Ultraviolet/blue light-emitting diodes based on single horizontal ZnO microrod/GaN heterojunction
© Du et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 15 August 2014
Published: 28 August 2014
We report electroluminescence (EL) from single horizontal ZnO microrod (MR) and p-GaN heterojunction light-emitting diodes under forward and reverse bias. EL spectra were composed of two blue emissions centered at 431 and 490 nm under forward biases, but were dominated by a ultraviolet (UV) emission located at 380 nm from n-ZnO MR under high reverse biases. Light-output-current characteristic of the UV emission reveals that the rate of radiative recombination is faster than that of the nonradiative recombination. Highly efficient ZnO excitonic recombination at reverse bias is caused by electrons tunneling from deep-level states near the n-ZnO/p-GaN interface to the conduction band in n-ZnO.
ZnO is one of the most potentially useful materials for near-ultraviolet photonic devices such as light-emitting diodes (LEDs) due to its direct wide bandgap energy of 3.37 eV and large exciton binding energy of 60 meV at room temperature (RT) [1–3]. Although ZnO p-n junction LEDs with low luminescence efficiency have recently been reported,  ZnO-based LEDs still suffer from difficulty in producing reliable and high-quality p-type doping materials [5–7]. Therefore, the n-ZnO and p-GaN heterojuction devices is suggested as an alternative approach due to their similar lattice structure (wurtzite) and electronic properties [8, 9]. Micro/nanostructure LEDs with good crystalline quality and superb waveguide properties are expected to provide an effective route for improving internal quantum efficiency as well as extraction efficiency . To date, various one-dimensional heterojuction micro/nanodevices have been fabricated . Among these structures, the heterojunction LEDs use vertically aligned one-dimensional ZnO structures such as microrods (MRs) and nanorods (NRs) which exhibit better electroluminescence (EL) performance than ZnO film LEDs because the carrier injection efficiency can be enhanced and structural defects are decreased in these micro/nanostructures [12–19]. Few studies have been reported concerning the EL from horizontal ZnO MRs/NRs [10, 20–22]. The UV electroluminescence centered around 390 nm in wavelength based on the single ZnO MR/p-GaN  and multiple ZnO MRs/p-GaN  heterojunction were realized under the forward injection current. In particular, the UV whispering-gallery-mode lasing in an individual ZnO MR-based diode has been demonstrated . A saturated blue emission around 460 nm caused by the interfacial radiative recombination in single ZnO MR/p-GaN at high forward bias was examined . Although those groups have produced the horizontal ZnO MR-based LEDs, a detailed investigation on the origins of the recombination processes is urgently needed for lighting applications. Here, we report one-dimensional hexagonal ZnO MR-based LEDs by simply transferring an individual ZnO MR onto p-type GaN thin film. Two obvious emission bands centered at 431 and 490 nm were obtained under both forward and reverse bias. The EL spectra were dominated by an intense UV emission band under higher reverse bias by reason of the tunneling electrons from GaN assisted by the deep-level states near the n-ZnO/p-GaN interface to the conduction band in n-ZnO. The origins of the distinct electron–hole recombination processes are discussed. Furthermore, the output light-current characteristic was determined to evaluate the high-efficiency electroluminescence performance of the diode.
