Ohmic contact formation to GaN by Ge+ implantation doping: Implantation fluence and encapsulation layer studies

This work investigates aspects of electrical doping of gallium nitride (GaN) by Ge+ ion implantation for low-resistivity ohmic contacts, with particular emphasis on reducing the level of defects produced. The 250 keV Ge+ ions were implanted through a retarding layer, with different effective fluences (2.4 × 1015–8.8 × 1015 cm−2), while dopant activation was done at a moderate temperature of 1100 °C using different encapsulation layers. Structural and electrical studies were performed: optical microscopy, Rutherford backscattering spectrometry in channeling geometry (with detailed analysis of point and extended defects using the McChasy software), and circular Transfer Line Method measurements were used. Implantation with the effective fluence of 4.8 × 1015 cm−2 allowed the formation of ohmic contacts with a specific contact resistance of (0.60 ± 0.07) Ω·mm while GaN sheet resistivity was (499 ± 15) Ω/sq., while implantations with both lower and higher fluences produced worse contacts. The topSiO2/AlN bilayer proved to be the best encapsulation layer, offering protection from surface degradation, good electrical parameters, and reproducibility. Although Ge+ implantation doping is promising, potentially introducing more dopants than respective Si+ doping, its drawback could be the higher concentrations of defects formed, thus damage prevention and removal (during both the implantation and activation annealing) is crucial.