Open Access
Add to BibSonomy
Abstract
Gallium oxide (Ga2O3) has been studied as one of the most promising wide bandgap semiconductors during the past decade. Here, we prepared high quality β-Ga2O3 films by pulsed laser deposition. β-Ga2O3 films of different thicknesses were achieved and their crystal properties were comprehensively studied. As thickness increases, grain size and surface roughness are both increased. Based on these β-Ga2O3 films, a series of ultraviolet (UV) photodetectors with interdigital electrodes structure were prepared. These devices embrace an ultralow dark current of 100 fA, and high photocurrent on/off ratio of 10E8 under UV light illumination. The photoresponse time is 4 ms which is faster than most of previous works. This work paves the way for the potential application of Ga2O3 in the field of UV detection.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Wide bandgap semiconductors, such as GaN [1–3] and SiC [4,5] have been studied for their widely applications in power electronics, UV photodetector, light emitting diode and so on. Recently, Gallium oxide (Ga2O3) has attracted a wide attention as the next generation semiconductor, which can be used as power electronic devices, deep ultraviolet photodetectors, gas sensors, solar cells and so on. Generally, Ga2O3 possess five common phases including corundum (α), monoclinic (β), defective spinel (γ), cubic (δ), and orthorhombic (ε) [6–12]. Among these phases, the most stable phase is β-Ga2O3, while other four phases tend to change to β-Ga2O3 under high temperature and high pressure [13–15]. β-Ga2O3 is a kind of wide bandgap semiconductors with bandgap of 4.9 eV approximately [16]. β-Ga2O3 has been widely studied and applied in electronic and photonic devices since the monoclinic phase of β-Ga2O3 is stable under normal conditions of temperature and pressure [13–15]. It is highly transparent under light illumination of wavelengths beyond 300 nm, [17] high critical breakdown field (T) of 8 MV/cm and high Baliga’s figure of merit Baliga figure-of-merit (BFOM) of 3444, [18] which is of huge potential in the future applications of gas sensors, power devices, and transparent conducting electrodes [19]. More importantly, β-Ga2O3 with a very strong photoresponse to ultraviolet (UV) near 254 nm, [20–22] is a powerful candidate for deep UV (DUV) detectors.
Light with a wavelength of 10 nm to 400 nm is called ultraviolet light, and the ultraviolet region with a wavelength of 200-280 nm is called “solar blind”.[23] This part of the wavelength is mainly absorbed by the ozone layer of the atmosphere, and there is no such radiation on the surface of the earth, which is the so-called “black background”. During the detection process, it will not be disturbed by solar radiation, which improves the accuracy of detection. Solar blind ultraviolet (UV) photodetectors have many applications such as missile tracking, ozone hole monitoring and fire detection.
There are already some UV detectors that have been studied based on the different materials, [24] such as ZnMgO, [25] GaN, [26] and AlGaN [27]. Achieving high quality film is still an unsolved problem thoroughly. Compared to these materials, β-Ga2O3 is easily to prepare and can detect light at deep ultraviolet wavelengths. High-quality β-Ga2O3 films can be prepare by many methods, such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and magnetron sputtering [28–30]. In this work, we have prepared high-quality β-Ga2O3 films using pulse laser deposition (PLD) and characterized the structural properties of the films. Besides, we have fabricated the interdigital electrodes structure and characterized photoresponse behaviors based on the β-Ga2O3 films with different thicknesses.
