The development of solar-blind ultraviolet detectors (SBUVDs) based on Ga₂O₃ is of interest primarily due to its suitable band gap energy (E_g = 4.5−5.3 eV), high thermal, chemical, and radiation stability, and the potential for depositing and growing high-quality films^[1−5].

Ga₂O₃-based SBUVDs show promise for wireless communication in the short-wavelength ultraviolet (UVC) range, laser radiation detection, astrophysical research, biomedical research, monitoring the effect of UVC on biological objects, flame sensors, and ozone layer monitoring systems^[6−11].

The metastable α-Ga₂O₃ polymorph has the largest band gap among other Ga₂O₃ polymorphs, with E_g ~ 5.0−5.3 eV^{[12, 13]}. The crystal lattice of the corundum-like α-Ga₂O₃ is similar to that of α-Al₂O₃^{[14, 15]}, enabling the growth of high-quality α-Ga₂O₃ films with low dislocation density on commercially available substrates through heteroepitaxy. Additionally, the α-Ga₂O₃ crystal lattice has higher symmetry, making it an attractive option for solar-blind ultraviolet detector development. For a considerable period, researches of the characteristics of α-Ga₂O₃ for the development of SBUVD were restricted due to the absence of methods to grow high-quality α-Ga₂O₃ films. SBUVDs developed to date, based on thin films and heterostructures of α-Ga₂O₃ obtained by mist chemical vapour deposition^[16], laser molecular beam epitaxy^[17], pulse laser deposition^[18], low temperature atomic layer deposition^[19], radio-frequency magnetron sputtering^[20] and sol-gel^[21], did not differ in high sensitivity of responsivity R_λ but were characterized by relatively high speed performance (low rise τ_r and decay τ_d times). The challenges in producing high-quality α-Ga₂O₃ layers likely contribute to the difficulty in developing SBUVDs based on α-Ga₂O₃ films and heterostructures. Additionally, the process of creating these heterostructures often involves additional operations.

Halide vapor phase epitaxy (HVPE) is growth method that enables to grow high-quality α-Ga₂O₃ films with controlled doping, thicknesses in the range from 0.1 to 10 µm, and is characterized by a relatively high growth rate^{[12, 13, 22−24]}. It is important to note that there are few studies on the photoelectric properties of structures based on HVPE grown α-Ga₂O₃ films.

Research^[25] demonstrated that SBUVDs based on HVPE grown α-Ga₂O₃ films exhibit exceptional high sensitivity to UVC radiation. The achieved responsivity R_λ was 4.24 × 10⁴ A/W correlated with low speed performance (long rise τ_r and decay τ_d times). Previously, we also investigated metal−semiconductor−metal (MSM) type SBUVDs based on high-quality HVPE grown α-Ga₂O₃ films with interdigitated ohmic Ti/Ni electrodes^[26]. The highest values of R_λ of 7.19 × 10⁴ A/W, the external quantum efficiency η of 3.79 × 10⁵ arb. un., and the detectivity D^* of 1.12 × 10¹⁸ Hz^0.5·cm/W were achieved at a wavelength λ of 235 nm and applied voltage of 10 V. However, these SBUVDs exhibited low speed performance too, with high τ_r and τ_d, and were unable to operate in self-powered mode.

The development of self-powered SBUVDs based on Ga₂O₃ has received much attention recently^{[27, 28]}. Such devices do not require external power and are characterized by high-speed performance and stable photoelectric properties. To implement the self-powered operation mode, it is necessary to create a built-in electric field in the semiconductor structures^[29]. MSM structures with a Schottky barrier are characterized by the existence of a built-in electric field at metal/semiconductor interfaces. Furthermore, these structures exhibit high-speed performance and low dark current I_D, which can be attributed to the presence of a built-in electric field and potential barrier at the metal/semiconductor interfaces. This, coupled with their relative ease of manufacture^[29−31], distinguishes them from other structures.

Previously, we investigated the photoelectric properties of the UVC detectors based on HVPE grown κ(ε)-Ga₂O₃ film with Pt contacts^[32]. A Schottky barrier is formed at the Pt/Ga₂O₃ interface. High values of photoelectric properties were achieved, including at self-powered operation mode, with R_λ and η values of 0.9 mA/W and 0.46%, respectively, at λ = 254 nm. The τ_r and τ_d did not exceed 100 ms. The potential for developing high-speed UVC detectors that operate autonomously has been demonstrated.

