We have demonstrated the electroluminescence (EL) of Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN n-i-p heterostructure light-emitting devices (LEDs). ZnO nano-clusters with sizes distributing from 2 to 7nm were found inside the co-sputtered i-ZnO-SiO2 nanocomposite layer under the observation of high-resolution transparent electron microscope. A clear UV EL at 376 nm from i-ZnO-SiO2 nanocomposite in these p-i-n heterostructure LEDs was observed under the forward current of 9 mA. The EL emission peak at 376 and 427nm of the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN n-i-p heterostructure LEDs were attributed to the radiative recombination from the ZnO clusters and the Mg acceptor levels in the p-GaN layer, respectively.
© 2011 OSA
ZnO has a direct bandgap of 3.37 eV at room temperature, a high free exciton-binding energy of 60 meV, and the likelihood of efficient excitonic optical transitions at elevated temperatures [1,2]. ZnO also offers several advantages: simple processing due to its compatibility with wet chemical etching, relatively low material costs, and long-term stability, among others. The natural n-type characteristic of ZnO from oxygen vacancies and Zn interstitials  is a great barrier to getting p-type ZnO. However, several reports have demonstrated advanced ZnO-based light emitting devices (LEDs) [4,5], causing ZnO to gain more attention for its applications in short-wavelength LEDs and laser diodes suitable for high-temperature operations. Up to now, although the homojunction or p-i-n structure of ZnO diodes are still being reported [4,5], difficulties in realizing reliable p-type ZnO [6,7] have driven most ZnO-based LEDs to develop on heterostructures by growing n-ZnO on p-type semiconductors, such as GaN, Si, AlGaN, and p-SrCu2O2 [8–11]. In general, the p-type GaN (p-GaN) is a good choice for the ZnO heterostructure LEDs because of the wurtzite structure of ZnO . However, not just the ultraviolet emission alone, several emissions including blue, orange and green bands because the presence of the zinc interstitials, oxygen vacancies or oxygen interstitials, are also observed from these homo or heterojunction devices [13,14]. This phenomenon is ascribed to the radiative recombination path involving the deep level states in the ZnO thin film device. Therefore, pure strongly ultraviolet emission compared to other emissions from ZnO light-emitting diodes is hard to achieve.
Besides, for improving the emission efficiency of the ZnO based LEDs, ZnO nanoparticles are of great interest 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 [15–23], such as sol–gel coating, sputtering technique, atomic layer deposition etc. For instance, Ma et al.  have successfully prepared ZnO nanoparticles by reactive magnetron sputtering and diffusion furnace method. Chen et al.  and Shih et al.  have reported that the ZnO was deposited in the small voids between SiO2 nanoparticles using atomic layer deposition (ALD) and spin coating methods. In this study, a simple and efficient co-sputtering method was proposed to fabricate ZnO nanoclusters in silica-based nanocomposites. ZnO-SiO2 nanocomposite and ZnO films were deposited on p-GaN as the active layer of the ultraviolet heterostructure ZnO-based LEDs. Furthermore, the optical and electrical characteristics of the fabricated heterostructure ZnO-based LEDs with ZnO-SiO2 nanocomposite and ZnO active layer are discussed as well.
