Enhancement of Responsivity in Solar-Blind UV Detector With Back-Gate MOS Structure Fabricated on β-Ga2O3 Films

Monoclinic Ga2O3 (β-Ga2O3) films were grown on Si/SiO2 by using MOCVD. Then, we fabricated the solar-blind photodetector with a back-gate MOS structure. The device exhibited obvious photoresponse under 254-nm UV light illumination, and the photocurrent increased by five orders of magnitude, which could be controlled by VGS. The current generated under dark conditions could also be regulated by VGS and tended to constant when the regulation of VGS was reaching saturation. Meanwhile, VGS was confirmed to have a certain ability to regulate the photocurrent. The present device demonstrated excellent stability and fast response (rise) and recovery (decay) times under the 254-nm light illumination as well as a responsivity of 417.5 A/W, suggesting a valuable application in solar-blind UV photodetectors.


INTRODUCTION
Owing to their wide bandgap (∼4.9 eV) and stable chemical and physical properties, β-Ga 2 O 3 thin films have been investigated for use as ultraviolet (UV) photodetectors (PDs) (Kokubun et al., 2007;Oshima et al., 2008). In the literature, PDs have received extensive attention for the applications in fire detection, missile tracking, radiation detection, and ozone hole monitoring (Monroy et al., 2003;Alaie et al., 2015). Meanwhile, the characteristics of the solar-blind UV PDs based on the β-Ga 2 O 3 films, such as responsivity, selectivity, quantum efficiency, and stability, have been reported. O. Sooyeoun et al. have demonstrated that the Si-implanted β-Ga 2 O 3 PD had a responsivity of 1.45 A/ W, and the rise and decay time constants were calculated to be 0.58/32.93 and 1.2/32.86 s, respectively (Sooyeoun et al., 2015). Y. Xu et al. have demonstrated that the β-Ga 2 O 3 PD grown on sapphire using the mist-CVD method exhibited a spectral responsivity of 150 A/W, and the external quantum efficiency was over 7 × 10 4 % (Xu et al., 2018). S. Nakagomi et al. fabricated a deep UV photodiode using a heterojunction between the β-Ga 2 O 3 layer and the 6H-SiC substrate and obtained blazing response speeds. The time constants were estimated to be 1.2 and 1.5 ms (Nakagomi et al., 2013), which were superior to the response time constants of all the above-mentioned reports, while the current was unstable in the range of light pulses.
Although many fabrication technologies have been used to improve the β-Ga 2 O 3 PDs' quality, the film-type PDs' photoresponses are still not their best performances, so that limits the practical application. In this work, we have prepared high-quality β-Ga 2 O 3 films using the MOCVD method. The most significant advantage of this technology is that a low-resistance Ohmic contact between the β-Ga 2 O 3 film and the electrode can be formed, thus reducing the influence on the flow of photoinduced carriers. Meanwhile, a solar-blind UV PD with a back-gate MOS structure was fabricated, and the photoresponse behaviors based on different applied voltages under dark and illumination conditions were characterized. Such detectors with a back-gate MOS structure fabricated on β-Ga 2 O 3 films show high responsivity and fast response times, which can also be regulated by adjusting the gate voltages and drain voltages.

EXPERIMENTAL DESIGN
The β-Ga 2 O 3 film was grown unintentionally doped (UID) on Si/ SiO 2 substrate with a diameter of 2 inches using the metal organic chemical vapor deposition (MOCVD) method. The growth temperature was set to 690°C, and the chamber pressure was held at 25 Torr. Trimethylgallium (TEGa) and high-purity oxygen were used as the gallium and oxygen precursors, respectively, and Ar was used as the carrier gas. Compared with other CVD growth methods, the MOCVD method could obtain high-quality films. All samples were cleaned successively with acetone, alcohol, and deionized water for 10 min to get ready for the transistor.
The interdigital Ti/Au electrodes were deposited on the β-Ga 2 O 3 film using a shadow mask by radio frequency magnetron sputtering, with a power of 100 W, a pressure of 1 Pa, and an argon gas ratio of 30 SCCM, and then we obtained the back-gate MOS structure PD. All films and devices were fabricated at Beijing Gallium Family Technology, Co., Ltd. Figure 1A shows the schematic cross section of the fabricated β-Ga 2 O 3 film device, including the SiO 2 dielectric layer of 100 nm, the grown β-Ga 2 O 3 film of 400 nm, and the deposition of interdigital Ti/Au (30/70 nm) electrodes. The schematic diagram of the fabricated β-Ga 2 O 3 film device with the back-gate MOS structure from the top-right view is shown in Figure 1B. This is a typical three-terminal MOS structure including the source side, which is generally used as a ground unit (GNDU); the gate, which is usually connected to an SMU power supply (SMU1); and the drain side (SMU2), which is also a power supply with working voltage. It is worth noting that the present device will convert into a typical MSM detector when the gate voltage is not applied (SMU1 0).
