Recently, solar-blind photodetectors (SBPDs) based on wide-bandgap semiconductors (e.g. Ga₂O₃,^[1–5] GaN,^[6,7] Al_xGa_1−xN,^[8,9] BN,^[10] diamond,^[11,12] and Zn_1−xMg_xO^[13]) have been extensively investigated owing to their wide range of civil and military applications, such as biomedical applications, flame detection, missile tracking, and intersatellite communication. However, for the Al_xGa_1−xN photodetectors that have been prepared, the device performance easily degrades as the Al concentration increases. The main drawback of BN and diamond is that their bandgaps are too large, approximately 6.3 eV and 5.5 eV, respectively, which makes their absorption edges fall at short wavelengths of 193 nm and 225 nm. Therefore, the full solar-blind UV wavelength cannot be effectively detected. For ZnMgO SBPDs,^[14–17] the main drawbacks of wurtzite ZnMgO (w-ZMO) and cubic ZnMgO (c-ZMO) are the relatively large dark current and low responsivity, respectively. Fortunately, high-performance SBPDs based on mixed-phase ZnMgO (m-ZMO) thin films have been recently reported, and these devices possess a dark current of 78 pA and a photoresponsivity of 434 A/W,^[15] which may be due to the large amounts of heterojunction interfaces between w-ZMO and c-ZMO. However, to obtain a high-performance m-ZMO SBPD, the Mg content of the film must be accurately controlled, which increases the difficulty of preparing the eligible film and device. Nevertheless, these results still provide a viable idea that better-performance SBPDs can be fabricated on gallium oxide (Ga₂O₃) thin films with mixed-phase structures.

It is surprising that among these materials, Ga₂O₃, which has a bandgap (E_g) of 4.4–5.15 eV, is intrinsically suitable for solar-blind photodetection. Amorphous Ga₂O₃ is easily prepared and has great potential for high-performance photodetection; therefore, it has received extensive attention. For instance, Kumar et al.^[18] comprehensively investigated the structure, morphology, and optical properties of amorphous Ga₂O₃ thin films. Qian et al.^[1] showed that the metal–semiconductor–metal (MSM) SBPD based on an amorphous Ga₂O₃ thin film has a responsivity as high as 70.26 A/W; this is because the amorphous Ga₂O₃ film possesses a high density of defects, oxygen vacancies, dangling bonds, etc., which contribute to the high internal gain. However, both the defects and oxygen vacancies act as trap centers and capture the photogenerated carriers, and thus hinder the recombination of hole–electron pairs, which would lead to persistent photoconductivity and ultimately a longer recovery time. Therefore, much of the work so far has focused on crystallized Ga₂O₃ thin films. Ga₂O₃ has five crystal structure phases, α, β, δ, ε, and γ.^[19] Among these five phases, monoclinic β-Ga₂O₃ has attracted much attention and been extensively investigated due to its good physical, chemical, and thermal stability. Additionally, the bandgap width of β-Ga₂O₃ is approximately 4.9 eV, corresponding to an absorption edge of approximately 250–280 nm, thus enabling effective detection of the whole solar-blind UV wavelength.^[20–22] For example, Yu et al.^[4] fabricated a β-Ga₂O₃ MSM SBPD with excellent performance by pulsed laser deposition (PLD) at 800 °C; the device possessed a very low dark current of approximately 1.2 × 10⁻¹¹ A, but a lower responsivity of 0.903 A/W. Interestingly, corundum α-Ga₂O₃ has a wider bandgap than β-Ga₂O₃, which is approximately 5.15 eV, and is intrinsically suitable for solar-blind photodetection. Recently, Guo et al.^[23] reported a device with an MSM structure based on an -Ga₂O₃ thin film, and investigated the UV photoresponse characteristics. The α-Ga₂O₃ SBPD possessed a very low dark current, but the responsivity was also relatively low. Consequently, it can be seen that the performance of both β-Ga₂O₃ and α-Ga₂O₃ SBPDs, such as the responsivity, remains lower than expected.^[4,24] To achieve a better performance in solar-blind photodetection, we employed the idea of preparing m-ZMO SBPDs; therefore, several mixed-phase Ga₂O₃ SBPDs were prepared, and the optoelectrical properties of the SBPDs were definitively studied.

