As an emerging wide band gap semiconductor material, β-Ga₂O₃ is considered as one of the most promising materials for power-electronic devices and solar-blind detectors. It is benefit from the excellent material properties of β-Ga₂O₃ including ultrawide bandgap (4.6−5.2 eV), high critical field (8 MV/cm), high Baliga’s figure-of merit (3214) and good chemical stability^[1−3]. β-Ga₂O₃ solar-blind ultraviolet (200−280 nm) photodetectors have wide application owing to their superiorities, such as low background noise, high response value and radioresistance^{[4, 5]}. In order to improve the photodetector performance, a series of methods are conducted to grow β-Ga₂O₃ films, including molecular beam epitaxy (MBE)^[6], halide vapor phase epitaxy (HVPE)^[7], low pressure chemical vapor deposition (LPCVD)^{[8, 9]}, mist chemical vapor deposition (mist CVD)^[10] and MOCVD^[11]. Metal−semiconductor−metal (MSM) structure has the advantages of simple fabrication process, convenient integration with readout circuits and high responsivity because of internal gain. Pratiyush et al.^[12] demonstrated MSM detector using heteroepitaxial β-Ga₂O₃ films with a high responsivity of 1.5 A/W. Qian et al.^[13] reported MSM detector based on amorphous Ga₂O₃ films with an ultrahigh response of 70.26 A/W and specific detection rate of 1.26 × 10¹⁴ Jones. By growing polycrystalline β-Ga₂O₃ film, Arora et al.^[14] had prepared the high-performance MSM detector with a response of 96.13 A/W.

Although β-Ga₂O₃ film grown by various technologies have been reported, there are still some difficulties in getting ideal quality β-Ga₂O₃ films. For example, the expensive substrate for homoepitaxial and the large lattice mismatch for heteroepitaxial. As low-cost substrates, sapphires are widely used in heteroepitaxial β-Ga₂O₃ films. However, the lattice mismatch between c-sapphire and β-Ga₂O₃ is 3.5%. The stress caused by lattice mismatch will lead to a high dislocation density, which is not conducive to the epitaxial growth of material. In order to alleviate lattice mismatch and improve crystalline quality of epitaxial films, a buffer layer between substrate and epitaxial layer can be applied. However, there are few reports about the effect of buffer layer on the quality of β-Ga₂O₃ film on c-sapphire^{[15, 16]}.

In this work, high-quality β-Ga₂O₃ films were grown by two-step MOCVD method on c-sapphire. By varying buffer layer growth temperature, β-Ga₂O₃ film with fullwidth at half maximum (FWHM) of 0.66° was obtained. The MSM solar-blind photodetector was fabricated to further verify the excellent performance of β-Ga₂O₃ film quality. The solar-blind photodetector exhibits high responsivity of 1422 A/W @ 254 nm. In addition, ultrahigh PDCR (10⁶), detectivity (2.5 × 10¹⁵ Jones) and respond speed (36 ms) were also achieved from the fabricated photodetector. This work provides an approach for the preparation of high-quality β-Ga₂O₃ films and the application of β-Ga₂O₃ solar-blind photodetectors.

Four β-Ga₂O₃ samples were prepared by depositing on c-plane (0001) sapphire in a MOCVD system by the two-step method. First, the thin buffer layer of Ga₂O₃ was deposited at low temperature (T_B), and then β-Ga₂O₃ film was grown on the buffer layer at 960 °C. Trimethylgallium (TMG) and O₂ were used as gallium and oxygen precursors, respectively. High-purity argon was used as carrier gas for TMG source. Four β-Ga₂O₃ films samples were grown at the same condition, except that T_B were at 650, 700, 750 and 800 °C. The crystalline quality of β-Ga₂O₃ films were characterized by high-resolution X-ray diffraction (HRXRD, X’Pert3 MRD). The surface morphologies of films were investigated by atomic force microscopy (AFM, SII Nanonavi) and scanning electron microscope (SEM, NanoSEM650). The photodetector with MSM structure was fabricated on the β-Ga₂O₃ film with T_B = 700 °C. Si ion implantation process^{[17, 18]} was performed and followed by deposition of Ti/Au electrode. After lift-off process, the interdigital electrodes were formed and the rapid thermal annealing at 475 °C for 1 min was conducted. The room-temperature optical responsivities of β-Ga₂O₃ PD were measured by an Agilent B1500A semiconductor parameter analyzer and ultraviolet light emitting diodes (UV LEDs) emitting 254 and 356 nm light. Spectral-response measurements at −1 V bias were performed by spectrometer equipped with Xe light source. An Si detector was tested for calibrating the spectrum response.

