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n-GaN/i-ZnO/p-GaN double heterojunction diodes were constructed by vertically binding p-GaN wafer on the tip of ZnO nanopencil arrays grown on n-GaN/sapphire substrates. An increased quantum confinement in the tip of ZnO nanopencils has been verified by photoluminescence measurements combined with quantitative analyses. Under forward bias, a sharp ultraviolet emission at ~375 nm due to localized excitons recombination can be observed in ZnO. The electroluminescence mechanism of the studied diode is tentatively elucidated using a simplified quantum confinement model. Additionally, the improved performance of the studied diode featuring an ultralow emission onset, a good operation stability and an enhanced ultraviolet emission shows the potential of our approach. This work provides a new route for the design and development of ZnO-based excitonic optoelectronic devices.
© 2016 Optical Society of America
1. Introduction
By virtue of its wide band gap (3.37 eV) and high exciton binding energy (60 meV), ZnO has been considered as one of important optoelectronic semiconductor materials [1–4], especially for the short wavelength excitonic devices [5–8]. However, the pursuit of stable and reproducible p-ZnO is still a challenge at present, which seriously restricts the development of ZnO-based homojunction devices [9,10]. Therefore, many studies have been devoted to development of ZnO-based p-n heterojunction structures by growing n-type ZnO on other p-type materials [11–16]. Among them, n-ZnO/p-GaN heterojunction is the most common configuration in consideration of the same crystalline structure, a small lattice mismatch (~1.8%), and a similar magnitude of the bandgap energy. Furthermore, it has been shown that one-dimensional (1D) ZnO nanostructures usually exhibit superior and additional properties compared to bulk and thin films because of a large surface-to-volume ratio, high crystalline quality and increased quantum confinement effect [17–19]. Thus, markedly improved performances are expected from 1D ZnO nanostructures-based light-emitting diodes (LEDs) [20]. However, to date, the electroluminescence (EL) performance of the proposed n-ZnO nanostructures/p-GaN heterojunction diode is still not so efficient as people imagined. On the one hand, the non-perfect ZnO/GaN interface with a high density defects leads to degraded internal quantum efficiency and operating stability [21]. On the other hand, although room temperature photonic devices benefit from the strong exciton localization, the fabrication of 1D ZnO nanomaterials with quantum confinement is still a challenge because the exciton Bohr radius (~2.4 nm for bulk ZnO) is in the range of few lattice constants [22]. As a result, rare EL emission associated with localized exciton has been achieved from 1D ZnO nanostructures-based LEDs. In addition, from an application point of view, high quality epitaxy and near-perfect interface are also cited as key factors in the quest to improve ZnO-based heterojunction LEDs because defects impurities usually act as nonradiative recombination centers. In order to enhance the carrier recombination efficiency, Li et al. have demonstrated a simple but feasible direct-contact method to fabricate n-ZnO/p-GaN heterojunction LEDs [23].
In this work, vertically aligned ZnO nanopencils (NPs) arrays were grown on n-GaN/sapphire substrates by photo-assisted metal-organic chemical vapor deposition (PA-MOCVD). An increased quantum confinement in the tip of ZnO NPs has been verified by photoluminescence (PL) measurements combined with quantitative analyses. When the ZnO NPs and p-GaN were fixed together, a ZnO NPs-based LED was constructed. Under forward bias, a sharp UV emission at ~375 nm has been achieved, and it is ascribed to the localized excitons recombination in the tip of ZnO NPs. Additionally, the EL performance of the proposed diode is remarkable in terms of its ultralow emission onset, good stability and enhanced ultraviolet emission.
