Sputtered ZnO–SiO2 nanocomposite light-emitting diodes (LEDs) were treated using a flat-top nanosecond laser (FTNL) under room temperature. The intensity of the 376 nm electroluminescence (EL) emission of ZnO–SiO2 nanocomposite LEDs at a current of 9 mA with FTNL treatment was approximately 1.4 times greater than LEDs without FTNL treatment. Furthermore, the FTNL-treated LEDs indicated a narrower full width at half maximum of the 376 nm EL emission than those of LEDs without FTNL treatment. Thus, FTNL treatment of ZnO–SiO2 nanocomposite LEDs could induce the recrystallization of distributed ZnO nanoclusters and reduce the defects in ZnO–SiO2 nanocomposite layers.
©2012 Optical Society of America
ZnO and related alloys have recently gained attention as promising materials for visible and ultraviolet light emissions. ZnO has a wide bandgap of 3.37 eV, a large free exciton-binding energy of 60 meV, and the likelihood of efficient excitonic optical transitions at elevated temperatures [1,2]. ZnO-based materials c as simple processing because of the compatibility of such materials with wet chemical etching, relatively low material costs, and long-term stability, among others. In terms of the improvement of the emission efficiency of ZnO-based light-emitting devices (LEDs), ZnO nanoparticles are advantageous because of their three-dimensional quantum confinement, which strongly enhances the excitation radiative recombination. Recently, nanoscale or submicron-sized ZnO materials have also been synthesized through various methods [3–11], such as sol–gel coating, sputtering technique, and atomic layer deposition (ALD). Ma et al.  successfully prepared ZnO nanoparticles using reactive magnetron sputtering and the diffusion furnace method. Chen et al.  and Shih et al.  reported that ZnO was deposited in the small voids between SiO2 nanoparticles using ALD and spin-coating methods. However, the electroluminance characteristics of ZnO nanoparticles in previous studies have not been reported. Recently, Chen et al.  studied the electroluminance characteristics of a ZnO cluster of co-sputtered ZnO–SiO2 nanocomposite p-i-n LEDs. However, the crystal quality of the room temperature (RT)-deposited ZnO–SiO2 nanocomposite p-i-n LEDs was not sufficient for the high-efficiency electroluminance. Therefore, a process after the RT co-sputtering, such as thermal annealing, might be necessary to improve the optoelectrical characteristics of ZnO–SiO2 nanocomposite p-i-n LEDs.
Compared with conventional thermal furnace, a laser annealing treatment has lower thermal budget, resulting in lesser opportunities for ZnO dots to merge. Many applications in laser manufacturing, such as semiconductor lithography, laser annealing treatment, micromachining, microstructuring, and material analysis, require a homogeneous intensity distribution of the laser beam instead of the traditional Gaussian distribution during fabrication processes [16–18]. Therefore, a laser beam with a flat-top scheme was used to offer high precision control of temperature uniformity in a defined region or effective thickness. In the current study, a flat-top nanosecond laser (FTNL) method was used on the ZnO–SiO2 nanocomposite film without affecting the quality of the p-GaN substrate. The post-FTNL treatment for improving the film quality of the ZnO–SiO2 nanocomposite layer is also demonstrated. An advantage of the square flat-top laser-beam generation scheme using a beam-shaping element is its potential for use in the design of high-power LED applications. Furthermore, ZnO–SiO2 nanocomposite and ZnO films were deposited on p-GaN substrate as the active layer of the UV-heterostructured ZnO-based LEDs. The optical and electrical characteristics of the fabricated heterostructure ZnO-based LEDs with ZnO–SiO2 nanocomposite and ZnO active layers with and without FTNL treatment were also discussed.
The ZnO–SiO2 nanocomposite thin films were deposited on a p-GaN/Al2O3 using an RF magnetron and a DC co-sputtering system. SiO2 and ZnO disks on the separated RF and DC sputtering guns were used as sputtering targets for the Si, Zn, and O elements, respectively. The detailed sputtering conditions of the ZnO–SiO2 nanocomposite and the Ga:ZnO were described in a previous publication . After the sputtering deposition of a 100 nm-thick ZnO–SiO2 nanocomposite layer on a p-GaN layer, the samples were post-treated with FTNL. The schematic of the FTNL system is shown in Fig. 1 . The laser source of the FTNL was a 355 nm nanosecond-pulse Nd:YAG laser (repetition rate of 40 kHz and laser fluence of 60 mJ/cm2). The circular Gaussian beam was transformed into a square flat-top intensity using an optical beam shaper. The square flat-top laser beam size of the FTNL is 50 × 50 μm2. The laser beam shot on the sample was moved along the x- and y-directions at 40 μm intervals. After the FTNL treatment, a 120 nm-thick Ga:ZnO was deposited on the ZnO–SiO2 nanocomposite. Photolithography and buffer oxide etching solution were subsequently used to partially etch out the Ga:ZnO/ZnO–SiO2 nanocomposite and the Ga:ZnO/ZnO until the p-GaN layer was exposed. Ni/Au (50 nm/200 nm) was deposited on a p-GaN substrate through evaporation to form an Ohmic contact with the LED p-electrodes. Cr/Au (50 nm/200 nm) was deposited on a Ga:ZnO through evaporation to form an Ohmic contact with the LED n-electrodes. The size of the fabricated Ga:ZnO/ZnO–SiO2 nanocomposite/p-GaN (LED I) and FTNL-treated Ga:ZnO/ZnO–SiO2 nanocomposite/p-GaN (LED II) was kept at 300 × 300 μm2. A semiconductor parameter analyzer (HP 4156) was then used to measure the current–voltage (I–V) characteristics of all fabricated LEDs. The electroluminescence (EL) spectra of all fabricated LEDs were also measured at RT. The microstructure of ZnO-SiO2 nanocomposite was examined using high-resolution transmission electron microscopy (HRTEM). The non-resonant and resonant Raman scattering spectra of the studied structures were recorded in the backscattering geometry under excitation through radiation from Ar+ ion and He–Cd lasers at wavelengths of 514.5 and 325 nm, respectively, and through Jobin–Yvon T64000 microspectrometer equipped with a liquid nitrogen-cooled charge-coupled device(CCD) detector at RT.
