapor phase epitaxy and the high-frequency magnetron sputtering, respectively. The κ(ε)-Ga₂O₃:Sn crystalline film has a bandgap of 4.61 ± 0.01 eV. The SnO₂ nanocrystalline film has a bandgap of 3.76 ± 0.01 eV and is characterized by a developed microrelief of the surface, represented by grains with a size of ~35 nm. Exposure to H₂ leads to an increase in electrical current and capacitance of SnO₂/κ(ε)-Ga₂O₃:Sn structures. The current response of heterostructures to H₂ significantly exceeds the capacitive one. Gas sensor response and speed of operation of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure under H₂ exposure overperform those of the single κ(ε)-Ga₂O₃:Sn and SnO₂ films in the temperature range of 25–175°C. This heterostructure demonstrates a low response to CO, NH₃, and CH₄ and a high response to NO₂ even at low concentrations. The current responses of SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure to 10⁴ ppm of H₂ and 100 ppm of NO₂ at 125 °C were 30–47 A.U. and 3.7 A.U., correspondingly. The sensory effect is realized mainly due to the chemisorption of gas molecules on the SnO₂ surface, which is the base region of the heterostructure. The κ(ε)-Ga₂O₃:Sn film is a source of electrons that are involved in reactions with gas molecules on the SnO₂ film surface. The SnO₂ film deposited on the κ(ε)-Ga₂O₃:Sn film is characterized by a more developed surface microstructure. This leads to an increase in the surface density of adsorption centers for gas molecules. The advantages of the SnO₂/κ(ε)-Ga₂O₃:Sn heterostructure for gas sensors are shown, the main one being high sensitivity at relatively low operating temperatures. Doubtfully, this structure has every chance of being the base of the sensor.