This work investigates the photoelectric properties of the UVC detectors based on HVPE grown α-Ga₂O₃ film with Pt contacts. The results show that these structures can function in self-powered operation mode and have a τ_r of 18 ms at this mode.

The α-Ga₂O₃ film was deposited with HVPE in a hot-wall reactor at 500 °C for a duration of 50 min. A semi-insulating α-Ga₂O₃ film was grown on a 2-inch (0001) sapphire epi-ready substrate using GaCl and O₂ as precursors at a Ⅵ/Ⅲ ratio of 2O₂/GaCl = 4.2. The rate of film growth was 0.1 µm/min.

Pt interdigital contacts were deposited onto the α-Ga₂O₃ surface by means of direct current magnetron sputtering and photolithography. The contacts had the geometry shown in Fig. 1 and an interelectrode distance of 300 μm. One ultraviolet (UV) sensitive element had a size of 5 × 5 mm² with an effective area S of 7.6 mm².

X-ray diffraction (XRD) spectrum was used to study the HVPE grown α-Ga₂O₃ films. A Dron 8H diffractometer (Bourevestnik, JSC) with CuKα radiation at λ = 1.5406 Å was used to register the XRD patterns in θ−2θ scanning mode. Analysis of the reflection peak positions was used to identify the phase composition of the films. Scanning electron microscopy (SEM) (Phenom ProX, The Netherlands) was used for analysis of the cross-sectional images of the samples at an acceleration voltage of 10 kV. Transmission spectra of HVPE grown α-Ga₂O₃ films were measured at room temperature using a DH-2000 deuterium/tungsten radiation source and Ocean Optics spectrometry system (Ocean Insight, Orlando, FL, USA). Measurements were performed using the Ocean Optics USB 2000+ spectrometer with an interval of λ = 320−517 nm and the Ocean Optics Flame spectrometer with an interval of λ = 200−850 nm, with an optical resolution of 1 nm for the λ.

A MonoScan 2000 monochromator (Ocean Optics) with an operating interval of Δλ = 200−850 nm was used to measure the spectral dependencies of the photoelectric properties. An optical radiation source was a DH-2000 Micropack halogen-deuterium lamp. The measurements were managed by the OceanView software. The method of measuring the photoelectric properties of UVC detectors and the corresponding system for this are described in Ref. [26].

The experiment utilized a krypton-fluorine lamp VL-6.C as the source of irradiation at λ = 254 nm and P = 620 μW/cm², where P is the light power density. The measurements were conducted automatically at room temperature and humidity. The I−V and I(t) curves of the UVC detectors were measured by Keithley 2636A, where I is the current, V is the applied voltage, and t is the time.

The formulas presented in Ref. [30] were used to calculate the dependencies of photo to dark current ratio (PDCR), R_λ, η, and D^* of UVC detectors on λ and V:

1PDCR=Iph/ID,

2Rλ=Iph/(P×S),

3η=[Rλ×h×c/(q×λ)]×100%,

4D*=Rλ×S1/2/(2×q×ID)1/2,

where I_ph is the photocurrent, I_ph = I_L − I_D, I_L is total current, h is Planck constant, and c is the speed of light in vacuum; q is the electron charge.

Fig. 2(a) shows the XRD pattern of the Ga₂O₃ film grown on the c-plane sapphire using HVPE. The peaks observed at 2θ = 40.15° and 86.85° correspond to the (0006) and (00012) planes of the α-Ga₂O₃ phase (JCPDS: 06-0503), respectively. Additionally, two other intense peaks observed at 2θ = 41.55° and 90.55° correspond to the (0006) and (00012) planes of the c-plane sapphire substrate (JCPDS: 46-1212), respectively.

SEM cross-sectional image of the HVPE grown α-Ga₂O₃ film on the c-plane sapphire substrate is presented in Fig. 2(b). The thickness of the α-Ga₂O₃ film is 3.3 µm. The α-Ga₂O₃ film exhibits a pure and uniform heterointerface with the sapphire substrate. The surface of the α-Ga₂O₃ film is smooth and does not show any microrelief features.