3. Results and discussions
Figure 2 indicates the I–V curves of the fabricated LEDs I and II. All samples exhibited a rectifying, diode-like behavior. The results reveal that on the one hand, the forward turn-on voltages of LEDs I and II were 4.16 and 4.02 V (at 50 μA), respectively. On the other hand, the reverse breakdown voltages of LEDs I and II were 13.6 and more than 20 V (at −50 μA), respectively. LEDs I and II indicated almost the same turn-on forward voltage. However, LED II showed a larger reverse breakdown voltage than LED I. FTNL treatment would effectively anneal the Ga:ZnO/ZnO–SiO2 nanocomposite/p-GaN layers. The annealed Ga:ZnO/ZnO–SiO2 nanocomposite induced the recrystallization of the Ga:ZnO and ZnO nanoclusters of the ZnO–SiO2 nanocomposite and improved the film quality of the Ga:ZnO and ZnO–SiO2 nanocomposites. The improved quality of the Ga:ZnO/ZnO–SiO2 nanocomposite films reduced the reverse leakage of the LED II.
Figures 3(a) and 3(b) show the 1 mA EL spectra of LEDs I and II, respectively, indicating the same main peak wavelength of 427 nm. The main 1-mA EL emission peaks with a full width at half maximum (FWHM) of about 60 nm for both LEDs I and II were attributed to the emission from the Mg acceptor levels in the p-GaN layer . The intensities of the 376 nm EL emission peaks under the 1 mA driving current of LEDs I and II were almost the same. Moreover, both LEDs I and II indicated a short-wavelength EL emission of 376 nm with intensities less than 427 nm. The short-wavelength 376 nm EL emission of both LEDs I and II could be attributed to the ZnO clusters inside the co-sputtered i-ZnO–SiO2 nanocomposite layer. However, the 1 mA EL spectrum of LED I also showed the third EL emission peak of 540 nm that was not found in the EL spectrum of LED II. The broad green–yellow emission of LED I might be attributed to the defect-related emission of the ZnO nanocluster in the as-deposited i-ZnO–SiO2 nanocomposite layer. The defects of the as-deposited i-ZnO–SiO2 nanocomposite layer were reduced using the FTNL treatment process, which also suppressed the defect-related emission of LED II.
The intensities of the 376 nm emissions of both LEDs I and II were greater than the 427 nm emissions with a driving current of 9 mA, as shown in Fig. 4 . The main peak EL emissions of LEDs I and II decreased from 427 nm to 376 nm with a higher driving current. This phenomenon could be attributed to the strong electron–hole recombination in the cluster size of the ZnO in the i-ZnO–SiO2 nanocomposite layer under high injection currents. Moreover, the intensity of the 376 nm EL emission with a current of 9 mA for LED II was higher than that of LED I. The 18 nm FWHM of the 376 nm EL emission of LED II was smaller than the 21 nm FWHM of LED I. The enhanced 376 nm EL intensity and FWHM might be attributed to the reduction of the defects of the ZnO nanocluster in the i-ZnO–SiO2 nanocomposite layer using the FTNL treatment. The intensities of the p-GaN emission of 427 nm of LEDs I and II were almost the same with a 9 mA driving current. This result could be attributed to the impurity-limited recombination and low carrier confinement in the p-GaN layer.
Both the 376 and 427 nm emission intensities of LEDs I and II increased with increasing driving current. Therefore, the intensity changes in LEDs I and II with driving currents of 376 and 427 nm EL emissions were observed by taking the intensity ratios of the 376 and 427 nm EL emissions, as indicated in the inset of Fig. 4. The intensity ratios of LEDs I and II were larger than 1 when the driving current was more than 5 mA. Moreover, the intensity ratio of LED II was higher than that of LED I when the current was more than 5 mA. The differences in the intensity ratios of LEDs I and II continued to increase as the current increased.