2. Experimental
2.1 Growth of ZnO NPs
ZnO NPs were grown on n-GaN/sapphire substrates by using PA-MOCVD system. The details about the reactor of PA-MOCVD system can be found elsewhere [24]. In this experiment, a two-stage growth method was employed. Diethylzinc (DEZn), high-purity oxygen (O2) were used as the source precursors for Zn and O, respectively. The argon was applied as the carrier gas during the epitaxial growth process. First, ZnO buffer layer was grown at a low temperature of 400 °C for 3 min, and the flow rates of DEZn and O2 were kept at 8.15 nmol/min and 6.64 μmol/min, respectively. Then, ZnO main layer was prepared at a high temperature of 550 °C for 30 min. The flow rates of DEZn and O2 were supplied at 8.15 nmol/min and 8.03 μmol/min, respectively. The reaction pressure during both growth stages was maintained at ~77 Pa. Consequently, vertically aligned ZnO NPs arrays were produced.
2.2 Fabrication of LEDs
n-GaN/i-ZnO NPs/p-GaN heterojunction LEDs were fabricated by vertically binding p-GaN wafer on the tip of ZnO NPs grown on n-GaN/sapphire substrates. Note that the electron and hole concentrations for n-type and p-type GaN films used here were 6.8 × 1017 cm−3 and 2.5 × 1017 cm−3, respectively. The detailed fabrication processes of the proposed LEDs were as follows. First, Ni (10 nm)/Au (80 nm) and Au (30 nm) were deposited by thermal evaporation method on p-GaN and n-GaN as contact electrodes, respectively. Subsequently, vertically aligned ZnO NPs grown on n-GaN/sapphire substrates were placed face to face on the p-GaN wafer, and then they were fixed together by binder clips to form a close and fine physical contact. The resulting sample was marked as device A. For comparison, a reference LED based on n-GaN/i-ZnO nanorods (NRs)/p-GaN structure, denoted as device B, was fabricated. Details on the growth of ZnO NRs arrays can be found elsewhere [25], and the fabrication method of device B is same as that of device A. The corresponding schematic diagrams of device A and B are shown in Figs. 1(a) and 1(b), respectively.
Fig. 1 Schematic diagrams of the fabricated n-GaN/i-ZnO/p-GaN double heterojunction diodes: (a) device A, (b) device B.
2.3 Characterizations
The crystallinity and morphology of the produced samples were characterized by X-ray diffractometer (XRD; Rigaku Ultima IV) and field emission scanning electron microscopy (SEM; Jeol-7500F), respectively. The PL spectra were recorded using a monochromator (Zolix Omni-λ 500) with a He-Cd laser (325 nm, 20 mW) as the excitation source. The current-voltage (I-V) characteristics of the devices were measured by using a semiconductor characterization analyser (Aligent B2900A) and EL spectra were recorded using a home-made acquisition equipment including a photomultiplier tube and a lock-in amplifier system.
3. Results and discussion
Figure 2(a) shows the cross-sectional SEM image of the ZnO NPs grown on n-GaN templates. The average height of ZnO NP is about 1 µm. As shown in the inset of Fig. 2(a), we can clearly see that the NP is composed of a large-diameter hexagonal stem and a small-diameter hexagonal tip. The diameter of the stem is about 70 nm and that of the top tip is about 10 nm. Thus, the density of the produced ZnO NPs is about 1010 cm−2. For comparison, the typical cross-sectional SEM image of the produced ZnO NRs is presented in Fig. 2(b). Well-aligned ZnO NRs, with an uniform diameter of about 90 nm and a length of about 2 µm, can be observed. By contrast with NPs, the NRs feature a typically columnar structures and an absence of the sharp tip. In order to further examine the structural properties of the produced ZnO NPs, XRD measurements were performed. Figure 2(c) shows XRD pattern of ZnO NPs grown on GaN/sapphire substrates. Only respective (0002) planes of ZnO and GaN, and (0006) plane of c-Al2O3 appear, indicating that ZnO NPs are preferentially oriented in c-axis direction. Meanwhile, XRD-ω scans are also performed to check the degree of alignment to the normal direction of the substrate surface. A relatively narrow full width at half-maximum (FWHM) of 1030 arcsec further implies the excellent ordering of the resulting ZnO NPs along the growth direction. In addition, the PL spectra of the produced NPs and NRs samples are shown in Fig. 2(d). Both samples present excellent optical quality with a dominant near-band-edge (NBE) excitonic emission as well as almost negligible defect-related deep level emission in the visible range. Additionally, compared with NRs, the NBE emission intensity of NPs is enhanced by a factor of around 