Mechanism of low-temperature-annealed Ohmic contacts to AlGaN/GaN heterostructures: A study via formation and removal of Ta-based Ohmic-metals

Abstract

We studied the mechanism of low-temperature-annealed Ohmic contacts to AlGaN/GaN heterostructures via formation and removal of Ta/Al/Ta Ohmic-metals. Multi-probe Hall device measurements show one order increase in the sheet electron concentration after Ohmic-metal formation compared with that before formation, indicating that high-density doping takes place in the AlGaN/GaN heterostructure under the Ohmic-metal. However, after Ohmic-metal removal, the increased sheet electron concentration returns to the value before formation. Moreover, we formed Ni/Au Schottky contacts on the AlGaN/GaN heterostructures before Ohmic-metal formation and after Ohmic-metal removal, and confirmed that the characteristics are almost the same. These results indicate that donors do not exist after Ohmic-metal removal, suggesting that, although high-density doping takes place, high-density donors are not formed under the Ohmic-metal. The high-density doping without high-density donors could be attributed to polarization doping, playing a significant role in Ohmic contact formation.