Thin-film temperature sensors based on LPD-fabricated β-Ga₂O₃ Schottky diodes

Numerous applications in the energy and transportation industries, e.g., those involving internal combustion, frequently demand electronic sensing and monitoring at high temperatures and pressures.¹ Conventional semiconductors, e.g., silicon (Si), are known to encounter limitations in such harsh environments. For example, the application of Si is limited below 200 °C because of its narrow bandgap (1.12 eV).² One of the technological solutions to overcome the high-temperature limit³ is to exploit the much lower intrinsic carrier density offered by wide-bandgap semiconductors, such as SiC (3.3 eV)⁴ and GaN (3.4 eV).⁵ Compared to SiC and GaN, the recently emerging high-bandgap Ga₂O₃ semiconductor^6,–8 holds even greater potential due to its wider bandgap of ∼4.9 eV, which has been exploited in multiple electronic and sensing devices, such as optical detectors,^9,–11 gas sensors,^12,13 and field-effect transistors.^14 –16 

The inorganic semiconductor compound Ga₂O₃ has six different polymorphs (α, β, γ, δ, ε, and κ-Ga₂O₃),^14,17,–19 among which β-Ga₂O₃ shows the best thermal stability suitable for high-temperature applications. Higashiwaki et al., in 2013, reported the fabrication of metal–oxide–semiconductor field-effect transistors based on β-Ga₂O₃, which can operate at high temperatures up to 250 °C without degradation of electrical properties.¹⁹ The majority of research and developments on utilizing β-Ga₂O₃ in electronic or sensing applications have been based on single-crystalline materials in substrate forms^17,18,20 (e.g., from Tamura Corporation, Japan²¹) or produced by epitaxial growth methods.^22,23 Recently, solution-processed β-Ga₂O₃ thin films have attracted research interest due to their significant cost and manufacturing advantages.^24,–26 In solution-based processing, the different crystal structures of α-, β-, γ-, δ-, ε-, and κ-Ga₂O₃, mainly thermally metastable α-Ga₂O₃²⁷ and stable β-Ga₂O₃, are obtained by combining different precursor solutions and thermal annealing steps.^28,29 

In this work, we report a low-cost and non-vacuum liquid phase deposition (LPD) technique to synthesize β-Ga₂O₃ thin films, which can be scaled to large-scale manufacturing, and demonstrate high-temperature sensing capability of Schottky devices fabricated from such solution-processed materials. We first investigate the morphology and crystal structures of the β-Ga₂O₃ materials synthesized with various precursor solutions under different processing conditions. We then fabricate Ni/β-Ga₂O₃ Schottky devices and characterize their electrical properties. From the temperature dependent current density vs voltage measurement, we extract the Ni/β-Ga₂O₃ Schottky barrier height and evaluate the effect of barrier inhomogeneity. Finally, the temperature-dependent electrical characterization in the range of 100–800 K is presented to demonstrate the promising potential of using solution-processed β-Ga₂O₃ thin-film devices as temperature sensors^30 –33 at high temperatures.

Substrate cleaning: In the synthesis process of nanocrystalline β-Ga₂O₃ films and Schottky devices, a single-side polished sapphire substrate was used. Prior to the deposition of β-Ga₂O₃ films and metal layers, the sapphire substrate underwent standard wet cleaning using organic solvents. To remove the organic residues, the surface of the sapphire substrate was treated with an ultraviolet (UV) ozone source (with emission wavelengths at 185 and 254 nm generated by a low-pressure mercury lamp). Both 185 and 254 nm photons from the UV ozone system can decompose common residual organic- and hydrogen-based compounds into free ions, radicals, and molecular (O₂, CO₂, and H₂O) formations.³⁴ As a result, the surface becomes hydrophilic and can be cleaned to facilitate subsequent manufacturing processes. As demonstrated in Fig. 1(a), the water contact angle of the sapphire substrate before UV ozone treatment irradiation was 40.40°. After UV ozone irradiation, the contact angle was radically reduced to 2.5° as shown in Fig. 1(b), indicating the hydrophilic improvement of the sapphire surface.