1.7. The cause will be explained later. From the normalized UV region spectrum shown in the inset of Fig. 2(d), it is interesting that the NBE emission of ZnO NPs is located at ~376 nm, with a blue shift of ~2 nm compared to that of the ZnO NRs at ~378 nm. Similar phenomena have been observed before and was attributed to various reasons such as the laser heating effect [26], the presence of uniaxial stress/strain [27], and the contributions of excitonic emission and their phonon replicas [28]. However, we consider that all of the existing explanations can’t account for our observation here, and the detailed discussions are as follows: (1) In our experiment, the light output power of the He-Cd laser was kept at 20 mW during the whole testing process. Thus, the effects of laser heating on both NPs and NRs samples are identical. (2) For both ZnO NPs and NRs, the uniaxial stress/strain can be well relaxed during 1D growth stage. Hence, the effect of uniaxial stress/strain on the position shift of PL peak can be ignored. (3) It has been demonstrated that the increased contributions of phonon replicas may lead to an obvious broadening of bandwidth [28,29]. However, as shown in the inset of Fig. 2(d), the FWHM of UV emission of ZnO NPs and NRs is ~11.4 nm and ~11.8 nm, respectively. This implies that the position shift of PL peak can’t be ascribed to the contributions of excitonic emission and their phonon replicas. Furthermore, He et al. have proposed that obvious quantum confinement can be observed in 1D ZnO nanowires when theirs diameter is less than 4ax (ax is the ground state excitonic radius and it is ~5 nm for bulk ZnO) [30]. As shown in Figs. 2(a) and 2(b), the tip diameter of the produced NPs and NRs is ~10 nm and ~90 nm, respectively. As a result, strong quantum confinement in the tip of NPs can be expected, thereby giving rise to an enhanced NBE emission and a blue-shifted peak position [25]. Taking into account of the impact of quantum confinement, the excitons transition energy () in the tip of ZnO NPs can be approximately described by the following equation [22]:
where Eg is band gap energy (~3.37 eV) of ZnO, is Planck’s constant, is the reduced mass, is the free space permittivity, is the relative permittivity, is the charge on an electron, and D is the tip diameter of ZnO NPs. From this equation, we work out that the value of in the tip of ZnO NPs is equal to ~3.34 eV (~371.2 nm), which is blue shift by ~3.4 nm with respect to the excitonic transition energy [Eg-60 meV, ~3.31 eV (~374.6 nm)] in the ZnO NRs. This is basically consistent with the PL measurement results as shown in Fig. 2(d).Fig. 2 Cross-section SEM images of ZnO NPs (a) and ZnO NRs (b). The inset of (a) shows the enlarged top-view image of ZnO NPs. (c) XRD patterns of ZnO NPs grown on n-GaN/sapphire substrates. The inset shows the (0002) ω-rocking curve of ZnO NPs. (d) PL spectra of ZnO NPs and NRs.
The inset of Fig. 3 presents I-V characteristics of device A and bias-dependence of the integrated EL intensity. The I-V curve exhibits an obvious rectifying behavior with a turn-on voltage of about 2.5 V. Under reverse bias, the current is effectively blocked even at a bias of −10 V (~4-times of the turn-on voltage) and is limited to ~0.8 μA. The forward current at the same voltage is ~120 μA. Therefore, device A is characteristic with a large rectification ratio (~150). The integrated EL intensity as a function of forward bias is also shown in the inset of Fig. 3. An almost linearly increasing trend suggests that the nonradiative recombination through defects do not increase in proportion with operating currents, indicating that device A behaves a good stability. Figure 3 shows the room temperature EL spectra of device A under different operating voltages. The EL onset is estimated to be ~3 V (~1.2 μA), which is the lowest value reported for LEDs using the similar structures [10–19]. The ultralow emission onset implies a near-perfect ZnO NPs/p-GaN interface with a low density of defect states and makes the studied LEDs interesting for further application. Additionally, the EL spectrum of device A shows an enhanced UV emission, featuring a dominant UV emission at ~388 nm and a sharp shoulder at ~376 nm. The detailed carrier recombination mechanisms of device A will be discussed later. As the forward bias increases from 3 to 12 V, the EL peak position is almost unchanged and the corresponding intensity increases quickly. Basing on the above results, we can consider that device A has a good performance of carrier injection and emission stability.