In this work, we explored the growth of Ga₂O₃ thin films on a-Al₂O₃ substrates under various oxygen pressures at 650 °C by PLD. Then, MSM SBPDs based on β-Ga₂O₃ and mixed-phase Ga₂O₃ thin films were fabricated. The UV photoresponse characteristics of the devices were analyzed in detail.

Ga₂O₃ thin films were grown on a-Al₂O₃ ( ) by the PLD technique using a KrF laser (λ = 248 nm) with a repetition rate of 2 Hz and a laser energy of 300 mJ. A stoichiometric ceramic Ga₂O₃ target (99.99% purity) with a 29.6 mm diameter was used for the Ga₂O₃ thin film deposition. The substrate was kept 60 mm away from the target, and the substrate was fixed at 650 °C during film growth. When the deposition chamber was pumped to a base pressure below 6.0 × 10⁻⁴ Pa, oxygen gas (99.999%) was introduced into the chamber, and the deposition oxygen pressure was maintained at 3.0 Pa, 2.0 Pa, 1.0 Pa, 0.5 Pa, or 0.09 Pa. The oxygen flow rate and the growth time of the Ga₂O₃ films were fixed at 30 sccm and 60 min, respectively, for all samples. After deposition, all the samples were naturally cooled down to 80 °C at a base pressure of 6.0 × 10⁻⁴ Pa, and then removed from the chamber. The thickness of the Ga₂O₃ layer was measured by an ET-4000M model high-precision probe profiler manufactured by Japan Kosaka Corporation. The crystal structure and quality of the films were investigated by x-ray diffraction (XRD) (D8 Advance SS 18KW) with Cu K_α radiation (λ = 1.5418 Å) as the x-ray source, and the structure of the Ga₂O₃ thin films was also confirmed by Raman spectroscopy. The transmittance spectra of the Ga₂O₃ films were analyzed by a UV–visible spectrophotometer (UV-2450) in the wavelengths ranging from 200 nm to 600 nm. The surface morphology of the film was characterized by field emission scanning electron microscopy (SEM, SU-70).

The process of preparing the SBPDs with an interdigitated MSM structure was as follows: a Au (50 nm) film was grown on the surface of the Ga₂O₃ film by thermal evaporation, finally forming a Schottky contact. Afterward, electrodes with an interdigitated structure were fabricated using standard photolithography and wet etching processes. Both the finger width and the interspacing were , and the finger length was . After fabrication of the device, several performance characteristics were analyzed. Spectral responsivity measurements were carried out using a Zolix solar cell scan 100 measurement system with a 150 W xenon lamp as the light source. In addition, a Keithely 2450 was used as a bias source for testing the photoresponsivity of the device under different bias voltages.

Figure 1 shows the XRD (θ–2θ) patterns of the Ga₂O₃ films grown on a-Al₂O₃ ( ) substrates at various oxygen pressures. When the oxygen pressure decreases from 3.0 Pa to 2.0 Pa, the a-Al₂O₃ ( ) diffraction peak and the characteristic diffraction peaks of ( ), (400), ( ), ( ), and ( ) belonging to the monoclinic phase of β-Ga₂O₃ films are observed, but no characteristic diffraction peak of α-Ga₂O₃ films is observed. However, upon further decreasing the oxygen pressure to 1.0 Pa, in addition to the aforementioned diffraction peaks, the characteristic (110) diffraction peak of α-Ga₂O₃ films also appears, which is consistent with the result reported in the literature that α-Ga₂O₃ can be grown on a-Al₂O₃ substrates.^[25] It is worth noting that when the oxygen pressure is further decreased to 0.5 Pa and 0.09 Pa, the results are basically the same as for the sample grown with an oxygen pressure of 1.0 Pa, which indicates that the mixed-phase Ga₂O₃ film with coexisting β-Ga₂O₃ and α-Ga₂O₃ can be obtained on an a-Al₂O₃ substrate. In addition, the intensities of the (400) and ( ) diffraction peaks are the strongest for the 1.0 Pa sample among all of the mixed-phase Ga₂O₃ thin films. The specific reason for this phenomenon will be further explained in detail below.