2. Experiment
A PLD (KrF excimer laser, 248 nm wavelength, 30 ns pulse width) method is used to epitaxially grow a β-Ga2O3 film on a sapphire substrate. In order to obtain a better quality β-Ga2O3, we have selected a sapphire substrate.[31] First, the single crystal sapphire (0001) plane substrate is cleaned by acetone, alcohol and deionized water. Then, we use silver paste to attach the substrate to the heating tray. During the heating process, the background vacuum of the PLD cavity is 5 × 10−5 Pa. The substrate temperature rises to 600 ℃ at a rate of 10 ℃ per minute. After oxygen is introduced, the partial pressure of oxygen changes to 4 Pa. The distance of target and substrate is 5 cm. Ga2O3 target (99%) is pulsed laser ablated with an energy density of 2 J/cm2 and a frequency of 2 Hz. After the process of deposition, we need to perform one hour of in-situ annealing to obtain crystals with good crystallinity. We obtained different thicknesses of Ga2O3 film by changing the number of laser pulse. After characterizing the structure and morphology of the material itself, we fabricated a light-guided UV detector using these high-quality Ga2O3 films. The structure of the device is metal-semiconductor-metal (MSM). We chose metal titanium (Ti), which is a good ohmic contact with the Ga2O3 film, as an electrode. For good conductivity during the test, we have coated a layer of gold (Au) on the top of the titanium. The electrode pattern of interdigitated electrode is obtained by spin coating photoresist and laser direct writing exposure. Ti/Au (15/45 nm) electrode was sputtered on the surface of the Ga2O3 by double ion beam sputtering. Finally, the device was immersed in acetone for 5 minutes and then lift off.
The crystal structure of Ga2O3 film is characterized by X-Ray diffraction (XRD). The surface morphology of the film is measured by atomic force microscope (AFM) (MFP-3D BIO). Film thickness was observed using scanning electron microscope (SEM) in the material section. Current-Voltage (I-V) characteristics and Current-Time (I-T) characteristics of devices are measured by a semiconductor analyzer (Agilent B1500A) and an ammeter (Agilent B2902A). To test the response of the device to light of different wavelengths, we use a monochromator (71SW301) to change the wavelength of light passing through a 350 W xenon lamp. Then a 5 V bias was applied to the device to monitor current changes under different wavelengths of light. A standard silicon-based detector was used to calibrate the power density. The responsivity of the device to different wavelengths of light is calculated. In the response time and photocurrent test of the device, UV source is provided by a deuterium lamp (THORLABS, SLS204). Deuterium lamp directly irradiates the sample surface through the optical fiber. The wavelength range is 200-700 nm. The bulb electrical power is 30 W. The fiber coupled output power is 0.2 mW. The response time measurement is obtained by using a chopper (MC2000) to obtain pulsed light. The frequency of the chopper is 1 Hz, and then the waveform of the pulse signal voltage signal is obtained by using an oscilloscope (MDO3014).
3. Results and discussion
Figure 1(a) shows the crystal structure of β-Ga2O3 and its (-201) crystal plane. We obtained films of different thicknesses by changing the number of pulse depositions. The details of the XRD patterns of samples with different thickness are shown in Fig. 1(b). It can be seen that there is no extra peak in addition to the peak of the (-201) and the peak of the substrate, indicating that the film is the preferred orientation crystal we need. (-402) and (-603) are parallel to (-201) crystal faces. We can clearly see that as the thickness increases, the crystallinity of the film gradually becomes better. According to the Scherrer formula:
Fig. 1. (a) Molecular structure diagram of β-Ga2O3 crystal and its (-201) surfaces. (b) XRD patterns of β-Ga2O3 films with different thicknesses. (c) XRD rocking curves for (–201) plane of β-Ga2O3 films. (d) HR-TEM bright field images of #4. #1, #2, #3, #4 and #5 are the sample labels.
Figures 2(a)–2(e) are SEM images of the cross section of the sample #1-#5. The thickness of the films are 90 nm, 167 nm, 281 nm, 298 nm, and 354 nm, respectively. Figures 2(f)-(j) shows the surface roughness of the films that become larger as the film thickness increases. The root-mean-square (RMS) values are 1.895 nm, 2.948 nm, 4.154 nm, 5.079 nm, and 6.360 nm, respectively. We can observe that as the thickness of the material increases, the roughness of the surface of the film becomes larger. The roughness of the surface is related to the crystallinity of the surface.[33,34]
Fig. 2. (a)-(e) The cross-sectional images of β-Ga2O3 films. (f)-(j) AFM surface images of β-Ga2O3 films. The scalebar of (a) is 200 nm. The scalebar of (b)-(e) is 500 nm. The scalebar of (f)-(j) is 400 nm.
