Blue light emission from the heterostructured ZnO/InGaN/GaN
© Wang et al.; licensee Springer. 2013
Received: 7 November 2012
Accepted: 19 December 2012
Published: 22 February 2013
ZnO/InGaN/GaN heterostructured light-emitting diodes (LEDs) were fabricated by molecular beam epitaxy and atomic layer deposition. InGaN films consisted of an Mg-doped InGaN layer, an undoped InGaN layer, and a Si-doped InGaN layer. Current-voltage characteristic of the heterojunction indicated a diode-like rectification behavior. The electroluminescence spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, the heterostructured LEDs had potential application in white LEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias.
Nowadays, white light-emitting diodes (WLEDs) have attracted significant interest for solid-state illumination due to their low power consumption, long operating time, and environmental benefits [1–3]. Hence, WLEDs are the most promising alternatives to replace conventional light sources, such as backlighting, interior lamps, and general lightings . Currently, the prevailing method is to use a blue LED coated with a yellow-emitting phosphor. However, during a long period of optical pumping, the degradation of the phosphor would decline the output efficiency of the WLEDs. Another way to obtain white light is to mix the emissions from different light sources . In particular, InGaN with a continuously variable bandgap from 0.7 to 3.4 eV has attracted considerable interest, and thus, InGaN/GaN WLEDs are regarded as the most promising solid-state lighting device which can work in the whole visible and part of the near UV spectral regions . Some groups have fabricated dichromatic InGaN-based WLEDs . However, compared with WLEDs with a mixture of blue, green and red emissions, they had lower color rendering index.
With a direct wide bandgap of 3.37 eV and high exciton binding energy of 60 meV, ZnO is considered as one of the best electroluminescent materials. However, herein lays an obstacle of ZnO homojunction diodes, which is p-type; it is a problem in obtaining high-quality and stable p-ZnO films. Although some p-n homojunction ZnO LEDs have been reported, their electroluminescence (EL) intensities were very weak [8–10]. As an alternative approach, heterostructured LEDs have been fabricated on top of a variety of p-type substrates, such as SrCu2O2, Si , and GaN . With the advantages of InGaN and ZnO, it is significant to fabricate ZnO/InGaN/GaN heterojunctions with blue, green, and red emissions to obtain white light.
In this work, we report the fabrication of ZnO/InGaN/GaN heterostructured LEDs. The EL spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, heterostructured LEDs have potential application in WLEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias.
Results and discussion
The photoluminescence (PL, HORIBA LabRAM HR800, HORIBA Jobin Yvon S.A.S., Longjumeau, Cedex, France) measurements were conducted at room temperature in the wavelength range of 350 to 700 nm to analyze the optical properties of n-ZnO films, InGaN films, and p-GaN substrates. In order to assess the performance of the heterostructured LEDs, current-voltage (I-V) and EL measurements were carried out at room temperature. The rectifying behavior with a turn-on voltage of about 2 V is observed in the I-V curve (Figure 1).
In conclusion, we have fabricated heterostructured ZnO/InGaN/GaN LEDs. The EL spectra under forward biases show a blue emission accompanied by a broad peak centered at 600 nm. The peak at 600 nm was deemed to be the combination of the emissions from Si-doped InGaN at 560 nm and Mg-doped InGaN at 610 nm. Counted with the CIE chromaticity diagram, white light can be observed in theory through the adjustment of the emission intensity ratio. Furthermore, a UV emission and an emission peak centered at 560 nm were observed under reverse bias. This work provides a simple way using the emission from ZnO, Mg-doped InGaN, Si-doped InGaN, and p-GaN to obtain white light in theory. With the appropriate emission intensity ratio, ZnO/InGaN/GaN heterostructured LEDs have potential application in WLEDs.
This work is supported by the National Natural Science Foundation of China (NSFC) under grant numbers 10904116, 11074192, 11175135, and J0830310, and by the foundation from CETC number 46 Research Institute. The authors would like to thank HH Huang and BR Li for their technical support.
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