Fig. 1.

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	Fig. 1. The XRD (θ–2θ) patterns of the Ga2O3 films with various oxygen pressures (3.0–0.09 Pa) grown on a-Al2O3 substrates; the inset is the partial enlargement of the XRD patterns.

Generally, regarding the crystal growth orientation of semiconductor material,^[26–28] the higher the dangling bond density of a certain crystal plane, the greater the attraction force to reactive atoms and the smaller the energy of reactive atoms that can migrate to this type of crystal surface. Conversely, if the dangling bond density of a certain crystal plane is smaller, then the ability to attract reactive atoms is weaker, and only the atoms with a large migration energy can migrate to the surface of this type of crystal plane. Figure 2(a) shows a diagram of the β-Ga₂O₃ unit cell; the atomic arrangements of the ( ) and (400) planes belonging to β-Ga₂O₃ are also displayed. The atomic density of the β-Ga₂O₃ (400) crystal plane is calculated as 14.462 nm⁻² by referring to the atomic arrangement of (400), and there are three dangling bonds around each atom according to the diagram of the β-Ga₂O₃ unit cell; therefore, the dangling bond density on the surface of the β-Ga₂O₃ (400) plane is 34.384 nm⁻². Similarly, the atomic density of the β-Ga₂O₃ ( ) plane is calculated as 13.867 nm⁻² by referring to the atomic arrangement of ( ), and there are three dangling bonds around each atom according to the diagram of the β-Ga₂O₃ unit cell; therefore, the dangling bond density on the surface of the β-Ga₂O₃ ( 02) plane is 41.601 nm⁻².

Fig. 2.

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	Fig. 2. (a) Diagram of β-Ga2O3 unit cell, and the atomic arrangement of (400) and ( ) planes; (b) diagram of α-Ga2O3 unit cell, and the atomic arrangement of the (110) plane.

Figure 3(b) shows a diagram of the α-Ga₂O₃ unit cell and the atomic arrangement of the (110) plane. According to Fig. 3(b), the number of dangling bonds around each atom is three, and the atomic and dangling bond densities of the α-Ga₂O₃ (110) plane are 10.157 nm⁻² and 30.471 nm⁻², respectively. The dangling bond densities of these three crystal planes are ranked in ascending order as follows: β-Ga₂O₃ ( -Ga₂O₃ (400) -Ga₂O₃ (110). Therefore, theoretically, the order of the three crystal planes in terms of the atomic attraction to reactive atoms is β-Ga₂O₃ ( -Ga₂O₃ (400) -Ga₂O₃ (110). The attraction force of the α-Ga₂O₃ (110) plane is the weakest, which implies that the growth of the α-Ga₂O₃ (110) plane requires higher migration energy. Both the probability of atoms colliding with each other and the number of collisions are small when the oxygen pressure in the chamber is low. As a result, more reactive atoms from the target can reach the surface of the substrate and migrate to the α-Ga₂O₃ (110) plane. In contrast, fewer Ga and O atoms from the target can reach the surface of the substrate and migrate to the α-Ga₂O₃ plane when the oxygen pressure in the chamber is high. It should be noted that this indicates good agreement between our research result and the theory of crystal growth. Coincidentally, the experimental method for and the theory of this difference in structure growth have been reported in the relevant literature for the growth of ZnMgO thin films.^[29]

Fig. 3.

