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Abstract
We construct the ZnO-based superluminescent light-emitting diodes (SLEDs) by spin-coating ZnO nano-particles onto p-GaN/sapphire substrate. By inserting another thin Al layer to form an n-ZnO/Al/n-ZnO/p-GaN sandwich structured SLD, the intensities of the photoluminescence and electroluminescence were greatly enhanced, which can be attributed to the surface plasmon resonance of this Al layer. The tendency of the intensities of the entire electroluminescence spectra shows a super-linearly behavior with increasing the forward bias. Besides, the spectral bandwidth is narrowed down enormously owing to the achievement of the SLD. Furthermore, the interfacial emissions between ZnO/GaN are effectively suppressed by partially oxidizing the Al layer.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As the world has hungered for a better performance of ultraviolet light-emitting diode (UV LED) devices, new materials created and engineered for specific applications to satisfy the demand, such as gallium nitride (GaN) and zinc oxide (ZnO). Zinc oxide is being developed and actively researched by both academic and commercial sectors recently due to its large exciton binding energy (60 meV) and wide direct band gap (3.37 eV) [1–4]. These properties make ZnO to be widely used as photonic devices within UV spectral range in room temperature (RT) [5–8]. However, the fabrication of homojunction ZnO LEDs is a challenge hindered by the bottleneck of p-type doping [9–11] and difficulty to achieve reproducible and stable p-type ZnO material [12-13]. In this situation, heterojunction LEDs based on n-ZnO and other p-type materials such as p-Cu2O [14], p-Si [15] and p-GaN [4,16,17,18,19] have been regarded as an alternative approach. Among them, n-ZnO/p-GaN heterojunction has mostly been suggested due to their virtue of similarity crystal structure (wurtzite) and closely matched lattice constant (∼1.8%).
However, the electroluminescence (EL) of the n-ZnO/p-GaN heterojunction LED is usually occupied by the blue emissions from the interface between GaN and ZnO and/or from the p-GaN region. This is because the higher mobility of electrons in ZnO than that of holes in p-GaN, electrons and holes are prone to recombine more in the p-GaN for the usual n-ZnO/p-GaN heterojunction LEDs. In other words, the EL emissions mainly originates from the p-GaN side rather than the n-ZnO since the electron injection from n-ZnO dominates over the hole injection from p-GaN. To increase the recombination rate of electron-hole in ZnO-side, several ways are recommended, such as surface plasmon resonance (SPR) and electrons blocking/retarding method [20–22]. For examples, the SPR of platinum (Pt) [23], gold (Au) [24,25], silver (Ag) [26,27] and aluminum (Al) [28] are undeniably an effective way to optimize the PL and EL performance of ZnO near band edge (NBE) emissions. There are a number of reports been conducted to enhance the NBE emissions from ZnO by using various metals. An improvement of lasing performance was also reported due to the synergistic energy coupling of graphene/Al surface plasmons with ZnO excitons [29]. A pure EL from the ZnO NBE was also observed from n-ZnO/AlN (20 nm) /p-GaN LEDs by blocking the flow of electrons from ZnO to p-GaN [30]. However, many researchers mostly use noble metals to excite surface plasmons (SP), which are always costly. On the other hand, Al accounts for great advantages, not only the SP energy of the Al/ZnO interface that is relatively close to NBE of ZnO than others [31], but also costs low and is abundant in the Earth. Besides, it stored and re-emitted a lot of energy with low absorption in the UV range [32]. Therefore, Al has been highlighted as an important candidate of plasmonic material in the blue or UV spectral range. Moreover, some ZnO-based emitting devices with advanced functionalities remain to be demonstrated. For instance, the superluminescent LED (SLED), an emitting device combining the beam directionality of laser diodes with the broadband emission of LEDs. SLEDs utilize the process of amplified spontaneous emission (ASE) where light of spontaneous emission can be amplified only within a single pass in the device. In this paper, heterojunction SLED devices decorated with aluminum had been fabricated. To assess the optical characteristics and crystalline qualities of ZnO NPs, PL measurements demonstrated that more than 20-fold intensity enhancement of the spontaneous emissions from Al-decorated ZnO NPs. Meanwhile, heterojunction SLED devices comprised of ZnO NPs, Al and p-GaN substrate show an excellent diode behavior. By increasing forward bias, the dominant EL emissions turned from n-ZnO/p-GaN interfacial emissions into typical ZnO NBE emissions, while the intensity was enhanced more than 24-fold by comparing with the device without Al decoration. Especially, by partially oxidizing the Al layer, the intensity of the p-GaN emission was effectively suppressed. A possible mechanism for the observed EL performance was proposed.
