Center for Nanoscale Characterization & Devices, Photonics for Energy Division
In its long evolution process, the human being has gotten with the white sunlight mixed by various visible lights and white lighting has been becoming very important. Today, white lighting has entered the 3rd era-white LED era after it passed through the 2nd era-the era of fluorescent lamp and began with the 1st era- era of incandescent lamp. The advantages of white LED, such as high efficiency(ten times more than incandescent lamp), environmental friendliness, wide range of application and long lifetime(fifty times of the incandescent lamp) promotes its use throughout the world.
The mixture of LEDs with three primary colors (red, green & blue) based on III-V semiconductor can produce white light with small energy loss, high luminous efficiency, and controllable and improvable color rendering index. However, the chips in a mixed white LED device have different luminous efficiency, driving voltage and temperature characteristics, leading to the difficulties in electronic circuit design and coordinated control. Besides, the light-emitting technology of a variety of chips has a high cost and is not conducive to commercial. Another white emitting strategy is very popular: On the basis of the chips of the blue and/or near UV LED, package of the chips by using yellow phosphor or three colors (red, green, blue) of phosphors can produce white light with generally high color rendering index and adjustable color temperature. However, this strategy is too dependent on the regulation of phosphors and limits the efficiency of white light emission.
The white LED technology based on ZnO can be a resolution of the above problems. As a wide and direct bandgap n-type semiconductor material, ZnO has stable excitons under room temperature (300 K, 26 meV) due to its high exciton binding energy (60 meV), which makes it a new generation semiconductor luminescence material. However, the p-type ZnO is difficult to be achieved due to the serious compensation effect of ZnO and increased Madelung Energy. The realization of p-type doping ZnO is still an international challenge, also a focus spot over all the world.
Recent years, Professor Yihua Gao’s group in Wuhan National Laboratory for Optoelectronics has being devoted themselves to the optoelectronic devices based on one-dimensional ZnO nanomaterials. Through a simple chemical vapor deposition method, they achieved one-step white LED based on high quality antimony (Sb) doped p-ZnO nanowire arrays/n-GaN thin film. By using X-ray photoelectron spectroscopy (XPS) and element mapping analysis, it is found that Sb is doped into ZnO arrays successfully. Room temperature PL spectra shows the change of band edge level before and after Sb-doping. Low temperature PL (4.65 K) spectra analysis exhibited that Sb doped ZnO can form stable acceptor level and deep levels, and further IV test showed that the p-type ZnO arrays is obtained.
Based on the obtained p-ZnO nanoarrays/n-GaN film structure, white LED was realized. The LED has a stable color temperature around 3500 K under 8.0-15.5 V. Compared with general white LED, the realization method of white LED is simple and easy to operate, and the warm white light is suitable for home lighting. This work is an important step for ZnO band engineering and optoelectronic device research, and provides a feasible strategy and material platform for white LED. Recently, this work was published in Adv. Funct. Mater. (IF:10.439, 25, 2182-2188 (2015)).
Figure 1. Characterization of the warm white LEDs device. a) I – V characteristics of the p-n heterojunction LED at RT. b) Voltage-dependent EL spectra of the device. c) The EL intensity of three obvious EL peaks with voltage. d) Schematic diagram of p-ZnO dominant energy level emission and interface recombination with GaN. e) Gaussian fi tting curves of EL spectra at 14 V. f) Photo galleries of room temperature EL emission from p-ZnO/n-GaN LEDs at the forward bias from 4 to 14 V. g) CIE chromaticity diagram of warm white LED with EL spectra at 14 V. h) Voltage-dependent color temperature changes from p-ZnO/n-GaN LEDs.
This work was finished in the Center for Nanoscale Characterization & Devices (CNCD) supported by WNLO and Prof. Zhong-Lin Wang. This work received the help of Dr. Junbo Han (Wuhan National High Magnetic Field Center), Dr. Jun Su, Dr. Luying Li and Dr. Nishuang Liu. This work was supported by the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11374110, 11074082, 11204093, 51371085, 11304106).
This work is the development of the research work of Prof. Yihua Gao’s group published in Vol. 8, 429-435 (2014) of Laser & Photonics Rev. (IF: 9.313, Top 3 in optics rank). In this work, Prof. Yihua Gao’s group demonstrated a wavelength-tunable LEDs of GaxZn1-xO nanowire arrays by a simple modified chemical vapor deposition heteroepitaxial growth on p-GaN substrate. The electroluminescence (EL) emission of GaxZn1-xO nanowire arrays LED continuously shifts with a wider range (～100 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The work widens the redshift of 40 nm of others work and made a big progress for bandgap engineering of ZnO and the development of LED by gallium doping.
Figure 2 (I) SEM images of grown GaxZn1-xO NW arrays with different Ga content, (a) pure ZnO, x = 0, (b) x =0.04, (c) x = 0.13, (d) x = 0.28, (e) x = 0.44, and (f) x = 0.66. Inset is the corresponding high-resolution SEM images, scale bars of the insets: 500 nm. (II) TEM characterization of heavily doped GZO NW (sample F), (a) HRTEM image of a GZO NW, (b) HAADF image of a GZO NW obtained from STEM, inset showing the EDS spectra of the GZO NW. (III) is the schematic of fabricated LED device. (IV) The EL emission of the n-GZO/p-GaN LEDs. (a) Normalized EL spectra recorded from the n-GZO/p-GaN LEDs under a forward bias voltage of 20 V, showing the emission peaks shift from UV to visible with increasing of Ga composition in GZO NWs. (b) The Gaussian curve fitting of the EL spectra of sample F. (c) Energy-band diagram of the n-GZO/p-GaN heterojunction under forward bias. (d) Photo galleries of room-temperature EL emission from n-GZO/p-GaN LEDs at a forward bias of 20 V.
 X. L. Ren, X. H. Zhang, N. S. Liu, L. Wen, L. W. Ding, Z. W. Ma, J. Su, L. Y. Li, J. B. Han & Y. H. Gao*. White Light-Emitting Diode From Sb-Doped p-ZnO Nanowire Arrays/n-GaN Film. Adv. Funct. Mater. 25, 2182-2188 (2015). Web address: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201404316/pdf X. H. Zhang, L. Y. Li, J. Su, Y. M. Wang, Y. L. Shi, X. L. Ren, N. S. Liu, A. Q. Zhang, J. Zhou & Y. H. Gao*. Bandgap Engineering of GaxZn1–xO Nanowire Arrays for Wavelength-tunable Light-emitting Diodes. Laser Photonics Rev. 8, 429-435 (2014). Web address: http://onlinelibrary.wiley.com/doi/10.1002/lpor.201300172/full； http://www.wnlo.cn/article.php?catPath=0,1,12,479,481&catID=482&articleID=4218