Table 1 compares the microstructures of the different films. By comparison, we can clearly see that the microstructure of the film changes regularly with the change of film thickness. As the thickness of the film increases, the grain size becomes significantly larger, and the FWHM of the rocking curve becomes narrower, indicating that the crystallinity gradually becomes better. But as the thickness increases, the roughness of the film surface increases significantly.
Table 1. Comparison of film structures with different thicknesses.
We began to design and fabricate a UV detector based on a gallium oxide film after characterizing the material. Structure of device is MSM (metal-semiconductor-metal). Ti/Au (15/45 nm). Interdigital electrode was fabricated on the surface of the material to increase the area of illumination. Figure 3(a) is a microscope image of the device. Figure 3(b) is a 3 Dimensions (3 D) image of the device structure. Channel width of the device is 100 µm, the overall length of the device is 2200 µm and the width is 2000 µm. The energy band diagrams of β-Ga2O3 MSM ultraviolet photodetector in dark and under illumination are shown in Fig. 3(c). In the dark state, there is a high barrier between the material and the electrode, and the barrier is significantly reduced after illumination. After the material is exposed to light, it absorbs photons, and the electrons in the valence band transition to the conduction band, leaving holes in the valence band, thereby increasing the carrier concentration in the material and generating a current under the action of the voltage across the device. Figure 3(d) shows the photocurrent (Ilight) dependence on bias of different devices. First, we can determine that Ti/Au is used as the electrode, which can achieve very good ohmic contact. The dark currents of the films we get are very small and are almost equal values. So, we only use a black curve to represent. The I-V curve is an almost linear relationship. The thickness of the film increases, the photocurrent tends to become larger, but such a trend does not continue. The Ilight is significantly reduced when a certain thickness is exceeded. Maximum current can be achieved with different thickness films at 20 V reflecting this trend. Figure 3(e) shows the current in the dark state of the film and the current under light illumination at 20 V. Current in the dark state is very small, which is smaller than the dark current of most devices reported in the past, [24,35–40,41,42] and the maximum photocurrent can reach 15 µA under illumination. Therefore, the switch ratio can reach 108, which is greater than most of the devices reported in the past. [28,35–37,39,40,42–46] Fig. 3(f) is transient response of β-Ga2O3 photodetector. During the test, the chopper frequency was set at 1 Hz, and bias was 20 V. We used the oscilloscope to test it more accurately under the same conditions and got the I-time curve of rise and fall as shown in Fig. 3(f). The current rise time after lighting is 4 ms, and the current drop time after light removal is 104 ms. The current rise time is shorter than the current drop time Due to the presence of defects states leads to carrier recombination slow process. Such response speed is faster than most past reported UV detectors. [14,16,17,22,23,24]
Fig. 3. (a) Optical image taken by microscope. (b) Schematic diagram of gallium oxide MSM ultraviolet photodetector. (c) The energy band diagrams of β-Ga2O3 MSM ultraviolet photodetector under dark and illumination conditions. (d) Current-voltage characteristics measured in dark and illumination for five photodetectors. (e) Photocurrent on/off ratio obtained as over 107 in logarithmic coordinates. (f) Transient response of β-Ga2O3 photodetectors, normalized response of photodetectors. During the test, the chopper frequency was set at 1 Hz, and bias was 20 V.
Figure 4(a) is an I-time graph showing samples with different thicknesses under the chopping effect of the chopper. The bias voltage is 20 V. We use a deuterium lamp to provide an ultraviolet light source with a total output of 0.2 mW. The illumination distance is controlled at 20 mm. We can see that the current rises and falls very rapidly.
Fig. 4. (a) Photocurrent switching characteristics of β-Ga2O3 photodetectors. During the test, the chopper frequency was set at 1 Hz, and bias was 20 V. (b) NPD varied with frequency (f) and was measured at room temperature, VSD = 3 V was set according to the working conditions of the device. (c) Spectral responsivities of the DUV detectors based on β-Ga2O3 photodetector. The pattern in the inset shows the optical power density of different wavelengths of light from a xenon lamp. P in the figure expresses the power density of light.