The optical absorption edge analysis (Fig. 2(c)) shows that E_g for films is 5.27 ± 0.05 eV, corresponding to λ = 235.3 nm, which is in the UVC interval and suitable for SBUVD. In Fig. 2(c), α represents the absorption coefficient. The linear section in Fig. 2(c) is more accurately approximated by the dependence of α² on the photon energy, indicating the implementation of direct optical transitions in α-Ga₂O₃ films.

The I−V curves of UVC detectors based on α-Ga₂O₃ films with Pt contacts are symmetrical under both dark and UV irradiation conditions (Fig. 3). The dark current of the structures increases from 0 to 6 nA as the voltage increases from 0 to 10 V. Exposure to ultraviolet irradiation at λ = 254 nm and P = 620 µW/cm² leads to a significant increase in current, from 6 nA to 1.4 mA at V = 10 V.

Fig. 4 displays the spectral dependencies of the photoelectric properties of the UVC detectors based on α-Ga₂O₃ films in the interval of λ = 200−370 nm and at V = 1 V. The PDCR, R_λ, D^* and η exhibit a sharp increase with a decrease in λ from 260 to 230 nm due to the interband photons absorption in the α-Ga₂O₃ film. The gradual decrease in PDCR, R_λ, D^* and η at λ > 230 nm is attributed to an increase in the influence of surface recombination. The photoelectric properties exhibited their maximum values at λ = 230 nm. At λ = 230 nm and V = 1 V, the PDCR, R_λ, D^*, and η were 1.16 × 10⁴ arb. un., 30.6 A/W, 6.95 × 10¹⁵ Hz^0.5·cm/W, and 1.65 × 10⁴%, respectively. The UVC detectors based on α-Ga₂O₃ films with Pt contacts are solar-blind and can also function as UVC bandpass detectors in the interval of 200 nm≤ λ ≤ 260 nm.

Fig. 5 displays the dependencies of the photoelectric properties of the UVC detectors based on α-Ga₂O₃ on the applied voltage. The values of PDCR, R_λ, D^*, and η increase significantly with an increase in applied voltage. The maximal values of PDCR, R_λ, D^*, and η at λ = 254 nm, P = 620 µW/cm² and V = 10 V were 2.6 × 10⁶ arb. un., 29.1 A/W, 6 × 10¹⁴ Hz^0.5·cm/W, and 1.4 × 10⁴%, respectively. The UVC detectors based on α-Ga₂O₃ film with Pt contacts exhibit sensitivity to UV irradiation at zero bias due to photovoltaic effect. R_λ = 1.3 × 10⁻⁴ A/W and η = 6.2 × 10⁻²% at zero bias.

The sections depicting the rise and decay of the total current of the SBUVDs in self-powered mode and at V = 10 V (as shown in Fig. 6) under UV exposure are approximated by biexponential functions:

5IL(t)=ILst−C1×exp[−t/τr1]−C2×exp[−t/τr2],

6IL(t)=ILst+K1×exp[−t/τd1]+K2×exp[−t/τd2],

where I_Lst is the steady-state total current; C₁, C₂, K₁ and K₂ are constants; τ_r1, τ_r2, τ_d1 and τ_d2 are the relaxation-time constants^[32]. It is important to note that τ_r1≫τ_r2 and τ_d1≫τ_d2. The fast-response components of τ_r1 and τ_d1 are typically a result of the rapid changes in carrier concentration caused by the generation and recombination of carrier charges during light on/off cycles. The slow-response component of τ_r2 and τ_d2 are due to the carrier trapping/releasing from defects^[33]. The fast response components largely determine the speed performance of the UVC detectors, so the equations τ_r = τ_r1 and τ_d = τ_d1 are fair.

The UVC detectors based on α-Ga₂O₃ films with Pt contacts in self-powered mode exhibit τ_r and τ_d values of 18 ms and 1.3 s, respectively, as shown in Figs. 6(b) and 6(d). At V = 10 V, the values are 300 and 110 ms for τ_r and τ_d, respectively. The times are notably lower than those obtained in our previous study^[26] on SBUVDs that were based on the HVPE grown α-Ga₂O₃ film with Ti/Ni contacts.

Compared to other works, the MSM type SBUVDs based on the HVPE-grown α-Ga₂O₃ film with Pt contacts exhibit low τ_r and τ_d, relatively high value of the responsivity in self-powered operation mode. Table 1 shows the following abbreviations: HJ is the heterojunction; SBD is the Schottky barrier diode; MLG is the multi-layer graphene; PEC is the photoelectrochemical photodetector.