We kept the above mixed growth solution at 60 °C for 24 h to obtain a white precipitate layer of GaO(OH), which serves as the precursor for β-Ga₂O₃. We discarded the un-reacted solution above the precipitation layer and rinsed the remaining precipitate with deionized water several times. Then, we placed the GaO(OH) precipitate on the cleaned sapphire substrate and spread it uniformly using the edge of a glass slide. A nano-grained GaO(OH) thin film structure formed when we annealed the GaO(OH) coated sapphire substrate at 80 °C in air for 15 min. Finally, we obtain a β-Ga₂O₃ thin film through the calcination of the nano-grained GaO(OH) film at 800 °C in atmospheric ambient for 3 h.

Electrical contact (Schottky and Ohmic) formation and device processing: To form Schottky and Ohmic contacts on the β-Ga₂O₃ surface, we used an electron-beam evaporation tool at a base pressure of 1.8–5 × 10⁻⁶ Torr. We deposited the metal films of Ni (80 nm) on the β-Ga₂O₃ surface through a shadow mask to form Schottky contacts [Fig. 2(a)]. We deposited a metal bilayer of Ti/Au (80/100 nm) on the β-Ga₂O₃ surface to form Ohmic contacts [Fig. 2(a)]. The contacts are circular in shapes of 2 mm diameter with an inter-contact spacing of 4.5 mm [Fig. 2(b)]. After the metal contact formation, the complete devices went through rapid thermal annealing at a vacuum pressure of 5 × 10⁻² Torr with the following thermal sequence: (1) ramp up from room temperature to 470 °C at a rate of 55 °C/min, (2) 1 min hold at 470 °C, and (3) ramp down from 470 °C to room temperature at a rate of −15 °C/min.

Materials and device characterization: The structural properties, phase purity, and crystallinity of the synthesized GaO(OH) precursor and β-Ga₂O₃ were investigated using a high-intensity x-ray thin-film micro-area diffractometer (Bruker D8 Discover; λ = 1.5406 Å) in θ–2θ scan mode. A high-resolution scanning electron microscope (SEM, Hitachi SU8000) was used to study the surface morphology and grain structure GaO(OH) precursor and β-Ga₂O₃. The current density–voltage (J–V) characteristics of the Ni/β-Ga₂O₃ Schottky devices were measured using a semiconductor device parameter analyzer (Agilent B1500A) in the bias range of −2–2 V in 100 mV steps. The relative error of current and voltage measurements in this range is less than ±0.1%.³⁶ For the temperature dependent electrical characterization, we used a closed-cycle helium-cooled cryostat system (Janis CCS-404) equipped with a 50 W resistive heater to stabilize the sample temperature in the range of 100–800 K in 50 K steps.

Figure 3(a) shows the x-ray diffraction (XRD) spectra of the nano-grained GaO(OH) film on the sapphire substrate. Multiple diffraction peaks are identified as originating from GaO(OH) according to the Inorganic Structure Database (ICSD, card number 409 671). The strongest three diffraction peaks are observed at 2θ = 21.37°, 33.66°, and 37.16°, which correspond to (110), (130), and (111) lattice planes, respectively. After calcination at 800 °C for 3 h, the GaO(OH) crystals were transformed to the monoclinic β-Ga₂O₃ form whose XRD spectra and indexing of identified peaks are shown in Fig. 3(b), which will be discussed later.

The SEM images in Figs. 4(a)–4(c) show the evolution of the morphology and structure of the GaO(OH) precipitate as a function of deposition time. The SEM image corresponding to a deposition time of 20 min indicates an amorphous phase for the GaO(OH) thin film. The GaO(OH) layer changed from an amorphous film to a spindle-shaped nanorod grain structure when the growth time increased from 20 min to 4 h. Further insights into the evolution of the GaO(OH) grain structure and morphology with deposition time can be collected through the x-ray diffraction measurement.

Figure 5 shows the XRD images of LPD grown GaO(OH) crystals as a function of growth durations of 20 min, 4, 8, 12, and 24 h. For the case of growth time of 20 min, the XRD signal is too weak to indicate distinctively identifiable peaks. This suggests that GaO(OH) is in the amorphous phase or amorphous/crystalline mixed phase with a very low crystalline volume fraction. GaO(OH) transformed from a colloidal film to a polycrystalline structure after the growth time of 4 h. With a further increase in growth time up to 24 h, the heights of the prominent peaks of (110), (130), and (111) planes of GaO(OH) gradually increased, signifying the crystallinity and purity improvement in GaO(OH).