The interdigital electrode patterns are 2 × 2 mm 2 . In detail, the metal electrode possession area is removed, and the effective photosensitive area is 1 mm 2 for the sample. As reported, the electron affinity of Ga 2 O 3 is 4.00 ± 0.05 eV, and the work function of Ti is 4.33 eV. Therefore, according to the Schottky-Mott rule, the metal-semiconductor (MS) interface barriers of Ti/β-Ga 2 O 3 are approximately 0.33 eV, suggesting an approximate Ohmic contact between metal Ti and semiconductor β-Ga 2 O 3 . It ensures that the electrode contact will not interfere with the photoelectric performance of the device as much as possible.
The current-voltage (I-V) and time-dependent photoresponse of the β-Ga 2 O 3 film detector were measured using a Keithely 4,200. The UV response measurement was performed using a 6-W UV lamp, which was used as a light source to provide illumination of wavelengths 254 and 365 nm. The time-dependent photoresponse measurement was performed at a fixed voltage of 10 V. All the measurements were carried out at room temperature.

RESULTS AND DISCUSSION
As shown in Figure 2, the grown layer exhibited a smooth surface morphology with a root mean square (RMS) roughness of 0.54 nm, which was determined from the 2D AFM image over a 5 × 5-μm 2 scan area. Obviously, the β-Ga 2 O 3 film has uniformly grown on the Si/SiO 2 substrate using the MOCVD growth method.
In order to investigate the UV photoresponse of the β-Ga 2 O 3 detector, the interdigital electrode was deposited by magnetron sputtering on the β-Ga 2 O 3 film. The light sources with the wavelengths of 365 and 254 nm were gained from the UV The I-V curves measured under dark conditions and 365-nm light do not show any obvious variations, and the currents remain ∼10 -9 and ∼10 -8 A at 10 V, respectively. With the applied voltage increasing, the current increases linearly both under dark and different illumination conditions ( Figure 3A), suggesting an Ohmic contact between Ti/Au and β-Ga 2 O 3 films. Similar results have been reported about the β-Ga 2 O 3 thin film grown on Al 2 O 3 with Ti/Au electrodes using laser molecular beam epitaxy technology (Guo et al., 2014a). Other than that, the current shows a sharp jump and increases constantly with the applied voltage intensity when the device is exposed under the 254-nm light (it is consistent with the bandgap ∼4.9 eV of β-Ga 2 O 3 ). The photo-to-dark current ratio >10 5 was obtained at 10 V, suggesting that the photocurrent increases by five orders of magnitude under 254-nm light illumination. The low dark current is important for the practical PDs for it implies a high sensitivity and low noise for a superior device (Guo et al., 2014b). The results suggest that the β-Ga 2 O 3 detector is not sensitive to 365nm light, while it exhibits obvious photoresponse under 254-nm UV light illumination. Besides, the device will show typical MSM detector I-V properties when the gate voltage is not applied.
The back-gate structure β-Ga 2 O 3 film detector was characterized under dark conditions by taking transport I DS -V GS ( Figure 4A) and output I d -V DS ( Figure 4B) tracings for different drain voltage (V DS ) and gate voltage (V GS ) values, respectively. Figure 4A shows that drain currents (I DS ) appear when the V GS is greater than 20 V for different V DS , and I DS tends to become larger with the increase in V DS . The results suggest that the MOS transistor becomes on-state when V GS > 20 V and the I DS can be controlled by V GS ; in other words, the typical three-terminal MOS structure fin field transistor (FET) can enhance the sensitiveness and responsivity with photoelectron  regulation by adjusting the drain voltage and gate voltage. The onstate behavior of the current may also indicate the existence of weak contact resistance, which can also be surmised from the nonlinear output characteristics I d -V DS at very low V DS , as shown in Figure 4B.
