Carrier transport analysis of n-ZnO:Al/p-GaN:Mg heterojunction light-emitting diodes

Carrier transport mechanisms of n-ZnO:Al/p-GaN:Mg heterojunction light-emitting diodes (HJ-LEDs) were investigated. HJ-LEDs exhibited a typical diode behavior with a large forward voltage of ∼6.0 V (due to the poor p-contact resistance and current crowding effect), a high reverse leakage current of -2.5 × 10^-4 A at -5 V, and injection-current-dependent electroluminescent spectra, which shifted from 430 (emitted from the p-GaN side at low currents) to 380 nm (from the n-ZnO side at high currents). Analysis of temperature-dependent reverse leakage current revealed that the predominant transport mechanism was variable range hopping conduction in the low temperature range (<260 K) and Poole-Frenkel conduction in the high temperature range (>260 K). The thermal activation energy at zero bias, which is comparable to the built-in potential, was as low as 114 meV, suggesting that tunneling played a crucial role in carrier transport under forward bias conditions.