2. Experimental section
The ZnO nanoparticles were synthesized using a two-stage reaction process sol-gel method with zinc acetate dehydrate (ZnAc2·2H2O) and diethylene glycol (DEG) [33,34]. In the first stage, 0.109755 g of ZnAc2·2H2O and 0.1 g NaOH were dissolved in ethanol solution. Then, in stage two, 1.0975 g of ZnAc2·2H2O was mixed with 50 ml diethylene glycol (DEG). They were subsequently heated until 180°C and fixed at 180°C for 1 hour under vigorous stirring. At the same time, 5 ml of the alkaline solution from the first stage was centrifuged (6000 rpm for 30 min) and the supernatant fluid was then slowly added into DEG solution when the temperature reached 150°C. The white suspension (ZnO NPs) was obtained and cooled to room temperature (RT) naturally when the reaction ended.
Three types of devices were fabricated. For the first type, n-ZnO/p-GaN LED device which referred to as ‘reference device’ was constructed by directly spin-coating the ZnO NPs on top of the commercially available p-GaN. The thickness of this ZnO NPs layer is around 400 nm. For the second device, a 5 nm aluminum layer which was deposited by PVD method and then straightforwardly nitrogen-annealed under 300°C for 10 minutes, was inserted between ZnO NPs layer, presenting a n-ZnO/Al/n-ZnO/p-GaN sandwich-structured device. As for the third device, the only difference from the second device was that the environment was surrounded by oxygen while annealing to partially oxidize the Al layer. Other parts of the device were the same as the one decorated with only Al layer, presenting a n-ZnO/Al@Al2O3/n-ZnO/p-GaN sandwich structured device. Finally, ITO glass was directly contacted with the ZnO NPs layer.
The morphology and structure of the samples were characterized by field emission scanning electron microscopy (FE-SEM). The EL and PL were collected via a 5X objective lens with a numerical aperture of 0.13 (LMU-5X-NUV, OFR) from the ITO cathode side. All spectra were measured under the same experimental conditions for comparison. The PL measurement were performed by excited He-Cd laser with wavelength of 325 nm and analyzed by a high-spectral-resolution spectrometer (HORIBA Jobin-Yvon iHR320) with a 600 mm−1 grating at the room temperature. The I-V curve characteristics and electrical properties were measured by Keithley 2400 source meter.
3. Results and discussion
Figure 1(a) shows the top view FE-SEM image of the as-grown ZnO NPs. These ZnO NPs were spin-coated on a clean Si substrate. They were identified with a mean diameter of about 12 nm [35,36]. Figure 1(b) plots the optical absorption and PL spectra of the as grown ZnO NPs. The sample showed a sharp absorption edge of 3.42 eV, which was greater than the bandgap value of ZnO bulk material due to the quantum confinement effect [37]. As a result, the PL spectra of these ZnO NPs showed a strong UV emission centered at 376 nm, which was originated from the NBE emissions of ZnO. Besides, there was an extremely weak green emission centered at ≈ 520 nm, which has been known to originate from the intrinsic defects of ZnO such as oxygen vacancies, oxygen interstitial, zinc vacancies, zinc interstitial and extrinsic impurities [38–40]. The full-width half-maximum (FWHM) of the UV peak was approximately 18 nm. In short, we could conclude that the ZnO NPs were guaranteed to have a high crystallinity.
