Other elements such as substrate orientation have also been reported to influence the Ohmic contact property. To form Ohmic contact, Ti/Au contacts were deposited, followed by an RTA process at 450 °C for 5 min, which was employed on both (
In recent years, researchers have investigated and optimized the Ohmic contact property of β-Ga
2O
3 by choosing a metal with a proper work function and investigating metal schemes, interfacial reactions between metal and semiconductors during the annealing process, and the doping concentration of Ga
2O
3 in the source/drain region, and they have achieved excellent results. Representative results with excellent Ohmic contact quality are summarized in
Figure 6 [23][27][28][32][34][43][44][45][46][47]. Despite the significant improvement in the Ohmic contact techniques for β-Ga
2O
3, there are also some questions that need to be solved before commercializing the devices. (1) For power device applications, contact performance in high-temperature, -current, and -voltage environments is another concern. Failure analyses of the electrical stress/cycling of Ohmic electrodes have been investigated for other WBG semiconductor systems
[48][49][50], while for β-Ga
2O
3, the research is lacking and efforts should be made to understand the degradation mechanism of Ohmic contacts under electrical stress. (2) To realize the integration of β-Ga
2O
3 semiconductors into Si CMOS technology, Au-free metal schemes should be investigated and proposed. The commonly used Ti/Au layer for Ga
2O
3 is not CMOS-compatible due to the existence of Au, which is a contaminant for Si fabrication lines
[51][52]. Au is used for oxidation protection. Some oxidation-resistant capping materials, such as TiN, which has been proven to realize low-R
C Ohmic contact in AlGaN/GaN HEMT
[53], can be substitutes for Ohmic contact realization in β-Ga
2O
3 devices. Related investigations should be carried out to prove the feasibility of Au-free schemes. (3) RIE, ion implantation, and epitaxial regrowth are used to achieve a low R
C. For the RIE process, the influences of plasma gas (including BCl
3/Ar, Ar, and CF
4 [54]), plasma power, bias power, etc., should be fully understood. For ion implantation, the high-temperature annealing used for impurity activation and damage recovery may cause unwanted effects, which should be noticed and further studied. In addition, the effect of substrate orientation should also be investigated. Other annealing techniques can also be used for Ohmic contact formation in β-Ga
2O
3 devices
[55][56].