The crystal property of β-Ga₂O₃ films was measured by XRD with ω−2θ scan mode. Fig. 1(a) shows the XRD spectra of Ga₂O₃ films with varying T_B from 650 to 800 °C. Three diffraction peaks at 18.9°, 38.3°, and 58.9° exhibit well-defined (−201), (−402), and (−603) planes of β-Ga₂O₃, respectively, indicating the (−201) oriented single crystal growth. This is owing to the similar oxygen atomic arrangement of (−201) plane β-Ga₂O₃ and c-sapphire. The diffraction peak intensity of β-Ga₂O₃ film with T_B = 650 °C is weak, which is likely due to the low migration rate of adsorbed atoms on substrate surface at low growth temperature. When increasing T_B, the crystalline quality of epitaxial layer is improved and the intensity of diffraction peak increases. Figs. 1(b) gives the ω scanning diffraction pattern of (−402) β-Ga₂O₃ planes and the FWHM values at different T_B. The FWHM values of films prepared at different T_B of 650, 700, 750, and 800 °C are 3.3°, 0.66°, 0.67°, and 0.93°, respectively. The β-Ga₂O₃ films in this work grown by two-step method have much lower FWHM values, compared with previously reported β-Ga₂O₃ films on sapphire^[19−21]. The sample grown at T_B = 700 °C shows the lowest FWHM value, indicating the optimized buffer layer growth condition is found.

AFM was performed to further investigate the surface morphology of films deposited on c-sapphire with different T_B. In Fig. 2(a), four samples were examined by AFM scan on a 5 × 5 μm scan area. The surfaces of four samples all show regular pseudo-hexagon structures, which are generated by the in-plane rotational domains and attributed to the symmetries of c-sapphire and β-Ga₂O₃^{[22, 23]}. The film with T_B = 650 °C presents the sparsest texture, which is owing to the low mobility of crystal nuclei during the initial growth and lead to sparse grains difficult to merge. The sample with T_B = 700 °C exhibits the densest texture, which is likely attributed to the moderate crystal nuclei mobility at this temperature. As the temperature rises (with T_B = 750 or 800 °C), the atom mobility increases, leading to the deposition of large crystal nuclei on substrates and resulting in rough surfaces. Fig. 2(b) shows the surface roughness of four samples, which are 19, 6.8, 7.5, and 12 nm, respectively. Fig. 2(c) gives the variation in film thickness with different T_B and inset shows the cross-sectional SEM image of sample grown with T_B = 700 °C. The thickest β-Ga₂O₃ film of 1 μm is obtained when T_B = 700 °C. These results indicate that the buffer layer temperature at 700 °C is favorable for β-Ga₂O₃ film two-step growth.

The MSM solar-blind photodetector was fabricated on β-Ga₂O₃ film with T_B = 700 °C. Fig. 3(a) shows the optical microscope diagram of device. Fig. 3(b) reveals the three-dimensional structure diagram of the device. Fig. 3(c) shows the I−V characteristic curve of MSM photodetector. As revealed in figure, the dark current is 400 pA at 10 V bias. Moreover, the I−V curve increases only a little under 365 nm light (light intensity of 15 mW/cm²), indicating that β-Ga₂O₃ film is not sensitive to 365 nm light. However, the I−V curve increases steeply under 254 nm light (light intensity of 1.9 mW/cm²), suggesting strong light response characteristics and excellent solar-blind photoelectric characteristics.