Fig. 1. (a) SEM images of the Sol-Gel synthesized ZnO NPs (b) Optical absorption (blue) and PL (black) spectra of the ZnO NPs. (c) Room temperature PL spectra of the ZnO decorated with (red) and without (black) Al layer. (d) The absorption spectrum of Al layer.
To enhance the optical property of ZnO NPs, ZnO NPs was deposited with a 5 nm Al layer. As shown in Fig. 1(c), the sample presented strong NBE emissions with more than 20-fold enhancement compared to the bare ZnO NPs. To understand the effect of Al layer on the enhancement ratio of the intensity of the NBE emissions, the optical transmittance spectrum of Al layer was measured. As shown in Fig. 1(d), there was an obvious transmittance deep existed in ZnO NBE region [41]. As aforementioned, the high quality ZnO NPs within the UV SLED devices, which can be improved by the surface plasmon effect of the Al layer.
Figure 2(a) and Fig. 2(b) show the room temperature EL spectra of the device without/with Al decoration under different forward bias voltages. These spectra were composed of the EL from n-ZnO and p-GaN layers under forward bias. The insets are the structure schematic diagram of devices and show the FWHM and integrated EL intensity of two devices as a function of forward bias. To construct the n-ZnO/p-GaN SLED device, ZnO NPs were directly spin-coated onto the p-GaN substrates, referred to as ‘reference device’. The thickness of ZnO NPs layer is about 400 nm. To fit the SPR between Al and ZnO NPs, a 5 nm Al thin layer, which was prepared by PVD method and was followed by a 300°C-annealing process for 10 minutes, was inserted into the ZnO NPs layer, presenting an n-ZnO/Al/n-ZnO/p-GaN sandwich structured device. The intensity of the emission did not increase rapidly with the voltage for n-ZnO/p-GaN SLED devices. As shown in Fig. 2(a), its EL intensity was relatively weak, leveled off and has almost none spectral narrowing under forward bias. However, the spectral intensity grew rapidly with the increment of forward bias from 10 V to 50 V for n-ZnO/Al/n-ZnO/p-GaN SLEDs, as shown in Fig. 2(b). We applied voltages as the common variable in this work. From the inset of Fig. 2(b), the EL intensity was increasing in a super-linear tendency under forward bias. In contrast with the above-mentioned structure, the I-V characteristic of the n-ZnO/p-GaN structure only shows approximately a linear behavior as shown in the inset of Fig. 2(a). In addition, the FWHM of its spectra decreased from 34 nm under 10 V to 17.5 nm under 50 V. This remarkable EL performance of sandwich structured n-ZnO/Al/n-ZnO/p-GaN devices showed a superluminescenct-diode behavior [42,43]. The corresponding current versus voltage (I-V) characteristics of the SLEDs before and after Al decoration were shown in Fig. 2(c), in order to visually illustrate the behavior of the heterojunction diode. Both SLEDs demonstrated typical rectifying diode-like behavior under applied bias. The forward bias turn-on voltages of these devices with and without Al decoration were 6 and 7.5 V, respectively. Both forward and reverse currents were increased under the same bias voltage after depositing Al. These results indicated that the decoration of Al layer on ZnO NPs has effectively formatted more conduction pathways in our semiconductor device, resulting in the decreased series resistance of the device decorated with Al layer. Figure 2(d) shows the room temperature EL emission spectra of the bare and Al-decorated devices under 35 V forward biases, inset is the normalized spectra of both devices. Both devices exhibited UV emission bands, which were shorter than 400 nm. However, for the bare devices, EL spectrum showed a dominant peak centered at 390 nm with FWHM of 35.4 nm. The EL spectra exhibited a 7 nm blue-shifted to 381 nm and the FWHM was narrowed down to 20 nm after the decoration of Al layer. By referring to the PL spectra, it was reasonable to assume that the NBE emission of ZnO NPs was evidently dominating in the device emission after decorated with Al layer.