In addition to the growth temperature and time, the microstructure of LPD grown GaO(OH) is sensitive to the pH value of Ga(NO₃)₃ and Na₂CO₃ mixed precursor solution.³⁷ The low and high-magnification top-view SEM images of GaO(OH) produced with different pH values (pH = 6.0, 7.5, and 9.0) at a growth temperature of 60 °C are shown in Fig. 6. When the pH value of the precursor solution was 6.0, the primary particles were elliptical spherical nanoparticles as shown in Figs. 6(a) and 6(d). When the pH value increased to 7.5, the crystal grains began to grow along the c axis [GaO(OH) is part of the orthorhombic crystal system] due to the increase in the hydroxide radicals. As a result, spherical nanoparticles coexisted with rod-like crystals as shown in Figs. 6(b) and 6(e). The spherical nanoparticles gradually disappeared with a further increase in pH to 9.0, leaving rod-shaped crystals and eventually quadrilateral nanorods as shown in Figs. 6(c) and 6(f).

Furthermore, we investigate the influence of the precursor solution temperature on GaO(OH) grain growth. At higher temperatures, it is expected that the increased kinetic energy of the radicals in the growth solution will lead to possible enhancement of the nucleation rate and therefore a positive correlation with the growth rate. Intending to enhance the growth rate, we experimented with three solution temperatures of 70, 80, and 90 °C in addition to 60 °C while keeping the pH value and growth time at 7.5 and 24 h, respectively. However, SEM investigation revealed (data not shown) that the growth rate of GaO(OH) crystal grains was relatively insensitive to the solution in the temperature range of 60–90 °C. Therefore, the GaO(OH) growth temperature was kept at 60 °C, which is far below the boiling point of the aqueous solution, thus assuring the stability of the GaO(OH) growth environment.

After the GaO(OH) precipitate was deposited and transferred to the surface of a sapphire substrate, β-Ga₂O₃ was obtained via calcination in a furnace in atmospheric ambient at 800 °C for 3 h. The surface morphology of the resultant β-Ga₂O₃ grains is shown in SEM images (Fig. 7), which revealed dehydrated holes on the surface while keeping the overall size and aspect ratio unchanged. X-ray diffraction [Fig. 3(b)] spectra confirmed the monoclinic crystal nature of the grown β-Ga₂O₃ with identified diffraction peaks from (002), (111), and (−311) crystal planes at 2θ = 31.68°, 35.22°, and 38.32°, respectively, identified according to the Joint Committee on Powder Diffraction Spectra (JCPDS, card number 43–1012), which are shown in Fig. 3(b). The XRD peaks and their locations (in the 2θ range of 20°–80°) for GaO(OH) and β-Ga₂O₃ are consistent with the previously reported XRD peak identification of nanostructured GaO(OH) and β-Ga₂O₃ reported by Huang et al.³⁸ and Shao et al.³⁹ 

Fitting the current density–voltage data to Eq. (3) with a theoretical Richardson constant of 41.11 A/cm²K² (based on the electron effective mass m* = 0.34m₀) yields the extraction of the Schottky barrier φ_B = 0.915 eV at room temperature (300 K). This estimated barrier height value is in agreement with the reported room-temperature Schottky barrier height of 1.05 eV by Armstrong et al.,⁴⁰ 0.95 eV by Oh et al.,⁴¹ and 1.07 eV by Jayawardena et al.⁴² Furthermore, the analysis shown in Fig. 9 indicates that in the temperature range from 300 to 800 K, the Schottky barrier height decreased with temperature. When the working temperature was greater than 550 K, the height of the barrier was close to being constant, which is advantageous for very high-temperature operations.

In Fig. 10, the temperature-dependent junction voltages were plotted at four different bias currents (10, 30, 50, and 70 µA) to demonstrate the temperature sensing functionality of the LPD thin-film Ni/β-Ga₂O₃ Schottky devices. The temperature sensitivity is −2.924, −1.68, −2.045, and −2.455 mV/K at the bias current of 10, 30, 50, and 70 µA, respectively. The highest temperature sensitivity of the junction voltage of the Schottky diodes reached −2.924 mV/K at a bias of 10 µA in the range between 300 and 500 K, which is comparable to that of a 4H–SiC Schottky diode-based temperature sensor (5.11 mV/°C from 30 to 300 °C) reported by Rao et al.⁴³ who demonstrated the potential of using the LPD-grown Ni/β-Ga₂O₃ Schottky diode for temperature sensing above room temperature. The thin-film Ni/β-Ga₂O₃ Schottky devices were not hermetically sealed, and the electrical measurement was conducted in vacuum. It is expected that the performance of these thin-film temperature sensors may depend on the device packaging and measurement environment,^44 –46 which is out of the scope of the current work and will be the subject of future investigation.

