High temperature instabilities of ohmic contacts on Mg-doped gallium nitride

This paper analyzes the high temperature (250 °C) long-term stability of ohmic contacts on p-type gallium nitride. The contributions of the ohmic contacts and semiconductor material degradation are separated by means of suitable test structures adopting the Transfer Length Method (TLM). Before stress, the current vs voltage curves measured at the pads of the TLMs showed linear shape, indicating good ohmic behaviour of the contacts. Average sheet resistance and contact resistivity were found to be equal to 53 kOhm/ and 2.7 mOhm•cm2. Thermal treatment at 250 °C was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist in a shift of the I-V curves towards higher voltages, and strong non-linearity of the characteristics around zero. This paper demonstrates that: (i) the electrical degradation is due to the worsening of the ohmic properties of the metal/semiconductor interface at the p-side of the diodes, implying the non-linear characteristics of the contacts and in a slight increase of the semiconductor sheet resistance; (ii) that degradation is reversible, after passivation removal and subsequent annealing; (iii) that sputtered passivation can be used as an alternative to usually adopted PECVD passivation for improving the high temperature stability of ohmic contacts on p-type GaN.