Effects of the Photoelectrochemical Etching in Hydrogen Fluride (HF) on the Optoelectrical Properties of Ga2O3

Photoelectrochemical (PEC) etching is preferred to produce micro-and nano-structures for constructing Ga2O3-based electronics and optoelectronics, owing to its numerous controllable parameters. During the devices fabrications, beyond the wet chemical and dry (plasma) etching produces, PEC etching also leads to device degradations inordinately. In this work, the Ga2O3 thin film was PEC etched by hydrogen fluride (HF) etchant, and its opto-electric deep-ultraviolet sensing performances, including photo-to-dark current ratio, responsivity, and response speed, before and after PEC etching were analyzed and discussed.


Introduction
Traditional narrow bandgap semiconductors, such as Si and GaAs, deep-UV photodetectors are not real solar blindness, as a result that additional visible light blocking filters are indispensable. As far as it goes, their relevant commercial solar-blind detectors are bulky, fragile and high voltage driven [1,2]. Wide and ultra-wide bandgap semiconductors, to some degree, could evade such a trouble in constructing photodetectors owing to their well-suited natural energy-band gaps [3][4][5]. Gallium trioxide (Ga2O3), as one of the ultra-wide bandgap semiconductors, is making an inroad in true solarblind ultraviolet (UV) photo-detecting devices [6,7], benefiting by its bandgap of ~4.9 eV [8,9], allowing its high solar-blind UV/visible light rejection ratio. The same as any semiconductor devices constructions [10,11], the etching procedures are required to perform patterning transfer in Ga2O3based electronics and optoelectronics [12].
In spite of that Ga2O3 is up against the etching difficulty due to its high chemical robustness and bonding strength. Wet chemical etching of Ga2O3 has been carried out in HCl [13], H2SO4 [13,14], KOH [13,15], HF [13,16], H3PO4 [14,17], NaOH [13] and HNO3 [13] solutions of a certain concentration. Of which, the etching rate and roughness are not very satisfied. In addition to that, plasma (dry) etching always cause the serious damage of the surface of the Ga2O3 that affect the carrier transports in relevant devices on account of the defect-related mechanisms [18][19][20]; such as etchants of BCl3/Cl2/Ar [21,22], Cl2/Ar [22][23][24], SF6/Ar [18,22], SF6/BCl3 [25], BCl3/Ar [22,24,26], CHF3/Ar [22], O2/Ar [22] inductively coupled discharged plasmas. Recently, photoelectrochemical (PEC) etching technique is performed to etch Ga2O3, of which the external voltages and light irradiations are assistant to finish the Ga2O3 etching processes. B. Alhalaili et al. [27]  UV photodetectors are placed huge hopes for applications in deep space detection and aviation communications; in which photodetectors may well encounter harsh detecting environments. For instance, strong acid exists extensively in the universe, which may corrode the photodetectors when it is detecting UV-ray and X-ray in the space station, leading to a degradation of device performances. Or to say, the developed wearable UV photodetectors are also in demands for its corrosion-resistant characterizations. Therefore, to investigate the corrosion-resistant property of detecting devices are urgently desired [29][30][31]. In this work, we perform PEC etching of Ga2O3 in HF etchant, and its deep-UV opto-electrical performances are analyzed. The electrical and optoelectrical characterizations before and after PEC etching are compared and discussed.

Experimental
The Ga2O3 thin film was deposited by using metal-organic chemical vapor deposition (MOCVD) method. When deposition, the triethylgallium (TEGa) and O2 gas (99.999% pure) were used as Ga and O sources for Ga2O3 growth at temperature of 735℃ and chamber pressure of 25 Torr [32,33]. Following which, the three-pair interdigital Ti/Au electrodes were patterned on the surface of the Ga2O3 films through direct-current radio-frequency magnetron sputtering. The electrodes are 3.9 mm long, 0.2 mm wide and 0.2 mm spacing distance, leading to an effective illuminated area of 0.043 cm 2 [34,35]. The Ga2O3 films were PEC etched under irradiation of 300 W xenon lamp in 4.6 M HF etchant, the light intensity is 1000 mW cm -2 and the anodic voltage is 10 V. After PEC etching, the Ga2O3 films were cleaned with deionized water and dried by N2 flow gun [36]. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) were used to verify and discuss the quality of the film. In addition, the current-voltage (I-V) and time-dependent currents (I-t) were measured by employing the Keithley 4200 semiconductor analyzer in air at room temperature.

Results and Discussion
As shown in figure 1 (a) and (b), the surface of the Ga2O3 thin film become more flat, due to the photoelectrochemical etching effect. Through X-ray (XRD) measurement as shown in Figure 1(c), it displayed (-201), (-402) and (-603) orientations for both as-deposited and HF-corroded the Ga2O3 thin film, along with sharp peaks. Figure 1 (d) is the X-ray photoelectron spectroscopy (XPS) of both asdeposited and HF-corroded the Ga2O3 thin film. In addition, as given in figure 1 (e) and (f), the binding energy of Ga3d shift from 19.94 eV to 20.21 eV after that the Ga2O3 thin film is corroded by HF; and that the binding energy of O1s change from 530.57 eV to 530.96 eV. As shown in figure 2 (a), it is the linear scale current-voltage (I-V) curves of the as-deposited and HF-corroded the Ga2O3 thin film with Ti/Au metal electrodes in the dark. Clearly, the I-V behavior is almost Ohmic contact after HF corrosion, while it is Schottky electrical performance, due to the low carrier concentration and high resistance of Ga2O3 [33]. Correspondingly, Figure 2 (b) is the semi-log scale I-V curves in the dark and under illuminations, from which we can see that the photo-to-dark current ratio (PDCR) of the as-deposited beta-Ga2O3 solar-blind photodetector is 8.12×10 5 , while it is 4.77×10 2 after HF corrosion. The high resolution HF has made a destroy for the Ga2O3 materials, leading to a decreased photo response.

Conclusion
In this work, we demonstrated the effects of Effects of the HF corrosion on the optoelectrical properties of Ga2O3 by photoelectrochemical (PEC) etching. The photo response and response speed for the as-deposited and HF corroded beta-Ga2O3 solar-blind photodetectors are all discussed in the paper.