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	Fig. 3. Raman spectra of Ga2O3 thin films with various oxygen pressures grown on a-plane a-Al2O3 ( ) substrates. The inset is the partial enlargement of the samples of 1.0 Pa, 0.5 Pa, and 0.09 Pa.

To further confirm that the Ga₂O₃ thin film is present in the form of a mixed-phase structure with coexisting α and β phases when the oxygen growth pressure does not exceed 1.0 Pa, Raman spectrum measurements were performed. Figure 3 shows the Raman spectra of Ga₂O₃ thin films grown with various oxygen pressures on a-Al₂O₃ ( ) substrates. The inset displays a partial enlargement for the 1.0 Pa, 0.5 Pa, and 0.09 Pa samples. It can be clearly seen that the characteristic Raman peaks of β-Ga₂O₃ films appear at 170 cm⁻¹, 200 cm⁻¹, and 350 cm⁻¹.^[30,31] Interestingly, in addition to the aforementioned characteristic Raman peaks belonging to the β-Ga₂O₃ film, another Raman peak for the Ga₂O₃ film appears at 251 cm⁻¹. According to the literature,^[32] β-Ga₂O₃ films have no Raman peaks within 200–300 cm⁻¹, and therefore the Raman peak at 251 cm⁻¹ belongs to α-Ga₂O₃. The same results are obtained when the oxygen pressure is further reduced to a pressure of 0.5 Pa or 0.09 Pa. This is consistent with the observation from XRD patterns. On the basis of this result, it can be confirmed that by reducing the oxygen growth pressure, it is possible to grow a mixed-phase Ga₂O₃ thin film with coexisting α-Ga₂O₃ and β-Ga₂O₃.

SEM was used to further elucidate the relationship between the surface morphology and crystal orientation. Figure 4 shows the SEM images of the Ga₂O₃ thin films grown at various oxygen pressures, with a magnification of 35 K. It can be clearly seen that the surface morphologies of the Ga₂O₃ thin films grown at 0.09 Pa, 0.5 Pa, and 1.0 Pa are rougher than those at 2.0 Pa and 3.0 Pa, which could be because the latter exhibit a single phase. The results are consistent with the observed XRD patterns and Raman spectra. Obviously, the result suggests that the surface morphology of the Ga₂O₃ thin film is rougher when the Ga₂O₃ thin film is mixed-phase because the growth rates of different crystal orientations are discrepant.

Fig. 4.

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	Fig. 4. SEM images of Ga2O3 thin films: (a) 3.0 Pa, (b) 2.0 Pa, (c) 1.0 Pa, (d) 0.5 Pa, (e) 0.09 Pa.

Figure 5 shows the absorbance spectrum of the Ga₂O₃ thin films deposited at different oxygen pressures, where the substrate temperature was 650 °C. The inset is the (α hν )² versus hν plot of Ga₂O₃ films grown with different oxygen pressures. It is evident that all the Ga₂O₃ thin films have a significant absorption cut-off edge, which is located at 250–300 nm. A further analysis of the optical spectra was performed to calculate the energy bandgap. For α and β phase Ga₂O₃ with a direct bandgap, the absorption follows the power law:^[2,33]

Fig. 5.

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	Fig. 5. UV–visible transmittance spectra of the Ga2O3 thin films grown at various oxygen pressures. The inset shows the plot of (αhv)2 versus hv used to estimate the bandgap.