To analyze the noise mechanism of the device, we tested the noise power density (NPD) in the dark state with a 3 V bias. The result is shown in Fig. 4(b), indicating that 1/f noise is the main source of noise for the device, and this noise is mainly due to material defects.[47] Fig. 4(c) shows the response of the device to different wavelengths in the UV region, ranging from 237 to 350 nm. It can be seen that the maximum responsivity is at 248 nm and the response cutoff wavelength is at 310 nm. For devices of different thicknesses, there is no significant difference in the response time.
The current under light conditions shows a tendency to increase first and then decrease, because the dark currents are very low, around 100 fA. The switch ratio shows the same trend. The responsivity of different thickness is measured at a wavelength of 254 nm under a bias of 20 V. We can see that the responsivity of films with different thickness, as reported in Table 1, to 254 nm light is different, the trend is increasing first and then decreasing, so the best responsive thickness is 298 nm according to Table 2.
Table 2. Performance comparison of gallium oxide photodetectors with different thicknesses.
Table 2 shows a comparison of the electrical properties of devices with different films. From the table we can see that the same structure of the device due to the difference in film thickness caused by changes in optoelectronic properties, and showed a certain regularity
4. Conclusion
In this work, we found a suitable condition for growing the preferred β-Ga2O3 films with (-201) facet-oriented by using the PLD method and studied the effect of thickness on the film properties. During the experiment, we found that an increase in film thickness caused a change in grain size and film surface roughness. In the test of MSM UV detector, as the thickness of the film increases, the resistance under light conditions decreases and then increases. The current in the dark state is only 100 fA at 20 V. The current under illumination can reach 16 µA at 20 V. The maximum photocurrent switching ratio can even reach 108, which is larger than most of the previous reports. The high-quality β-Ga2O3 film exhibits a very short response time under the illumination of a deuterium lamp, with a maximum rise time of 4 ms and a fall time of 104 ms. This work demonstrates the promising applications of Ga2O3 film and its UV detectors. Overall, we got high-quality β-Ga2O3 films and proved great potentials of β-Ga2O3 devices.
Funding
Key Research Project of Frontier Sciences of Chinese Academy of Sciences (QYZDB-SSW-JSC016, QYZDY-SSW-JSC042).
Acknowledgment
We are very grateful for the support provided by the Key Research Project of Frontier Sciences of Chinese Academy of Sciences.
Disclosures
The authors declare no conflicts of interest.
References
1. A. Gundimeda, S. Krishna, N. Aggarwal, A. Sharma, N. D. Sharma, K. K. Maurya, S. Husale, and G. Gupta, “Fabrication of non-polar GaN based highly responsive and fast UV photodetector,” Appl. Phys. Lett. 110(10), 103507 (2017). [CrossRef]
2. B. Potì, M. T. Todaro, M. C. Frassanito, A. Pomarico, A. Passaseo, M. Lomascolo, R. Cingolani, and M. De Vittorio, “High responsivity GaN-based UV detectors,” Electron. Lett. 39(24), 1747 (2003). [CrossRef]