The ZnO-SiO2 nanocomposite and ZnO thin films were deposited on p-GaN substrate by RF magnetron and 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 sputtering chamber was pumped to a high vacuum of 5x10−6 torr using a turbo molecular pump. Afterwards, Ar gas was introduced into the sputtering chamber through a set of mass flow controllers using the flow rate of 10 sccm (standard cubic centimeters per minute). The working pressure was set at 5 mtorr, and all the samples were deposited at room temperature during the sputtering process. To form the 100 nm-thick ZnO-SiO2 nanocomposite films, SiO2 and ZnO were sputtered and deposited simultaneously on the p-GaN substrate. The RF power of the SiO2 target and DC power of the ZnO target were 125 and 75 W, respectively. A 100 nm-thick ZnO film was deposited on p-GaN with 75 W DC power as an active layer of the ZnO-based heterostructure LEDs. On the other hand, a 120 nm-thick Ga:ZnO was deposited on the ZnO-SiO2 nanocomposite, ZnO, and a p-GaN layer on the n-type layer of LEDs for comparison. Photolithography and buffer oxide etching solution were subsequently used to partially etch out the Ga:ZnO/ZnO-SiO2 nanocomposite, Ga:ZnO/ZnO, and Ga:ZnO until the p-GaN layer was exposed. Ni/Au (50 nm/200 nm) was deposited on p-GaN by evaporation to form ohmic contact with the p-electrode of LEDs. On the other hand, Cr/Au (50 nm/200 nm) was deposited on Ga:ZnO by evaporation to form ohmic contact with the n-electrode of LEDs. The microstructure of ZnO-SiO2 nanocomposite was examined using high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 1 , the size of all fabricated Ga:ZnO/ZnO-SiO2 nanocomposite/p-GaN (i.e., LED I), Ga:ZnO/ZnO/p-GaN (i.e., LED II), and Ga:ZnO/p-GaN (i.e., LED III) LEDs was kept at 300 x 300 μm2. An HP 4156 semiconductor parameter analyzer was then used to measure the current-voltage (I-V) characteristics of all fabricated LEDs. Electroluminescence (EL) spectra of all fabricated LEDs were also measured at room temperature.
3. Results and discussions
Figure 2 indicates the I-V curves of the fabricated LEDs I, II, and III. All samples exhibited a rectifying, diode-like behavior. Results revealed that the forward turn-on voltages of LED I, II, and III were 4.16, 3.70, and 3.14 V (at 50μA), respectively. On the other hand, the reverse breakdown voltages of LEDs I, II, and III were −13.6, −10.0, and −4.1 V (at −50μA), respectively. The Ga:ZnO/p-GaN heterojunction LEDs showed that the lowest turn-on voltage compared to the Ga:ZnO/ZnO/p-GaN and Ga:ZnO/ZnO-SiO2 nanocomposite/p-GaN p-i-n heterojunction LEDs. This phenomenon may due to the high resistance of the i-ZnO and i-ZnO-SiO2 nanocomposite layer increased the turn-on voltage of the p-i-n heterojunction LEDs. Meanwhile, the ZnSiOx compound might be formed in the ZnO and SiO2 of the co-sputtered ZnO-SiO2nanocomposite layer. The ZnSiOx compound in the ZnO-SiO2 nanocomposite layer might result in higher resistivity than in the i-ZnO layer, causing the Ga:ZnO/ZnO-SiO2 nanocomposite/p-GaN p-i-n heterojunction LEDs to have the highest turn-on voltage.
Figure 3 demonstrates the EL spectra of LEDs I, II, and III driven at 9 mA. The main peak wavelength of EL spectra of LEDs I, II, and III were 376, 415, and 423 nm, respectively. LEDs II and III indicated single peak broadband emission from 360 to 550 nm. Meanwhile, the broadband EL emissions of LEDs II and III were from the combination of the deep-level carrier recombination in the p-GaN layer and the carrier recombination between n-ZnO and p-GaN. On the other hand, LED I indicated multi-peak EL emission with peak wavelengths EL of 376, 391, and around 420 nm, which were close to the near-band edge emission of the ZnO and deep-level carrier recombination in the p-GaN layer, respectively. However, the co-sputtered i-ZnO-SiO2 nanocomposite layer had an optical bandgap of 4.92 eV. The emission of the 376 and 391 nm were both less than the 4.92 eV of the ZnO-SiO2 nanocomposite layer optical bandgap, thereby implying that the ZnO nanosized clusters might form in the ZnO-SiO2 nanocomposite layer. Chen et al.  and Shih et al.  has reported that the ZnO-SiO2 nanocomposite layer was formed by ALD ZnO on spin coated SiO2 nanoparticles. ZnO nanodots could be form in the voids between SiO2 nanoparticles because that the precursors in the ALD process can penetrate through the thick closely stacked SiO2 nanoparticle layer to fill the small voids near the substrate. They found that ZnO nanodots exhibited a photoluminescence (PL) peak at 370 nm and the PL intensity of ZnO nanodots is less than the n-ZnO layer. The EL peak wavelength of 376 nm form cosputtered i-ZnO-SiO2 nanocomposite in our present study is close to the PL peak of ZnO nanodots of 370nm. Therefore, EL emission at 376 nm should be related to the ZnO nanoclusters in the cosputtered i-ZnO-SiO2 nanocomposite layer. However, Chen et al. did not observe the EL emission from ZnO nanodots in the ZnO-SiO2 nanocomposite layer but strong EL emission peak at 387nm from n-ZnO.