The ZnO MRs were grown on Si (100) substrates by a high-temperature thermal evaporation process. A mixture of ZnO and graphite powders (1:1 in weight ratio) was loaded in an alumina boat serving as the source material. The boat was centered inside a 2.5-cm quartz tube in a tube furnace. A clean Si substrate was placed on top of the Al2O3 boat to collect samples. The furnace was heated to 1,050°C at a rate of 20°C/min and kept at that temperature for 60 min. After the furnace had naturally cooled down to room temperature, the ZnO MRs were deposited on the Si substrate. To construct the LED, a p-type GaN layer was grown on a (0001) sapphire substrate with hole concentration and mobility of 1017 cm−3 and 10 cm2/V-s, respectively, was used as the hole injection layer. A thin layer of PMMA was partly coated on the p-type GaN film to serve as an insulating layer. After the substrate was heated at 50°C for 20 min to improve the quality of the PMMA, a single ZnO MR was transferred to the prepared p-GaN substrate and crossed the boundary with the p-GaN and PMMA. Finally, the ZnO MR was fixed by Ag paste which served as the cathode, while another Ag electrode on the GaN film worked as the anode. The sample morphology was examined with a high-resolution Zeiss FEG scanning electron microscope (SUPRA 55, Carl Zeiss, Oberkochen, Germany). The polarized micro-Raman spectra of the individual ZnO MR were measured using a Horiba Jobin-Yvon iHR320 spectrometer (Horiba, Kyoto, Japan) in a backscattering configuration. The 532-nm line of a frequency-doubled Nd:YAG laser with 4.2-mW power was used for off-resonance excitation. The I-V measurements were carried out with a Keithley 2400 source meter (Cleveland, OH, USA). Micro-photoluminescence (μ-PL) and EL measurements were conducted by the above spectrometer. The optical source was provided by a 0.3-mW He-Cd laser with the wavelength of 325 nm. All measurements were performed at room temperature.
Results and discussion
More importantly, the excitonic emission of ZnO MR dramatically increases and becomes a distinct peak as the applied reversed biases increase as shown in Figure 3b. The EL spectra are dominated by the p-GaN emission under forward biases, whereas they are dominated by the n-ZnO emission under reverse biases. The inset in Figure 3a,b shows the EL image of the LED under the biases in a dark room, emitting bright blue and white light, respectively. Note that they are visible to the naked eye. The mechanism of carrier recombination of EL can be interpreted by the energy band diagram as shown in Figure 3c. Figure 3d displays the intensity of the three emission peaks as a function of the reverse bias. Under low reverse bias current, due to the lower mobility in the p-GaN, all of the radiative recombination mainly occurs in the p-GaN and interfacial layer. When the reverse bias current increases, the radiative recombination occurs in three places - the p-GaN, interfacial layer, and ZnO MR. Until the applied current exceeds a certain value, the carrier recombination in the p-GaN no longer increases because of the limited hole concentration in the p-GaN thin film. Finally, the excitonic emission of ZnO MR dramatically increases and becomes a distinct peak as the applied reversed bias current increases. The three peak intensities of the ZnO emission under reverse bias are depicted as a function of injection current in a log-log scale. Using the formula Iem ~ Im, where Iem is the peak intensity, I is the injection current, m is an index, the dependence curve can be fitted, and the fitting results reveal that the device shows a superlinear relationship with m = 2. This implies that, compared to the reported heterojunction device , the effect of defect-related nonradiative recombination is negligible and almost every injected carrier leads to the emission of a photon under reverse bias. In contrast, the emissions from GaN and interfacial recombination both show superlinear dependence under low current injection; however, the luminescence peak intensities increase sublinearly at higher injected currents (I > 7 mA). This indicates that nonradiative recombination is responsible for the output saturation.
In summary, we have obtained UV and blue dual-color LED based on single ZnO MR and p-GaN heterojunction under forward and reverse biases, respectively. The origin of the EL emission was confirmed by comparing the EL and PL spectra. For the excitonic ZnO emission, the rate of radiative recombination is faster than that of the nonradiative recombination under reverse bias. The tunneling electrons assisted by the deep-level states near the n-ZnO/p-GaN interface to the conduction band in n-ZnO result in the efficient ZnO excitonic luminescence under reverse bias. This stable UV/violet EL device should have potential applications in many areas, including multicolor lighting, displays, and lighting decoration.
This research is financially supported by the National Science Council of Taiwan under grants NSC-102-2112-M-006-012-MY3 and the Aim for the Top University Project of the Ministry of Education.
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