Fig. 3 EL spectra of device A under different forward biases. The inset shows the driving current and emission intensity as a function of the applied voltage.
The inset of Fig. 4(a) presents a colored photo taken from device A at 12 V using a digital camera. From the photograph, numerous light spots can be seen over the active region, indicating that lots of nanoscale heterojunction diodes are formed between the tips of ZnO NPs and the underlying p-GaN wafer. Because the tip diameter of ZnO NPs is ~10 nm, the contact area (S1) of single ZnO NP/p-GaN heterojunction is estimated to be ~78.5 nm2 (assuming top tip of ZnO NPs is a circle). As shown in the inset of Fig. 4(a), the area (S2) of the active region of device A is ~5 × 10−2 cm2 (1 cm × 0.05 cm), while the density (n) of the formed nanoscale heterojunctions should be roughly equal to that of ZnO NPs (~1010 cm−2). Therefore, we can work out that the total contact area between ZnO NPs and p-GaN films in device A is estimated to be ~3.93 × 10−5 cm2 (n × S2 × S1). To clarify the carriers recombination mechanism of device A, a representative EL spectrum measured at 12 V is decomposed with Gaussian functions. As shown in Fig. 4(a), the EL spectrum can be well-fitted by four individual Gaussion peaks, which results in L1 emission at ~375 nm, L2 emission at ~387 nm, L3 emission at ~405 nm, and L4 emission at ~432 nm. By comparing with the PL spectrum of p-GaN films as shown in Fig. 4(b), The L4 emission should be from the transitions in p-GaN associated with Mg-acceptor level. As previously reported [31, 32], the L3 emission can be ascribed to the radiative interfacial recombination of the electrons from ZnO and the holes from p-GaN. The L2 emission located at ~387 nm has also been observed frequently from the EL spectra of n-ZnO/p-GaN heterojunction diode and can be attributed to the transmission from shallow donors to the valence band in ZnO [33]. In addition, the peak position of L1 emission (~375 nm) is almost same as that observed in ZnO NPs PL (~376 nm), which suggests that the rediative recombination occurs in ZnO NPs, and that the L1 emission should be excitonic luminescence in nature. To further clarify the carriers transport and recombination processes in device A, a simplified schematic diagram of ZnO NP/p-GaN heterojunction is presented in Fig. 4(c). Basing on the above results, we can approximatively model the tip of ZnO NPs as cylindrical quantum well (CQW) with different well diameters. The region I represents the CQW with the diameter less than 4ax, behaving an enhanced exciton localization and exciton binding energy due to 1D confinement. The region II represents the CQW with the diameter larger than 4ax. In this region, quantum confinement effect does not play any role. Under forward bias, the holes in p-GaN can transfer into the tip of ZnO NPs and preferentially recombine with electrons in region I. The quantum confinement in region I can convert the injected carriers to localized excitons, contributing to the L1 emission. Besides, the injected holes can also traverse the region I and recombine with electrons in region II, leading to the L2 emission. Meanwhile, the injected electrons can recombine with holes at heterojunction interface and p-GaN side, resulting in the L3 and L4 emissions, respectively.
Fig. 4 (a) Gaussian deconvolution of a representative EL spectrum measured at 12 V. The inset shows the corresponding light emission image. (b) Room temperature PL spectrum of p-GaN films. (c) Schematic diagram of nanoscale ZnO NP/p-GaN heterojunction.