Figure 6 shows the spectroscopic photoresponsivity of the fabricated Ga₂O₃ SBPDs measured at a bias of 25.0 V at different wavelengths ranging from 200 nm to 600 nm, and the inset is a schematic diagram of the device structure of the SBPD. It is evident that the photoresponsivity exhibits a significant increase of approximately two orders of magnitude when the oxygen pressure decreases from 2.0 Pa to 1.0 Pa. Moreover, the photoresponsivity values of all mixed-phase Ga₂O₃ SBPDs are approximately two orders of magnitude higher than those of the single β-Ga₂O₃ SBPDs, and the photoresponsivity reaches a maximum of 12 A/W; the I_light/I_dark reaches a maximum of 191.3. Interestingly, the crystal structure of the Ga₂O₃ thin film changed from a single β phase to a mixed-phase when the oxygen pressure was reduced from 2.0 Pa to 1.0 Pa. As has been reported, the dangling bonds and interface states could cause a high internal gain that will lead to a high responsivity and a persistent photocurrent effect.^[1,15] A detailed analysis, according to previous reports, is as follows. The responsivity (R) of photodetectors can be expressed as^[34]

Fig. 6.

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	Fig. 6. Spectroscopic photoresponsivity of Ga2O3 SBPDs measured at a bias of 25.0 V at different wavelengths ranging from 200 nm to 600 nm. The inset is a schematic diagram of the fabricated Ga2O3 thin film MSM structure photodetector.

Apparently, at a fixed bias voltage, the photoresponsivity of the UV detector is determined by the photocurrent, and the photocurrent is proportional to the number of carriers generated under illumination and their mobility. Furthermore, the number of carriers and their mobility are related to the UV absorption coefficient of Ga₂O₃ thin films and the ability of photocarriers to transmit in Ga₂O₃ thin films.^[34] As shown in Fig. 5, the maximum value of the light absorption coefficients of different samples is approximately two times the minimum value, but the maximum value of the photoresponsivity of the samples is approximately two orders of magnitude higher than the minimum value. Therefore, the high responsivity of the mixed-phase Ga₂O₃ SBPDs can be ascribed to the high internal gain, which is caused by the large number of dangling bonds in the interfaces between the α and β phases in Ga₂O₃. The relationship between the responsivity (R) and internal gain (G) can be expressed as^[1,15,33]

As mentioned above, there are a large number of dangling bonds in the mixed-phase Ga₂O₃ films, which cause a great number of heterojunction interfaces between monoclinic β-Ga₂O₃ and corundum α-Ga₂O₃. Theoretically, the many heterojunction interfaces are responsible for the high responsivity and low dark current of the photodetector devices.^[15] Apparently, high-responsivity devices have been fabricated on mixed-phase Ga₂O₃ thin films, but unfortunately, an undesired high dark current has also been acquired. The detailed results are as follows. Table 1 shows a comparison of the I_light, I_dark, and I_light/I_dark for the devices with different oxygen pressures. Both the light and dark currents of the mixed-phase Ga₂O₃ SBPDs are approximately three orders of magnitude greater than those of the single β-Ga₂O₃ SBPDs. This phenomenon is likely caused by the large number of dangling bonds between monoclinic β-Ga₂O₃ and corundum α-Ga₂O₃, which cause a high interface density. Furthermore, due to the dangling bonds, defects such as phase transformation dislocations are easily generated. These defects act as hole traps and can trap hole carriers, which would reduce the probability of recombination of non-balanced carriers and prolong carrier life; therefore, the light and dark currents of the devices are eventually larger. Another possible reason for this is that when the oxygen pressure in the cavity is low, the Ga₂O₃ thin film is in a gallium-rich and oxygen-deficient state. Furthermore, oxygen vacancies are generated in the film, and oxygen vacancies can provide free electrons as donor levels, and finally result in a higher dark current of the mixed-phase Ga₂O₃. Additionally, it can be seen that the dark current decreases slightly with a reduction in oxygen pressure, probably because the proportion of α-Ga₂O₃ increases and the proportion of β-Ga₂O₃ decreases. It must be noted that the I_light/I_dark of the device reaches 191.3 for the 1.0 Pa film, which is much higher than that of other mixed-phase Ga₂O₃ SBPDs with the 0.5 Pa and 0.09 Pa films. According to previous research results of our research group, the cause of this phenomenon is that the mixed-phase Ga₂O₃ thin film grown at 1.0 Pa mainly grows along (400) and planes, with the wider bandgap of (400) β-Ga₂O₃ and relatively high density of boundaries at the interfaces between the (400) and planes leading to a higher I_light/I_dark.^[35]

Table 1.

