Implantation and Activation of Ge and Si Shallow Donors in β-Ga₂O₃ for Improved Ohmic Contact Formation

The recent developments in the fabrication of electronic devices based on the semiconductor β-Ga₂O₃ have demonstrated the high potential of this material to be used in next-generation power electronics applications. Due to the ultra-wide bandgap of 4.8 eV, a high breakdown strength of 8 MV/cm is expected, which could pave the way for power devices with even higher breakdown voltages and efficiencies than are possible with the SiC and GaN counterparts. One major key requirement for high performance electronic devices based on ultrawide bandgap semiconductors such as β-Ga₂O₃ is the formation of low ohmic contact resistances in order to reduce conduction losses inside the devices. This becomes even more important for RF devices, where the channel resistance is much lower as in high-voltage devices. An effective way to reduce ohmic contact resistances is the use of ion implantation of shallow donors for local n-type doping of the contact region prior to ohmic metal deposition.<br/>In this presentation, we report on the optimum annealing conditions for the activation of Ge-implanted (100) β-Ga₂O₃ in order to reach low ohmic contact resistances. In this regard, the influence of rapid thermal annealing treatment at temperatures between 900 and 1200 °C on the structural and electrical properties were analyzed. Our studies reveal significant changes in the surface morphology after high-temperature annealing above 1000 °C involving an increase in the surface roughness which is accompanied with a steady increase of the activation efficiency up to almost 20% at annealing temperatures of 1200 °C. Optimum annealing conditions were identified at 1100 °C at which the specific contact resistance is reduced by one order of magnitude down to 4.8 × 10^-7 Ωcm² in comparison to non-implanted samples. In contrast to these findings, the surface morphology of Si-implanted (010) β-Ga₂O₃ with a comparable doping profile remains almost unchanged even after annealing at 1100 °C. Furthermore, the highest activation efficiency of around 60 % is reached at annealing temperatures between 900 and 1000 °C whereas higher temperatures lead to significant reduction. Our investigations verify the severe impact of the anisotropic nature of β-Ga₂O₃ on implantation processes of shallow donors which is particularly emphasized by the implantation instability of (100) β-Ga₂O₃ at high beam currents. Comparative experiments on β-Ga₂O₃ samples with different crystal orientations revealed a massive destruction of the (100) β-Ga₂O₃ surface after Ge implantation using beam currents around 1 µA whereas no damage was observed for (010) β-Ga₂O₃ using the same implantation conditions.<br/>In summary, our results will reveal and identify important challenges but also the huge potential of ion implantation of shallow donors into β-Ga₂O₃in order to reach low ohmic contact resistances.