3. S.-J. Chang, Y. D. Jhou, Y. C. Lin, S. L. Wu, C. H. Chen, T. C. Wen, and L. W. Wu, “GaN-Based MSM Photodetectors Prepared on Patterned Sapphire Substrates,” IEEE Photonics Technol. Lett. 20(22), 1866–1868 (2008). [CrossRef]
4. L. A. Kosyachenko, V. M. Sklyarchuk, and Y. F. Sklyarchuk, “Electrical and photoelectric properties of au–sic schottky barrier diodes,” Solid-State Electron. 42(1), 145–151 (1998). [CrossRef]
5. S. Nishino, J. A. Powell, and H. A. Will, “Production of large-area single-crystal wafers of cubic SiC for semiconductor devices,” Appl. Phys. Lett. 42(5), 460–462 (1983). [CrossRef]
6. S. J. Pearton, J. Yang, P. H. Cary, F. Ren, J. Kim, M. J. Tadjer, and M. A. Mastro, “A review of Ga2O3 materials, processing, and devices,” Appl. Phys. Rev. 5(1), 011301 (2018). [CrossRef]
7. H. He, R. Orlando, M. A. Blanco, R. Pandey, E. Amzallag, I. Baraille, and M. Rérat, “First-principles study of the structural, electronic, and optical properties of Ga2O3 in its monoclinic and hexagonal phases,” Phys. Rev. B 74(19), 195123 (2006). [CrossRef]
8. P. Kroll, R. Dronskowski, and M. Martin, “Formation of spinel-type gallium oxynitrides: a density-functional study of binary and ternary phases in the system Ga–O–N,” J. Mater. Chem. 15(32), 3296–3302 (2005). [CrossRef]
9. H. He, M. A. Blanco, and R. Pandey, “Electronic and thermodynamic properties of β-Ga2O3,” Appl. Phys. Lett. 88(26), 261904 (2006). [CrossRef]
10. H. Y. Playford, A. C. Hannon, E. R. Barney, and R. I. Walton, “Structures of uncharacterised polymorphs of gallium oxide from total neutron diffraction,” Chem. Eur. J. 19(8), 2803–2813 (2013). [CrossRef]
11. H. von Wenckstern, “Group-III Sesquioxides: Growth, Physical Properties and Devices,” Adv. Electron. Mater. 3(9), 1600350 (2017). [CrossRef]
12. S. Yoshioka, H. Hayashi, A. Kuwabara, F. Oba, K. Matsunaga, and I. Tanaka, “Structures and energetics of Ga2O3 polymorphs,” J. Phys.: Condens. Matter 19(34), 346211 (2007). [CrossRef]
13. J. P. Remeika and M. Marezio, “GROWTH OF α-Ga2O3 SINGLE CRYSTALS AT 44 KBARS,” Appl. Phys. Lett. 8(4), 87–88 (1966). [CrossRef]
14. M. Marezio and J. P. Remeika, “Bond Lengths in the α-Ga2O3 Structure and the High-Pressure Phase of Ga2−xFexO3,” J. Chem. Phys. 46(5), 1862–1865 (1967). [CrossRef]
15. N. D. Cuong, Y. W. Park, and S. G. Yoon, “Microstructural and electrical properties of Ga2O3 nanowires grown at various temperatures by vapor–liquid–solid technique,” Sens. Actuators, B 140(1), 240–244 (2009). [CrossRef]
16. R. Suzuki, S. Nakagomi, Y. Kokubun, N. Arai, and S. Ohira, “Enhancement of responsivity in solar-blind β-Ga2O3 photodiodes with a Au Schottky contact fabricated on single crystal substrates by annealing,” Appl. Phys. Lett. 94(22), 222102 (2009). [CrossRef]
17. Z. Feng, L. Huang, Q. Feng, X. Li, H. Zhang, W. Tang, J. Zhang, and Y. Hao, “Influence of annealing atmosphere on the performance of a β-Ga2O3 thin film and photodetector,” Opt. Mater. Express 8(8), 2229–2237 (2018). [CrossRef]
18. H. Zhou, M. Si, S. Alghamdi, G. Qiu, L. Yang, and P. D. Ye, “High-Performance Depletion/Enhancement-ode β-Ga2O3 on Insulator (GOOI) Field-Effect Transistors With Record Drain Currents of 600/450 mA/mm,” IEEE Electron Device Lett. 38(1), 103–106 (2017). [CrossRef]