That is to say, the contact between Ti/Au and the β-Ga 2 O 3 film still needs to be optimized. The same inference has been made on the back-gate a-MoS 2 thin film transistors (Kouvatsos et al., 2015). The output characteristics I d -V DS exhibit the dark currents (I d ) that are closely related to V GS . With the V GS changed from 30 to 0 V (the step is −5 V) as shown in Figure 4B, I d is reduced by two orders of magnitude. This behavior indicates that V GS has a certain ability to regulate the dark current I d (under dark conditions). Along with the further decrease of V GS to −30 V ( Figure 4C), I d almost tends to constant, that is, the regulation of V GS reaching saturation. The results exhibit typical Schottky barrier characteristics. Figure 4D shows the characteristics of I p -V DS under dark and 254nm light illumination with a light intensity of 40 μW/cm 2 . Under the dark condition, the current-voltage characteristic showed good rectifying properties, and the rectification ratio reached 10 4 at V DS ±10 V (Figures 4B,C). When the 254-nm light illuminated toward the PD, the generation of photocurrent (I p ) was clearly observed and increases rapidly vs. the drain voltage. What is more, I p is affected by the gate voltage V GS , suggesting that V GS has a certain ability to regulate the photocurrent I p regardless of the irradiation intensity. This indicates that this PD works properly as a Schottky barrier phototransistor. Similar three-terminal PDs have been constructed on MgZnO/ZnO/MgZnO/Si, which showed the same modulation (Ye et al., 2016). Furthermore, we find that I p is proportional to the light intensity when the device is exposed to different light powers (the result is not shown here), that is to say, the device performance is affected by the tested light intensity, as reported on the similar PDs .
The time-dependent photoresponse of the β-Ga 2 O 3 PD was measured to characterize the response speed using 254-nm light illumination under a fixed grain voltage V GS 10 V for various drain voltages, as shown in Figure 5A. The measurements show that the detector still exhibits an identical response after several illumination cycles, indicating that the present device has excellent stability and good reversibility.
From the measurements at different V DS , one can see that the photocurrents rise and fall very rapidly and are obviously affected by V DS . Under 254-nm light illumination, the photocurrent instantaneously increases to a stable value of 7.1 × 10 -4 A (V DS 30 V), and it rapidly decreases to ∼10 -8 A when the light turns off; the latter is quite close to the initial dark current value. Compared with the Al 2 O 3 /β-Ga 2 O 3 UV light PD (Sooyeoun et al., 2015;Wang et al., 2018) and other technological β-Ga 2 O 3 PDs (Guo et al., 2014a;Xu et al., 2018;Tang et al., 2020) which show obvious relaxation phenomena, the present MOS structure device greatly reduces the rise and fall times, improving the sensitivity of the PD. For a more detailed study of the response time, the quantitative analysis of the current during the rise and decay processes involves the fitting of the photoresponse curve with an exponential relaxation equation of the following type (Ramana et al., 2014;Wang et al., 2018): where I 0 is the steady-state photocurrent, t is the time, A is a constant, and τ is the relaxation time constant. The enlarged view of the rise/decay processes and the corresponding fitting curves are shown in Figure 5B, and τ r and τ d denote the rise and decay time constants, respectively. The current of the rise process is steeper with τ r 0.23 ± 0.02 s and that of the decay process is τ d 0.13 ± 0.02 s, which implies that there are fewer oxygen vacancies or other defects . The present time constants are comparable to the available Ga 2 O 3 PDs as listed in the study by Dong et al. (2019), although we remain skeptical of the fitted results of 0.03/0.08 s. Obviously, the present MOS The spectral responsivity (R) is a critical parameter to evaluate the sensitivity of the PDs. According to the definition, the value of R is equal to the photocurrent generated by R where P is the incident light intensity and S is the effective illuminated area on a device. In the present work, the UV light (40 μW/cm 2 ) was vertically irradiated on the device, and the effective irradiated area was 1 mm 2 , and then the photoresponsivity at a 10-V applied voltage was confirmed to be 417.5 A/W by the definition equation above. This is comparable to the reported values (1.45-150 A/W) for β-Ga 2 O 3 PDs (Kokubun et al., 2007;Suzuki et al., 2011;Hu et al., 2015;Sooyeoun et al., 2015;Qian et al., 2017;Xu et al., 2018). Such high R indicates that the photogenerated electrons converted efficiently into photocurrent (I p ). Higher responsivity was demonstrated by using a carrier multiplication process or a high resistive cap layer (Suzuki et al., 2011;Hu et al., 2015). Another parameter-external quantum efficiency (η ext )-is often used to evaluate the sensitivity at the same time. It is defined as the number of electrons detected per incident photon, which can be expressed as follows (Suzuki et al., 2009): where h is the Plank constant, c is the velocity of light, e is the electronic charge, λ is the wavelength of light, and Γ is the internal gain. For the present work, the value of η ext Γ is estimated to be 0.47 × 10 4 % under 254-nm light illumination, which exhibits the existence of internal gain. Although the current performance is not the best, we believe the present device with a back-gate structure on β-Ga 2 O 3 films can be further improved through minimizing the defects in substrates and optimizing the manufacturing process of the detectors, promoting the commercialization of high-performing solar-blind PDs. Compared with available β-Ga 2 O 3 and other PDs listed in Table 1, the present three-terminal PD exhibited superiority whether in spectral responsivity or in response time, although some of the listed results are questionable.