To further understand the effect of FTNL treatment on ZnO-SiO2 nanocomposite, the HRTEM and Raman spectra test were performed on the ZnO-SiO2 nanocomposite layer with and without the FTNL treatment. Figure 5 shows the non-resonant Raman spectra of the ZnO-SiO2 nanocomposite films with and without FTNL treatment. The peaks located at 325, 435, and 584 cm−1 can be observed for both ZnO-SiO2 nanocomposite samples. The Raman spectrum of the as-grown ZnO film, which is not shown in the figure, prominently peaks at 330, 439, and 584 cm−1, which correspond to the multiphonon, E2(high), and E1(LO) modes, respectively. Therefore, the 435 cm−1 E2(high) peak of ZnO-SiO2 nanocomposite is about 4 cm−1 shorter than the 439 cm−1 E2(high) peak of the as-grown ZnO film. This phenomenon may be due to the optical phonon confinement by nanostructures [20,21], and is usually identified as the hexagonal phase of ZnO [21,22]. The HRTEM picture of the as-grown ZnO-SiO2 nanocomposite layer, as shown in the inset of Fig. 5, indicates the ZnO nanoclusters and E2(high) peak wavenumber shift of the Raman measurement. Moreover, Cheng et al.  presented the size dependency of the intensity ratio between the second- and first-order LO Raman scatterings (resonant Raman scattering). In the present study, the ratio between the second- and first-order Raman scattering cross sections of the samples with and without FTNL treatment was found to be 0.48 and 0.45, respectively. Therefore, the fitting ZnO crystallite sizes are smaller than 10 nm. This result is in agreement with the dot size derived from the HRTEM images.
Meanwhile, the E2(high) peak of the Raman spectrum of the ZnO-SiO2 nanocomposite with FTNL treatment indicates stronger intensity and narrower FWHM than the as-grown ZnO-SiO2 nanocomposite. The intensity ratio of the E1(LO) peak to the E2(high) peak was about 0.08 and 0.36 with and without FTNL treatment, respectively. The 584 cm−1 Raman peak was mostly ascribed to the LO phonon of A1 or E1 symmetry or the defect-induced mode, such as oxygen vacancies (Vo), zinc interstitials, or surface hydroxide (OH−) [24–26]. The green–yellow emission comes from Vo as the previous reports indicate [27–29], and the results also agree with the previous discussion on Raman spectra. Furthermore, Yang et al.  proposed that the surface-trapping center is OH− because green–yellow emission intensities show a direct correlation with surface OH− concentrations. The decreases of LO Raman scatterings and green–yellow emission intensities are accompanied with a UV peak shift, which suggests that they are related to surface OH−. The enhanced intensity and FWHM of the E2(high) peak and intensity ratio of the E2(high) peak to E1(LO) peak would imply the improvement of crystallization or the increase of ZnO nanocluster density of the FTNL-treated ZnO-SiO2 nanocomposite. Besides, the HRTEM image of the FTNL-treated ZnO-SiO2 nanocomposite also indicates the larger ZnO nanocluster density of 2.2 × 1010 cm−2 than the as-grown ZnO-SiO2 nanocomposite with ZnO nanocluster density of 1.4 × 1010 cm−2. Therefore, we deduce that the increase of EL emission of the FTNL-treated ZnO-SiO2 nanocomposite comes from the increase of embedded ZnO nanocluster density and improved film quality of ZnO-SiO2 nanocomposite.
The advantage of the FTNL treatment was the annealing of the i-ZnO–SiO2 nanocomposite layer under RT. A Gaussian beam laser was transferred to the flat-top beam through a beam shaper. A uniform annealing beam shot was achieved. Therefore, the laser shot annealing uniformity was improved. Moreover, the FTNL treatment was able to heat the samples near the surface rather than all the samples, as shown in Fig. 1. The treatment allowed the annealing of the samples under low temperature such as at RT.
In summary, the current study demonstrated the FTNL treatment of the ZnO–SiO2 nanocomposite LEDs under RT. The defects related to the green–yellow emission of LED I with a low driving current was suppressed after the FTNL treatment. Moreover, the intensity of the 376 nm EL emission of the ZnO–SiO2 nanocomposite LEDs with a current of 9 mA and FTNL treatment was higher than that of the LED without FTNL treatment. The 18 nm FWHM of the 376 nm EL emission of LED II was smaller than that of the 21 nm of LED I. Thus, the FTNL treatment on the ZnO–SiO2 nanocomposite LEDs recrystallizes the distributed ZnO nanocluster and reduces the defects in the ZnO–SiO2 nanocomposite layer. It also enhances the EL emission efficiency of the ZnO nanocluster in the ZnO–SiO2 nanocomposite layer.
The authors would like to acknowledge the financial support from the National Science Council (Grant Number NSC 97-2221-E-006-242-MY3). The present work was also supported in part by the Center for Frontier Materials and Micro/Nano Science and Technology and by the Advanced Optoelectronic Technology Center of the National Cheng Kung University under the projects supervised by the Ministry of Education.
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