Optoelectronic characteristics of YAG phosphor-incorporated ZnO films deposited by ultrasonic spray pyrolysis
© Chen and Huang; licensee Springer. 2012
Received: 4 October 2012
Accepted: 1 November 2012
Published: 15 November 2012
This work presents a novel white light device. An yttrium aluminum garnet (YAG) phosphor-incorporated zinc oxide (ZnO) film is deposited on a slide glass substrate by ultrasonic spray pyrolysis. A nanoflower consisting of a hexagonal nanopetal is formed on the surfaces of the samples, and the sizes of the nanopetal are approximately 200 to 700 nm. Additionally, the nanopetal becomes blunted with an increasing incorporated amount of YAG. As the incorporated amount is 1.5 and 2.5 wt.%, the photoluminescence color of the YAG-incorporated ZnO film is nearly white, possibly contributing to the YAG emission and the band-to-deep level transition in the ZnO film.
Solid state lighting technology is the next generation light source, owing to its potential luminescence efficiency. GaN-based white light-emitting diode (LED) is the most widely used solid state light source because of its long lifetime, high energy efficiency, small size, ability to produce color light directly without filtering, and integration with other semiconductor electronic elements [1–6]. However, prohibitively high manufacturing costs, unstable production in run-to-run and wafer-to-wafer, inadequate light uniformity, and inferior color rendering index represent major obstacles to GaN-based LED as a main light source in the future. The first two obstacles are attributed to the expensive growth technology of metal-organic chemical vapor deposition and the Ga-N bonding mechanism. The latter two obstacles are caused by a mixture of blue and yellow lights. White LED consists of a blue LED chip and a yellow phosphor coating. A non-conformal phosphor coating causes thickness of phosphor coating layer variation and inadequate color uniformity.
Therefore, this study describes a novel white light device for light source. ZnO is commonly used as a material for optical device applications in the UV range owing to its wide direct bandgap (3.37 eV) [7–10]. An n-ZnO/p-GaN heterostructure LED was reported . However, that is a UV range structure, and no any optical characteristics were demonstrated. In this work, an yttrium aluminum garnet (YAG) phosphor-incorporated ZnO film is deposited by ultrasonic spray pyrolysis. The optoelectronic characteristics are also studied. Additionally, the crystallinity of YAG phosphor-incorporated into the ZnO films is studied using X-ray diffraction (XRD) analysis.
YAG phosphor-incorporated ZnO film was deposited by ultrasonic spray pyrolysis on slide glass substrates at atmospheric pressure in nitrogen (N2) gas, at a flow rate of 100 sccm for 30 min. The YAG phosphor-incorporated ZnO film (YAG phosphor at 0, 0.5, 1.5, and 2.5 wt.%; NYAG4156 phosphor, INTEMATIX, Fremont, CA, USA) was produced by spraying aqueous solutions. The spraying aqueous solution preparation is as follows: (1) a solution consisted of Zn(CH3COO)2·2H2O (0.2 M) and CH3COONH4·2H2O (0.2 M) with 1:3 proportional ratio which were used as sources of ZnO, and (2) the NYAG4156 phosphor powder was added into the ZnO source solution to form the spraying aqueous solution, where the NYAG4156 powder is suspendible. A slide glass was used as the substrate, which was etched with HCl for 5 min before deposition. An aerosol of the precursor solution was then generated using a commercial ultrasonic nebulizer. Next, the morphology of the film was studied by field-emission scanning electron microscope (FESEM). The resistivity and the mobility of the film were studied by Hall measurement. The crystallinity was investigated by XRD using a rotating anode Rigaku X-ray diffractometer (Mac Science Corporation, Yokohama, Japan) with Cu-Kα1 radiation at a wavelength of 1.54 Å, where the radiation was generated at 45 kV and 40 mA. Notably, the film had a polycrystalline structure. Additionally, photoluminescence (PL) was measured at room temperature (RT). The excitation source for photoluminescence was a frequency-quadrupled Nd:YAG laser, which emitted 266 nm, 6 ns pulses at a 5-Hz repetition rate.
Results and discussion
In summary, an YAG phosphor-incorporated ZnO film is deposited on a slide glass substrate by ultrasonic spray pyrolysis. A nanoflower consisting of a hexagonal nanopetal is formed on the surface of the samples; in addition, the sizes of the nanopetal are approximately 200 to 700 nm. The origin of the hexagonal nanoflower may contribute to the three-dimensional growth becoming two-dimensional, owing to the reduction of the source and aggregation of the residue precursor as the growth process is terminated. Consequently, the hexagonal nanopetal is formed on the surface of the sample. This study also examines the PL spectra of the samples. As the incorporated amount of the YAG is 1.5 and 2.5 wt.%, the photoluminescence color of the YAG-incorporated ZnO films is nearly white. This color may contribute to the wide emission band ranging from 420 to 650 nm, as caused by the YAG emission and the band-to-deep level transition in the ZnO film.
Financial support of this work was provided by the National Science Council of the Republic of China under contract number NSC 101-2221-E-027-054.
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