Blue-violet heterojunction LEDs based on hydrothermally synthesized ZnO nanorods

Abstract

Zinc oxide nanorods have great potential for the realization of high efficiency heterostructure LEDs based on p-doped GaN. Well-aligned vertical nanorods are desirable to enhance the LED outcoupling performances due to a better confinement of the light [1]. However, due to the lack of reproducible p-type ZnO, a p-GaN substrate is still needed. This work reports on the fabrication of n-ZnO/p-GaN heterojunction LEDs based on vertical ZnO nanorods grown by hydrothermal method. The chemical reaction in the hydrothermal growth of ZnO is based on the decomposition of zinc nitrate, (Zn(NO3)2) with resulting Zn2+ ions reacting with the hydroxyl ions obtained by the thermal degradation of hexamethylenetetramine (HMTA, (CH2)6N4). Samples, consisting in n.i.d. GaN (2 Pm)/pGaN (1 Pm) grown on a (0001) sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), were immersed in the nutrient solution (50 mM) at 80°C for 3 hours. The obtained nanorods follow the crystalline growth direction of the GaN layer along the caxis. The (002) and (004) diffraction peaks are visible in the XRD T–2T scan shown in Fig. 1. The n-ZnO layer consists of a dense collection of coalesced ZnO nanorods, as shown in the scanning-electron microscope (SEM) image of Fig. 2. The coalescence of the nanorods is high when using highly-concentrated solutions. A dense nanorods layer is less fragile and eases handling during the subsequent metal contacts fabrication. The LEDs are fabricated by standard photolithography using direct laser writing for the metal contact patterning. The diameter of the LEDs’ active region spans from 100 to 400 mm. The proposed layout is sketched in Fig. 3: the inset shows an optical microscope image of the fabricated device. Al was used as p electrode metal whereas Cu for the n electrode. A turn-on voltage of approximately 3.5 V was measured, which is consistent with the band structure of the heterojunction. Optical powers of 2 PW were measured at 20 mA of driving current (for LEDs of 400 Pm of diameter). This relatively low value of optical power is due to leakage current paths. The I-V characteristics of two typical LEDs are shown in Fig. 4. Photoluminescence (PL) and electroluminescence (EL) spectra shown in Fig. 5 (the inset shows the blue-violet emission of an LED), prove that the recombination largely occurs in the ZnO side. Both electrical and optical characteristics strongly depend on the carrier injection, in particular the lateral hole injection, due to the poor conductivity of the p-GaN layer. In conclusion, this work demonstrates the fabrication and characterization of n-ZnO nanorods/p-GaN heterojunction LEDs. The recombination mainly occurs in ZnO nanorods side. Different layouts should be investigated in order to improve both the lateral current injection and the efficiency of the LEDs.