Figure 4 displays the room temperature EL spectra of the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN LEDs for five different injection currents. The double EL peak emission of LED I was observed for all the driving currents. The EL emission of LED I with wavelengths of 376 and 427 nm was found at 1 mA driving current. However, the intensity of the 376 nm emission at 1 mA was much less than for the 427 nm emission. The short wavelength EL emission of 376 nm could be from the ZnO clusters inside the co-sputtered i-ZnO-SiO2 nanocomposite layer. Both the 376 and 427 nm emission intensities of LED I increased with higher driving current. However, the intensity of the 376 nm emission of LED I was greater than that of the 427 nm emission when the driving current was larger than 7 mA. Under low driving current, such as 1 mA, the main EL emission peak at 427 nm of LED I could be attributed to the emission from the Mg acceptor levels in the p-GaN layer . The EL main peak emissions of LED I turned from 427 to 376 nm with the high driving current, which could be attributed to the strong electron-hole recombination in the cluster size of the ZnO in i-ZnO-SiO2 nanocomposite layer under high injection current.
The inset presents the light output-current (L-I) characteristics of the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN LEDs, which were obtained from a direct measurement of the peak emission intensity at 376 nm. As shown in a log-log scale, the L-I results can be fitted with the power law L~Im. The power exponent m is ~1. The obtained linear value of m is the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN LEDs, and this result reveals the high efficiency of the carrier recombination that occurred in the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN LEDs . Meanwhile, the structure of ZnO nanodots embedded in the amorphous SiOx matrix provided a larger energy barrier (ZnSiOx or SiOx), which improved the carrier confinement phenomenon in the i-ZnO-SiO2 nanocomposite layer.
From the EL emission results, the ultraviolet emission occurred from the ZnO clusters embedded in the ZnO-SiO2 nanocomposite can be realized. To understand clearly whether ZnO nanoclusters were in the i-ZnO-SiO2 nanocomposite layer, an HRTEM was used on the ZnO and SiO2 that co-sputtered the i-ZnO-SiO2 nanocomposite layer, as shown in Fig. 5 . A nanocrystallized material was found inside the i-ZnO-SiO2 nanocomposite layer in the HRTEM picture, as shown in Fig. 5 as well. The present study shows that the regular atom spacing of the nanocrystallized material was 0.248 nm, which should be correlated with the spacing of the (101) ZnO. Moreover, the sizes of the ZnO nanoclusters were distributed from 2 to 7 nm. Under low current injection, the injected carriers in the i-ZnO-SiO2 nanocomposite layer recombined in the ZnO nanocluster and came out the wide EL emission spectra in wavelength range of 360 to 400nm. However, under high current injection the injected carriers in the i-ZnO-SiO2 nanocomposite layer recombined efficiently in the high quantum confined ZnO nanoclusters and turned out the strong and sharp EL emission spectra with a peak wavelength of 376 nm.
In summary, we have observed the EL emission of Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN LEDs, and the double EL peak emission of LED I has been observed for all the driving currents when compared with LED II and III. The EL emission of LED I with wavelength 376 and 427 nm was found under low driving current. The long wavelength EL emission of 427 nm and the short wavelength EL emission of 376 nm should be attributed to the illumination of the p-GaN layer and the ZnO clusters inside the co-sputtered i-ZnO-SiO2 nanocomposite layer, respectively. The short EL wavelength emission of LED I increased in intensity as the driving current was increased. Meanwhile, nanocrystallized material was found inside the i-ZnO-SiO2 nanocomposite layer in the HRTEM picture, which revealed that the sizes of the ZnO nanoclusters were distributed from 2 to 7 nm. The distributed size of ZnO nanoclusters caused the wide EL spectra in the short wavelength range of LED I under the low current injection. However, the intensity of ZnO nanoclusters related 376 nm EL emission of LED I is greater than that of the 427 nm by driving at 9 mA.
The authors would like to acknowledge the financial support of the National Science Council for the research Grant Nos. 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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