Table 1.

Table 1. Comparison of Ilight, Idark, and Ilight/Idark for the devices with different oxygen pressures. . Oxygen pressure/Pa Structure Ilight/A Idark/A Ilight/Idark 3.0 β-Ga2O3 7.25 × 10−8 2.39 × 10−10 303.1 2.0 3.01 × 10−7 5.04 × 10−9 173.3 1.0 mixed-phase 6.03 × 10−5 1.78 × 10−6 191.3 0.5 Ga2O3 3.08 × 10−5 1.65 × 10−6 55.2 0.09 4.01 × 10−6 1.55 × 1016 39.2

Table 1. Comparison of Ilight, Idark, and Ilight/Idark for the devices with different oxygen pressures. .

To deeply investigate the response speed characteristics of the device, 266 nm wavelength light was used to illuminate the device, and a transient response time test was performed. Figure 7(a) shows the transient response of Ga₂O₃ SBPDs with various oxygen growth pressures at a bias of 25 V under 254 nm light illumination. We observed that the increment of the signal intensity was rapid when the excitation light was turned on. However, by turning off the excitation light, a relatively slow response occurred in the device. This slowly decaying response was ascribed to the oxygen-related hole trap states generated at the surface of the Ga₂O₃ thin film.^[24] These hole trap states would reduce charge carrier recombination because some carriers are captured as the traps empty. Therefore, a slow decay time occurred when the incident light was turned off (Fig. 7(a)). Additionally, the rise time and decay time of the mixed-phase Ga₂O₃ SBPDs are longer than those of the single β-Ga₂O₃ SBPDs, as shown in Figs. 7(a) and 7(b). This is due to the high density of dangling bonds at the junction of α-Ga₂O₃ and β-Ga₂O₃ in the mixed-phase Ga₂O₃ thin film, and higher-density acceptor interface states can be introduced. These acceptor interface states can capture a large number of photogenerated holes and reduce the carrier recombination to prolong the lifetime of the photogenerated carriers.

Fig. 7.

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	Fig. 7. The transient response of Ga2O3 SBPDs with various oxygen pressures at the bias of 25 V under 254 nm light illumination: (a) the drop time, and (b) the rise time.

In summary, Ga₂O₃ thin films were deposited on a-Al₂O₃ substrates by PLD with different oxygen pressures at 650 °C. By observing the XRD patterns and Raman spectra, it can be found that as the oxygen pressure in the chamber decreases from 3.0 Pa to 0.09 Pa, the crystal structure of the Ga₂O₃ thin film changes from a single β phase to mixed-phase. Due to the presence of a large number of dangling bonds at the interface of different internal structures of the mixed-phase Ga₂O₃ thin film, a high internal gain can be obtained, and therefore, the photoresponsivity can be greatly improved. The results of spectral responsivity tests show that the maximum responsivity of mixed-phase Ga₂O₃ SBPDs reached 12 A/W at a bias of 25 V, which is approximately two orders of magnitude higher than that of the single β-Ga₂O₃ SBPDs, with the response wavelength located at 260 nm. Additionally, the I_light/I_dark reached two orders of magnitude higher in the mixed-phase than in the single β-phase Ga₂O₃. In fact, there is no denying that further effort should be focused on reducing the dark current and improving the response speed of the mixed-phase Ga₂O₃ SBPDs. The significance of our research work lies in the exploration of the feasibility of preparing a mixed-phase structure Ga₂O₃ film via the PLD method and further fabrication of a mixed-phase structure Ga₂O₃ thin film-type SBPD with excellent performance.