Exposure to UVC radiation results in the bipolar generation of charge carriers in α-Ga₂O₃. In general terms the self-powered mode in MSM structures based on α-Ga₂O₃ is attributed to the built-in electric field at the Pt/α-Ga₂O₃ interfaces. At the interfaces of Pt/α-Ga₂O₃, photogenerated charge carriers in the charge space regions are separated by a built-in electric field. At zero bias, the electric current should ideally be zero, as the potential barriers at the Pt/α-Ga₂O₃ interfaces compensate each other. However, the Keithley 2636 A high-precision source-meter supplies a minimum voltage of V = 0.1 mV for measurements. This can cause one potential barrier to increase and the other to decrease for charge carriers. A larger number of charge carriers flow through the potential barrier at the Pt/α-Ga₂O₃ interface with a lower height, causing an imbalance in the current. It is important to consider the potential imperfections and asymmetry of the contacts, as well as any defects in the semiconductor material. These factors can impact the transport of charge carriers in self-powered SBUVD^[28]. The low sensitivity of detectors to radiation at λ ≥ 260 nm is due to the large value of E_g = 5.27 ± 0.05 eV of the α-Ga₂O₃ film, which corresponds to a λ = 235 nm. Fig. 7 illustrates the operational modes of the developed SBUVD.

The rise time decreases from 300 to 18 ms and the decay time increases from 110 ms to 1.3 s at λ = 254 nm at reducing of V from 10 to 0 V. Applying voltage to SBUVD causes a shift in the Fermi level of the semiconductor. This process involves additional defects in generating charge carriers, resulting in an increase in τ_r. Additionally, the electric field present in the entire structure, including the quasi-neutral bulk of the semiconductor and not just the space charge regions at the Pt/α-Ga₂O₃ interfaces, accelerates the drift of charge carriers, leading to a decrease in τ_d. This phenomenon has been previously observed in the Refs. [43, 44].

The SBUVDs exhibit a very high η of 1.4 × 10⁴% at λ = 254 nm, P = 620 µW/cm² and V = 10 V, which is caused by the strong trapping of holes in semiconductors^[45]. This results in an increased lifetime of non-equilibrium electrons that contribute to the photocurrent until the trapped holes are released and available for recombination. The detectors' R_λ and η in self-powered mode are relatively low due to their design. As previously reported^{[46, 47]}, the diffusion length for both holes and electrons in Ga₂O₃ is limited to 400−500 nm. To enhance the R_λ and η of detectors, it is possible to reduce the interelectrode distance^[46].

Detectors based on α-Ga₂O₃ films with Pt interdigital contacts were developed to detect shortwave ultraviolet irradiation. α-Ga₂O₃ films, 3.3 μm in thick, were grown by halide vapor phase epitaxy on c-plane planar sapphire substrates. The spectral dependencies of the photo-to-dark current ratio, responsivity, external quantum efficiency, and detectivity of the structures were studied in the wavelength interval of 200−370 nm. The detectors showed a significant response to ultraviolet radiation with a wavelength of no more than 260 nm. The maximum of photo to dark current ratio, responsivity, external quantum efficiency, and detectivity of the structures were 1.16 × 10⁴ arb. un., 30.6 A/W, 1.65 × 10⁴%, and 6.95 × 10¹⁵ Hz^0.5·cm/W, respectively, at a wavelength of 230 nm and an applied voltage of 1 V. The high values of the photoelectric properties were due to the internal enhancement of the photoresponse, mainly related to strong trapping of holes under exposure to ultraviolet irradiation. The α-Ga₂O₃ film-based metal–semiconductor–metal type detectors with Pt contacts can function as ultraviolet bandpass detectors in the interval of 200 nm ≤ λ ≤ 260 nm. The detectors are sensitive to ultraviolet irradiation at zero applied voltage and can function in self-powered operation mode due to the built-in electric field at the Pt/α-Ga₂O₃ interfaces. The structures' responsivity and external quantum efficiency at a wavelength of 254 nm and zero applied voltage were 0.13 mA/W and 6.2 × 10⁻²%, respectively. The rise and decay times were18 ms and 1.3 s, respectively, in self-powered mode.