To further study the uniquely spectral behavior, each EL spectrum of device A is separated into four distinct sub-bands according to Gaussian functions as shown in Fig. 5(a). Figure 5(b) presents the intensity ratios of L1/L2 at different biases. It has been reported that the localized excitons can prevent carrier migration toward nonradiative defects and contribute to the higher optical gain because of the enhanced optical efficiency [34]. Therefore, in our case, the injected nonequilibrium carriers may possess higher radiative recombination efficiency in region I, thereby resulting in an increased L1/L2 intensity ratio. Figure 5(c) shows the peak positions of L1, L2, L3 and L4 emissions as function of forward bias. It can be seen that, as the forward bias increases from 3 to 12 V, the peak positions of L1, L2, L3, and L4 emissions are almost unchanged, indicating a good operating stability of the device. In addition, the integrated emission bands intensity as a function of forward bias is plotted in Fig. 5(d). It is found that the intensity of ZnO emission (L1 + L2) increases much faster than other two parts and acts as dominant role under all comparable biases. This further demonstrates that device A possesses a high UV emission efficiency.
Fig. 5 (a) Gaussian deconvolution for all EL spectra of device A at different forward biases. (b) Intensity ratio of L1/L2 at different forward biases. (c) Peak position shift as a function of the operating bias for L1, L2, L3, and L4 emissions. (d) Integrated emission intensity as a function of the operating voltage.
For comparison, a reference LED (device B) was made by growing ZnO NRs arrays on n-GaN/sapphire substrates and then binding the p-GaN wafer on the tips of ZnO NRs. It is noteworthy that both device A and B possess an identical device structure but different active layer size and contact area at the heterointerface. Figure 6(a) shows the EL spectra of device B under different forward biases, which display a dominant UV-blue emission centered at ~396 nm. With increasing the operating voltage, the corresponding EL intensity increases gradually, and meanwhile the dominant UV-blue emission peak is blue shifted obviously. The inset of Fig. 6(a) shows the I-V characteristic curve of device B. It can be seen that the rectification ratio of device B is only ~36 at ± 10 V, which is much smaller than that of device A (~150 at ± 10 V). The degraded rectification behavior of device B can be attributed to the increased contact area at ZnO NPs/p-GaN heterointerface, which increases the interfacial defects [35]. Figure 6(b) shows the Gaussian fitting results of device B under 20 V bias. The emission peak also can be divided into three parts: the peak at ~388 nm is originating from ZnO NBE emission (L2), the peak centered at ~401 nm can be attributed to the recombination of carriers in the interface of ZnO and p-GaN (L3), and the peak located at ~421 nm is from the transitions in p-GaN associated with Mg-acceptor level (L4). Obviously, for device B, the interfacial emission (L3) dominates the EL spectrum, and the localized exciton emission is absent due to a decreased carrier confinement in the tip of ZnO NRs.
Fig. 6 (a) EL spectra of device B under different forward biases. The inset is the I-V curve of device B. (b) Gaussian fitting results of device B under 20 V bias.
4. Conclusions
Heterojunction LEDs based on n-GaN/i-ZnO NPs/p-GaN are designed and fabricated. An increased quantum confinement in the sharp tip of ZnO NPs has been verified by optical methods combined with quantitative analyses. The proposed diode is characterized by ultralow emission onset of ~1.2 μA, large rectification ratio of ~150, good operating stability and highly efficient UV emission. Analysis of EL spectra and carriers recombination mechanism indicates that the quantum confinement in the tip of ZnO NPs can convert the injected carriers to localized excitons, which should account for the ~375 nm EL emission. Our investigation on the unique emission properties of localized exciton in ZnO NPs shed light on developing stable and high-efficiency ZnO-based excitonic optoelectronic devices.
Funding
This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 61106003, 61274023, 61223005 and 61376046), the Science and Technology Developing Project of Jilin Province (20130204032GX, 20150519004JH and 20160101309JC), and the Program for New Century Excellent Talents in University (NCET-13-0254).
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