19. Z. Liu, T. Yamazaki, Y. Shen, T. Kikuta, N. Nakatani, and Y. Li, “O2 and CO sensing of Ga2O3 multiple nanowire gas sensors,” Sens. Actuators, B 129(2), 666–670 (2008). [CrossRef]
20. T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, and S. Fujita, “Vertical Solar-Blind Deep-Ultraviolet Schottky Photodetectors Based on β-Ga2O3 Substrates,” Appl. Phys. Express 1(1), 011202 (2008). [CrossRef]
21. D. Y. Guo, Z. P. Wu, Y. H. An, X. C. Guo, X. L. Chu, C. L. Sun, L. H. Li, P. G. Li, and W. H. Tang, “Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors,” Appl. Phys. Lett. 105(2), 023507 (2014). [CrossRef]
22. W. E. Mahmoud, “Solar blind avalanche photodetector based on the cation exchange growth of β-Ga2O3/SnO2 bilayer heterostructure thin film,” Sol. Energy Mater. Sol. Cells 152, 65–72 (2016). [CrossRef]
23. X. Chen, F. Ren, S. Gu, and J. Ye, “Review of gallium-oxide-based solar-blind ultraviolet photodetectors,” Photonics Res. 7(4), 381–415 (2019). [CrossRef]
24. Y. Xu, Z. An, L. Zhang, Q. Feng, J. Zhang, C. Zhang, and Y. Hao, “Solar blind deep ultraviolet β-Ga2O3 photodetectors grown on sapphire by the Mist-CVD method,” Opt. Mater. Express 8(9), 2941–2947 (2018). [CrossRef]
25. T. Gruber, C. Kirchner, R. Kling, F. Reuss, and A. Waag, “ZnMgO epilayers and ZnO–ZnMgO quantum wells for optoelectronic applications in the blue and UV spectral region,” Appl. Phys. Lett. 84(26), 5359–5361 (2004). [CrossRef]
26. Y. Huang, X. Duan, Y. Cui, and M. Lieber, “Gallium nitride nanowire nanodevices,” Nano Lett. 2(2), 101–104 (2002). [CrossRef]
27. C.-Y. Cho, Y. Zhang, E. Cicek, B. Rahnema, Y. Bai, R. McClintock, and M. Razeghi, “Surface plasmon enhanced light emission from AlGaN-based ultraviolet light-emitting diodes grown on Si (111),” Appl. Phys. Lett. 102(21), 211110 (2013). [CrossRef]
28. T. Oshima, T. Okuno, and S. Fujita, “Ga2O3 Thin Film Growth onc-Plane Sapphire Substrates by Molecular Beam Epitaxy for Deep-Ultraviolet Photodetectors,” Jpn. J. Appl. Phys. 46(11), 7217–7220 (2007). [CrossRef]
29. M. Ogita, K. Higo, Y. Nakanishi, and Y. Hatanaka, “Ga2O3 thin film for oxygen sensor at high temperature,” Appl. Surf. Sci. 175-176, 721–725 (2001). [CrossRef]
30. G. Wagner, M. Baldini, D. Gogova, M. Schmidbauer, R. Schewski, M. Albrecht, Z. Galazka, D. Klimm, and R. Fornari, “Homoepitaxial growth of β-Ga2O3 layers by metal-organic vapor phase epitaxy,” Phys. Status Solidi A 211(1), 27–33 (2014). [CrossRef]
31. S. J. Hao, M. Hetzl, F. Schuster, K. Danielewicz, A. Bergmaier, G. Dollinger, Q. L. Sai, C. T. Xia, T. Hoffmann, M. Wiesinger, S. Matich, W. Aigner, and M. Stutzmann, “Growth and characterization of β-Ga2O3 thin films on different substrates,” J. Appl. Phys. 125(10), 105701 (2019). [CrossRef]
32. W. S. Cho and M. Kakihana, “Crystallization of ceramic pigment CoAl2O4 nanocrystals from Co–Al metal organic precursor,” J. Alloys Compd. 287(1-2), 87–90 (1999). [CrossRef]
33. W. Feng, H. Zhou, and F. Chen, “Impact of thickness on crystal structure and optical properties for thermally evaporated PbSe thin films,” Vacuum 114, 82–85 (2015). [CrossRef]
