Figure 4

Performance of SnO₂-based heterojunction light-emitting diodes (LEDs). (a) I–V curves of the SnO₂/p-GaN and the SnO₂/MgO/p-GaN heterojunctions. The turn-on voltages are about 6 and 9 V for SnO₂/p-GaN and SnO₂/MgO/p-GaN, respectively. Inset in (a) shows the ohmic behavior of the n-n and the p-p contacts in the LED device. (b) Normalized room temperature (RT) photoluminescence spectra of the p-GaN substrate, the SnO₂/p-GaN and the SnO₂/MgO/p-GaN heterojunctions. (c and d) RT electroluminescence (EL) spectra of the SnO₂/p-GaN and the SnO₂/MgO/p-GaN heterojunction LEDs under various forward excitation currents. Inset in (d) shows the photograph of the SnO₂/MgO/p-GaN LEDs biased under a forward current of 6 mA. (e and f) Schematics illustrating the band alignment of the SnO₂/p-GaN and the SnO₂/MgO/p-GaN heterojunctions under a forward bias. In the SnO₂/p-GaN junction, the charge leakage makes the GaN-related emission dominate the spectra. On the other hand, in the SnO₂/MgO/p-GaN junction, electrons in the SnO₂ layer are blocked by the high barrier at the SnO₂/MgO interface, giving rise to the efficient SnO₂-related EL in the ultraviolet regime.