The correlation between radiative surface defect states and high color rendering index from ZnO nanotubes
© Sadaf et al; licensee Springer. 2011
Received: 15 May 2011
Accepted: 30 August 2011
Published: 30 August 2011
Combined surface, structural and opto-electrical investigations are drawn from the chemically fashioned ZnO nanotubes and its heterostructure with p-GaN film. A strong correlation has been found between the formation of radiative surface defect states in the nanotubes and the pure cool white light possessing averaged eight color rendering index value of 96 with appropriate color temperature. Highly important deep-red color index value has been realized > 95 which has the capability to render and reproduce natural and vivid colors accurately. Diverse types of deep defect states and their relative contribution to the corresponding wavelengths in the broad emission band is suggested.
KeywordsZnO nanotubes ZnO/GaN heterostructure radiative surface defects color rendering index R9 color indexed
The solid-state lighting holds tremendous prospective for future illumination, backlight panel display industry and biomedical applications due to their brightness and durability [1–3]. Over the past decade, much attention has been drawn towards white-light-emitting diodes (WLEDs) as new light sources due to their reliability with great economic and ecological consequences. So far, different materials and a number of nanostructures are being used to fabricate WLEDs such as phosphors, nanocrystals, polymers, and nanocrystal-polymer combination [4–7]. To this end, phosphor and polymers are being studied comprehensively for wavelength conversion and to generate full-color emission but still much efforts are required to achieve the light-emitting devices with high color rendering index (CRI) value approaching 100 for future lighting.
During the last years, zinc oxide (ZnO) material has been extensively investigated as a suitable contender for new-generation photonic devices. ZnO contains a promising emission tendency for blue/ultraviolet and full-color lighting, owing to the wide band gap, large exciton binding energy and many radiative deep levels depending on its synthesizing techniques [8, 9]. The ease in the fabrication of nanoscale structures with huge diversity in shape and size is another advantageous characteristic of the ZnO material. However, the self-compensation feature of p-ZnO exists as a real hurdle in the pursuit of stable homojunctions of ZnO . In this regard, GaN provides a suitable replacement of the p-ZnO for the fabrication of pn-heterostructures due to their better match in crystal structure, wide band gap and opto-electronic properties compared to other p-type materials. Among a variety of nanoscale structures of ZnO, nanotubes along with p-GaN have the potential to provide a heterostructure with substantial advantages and the conjunction of high surface to volume ratio with huge number of intrinsic and extrinsic defects could culminate a full-color illumination. Moreover, ZnO-nanotubes/GaN heterostructure have an aptitude to produce an environmentally benign alternative of the traditional lighting sources with high CRI value encompassing the diverse applications. Along with the first eight colors rendering indices of CRI (Ra), deep-red rendering index R9 contains a significant importance for the reproduction of the original colors of different objects. Furthermore, the heterostructure under investigation is based on simple manufacturing technique and offers high stability of the CRI with increasing temperature which is the main dilemma of the polymeric and phosphoric-based light-emitting devices. Here, a heterostructure fashioned with the combination of chemically fabricated ZnO nanotubes and Mg-doped GaN thin film has been used to unreveal the defect-related broad visible emission mechanism. Transmission electron microscope (TEM), cathodo- and electroluminescence (CL and EL) techniques have been utilized to observe the influence of the etching mechanism on the defect states in the nanotubes. Moreover, the corresponding impact of chemical etching on the radiative and non-radiative recombination has been studied which play a crucially important role in the production of high CRI and R9 values.
To make n-ZnO nanotubes/p-GaN heterostructure structure, vertically well-aligned ZnO nanorods have been grown on p-GaN thin film employing a low-temperature aqueous chemical synthesis technique. These nanorods have been further dipped in potassium chloride solution with concentration of 5 M for 10 h for the fabrication of nanotubes under the process of the wet chemical etching . An insulating layer of Shipley 1805 (Shipley Company, Marlborough, MA, USA) has been spun coated to fill the space between the nanotubes for the isolation of electrical contacts followed by reactive ion etching to expose the tips of nanotubes. Finally ohmic contacts on p-GaN and n-ZnO have been made by thermal evaporation of the Ni/Au and Ti/Au bilayer electrodes, respectively.
Results and discussion
Color rendering index, color temperature, R9 and x, y coordinates values corresponding to different injection currents
Injected current (mA)
Color rendering index
In summary, we have correlated the removal of non-radiative recombination centers present in the core of nanorods as well as the production of surface defect states as radiative recombination centers in nanotubes and their role in the enhancement in the emission intensity and CRI value of the heterostructure. The broad band emission spectrum is suggested as a result of the superposition of different emission peaks corresponding to the diversity of the deep level defect states. A high value of R9 > 95 has been achieved which could uncover the device applications in the fields of decorative industry and medical surgery.
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