On the origin of annealing-induced degradation in Al-based ohmic contacts to N-Face n-GaN

On the origin of annealing-induced degradation in Al-based ohmic contacts to N-Face n-GaN
Issue Date
International Conference on Electronic Materials and Nanotechnology for Green Environment 2010
Thin-film vertical InGaN/GaN light-emitting diodes (LEDs) fabricated by the laser lift-off (LLO) process1 have been demonstrated to be very effective for high power operations, which are required to realize solid-state lighting. In the vertical LEDs, an ohmic contact should be formed on the laserirradiated N-face n-GaN epilayer. However, achieving reliable ohmic contacts in the laser-irradiated N-face n-GaN is very challenging due to thermal degradation at a wide range of temperatures (200– 700°C), whereas ohmic contacts to Ga-face GaN can be easily formed using conventional Al-based metallization after annealing at temperatures over 400°C. Although the changes in surface states, point defects, and interfacial compounds have been reported to cause the annealinginduced, degradation of ohmic contacts to the N-face GaN, the exact origin is still unclear. Developing thermally stable metallization on the laser-irradiated N-face n-GaN seems to be possible only on the basis of a precise understanding of the band bending at the metal/GaN interface, but no work has been reported yet about the change in the interfacial band bending with annealing in metal contacts to the N-face n- GaN.In this work, we investigate band bendings in Al ohmic contacts to laser-irradiated Ga- and Nface n-GaN as a function of annealing temperature using synchrotron radiation photoemission spectroscopy (SRPES). The SRPES results are then compared with electrical properties of the contacts. The origin of the different behaviors in band bendings between both samples is discussed in terms of point defects and polarity-dependent polarization charges at the interfacial regions. Finally we propose promising methods and metallization schemes by which reliable ohmic contacts on the N-face GaN can be obtained.
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