To better understand the mechanism of electron transfer on the MSM and back-gate MOS structure PD under the dark and light conditions, the schematic energy band diagrams are displayed in Figure 6. The Ti/Au contributes to a small MS interface barrier of 0.33 eV with close work function, suggesting approximate Ohmic contacts. For the Ohmic contact PD, the current path is directly formed while seldom affected by MS contact resistance, so that the material's natural photoelectric performance can be better reflected.
As shown in Figure 6A, the dark currents remain at a low value with few separated hole-electron pairs, suggesting that the β-Ga 2 O 3 film has few intrinsic defects. Under the 254-nm illumination, the photo-generated electrons jump to the valence band (VB) from the conduction band (CB) and gather on the β-Ga 2 O 3 film surface, where electrons will transport under forward voltage, as shown in Figure 6B. By contrast, the MOS structure has a strong switching regulation effect on the channel layer to collect or deplete electrons; therefore, it exhibits boost forward current and low dark leakage current, compared with the MSM PD of the same size. For the three-terminal MOS structure, the SiO 2 layer has strong dielectric properties that can effectively stop the electrons and holes from transporting across the SiO 2 /β-Ga 2 O 3 interface and then dramatically reduce the vertical leakage loss and more effectively control the lateral movement of electrons in the β-Ga 2 O 3 channel layer. This is a typical advantage of the MOS structure PD in regulating responsitivity and sensitivity over the MSM structure. When forward voltage was forced onto the SiO 2 dielectric layer, the SiO 2 /β-Ga 2 O 3 layer would induce a change in electric potential under the external electric field, as shown in Figure 6C and Figure 6D. It is obvious that a top-down high electric field has formed in the β-Ga 2 O 3 film and varies with the external electric field, so that more electrons will obtain more kinetic energies and then jump into the VB. Eventually, more electrons will gather on the channel, which will form a directional movement current under forward drain bias voltage. Under 254-nm irradiation, a large number of photo-generated electrons rapidly accumulated in the channel layer under the action of the electric field from the β-Ga 2 O 3 film. This is why the MOS structure PD displayed much weaker relaxation phenomena and much shorter rise and fall photoresponse times.
In this case, the composition of the channel electrons consists of photo-generated electrons, intrinsic electrons, and electric field-generated electrons; therefore, the MOS structure possesses enhanced responsivity compared with the MSM structure. On the contrary, if the gate is forced on reverse bias, an opposite internal electric field will be generated onto the β-Ga 2 O 3 film and it gradually depletes the channel electrons, presenting a low leakage current. In general, the advantages of the back-gate MOS structure PD are as follows: for one thing, it keeps the design of the top interface on the MSM structure, ensuring the photosensitive area is sufficiently exposed to illumination; for another, the back-gate structure is made at the bottom, realizing the responsive regulation and reducing the relaxation phenomena.

CONCLUSION
In conclusion, the solar-blind PD with the back-gate MOS structure was fabricated on the β-Ga 2 O 3 film, which exhibited obvious photoresponse under 254-nm UV light illumination, and the photocurrent could be controlled by V GS . Otherwise, the dark current could also be regulated by V GS and tended to constant when the regulation of V GS was reaching saturation. What is more, V GS had a certain ability to regulate the photocurrent. The present device demonstrated excellent stability and fast response (rise) and recovery (decay) times under the 254-nm light illumination. All these excellent characteristics indicate that the back-gate MOS structure β-Ga 2 O 3 film has great potential applications on the solar-blind UV PDs.

DATA AVAILABILITY STATEMENTS
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
XJ fabricated the material and characterized the detector. DM analyzed the experimental results. All authors contributed to the writing of the article and evaluation of the results.