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). 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.