The light/dark current rejection ratio (PDCR)^[24], which is defined as (I_photo − I_dark )/I_dark, achieves an extremely large value of more than 10⁶, indicating that the device has extremely high sensitivity. The responsivity of MSM detector with effective illumination area about 4.55 × 10⁴μm² reaches 1422 A/W under 10 V bias. Detectivity (D*) can be used to evaluate the ability to perceive the lowest detectable signal, which is related to background noise. Assuming that dark current is the main source of device noise, D* can be expressed as: RS^1/2/(2qI_dark)^{1/2 [24]}. The photodetection based on high-quality β-Ga₂O₃ film has a D* of 2.5 × 10¹⁵ Jones under 254 nm light, indicating the ultra-high signal-to-noise ratio. The excellent wavelength selectivity and ultrahigh D* indicate that high sensitivity solar-blind detector is achieved.

Fig. 3(d) is energy band diagram and photogenerated carriers of the fabricated MSM photodetector under 254 nm light with external bias. It is determined from carrier transport mechanism that the optical response of photodetector is a complex process. Photogenerated carriers will be captured by deep-level defects so the lifetime is longer than transmission time. Moreover, electrons are quickly swept-out and reinjected to remain electric neutrality, generating multiple electrons and huge photoconductive gain.

The transient response of MSM photodetector under 254 nm wavelength illumination was tested by controlling the turning on and off for the light source, as shown in Fig. 4(a). When the light source is turned off, the current of the device decays quickly, showing a fast decay speed. In order to quantitatively analyze the response process of current rise and fall, the time responding characteristics are amplified as shown in Fig. 4(b). Then the double exponential relaxation equation is used to fit the I−T curve, and the equation is as follows^[25]: I = I₀ + Aexp(−t/τ₁) + Bexp(−t/τ₂), where I₀ is the photocurrent in the stable state, t is the time, A and B are constants, τ₁ and τ₂ denote two relaxation time constants. τ₁ reflects the rapid change of carrier concentration after the light source is turned on or off, and τ₂ is caused by carrier capture/release due to defects in β-Ga₂O₃ film. By fitting the photoelectric response curve, the time constants of rising edge are τ_r1 = 35 ms and τ_r2 = 181 ms. The decay time constants of falling edge (i.e. the decay time) are: τ_d1 = 4 ms and τ_d2 = 32 ms. These values are faster than Ga₂O₃ photodetectors reported before^{[24, 26−29]}. Furthermore, such fast decay speed (τ_d = 36 ms) indicates weak persistent photoconductivity (PPC) behavior, which can be caused by the reduced defect density in two-step growth film^[29].

Fig. 5(a) shows the spectral response of the β-Ga₂O₃ MSM photodetector @−1 V and the peak wavelength is located at ~250 nm (4.97 eV), which is close to the bandgap of β-Ga₂O₃. The increased responsivity after 400 nm wavelength may be due to the absorption corresponding to the defect energy level. The rejection ratio (R_{250 nm}/R_{400 nm}) of 10⁵ shows a higher response wavelength selectivity than previously reported results^{[30, 31]}. Excellent solar-blind ultraviolet detection characteristic demonstrates the high-quality film and strong interband excited transitions. Fig. 5(b) gives the responsivities and detectivities of the reported deep ultraviolet photodetectors based on Ga₂O₃ films. It showed clearly that the obtained solar-blind photodetector displays the better comprehensive performance. In addition, the actual responsivity (R_actual) is given by R_actual = GR_ideal = G(ηq/hν)^[6], where η is EQE and ideal responsivity (R_ideal) is β-Ga₂O₃ photodetector detected at 254 nm. Assuming η is 100% and R_ideal is 0.2 A/W, the gain (G) is equal to 7110 indicating huge internal gain in β-Ga₂O₃ photodetector.

In summary, a Ga₂O₃ buffer layer is introduced to reduce lattice-mismatch-related defects using MOCVD method. It is found that T_B will obviously affect crystalline quality and the optimized buffer layer temperature for two step method is 700 °C. The MSM solar-blind photodetector shows ultrahigh responsivity of 1422 A/W and superior detectivity of 2.5× 10¹⁵ Jones. Besides, a notable rejection ratio (R_{250 nm}/R_{400 nm}) of 10⁵ is realized, indicating the excellent wavelength selectivity. A remarkable gain of 7110 further demonstrates high internal gain in the β-Ga₂O₃ MSM photodetector. The outstanding photodetector performance suggests that two-step growth method has great potential for preparing high-quality β-Ga₂O₃ film on sapphire.