Inhomogeneous carrier transport at contact/GaN interfaces and thermal instability of vertical light-emitting diodes

The carrier-transport behavior at the interface of a contact and n-type GaN was investigated for group III nitride vertical light-emitting diodes (LEDs). Three types of samples were investigated including dry-etched flat Ga-polar n-GaN (GA), dry-etched flat N-polar n-GaN (NF), and wet-etched roughened N-polar n-GaN (NR). Schottky diodes fabricated using a palladium contact revealed that carrier transport at the contact/GaN interface could be understood in terms of the barrier inhomogeneity model, in which the local shallow barriers dominated the overall carrier transport. Kelvin probe force microscopy showed that the peculiar crystallographic structures and native defects (acting as the local shallow barriers) led to the largest tunneling probability of the NR samples. The study was followed up by forming a TiN/Al contact on the NR surfaces to study the thermal instability of the vertical LEDs, revealing that the vertical LEDs were degraded after a thermal annealing process. This could be related to the n-contact size, indicating that the degradation of vertical LEDs was likely due to the failure of Ohmic patches (contacts formed on the defects and/or the tip/edge of hexagonal cones) at elevated temperature.