Figure 11(a) shows the low-temperature J–V characteristics of Ni/β-Ga₂O₃ from 100 to 250 K. We extracted the Schottky barrier height [Fig. 11(b)], showing its rise with temperature. This feature is opposite to what we observed in the high-temperature measurements as previously presented (T > 500 K). This can be explained using the thermal energy (k_BT), and the current conduction is unfavorable due to low energy electrons or even freezing of charge electrons. In this case, the charge transport mechanism may be in combination with thermionic field emission (TFE) and field emission (FE) related to the tunneling of electrons in addition to the thermionic emission (TE).⁴⁷ The structural evaluation using x-ray diffraction [Fig. 3(b)] indicates that the LPD grown β-Ga₂O₃ is polycrystalline in nature. This leads to the incorporation of defects in the interface between the β-Ga₂O₃ and Ni metal interface, resulting in Schottky barrier inhomogeneities.^48 –50 

Figure 11(b) shows the temperature-dependent plots (φ_bn vs $12kBT$⁠) following Eq. (5), where the uniformity of the barrier height depends on the fluctuation with the standard deviation near $φbn̄$⁠. Data fitting with Eq. (5) results in $φbn̄$ = 0.927 eV and σ_s = 99.8 meV (refer to Table I). The larger standard deviation of about 0.1 eV indicates that the barrier height at the Ni/β-Ga₂O₃ interface is more uneven (i.e., with increased inhomogeneities), and the current transport at low temperature is strictly affected by the barrier unevenness introduced by defects.⁵² Apart from the roughness from the interface, the polycrystalline nature of LPD grown β-Ga₂O₃ (as evident from x-ray diffraction and scanning electron microscopic evaluation) introduces defective grain boundaries, which can be responsible for the potential fluctuation. In addition, during the solution processing of β-Ga₂O₃, it is always possible to have the chemical impurities (act as trap centers) between the LPD grown β-Ga₂O₃ and metal contact.

We successfully synthesized β-Ga₂O₃ by a low-temperature and non-vacuum-based LPD method. From a Ga(NO₃)₃–Na₂CO₃ aqueous solution, through pH adjustment and control processing temperature and time, we synthesized different shapes of uniform GaO(OH) particles. The resultant GaO(OH) nanoparticles were calcined at 800 °C through a high-temperature annealing process to transform them into a monoclinic β-Ga₂O₃ nanograin structure. Through the temperature-dependent electrical analysis of fabricated horizontal-structured Ni/β-Ga₂O₃ Schottky diodes, we estimated the Schottky barrier height and explored the Schottky barrier inhomogeneity. We also discussed the utilization of Ni/β-Ga₂O₃ Schottky diodes for high-temperature sensing applications. At room temperature, the Schottky barrier height was 0.915 eV, and the Richardson constant was 43.04 ± 0.01 A/cm² K², which was very close to the theoretical value of 41.11 A/cm² K². Through the temperature (300–500 K) sensitivity analysis, the highest temperature sensitivity of the junction voltage of the Schottky diodes reached −2.924 mV/K with a fixed bias current at 10 µA. Through the surface potential fluctuation model, it was verified that the inhomogeneity of the barrier causes the non-ideal effect at low temperature, and the probable reason is the inhomogeneous defect distribution at the Ni/β-Ga₂O₃ interface. These results indicate that under high-temperature conditions, β-Ga₂O₃ produced using the LPD method has good potential for thin-film temperature sensors.

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under Contract No. MOST 109-2221-E-006-148. This research was, in part, supported by the Ministry of Science and Technology, Taiwan, through Grant No. MOST 110-2221-E-006-175. J.V.L. also acknowledges support from United States Air-Force Office Science and Research through Award Nos. FA2386-21-1-4071 and FA2386-22-1-4006.

The authors have no conflicts to disclose.

S.P. and T.-S.C. contributed equally to this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.