White Electroluminescence Using ZnO Nanotubes/GaN Heterostructure Light-Emitting Diode
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We report the fabrication of heterostructure white light–emitting diode (LED) comprised of n-ZnO nanotubes (NTs) aqueous chemically synthesized on p-GaN substrate. Room temperature electroluminescence (EL) of the LED demonstrates strong broadband white emission spectrum consisting of predominating peak centred at 560 nm and relatively weak violet–blue emission peak at 450 nm under forward bias. The broadband EL emission covering the whole visible spectrum has been attributed to the large surface area and high surface states of ZnO NTs produced during the etching process. In addition, comparison of the EL emission colour quality shows that ZnO nanotubes have much better quality than that of the ZnO nanorods. The colour-rendering index of the white light obtained from the nanotubes was 87, while the nanorods-based LED emit yellowish colour.
KeywordsZnO nanotubes Light-emitting diodes Electroluminescence Lightning White light sources
Zinc oxide has substantive advantages including the quests for LEDs and laser diodes [1, 2] because of its wide band gap (~3.37 eV) and large exciton-binding energy (60 meV). Also, deep level defects (intrinsic and extrinsic) in ZnO can lead to an emission band covering the whole visible spectrum . Furthermore, the optoelectronic properties of ZnO can be tuned by changing its morphology, composition, crystalline structure, orientation and the growth conditions [4, 5]. The development of ZnO homojunction diodes is rare due to the lack of authentic procedure to grow high-quality p-type ZnO [6, 7]. As an interesting alternate, the deposition of n-ZnO nanostructures on different p-type semiconductors is quite frequent to utilize the electronic and optical advantages offered by heterostructure devices. Gallium nitride (GaN) is the most suitable p-type material having wide band gap, wurtzite crystal structure and least mismatching (~1.8%) in lattice constants . Till date, white LEDs are being fabricated either by using phosphors or by mixing several coloured light. However, there are significant barriers for the large-scale applications of both processes. The former is associated with a strong reduction in device efficiency due to the problems such as low durability and requirement of relatively high fabrication temperatures for organic and inorganic phosphors, respectively. On the other hand, the latter requires the complex mixing or doping schemes, and exponential decay in emission power occurs with the increase in temperature, resulting in substantial change in colour stability [9–11]. Recently, ZnO NTs/Au Schottky LED has been reported covering an emission spectrum between 400 and 600 nm . Here, we report a promising route towards the fabrication of heterostructure LED by exploiting the ZnO NTs for white light–emitting applications. As ZnO NTs have excellent optoelectronic properties due to their large surface to bulk ratio with high porosity and significant amount of surface states giving rise to broadband emission covering the whole visible spectrum. This technique has several advantages, e.g. cheap, simple and environmentally benign in comparison with other techniques.
ZnO nanorods were aqueous chemically synthesized on p-GaN substrate by using low-temperature approach. The GaN substrate containing ZnO nanorods was cut into two pieces. One of them was used as a reference sample, while the second half was dipped in potassium chloride (KCl) aqueous solution for the trimming of ZnO nanorods into nanotubes using optimized parameters, as previously reported , except the immersion time which was shortened intentionally to reduce the etching depth of ZnO NTs. Both samples were further processed to fabricate the LEDs by depositing the ohmic contact on p-GaN film by evaporating Ni/Au bilayers of thicknesses 20/40 nm, respectively. Shipley-1805 photoresist film was homogeneously deposited as an insulating layer by spin coating to fill the space between the ZnO NTs and uncovered area of the p-GaN substrate. Selective etching of the photoresist was carried out using a reactive ion etching in order to expose the tip of ZnO NTs ~ few nanometres. Finally, top contacts on ZnO were made by evaporating Ti/Au bilayers of thicknesses 20/30 nm, respectively.
Results and Discussion
In conclusion, white light–emitting diode based on n-ZnO NTs/p-GaN was developed by using simple, cost-efficient and low-temperature approach. The fabricated LED showed higher current density and stronger EL emission peaks centred at 450 and 560 nm when compared to the LED comprised of ZnO nanorods. The enhanced EL intensity of the visible band (425–750 nm) emission from the ZnO NTs is due to their high porosity, large surface area and the high density of recombination centres at the surface. The use of ZnO NTs provided the prospects to explore the potential of white inorganic light–emitting diode with superior performance.
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