Study Of Heterostructure Ultraviolet Light-emitting Device Based On ZnO Nanowire

Wide band gap (3.37eV) and high exciton binding energy (60meV) make ZnO apromising candidate for ultraviolet (UV) light-emitting diodes (LEDs) and low-thresholdlasing diodes (LDs). Single-crystalline ZnO nanowire (NW) has superior electrical transportand optical properties, and their incorporation into light-emitting devices is expected to offerimproved performance. Meanwhile, metal localized surface plasmons (LSPs) with suitableenergy can couple with ZnO excitons resonantly, and enhance the electroluminescence (EL)of LEDs effectively by improving internal quantum efficiency and light-extraction efficiencyof active layer. Herein, our research work focuses on the design and development ofheterojunction ultraviolet LEDs based on ZnO nanowire. I will introduce our work detailedlyfrom aspects of the nanowire growth, optical property, LED fabrication&characterization andperformance improvement. The specific content is as follows:First, we employed catalyst-free high-pressure pulsed laser deposition (PLD) techniqueand hydrothermal synthesis method respectively to achieve size-controlled growth ofsingle-crystalline ZnO NWs. The effect of preparation conditions such as growth pressure,substrate temperature in PLD system, and precursor concentration, seed-layer quality inhydrothermal synthesis, on the growth of ZnO NWs were discussed in detail.Based on this, ZnO/MgZnO core/shell heterostructure NW arrays were also fabricated.Especially, the growth of MgZnO shell on ZnO NW is heteroepitaxial with a high-qualityinterface. The optical properties of single ZnO NWs were studied in detail by spatiallyresolved cathodoluminescence. The results indicate that the MgZnO shell layer can effectivelyconfine photogenerated carriers and passivate surface states of ZnO NWs, and enhance theUV near-band-edge emission efficiency of the core/shell NW arrays.Based on these NW heterostructures, the p-n heterojunction LEDs using MgZnO-coatedand bare ZnO NW arrays as active layers were manufactured on p-GaN substrates. Bothdevices were exposed to ambient air over a1-yr period to assess their stability. The resultsrevealed that surface-adsorbed O2and OH-species, as acceptor and donor surface states, canquench UV EL and favor undesirable surface-mediated nonradiative and deep-levelrecombinations. The MgZnO coating prevents surface adsorptions, and so the coated NWdevice showed higher efficiency and stability than the uncoated one.On the other hand, to further improve the quantum efficiency and light-extraction efficiency of heterojunction LEDs, the LSPs of Ag nanoparticles (NPs) were incorporated intothe ZnO NW arrays. By combining the advantages of both nanomaterial and metal LSP, asignificant enhancement (~7-fold) and extended spatial distribution of UV EL were obtainedfrom Ag NPs decorated ZnO NWs array LED. In addition, to accurately control the LSPextinction property and spacer-layer thickness, the Ag NPs and MgO spacer-layer wereintroduced into a p-GaN/i-ZnO/n-ZnO film heterostructure device. By optimizing the MgOthickness, which can suppress the unwanted charge transfer and nonradiative F rster resonantenergy transfer between Ag and ZnO, a7-fold EL enhancement was achieved. Time-resolvedand temperature-dependent photoluminescence measurements reveal that the luminescenceenhancements mainly result from the following two points:(1) resonant coupling betweenZnO excitons and Ag LSPs;(2) high light-extraction efficiency from LSP mode. Especiallythe former one increases the spontaneous emission rate and internal quantum efficiency ofZnO material.