34. M. Bender, N. Katsarakis, E. Gagaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, and G. Kiriakidis, “Dependence of the photoreduction and oxidation behavior of indium oxide films on substrate temperature and film thickness,” J. Appl. Phys. 90(10), 5382–5387 (2001). [CrossRef]
35. L.-X. Qian, H.-F. Zhang, P. T. Lai, Z.-H. Wu, and X.-Z. Liu, “High-sensitivity β-Ga2O3 solar-blind photodetector on high-temperature pretreated c-plane sapphire substrate,” Opt. Mater. Express 7(10), 3643–3653 (2017). [CrossRef]
36. S. Oh, Y. Jung, M. A. Mastro, J. K. Hite, C. R. Eddy Jr., and J. Kim, “Development of solar-blind photodetectors based on Si-implanted β-Ga2O3,” Opt. Express 23(22), 28300–28305 (2015). [CrossRef]
37. D. Guo, Z. Wu, P. Li, Y. An, H. Liu, X. Guo, H. Yan, G. Wang, C. Sun, L. Li, and W. Tang, “Fabrication of β-Ga2O3 thin films and solar-blind photodetectors by laser MBE technology,” Opt. Mater. Express 4(5), 1067–1076 (2014). [CrossRef]
38. G. C. Hu, C. X. Shan, N. Zhang, M. M. Jiang, S. P. Wang, and D. Z. Shen, “High gain Ga2O3 solar-blind photodetectors realized via a carrier multiplication process,” Opt. Express 23(10), 13554–13561 (2015). [CrossRef]
39. F.-P. Yu, S.-L. Ou, and D.-S. Wuu, “Pulsed laser deposition of gallium oxide films for high performance solar-blind photodetectors,” Opt. Mater. Express 5(5), 1240–1249 (2015). [CrossRef]
40. P. Ravadgar, R. H. Horng, S. D. Yao, H. Y. Lee, B. R. Wu, S. L. Ou, and L. W. Tu, “Effects of crystallinity and point defects on optoelectronic applications of β-Ga2O3 epilayers,” Opt. Express 21(21), 24599–24610 (2013). [CrossRef]
41. Y. H. An, Y. S. Zhi, W. Cui, X. L. Zhao, Z. P. Wu, D. Y. Guo, P. G. Li, and W. H. Tang, “Thickness Tuning Photoelectric Properties of β-Ga2O3 Thin Film Based Photodetectors,” J. Nanosci. Nanotechnol. 17(12), 9091–9094 (2017). [CrossRef]
42. A. Singh Pratiyush, S. Krishnamoorthy, S. Vishnu Solanke, Z. Xia, R. Muralidharan, S. Rajan, and D. N. Nath, “High responsivity in molecular beam epitaxy grown β-Ga2O3 metal semiconductor metal solar blind deep-UV photodetector,” Appl. Phys. Lett. 110(22), 221107 (2017). [CrossRef]
43. L. X. Qian, Y. Wang, Z. H. Wu, T. Sheng, and X. Z. Liu, “β-Ga2O3 solar-blind deep-ultraviolet photodetector based on annealed sapphire substrate,” Vacuum 140, 106–110 (2017). [CrossRef]
44. X. H. Chen, S. Han, Y. M. Lu, P. J. Cao, W. J. Liu, Y. X. Zeng, F. Jia, W. Y. Xu, X. K. Liu, and D. L. Zhu, “High signal/noise ratio and high-speed deep UV detector on β-Ga2O3 thin film composed of both (400) and (-201) orientation β-Ga2O3 deposited by the PLD method,” J. Alloys Compd. 747, 869–878 (2018). [CrossRef]
45. W. Cui, D. Guo, X. Zhao, Z. Wu, P. Li, L. Li, C. Cui, and W. Tang, “Solar-blind photodetector based on Ga2O3 nanowires array film growth from inserted Al2O3 ultrathin interlayers for improving responsivity,” RSC Adv. 6(103), 100683 (2016). [CrossRef]
46. X. Wang, Z. Chen, D. Guo, X. Zhang, Z. Wu, P. Li, and W. Tang, “Optimizing the performance of a β-Ga2O3 solar-blind UV photodetector by compromising between photoabsorption and electric field distribution,” Opt. Mater. Express 8(9), 2918–2927 (2018). [CrossRef]
47. A. A. Balandin, “Low-frequency 1/f noise in graphene devices,” Nat. Nanotechnol. 8(8), 549–555 (2013). [CrossRef]
