Among the five polymorphs of Ga₂O₃, β?Ga₂O₃ is notable for its superior thermal stability and a breakdown field strength of approximately 8 MV/cm. In addition, β?Ga₂O₃ demonstrates high selectivity and exceptional photoresponse characteristics towards solar-blind ultraviolet light, making it an ideal candidate for solar-blind photodetectors (SBPDs). In contrast, ε?Ga₂O₃, the second most stable polymorph, exhibits ferroelectric properties and a significant spontaneous polarization coefficient of 24.44 μC/cm2. However, a major challenge is the lack of mature commercial metal-organic chemical vapor deposition (MOCVD) equipment specifically designed for Ga₂O₃, as well as the absence of suitable epitaxial growth substrates. In this paper, we utilize a self-developed MOCVD system, which demonstrates stability and reliability, enabling significant advancements in Ga₂O₃ growth. This leads to high system stability and moderate-scale production capabilities. Recent studies employ c-plane sapphire as the substrate for heteroepitaxial growth of Ga₂O₃, with controlled growth conditions such as temperature, gas flow rates, and pressure to yield various Ga₂O₃ polymorphs. Lower growth temperatures favor the formation of hexagonal ε?Ga₂O₃, while temperatures above 500 ℃ promote a gradual transition to β-Ga₂O₃. In this paper, we investigate the influence of growth temperature on the properties of Ga₂O₃ thin films and SBPDs at temperatures below 700 ℃, providing a feasible approach for the heteroepitaxial MOCVD growth of Ga₂O₃ thin films. We aim to overcome existing limitations by offering practical insights into optimizing growth conditions for enhanced material quality and device performance.

During epitaxial growth, the MOCVD system’s reactor pressure is maintained at 50 mBar while the temperature is gradually increased to the target level. Upon reaching the target temperature, oxygen (O₂) and triethylgallium (TEGa) are introduced as O and Ga sources onto a (002) c-plane sapphire substrate with a molar flow ratio of O to Ga of 1600 for a growth duration of 1 hour. Five sets of Ga₂O₃ thin films are grown at temperatures of 500, 530, 580, 630, and 680 ℃, labeled as T₅₀₀, T₅₃₀, T₅₈₀, T₆₃₀, and T₆₈₀, respectively. Post-growth characterization is performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible spectrophotometry to analyze the structural, surface, elemental composition, and optical properties of the Ga₂O₃ thin films. Metal hard masks are used for pattern transfer, and Ag interdigitated electrodes are sputtered onto the films to fabricate planar metal-semiconductor-metal (MSM) SBPDs. The current-voltage characteristics of the detectors are measured under dark conditions and illumination at 254 nm and 365 nm wavelengths (both with a light power density of 600 μW/cm2) using a Keithley 2450 SourceMeter. The transient photoresponse characteristics are evaluated by recording current-time (I-t) curves while periodically turning the 254 nm light source on and off at 60-second intervals.

XRD analysis reveals that T₅₀₀ predominantly consists of ε-Ga₂O₃, T₅₃₀ is a mixture of ε and β phases, while T₅₈₀, T₆₃₀, and T₆₈₀ are composed entirely of the β?Ga₂O₃ [Fig. 1(a)]. This indicates that temperature is a critical factor influencing the crystalline phase of Ga₂O₃, with a transition from ε to β phase occurring around 530 ℃. The XRC full width at half maximum (FWHM) results for all films align with those typically reported for heteroepitaxial growth [Figs. 1(b)?(f)]. All Ga₂O₃ thin films exhibit high crystalline quality, as evidenced by a sharp absorption edge near 255 nm in their transmission spectra [Fig. 3(a)]. SEM images show that film surfaces become smoother at lower temperatures but increasingly rougher as the growth temperature rises [Figs. 2(b)?(f)], likely due to the stronger crystal orientation of β-Ga₂O₃ at higher temperatures. The bandgap, derived from the absorption spectrum using the Tauc plot, increases with temperature [Figs. 3(b) and (c)], attributed to a reduction in oxygen vacancy defects and improved stoichiometry relative to bulk Ga₂O₃. XPS results show that the binding energies of Ga 2p_1/2, Ga 2p_3/2, and Ga 3d increase and then decrease with temperature [Fig. 4(b)], indicating that lower binding energies suggest more Ga—O bond formation. A similar trend is observed in the relative concentration of oxygen vacancies [Fig. 4(c)], suggesting that the T₅₀₀ and T₆₈₀ films exhibit enhanced structural stability. The fabricated SBPDs demonstrate excellent overall performance, particularly the device based on T₆₈₀, which achieves a light-dark current ratio (PDCR) of 3.75×10⁷, a responsivity (R) of 0.5 A/W, a detectivity (D) of 3.11×10¹² Jones, an external quantum efficiency (EQE) of 242.4%, and an R_{254 nm}/R_{365 nm} of 1.35×10⁶. In transient photoresponse testing, reduced oxygen vacancy defect concentrations result in diminished photoconductivity effects and enhanced response speeds. For the SBPD based on T₆₈₀, the response time constants are τ_r1=0.067 s, τ_r2=0.366 s, τ_d1=0.008 s, and τ_d2=0.063 s [Fig. 5(b)].

Using the MOCVD system, pure ε-phase Ga₂O₃ can be grown heteroepitaxially at 500 ℃. At 530 ℃, the onset of β?phase Ga₂O₃ occurs, and further heating to 580 ℃ results in the formation of pure β?phase Ga₂O₃. Simultaneously, the bandgap of Ga₂O₃ increases from 4.83 eV to 5.00 eV with rising temperature. All Ga₂O₃ thin films exhibit high crystalline quality, as evidenced by a sharp absorption edge near 255 nm in their transmission spectra. In addition, XPS analysis indicates that the crystal structures of the pure ε?phase Ga₂O₃ film (T₅₀₀) and the highest-temperature-grown pure β?phase Ga₂O₃ film (T₆₈₀) are relatively more stable. The fabricated MSM-type SBPDs demonstrate rapid photoresponse characteristics, along with good repeatability and stability. Specifically, under a bias voltage of 5 V and a light power density of 600 μW/cm2, the SBPD based on T₆₈₀ achieves a PDCR of 3.75×10⁷, a D of 3.11×10¹² Jones, an EQE of 242.4%, and an R_{254 nm}/R_{365 nm} of 1.35×10⁶, indicating excellent overall performance. In transient photoresponse tests conducted at a light power density of 600 μW/cm2, the rise time and fall time are measured to be 0.067 and 0.008 s, respectively.