Fig. 2. RT EL spectra without (a) and with (b) Al decoration. The insets are the structure of devices, and the FWHM and integrated EL intensity under forward bias. (c) The I-V curves decorated with (red) and without (black) Al. (d) RT EL (35 V) spectra of the devices decorated with (red) and without (black) Al; the inset shows the normalized EL spectra of both devices.
We further analyzed each specific luminescence components more systematically under different forward bias by Gaussian deconvolution fittings. As shown in Fig. 3(a), the typical EL spectra of n-ZnO/p-GaN and n-ZnO/Al/n-ZnO/p-GaN were decomposed into three Gaussian parts, which were the NBE recombination of ZnO, interface carrier recombination of ZnO/GaN, and the carrier recombination in p-GaN [15]. It could clearly be seen in Fig. 3(b) that the EL intensity from the p-GaN increases slightly as a function of the applied voltage and saturates as applied voltage higher than ∼40 V. On the other hand, the increasing trend of the interface emission can be obtained, while the intensity of ZnO NBE emissions increase rapidly with the following forward bias. The growth tendency of ZnO NBE emissions was almost resembled to the entire EL spectrum, which can attribute to the resonant coupling between excitons of ZnO and surface plasmons of Al. In addition, the FWHM of the ZnO NBE emissions decreases from 18.5 nm at 10 V to 10.8 nm at 50 V with an increasing percentage of integrated intensity, as shown in the inset of Fig. 3(b), which indicates the onset of amplified spontaneous emission of ZnO. In the case of n-ZnO/Al/n-ZnO/p-GaN sandwich structured device structured device, the couple between excitons of ZnO NPs and surface plasmons of Al induces surface plasmon amplification from gain medium. Consequently, the spectral bandwidth narrows down enormously. Furthermore, the domination of ZnO NBE emissions diminished the recombination of other sources in the device, which lead to the blue shifting of the EL spectrum.
Fig. 3. (a) Gaussian fitting result of the device decorated with Al layer under 35 V bias. (b) The integrated intensity of each sub-component. The inset is the FWHM and integrated intensity of ZnO NBE under forward bias.
However, Al is an active metal, which can react spontaneously with water and air to form aluminum oxide [44]. There were some groups indicated that aluminum oxide (Al2O3) is a dielectric material that is suitable to be used as an electron blocking layer in the n-ZnO/Al@Al2O3/n-ZnO/p-GaN heterojunction because Al2O3 could become a tunnel to retard or even block the electrons [22,45,46]. Even though most of those electrons could still tunnel through the thin Al2O3 layer, their kinetic energy reduces during the tunneling process as electrons experienced a strong scattering with Al2O3 layer. As a result, the location for electrons and holes to recombine could be close to n-ZnO region. In short, Al layer in its nature transforms itself to Al2O3 easily. We thus get a step further to analyze the effect of Al2O3.
Figure 4(a) illustrates the comparison between PL spectrum of ZnO NPs and EL spectrum of the n-ZnO/Al@Al2O3/n-ZnO/p-GaN SLED. The inset of Fig. 4(a) is the structure schematic diagram of the n-ZnO/Al@Al2O3/n-ZnO/p-GaN SLED, which was no different from the device decorated with Al layer, except that the Al layer was surrounded by oxygen during the annealing process. Something noteworthy was that the shape of EL spectrum is very much alike to the PL of ZnO NBE emissions, but in a longer wavelength due to Joule’s heating effect. By partially oxidizing the Al layer, both the forward and reverse currents gradually decreased. The device exhibited a larger turn-on voltage (10 V) and series resistance (22 kΩ), as shown in the inset of Fig. 4(b). These phenomena indicated that the Al layer was indeed oxidized and the introduced thin Al2O3 layer has played a role of the charge blocking layer. Despite the increment of resistivity, the EL spectra changed in a good way. As mentioned previously in Fig. 2(b), the FWHM of the device decorated with Al layer was 19.6 nm under 35 V forward bias. As shown in Fig. 4(c), its FWHM got even narrower (18.35 nm) when the Al layer was partially oxidized. Besides, the emissions from 410 nm to 440 nm became weaker. However, after partially oxidizing the Al layer, the emission peak appeared at a longer wavelength by comparing to the device without oxidizing that Al layer. The redshift was deduced by the Joule’s heating effect because the resistivity of device increases after oxidizing the Al layer. There were other reports indicated a similar EL redshift phenomenon when they studied p-GaN/insulating layer/n-ZnO heterojunction LEDs [41,47,48].
Fig. 4. (a) The PL spectrum of ZnO (black) and the EL spectrum (red) of the device with partially oxidized Al layer under 35 V. Inset is the structure of the device with partially oxidized Al layer. (b) The inset is the I-V curve of the devices decorated with (blue), without (black) Al layer, and with partially oxidized Al layer (red). (c) Normalized EL spectra of three devices under 35 V bias.
To understand the details about the emission behavior, the emission peak deconvolution was executed. As shown in Fig. 5(a), each Gaussian curves fitted well with experimental curves. Figure 5(b) shows the analysis of fitting results in Fig. 5(a). The ratio of the interfacial recombination to the ultraviolet emission from ZnO NPs decreased when the device decorated with Al layer, which meant that an effective SPR coupling has occurred between SP of Al layer and the NBE of ZnO [28]. Nevertheless, ZnO emissions occupied almost the whole of the device emission after partially oxidized the Al layer.
Fig. 5. (a) Gaussian fitting result of three devices under 35 V bias. (b) The ratio of each sub-component in the entire emission with partially oxidized Al layer (red), only Al layer (blue) and without any decoration (black).
The mechanism could be understood in terms of energy alignment in Fig. 6. Under forward bias, the injected electrons and holes could easily enter into the p-GaN and n-ZnO respectively. Then, radiative transitions could occur in both materials. However, as for a bare device, the entire EL performance was mostly brought out by the carrier transport and radiative transitions in the p-GaN region. This was because the electron mobility was higher than the hole mobility owing to the fact that electron has a smaller effective mass. As a result, the injected electrons from n-ZnO to p-GaN would overbear the injected holes from p-GaN to n-ZnO, resulting in a greater radiation recombination rate happened in the p-GaN region and the interface between GaN/ZnO. Thus, there was only a small percentage of ZnO NBE emissions detected during the measurement, which agreed with our experimental results that the bare device peaked at 390 nm. However, the introduction of partially oxidized Al leads to changes of valence band and conduction band offsets between two ZnO layers, which retarded the flow of electrons and waited for holes to enter into ZnO region [45]. Thus, electron-hole recombination occurred mostly in the ZnO layer. In the meantime, the leftover of Al could act as surface plasma to enhance the emissions of ZnO.
Fig. 6. The energy band alignments and carrier dynamic behaviors of the n-ZnO/p-GaN (left) and n-ZnO/Al@Al2O3/n-ZnO/p-GaN (right) devices under forward bias, respectively.
4. Conclusion
In summary, we have demonstrated more than 20-fold intensity enhancement of the NBE emissions of ZnO NPs based heterojunction SLED by decorating Al layer. By partially oxidized this Al layer, the device can be optimized to effectively suppress the growth of interfacial GaN/ZnO and p-GaN emissions. Meanwhile, the partially oxidized Al layer can still provide surface plasma to enhance the NBE emissions of ZnO. We believe that this intuitive fabrication process will become a promising method for developing a highly efficient pure ZnO-based emission devices in related applications.
Funding
Ministry of Science and Technology, Taiwan (105-2112-M-006-004-MY3, 108-2112-M-006-005).
Acknowledgments
This research was also, in part, supported by the Ministry of Education, Taiwan. Headquarters of University Advancement to the National Cheng Kung University (NCKU). The authors thank Prof. Jinn-Kong Sheu and Prof. Wei-Chih Lai for preparation of GaN epitaxial films.
Disclosures
The authors declare no conflicts of interest.
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